Postnatal development of the molecular complex underlying astrocyte polarization

Abstract

Astrocytes are highly polarised cells with processes that ensheath microvessels, cover the brain surface, and abut synapses. The endfoot membrane domains facing microvessels and pia are enriched with aquaporin-4 water channels (AQP4) and other members of the dystrophin associated protein complex (DAPC). Several lines of evidence show that loss of astrocyte polarization, defined by the loss of proteins that are normally enriched in astrocyte endfeet, is a common denominator of several neurological diseases such as mesial temporal lobe epilepsy, Alzheimer’s disease, and stroke. Little is known about the mechanisms responsible for inducing astrocyte polarization in vivo. Here we introduce the term endfoot-basal lamina junctional complex (EBJC) to denote the proteins that consolidate and characterize the gliovascular interface. The present study was initiated in order to resolve the developmental profile of the EBJC in mouse brain. We show that the EBJC is established after the first week postnatally. Through a combination of methodological approaches, including light microscopic and high resolution immunogold cytochemistry, quantitative RT-PCR, and Western blotting, we demonstrate that the different members of this complex exhibit distinct ontogenic profiles––with the extracellular matrix (ECM) proteins laminin and agrin appearing earlier than the other members of the complex. Specifically, while laminin and agrin expression peak at P7, quantitative immunoblot analyses indicate that AQP4, α-syntrophin, and the inwardly rectifying K+ channel Kir4.1 expression increases towards adulthood. Our findings are consistent with ECM having an instructive role in establishing astrocyte polarization in postnatal development and emphasize the need to explore the involvement of ECM in neurological disease.Electronic supplementary materialThe online version of this article (doi:10.1007/s00429-014-0775-z) contains supplementary material, which is available to authorized users.

Developmental expression of Kir4.1 in astrocytes and oligodendrocytes of rat somatosensory cortex and hippocampus

“…In accordance with recent studies (Lunde et al, 2014; Nwaobi et al, 2014), we found a continuous increase of Kir4.1 expression from being close to undetectable at E20 to being strongly expressed at adult stages (P60). These data are in agreement with studies suggesting that K ir channel expression correlates positively with cell differentiation (MacFarlane and Sontheimer 1997; Sontheimer et al, 1989; Barres et al, 1990; Olsen and Sontheimer, 2004) and with electrophysiological studies demonstrating that Kir4.1 channel activity increases during the first several postnatal weeks (Bordey and Sontheimer, 1997; Seifert et al, 2009).…”

Translation in astrocyte distal processes sets molecular heterogeneity at the gliovascular interface

“…Among the endfeetome, we found molecules already known to control cerebrovascular functions such as Cx43 (Gja1), which is implicated in BBB immune quiescence [8, 34], Agt, which plays a role in BBB integrity [35], or Aqp4, Kir4.1 (Kncj10), Hepacam and Mlc1, which are all involved in perivascular homeostasis [13, 30, 36]. Their presence strongly suggested that the endfeetome represents important astroglial functions for the regulation of the brain vascular physiology.…”

Mechanisms underlying AQP4 accumulation in astrocyte endfeet

“…An obvious candidate is laminin. Laminin is a main constituent of the endothelial basal lamina and is among the first matrix molecules to appear at the endothelial‐endfoot interface (Lunde et al, ). However, in the present experiments, laminin did not induce any polarization on its own, nor did it enhance the polarizing effect of brain endothelial cells.…”