Research

Hedgehog (Hh) signaling is conserved in all metazoan animals, and is crucial for normal embryonic development and tissue homeostasis.   Mis-regulation of the Hh pathway results in a variety of birth defects, including holoprosencephaly, polydactyly, spinal bifida, and heart and skeletal malformations.   On the other hand, failure of normal restraints in Hh mitogenic activity has been associated to up to 25% of cancer, with the basal cell carcinoma and medulloblastoma being the most famous examples.   Thus, an understanding of how Hh signaling activities are precisely modulated is likely to provide clues for preventing or ameliorating Hh-related birth defects and tumor formation.   Hh signaling is a complex biological process requiring two transmembrane proteins, Patched (Ptc) and Smoothened (Smo).   The goal of our research is to understand the molecular and cellular function of Ptc and Smo in intiating Hh signal transduction.   Our lab is using Drosophila melanogaster as the principle model system based on its wealth of sophisticated genetic tools and depth of comparative genomic information.   We are also starting to explore mammalian systems to advance our knowledge of the Hh pathway gained from the Drosophila system.   Currently, we focus on the following three areas to understand the significance of the movement of Smo-containing vesicles and the molecular function of Ptc and related proteins during animal development.

Regulators for Smo protein trafficking

We showed previously that movement of Smo protein to the plasma membrane is a pre-requisite for activating Hh signaling in vivo .  Since hyper-activation of the Hh pathway causes cancer, a good understanding of Smo protein trafficking is likely to provide clues for improved treatment for human disorders.   We are conducting a biased candidate gene screen to look for interactions between Smo and members of Rab proteins, exocyst complex and cytoskeletal remodeling factors.   Furthermore, we are starting an unbiased, cell-based genome-wide RNA interference screen to identify novel proteins regulating Smo transport.

Transporter activity of Ptc protein

The Hh receptor Ptc antagonizes Smo activity, but the exact mechanism remains a mystery.   Ptc belongs to a protein family that shares a sterol-sensing domain (SSD).   The topology of Ptc protein SSD is related to bacterial RND transporters.   We have shown that Ptc may behave as a transporter when assayed in vitro .   Currently we are testing Ptc transporter activity in vivo , using transgenic Drosophila lines expressing various ptc SSD mutations.   The identification of Ptc as a transporter opens up a whole new set of opportunities for dissecting Hh signal transduction.

Membrane trafficking regulated by a ptc-related gene

We have cloned a ptc-related gene (mde), which plays an essential role in early embryogenesis in Drosophila .   Mde protein localization patterns are highly dynamic that correlates with membrane addition events during cellularization in early embryos. We are currently testing whether Mde is associated with secretion vesicles and/or the cytoskeleton, and whether it plays a regulatory role in targeted membrane trafficking.