Since the first genetic description of Smads in 1995 and the initial biochemical characterizations of the proteins in 1996, our understanding of how Smads function to mediate TGFβ signaling has grown considerably. The availability of complete human and Drosophila
genome sequences has confirmed that the full complement of Smads is now known. In addition, structural, biochemical and cell-biological approaches have culminated in the development of a model that provides a molecular description of how Smads transmit TGFβ superfamily signals (Figure ). With this basic framework in hand, current research efforts are directed towards reaching a more detailed mechanistic understanding of the signaling process. An area of particular interest is how localization of Smads and their association with other proteins is controlled. The phosphorylation of Smads by the TGFβ receptor complex is essential for initiating the signaling cascade, and recruitment of R-Smads to the receptor is thought to be facilitated by SARA, but the subcellular compartment in which these events occur, and even whether there is a SARA-like protein for the BMP-regulated Smads, is not known. Insights into what determines the subcellular localization and nuclear accumulation of Smads will also be invaluable for enhancing our understanding of how their nuclear activities are manifested. Recent evidence has shown that Smads associate with E3 ubiquitin-ligases, are themselves ubiquitinated and degraded, and can serve as adapters to mediate ubiquitin-mediated degradation of other proteins [22
]. Thus, it will be important to understand how a cell maintains the delicate balance between Smad and ubiquitin-ligase protein levels to ensure appropriate responsiveness to TGFβ-superfamily signals. Although Smads are known to function in the nucleus as transcriptional regulators, little is understood of what determines whether Smads positively or negatively regulate transcription. Furthermore, very few DNA-binding partners for the BMP-regulated Smads are known.
Cells receive multiple simultaneous signals, and the interaction of the TGFβ pathway components with effectors of other signaling pathways has been described. Thus, future efforts will also focus on developing a better understanding of how and whether Smads cross-talk with other signaling pathways. Current research efforts, including the search for novel Smad-interacting proteins, will undoubtedly shed light on these questions and may reveal new insights that challenge existing paradigms.
TGFβ superfamily members play critical roles in numerous developmental events from cell-fate determination to organogenesis, and there is great interest in understanding these events. Examination of the effects of gene disruption in mice has revealed important information on the role Smads play in the earliest events. With the exception of Smad3 and Smad6, however, mice deficient in Smads die early in embryonic life; future work directed towards understanding their role in later development will thus require the generation of conditional alleles. In addition, the phenotype of mice lacking Smad1, Smad7 or Smad8 is eagerly awaited. These studies in mice will be bolstered by the analysis of Smad function in several other genetically manipulatable model systems, including C. elegans, Drosophila and zebrafish.
TGFβ signaling has been implicated in a wide variety of human disorders, including fibrosis, hypertension, osteoporosis, atherosclerosis and cancer, making this pathway an excellent target for therapeutic intervention [2
]. Furthermore, mutations in components of the TGFβ signaling pathway have been associated with a number of hereditary diseases including persistent Miillerian duct syndrome, hereditary hemorrhagic telangiactasia, hereditary chondrodysplasia, familial primary pulmonary hypertension and hereditary non-polyposis colorectal cancer [2
]. Of the Smads, only Smad4 has been shown to be associated with a hereditary disease, namely juvenile polyposis syndrome. Thus, it will be important to determine whether other hereditary syndromes can be attributed to mutations in Smads. In addition, the pathological implications of Smad hemizygosity or Smad dysfunction in other diseases, including cancer, is a worthy undertaking, as this may provide a target for the development of novel clinical treatments.