Regulation of endosome trafficking

Endocytosis is a process by which cells internalize extracellular molecules and proteins on the plasma membrane into lipid vesicles or endosomes. Internalized cargo enters into early endosomes, from which it can be recycled back to the plasma membrane, or degraded by the lysosome. Small GTPases of the Rab family are key regulators of vesicular trafficking. Rab5 and Rab7 localize to early and late endosomes, respectively, where they regulate cargo delivery to the lysosome. We identified TBC-2 as an important regulator of early to late endosome trafficking in C. elegans. TBC-2 localizes to endosome membranes and functions as a Rab5 GTPase Activating Protein. Projects in the lab are currently underway to further define how TBC-2 regulates trafficking, understand the mechanisms that regulate TBC-2, further understand the trafficking defects caused by tbc-2 mutations, and identify additional factors that function with TBC-2.

Regulation of RTK/Ras/MAPK signaling

Receptor Tyrosine Kinase (RTK)/Ras GTPase/Mitogen Activated Protein Kinase (MAPK) signaling is a highly conserved signaling pathway that relays extracellular signals into the cell to elicit signal and cell specific responses such as cell proliferation, migration, and differentiation. RTK/Ras/MAPK signaling is essential for metazoan development and mutations in components of this pathway underlie several human developmental disorders and are commonly found in human cancers.

Negative regulators: As a mechanism of signal downregulation, activated RTKs are internalized and undergo endosomal trafficking to the lysosome. We identified Rab7, the late endosomal Rab GTPase, as a negative regulator of RTK signaling during C. elegans vulval cell fate specification. Our data and those of others working in mammalian systems would suggest that Rab7 acts by regulating RTK degradation. To identify additional factors that function with Rab7 to regulate RTK signaling, we performed a classical forward genetic screen for mutations with shared phenotypes. Thus far we have two mutants that are strong candidates for being both new regulators of RTK/Ras/MAPK signaling and regulators of vesicular trafficking.

Positive regulators: Scaffold proteins, by grouping signaling proteins, promote the efficiency and specificity of cell signaling. Kinase Suppressor of Ras (KSR) and Connector eNhancer of Ksr (CNK) are scaffold proteins that promote of RTK/Ras/MAPK signaling in worms, flies and mammals. In C. elegans, there are two KSRs, KSR-1 and KSR-2, that have both unique and redundant tissue specific functions. KSR-1 and KSR-2 are redundantly required for the specification of the excretory duct cell fate and larval viability. Loss of KSR-1 alone has little effect, however loss of KSR-2 or the lone CNK homolog, CNK-1 in combination with loss of KSR-1, results in a robust larval lethal phenotype. To identify additional factors that function with CNK-1 and the KSRs, we performed a genome-wide RNAi screen for enhancers of ksr-1 lethality. This screen identified 16 genes involved in protein ubiquitination, mRNA degradation, chromatin remodeling/histone acetylation, RNAi/small RNA regulation, as well as some novel genes. We are interested in understanding how this diverse set of genes impact RTK/Ras/MAPK signaling.