Figure 6. Hypothetical model.

Id1 prevents myogenic differentiation by interfering with the transcriptional activity of E2A/MyoD complexes [16]. During early stages of osteoblast differentiation, Runx2 is expressed. Expression levels of Runx2 need to be tightly controlled during later stages of differentiation, as Runx2 overexpression prevents late osteoblast differentiation [52]. In addition, it was demonstrated that Id2 prevents Runx2 binding to the late osteoblast marker promoter of OCN [38]. We suggest that differential gene expression regulation reliant on endocytic signal transduction provides an additional mechanism for the cell to regulate differentiation processes.