p38 MAPK Signaling in Postnatal Tendon Growth and Remodeling

Abstract

Tendon is a dynamic tissue whose structure and function is influenced by mechanical loading, but little is known about the fundamental mechanisms that regulate tendon growth and remodeling in vivo. Data from cultured tendon fibroblasts indicated that the p38 MAPK pathway plays an important role in tendon fibroblast proliferation and collagen synthesis in vitro. To gain greater insight into the mechanisms of tendon growth, and explore the role of p38 MAPK signaling in this process, we tested the hypotheses that inducing plantaris tendon growth through the ablation of the synergist Achilles tendon would result in rapid expansion of a neotendon matrix surrounding the original tendon, and that treatment with the p38 MAPK inhibitor SB203580 would prevent this growth. Rats were treated with vehicle or SB203580, and subjected to synergist ablation by bilateral tenectomy of the Achilles tendon. Changes in histological and biochemical properties of plantaris tendons were analyzed 3, 7, or 28 days after overload, and comparisons were made to non-overloaded animals. By 28 days after overload, tendon mass had increased by 30% compared to non-overloaded samples, and cross-sectional area (CSA) increased by around 50%, with most of the change occurring in the neotendon. The expansion in CSA initially occurred through the synthesis of a hyaluronic acid rich matrix that was progressively replaced with mature collagen. Pericytes were present in areas of active tendon growth, but never in the original tendon ECM. Inhibition of p38 MAPK resulted in a profound decrease in IL6 expression, and had a modest effect on the expression of other ECM and cell proliferation genes, but had a negligible impact on overall tendon growth. The combined results from this study provided novel insights into tendon mechanobiology, and suggest that p38 MAPK signaling does not appear to be necessary for tendon growth in vivo.

Platelet-Rich Plasma Activates Proinflammatory Signaling Pathways and Induces Oxidative Stress in Tendon Fibroblasts

“…PRP also induced the expression of several major MMPs, including MMP3, MMP9, MMP10 and MMP13 which together degrade the major fibrillar collagens, minor collagens and other ECM structural proteins 8 . While we are still in the early stages of understanding the networks of transcription factors and signaling pathways that regulate tendon fibroblast specification and proliferation, EGR1, EGR2 and scleraxis are transcription factors known to play crucial roles in tendon development, growth and remodeling 16,20,36 , and in the current study PRP downregulated the expression of all three of these genes. Tenomodulin, which is a marker of differentiated fibroblasts 20 , was also downregulated by PRP.…”

“…Hyaluronic acid (HA) is a glycosaminoglycan that serves as a template for new ECM synthesis 3,36 , and PRP treatment had no effect on the expression of the major HA synthesis enzymes, HAS1 and HAS2. PRP downregulated the expression of the major collagens in tendon, collagen 1 and 3 as well as elastin, which has an important role in restoring ECM organization after being stretched.…”

“…Tendon fibroblasts or macrophages were treated with PPP or PRP for 24 hours, and RNA was isolated as previously described 16,36 using a miRNeasy micro kit (Qiagen, Valencia, CA). All RNA had an A 260 /A 280 ratio > 1.8 (Nanodrop, ThermoFisher, Waltham, MA) and RIN values > 8.0 measured (Bioanalyzer, Agilent, Santa Clara, CA).…”

Pathogenesis and management of tendinopathies in sports medicine

“…34 Furthermore, mechanical loading induces the expression of genes such as scleraxis and tenomodulin that are associated with the accumulation of new tenocytes and the production of healthy ECM. 15,16 A particular form of eccentric exercise, termed "heavy slow resistance" training, uses heavy weight with slow repetition and has been notably effective for the treatment of patellar tendinopathy. 33 The combined efficacy of eccentric training from clinical research studies, the low cost of therapy, and the compelling evidence of improved ECM quality at the cellular and molecular levels have resulted in eccentric exercises being frequently considered as a first-line therapy in the management of tendinopathy.…”

“…23 This is supported by animal studies in rodents that demonstrate tendon growth mostly occurs in an outward direction from the epitenon. 15,16 However, the lack of a discernible endotenon in rodents makes it difficult to rule out the possibility of intrafascicular growth between tendon fascicles. Together, these findings suggest that the lack of tissue renewal in the core of adult tendon tissue may contribute to the protracted course that is characteristic of many tendinopathies, and perhaps cell-based therapies that selectively utilize a fraction of cells from those connective tissue layers known to be enriched in tendon stem cells may enhance the regeneration of injured tendon tissue.…”