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 fun ctions of osteoclasts, a description of the gene expression profile of osteoclasts would be beneficial. We have sought to identify genes tha t are highly expressed in osteoclasts by partially sequencing 194 rand omly chosen cDNA clones from a representative rabbit osteoclast cDNA l ibrary. Comparison to nucleic acid and protein sequence databases indi cates that 135 of these cDNAs are identical to or homologous to known mammalian genes. Reverse transcription-polymerase chain reaction (RT-P CR) assays with microisolated osteoclasts were used to verify the oste oclast expression of some of these genes. Fifty-nine cDNAs, including two abundantly expressed species, have no significant similarity to th e sequence databases and likely represent novel genes. The most abunda nt of the osteoclast expressed genes encode cofilin and the vacuolar H +-ATPase 16 kd subunit. Each were represented at a frequency of 4.1% o f the clones in the library (95% confidence interval = 2.4-6.6%). The high expression of these gene products is consistent with the high mot ility of osteoclasts and their very-active hydrogen ion secretion, Oth er 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 pro tein 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%).