Thrombospondin 2, a matricellular protein with diverse functions

Abstract

Thrombospondin (TSP) 2 is a close relative of TSP1 but differs in its temporal and spatial distribution in the mouse. This difference in expression undoubtedly reflects the marked disparity in the DNA sequences of the promoters in the genes encoding the two proteins. The synthesis of TSP2 occurs primarily in connective tissues of the developing and growing mouse. In the adult animal the protein is again produced in response to tissue injury and in association with the growth of tumors. Despite the abnormalities in collagen fibrillogenesis, fragility of skin, and laxity of tendons and ligaments observed in the TSP2-null mouse, TSP2 does not appear to contribute directly to the structural integrity of connective tissue elements. Instead, emerging evidence supports a mode of action of TSP2 ‘at a distance’, i.e. by modulating the activity and bioavailability of proteases and growth factors in the pericellular environment and, very likely, by interaction with cell-surface receptors. Thus, TSP2 qualifies as a matricellular protein, as defined in the introduction to this minireview series. The phenotype of TSP2-null mice has been very helpful in providing clues to the functions of TSP2. In addition to histological and functional abnormalities in connective tissues, these mice display an increased vascularity of the dermis and subdermal tissues, increased endosteal bone growth, a bleeding defect, and a marked adhesive defect of dermal fibroblasts. Our laboratory has established that TSP2 binds matrix metalloproteinase 2 (MMP2) and that the adhesive defect in TSP2-null fibroblasts results from increased MMP2 activity. The investigation of the basis for the other defects in the TSP2-null mouse is likely to yield equally interesting results.

Introduction

The existence of a second member of the thrombospondin (TSP) family was first detected in 1991, when it was found that the sequence of a cDNA clone that had been isolated from an NIH 3T3 cell λgt11 library was homologous to, but different from, the coding sequence of a previously isolated mouse TSP1 genomic clone (Bornstein et al., 1991b). TSP2 and TSP1 are encoded by different genes, as indicated by Southern blot and cytogenetic analyses, and RNAse protection analyses revealed that the tissue-specific expression of the two genes is also different (Bornstein et al., 1991b). The identification of a second TSP gene raised the hope that part of the increasing complexity of the TSP literature could be explained by the failure to recognize the presence of the second protein. An inability to distinguish between TSP1 and TSP2 seemed particularly likely in studies that depended on the specificity of anti-TSP antibodies, since the sequences of the two proteins were found to be very similar (Bornstein, 1992, Bornstein and Sage, 1994). However, as more studies of both proteins have been performed during the past decade, the innate functional complexity of both TSPs, and the different roles that they play in mammalian biology, have become apparent. Indeed, it would appear that polyvalent antibodies to either TSP are monospecific (O'Rourke et al., 1992, Tooney et al., 1998), perhaps because the domains that differ most in amino acid sequence among orthologs of TSP1 and TSP2 are also most different between the two paralogs (Bornstein and Sage, 1994).

TSP1 is a major component of α granules in platelets and a convenient natural source of the protein. In contrast, the extraction of useful amounts of TSP2 from tissues has not been feasible to date. Production of recombinant TSP2 in mammalian cells has not been reported, and synthesis in insect cells, while feasible (Chen et al., 1994, Kyriakides et al., 1998b), is limited by the low yields of the secreted trimeric protein and its sensitivity to proteolytic degradation (unpublished observations). The lack of availability of adequate amounts of pure TSP2 has been a major factor in the slow pace of research on the interactions and functions of this protein.