Inverse correlation between mitochondrial size and metabolic competence: a quantitative cytochemical study of cytochrome oxidase activity

Abstract

Mitochondria are topologically closed bilayered systems where the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate occurs via oxidative phosphorylation. The ordered architecture (and its extension) of the mitochondria (i.e. inner membrane, outer membrane and cristae) constitutes a critical topographic arrangement for their energy-providing mechanisms. Thus, quantitative estimations of the ultrastructural features of organelles preferentially stained by means of function-related cytochemical reactions reliably report on their potential to supply adequate amounts of ATP. On the basis of this rationale, we carried out a computer-assisted cytochemical study of cytochrome oxidase (COX) activity on mitochondria of different size in the cerebellar cortex of adult rats. The total intra-mitochondrial area of the cytochemical precipitates (CPA)/mitochondrion, the area (MA) and the longer diameter (F max ) of COX-positive organelles were measured. The ratio (R): CPA/MA was also calculated and referred to as the percentage of mitochondrial inner membrane area involved in COX activity. The regression analysis of R vs MA showed a significant inverse correlation (r=0.905). The fourfold increase in MA from quartiles I to IV was matched by increases in F max and CPA, respectively, but it was also related to a 25% decrease in R. By matching quantitative cytochemical estimations of COX activity within mitochondria with the morphometric assessment of their ultrastructural features, the present study correlates size to the metabolic competence of COX-positive organelles. Quantitative cytochemistry of COX activity is currently regarded as a reliable marker of cellular metabolism; thus our findings support the hypothesis that enlargements in size are inversely correlated with the mitochondrial metabolic competence.

Experimental Apoptosis Provides Clues about the Role of Mitochondrial Changes in Neuronal Death

“…A recent report from our laboratory has clearly shown that the small and medium-sized organelles are much more efficient in providing adequate ATP amounts than are the organelles of larger size. 17,18 Considering our findings altogether, the present investigation confirms and strengthens the notion that mitochondria play a central role in regulating the processes involved in programmed cell death. In turn, this suggests that mitochondria may be considered reliable targets for intervention strategies in several pathological conditions (such as the neurodegenerative diseases typical of aging), for which cell death constitutes a well-documented step in the progression of the pathology, that is, Alzheimer's and Parkinson's disease.…”

Mitochondrial Dynamics Associated with Oxygen-Glucose Deprivation in Rat Primary Neuronal Cultures

“…Although mitochondrial fragmentation reduces ATP production, enlarged mitochondria due to an imbalance favoring fusion over fission, produce more energy compared with normal mitochondria [11], [12]. However, the opposite has also been reported [13], [14].…”

Neuronal Apoptosis in Alzheimer's Disease

“…COX activity was cytochemically evidenced as reported in our previous papers. [6][7][8] Briefly, the freshly excised tissue samples (40-60-μm thickness) were fixed 4-5 min in the following solution: 2.5% paraformaldehyde, 1.5% glutaraldehyde in 0.1 mol/L phosphate buffer, pH 7.4 + 4% sucrose. Following overnight rinse in the buffer sucrose solution, the samples were incubated for 4 h at 37 • C in the dark in the following solution (100 mL): 50 mg DAB, 27 mg cytochrome c, 4 g sucrose.…”

“…Following overnight rinse in the buffer sucrose solution, the samples were incubated for 4 h at 37 • C in the dark in the following solution (100 mL): 50 mg DAB, 27 mg cytochrome c, 4 g sucrose. [6][7][8] Postfixation in osmium tetroxide was followed by embedding, sectioning, and contrasting procedures as for conventional electron microscopic procedures. According to the above-described procedure, in the DAB-COX reaction the dark precipitate from COX activity is markedly evident on the cristae and inner mitochondrial membrane and can be readily observed with the electron microscope.…”

“…In turn, mitochondrial metabolic competence (MMC), defined as the organelle's capacity to provide adequate amounts of adenosintriphosphate in due time, might represent a critical determinant, playing a pivotal role in several biological processes and pathological conditions. [3][4][5][6][7][8] Cytochrome oxidase (COX) (complex IV of the mitochondrial respiratory chain) is the terminal electron acceptor in oxidative phosphorylation and, for this reason, has been considered a reliable cellular marker of MMC; [6][7][8] thus, quantitative estimations of the activity of this key enzyme for cellular respiration may provide information on the functional capacities of selected populations of organelles. To further investigate the role of precocious agerelated mitochondrial dysfunction in the development of senile dementia, we estimated MMC in a monkey model of AD (Macaca fascicularis) by measuring some parameters, functional to COX activity, in different CNS areas of animals at different ages.…”