Selective Distribution of Lactate Dehydrogenase Isoenzymes in Neurons and Astrocytes of Human Brain

Abstract

In vertebrates, the interconversion of lactate and pyruvate is catalyzed by the enzyme lactate dehydrogenase. Two distinct subunits combine to form the five tetrameric isoenzymes of lactate dehydrogenase. The LDH-5 subunit (muscle type) has higher maximal velocity (Vmax) and is present in glycolytic tissues, favoring the formation of lactate from pyruvate. The LDH-1 subunit (heart type) is inhibited by pyruvate and therefore preferentially drives the reaction toward the production of pyruvate. There is mounting evidence indicating that during activation the brain resorts to the transient glycolytic processing of glucose. Indeed, transient lactate formation during physiological stimulation has been shown by 1H-magnetic resonance spectroscopy. However, since whole-brain arteriovenous studies under basal conditions indicate a virtually complete oxidation of glucose, the vast proportion of the lactate transiently formed during activation is likely to be oxidized. These in vivo data suggest that lactate may be formed in certain cells and oxidized in others. We therefore set out to determine whether the two isoforms of lactate dehydrogenase are localized to selective cell types in the human brain. We report here the production and characterization of two rat antisera, specific for the LDH-5 and LDH-1 subunits of lactate dehydrogenase, respectively. Immunohistochemical, immunodot, and western-blot analyses show that these antisera specifically recognize their homologous antigens. Immunohistochemistry on 10 control cases demonstrated a differential cellular distribution between both subunits in the hippocampus and occipital cortex: neurons are exclusively stained with the anti-LDH1 subunit while astrocytes are stained by both antibodies. These observations support the notion of a regulated lactate flux between astrocytes and neurons.

Physiological and Molecular Characteristics of Rat Hypothalamic Ventromedial Nucleus Glucosensing Neurons

“…Lactate transport into neurons produced by astrocytes has been proposed as an alternate source of energy for neuronal metabolism (8,42). For this to occur, lactate must be transported using MCT (55) followed by conversion to pyruvate using lactate dehydrogenase (LDH) (42,56).…”

“…Finally, the activity of glucosensing neurons is affected by both metabolites (i.e., lactate, ketone bodies) (39,40) and peripheral hormones (e.g., insulin, leptin) that circulate in proportion to the amount of stored adipose tissue (41). Lactate, produced largely by astrocytes (42), and ketone bodies from the periphery can serve as alternate energy sources or signaling molecules to glucose once it is transported into neurons by monocarboxylate transporters (MCTs) (43). Current evidence suggests that neurons express mainly the MCT-2 isoform (44).…”

Astrocytes and Epilepsy

“…Of these LDH5 is found exclusively in astrocytes. 49 Whether these subunits are altered in the sclerotic hippocampus is not yet known. The changes in pyruvate carboxylase and cytosolic malic enzyme (two other enzymes exclusive to astrocytes in the sclerotic hippocampus) have not been investigated yet.…”

Effects of continuous hypoxia on energy metabolism in cultured cerebro-cortical neurons

“…Among the five investigated enzymes LDH was most significantly (p<0.001) increased (251%; 515.6 ± 9.5 versus 175.7 ± 34 mU/mg protein) above control in hypoxic neurons (Table 1, Figure 2). To determine whether this increase in LDH activity was a result of increased enzyme expression, we measured the LDH-1 isoform by Western blot since this isoform is predominantly localized in neurons (Venkov et al, 1976;Tholey et al, 1981;Bittar et al, 1996). LDH-1 expression was increased by 119% in hypoxic neurons as compared to that in control normoxic neurons (Figure 3).…”