Group I metabotropic glutamate receptors control proliferation, survival and differentiation of cultured neural progenitor cells isolated from the subventricular zone of adult mice

Abstract

Neural progenitor cells (NPCs) are found in the subventricular zone (SVZ) of the adult brain, a specialized neurogenic niche that might provide a substrate for brain repair after injury. The incomplete knowledge of how NPCs in the niche respond to local signals limits the use of cultured NPCs in the development of cell transplantation strategies. We show that neurospheres obtained from the SVZ of the adult mouse expressed functional mGlu1 and mGlu5 metabotropic glutamate receptors. Pharmacological blockade of mGlu5 receptors promoted the apoptotic death of progenitors undergoing differentiation into neurons (PSA/NCAM⁺ cells for the most part), whereas blockade of mGlu1 receptors reduced the proliferation rate of NPCs, and promoted their differentiation towards the neuronal lineage. We conclude that endogenous activation of mGlu5 receptors might support specifically the survival of neuronal-restricted precursors, whereas endogenous activation of mGlu1 receptors might sustain the proliferation of earlier progenitors. Moreover, mGlu1 receptor antagonists increased the survival of NPCs, suggesting that endogenously activated mGlu1 receptors might play a role in the natural cell loss regulating the number or the type of progenitors.

Introduction

Neural progenitor cells (NPCs) of the adult central nervous system (CNS) have a limited capacity of self-renewal, but can generate neurons and glia (Gage, 2000). In the adult mammalian brain, a large number of neurons are produced both in the subgranular zone (SGZ) of the hippocampus (Altman and Das, 1965, Kornack and Rakic, 1999), and in the subventricular zone (SVZ) of the lateral ventricle (reviewed in Alvarez-Buylla and Lim, 2004). The extracellular signals that regulate neurogenesis in these areas are only partially identified. The neurotransmitter GABA is emerging as a key signal within the neurogenic niches. In the SVZ, GABA that is released by neuroblasts seems to act as a negative feedback to limit the proliferation of GFAP-expressing progenitors (Liu et al., 2005). In addition, GABA appears to regulate the migration rate of neuroblasts during adult SVZ neurogenesis (Ge et al., 2007).

Recent evidence indicates that glutamate, the major excitatory neurotransmitter in mature neurons, can also influence neurogenesis (reviewed in Schlett, 2006). Interestingly, brain injuries that involve glutamate excitotoxicity, such as brain ischemia and epileptic seizures, all stimulate neurogenesis (Ribak and Dashtipour, 2002, Sharp et al., 2002); hence, glutamate receptor signaling might regulate the proliferative response to brain insults.

Glutamate acts through a variety of ionotropic (iGlu) and metabotropic (mGlu) receptors (Rao and Finkbeiner, 2007, Recasens et al., 2007). Blockade of NMDA iGlu receptors has been shown to sustain NPC proliferation both in the adult hippocampus (Cameron et al., 1995, Nacher et al., 2001) and in the neonatal neocortex (Hirasawa et al., 2003), and to arrest the migration of NPCs from the SVZ to the cortical plate (Behar et al., 1999). The activity of AMPA iGlu receptors both reduces NPC proliferation in the embryonic SVZ (Haydar et al., 2000) and neocortex (LoTurco et al., 1995), and increases proliferation in the embryonic ventricular zone (Haydar et al., 2000) and in the adult hippocampus (Bai et al., 2003). The influence of mGlu receptors on neurogenesis has been less studied. The mGlu receptor family includes eight subtypes subdivided into three groups on the basis of their sequence, pharmacological profile, and transduction pathways. Group I includes mGlu1 and mGlu5 receptors, which are coupled to Gq proteins; group II includes mGlu2 and mGlu3 receptors, which are coupled to Gi/Go; group III includes mGlu4, mGlu6, mGlu7, and mGlu8 receptors, which are also coupled to Gi/Go (Recasens et al., 2007). mGlu receptors have been implicated in the regulation of brain plasticity (Nicoletti et al., 1986, Dudek and Bear, 1989, Flint et al., 1999), neuronal survival (Copani et al., 1998, Catania et al., 2001) and differentiation (Catania et al., 2001, Canudas et al., 2004), and at least the mGlu5 subtype has been found to be expressed in zones of active neurogenesis of the embryonic and postnatal rat brain (Di Giorgi-Gerevini et al., 2005). Hence, mGlu receptors might function as endogenous regulators of neurogenesis. Accordingly, activation of mGlu3 receptors reduces basal levels of apoptosis and increases the proliferation of NPCs from the SVZ of newborn rats (Brazel et al., 2005). Both mGlu3 and mGlu5 receptors support proliferation and survival of NPCs isolated from the forebrain of mouse embryos (Di Giorgi-Gerevini et al., 2005), whereas activation of mGlu1 receptors promotes neurogenesis in organotypic cultures prepared from neonate rat hippocampus (Baskys et al., 2005).

Besides the demonstration that a chronic treatment with the mGlu2/3 receptor antagonist, MGS0039, enhances cell proliferation in the adult hippocampus (Yoshimizu and Chaki, 2004), the function of mGlu receptors in NPCs of the adult brain remains uncharacterized. Here, we examined the role of group I mGlu receptors in the proliferation, differentiation and survival of NPCs isolated from the adult mouse SVZ.