Estimating the glutamate transporter surface density in distinct sub-cellular compartments of mouse hippocampal astrocytes

Abstract

Glutamate transporters preserve the spatial specificity of synaptic transmission by limiting glutamate diffusion away from the synaptic cleft, and prevent excitotoxicity by keeping the extracellular concentration of glutamate at low nanomolar levels. Glutamate transporters are abundantly expressed in astrocytes, and previous estimates have been obtained about their surface expression in astrocytes of the rat hippocampus and cerebellum. Analogous estimates for the mouse hippocampus are currently not available. In this work, we derive the surface density of astrocytic glutamate transporters in mice of different ages via quantitative dot blot. We find that the surface density of glial glutamate transporters is similar in 7-8 week old mice and rats. In mice, the levels of glutamate transporters increase until about 6 months of age and then begin to decline slowly. Our data, obtained from a combination of experimental and modeling approaches, point to the existence of stark differences in the density of expression of glutamate transporters across different sub-cellular compartments, indicating that the extent to which astrocytes limit extrasynaptic glutamate diffusion depends not only on their level of synaptic coverage, but also on the identity of the astrocyte compartment in contact with the synapse. Together, these findings provide information on how heterogeneity in the spatial distribution of glutamate transporters in the plasma membrane of hippocampal astrocytes my alter glutamate receptor activation out of the synaptic cleft.

Astrocyte‐ and NMDA receptor‐dependent slow inward currents differently contribute to synaptic plasticity in an age‐dependent manner in mouse and human neocortex

“…(b) Age‐related decline of astrocyte‐specific EAAT‐1 and EAAT‐2 glutamate transporters was seen on the rodent hippocampus which increases ambient glutamate levels and thus weakens synaptic neurotransmission (Potier et al, 2010 ). Initially, there is an increase in glutamate transporter surface expression until it reaches its maximum in adults, followed by a slow decline with aging (Rǎdulescu et al, 2022 ). (c) According to the diffusion properties of glutamate, it is known that the increased tortuosity of the diffusion pathways can alter neuronal currents elicited by ambient glutamate (Syková, 2004 ; Syková & Vargová, 2008 ).…”

“…The charge transfer and kinetic parameters have a strong decline in humans and SIC activity completely disappears after 70. This might be due to an alteration of astrocytic glutamate release or uptake, as well as extrasynaptic NMDA receptors (Gramuntell et al, 2021 ; Law et al, 2003 ; Liu et al, 2004 ; Potier et al, 2010 ; Rǎdulescu et al, 2022 ).…”

“…For aging of SICs, the following parameters should be con- ent glutamate levels and thus weakens synaptic neurotransmission (Potier et al, 2010). Initially, there is an increase in glutamate transporter surface expression until it reaches its maximum in adults, followed by a slow decline with aging (Rǎdulescu et al, 2022). (c)…”

Revealing the contribution of astrocytes to glutamatergic neuronal transmission

“…GLAST and GLT-1 are highly expressed in astrocytes of the hippocampus, striatum, and cerebral cortex and oversee glutamate uptake at the synapse (Levy et al, 1993 ; Lehre et al, 1995 ; Bergles and Jahr, 1997 ; Mennerick et al, 1998 ). According to studies focused on evaluating the subcellular distribution of GLAST and GLT-1, both transporters are highly expressed in hippocampal astrocytes with a predominant presence of GLT-1 in the filopodium and perivascular end-feet, and GLAST is mostly present in the soma and processes (Schreiner et al, 2014 ; Radulescu et al, 2022 ). However, the GLT-1 isoforms (GLT-1a and b) are expressed in neurons from the hippocampus, cerebral cortex, striatum, thalamus, and midbrain (Chen et al, 2002 , 2004 ; Berger et al, 2005 ).…”