Isolation and characterization of a novel 45 kDa calponin-like protein from anterior byssus retractor muscle of the mussel Mytilus galloprovincialis

SUMMARY: A calponin-like protein of 45 kDa was isolated from mussel anterior byssus retractor muscle (ABRM) and its inhibitory effects on actomyosin Mg²⁺-ATPase was demonstrated. The 2-D electrophoresis for ABRM myofibrils gave a spot of 45 kDa protein in addition to myofibrillar proteins such as myosin and actin. The 45 kDa protein, which was more basic and showed a slightly higher molecular weight than actin, was isolated by ion-exchange chromatography and subjected to chymotryptic digestion. N-terminal amino acid sequencing of polypeptide fragments produced gave two sequences, ASQKGMTSFGAVRHH and GMDRALISKMGSKYDSGL, both of which showed a high homology to those of vertebrate calponins and invertebrate calponin-related proteins. Furthermore, the 45 kDa protein strongly reacted with commercially available antibody raised against chicken smooth muscle calponin, demonstrating that the mussel ABRM 45 kDa protein is a new member of the calponin family. Then, actomyosin Mg²⁺-ATPase activity of ABRM was measured in the presence and absence of the 45 kDa protein. The 45 kDa protein clearly inhibited actomyosin Mg²⁺-ATPase activity in a dose-dependent manner as in the case of other vertebrate calponins. These results indicate that the 45 kDa calponin-like protein is involved in the thin filament-associated regulation of molluscan smooth muscle contraction, possibly of a unique contraction called catch.     

Calponin Expression in Renal Tubulointerstitial Fibrosis Induced in Rats by Cisplatin

Renal tubulointerstitial fibrosis is the common feature of chronic renal failure, regardless of its etiology. Myofibroblasts play important roles in progression of the fibrosis and are characterized by expressions of various cytoskeletons such as vimentin, desmin and α-smooth muscle actin (α-SMA). To pursue the characteristics of the cells, we immunohistochemically investigated the relationship between calponin (a marker of terminal smooth muscles) expression and myofibroblasts in cisplatin-induced rat renal tubulointerstitial fibrosis. Calponin-expressing interstitial cells increased with fibrosis and reacted simultaneously to vimentin or α-SMA (a marker of well-differentiated myofibroblasts) but not desmin or Thy-1 (a marker of myofibroblasts at the early stage). The present study shows that calponin may be expressed transiently in relatively well-developed myofibroblasts in rat renal fibrosis. Calponin could become a marker for myofibroblast development in chronic renal toxicity in rats.     

Calponin expression and myoepithelial cell differentiation in canine, feline and human mammary simple carcinomas

Calponin is a 34‐kDa smooth muscle‐specific protein that has been shown to be a highly sensitive marker of myoepithelial cells in canine, feline and human mammary tissue and tumours. The expression of calponin was studied in 15 canine, 32 feline and 28 human simple mammary carcinomas using a monoclonal mouse antihuman calponin antibody and the avidin–biotin peroxidase complex (ABC) immunohistochemical technique. Calponin expression was compared with the expression of cytokeratin 14, a marker of normal mammary myoepithelial cells in the three species. Four different types of calponin‐positive cells were identified: (1) Type 1: cytokeratin‐14‐positive pre‐existing myoepithelial cells forming a continuous layer with images of focal disruptions; (2) Type 2: cytokeratin‐14‐positive isolated nests of fusiform, polygonal or round cells without atypia; (3) Type 3: cytokeratin‐14‐positive atypical cells indistinguishable from non‐reactive atypical cells, which should have never been detected in haematoxylin and eosin‐stained sections and (4) Type 4: cytokeratin‐14‐negative stromal fusiform cells around the neoplastic growth or cell nests, identified as myofibroblasts. Calponin‐negative and cytokeratin‐14‐positive atypical neoplastic cells were observed in three canine, 28 feline and two human carcinomas. The latter were indicative of altered expression of high‐molecular‐weight cytokeratins in luminal epithelial‐type simple carcinomas. Our findings show that calponin is a good marker of myoepithelial cell differentiation in feline, human and, particularly, canine simple carcinomas. The high number (six out of 15) of canine tumours with type 3 cells points to the need of both introducing calponin examination in the routine diagnostic schedule and performing further studies on its prognostic significance.