Localization of Hexokinase in Neural Tissue: Electron Microscopic Studies of Rat Cerebellar Cortex

Abstract

The distribution of hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) in the rat cerebellar cortex has been studied at the electron microscopic level using the peroxidase-antiperoxidase procedure. Extensive staining of cytoplasmic regions, with some increased staining at mitochondrial profiles, was seen in the cell bodies of both neurons (basket, stellate, Lugaro, Golgi, and granule cells) and astrocytes. Oligodendrocytes showed little or no detectable staining. Purkinje cell perikarya were much less intensely stained than were the perikarya of other neurons. The initial portion of the Purkinje dendrite was, like the perikaryon from which it emerged, lightly stained. More intense staining was seen in the secondary and tertiary branches of the Purkinje dendrite, but the terminal branches were devoid of stain. Granule cell dendrites were well stained in their initial portions but devoid of stain in their terminal dendritic digits which form part of the cerebellar glomeruli. In contrast to the unstained granule cell dendritic digits, the central mossy fiber nerve terminal of the glomerulus exhibited intense staining of the mitochondrial profiles and of synaptic vesicles adjacent to the mitochondria. Axons of basket cells showed intense staining in the segments adjacent to the Purkinje cell soma, while terminal twigs of the basket axons in the pinceau surrounding the (unstained) initial segment of the Purkinje axon showed markedly decreased staining intensity. These results indicate that there may be substantial variation in hexokinase levels between the various regions of neuronal processes. Hexokinase was seen at both cytoplasmic and mitochondrial locations in a variety of cells. It does not appear likely that location of hexokinase can be directly correlated with cell type, i.e., with neurons versus glia.

Metabolism of [U-13C]Leucine in Cultured Astroglial Cells

“…Also in the instances of energy substrates such as sugars (36,37) and fatty acids (38) astroglial cells have been found to generate standard fuel molecules (36), i.e., lactate and ketone bodies, respectively. These compounds are released from the astrocytes and could be taken up by neighbouring cells, especially by those which contain hexokinase only at low activity or not at all (39)(40)(41)(42)(43). Among the fuel molecules forwarded by astrocytes to adjacent neurons could also be the 2-oxoisocaproate derived from leucine, since at least some neurons are equipped with the enzymes required for introducing the keto acid into energy metabolism (14,44).…”

Glial Metabolism of Isoleucine

“…Like the energy metabolism of neurons [49] also that of oligodendrocytes may depend to a certain degree on the uptake of astrocyte-born substances. Oligodendrocytes express hexokinase only at low level and they are supposed to utilize in their energy metabolism substrates other than glucose [50,51]. Therefore, oligodendrocytes may well utilize BCAAs and substances derived from them for their energy metabolism.…”

ATP and Brain Function

“…This assumption is supported by the finding (Pysh and Khan, 1972), that although these two types of cells have about the same number of mitochondria, the crystal vol ume fraction of these organelles (i.e., crystal pack ing density) is considerably higher in neurons (17.3%) than in either astrocytes (11%) or oligoden drocytes (11.3%). Furthermore, whereas most of the glycolytic activity appears to be localized within neuronal perikarya and proximal dendrites (Friede, 1966;Kao-Jen and Wilson, 1980), the distal seg ments of dendrites often contain numerous mito chondria and the number of mitochondrial profiles per unit of cross-sectional area tends to be greatest in the smaller dendritic branches and axon termi nals (Nafstad and Blackstad, 1966). …”