Promotion of Bone Morphogenetic Protein Signaling by Tetraspanins and Glycosphingolipids

Abstract

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor β (TGFβ) 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. We have used a unique and sensitive genetic screen to identify the plasma membrane-localized tetraspanin TSP-21 as a key new factor in the C. elegans BMP-like “Sma/Mab” signaling pathway that controls body size and postembryonic M lineage development. We showed that TSP-21 acts in the signal-receiving cells and genetically functions at the ligand-receptor level. We further showed that TSP-21 can associate with itself and with two additional tetraspanins, TSP-12 and TSP-14, which also promote Sma/Mab signaling. TSP-12 and TSP-14 can also associate with SMA-6, the type I receptor of the Sma/Mab pathway. Finally, we found that glycosphingolipids, major components of the tetraspanin-enriched microdomains, are required for Sma/Mab signaling. Our findings suggest that the tetraspanin-enriched membrane microdomains are important for proper BMP signaling. As tetraspanins have emerged as diagnostic and prognostic markers for tumor progression, and TSP-21, TSP-12 and TSP-14 are all conserved in humans, we speculate that abnormal BMP signaling due to altered expression or function of certain tetraspanins may be a contributing factor to cancer development.

Two Paralogous Tetraspanins TSP-12 and TSP-14 Function with the ADAM10 Metalloprotease SUP-17 to Promote BMP Signaling in Caenorhabditis elegans

“…We have previously reported a weak Susm ( su ppression of the sm a-9 M lineage) phenotype in the tsp-12(ok239) null mutants, which is enhanced by tsp-14(RNAi) , but not by RNAi of any of the remaining 19 tsp genes in C . elegans [7]. To further determine mechanistically how TSP-12 and TSP-14 function in Sma/Mab signaling, we generated three deletion alleles of tsp-14 ( jj95 , jj96 and jj97 ) using the CRISPR/Cas9 system (Fig 1C; see Materials and Methods).…”

“…In particular, we have previously shown that the small transmembrane tetraspanin proteins are important in promoting BMP signaling in C . elegans [7]. …”

“…Previously, we have shown that in C . elegans , the TspanC8 tetraspanins TSP-12 and TSP-14 function in modulating BMP signaling [7]. However, whether they exert their functions in BMP regulation by acting through the C .…”

“…Thus, Sma/Mab pathway mutants exhibit a Susm ( su ppression of the sm a-9 M lineage) phenotype ([19]; Fig 1A and 1E). We have previously shown that the Susm phenotype can be used as a specific and sensitive assay to screen efficiently for mutants with defects in Sma/Mab signaling [7]. Using the Susm assay, we have identified several highly conserved factors that function to modulate BMP signaling at the level of the ligand-receptor complex.…”

“…Using the Susm assay, we have identified several highly conserved factors that function to modulate BMP signaling at the level of the ligand-receptor complex. These include the RGM protein DRAG-1 [20], the neogenin/DCC homolog UNC-40 [18], the tetraspanin TSP-21 [7], as well as two redundant tetraspanins, TSP-12 and TSP-14.…”