Understanding the pathophysiological mechanisms of Rett syndrome (RTT) at the cellular and molecular levels, and establishing successful bioassays for evaluation of potential therapeutic strategies take priority in the path of research on this neurodevelopmental disorder. RTT, an autism spectrum disorder, is a devastating childhood disability due to its impact on individuals (1:10,000 births worldwide), their families and society RTT is caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2), a transcriptional regulator that binds to methylated CpG sites in promoter regions of several genes, including the neurotrophin Bdnf, and the Ca2+-permeable non-selective cationic channel subunits Trpc3 and Trpc6. The availability of endogenously expressed BDNF for its activity-dependent release can be monitored with membrane currents and dendritic Ca2+ signals mediated by TRPC channels. Preliminary Results demonstrate that TRPC currents and Ca2+ signals evoked in CA3 pyramidal neurons by stimulation of presynaptic mossy fiber (MF) are smaller in symptomatic Mecp2 mutant mice. Responses evoked by either recombinant BDNF or a non-hydrolyzable DAG analog (to activate TRPC channels) are also impaired in Mecp2 mutant neurons. Consistently, mRNA and protein levels of both BDNF and TRPC3 are lower in Mecp2 mutant hippocampus. Preliminary Results show that the TRPC6 channel activator hyperforin evokes membrane currents and Ca2+ signals, and promotes dendritic spine maturation in CA3 pyramidal neurons, resembling well-known actions of BDNF. Based on these Preliminary Results and since TRPC3 and TRPC6 form heteromultimers, our hypothesis is that impaired BDNF signaling through TRPC3/6 channels in Mecp2 mutant mice can be overcome by treatment with the selective TRPC6 activator hyperforin to reverse two RTT-like phenotypes: hippocampal network hyperactivity and immature dendritic spines. We propose two Specific Aims: 1. Test whether membrane currents and Ca2+ signals evoked by BDNF and mediated by TRPC channels in CA3 pyramidal neurons and GABAergic interneurons are impaired in Mecp2 mutant mice;and 2. Test whether treatment with hyperforin reverses hippocampal phenotypes in Mecp2 mutant mice, i.e. hippocampal network hyperactivity and immature dendritic spines. Identifying TRPC3/6 channels as novel targets for pharmacological intervention is necessary for pre-clinical trials leading to rational treatments for RTT and other neurodevelopmental disorders associated with MECP2 mutations and impaired BDNF signaling.

Public Health Relevance

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder of the autism spectrum associated with intellectual disabilities, epilepsy and seizure disorders, and associated with mutations in MECP2, a methylated DNA-binding protein that regulates transcription of multiple genes, including Bdnf, Trpc3 and Trpc6. To provide pre-clinical data in support of new therapies for RTT, we will test whether impaired BDNF signaling through TRPC3/6 channels in hippocampal neurons from Mecp2 mutant mice can be reversed by selective activators of these PLC-coupled channels.

National Institute of Health (NIH)
Eunice Kennedy Shriver National Institute of Child Health & Human Development (NICHD)
Exploratory/Developmental Grants (R21)
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Special Emphasis Panel (ZRG1-MDCN-N (04))
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Oster-Granite, Mary Lou
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University of Alabama Birmingham
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Li, Wei; Xu, Xin; Pozzo-Miller, Lucas (2016) Excitatory synapses are stronger in the hippocampus of Rett syndrome mice due to altered synaptic trafficking of AMPA-type glutamate receptors. Proc Natl Acad Sci U S A 113:E1575-84
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