TRPV4-mediated mechanotransduction regulates the metabolic response of chondrocytes to dynamic loading

Nutrient channels and stirring enhanced the composition and stiffness of large cartilage constructs

A significant challenge in cartilage tissue engineering is to successfully culture functional tissues that are sufficiently large to treat osteoarthritic joints. Transport limitations due to nutrient consumption by peripheral cells produce heterogeneous constructs with matrix-deficient centers. Incorporation of nutrient channels into large constructs is a promising technique for alleviating transport limitations, in conjunction with simple yet effective methods for enhancing media flow through channels. Cultivation of cylindrical channeled constructs flat in culture dishes, with or without orbital shaking, produced asymmetric constructs with poor tissue properties. We therefore explored a method for exposing the entire construct surface to the culture media, while promoting flow through the channels. To this end, chondrocyte-seeded agarose constructs (∅10mm, 2.34mm thick), with zero or three nutrient channels (∅1mm), were suspended on their sides in custom culture racks and subjected to three media stirring modes for 56 days: uniaxial rocking, orbital shaking, or static control. Orbital shaking led to the highest construct EY, sulfated glycosaminoglycan (sGAG), and collagen contents, whereas rocking had detrimental effects on sGAG and collagen versus static control. Nutrient channels increased EY as well as sGAG homogeneity, and the beneficial effects of channels were most marked in orbitally shaken samples. Under these conditions, the constructs developed symmetrically and reached or exceeded native levels of EY (~400kPa) and sGAG (~9%/ww). These results suggest that the cultivation of channeled constructs in culture racks with orbital shaking is a promising method for engineering mechanically competent large cartilage constructs.

Promoting increased mechanical properties of tissue engineered neocartilage via the application of hyperosmolarity and 4α-phorbol 12,13-didecanoate (4αPDD)

Osteoarthritis, a degenerative disease of the load-bearing joints, greatly reduces quality of life for millions of Americans and places a tremendous cost on the American healthcare system. Due to limitations of current treatments, tissue engineering of articular cartilage may provide a promising therapeutic option to treat cartilage defects. However, cartilage tissue engineering has yet to recapitulate the functional properties of native tissue. During normal joint loading, cartilage tissue experiences variations in osmolarity and subsequent changes in ionic concentrations. Motivated by these known variations in the cellular microenvironment, this study sought to improve the mechanical properties of neocartilage constructs via the application of hyperosmolarity and transient receptor potential vanilloid 4 (TRPV4) channel activator 4α-phorbol 12,13-didecanoate (4αPDD). It was shown that 4αPDD elicited significant increases in compressive properties. Importantly, when combined, 4αPDD positively interacted with hyperosmolarity to modulate its effects on tensile stiffness and collagen content. Thus, this study supports 4αPDD-activated channel TRPV4 as a purported mechanosensor and osmosensor that can facilitate the cell and tissue level responses to improve the mechanical properties of engineered cartilage. To our knowledge, this study is the first to systematically evaluate the roles of hyperosmolarity and 4αPDD on the functional (i.e., mechanical and biochemical) properties of self-assembled neotissue. Future work may combine 4αPDD-induced channel activation with other chemical and mechanical stimuli to create robust neocartilages suitable for treatment of articular cartilage defects.

The structure and function of the pericellular matrix of articular cartilage

Chondrocytes in articular cartilage are surrounded by a narrow pericellular matrix (PCM) that is both biochemically and biomechanically distinct from the extracellular matrix (ECM) of the tissue. While the PCM was first observed nearly a century ago, its role is still under investigation. In support of early hypotheses regarding its function, increasing evidence indicates that the PCM serves as a transducer of biochemical and biomechanical signals to the chondrocyte. Work over the past two decades has established that the PCM in adult tissue is defined biochemically by several molecular components, including type VI collagen and perlecan. On the other hand, the biomechanical properties of this structure have only recently been measured. Techniques such as micropipette aspiration, in situ imaging, computational modeling, and atomic force microscopy have determined that the PCM exhibits distinct mechanical properties as compared to the ECM, and that these properties are influenced by specific PCM components as well as disease state. Importantly, the unique relationships among the mechanical properties of the chondrocyte, PCM, and ECM in different zones of cartilage suggest that this region significantly influences the stress-strain environment of the chondrocyte. In this review, we discuss recent advances in the measurement of PCM mechanical properties and structure that further increase our understanding of PCM function. Taken together, these studies suggest that the PCM plays a critical role in controlling the mechanical environment and mechanobiology of cells in cartilage and other cartilaginous tissues, such as the meniscus or intervertebral disc.

The chondrocyte primary cilium

The presence and role of primary, or non-motile, cilia on chondrocytes has confused cartilage researchers for decades. Initial explanations attributed a vestigial nature to chondrocyte cilia. Evidence is now emerging that supports the role of the chondrocyte primary cilium as a sensory organelle, in particular, in mechanotransduction and as a compartment for signaling pathways. Early electron microscopy images depicted bent cilia aligned with the extracellular matrix (ECM) in a manner that suggested a response to mechanical forces. Molecules known to be mechanotransducers in other cell types, including integrins and proteoglycans, are present on chondrocyte cilia. Further, chondrocytes which lack cilia fail to respond to mechanical forces in the same manner that chondrocytes with intact cilia respond. From a clinical perspective, chondrocytes from osteoarthritic (OA) cartilage have cilia with different characteristics than cilia found on chondrocytes from healthy cartilage.

OBJECTIVE

This review examines the evidence supporting the function of chondrocyte cilia and briefly speculates on the involvement of intraflagellar transport (IFT) in the signaling pathway of mechanotransduction through the cilium.

CONCLUSIONS

Emerging evidence suggests cilia may be a promising target for preventing and treating OA.

Impact of cellular microenvironment and mechanical perturbation on calcium signalling in meniscus fibrochondrocytes

Mechanical signals regulate a multitude of cell functions and ultimately govern fibrous tissue growth, maintenance and repair. Such mechanotransduction processes often involve modulation of intracellular calcium concentration ([Ca2+]i). However, most studies interrogate these responses in cells in simplified culture systems, thereby removing potentially important inputs from the native extracellular microenvironment. The objective of this study was to test the hypothesis that the intracellular calcium response of meniscus fibrochondrocytes (MFCs) is dependent on both the microenvironmental context in which this perturbation is applied and on the tensile deformation. Using a custom micro-mechanical tester mounted on a confocal microscope, intracellular calcium activity in MFCs in response to incremental tissue strains (0, 3, 6 and 9 %) was monitored in situ (i.e., in the native tissues) on MFC-seeded aligned scaffolds and MFC-seeded silicone membranes. The [Ca2+]i regulation by MFCs within the native meniscus tissue microenvironment was considerably different from [Ca2+]i regulation by MFCs on either aligned nanofibrous scaffolds or flat silicone membranes. Additionally, increasing levels of tensile deformation resulted in a greater number of responding cells, both in situ and in vitro, while having no effects on temporal characteristics of [Ca2+]i signalling. Collectively, these findings have significant implications for mechanobiology of load-bearing fibrous tissues and their responses to injury and degeneration. In addition, from a tissue engineering perspective, the findings establish cellular benchmarks for maturing engineered constructs, where native tissue-like calcium mechano-regulation may be an important outcome parameter to achieve mechanical functionality comparable to native tissue.

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

Intracellular Ca(2+) transient is crucial in initiating the differentiation of mesenchymal cells into chondrocytes, but whether voltage-gated Ca(2+) channels are involved remains uncertain. Here, we show that the T-type voltage-gated Ca(2+) channel Cav3.2 is essential for tracheal chondrogenesis. Mice lacking this channel (Cav3.2(-/-)) show congenital tracheal stenosis because of incomplete formation of cartilaginous tracheal support. Conversely, Cav3.2 overexpression in ATDC5 cells enhances chondrogenesis, which could be blunted by both blocking T-type Ca(2+) channels and inhibiting calcineurin and suggests that Cav3.2 is responsible for Ca(2+) influx during chondrogenesis. Finally, the expression of sex determination region of Y chromosome (SRY)-related high-mobility group-Box gene 9 (Sox9), one of the earliest markers of committed chondrogenic cells, is reduced in Cav3.2(-/-) tracheas. Mechanistically, Ca(2+) influx via Cav3.2 activates the calcineurin/nuclear factor of the activated T-cell (NFAT) signaling pathway, and a previously unidentified NFAT binding site is identified within the mouse Sox9 promoter using a luciferase reporter assay and gel shift and ChIP studies. Our findings define a previously unidentified mechanism that Ca(2+) influx via the Cav3.2 T-type Ca(2+) channel regulates Sox9 expression through the calcineurin/NFAT signaling pathway during tracheal chondrogenesis.

Biomechanics and mechanobiology in functional tissue engineering

The field of tissue engineering continues to expand and mature, and several products are now in clinical use, with numerous other preclinical and clinical studies underway. However, specific challenges still remain in the repair or regeneration of tissues that serve a predominantly biomechanical function. Furthermore, it is now clear that mechanobiological interactions between cells and scaffolds can critically influence cell behavior, even in tissues and organs that do not serve an overt biomechanical role. Over the past decade, the field of "functional tissue engineering" has grown as a subfield of tissue engineering to address the challenges and questions on the role of biomechanics and mechanobiology in tissue engineering. Originally posed as a set of principles and guidelines for engineering of load-bearing tissues, functional tissue engineering has grown to encompass several related areas that have proven to have important implications for tissue repair and regeneration. These topics include measurement and modeling of the in vivo biomechanical environment; quantitative analysis of the mechanical properties of native tissues, scaffolds, and repair tissues; development of rationale criteria for the design and assessment of engineered tissues; investigation of the effects biomechanical factors on native and repair tissues, in vivo and in vitro; and development and application of computational models of tissue growth and remodeling. Here we further expand this paradigm and provide examples of the numerous advances in the field over the past decade. Consideration of these principles in the design process will hopefully improve the safety, efficacy, and overall success of engineered tissue replacements.

Mouse models of osteoarthritis: modelling risk factors and assessing outcomes

Osteoarthritis (OA) is a prevalent musculoskeletal disease that results in pain and low quality of life for patients, as well as enormous medical and socioeconomic burdens. The molecular mechanisms responsible for the initiation and progression of OA are still poorly understood. As such, mouse models of the disease are having increasingly important roles in OA research owing to the advancements of microsurgical techniques and the use of genetically modified mice, as well as the development of novel assessment tools. In this Review, we discuss available mouse models of OA and applicable assessment tools in studies of experimental OA.

Follistatin in chondrocytes: the link between TRPV4 channelopathies and skeletal malformations

Point mutations in the calcium-permeable TRPV4 ion channel have been identified as the cause of autosomal-dominant human motor neuropathies, arthropathies, and skeletal malformations of varying severity. The objective of this study was to determine the mechanism by which TRPV4 channelopathy mutations cause skeletal dysplasia. The human TRPV4(V620I) channelopathy mutation was transfected into primary porcine chondrocytes and caused significant (2.6-fold) up-regulation of follistatin (FST) expression levels. Pore altering mutations that prevent calcium influx through the channel prevented significant FST up-regulation (1.1-fold). We generated a mouse model of the TRPV4(V620I) mutation, and found significant skeletal deformities (e.g., shortening of tibiae and digits, similar to the human disease brachyolmia) and increases in Fst/TRPV4 mRNA levels (2.8-fold). FST was significantly up-regulated in primary chondrocytes transfected with 3 different dysplasia-causing TRPV4 mutations (2- to 2.3-fold), but was not affected by an arthropathy mutation (1.1-fold). Furthermore, FST-loaded microbeads decreased bone ossification in developing chick femora (6%) and tibiae (11%). FST gene and protein levels were also increased 4-fold in human chondrocytes from an individual natively expressing the TRPV4(T89I) mutation. Taken together, these data strongly support that up-regulation of FST in chondrocytes by skeletal dysplasia-inducing TRPV4 mutations contributes to disease pathogenesis.