Cell-specific abnormalities of glutamate transporters in schizophrenia: sick astrocytes and compensating relay neurons?

Abstract

Excitatory amino-acid transporters (EAATs) bind and transport glutamate, limiting spillover from synapses due to their dense perisynaptic expression primarily on astroglia. Converging evidence suggests that abnormalities in the astroglial glutamate transporter localization and function may underlie a disease mechanism with pathological glutamate spillover as well as alterations in the kinetics of perisynaptic glutamate buffering and uptake contributing to dysfunction of thalamo-cortical circuits in schizophrenia. We explored this hypothesis by performing cell- and region-level studies of EAAT1 and EAAT2 expression in the mediodorsal nucleus of the thalamus in an elderly cohort of subjects with schizophrenia. We found decreased protein expression for the typically astroglial-localized glutamate transporters in the mediodorsal and ventral tier nuclei. We next used laser-capture microdissection and quantitative polymerase chain reaction to assess cell-level expression of the transporters and their splice variants. In the mediodorsal nucleus, we found lower expression of transporter transcripts in a population of cells enriched for astrocytes, and higher expression of transporter transcripts in a population of cells enriched for relay neurons. We confirmed expression of transporter protein in neurons in schizophrenia using dual-label immunofluorescence. Finally, the pattern of transporter mRNA and protein expression in rodents treated for 9 months with antipsychotic medication suggests that our findings are not due to the effects of antipsychotic treatment. We found a compensatory increase in transporter expression in neurons that might be secondary to a loss of transporter expression in astrocytes. These changes suggest a profound abnormality in astrocyte functions that support, nourish and maintain neuronal fidelity and synaptic activity.

Cell-subtype-specific changes in adenosine pathways in schizophrenia

“…As most schizophrenia subjects were on antipsychotic medications like haloperidol, it is not possible to meaningfully compare the effects with subjects off antipsychotics in this dataset. An additional limitation is that gene expression changes are measured in populations of cells that are highly enriched [24][25][26], but may contain neuropil, processes, or other small cells including interneurons. ENT1 expression is sensitive to hypoxic conditions.…”

“…Cell capture and quantitative polymerase chain reaction (qPCR) Laser capture microdissection (LCM) and qPCR methods are described in detail in the Supplemental Information and were conducted and validated as previously described [24][25][26]. Briefly, for region-level studies, RNA was extracted from 14 µm DLPFC cryosections using the RNeasy Minikit (Qiagen, NL) according to the manufacturer's instructions.…”

“…Quality control studies confirming sample enrichment are described in Supplementary Fig. 8, as well as in prior publications [24][25][26].…”

Understanding the genetic liability to schizophrenia through the neuroepigenome

“…Within brain tissue, however, neurons are intermingled with, and outnumbered by, multiple types of non-neuronal cells (microglia, oligodendrocytes and astrocytes). This is important, since SCZ-associated abnormalities have been demonstrated in specific cellular populations, including neocortical neurons (Benes et al, 2001; Chen et al, 2014b; Rajkowska et al, 2001), astrocytes (McCullumsmith et al, 2015; Schnieder and Dwork, 2011), oligodendrocytes (Haroutunian et al, 2014; Mighdoll et al, 2015; Roussos and Haroutunian, 2014) and microglia (Bernstein et al, 2015). Furthermore, CRE-mediated epigenetic regulation shows cell type specificity (Cheung et al, 2010; Heintzman et al, 2009; Maurano et al, 2012; Roadmap Epigenomics et al, 2015).…”

Connectivity analyses of bioenergetic changes in schizophrenia: Identification of novel treatments

“…LCM was performed as previously described (11,18,59,60). Briefly, 14 µm frozen formerly Arcturus, Mountain View, CA, USA) (11,59,60). Laser settings were adjusted prior to each session to produce optimal cutting and capturing with laser settings ranging from 70-100 mW in power, and 2,000-3,000 µsec in duration.…”