Tendon stem progenitor cells: Understanding the biology to inform therapeutic strategies for tendon repair.

Tendon and ligament injuries are a leading cause of healthcare visits with significant impact in terms of economic cost and reduced quality of life. To date, reparative strategies remain largely restricted to conservative treatment or surgical repair. However, these therapies fail to restore native tendon structure and function; thus, the tissue may re-rupture or degenerate with time. To improve tendon healing, one promising strategy may be harnessing the innate potential of resident tendon stem/progenitor cells (TSPCs) to guide tenogenic regeneration. In this review, we outline recent advances in the identification and characterization of putative TSPC populations, and discuss biochemical, biomechanical, and biomaterial methods employed for their culture and differentiation. Finally, we identify limitations in our current understanding of TSPC biology, key challenges for their use, and potential therapeutic strategies to inform cell-based tendon repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1270-1280, 2019.

A novel role for cathepsin K in periosteal osteoclast precursors during fracture repair.

Osteoporosis management is currently centered around bisphosphonates, which inhibit osteoclast (OC) bone resorption but do not affect bone formation. This reduces fracture risk, but fails to restore healthy bone remodeling. Studies in animal models showed that cathepsin K (CatK) inhibition by genetic deletion or chemical inhibitors maintained bone formation while abrogating resorption during bone remodeling and stimulated periosteal bone modeling. Recently, periosteal mononuclear tartrate-resistant acid phosphatase-positive (TRAP+ ) osteoclast precursors (OCPs) were shown to augment angiogenesis-coupled osteogenesis. CatK gene deletion increased osteoblast differentiation via enhanced OCP and OC secretion of platelet-derived growth factor (PDGF)-BB and sphingosine 1 phosphate. The effects of periosteum-derived OCPs on bone remodeling are unknown, particularly with regard to fracture repair. We hypothesized that periosteal OCPs derived from CatK-null (Ctsk-/- ) mice may enhance periosteal bone formation during fracture repair. We found fewer periosteal OCPs in Ctsk-/- mice under homeostatic conditions; however, after fracture, this population increased in number relative to that seen in wild-type (WT) mice. Enhanced TRAP staining and greater expression of PDGF-BB were observed in fractured Ctsk-/- femurs relative to WT femurs. This early pattern of augmented PDGF-BB expression in Ctsk-/- mice may contribute to improved fracture healing by enhancing callus mineralization in Ctsk-/- mice.

The Hajdu Cheney Mutation Is a Determinant of B-Cell Allocation of the Splenic Marginal Zone.

The neurogenic locus notch homolog protein (Notch)-2 receptor is a determinant of B-cell allocation, and gain-of-NOTCH2-function mutations are associated with Hajdu-Cheney syndrome (HCS), a disease presenting with osteoporosis and acro-osteolysis. We generated a mouse model reproducing the HCS mutation (Notch2HCS), and heterozygous global mutant mice displayed gain-of-Notch2 function. In the mutant spleen, the characteristic perifollicular rim marking the marginal zone (MZ), which is the interface between the nonlymphoid red pulp and the lymphoid white pulp, merged with components of the white pulp. As a consequence, the MZ of Notch2HCS mice occupied most of the splenic structure. To explore the mechanisms involved, lymphocyte populations from the bone marrow and spleen were harvested from heterozygous Notch2HCS mice and sex-matched control littermates and analyzed by flow cytometry. Notch2HCS mice had an increase in CD21/35highCD23- splenic MZ B cells of approximately fivefold and a proportional decrease in splenic follicular B cells (CD21/35intCD23+) at 1, 2, and 12 months of age. Western blot analysis revealed that Notch2HCS mutant splenocytes had increased phospho-Akt and phospho-Jun N-terminal kinase, and gene expression analysis of splenic CD19+ B cells demonstrated induction of Hes1 and Hes5 in Notch2HCS mutants. Anti-Notch2 antibodies decreased MZ B cells in control and Notch2HCS mice. In conclusion, Notch2HCS mutant mice have increased mature B cells in the MZ of the spleen.

Loss of Cbl-PI3K interaction modulates the periosteal response to fracture by enhancing osteogenic commitment and differentiation.

The periosteum contains multipotent skeletal progenitors that contribute to bone repair. The signaling pathways regulating the response of periosteal cells to fracture are largely unknown. Phosphatidylinositol-3 Kinase (PI3K), a prominent lipid kinase, is a major signaling protein downstream of several factors that regulate osteoblast differentiation. Cbl is an E3 ubiquitin ligase and a major adaptor protein that binds to the p85 regulatory subunit and modulates PI3K activity. Substitution of tyrosine 737 to phenylalanine (Y737F) in Cbl abolishes the interaction between Cbl and p85 subunit without affecting the Cbl's ubiquitin ligase function. Here, we investigated the role of PI3K signaling during the very early stages of fracture healing using OsterixRFP reporter mice. We found that the absence of PI3K regulation by Cbl resulted in robust periosteal thickening, with increased proliferation of periosteal cells. While the multipotent properties of periosteal progenitors to differentiate into chondrocytes and adipocytes did not change, osteogenic differentiation in the absence of Cbl-PI3K interaction was highly augmented. The increased stability and nuclear localization of Osterix observed in periosteal cells lacking Cbl-PI3K interaction may explain this enhanced osteogenic differentiation since the expression of Osterix transcriptional target genes including osteocalcin and BSP are increased in YF cells. Overall, our findings highlight a hitherto unexplored and novel role for Cbl and PI3K in modulating the osteogenic response of periosteal cells during the early stages of fracture repair.

Induced pluripotent stem cells: fundamentals and applications of the reprogramming process and its ramifications on regenerative medicine.

OBJECTIVE: To provide a comprehensive source of information about the reprogramming process and induced pluripotency. BACKGROUND: The ability of stem cells to renew their own population and to differentiate into specialized cell types has always attracted researchers looking to exploit this potential for cellular replacement therapies, pharmaceutical testing and studying developmental pathways. While adult stem cell therapy has already been brought to the clinic, embryonic stem cell research has been beset with legal and ethical impediments. FOCUS: The conversion of human somatic cells to human induced pluripotent stem cells (hiPSCs), which are equivalent to human embryonic stem cells (hESCs), provides a system to sidestep these barriers and expedite pluripotent stem cell research for the aforementioned purposes. However, being a very recent discovery, iPSCs have yet to overcome many other obstacles and criticism to be proven safe and feasible for clinical use. METHODOLOGY: This review introduces iPSC, the various methods that have been used to generate them and their pros and cons. It also covers in detail the pluripotency factors responsible for iPSC generation as well as the signaling pathways, epigenetic modifications and miRNA regulation implicated in the reprogramming process. The known molecular crosstalk between these reprogramming regulators is also illuminated. We will also mention the molecular compounds which have been shown to either replace one or more genetic factors or improve overall efficiency and kinetics of iPSC induction. CONCLUSION: To conclude, we will briefly discuss the current problems that hinder bench to bedside translation of iPSC research as well as the possible steps that can bring iPSC therapy and other potential applications closer to fruition.