Transmembrane proteoglycans syndecan-2, 4, receptor candidates for the impact of HGF and FGF2 on semaphorin 3A expression in early-differentiated myoblasts

Unravelling the Effects of Syndecan-4 Knockdown on Skeletal Muscle Functions

The remodelling of the extracellular matrix plays an important role in skeletal muscle development and regeneration. Syndecan-4 is a cell surface proteoglycan crucial for muscle differentiation. Syndecan-4^-/- mice have been reported to be unable to regenerate following muscle damage. To investigate the consequences of the decreased expression of Syndecan-4, we have studied the in vivo and in vitro muscle performance and the excitation-contraction coupling machinery in young and aged Syndecan-4^+/- (SDC4) mice. In vivo grip force was decreased significantly as well as the average and maximal speed of voluntary running in SDC4 mice, regardless of their age. The maximal in vitro twitch force was reduced in both EDL and soleus muscles from young and aged SDC4 mice. Ca²⁺ release from the sarcoplasmic reticulum decreased significantly in the FDB fibres of young SDC4 mice, while its voltage dependence was unchanged regardless of age. These findings were present in muscles from young and aged mice as well. On C2C12 murine skeletal muscle cells, we have also found altered calcium homeostasis upon Syndecan-4 silencing. The decreased expression of Syndecan-4 leads to reduced skeletal muscle performance in mice and altered motility in C2C12 myoblasts via altered calcium homeostasis. The altered muscle force performance develops at an early age and is maintained throughout the life course of the animal until old age.

Syndecan-4-/- Mice Have Smaller Muscle Fibers, Increased Akt/mTOR/S6K1 and Notch/HES-1 Pathways, and Alterations in Extracellular Matrix Components

Background

Extracellular matrix (ECM) remodeling is essential for skeletal muscle development and adaption in response to environmental cues such as exercise and injury. The cell surface proteoglycan syndecan-4 has been reported to be essential for muscle differentiation, but few molecular mechanisms are known. Syndecan-4^–/– mice are unable to regenerate damaged muscle, and display deficient satellite cell activation, proliferation, and differentiation. A reduced myofiber basal lamina has also been reported in syndecan-4^–/– muscle, indicating possible defects in ECM production. To get a better understanding of the underlying molecular mechanisms, we have here investigated the effects of syndecan-4 genetic ablation on molecules involved in ECM remodeling and muscle growth, both under steady state conditions and in response to exercise.

Methods

Tibialis anterior (TA) muscles from sedentary and exercised syndecan-4^–/– and WT mice were analyzed by immunohistochemistry, real-time PCR and western blotting.

Results

Compared to WT, we found that syndecan-4^–/– mice had reduced body weight, reduced muscle weight, muscle fibers with a smaller cross-sectional area, and reduced expression of myogenic regulatory transcription factors. Sedentary syndecan-4^–/– had also increased mRNA levels of syndecan-2, decorin, collagens, fibromodulin, biglycan, and LOX. Some of these latter ECM components were reduced at protein level, suggesting them to be more susceptible to degradation or less efficiently translated when syndecan-4 is absent. At the protein level, TRPC7 was reduced, whereas activation of the Akt/mTOR/S6K1 and Notch/HES-1 pathways were increased. Finally, although exercise induced upregulation of several of these components in WT, a further upregulation of these molecules was not observed in exercised syndecan-4^–/– mice.

Conclusion

Altogether our data suggest an important role of syndecan-4 in muscle development.

Premature satellite cell activation before injury accelerates myogenesis and disrupts neuromuscular junction maturation in regenerating muscle

Satellite cell (SC) activation, mediated by nitric oxide (NO), is essential to myogenic repair, whereas myotube function requires innervation. Semaphorin (Sema) 3A, a neuro-chemorepellent, is thought to regulate axon guidance to neuromuscular junctions (NMJs) during myotube differentiation. We tested whether "premature" SC activation (SC activation before injury) by a NO donor (isosorbide dinitrate) would disrupt early myogenesis and/or NMJs. Adult muscle was examined during regeneration in two models of injury: myotoxic cardiotoxin (CTX) and traumatic crush (CR) (n = 4-5/group). Premature SC activation was confirmed by increased DNA synthesis by SCs immediately in pretreated mice after CTX injury. Myotubes grew faster after CTX than after CR; growth was accelerated by pretreatment. NMJ maturation, classified by silver histochemistry (neurites) and acetylcholinesterase (AchE), and α-bungarotoxin staining (Ach receptors, AchRs) were delayed by pretreatment, consistent with a day 6 rise in the denervation marker γ-AchR. With pretreatment, S100B from terminal Schwann cells (TSCs) increased 10- to 20-fold at days 0 and 10 after CTX and doubled 6 days after CR. Premature SC activation disrupted motoneuritogenesis 8-10 days post-CTX, as pretreatment reduced colocalization of pre- and postsynaptic NMJ features and increased Sema3A-65. Premature SC activation before injury both accelerated myogenic repair and disrupted NMJ remodeling and maturation, possibly by reducing Sema3A neuro-repulsion and altering S100B. This interpretation extends the model of Sema3A-mediated motoneuritogenesis during muscle regeneration. Manipulating the timing and type of Sema3A by brief NO effects on SCs suggests an important role for TSCs and Sema3A-65 processing in axon guidance and NMJ restoration during muscle repair.

Semaphorin 3A potentiates the profibrotic effects of transforming growth factor-β1 in the cornea

Corneal scarring is a major cause of blindness worldwide with few effective therapeutic options. Finding a treatment would be of tremendous public health benefit, but requires a thorough understanding of the complex interactions that underlie this phenomenon. Here, we tested the hypothesis that the large increase in expression of Semaphorin 3A (SEMA3A) in corneal wounds contributes to the development of stromal fibrosis. We first verified this increased expression in vivo, in a cat model of photorefractive keratectomy-induced corneal wounding. We then examined the impact of adding exogenous SEMA3A to cultured corneal fibroblasts, and assessed how this affected the ability of transforming growth factor-beta1 (TGF-β1) to induce their differentiation into myofibroblasts. Finally, we examined how siRNA knockdown of endogenous SEMA3A affected these same phenomena. We found exogenous SEMA3A to significantly potentiate TGF-β1's profibrotic effects, with only a minimal contribution from cell-intrinsic SEMA3A. Our results suggest a previously unrecognized interaction between SEMA3A and TGF-β1 in the wounded cornea, and a possible contribution of SEMA3A to the regulation of tissue fibrosis and remodeling in this transparent organ.

A mechanism for semaphorin-induced apoptosis: DNA damage of endothelial and myogenic cells in primary cultures from skeletal muscle

One hallmark of cancer is its ability to recruit a vascular supply to support rapid growth. Suppression of angiogenesis holds potential as a second-line or adjuvant therapy to stunt cancer growth, progression, metastasis, and post-resection regeneration. To begin to test the hypothesis that semaphorin 3A and 3F together, will induce endothelial cell apoptosis by inducing DNA damage, mixed primary cultures isolated from normal adult mouse skeletal muscle were treated for 48 hr with Sema3A ± Sema3F (100ng/mL). Changes in surviving-cell density, DNA synthesis, DNA repair (gamma-Histone 2AX, γH2AX, an indirect measure for DNA damage), and apoptotic DNA fragmentation (TUNEL staining) were assayed in cultures of CD31+ endothelial and desmin+ muscle cells. Sema3F increased DNA damage-associated DNA repair in both cell types. Co-treatment with Sema3A+3F increased γH2AX staining ~25-fold over control levels, and further increased apoptosis compared to control and Sema3A alone. Results were negated by treatment with neutralizing anti-semaphorin antibodies and are interpreted as suggesting that Sema3A may sensitize endothelial but not muscle cells to Sema3F-induced DNA damage. These preliminary findings on a complex system of interacting cells may contribute to developing applications that could target angiogenic regulatory mechanisms for their therapeutic potential against cancer progression and metastasis.

Emerging roles of syndecan 2 in epithelial and mesenchymal cancer progression

Syndecan 2 (SDC2) belongs to a four-member family of evolutionary conserved small type I transmembrane proteoglycans consisting of a protein core to which glycosaminoglycan chains are covalently attached. SDC2 is a cell surface heparan sulfate proteoglycan, which is increasingly drawing attention for its distinct characteristics and its participation in numerous cell functions, including those related to carcinogenesis. Increasing evidence suggests that the role of SDC2 in cancer pathogenesis is dependent on cancer tissue origin rendering its use as a biomarker/therapeutic target feasible. This mini review discusses the mechanisms, through which SDC2, in a distinct manner, modulates complex signalling networks to affect cancer progression. © 2017 IUBMB Life, 69(11):824-833, 2017.

Slow-Myofiber Commitment by Semaphorin 3A Secreted from Myogenic Stem Cells

Recently, we found that resident myogenic stem satellite cells upregulate a multi-functional secreted protein, semaphorin 3A (Sema3A), exclusively at the early-differentiation phase in response to muscle injury; however, its physiological significance is still unknown. Here we show that Sema3A impacts slow-twitch fiber generation through a signaling pathway, cell-membrane receptor (neuropilin2-plexinA3) → myogenin-myocyte enhancer factor 2D → slow myosin heavy chain. This novel axis was found by small interfering RNA-transfection experiments in myoblast cultures, which also revealed an additional element that Sema3A-neuropilin1/plexinA1, A2 may enhance slow-fiber formation by activating signals that inhibit fast-myosin expression. Importantly, satellite cell-specific Sema3A conditional-knockout adult mice (Pax7CreER^T2 -Sema3A^fl °^x activated by tamoxifen-i.p. injection) provided direct in vivo evidence for the Sema3A-driven program, by showing that slow-fiber generation and muscle endurance were diminished after repair from cardiotoxin-injury of gastrocnemius muscle. Overall, the findings highlight an active role for satellite cell-secreted Sema3A ligand as a key "commitment factor" for the slow-fiber population during muscle regeneration. Results extend our understanding of the myogenic stem-cell strategy that regulates fiber-type differentiation and is responsible for skeletal muscle contractility, energy metabolism, fatigue resistance, and its susceptibility to aging and disease. Stem Cells 2017;35:1815-1834.

The role of semaphorin3A in myogenic regeneration and the formation of functional neuromuscular junctions on new fibres

Current research on skeletal muscle injury and regeneration highlights the crucial role of nerve-muscle interaction in the restoration of innervation during that process. Activities of muscle satellite or stem cells, recognized as the 'currency' of myogenic repair, have a pivotal role in these events, as shown by ongoing research. More recent investigation of myogenic signalling events reveals intriguing roles for semaphorin3A (Sema3A), secreted by activated satellite cells, in the muscle environment during development and regeneration. For example, Sema3A makes important contributions to regulating the formation of blood vessels, balancing bone formation and bone remodelling, and inflammation, and was recently implicated in the establishment of fibre-type distribution through effects on myosin heavy chain gene expression. This review highlights the active or potential contributions of satellite-cell-derived Sema3A to regulation of the processes of motor neurite ingrowth into a regenerating muscle bed. Successful restoration of functional innervation during muscle repair is essential; this review emphasizes the integrative role of satellite-cell biology in the progressive coordination of adaptive cellular and tissue responses during the injury-repair process in voluntary muscle.