Hydra metalloproteinase 1: a secreted astacin metalloproteinase whose apical axis expression is differentially regulated during head regeneration.

The newly emerging astacin metalloproteinase family comprises multiple members with diverse functions. Most recently, the development-related functions have been attributed to both (1) proteolytic cleavage and subsequent release of active TGF-beta-like growth factors from latent inhibitory complexes and (2) modification of extracellular matrix (ECM) assembly and composition. We previously identified and purified hydra metalloproteinase 1 (HMP-1), a developmentally important astacin proteinase that functions in head regeneration and transdifferentiation of tentacle battery cells (L. Yan et al., 1995, Development 121, 1591-1602). In the present study, further cloning revealed that HMP-1 is produced as a secreted zymogen with a conserved hydrophobic signal sequence and a putative propeptide. The processed HMP-1 is composed of a characteristic astacin proteinase domain and a unique Cys-rich C-terminus. With this simple domain structure, HMP-1 represents an ancestral astacin proteinase. Consistent with its role in head regeneration, HMP-1 mRNA is expressed at highest levels by endodermal cells at the apical pole of the body column just inferior to the base of tentacles, the region of active cell differentiation or transdifferentiation. A modified immunocytochemical procedure demonstrated that HMP-1 protein can be localized not only to ECM of tentacles as we previously reported, but also to endodermal cells of the body column in a pattern similar to its mRNA distribution. The localization of HMP-1 protein in tentacles was confirmed using an enzymatic approach. A translocation of HMP-1 protein from cells in the body column to the extracellular milieu in tentacles further suggests that HMP-1 is a secreted protein. HMP-1 expression undergoes extensive regulation at the transcriptional level both temporally and spatially during head regeneration. The involvement of HMP-1 in this morphogenetic process is further supported by the blockage of head regeneration with localized antisense treatment. Taken together, these results suggest that HMP-1 is a secreted astacin metalloproteinase that has an important role in regulating hydra head morphogenesis potentially through its differential expression along the body axis.