Mini reviewThrombospondin 2, a matricellular protein with diverse functions
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1998b), is limited by the low yields of the secreted trimeric protein and its sensitivity to proteolytic degradation (unpublished observations).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.
Section snippets
Homologs of TSP2
Thbs2 is a member of a family of five unlinked genes that encode TSPs 1–4 and TSP5/COMP. The five proteins can be grouped into one subfamily consisting of the two homotrimers, TSP1 and 2, and another with the three homopentamers, TSP3–5. The two subfamilies differ further in that TSP1 and 2 contain a procollagen domain and three type I repeats that are missing in TSP3–5, and the latter proteins contain four type II repeats rather than the three present in TSP 1 and 2 (Fig. 1).
The type I or
Gene structure and regulation
The exon/intron structure of the mouse Thbs2 gene is very similar to that of Thbs1, and the correlation between coding exons and the modular structure of the proteins is also very similar in the two genes (Bornstein et al., 1990, Lawler et al., 1991, Bornstein, 1992, Shingu and Bornstein, 1993). In contrast, the sequences of the proximal promoter regions differ markedly. In particular, a distal serum response element at −1280 in the human gene and an NF-Y binding site at −65 have been
Structure and biochemical properties
The structure of a TSP2 chain is shown schematically in Fig. 1. The modular domain structure and a comparison with TSP1 have been described in some detail (Laherty et al., 1992, Bornstein and Sage, 1994). Judging from the domain structures depicted in Fig. 1 and from sequence-generated evolutionary trees of members of the TSP gene family (Lawler et al., 1993), TSP1 and TSP2 are clearly more closely related to each other than either is to TSP3–5. The mass of a mouse TSP2 chain, based on a
Tissue distribution of TSP2 in the developing and adult mouse.
TSP2 has been detected in human tumors by RT-PCR (Tokunaga et al., 1999), in the chicken embryo by in situ hybridization (Tucker, 1993, Tucker et al., 1995), and in bovine adrenal cortex by immunocytochemistry (Danik et al., 1999), but systematic analyses of the cellular and tissue distribution of TSP2 have been limited to the mouse. Iruela-Arispe et al. (1993) published an extensive analysis of the distribution of TSP1, 2, and 3 mRNAs, as detected by in situ hybridization and RNAse protection,
The phenotype of the TSP2-null mouse
The broad distribution of expression of TSP2 (see Section 5) and the relative paucity of information regarding its function provided an impetus for the generation of TSP2-null mice by targeted disruption of the Thbs2 gene in embryonic stem cells (Kyriakides et al., 1998a). The mutation deleted exon 2 (which contains the translation start site) and exon 3. Although low levels of an aberrant lower MW TSP2 mRNA were detected, there was no evidence for production of either TSP2 or a TSP2 fragment
Conclusions and directions for future research
Much of our thinking about TSP2 tends to be influenced by the more extensive information available for its close relative, TSP1. This information is almost certainly pertinent to the structure and biochemistry of TSP2 as a protein, but must be regarded cautiously when considered as a guide to its function in vivo. This reservation derives from an awareness of the largely different distributions of the two proteins in the mouse, and from the distinct phenotypes of TSP1- and TSP2-null mice.
Acknowledgements
Contributions from our laboratory were supported by National Institutes of Health grants HL 18645 and AR 45418 and by National Science Foundation grant EEC9529161. L.C.A. is supported by an American Heart Association fellowship (9920418Z) and K.D.H. by National Institutes of Health Training Grant (DE 07063).
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2022, Pathology Research and PracticeCitation Excerpt :As a result, TSP1 and TSP2 can inhibit angiogenesis, tumor cell growth, and metastasis due to their similar biostructure and biofunction. Moreover, TSP-linked signaling pathway adaptor proteins, including CD36, cytoplasmic tyrosine kinase p59fyn, caspase-3-like proteases, and p38 mitogen-activated protein kinase (MAPK), participate in the initiation of signaling and inducing endothelial cell apoptosis [94,98,99]. A variety of factors can affect TSP1 gene expression.