TGF-beta family responsiveness can be modulated by proteases that target TGF-beta receptors and co-receptors. These proteolytic activities add to the regulatory complexity of TGF-beta family signaling. For instance, thrombin cleavage of PAR1 leads to internalization of the co-receptor endoglin, and MMP-14 and MMP-16 mediate biglycan shedding.1, 2 Members of the BMP-1 protease family also play important roles in the regulation of TGF-beta family activities. The highly conserved BMP-1/PCP subgroup of the astacin family includes BMP-1 and alternatively spliced mammalian tolloid (mTLD), and mammalian tolloid-like (mTLL) 1 and mTLL2. These enzymes contain an astacin protease domain followed by variable numbers of CUB and EGF-like domains. They cleave N-terminal to an aspartic acid residue in multiple extracellular matrix (ECM) components (collagens, laminins, small leucine-rich proteoglycans, and SIBLING proteins), lysyl oxidases, and growth factor-related molecules.3
TGF-beta family proproteins are cleaved in the trans-Golgi between the N-terminal propeptide and the mature growth factor. For TGF-beta1, -beta2, -beta3, GDF-8, and GDF-11, the prodomain is secreted in association with the growth factor and maintains the growth factor in an inactive state. BMP-1 family of proteases regulate the activation of these latent complexes by several mechanisms.
Large latent complexes consisting of TGF-beta, latency-associated peptide (LAP), and latent TGF-beta-binding protein (LTBP) are anchored to the ECM by the LTBP. A recent report by Ge et al.4 describes how BMP-1 cleaves LTBP1 at two positions, leaving its central portion associated with TGF-beta/LAP and severing the connection of the large latent complexes to the ECM (Figure 1A). This processing of LTBP1 is required for efficient MMP-2-mediated liberation of TGF-beta from LAP. Knockout mice deficient in BMP-1, mTLD, and mTLL1 have greatly increased amounts of large latent complexes associated with the ECM and significantly reduced levels of active TGF-beta. One of the many effects of TGF-beta is to induce further expression of BMP-1, resulting in positive feedback regulation of TGF-beta activity. Certain TGF-beta family members retain non-covalent associations with propeptides following cleavage from the latent protein. For instance, BMP-1 family proteases cleave at single positions within the non-covalently associated GDF-8 and GDF-11 prosegments, resulting in the release of active growth factors (Figure 1B).5, 6 BMP-1, mTLL-1, and mTLL-2 are comparably effective in this activity.3 Prodomains of GDF-11 with a substitution of the Asp in the cleavage sites can associate with mature GDF-11 and block its activity.6
||Figure 1. Three mechanisms for BMP-1-related activation of latent TGF-beta family complexes. A: BMP-1 cleaves LTBP at two positions in a process required for efficient MMP-2-mediated liberation of TGF-beta from LAP. B: BMP-1 family proteases cleave at single positions within the GDF-8 and GDF-11 propeptides, releasing the active growth factor dimers. C: Activation of BMP in complex with chordin is mediated by BMP-1 proteolysis at two sites within chordin. Figures adapted from references 1 and 2.
Other BMP homodimers and heterodimers are not secreted in complex with their prosegments, but are held in latent complexes by subsequent association with chordin. Activation of these complexes is achieved by BMP-1 mediated proteolysis at two sites within chordin (Figure 1C).7 Chordin recognition is conferred by the first CUB domain of BMP-1, as mTLL-2 does not cleave chordin unless its first CUB domain is swapped with that of BMP-1.8 PCPE-1, which enhances BMP-1 in the removal of the C-terminal propeptide of procollagen, does not affect chordinase activity.8, 9
The activity of BMP-1 family proteases is required during embryonic development. Genetic knockout results in embryonic or perinatal lethal defects in skull, heart, and abdominal wall formation. In these experiments, the liberation of TGF-beta from the large latent complexes is reduced, chordin processing is inefficient, and collagen fibrillogenesis is aberrant.4, 10, 11
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