Hypoxic Conditions Modulate Chondrogenesis through the Circadian Clock: The Role of Hypoxia-Inducible Factor-1α.

Hypoxia-inducible factor-1 (HIF-1) is a heterodimer transcription factor composed of an alpha and a beta subunit. HIF-1α is a master regulator of cellular response to hypoxia by activating the transcription of genes that facilitate metabolic adaptation to hypoxia. Since chondrocytes in mature articular cartilage reside in a hypoxic environment, HIF-1α plays an important role in chondrogenesis and in the physiological lifecycle of articular cartilage. Accumulating evidence suggests interactions between the HIF pathways and the circadian clock. The circadian clock is an emerging regulator in both developing and mature chondrocytes. However, how circadian rhythm is established during the early steps of cartilage formation and through what signaling pathways it promotes the healthy chondrocyte phenotype is still not entirely known. This narrative review aims to deliver a concise analysis of the existing understanding of the dynamic interplay between HIF-1α and the molecular clock in chondrocytes, in states of both health and disease, while also incorporating creative interpretations. We explore diverse hypotheses regarding the intricate interactions among these pathways and propose relevant therapeutic strategies for cartilage disorders such as osteoarthritis.

Isolation and Culturing of Primary Murine Chondroprogenitor Cells: A Mammalian Model of Chondrogenesis.

Embryonic limb bud-derived micromass cultures are valuable tools for investigating cartilage development, tissue engineering, and therapeutic strategies for cartilage-related disorders. This collection of fine-tuned protocols used in our laboratories outlines step-by-step procedures for the isolation, expansion, and differentiation of primary mouse limb bud cells into chondrogenic micromass cultures. Key aspects covered in these protocols include synchronized fertilization of mice (Basic Protocol 1), tissue dissection, cell isolation, micromass formation, and culture optimization parameters, such as cell density and medium composition (Basic Protocol 2). We describe techniques for characterizing the chondrogenic differentiation process by histological analysis (Basic Protocol 3). The protocols also address common challenges encountered during the process and provide troubleshooting strategies. This fine-tuned comprehensive protocol serves as a valuable resource for scientists working in the fields of developmental biology, cartilage tissue engineering, and regenerative medicine, offering an updated methodology for the study of efficient chondrogenic differentiation and cartilage tissue regeneration. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Synchronized fertilization of mice Basic Protocol 2: Micromass culture of murine embryonic limb bud-derived cells Basic Protocol 3: Qualitative assessment of cartilage matrix production using Alcian blue staining.

Combining biomechanical stimulation and chronobiology: a novel approach for augmented chondrogenesis?

The unique structure and composition of articular cartilage is critical for its physiological function. However, this architecture may get disrupted by degeneration or trauma. Due to the low intrinsic regeneration properties of the tissue, the healing response is generally poor. Low-grade inflammation in patients with osteoarthritis advances cartilage degradation, resulting in pain, immobility, and reduced quality of life. Generating neocartilage using advanced tissue engineering approaches may address these limitations. The biocompatible microenvironment that is suitable for cartilage regeneration may not only rely on cells and scaffolds, but also on the spatial and temporal features of biomechanics. Cell-autonomous biological clocks that generate circadian rhythms in chondrocytes are generally accepted to be indispensable for normal cartilage homeostasis. While the molecular details of the circadian clockwork are increasingly well understood at the cellular level, the mechanisms that enable clock entrainment by biomechanical signals, which are highly relevant in cartilage, are still largely unknown. This narrative review outlines the role of the biomechanical microenvironment to advance cartilage tissue engineering via entraining the molecular circadian clockwork, and highlights how application of this concept may enhance the development and successful translation of biomechanically relevant tissue engineering interventions.

Isolation and Micromass Culturing of Primary Chicken Chondroprogenitor Cells for Cartilage Regeneration.

Much of the skeletal system develops by endochondral ossification, a process that takes place in early fetal life. This makes the early stages of chondrogenesis, i.e., when chondroprogenitor mesenchymal cells differentiate to chondroblasts, challenging to study in vivo. In vitro methods for the study of chondrogenic differentiation have been available for some time. There is currently high interest in developing fine-tuned methodology that would allow chondrogenic cells to rebuild articular cartilage and restore joint functionality. The micromass culture system that relies on embryonic limb bud-derived chondroprogenitor cells is a popular method for the study of the signaling pathways that control the formation and maturation of cartilage. In this protocol, we describe a technique fine-tuned in our laboratory for culturing limb bud-derived mesenchymal cells from early-stage chick embryos in high density (Basic Protocol 1). We also provide a fine-tuned method for high-efficiency transient transfection of cells before plating using electroporation (Basic Protocol 2). In addition, protocols for histochemical detection of cartilage extracellular matrix using dimethyl methylene blue, Alcian blue, and safranin O are also provided (Basic Protocol 3 and Alternate Protocols 1 and 2, respectively). Finally, a step-by-step guide on a cell viability/proliferation assay using MTT reagent is also described (Basic Protocol 4). © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Micromass culture of chick embryonic limb bud-derived cells Basic Protocol 2: Transfection of cells with siRNA constructs using electroporation prior to micromass culturing Basic Protocol 3: Qualitative and quantitative assessment of cartilage matrix production using dimethyl methylene blue staining and image analysis Alternate Protocol 1: Qualitative assessment of cartilage matrix production using Alcian blue staining Alternate Protocol 2: Qualitative assessment of cartilage matrix production using safranin O staining Basic Protocol 4: Measurement of mitochondrial activity with the MTT assay.

Ion channels involved in inflammation and pain in osteoarthritis and related musculoskeletal disorders.

Osteoarthritis (OA) is a currently incurable, chronic, progressive, and debilitating musculoskeletal (MSK) condition. One of its hallmark symptoms is chronic nociceptive and neuropathic pain, which significantly reduces the quality of life of patients with OA. Although research into the pathomechanisms of OA pain is ongoing and several pain pathways are well understood, the true source of OA pain remains unclear. Ion channels and transporters are key mediators of nociceptive pain. In this narrative review article, we summarize the state-of-the-art in relation to the distribution and function of ion channels in all major synovial joint tissues in the context of pain generation. We provide an update on the ion channels likely involved in mediating peripheral and central nociceptive pathways in the nervous system in OA pain, including voltage-gated sodium and potassium channels, members of the transient receptor potential (TRP) channel family, and purinergic receptor complexes. We focus on ion channels and transporters that have the potential to be candidate drug targets for pain management in patients with OA. We propose that ion channels expressed by the cells of constituent tissues of OA-afflicted synovial joints including cartilage, bone, synovium, ligament, and muscle, should be more thoroughly investigated and targeted in the context of OA pain. Based on key findings from recent basic research articles as well as clinical trials, we propose novel directions for the development of future analgesic therapies to improve the quality of life of patients with OA.

The clusterin connectome: Emerging players in chondrocyte biology and putative exploratory biomarkers of osteoarthritis.

Introduction: Clusterin is a moonlighting protein that has many functions. It is a multifunctional holdase chaperone glycoprotein that is present intracellularly and extracellularly in almost all bodily fluids. Clusterin is involved in lipid transport, cell differentiation, regulation of apoptosis, and clearance of cellular debris, and plays a protective role in ensuring cellular survival. However, the possible involvement of clusterin in arthritic disease remains unclear. Given the significant potential of clusterin as a biomarker of osteoarthritis (OA), a more detailed analysis of its complex network in an inflammatory environment, specifically in the context of OA, is required. Based on the molecular network of clusterin, this study aimed to identify interacting partners that could be developed into biomarker panels for OA. Methods: The STRING database and Cytoscape were used to map and visualize the clusterin connectome. The Qiagen Ingenuity Pathway Analysis (IPA) software was used to analyze and study clusterin associated signaling networks in OA. We also analyzed transcription factors known to modulate clusterin expression, which may be altered in OA. Results: The top hits in the clusterin network were intracellular chaperones, aggregate-forming proteins, apoptosis regulators and complement proteins. Using a text-mining approach in Cytoscape, we identified additional interacting partners, including serum proteins, apolipoproteins, and heat shock proteins. Discussion: Based on known interactions with proteins, we predicted potential novel components of the clusterin connectome in OA, including selenoprotein R, semaphorins, and meprins, which may be important for designing new prognostic or diagnostic biomarker panels.

Ca2+-Activated K+ Channels in Progenitor Cells of Musculoskeletal Tissues: A Narrative Review.

Musculoskeletal disorders represent one of the main causes of disability worldwide, and their prevalence is predicted to increase in the coming decades. Stem cell therapy may be a promising option for the treatment of some of the musculoskeletal diseases. Although significant progress has been made in musculoskeletal stem cell research, osteoarthritis, the most-common musculoskeletal disorder, still lacks curative treatment. To fine-tune stem-cell-based therapy, it is necessary to focus on the underlying biological mechanisms. Ion channels and the bioelectric signals they generate control the proliferation, differentiation, and migration of musculoskeletal progenitor cells. Calcium- and voltage-activated potassium (KCa) channels are key players in cell physiology in cells of the musculoskeletal system. This review article focused on the big conductance (BK) KCa channels. The regulatory function of BK channels requires interactions with diverse sets of proteins that have different functions in tissue-resident stem cells. In this narrative review article, we discuss the main ion channels of musculoskeletal stem cells, with a focus on calcium-dependent potassium channels, especially on the large conductance BK channel. We review their expression and function in progenitor cell proliferation, differentiation, and migration and highlight gaps in current knowledge on their involvement in musculoskeletal diseases.

The temporal transcriptomic signature of cartilage formation.

Chondrogenesis is a multistep process, in which cartilage progenitor cells generate a tissue with distinct structural and functional properties. Although several approaches to cartilage regeneration rely on the differentiation of implanted progenitor cells, the temporal transcriptomic landscape of in vitro chondrogenesis in different models has not been reported. Using RNA sequencing, we examined differences in gene expression patterns during cartilage formation in micromass cultures of embryonic limb bud-derived progenitors. Principal component and trajectory analyses revealed a progressively different and distinct transcriptome during chondrogenesis. Differentially expressed genes (DEGs), based on pairwise comparisons of samples from consecutive days were classified into clusters and analysed. We confirmed the involvement of the top DEGs in chondrogenic differentiation using pathway analysis and identified several chondrogenesis-associated transcription factors and collagen subtypes that were not previously linked to cartilage formation. Transient gene silencing of ATOH8 or EBF1 on day 0 attenuated chondrogenesis by deregulating the expression of key osteochondrogenic marker genes in micromass cultures. These results provide detailed insight into the molecular mechanism of chondrogenesis in primary micromass cultures and present a comprehensive dataset of the temporal transcriptomic landscape of chondrogenesis, which may serve as a platform for new molecular approaches in cartilage tissue engineering.

Neuronal P2X4 receptor may contribute to peripheral inflammatory pain in rat spinal dorsal horn.

Objective: Intense inflammation may result in pain, which manifests as spinal central sensitization. There is growing evidence that purinergic signaling plays a pivotal role in the orchestration of pain processing. Over the last decade the ionotropic P2X purino receptor 4 (P2X4) got into spotlight in neuropathic disorders, however its precise spinal expression was scantily characterized during inflammatory pain. Thus, we intended to analyze the receptor distribution within spinal dorsal horn and lumbar dorsal root ganglia (DRG) of rats suffering in inflammatory pain induced by complete Freund adjuvant (CFA). Methods: CFA-induced peripheral inflammation was validated by mechanical and thermal behavioral tests. In order to ensure about the putative alteration of spinal P2X4 receptor gene expression qPCR reactions were designed, followed by immunoperoxidase and Western blot experiments to assess changes at a protein level. Colocalization of P2X4 with neuronal and glial markers was investigated by double immunofluorescent labelings, which were subsequently analyzed with IMARIS software. Transmission electronmicroscopy was applied to study the ultrastructural localization of the receptor. Concurrently, in lumbar DRG cells similar methodology has been carried out to complete our observations. Results: The figures of mechanical and thermal behavioral tests proved the establishment of CFA-induced inflammatory pain. We observed significant enhancement of P2X4 transcript level within the spinal dorsal horn 3 days upon CFA administration. Elevation of P2X4 immunoreactivity within Rexed lamina I-II of the spinal gray matter was synchronous with mRNA expression, and confirmed by protein blotting. According to IMARIS analysis the robust protein increase was mainly detected on primary afferent axonterminals and GFAP-labelled astrocyte membrane compartments, but not on postsynaptic dendrites was also validated ultrastructurally within the spinal dorsal horn. Furthermore, lumbar DRG analysis demonstrated that peptidergic and non-peptidergic nociceptive subsets of ganglia cells were also abundantly positive for P2X4 receptor in CFA model. Conclusion: Here we provide novel evidence about involvement of neuronal and glial P2X4 receptor in the establishment of inflammatory pain.

Isolation of High-Quality Total RNA from Small Animal Articular Cartilage for Next-Generation Sequencing.

Articular cartilage is characterized by a low density of chondrocytes surrounded by an abundant extracellular matrix (ECM) consisting of a dense mixture of collagens, proteoglycans, and glycosaminoglycans. Due to its low cellularity and high proteoglycan content, it is particularly challenging to extract high-quality total RNA suitable for sensitive high-throughput downstream applications such as RNA sequencing (RNA-Seq). Available protocols for high-quality RNA isolation from articular chondrocytes are inconsistent, resulting in suboptimal yield and compromised quality. This poses a significant difficulty in the application of RNA-Seq to study the cartilage transcriptome. Current protocols rely either on dissociation of cartilage ECM by collagenase digestion or pulverizing cartilage using various methods prior to RNA extraction. However, protocols for cartilage processing vary significantly depending on the species and source of cartilage within the body. Protocols for isolating RNA from human or large mammal (e.g., horse or cattle) cartilage samples are available, but this is not the case for chicken cartilage, despite the species being extensively used in cartilage research. Here, we present two improved RNA isolation protocols based on pulverization of fresh articular cartilage using a cryogenic mill or on enzymatic digestion using 1.2% (w/v) collagenase II. Our protocols optimize the collection and tissue processing steps to minimize RNA degradation and enhance RNA purity. Our results show that RNA purified from chicken articular cartilage using these methods has appropriate quality for RNA-Seq experiments. The procedure is applicable for RNA extraction from cartilage from other species such as dog, cat, sheep, and goat. The workflow for RNA-Seq analysis is also described here. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Extraction of total RNA from pulverized chicken articular cartilage Alternate Protocol: Extraction of total RNA from collagen-digested articular cartilage Support Protocol: Dissection of chicken articular cartilage from the knee joint Basic Protocol 2: RNA sequencing of total RNA from chicken articular cartilage.

Cyclic uniaxial mechanical load enhances chondrogenesis through entraining the molecular circadian clock.

The biomechanical environment plays a key role in regulating cartilage formation, but the current understanding of mechanotransduction pathways in chondrogenic cells is incomplete. Among the combination of external factors that control chondrogenesis are temporal cues that are governed by the cell-autonomous circadian clock. However, mechanical stimulation has not yet directly been proven to modulate chondrogenesis via entraining the circadian clock in chondroprogenitor cells. The purpose of this study was to establish whether mechanical stimuli entrain the core clock in chondrogenic cells, and whether augmented chondrogenesis caused by mechanical loading was at least partially mediated by the synchronised, rhythmic expression of the core circadian clock genes, chondrogenic transcription factors, and cartilage matrix constituents at both transcript and protein levels. We report here, for the first time, that cyclic uniaxial mechanical load applied for 1 h for a period of 6 days entrains the molecular clockwork in chondroprogenitor cells during chondrogenesis in limb bud-derived micromass cultures. In addition to the several core clock genes and proteins, the chondrogenic markers SOX9 and ACAN also followed a robust sinusoidal rhythmic expression pattern. These rhythmic conditions significantly enhanced cartilage matrix production and upregulated marker gene expression. The observed chondrogenesis-promoting effect of the mechanical environment was at least partially attributable to its entraining effect on the molecular clockwork, as co-application of the small molecule clock modulator longdaysin attenuated the stimulatory effects of mechanical load. This study suggests that an optimal biomechanical environment enhances tissue homoeostasis and histogenesis during chondrogenesis at least partially through entraining the molecular clockwork.

Analysis of Gene Expression Patterns of Epigenetic Enzymes Dnmt3a, Tet1 and Ogt in Murine Chondrogenic Models.

We investigated the gene expression pattern of selected enzymes involved in DNA methylation and the effects of the DNA methylation inhibitor 5-azacytidine during in vitro and in vivo cartilage formation. Based on the data of a PCR array performed on chondrifying BMP2-overexpressing C3H10T1/2 cells, the relative expressions of Tet1 (tet methylcytosine dioxygenase 1), Dnmt3a (DNA methyltransferase 3), and Ogt (O-linked N-acetylglucosamine transferase) were further examined with RT-qPCR in murine cell line-based and primary chondrifying micromass cultures. We found very strong but gradually decreasing expression of Tet1 throughout the entire course of in vitro cartilage differentiation along with strong signals in the cartilaginous embryonic skeleton using specific RNA probes for in situ hybridization on frozen sections of 15-day-old mouse embryos. Dnmt3a and Ogt expressions did not show significant changes with RT-qPCR and gave weak in situ hybridization signals. The DNA methylation inhibitor 5-azacytidine reduced cartilage-specific gene expression and cartilage formation when applied during the early stages of chondrogenesis. In contrast, it had a stimulatory effect when added to differentiated chondrocytes, and quantitative methylation-specific PCR proved that the DNA methylation pattern of key chondrogenic marker genes was altered by the treatment. Our results indicate that the DNA demethylation inducing Tet1 plays a significant role during chondrogenesis, and inhibition of DNA methylation exerts distinct effects in different phases of in vitro cartilage formation.

Pituitary Adenylate Cyclase Activating Polypeptide Has Inhibitory Effects on Melanoma Cell Proliferation and Migration In Vitro.

Pituitary adenylate cyclase activating polypeptide (PACAP) is an endogenous neuropeptide which is distributed throughout the body. PACAP influences development of various tissues and exerts protective function during cellular stress and in some tumour formation. No evidence is available on its role in neural crest derived melanocytes and its malignant transformation into melanoma. Expression of PACAP receptors was examined in human skin samples, melanoma lesions and in a primary melanocyte cell culture. A2058 and WM35 melanoma cell lines, representing two different stages of melanoma progression, were used to investigate the effects of PACAP. PAC1 receptor was identified in melanocytes in vivo and in vitro and in melanoma cell lines as well as in melanoma lesions. PACAP administration did not alter viability but decreased proliferation of melanoma cells. With live imaging random motility, average speed, vectorial distance and maximum distance of migration of cells were reduced upon PACAP treatment. PACAP administration did not alter viability but decreased proliferation capacity of melanoma cells. On the other hand, PACAP administration decreased the migration of melanoma cell lines towards fibronectin chemoattractant in the Boyden chamber. Furthermore, the presence of the neuropeptide inhibited the invasion capability of melanoma cell lines in Matrigel chambers. In summary, we provide evidence that PACAP receptors are expressed in melanocytes and in melanoma cells. Our results also prove that various aspects of the cellular motility were inhibited by this neuropeptide. On the basis of these results, we propose PACAP signalling as a possible target in melanoma progression.

A Synchronized Circadian Clock Enhances Early Chondrogenesis.

OBJECTIVE: Circadian rhythms in cartilage homeostasis are hypothesized to temporally segregate and synchronize the activities of chondrocytes to different times of the day, and thus may provide an efficient mechanism by which articular cartilage can recover following physical activity. While the circadian clock is clearly involved in chondrocyte homeostasis in health and disease, it is unclear as to what roles it may play during early chondrogenesis. DESIGN: The purpose of this study was to determine whether the rhythmic expression of the core circadian clock was detectable at the earliest stages of chondrocyte differentiation, and if so, whether a synchronized expression pattern of chondrogenic transcription factors and developing cartilage matrix constituents was present during cartilage formation. RESULTS: Following serum shock, embryonic limb bud-derived chondrifying micromass cultures exhibited synchronized temporal expression patterns of core clock genes involved in the molecular circadian clock. We also observed that chondrogenic marker genes followed a circadian oscillatory pattern. Clock synchronization significantly enhanced cartilage matrix production and elevated SOX9, ACAN, and COL2A1 gene expression. The observed chondrogenesis-promoting effect of the serum shock was likely attributable to its synchronizing effect on the molecular clockwork, as co-application of small molecule modulators (longdaysin and KL001) abolished the stimulating effects on extracellular matrix production and chondrogenic marker gene expression. CONCLUSIONS: Results from this study suggest that a functional molecular clockwork plays a positive role in tissue homeostasis and histogenesis during early chondrogenesis.

N-methyl-D-aspartate (NMDA) receptor expression and function is required for early chondrogenesis.

BACKGROUND: In vitro chondrogenesis depends on the concerted action of numerous signalling pathways, many of which are sensitive to the changes of intracellular Ca2+ concentration. N-methyl-D-aspartate (NMDA) glutamate receptor is a cation channel with high permeability for Ca2+. Whilst there is now accumulating evidence for the expression and function of NMDA receptors in non-neural tissues including mature cartilage and bone, the contribution of glutamate signalling to the regulation of chondrogenesis is yet to be elucidated. METHODS: We studied the role of glutamatergic signalling during the course of in vitro chondrogenesis in high density chondrifying cell cultures using single cell fluorescent calcium imaging, patch clamp, transient gene silencing, and western blotting. RESULTS: Here we show that key components of the glutamatergic signalling pathways are functional during in vitro chondrogenesis in a primary chicken chondrogenic model system. We also present the full glutamate receptor subunit mRNA and protein expression profile of these cultures. This is the first study to report that NMDA-mediated signalling may act as a key factor in embryonic limb bud-derived chondrogenic cultures as it evokes intracellular Ca2+ transients, which are abolished by the GluN2B subunit-specific inhibitor ifenprodil. The function of NMDARs is essential for chondrogenesis as their functional knock-down using either ifenprodil or GRIN1 siRNA temporarily blocks the differentiation of chondroprogenitor cells. Cartilage formation was fully restored with the re-expression of the GluN1 protein. CONCLUSIONS: We propose a key role for NMDARs during the transition of chondroprogenitor cells to cartilage matrix-producing chondroblasts.

Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) Pathway Is Induced by Mechanical Load and Reduces the Activity of Hedgehog Signaling in Chondrogenic Micromass Cell Cultures.

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neurohormone exerting protective function during various stress conditions either in mature or developing tissues. Previously we proved the presence of PACAP signaling elements in chicken limb bud-derived chondrogenic cells in micromass cell cultures. Since no data can be found if PACAP signaling is playing any role during mechanical stress in any tissues, we aimed to investigate its contribution in mechanotransduction during chondrogenesis. Expressions of the mRNAs of PACAP and its major receptor, PAC1 increased, while that of other receptors, VPAC1, VPAC2 decreased upon mechanical stimulus. Mechanical load enhanced the expression of collagen type X, a marker of hypertrophic differentiation of chondrocytes and PACAP addition attenuated this elevation. Moreover, exogenous PACAP also prevented the mechanical load evoked activation of hedgehog signaling: protein levels of Sonic and Indian Hedgehogs and Gli1 transcription factor were lowered while expressions of Gli2 and Gli3 were elevated by PACAP application during mechanical load. Our results suggest that mechanical load activates PACAP signaling and exogenous PACAP acts against the hypertrophy inducing effect of mechanical load.

Purinergic signalling is required for calcium oscillations in migratory chondrogenic progenitor cells.

Osteoarthritis (OA) is the most common form of chronic musculoskeletal disorders. A migratory stem cell population termed chondrogenic progenitor cells (CPC) with in vitro chondrogenic potential was previously isolated from OA cartilage. Since intracellular Ca(2+) signalling is an important regulator of chondrogenesis, we aimed to provide a detailed understanding of the Ca(2+) homeostasis of CPCs. In this work, CPCs immortalised by lentiviral administration of the human telomerase reverse transcriptase (hTERT) and grown in monolayer cultures were studied. Expressions of all three IP3Rs were confirmed, but no RyR subtypes were detected. Ca(2+) oscillations observed in CPCs were predominantly dependent on Ca(2+) release and store replenishment via store-operated Ca(2+) entry; CPCs express both STIM1 and Orai1 proteins. Expressions of adenosine receptor mRNAs were verified, and adenosine elicited Ca(2+) transients. Various P2 receptor subtypes were identified; P2Y1 can bind ADP; P2Y4 is targeted by UTP; and ATP may evoke Ca(2+) transients via detected P2X subtypes, as well as P2Y1 and P2Y2. Enzymatic breakdown of extracellular nucleotides by apyrase completely abrogated Ca(2+) oscillations, suggesting that an autocrine/paracrine purinergic mechanism may drive Ca(2+) oscillations in these cells. As CPCs possess a broad spectrum of functional molecular elements of Ca(2+) signalling, Ca(2+)-dependent regulatory mechanisms can be supposed to influence their differentiation potential.

Pituitary adenylate cyclase-activating polypeptide (PACAP) signalling enhances osteogenesis in UMR-106 cell line.

Presence of the pituitary adenylate cyclase-activating polypeptide (PACAP) signalling has been proved in various peripheral tissues. PACAP can activate protein kinase A (PKA) signalling via binding to pituitary adenylate cyclase-activating polypeptide type I receptor (PAC1), vasoactive intestinal polypeptide receptor (VPAC) 1 or VPAC2 receptor. Since little is known about the role of this regulatory mechanism in bone formation, we aimed to investigate the effect of PACAP on osteogenesis of UMR-106 cells. PACAP 1-38 as an agonist and PACAP 6-38 as an antagonist of PAC1 were added to the culture medium. Surprisingly, both substances enhanced protein expressions of collagen type I, osterix and alkaline phosphatase, along with higher cell proliferation rate and an augmented mineralisation. Although expression of PKA was elevated, no alterations were detected in the expression, phosphorylation and nuclear presence of CREB, but increased nuclear appearance of Runx2, the key transcription factor of osteoblast differentiation, was shown. Both PACAPs increased the expressions of bone morphogenetic proteins (BMPs) 2, 4, 6, 7 and Smad1 proteins, as well as that of Sonic hedgehog, PATCH1 and Gli1. Data of our experiments indicate that activation of PACAP pathway enhances bone formation of UMR-106 cells and PKA, BMP and Hedgehog signalling pathways became activated. We also found that PACAP 6-38 did not act as an antagonist of PACAP signalling in UMR-106 cells.

Pituitary adenylate cyclase activating polypeptide (PACAP) signalling exerts chondrogenesis promoting and protecting effects: implication of calcineurin as a downstream target.

Pituitary adenylate cyclase activating polypeptide (PACAP) is an important neurotrophic factor influencing differentiation of neuronal elements and exerting protecting role during traumatic injuries or inflammatory processes of the central nervous system. Although increasing evidence is available on its presence and protecting function in various peripheral tissues, little is known about the role of PACAP in formation of skeletal components. To this end, we aimed to map elements of PACAP signalling in developing cartilage under physiological conditions and during oxidative stress. mRNAs of PACAP and its receptors (PAC1,VPAC1, VPAC2) were detectable during differentiation of chicken limb bud-derived chondrogenic cells in micromass cell cultures. Expression of PAC1 protein showed a peak on days of final commitment of chondrogenic cells. Administration of either the PAC1 receptor agonist PACAP 1-38, or PACAP 6-38 that is generally used as a PAC1 antagonist, augmented cartilage formation, stimulated cell proliferation and enhanced PAC1 and Sox9 protein expression. Both variants of PACAP elevated the protein expression and activity of the Ca-calmodulin dependent Ser/Thr protein phosphatase calcineurin. Application of PACAPs failed to rescue cartilage formation when the activity of calcineurin was pharmacologically inhibited with cyclosporine A. Moreover, exogenous PACAPs prevented diminishing of cartilage formation and decrease of calcineurin activity during oxidative stress. As an unexpected phenomenon, PACAP 6-38 elicited similar effects to those of PACAP 1-38, although to a different extent. On the basis of the above results, we propose calcineurin as a downstream target of PACAP signalling in differentiating chondrocytes either in normal or pathophysiological conditions. Our observations imply the therapeutical perspective that PACAP can be applied as a natural agent that may have protecting effect during joint inflammation and/or may promote cartilage regeneration during degenerative diseases of articular cartilage.

Mechanical loading stimulates chondrogenesis via the PKA/CREB-Sox9 and PP2A pathways in chicken micromass cultures.

Biomechanical stimuli play important roles in the formation of articular cartilage during early foetal life, and optimal mechanical load is a crucial regulatory factor of adult chondrocyte metabolism and function. In this study, we undertook to analyse mechanotransduction pathways during in vitro chondrogenesis. Chondroprogenitor cells isolated from limb buds of 4-day-old chicken embryos were cultivated as high density cell cultures for 6 days. Mechanical stimulation was carried out by a self-designed bioreactor that exerted uniaxial intermittent cyclic load transmitted by the culture medium as hydrostatic pressure and fluid shear to differentiating cells. The loading scheme (0.05 Hz, 600 Pa; for 30 min) was applied on culturing days 2 and 3, when final commitment and differentiation of chondroprogenitor cells occurred in this model. The applied mechanical load significantly augmented cartilage matrix production and elevated mRNA expression of several cartilage matrix constituents, including collagen type II and aggrecan core protein, as well as matrix-producing hyaluronan synthases through enhanced expression, phosphorylation and nuclear signals of the main chondrogenic transcription factor Sox9. Along with increased cAMP levels, a significantly enhanced protein kinase A (PKA) activity was also detected and CREB, the archetypal downstream transcription factor of PKA signalling, exhibited elevated phosphorylation levels and stronger nuclear signals in response to mechanical stimuli. All the above effects were diminished by the PKA-inhibitor H89. Inhibition of the PKA-independent cAMP-mediators Epac1 and Epac2 with HJC0197 resulted in enhanced cartilage formation, which was additive to that of the mechanical stimulation, implying that the chondrogenesis-promoting effect of mechanical load was independent of Epac. At the same time, PP2A activity was reduced following mechanical load and treatments with the PP2A-inhibitor okadaic acid were able to mimic the effects of the intervention. Our results indicate that proper mechanical stimuli augment in vitro cartilage formation via promoting both differentiation and matrix production of chondrogenic cells, and the opposing regulation of the PKA/CREB-Sox9 and the PP2A signalling pathways is crucial in this phenomenon.