http://onlinelibrary.wiley.com/doi/10.1002/glia.22898/full
Article first published online: 6 AUG 2015
DOI: 10.1002/glia.22898
© 2015 The Authors. Glia Published by Wiley Periodicals, Inc.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs
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Keywords:
- glia proliferation;
- neurosphere;
- genomewide analysis
Abstract
Astrocytes
react to brain injury in a heterogeneous manner with only a subset
resuming proliferation and acquiring stem cell properties in vitro.
In order to identify novel regulators of this subset, we performed
genomewide expression analysis of reactive astrocytes isolated 5 days
after stab wound injury from the gray matter of adult mouse cerebral
cortex. The expression pattern was compared with astrocytes from intact
cortex and adult neural stem cells (NSCs) isolated from the subependymal
zone (SEZ). These comparisons revealed a set of genes expressed at
higher levels in both endogenous NSCs and reactive astrocytes, including
two lectins—Galectins 1 and 3. These results and the pattern of
Galectin expression in the lesioned brain led us to examine the
functional significance of these lectins in brains of mice lacking
Galectins 1 and 3. Following stab wound injury, astrocyte reactivity
including glial fibrillary acidic protein expression, proliferation and
neurosphere-forming capacity were found significantly reduced in mutant
animals. This phenotype could be recapitulated in vitro and was
fully rescued by addition of Galectin 3, but not of Galectin 1. Thus,
Galectins 1 and 3 play key roles in regulating the proliferative and NSC
potential of a subset of reactive astrocytes. GLIA 2015.
Introduction
Reactive
gliosis is a widespread reaction of glial cells to pathological
processes in the brain. It involves astrocytes, NG2 glia, and microglia
that mediate beneficial and adverse effects, such as wound closure and
scar formation, respectively (Kettenmann et al., 2011). Recent work indicates a striking heterogeneity in the reaction of each type of glial cells (Anderson et al., 2014; Burda and Sofroniew, 2014; Dimou and Götz, 2014). This could be best demonstrated by live in vivo
imaging, which revealed a surprisingly heterogeneous reaction of
astrocytes reacting to stab wound injury in the adult murine cerebral
cortex gray matter (GM), with some astrocytes hardly reacting at all,
others polarizing toward the injury site and yet others proliferating
and generating two daughter astrocytes (Bardehle et al., 2013).
Furthermore, clonal analysis demonstrated that the differential
reaction of astrocyte subtypes is seemingly related to their distinct
developmental origin (Martín-López et al., 2013).
In view of this heterogeneity, it is now important to address the
mechanisms regulating the reaction of these distinct astrocyte subsets
after brain injury.
Astrocytes resuming
cell division after lesion are of particular importance, as
proliferation is the only means to increase astrocyte numbers at the
injury site in the cerebral cortex GM (Bardehle et al., 2013).
Indeed, proliferating astrocytes are critical for restricting the
injury size and the number of infiltrating cells and inflammation, since
their elimination has been shown to aggravate brain damage after lesion
(Burda and Sofroniew, 2014).
Interestingly, astrocyte proliferation in the GM is highly
injury-dependent and does not occur upon amyloid plaque deposition or
even pronounced neuronal cell death, in spite of profound microglia
activation and proliferation (Behrendt et al., 2013; Sirko et al., 2013).
Instead, it is selectively elicited upon injury involving alterations
of the blood brain barrier, such as traumatic, ischemic, and
demyelinating injuries (Behrendt et al., 2013; Dimou and Götz, 2014; Gadea et al., 2008; Götz and Sirko, 2013; Kamphuis et al., 2012).
These injury-specific differences led to the identification of signals
regulating reactive astrocyte proliferation, including endothelin-1,
sonic hedgehog and fibroblast growth factor (FGF) signaling (Gadea et
al., 2008; Kang et al., 2014; Sirko et al., 2013; Zamanian et al., 2012).
To obtain a more comprehensive view on the key regulators of reactive
astrocyte proliferation, we set out to examine the pattern of gene
expression in reactive astrocytes at the peak of their proliferation
following stab wound injury in comparison to nonproliferative astrocytes
in the intact adult cerebral cortex GM.
As
a subset of proliferating reactive astrocytes acquire neural stem cell
(NSC) potential after injury, monitored by the ability to form
multipotent, self-renewing neurospheres (Buffo et al., 2008; Grande et al, 2013; Sirko et al., 2013),
this prompts the question how much of the gene expression changes of
reactive astrocytes may be shared with NSCs. Only genomewide expression
analysis comparing reactive astrocytes, NSCs and nonreactive astrocytes
allow determining the degree of similarity between NSCs and reactive
astrocytes and the extent of injury-specific gene expression.
A
small number of candidates shared by reactive astrocytes and endogenous
NSCs have already been identified and tested, including glial
fibrillary acidic protein (GFAP), Nestin, Musashi, DSD1-proteoglycan,
and Tenascin-C (for review, see Götz et al., 2015; Robel et al., 2011; Sirko et al., 2009).
However, these proteins also appear in injury conditions without
reactive proliferation of astrocytes and/or neurosphere formation
(Kamphuis et al., 2012;
Robel at al., 2011), thus emphasizing the need for additional molecular
insights. Toward this aim, we compared genomewide expression of
astrocytes reacting to stab wound with astrocytes from the intact adult
GM, as well as an existing expression profile of endogenous NSCs located
in the adult SEZ (Beckervordersandforth et al., 2010).
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