Human truncated Smad 6 (Smad 6s) inhibits the BMP pathway in Xenopus laevis

A previously identified truncated form of the human Smad 6 gene containing a unique 12 amino acid motif at its N-terminus was studied. We have named this truncated form of the gene Smad 6s, for 'short-form', to distinguish it from the full-length form (Smad 6fl). Reverse transcription-polymerase chain reaction and immunohistochemistry revealed that Smad 6s has a unique pattern of expression in human coronary tissue and is upregulated in diseased heart tissue. We used the expression of human Smad 6s in Xenopus laevis as a model system to assess Smad 6s function. Injection of Smad 6fl RNA (4-cell embryos, 2 x ventral) produced tadpoles with partial secondary axes. In contrast, Smad 6s RNA injected in a similar manner produced tadpoles with a severe 'head-only' phenotype with no morphological appearance of a secondary axis. Mutant Smad 6s RNA lacking the unique 12 amino acids at the N-terminus of the Smad 6s isoform produced no embryonic phenotype, suggesting that this region is important in conferring biological activity. Ectodermal explant assays show that Smad 6s has activity consistent with being a BMP antagonist and can synergize with and enhance the activities of the activin and fibroblast growth factor pathways, all of which are novel findings in this study.