Mechanism of Par3-mediated regulation of dendritic spine plasticity

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Project Summary/Abstract Dendritic spines are small, highly polarized protrusions on excitatory neurons serving as sites of postsynaptic input. Plasticity of dendritic spines is necessary for learning, while stable dendritic spines are thought to encode long-term memories. The polarized nature of dendritic spines suggests their plasticity and stability may be mediated by polarity proteins. The Partitioning defective (Par) polarity protein 3 (Par3) regulates mature dendritic spine formation in vitro, and several single nucleotide polymorphisms (SNPs) and copy number variation (CNV) of Pard3, which encodes Par3, are associated with intelligence, schizophrenia, and autism spectrum disorder (ASD). Moreover, Par3 forms a complex with atypical protein kinase C (aPKC). A constitutively active truncated aPKC variant has been proposed as a “memory molecule,” while a full length aPKC variant is implicated in long term potentiation (LTP). Together, these data implicate Par3 in mature dendritic spine stabilization, which may play a role in cognition and social interactions. However, the in vivo mechanisms of Par3 in dendritic spine plasticity and stability remains completely unknown. To investigate the mechanisms, we developed a novel conditional knockout line that depletes Par3 in postnatal forebrain pyramidal neurons (Par3-/-). Par3-/- exhibits increased dendritic spine density and increased immature dendritic spine morphology. Phosphoproteomic analysis of Par3-/- hippocampal tissue revealed increased phosphorylation of CAMSAPs, which bind and stabilize microtubule (MT) minus-ends. Our central hypothesis is that Par3 regulates MT stability through CAMSAPs to stabilize dendritic spines, which is necessary for normal cognition and social interactions. In the absence of Par3, aPKC becomes abnormally activated leading to increased CAMSAP phosphorylation and decreased MT stability. Aim 1 investigates the hypothesis that loss of Par3 destabilizes dendritic spines leading to impaired cognitive functions and social behavior in mice. Aim 2 uses biochemical assays and live cell imaging to test the hypothesis that Par3 regulates aPKC phosphorylation of CAMSAP2 at S992, influencing MT and dendritic spine stability. Together, these experiments will elucidate the in vivo mechanisms of Par3 regulation of dendritic spine stability, cognition, and social behavior. This may provide important insight to further understand the mechanisms of neurodevelopmental disorders, such as schizophrenia and ASD. The proposed fellowship will also train the applicant in several innovative techniques in biochemical, cellular, molecular, and behavioral neuroscience. The established faculty-student mentorships will ensure proper scientific and professional training necessary to become a successful, independent neurobiologist of learning and memory.