Scleraxis is required for maturation of tissue domains for proper integration of the musculoskeletal system

TLDR

Scleraxis (Scx) is a basic helix‑loop‑helix transcription factor expressed persistently in tendons and ligaments and transiently in entheseal cartilage. The study generated a novel Scx Cre knock‑in allele to drive Cre expression under the Scx promoter and inactivate endogenous Scx. Cre‑mediated excision occurs in tendinous and ligamentous tissues that persistently express Scx, reflecting the intensity and duration of endogenous expression. Loss of Scx in Scx Cre/Cre KI mice caused defective maturation of tendons, ligaments, and entheseal cartilage, as shown by absent tenomodulin, reduced Sox9, and decreased Smad phosphorylation, highlighting Scx’s essential role in musculoskeletal integration.

Abstract

Abstract Scleraxis (Scx) is a basic helix-loop-helix transcription factor that is expressed persistently in tendons/ligaments, but transiently in entheseal cartilage. In this study, we generated a novel Scx Cre knock-in (KI) allele, by in-frame replacement of most of Scx exon 1 with Cre recombinase (Cre ), to drive Cre expression using Scx promoter and to inactivate the endogenous Scx . Reflecting the intensity and duration of endogenous expression, Cre-mediated excision occurs in tendinous and ligamentous tissues persistently expressing Scx . Expression of tenomodulin, a marker of mature tenocytes and ligamentocytes, was almost absent in tendons and ligaments of Scx Cre / Cre KI mice lacking Scx to indicate defective maturation. In homozygotes, the transiently Scx -expressing entheseal regions such as the rib cage, patella cartilage, and calcaneus were small and defective and cartilaginous tuberosity was missing. Decreased Sox9 expression and phosphorylation of Smad1/5 and Smad3 were also observed in the developing entheseal cartilage, patella, and deltoid tuberosity of Scx Cre / Cre KI mice. These results highlighted the functional importance of both transient and persistent expression domains of Scx for proper integration of the musculoskeletal components.