Bone morphogenetic proteins (BMPs) belong to the transforming growth factor beta (TGFbeta) superfamily of secreted molecules. BMPs play essential roles in multiple developmental and homeostatic processes in metazoans. Malfunction of the BMP pathway can cause a variety of diseases in humans, including cancer, skeletal disorders and cardiovascular diseases. Identification of factors that ensure proper spatiotemporal control of BMP signaling is critical for understanding how this pathway is regulated. In C. elegans, mutations in the BMP-like Sma/Mab pathway exhibit altered body sizes and the suppression of the dorsoventral patterning defects of sma-9(0) mutants in the postembryonic M lineage, among a number of phenotypes. Our lab has been using the sma-9(0) suppression assay to identify new players in the Sma/Mab pathway. We recently identified three conserved tetraspanin proteins, TSP-12, TSP-14 and TSP-21, as positive modulators of the Sma/Mab pathway. In addition to suppressing the sma-9(0) M lineage defect, tsp-21(0) single mutants or tsp-12(0); tsp-14(0) double mutants exhibit other defects in Sma/Mab signaling, including a small body size and reduced RAD-SMAD reporter expression. Genetic epistasis and cell type-specific rescue experiments showed that TSP-21 functions in the signal-receiving cells and genetically at the level of the ligand-receptor complex in the Sma/Mab pathway. A functional TSP-21::GFP generated via CRISPR/Cas9-mediated homologous recombination is plasma membrane-localized in a number of tissues, including the hypodermal, intestinal and M lineage cells. Using the split-ubiquitin yeast two-hybrid system, we found that TSP-21 can associate with itself and with TSP-12 and TSP-14. Furthermore, TSP-12 and TSP-14 can also associate with, and promote the cell surface localization of, SUP-17, an ortholog of the ADAM protease (a disintegrin and metalloprotease) ADAM10, which we have found to function in the Sma/Mab pathway as well. Because TSP-12 and TSP-14 can also associate with SMA-6, the type I receptor of the Sma/Mab pathway, in the yeast two-hybrid system, we are currently determining whether TSP-12 and TSP-14 may be required for the proper localization of SMA-6. Altered BMP signaling is known to play a role in cancer progression. In recent years, tetraspanins have emerged as diagnostic and prognostic markers for tumor progression, we speculate that abnormal BMP signaling due to altered expression or function of certain tetraspanins may be a contributing factor to cancer development. We therefore plan to determine if the roles of TSP-12, TSP-14 and TSP-21 in promoting BMP signaling are conserved in mammals.

Affiliation:
- Department of Molecular Biology & Genetics, Cornell University Ithaca, NY 14853