Scott Atwood, PhD

Hedgehog (Hh) signaling is essential for development of all vertebrates and drives proliferation, migration, and differentiation of progenitor cells to pattern organ development. In vertebrates, Hh signals through an organelle called the primary cilium. Cilia are polarized microtubule-based signaling projections that nucleate from the centrosome-derived basal body and are found on almost every vertebrate cell, including epithelial and mesenchymal cells that make up the skin and hair follicle. Hh pathway activation begins when Hh ligand binds to and inhibits the transmembrane receptor Ptch1, allowing the signal transducer Smo to activate Gli transcription factors and amplify Hh target gene expression. Although the main players have been identified, how Hh pathway components work together and how they are regulated remains unclear. My colleagues and I have found that Missing-in-Metastasis (MIM), a multi-domain scaffold protein that links membrane dynamics with the actin cytoskeleton, regulates primary cilia formation and Hh responsiveness at the basal body to promote de novo hair follicle formation. MIM functions in part by inhibiting the activity of the oncogenes Src kinase and Cortactin to prevent actin polymerization at the base of the cilium. MIM also interacts with the oncogenic kinase aPKC to facilitate Hh signaling and ciliogenesis in hair follicles and basal cell carcinoma. How MIM mediates its effects on kinases remains poorly understood and we are currently identifying new interaction partners through tandem-affinity proteomic screens to gain insight into the underlying mechanism of scaffold-kinase regulation.

Complex tissues and cellular architecture develop and are maintained by a process known as cell polarity. Individual cells set up a gradient of components that function to organize interior and exterior structures and signaling factors, allowing the cell to adopt specific fates and perform specialized functions. Cell polarity drives a diverse range of processes such as epithelial barrier functions, cell-to-cell communication, motility, localized signaling, as well as stem cell self-renewal and differentiation. The conserved oncogene atypical Protein Kinase C (aPKC) is a master regulator of cell polarity that is found in virtually all polarized systems and plays a critical role in organizing the cell cortex by segregating cellular components through phosphorylation. aPKC is part of a complex of proteins that include the PDZ domain protein Par6 and Rho GTPase Cdc42. I have shown that Cdc42 recruits and activates aPKC at the apical cortex of Drosophila neural stem cells. Once there, aPKC maintains the stem cell state in part by phosphorylating and segregating fate determinants such as Miranda and Numb to the differentiating daughter cell. In murine skin, aPKC controls polarized structures such as primary cilia and directs their signaling. I'm currently exploring aPKC's role in polarizing murine basal keratinocytes, dermal cells, and how this signal gives rise to cycling hair follicles.

One of the emerging themes in cancer biology is the dependence of cancer subtypes on certain signaling pathways for continued tumor growth. For example, mutations that activate the Hh signaling pathway drive growth of a variety of cancers including basal cell carcinoma (BCC), medulloblastoma, pancreatic, prostate, and small cell lung cancer that account for up to 25% of all human cancer deaths. Despite the critical nature of Hh signaling, how Hh mediates tumor proliferation remains poorly understood. Use of Smo antagonists are effective in treating late advanced or metastatic BCC, however early tumor resistance in about 20% of patients illustrates the need for additional targets for therapy. I have identified aPKC as a novel Hh target gene and activator of Hh signaling. aPKC forms a positive feedback loop by phosphorylating and activating Gli1, resulting in an increase in DNA binding and transcriptional activity. Smo antagonist-sensitive and resistant BCCs upregulate aPKC activity to drive high levels of Hh signaling for continued growth. Application of topical aPKC inhibitors suppress signaling and growth of murine tumors and Smo-resistant BCC cell lines, implicating aPKC as a new, tumor-selective therapeutic target for the treatment of Hh-dependent cancers. I am currently identifying new aPKC inhibitors, determining how cancer promotes kinase activity, and how substrate phosphorylation drives tumor growth.