TRPM7 channels mediate spontaneous Ca2+ fluctuations in growth plate chondrocytes that promote bone development

Nav1.7 as a chondrocyte regulator and therapeutic target for osteoarthritis

Osteoarthritis (OA) is the most common joint disease. Currently there are no effective methods that simultaneously prevent joint degeneration and reduce pain1. Although limited evidence suggests the existence of voltage-gated sodium channels (VGSCs) in chondrocytes2, their expression and function in chondrocytes and in OA remain essentially unknown. Here we identify Nav1.7 as an OA-associated VGSC and demonstrate that human OA chondrocytes express functional Nav1.7 channels, with a density of 0.1 to 0.15 channels per µm2 and 350 to 525 channels per cell. Serial genetic ablation of Nav1.7 in multiple mouse models demonstrates that Nav1.7 expressed in dorsal root ganglia neurons is involved in pain, whereas Nav1.7 in chondrocytes regulates OA progression. Pharmacological blockade of Nav1.7 with selective or clinically used pan-Nav channel blockers significantly ameliorates the progression of structural joint damage, and reduces OA pain behaviour. Mechanistically, Nav1.7 blockers regulate intracellular Ca2+ signalling and the chondrocyte secretome, which in turn affects chondrocyte biology and OA progression. Identification of Nav1.7 as a novel chondrocyte-expressed, OA-associated channel uncovers a dual target for the development of disease-modifying and non-opioid pain relief treatment for OA.

Atypical cell death and insufficient matrix organization in long-bone growth plates from Tric-b-knockout mice

TRIC-A and TRIC-B proteins form homotrimeric cation-permeable channels in the endoplasmic reticulum (ER) and nuclear membranes and are thought to contribute to counterionic flux coupled with store Ca2+ release in various cell types. Serious mutations in the TRIC-B (also referred to as TMEM38B) locus cause autosomal recessive osteogenesis imperfecta (OI), which is characterized by insufficient bone mineralization. We have reported that Tric-b-knockout mice can be used as an OI model; Tric-b deficiency deranges ER Ca2+ handling and thus reduces extracellular matrix (ECM) synthesis in osteoblasts, leading to poor mineralization. Here we report irregular cell death and insufficient ECM in long-bone growth plates from Tric-b-knockout embryos. In the knockout growth plate chondrocytes, excess pro-collagen fibers were occasionally accumulated in severely dilated ER elements. Of the major ER stress pathways, activated PERK/eIF2α (PKR-like ER kinase/ eukaryotic initiation factor 2α) signaling seemed to inordinately alter gene expression to induce apoptosis-related proteins including CHOP (CCAAT/enhancer binding protein homologous protein) and caspase 12 in the knockout chondrocytes. Ca2+ imaging detected aberrant Ca2+ handling in the knockout chondrocytes; ER Ca2+ release was impaired, while cytoplasmic Ca2+ level was elevated. Our observations suggest that Tric-b deficiency directs growth plate chondrocytes to pro-apoptotic states by compromising cellular Ca2+-handling and exacerbating ER stress response, leading to impaired ECM synthesis and accidental cell death.

Interaction of Calmodulin with TRPM: An Initiator of Channel Modulation

Transient receptor potential melastatin (TRPM) channels, a subfamily of the TRP superfamily, constitute a diverse group of ion channels involved in mediating crucial cellular processes like calcium homeostasis. These channels exhibit complex regulation, and one of the key regulatory mechanisms involves their interaction with calmodulin (CaM), a cytosol ubiquitous calcium-binding protein. The association between TRPM channels and CaM relies on the presence of specific CaM-binding domains in the channel structure. Upon CaM binding, the channel undergoes direct and/or allosteric structural changes and triggers down- or up-stream signaling pathways. According to current knowledge, ion channel members TRPM2, TRPM3, TRPM4, and TRPM6 are directly modulated by CaM, resulting in their activation or inhibition. This review specifically focuses on the interplay between TRPM channels and CaM and summarizes the current known effects of CaM interactions and modulations on TRPM channels in cellular physiology.

The mechanosensitive ion channel PIEZO1 promotes satellite cell function in muscle regeneration

Muscle satellite cells (MuSCs), myogenic stem cells in skeletal muscles, play an essential role in muscle regeneration. After skeletal muscle injury, quiescent MuSCs are activated to enter the cell cycle and proliferate, thereby initiating regeneration; however, the mechanisms that ensure successful MuSC division, including chromosome segregation, remain unclear. Here, we show that PIEZO1, a calcium ion (Ca²⁺)-permeable cation channel activated by membrane tension, mediates spontaneous Ca²⁺ influx to control the regenerative function of MuSCs. Our genetic engineering approach in mice revealed that PIEZO1 is functionally expressed in MuSCs and that Piezo1 deletion in these cells delays myofibre regeneration after injury. These results are, at least in part, due to a mitotic defect in MuSCs. Mechanistically, this phenotype is caused by impaired PIEZO1-Rho signalling during myogenesis. Thus, we provide the first concrete evidence that PIEZO1, a bona fide mechanosensitive ion channel, promotes proliferation and regenerative functions of MuSCs through precise control of cell division.

Mesenchymal cell TRPM7 expression is required for bone formation via the regulation of chondrogenesis

Transient receptor potential melastatin-subfamily member 7 (TRPM7) is a bifunctional protein containing a kinase fused to an ion channel permeated with cations, including Ca²⁺ and Mg²⁺. Trpm7-null mice show embryonic lethality. Paired related homeobox 1 (Prx1) is expressed in undifferentiated mesenchymal cells such as the progenitor cells of both chondrocytes and osteoblasts involved in limb skeleton formation. Prx1-Cre-dependent Trpm7 mesenchymal-deleted mice were generated to examine the role of TRPM7 in bone development. We found that Prx1-Cre;Trpm7^fl/fl mice had shortened bones and impaired trabecular bone formation. Trabecular bone parameters, such as the bone volume (BV/TV), and trabecular number (Tb.N), were decreased in Prx1-Cre;Trpm7^fl/fl mice. The cortical bone parameters of cortical bone area (Ct.Ar) and cortical bone thickness (Ct.Th) were also down-regulated in these mice. The bone formation rate in Prx1-Cre;Trpm7^fl/fl mice was unchanged, but the hypertrophic area and cell size of the zone were smaller, and the expression of Col2a1, Col10a1 and Mmp13 was downregulated compared with control mice. These findings suggest impaired chondrogenesis in Prx1-Cre;Trpm7^fl/fl mice compared to control mice. The receptor activator of nuclear factor-kappa B ligand (RANKL) expression was increased, and RANKL-positive cells and osteoclasts were markedly accumulated in the boundary region between the growth plate and trabecular bone. In contrast, TRPM7 KR mice, which are kinase-dead mutants in which the TRPM7 ion channel function has not been altered, showed no marked differences in trabecular or cortical bone parameters compared to wild-type mice. These findings suggest that TRPM7 is critical as a cation channel rather than as a kinase in bone development via the regulation of chondrogenesis.

C-type natriuretic peptide facilitates autonomic Ca2+ entry in growth plate chondrocytes for stimulating bone growth

The growth plates are cartilage tissues found at both ends of developing bones, and vital proliferation and differentiation of growth plate chondrocytes are primarily responsible for bone growth. C-type natriuretic peptide (CNP) stimulates bone growth by activating natriuretic peptide receptor 2 (NPR2) which is equipped with guanylate cyclase on the cytoplasmic side, but its signaling pathway is unclear in growth plate chondrocytes. We previously reported that transient receptor potential melastatin-like 7 (TRPM7) channels mediate intermissive Ca²⁺ influx in growth plate chondrocytes, leading to activation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) for promoting bone growth. In this report, we provide evidence from experiments using mutant mice, indicating a functional link between CNP and TRPM7 channels. Our pharmacological data suggest that CNP-evoked NPR2 activation elevates cellular cGMP content and stimulates big-conductance Ca²⁺-dependent K⁺ (BK) channels as a substrate for cGMP-dependent protein kinase (PKG). BK channel-induced hyperpolarization likely enhances the driving force of TRPM7-mediated Ca²⁺ entry and seems to accordingly activate CaMKII. Indeed, ex vivo organ culture analysis indicates that CNP-facilitated bone growth is abolished by chondrocyte-specific Trpm7 gene ablation. The defined CNP signaling pathway, the NPR2-PKG-BK channel–TRPM7 channel–CaMKII axis, likely pinpoints promising target proteins for developing new therapeutic treatments for divergent growth disorders.

Regulation of chondrocyte biosynthetic activity by dynamic hydrostatic pressure: the role of TRP channels

INTRODUCTION

Chondrocytes perceive and respond to mechanical loading as signals that regulate their metabolism. Joint loading exposes chondrocytes to multiple modes of mechanical stress, including hydrostatic pressure; however, the mechanisms by which chondrocytes sense physiologically relevant levels of hydrostatic pressure are not well understood. We hypothesized that hydrostatic pressure is transduced to an intracellular signal through mechanosensitive membrane ion channels of chondrocytes. The goals of this study were to examine the effect of hydrostatic loading on the development of engineered cartilage tissue and the contribution of mechanosensitive ion channels on these hydrostatic loading effects.

METHODS

Using a 3D model of porcine chondrocytes in agarose, we applied specific chemical inhibitors to determine the role of transient receptor potential (TRP) ion channels TRPV1, TRPV4, TRPC3, and TRPC1 in transducing hydrostatic pressure.

RESULTS

Hydrostatic loading caused a frequency and magnitude-dependent decrease in sulfated glycosaminoglycans (S-GAG), without changes in DNA content. Inhibiting TRPC3 and TRPV4 decreased S-GAG content; however, only the inhibition of TRPV1 partially attenuated the hydrostatic loading-induced reduction in S-GAG content.

CONCLUSIONS

Our findings indicate that TRPV1 may serve as a transducer of hydrostatic pressure in chondrocytes, and provide further support for the role of TRPV4 in regulating chondrocyte anabolism, as well as initial evidence implicating TRPC3 in chondrogenesis. These findings add to our further understanding of the chondrocyte "channelome" and suggest that a range of ion channels mediate the transduction of different biophysical stimuli such as hydrostatic pressure, membrane stretch, or osmotic stress.

Modulators of TRPM7 and its potential as a drug target for brain tumours

TRPM7 is a non-selective divalent cation channel with an alpha-kinase domain. Corresponding with its broad expression, TRPM7 has a role in a wide range of cell functions, including proliferation, migration, and survival. Growing evidence shows that TRPM7 is also aberrantly expressed in various cancers, including brain cancers. Because ion channels have widespread tissue distribution and result in extensive physiological consequences when dysfunctional, these proteins can be compelling drug targets. In fact, ion channels comprise the third-largest drug target type, following enzymes and receptors. Literature has shown that suppression of TRPM7 results in inhibition of migration, invasion, and proliferation in several human brain tumours. Therefore, TRPM7 presents a potential target for therapeutic brain tumour interventions. This article reviews current literature on TRPM7 as a potential drug target in the context of brain tumours and provides an overview of various selective and non-selective modulators of the channel relevant to pharmacology, oncology, and ion channel function.

Elucidation of the Physiological Functions of Membrane Proteins as Novel Drug Target Candidate Molecules

For pharmaceutical research focused on identifying novel drug target candidate molecules, it is essential to explore unknown biological phenomena, elucidate underlying molecular mechanisms and regulate biological processes based on these findings. Proteins expressed on the plasma membrane and endoplasmic reticulum (ER) membrane play important roles in linking extracellular environmental information to intracellular processes. Stimulating membranous proteins induces various kinds of changes in cells, such as alterations in gene expression levels and enzymatic activities. However, the physiological functions and endogenous ligands of many G-protein-coupled receptors (GPCRs) have not been determined, although GPCRs already constitute a large class of drug-target membrane proteins. Furthermore, the precise physiological roles played by many ER membrane proteins have not been elucidated to date. In this review article, I summarize the results of our recent studies, including the observations that the lipid sensor FFAR4/GPR120 controlled systemic energy homeostasis and that the ER membrane monovalent cation channel trimeric intracellular cation (TRIC)-B and the plasma membrane divalent cation channel transient receptor potential melastatin 7 (TRPM7) regulated bone formation. I further describe the therapeutic significance of these membranous protein-related biological processes.

Calcium calmodulin kinase II activity is required for cartilage homeostasis in osteoarthritis

WNT ligands can activate several signalling cascades of pivotal importance during development and regenerative processes. Their de-regulation has been associated with the onset of different diseases. Here we investigated the role of the WNT/Calcium Calmodulin Kinase II (CaMKII) pathway in osteoarthritis. We identified Heme Oxygenase I (HMOX1) and Sox-9 as specific markers of the WNT/CaMKII signalling in articular chondrocytes through a microarray analysis. We showed that the expression of the activated form of CaMKII, phospho-CaMKII, was increased in human and murine osteoarthritis and the expression of HMOX1 was accordingly reduced, demonstrating the activation of the pathway during disease progression. To elucidate its function, we administered the CaMKII inhibitor KN93 to mice in which osteoarthritis was induced by resection of the anterior horn of the medial meniscus and of the medial collateral ligament in the knee joint. Pharmacological blockade of CaMKII exacerbated cartilage damage and bone remodelling. Finally, we showed that CaMKII inhibition in articular chondrocytes upregulated the expression of matrix remodelling enzymes alone and in combination with Interleukin 1. These results suggest an important homeostatic role of the WNT/CaMKII signalling in osteoarthritis which could be exploited in the future for therapeutic purposes.

Enhanced Ca2+ handling in thioglycolate-elicited peritoneal macrophages

In macrophage biology, resident peritoneal macrophages (RPMs) and thioglycolate-elicited peritoneal macrophages (TGPMs) have been traditionally utilized as primary cultured models. RPMs and TGPMs exhibit distinct morphological, functional and metabolic characteristics, although it remains unclear how cellular Ca²⁺ handling differs between them. In our Fura-2 Ca²⁺ imaging, TGPMs displayed elevated resting Ca²⁺ levels, increased store Ca²⁺ contents and facilitated store-operated Ca²⁺ entry (SOCE) compared with RPMs. The intensified intracellular Ca²⁺ stores were enriched with major luminal Ca²⁺-binding proteins inducibly expressed in TGPMs. The elevated resting Ca²⁺ level was predominantly maintained by constitutive Ca²⁺ influx, probably through the transient receptor potential (TRP) family members TRPP2, TRPM7 and TRPA1. These TRP family channels seemed to be largely activated in a manner dependent on phospholipase C activity, and together with Orai channels, contributed to SOCE. Moreover, Ca²⁺-dependent K⁺ channels efficiently facilitated SOCE by enhancing the Ca²⁺ driving force in TGPMs. The consolidated cellular Ca²⁺ handling described may underlie the specialized cell-physiological features of TGPMs, such as vital proliferation, active migration and avid phagocytosis.

Carvacrol affects breast cancer cells through TRPM7 mediated cell cycle regulation

As the most prevalent cancer for females, breast cancer is also the second most popular cancer type overall. More efforts are needed to research new drugs and combination therapies for this disease. A naturally derived transient receptor potential melastatin-like 7 channel (TRPM7) inhibitor, carvacrol, was found to have anti-cancer potentials. We hypothesized that carvacrol affects breast cancer cells through TRPM7 mediated cell cycle regulation. Cell viability and apoptosis of breast cancer cell lines BT-483, BT-474, MCF-7, MDA-MB-231, and MDA-MB-453 were determined using the CCK-8 assay and ELISA respectively. TRPM7 in MDA-MB-231, MCF-7 was knocked down. Functional TRPM7 in MDA-MB-231, MCF-7, and HEK293 cells were tested with western blotting, patch-clamp, and fura-2 quench assay. The cell cycle and the regulatory proteins were determined by flow cytometry and western blotting. Results showed that carvacrol inhibited the viability of breast cancer cells with different potency. At 200 μM, MDA-MB-231 was the most sensitive, and MCF-7 was the least sensitive. At >200 μM, the apoptosis was dramatically induced. Carvacrol inhibited TRPM7 functions in MDA-MB-231, MCF-7, and HEK293. Carvacrol at 200 μM increased cells in the G1/G0 phase and decreased cells in the S and G2/M phase by regulating some cyclin proteins in MDA-MB-231. These effects were blocked by the knockdown of TRPM7. This study demonstrated that carvacrol suppresses breast cancer cells by cell cycle regulation and the TRPM7 pathway is one of the pharmacological mechanisms.

Acid- and Volume-Sensitive Chloride Currents in Human Chondrocytes

Chondrocytes face extreme alterations of extracellular osmolarity and pH, which force them to appropriately regulate their cell volume (CV) and cellular pH. Perturbations of these mechanisms lead to chondrocyte death and ultimately to osteoarthritis (OA), the most common chronic joint diseases worldwide. OA hallmarks are altered cartilage hydration and severe fluid acidification. Impaired CV regulation and acidotoxicity contribute to disease progression and volume-sensitive anion channels are upregulated in OA. This study assessed the effect of hypotonicity and extracellular acidification on chondrocyte Cl^– conductances and CV regulation. Cl^– currents and membrane potentials were measured in human C28/I2 cells and primary human chondrocytes using the patch clamp technique. Intracellular pH was assessed by BCECF fluorescence, CV measurements were performed using the Coulter method, and cell viability/cell death by a resazurin assay. Hypotonic cell swelling caused activation of a volume-sensitive outwardly rectifying (VSOR) Cl^– current followed by a regulatory volume decrease (RVD), which was attenuated by the Cl^– channel blocker DCPIB. Extracellular, but not intracellular acidification to pH ≤ 5.0 elicited an acid-sensitive outwardly rectifying (ASOR) Cl^– conductance. Activation of either current depolarized the cell membrane potential. Under simultaneous hypotonic and acidic stimulation, VSOR and ASOR currents transiently coactivated, giving rise to a mixed current phenotype. Over time the VSOR current gradually vanished and the residual conductance showed a pure ASOR current phenotype. Extracellular acidification caused an isotonic CV gain and a complete suppression of RVD under hypotonic conditions. The results suggest that deactivation of the VSOR current under acidic conditions impairs CV regulation in chondrocytes, which is likely to compromise chondrocyte viability.

Integrating Bioelectrical Currents and Ca2+ Signaling with Biochemical Signaling in Development and Pathogenesis

Roles of bioelectrical signals are increasingly recognized in excitable and nonexcitable non-neural tissues. Diverse ion-selective channels, pumps, and gap junctions participate in bioelectrical signaling, including those transporting calcium ions (Ca2+). Ca2+ is the most versatile transported ion, because it serves as an electrical charge carrier and a biochemical regulator for multiple molecular binding, enzyme, and transcription activities. We aspire to learn how bioelectrical signals crosstalk to biochemical/biomechanical signals. In this study, we review four recent studies showing how bioelectrical currents and Ca2+ signaling affect collective dermal cell migration during feather bud elongation, affect chondrogenic differentiation in limb development, couple with mechanical tension in aligning gut smooth muscle, and affect mitochondrial function and skeletal muscle atrophy. We observe bioelectrical signals involved in several developmental and pathological conditions in chickens and mice at multiple spatial scales: cellular, cellular collective, and subcellular. These examples inspire novel concept and approaches for future basic and translational studies.

Functional expression of TRPM7 as a Ca2+ influx pathway in adipocytes

In adipocytes, intracellular Ca²⁺ and Mg²⁺ modulates physiological functions, such as insulin action and the secretion of adipokines. TRPM7 is a Ca²⁺/Mg²⁺‐permeable non‐selective cation channel. TRPM7 mRNA is highly expressed in adipose tissue, however, its functional expression in adipocytes remains to be elucidated. In this study, we demonstrated for the first time that TRPM7 was functionally expressed in both freshly isolated white adipocytes and in 3T3‐L1 adipocytes differentiated from a 3T3‐L1 pre‐adipocyte cell line by whole‐cell patch‐clamp recordings. Consistent with known properties of TRPM7 current, the current in adipocytes was activated by the elimination of extracellular divalent cations and the reduction of intracellular free Mg²⁺ concentrations, and was inhibited by the TRPM7 inhibitors, 2‐aminoethyl diphenylborinate (2‐APB), hydrogen peroxide (H₂O₂), N‐methyl maleimide (NMM), NS8593, and 2‐amino‐2‐[2‐(4‐octylphenyl)ethyl]‐1,3‐propanediol (FTY720). Treatment with small‐interfering (si) RNA targeting TRPM7 resulted in a reduction in the current to 23 ± 7% of nontargeting siRNA‐treated adipocytes. Moreover a TRPM7 activator, naltriben, increased the TRPM7‐like current and [Ca²⁺]_i in 3T3‐L1 adipocytes but not in TRPM7‐knockdown adipocytes. These findings indicate that TRPM7 is functionally expressed, and plays a role as a Ca²⁺ influx pathway in adipocytes.

Hypo-Osmotic Loading Induces Expression of IL-6 in Nucleus Pulposus Cells of the Intervertebral Disc Independent of TRPV4 and TRPM7

Painful intervertebral disc (IVD) degeneration is an age-related process characterized by reduced tissue osmolarity, increased catabolism of the extracellular matrix, and elevated levels of pro-inflammatory molecules. With the aging population and constantly rising treatment costs, it is of utmost importance to identify potential therapeutic targets and new pharmacological treatment strategies for low back pain. Transient receptor potential (TRP) channels are a family of Ca²⁺ permeable cell membrane receptors, which can be activated by multitude of stimuli and have recently emerged as contributors to joint disease, but were not investigated closer in the IVD. Based on the gene array screening, TRPC1, TRPM7, and TRPV4 were overall the most highly expressed TRP channels in bovine IVD cells. We demonstrated that TRPV4 gene expression was down-regulated in hypo-osmotic condition, whereas its Ca²⁺ flux increased. No significant differences in Ca²⁺ flux and gene expression were observed for TRPM7 between hypo- and iso-osmotic groups. Upon hypo-osmotic stimulation, we overall identified via RNA sequencing over 3,000 up- or down-regulated targets, from which we selected aggrecan, ADAMTS9, and IL-6 and investigated whether their altered gene expression is mediated through either the TRPV4 or TRPM7 channel, using specific activators and inhibitors (GSK1016790A/GSK2193874 for TRPV4 and Naltriben/NS8593 for TRPM7). GSK1016790A induced the expression of IL-6 under iso-osmotic condition, alike to hypo-osmotic stimulation alone, indicating that this effect might be TRPV4-mediated. However, using the TRPV4 blocker GSK2193874 failed to prevent the increase of IL-6 under hypo-osmotic condition. A treatment with TRPM7-activator did not cause significant changes in the gene expression of tested targets. In conclusion, while TRPV4 and TRPM7 are likely involved in osmosensing in the IVD, neither of them mediates hypo-osmotically-induced gene expression changes of aggrecan, ADAMTS9, and IL-6.

High-Level Expression of Alkaline Phosphatase by Adeno-Associated Virus Vector Ameliorates Pathological Bone Structure in a Hypophosphatasia Mouse Model

Hypophosphatasia (HPP) is a systemic skeletal disease caused by mutations in the gene encoding tissue-nonspecific alkaline phosphatase (TNALP). We recently reported that survival of HPP model mice can be prolonged using an adeno-associated virus (AAV) vector expressing bone-targeted TNALP with deca-aspartate at the C terminus (TNALP-D₁₀); however, abnormal bone structure and hypomineralization remained in the treated mice. Here, to develop a more effective and clinically applicable approach, we assessed whether transfection with TNALP-D₁₀ expressing virus vector at a higher dose than previously used would ameliorate bone structure defects. We constructed a self-complementary AAV8 vector expressing TNALP driven by the chicken beta-actin (CBA) promoter (scAAV8-CB-TNALP-D₁₀). The vector was injected into both quadriceps femoris muscles of newborn HPP mice at a dose of 4.5 × 10¹² vector genome (v.g.)/body, resulting in 20 U/mL of serum ALP activity. The 4.5 × 10¹² v.g./body-treated HPP mice grew normally and displayed improved bone structure at the knee joints in X-ray images. Micro-CT analysis showed normal trabecular bone structure and mineralization. The mechanical properties of the femur were also recovered. Histological analysis of the femurs demonstrated that ALP replacement levels were sufficient to promote normal, growth plate cartilage arrangement. These results suggest that AAV vector-mediated high-dose TNALP-D₁₀ therapy is a promising option for improving the quality of life (QOL) of patients with the infantile form of HPP.

A Comparison of Osteoblast and Osteoclast In Vitro Co-Culture Models and Their Translation for Preclinical Drug Testing Applications

As the population of western societies on average ages, the number of people affected by bone remodeling-associated diseases such as osteoporosis continues to increase. The development of new therapeutics is hampered by the high failure rates of drug candidates during clinical testing, which is in part due to the poor predictive character of animal models during preclinical drug testing. Co-culture models of osteoblasts and osteoclasts offer an alternative to animal testing and are considered to have the potential to improve drug development processes in the future. However, a robust, scalable, and reproducible 3D model combining osteoblasts and osteoclasts for preclinical drug testing purposes has not been developed to date. Here we review various types of osteoblast-osteoclast co-culture models and outline the remaining obstacles that must be overcome for their successful translation.

The Role of Transient Receptor Potential Melastatin 7 (TRPM7) in Cell Viability: A Potential Target to Suppress Breast Cancer Cell Cycle

The divalent cation-selective channel transient receptor potential melastatin 7 (TRPM7) channel was shown to affect the proliferation of some types of cancer cell. However, the function of TRPM7 in the viability of breast cancer cells remains unclear. Here we show that TRPM inhibitors suppressed the viability of TRPM7-expressing breast cancer cells. We first demonstrated that the TRPM7 inhibitors 2-aminoethyl diphenylborinate (2-APB), ginsenoside Rd (Gin Rd), and waixenicin A preferentially suppressed the viability of human embryonic kidney HEK293 overexpressing TRPM7 (HEK-M7) cells over wildtype HEK293 (WT-HEK). Next, we confirmed the effects of 2-APB on the TRPM7 channel functions by whole-cell currents and divalent cation influx. The inhibition of the viability of HEK-M7 cells by 2-APB was not mediated by the increase in cell death but by the interruption of the cell cycle. Similar to HEK-M7 cells, the viability of TRPM7-expressing human breast cancer MDA-MB-231, AU565, and T47D cells were also suppressed by 2-APB by arresting the cell cycle in the S phase. Furthermore, in a novel TRPM7 knock-out MDA-MB-231 (KO-231) cell line, decreased divalent influx and reduced proliferation were observed compared to the wildtype MDA-MB-231 cells. 2-APB and Gin Rd preferentially suppressed the viability of wildtype MDA-MB-231 cells over KO-231 by affecting the cell cycle in wildtype but not KO-231 cells. Our results suggest that TRPM7 regulates the cell cycle of breast cancers and is a potential therapeutic target.

L-type voltage-gated Ca2+ channel CaV1.2 regulates chondrogenesis during limb development

All cells, including nonexcitable cells, maintain a discrete transmembrane potential (V _mem), and have the capacity to modulate V _mem and respond to their own and neighbors' changes in V _mem Spatiotemporal variations have been described in developing embryonic tissues and in some cases have been implicated in influencing developmental processes. Yet, how such changes in V _mem are converted into intracellular inputs that in turn regulate developmental gene expression and coordinate patterned tissue formation, has remained elusive. Here we document that the V _mem of limb mesenchyme switches from a hyperpolarized to depolarized state during early chondrocyte differentiation. This change in V _mem increases intracellular Ca²⁺ signaling through Ca²⁺ influx, via Ca_V1.2, 1 of L-type voltage-gated Ca²⁺ channels (VGCCs). We find that Ca_V1.2 activity is essential for chondrogenesis in the developing limbs. Pharmacological inhibition by an L-type VGCC specific blocker, or limb-specific deletion of Ca_V1.2, down-regulates expression of genes essential for chondrocyte differentiation, including Sox9, Col2a1, and Agc1, and thus disturbs proper cartilage formation. The Ca²⁺-dependent transcription factor NFATc1, which is a known major transducer of intracellular Ca²⁺ signaling, partly rescues Sox9 expression. These data reveal instructive roles of Ca_V1.2 in limb development, and more generally expand our understanding of how modulation of membrane potential is used as a mechanism of developmental regulation.