The ciliary protein IFT88 controls post-natal cartilage thickness and influences development of osteoarthritis

Critical signaling molecules in the temporomandibular joint osteoarthritis under different magnitudes of mechanical stimulation

The mechanical stress environment in the temporomandibular joint (TMJ) is constantly changing due to daily mandibular movements. Therefore, TMJ tissues, such as condylar cartilage, the synovial membrane and discs, are influenced by different magnitudes of mechanical stimulation. Moderate mechanical stimulation is beneficial for maintaining homeostasis, whereas abnormal mechanical stimulation leads to degeneration and ultimately contributes to the development of temporomandibular joint osteoarthritis (TMJOA), which involves changes in critical signaling molecules. Under abnormal mechanical stimulation, compensatory molecules may prevent degenerative changes while decompensatory molecules aggravate. In this review, we summarize the critical signaling molecules that are stimulated by moderate or abnormal mechanical loading in TMJ tissues, mainly in condylar cartilage. Furthermore, we classify abnormal mechanical stimulation-induced molecules into compensatory or decompensatory molecules. Our aim is to understand the pathophysiological mechanism of TMJ dysfunction more deeply in the ever-changing mechanical environment, and then provide new ideas for discovering effective diagnostic and therapeutic targets in TMJOA.

Skeletal ciliopathy: pathogenesis and related signaling pathways

Cilia are tiny organelles with conserved structures and components in eukaryotic cells. Ciliopathy is a set of diseases resulting from cilium dysfunction classified into first-order and second-order ciliopathy. With the advancement of clinical diagnosis and radiography, numerous skeletal phenotypes, including polydactyly, short limbs, short ribs, scoliosis, a narrow thorax, and numerous anomalies in bone and cartilage, have been discovered in ciliopathies. Mutation in genes encoding cilia core components or other cilia-related molecules have been found in skeletal ciliopathies. Meanwhile, various signaling pathways associated with cilia and skeleton development have been deemed to be significant for the occurrence and progression of diseases. Herein, we review the structure and key components of the cilium and summarize several skeletal ciliopathies with their presumable pathology. We also emphasize the signaling pathways involved in skeletal ciliopathies, which may assist in developing potential therapies for these diseases.

Cilia loss on distinct neuron populations differentially alters cocaine-induced locomotion and reward

BACKGROUND

Neuronal primary cilia are being recognized for their role in mediating signaling associated with a variety of neurobehaviors, including responses to drugs of abuse. They function as signaling hubs, enriched with a diverse array of G-protein coupled receptors (GPCRs), including several associated with motivation and drug-related behaviors. However, our understanding of how cilia regulate neuronal function and behavior is still limited.

AIMS

The objective of the current study was to investigate the contributions of primary cilia on specific neuronal populations to behavioral responses to cocaine.

METHODS

To test the consequences of cilia loss on cocaine-induced locomotion and reward-related behavior, we selectively ablated cilia from dopaminergic or GAD2-GABAergic neurons in mice.

RESULTS

Cilia ablation on either population of neurons failed to significantly alter acute locomotor responses to cocaine at a range of doses. With repeated administration, mice lacking cilia on GAD2-GABAergic neurons showed no difference in locomotor sensitization to cocaine compared to wild-type (WT) littermates, whereas mice lacking cilia on dopaminergic neurons exhibited reduced locomotor sensitization to cocaine at 10 and 30 mg/kg. Mice lacking cilia on GAD2-GABAergic neurons showed no difference in cocaine conditioned place preference (CPP), whereas mice lacking cilia on dopaminergic neurons exhibited reduced CPP compared to WT littermates.

CONCLUSIONS

Combined with previous findings using amphetamine, our results show that behavioral effects of cilia ablation are cell- and drug type-specific, and that neuronal cilia contribute to modulation of both the locomotor-inducing and rewarding properties of cocaine.

Double-edged role of mechanical stimuli and underlying mechanisms in cartilage tissue engineering

Mechanical stimuli regulate the chondrogenic differentiation of mesenchymal stem cells and the homeostasis of chondrocytes, thus affecting implant success in cartilage tissue engineering. The mechanical microenvironment plays fundamental roles in the maturation and maintenance of natural articular cartilage, and the progression of osteoarthritis Hence, cartilage tissue engineering attempts to mimic this environment in vivo to obtain implants that enable a superior regeneration process. However, the specific type of mechanical loading, its optimal regime, and the underlying molecular mechanisms are still under investigation. First, this review delineates the composition and structure of articular cartilage, indicating that the morphology of chondrocytes and components of the extracellular matrix differ from each other to resist forces in three top-to-bottom overlapping zones. Moreover, results from research experiments and clinical trials focusing on the effect of compression, fluid shear stress, hydrostatic pressure, and osmotic pressure are presented and critically evaluated. As a key direction, the latest advances in mechanisms involved in the transduction of external mechanical signals into biological signals are discussed. These mechanical signals are sensed by receptors in the cell membrane, such as primary cilia, integrins, and ion channels, which next activate downstream pathways. Finally, biomaterials with various modifications to mimic the mechanical properties of natural cartilage and the self-designed bioreactors for experiment in vitro are outlined. An improved understanding of biomechanically driven cartilage tissue engineering and the underlying mechanisms is expected to lead to efficient articular cartilage repair for cartilage degeneration and disease.

Primary cilium-mediated signaling cascade suppresses age-related biliary fibrosis

The primary cilium is increasingly recognized as a crucial player in the physiology of biliary epithelial cells (BECs). However, the precise role of primary cilia in the development of age-related biliary fibrosis remains unclear. Herein, using cilium-deficient mice, we demonstrate that disruption of ciliary homeostasis in BECs in aged mice leads to significant bile duct proliferation, augmented biliary fibrosis, and heightened indicators of liver injury. Our RNA-sequencing data revealed a dysregulation in genes associated with various biological processes such as bile secretion, fatty acid metabolism, and inflammation. Loss of primary cilia also significantly enhanced signaling pathways driving the development of biliary fibrosis. Our findings collectively suggest that loss of primary cilia in the BECs of aged mice initiates a cascade of signaling events that contribute to biliary fibrosis, highlighting the primary cilium as a potential therapeutic target in the treatment of fibrosing cholangiopathies.

Adseverin, an actin-binding protein, modulates hypertrophic chondrocyte differentiation and osteoarthritis progression

In osteoarthritis (OA), a disease characterized by progressive articular cartilage degradation and calcification, the articular chondrocyte phenotype changes and this correlates with actin cytoskeleton alterations suggesting that it regulates gene expression essential for proper phenotype. This study reports that OA is associated with the loss of adseverin, an actin capping and severing protein. Adseverin deletion (Adseverin-/-) in mice compromised articular chondrocyte function, by reducing F-actin and aggrecan expression and increasing apoptosis, Indian hedgehog, Runx2, MMP13, and collagen type X expression, and cell proliferation. This led to stiffer cartilage and decreased hyaline and increased calcified cartilage thickness. Together, these changes predisposed the articular cartilage to enhanced OA severity in Adseverin-/- mice who underwent surgical induction of OA. Adseverin-/- chondrocyte RNA sequencing and in vitro studies together suggests that adseverin modulates cell viability and prevents mineralization. Thus, adseverin maintains articular chondrocyte phenotype and cartilage tissue homeostasis by preventing progression to hypertrophic differentiation in vivo. Adseverin may be chondroprotective and a potential therapeutic target.

The factory, the antenna and the scaffold: the three-way interplay between the Golgi, cilium and extracellular matrix underlying tissue function

The growth and development of healthy tissues is dependent on the construction of a highly specialised extracellular matrix (ECM) to provide support for cell growth and migration and to determine the biomechanical properties of the tissue. These scaffolds are composed of extensively glycosylated proteins which are secreted and assembled into well-ordered structures that can hydrate, mineralise, and store growth factors as required. The proteolytic processing and glycosylation of ECM components is vital to their function. These modifications are under the control of the Golgi apparatus, an intracellular factory hosting spatially organised, protein-modifying enzymes. Regulation also requires a cellular antenna, the cilium, which integrates extracellular growth signals and mechanical cues to inform ECM production. Consequently, mutations in either Golgi or ciliary genes frequently lead to connective tissue disorders. The individual importance of each of these organelles to ECM function is well-studied. However, emerging evidence points towards a more tightly linked system of interdependence between the Golgi, cilium and ECM. This review examines how the interplay between all three compartments underpins healthy tissue. As an example, it will look at several members of the golgin family of Golgi-resident proteins whose loss is detrimental to connective tissue function. This perspective will be important for many future studies looking to dissect the cause and effect of mutations impacting tissue integrity.

Osteoarthritis year in review 2022: biology

The field of osteoarthritis (OA) biology is rapidly evolving and brilliant progress has been made this year as well. Landmark studies of OA biology published in 2021 and early 2022 were selected through PubMed search by personal opinion. These papers were classified by their molecular mechanisms, and it was largely divided into the intracellular signaling mechanisms and the inter-compartment interaction in chondrocyte homeostasis and OA progression. The intracellular signaling mechanisms involving OA progression included (1) Piezo1/transient receptor potential channels of the vanilloid subtype (TRPV) 4-mediated calcium signaling, (2) mechanical load-F-box and WD repeat domain containing 7 (FBXW7) in chondrocyte senescence, (3) mechanical loading-primary cilia-hedgehog signaling, (4) low grade inflammation by toll-like receptor (TLR)-CD14-lipopolysaccharide-binding protein (LBP) complex and inhibitor of NF-κB kinase (IKK) β-nuclear factor kappa B (NF-κB) signaling, (5) selenium pathway and reactive oxygen species (ROS) production, (6) G protein-coupled receptor (GPCR) and cyclic adenosine monophosphate (cAMP) signaling, (7) peroxisome proliferator-activated receptor α (PPARα)-acyl-CoA thioesterase 12 (ACOT12)-mediated de novo lipogenesis and (8) hypoxia-disruptor of telomeric silencing 1-like (DOT1L)-H3-lysine 79 (H3K79) methylation pathway. The studies on inter-compartment or intercellular interaction in OA progression included the following subjects; (1) the anabolic role of lubricin, glycoprotein from superficial zone cells, (2) osteoclast-chondrocyte interaction via exosomal miRNA and sphingosine 1-phosphate (S1P), (3) senescent fibroblast-like synoviocyte and chondrocyte interaction, (4) synovial macrophage and chondrocyte interaction through Flightless I, (5) αV integrin-mediated transforming growth factor beta (TGFβ) activation by mechanical loading, and (6) osteocytic TGFβ in subchondral bone thickening. Despite the disastrous Covid-19 pandemic, many outstanding studies have expanded the boundary of OA biology. They provide both critical insight into the pathophysiology as well as clues for the treatment of OA.

Roles of the calcified cartilage layer and its tissue engineering reconstruction in osteoarthritis treatment

Sandwiched between articular cartilage and subchondral bone, the calcified cartilage layer (CCL) takes on both biomechanical and biochemical functions in joint development and ordinary activities. The formation of CCL is not only unique in articular cartilage but can also be found in the chondro-osseous junction adjacent to the growth plate during adolescence. The formation of CCL is an active process under both cellular regulation and intercellular communication. Abnormal alterations of CCL can be indications of degenerative diseases including osteoarthritis. Owing to the limited self-repair capability of articular cartilage and core status of CCL in microenvironment maintenance, tissue engineering reconstruction of CCL in damaged cartilage can be of great significance. This review focuses on possible tissue engineering reconstruction methods targeting CCL for further OA treatment.

Role of Primary Cilia in Skeletal Disorders

Primary cilia are highly conserved microtubule-based organelles that project from the cell surface into the extracellular environment and play important roles in mechanosensation, mechanotransduction, polarity maintenance, and cell behaviors during organ development and pathological changes. Intraflagellar transport (IFT) proteins are essential for cilium formation and function. The skeletal system consists of bones and connective tissue, including cartilage, tendons, and ligaments, providing support, stability, and movement to the body. Great progress has been achieved in primary cilia and skeletal disorders in recent decades. Increasing evidence suggests that cells with cilium defects in the skeletal system can cause numerous human diseases. Moreover, specific deletion of ciliary proteins in skeletal tissues with different Cre mice resulted in diverse malformations, suggesting that primary cilia are involved in the development of skeletal diseases. In addition, the intact of primary cilium is essential to osteogenic/chondrogenic induction of mesenchymal stem cells, regarded as a promising target for clinical intervention for skeletal disorders. In this review, we summarized the role of primary cilia and ciliary proteins in the pathogenesis of skeletal diseases, including osteoporosis, bone/cartilage tumor, osteoarthritis, intervertebral disc degeneration, spine scoliosis, and other cilium-related skeletal diseases, and highlighted their promising treatment methods, including using mesenchymal stem cells. Our review tries to present evidence for primary cilium as a promising target for clinical intervention for skeletal diseases.

Primary Cilia Direct Murine Articular Cartilage Tidemark Patterning Through Hedgehog Signaling and Ambulatory Load

Articular cartilage (AC) is essential for body movement but is highly susceptible to degenerative diseases and has poor self-repair capacity. To improve current subpar regenerative treatments, developmental mechanisms of AC should be clarified and, specifically, how its postnatal multizone organization is acquired. Primary cilia are cell surface organelles crucial for mammalian tissue morphogenesis. Although their importance for chondrocyte function is appreciated, their specific roles in postnatal AC morphogenesis remain unclear. To explore these mechanisms, we used a murine conditional loss-of-function approach (Ift88-flox) targeting joint-lineage progenitors (Gdf5Cre) and monitored postnatal knee AC development. Joint formation and growth up to juvenile stages were largely unaffected. However, mature AC (aged 2 months) exhibited disorganized extracellular matrix, decreased aggrecan and collagen II due to reduced gene expression (not increased catabolism), and marked reduction of AC modulus by 30%-50%. In addition, and unexpectedly, we discovered that tidemark patterning was severely disrupted, as was hedgehog signaling, and exhibited specificity based on regional load-bearing functions of AC. Interestingly, Prg4 expression was markedly increased in highly loaded sites in mutants. Together, our data provide evidence that primary cilia orchestrate postnatal AC morphogenesis including tidemark topography, zonal matrix composition, and ambulation load responses. © 2022 American Society for Bone and Mineral Research (ASBMR).

Ciliary IFT88 Protects Coordinated Adolescent Growth Plate Ossification From Disruptive Physiological Mechanical Forces

Compared with our understanding of endochondral ossification, much less is known about the coordinated arrest of growth defined by the narrowing and fusion of the cartilaginous growth plate. Throughout the musculoskeletal system, appropriate cell and tissue responses to mechanical force delineate morphogenesis and ensure lifelong health. It remains unclear how mechanical cues are integrated into many biological programs, including those coordinating the ossification of the adolescent growth plate at the cessation of growth. Primary cilia are microtubule-based organelles tuning a range of cell activities, including signaling cascades activated or modulated by extracellular biophysical cues. Cilia have been proposed to directly facilitate cell mechanotransduction. To explore the influence of primary cilia in the mouse adolescent limb, we conditionally targeted the ciliary gene Intraflagellar transport protein 88 (Ift88^fl/fl ) in the juvenile and adolescent skeleton using a cartilage-specific, inducible Cre (AggrecanCreER^T2 Ift88^fl/fl ). Deletion of IFT88 in cartilage, which reduced ciliation in the growth plate, disrupted chondrocyte differentiation, cartilage resorption, and mineralization. These effects were largely restricted to peripheral tibial regions beneath the load-bearing compartments of the knee. These regions were typified by an enlarged population of hypertrophic chondrocytes. Although normal patterns of hedgehog signaling were maintained, targeting IFT88 inhibited hypertrophic chondrocyte VEGF expression and downstream vascular recruitment, osteoclastic activity, and the replacement of cartilage with bone. In control mice, increases to physiological loading also impair ossification in the peripheral growth plate, mimicking the effects of IFT88 deletion. Limb immobilization inhibited changes to VEGF expression and epiphyseal morphology in Ift88cKO mice, indicating the effects of depletion of IFT88 in the adolescent growth plate are mechano-dependent. We propose that during this pivotal phase in adolescent skeletal maturation, ciliary IFT88 protects uniform, coordinated ossification of the growth plate from an otherwise disruptive heterogeneity of physiological mechanical forces. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

The Extracellular Matrix of Articular Cartilage Controls the Bioavailability of Pericellular Matrix-Bound Growth Factors to Drive Tissue Homeostasis and Repair

The extracellular matrix (ECM) has long been regarded as a packing material; supporting cells within the tissue and providing tensile strength and protection from mechanical stress. There is little surprise when one considers the dynamic nature of many of the individual proteins that contribute to the ECM, that we are beginning to appreciate a more nuanced role for the ECM in tissue homeostasis and disease. Articular cartilage is adapted to be able to perceive and respond to mechanical load. Indeed, physiological loads are essential to maintain cartilage thickness in a healthy joint and excessive mechanical stress is associated with the breakdown of the matrix that is seen in osteoarthritis (OA). Although the trigger by which increased mechanical stress drives catabolic pathways remains unknown, one mechanism by which cartilage responds to increased compressive load is by the release of growth factors that are sequestered in the pericellular matrix. These are heparan sulfate-bound growth factors that appear to be largely chondroprotective and displaced by an aggrecan-dependent sodium flux. Emerging evidence suggests that the released growth factors act in a coordinated fashion to drive cartilage repair. Thus, we are beginning to appreciate that the ECM is the key mechano-sensor and mechano-effector in cartilage, responsible for directing subsequent cellular events of relevance to joint health and disease.

Isolation, profiling, and tracking of extracellular vesicle cargo in Caenorhabditis elegans

Extracellular vesicles (EVs) may mediate intercellular communication by carrying protein and RNA cargo. The composition, biology, and roles of EVs in physiology and pathology have been primarily studied in the context of biofluids and in cultured mammalian cells. The experimental tractability of C. elegans makes for a powerful in vivo animal system to identify and study EV cargo from its cellular source. We developed an innovative method to label, track, and profile EVs using genetically encoded, fluorescent-tagged EV cargo and conducted a large-scale isolation and proteomic profiling. Nucleic acid binding proteins (∼200) are overrepresented in our dataset. By integrating our EV proteomic dataset with single-cell transcriptomic data, we identified and validated ciliary EV cargo: CD9-like tetraspanin (TSP-6), ectonucleotide pyrophosphatase/phosphodiesterase (ENPP-1), minichromosome maintenance protein (MCM-3), and double-stranded RNA transporter SID-2. C. elegans EVs also harbor RNA, suggesting that EVs may play a role in extracellular RNA-based communication.

Highly efficient CRISPR-Cas9-mediated editing identifies novel mechanosensitive microRNA-140 targets in primary human articular chondrocytes

OBJECTIVE

MicroRNA 140 (miR-140) is a chondrocyte-specific endogenous gene regulator implicated in osteoarthritis (OA). As mechanical injury is a primary aetiological factor in OA, we investigated miR-140-dependent mechanosensitive gene regulation using a novel CRISPR-Cas9 methodology in primary human chondrocytes.

METHOD

Primary (passage 1/2) human OA chondrocytes were isolated from arthroplasty samples (six donors) and transfected with ribonuclear protein complexes or plasmids using single guide RNAs (sgRNAs) targeting miR-140, in combination with Cas9 endonuclease. Combinations of sgRNAs and single/double transfections were tested. Gene editing was measured by T7 endonuclease 1 (T7E1) assay. miRNA levels were confirmed by qPCR in chondrocytes and in wild type murine femoral head cartilage after acute injury. Predicted close match off-targets were examined. Mechanosensitive miR-140 target validation was assessed in 42 injury-associated genes using TaqMan Microfluidic cards in targeted and donor-matched control chondrocytes. Identified targets were examined in RNAseq data from costal chondrocytes from miR-140^-/- mice.

RESULTS

High efficiency gene editing of miR-140 (90-98%) was obtained when two sgRNAs were combined with double RNP-mediated CRISPR-Cas9 transfection. miR-140 levels fell rapidly after femoral cartilage injury. Of the top eight miR-140 gene targets identified (P < 0.01), we validated three previously identified ones (septin 2, bone morphogenetic protein 2 and fibroblast growth factor 2). Novel targets included Agrin, a newly recognised pro-regenerative cartilage agent, and proteins associated with retinoic acid signalling and the primary cilium.

CONCLUSION

We describe a highly efficient CRISPR-Cas9-mediated strategy for gene editing in primary human chondrocytes and identify several novel mechanosensitive miR-140 targets of disease relevance.