Ketogenic Diet-Induced Alterations in Neuronal Signaling-Related Proteins are Not Due to Differences in Synaptosome Protein Levels

Abstract

Impaired cognitive function is a hallmark of advancing age, and the potential to reverse or delay these cognitive deficits through dietary intervention holds promise for improving quality of life for older adults. Specifically, ketogenic diets (KDs) have now been widely used for the treatment of several neurological and peripheral disorders, including diseases profoundly affecting cognitive health, of which advanced age is the single greatest risk factor. However, the precise mechanisms of the efficacy of KD-based interventions to reverse age-related cognitive and neurobiological declines are not fully elucidated. We have previously demonstrated that a KD improves cognitive function, with concurrent increases in global levels of proteins related to synaptic signaling in the aging hippocampus (HPC) and prefrontal cortex (PFC). Despite these advances, it remains unclear as to whether these changes reflect biochemical modifications specifically localized to synaptic terminals. To address this important, unanswered question, we purified synaptosomes from the HPC and PFC of aging rats fed a KD or control diet (CD) for a minimum of 4 months and quantified 10 proteins related to synaptic transmission. In contrast to previous studies of global protein expression, the signaling proteins measured did not show significant differences between diet groups in synaptosomes isolated from either region. When pre-mortem performance on an Object-Place Paired Association task was considered, we found a significant correlation between several proteins within the HPC and PFC synaptosomes of CD-fed rats, more pronounced in CD-fed aged rats, that are conspicuously absent in KD-fed rats from both age groups. Moreover, there is a significant alteration in the ratio of VGAT/VGluT1, markers of excitatory and inhibitory synaptic vesicles, in the PFC with dietary intervention that is absent in the HPC, confirming prior reports of regionally specific alterations in excitatory and inhibitory signaling post KD. These new and extended findings reveal important, naturally occurring associations between protein levels localized to synaptic terminals, while clarifying that effects KD likely increase synaptic abundance without altering the biochemical composition of isolated synapses.

Keywords: Aging; GABA; Glutamate; Receptor; Synapse.

Fig. 1

GABAergic signaling related protein expression within hippocampal (HPC) and prefrontal cortical (PFC) synaptosome isolates. (A) There was a significant main effect of age on HPC VGAT expression (p = 0.05), (B-H) but no other effects of age or diet on any other GABAergic signaling related proteins within the HPC or PFC (p ≥ 0.05 for all comparisons). Data are represented as group means ± 1 SEM with individual dots representing each biological replicate (black = young, gray = aged, triangle = female), * = p < 0.05

Fig. 2

Glutamatergic signaling related protein expression within hippocampal (HPC) and prefrontal cortical (PFC) synaptosome isolates. (A-H) There were no significant effects in either of the brain regions of age (p > 0.25 for all comparisons), diet (p > 0.46 for all comparisons) or the interaction between diet and age (p > 0.17 for all comparisons) on the glutamatergic signaling-related proteins. Data are represented as group means ± 1 SEM with individual dots representing each biological replicate (black = young, gray = aged, triangle = female)

Fig. 3

Correlation matrix for (A) hippocampal and (B) prefrontal cortical synaptosomal protein concentrations with each other and with object-place paired association (OPPA) task performance in young and aged rats on the control (left) and ketogenic (right) diets. Values in the lower left quadrant indicate the correlation coefficient, whereas values in the upper right quadrant indicate correlation direction (color coded using scale shown on the right) and strength (indicated by circle size), * indicates p < 0.05