Characterization of Vimentin-Immunoreactive Astrocytes in the Human Brain

Abstract

Astrocytes are commonly identified by their expression of the intermediate filament protein glial fibrillary acidic protein (GFAP). GFAP-immunoreactive (GFAP-IR) astrocytes exhibit regional heterogeneity in density and morphology in the mouse brain as well as morphological diversity in the human cortex. However, regional variations in astrocyte distribution and morphology remain to be assessed comprehensively. This was the overarching objective of this postmortem study, which mainly exploited the immunolabeling of vimentin (VIM), an intermediate filament protein expressed by astrocytes and endothelial cells which presents the advantage of more extensively labeling cell structures. We compared the densities of vimentin-immunoreactive (VIM-IR) and GFAP-IR astrocytes in various brain regions (prefrontal and primary visual cortex, caudate nucleus, mediodorsal thalamus) from male individuals having died suddenly in the absence of neurological or psychiatric conditions. The morphometric properties of VIM-IR in these brain regions were also assessed. We found that VIM-IR astrocytes generally express the canonical astrocytic markers Aldh1L1 and GFAP but that VIM-IR astrocytes are less abundant than GFAP-IR astrocytes in all human brain regions, particularly in the thalamus, where VIM-IR cells were nearly absent. About 20% of all VIM-IR astrocytes presented a twin cell morphology, a phenomenon rarely observed for GFAP-IR astrocytes. Furthermore VIM-IR astrocytes in the striatum were often seen to extend numerous parallel processes which seemed to give rise to large VIM-IR fiber bundles projecting over long distances. Moreover, morphometric analyses revealed that VIM-IR astrocytes were more complex than their mouse counterparts in functionally homologous brain regions, as has been previously reported for GFAP-IR astrocytes. Lastly, the density of GFAP-IR astrocytes in gray and white matter were inversely correlated with vascular density, but for VIM-IR astrocytes this was only the case in gray matter, suggesting that gliovascular interactions may especially influence the regional heterogeneity of GFAP-IR astrocytes. Taken together, these findings reveal special features displayed uniquely by human VIM-IR astrocytes and illustrate that astrocytes display important region- and marker-specific differences in the healthy human brain.

Widespread decrease of cerebral vimentin-immunoreactive astrocytes in depressed suicides

“…Fresh-frozen 1cm 3 tissue blocks from each region were fixed overnight in 10% formalin, suspended in 30% sucrose solution until equilibrium was reached, flash-frozen in -35°C isopentane, and cut on a sliding microtome into 50 µm-thick serial sections that were stored at -20°C in a cryoprotectant solution until processing for immunohistochemistry (IHC). Immunolabeling involved independently using each antibody listed in Table 2 within a conventional DAB IHC protocol and a separate stereological series of sections, as described previously (O'Leary et al, 2020).…”

“…Stereological cell counting and live tracing was performed as described previously (O'Leary et al, 2020), while additionally blinded to subject identities. Reliable stereological estimates, with a Gunderson coefficient of error (CE, m=1) < 0.10, were obtained by sampling the following percentage areas from section contours: 5% for GFAP in all regions; and 10% for VIM in the PFC GM and the caudate nucleus, 25% in the PFC WM, and 100% in the mediodorsal thalamus.…”

“…Animal studies have since revealed that astrocytes can strongly modulate most facets of neuronal activity, including neuronal firing, neurotransmitter synthesis, neurotransmitter reuptake, and synaptic transmission (Norenberg & Martinez-Hernandez, 1979;Rothstein et al, 1996;Duan et al, 1999;Cui et al, 2018;Covelo & Araque, 2018). Astrocytes might especially influence neuronal activity in the human brain, as they are almost threefold larger in volume and fourfold faster at signaling in the human cortex than in the mouse cortex (Oberheim et al, 2009;O'Leary et al, 2020).…”

Widespread Decrease of Cerebral Vimentin-Immunoreactive Astrocytes in Depressed Suicides

“…Animal studies have since revealed that astrocytes can strongly modulate most facets of neuronal activity, including neuronal firing, neurotransmitter synthesis, neurotransmitter reuptake, and synaptic transmission ( 2 – 6 ). Astrocytes might especially influence neuronal activity in the human brain, as they are almost three-fold larger in volume and four-fold faster at signaling in the human cortex than in the mouse cortex ( 7 , 8 ).…”

“…A previous post-mortem study has found qualitative differences in VIM-IR astrocytes in neurological conditions ( 35 ). In a recent post-mortem study, we characterized VIM immunoreactivity in different cortical and subcortical brain regions using samples from healthy individuals having died suddenly, and found that VIM-IR astrocytes had different densities from GFAP-IR astrocytes, but that both GFAP-IR and VIM-IR astrocyte density inversely correlated with CD31-IR vascular density ( 8 ). We were then interested in how these findings might relate to depression.…”