EXPRESSION OF TENASCIN-C IN BONES RESPONDING TO MECHANICAL LOAD

A number of early biochemical responses of bane cells to mechanical lo ading have been identified, but the full sequence of events from the s ensing of strain to the formation of new bone is poorly characterized, Extracellular matrix proteins can modulate cell behavior and would be ideal molecules to amplify the early response to loading, The extrace llular matrix protein, tenascin-C, supports differentiation of culture d osteoblast-like cells. The current study was carried out to investig ate whether expression patterns of tenascin-C in loaded bones support a role for this protein as a mediator of the osteoregulatory response to loading. Tenascin-C expression was investigated by Northern blot an alysis in rat ulnae subjected to an established noninvasive loading re gimen engendering physiological strain levels, RNA extracted from load ed compared with contralateral control bones 6 h after loading showed a significant increase in tenascin-C transcript expression. The presen ce of tenascin-C was investigated by immunohistochemistry in bones of animals killed 3, 5, or 15 days after the initiation of daily loading. In animals killed at 3 or 5 days, periosteal surfaces undergoing load -induced reversal from resorption to formation showed enhanced tenasci n-C staining. In animals killed at 15 days, the hone formed in respons e to loading was clearly demarcated from old bone by strong tenascin-C staining of reversal lines. Within this new bone, tenascin-C staining was seen in the lacunae of older but not more recently embedded osteo cytes. The results presented here indicate that tenascin-C expression by bone cells is enhanced in the early osteogenic response to loading. This may indicate that tenascin-C acts as a mediator of the mechanica lly adaptiveresponse.