Osteogenic differentiation: a universal cell program of heterogeneous mesenchymal cells or a similar extracellular matrix mineralizing phenotype?

"A Friend Among Strangers" or the Ambiguous Roles of Runx2

The transcription factor Runx2 plays a crucial role in regulating osteogenic differentiation and skeletal development. This factor not only controls the expression of genes involved in bone formation, but also interacts with signaling pathways such as the Notch pathway, which are essential for body development. However, studies have produced conflicting results regarding the relationship between Runx2 and the Notch pathway. Some studies suggest a synergistic interaction between these molecules, while others suggest an inhibitory one, for example, the interplay between Notch signaling, Runx2, and vitamin D3 in osteogenic differentiation and bone remodeling. The findings suggest a complex relationship between Notch signaling and osteogenic differentiation, with ongoing research needed to clarify the mechanisms involved and resolve existing contradictions regarding role of Notch in this process. Additionally, there is increasing evidence of contradictory roles for Runx2 in various tissues and organs, both under normal conditions and in pathological states. This diversity of roles makes Runx2 a potential therapeutic target, offering new directions for research. In this review, we have discussed the mechanisms of osteogenic differentiation and the important role of Runx2 in this process. We have also examined its relationship with different signaling pathways. However, there are still many uncertainties and inconsistencies in our current understanding of these interactions. Additionally, given that Runx2 is also involved in numerous other events in various tissues, we have tried to comprehensively examine its functions outside the skeletal system.

Assembling the Puzzle Pieces. Insights for in Vitro Bone Remodeling

As a highly dynamic organ, bone changes during throughout a person's life. This process is referred to as 'bone remodeling' and it involves two stages - a well-balanced osteoclastic bone resorption and an osteoblastic bone formation. Under normal physiological conditions bone remodeling is highly regulated that ensures tight coupling between bone formation and resorption, and its disruption results in a bone metabolic disorder, most commonly osteoporosis. Though osteoporosis is one of the most prevalent skeletal ailments that affect women and men aged over 40 of all races and ethnicities, currently there are few, if any safe and effective therapeutic interventions available. Developing state-of-the-art cellular systems for bone remodeling and osteoporosis can provide important insights into the cellular and molecular mechanisms involved in skeletal homeostasis and advise better therapies for patients. This review describes osteoblastogenesis and osteoclastogenesis as two vital processes for producing mature, active bone cells in the context of interactions between cells and the bone matrix. In addition, it considers current approaches in bone tissue engineering, pointing out cell sources, core factors and matrices used in scientific practice for modeling bone diseases and testing drugs. Finally, it focuses on the challenges that bone regenerative medicine is currently facing.

Purinergic Signaling in Pathologic Osteogenic Differentiation of Aortic Valve Interstitial Cells from Patients with Aortic Valve Calcification

Purinergic signaling is associated with a vast spectrum of physiological processes, including cardiovascular system function and, in particular, its pathological calcifications, such as aortic valve stenosis. Aortic valve stenosis (AS) is a degenerative disease for which there is no cure other than surgical replacement of the affected valve. Purinergic signaling is known to be involved in the pathologic osteogenic differentiation of valve interstitial cells (VIC) into osteoblast-like cells, which underlies the pathogenesis of AS. ATP, its metabolites and related nucleotides also act as signaling molecules in normal osteogenic differentiation, which is observed in pro-osteoblasts and leads to bone tissue development. We show that stenotic and non-stenotic valve interstitial cells significantly differ from each other, especially under osteogenic stimuli. In osteogenic conditions, the expression of the ecto-nucleotidases ENTPD1 and ENPP1, as well as ADORA2b, is increased in AS VICs compared to normal VICs. In addition, AS VICs after osteogenic stimulation look more similar to osteoblasts than non-stenotic VICs in terms of purinergic signaling, which suggests the stronger osteogenic differentiation potential of AS VICs. Thus, purinergic signaling is impaired in stenotic aortic valves and might be used as a potential target in the search for an anti-calcification therapy.