A metabotropic glutamate receptor 3 (mGlu3R) isoform playing neurodegenerative roles in astrocytes is prematurely up-regulated in an Alzheimer's model.

Subtype 3 metabotropic glutamate receptor (mGlu3R) displays a broad range of neuroprotective effects. We previously demonstrated that mGlu3R activation in astrocytes protects hippocampal neurons from Aß neurotoxicity through stimulation of both neurotrophin release and Aß uptake. Alternative-spliced variants of mGlu3R were found in human brains. The most prevalent variant, mGlu3Δ4, lacks exon 4 encoding the transmembrane domain and can inhibit ligand binding to mGlu3R. To date, neither its role in neurodegenerative disorders nor its endogenous expression in CNS cells has been addressed. The present paper describes for the first time an association between altered hippocampal expression of mGlu3Δ4 and Alzheimer's disease (AD) in the preclinical murine model PDAPP-J20, as well as a deleterious effect of mGlu3Δ4 in astrocytes. As assessed by western blot, hippocampal mGlu3R levels progressively decreased with age in PDAPP-J20 mice. On the contrary, mGlu3Δ4 levels were drastically increased with aging in nontransgenic mice, but prematurely over-expressed in 5-month-old PDAPP-J20-derived hippocampi, prior to massive senile plaque deposition. Also, we found that mGlu3Δ4 co-precipitated with mGlu3R mainly in 5-month-old PDAPP-J20 mice. We further showed by western blot that primary cultured astrocytes and neurons expressed mGlu3Δ4, whose levels were reduced by Aß, thereby discouraging a causal effect of Aß on mGlu3Δ4 induction. However, heterologous expression of mGlu3Δ4 in astrocytes induced cell death, inhibited mGlu3R expression, and prevented mGlu3R-dependent Aß glial uptake. Indeed, mGlu3Δ4 promoted neurodegeneration in neuron-glia co-cultures. These results provide evidence of an inhibitory role of mGlu3Δ4 in mGlu3R-mediated glial neuroprotective pathways, which may lie behind AD onset.

Astrocytes from cortex and striatum show differential responses to mitochondrial toxin and BDNF: implications for protection of striatal neurons expressing mutant huntingtin.

BACKGROUND: Evidence shows significant heterogeneity in astrocyte gene expression and function. We previously demonstrated that brain-derived neurotrophic factor (BDNF) exerts protective effects on whole brain primary cultured rat astrocytes treated with 3-nitropropionic acid (3NP), a mitochondrial toxin widely used as an in vitro model of Huntington's disease (HD). Therefore, we now investigated 3NP and BDNF effects on astrocytes from two areas involved in HD: the striatum and the entire cortex, and their involvement in neuron survival. METHODS: We prepared primary cultured rat cortical or striatal astrocytes and treated them with BDNF and/or 3NP for 24 h. In these cells, we assessed expression of astrocyte markers, BDNF receptor, and glutamate transporters, and cytokine release. We prepared astrocyte-conditioned medium (ACM) from cortical and striatal astrocytes and tested its effect on a cellular model of HD. RESULTS: BDNF protected astrocytes from 3NP-induced death, increased expression of its own receptor, and activation of ERK in both cortical and striatal astrocytes. However, BDNF modulated glutamate transporter expression differently by increasing GLT1 and GLAST expression in cortical astrocytes but only GLT1 expression in striatal astrocytes. Striatal astrocytes released higher amounts of tumor necrosis factor-α than cortical astrocytes in response to 3NP but BDNF decreased this effect in both populations. 3NP decreased transforming growth factor-ß release only in cortical astrocytes, whereas BDNF treatment increased its release only in striatal astrocytes. Finally, we evaluated ACM effect on a cellular model of HD: the rat striatal neuron cell line ST14A expressing mutant human huntingtin (Q120) or in ST14A cells expressing normal human huntingtin (Q15). Neither striatal nor cortical ACM modified the viability of Q15 cells. Only ACM from striatal astrocytes treated with BDNF and ACM from 3NP + BDNF-treated striatal astrocytes protected Q120 cells, whereas ACM from cortical astrocytes did not. CONCLUSIONS: Data suggest that cortical and striatal astrocytes respond differently to mitochondrial toxin 3NP and BDNF. Moreover, striatal astrocytes secrete soluble neuroprotective factors in response to BDNF that selectively protect neurons expressing mutant huntingtin implicating that BDNF modulation of striatal astrocyte function has therapeutic potential against neurodegeneration.

Melanocortin-receptor 4 activation modulates proliferation and differentiation of rat postnatal hippocampal neural precursor cells.

Postnatal hippocampal neurogenesis is essential for learning and memory. Hippocampal neural precursor cells (NPCs) can be induced to proliferate and differentiate into either glial cells or dentate granule cells. Notably, hippocampal neurogenesis decreases dramatically with age, partly due to a reduction in the NPC pool and a decrease in their proliferative activity. Alpha-melanocyte-stimulating hormone (α-MSH) improves learning, memory, neuronal survival and plasticity. Here, we used postnatally-isolated hippocampal NPCs from Wistar rat pups (male and female combined) to determine the role of the melanocortin analog [Nle4, D-Phe7]-α-MSH (NDP-MSH) in proliferation and fate acquisition of NPCs. Incubation of growth-factor deprived NPCs with 10 nM NDP-MSH for 6 days increased the proportion of Ki-67- and 5-bromo-2'-deoxyuridine (BrdU)-positive cells, compared to the control group, and these effects were blocked by the MC4R antagonist JKC-363. NDP-MSH also increased the proportion of glial fibrillar acidic protein (GFAP)/Ki-67, GFAP/sex-determining region Y-box2 (SOX2) and neuroepithelial stem cell protein (NESTIN)/Ki-67-double positive cells (type-1 and type-2 precursors). Finally, NDP-MSH induced peroxisome proliferator-activated receptor (PPAR)-γ protein expression, and co-incubation with the PPAR-γ inhibitor GW9662 prevented the effect of NDP-MSH on NPC proliferation and differentiation. Our results indicate that in vitro activation of MC4R in growth-factor-deprived postnatal hippocampal NPCs induces proliferation and promotes the relative expansion of the type-1 and type-2 NPC pool through a PPAR-γ-dependent mechanism. These results shed new light on the mechanisms underlying the beneficial effects of melanocortins in hippocampal plasticity and provide evidence linking the MC4R and PPAR-γ pathways in modulation of hippocampal NPC proliferation and differentiation.