Localization and expression of CSF-1 receptor in rat dental follicle cells

Colony-stimulating factor-1 (CSF-1) accelerates tooth eruption in rats and is localized in the dental follicle, a loose connective tissue sac that is necessary for eruption to occur. CSF-1 enhances the cellular events that occur in the follicle prior to eruption--namely, an influx of monocytes into the follicle early post-natally to form the osteoclasts needed to resorb bone for the eruption pathway. Because CSF-1 levels are at a peak at day 3 post-natally, and because CSF-1 has an autocrine effect on its own gene expression, the question remains as to what causes the subsequent decline in CSF-1 protein and mRNA after day 3 post-natally. To determine if the autocrine effect is inhibited through the CSF-1 receptor, analysis of the CSF-1 receptor mRNA levels in cultured dental follicle cells reveals that high concentrations of CSF-1 reduce the gene expression of the CSF-1 receptor. Interleukin 1 alpha, a molecule that enhances CSF-1 gene expression, has no effect on CSF-1 receptor mRNA levels. Immunostaining for the CSF-1 receptor protein shows that it is present in the dental follicle early post-natally and is either absent or greatly reduced by day 10 post-natally. Earlier studies showed that the mRNA levels of the CSF-1 receptor also parallel this time course. Thus, the above results suggest that the feedback inhibition of the autocrine effect of CSF-1 on its own expression is through the effect of CSF-1 inhibiting the translation and transcription of its receptor. In turn, these molecular interactions possibly regulate the cellular events that occur in the follicle prior to and during eruption.