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Although mitochondria have been the object of intensive study over many decades, some aspects of their metabolism remain to be fully elucidated, including the L L-lactate metabolism. We review here the novel insights arisen from investigations on [7]. Our present position, based on our own work and that of others reviewed here is that the evidence for mitochondrial metabolism of L L-LAC, due to the existence of both L L-LAC carrier-mediated transport processes and of the mitochondrial L L-lactate dehydrogenase (mL-LDH) is now compelling. SpermatozoaThe first evidence in favour of the presence of a putative mL-LDH was from Clausen who found that about 40% of total activity in sperm cells was present in a particulate fraction showing high succinate dehydrogenase activity [8].The existence of the mL-LDH which can reduce pyridine nucleotide in the mitochondrial matrix was demonstrated in hypotonically treated rabbit epididymal spermatozoa by means of oxygen uptake and fluorimetric studies [9]. It was also shown that the mL-LDH functions actively in these cells. In sperm cells the cytosolic L L-lactate dehydrogenase (cL-LDH) and mL-LDH appear to be the same isoenzyme unique to those cells: L-LDH-X. The dual localization of the enzyme enables mammalian spermatozoa to exchange cytosolic and mitochondrial reducing equivalents by means of a L L-LAC/ pyruvate (PYR) shuttle, as suggested by Blanco et al. [10] and reconstituted later [11]. LiverSince liver possesses the enzymatic machinery for gluconeogenesis (GNG) and since L L-LAC is a major substrate for GNG, liver play a major role in L L-LAC metabolism. Indeed, metabolism of L L-LAC has traditionally been considered solely as a function of the cL-LDH in spite of the fact that the L L-LAC oxidation by mitochondria and the existence of an mL-LDH in the inner compartments of isolated rat liver mitochondria (RLM) has been widely reported [12][13][14][15]. In a recent detailed investigation de Bari et al. [16] showed that externally added L L-LAC can be oxidized in the matrix by an mL-LDH, with reduction of intramitochondrial NAD(P) +

A critical role for mitochondrial dysfunction has been proposed in the pathogenesis of Down's syndrome (DS), a human multifactorial disorder caused by trisomy of chromosome 21, associated with mental retardation and early neurodegeneration. Previous studies from our group demonstrated in DS cells a decreased capacity of the mitochondrial ATP production system and overproduction of reactive oxygen species (ROS) in mitochondria. In this study we have tested the potential of epigallocatechin-3-gallate (EGCG) - a natural polyphenol component of green tea - to counteract the mitochondrial energy deficit found in DS cells. We found that EGCG, incubated with cultured lymphoblasts and fibroblasts from DS subjects, rescued mitochondrial complex I and ATP synthase catalytic activities, restored oxidative phosphorylation efficiency and counteracted oxidative stress. These effects were associated with EGCG-induced promotion of PKA activity, related to increased cellular levels of cAMP and PKA-dependent phosphorylation of the NDUFS4 subunit of complex I. In addition, EGCG strongly promoted mitochondrial biogenesis in DS cells, as associated with increase in Sirt1-dependent PGC-1α deacetylation, NRF-1 and T-FAM protein levels and mitochondrial DNA content. In conclusion, this study shows that EGCG is a promoting effector of oxidative phosphorylation and mitochondrial biogenesis in DS cells, acting through modulation of the cAMP/PKA- and sirtuin-dependent pathways. EGCG treatment promises thus to be a therapeutic approach to counteract mitochondrial energy deficit and oxidative stress in DS.

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Mitochondrial dysfunctions critically impair nervous system development and are potentially involved in the pathogenesis of various neurodevelopmental disorders, including Down syndrome (DS), the most common genetic cause of intellectual disability. Previous studies from our group demonstrated impaired mitochondrial activity in peripheral cells from DS subjects and the efficacy of epigallocatechin-3-gallate (EGCG) - a natural polyphenol major component of green tea - to counteract the mitochondrial energy deficit. In this study, to gain insight into the possible role of mitochondria in DS intellectual disability, mitochondrial functions were analyzed in neural progenitor cells (NPCs) isolated from the hippocampus of Ts65Dn mice, a widely used model of DS which recapitulates many major brain structural and functional phenotypes of the syndrome, including impaired hippocampal neurogenesis. We found that, during NPC proliferation, mitochondrial bioenergetics and mitochondrial biogenic program were strongly compromised in Ts65Dn cells, but not associated with free radical accumulation. These data point to a central role of mitochondrial dysfunction as an inherent feature of DS and not as a consequence of cell oxidative stress. Further, we disclose that, besides EGCG, also the natural polyphenol resveratrol, which displays a neuroprotective action in various human diseases but never tested in DS, restores oxidative phosphorylation efficiency and mitochondrial biogenesis, and improves proliferation of NPCs. These effects were associated with the activation of PGC-1α/Sirt1/AMPK axis by both polyphenols. This research paves the way for using nutraceuticals as a potential therapeutic tool in preventing or managing some energy deficit-associated DS clinical manifestations.

In the present study we investigated whether isolated rat liver mitochondria can take up and metabolize D-lactate. We found the following: (1) externally added D-lactate causes oxygen uptake by mitochondria [P/O ratio (the ratio of mol of ATP synthesized to mol of oxygen atoms reduced to water during oxidative phosphorylation)=2] and membrane potential (Delta(psi)) generation in processes that are rotenone-insensitive, but inhibited by antimycin A and cyanide, and proton release from coupled mitochondria inhibited by alpha-cyanocinnamate, but not by phenylsuccinate; (2) the activity of the putative flavoprotein (D-lactate dehydrogenase) was detected in inside-out submitochondrial particles, but not in mitochondria and mitoplasts, as it is localized in the matrix phase of the mitochondrial inner membrane; (3) three novel separate translocators exist to mediate D-lactate traffic across the mitochondrial inner membrane: the D-lactate/H(+) symporter, which was investigated by measuring fluorimetrically the rate of endogenous flavin reduction, the D-lactate/oxoacid antiporter (which mediates both the D-lactate/pyruvate and D-lactate/oxaloacetate exchanges) and D-lactate/malate antiporter studied by monitoring photometrically the appearance of the D-lactate counteranions outside mitochondria. The D-lactate translocators, in the light of their different inhibition profiles separate from the monocarboxylate carrier, were found to differ from each other in the V(max) values and in the inhibition and pH profiles and were shown to regulate mitochondrial D-lactate metabolism in vitro. The D-lactate translocators and the D-lactate dehydrogenase could account for the removal of the toxic methylglyoxal from cytosol, as well as for D-lactate-dependent gluconeogenesis.

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