A key component of neural circuit formation is the elaboration of complex dendritic arbors, the pattern of which constrains inputs to the neuron and thus, the information it processes. As such, many neurodevelopmental disorders such as autism and Down, Rett, and Fragile X Syndromes are associated with reduced forebrain dendrite arborization. Identifying molecules involved in regulating dendrite arborization and neural circuitry formation therefore, is a start to understanding these disorders.
Nearly 70 cadherin superfamily adhesion molecules are encoded by the Pcdha, Pcdhb, and Pcdhg gene clusters. These so-called clustered protocadherins (Pcdhs) are broadly expressed throughout the nervous system, with lower levels found in a few non-neuronal tissues. Each neuron expresses a limited repertoire of clustered Pcdh genes, a complicated process controlled by differential methylation and promoter choice. The clustered Pcdh proteins interact homophilically in trans as cis-multimers, which has the potential to generate a combinatorially explosive number of distinct adhesive interfaces that may give neurons unique molecular identities important for circuit formation. Functional studies of animals in which clustered Pcdhs have been deleted or disrupted demonstrate that these proteins play critical roles in neuronal survival, axon and dendrite arborization, and synaptogenesis. Additionally, they have been implicated in the progression of several cancers, suggesting that basic studies of their function and signaling pathways will have important future clinical applications.
Recent work has shown that γ-Pcdhs can regulate the Wnt signaling pathway, a common tumorigenic pathway which also play roles in neurodevelopment, but the molecular mechanisms remain unknown. I determined that γ-Pcdhs differentially regulate Wnt signaling: the C3 isoform uniquely inhibits the pathway while 13 other isoforms upregulate Wnt signaling. Focusing on γ-Pcdh-C3, I show that the variable cytoplasmic domain (VCD) is critical for Wnt signaling inhibition. γ-Pcdh-C3, but not other isoforms, physically interacts with Axin1, a key component of the canonical Wnt pathway. The C3 VCD competes with Dishevelled for binding to the DIX domain of Axin1, which stabilizes Axin1 at the membrane and leads to reduced phosphorylation of Wnt co-receptor Lrp6. I also present evidence that the Wnt pathway can be modulated up (by γ-Pcdh-A1) or down (by γ-Pcdh-C3) in the cerebral cortex in vivo, using conditional transgenic alleles.
Studies have implicated γ-Pcdhs as a whole, in many neurodevelopmental processes but little is known if distinct roles exists for individual isoforms. By using a specific C3-isoform knockout mouse line engineered in collaboration with Dr. Robert Burgess of The Jackson Laboratory, I was able to uncover a unique role for the C3-isoform in the regulation of dendrite arborization. Mice without γ-Pcdh-C3 exhibit significantly reduced dendrite complexity in cortical neurons. This phenotype was recapitulated in cultured cortical neurons in vitro, which can be rescued by reintroducing the C3-isoform. The ability of γ-Pcdh-C3 to promote dendrite arborization cell-autonomously was abrogated when Axin1 was depleted with an shRNA, indicating that this process by which γ-Pcdh-C3 regulates dendrite arborization is mediated by its interaction with Axin1, which I had previously demonstrated. Together, these data suggest that γ-Pcdh-C3 has unique roles distinct from other γ-Pcdhs, in the regulation of Wnt signaling and dendrite arborization, both of which are mediated by interaction with Axin1.
| Identifer | oai:union.ndltd.org:uiowa.edu/oai:ir.uiowa.edu:etd-7445 |
| Date | 01 December 2017 |
| Creators | Mah, Kar Men |
| Contributors | Weiner, Joshua A. |
| Publisher | University of Iowa |
| Source Sets | University of Iowa |
| Language | English |
| Detected Language | English |
| Type | dissertation |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | Copyright © 2017 Kar Men Mah |
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