FAK Promotes Early Osteoprogenitor Cell Proliferation by Enhancing mTORC1 Signaling

Regulation of intracellular cAMP levels in osteocytes by mechano-sensitive focal adhesion kinase via PDE8A

Osteocytes are the primary mechano-sensitive cell type in bone. Mechanical loading is sensed across the dendritic projections of osteocytes leading to transient reductions in focal adhesion kinase (FAK) activity. Knowledge regarding the signaling pathways downstream of FAK in osteocytes is incomplete. We performed tyrosine-focused phospho-proteomic profiling in osteocyte-like Ocy454 cells to identify FAK substrates. Gsα, parathyroid hormone receptor (PTH1R), and phosphodiesterase 8A (PDE8A), all proteins associated with cAMP signaling, were found as potential FAK targets based on their reduced tyrosine phosphorylation in both FAK- deficient or FAK inhibitor treated cells. Real time monitoring of intracellular cAMP levels revealed that FAK pharmacologic inhibition or gene deletion increased basal and GPCR ligand-stimulated cAMP levels and downstream phosphorylation of protein kinase A substrates. Mutating FAK phospho-acceptor sites in Gsα and PTH1R had no effect on PTH- or FAK inhibitor-stimulated cAMP levels. Since FAK inhibitor treatment augmented cAMP levels even in the presence of forskolin, we focused on potential FAK substrates downstream of cAMP generation. Indeed, PDE8A inhibition mimicked FAK inhibition at the level of increased cAMP, PKA activity, and expression of cAMP-regulated target genes. In vitro kinase assay showed that PDE8A is directly phosphorylated by FAK while immunoprecipitation assays revealed intracellular association between FAK and PDE8A. Thus, FAK inhibition in osteocytes acts synergistically with signals that activate adenylate cyclase to increase intracellular cAMP. Mechanically-regulated FAK can modulate intracellular cAMP levels via effects on PDE8A. These data suggest a novel signal transduction mechanism that mediates crosstalk between mechanical and cAMP-linked hormonal signaling in osteocytes.

Preliminary study of the homeostatic regulation of osseointegration by nanotube topology

The ideal implant surface plays a substantial role in maintaining bone homeostasis by simultaneously promoting osteoblast differentiation and limiting overactive osteoclast activity to a certain extent, which leads to satisfactory dynamic osseointegration. However, the rational search for implant materials with an ideal surface structure is challenging and a hot research topic in the field of tissue engineering. In this study, we constructed titanium dioxide titanium nanotubes (TNTs) by anodic oxidation and found that this structure significantly promoted osteoblast differentiation and inhibited osteoclast formation and function while simultaneously inhibiting the total protein levels of proline-rich tyrosine kinase 2 (PYK2) and focal adhesion kinase (FAK). Knockdown of the PYK2 gene by siRNA significantly suppressed the number and osteoclastic differentiation activity of mouse bone marrow mononuclear cells (BMMs), while overexpression of PYK2 inhibited osteogenesis and increased osteoclastic activity. Surprisingly, we found for the first time that neither knockdown nor overexpression of the FAK gene alone caused changes in osteogenesis or osteoclastic function. More importantly, compared with deletion or overexpression of PYK2/FAK alone, coexpression or cosilencing of the two kinases accelerated the effects of TNTs on osteoclastic and osteogenic differentiation on the surface of cells. Furthermore, in vivo experiments revealed a significant increase in positiveexpression-PYK2 cells on the surface of TNTs, but no significant change in positiveexpression -FAK cells was observed. In summary, PYK2 is a key effector molecule by which osteoblasts sense nanotopological mechanical signals and maintain bone homeostasis around implants. These results provide a referable molecular mechanism for the future development and design of homeostasis-based regulatory implant biomaterials.

Inhibition of focal adhesion kinase 2 results in a macrophage polarization shift to M2 which attenuates local and systemic inflammation and reduces heterotopic ossification after polysystem extremity trauma

Introduction

Heterotopic ossification (HO) is a complex pathology often observed in combat injured casualties who have sustained severe, high energy polytraumatic extremity injuries. Once HO has developed, prophylactic therapies are limited outside of surgical excision. Tourniquet-induced ischemia injury (IR) exacerbates trauma-mediated musculoskeletal tissue injury, inflammation, osteogenic progenitor cell development and HO formation. Others have shown that focal adhesion kinase-2 (FAK2) plays a key role in regulating early inflammatory signaling events. Therefore, we hypothesized that targeting FAK2 prophylactically would mitigate extremity trauma induced IR inflammation and HO formation.

Methods

We tested whether the continuous infusion of a FAK2 inhibitor (Defactinib, PF-573228; 6.94 µg/kg/min for 14 days) can mitigate ectopic bone formation (HO) using an established blast-related extremity injury model involving femoral fracture, quadriceps crush injury, three hours of tourniquet-induced limb ischemia, and hindlimb amputation through the fracture site. Tissue inflammation, infiltrating cells, osteogenic progenitor cell content were assessed at POD-7. Micro-computed tomography imaging was used to quantify mature HO at POD-56.

Results

In comparison to vehicle control-treated rats, FAK2 administration resulted in no marked wound healing complications or weight loss. FAK2 treatment decreased HO by 43%. At POD-7, marked reductions in tissue proinflammatory gene expression and assayable osteogenic progenitor cells were measured, albeit no significant changes in expression patterns of angiogenic, chondrogenic and osteogenic genes. At the same timepoint, injured tissue from FAK-treated rats had fewer infiltrating cells. Additionally, gene expression analyses of tissue infiltrating cells resulted in a more measurable shift from an M1 inflammatory to an M2 anti-inflammatory macrophage phenotype in the FAK2 inhibitor-treated group.

Discussion

Our findings suggest that FAK2 inhibition may be a novel strategy to dampen trauma-induced inflammation and attenuate HO in patients at high risk as a consequence of severe musculoskeletal polytrauma.

Focal adhesion kinase (FAK): its structure, characteristics, and signaling in skeletal system

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase and distributes important regulatory functions in skeletal system. Mesenchymal stem cell (MSC) possesses significant migration and differentiation capacity, is an important source of distinctive bone cells production and a prominent bone development pathway. MSC has a wide range of applications in tissue bioengineering and regenerative medicine, and is frequently employed for hematopoietic support, immunological regulation, and defect repair, although current research is insufficient. FAK has been identified to cross-link with many other keys signaling pathways in bone biology and is considered as a fundamental "crossroad" on the signal transduction pathway and a "node" in the signal network to mediate MSC lineage development in skeletal system. In this review, we summarized the structure, characteristics, cellular signaling, and the interactions of FAK with other signaling pathways in the skeletal system. The discovery of FAK and its mediated molecules will lead to a new knowledge of bone development and bone construction as well as considerable potential for therapeutic use in the treatment of bone-related disorders such as osteoporosis, osteoarthritis, and osteosarcoma.

Chondrocyte Tsc1 controls cranial base bone development by restraining the premature differentiation of synchondroses

Cranial base bones are formed through endochondral ossification. Synchondroses are growth plates located between cranial base bones that facilitate anterior-posterior growth of the skull. Coordinated proliferation and differentiation of chondrocytes in cranial base synchondroses is essential for cranial base bone growth. Herein, we report that constitutive activation of the mechanistic target of rapamycin complex 1 (mTORC1) signaling via Tsc1 (Tuberous sclerosis 1) deletion in chondrocytes causes abnormal skull development with decreased size and rounded shape. In contrast to decreased anterior-posterior growth of the cranial base, mutant mice also exhibited significant expansion of cranial base synchondroses including the intersphenoid synchondrosis (ISS) and the spheno-occipital synchondrosis (SOS). Cranial base synchondrosis expansion in TSC1-deficient mice was accounted for by an expansion of the resting zone due to increased cell number and size without alteration in cell proliferation. Furthermore, our data showed that mTORC1 activity is inhibited in the resting and proliferating zone chondrocytes of wild type mice, and Tsc1 deletion activated mTORC1 signaling of the chondrocytes in the resting zone area. Consequently, the chondrocytes in the resting zone of TSC1-deficient mice acquired characteristics generally attributed to pre-hypertrophic chondrocytes including high mTORC1 activity, increased cell size, and increased expression level of PTH1R (Parathyroid hormone 1 receptor) and IHH (Indian hedgehog). Lastly, treatment with rapamycin, an inhibitor of mTORC1, rescued the abnormality in synchondroses. Our results established an important role for TSC1-mTORC1 signaling in regulating cranial base bone development and showed that chondrocytes in the resting zone of synchondroses are maintained in an mTORC1-inhibitory environment.

Biophysical Modulation of the Mitochondrial Metabolism and Redox in Bone Homeostasis and Osteoporosis: How Biophysics Converts into Bioenergetics

Bone-forming cells build mineralized microstructure and couple with bone-resorbing cells, harmonizing bone mineral acquisition, and remodeling to maintain bone mass homeostasis. Mitochondrial glycolysis and oxidative phosphorylation pathways together with ROS generation meet the energy requirement for bone-forming cell growth and differentiation, respectively. Moderate mechanical stimulations, such as weight loading, physical activity, ultrasound, vibration, and electromagnetic field stimulation, etc., are advantageous to bone-forming cell activity, promoting bone anabolism to compromise osteoporosis development. A plethora of molecules, including ion channels, integrins, focal adhesion kinases, and myokines, are mechanosensitive and transduce mechanical stimuli into intercellular signaling, regulating growth, mineralized extracellular matrix biosynthesis, and resorption. Mechanical stimulation changes mitochondrial respiration, biogenesis, dynamics, calcium influx, and redox, whereas mechanical disuse induces mitochondrial dysfunction and oxidative stress, which aggravates bone-forming cell apoptosis, senescence, and dysfunction. The control of the mitochondrial biogenesis activator PGC-1α by NAD+-dependent deacetylase sirtuins or myokine FNDC/irisin or repression of oxidative stress by mitochondrial antioxidant Nrf2 modulates the biophysical stimulation for the promotion of bone integrity. This review sheds light onto the roles of mechanosensitive signaling, mitochondrial dynamics, and antioxidants in mediating the anabolic effects of biophysical stimulation to bone tissue and highlights the remedial potential of mitochondrial biogenesis regulators for osteoporosis.

Osteoblast Differentiation and Signaling: Established Concepts and Emerging Topics

Osteoblasts, the cells that build up our skeleton, are remarkably versatile and important cells that need tight regulation in all the phases of their differentiation to guarantee proper skeletal development and homeostasis. Although we know many of the key pathways involved in osteoblast differentiation and signaling, it is becoming clearer and clearer that this is just the tip of the iceberg, and we are constantly discovering novel concepts in osteoblast physiology. In this review, we discuss well-established pathways of osteoblastic differentiation, i.e., the classical ones committing mesenchymal stromal cells to osteoblast, and then osteocytes as well as recently emerged players. In particular, we discuss micro (mi)RNAs, long non-coding (lnc)RNAs, circular (circ)RNAs, and extracellular vesicles, focusing on the mechanisms through which osteoblasts are regulated by these factors, and conversely, how they use extracellular vesicles to communicate with the surrounding microenvironment.