Figure 1. Spectral display of high-energy phosphate and membrane phospholipid phosphorus metabolites from the parieto-occipital lobe of amnestic mild cognitive impairment individual. PE, phosphoethanolamine; PC, phosphocholine; Piex and Piin inorganic phosphate external and internal, respectively; GPE, glycerophosphoethanolamine; GPC, glycerophosphocholine; PCr, phosphocreatine; ATP forms: α, β, and δ adenosine triphosphate; NAD, nicotinamide adenine dinucleotide; UDPG, uridine diphosphate glucose.
We investigated the relationships across BEM indices measured using PCr/t-ATP (reflective of energy reserve), Pi/t-ATP (reflective of energy consumption: extracellular-Pi/t-ATP and intracellular-Pi/t-ATP), and PCr/Pi (reflective of a metabolic state) and its regulatory co-factors (intracellular Mg2+ and pH) followed by membrane phospholipid metabolite index (PMEs/PDEs) in the parieto-occipital lobes with cognitive performance of EF (complex abstraction, innovation, inhibition and switching, conceptual reasoning, and working memory), memory (episodic), attention, and visuo-spatial skills. Age and education had no appreciable effects on the high-energy phosphate and membrane phospholipid indices, nor on the cognitive measures in our sample (Supplementary Table S1). Therefore, we did not adjust for age and education in our final analyses of BEM indices and cognitive performance.
In the BEM index and neurocognitive relationship, we found four significant results. First, the extracellular-Pi/t-ATP ratio was inversely related with performance of attention [selective auditory attention-trail 2: (b = −0.0018, t = −2.254, p = 0.038)]; whereas intracellular-Pi/t-ATP ratio was trending to be inversely proportional with the performance on the EF domain of inhibition and switching [Trails B: (b = −13.867, t = −2.029, p = 0.0585)]. Second, the energy reserve indexed by PCr/t-ATP was inversely related with performance on immediate recall of cognitive memory screen [CVLT: (b = −0.0138, t = −3.272, p = 0.004)]. Finally, the regulatory co-factors of BEM index intracellular pH was also inversely associated with attention [selective auditory attention-trail 2: (b = −285.95, t = −2.172, p = 0.044)], whereas Mg2+ was positively associated with visual-spatial skills [Trails A: (b = −12.471, t = 2.219, p = 0.040)].
In view of a previous study by Rijpma et al. (2018) in AD for observation of a reduced level of PCr/Pi, an indicator of metabolic state, we also investigated the relationship of PCr/Pi ratio with cognition. A significant inverse relationship was found between the PCr/Pi index and performances on EF of innovation [TOSL: (b = −0.78, t = −2.18, p = 0.044)], and delayed recall of cognitive memory screen [CVLT: (b = −2.43, t = −2.278, p = 0.007)].
See Figure 2 and Table 5 for details.
Figure 2. Association of BEM, membrane phospholipid indices, and regulator co-factors in the parieto-occipital brain region with cognitive performance in amnestic mild cognitive impairment (p ≤ 0.05). TOSL, Test of Strategic Learning; CVLT, California Verbal Learning test; intracellular_Pi, inorganic phosphate (intracellular); extracellular_Pi, inorganic phosphate (extracellular); PCr, phosphocreatine; t-ATP (total adenosine triphosphate: sum of α-ATP, β-ATP, and γ-ATP); PMEs, phosphomonoesters; PDEs, phosphodiesters.
Table 5. Linear regression model: association of BEM and membrane phospholipid indices in the parieto-occipital brain areas with neurocognitive and screening cognitive measures in mild cognitive impairment.
The membrane phospholipid metabolite index (PMEs/PDEs) was positively associated with cognitive domain of attention [selective auditory attention-Trail 1: (b = 5.92, t = 2.14, p = 0.047)].
The present exploratory pilot study investigated two main goals in aMCI, a heterogeneous and unstable clinical population. This group is at higher risk of developing AD/dementia than the cognitively normal population, although many with aMCI may not progress. Our main goal was to establish a methodology and test the feasibility to measure BEM related high-energy phosphate and membrane phospholipid metabolites precisely at UHF magnetic strength 7-Tesla using a partial volume-coil 31P MRS technique in the parieto-occipital region of the brain in a population with variability as defined by its nature. Our results showed that we were able to resolve 13 of 31P peaks with well-separated spectral resolution of high-energy phosphate and membrane phospholipid metabolites. The enhanced spectral resolution suggests this methodology may provide a feasible way to further explore BEM in more extensive comparative studies with healthy controls and those affected with AD in the future at UHF magnetic strength. Next, we investigated if the BEM indices derived from measuring the high-energy phosphates and membrane phospholipid metabolites at 7T showed a correspondence with cognitive performance (BEM-Cognitive correlates). Overall, we hypothesized an inverse relationship across BEM indices—energy reserve, energy consumption, and metabolic state—along with regulatory co-factors pH and Mg2+ and membrane phospholipid index with cognitive performance supported by previous work from Ashraf et al. (2015), Croteau et al. (2018), and Corriveau-Lecavalier et al. (2019). Our results support our assumptions of an inverse relationship in aMCI with the exception of Mg2+ and the membrane phospholipid index, which were positively associated with visuospatial domain and attention, respectively.
An increasing body of research is focused on investigating pathophysiologic changes in the in vivo human brain with the goal to track and diagnose the onset of dementia as early as possible, particularly in populations at higher risk, such as aMCI. Although this cohort has an elevated risk for developing Alzheimer’s, the evidence that many will not develop dementia may make this an informative population to study, as a preliminary step. Due to variability of the disease progression, identification of early biological markers is an important goal in developing effective therapeutics. In prior research, seeking to better understand BEM, the research to date focused predominantly on either healthy controls or disease populations like AD at lower magnetic strength, with a knowledge gap regarding the potential role of BEM on cognition in aMCI. The current work expands on these efforts by reporting on the first, albeit small, pilot study in aMCI to explore the feasibility of acquiring precise data at UHF magnetic strength 7T using 31P MRS to study BEM-cognitive correlations.
Our first goal of this study was to identify a distinct spectral display of high-energy phosphate and membrane phospholipid metabolites using 31P MRS at 7T. The results revealed it was possible to resolve and identify a high-quality spectral display with improved SNR of the high-energy phosphate and membrane phosphorus metabolites in the parieto-occipital lobes using the UHF MRI. What is interesting to note is that the spectral resolution of brain phosphorus metabolites in this study was similar to previous studies in healthy young adult brains and muscle (Ren et al., 2015) and heart (Rodgers et al., 2014) at 7T. Moreover, the use of 7T allowed clear separation of multiple phosphorus metabolites with similar chemical shifts in the spectra collected from the parieto-occipital region. With improved SNR in the summed spectra, we were able to fully resolve PMEs into PE and PC, PDEs into GPC and GPE, and Pi into intra- and extracellular compartments, along with observation of NAD(H) and UDP(G). At UHF MRI, the α- and β-ATP signal appeared to be an easy-to-fit single peak on the spectral display due to improved SNR of the phosphorus metabolites. It is worthy to note that the α-ATP chemical shift can be affected by the NAD(H) signal, a shift that cannot be resolved at a lower magnetic field. Thereby, the easy-to-fit singlet of α- and β-ATP allowed us to calculate the concentration of free Mg2+, an important regulatory co-factor of BEM in the in vivo human brain. Overall, the 31P MRS data acquisition methodology with improved SNR provided indirect help in accurately measuring the regulatory co-factors (intracellular Mg2+ and pH), which are essential metabolic sensors to provide neuronal environmental cues alterations in health and disease progression (Rabhi et al., 2017). This capability was previously limited due to the use and insensitivity of lower magnetic strengths such as 1.5-Tesla (Smith et al., 1995; Lotti et al., 1996; Forlenza et al., 2005) or 3-Tesla (Mandal et al., 2012; Rijpma et al., 2018).
Another methodological contribution of this study was the resolution of Pi into two separate peaks, intracellular and extracellular compartments (Ren et al., 2018). The extracellular Pi pool includes the signal of Pi from both the vascular and interstitial space plus the space occupied by cerebrospinal fluid (CSF). One hypothesis that could be explored in subsequent studies is whether brain atrophy leads to increased extracellular space and reduced intracellular space, assuming invariant extracellular-Pi is