Disruption of striatal glutamatergic transmission induced by mutant huntingtin involves remodeling of both postsynaptic density and NMDA receptor signaling

Abstract

We study the striatal susceptibility to NMDA receptor (NMDAR)-mediated injury of two Huntington’s disease (HD) transgenic mice: R6/1 and R6/1:BDNF^+/−. We found that R6/1:BDNF^+/− mice – which express reduced levels of BDNF – were more resistant than R6/1 mice to intrastriatal injection of quinolinate. This increased resistance is related to a differential reduction in expression of NMDAR scaffolding proteins, MAGUKs (PSD-95, PSD-93, SAP-102 and SAP-97) but not to altered levels or synaptic location of NMDAR. A robust reorganization of postsynaptic density (PSD) was detected in HD transgenic mice, shown by a switch of PSD-93 by PSD-95 in PSD. Furthermore, NMDAR signaling pathways were affected by different BDNF levels in HD mice; we found a reduction of synaptic αCaMKII (but not of nNOS) in R6/1:BDNF^+/− compared to R6/1 mice. The specific regulation of MAGUKs and αCaMKII in striatal neurons may reflect a protective mechanism against expression of mutant huntingtin exon-1.

Introduction

Huntington’s disease (HD) is due to an abnormal expansion of CAG codon in exon-1 of the huntingtin (htt) gene, resulting in a devastating cognitive and motor disorder (HDCRG, 1993) which is mainly attributable to a selective degeneration of striatal projection neurons (Reiner et al., 1988). Growing evidence links NMDA receptor (NMDAR)-mediated excitotoxicity with the selective degeneration in HD (Perez-Navarro et al., 2006). Presymptomatic HD transgenic mice display increased NMDAR-mediated current densities (Starling et al., 2005) and higher intracellular Ca²⁺ overload after glutamate application (Tang et al., 2005). These characteristics are concomitant with enhanced susceptibility to NMDAR-mediated excitotoxicity of striatal neurons induced by mutant htt in vitro (Zeron et al., 2002). However, in spite of sustained NMDAR overactivity, the loss of striatal neurons is a late event in some HD transgenic mice (Mangiarini et al., 1996, Canals et al., 2004, Diaz-Hernandez et al., 2005). It has therefore been suggested that the chronic excitotoxic stress initiated by mutant htt could be modulated by compensatory mechanisms that involve changes in NMDAR-mediated neurotoxic pathways (Jarabek et al., 2004).

Synaptic targeting and activity of NMDAR are modulated by members of the membrane-associated guanylate kinase (MAGUK) family of proteins (PSD-95, PSD-93, SAP-102 and SAP-97; Kim and Sheng, 2004). MAGUKs also mediate the interaction of NMDAR with signaling proteins, such as nNOS (Brenman et al., 1996, Sattler et al., 1999). This interaction facilitates the NMDAR-mediated Ca²⁺ influx activation of nNOS (Sattler et al., 1999), which in turn activates the p38-kinase neurotoxic pathway (Cao et al., 2005). Altered striatal MAGUK expression has been related to the pathophysiology of neurodegenerative disorders of the basal ganglia (Gardoni et al., 2006, Nash et al., 2005) including HD (Jarabek et al., 2004, Luthi-Carter et al., 2002). Moreover, mutant htt increases NMDAR phosphorylation via PSD-95 (Song et al., 2003). On the other hand, αCaMKII also modulates NMDAR function and participates in NMDAR-mediated neurotoxicity (Hajimohammadreza et al., 1995, Colbran and Brown, 2004). Following glutamate treatment, αCaMKII translocates to the postsynaptic density (PSD) (Dosemeci et al., 2001) where it phosphorylates NMDAR (Gardoni et al., 1998) and inhibits the pro-survival RAS/ERK pathway (Oh et al., 2004). Thus, pretreatment with a CaMKII inhibitor prevents neuronal death following ischemia (Hajimohammadreza et al., 1995).

Another mechanism involved in the specific vulnerability of striatal neurons in HD is the regulation of the activity of neurotrophic factors (Zuccato et al., 2001). BDNF is the most potent trophic factor for striatal projection neurons in the excitotoxic model (Perez-Navarro et al., 2000) and in transgenic mice (Canals et al., 2004). Furthermore, reduced levels of endogenous BDNF increase mutant htt-induced degeneration of striatal neurons inducing advanced onset and increased severity of motor abnormalities (Canals et al., 2004). However, the mechanism by which BDNF levels modulate mutant htt-induced striatal neuropathology remains unclear. We therefore studied whether impaired BDNF trophic support in HD modulates abnormalities of NMDAR in striatal neurons, and the possible mechanism involved in these changes. Our results show that the reduction of BDNF levels in mice expressing mutant htt exon-1 increases the disruption of NMDAR signaling in striatal neurons by selectively modulating scaffolding and signaling proteins of NMDAR in the PSD.