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

Transient receptor potential vanilloid 4 as a regulator of induced pluripotent stem cell chondrogenesis

Transient receptor potential vanilloid 4 (TRPV4) is a polymodal calcium-permeable cation channel that is highly expressed in cartilage and is sensitive to a variety of extracellular stimuli. The expression of this channel has been associated with the process of chondrogenesis in adult stem cells as well as several cell lines. Here, we used a chondrogenic reporter (Col2a1-GFP) in murine induced pluripotent stem cells (iPSCs) to examine the hypothesis that TRPV4 serves as both a marker and a regulator of chondrogenesis. Over 21 days of chondrogenesis, iPSCs showed significant increases in Trpv4 expression along with the standard chondrogenic gene markers Sox9, Acan, and Col2a1, particularly in the green fluorescent protein positive (GFP+) chondroprogenitor subpopulation. Increased gene expression for Trpv4 was also reflected by the presence of TRPV4 protein and functional Ca²⁺ signaling. Daily activation of TRPV4 using the specific agonist GSK1016790A resulted in significant increases in cartilaginous matrix production. An improved understanding of the role of TRPV4 in chondrogenesis may provide new insights into the development of new therapeutic approaches for diseases of cartilage, such as osteoarthritis, or channelopathies and hereditary disorders that affect cartilage during development. Harnessing the role of TRPV4 in chondrogenesis may also provide a novel approach for accelerating stem cell differentiation in functional tissue engineering of cartilage replacements for joint repair.

Challenges in Fabrication of Tissue-Engineered Cartilage with Correct Cellular Colonization and Extracellular Matrix Assembly

A correct articular cartilage ultrastructure regarding its structural components and cellularity is important for appropriate performance of tissue-engineered articular cartilage. Various scaffold-based, as well as scaffold-free, culture models have been under development to manufacture functional cartilage tissue. Even decellularized tissues have been considered as a potential choice for cellular seeding and tissue fabrication. Pore size, interconnectivity, and functionalization of the scaffold architecture can be varied. Increased mechanical function requires a dense scaffold, which also easily restricts cellular access within the scaffold at seeding. High pore size enhances nutrient transport, while small pore size improves cellular interactions and scaffold resorption. In scaffold-free cultures, the cells assemble the tissue completely by themselves; in optimized cultures, they should be able to fabricate native-like tissue. Decellularized cartilage has a native ultrastructure, although it is a challenge to obtain proper cellular colonization during cell seeding. Bioprinting can, in principle, provide the tissue with correct cellularity and extracellular matrix content, although it is still an open question as to how the correct molecular interaction and structure of extracellular matrix could be achieved. These are challenges facing the ongoing efforts to manufacture optimal articular cartilage.

Tension stimulation drives tissue formation in scaffold-free systems

Scaffold-free systems have emerged as viable approaches for engineering load-bearing tissues. However, the tensile properties of engineered tissues have remained far below the values for native tissue. Here, by using self-assembled articular cartilage as a model to examine the effects of intermittent and continuous tension stimulation on tissue formation, we show that the application of tension alone, or in combination with matrix remodelling and synthesis agents, leads to neocartilage with tensile properties approaching those of native tissue. Implantation of tension-stimulated tissues results in neotissues that are morphologically reminiscent of native cartilage. We also show that tension stimulation can be translated to a human cell source to generate anisotropic human neocartilage with enhanced tensile properties. Tension stimulation, which results in nearly sixfold improvements in tensile properties over unstimulated controls, may allow the engineering of mechanically robust biological replacements of native tissue.

TRPV4: Molecular Conductor of a Diverse Orchestra

Transient receptor potential vanilloid type 4 (TRPV4) is a calcium-permeable nonselective cation channel, originally described in 2000 by research teams led by Schultz (Nat Cell Biol 2: 695-702, 2000) and Liedtke (Cell 103: 525-535, 2000). TRPV4 is now recognized as being a polymodal ionotropic receptor that is activated by a disparate array of stimuli, ranging from hypotonicity to heat and acidic pH. Importantly, this ion channel is constitutively expressed and capable of spontaneous activity in the absence of agonist stimulation, which suggests that it serves important physiological functions, as does its widespread dissemination throughout the body and its capacity to interact with other proteins. Not surprisingly, therefore, it has emerged more recently that TRPV4 fulfills a great number of important physiological roles and that various disease states are attributable to the absence, or abnormal functioning, of this ion channel. Here, we review the known characteristics of this ion channel's structure, localization and function, including its activators, and examine its functional importance in health and disease.