Astroglial glutamate transporters coordinate excitatory signaling and brain energetics
Section snippets
Introduction to glutamate transporters
Glutamate is arguably the most important neurotransmitter in mammals; it mediates the fast excitatory signaling that is needed for essentially all motor, sensory, and autonomic processing (for reviews, see McDonald and Johnston, 1990, Robinson and Coyle, 1987). The plasticity of excitatory signaling also underlies memory formation (for review, see Ho et al., 2011). ‘Appropriate’ excitatory signaling is required for normal brain development and abnormal excitatory signaling contributes to many
Model systems to study GLT-1- or GLAST-mediated uptake
Although astroglial GLT-1 arguably represents the predominant route for glutamate clearance, it is not clear that there is a system or technique that can be readily used to assay this pool of transporter for biochemical analyses. Compared to brain tissue, astrocytes in culture express very little or no GLT-1 and there is essentially no dihydrokainate-sensitive (GLT-1-mediated) uptake. In contrast, cultured astrocytes express high levels of GLAST protein and the pharmacology of uptake is
Glutamate transporter interactions (co-immunoprecipitating proteins)
A variety of hypothesis-driven or yeast two-hybrid approaches were used to identify proteins that co-immunoprecipitate with one or more of the Na+-dependent glutamate transporters, including Ajuba (Marie et al., 2002), glutamate transporter associated proteins (GTRAPs) (Jackson et al., 2001, Lin et al., 2001), protein kinase Cα (González et al., 2003, González et al., 2005), a septin GTPase (Sept 2) (Kinoshita et al., 2004), syntaxin 1A (Yu et al., 2006), PSD-95 (Gonzalez-Gonzalez et al., 2008,
Coupling to Na+/K+-ATPase
Of all of the proteins known/thought to interact with the glial glutamate transporters GLT-1 and GLAST, the interactions with the Na+/K+-ATPase are the best characterized and conceptually the most straight-forward. Glutamate entry through GLT-1/GLAST is steeply dependent upon the Na+ gradient, with each glutamate molecule accompanied by the influx of 3 Na+ ions (and a proton) and the efflux of a K+ ion (Zerangue and Kavanaugh, 1996). Glutamate uptake is associated with a sharp increase in the
Coupling to the Na+/Ca2+ exchanger
The Na+/Ca2+ exchanger (NCX) is a bidirectional exchanger that, in its so-called ‘forward mode’, couples the inward movement of three Na+ ions to the outward movement of one Ca2+ ion (Blaustein and Lederer, 1999). There are three isoforms (NCX1-3) that are the product of distinct genes. All three isoforms are expressed in astrocytes (Minelli et al., 2007, Pappalardo et al., 2014, Zhang et al., 2014).
There are multiple lines of evidence suggesting physical and functional interactions between the
Coupling of glutamate transport to glycogen phosphorylase
Glycogen phosphorylase was also identified in co-immunoprecipitates with GLT-1 (Genda et al., 2011). In the nervous system, glycogen and glycogen phosphorylase are almost exclusively found in astrocytes (Pfeiffer et al., 1992, Pfeiffer-Guglielmi et al., 2003, Richter et al., 1996). There are two forms of glycogen phosphorylase, a brain form and a muscle form (for recent discussions, see Brown and Ransom, 2015, Dienel and Cruz, 2015, DiNuzzo et al., 2013, Hertz et al., 2015). These two forms are
Coupling of glutamate transport to glycolysis
We identified five glycolytic enzymes in the proteomic analysis of GLT-1 immunoprecipitates; this would seemingly support functional coupling between GLT-1 and glycolysis (Genda et al., 2011). In fact almost 20 years earlier, Pellerin and Magistretti demonstrated that glutamate stimulates glucose uptake and increases extracellular lactate in astrocyte cultures. They showed that the effects of glutamate were mimicked by a non-metabolizable transporter substrate (d-aspartate) and blocked by an
Coupling of glutamate transport to mitochondria
Of the 73 proteins that co-immunoprecipitate with GLT-1, 25 were categorized as mitochondrial (Genda et al., 2011). Several of the proteins are found on the outer mitochondrial membrane, including voltage dependent anion channel isoforms 2 and 3 (VDAC2, VDAC3), mitochondrial glutamate carrier 1 (GC1, Slc25a22), the glutamate-aspartate exchanger (Aralar, Slc25a12), and the α-ketoglutarate/malate exchanger (Slc25a11). Several other proteins found on the inner mitochondrial membrane or in the
Implications for glutamate metabolism
Generally, recycling of transmitter pools of glutamate is simplified into a glutamate–glutamine cycle (Fig. 3). In this cycle, glutamate is cleared by astroglial transporters. After transport, glutamate is converted to glutamine by glutamine synthetase, an enzyme that is almost exclusively expressed in astrocytes (Norenberg and Martinez-Hernandez, 1979). This glutamine is then exported from astrocytes by transporters (SNATs) and the extracellular glutamine is then imported into neurons for
Glutamate transport and the neurovascular response
The human brain, which represents ∼2% of body weight, consumes a disproportionate amount of the body's basal energy budget (roughly 20%) (for discussions, see Harris et al., 2012, Hertz et al., 2007, Stobart and Anderson, 2013, Weber and Barros, 2015). Unlike other biological systems that can use fats, amino acids, or carbohydrates/glucose for fuel, the brain is essentially dependent upon glucose for fuel (Stanley et al., 2014, Weber and Barros, 2015). While glycogen and glutamate provide local
Summary and future directions
In summary, several studies have demonstrated that the astroglial glutamate transporters, GLT-1 and GLAST, co-compartmentalize with and functionally couple to diverse sources of ‘energy’, including the Na+/K+-ATPase, Na+/Ca2+ exchanger, glycogen, glycolysis, and mitochondria. Many of these sources of ‘energy’ either directly or indirectly support glutamate uptake by maintaining the Na+ gradient required for active transport or by generating ATP. All of these processes are also functionally
Conflicts of interests
Michael B. Robinson is the Editor-in-Chief for Neurochemistry International and is financially reimbursed for his work as Editor. The workflow for the review process was modified so that M.B.R. did not have editorial access and was kept blinded to the reviewers’ identities.
Acknowledgments
The authors are supported by the National Institutes of Neurologic Disease and Stroke (R01 NS077773). Joshua Jackson is also partially supported by a Foerderer award from Children's Hospital of Philadelphia. The authors would like to thank John O'Donnell and Zila Martinez-Lozado for their comments that were used to improve the manuscript. The authors would also like to thank Elizabeth Krizman who helped with the preparation of the figures.
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