alzheimer 's disease ( ad ) is a progressive neurodegenerative disease , which is thought to initiate by accumulation of a peptide and synaptic dysfunctions . ad is primarily characterized by impairment in memory formation , but the disease accompanies failure in sensory processing as well as cognitive abilities . impairment in visual processing is sometimes observed in ad patients [ 26 ] and in particular severe deficits in visuospatial function are observed in a subpopulation of ad with blint 's syndrome [ 79 ] . while primary sensory cortices are thought to be less affected by ad [ 3 , 10 , 11 ] , a plaques and neurofibrillary tangle  hallmarks of ad  are present in the superficial layers of primary visual cortex ( v1 ) from postmortem ad patients [ 7 , 12 , 13 ] and mouse models of ad [ 14 , 15 ] . in a recent study using in vivo 2 photon ca imaging in v1 of a mouse model of ad ( app23xps45 ) , an age - dependent progressive loss of neuronal orientation tuning paralleled the increase in a load . the orientation tuning defects were limited to neurons with hyperactivity under basal conditions , which are often found in close proximity to a plaques . furthermore , the loss of orientation tuning accompanied a progressive deficit in a visual pattern discrimination task , which suggests that the neuronal dysfunction may lead to functional decline in visual processing . in addition , ocular dominance plasticity in v1 is impaired in a different mouse model of ad ( appswe ; ps1e9 ) at an early developmental age before excessive a accumulation is observed , which suggests that app and/or moderate production of a may negatively impact sensory cortex plasticity . recently manipulations that prevent a production have revealed a crucial role of a in normal synaptic function ( reviewed in ) , particularly homeostatic processes that provide stability to neuronal activity [ 1921 ] . a is produced in an activity - dependent manner by sequential cleavage of the amyloid precursor protein ( app ) by - and -secretases [ 20 , 22 ] , which happens mainly in endosomes [ 2325 ] . in hippocampal neurons , overproduction or exogenous application of a can induce endocytosis of ampa receptors to reduce synaptic drive [ 19 , 20 , 26 ] . notably , the activity dependence of a production requires the immediate early gene arc , which is critical for activity - dependent endocytosis of ampa receptors   and homeostatic synaptic scaling in both hippocampus   and v1 . in v1 , a few days of visual deprivation homeostatically scale up the strength of excitatory synapses [ 2934 ] , which is observed during the critical period through adulthood at least in the superficial layers . this process is readily reversed by a few hours of visual experience [ 29 , 30 , 33 , 35 ] . bidirectional homeostatic synaptic plasticity induced by changes in visual experience recruits distinct molecular signalings , which are not exactly the inverse of each other . for example , upscaling induced by losing vision requires phosphorylation of ampa receptors and synaptic expression of ca - permeable ampa receptors [ 30 , 33 ] , while downscaling by visual experience depends on arc . despite the wealth of knowledge suggesting that visual cortex is an excellent system to study experience - dependent homeostatic synaptic plasticity ( reviewed in ) , a direct test of whether a plays a role in this process is lacking . to address this , we used a knockout ( ko ) mouse , which lacks the major neuronal -secretase ( bace1 ) and hence a [ 37 , 38 ] ( but see ) . we found that bace1 kos exhibit abnormally enhanced basal excitatory synaptic transmission in layer 2/3 ( l2/3 ) pyramidal neurons of v1 and lack sensory experience - dependent homeostatic synaptic plasticity . our results suggest a novel role of bace1 in homeostatic regulation of excitatory synaptic transmission in v1 . male mice were derived from heterozygous breeders and identified as bace1 ( ko ) or bace1 ( wt ) via polymerase chain reaction ( pcr ) analysis as described previously . mice were raised in 12-hour light/12-hour dark cycle until postnatal days 2226 ( p22p26 ) , at which point some mice were dark - exposed ( de ) for 2 days while others remained in a normal lighted environment ( normal reared , nr ) . some de mice were returned to normal light conditions for 2 hours to study the effect of light reexposure ( le ) . all experiments were approved by the johns hopkins university institutional animal care and use committee ( iacuc ) and followed the guidelines of the animal welfare act . mice between p24 and p28 were deeply anesthetized with isofluorane vapors in a chamber placed inside a chemical fume hood . after absence of corneal reflex , mice were decapitated and the brain was isolated in ice - cold dissection buffer ( in mm : 212.7 sucrose , 5 kcl , 1.25 nah2po4 , 26 nahco3 , 10 glucose , 3 mgcl2 , and 1 cacl2 ) , which was bubbled with 95% o2/5% co2 gas . primary visual cortex blocks were rapidly dissected and coronally sectioned into 300  m thick slices using a vibratome 3000 plus microslicer ( ted pella ) . slices were gently transferred to a submersion - type holding chamber filled with artificial cerebral spinal fluid ( acsf , in mm : 125 nacl , 5 kcl , 1.25 nah2po4 , 26 nahco3 , 10 glucose , 1 mgcl2 , and 2 cacl2 ) , which was saturated with 95% o2/5% co2 . for mepsc recording , slices were transferred to a submersion - type recording chamber mounted on the fixed stage of an upright microscope ( e600 fn ; nikon , tokyo , japan ) with oblique infrared ( ir ) illumination . ampa receptor - mediated miniature excitatory postsynaptic currents were isolated pharmacologically with 1  m tetrodotoxin ( ttx ) , 20  m bicuculline , and 100  m dl-2-amino-5 phosphonopentanoic acid ( dl - apv ) . these agents were added to acsf ( bubbled with 95% o2/5% co2 and maintained at 30  1c ) , which was continually perfused at a rate of 2  ml / min . cells in layers 2/3 of primary visual cortex were identified by their pyramidal - shaped soma and apical dendrite pointing towards the pia . pyramidal neurons were patched using a whole - cell patch pipette with a tip resistance between 3 and 5  m , which was filled with internal solution containing in mm : 130 cs - gluconate , 8 kcl , 1 egta , 10 hepes , 4 atp , 5 qx-314 ; ph 7.4 , 285295  mosm . 46 minutes were recorded from each cell initiated about 2 - 3 minutes after cell break - in . axon patch - clamp amplifier 700b ( molecular devices , union city , ca ) was used for voltage - clamp recordings . cells were held at 80  mv and the recorded mepsc data was digitized at 10  khz by a data acquisition board ( national instruments , austin , tx ) and acquired through custom - made igor pro software ( wavemetrics , lake oswego , or ) . for intrinsic excitability measures , we performed current clamp recordings with k - gluconate internal solution ( in mm : 130  mm k - gluconate , 10  mm kcl , 10  mm hepes , 0.2 egta , 0.5 na3gtp , 4  mm mgatp , 10  mm na - phosphocreatine , ph 7.25 , mosm 290 ) . drugs were added to the acsf to block synaptic transmission mediated by ampa receptors ( 10  m 2,3-dihydroxy-6-nitro-7-sulfamoyl - benzo[f]quinoxaline-2,3-dione ; nbqx ) , nmdar receptors ( 100  m dl-2-amino-5-phosphonopentanoic acid ; apv ) , and gabaa receptors ( 10  m picrotoxin ) . neurons were patched in current clamp and a small amount of current was injected to keep the resting membrane potential at 75  mv . current pulses of 500 msec duration were injected at 10-second intervals with increasing amplitudes at 40 pa steps until the responses reached an asymptote ( between 40 and 600 pa , or 7 steps ) . from the data collected , the rheo base ( minimum current needed to produce a single action potential ) and average numbers of spikes per step increase in current from rheo base were calculated for bace ko and wt groups . acquired mepscs were analyzed with a mini analysis program ( synaptosoft , decatur , ga ) , with a detection threshold set at 3 times the root mean square ( rms ) noise level . recordings were excluded from analysis if the rms noise was greater than 2 , series resistance larger than 25 m , and input resistance less than 100 m. there was no significant difference in rms noise across the experimental groups ( table 1 ) . we also excluded all mepscs with a rise time greater than 3  ms and those that showed a negative correlation between amplitude and rise time , which may reflect dendritic filtering . 200 consecutive mepscs were analyzed from each cell , and the data are expressed as mean  standard error of the mean . one - factor analysis of variance ( anova ) followed by newman - keuls multiple comparison post hoc test was used to statistically compare data across multiple groups and kolmogorov - smirnov test was used for cumulative probabilities . two - factor anova was used to test interaction between genotype and the experimental variable and student 's t - test was used to compare measurements between wt and ko . for all tests to examine the role of bace1 in visual cortex synaptic function and plasticity , we recorded mepscs in l2/3 pyramidal neurons of v1 in bace1 wt and ko mice . to alter visual experience , mice were dark - exposed ( de ) between postnatal days 2224 ( p2224 ) for 2 days and a subset of them was returned to a lighted environment for 2 hours ( light exposed , le ) . age - matched control mice ( normal reared , nr ) were kept in a normal light / dark cycle . as reported previously , in wts 2 days of de scaled up the amplitude of mepscs , which then returned to nr values after 2 hours of le ( figure 1(a ) ) . we found that bace1 ko mice have significantly larger mepscs compared to bace1 wts under normal conditions ( figure 1(b ) ) . this is consistent with a potential deficit in downscaling mechanisms in bace1 kos , which would result in larger basal mepscs . furthermore , bace1 kos failed to significantly increase or decrease mepsc amplitude with de or le , respectively ( one - factor anova : p = 0.4 ; figure 1(c ) ) , which suggests a lack of experience - dependent homeostatic synaptic plasticity . there was no statistically significant difference in mepsc frequency or kinetics across genotype or experimental conditions ( figure 2 , table 1 ) . previous studies showed that bace1 kos display heightened spontaneous seizure - like activity [ 40 , 41 ] and display alterations in voltage - gated na channel density [ 40 , 42 ] ( but see ) . therefore , the increase in basal mepscs of bace1 kos could have been due to increased spontaneous activity . however , we did not find significant difference in the intrinsic excitability of l2/3 neurons in v1 of bace1 kos compared to bace1 wts ( figure 3(a ) ) . furthermore , there was no difference in the rheo base ( figure 3(b ) ) , resting membrane potential ( figure 3(c ) ) , action potential threshold ( figure 3(d ) ) , or the input resistance ( figure 3(e ) ) measured in current clamp from neurons of bace1 wt and ko . our data suggest that lacking bace1 , which abolishes a production [ 37 , 38 ] and several other bace1 substrates [ 4248 ] , results in basal potentiation of excitatory synaptic transmission in l2/3 of v1 and prevents homeostatic regulation by changes in sensory experience . this phenotype mirrors that observed in arc kos , which we have previously reported to lack experience - dependent downscaling of mepscs in the same population of neurons with visual experience . the larger basal mepsc amplitude in bace1 kos would then reflect a lack of homeostatic downscaling with normal visual activity , and arc kos also exhibit a similar increase in basal mepsc amplitude . a recent study links arc with bace1 by placing arc as an activity  sensor  that interacts with presenilin-1 ( psen1 ) , which forms the active core of -secretase , and acts downstream of bace1 to produce a . the similar phenotype seen in v1 of arc ko and bace1 ko further supports this idea and suggests that activity - dependent production of a may play a role in homeostatic downscaling of excitatory synapses in vivo . this interpretation apparently contradicts a recent study using mice lacking psen1 , which can not undergo inactivity - induced scaling up of excitatory synapses in hippocampal neurons . however , this study showed that -secretase inhibitor does not block inactivity - induced scaling up of mepscs , which suggests that psen1 may have an additional function in scaling up synapses separate from its role on a production . while our data are consistent with the proposed function of activity - dependent a production in homeostatic control of excitatory synapses , we can not rule out the possibility that the phenotype seen in bace1 ko may stem from lacking other products of this enzyme . bace1 acts on substrates other than app , such as subunits of voltage - gated na channels and neuregulin-1 ( nrg1 ) ( reviewed in ) . as such , bace1 ko mice exhibit excitability issues and defects in axon myelination [ 40 , 47 , 48 ] . however , the lack of a change in intrinsic excitability or spike threshold in v1 l2/3 neurons of bace1 ko ( figure 3 ) suggests that altered regulation of voltage - gated na channels [ 40 , 41 ] may not be likely , at least in this brain area . also , it suggests that the increased incidence of seizures in bace1 kos may reflect the increase in basal excitatory synaptic drive that we report here ( figure 1 ) or may be due to impaired cleaving of other bace1 substrates such as seizure protein 6 ( sez6 ) . the increase in basal mepscs seen in bace1 kos may also be a result of increased app processing through the -secretase pathway in the absence of bace1 . soluble app fragment ( sapp- ) , which is produced by -secretase cleavage of app , has known effects on synaptic function such as facilitation of ltp [ 51 , 52 ] . however , this can not account for the lack of scaling down in bace1 ko when reexposed to light following de , unless facilitation of ltp would somehow counteract the effects of scaling down with increased visual activity . in addition , the locus of bace1 expression that is critical for synaptic homeostasis is unclear and would need to be determined experimentally . it is possible that the defect in homeostatic synaptic plasticity in v1 of bace1 ko may reflect abnormal vasculature in the retina . however , this retinal phenotype was observed in older bace1 ko ; hence , it is unclear whether this phenotype is present in the younger mice used in our study or how it affects vision . even in older bace1 kos there was about a 20% reduction in photopic electroretinography ( erg ) without changes in scotopic erg . we have reported previously that incomplete loss of vision by bilateral eye lid suture , which reduces visually evoked responses in v1 to about 10% of normal responses , is ineffective at scaling up mepscs in the l2/3 of v1 . therefore , even with the magnitude of change in retinal function seen in older bace1 kos , it is unlikely that reduced visual inputs would scale up basal mepscs as seen in our study . a has been shown to increase neuronal excitability via oxidative stress by altering intracellular ca dynamics ( reviewed in [ 55 , 56 ] ) . therefore , we can not rule out whether altered ca dynamics could have contributed to the bace1 ko synaptic phenotype seen here , especially because homeostatic regulation of ampa receptors is known to depend on changes in intracellular ca signals ( reviewed in ) . furthermore , because the bace1 kos develop without the bace1 gene , it is possible that the phenotype seen here may be due to compensatory changes . hence , it is pertinent to test whether acute block of bace1 would also result in similar synaptic deficits in future studies . considering that even with bace1 inhibitors , the treatment regime is likely of long duration , studying the bace1 ko phenotype would still be of relevance . in any case , our results underscore a novel function of bace1 enzyme in regulating experience - dependent homeostatic synaptic plasticity in neocortex . this adds to the growing list of normal physiological functions mediated by the enzyme and cautions the use of bace1 inhibitors as ad therapeutics .
alzheimer 's disease ( ad ) is the most common form of age - related dementia , which is thought to result from overproduction and/or reduced clearance of amyloid - beta ( a ) peptides . 
 studies over the past few decades suggest that a is produced in an activity - dependent manner and has physiological relevance to normal brain functions . 
 similarly , physiological functions for - and -secretases , the two key enzymes that produce a by sequentially processing the amyloid precursor protein ( app ) , have been discovered over recent years . in particular , activity - dependent production of a has been suggested to play a role in homeostatic regulation of excitatory synaptic function . 
 there is accumulating evidence that activity - dependent immediate early gene arc is an activity  sensor ,  which acts upstream of a production and triggers ampa receptor endocytosis to homeostatically downregulate the strength of excitatory synaptic transmission . 
 we previously reported that arc is critical for sensory experience - dependent homeostatic reduction of excitatory synaptic transmission in the superficial layers of visual cortex . here 
 we demonstrate that mice lacking the major neuronal -secretase , bace1 , exhibit a similar phenotype : stronger basal excitatory synaptic transmission and failure to adapt to changes in visual experience . 
 our results indicate that bace1 plays an essential role in sensory experience - dependent homeostatic synaptic plasticity in the neocortex .