Ketogenic diets cause opposing changes in synaptic morphology in CA1 hippocampus and dentate gyrus of late-adult rats.

Ketogenic diets (KDs) have beneficial effects on several diseases, such as epilepsy, mitochondriopathies, cancer, and neurodegeneration. However, little is known about their effects on aging individuals. In the present study, late-adult (19-month-old) rats were fed for 8 weeks with two medium chain triglycerides (MCT)-KDs, and the following morphologic parameters reflecting synaptic plasticity were evaluated in stratum moleculare of hippocampal CA1 region (SM CA1) and outer molecular layer of hippocampal dentate gyrus (OML DG): average area (S), numeric density (Nv(s)), and surface density (Sv) of synapses, and average volume (V), numeric density (Nv(m)), and volume density (Vv) of synaptic mitochondria. In SM CA1, MCT-KDs induced the early appearance of the morphologic patterns typical of old animals (higher S and V, and lower Nv(s) and Nv(m)). On the contrary, in OML DG, Sv and Vv of MCT-KDs-fed rats were higher (as a result of higher Nv(s) and Nv(m)) versus controls; these modifications are known to improve synaptic function and metabolic supply. The opposite effects of MCT-KDs might reflect the different susceptibility to aging processes: OML DG is less vulnerable than SM CA1, and the reactivation of ketone bodies uptake and catabolism might occur more efficiently in this region, allowing the exploitation of their peculiar metabolic properties. Present findings provide the first evidence that MCT-KDs may cause opposite morphologic modifications, being potentially harmful for SM CA1 and potentially advantageous for OML DG. This implies risks but also promising potentialities for their therapeutic use during aging.

Selective decline of the metabolic competence of oversized synaptic mitochondria in the old monkey cerebellum.

The morphofunctional features of synaptic mitochondria, positive to the activity of cytochrome oxidase (COX), were investigated in the cerebellar cortex of adult and old monkeys (Macaca fascicularis) to assess the potential age-related changes in the energy metabolism occurring at the neuronal synaptic compartment. The following mitochondrial ultrastructural parameters-numeric density (Nv), volume density (Vv), average volume (V), and average length (Fmax)-were measured by computer-assisted morphometric methods. The ratio (R) area of the COX cytochemical precipitate/area of the mitochondrion was semi-automatically calculated and considered as an estimation of the mitochondrial metabolic competence (MMC), that is, the capacity of single organelles to provide adequate amounts of adenosinetriphosphate. No age-related significant differences were found in any of the ultrastructural parameters taken into account, whereas a significant decrease of R was observed in old animals. In these animals, the quartile distribution of the COX-positive organelles, according to their respective cross-sectional area, showed no significant difference of R when comparing small (I quartile), medium-sized (II quartile), and large (III quartile) mitochondria, while a significant decrease of R was evident in oversized mitochondria (IV quartile). Although our data document an age-related preservation of the morphological features of COX-positive mitochondria in the monkey cerebellum, the significant decrease of R in old animals needs to be considered from the functional standpoint. Since COX is the terminal enzyme of the mitochondrial respiratory chain, the estimation of its activity is regarded as a reliable MMC index; thus our findings, by matching preferential cytochemistry and morphometry, support the hypothesis that the specific functional impairment of enlarged synaptic mitochondria may seriously affect information processing and cell-to-cell communication at synaptic junctional areas with aging.

Experimental apoptosis provides clues about the role of mitochondrial changes in neuronal death.

A quantitative morphometric study has been carried out in human neuroblastoma SK-N-BE cells to evaluate the ultrastructural features and the metabolic efficiency of mitochondria involved in the early steps of apoptosis. In mitochondria from control and apoptotic cells cytochrome oxidase (COX) activity was estimated by preferential cytochemistry. Number of mitochondria (numeric density: Nv), volume fraction occupied by mitochondria/microm3 of cytoplasm (volume density: Vv), and average mitochondrial volume (V) were calculated for both COX-positive and -negative organelles. The ratio (R) of the cytochemical precipitate area to the overall area of each mitochondrion was evaluated on COX-positive organelles to estimate the inner mitochondrial membrane fraction actively involved in cellular respiration. Following apoptotic stimulus, the whole mitochondrial population showed a significant increase of Nv and Vv, while V was significantly decreased. In COX-positive organelles higher values of Nv were found, V appeared significantly reduced, and Vv was unchanged. R was increased at a nonsignificant extent in apoptotic cells. COX-positive mitochondria accounted for 21% and 35% of the whole population in control and in apoptotic cells, respectively. These findings document that in the early stages of apoptosis the increased fraction of small mitochondria provides an adequate amount of ATP for progression of the programmed cell death and these more efficient organelles appear to represent a reactive response to the loss of metabolically impaired mitochondria. A better understanding of the mitochondrial role in neuronal apoptosis may suggest potential interventions to prevent the extensive nerve cell death typical of neurodegenerative diseases.