Transforming Growth Factorβ-Induced Superficial Zone Protein Accumulation in the Surface Zone of Articular Cartilage Is Dependent on the Cytoskeleton

Abstract

The phenotype of articular chondrocytes is dependent on the cytoskeleton, specifically the actin microfilament architecture. Articular chondrocytes in monolayer culture undergo dedifferentiation and assume a fibroblastic phenotype. This process can be reversed by altering the actin cytoskeleton by treatment with cytochalasin. Whereas dedifferentiation has been studied on chondrocytes isolated from the whole cartilage, the effects of cytoskeletal alteration on specific zones of cells such as superficial zone chondrocytes are not known. Chondrocytes from the superficial zone secrete superficial zone protein (SZP), a lubricating proteoglycan that reduces the coefficient of friction of articular cartilage. A better understanding of this phenomenon may be useful in elucidating chondrocyte dedifferentiation in monolayer and accumulation of the cartilage lubricant SZP, with an eye toward tissue engineering functional articular cartilage. In this investigation, the effects of cytoskeletal modulation on the ability of superficial zone chondrocytes to secrete SZP were examined. Primary superficial zone chondrocytes were cultured in monolayer and treated with a combination of cytoskeleton modifying reagents and transforming growth factor β (TGFβ) 1, a critical regulator of SZP production. Whereas cytochalasin D maintains the articular chondrocyte phenotype, the hallmark of the superficial zone chondrocyte, SZP, was inhibited in the presence of TGFβ1. A decrease in TGFβ1-induced SZP accumulation was also observed when the microtubule cytoskeleton was modified using paclitaxel. These effects of actin and microtubule alteration were confirmed through the application of jasplakinolide and colchicine, respectively. As Rho GTPases regulate actin organization and microtubule polymerization, we hypothesized that the cytoskeleton is critical for TGFβ-induced SZP accumulation. TGFβ-mediated SZP accumulation was inhibited by small molecule inhibitors ML141 (Cdc42), NSC23766 (Rac1), and Y27632 (Rho effector Rho Kinase). On the other hand, lysophosphatidic acid, an upstream activator of Rho, increased SZP synthesis in response to TGFβ1. These results suggest that SZP production is dependent on the functional cytoskeleton, and Rho GTPases contribute to SZP accumulation by modulating the actions of TGFβ.

Biomimetic multidirectional scaffolds for zonal osteochondral tissue engineering via a lyophilization bonding approach

“…Furthermore, the smaller elongated pore channels [48 µm, Fig. (A2)] may encourage infiltrating MSCs to adopt an elongated morphology which may help establish a superficial zone phenotype …”

Mechanical microenvironments and protein expression associated with formation of different skeletal tissues during bone healing

“…CDC42 has been shown to be necessary for terminal differentiation of cartilage and to mediate the upregulation of MMP-9 in articular chondrocytes by fluid-induced shear stress in vitro (Aizawa 2012; Jin et al 2000; Wang and Beier 2005). Yet, inhibiting each of CDC42 and RhoA inhibits the production of superficial zone protein—a lubricating proteoglycan—by human articular chondrocytes treated with TGF-beta (McNary et al 2014). Collectively, these data demonstrate that the roles of RhoA and CDC42 differ among cell types and between in vitro and in vivo contexts.…”

Surface Zone Articular Chondrocytes Modulate the Bulk and Surface Mechanical Properties of the Tissue-Engineered Cartilage

“…Induction of cell rounding through 3D culture 58 or cytoskeletal modulation 31 is not conducive toward SZP synthesis. In this self-assembly system, SZ chondrocytes do not retain their native cell morphology and appear round like MZ chondrocytes (Fig.…”

“…As described previously, bovine chondrocytes were isolated 14,31 and self-assembled. 8,28 Briefly, articular cartilage was harvested from 1-to 3-month-old bovine stifle joints obtained from a local abattoir.…”