Signal Transduction Mechanisms

Phosphatidylinositol 3-kinase (Ptdins-3K) is an important enzyme in signal transduction pathways and regulates a variety of biological responses including glucose uptake, cell survival, cell proliferation, motility, endocytosis, vesicle trafficking, and autophagy. Ptdins-3K mediates these responses by generating the second messengers Ptdins(3)P, Ptdins(3,4)P2, and Ptdins(3,4,5)P3. which then activate downstream signaling responses by mediating protein-lipid interactions; Ptdins(3,4)P2, and Ptdins(3,4,5)P3 activate downsteam signaling molecules by binding a subset of proteins with pleckstrin homology (PH) domains and Ptdins(3)P functions by binding a subset of proteins with PX or FYVE domains.

Elucidate the function of novel PI3K targets

To understand additional mechanisms whereby PI3-kinase regulates these responses, we identified and cloned several new Ptdins-3K targets and are working to elucidate the function of these proteins in Ptdins-3K signaling. In this regard, we recently studied mice deficient in BAM32 (B Cell Adaptor Molecule of 32 kDa), one of the PH-domain containing molecules we identified. We found that Bam32 was essential for normal B-cell proliferation following B cell receptor (BCR) activation and in generating T-independent II responses, the latter leading to a markedly increased susceptibility to infection by Streptococcus pneumonia We are now working to identify the signaling pathways regulated by BAM32 and whether mutation in BAM32 may account for increased susceptibility to encapusulated organisms in a subset of immunodeficient patients.

Function and regulation of myotubularins (MTM)

Myotubularins constitute a large family of lipid phosphatases that specifically dephosphorylate Ptdins(3)P. MTM1 is mutated in X-linked myotubular myopathy and MTMR2, and MTMR13 are mutated in Charcot-Marie-Tooth disease (CMT type 4B), although the mechanisms whereby MTM dysfunction leads to these diseases is unknown. To gain insight into MTM function, we undertook the study of MTMs in the nematode C. elegans and in mammalian cells. Our initial studies provided genetic evidence that MTMs function non-redundantly to dephosphorylate Ptdins(3)P and negatively regulate the Class III PI3K. In addition, we found that MTMs also play essential roles in endocytosis. We recently identified several proteins that bind specifically to a single class of MTMs and are currently working to understand the roles of these interacting proteins in MTM function and regulation using C. Elegans and mammalian cells.

Function and regulation of Sterile (Ste 20) serine/threonine kinases

We previously identified a Ste20 kinase that interacts with the SH3 domains of Nck, termed NIK (Nck Interacting Kinase). We found that NIK and its Drosophila ortholog termed misshapen (msn) specifically activate the JNK map kinase pathway and are essential genes in development. Using genetics in Drosophila and experiments in mammalian cells and knock out mice, we have mapped out some of the pathways in which NIK and msn function. We are currently continuing to use these systems to further understand the biological roles of this family of kinases. More recently, we have begun to study the role of NIK in cancer. NIK protein expression is upregulated in a number of cancers and may contribute to oncogenesis by integrating growth signals from integrins.