Restoring endogenous Dlg4/PSD95 expression by an artificial transcription factor ameliorates cognitive and motor learning deficits in the R6/2 mouse model of Huntington's disease.

Background: Huntington's disease (HD) is an incurable hereditary disorder caused by an expansion of CAG repeats in exon 1 of the Huntingtin gene (HTT). HD is characterized by motor dysfunction and cognitive decline. The pathophysiology of HD begins in cortico-striatal circuits and later spreads to other brain regions, notably the hippocampus. At the cellular level, structural changes in synapses have been observed prior to neuronal degeneration, significantly disrupting the formation and maintenance of neuronal circuits. The postsynaptic density protein 95 (PSD-95, hereafter Dlg4/PSD95) is a key synaptic plasticity protein reduced in HD and other neurodegenerative diseases such as Alzheimer's disease (AD). Epigenetic silencing of plasticity and memory genes contributes to AD pathology and cognitive impairment. To restore endogenous Dlg4/PSD95 expression in AD, we previously developed an epigenetic editing strategy where a zinc finger DNA-binding domain targeting the Dlg4/PSD95 gene promoter was fused to the transactivation domain VP64 and driven under a CMV promoter. AAV-PhP.B-mediated delivery of this artificial transcription factor (ATF) CMV-PSD95-6ZF-VP64 improved cognition in an AD mouse model. Here, we assessed the therapeutic potential of AAV9-mediated delivery of the synapsin-driven ATF PSD95-6ZF-VP64 in the R6/2 HD mouse model.

Results: Consistent with the previous studies, R6/2 mice exhibited reduced hippocampal Dlg4/PSD95 mRNA and protein levels in young adulthood (7 weeks), which persisted into early adulthood (14 weeks). Starting at adolescents (4 weeks), the R6/2 mice also displayed motor (i.e., accelerated rotarod) and cognitive (i.e., Barnes maze and object location memory) impairments. In wild-type primary hippocampal cultures, AAV9-PSD95-6ZF-VP64 led to an increase in synaptic PSD-95 clusters and spine size. Intracerebroventricular injections of neonatal R6/2 mice with AAV9-PSD95-6ZF-VP64 elevated hippocampal Dlg4/PSD95 expression levels to those observed in control non-transgenic mice. Importantly, AAV9-PSD95-6ZF-VP64 effectively improved hippocampal-dependent deficits in spatial learning and memory in young adult HD mice, as well as impairments in motor coordination and motor skill learning, with these benefits persisting into adulthood.

Conclusion: This work validates Dlg4/PSD95 as a key player in the prodromal phase of HD pathology and establishes the ATF PSD95-6ZF-VP64 as an attractive therapeutic tool for treating the disease's early phase.