Differential Regulation of Glutamate Transporter Subtypes by Pro-Inflammatory Cytokine TNF-α in Cortical Astrocytes from a Rat Model of Amyotrophic Lateral Sclerosis

Abstract

Dysregulation of the astroglial glutamate transporters GLAST and GLT-1 has been implicated in several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) where a loss of GLT-1 protein expression and activity is reported. Furthermore, the two principal C-terminal splice variants of GLT-1 (namely GLT-1a and GLT-1b) show altered expression ratio in animal models of this disease. Considering the putative link between inflammation and excitotoxicity, we have here characterized the influence of TNF-α on glutamate transporters in cerebral cortical astrocyte cultures from wild-type rats and from a rat model of ALS (hSOD1G93A). Contrasting with the down-regulation of GLAST, a 72 h treatment with TNF-α substantially increased the expression of GLT-1a and GLT-1b in both astrocyte cultures. However, as the basal level of GLT-1a appeared considerably lower in hSOD1G93A astrocytes, its up-regulation by TNF-α was insufficient to recapitulate the expression observed in wild-type astrocytes. Also the glutamate uptake activity after TNF-α treatment was lower for hSOD1G93A astrocytes as compared to wild-type astrocytes. In the presence of the protein synthesis inhibitor cycloheximide, TNF-α did not influence GLT-1 isoform expression, suggesting an active role of dynamically regulated protein partners in the adaptation of astrocytes to the inflammatory environment. Confirming the influence of inflammation on the control of glutamate transmission by astrocytes, these results shed light on the regulation of glutamate transporter isoforms in neurodegenerative disorders.

Constitutive downregulation protein kinase C epsilon in hSOD1G93A astrocytes influences mGluR5 signaling and the regulation of glutamate uptake

“…Indeed, deficient glutamate clearance has been reported in synaptosomes prepared from several affected brain structures and spinal cords derived from sporadic ALS patients (Rothstein, Martin, & Kuncl, 1992;Grosskreutz, Van Den Bosch, & Keller, 2010). Earlier publications from our group (Dumont, Goursaud, Desmet, & Hermans, 2014;Goursaud, Maloteaux, & Hermans, 2009) and the present results also support such suggestions by showing a reduced aspartate uptake velocity in hSOD1 G93A astrocytes. the human equivalent of GLT-1), as has been observed in the motor cortex and spinal cord of familial and sporadic ALS patients (Rothstein, Van, Levey, Martin, & Kuncl, 1995;Fray et al, 1998;Grosskreutz et al, 2010).…”

Prokineticin‐2 promotes chemotaxis and alternative A2 reactivity of astrocytes

“…Astrocyte uptake of glutamate not only reduces the extracellular levels but can also induce astrocyte reactivity and alter multiple astrocyte functions (Morales & Rodriguez, 2012), so it is important to examine astrocyte-mediated glutamate uptake. Interestingly, proinflammatory cytokines, such as TNFα, decrease the expression of glutamate transporters, especially GLAST (Dumont, Goursaud, Desmet, & Hermans, 2014). Therefore, we used radiolabeled glutamate to determine whether rPK2 treatment could increase glutamate uptake by primary mouse astrocytes.…”

“Targeting astrocytes in CNS injury and disease: A translational research approach”

“…Patients with ALS have been reported to have impaired glutamate metabolism, caused by reduced glutamate uptake by astrocyte-associated glutamate transporters (Rothstein et al, 1992). A recent study found that the inflammatory cytokine TNF-α is able to increase the expression of GLT-1 in astrocytes cultured from wild-type rats, but not those cultured from a rat model of ALS (Dumont et al, 2014). Another study has linked astrocytes, inflammation, and ALS pathology, reporting that in both mouse and human ALS, astrocytes have upregulated expression of the anti-inflammatory cytokine transforming growth factor-β1 (TGF-β1), which prevents microglial and T cell production of IGF-1, leading to accelerated disease progression due to the loss of inflammatory-mediated neuroprotection (Endo et al, 2015).…”