Cav3.2 T-type calcium channel is required for the NFAT-dependent Sox9 expression in tracheal cartilage

A CatSper-Uninvolved Mechanism to Induce Forward Sperm Motility in the Internal Fertilization

Sperm-specific cation channel (CatSper), sperm-specific Na + /H + exchanger (sNHE), and soluble adenylyl cyclase (sAC) are necessary in the signaling pathways to control sperm motility in many animals, whereas some animals have lost some or all of them. In the present study, we examined CatSper-uninvolved signaling for vigorous undulation of the undulating membrane that is attached to the sperm tail and gives thrust for forward motility in the internally fertilizing newt Cynops pyrrhogaster. Reverse-transcription PCR failed to detect sNHE in the newt sperm. However, the pH of sperm cytoplasm was raised under a high extracellular pH equivalent to that of egg jelly, where sperm motility is initiated by sperm motility-initiating substance (SMIS). Carbonic anhydrase XII/ XVI and SLC4A4/8 were suggested to be present in the sperm, and transported bicarbonates raised the intracellular pH. In egg jelly extract that contained SMIS, the anion transporter inhibitor DIDS weakened the undulation of the undulating membrane, while bicarbonates enhanced it. The cyclic AMP concentration was found to increase in sperm cytoplasm in the egg-jelly extract. An inhibitor of sAC (KH7) weakened the undulation of the undulating membrane, and dibutyryl cyclic AMP blocked the inhibitory effect. Inhibitor of transmembrane AC (DDA) limitedly affected the undulation. The undulation was weakened by an inhibitor of protein kinase A (H89), and by an inhibitor of transient receptor potential (TRP) channels (RN1747). Our results support the conclusions that the high pH of the egg jelly triggers a signaling pathway through sAC, PKA, and TRP channels, and coacts with SMIS to induce forward sperm motility.

The T-type calcium channelosome

T-type calcium channels perform crucial physiological roles across a wide spectrum of tissues, spanning both neuronal and non-neuronal system. For instance, they serve as pivotal regulators of neuronal excitability, contribute to cardiac pacemaking, and mediate the secretion of hormones. These functions significantly hinge upon the intricate interplay of T-type channels with interacting proteins that modulate their expression and function at the plasma membrane. In this review, we offer a panoramic exploration of the current knowledge surrounding these T-type channel interactors, and spotlight certain aspects of their potential for drug-based therapeutic intervention.

Wnt signaling regulates ion channel expression to promote smooth muscle and cartilage formation in developing mouse trachea

Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in noncontractile tissue and embryonic development has yet to be understood. Tracheobronchomalacia (TBM) and complete tracheal rings (CTR) are disorders affecting the muscle and cartilage of the trachea and bronchi, whose etiology remains poorly understood. We demonstrated that trachealis muscle organization and polarity are disrupted after epithelial ablation of Wntless (Wls), a cargo receptor critical for the Wnt signaling pathway, in developing trachea. The phenotype resembles the anomalous trachealis muscle observed after deletion of ion channel encoding genes in developing mouse trachea. We sought to investigate whether and how the deletion of Wls affects ion channels during tracheal development. We hypothesize that Wnt signaling influences the expression of ion channels to promote trachealis muscle cell assembly and patterning. Deleting Wls in developing trachea causes differential regulation of genes mediating actin binding, cytoskeleton organization, and potassium ion channel activity. Wnt signaling regulates the expression of Kcnj13, Kcnd3, Kcnj8, and Abcc9 as demonstrated by in vitro studies and in vivo analysis in Wnt5a and β-catenin-deficient tracheas. Pharmacological inhibition of potassium ion channels and Wnt signaling impaired contractility of developing trachealis smooth muscle and formation of cartilaginous mesenchymal condensation. Thus, in mice, epithelial-induced Wnt/β-catenin signaling mediates trachealis muscle and cartilage development via modulation of ion channel expression, promoting trachealis muscle architecture, contractility, and cartilaginous extracellular matrix. In turn, ion channel activity may influence tracheal morphogenesis underlying TBM and CTR.NEW & NOTEWORTHY Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in noncontractile tissue and embryonic development has yet to be understood. In this study, we focused on the role of ion channels in the differentiation and patterning of the large airways of the developing respiratory tract. We identify a mechanism by which Wnt-beta-catenin signaling controls levels of ion channel-encoding genes to promote tracheal differentiation.

Contractility-Induced Phase Separation in Active Solids

Experiments over many decades are suggestive that the combination of cellular contractility and active phase separation in cell-matrix composites can enable spatiotemporal patterning in multicellular tissues across scales. To characterize these phenomena, we provide a general theory that incorporates active cellular contractility into the classical Cahn-Hilliard-Larché model for phase separation in passive viscoelastic solids. Within this framework, we show how a homogeneous cell-matrix mixture can be destabilized by activity via either a pitchfork or Hopf bifurcation, resulting in stable phase separation and/or traveling waves. Numerical simulations of the full equations allow us to track the evolution of the resulting self-organized patterns in periodic and mechanically constrained domains, and in different geometries. Altogether, our study underscores the importance of integrating both cellular activity and mechanical phase separation in understanding patterning in soft, active biosolids in both in vivo and in vitro settings.

Wnt signaling regulates ion channel expression to promote smooth muscle and cartilage formation in developing mouse trachea

Ion channels play critical roles in the physiology and function of the nervous system and contractile tissue; however, their role in non-contractile tissue and embryonic development has yet to be understood. Tracheobronchomalacia (TBM) and complete tracheal rings (CTR) are disorders affecting the muscle and cartilage of the trachea and bronchi, whose etiology remains poorly understood. We demonstrated that trachealis muscle organization and polarity are disrupted after epithelial ablation of Wls, a cargo receptor critical for the Wnt signaling pathway, in developing trachea. The phenotype resembles the anomalous trachealis muscle observed after deletion of ion channel encoding genes in developing mouse trachea. We sought to investigate whether and how the deletion of Wls affects ion channels during tracheal development. We hypothesize that Wnt signaling influences the expression of ion channels to promote trachealis muscle cell assembly and patterning. Deleting Wls in developing trachea causes differential regulation of genes mediating actin binding, cytoskeleton organization, and potassium ion channel activity. Wnt signaling regulated expression of Kcnj13, Kcnd3, Kcnj8, and Abcc9 as demonstrated by in vitro studies and in vivo analysis in Wnt5a and β-catenin deficient tracheas. Pharmacological inhibition of potassium ion channels and Wnt signaling impaired contractility of developing trachealis smooth muscle and formation of cartilaginous mesenchymal condensation. Thus, in mice, epithelial-induced Wnt/β-catenin signaling mediates trachealis muscle and cartilage development via modulation of ion channel expression, promoting trachealis muscle architecture, contractility, and cartilaginous extracellular matrix. In turn, ion channel activity may influence tracheal morphogenesis underlying TBM and CTR.

Systematic Analysis of Smooth Muscle and Cartilage Ring Formation during Mouse Tracheal Tubulogenesis

The trachea tube is the exclusive route to allow gas exchange between the external environment and the lungs. Recent studies have shown the critical role of mesenchymal cells in tracheal tubulogenesis. Improved methods for studying the dynamics of the tracheal mesenchyme development are needed to investigate the cellular and molecular mechanisms during tracheal tubulogenesis. Here, we describe a detailed protocol for a systematic analysis of tracheal tube development to enable observing tracheal smooth muscle (SM) and cartilage ring formation. We describe immunostaining, confocal and stereomicroscopy imaging, and quantitative methods to study the process of tracheal SM and cartilage ring development, including SM cell alignment, polarization, and changes in cell shape as well as mesenchymal condensation. The technologies and approaches described here not only improve analysis of the patterning of the developing trachea but also help uncover the mechanisms underlying airway disease. This protocol also provides a useful technique to analyze cell organization, polarity, and nuclear shape in other organ systems.

Regulation of pain neurotransmitters and chondrocytes metabolism mediated by voltage-gated ion channels: A narrative review

Osteoarthritis (OA) is one of the leading causes of chronic pain and dysfunction. It is essential to comprehend the nature of pain and cartilage degeneration and its influencing factors on OA treatment. Voltage-gated ion channels (VGICs) are essential in chondrocytes and extracellular matrix (ECM) metabolism and regulate the pain neurotransmitters between the cartilage and the central nervous system. This narrative review focused primarily on the effects of VGICs regulating pain neurotransmitters and chondrocytes metabolism, and most studies have focused on voltage-sensitive calcium channels (VSCCs), voltage-gated sodium channels (VGSCs), acid-sensing ion channels (ASICs), voltage-gated potassium channels (VGKCs), voltage-gated chloride channels (VGCCs). Various ion channels coordinate to maintain the intracellular environment's homeostasis and jointly regulate metabolic and pain under normal circumstances. In the OA model, the ion channel transport of chondrocytes is abnormal, and calcium influx is increased, which leads to increased neuronal excitability. The changes in ion channels are strongly associated with the OA disease process and individual OA risk factors. Future studies should explore how VGICs affect the metabolism of chondrocytes and their surrounding tissues, which will help clinicians and pharmacists to develop more effective targeted drugs to alleviate the progression of OA disease.

Cav 3.2 T-type calcium channel regulates mouse platelet activation and arterial thrombosis

BACKGROUND

Cav 3.2 is a T-type calcium channel that causes low-threshold exocytosis. T-type calcium channel blockers reduce platelet granule exocytosis and aggregation. However, studies of the T-type calcium channel in platelets are lacking.

OBJECTIVE

To examine the expression and role of Cav 3.2 in platelet function.

METHODS

Global Cav 3.2-/- and platelet-specific Cav 3.2-/- mice and littermate controls were used for this study. Western blot analysis was used to detect the presence of Cav 3.2 and activation of the calcium-responsive protein extracellular signal-regulated kinase (ERK). Fura-2 dye was used to assess platelet calcium. Flow cytometry and light transmission aggregometry were used to evaluate platelet activation markers and aggregation, respectively. FeCl3 -induced thrombosis and a microfluidic flow device were used to assess in vivo and ex vivo thrombosis, respectively.

RESULTS

Cav 3.2 was expressed in mouse platelets. As compared with wild-type controls, Cav 3.2-/- mouse platelets showed reduced calcium influx. Similarly, treatment with the T-type calcium channel inhibitor Ni2+ decreased the calcium influx in wild-type platelets. As compared with controls, both Cav 3.2-/- and Ni2+ -treated wild-type platelets showed reduced activation of ERK. ATP release, P-selectin exposure, and αIIb β3 activation were reduced in Cav 3.2-/- and Ni2+ -treated wild-type platelets, as was platelet aggregation. On in vivo and ex vivo thrombosis assay, Cav3.2 deletion caused delayed thrombus formation. However, tail bleeding assay showed intact hemostasis.

CONCLUSION

These results suggest that Cav 3.2 is required for the optimal activation of platelets.

Targeting T-type channels in cancer: What is on and what is off?

Over the past 20 years, various studies have demonstrated a pivotal role of T-type calcium channels (TTCCs) in tumor progression. Cytotoxic effects of TTCC pharmacological blockers have been reported in vitro and in preclinical models. However, their roles in cancer physiology are only beginning to be understood. In this review, we discuss evidence for the signaling pathways and cellular processes stemming from TTCC activity, mainly inferred by inverse reasoning from pharmacological blocks and, only in a few studies, by gene silencing or channel activation. A thorough analysis indicates that drug-induced cytotoxicity is partially an off-target effect. Dissection of on/off-target activity is paramount to elucidate the physiological roles of TTCCs, and to deliver efficacious therapies suited to different cancer types and stages.

Targeted Activation of GPCR-Mediated Ca2+ Signaling Drives Enhanced Cartilage-Like Matrix Formation

Intracellular calcium ([Ca²⁺]_i) signaling is a critical regulator of chondrogenesis, chondrocyte differentiation, and cartilage development. Calcium (Ca²⁺) signaling is known to direct processes that govern chondrocyte gene expression, protein synthesis, cytoskeletal remodeling, and cell fate. Control of chondrocyte/chondroprogenitor Ca²⁺ signaling has been attempted through mechanical and/or pharmacological activation of endogenous Ca²⁺ signaling transducers; however, such approaches can lack specificity and/or precision regarding Ca²⁺ activation mechanisms. Synthetic signaling platforms permitting precise and selective Ca²⁺ signal transduction can improve dissection of the roles that [Ca²⁺]_i signaling play in chondrocyte behavior. One such platform is the chemogenetic hM3Dq DREADD (designer receptor exclusively activated by designer drugs) that activates [Ca²⁺]_i signaling via the Gα_q-PLCβ-IP₃-ER pathway upon clozapine N-oxide (CNO) administration. We previously demonstrated hM3Dq's ability to precisely and synthetically initiate robust [Ca²⁺]_i transients and oscillatory [Ca²⁺]_i signaling in chondrocyte-like ATDC5 cells. Here, we investigate the effects that long-term CNO stimulatory culture have on hM3Dq [Ca²⁺]_i signaling dynamics, proliferation, and protein deposition in 2D ATDC5 cultures. Long-term culturing under repeated CNO stimulation modified the temporal dynamics of hM3Dq [Ca²⁺]_i signaling, increased cell proliferation, and enhanced matrix production in a CNO dose- and frequency-dependent manner, and triggered the formation of cell condensations that developed aligned, anisotropic neotissue structures rich in cartilaginous proteoglycans and collagens, all in the absence of differentiation inducers. This study demonstrated Gα_q-GPCR-mediated [Ca²⁺]_i signaling involvement in chondroprogenitor proliferation and cartilage-like matrix production, and established hM3Dq as a powerful tool for elucidating the role of GPCR-mediated Ca²⁺ signaling in chondrogenesis and chondrocyte differentiation.

Neuromedin B promotes chondrocyte differentiation of mesenchymal stromal cells via calcineurin and calcium signaling

Background

Articular cartilage is a complex tissue with poor healing capacities. Current approaches for cartilage repair based on mesenchymal stromal cells (MSCs) are often disappointing because of the lack of relevant differentiation factors that could drive MSC differentiation towards a stable mature chondrocyte phenotype.

Results

We used a large-scale transcriptomic approach to identify genes that are modulated at early stages of chondrogenic differentiation using the reference cartilage micropellet model. We identified several modulated genes and selected neuromedin B (NMB) as one of the early and transiently modulated genes. We found that the timely regulated increase of NMB was specific for chondrogenesis and not observed during osteogenesis or adipogenesis. Furthermore, NMB expression levels correlated with the differentiation capacity of MSCs and its inhibition resulted in impaired chondrogenic differentiation indicating that NMB is required for chondrogenesis. We further showed that NMB activated the calcineurin activity through a Ca²⁺-dependent signaling pathway.

Conclusion

NMB is a newly described chondroinductive bioactive factor that upregulates the key chondrogenic transcription factor Sox9 through the modulation of Ca²⁺ signaling pathway and calcineurin activity.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00695-1.

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.

Mammalian tracheal development and reconstruction: insights from in vivo and in vitro studies

The trachea delivers inhaled air into the lungs for gas exchange. Anomalies in tracheal development can result in life-threatening malformations, such as tracheoesophageal fistula and tracheomalacia. Given the limitations of current therapeutic approaches, development of technologies for the reconstitution of a three-dimensional trachea from stem cells is urgently required. Recently, single-cell sequencing technologies and quantitative analyses from cell to tissue scale have been employed to decipher the cellular basis of tracheal morphogenesis. In this Review, recent advances in mammalian tracheal development and the generation of tracheal tissues from pluripotent stem cells are summarized.

Voltage-Gated Calcium Channels in Nonexcitable Tissues

The identification of a gain-of-function mutation in CACNA1C as the cause of Timothy syndrome, a rare disorder characterized by cardiac arrhythmias and syndactyly, highlighted roles for the L-type voltage-gated Ca2+ channel CaV1.2 in nonexcitable cells. Previous studies in cells and animal models had suggested that several voltage-gated Ca2+ channels (VGCCs) regulated critical signaling events in various cell types that are not expected to support action potentials, but definitive data were lacking. VGCCs occupy a special position among ion channels, uniquely able to translate membrane excitability into the cytoplasmic Ca2+ changes that underlie the cellular responses to electrical activity. Yet how these channels function in cells not firing action potentials and what the consequences of their actions are in nonexcitable cells remain critical questions. The development of new animal and cellular models and the emergence of large data sets and unbiased genome screens have added to our understanding of the unanticipated roles for VGCCs in nonexcitable cells. Here, we review current knowledge of VGCC regulation and function in nonexcitable tissues and cells, with the goal of providing a platform for continued investigation.

Targeted Gq-GPCR activation drives ER-dependent calcium oscillations in chondrocytes

The temporal dynamics of calcium signaling are critical regulators of chondrocyte homeostasis and chondrogenesis. Calcium oscillations regulate differentiation and anabolic processes in chondrocytes and their precursors. Attempts to control chondrocyte calcium signaling have been achieved through mechanical perturbations and synthetic ion channel modulators. However, such stimuli can lack both local and global specificity and precision when evoking calcium signals. Synthetic signaling platforms can more precisely and selectively activate calcium signaling, enabling improved dissection of the roles of intracellular calcium ([Ca²⁺]_i) in chondrocyte behavior. One such platform is hM3Dq, a chemogenetic DREADD (Designer Receptors Exclusively Activated by Designer Drugs) that activates calcium signaling via the Gα_q-PLCβ-IP₃-ER pathway upon administration of clozapine N-oxide (CNO). We previously described the first-use of hM3Dq to precisely mediate targeted, synthetic calcium signals in chondrocyte-like ATDC5 cells. Here, we generated stably expressing hM3Dq-ATDC5 cells to investigate the dynamics of Gα_q-GPCR calcium signaling in depth. CNO drove robust calcium responses in a temperature- and concentration-dependent (1 pM-100 μM) manner and elicited elevated levels of oscillatory calcium signaling above 10 nM. hM3Dq-mediated calcium oscillations in ATDC5 cells were reliant on ER calcium stores for both initiation and sustenance, and the downregulation and recovery dynamics of hM3Dq after CNO stimulation align with traditionally reported GPCR recycling kinetics. This study successfully generated a stable hM3Dq cell line to precisely drive Gα_q-GPCR-mediated and ER-dependent oscillatory calcium signaling in ATDC5 cells and established a novel tool to elucidate the role that GPCR-mediated calcium signaling plays in chondrocyte biology, cartilage pathology, and cartilage tissue engineering.

HEMA and alginate-based chondrogenic semi-interpenetrated hydrogels: synthesis and biological characterization

Cartilage tissue engineering (CTE) has the general objective of restoring and improving damaged cartilage. A very interesting strategy of CTE is to combine different polymers to obtain a viscoelastic material. In this study, we have evaluated the applicability of poly(2-hydroxyethyl methacrylate) networks semi-interpenetrated with sodium alginate for CTE. Alginate-containing hydrogels show an increase in scaffold porosity and swelling capacity, when compared with nonporous poly(2-hydroxyethyl methacrylate) scaffolds. Primary chondrocytes from young rats were cultured on the hydrogels, and an increase in chondrocyte proliferation and chondrocytic markers was observed in alginate-containing hydrogels. Chondrocytic phenotype was preserved on hydrogels containing the lowest amount of crosslinker and initiator (SEMI 3 and SEMI 4). In addition, nitric oxide production by RAW264.7 macrophages grown on hydrogels was tested, and none of the hydrogels showed high levels of this inflammatory marker after 2 days. These results indicate that our alginate-containing hydrogels could be useful for CTE.

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.

The RCAN1.4-calcineurin/NFAT signaling pathway is essential for hypoxic adaption of intervertebral discs

Calcipressin-1, also known as regulator of calcineurin 1 (RCAN1), can specifically bind calcineurin at or near the calcineurin A catalytic domain and downregulate calcineurin activity. However, whether RCAN1 affects the hypoxic intervertebral disc (IVD) phenotype through the calcineurin/NFAT signaling pathway remains unclear. First, we confirmed the characteristics of the degenerative nucleus pulposus (NP) by H&E, safranin O/fast green and Alcian blue staining, and detected increased RCAN1 levels in the degenerative NP by immunohistochemistry. Then, we demonstrated that the protein level of RCAN1.4 was higher than that of RCAN1.1 and progressively elevated from the control group to the Pfirrmann grade V group. In vitro, both hypoxia (1% O₂) and overexpression of HIF-1α reduced the protein level of RCAN1.4 in rat NP cells in a dose- and time-dependent manner. We further found that miRNA-124, through a nondegradative pathway (without the proteasome or lysosome), suppressed the expression of RCAN1.4. As expected, calcineurin in NP cells was activated and primarily promoted nuclear translocation of NFATc1 under hypoxia or RCAN1.4 siRNA transfection. Furthermore, SOX9, type II collagen and MMP13 were elevated under hypoxia, RCAN1.4 siRNA transfection or NFATc1 overexpression. Using chromatin immunoprecipitation (ChIP) and a luciferase reporter assay (with mutation), we clarified that NFATc1 increasingly bound the SOX9 promotor region (bp −367~−357). Interaction of HIF-1α and NFATc1 promoted MMP13 transcription. Finally, we found that FK506 reversed hypoxia-induced activation of the calcineurin/NFAT signaling pathway in NP cells and an ex vivo model. Together, these findings show that the RCAN1.4-calcineurin/NFAT signaling pathway has a vital role in the hypoxic phenotype of NP cells. RCAN1.4 might be a therapeutic target for degenerative disc diseases.

Treatments targeting a protein that is overexpressed in damaged spinal cartilage could ease degenerative conditions associated with lower back pain. The intervertebral discs are complex cartilage tissues that absorb forces while allowing the motion of our spines. An immune-promoting enzyme called calcineurin is important in maintaining the supple, gel-like structure of the central part of each disc, the nucleus pulposus (NP). Fendong Zhao and Jian Chen at Zhejiang University School of Medicine Hangzhou, China and co-workers showed that RCAN1.4, a protein known to suppress calcineurin, is overexpressed in damaged human NPs. The team further revealed how a signaling pathway starting with RCAN1.4 suppresses key genes involved in forming the collagen fibers that hold the NP together. They therefore suggest that therapies targeting this protein could benefit patients suffering from disc degeneration diseases.

T-type, but not L-type, voltage-gated calcium channels are dispensable for lymphatic pacemaking and spontaneous contractions

The spontaneous contractions of collecting lymphatic vessels provide an essential propulsive force to return lymph centrally. These contractions are driven by an intrinsic electrical pacemaker, working through an unknown underlying ionic mechanism that becomes compromised in some forms of lymphedema. In previous studies, T-type voltage-gated Ca2+ channels (VGCCs) were implicated in this pacemaking mechanism, based on the effects of the reputedly selective T-type VGCC inhibitors mibefradil and Ni2+. Our goal was to test this idea in a more definitive way using genetic knock out mice. First, we demonstrated through both PCR and immunostaining that mouse lymphatic muscle cells expressed Cav3.1 and Cav3.2 and produced functional T-type VGCC currents when patch clamped. We then employed genetic deletion strategies to selectively test the roles of each T-type VGCC isoform in the regulation of lymphatic pacemaking. Surprisingly, global deletion of either, or both, isoform(s) was without significant effect on either the frequency, amplitude, or fractional pump flow of lymphatic collectors from two different regions of the mouse, studied ex vivo. Further, both WT and Cav3.1-/-; 3.2-/- double knock-out lymphatic vessels responded similarly to mibefradil and Ni2+, which substantially reduced contraction amplitudes and slightly increased frequencies at almost all pressures in both strains: a pattern consistent with inhibition of L-type rather than T-type VGCCs. Neither T-type VGCC isoform was required for ACh-induced inhibition of contraction, a mechanism by which those channels in smooth muscle are thought to be targets of endothelium-derived nitric oxide. Sharp intracellular electrode measurements in lymphatic smooth muscle revealed only subtle, but not significant, differences in the resting membrane potential and action potential characteristics between vessels from wild-type and Cav3.1-/-; 3.2-/- double knock-out mice. In contrast, smooth-muscle specific deletion of the L-type VGCC, Cav1.2, completely abolished all lymphatic spontaneous contractions. Collectively our results suggest that, although T-type VGCCs are expressed in mouse lymphatic smooth muscle, they do not play a significant role in modulating the frequency of the ionic pacemaker or the amplitude of spontaneous contractions. We conclude that the effects of mibefradil and Ni2+ in other lymphatic preparations are largely or completely explained by off-target effects on L-type VGCCs, which are essential for controlling both the frequency and strength of spontaneous contractions.

Complete Tracheal Ring Deformity. A Translational Genomics Approach to Pathogenesis

Rationale: Complete tracheal ring deformity (CTRD) is a rare congenital abnormality of unknown etiology characterized by circumferentially continuous or nearly continuous cartilaginous tracheal rings, variable degrees of tracheal stenosis and/or shortening, and/or pulmonary arterial sling anomaly.Objectives: To test the hypothesis that CTRD is caused by inherited or de novo mutations in genes required for normal tracheal development.Methods: CTRD and normal tracheal tissues were examined microscopically to define the tracheal abnormalities present in CTRD. Whole-exome sequencing was performed in children with CTRD and their biological parents ("trio analysis") to identify gene variants in patients with CTRD. Mutations were confirmed by Sanger sequencing, and their potential impact on structure and/or function of encoded proteins was examined using human gene mutation databases. Relevance was further examined by comparison with the effects of targeted deletion of murine homologs important to tracheal development in mice.Measurements and Main Results: The trachealis muscle was absent in all of five patients with CTRD. Exome analysis identified six de novo, three recessive, and multiple compound-heterozygous or rare hemizygous variants in children with CTRD. De novo variants were identified in SHH (Sonic Hedgehog), and inherited variants were identified in HSPG2 (perlecan), ROR2 (receptor tyrosine kinase-like orphan receptor 2), and WLS (Wntless), genes involved in morphogenetic pathways known to mediate tracheoesophageal development in mice.Conclusions: The results of the present study demonstrate that absence of the trachealis muscle is associated with CTRD. Variants predicted to cause disease were identified in genes encoding Hedgehog and Wnt signaling pathway molecules, which are critical to cartilage formation and normal upper airway development in mice.

A novel phospho-modulatory mechanism contributes to the calcium-dependent regulation of T-type Ca2+ channels

Ca_v3 / T-type Ca²⁺ channels are dynamically regulated by intracellular Ca²⁺ ions, which inhibit Ca_v3 availability. Here, we demonstrate that this inhibition becomes irreversible in the presence of non-hydrolysable ATP analogs, resulting in a strong hyperpolarizing shift in the steady-state inactivation of the residual Ca_v3 current. Importantly, the effect of these ATP analogs was prevented in the presence of intracellular BAPTA. Additional findings obtained using intracellular dialysis of inorganic phosphate and alkaline phosphatase or NaN₃ treatment further support the involvement of a phosphorylation mechanism. Contrasting with Ca_v1 and Ca_v2 Ca²⁺ channels, the Ca²⁺-dependent modulation of Ca_v3 channels appears to be independent of calmodulin, calcineurin and endocytic pathways. Similar findings were obtained for the native T-type Ca²⁺ current recorded in rat thalamic neurons of the central medial nucleus. Overall, our data reveal a new Ca²⁺ sensitive phosphorylation-dependent mechanism regulating Ca_v3 channels, with potentially important physiological implications for the multiple cell functions controlled by T-type Ca²⁺ channels.

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.

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

During endochondral ossification of long bones, the proliferation and differentiation of chondrocytes cause them to be arranged into layered structures constituting the epiphyseal growth plate, where they secrete the cartilage matrix that is subsequently converted into trabecular bone. Ca²⁺ signaling has been implicated in chondrogenesis in vitro. Through fluorometric imaging of bone slices from embryonic mice, we demonstrated that live growth plate chondrocytes generated small, cell-autonomous Ca²⁺ fluctuations that were associated with weak and intermittent Ca²⁺ influx. Several genes encoding Ca²⁺-permeable channels were expressed in growth plate chondrocytes, but only pharmacological inhibitors of transient receptor potential cation channel subfamily M member 7 (TRPM7) reduced the spontaneous Ca²⁺ fluctuations. The TRPM7-mediated Ca²⁺ influx was likely activated downstream of basal phospholipase C activity and was potentiated upon cell hyperpolarization induced by big-conductance Ca²⁺-dependent K⁺ channels. Bones from embryos in which Trpm7 was conditionally knocked out during ex vivo culture exhibited reduced outgrowth and displayed histological abnormalities accompanied by insufficient autophosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) in the growth plate. The link between TRPM7-mediated Ca²⁺ fluctuations and CaMKII-dependent chondrogenesis was further supported by experiments with chondrocyte-specific Trpm7 knockout mice. Thus, growth plate chondrocytes generate spontaneous, TRPM7-mediated Ca²⁺ fluctuations that promote self-maturation and bone development.

The calcium channel subunit gamma-4 is regulated by MafA and necessary for pancreatic beta-cell specification

Voltage-gated Ca2+ (CaV) channels trigger glucose-induced insulin secretion in pancreatic beta-cell and their dysfunction increases diabetes risk. These heteromeric complexes include the main subunit alpha1, and the accessory ones, including subunit gamma that remains unexplored. Here, we demonstrate that CaV gamma subunit 4 (CaVγ4) is downregulated in islets from human donors with diabetes, diabetic Goto-Kakizaki (GK) rats, as well as under conditions of gluco-/lipotoxic stress. Reduction of CaVγ4 expression results in decreased expression of L-type CaV1.2 and CaV1.3, thereby suppressing voltage-gated Ca2+ entry and glucose stimulated insulin exocytosis. The most important finding is that CaVγ4 expression is controlled by the transcription factor responsible for beta-cell specification, MafA, as verified by chromatin immunoprecipitation and experiments in beta-cell specific MafA knockout mice (MafA Δβcell ). Taken together, these findings suggest that CaVγ4 is necessary for maintaining a functional differentiated beta-cell phenotype. Treatment aiming at restoring CaVγ4 may help to restore beta-cell function in diabetes.

Fibrillin-2 is a key mediator of smooth muscle extracellular matrix homeostasis during mouse tracheal tubulogenesis

Epithelial tubes, comprised of polarised epithelial cells around a lumen, are crucial for organ function. However, the molecular mechanisms underlying tube formation remain largely unknown. Here, we report on the function of fibrillin (FBN)2, an extracellular matrix (ECM) glycoprotein, as a critical regulator of tracheal tube formation.We performed a large-scale forward genetic screen in mouse to identify regulators of respiratory organ development and disease. We identified Fbn2 mutants which exhibit shorter and narrowed tracheas as well as defects in tracheal smooth muscle cell alignment and polarity.We found that FBN2 is essential for elastic fibre formation and Fibronectin accumulation around tracheal smooth muscle cells. These processes appear to be regulated at least in part through inhibition of p38-mediated upregulation of matrix metalloproteinases (MMPs), as pharmacological decrease of p38 phosphorylation or MMP activity partially attenuated the Fbn2 mutant tracheal phenotypes. Analysis of human tracheal tissues indicates that a decrease in ECM proteins, including FBN2 and Fibronectin, is associated with tracheomalacia.Our findings provide novel insights into the role of ECM homeostasis in mesenchymal cell polarisation during tracheal tubulogenesis.

Gene expression profiling: identification of gene expression in human MSC chondrogenic differentiation

Understanding the mechanisms that govern cell fate will lead to the development of techniques for the induction of human mesenchymal stem cell differentiation into desired cell outcomes and the production of an autologous source of tissue for regenerative medicine. Here, we demonstrate that stem cells derived from adult bone marrow grown with 3D pellets take on characteristics similar to human cartilage. The NFAT signaling pathway is primarily linked to cell differentiation and influences chondrogenic differentiation. Based on our previous results that alterations in the expression of the NFATc1 gene affect chondrogenesis, we screened a microarray and identified 29 genes with altered expression, including 13 up-regulated (fold change ≥ 2) and 16 down-regulated (fold change ≤ 2) genes, compared with the control group. We then used RT-PCR to validate the chip data. Gene ontology and pathway analyses were performed on these altered genes. We found that these altered genes function in the complement and coagulation cascades, metabolism, biosynthesis, transcriptional regulation, proteolysis, and intracellular signaling pathways, such as the cytoplasmic calcineurin-dependent signaling pathway, the cyclin-dependent kinase inhibitor 2C signaling pathway, the MAPK signaling pathway, and the insulin signaling pathway. Our study suggests that these pathways may play important roles in chondrogenesis, which could be useful in the design of biomaterials.

Zyxin-involved actin regulation is essential in the maintenance of vinculin focal adhesion and chondrocyte differentiation status

Objectives

To investigate the role of zyxin‐involved actin regulation in expression level of vinculin focal adhesion and collagen production of chondrocyte and its possible underlying mechanism.

Materials and methods

Chondrocytes obtained from rabbit articular cartilage were used in this study. The expression of zyxin, actin and vinculin, as well as the extracellular matrix (ECM) protein collagen type I, II and X (COL I, II and X) of chondrocytes were compared between zyxin‐knockdown group and negative control group, and between transforming growth factor‐β1 (TGF‐β1) treatment group and non‐treatment group, respectively.

Results

Knockdown of zyxin increased the ratio of globular actin (G‐actin) to filamentous actin (F‐actin) of chondrocyte, which further inhibited expression of vinculin and chondrogenic marker COL II as well as hypertrophy marker COL X. On the other hand, chondrocytes treated with TGF‐β1 showed an enhanced expression of F‐actin, and a lower expression of zyxin compared to non‐treatment group. In response to TGF‐β1‐induced actin polymerization, expression of vinculin and COL I was increased, while expression of COL II and aggrecan was decreased.

Conclusions

These results demonstrate supporting evidence that in chondrocytes the level of zyxin is closely associated with the state of actin polymerization. In particular, the change of zyxin and F‐actin parallels with the change of COL II and vinculin, respectively, indicating a major role of zyxin‐actin interaction in the synthesis of collagen ECM and the remodelling of cytoskeleton‐ECM adhesion.

The Role of Physical Stimuli on Calcium Channels in Chondrogenic Differentiation of Mesenchymal Stem Cells

Human mesenchymal stem cells (hMSC) are becoming increasingly popular in tissue engineering. They are the most frequently used stem cell source for clinical applications due to their high potential to differentiate into several lineages. Cartilage is known for its low capacity for self-maintenance and currently there are no efficient methods to improve cartilage repair. Chondrogenic differentiation of hMSC isolated from different tissues is widely employed due to a high clinical demand for the improvement of cartilage regeneration. Calcium channels that are regulated by physical stimuli seem to play a pivotal role in chondrogenic differentiation of MSCs. These channels increase intracellular calcium concentration, which leads to the initiation of the relevant cellular processes that are required for differentiation. This review will focus on the impact of different physical stimuli, including electrical, electromagnetic/magnetic and mechanical on various calcium channels and calcium signaling mechanisms during chondrogenic differentiation of hMSC.

The chondrocyte channelome: A narrative review

Chondrocytes are the main cells in the extracellular matrix (ECM) of articular cartilage and possess a highly differentiated phenotype that is the hallmark of the unique physiological functions of this specialised load-bearing connective tissue. The plasma membrane of articular chondrocytes contains a rich and diverse complement of membrane proteins, known as the membranome, which defines the cell surface phenotype of the cells. The membranome is a key target of pharmacological agents and is important for chondrocyte function. It includes channels, transporters, enzymes, receptors, and anchors for intracellular, cytoskeletal and ECM proteins and other macromolecular complexes. The chondrocyte channelome is a sub-compartment of the membranome and includes a complete set of ion channels and porins expressed in these cells. Many of these are multi-functional proteins with "moonlighting" roles, serving as channels, receptors and signalling components of larger molecular assemblies. The aim of this review is to summarise our current knowledge of the fundamental aspects of the chondrocyte channelome, discuss its relevance to cartilage biology and highlight its possible role in the pathogenesis of osteoarthritis (OA). Excessive and inappropriate mechanical loads, an inflammatory micro-environment, alternative splicing of channel components or accumulation of basic calcium phosphate crystals can result in an altered chondrocyte channelome impairing its function. Alterations in Ca²⁺ signalling may lead to defective synthesis of ECM macromolecules and aggravated catabolic responses in chondrocytes, which is an important and relatively unexplored aspect of the complex and poorly understood mechanism of OA development.

Altered spontaneous calcium signaling of in situ chondrocytes in human osteoarthritic cartilage

Intracellular calcium ([Ca²⁺]_i) signaling is an essential universal secondary messenger in articular chondrocytes. However, little is known about its spatiotemporal features in the context of osteoarthritis (OA). Herein, by examining the cartilage samples collected from patients undergoing knee arthroscopic surgery, we investigated the spatiotemporal features of spontaneous [Ca²⁺]_i signaling in in situ chondrocytes at different OA stages. Our data showed zonal dependent spontaneous [Ca²⁺]_i signaling in healthy cartilage samples under 4 mM calcium environment. This signal was significantly attenuated in healthy cartilage samples but increased in early-degenerated cartilage when cultured in 0 mM calcium environment. No significant difference was found in [Ca²⁺]_i intensity oscillation in chondrocytes located in middle zones among ICRS 1–3 samples under both 4 and 0 mM calcium environments. However, the correlation was found in deep zone chondrocytes incubated in 4 mM calcium environment. In addition, increased protein abundance of Ca_v3.3 T-type voltage dependent calcium channel and Nfatc2 activity were observed in early-degenerated cartilage samples. The present study exhibited OA severity dependent spatiotemporal features of spontaneous [Ca²⁺]_i oscillations of in situ chondrocytes, which might reflect the zonal specific role of chondrocytes during OA progression and provide new insight in articular cartilage degradation during OA progression.

Methylation-mediated silencing of miR-124 facilitates chondrogenesis by targeting NFATc1 under hypoxic conditions

Chondrogenic differentiation of mesenchymal stem cells is regulated by many different pathways. Recent studies have established that hypoxia and epigenetic alterations potently affect expression of chondrogenesis marker genes. Sox9 is generally regarded as a master regulator of chondrogenesis and microRNA-124 (miRNA-124) regulates gene expression in murine bone marrow-derived mesenchymal stem cells. Therefore, in this study we investigated whether epigenetic regulation of miRNA-124 could affect the expression of Sox9 and thereby regulate chondrogenesis. A cell pellet culture model was used to induce chondrogenesis in C3H10T1/2 cells under hypoxic conditions (2% O₂) to determine the effects of hypoxia on miR-124 expression and DNA methylation. The expression of miR-124 was significantly downregulated under hypoxic conditions compared to normoxic conditions (21% O₂). The expression of chondrogenesis marker genes was significantly increased under hypoxic conditions. Bisulfite sequencing of the CpG islands in the promoter region of miR-124-3 showed that CpG methylation was significantly increased under hypoxic conditions. Treating the cells with the DNA demethylating agent 5'-AZA significantly increased miR-124 expression and decreased expression of markers of chondrogenesis. Overexpressing miR-124 under hypoxic conditions inhibited NFATc1 reporter activity. NFATc1 was shown to bind to the promoter region of Sox9. Taken together, our data provide evidence that miR-124 acts as an inhibitor of NFATc1. Under hypoxic conditions when miR-124 is downregulated by methylation of CpG islands in the promoter, NFATc1 can bind to the Sox9 promoter and induce the expression of Sox9 leading to chondrogenesis. These results support the role of epigenetic regulation in establishing and maintaining a chondrogenic phenotype.

Enhancement of mesenchymal stem cell chondrogenesis with short-term low intensity pulsed electromagnetic fields

Pulse electromagnetic fields (PEMFs) have been shown to recruit calcium-signaling cascades common to chondrogenesis. Here we document the effects of specified PEMF parameters over mesenchymal stem cells (MSC) chondrogenic differentiation. MSCs undergoing chondrogenesis are preferentially responsive to an electromagnetic efficacy window defined by field amplitude, duration and frequency of exposure. Contrary to conventional practice of administering prolonged and repetitive exposures to PEMFs, optimal chondrogenic outcome is achieved in response to brief (10 minutes), low intensity (2 mT) exposure to 6 ms bursts of magnetic pulses, at 15 Hz, administered only once at the onset of chondrogenic induction. By contrast, repeated exposures diminished chondrogenic outcome and could be attributed to calcium entry after the initial induction. Transient receptor potential (TRP) channels appear to mediate these aspects of PEMF stimulation, serving as a conduit for extracellular calcium. Preventing calcium entry during the repeated PEMF exposure with the co-administration of EGTA or TRP channel antagonists precluded the inhibition of differentiation. This study highlights the intricacies of calcium homeostasis during early chondrogenesis and the constraints that are placed on PEMF-based therapeutic strategies aimed at promoting MSC chondrogenesis. The demonstrated efficacy of our optimized PEMF regimens has clear clinical implications for future regenerative strategies for cartilage.

Ubiquitin-Dependent Regulation of Stem Cell Biology

The growth of a metazoan body relies on a series of highly coordinated cell-fate decisions by stem cells which can undergo self-renewal, reversibly enter a quiescent state, or terminally commit to a cell specification program. To guide their decisions, stem cells make frequent use of ubiquitylation, a post-translational modification that can affect the activity, interaction landscape, or stability of stem cell proteins. In this review we discuss novel findings that have provided insight into ubiquitin-dependent mechanisms of stem cell control and revealed how an essential and highly conserved protein modification can shape metazoan development.

miR-146a facilitates osteoarthritis by regulating cartilage homeostasis via targeting Camk2d and Ppp3r2

Osteoarthritis (OA), characterized by insufficient extracellular matrix synthesis and cartilage degeneration, is known as an incurable disease because its pathogenesis is poorly elucidated. Thus far, limited information is available regarding the pathophysiological role of microRNAs (miRNAs) in OA. In this study, we investigated the specific function of miR-146a in OA pathophysiology using mouse OA models. We found that the articular cartilage degeneration of miR-146a knockout (KO) mice was alleviated compared with that of the wild-type (WT) mice in spontaneous and instability-induced OA models. We demonstrate that miR-146a aggravated pro-inflammatory cytokines induced suppressing the expression of cartilage matrix-associated genes. We further identified calcium/calmodulin-dependent protein kinase II delta (Camk2d) and protein phosphatase 3, regulatory subunit B, beta isoform (Ppp3r2, also known as calcineurin B, type II) were essential targets of miR-146a in regulating cartilage homeostasis. Moreover, we found that surgical-induced OA mice treated with a miR-146a inhibitor significantly alleviated the destruction of articular cartilage via targeting Camk2d and Ppp3r2. These results suggested that miR-146a has a crucial role in maintaining cartilage homeostasis. MiR-146a inhibition in chondrocytes can be a potential therapeutic strategy to ameliorate OA.

Cartilage rings contribute to the proper embryonic tracheal epithelial differentiation, metabolism, and expression of inflammatory genes

The signaling cross talk between the tracheal mesenchyme and epithelium has not been researched extensively, leaving a substantial gap of knowledge in the mechanisms dictating embryonic development of the proximal airways by the adjacent mesenchyme. Recently, we reported that embryos lacking mesenchymal expression of Sox9 did not develop tracheal cartilage rings and showed aberrant differentiation of the tracheal epithelium. Here, we propose that tracheal cartilage provides local inductive signals responsible for the proper differentiation, metabolism, and inflammatory status regulation of the tracheal epithelium. The tracheal epithelium of mesenchyme-specific Sox9^Δ/Δ mutant embryos showed altered mRNA expression of various epithelial markers such as Pb1fa1, surfactant protein B (Sftpb), secretoglobulin, family 1A, member 1 (Scgb1a1), and trefoil factor 1 (Tff1). In vitro tracheal epithelial cell cultures confirmed that tracheal chondrocytes secrete factors that inhibit club cell differentiation. Whole gene expression profiling and ingenuity pathway analysis showed that the tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), and transforming growth factor-β (TGF-β) signaling pathways were significantly altered in the Sox9 mutant trachea. TNF-α and IFN-γ interfered with the differentiation of tracheal epithelial progenitor cells into mature epithelial cell types in vitro. Mesenchymal knockout of Tgf-β1 in vivo resulted in altered differentiation of the tracheal epithelium. Finally, mitochondrial enzymes involved in fat and glycogen metabolism, cytochrome c oxidase subunit VIIIb (Cox8b) and cytochrome c oxidase subunit VIIa polypeptide 1 (Cox7a1), were strongly upregulated in the Sox9 mutant trachea, resulting in increases in the number and size of glycogen storage vacuoles. Our results support a role for tracheal cartilage in modulation of the differentiation and metabolism and the expression of inflammatory-related genes in the tracheal epithelium by feeding into the TNF-α, IFN-γ, and TGF-β signaling pathways.

Membrane channel gene expression in human costal and articular chondrocytes

Chondrocytes are the uniquely resident cells found in all types of cartilage and key to their function is the ability to respond to mechanical loads with changes of metabolic activity. This mechanotransduction property is, in part, mediated through the activity of a range of expressed transmembrane channels; ion channels, gap junction proteins, and porins. Appropriate expression of ion channels has been shown essential for production of extracellular matrix and differential expression of transmembrane channels is correlated to musculoskeletal diseases such as osteoarthritis and Albers-Schönberg. In this study we analyzed the consistency of gene expression between channelomes of chondrocytes from human articular and costal (teenage and fetal origin) cartilages. Notably, we found 14 ion channel genes commonly expressed between articular and both types of costal cartilage chondrocytes. There were several other ion channel genes expressed only in articular (6 genes) or costal chondrocytes (5 genes). Significant differences in expression of BEST1 and KCNJ2 (Kir2.1) were observed between fetal and teenage costal cartilage. Interestingly, the large Ca²⁺ activated potassium channel (BKα, or KCNMA1) was very highly expressed in all chondrocytes examined. Expression of the gap junction genes for Panx1, GJA1 (Cx43) and GJC1 (Cx45) was also observed in chondrocytes from all cartilage samples. Together, this data highlights similarities between chondrocyte membrane channel gene expressions in cells derived from different anatomical sites, and may imply that common electrophysiological signaling pathways underlie cellular control. The high expression of a range of mechanically and metabolically sensitive membrane channels suggest that chondrocyte mechanotransduction may be more complex than previously thought.

Voltage-dependent calcium channels in chondrocytes: roles in health and disease

Chondrocytes, the single cell type in adult articular cartilage, have conventionally been considered to be non-excitable cells. However, recent evidence suggests that their resting membrane potential (RMP) is less negative than that of excitable cells, and they are fully equipped with channels that control ion, water and osmolyte movement across the chondrocyte membrane. Amongst calcium-specific ion channels, members of the voltage-dependent calcium channel (VDCC) family are expressed in chondrocytes where they are functionally active. L-type VDCC inhibitors such as nifedipine and verapamil have contributed to our understanding of the roles of these ion channels in chondrogenesis, chondrocyte signalling and mechanotransduction. In this narrative review, we discuss published data indicating that VDCC function is vital for chondrocyte health, especially in regulating proliferation and maturation. We also highlight the fact that activation of VDCC function appears to accompany various inflammatory aspects of osteoarthritis (OA) and, based on in vitro data, the application of nifedipine and/or verapamil may be a promising approach for ameliorating OA severity. However, very few studies on clinical outcomes are available regarding the influence of calcium antagonists, which are used primarily for treating cardiovascular conditions in OA patients. This review is intended to stimulate further research on the chondrocyte 'channelome', contribute to the development of novel therapeutic strategies and facilitate the retargeting and repositioning of existing pharmacological agents currently used for other comorbidities for the treatment of OA.

The chondrocyte clock gene Bmal1 controls cartilage homeostasis and integrity

Osteoarthritis (OA) is the most prevalent and debilitating joint disease, and there are currently no effective disease-modifying treatments available. Multiple risk factors for OA, such as aging, result in progressive damage and loss of articular cartilage. Autonomous circadian clocks have been identified in mouse cartilage, and environmental disruption of circadian rhythms in mice predisposes animals to OA-like damage. However, the contribution of the cartilage clock mechanisms to the maintenance of tissue homeostasis is still unclear. Here, we have shown that expression of the core clock transcription factor BMAL1 is disrupted in human OA cartilage and in aged mouse cartilage. Furthermore, targeted Bmal1 ablation in mouse chondrocytes abolished their circadian rhythm and caused progressive degeneration of articular cartilage. We determined that BMAL1 directs the circadian expression of many genes implicated in cartilage homeostasis, including those involved in catabolic, anabolic, and apoptotic pathways. Loss of BMAL1 reduced the levels of phosphorylated SMAD2/3 (p-SMAD2/3) and NFATC2 and decreased expression of the major matrix-related genes Sox9, Acan, and Col2a1, but increased p-SMAD1/5 levels. Together, these results define a regulatory mechanism that links chondrocyte BMAL1 to the maintenance and repair of cartilage and suggest that circadian rhythm disruption is a risk factor for joint diseases such as OA.

Mesodermal lineages in the developing respiratory system

The life-sustaining air-blood interface of the respiratory system requires the exquisite integration of the epithelial lining with the mesenchymal capillary network, all supported by elastic smooth muscle and rigid cartilage keeping the expandable airways open. These intimate tissue interactions originate in the early embryo, where bidirectional paracrine signaling between the endoderm epithelium and adjacent mesoderm orchestrates lung and trachea development and controls the stereotypical branching morphogenesis. Although much attention has focused on how these interactions impact the differentiation of the respiratory epithelium, relatively less is known about the patterning and differentiation of the mesenchyme. Endothelial cells, smooth muscle cells, and chondrocytes together with other types of mesenchymal cells are essential components of a functional respiratory system, and malformation of these cells can lead to various congenital defects. In this review, we summarize the current understanding of mesenchymal development in the fetal trachea and lung, focusing on recent findings from animal models that have begun to shed light on the poorly understood respiratory mesenchyme lineages.

T-type Calcium Channel Regulation of Neural Tube Closure and EphrinA/EPHA Expression

A major class of human birth defects arise from aberrations during neural tube closure (NTC). We report on a NTC signaling pathway requiring T-type calcium channels (TTCCs) that is conserved between primitive chordates (Ciona) and Xenopus. With loss of TTCCs, there is a failure to seal the anterior neural folds. Accompanying loss of TTCCs is an upregulation of EphrinA effectors. Ephrin signaling is known to be important in NTC, and ephrins can affect both cell adhesion and repulsion. In Ciona, ephrinA-d expression is downregulated at the end of neurulation, whereas, with loss of TTCC, ephrinA-d remains elevated. Accordingly, overexpression of ephrinA-d phenocopied TTCC loss of function, while overexpression of a dominant-negative Ephrin receptor was able to rescue NTC in a Ciona TTCC mutant. We hypothesize that signaling through TTCCs is necessary for proper anterior NTC through downregulation of ephrins, and possibly elimination of a repulsive signal.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Endodermal Wnt signaling is required for tracheal cartilage formation

Tracheobronchomalacia is a common congenital defect in which the walls of the trachea and bronchi lack of adequate cartilage required for support of the airways. Deletion of Wls, a cargo receptor mediating Wnt ligand secretion, in the embryonic endoderm using ShhCre mice inhibited formation of tracheal-bronchial cartilaginous rings. The normal dorsal-ventral patterning of tracheal mesenchyme was lost. Smooth muscle cells, identified by Acta2 staining, were aberrantly located in ventral mesenchyme of the trachea, normally the region of Sox9 expression in cartilage progenitors. Wnt/β-catenin activity, indicated by Axin2 LacZ reporter, was decreased in tracheal mesenchyme of Wls(f/f);Shh(Cre/+) embryos. Proliferation of chondroblasts was decreased and reciprocally, proliferation of smooth muscle cells was increased in Wls(f/f);Shh(Cre/+) tracheal tissue. Expression of Tbx4, Tbx5, Msx1 and Msx2, known to mediate cartilage and muscle patterning, were decreased in tracheal mesenchyme of Wls(f/f);Shh(Cre/+) embryos. Ex vivo studies demonstrated that Wnt7b and Wnt5a, expressed by the epithelium of developing trachea, and active Wnt/β-catenin signaling are required for tracheal chondrogenesis before formation of mesenchymal condensations. In conclusion, Wnt ligands produced by the tracheal epithelium pattern the tracheal mesenchyme via modulation of gene expression and cell proliferation required for proper tracheal cartilage and smooth muscle differentiation.

NFATc1 Links EGFR Signaling to Induction of Sox9 Transcription and Acinar-Ductal Transdifferentiation in the Pancreas

BACKGROUND & AIMS

Oncogenic mutations in KRAS contribute to the development of pancreatic ductal adenocarcinoma, but are not sufficient to initiate carcinogenesis. Secondary events, such as inflammation-induced signaling via the epidermal growth factor receptor (EGFR) and expression of the SOX9 gene, are required for tumor formation. Herein we sought to identify the mechanisms that link EGFR signaling with activation of SOX9 during acinar-ductal metaplasia, a transdifferentiation process that precedes pancreatic carcinogenesis.

METHODS

We analyzed pancreatic tissues from Kras(G12D);pdx1-Cre and Kras(G12D);NFATc1(Δ/Δ);pdx1-Cre mice after intraperitoneal administration of caerulein, vs cyclosporin A or dimethyl sulfoxide (controls). Induction of EGFR signaling and its effects on the expression of Nuclear factor of activated T cells c1 (NFATc1) or SOX9 were investigated by quantitative reverse-transcription polymerase chain reaction, immunoblot, and immunohistochemical analyses of mouse and human tissues and acinar cell explants. Interactions between NFATc1 and partner proteins and effects on DNA binding or chromatin modifications were studied using co-immunoprecipitation and chromatin immunoprecipitation assays in acinar cell explants and mouse tissue.

RESULTS

EGFR activation induced expression of NFATc1 in metaplastic areas from patients with chronic pancreatitis and in pancreatic tissue from Kras(G12D) mice. EGFR signaling also promoted formation of a complex between NFATc1 and C-JUN in dedifferentiating mouse acinar cells, leading to activation of Sox9 transcription and induction of acinar-ductal metaplasia. Pharmacologic inhibition of NFATc1 or disruption of the Nfatc1 gene inhibited EGFR-mediated induction of Sox9 transcription and blocked acinar-ductal transdifferentiation and pancreatic cancer initiation in mice.

CONCLUSIONS

EGFR signaling induces expression of NFATc1 and Sox9, leading to acinar cell transdifferentiation and initiation of pancreatic cancer. Strategies designed to disrupt this pathway might be developed to prevent pancreatic cancer initiation in high-risk patients with chronic pancreatitis.

Physicochemical and biomechanical stimuli in cell-based articular cartilage repair

Articular cartilage is a unique load-bearing connective tissue with a low intrinsic capacity for repair and regeneration. Its avascularity makes it relatively hypoxic and its unique extracellular matrix is enriched with cations, which increases the interstitial fluid osmolarity. Several physicochemical and biomechanical stimuli are reported to influence chondrocyte metabolism and may be utilized for regenerative medical approaches. In this review article, we summarize the most relevant stimuli and describe how ion channels may contribute to cartilage homeostasis, with special emphasis on intracellular signaling pathways. We specifically focus on the role of calcium signaling as an essential mechanotransduction component and highlight the role of phosphatase signaling in this context.

Ser/Thr-phosphoprotein phosphatases in chondrogenesis: neglected components of a two-player game

Protein phosphorylation plays a determining role in the regulation of chondrogenesis in vitro. While signalling pathways governed by protein kinases including PKA, PKC, and mitogen-activated protein kinases (MAPK) have been mapped in great details, published data relating to the specific role of phosphoprotein phosphatases (PPs) in differentiating chondroprogenitor cells or in mature chondrocytes is relatively sparse. This review discusses the known functions of Ser/Thr-specific PPs in the molecular signalling pathways of chondrogenesis. PPs are clearly equally important as protein kinases to counterbalance the effect of reversible protein phosphorylation. Of the main Ser/Thr PPs, some of the functions of PP1, PP2A and PP2B have been characterised in the context of chondrogenesis. While PP1 and PP2A appear to negatively regulate chondrogenic differentiation and maintenance of chondrocyte phenotype, calcineurin is an important stimulatory mediator during chondrogenesis but becomes inhibitory in mature chondrocytes. Furthermore, PPs are implicated to be mediators during the pathogenesis of osteoarthritis that makes them potential therapeutic targets to be exploited in the close future. Among the many yet unexplored targets of PPs, modulation of plasma membrane ion channel function and participation in mechanotransduction pathways are emerging novel aspects of signalling during chondrogenesis that should be further elucidated. Besides the regulation of cellular ion homeostasis, other potentially significant novel roles for PPs during the regulation of in vitro chondrogenesis are discussed.