Changes in the proliferative capacity of NG2 cell subpopulations during postnatal development of the mouse hippocampus

Abstract

Besides astrocytes and oligodendrocytes, NG2 proteoglycan-expressing cells (NG2 glia) represent a third subtype of macroglia in the brain. Originally described as oligodendrocyte precursor cells, they feature several characteristics not expected from mere progenitor cells, including synaptic connections with neurons. There is accumulating evidence that the properties of NG2 glia differ between different brain regions and developmental stages. To further analyze this proposed heterogeneity, we studied electrophysiological properties, transcript and protein expression, distribution and proliferative capacity of NG2 glia during postnatal development, focusing on the hippocampus and corpus callosum. All NG2 glia displayed a 'complex' current pattern consisting of voltage- and time-dependent in- and outward currents. In juvenile mice, Kir current densities and rectification index were highly variable and on average significantly lower than in adult animals. Single cell RT-PCR analyses of electrophysiologically characterized cells demonstrated that different glial genes were expressed at variable extent, independent of developmental stage and genetic background. In the hippocampus proper and the corpus callosum, the density of NG2 glia was highest at postnatal days (P)10-12, decreased by ~50 % at P25-35 and then remained stable in adults (P80-90). Interestingly, co-expression of NG2 and S100β, a marker for mature astrocytes, increased from 7 % at P10-12 to 27 % at P25-35 in the hippocampus proper, and then dropped again in the stratum radiatum at P80-90. In the dentate gyrus and corpus callosum, co-expression of NG2 and S100β was very low (3 %) and constant throughout development. Age-related differences were also observed with Ki-67, a proliferation marker. In NG2 glia of the CA1 region, its expression decreased from 16 % at P10-12 to 9 % (P25-35) and then 3 % (P80-90). Triple-stainings revealed that Ki-67 was also expressed in 2-3 % of NG2/S100β-positive cells in the juvenile and mature stratum radiatum, indicating that the latter, in contrast to S100β-positive astrocytes, still host proliferative potential. Taken together, we found significant differences in transcript and protein expression, electrophysiological properties and proliferative capacity of NG2 glia in the mouse forebrain, suggesting the co-existence of several subpopulations of NG2 glia. Our data thus support the idea of a substantial regional and developmental heterogeneity in this subtype of macroglia.

Properties of human astrocytes and NG2 glia

“…Like in rodents, human NG2 glia consistently coexpress NG2 and PDGFα receptors but not GFAP (Bedner et al, ; Wilson, Scolding, & Raine, ). In contrast, the Ca 2+ binding protein S100β was expressed in almost all NG2 glial cells of human hippocampus (Bedner et al, ), but only in a subset of mouse hippocampal NG2 glia (Jabs et al, ; Karram et al, ; Moshrefi‐Ravasdjani et al, ). Interestingly, in the human nonsclerotic hippocampus the density of NG2 glial cells was higher than in the hippocampus of mice (~11 × 10 3 vs. ~1.5 × 10 3 cells/mm 3 ).…”

“…For instance, they express functional voltage-gated K + , Na + , and Ca 2+ channels but are unable to generate action potentials (Haberlandt et al, 2011;Nishiyama et al, 2009). Glial cells expressing the proteoglycan NG2 are abundantly distributed in white and gray matter areas of the developing and adult brain, where they make up 5-10% of all glia and are heterogeneous with regard to their antigen profile (Degen et al, 2012;Moshrefi-Ravasdjani et al, 2017;Trotter, Karram, & Nishiyama, 2010). They sustain the capacity to proliferate and differentiate throughout life and constitute therefore the largest proliferative cell population outside the adult neurogenic niches in the CNS (Dawson, Polito, Levine, & Reynolds, 2003;Moshrefi-Ravasdjani et al, 2017).…”

“…Glial cells expressing the proteoglycan NG2 are abundantly distributed in white and gray matter areas of the developing and adult brain, where they make up 5-10% of all glia and are heterogeneous with regard to their antigen profile (Degen et al, 2012;Moshrefi-Ravasdjani et al, 2017;Trotter, Karram, & Nishiyama, 2010). They sustain the capacity to proliferate and differentiate throughout life and constitute therefore the largest proliferative cell population outside the adult neurogenic niches in the CNS (Dawson, Polito, Levine, & Reynolds, 2003;Moshrefi-Ravasdjani et al, 2017). Fate mapping studies revealed that in the postnatal white matter, NG2 glial cells mainly differentiate into myelinating oligodendrocytes (Dimou, Simon, Kirchhoff, Takebayashi, & Götz, 2008;Kang, Fukaya, Yang, Rothstein, & Bergles, 2010;Zhu et al, 2011;Zhu, Bergles, & Nishiyama, 2008), a process that is enhanced following demyelination, CNS injury, and in chronic neurodegenerative disease (Kang et al, 2010;Levine & Reynolds, 1999;Magnus et al, 2008;McTigue, Wei, & Stokes, 2001).…”

“…In contrast, most of the grey matter NG2 glia do not differentiate into oligodendrocyte but maintain their phenotype throughout postnatal life (Dimou et al, 2008;Rivers et al, 2008;Zhu et al, 2008). In several brain regions, the density of NG2 glia decreases with increasing age (Moshrefi-Ravasdjani et al, 2017) and their loss through death, differentiation, or ablation induces rapid proliferation and migration of adjacent cells to reestablish their density (Hughes, Kang, Fukaya, & Bergles, 2013). Intriguingly, rodent NG2 glia receive direct synaptic inputs from glutamatergic and GABAergic neurons, a feature that is unique among glial cells (Bergles, Roberts, Somogyi, & Jahr, 2000;Jabs et al, 2005;Lin & Bergles, 2004).…”

Oligodendroglia-lineage cells in brain plasticity, homeostasis and psychiatric disorders

“…Lineage tracing studies indicate that approximately half of the resident NG2-glia cells do not give rise to any other cell types [63,64]. Recently, NG2-glial subpopulations have been described in the mouse hippocampus based on their proliferative capacities [65] and migration [66]. In addition to the regional differences (i.e.…”