Syndecan-4 affects myogenesis via Rac1-mediated actin remodeling and exhibits copy-number amplification and increased expression in human rhabdomyosarcoma tumors

Abstract

Skeletal muscle demonstrates a high degree of regenerative capacity repeating the embryonic myogenic program under strict control. Rhabdomyosarcoma is the most common sarcoma in childhood and is characterized by impaired muscle differentiation. In this study, we observed that silencing the expression of syndecan-4, the ubiquitously expressed transmembrane heparan sulfate proteoglycan, significantly enhanced myoblast differentiation, and fusion. During muscle differentiation, the gradually decreasing expression of syndecan-4 allows the activation of Rac1, thereby mediating myoblast fusion. Single-molecule localized superresolution direct stochastic optical reconstruction microscopy (dSTORM) imaging revealed nanoscale changes in actin cytoskeletal architecture, and atomic force microscopy showed reduced elasticity of syndecan-4-knockdown cells during fusion. Syndecan-4 copy-number amplification was observed in 28% of human fusion-negative rhabdomyosarcoma tumors and was accompanied by increased syndecan-4 expression based on RNA sequencing data. Our study suggests that syndecan-4 can serve as a tumor driver gene in promoting rabdomyosarcoma tumor development. Our results contribute to the understanding of the role of syndecan-4 in skeletal muscle development, regeneration, and tumorigenesis.

Overexpression of Syndecan-4 inhibits myogenesis by regulating the expression of myogenic regulatory factors

“…Similarly, a previous study showed that myotube formation, which is increased by SDC4 knockout in bovine cells and the cytoplasmic domain of SDC4, inhibits myoblast fusion [22]. Szabo et al reported that SDC4 expression gradually decreases during muscle differentiation, while high expression of SDC4 hinders myogenesis [27]. Decreasing SDC4 expression during myogenesis is crucial for myotube formation.…”

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

“…dSTORM super-resolution microscopy results have revealed that Syndecan-4 affects the nanoscale structure of the actin network in the lamellipodia of C2C12 myoblasts during migration [ 9 ] and also in differentiating cell cultures [ 14 ] , thereby influencing cell migration and cell–cell fusion. Since Syndecan-4 regulates the activity of small GTP-ase Rac1 [ 22 ], the Syndecan-4/Rac1-mediated actin remodelling [ 14 ] plays a role in this phenomenon. Moreover, Syndecan-4 directly binds alpha-actinin, a cross-linking protein between F-actin filaments [ 23 ].…”

“…To carry out further studies, cells were seeded on glass coverslips for 24 h, and differentiation was then induced by replacing the growth medium with a differentiation medium containing 2% horse serum (Gibco/Life Technologies, New Zealand). The Syndecan-4 expression of normal and mutated C2C12 cells was compared using Western blot and qPCR experiments [ 9 , 14 , 22 ]. The silencing of Syndecan-4 decreased its level by ~50% in the KD cells.…”

“…Additionally, the Syndecan-4 level and its localization were acknowledged to be regulated by the electrical activity of the cells, suggesting a controlling mechanism influencing the adhesion of skeletal myotubes during differentiation [ 10 ]. An interesting finding was that all syndecans are present in muscles during development [ 4 , 11 , 12 ] and also in proliferating myoblasts, but their expression level decreases with time during the muscle maturation process [ 13 , 14 ]. Thus, its expression is high in myoblasts and young myotubes at the embryonic stage, but its amount starts to decline in myotubes after birth and it is present only in satellite cells in adults [ 4 ].…”

Extracellular matrix: Brick and mortar in the skeletal muscle stem cell niche

“…In MuSCs, syndecans-3 and -4 have important nonredundant roles in transducing and integrating signals from the ECM as well as from transmembrane signaling proteins ( Cornelison et al, 2001 ). Loss of syndecan-3 leads to diminished proliferation, differentiation defects, and impaired self-renewal due to aberrant niche interactions, while loss of syndecan-4, which is highly expressed by MuSCs ( Figures 2A,F ), impairs activation, proliferation, migration, and differentiation via multiple mechanisms ( Cornelison et al, 2004 ; Munoz et al, 2006 ; Pisconti et al, 2010 ; Shin et al, 2012 ; Bentzinger et al, 2013 ; Becsky et al, 2020 ; Ronning et al, 2020 ; Szabo et al, 2022 ). Through their extracellular domain and its associated heparan sulfate or chondroitin sulfate carbohydrate chains, syndecans form ternary structures with growth factors and their receptors including FGFs, transforming growth factor-β family members, Wnts, and small chemokines such as SDF-1/CXCL12 and RANTES/CCL5 ( Allen et al, 2001 ; Slimani et al, 2003 ; Chen et al, 2004 ; Charnaux et al, 2005 ; Munoz et al, 2006 ).…”