Osteoblasts arise from common progenitors with chondrocytes, muscle and adipocytes, and various hormones and local factors regulate their differentiation. We review here regulation of osteoblast differentiation mediated by the local factors such as bone morphogenetic proteins (BMPs) and hedgehogs and the transcription factor, core-binding factor alpha-1 (Cbfa1). BMPs are the most potent regulators of osteoblast differentiation among the local factors. Sonic and Indian hedgehogs are involved in osteoblast differentiation by interacting with BMPs. Cbfa1, a member of the runt domain gene family, plays a major role in the processes of a determination of osteoblast cell lineage and maturation of osteoblasts. Cbfa1 is an essential transcription factor for osteoblast differentiation and bone formation, because Cbfa1-deficient mice completely lacked bone formation due to maturation arrest ofosteoblasts. Although the regulatory mechanism of Cbfa1 expression has not been fully clarified, BMPs are an important local factor that up-regulates Cbfa1 expression. Thus, the intimate interaction between local factors such as BMPs and hedgehogs and the transcription factor, Cbfa1, is important to osteoblast differentiation and bone formation.

PGAM5 is a unique type of protein phosphatase that exists in mitochondria. It has been shown to exist in the inner mitochondrial membrane through its transmembrane domain and to be cleaved within the transmembrane domain upon mitochondrial dysfunction. However, its submitochondrial localization remains controversial; many researchers claim that PGAM5 localizes to the outer mitochondrial membrane based on the findings that PGAM5 associates with many cytoplasmic proteins. Here, we found that cleaved PGAM5 was released from mitochondria during mitophagy, a selective form of autophagy specific for mitochondria, and that the release was inhibited by proteasome inhibitors in HeLa cells stably expressing the E3 ubiquitin ligase Parkin. However, treatment of parental HeLa cells lacking Parkin with mitophagy-inducing agents caused PGAM5 cleavage but did not cause its release from mitochondria. Thus, cleaved PGAM5 appears to be released from mitochondria depending on proteasome-mediated rupture of the outer membrane during mitophagy, which has been previously shown to precede autophagy-mediated degradation of whole mitochondria. This study suggests that PGAM5 senses mitochondrial dysfunction in the inner mitochondrial membrane and serves as a signaling intermediate that regulates the cellular response to mitochondrial stress upon its cleavage and release from mitochondria.

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