The Mohawk homeobox gene is a critical regulator of tendon differentiation

Abstract

Mohawk (Mkx) is a member of the Three Amino acid Loop Extension superclass of atypical homeobox genes that is expressed in developing tendons. To investigate the in vivo functions of Mkx, we generated Mkx −/− mice. These mice had hypoplastic tendons throughout the body. Despite the reduction in tendon mass, the cell number in tail tendon fiber bundles was similar between wild-type and Mkx −/− mice. We also observed small collagen fibril diameters and a down-regulation of type I collagen in Mkx −/− tendons. These data indicate that Mkx plays a critical role in tendon differentiation by regulating type I collagen production in tendon cells.T endons are dense, fibrous connective tissues that connect muscle to bone, transmitting the forces that allow for body movement (1). Tendon damage from overuse or degeneration due to aging is a common clinical problem because damaged tendon tissue heals very slowly and rarely recovers completely (2). The establishment of new therapies, such as regenerative medicine, for injured tendons has been delayed by a limited understanding of tendon biology (1, 3).Tendons are composed primarily of collagen fibrils that cross-link to each other to form fibers (4). A small number of tendon cells reside between parallel chains of these fibrils and synthesize the specific ECM that contains collagens and proteoglycans (4, 5). The elasticity of tendons is provided by the large amount of collagen, predominantly type I collagen and small amounts of other collagens, including types III, IV, V, and VI (4, 6-9). The proteoglycans found in tendons, including decorin, fibromodulin, biglycan, and lumican, act to lubricate and organize collagen fiber bundles (4, 5). Targeted disruption of these proteoglycans in mice leads to abnormal collagen fibrils in tendons (3, 10-13). Tendon disruptions have also been described in patients with defects in collagen production, such as Ehlers-Danlos Syndrome, in which the type I collagen gene is mutated (14). These studies indicate that the ability of tendon cells to produce ECM is important for tendon formation.Recently, it was reported that Scleraxis (Scx), a basic helix-loophelix (bHLH) transcription factor expressed in the tendon progenitors and cells of all tendon tissues (15, 16), is essential for tendon differentiation. Scx knockout mice show severe disruption of force-transmitting tendons, although ligaments, which are tissues connecting bone to bone that closely resemble tendons in their components, and short-range anchoring tendons are not affected (17). It was also reported that Scx positively regulates the expression of type I collagen, a main ECM component of tendons (18). However, the type I collagen does not completely disappear from the tendons of Scx knockout mice (17), suggesting the presence of other regulatory factors for type I collagen. The tendon differentiation mechanisms remain largely unknown, with Scx being the only known transcription factor regulating tendon differentiation.Mohawk (Mkx; also known as Irxl1) is the sole member of a newly c...

Characterization of the DNA-binding Properties of the Mohawk Homeobox Transcription Factor

“…The Mkx homeobox gene has been shown to be an important regulator of skeletal muscle and tendon differentiation (5,6). We have characterized the DNA recognition motif of the mouse Mkx homeodomain and identified an Mkx-responsive element within the proximal promoter of Mkx.…”

“…Mkx is a conserved homeobox gene that has recently been shown to be a key regulator of skeletal muscle and tendon differentiation (2,5,6). Targeted disruption of Mkx in the mouse results in severe tendon hypoplasia and a significant decrease in the expression of extracellular matrix proteoglycans critical for musculoskeletal function, including collagen type I and decorin (5). Parsing the mechanism(s) by which Mkx regulates the development of the musculoskeletal system is dependent on characterizing its DNA-binding properties and ultimately identifying its direct genetic targets.…”

Three-Dimensional Culture and Transforming Growth Factor Beta3 Synergistically Promote Tenogenic Differentiation of Equine Embryo-Derived Stem Cells

“…More robust markers of tenocytes include Scleraxis (SCX), Tenomodulin (TNMD), Tenascin-C (TNC), Cartilage oligomeric matrix protein (COMP), and Thrombospondin-4 (THBS4), which are not only collectively expressed in tendons, but individually also present in other tissue types. [22][23][24][25][26] Developmental studies have also implicated genes such as Six1/2, Eph-A4, Eya1/2, Egr1/2, and Mohawk (Mkx), 25,[27][28][29][30] although it has not been established whether these genes show temporally restricted expression in tendon tissue.…”

“Soft” tissue patterning: Muscles and tendons of the limb take their form

“…Recently, the involvement of additional transcription factors (Mohawk, Egr1, and Egr2) in tendon development and morphogenesis has been demonstrated (Ito et al, 2010;Lejard et al, 2010;Liu et al, 2010). Although these factors do not participate in tendon patterning, they may be required alongside scx to regulate tendon development.…”

“…The atypical homeobox gene Mohawk (Mkx), is expressed in differentiating limb tendon progenitors from E12.5 as they align between the forming muscles, and is upregulated once the progenitors undergo condensation and further differentiation (Liu et al, 2010). Mkx null mice display a reduction in type I collagen, the main collagen expressed in tendons (Ito et al, 2010;Liu et al, 2010) but no reduction in scx expression, suggesting that Mkx is required for tendon differentiation and morphogenesis but is not involved in their specification (Liu et al, 2010). Two additional transcription factors, members of the Early growth response-like family genes, Egr1 and Egr2, were similarly shown to be expressed in developing and differentiating tendons (Lejard et al, 2010).…”