Osteoclast molecular phenotyping by random cDNA sequencing

The osteoclast is a cell type that is highly specialized for its bone resorption function. In order to decipher the numerous biochemical functions of osteoclasts, a description of the gene expression profile of osteoclasts would be beneficial. We have sought to identify genes that are highly expressed in osteoclasts by partially sequencing 194 randomly chosen cDNA clones from a representative rabbit osteoclast cDNA library. Comparison to nucleic acid and protein sequence databases indicates that 135 of these cDNAs are identical to or homologous to known mammalian genes. Reverse transcription-polymerase chain reaction (RT-PCR) assays with microisolated osteoclasts were used to verify the osteoclast expression of some of these genes. Fifty-nine cDNAs, including two abundantly expressed species, have no significant similarity to the sequence databases and likely represent novel genes. The most abundant of the osteoclast expressed genes encode cofilin and the vacuolar H(+)-ATPase 16 kd subunit. Each were represented at a frequency of 4.1% of the clones in the library (95% confidence interval = 2.4-6.6%). The high expression of these gene products is consistent with the high motility of osteoclasts and their very active hydrogen ion secretion. Other abundantly expressed sequences include beta-actin (95% C.I. = 2.0-6.0%), creatine kinase B (95% C.I. = 1.2-4.9%), c-fms and ribosomal protein L18 (95% C.I. = 0.8-4.3%), and cathepsin-OC2, cyclophilin, delta-aminolevulinate synthetase, 16S mitochondrial rRNA, and two novel gene sequences (95% C.I. = 0.5-3.6%).