Extensive clear zone and defective ruffled border formation in osteoclastsof osteopetrotic (ia/ia) rats: Implications for secretory function

The cellular distribution of osteoclast integrin subunits alpha(v) and beta (3), the tissue distribution, and level of the apparent ligand osteopontin (OPN) as well as of the putative regulatory enzyme tartrate-resistant acid phosphatase (TRAP) were studied along with the intracellular distribution o f the activation marker c-src in osteopetrotic ia/ia (incisors-absent) muta nt rats and their normal littermates. In ia/ia rats, the osteoclasts are in capable of bone matrix resorption, Ultrastructurally the cells exhibit exte nded clear zones at the expense of ordinary ruffled borders. A secretory dy sfunction in the mutant is strongly suggested by the absence of detectable extracellular TRAP, concomitant with an accumulation of the enzyme in abund ant small cytoplasmic vesicles. Moreover, TRAP mRNA, protein content, as we ll as enzymatic activity were elevated. Furthermore, increased levels of in tegrin subunits alpha(v) and beta(3) were detected at the clear zone of mut ant osteoclasts. OPN mRNA levels were elevated in long bones from mutants. In ia/ia rats, immunolabeling for OPN was homogeneously distributed at the surface facing osteoclasts, while in normal littermates it was concentrated at the clear zone area and barely detectable at ruffled borders. The absen ce of OPN labeling in the abundant, putative intracellular secretory vesicl es in mutant osteoclasts suggests that these cells do not produce OPN, The osteoclasts of ia/ia rats appeared to produce and translocate the c-src pro tein to the cell membrane. In ia/ia a defect ruffled border-formation is ob served along with extensive clear zone formation and decreased secretory fu nction. The lesion may be due to a signaling defect, but in that case the d efect seems to be located downstream to or not involving the c-src pathway. Our results illustrate the close relationship between secretory function a nd ruffled border formation in osteoclasts, a relationship that appears to be necessary for proper resorptive function. (C) 1999 Academic Press.