TRPV4 calcium influx controls sclerostin protein loss independent of purinergic calcium oscillations

Perception and response of skeleton to mechanical stress

Mechanical stress stands as a fundamental factor in the intricate processes governing the growth, development, morphological shaping, and maintenance of skeletal mass. The profound influence of stress in shaping the skeletal framework prompts the assertion that stress essentially births the skeleton. Despite this acknowledgment, the mechanisms by which the skeleton perceives and responds to mechanical stress remain enigmatic. In this comprehensive review, our scrutiny focuses on the structural composition and characteristics of sclerotin, leading us to posit that it serves as the primary structure within the skeleton responsible for bearing and perceiving mechanical stress. Furthermore, we propose that osteocytes within the sclerotin emerge as the principal mechanical-sensitive cells, finely attuned to perceive mechanical stress. And a detailed analysis was conducted on the possible transmission pathways of mechanical stress from the extracellular matrix to the nucleus.

Osteocyte-mediated mechanical response controls osteoblast differentiation and function

Low bone mass is a pervasive global health concern, with implications for osteoporosis, frailty, disability, and mortality. Lifestyle factors, including sedentary habits, metabolic dysfunction, and an aging population, contribute to the escalating prevalence of osteopenia and osteoporosis. The application of mechanical load to bone through physical activity and exercise prevents bone loss, while sufficient mechanical load stimulates new bone mass acquisition. Osteocytes, cells embedded within the bone, receive mechanical signals and translate these mechanical cues into biological signals, termed mechano-transduction. Mechano-transduction signals regulate other bone resident cells, such as osteoblasts and osteoclasts, to orchestrate changes in bone mass. This review explores the mechanisms through which osteocyte-mediated response to mechanical loading regulates osteoblast differentiation and bone formation. An overview of bone cell biology and the impact of mechanical load will be provided, with emphasis on the mechanical cues, mechano-transduction pathways, and factors that direct progenitor cells toward the osteoblast lineage. While there are a wide range of clinically available treatments for osteoporosis, the majority act through manipulation of the osteoclast and may have significant disadvantages. Despite the central role of osteoblasts to the deposition of new bone, few therapies directly target osteoblasts for the preservation of bone mass. Improved understanding of the mechanisms leading to osteoblastogenesis may reveal novel targets for translational investigation.

The Multifaceted Functions of TRPV4 and Calcium Oscillations in Tissue Repair

The transient receptor potential vanilloid 4 (TRPV4) specifically functions as a mechanosensitive ion channel and is responsible for conveying changes in physical stimuli such as mechanical stress, osmotic pressure, and temperature. TRPV4 enables the entry of cation ions, particularly calcium ions, into the cell. Activation of TRPV4 channels initiates calcium oscillations, which trigger intracellular signaling pathways involved in a plethora of cellular processes, including tissue repair. Widely expressed throughout the body, TRPV4 can be activated by a wide array of physicochemical stimuli, thus contributing to sensory and physiological functions in multiple organs. This review focuses on how TRPV4 senses environmental cues and thereby initiates and maintains calcium oscillations, critical for responses to organ injury, tissue repair, and fibrosis. We provide a summary of TRPV4-induced calcium oscillations in distinct organ systems, along with the upstream and downstream signaling pathways involved. In addition, we delineate current animal and disease models supporting TRPV4 research and shed light on potential therapeutic targets for modulating TRPV4-induced calcium oscillation to promote tissue repair while reducing tissue fibrosis.

Spectrum of PHEX Mutations and FGF23 Profiles in a Taiwanese Cohort With X-Linked Hypophosphatemia Including 102 Patients

BACKGROUND/AIM

X-linked hypophosphatemia (XLH), the most common form of hereditary rickets, results from loss-of-function mutations in the phosphate-regulating PHEX gene. Elevated fibroblast growth factor 23 (FGF23) contributes to hypophosphatemia in XLH. This study aimed to characterize PHEX variants and serum FGF23 profiles in Taiwanese patients with XLH.

PATIENTS AND METHODS

We retrospectively reviewed the records of 102 patients clinically suspected of having hypophosphatemic rickets from 2006 to 2022. Serum intact Fibroblast growth factor-23 (iFGF23) levels were measured on clinic visit days. PHEX mutations were identified using Sanger sequencing, and negative cases were analyzed using whole-exome sequencing.

RESULTS

The majority (92.1%) of patients exhibited elevated FGF23 compared with normal individuals. Among 102 patients, 44 distinct PHEX mutations were identified. Several mutations recurred in multiple unrelated Taiwanese families. We discovered a high frequency of novel PHEX mutations and identified variants associated with extreme FGF23 elevation and tumorigenesis.

CONCLUSION

Our findings revealed the PHEX genotypic variants and FGF23 levels in Taiwanese patients with XLH. These results are crucial given the recent approval of burosumab, a monoclonal FGF23 antibody, for XLH therapy. This study provides key insights into the clinical management of XLH in Taiwan.

TRPV4 differentially controls inflammatory cytokine networks during static and dynamic compression of the intervertebral disc

Background

The ion channel transient receptor potential vanilloid 4 (TRPV4) critically transduces mechanical forces in the IVD, and its inhibition can prevent IVD degeneration due to static overloading. However, it remains unknown whether different modes of loading signals through TRPV4 to regulate the expression of inflammatory cytokines. We hypothesized that TRPV4 signaling is essential during static and dynamic loading to mediate homeostasis and mechanotransduction.

Methods

Mouse functional spine units were isolated and either cyclically compressed for 5 days (1 Hz, 1 h, 10% strain) or statically compressed (24 h, 0.2 MPa). Conditioned media were monitored at 6 h, 24 h, 2 days, and 5 days, with and without TRPV4 inhibition. Effects of TRPV4 activation was also evaluated without loading. The media was analyzed for a panel of 44 cytokines using a microbead array and then a correlative network was constructed to explore the regulatory relationships during loading and TRPV4 inhibition. After the loading regimen, the IVDs were evaluated histologically for degeneration.

Results

Activation of TRPV4 led to an increase interleukin-6 (IL-6) family of cytokines (IL-6, IL-11, IL-16, and leukemia inhibitory factor [LIF]) and decreased the T-cell (CCL3, CCL4, CCL17, CCL20, CCL22, and CXCL10) and monocyte (CCL2 and CCL12) recruiting chemokines by the IVD. Dynamic and static loading each provoked unique chemokine correlation networks. The inhibition of TRPV4 during dynamic loading dysregulated the relationship between LIF and other cytokines, while the inhibition of TRPV4 during static loading disrupted the connectivity of IL-16 and VEGFA.

Conclusions

We demonstrated that TRPV4 critically mediates the cytokine production following dynamic and static loading. The activation of TRPV4 upregulated a diverse set of cytokines that may suppress the chemotaxis of T-cells and monocytes, implicating the role of TRPV4 in maintaining the immune privilege of healthy IVD.

Osteoblast-lineage calcium/calmodulin-dependent kinase 2 delta and gamma regulates bone mass and quality

Bone regulates its mass and quality in response to diverse mechanical, hormonal, and local signals. The bone anabolic or catabolic responses to these signals are often received by osteocytes, which then coordinate the activity of osteoblasts and osteoclasts on bone surfaces. We previously established that calcium/calmodulin-dependent kinase 2 (CaMKII) is required for osteocytes to respond to some bone anabolic cues in vitro. However, a role for CaMKII in bone physiology in vivo is largely undescribed. Here, we show that conditional codeletion of the most abundant isoforms of CaMKII (delta and gamma) in mature osteoblasts and osteocytes [Ocn-cre:Camk2d/Camk2g double-knockout (dCKO)] caused severe osteopenia in both cortical and trabecular compartments by 8 wk of age. In addition to having less bone mass, dCKO bones are of worse quality, with significant deficits in mechanical properties, and a propensity to fracture. This striking skeletal phenotype is multifactorial, including diminished osteoblast activity, increased osteoclast activity, and altered phosphate homeostasis both systemically and locally. These dCKO mice exhibited decreased circulating phosphate (hypophosphatemia) and increased expression of the phosphate-regulating hormone fibroblast growth factor 23. Additionally, dCKO mice expressed less bone-derived tissue nonspecific alkaline phosphatase protein than control mice. Consistent with altered phosphate homeostasis, we observed that dCKO bones were hypo-mineralized with prominent osteoid seams, analogous to the phenotypes of mice with hypophosphatemia. Altogether, these data reveal a fundamental role for osteocyte CaMKIIδ and CaMKIIγ in the maintenance of bone mass and bone quality and link osteoblast/osteocyte CaMKII to phosphate homeostasis.

The Mechanotransduction Signaling Pathways in the Regulation of Osteogenesis

Bones are constantly exposed to mechanical forces from both muscles and Earth's gravity to maintain bone homeostasis by stimulating bone formation. Mechanotransduction transforms external mechanical signals such as force, fluid flow shear, and gravity into intracellular responses to achieve force adaptation. However, the underlying molecular mechanisms on the conversion from mechanical signals into bone formation has not been completely defined yet. In the present review, we provide a comprehensive and systematic description of the mechanotransduction signaling pathways induced by mechanical stimuli during osteogenesis and address the different layers of interconnections between different signaling pathways. Further exploration of mechanotransduction would benefit patients with osteoporosis, including the aging population and postmenopausal women.

The regulation and functions of the matricellular CCN proteins induced by shear stress

Shear stress is a frictional drag generated by the flow of fluid, such as blood or interstitial fluid, and plays a critical role in regulating cellular gene expression and functional phenotype. The matricellular CCN family proteins are dynamically regulated by shear stress of different flow patterns, and their expression significantly alters the microenvironment of cells. Secreted CCN proteins mainly bind to several cell surface integrin receptors to mediate their diverse functions in regulating cell survival, function, and behavior. Gene-knockout studies indicate major functions of CCN proteins in the cardiovascular and skeletal systems, the two primary systems in which CCN expressions are regulated by shear stress. In the cardiovascular system, the endothelium is directly exposed to vascular shear stress. Unidirectional laminar blood flow generates laminar shear stress, which promotes a mature endothelial phenotype and upregulates anti-inflammatory CCN3 expression. In contrast, disturbed flow generates oscillatory shear stress, which induces endothelial dysfunction through the induction of CCN1 and CCN2. Shear-induced CCN1 binds to integrin α6β1 and promotes superoxide production, NF-κB activation, and inflammatory gene expression in endothelial cells. Although the interaction between shear stress and CCN4-6 is not clear, CCN 4 exhibits a proinflammatory property and CCN5 inhibits vascular cell growth and migration. The crucial roles of CCN proteins in cardiovascular development, homeostasis, and disease are evident but not fully understood. In the skeletal system, mechanical loading on bone generates shear stress from interstitial fluid in the lacuna-canalicular system and promotes osteoblast differentiation and bone formation. CCN1 and CCN2 are induced and potentially mediate fluid shear stress mechanosensing in osteocytes. However, the exact roles of interstitial shear stress-induced CCN1 and CCN2 in bone are still not clear. In contrast to other CCN family proteins, CCN3 inhibits osteoblast differentiation, although its regulation by interstitial shear stress in osteocytes has not been reported. The induction of CCN proteins by shear stress in bone and their functions remain largely unknown and merit further investigation. This review discusses the expression and functions of CCN proteins regulated by shear stress in physiological conditions, diseases, and cell culture models. The roles between CCN family proteins can be compensatory or counteractive in tissue remodeling and homeostasis.

Modulation of TRPV4 protects against degeneration induced by sustained loading and promotes matrix synthesis in the intervertebral disc

While it is well known that mechanical signals can either promote or disrupt intervertebral disc (IVD) homeostasis, the molecular mechanisms for transducing mechanical stimuli are not fully understood. The transient receptor potential vanilloid 4 (TRPV4) ion channel activated in isolated IVD cells initiates extracellular matrix (ECM) gene expression, while TRPV4 ablation reduces cytokine production in response to circumferential stretching. However, the role of TRPV4 on ECM maintenance during tissue-level mechanical loading remains unknown. Using an organ culture model, we modulated TRPV4 function over both short- (hours) and long-term (days) and evaluated the IVDs' response. Activating TRPV4 with the agonist GSK101 resulted in a Ca²⁺ flux propagating across the cells within the IVD. Nuclear factor (NF)-κB signaling in the IVD peaked at 6 h following TRPV4 activation that subsequently resulted in higher interleukin (IL)-6 production at 7 days. These cellular responses were concomitant with the accumulation of glycosaminoglycans and increased hydration in the nucleus pulposus that culminated in higher stiffness of the IVD. Sustained compressive loading of the IVD resulted in elevated NF-κB activity, IL-6 and vascular endothelial growth factor A (VEGFA) production, and degenerative changes to the ECM. TRPV4 inhibition using GSK205 during loading mitigated the changes in inflammatory cytokines, protected against IVD degeneration, but could not prevent ECM disorganization due to mechanical damage in the annulus fibrosus. These results indicate TRPV4 plays an important role in both short- and long-term adaptations of the IVD to mechanical loading. The modulation of TRPV4 may be a possible therapeutic for preventing load-induced IVD degeneration.

In vitromodel to study confined osteocyte networks exposed to flow-induced mechanical stimuli

Osteocytes are considered the primary mechanical sensor in bone tissue and orchestrate the coupled bone remodeling activity of adjacent osteoblast and osteoclast cells.In vivoinvestigation of mechanically induced signal propagation through networks of interconnected osteocytes is confounded by their confinement within the mineralized bone matrix, which cannot be modeled in conventional culture systems. In this study, we developed a new model that mimics thisin vivoconfinement using gelatin methacrylate (GelMA) hydrogel or GelMA mineralized using osteoblast-like model cells. This model also enables real-time optical examination of osteocyte calcium (Ca2+) signaling dynamics in response to fluid shear stimuli cultured under confined conditions. Using this system, we discovered several distinct and previously undescribed patterns of Ca2+responses that vary across networks of interconnected osteocytes as a function of space, time and connectivity. Heterogeneity in Ca2+signaling may provide new insights into bone remodeling in response to mechanical loading. Overall, such a model can be extended to study signaling dynamics within cell networks exposed to flow-induced mechanical stimuli under confined conditions.

Seasonal airway microbiome and transcriptome interactions promote childhood asthma exacerbations

BACKGROUND

Seasonal variation in respiratory illnesses and exacerbations in pediatric populations with asthma is well described, though whether upper airway microbes play season-specific roles in these events is unknown.

OBJECTIVE

We hypothesized that nasal microbiota composition is seasonally dynamic and that discrete microbe-host interactions modify risk of asthma exacerbation in a season-specific manner.

METHODS

Repeated nasal samples from children with exacerbation-prone asthma collected during periods of respiratory health (baseline; n = 181 samples) or first captured respiratory illness (n = 97) across all seasons, underwent bacterial (16S ribosomal RNA gene) and fungal (internal transcribed spacer region 2) biomarker sequencing. Virus detection was performed by multiplex PCR. Paired nasal transcriptome data were examined for seasonal dynamics and integrative analyses.

RESULTS

Upper airway bacterial and fungal microbiota and rhinovirus detection exhibited significant seasonal dynamics. In seasonally adjusted analysis, variation in both baseline and respiratory illness microbiota related to subsequent exacerbation. Specifically, in the fall, when respiratory illness and exacerbation events were most frequent, several Moraxella and Haemophilus members were enriched both in virus-positive respiratory illnesses and those that progressed to exacerbations. The abundance of 2 discrete bacterial networks, characteristically comprising either Streptococcus or Staphylococcus, exhibited opposing interactions with an exacerbation-associated SMAD3 nasal epithelial transcriptional module to significantly increase the odds of subsequent exacerbation (odds ratio = 14.7, 95% confidence interval = 1.50-144, P = .02; odds ratio = 39.17, 95% confidence interval = 2.44-626, P = .008, respectively).

CONCLUSIONS

Upper airway microbiomes covary with season and with seasonal trends in respiratory illnesses and asthma exacerbations. Seasonally adjusted analyses reveal specific bacteria-host interactions that significantly increase risk of asthma exacerbation in these children.

Sclerostin: From Molecule to Clinical Biomarker

Sclerostin, a glycoprotein encoded by the SOST gene, is mainly produced by mature osteocytes and is a critical regulator of bone formation through its inhibitory effect on Wnt signaling. Osteocytes are differentiated osteoblasts that form a vast and highly complex communication network and orchestrate osteogenesis in response to both mechanical and hormonal cues. The three most commonly described pathways of SOST gene regulation are mechanotransduction, Wnt/β-catenin, and steroid signaling. Downregulation of SOST and thereby upregulation of local Wnt signaling is required for the osteogenic response to mechanical loading. This review covers recent findings concerning the identification of SOST, in vitro regulation of SOST gene expression, structural and functional properties of sclerostin, pathophysiology, biological variability, and recent assay developments for measuring circulating sclerostin. The three-dimensional structure of human sclerostin was generated with the AlphaFold Protein Structure Database applying a novel deep learning algorithm based on the amino acid sequence. The functional properties of the 3-loop conformation within the tertiary structure of sclerostin and molecular interaction with low-density lipoprotein receptor-related protein 6 (LRP6) are also reviewed. Second-generation immunoassays for intact/biointact sclerostin have recently been developed, which might overcome some of the reported methodological obstacles. Sclerostin assay standardization would be a long-term objective to overcome some of the problems with assay discrepancies. Besides the use of age- and sex-specific reference intervals for sclerostin, it is also pivotal to use assay-specific reference intervals since available immunoassays vary widely in their methodological characteristics.

Activation of TRPV4 stimulates transepithelial K+ secretion in rat epididymal epithelium

The maturation of sperms is dependent on the coordinated interactions between sperm and the unique epididymal luminal milieu, which is characterized by high K+ content. This study investigated the involvement of transient receptor potential vanilloid 4 (TRPV4) in the K+ secretion of epididymal epithelium. The expression level and cellular localization of TRPV4 and Ca2+- activated K+ channels (KCa) were analyzed via RT-PCR, real-time quantitative PCR, western blot, and immunofluorescence. The functional role of TRPV4 was investigated using short circuit current (ISC) and intracellular Ca2+ imaging techniques. We found a predominant expression of TRPV4 in the corpus and cauda epididymal epithelium. Activation of TRPV4 with a selective agonist, GSK1016790A, stimulated a transient decrease in the ISC of the epididymal epithelium. The ISC response was abolished by either the TRPV4 antagonists, HC067047 and RN-1734, or the removal of basolateral K+. Simultaneously, the application of GSK1016790A triggered Ca2+ influx in epididymal epithelial cells. Our data also indicated that the big conductance KCa (BK), small conductance KCa (SK), and intermediate conductance KCa (IK) were all expressed in rat epididymis. Pharmacological studies revealed that BK, but not SK and IK, mediated TRPV4-elicited transepithelial K+ secretion. Finally, we demonstrated that TRPV4 and BK were localized in the epididymal epithelium, which showed an increased expression level from caput to cauda regions of rat epididymis. This study implicates that TRPV4 plays an important role in the formation of high K+ concentration in epididymal intraluminal fluid via promoting transepithelial K+ secretion mediated by BK.

TRPV4 inhibitor HC067047 produces antidepressant-like effect in LPS-induced depression mouse model

Inflammation is a crucial component that contributes to the pathogenesis of major depressive disorder. It has been revealed that the nonselective cation channel transient receptor potential vanilloid 4 (TRPV4) profoundly affects a variety of physiological processes, including inflammation. However, its roles and mechanisms in LPS-induced depression are still unclear. Here, for the first time, we found that there was a significant increase in TRPV4 in the hippocampus in a depression mouse model induced by LPS. TRPV4 inhibitor HC067047 or knockdown the hippocampal TRPV4 with TRPV4 shRNA could effectively rescue the aberrant behaviors. Furthermore, TRPV4 inhibitor HC067047 reduced the activation of astrocyte and microglia, decreased expression of CaMKII-NLRP3 inflammasome and increased the expression of neurogenesis marker DCX in the hippocampus. In addition, enhanced neuroinflammation in the serum was also reversed by TRPV4 inhibitor HC067047. Thus, we consider that TRPV4 has an important role in contributing to the depression-like behavior following LPS-induced systemic inflammation.

In vitro Fluid Shear Stress Induced Sclerostin Degradation and CaMKII Activation in Osteocytes

Bone is a dynamic tissue that adapts to changes in its mechanical environment. Mechanical stimuli pressurize interstitial fluid in the lacunar-canalicular system within the bone matrix, causing fluid shear stress (FSS) across bone embedded, mechano-sensitive osteocytes. Therefore, modeling this mechanical stimulus in vitro is vital for identifying mechano-transduction cascades that contribute to the regulation of mechano-responsive proteins, such as the Wnt/β-catenin antagonist, sclerostin, which is reduced in response to FSS. Recently, we reported the rapid post-translational degradation of sclerostin protein in bone cells following FSS. Given the fundamental nature of sclerostin to bone physiology and the nuances of studying its rapid post-translational control, here, we detail our FSS protocol, and adaptations that can be made, to stimulate Ocy454 osteocyte-like cells to study sclerostin protein in vitro. While this protocol is optimized for detecting sclerostin degradation by western blot, this protocol can be adapted to examine transcriptional changes with RT-qPCR, cellular dynamics with live cell imaging, or secreted factors in the FSS buffer. This protocol utilizes 3D-printed FSS tips that are compatible with commercially available 96-well plates, allowing for high experimental accessibility, versatility, and throughput. However, this protocol can be adapted for any FSS chamber. It can also be combined with pharmacological inhibitors or genetic manipulations to interrogate the role of specific cellular components. In all, this experimental set-up and protocol is highly adaptable to allow for many experimental outcomes to examine many aspects of cell mechano-transduction.

Osteocyte calcium signaling - A potential translator of mechanical load to mechanobiology

Osteocytes are embedded dendritic bone cells; by virtue of their position in bone tissue, ability to coordinate bone building osteoblasts and resorbing osteoclasts, and sensitivity to tissue level mechanical loading, they serve as the resident bone mechanosensor. The mechanisms osteocytes use to change mechanical loading into biological signals that drive tissue level changes has been well studied over the last 30 years, however the ways loading parameters are encoded at the cellular level are still not fully understood. Calcium signaling is a first messenger signal exhibited by osteocytes in response to mechanical forces. A body of work interrogating the mechanisms of osteocyte calcium signaling exists and is presently expanding, presenting the opportunity to better understand the relationship between calcium signaling characteristics and tuned osteocyte responses to tissue level strain features (e.g. magnitude, duration, frequency). This review covers the history of osteocyte load induced calcium signaling and highlights potential cellular mechanisms used by osteocytes to turn details about loading parameters into biological events.

Tunnels in the rock: Dynamics of osteocyte morphogenesis

Osteocytes are dynamic, bone matrix-remodeling cells that form an intricate network of interconnected projections through the bone matrix, called the lacunar-canalicular system. Osteocytes are the dominant mechanosensory cells in bone and their mechanosensory and mechanotransductive functions follow their morphological form. During osteocytogenesis and development of the osteocyte lacunar-canalicular network, osteocytes must dramatically remodel both their cytoskeleton and their extracellular matrix. In this review, we summarize our current understanding of the mechanisms that govern osteocyte differentiation, cytoskeletal morphogenesis, mechanotransduction, and matrix remodeling. We postulate that the physiologic activation of matrix remodeling in adult osteocytes, known as perilacunar/canalicular remodeling (PLR) represents a re-activation of the developmental program by which the osteocyte network is first established. While much of osteocyte biology remains unclear, new tools and approaches make the present moment a particularly fruitful and exciting time to study the development of these remarkable cells.

The role of TRPV channels in osteoporosis

As the world's population ages, the treatment of osteoporosis is a major problem to be addressed. The cause of osteoporosis remains unclear. Ca²⁺ is not only an important component of bones but also plays a key role in osteoporosis treatment. Transient receptor potential vanilloid (TRPV) channels are one of the TRP channel families that is widely distributed in various organs, playing an important role in the physiological regulation of the human body. Bone formation and bone absorption may require Ca²⁺ transport via TRPV channels. It has been proven that the TRPV subtypes 1, 2, 4, 5, 6 (TRPV1, TRPV2, TRPV4, TRPV5, TRPV6) may affect bone metabolism balance through selective regulation of Ca²⁺. They significantly regulate osteoblast/osteoclast proliferation, differentiation and function. The purpose of this review is to explore the mechanisms of TRPV channels involved in regulation of the differentiation of osteoblasts and osteoclasts, as well as to discuss the latest developments in current researches, which may provide new clues and directions for an in-depth study of osteoporosis and other related bone metabolic diseases.

Amelioration of Scopolamine-induced Learning and Memory Impairment by the TRPV4 Inhibitor HC067047 in ICR Mice

Alzheimer's disease (AD) is one of the most common causes of neurodegenerative diseases in the elderly. Cholinergic dysfunction is one of the pathological hallmarks of AD and leads to learning and memory impairment. Transient receptor potential vanilloid 4(TRPV4), a nonselective cation channel, is involved in learning and memory functions. HC067047, a TRPV4 specific inhibitor, has been reported to protect neurons against cerebral ischemic injury and amyloid-β -(Aβ) 40-induced hippocampal cell death. However, whether HC067047 could improve scopolamine (SCP)-induced cognitive dysfunction in mice is still unknown. The aims of this study were to verify whether HC067047 could ameliorate the SCP-induced learning and memory impairments in mice and to elucidate its underlying mechanisms of action. In this study, we examined the neuroprotective effect of the HC067047 against cognitive dysfunction induced by SCP (5 mg/kg, i.p.), a muscarinic receptor antagonist. The results showed that administration of HC067047(10 mg/kg, i.p.) significantly ameliorated SCP-induced cognitive dysfunction as assessed by the novel place recognition test (NPRT) and novel object recognition test (NORT). In the Y-maze test, HC067047 significantly enhanced the time spent in the novel arm in SCP mice. To further investigate the molecular mechanisms underlying the neuroprotective effect of HC067047, expression of several proteins involved in apoptosis was examined. The results demonstrated that HC067047 treatment decreased the protein levels of proapoptotic proteins such as Bax and caspase-3 in the hippocampus of SCP mice. In addition, HC067047 enhanced expression of the neurogenesis marker DCX and improved levels of the mature neuronal marker NeuN in SCP mice. These findings suggest the neuroprotective potential of the TRPV4 inhibitor HC067047 for the management of dementia with learning and memory loss.

The cytoskeleton and connected elements in bone cell mechano-transduction

Bone is a mechano-responsive tissue that adapts to changes in its mechanical environment. Increases in strain lead to increased bone mass acquisition, whereas decreases in strain lead to a loss of bone mass. Given that mechanical stress is a regulator of bone mass and quality, it is important to understand how bone cells sense and transduce these mechanical cues into biological changes to identify druggable targets that can be exploited to restore bone cell mechano-sensitivity or to mimic mechanical load. Many studies have identified individual cytoskeletal components - microtubules, actin, and intermediate filaments - as mechano-sensors in bone. However, given the high interconnectedness and interaction between individual cytoskeletal components, and that they can assemble into multiple discreet cellular structures, it is likely that the cytoskeleton as a whole, rather than one specific component, is necessary for proper bone cell mechano-transduction. This review will examine the role of each cytoskeletal element in bone cell mechano-transduction and will present a unified view of how these elements interact and work together to create a mechano-sensor that is necessary to control bone formation following mechanical stress.

Disparate bone anabolic cues activate bone formation by regulating the rapid lysosomal degradation of sclerostin protein

The downregulation of sclerostin in osteocytes mediates bone formation in response to mechanical cues and parathyroid hormone (PTH). To date, the regulation of sclerostin has been attributed exclusively to the transcriptional downregulation of the Sost gene hours after stimulation. Using mouse models and rodent cell lines, we describe the rapid, minute-scale post-translational degradation of sclerostin protein by the lysosome following mechanical load and PTH. We present a model, integrating both new and established mechanically and hormonally activated effectors into the regulated degradation of sclerostin by lysosomes. Using a mouse forelimb mechanical loading model, we find transient inhibition of lysosomal degradation or the upstream mechano-signaling pathway controlling sclerostin abundance impairs subsequent load-induced bone formation by preventing sclerostin degradation. We also link dysfunctional lysosomes to aberrant sclerostin regulation using human Gaucher disease iPSCs. These results reveal how bone anabolic cues post-translationally regulate sclerostin abundance in osteocytes to regulate bone formation.

New Advances in Osteocyte Mechanotransduction

PURPOSE OF REVIEW

Skeletal adaptation to mechanical loading plays a critical role in bone growth and the maintenance of bone homeostasis. Osteocytes are postulated to serve as a hub orchestrating bone remodeling. The recent findings on the molecular mechanisms by which osteocytes sense mechanical loads and the downstream bone-forming factors are reviewed.

RECENT FINDINGS

Calcium channels have been implicated in mechanotransduction in bone cells for a long time. Efforts have been made to identify a specific calcium channel mediating the skeletal response to mechanical loads. Recent studies have revealed that Piezo1, a mechanosensitive ion channel, is critical for normal bone growth and is essential for the skeletal response to mechanical loading. Identification of mechanosensors and their downstream effectors in mechanosensing bone cells is essential for new strategies to modulate regenerative responses and develop therapies to treat the bone loss related to disuse or advanced age.