Glutamine cycle enzymes in the crayfish giant nerve fiber: Implications for axon‐to‐glia signaling

Abstract

Two of the key enzymes involved in glutamate metabolism, glutaminase and glutamine synthetase, were quantitatively localized to axons and glia of the crayfish giant nerve fiber by immunocytochemistry and electron microscopy of antibody-linked gold microspheres. In Western blots, rabbit antisera for glutamine synthetase and glutaminase specifically recognized crayfish polypeptides corresponding approximately in size to subunits of purified mammalian brain enzymes. Glutamine synthetase immunoreactivity was found to be 11 times greater in the adaxonal glial cells than in the axon. Glutaminase immunoreactivity was found in somewhat greater concentration (2.5:1) in glia as compared to axoplasm. Glutamate immunoreactivity also was evaluated and found to be present in high concentration in both glia and axons, as might be expected for an important substrate of cellular metabolism. Using radiolabeled substrates it was demonstrated that glutamine and glutamate were interconverted by the native enzymes in the intact crayfish giant nerve fiber and that the formation of glutamine from glutamate occurred in the axoplasm-free nerve fiber, the cellular component of which is primarily periaxonal glia. The results of this investigation provide immunocytochemical and metabolic evidence consistent with an intercellular glutamine cycle that modulates the concentration of periaxonal glutamate and glutamine in a manner similar to that described for perisynaptic regions of the vertebrate central nervous system. These findings further corroborate previous electrophysiological evidence that glutamate serves as the axon-to-glial cell neurochemical signal that activates glial cell mechanisms responsible for periaxonal ion homeostasis.

Glutamine synthetase is a glial-specific marker in the olfactory regions of the lobster (Panulirus argus) nervous system

“…Without the production of an antiserum to purified lobster GS, the results presented here are the best analysis of GS in a non-vertebrate nervous system reported to date. Recently, analyses of the crayfish peripheral nervous system giant axon using rabbit antisera against mammalian GS found glial-specific localization of the antigen (McKinnon et al, 1995). Therefore, the findings in crayfish peripheral nervous system and our analyses of the lobster central nervous system are consistent with glial-specific expression of GS in arthropods much as it is in vertebrates.…”

“…The functional significance of glial compartmentalization of GS in vertebrates reflects the role of these cells in uptake and metabolization of neuroactive amino acids such as glutamate and GABA (Derouiche and Rauen, 1995). Glial uptake and compartmentalization of neuroactive amino acids released as transmitter substances by neurons has been reported in arthropod neural tissue as well (Orkand and Kravitz, 1971;McKinnon et al, 1995). Thus, it is tempting to suggest that a functional parallelism truly exists between glia of vertebrates and invertebrates.…”

“…Whether or not this is a characteristic of more primitive nervous systems has not been addressed previously in any great detail. Recently, McKinnon et al (1995) reported the identification of GS in certain glia associated with the crayfish giant axon of the peripheral nervous system. It is well established that amino acids such as glutamate and GABA are common neurotransmitters in many invertebrates, including arthropods.…”

Synthesis and release of N-acetylaspartylglutamate (NAAG) by crayfish nerve fibers: implications for axon–glia signaling

“…Stimulation also decreased the relative contribution of glutamate to axoplasmic and glial radiolabel while increasing the contribution of glutamine. This may re£ect activation of a glutamate^glutamine cycle between axons and their associated glia in the cray¢sh (McKinnon et al, 1995) similar to that described for synaptic regions of the nervous system of a variety of animals (Baslow, 2000).…”

“…Enzymes required for a`glutamine^glutamate cycle' between axons and glia, as described for synaptic regions of vertebrate CNS, have been described in the cray¢sh MGNF. Glutamine synthetase is primarily present in periaxonal glia, whereas glutaminase is equally distributed between the axon and glia (McKinnon et al, 1995). Taken together with the ¢nding of GCP II activity in the giant nerve ¢ber, the mechanism for recycling glutamate, glutamine, and NAAG between axons and their associated glia is present and appears to be active in this axon^glia system (Baslow, 2000).…”