Regulation of tendon differentiation by scleraxis distinguishes force-transmitting tendons from muscle-anchoring tendons

Abstract

The scleraxis (Scx) gene, encoding a bHLH transcription factor, is expressed in the progenitors and cells of all tendon tissues. To determine Scx function, we produced a mutant null allele. Scx -/-mice were viable, but showed severe tendon defects, which manifested in a drastically limited use of all paws and back muscles and a complete inability to move the tail. Interestingly, although the differentiation of all force-transmitting and intermuscular tendons was disrupted, other categories of tendons, the function of which is mainly to anchor muscles to the skeleton, were less affected and remained functional, enabling the viability of Scx -/-mutants. The force-transmitting tendons of the limbs and tail varied in the severity to which they were affected, ranging from dramatic failure of progenitor differentiation resulting in the loss of segments or complete tendons, to the formation of small and poorly organized tendons. Tendon progenitors appeared normal in Scx -/-embryos and a phenotype resulting from a failure in the condensation of tendon progenitors to give rise to distinct tendons was first detected at embryonic day (E)13.5. In the tendons that persisted in Scx -/-mutants, we found a reduced and less organized tendon matrix and disorganization at the cellular level that led to intermixing of tenocytes and endotenon cells. The phenotype of Scx -/-mutants emphasizes the diversity of tendon tissues and represents the first molecular insight into the important process of tendon differentiation.

Expression analysis of limb element markers during mouse embryonic development

“…Gdf5 is expressed across developing joint sites, including interphalangeal joints, from E14.5 (Chen et al, 2016; Houweling et al, 2001; Huang et al, 2016; Sohaskey et al, 2009). Our findings for Scleraxis expression are in agreement with previously published analyses of Scleraxis , indicating expression in the autopod and in developing digit rays and cartilage condensations at E12.5, as well as defined expression in developing tendons at E13.5 (Huang et al, 2015; McGlinn et al, 2005; Schweitzer et al, 2001) and along developing digits, with strongest expression across sites of developing interzones at E14.5 (Murchison et al, 2007; Schweitzer et al, 2001). MyoD1 is expressed in muscle progenitors in the zeugopod, not the autopod, at E12.5.…”

“…MyoD1 expression, a myogenic helix‐loop‐helix protein family restricted to differentiating skeletal muscle (reviewed in Tapscott, 2005), has been examined in the context of comparison to mutant models but infrequently as a marker of muscle development alone (Grifone et al, 2005; Kablar et al, 1997; Laclef et al, 2003; L'honoré et al, 2010; Mayeuf‐Louchart et al, 2016), highlighting the requirement for a summative analysis of expression during mouse limb development. Scleraxis , a basic helix‐loop‐helix factor expressed in connective tissues that form muscle attachments (Schweitzer et al, 2001), is reexamined here alongside other limb element development markers to allow a comparison to the development of the other tissues (Murchison et al, 2007; Schweitzer et al, 2001). Cdh5 expression, an endothelial‐specific cadherin expressed during early vasculogenesis with expression maintained throughout adult life (Neuhaus et al, 2014), has not, as far as we are aware, been studied throughout mouse limb development, and indeed whole‐mount in situ analysis of blood vessel markers is rare, with the majority of studies describing expression at one or two time points as a comparison to mutant models (Eshkar‐Oren et al, 2009; Eshkar‐Oren et al, 2015; Ota et al, 2007; Vieira et al, 2007).…”

Scleraxis is Required for Differentiation of the Stapedius and Tensor Tympani Tendons of the Middle Ear

“…Scx was also previously shown to be expressed in tendon progenitor cells in all developing tendons of the axial and appendicular skeleton (Schweitzer et al 2001;Murchison et al 2007;Pryce et al 2007). To identify the early stages in the formation of middle ear tendons, we analyzed Scx-GFP and Scx mRNA expression at E14.5 and E16.5 when middle ear tendon progenitors FIG.…”

Spatiotemporal protein distribution of TGF‐βs, their receptors, and extracellular matrix molecules during embryonic tendon development

“…The function of scleraxis during tendon development has yet to be elucidated. However, a necessary role for scleraxis is suggested by the disruption in development of force-transmitting and intermuscular tendons in scleraxis-null mice (Murchison et al, 2007). Ultimately, the bulk of our understanding of tendon development derives from collagen fibrillogenesis studies.…”