High brain lactate is a hallmark of aging and caused by a shift in the lactate dehydrogenase A/B ratio

Abstract

At present, there are few means to track symptomatic stages of CNS aging. Thus, although metabolic changes are implicated in mtDNA mutation-driven aging, the manifestations remain unclear. Here, we used normally aging and prematurely aging mtDNA mutator mice to establish a molecular link between mitochondrial dysfunction and abnormal metabolism in the aging process. Using proton magnetic resonance spectroscopy and HPLC, we found that brain lactate levels were increased twofold in both normally and prematurely aging mice during aging. To correlate the striking increase in lactate with tissue pathology, we investigated the respiratory chain enzymes and detected mitochondrial failure in key brain areas from both normally and prematurely aging mice. We used in situ hybridization to show that increased brain lactate levels were caused by a shift in transcriptional activities of the lactate dehydrogenases to promote pyruvate to lactate conversion. Separation of the five tetrameric lactate dehydrogenase (LDH) isoenzymes revealed an increase of those dominated by the Ldh-A product and a decrease of those rich in the Ldh-B product, which, in turn, increases pyruvate to lactate conversion. Spectrophotometric assays measuring LDH activity from the pyruvate and lactate sides of the reaction showed a higher pyruvate → lactate activity in the brain. We argue for the use of lactate proton magnetic resonance spectroscopy as a noninvasive strategy for monitoring this hallmark of the aging process. The mtDNA mutator mouse allows us to conclude that the increased LDH-A/LDH-B ratio causes high brain lactate levels, which, in turn, are predictive of aging phenotypes.mtDNA mutator mouse | proton magnetic resonance spectroscopy | in situ hybridization | COX/SDH enzyme histochemistry | HPLC

High brain lactate is not caused by a shift in the lactate dehydrogenase A/B ratio

“…Under such conditions, it cannot be excluded that isoenzyme pattern may influence the time course of the lactate concentration. However, because the LDH activity is high, such transitions in net flux of the LDH reaction must be short-lived and are unlikely to explain the observations by Ross et al (1).…”

“…In their paper, Ross et al (1) suggested that high brain lactate is a marker of aging and that this lactate accumulation is explained by a shift in the lactate dehydrogenase (LDH) isoenzyme pattern also found to be associated with aging. Although the brain lactate increase with age is highly interesting, the suggested explanation may be problematic.…”

Defects in β-Cell Ca2+ Dynamics in Age-Induced Diabetes

“…Cooperation between brain and islet plays a critical role in the control of both energy and glucose homeostasis (52). Therefore, mitochondrial mutations/deletions in the central nervous system and other organs in mtDNA mutator mice might be crucial in the brain-centered regulation of glucose homeostasis via insulin-independent mechanisms (53).…”

Visualization of Mitochondrial Respiratory Function using Cytochrome <em>C</em> Oxidase / Succinate Dehydrogenase (COX/SDH) Double-labeling Histochemistry

“…This technique, with early studies dating back to 1968, remains popular, with many considering it the "gold standard" for identifying mitochondrial diseases in patients 14,19,26,27 . It is now frequently used to investigate mtDNA mutation-driven aging and aging-related disorders 12,13,18,20,21,24 . The COX/SDH double-labeling method is often used in parallel with other techniques to identify specific mtDNA mutations and to further investigate the mitochondrial respiratory enzymes, such as oximetric measurements and spectrophotometric enzyme analysis 28,29 .…”

“…Although other respiratory complexes could be investigated, Complexes IV and II are the most amenable to histochemical examination 8,9 . Complex II, or succinate dehydrogenase (SDH), is entirely encoded by nuclear DNA (Figure 1), and its activity is typically not affected by impaired mtDNA, although an increase might indicate mitochondrial biogenesis [10][11][12] . The impaired mtDNA observed in mitochondrial diseases, aging, and age-related diseases often leads to the presence of cells with low or absent COX activity 2,12-14 .…”