BIOLOGICAL OVERVIEW

Tensin is an actin-binding protein that is localized in focal adhesions. At focal adhesion sites, tensin participates in the protein complex that establishes transmembrane linkage between the extracellular matrix and cytoskeletal actin filaments. Even though there have been many studies on tensin as an adaptor protein, the role of tensin during development has not yet been clearly elucidated. The developmental role of tensin was dissected by isolating Drosophila tensin mutants and characterizing its role in wing development. The Drosophila tensin loss-of-function mutations resulted in the formation of blisters on the wings, due to a defective wing unfolding process. Interestingly, by¹ -- the mutant allele of the gene blistery (by) -- also shows a blistered wing phenotype, but fails to complement the wing blister phenotype of the Drosophila tensin mutants, implying that they are alleles of the same gene. These results demonstrate that by encodes Drosophila tensin protein and that the Drosophila tensin mutants are alleles of by. Using a genetic approach, it has been demonstrated that tensin interacts with integrin and also with the components of the JNK signaling pathway during wing development; overexpression of by in wing imaginal discs significantly increases JNK activity and induces apoptotic cell death. Collectively, these data suggest that tensin relays signals from the extracellular matrix to the cytoskeleton through interaction with integrin, and through the modulation of the JNK signal transduction pathway during Drosophila wing development (Lee, 2003).

Tensin is a focal adhesion molecule that binds to actin filaments through its N terminus (Lo, 1994a; Lo, 1994b; Chuang, 1995). In addition, it contains two functional motifs, including a Src homology domain 2 (SH2) and a phosphotyrosine binding (PTB) domain (Davis, 1991; Chen, 2000; Chen, 2002). This conserved domain structure gives significant clues to its possible function, including potential roles in cell signaling (Lee, 2003 and references therein).

Tensin is best known as an adaptor protein linking integrin to the actin cytoskeleton. Integrins are a family of transmembrane receptors that are localized in focal adhesions. The extracellular domain of integrin interacts with the extracellular matrix, and its cytoplasmic domain anchors actin filaments to the plasma membrane through the focal adhesion protein complex. Integrin is also believed to participate in diverse biological events such as cytoskeletal restructuring, cell motility and even cell survival via focal adhesion complexes that include tensin, focal adhesion kinase (FAK), Src kinase and protein kinase C. Most interesting is the fact that integrin is involved in various cell signaling pathways through its association with focal adhesion proteins. For example, integrin activates ERKMAP kinase by promoting the SH2 domain-mediated association of Grb2 with tyrosine kinases such as FAK and c-Src in focal adhesions. The phosphoinositide 3 kinase (PI3K)-dependent signaling pathway is also activated by integrin through a FAK-dependent mechanism. In addition, JNK is also activated by integrins when cells are attached to the extracellular matrix (Lee, 2003 and references therein).

The role of tensin as an adaptor for integrin and as a required component in focal adhesions suggests the possibility that it may act as a mediator of integrin signaling. In support of this idea, much indirect and direct evidence has been collected. Previously, the SH2 and PTB domains in the C terminus of tensin have been demonstrated to bind tyrosine phosphorylated proteins such as PI3K and p130 CAS (Salgia, 1995; Salgia, 1996; Auger, 1996). In addition, tensin itself is phosphorylated at serine, threonine and tyrosine residues when cells are stimulated by either cell adhesion (Bockholt, 1993), growth factors (Jiang, 1996) or oncogenes, including v-Src and Bcr/Abl (Davis, 1991; Salgia, 1995). Indeed, a recent study has shown that overexpression of tensin alone can activate JNK in human embryonic kidney 293T cells (Katz, 2000). According to this report, the tensin-mediated JNK activities are independent of the activities of small GTP-binding proteins such as Rac and Cdc42, but dependent on the activity of SEK (Lee, 2003 and references therein). blistery, one of the genes which, when disrupted, results in a blistered wing phenotype, encodes the Drosophila ortholog of mammalian tensins; with by mutants, the functions of tensin have been characterized in vivo. The blistered wing phenotype of the by loss-of-function flies is caused by a defective wing unfolding process after eclosion. Additionally, tensin functionally interacts with integrin and the JNK signaling pathway. These results demonstrate the in vivo roles of tensin in development and suggest that tensin might be a transducer of signals from integrin to the JNK signaling pathway (Lee, 2003).

During Drosophila wing development, the dorsal and ventral wing epithelia are fused together by highly specified cell-cell adhesions, and defects in this process result in blistered wings. The blistered wing phenotype observed in the by mutants indicates that tensin functions in such a cell adhesion process. In Drosophila, integrin is well known as a central molecule that mediates adhesion between wing layers. The loss of PS integrin function in the wings causes the formation of a fluid-filled blister. Moreover, the loss of adaptor proteins such as short stop and integrin-linked kinase that mediate attachment of extracellular matrix (ECM)-integrin complexes to cytoskeleton also result in a wing blister phenotype, implying that integrin and its adaptor protein complexes are indispensable in wing layer adhesion. Because tensin is a major component of the ECM-integrin complex, and because the wing blister phenotype of an integrin mutant is dramatically enhanced by additional loss of by, it is believed that improperly mediated integrin signaling caused by the loss of tensin results in the wing blister phenotype in the by mutants. This idea is supported by mammalian cell studies showing that tensin acts as a molecular linker between actin cytoskeleton and integrin, and plays a crucial function (Pankov, 2000) when integrin translocates from focal adhesions to fibrillar adhesions (Lee, 2003).

Besides the defects in wing cell adhesion process, another distinct mutant phenotype in the by mutants was observed; they lay rounded eggs due to defective oocyte elongation during oogenesis. Genetics data suggest the possibility of a functional interaction between tensin and integrin during oogenesis. Interestingly, a similar phenotype has been reported recently in the studies with follicle cell clones lacking PS integrin. Taken together, these findings suggest that tensin and integrins are tightly linked together during most, if not all, of their various functions in Drosophila development (Lee, 2003).

Tensin genetically interacts with the components of the JNK signaling pathway, and regulates JNK activity during wing development. The supporting evidence for the engagement of tensin in the JNK signaling pathway comes from a recent report (Katz, 2000) that transfected mammalian tensin activates JNK signaling in HEK 293T cells (Lee, 2003).

Interestingly, in mammalian cells, JNK is also activated via adaptor proteins p130 CAS and Crk which receive a signal from the FAK/Src tyrosine kinase complex in the cell adhesion sites when cells attach to the ECM. Since tensin is a possible substrate for FAK (Guan, 1997), and p130 CAS is able to interact with the C terminus of tensin (Salgia, 1995; Salgia, 1996), it is highly possible that tensin is involved in this signaling cascade and mediates signals from integrin and FAK to the JNK signaling pathway (Lee, 2003).

In addition, tensin does not interact genetically with other signaling pathways known to interact with integrins such as the ERK-MAPK and the PI3K signaling pathways. Integrin signaling is mediated mainly by protein complexes including tensin in focal adhesions. Thus, it is thought that focal adhesion molecules related to integrin such as tensin are important for directing integrin mediated extracellular signals to a specific signaling pathway. Consequently, it is suggested that at least during Drosophila wing development, tensin has an ability to drive signals from integrin selectively to the JNK signaling pathway. However, further studies are required to confirm this hypothesis and determine the details behind the connection between focal adhesion proteins and related intracellular signaling (Lee, 2003).

In summary, evidence has been found for the functional interaction between integrin and tensin, and for the modulation of JNK signaling by tensin during Drosophila wing development. These results strongly suggest that tensin is not merely an adaptor protein in focal adhesions, but also an important mediator of signal transduction in Drosophila. The Drosophila model will be useful in future studies that address the function of tensin as a signaling molecule (Lee, 2003).

GENE STRUCTURE

cDNA clone length - 2995 base pairs

Bases in 5' UTR - 18

Exons - 2

Bases in 3' UTR - 804

PROTEIN STRUCTURE

Amino Acids - 720

Structural Domains

The sole Drosophila ortholog of mammalian tensins, CG9379, was identified in the Drosophila genome data bank by BLAST search, and corresponds to the open reading frame deduced from Drosophila EST clone RH56077. Drosophila tensin has a predicted molecular weight of 79 kDa, with a much shorter N-terminal region than that of its mammalian counterpart. Although the N-terminal sequence of tensin is not conserved, its C-terminal region of about 350 amino acids, which includes the SH2 and PTB domains, exhibits significant homology to the human homologs, with about 40% amino acid identity (Lee, 2003).

The by coding region consists of a single exon, confirmed by several ESTs from the Berkeley Drosophila Genome Project. It encodes a protein that is clearly the Drosophila ortholog of vertebrate tensin, despite being significantly shorter. The SH2 and PTB domains are more similar to these domains in tensin orthologs compared with other proteins, such as Shc, that share these domains. The region N-terminal to the SH2 domain is much shorter in Drosophila tensin compared to human or chicken tensin (456 residues versus 1461 or 1470) and does not contain a phosphatase domain. In the mosquito A. gambia, it is possible to find exons encoding a phosphatase domain upstream of the annotated tensin gene, suggesting mosquito tensin is more similar to vertebrate tensin. However, extensive searches failed to identify potential exons upstream of the Drosophila gene that would encode a phosphatase domain. Furthermore, the genome of the sibling species D. pseudoobscura contains a gene very similar to that in D. melanogaster, suggesting that the shortening of tensin occurred after the divergence of mosquito and fly. Despite the lack of sequence similarity, the N terminus of Drosophila tensin is also able to bind filamentous actin with the binding of the shorter degradation product showing that the binding site is within the first 100 residues (Torgler, 2004).

date revised: 10 June 2004

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