Impairment and Restoration of Homeostatic Plasticity in Cultured Cortical Neurons From a Mouse Model of Huntington Disease

Huntington disease (HD) is an inherited neurodegenerative disorder caused by a mutation in the huntingtin gene. The onset of symptoms is preceded by synaptic dysfunction. Homeostatic synaptic plasticity (HSP) refers to processes that maintain the stability of networks of neurons, thought to be required to enable new learning and cognitive flexibility. One type of HSP is synaptic scaling, in which the strength of all of the synapses onto a cell increases or decreases following changes in the cell’s level of activity. Several pathways implicated in synaptic scaling are dysregulated in HD, including brain-derived neurotrophic factor (BDNF) and calcium signaling. Here, we investigated whether HSP is disrupted in cortical neurons from an HD mouse model. We treated cultured cortical neurons from wild-type (WT) FVB/N or YAC128 HD mice with tetrodotoxin (TTX) for 48 h to silence action potentials and then recorded miniature excitatory postsynaptic currents. In WT cultures, these increased in both amplitude and frequency after TTX treatment, and further experiments showed that this was a result of insertion of AMPA receptors and formation of new synapses, respectively. Manipulation of BDNF concentration in the culture medium revealed that BDNF signaling contributed to these changes. In contrast to WT cortical neurons, YAC128 cultures showed no response to action potential silencing. Strikingly, we were able to restore the TTX-induced changes in YAC128 cultures by treating them with pridopidine, a drug which enhances BDNF signaling through stimulation of the sigma-1 receptor (S1R), and with the S1R agonist 3-PPP. These data provide evidence for disruption of HSP in cortical neurons from an HD mouse model that is restored by stimulation of S1R. Our results suggest a potential new direction for developing therapy to mitigate cognitive deficits in HD.

Alterations in STriatal-Enriched protein tyrosine Phosphatase expression, activation, and downstream signaling in early and late stages of the YAC128 Huntington's disease mouse model

Striatal neurodegeneration and synaptic dysfunction in Huntington's disease are mediated by the mutant huntingtin (mHtt) protein. MHtt disrupts calcium homeostasis and facilitates excitotoxicity, in part by altering NMDA receptor (NMDAR) trafficking and function. Pre-symptomatic (excitotoxin-sensitive) transgenic mice expressing full-length human mHtt with 128 polyglutamine repeats (YAC128 Huntington's disease mice) show increased calpain activity and extrasynaptic NMDAR (Ex-NMDAR) localization and signaling. Furthermore, Ex-NMDAR stimulation facilitates excitotoxicity in wild-type cortical neurons via calpain-mediated cleavage of STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61). The cleavage product, STEP33, cannot dephosphorylate p38 mitogen-activated protein kinase (MAPK), thereby augmenting apoptotic signaling. Here, we show elevated extrasynaptic calpain-mediated cleavage of STEP61 and p38 phosphorylation, as well as STEP61 inactivation and reduced extracellular signal-regulated protein kinase 1/2 phosphorylation (ERK1/2) in the striatum of 6-week-old, excitotoxin-sensitive YAC128 mice. Calpain inhibition reduced basal and NMDA-induced STEP61 cleavage. However, basal p38 phosphorylation was normalized by a peptide disrupting NMDAR-post-synaptic density protein-95 (PSD-95) binding but not by calpain inhibition. In 1-year-old excitotoxin-resistant YAC128 mice, STEP33 levels were not elevated, but STEP61 inactivation and p38 and ERK 1/2 phosphorylation levels were increased. These results show that in YAC128 striatal tissue, enhanced NMDAR-PSD-95 interactions contributes to elevated p38 signaling in early, excitotoxin-sensitive stages, and suggest that STEP61 inactivation enhances MAPK signaling at late, excitotoxin-resistant stages. The YAC128 Huntington's disease mouse model shows early, enhanced susceptibility to NMDA receptor-mediated striatal apoptosis, progressing to late-stage excitotoxicity resistance. This study shows that elevated NMDA receptor-PSD-95 interactions as well as decreased extrasynaptic STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61) activation may contribute to early enhanced apoptotic signaling. In late-stage YAC128 mice, reduced STEP61 levels and activity correlate with elevated MAPK signaling, consistent with excitotoxicity resistance. Solid and dotted arrows indicate conclusions drawn from the current study and other literature, respectively.

Calpain and STriatal-Enriched protein tyrosine phosphatase (STEP) activation contribute to extrasynaptic NMDA receptor localization in a Huntington's disease mouse model

In Huntington's disease (HD), the mutant huntingtin (mhtt) protein is associated with striatal dysfunction and degeneration. Excitotoxicity and early synaptic defects are attributed, in part, to altered NMDA receptor (NMDAR) trafficking and function. Deleterious extrasynaptic NMDAR localization and signalling are increased early in yeast artificial chromosome mice expressing full-length mhtt with 128 polyglutamine repeats (YAC128 mice). NMDAR trafficking at the plasma membrane is regulated by dephosphorylation of the NMDAR subunit GluN2B tyrosine 1472 (Y1472) residue by STriatal-Enriched protein tyrosine Phosphatase (STEP). NMDAR function is also regulated by calpain cleavage of the GluN2B C-terminus. Activation of both STEP and calpain is calcium-dependent, and disruption of calcium homeostasis occurs early in the HD striatum. Here, we show increased calpain cleavage of GluN2B at both synaptic and extrasynaptic sites, and elevated extrasynaptic total GluN2B expression in the YAC128 striatum. Calpain inhibition significantly reduced extrasynaptic GluN2B expression in the YAC128 but not wild-type striatum. Furthermore, calpain inhibition reduced whole-cell NMDAR current and the surface/internal GluN2B ratio in co-cultured striatal neurons, without affecting synaptic GluN2B localization. Synaptic STEP activity was also significantly higher in the YAC128 striatum, correlating with decreased GluN2B Y1472 phosphorylation. A substrate-trapping STEP protein (TAT-STEP C-S) significantly increased VGLUT1-GluN2B colocalization, as well as increasing synaptic GluN2B expression and Y1472 phosphorylation. Moreover, combined calpain inhibition and STEP inactivation reduced extrasynaptic, while increasing synaptic GluN2B expression in the YAC128 striatum. These results indicate that increased STEP and calpain activation contribute to altered NMDAR localization in an HD mouse model, suggesting new therapeutic targets for HD.

N-methyl-D-aspartate receptor subunit- and neuronal-type dependence of excitotoxic signaling through post-synaptic density 95

NMDA receptors (NMDARs) mediate excitatory synaptic transmission during repetitive or prolonged glutamate release, playing a critical role in synaptic plasticity or cell death, respectively. Evidence indicates that a major pathway of NMDAR signaling to cell death in cortical and hippocampal neurons requires the scaffolding protein post-synaptic density 95 (PSD-95) and activation of neuronal nitric oxide synthase. However, it is not known if this PSD-95-dependent pathway contributes to excitotoxicity in other brain regions. It is also unclear whether the neuroprotective effects of Tat-NR2B9c, a membrane-permeant peptide that disrupts PSD-95/NMDAR binding, correlate with uncoupling NR2B- and/or NR2A-type NMDARs from PSD-95. In this study, we used cultured hippocampal and striatal neurons to test the potency of Tat-NR2B9c on uncoupling NR2 subunits from PSD-95 and protecting against NMDA-induced excitotoxicity. We found that the concentration of Tat-NR2B9c required to dissociate 50% of PSD-95 was fourfold lower for NR2B than NR2A in cultured hippocampal and striatal neurons, and that this concentration correlated tightly with protection against NMDA-induced toxicity in hippocampal neurons without altering NMDAR current. In contrast, NMDAR signaling to cell death in cultured striatal neurons occurred independently of the NR2B/PSD-95 interaction or neuronal nitric oxide synthase activation. These results will facilitate development of neuronal type-specific protective therapies.

Altered NMDA receptor trafficking in a yeast artificial chromosome transgenic mouse model of Huntington's disease

Overactivation of NMDA receptors (NMDARs) is believed to play a role in degeneration of striatal medium-sized spiny neurons (MSNs) in Huntington's disease (HD). This hereditary disorder is caused by an expansion >35 in the polyglutamine (polyQ) region of the protein huntingtin (htt). Previous work has shown that NMDAR current, calcium signaling, and/or toxicity are enhanced in striatal MSNs in a variety of transgenic mice and cellular models of HD, but whether the enhancement is specific for MSNs or correlated with mutant htt (mhtt) polyQ length is not known. Furthermore, the mechanism underlying the increase in NMDAR activity has not been elucidated. Here we report polyQ length-dependent enhancement of peak NMDAR current density by mhtt in cultured MSNs, but not cortical neurons, from the yeast artificial chromosome (YAC) transgenic HD mouse model. We also observed a shift of NMDAR subunits NR1 and NR2B from internal pools to the plasma membrane and a significantly faster rate of NMDAR insertion to the surface in YAC72 MSNs. In comparing YAC72 with wild-type striatal tissue, subcellular fractionation revealed a relative enrichment of NR1 C2'-containing NMDARs in the vesicle/microsome-enriched fraction, and coimmunoprecipitation experiments demonstrated an increased proportion of NR1 C2' isoforms associated with NR2 subunits, which may contribute to faster forward trafficking of these receptors. Our results suggest that altered NMDAR trafficking may underlie potentiation of NMDAR-mediated current and toxicity in the YAC72 HD mouse model. This polyQ length-dependent, neuronal-specific change in NMDAR activity induced by mhtt may contribute to selective neuronal degeneration in HD.