Polyglutamine-expanded Huntingtin Promotes Sensitization of N-Methyl-d-aspartate Receptors via Post-synaptic Density 95

Abstract

Huntington's disease (HD) 1 is a dominant inherited neurodegenerative disorder characterized by choreiform movement, psychiatric disturbance, and cognitive decline (2). The HD gene encodes a 350-kDa protein designated as huntingtin (3), which is richly expressed in dendrites and nerve terminals, where huntingtin is associated with synaptic vesicles and microtubule complexes (4 -5). The defect of the HD gene is the expansion of a CAG repeat encoding polyglutamine at its 5Ј end, and the length of the repeat is correlated with the age of onset and the severity of the disease (6).Although the HD gene has been identified for many years, how polyglutamine-expanded huntingtin causes neurons to die remains unclear. Increased glutamate-mediated excitotoxicity in HD has been a very popular hypothesis for the last 25 years (1). The hypothesis is generated from findings that the intrastriatal injection of glutamate or kainic acid in rat causes selective loss of medium spiny neurons that are also selectively affected in HD (7-8). This hypothesis is supported further by the findings that NMDA receptors are hyperactive, and excitotoxicity mediated by these receptors is enhanced significantly in HD transgenic mice (9 -10). These results suggest that overactivation of glutamate receptors may play a significant role in the pathogenesis of HD. However, over 95% of normal or polyglutamine-expanded huntingtin is located in the cytoplasm (6 -7), whereas glutamate receptors are cell surface receptors. How a cytoplasmic protein alters glutamate receptors on the cell surface membrane is an intriguing question.PSD-95 is a scaffold protein that contains an SH3 domain, a GK domain, and three PDZ domains that bind to the NMDA receptor NR2 subunits and kainate receptor GluR6 subunit (11)(12). The binding of PSD-95 to NMDA or kainate receptors causes the clustering of the receptors in the postsynaptic membrane and regulates NMDA-dependent long term potentiation and long term depression (12). PSD-95 also binds to cytoplasmic signaling proteins and links the receptors to cellular signaling cascades (13)(14). In transgenic mice lacking PSD-95, the frequency function of NMDA-dependent long term potentiation and long term depression is shifted, and spatial learning is impaired severely (15). Suppression of PSD-95 expression inhibits NMDA receptor-mediated activation of nitric-oxide synthase and excitotoxicity (16). These studies suggest that PSD-95 regulates glutamate receptor-mediated excitotoxicity and plays an important role in spatial learning, which is severely impaired in HD patients and HD transgenic mice (17)(18)(19). In previous studies, we observed that the overexpression of polyglutamine-expanded huntingtin caused neuronal apoptosis via activation of the mixed lineage kinase/c-Jun N-terminal kinase signaling pathway (20 -21), and mixed lineage kinase is also involved in neuronal toxicity mediated by GluR6 receptors via interaction with . The present study is intended to investigate the "missing link" protein between glutamate receptor...

Mutant huntingtin can paradoxically protect neurons from death

“…[29][30][31][32][33] Other candidates that possibly could interact with the domain of the protein encoded by exon 1, but have not been shown to interact only with this short sequence, include huntingtin interacting protein 1 (HIP1), Rab11 familyinteracting protein 2 (FIP2), huntingtin interacting protein 14 (HIP14), nuclear receptor corepressor 1 (N-CoR) and cystathionine beta-synthase (CBS) [34][35][36][37][38] (for overview see Figure 2). …”

“…The N171-82Q transgenic HD mouse, which exhibits reduced sensitivity to excitotoxin, has reduced levels of PSD95. 15 Sun et al 33 reported that a pathogenic poly-Q stretch (48 Q) impairs PSD95 binding to Huntingtin. In the same study, a Huntingtin construct with a non-pathogenic, 16 glutamine long poly-Q stretch protected cells in vitro from glutamate-mediated neuronal death by 82%.…”

Molecular mechanisms of excitotoxicity and their relevance to pathogenesis of neurodegenerative diseases

“…PSD-95 itself has been shown to bind the C-terminal tail of NR2 subunits and helps to stabilize NMDARs in cell surface clusters [67] . Htt was associated with PSD-95 in transfected 293T cells, and co-immunoprecipitation studies in human cortical tissue revealed interactions of htt with both PSD-95 and NMDAR subunits NR1, NR2A and NR2B [68] , suggesting that htt may associate with NMDARs via PSD-95. Recent research has shown that the polyQ expansion in htt is associated with an increase in NR1A/NR2B-mediated excitotoxic cell death.…”

Expression of Polyglutamine-expanded Huntingtin Induces Tyrosine Phosphorylation of N-Methyl-D-aspartate Receptors

“…TUNEL Staining-36 h post-transfection, HN33 cells were fixed with 4% paraformaldehyde and then permeabilized with 0.1% Triton X-100 for 2 min on ice, and TUNEL staining was performed as described previously (1). Most apoptotic HN33 cells were detached from the slides, and TUNEL staining was performed on the remaining attached cells.…”

“…In neuronal cells expressing the mutated huntingtin or mice transgenic for HD, NMDA receptors are highly responsive to the receptor agonists, and the receptor-mediated current is also significantly increased (18 -20). In our previous studies, we observed that expression of polyglutamine-expanded huntingtin induced sensitization of NMDA receptor via PSD-95 (1). In the present studies, we examined whether tyrosine phosphorylation of NMDA receptors may contribute to the sensitization of the receptors in HN33 cells.…”