Neuronal activity regulates glutamate transporter dynamics in developing astrocytes

Abstract

Glutamate transporters maintain a low ambient level of glutamate in the CNS and shape the activation of glutamate receptors at synapses. Nevertheless, the mechanisms that regulate the trafficking and localization of transporters near sites of glutamate release are poorly understood. Here we examined the subcellular distribution and dynamic remodeling of the predominant glutamate transporter GLT-1 (EAAT2) in developing hippocampal astrocytes. Immunolabeling revealed that endogenous GLT-1 is concentrated into discrete clusters along branches of developing astrocytes that were apposed preferentially to synapsin-1 positive synapses. GFP-GLT-1 fusion proteins expressed in astrocytes also formed distinct clusters that lined the edges of astrocyte processes, as well as the tips of filopodia and spine-like structures. Time-lapse 3D confocal imaging in tissue slices revealed that GFP-GLT-1 clusters were dynamically remodeled on a timescale of minutes. Some transporter clusters moved within developing astrocyte branches as filopodia extended and retracted, while others maintained stable positions at the tips of spine-like structures. Blockade of neuronal activity with tetrodotoxin reduced both the density and perisynaptic localization of GLT-1 clusters. Conversely, enhancement of neuronal activity increased the size of GLT-1 clusters and their proximity to synapses. Together, these findings indicate that neuronal activity influences both the organization of glutamate transporters in developing astrocyte membranes and their position relative to synapses.

Morphological plasticity of astroglia: Understanding synaptic microenvironment

“…The PAP remodeling, with or without local rearrangement of glutamate transporter proteins, is capable of altering the efficiency of extrasynaptic glutamate clearance or escape (Benediktsson et al, 2012; Danbolt, 2001; Panatier et al, 2006; Zheng et al, 2008) (although (Diamond et al, 1998)) and hence affecting the modus operandi for local excitatory circuitry. Altered glial coverage could also affect astrocyte‐neuron lactate transport contributing to synaptic efficacy (Suzuki et al, 2011) and local water homeostasis (Simard and Nedergaard, 2004).…”

“…In contrast, astrocytes in the hippocampus have very high levels of GLT‐1 and hardly any GLAST (Chaudhry et al, 1995; Furuta et al, 1997; Tzingounis and Wadiche, 2007). Furthermore, expression of glutamate transporters seems to depend on neuronal activity: decreased synaptic activity appears to downregulate levels (Benediktsson et al, 2012; Yang et al, 2009). There seems to be further differentiation of astrocytes within regions.…”

Riluzole effects on behavioral sensitivity and the development of axonal damage and spinal modifications that occur after painful nerve root compression

“…11,35,38,69 GLT-1 is only expressed by glial cells in the rat spinal cord, 70 and astrocytes require neuronal signaling to express GLT-1. 6,23,77 The downregulation of GLT-1 after a painful nerve root compression may be due to impaired afferent neurotransmission in the superficial laminae that is associated with the overall decrease in neuropeptides in this region (Figs. 3 and 4).…”

Deriving the Time Course of Glutamate Clearance with a Deconvolution Analysis of Astrocytic Transporter Currents

“…The estimated time course is an average measure of the time course of glutamate clearance at all areas of the astrocytic membrane to which we have electrophysiological access: it is not sensitive to heterogeneities in glutamate uptake at different sites within the astrocytic membrane 19 . This information is more likely to be obtained with high-resolution microscopy of glutamate-sensitive fluorescent reporters 20 .…”