Title of Invention: Modulation of Axon Degeneration Abstract: The invention relates generally to treatment of neurological disorders and nervous system injuries. The invention specifically provides methods of using modulators of particular target proteins to modulate degeneration of neurons or portions thereof, such as axons. Detailed Description: FIELD OF THE INVENTION This invention relates generally to treatment of neurological disorders and nervous system injuries. The invention specifically concerns the use of modulators of particular target proteins and processes in methods to inhibit neuron and axon degeneration. BACKGROUND OF THE INVENTION Neuron or axon degeneration plays a central role in the proper development of the nervous system and is a hallmark of many neurodegenerative diseases including, for example, amyotrophic lateral sclerosis (ALS), Alzheimer's disease, and Parkinson's disease, as well as traumatic injury to the brain and spinal cord. These diseases and injuries are devastating to patients and caregivers, and also result in great financial burdens, with annual costs currently exceeding several hundred billion dollars in the United States alone. Most current treatments for these diseases and conditions are inadequate. Adding to the urgency of the problems created by these diseases is the fact that many such diseases are age-related, and thus their incidence is increasing rapidly as population demographics change. There is a great need for the development of effective approaches to treating neurodegenerative diseases and nervous system injuries. SUMMARY OF THE INVENTION The invention provides methods for inhibiting degeneration of a neuron or a portion thereof (e.g., the neuron cell body, an axon, or a dendrite). The methods involve administering to the neuron or portion thereof an agent that modulates: (i) the activity or expression of a target protein in the neuron or portion thereof, or (ii) a process in the neuron or portion thereof. Examples of proteins that can be targeted in the methods of the invention include dual leucine zipper-bearing kinase (DLK), glycogen synthase kinase 3β (GSK3β), p38 mitogen-activated protein kinase (p38 MAPK), β-catenin, transcription factor 4 (TCF4), epidermal growth factor receptor (EGFR), phosphoinositide 3-kinase (PI3K), cyclin-dependent kinase 5 (cdk5), adenylyl cyclase, c-Jun N-terminal kinase (JNK), BCL2-associated X protein (Bax), Ih channel, calcium/calmodulin-dependent protein kinase kinase (CaMKK), G-proteins, G-protein coupled receptors, transcription factor 4 (TCF4), or β-catenin, while examples of processes that can be targeted are transcription and protein synthesis. The neuron or portion thereof can consist of or can be within a neuron selected from the group consisting of a cerebellar granule neuron, a dorsal root ganglion neuron, a cortical neuron, a sympathetic neuron, and a hippocampal neuron. The agents can be, for example, inhibitors of the target protein or process. Further, the agents can be, for example, selected from the group consisting of antibodies, polypeptides, peptides, peptibodies, nucleic acid molecules, short interfering RNAs (siRNAs), polynucleotides, aptamers, small molecules, and polysaccharides. In the case of antibodies, the antibodies can be monoclonal antibodies, chimeric antibodies, humanized antibodies, human antibodies, or antibody fragments (e.g., an Fv, Fab, Fab′, or F(ab′)2 fragment). In the case of small molecules, the agent can be, for example, selected from the group consisting of MG132, SB 415286, GSK3β inhibitor I, GSK3β inhibitor VII, GSK3β inhibitor VIII, GSK3β inhibitor XII, Lithium Chloride, SB 202190, SB 239063, SB 239069, SB 203580, SB 203580HCl, AG 556, AG 555, AG 494, PD168393, Tyrphostin B44, Tyrphostin B42 (AG 490), LY 294022, Anisomycin, Cycloheximide, Roscovitine, Forskolin, NKH 477, Actinomycin D, SP600125, Bax Channel Blocker, ZD7288, STO-609, bortezomid, disulfuram, pamapimod, gefitinib, erlotinib, lapatinib ditosylate, demeclocycline hydrochloride, gentamicin sulfate, neomycin sulfate, paromomycin sulfate, and pharmaceutically acceptable salts thereof. The neuron or portion thereof can be present in a subject, such as a human subject. The subject can, for example, have or be at risk of developing a disease or condition selected from the group consisting of (i) disorders of the nervous system, (ii) conditions of the nervous system that are secondary to a disease, condition, or therapy having a primary effect outside of the nervous system, (iii) injuries to the nervous system caused by physical, mechanical, or chemical trauma, (iv) pain, (v) ocular-related neurodegeneration, (vi) memory loss, and (vii) psychiatric disorders. Examples of disorders of the nervous system include amyotrophic lateral sclerosis (ALS), trigeminal neuralgia, glossopharyngeal neuralgia, Bell's Palsy, myasthenia gravis, muscular dystrophy, progressive muscular atrophy, primary lateral sclerosis (PLS), pseudobulbar palsy, progressive bulbar palsy, spinal muscular atrophy, inherited muscular atrophy, invertebrate disk syndromes, cervical spondylosis, plexus disorders, thoracic outlet destruction syndromes, peripheral neuropathies, prophyria, Alzheimer's disease, Huntington's disease, Parkinson's disease, Parkinson's-plus diseases, multiple system atrophy, progressive supranuclear palsy, corticobasal degeneration, dementia with Lewy bodies, frontotemporal dementia, demyelinating diseases, Guillain-Barré syndrome, multiple sclerosis, Charcot-Marie-Tooth disease, prion disease, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome (GSS), fatal familial insomnia (FFI), bovine spongiform encephalopathy, Pick's disease, epilepsy, and AIDS demential complex. Examples of pain include chronic pain, fibromyalgia, spinal pain, carpel tunnel syndrome, pain from cancer, arthritis, sciatica, headaches, pain from surgery, muscle spasms, back pain, visceral pain, pain from injury, dental pain, neuralgia, such as neuogenic or neuropathic pain, nerve inflammation or damage, shingles, herniated disc, torn ligament, and diabetes. Examples of conditions of the nervous system that are secondary to a disease, condition, or therapy having a primary effect outside of the nervous system include peripheral neuropathy or neuralgia caused by diabetes, cancer, AIDS, hepatitis, kidney dysfunction, Colorado tick fever, diphtheria, HIV infection, leprosy, lyme disease, polyarteritis nodosa, rheumatoid arthritis, sarcoidosis, Sjogren syndrome, syphilis, systemic lupus erythematosus, and amyloidosis. Examples of injuries to the nervous system caused by physical, mechanical, or chemical trauma include nerve damage caused by exposure to toxic compounds, heavy metals, industrial solvents, drugs, chemotherapeutic agents, dapsone, HIV medications, cholesterol lowering drugs, heart or blood pressure medications, and metronidazole. Additional examples include burn, wound, surgery, accidents, ischemia, prolonged exposure to cold temperature, stroke, intracranial hemorrhage, and cerebral hemorrhage. Examples of psychiatric disorders include schizophrenia, delusional disorder, schizoaffective disorder, schizopheniform, shared psychotic disorder, psychosis, paranoid personality disorder, schizoid personality disorder, borderline personality disorder, anti-social personality disorder, narcissistic personality disorder, obsessive-compulsive disorder, delirium, dementia, mood disorders, bipolar disorder, depression, stress disorder, panic disorder, agoraphobia, social phobia, post-traumatic stress disorder, anxiety disorder, and impulse control disorders. Examples of ocular-related neurodegeneration include glaucoma, lattice dystrophy, retinitis pigmentosa, age-related macular degeneration (AMD), photoreceptor degeneration associated with wet or dry AMD, other retinal degeneration, optic nerve drusen, optic neuropathy, and optic neuritis. Examples of glaucoma include primary glaucoma, low-tension glaucoma, primary angle-closure glaucoma, acute angle-closure glaucoma, chronic angle-closure glaucoma, intermittent angle-closure glaucoma, chronic open-angle closure glaucoma, pigmentary glaucoma, exfoliation glaucoma, developmental glaucoma, secondary glaucoma, phacogenic glaucoma, glaucoma secondary to intraocular hemorrhage, traumatic glaucoma, neovascular glaucoma, drug-induced glaucoma, toxic glaucoma, and glaucoma associated with intraocular tumors, retinal deatchments, severe chemical burns of the eye, and iris atrophy. Contacting of the neuron or portion thereof with the agent, according to the methods of the invention, can involve administering to a subject a pharmaceutical composition including the agent. The administering can be carried out by, for example, intravenous infusion; injection by intravenous, intraperitoneal, intracerebral, intramuscular, intraocular, intraarterial or intralesional routes; or topical or ocular application. Further, the methods of the invention can include administering to a subject one or more additional pharmaceutical agents. In other examples, the neuron or portion thereof treated according to the methods of the invention is ex vivo or in vitro (e.g., a nerve graft or nerve transplant). In the above-described methods of the invention, the DLK signaling inhibitor results in a decrease in JNK phosphorylation (e.g., a decrease in JNK2 and/or JNK3 phosphorylation), JNK activity (e.g., a decrease in JNK2 and/or JNK3 activity), and/or JNK expression (e.g., a decrease in JNK2 and/or JNK3 expression). In these methods, the DLK signaling inhibitor can be any of the DLK signaling inhibitors described herein (e.g., a small molecule, such as JNKV or JNKVII, or an siRNA). In the above-described methods of the invention, the agent can be a G-protein inhibitor. The G-protein inhibitor can be any of the G-protein inhibitors described herein (e.g., a small molecule). The invention also includes methods of identifying agents for use in inhibiting degeneration of a neuron or a portion thereof. These methods involve contacting a neuron or portion thereof with a candidate agent in an assay of axon or neuron degeneration (e.g., anti-nerve growth factor (NGF) antibodies, serum deprivation/KCl reduction, and/or rotenone treatment). Detection of reduced degeneration of the neuron or portion thereof in the presence of the candidate agent, relative to a control, indicates the identification of an agent for use in inhibiting degeneration of a neuron or portion thereof. The candidate agent can be, for example, selected from the group consisting of antibodies, polypeptides, peptides, peptibodies, nucleic acid molecules, short interfering RNAs (siRNAs), polynucleotides, aptamers, small molecules, and polysaccharides. The invention also includes pharmaceutical compositions and kits that contain one or more agent that can be used to inhibit degeneration of a neuron or a portion thereof, as described herein. The pharmaceutical compositions and kits can include, for example, one or more agent selected from the group consisting of MG132, SB 415286, GSK3β inhibitor I, GSK3β inhibitor VII, GSK3β inhibitor VIII, GSK3β inhibitor XII, Lithium Chloride, SB 202190, SB 239063, SB 239069, SB 203580, SB 203580HCl, AG 556, AG 555, AG 494, PD168393, Tyrphostin B44, Tyrphostin B42 (AG 490), LY 294022, Anisomycin, Cycloheximide, Roscovitine, Forskolin, NKH 477, Actinomycin D, SP600125, Bax Channel Blocker, ZD7288, STO-609, bortezomid, disulfuram, pamapimod, gefitinib, erlotinib, lapatinib ditosylate, demeclocycline hydrochloride, gentamicin sulfate, neomycin sulfate, paromomycin sulfate, and pharmaceutically acceptable salts thereof. The pharmaceutical compositions and kits can optionally include one or more pharmaceutically acceptable excipients. Further, the pharmaceutical compositions and kits can optionally include instructions for use of the compositions and kits in methods for inhibiting degeneration of a neuron or portion thereof. In any of the methods, compositions, and kits of the invention, the agent may be a DLK signaling inhibitor (e.g., a siRNA molecule targeting DLK comprising, e.g., the sequence of, desirably, GCACTGAATTGGACAACTCTT (SEQ ID NO: 1), GAGTTGTCCAATTCAGTGCTT (SEQ ID NO: 2), GACATCGCCTCCGCTGATTT (SEQ ID NO: 3), ATCAGCGGAGGCGATGTCCTT (SEQ ID NO: 4), GCAAGACCCGTCACCGAAATT (SEQ ID NO: 5), TTTCGGTGACGGGTCTTGCTT (SEQ ID NO: 6), GCGGTGTCCTGGTCTACTATT (SEQ ID NO: 7), or TAGTAGACCAGGACACCGCTT (SEQ ID NO: 8); an antibody, such as, antibody 317, antibody 318, antibody 319, antibody 320, antibody 321, antibody 322, an siRNA molecule targeting the JNK1 sequence of TTGGATGAAGCCATTAGACTA (SEQ ID NO: 9), an siRNA molecule targeting the JNK2 sequence of ACCTTTAATGGACAACATTAA (SEQ ID NO: 10) or AAGGATTAGCTTTGTATCATA (SEQ ID NO: 11), an siRNA targeting the JNK3 sequence of CCCGCATGTGTCTGTATTCAA (SEQ ID NO: 12), SP600125, JNKV inhibitor, JNKVIII inhibitor, SC-202673, SY-CC-401, SP600125, As601245, XG-102, myricetin, T278A DLK, S281A DLK, K152A DLK, and the leucine zipper domain of DLK), a GSK3β inhibitor (e.g., SB415287, GSK3β inhibitor I, GSK3β inhibitor VII, GSK3β inhibitor VIII, GSK3β inhibitor XII, and lithium chloride), an EGFR pathway inhibitor (e.g., erlotinib, tyrphostin B44, tyrphostin B42/AG490, AG555, AG494, PD168393, SB203580, SB239063, SB202190, SB239069, STO-609, and SP600125), or a G-protein inhibitor (e.g., SCG292676 and pertussis toxin). In any of the above-described methods, the administering of an agent results in at least a 10% decrease (e.g., at least 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or even 100% decrease) in one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10) symptoms of a disorder of the nervous system; condition of the nervous system that is secondary to a disease, condition, or therapy having a primary effect outside of the nervous system; injury to the nervous system caused by physical, mechanical, or chemical trauma; pain; ocular-related neurodegeneration; memory loss; or psychiatric disorder. Non-limiting examples of such symptoms include tremors, slowness of movement, ataxia, loss of balance, depression, decreased cognitive function, short-term memory loss, long-term memory loss, confusion, changes in personality, language difficulties, loss of sensory perception, sensitivity to touch, numbness in extremities, muscle weakness, muscle paralysis, muscle cramps, muscle spasms, significant changes in eating habits, excessive fear or worry, insomnia, delusions, hallucinations, fatigue, back pain, chest pain, digestive problems, headache, rapid heart rate, dizziness, blurred vision, shadows or missing areas of vision, metamorphopsia, impairment in color vision, decreased recovery of visual function after exposure to bright light, and loss in visual contrast sensitivity. In any of the above-described methods, the administering results in at least a 10% decrease (e.g., at least 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or even 100% decrease) in the likelihood of developing a disorder of the nervous system; condition of the nervous system that is secondary to a disease, condition, or therapy having a primary effect outside of the nervous system; injury to the nervous system caused by physical, mechanical, or chemical trauma; pain; ocular-related neurodegeneration; memory loss; or psychiatric disorder, compared to a control population of subjects that are not administered said agent. The invention also provides methods for activating degeneration of a neuron or portion thereof. These methods involve administering to a neuron or portion thereof an agent that modulates: (i) the activity of a target protein in the neuron or portion thereof, or (ii) a process in the neuron or portion thereof. The target protein can be, for example, selected from the group consisting of dual leucine zipper-bearing kinase (DLK), glycogen synthase kinase 3β (GSK3β), p38 mitogen-activated protein kinase (p38 MAPK), epidermal growth factor receptor (EGFR), phosphoinositide 3-kinase (PI3K), cyclin-dependent kinase 5 (cdk5), adenylyl cyclase, c-Jun N-terminal kinase (JNK), BCL2-associated X protein (Bax), Ih channel, calcium/calmodulin-dependent protein kinase kinase (CaMKK), a G-protein, a G-protein coupled receptor, transcription factor 4 (TCF4), or β-catenin, while the process can be, for example, transcription or protein synthesis. Further, the agent can be, for example, an activator of the target protein or process (as is the case for the targets listed above, with the exception of adenylyl cyclase). In the methods of activiating degeneration of a neuron or portion thereof described above, the modulation of a target protein may be an increase in the activity or expression of GSK3β, a decrease in the activity or expression of β-catenin, and/or a loss in the activity or expression of TCF4. The invention also provides purified antibodies that specifically bind to the phosphorylated form of DLK (e.g., antibody 318, antibody 319, antibody 320, antibody 321, or antibody 322) and inhibitory nucleic acids (e.g., siRNA) that comprise the sequence of, desirably, GCACTGAATTGGACAACTCTT (SEQ ID NO: 1), GAGTTGTCCAATTCAGTGCTT (SEQ ID NO: 2), GGACATCGCCTCCGCTGATTT (SEQ ID NO: 3), ATCAGCGGAGGCGA TGTCCTT (SEQ ID NO: 4), GCAAGACCCGTCACCGAAATT (SEQ ID NO: 5), TTTCGGTGACGGGTCTTGCTT (SEQ ID NO: 6), GCGGTGTCCTGGTCTACTATT (SEQ ID NO: 7), or TAGTAGACCAGGACACCGCTT (SEQ ID NO: 8) that mediate a decrease in the expression of DLK. The invention further provides methods of modulating the growth, cell size, and/or proliferation of a neuron (e.g., cerebellar granule neuron, a dorsal root ganglion neuron, a cortical neuron, or a sympathetic neuron) by contacting a neuron with a DLK signaling inhibitor. The DLK signaling inhibitor used in these methods can be any of the DLK signaling inhibitors described herein (e.g., an siRNA molecule targeting DLK, an siRNA molecule targeting JNK1, an siRNA molecule targeting JNK2, an siRNA molecule targeting JNK3, a small molecule, T278A DLK, S281A DLK, K152A DLK, and the leucine zipper domain of DLK). In certain examples, the invention provides methods for inhibiting or preventing degeneration of a central nervous system (CNS) neuron, or a portion thereof (e.g., the axon or the cell body portion of the CNS neuron). These methods can include administering to the CNS neuron an agent that inhibits the activity (e.g., signaling activity) or expression of the dual leucine zipper-bearing kinase (DLK). In certain examples, the CNS neuron or portion thereof is, for example, a CNS neuron selected from a cerebellar granule neuron, a cortical neuron, and a hippocampal neuron. In various examples, the CNS neuron is not a dopaminergic neuron. According to the methods of the invention, the administering can result in, for example, at least a 10% decrease (e.g., at least 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or even 100% decrease) in the degeneration of a population of neurons as compared to a population of neurons not treated with the agent. The agent noted above can be selected from, for example, antibodies, short interfering RNAs (siRNA), polypeptides, peptibodies, antisense polynucleotides, aptamers, small molecules, and polysaccharides. In the case of an antibody, the antibody can be, for example, selected from a polyclonal antibody, monoclonal antibody, chimeric antibody, humanized antibody, Fv fragment, Fab fragment, Fab′ fragment, and F(ab′)2 fragment. In more specific examples, the agent can be selected from an siRNA molecule targeting DLK, an siRNA molecule targeting JNK 1, an siRNA molecule targeting JNK2, an siRNA molecule targeting JNK3, a small molecule, a dominant negative form of DLK, a kinase-dead form of DLK, and combinations thereof. The methods of the invention can involve administration of the agent to a CNS neuron ex vivo. Such methods can further include grafting or implanting the CNS neuron into a human patient after administration of the agent. In other examples of the methods of the invention, the CNS neuron is present in a human patient. In such methods, the agent can be administered in a pharmaceutically acceptable carrier. Further, the administration of the agent can be carried out by, for example, a route selected from parenteral, subcutaneous, intravenous, intraperitoneal, intracerebral, intralesional, intramuscular, intraocular, and intraarterial routes, or by interstitial infusion or use of an implanted delivery device. Further, the methods of the invention can include administering one or more additional pharmaceutical agents. The patients noted above with respect to the methods of the invention can have or be at risk of developing a neurodegenerative disease or condition, such as one or more of the neurodegenerative diseases or conditions listed above or elsewhere herein. Administering an agent or agents according to the methods of the invention can result in (i) a decrease in JNK phosphorylation, JNK activity, and/or JNK expression, (ii) a decrease in cJun phosphorylation, cJun activity, and/or cJun expression, and/or (iii) a decrease in p38 phosphorylation, p38 activity, and/or p38 expression. The invention also provides methods for decreasing or preventing one or more symptoms, or decreasing the progression, of a neurodegenerative disease or condition, such as one or more of the neurodegenerative diseases or conditions listed above or elsewhere herein. These methods include administering to a patient an agent that inhibits the activity or expression of the dual leucine zipper-bearing kinase (DLK), as described herein. Further, the invention includes the use of one or more agents (see, e.g., the agents listed above and elsewhere herein) that inhibit the activity or expression of the dual leucine zipper-bearing kinase (DLK) in the manufacture of a medicament that inhibits or prevents degeneration of a central nervous system (CNS) neuron, or a portion thereof. The medicament can, for example, decrease one or more symptoms of, or the likelihood of developing, a neurodegenerative disease or condition such as, for example, one or more of the neurodegenerative diseases or conditions listed above and elsewhere herein. The medicament can be formulated for administration by, for example, a route selected from parenteral, subcutaneous, intravenous, intraperitoneal, intracerebral, intralesional, intramuscular, intraocular, and intraarterial routes, or by interstitial infusion or use of an implanted delivery device. Further, the medicament can also include one or more additional pharmaceutical agents. Use of an agent or agents as described herein according to the invention can result in (i) a decrease in JNK phosphorylation, JNK activity, and/or INK expression, (ii) a decrease in cJun phosphorylation, cJun activity, and/or cJun expression, and/or (iii) a decrease in p38 phosphorylation, p38 activity, and/or p38 expression. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows that treatment of neurons for 20 hours with anti-NGF antibodies results in axon degeneration. The two images in the top row show neurons visualized with Tuj1 (neuron specific β-tubulin) antibodies, with and without 20 hours of treatment with anti-NGF antibodies. The two images in the bottom row show neurons visualized with actin antibodies incubated with or without (control) 50 μg/ml anti-NGF antibodies. FIG. 2 shows that varicosities form in neurons cultured with anti-NGF antibodies for 1, 3, 6, 9, 12, or 16 hours. FIG. 3 shows that axons cultured with anti-NGF antibodies for 16 hours lack elongated mitochondria and show accumulation of mitochondria in varicosities. FIG. 4 shows that, in axons cultured with anti-NGF antibodies for 0 to 48 hours, the microtubule network is not disassembled before the actin or neurofilament networks. FIG. 5 illustrates Wallerian degeneration, which takes place in axons severed from neuron cell bodies (top panel; Raff et al., Science 296(5569):868-871, 2002), and shows that there is a significant delay in axon degeneration after lesion in Wallerian Degeneration Slow (WldS) mutants, as compared to controls (bottom panel; Araki et al., Science 305(5686):1010-1013, 2004). FIGS. 6A-6D show that a proteasome inhibitor and a GSK inhibitor prevent axon degeneration in an anti-NGF antibody-based NGF withdrawal assay. FIGS. 7A-7D show that a p38 MAPK inhibitor and an adenylyl cyclase activator prevent axon degeneration in an anti-NGF antibody-based NGF withdrawal assay. FIGS. 8A-8D show that a transcription inhibitor and an EGFR kinase inhibitor prevent axon degeneration in an anti-NGF antibody-based NGF withdrawal assay. FIGS. 9A-9D show that a JNK inhibitor and Bax Channel Blocker prevent axon degeneration in an anti-NGF antibody-based NGF withdrawal assay. FIGS. 10A-10D shows that an Ih channel blocker and a CAMKK inhibitor prevent axon degeneration in an anti-NGF antibody-based NGF withdrawal assay. FIG. 11 is a graph showing the activities of inhibitors of GSK3 (30 μM SB415286), EGFR kinase (10 μM AG555), p38 MAPK (30 μM SB239063), CAMKK (15 μM STO-609), and INK (10 μM SP600125) when added at the time of NGF withdrawal (t=0) or at 3, 6, 9, or 12 hours after NGF withdrawal. FIG. 12 illustrates a Campenot chamber, in which somal (cell body) and axonal environments are separated. FIG. 13 shows that axon degeneration is localized and proceeds without apoptosis in Campenot chamber studies in which NGF withdrawal took place in the axon-containing chamber. Degeneration is visualized by tubulin immunofluorescence. FIG. 14 shows that there are no signs of degeneration in the cell body compartment in the Campenot chamber-based assay illustrated in FIG. 13. FIG. 15 shows that cell bodies (left panels), but not axons (right panels), in the presence of 30 μM SB415 (GSK inhibitor; GSKi) or 15 μM Act D (transcription inhibitor; TXNi) were protected from local degeneration. FIG. 16 shows that axons (right panels), but not cell bodies (left panels), exposed to anti-NGF antibodies in the presence of 10 μM AG555 (EGFR inhibitor; EGFRi) or 30 μM SB239 (p38 inhibitor; p38i) were protected from local degeneration. FIG. 17 is a graph showing quantification of axon degeneration in Campenot chambers in which anti-NGF antibodies were added to the axonal environment in the presence or absence (DMSO) of 15 μM actinomycin D (ActD), 30 μM SB415286 (SB415), 10 μM AG555, or 30 μM SB239063 (SB239) in the axonal (Axon) or the somal environment (Cell). FIG. 18 is a model based on data from the screens described herein. FIG. 19 shows that cell bodies appear smaller when NGF is removed from the axon compartment in a Campenot chamber. FIG. 20 shows that many neurons deprived of NGF in the axon compartment have increased cleavage of caspase-3 and show nuclear condensation. (Neuron health may be affected by the membrane stain DiI.) FIG. 21 is a graph showing GSK3 activity (as measured by decreased levels of phosphorylated GSK3β) at the start of NGF withdrawal (t=0) and after 1, 3, 6, 9, and 12 hours of NGF withdrawal in the cell body or axon compartment. FIG. 22 is a graph showing INK activity (as measured by increased levels of phosphorylated JNK) at the start of NGF withdrawal (t=0) and after 1, 3, 6, 9, and 12 hours of NGF withdrawal in the cell body or axon compartment. FIG. 23 shows that a large number of axons with varicosities, as well as fragmented axons, were observed when the cell body inhibitors (30 μM SB415 (GSKi) and 15 μM Act D (TXNi)) were added to the cell body compartment. The addition of the axon inhibitors (10 μM AG555 (EGFRi) and 30 μM SB239 (p38i)) to the axon compartment showed fewer varicosities, and the axons seemed to go straight to fragmentation. FIG. 24 shows that there may be more functional mitochondria, but still no elongated mitochondria, in NGF-deprived neurons treated with GSK, EGFR, and p38 inhibitors. FIG. 25 shows that the GSK inhibitor SB415 can delay axon degeneration after lesion. FIG. 26 shows that, after global NGF withdrawal, 10 μM or 25 μM GSK inhibitor blocks axon degeneration, but does not block cell death. FIG. 27 shows that EGFR expression is increased in a section of SOD1 mouse (Tg) spinal cord stained with anti-EGFR antibodies (right panel), as compared to a non-transgenic control (NTG; left panel). FIG. 28 shows that EGFR is normally expressed in neurons (motor neurons) and that the level of phosphorylated EGFR (pEGFR) is increased in ALS SOD1 mouse model (SOD1-Tg) compared to a non-transgenic control (Non-Tg). FIG. 29 shows that the number of axons is decreased in the ALS SOD1 mouse model (SOD1-Tg) as compared to a non-transgenic control (Non-Tg), and that the phosphorylated EGFR (pEGFR) in the ALS SOD1 model partially co-localizes with axons. FIG. 30 shows that the small molecule inhibitors of JNK (5 μM SP600125), CaMKK (5 μM STO-609), EGFR (1 μM or 10 μM AG555), p38 (5 μM SB239063), and GSK (10 μM SB415286) protect cerebellar granule neurons from serum deprivation/KCl reduction. FIG. 31 shows that the small molecule inhibitors of EGFR, GSK, CaMKK, JNK, and p38 protect hippocampal neurons against 10 rotenone. FIG. 32 shows that the small molecule inhibitors of EGFR, GSK, CaMKK, JNK, and p38 protect cortical neurons against 10 μM rotenone. FIG. 33 shows that ErbB receptors are detected on axons in dorsal root ganglion neurons by immunocytochemistry using antibodies specific for EGFR (top left panel), Her2 (top right panel), Her3 (bottom left panel), and Her4 (bottom right panel). FIG. 34 shows that EGFR is expressed in axons of dorsal root ganglion neurons using immunocytochemistry. FIG. 35 shows that 100 μg/mL EGF does not induce axon degeneration when added to dorsal root ganglion neurons and that addition of 100 μg/mL EGF induces phosphorylation of ERK in the treated neurons. FIG. 36 shows that the EGFR ectodomain (50 μg/mL) does not block axon degeneration induced by NGF withdrawal in dorsal root ganglion neurons. FIG. 37 shows that 3.4 μM, 11.1 μM, 33.3 μM, and 100 μM Tarceva® (erlotinib) blocks degeneration in dorsal spinal cord explants. FIG. 38 shows that dual leucine zipper-bearing kinase (DLK) acts upstream from JNK in axon degeneration. Transfection of a plasmid encoding wild type DLK in 293 cells results in JNK activation (as measured by increased levels of phosphorylated JNK) compared to cells mock-transfected with a control plasmid or a plasmid encoding kinase-dead DLK (DLK-KD). Knockdown of DLK expression by siRNA in dorsal root ganglion, peripheral system neurons, protects axons from degeneration induced by NGF withdrawal. The knockdown of DLK expression using DLK siRNA was confirmed using quantitative PCR as compared to a control siRNA (bottom right panel). FIG. 39 shows that siRNA knockdown of DLK signaling delays local axon degeneration. FIGS. 40A and 40B depict the results of experiments assessing the impact of DLK knockdown on NGF withdrawal-induced sympathetic neuron degeneration using phase contrast microscopy to visualize neurons. FIG. 41 shows that knockdown of DLK expression using DLK siRNA (DLK) protects sympathetic neurons from camptothecin- and vincristine-induced apoptosis compared to neurons treated with a control siRNA (NT). FIG. 42 shows that transfection of sympathetic neurons with a plasmid encoding kinase-dead DLK (KD) protects the neurons from NGF withdrawal-induced apoptosis compared to neurons transfected with a plasmid encoding wild type DLK (DLK). FIGS. 43A-43D show the results of experiments assessing the binding specificities of the anti-pDLK antibodies described in Example 15A. FIG. 43A shows Western blot analyses of the binding of each of the anti-pDLK antibodies described herein to DLK, DLK in the presence of a dominant negative DLK, and control kinase MLK3. FIG. 43B shows immunofluorescent microscopic images of the binding of anti-pDLK antibodies 318 and 319 to cultured 293T cells transformed with DLK (upper two images) or control kinase MLK3 (lower two images). FIG. 43C shows a Western blot detecting JNK and phospho-JNK antibodies in 293 cells transfected with DLK and DLK mutants which lack activity (T278A and S281A). FIG. 43D is a Western blot which shows that the level of phosphorylated DLK is elevated in cortical neurons treated with vincristine, compared to untreated (control) cortical neurons. FIGS. 44A and 44B show binding of anti-pDLK antibodies (antibody 318) to spinal cord sections in wild type and SOD1 mutant mice at end stage of disease (FIG. 44A) and at the onset of symptoms (FIG. 44B). FIG. 44C depicts Western blot analyses of pDLK, pJNK, and pcJUN levels in human Alzheimer's disease patient cortical samples. FIGS. 45A and 45B depict the results of experiments assessing the impact of DLK silencing on phosphorylation of JNK in response to NGF withdrawal stress in sympathetic neurons and dorsal root ganglion neurons; and vincristine-induced stress in cortical neurons, as described in Example 15C and Example 14B. FIG. 46 shows the protective effect of JNK inhibitors on DRG explants subjected to NGF withdrawal stress, as described in Example 15C. FIG. 47 depicts the results of experiments assessing the impact of silencing JNK1, JNK2, JNK3, individually, and JNK2 and JNK3 together in DRG neurons on axon degeneration observed upon NGF withdrawal stress, as described in Example 15C. FIG. 48A shows the affect of DLK siRNA and control siRNA on the survival of cortical neurons. FIG. 48B shows the affect of DLK siRNA and control siRNA on the survival of sympathetic neurons. FIG. 49 is a set of immunomicrographs showing the ability of an inhibitor of G-coupled protein receptors (SCH 202676; 10 μM or 100 μM) to prevent NGF withdrawal-induced degeneration in DRGs. FIG. 50 is a set of immunomicrographs showing the ability of SCH 202676 (0.1 μM or 1 μM) to prevent NGF withdrawal-induced degeneration in DRGs. FIG. 51 is a set of immunomicrographs showing the ability of 0.01 μg/mL, 0.1 μg/mL, or 1 μg/mL pertussis toxin (an inhibitor of G-protein signaling) to prevent NGF withdrawal-induced degeneration in DRGs. FIG. 52 is a set of immunomicrographs of rat hippocampal neurons showing the effect of expression of active mutant GSK (GSK3S9A), wild type TCF4, and mutant inactive TCF4 on degeneration. FIG. 53 shows reduced degeneration of cortical neurons in DLK knock-out mice in response to N-methyl-D-aspartic acid (NMDA) treatment as described in Example 18. Specifically, FIG. 53(A) is a Western blot showing the levels of DLK, phosphorylated cJun (P CJUN), phosphorylated p38 (P P38), and phosphorylated JNK (P JNK) in cortical neurons from wild-type mice (WT), mice heterozygous for DLK (dlk+/−) (HET), and DLK knock-out mice (DLK KO), with or without NMDA treatment. FIG. 53(B) is a graph showing the results of an LDH release assay. Increased survival of cortical neurons from DLK knock-out mice following NMDA treatment was observed compared to WT and HET neurons. FIG. 54 shows the effect of a dominant negative form of DLK on cJun activation as described in Example 19. Briefly, mice were injected with adeno-associated virus vectors expressing GFP (AAV GFP) or a dominant negative form of DLK (AAV DLK-LZ). Neurons expressing phosphorylated cJun (p-cJun) or p-cJun and GFP were quantified using immunohistochemistry and fluorescence microscopy methods. FIG. 55 shows the results of experiments in which mice were injected with AAV GFP or AAV DLK-LZ and then subjected to kainic acid induced seizure as described in Example 20. The severity of the seizure was measured over a three hour period. DETAILED DESCRIPTION OF THE INVENTION A. Definitions The term “target” is used herein to refer to proteins and processes that, when modulated by agents impacting their activities, inhibit or decrease axon degeneration. Most of the targets described herein, when contacted with an agent that inhibits their activity, inhibit or decrease axon degeneration, but the targets of the present invention also include proteins and processes that, when activated, inhibit or decrease axon degeneration. Exemplary targets of the invention are as follows: dual leucine zipper-bearing kinase (DLK), glycogen synthase kinase 3β (GSK3β), p38 mitogen-activated protein kinase (p38 MAPK), epidermal growth factor receptor (EGFR), phosphoinositide 3 kinase (PI3K), cyclin-dependent kinase 5 (Cdk5), adenylyl cyclase, c-Jun N-terminal kinase (JNK), BCL2-associated X protein (Bax), Ih channel, calcium/calmodulin-dependent protein kinase kinase (CaMKK), a G-protein, a G-protein coupled receptor, transcription factor 4 (TCF4), β-catenin, transcription, and protein synthesis. A selection of common alternate designations for several of these targets is listed in Table 1. The targets include native, human sequences and homologues of these sequences from monkeys, mice, rats, and other non-human mammals, including all naturally occurring variants, such as alternatively spliced and allelic variants and isoforms, as well as soluble forms thereof. Exemplary, non-limiting sequence references are also provided in Table 1. Additional sequences, including sequences of various target isoforms, variants, homologues, and fragments may also be considered as targets, according to the present invention. TABLE 1 Exemplary Genbank Accession Target Alternate names numbers Gycogen synthase GSK-3β; GSK-3 beta 3; GSK3beta isoform; GSK-3α, CAG38748 kinase 3 beta (GSK3β) GSK-3b2 NP_002084 NP_063937 β-catenin Catenin beta-1, beta-catenin, CTNNB, CTNB1 NP_001091679 NP_001091680 NP_001895 TCF4 Transcription factor 4, E2-2, ITF2, SEF2, SEF2-1, SEF2- AAI25085 1A, SEF2-1B NP_001077431 NP_003190 EAW63024 EAW63023 EAW63022 EAW63021 EAW63020 EAW63019 EAW63018 EAW63017 AAI25086 Q9NQB0 p38 Mitogen-activated p38alpha (MAPK14, CSBP2, Crk1, Csbp1, MGC102436, NP_002736.3 protein kinase Mxi2, PRKM14, PRKM15, p38, p38-alpha, p38MAPK, NP_620407 p38a, p38alpha, OTTMUSP00000021706; cytokine suppressive anti-inflammatory drug binding protein 1; mitogen activated protein kinase 14; p38 MAP kinase alpha; p38 MAPK; p38 alpha; tRNA synthetase cofactor p38) p38beta (MAPK11, DKFZp586C1322, P38b, Prkm11, Sapk2, Sapk2b, p38-2, p38beta2, mitogen activated protein kinase 11; protein kinase, mitogen activated kinase, 11, p38beta) p38delta (MAPK13, SAPK4, Serk4, OTTMUSP00000028863; SAPK/Erk/kinase 4; mitogen activated protein kinase 13; p38 delta MAP kinase) p38gamma (MAPK12, AW123708, Erk6, Prkm12, Sapk3, mitogen activated protein kinase 12; stress activated protein kinase 3) Epidermal growth factor EGFR (ERBB, ERBB1, HER1, PIG61, mENA, avian AAG35789 receptor (EGFR) erythroblastic leukemia viral (v-erb-b) oncogene homolog; NP_005219 cell growth inhibiting protein 40; cell proliferation- NP_958439 inducing protein 61; epidermal growth factor receptor) NP_958440 ErbB2 (CD340, HER-2, HER-2/neu, HER2, NEU, NGL, NP_958441 TKR1, c-erb B2/neu protein; erbB-2; herstatin; neuroblastoma/glioblastoma derived oncogene homolog; v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2 (neuro/glioblastoma derived oncogene homolog)) ErbB3 (ErbB-3, HER3, LCCS2, MDA-BF-1, MGC88033, c-erbB-3, c-erbB3, erbB3-S, p180-ErbB3, p45-sErbB3, p85-sErbB3, erbB-3; lethal congenital contracture syndrome 2; v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 3) ErbB4 (HER4, MGC138404, p180erbB4, avian erythroblastic leukemia viral (v-erb-b2) oncogene homolog 4; receptor tyrosine-protein kinase erbB-4; tyrosine kinase-type cell surface receptor HER4; v-erb-a avian erythroblastic leukemia viral oncogene homolog-like 4; v-erb-a erythroblastic leukemia viral oncogene homolog 4) Mitogen-activated Jun N-terminal kinase (1); JNK; JNK1; PRKM8; SAPK1; NP_003609 protein kinase 8 (JNK) JNK1A2; JNK21B1/2; mitogen-activated protein kinase-8; CAG38817 MAPK8; JNK-46 AAH65516 JNK-2; Jun N-terminal kinase (2); MAPK9; JNK2; SAPK; NP_002741 p54a; JNK2A; JNK2B; PRKM9; JNK-55; JNK2BETA; NP_620634 p54aSAPK; JNK2ALPHA NP_620635 JNK-3; Jun N-terminal kinase (3); MAPK10; JNK3; NP_620637 JNK3A; PRKM10; p493F12; FLJ12099; FLJ33785; MGC50974; p54bSAPK Calcium/calmodulin- CAMKKA; CAMKK alpha protein; CaM-KK alpha; NP_115670 dependent protein CaM-kinase IV kinase; CaM-kinase kinase alpha; CaMKK NP_757343 kinase beta (CaMKbeta) 1; CaMKK alpha; calcium/calmodulin-dependent protein NP_757344 kinase kinase alpha; calcium/calmodulin-dependent protein kinase 1 alpha; calcium/calmodulin-dependent protein kinase kinase 1, alpha CAMKKB; CAMKK beta protein; CaM-KK beta; CaM-kinase kinase beta; CaMKK beta; NP_006540 calcium/calmodulin-dependent protein kinase kinase beta; NP_705719 calcium/calmodulin-dependent protein kinase beta; NP_705720 calcium/calmodulin-dependent protein kinase kinase 2, NP_757363 beta; calcium/calmodulin-dependent protein kinase kinase NP_757364 2, beta NP_757365 NP_757380 dual leucine zipper- DLK, DLK1, fetal antigen 1 (FA1), PG2, PREF-1, PREF1, NP_003827 bearing kinase ZOG, delta-lika protein dlk, pG2, preadipocyte factor 1 ABC26857 EAW81713 EAW81712 EAW81711 AAH14015 AAH13197 AAH07741 P80370 Phosphoinositide 3 PI3-kinase, PI3K, Phosphotidylinositol 3 kinase CAA74194 kinase Cyclin-dependent kinase Cdk5 NP_004926 5 Adenylyl cyclase Adenylyl cyclase 1 (ADCY1); 3′,5′-cyclic AMP synthetase NM_021116 (1); ATP pyrophosphate-lyases (1); Ca2+/calmodulin- activated adenylyl cyclase; Adenylate cyclase type I; Brain adenylate cyclase 1 Adenylyl cyclase 2 (ADCY2); 3′,5′-cyclic AMP synthetase (2); ATP pyrophosphate-lyase (2); adenylate cyclase type NP_065433 II; adenylate cyclase 2 (brain); Adenylate cyclase II; Type II adenylate cyclase Adenylyl cyclase 8 (ADCY8); ATP pyrophosphate-lyase 8; adenylate cyclase type VIII; adenylyl cyclase 8; Ca(2+)/calmodulin-activated adenylyl cyclase; NP_001106 adenylate cyclase 8 (brain); adenylyl cyclase-8, brain adenylate cyclase 3 (Adcy3, AC3, mKIAA0511, adenylyl cyclase 3) adenylate cyclase 4 (Adcy4, KIAA4004, mKIAA4004) adenylate cyclase 5 (Adcy5, AW121902, Ac5) adenylate cyclase 6 (Adcy6, mKIAA0422) adenylate cyclase 7 (Adcy7, AA407758, MGC141539, adenylyl cyclase type VII) adenylate cyclase 9 (Adcy9, AW125421, D16Wsu65e, mKIAA0520) adenylate cyclase 10 (Adcy10, 4930431D04Rik, 4931412F17, Sacy, sAC, OTTMUSP00000023839; soluble adenylyl cyclase; testicular soluble adenylyl cyclase) Ih Channel HCN1; hyperpolarization activated cyclic nucleotide-gated NP_066550 potassium channel 1; brain cyclic nucleotide-gated channel 1; BCNG-1; HAC-2 HCN2; hyperpolarization activated cyclic nucleotide-gated NP_001185 potassium channel 2; brain cyclic nucleotide-gated channel 2; BCNG-2; HAC-1 HCN3; KIAA1535; MGC131493; OTTHUMP00000034062; hyperpolarization activated cyclic nucleotide-gated potassium channel 3; NP_065948 potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 3 HCN4; hyperpolarization activated cyclic nucleotide-gated potassium channel 4 NP_005468 BCL2-associated X apoptosis regulator BAX; BCL2L4 Q07812 protein (Bax) NP_004315 NP_620116 NP_620118 NP_620119 NP_620120 Transcription Actinomycin D “Isolated” when used to describe the various proteins disclosed herein, means a protein that has been identified and separated and/or recovered from a component of its natural environment. Contaminant components of its natural environment are materials that may interfere with uses (e.g., uses in therapy or antibody production) for the protein, and may include enzymes, hormones, and other proteinaceous or non-proteinaceous solutes. In various embodiments, the protein will be purified (i) to a degree sufficient to obtain at least 15 residues of N-terminal or internal amino acid sequence by use of a spinning cup sequenator, and/or (ii) to homogeneity by SDS-PAGE under non-reducing or reducing conditions using Coomassie blue or silver stain, and/or (iii) to homogeneity by mass spectroscopic or peptide mapping techniques. Isolated protein includes protein in situ within recombinant cells, as at least one component of the natural environment of the protein in question will not be present. Ordinarily, however, isolated protein will be prepared by at least one purification step. Isolated target proteins as described herein (or fragments thereof) can be used to make antibodies as described herein against the target proteins. An “isolated” nucleic acid molecule is a nucleic acid molecule that is identified and separated from at least one contaminant nucleic acid molecule with which it is ordinarily associated in the natural source of the nucleic acid in question. An isolated nucleic acid molecule is other than in the form or setting in which it is found in nature. Isolated nucleic acid molecules therefore are distinguished from the nucleic acid molecules as they exist in natural cells. However, an isolated nucleic acid molecule includes nucleic acid molecules contained in cells that ordinarily express such nucleic acid molecule where, for example, the nucleic acid molecule is in a chromosomal location different from that of natural cells. An example of an isolated nucleic acid molecule is one lacking 5′ and/or 3′ flanking sequences with which it is contiguous in a natural setting. As used herein, the terms “antagonist” and “inhibitor” refer to agents capable of blocking, neutralizing, inhibiting, abrogating, reducing and/or interfering with one or more of the activities of targets and/or reducing the expression of one or more target proteins (or the expression of nucleic acids encoding one or more target proteins) as described herein. They include, for example, antibodies, polypeptides, peptides, nucleic acid molecules, short interfering RNAs (siRNAs) and other inhibitory RNAs, small molecules (e.g., small inorganic molecules), polysaccharides, polynucleotides, aptamers, and peptibodies. Antagonists or inhibitors of particular targets as described herein (i.e., targets other than adenylyl cyclase) generally inhibit or decrease axon degeneration (e.g., by at least 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or even 100% decrease compared to a control that is untreated with the inhibitor), as described herein. An inhibitor may decrease the activity and/or expression of a target protein by at least 10% (e.g., by at least 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or even 100% decrease) as compared to the expression and/or activity of the target protein that is untreated with the inhibitor. A “DLK signaling inhibitor” is an agent capable of decreasing the activity (e.g., kinase activity) or the expression of a DLK protein (or a nucleic acid encoding a DLK protein) and/or deceasing the activity and/or expression of one or more proteins involved in a DLK signaling pathway (e.g., JNK1, JNK2, JNK3, cJun (e.g., cJun-63 and cJun-73), MKK4, and MKK7). Examples of DLK signaling inhibitors include siRNA molecules that decrease the expression of a nucleic acid encoding DLK (e.g., desirably, a sequence of GCACTGAATTGGACAACTCTT (SEQ ID NO: 1), GAGTTGTCCAATTCAGTGCTT (SEQ ID NO: 2), GGACATCGCCTCCGCTGATTT (SEQ ID NO: 3), ATCAGCGGAGGCGATGTCCTT (SEQ ID NO: 4), GCAAGACCCGTCACCGAAATT (SEQ ID NO: 5), TTTCGGTGACGGGTCTTGCTT (SEQ ID NO: 6), GCGGTGTCCTGGTCTACTATT (SEQ ID NO: 7), or TAGTAGACCAGGACACCGCTT (SEQ ID NO: 8)), JNK1 (e.g., a sequence targeting the JNK1 sequence of TTGGATGAAGCCATTAGACTA (SEQ ID NO: 9)), JNK2 (e.g., a sequence targeting the JNK2 sequence of ACCTTTAATGGACAACATTAA (SEQ ID NO: 10) or AAGGATTAGCTTTGTATCATA (SEQ ID NO: 11)), JNK3 (e.g., a sequence targeting the JNK3 sequence of CCCGCATGTGTCTGTATTCAA (SEQ ID NO: 12)), cJun (e.g., cJun-63 and cJun-73), MKK4, and MKK7. Additional examples of DLK inhibitors include antibodies that bind to a DLK protein (e.g., antibodies that recognize unphosphorylated or phosphorylated DLK, such as the 317, 318, 319, 320, 321, and 322 antibodies described herein), JNK1, JNK2, JNK3, cJun (e.g., cJun-63 and cJun-73), MKK4, and/or MKK7; inhibitors of JNK activity (e.g., SC-202673, SY-CC-401, SP600125, JNKV inhibitor, JNKVIII inhibitor, AS601245, and XG-102, as well as Catalog Nos. 420119, 420130, 420131, 420123, 420116, 420118, 420136, 420129, 420135, 420134, 420133, 420140, and 420128 from EMD Biosciences); inhibitors of MKK4 activity (e.g., myricetin and the inhibitors described in WO 04/058764), and inhibitors of MKK7 activity (e.g., inhibitors described in U.S. Pat. No. 7,195,894 and WO 04/002532). A DLK inhibitor may also be a dominant negative form or kinase-dead form of DLK protein (or a nucleic acid encoding a dominant negative form or a kinase-dead form of DLK protein), such as 1278A DLK, S281A DLK, S152A DLK, and the leucine zipper domain of DLK. Another example of an inhibitor is a “GSK3β inhibitor.” GSK3β inhibitor refers to an agent capable of decreasing the activity and/or expression of GSK3β (or a nucleic acid encoding GSK3β) and/or decreasing the activity and/or the expression of one or more proteins (or a nucleic acid encoding the one or more proteins) that activate GSK3β or the expression or activity of one or more substrates of GSK3β. Non-limiting examples of GSK3β inhibitors include SB415286, GSK3β inhibitor I, GSK3β inhibitor VII, GSK3β inhibitor VIII, GSK3β inhibitor XII, and lithium chloride. An additional example of an inhibitor is a “G-protein inhibitor.” A G-protein inhibitor refers to an agent capable of decreasing the activity and/or expression of one or more G-proteins or G-protein coupled receptors (GPCRs) (or the expression of one or more nucleic acids encoding a G-protein or a GPCR), and/or decreasing the activity and/or expression of one or more proteins downstream of a G-protein or a GPCR. Non-limiting examples of a G-protein inhibitor include siRNA molecules that decrease the level of expression of a nucleic acid encoding a G-protein or a GPCR, an antibody or peptibody that binds to a G-protein or GPCR, or a small molecule or peptide that inhibits the activity of a G-protein or GPCR (e.g., SCH202676 and pertussis toxin). Another example of an inhibitor is a “EGFR pathway inhibitor.” EGFR pathway inhibitor refers to an agent capable of decreasing the activity and/or expression of EGFR protein (or a nucleic acid encoding EGFR) and/or decreasing the activity and/or the expression of one or more proteins that function downstream of EGFR in the cell (e.g., p38 MAPK, CAMKK, and JNK). Non-limiting examples of EGFR pathway inhibitors include inhibitors of EGFR (e.g., erlotinib, tyrphostin B44, tyrphostin B42/AG 490, AG555, AG494, and PD168393), inhibitors of p38 MAPK (e.g., SB203580, SB239063, SB202190, and SB239069), inhibitors of CAMKK (e.g., STO-609), and inhibitors of JNK (e.g., SP600125). Additional examples of EGFR pathway inhibitors include antibodies and peptibodies that bind to EGFR, p38 MAPK, CAMKK, and/or JNK; and siRNA molecules that decrease the expression of one or more nucleic acids that encode a protein that functions downstream of EGFR in the cell (e.g., EGFR, p38 MAPK, CAMKK, and/or JNK). An additional example of an inhibitor is a “CAMKβ inhibitor.” A CAMKβ inhibitor refers to an agent capable of decreasing the activity and/or expression of CAMKβ protein (or a nucleic acid encoding CAMKβ) and/or decreasing the activity and/or expression of one or more proteins that function downstream of CAMKβ in the cell. Non-limiting examples of CAMKβ inhibitors include antibodies and peptibodies that specifically bind to CAMKβ, and siRNA molecules that decrease the expression of one or more nucleic acids that encode CAMKβ or a protein that functions downstream of CAMKβ. Another example of an inhibitor is a “cdk5 inhibitor.” A cdk5 inhibitor refers to an agent capable of decreasing the activity and/or expression of cdk5 protein (or a nucleic acid encoding cdk5) and/or decreasing the activity and/or expression of one or more proteins that function downstream of cdk5 in the cell. Non-limiting examples of cdk5 inhibitors include antibodies and peptibodies that specifically bind to cdk5, and siRNA molecules that decrease the expression of one or more nucleic acids that encode cdk5 or a protein that functions downstream of cdk5. An additional example of an inhibitor is a “TCF4 inhibitor.” A TCF4 inhibitor refers to an agent capable of decreasing the activity and/or expression of TCF4 protein (or a nucleic acid encoding TCF4) and/or decreasing the activity and/or expression of a gene regulated by TCF4 protein. Non-limiting examples of TCF4 inhibitors include antibodies and peptibodies that specifically bind to TCF4 or a protein encoded by a gene regulated by TCF4, and siRNA molecules that decrease the expression of one or more nucleic acids that encode TCF4 or decrease the expression of an mRNA encoded by a gene regulated by TCF4. An additional example of an inhibitor is a “β-catenin inhibitor.” A (β-catenin refers to an agent capable of decreasing the activity and/or expression of “β-catenin protein (or a nucleic acid encoding “β-catenin) and/or decreasing the activity and/or expression of a gene regulated by “β-catenin protein. Non-limiting examples of β-catenin inhibitors include antibodies and peptibodies that specifically bind to β-catenin or a protein encoded by a gene regulated by β-catenin, and siRNA molecules that decrease the expression of one or more nucleic acids that encode β-catenin or decrease the expression of an mRNA encoded by a gene regulated by β-catenin. An additional example of an inhibitor is an “adenyl cyclase inhibitor.” An adenyl cyclase inhibitor refers to an agent capable of decreasing the activity and/or expression of adenyl cyclase protein (or a nucleic acid encoding adenyl cyclase) and/or decreasing the activity and/or expression of one or more proteins that function downstream of adenyl cyclase in the cell. Non-limiting examples of adenyl cyclase inhibitors include antibodies and peptibodies that specifically bind to adenyl cyclase, and siRNA molecules that decrease the expression of one or more nucleic acids that encode adenyl cyclase or a protein that functions downstream of adenyl cyclase. Additional examples of adenyl cyclase inhibitors include small molecules that inhibit the activity of adenyl cyclase (e.g., forksolin and NKH 477). The terms “agonist” or “activator” as used herein refer to agents capable of increasing or activating one or more of the activities of targets as described herein, and include, for example, antibodies, polypeptides, peptides, nucleic acid molecules, short interfering RNAs (siRNAs) or other inhibitory RNAs, small molecules (e.g., small inorganic molecules), polysaccharides, polynucleotides, aptamers, and peptibodies. Agonists or activators of adenylyl cyclase as described herein generally inhibit or decrease axon degeneration, while agonists or activators of the other particular targets described herein can be considered to activate axon degeneration. The term “antibody” herein is used in the broadest sense understood in the art and specifically covers, for example, intact antibodies, monoclonal antibodies, polyclonal antibodies, monospecific antibodies, multispecific antibodies (e.g., bispecific antibodies) formed from at least two intact antibodies, antibody fragments, provided that they exhibit the desired biological activity, and intrabodies. The term “monoclonal antibody” as used herein refers to an antibody obtained from a population of substantially homogeneous antibodies, i.e., the individual antibodies comprising the population are identical except for possible naturally occurring mutations that may be present in minor amounts. Monoclonal antibodies are highly specific, being directed against a single antigenic site or epitope. Furthermore, in contrast to polyclonal antibody preparations, which include different antibodies directed against different determinants (epitopes), each monoclonal antibody is directed against a single determinant on an antigen. In addition to their specificity, monoclonal antibodies are advantageous in that they may be synthesized so that they are uncontaminated by other antibodies. The modifier “monoclonal” indicates the character of the antibody as being obtained from a substantially homogeneous population of antibodies, and is not to be construed as
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