Trans-regulation of Syndecan Functions by Hetero-oligomerization

Characterization of wooden breast myopathy: a focus on syndecans and ECM remodeling

Introduction: The skeletal muscle deformity of commercial chickens (Gallus gallus), known as the wooden breast (WB), is associated with fibrotic myopathy of unknown etiology. For future breeding strategies and genetic improvements, it is essential to identify the molecular mechanisms underlying the phenotype. The pathophysiological hallmarks of WB include severe skeletal muscle fibrosis, inflammation, myofiber necrosis, and multifocal degeneration of muscle tissue. The transmembrane proteoglycans syndecans have a wide spectrum of biological functions and are master regulators of tissue homeostasis. They are upregulated and shed (cleaved) as a regulatory mechanism during tissue repair and regeneration. During the last decades, it has become clear that the syndecan family also has critical functions in skeletal muscle growth, however, their potential involvement in WB pathogenesis is unknown. Methods: In this study, we have categorized four groups of WB myopathy in broiler chickens and performed a comprehensive characterization of the molecular and histological profiles of two of them, with a special focus on the role of the syndecans and remodeling of the extracellular matrix (ECM). Results and discussion: Our findings reveal differential expression and shedding of the four syndecan family members and increased matrix metalloproteinase activity. Additionally, we identified alterations in key signaling pathways such as MAPK, AKT, and Wnt. Our work provides novel insights into a deeper understanding of WB pathogenesis and suggests potential therapeutic targets for this condition.

Conformations, interactions and functions of intrinsically disordered syndecans

Syndecans are transmembrane heparan sulfate proteoglycans present on most mammalian cell surfaces. They have a long evolutionary history, a single syndecan gene being expressed in bilaterian invertebrates. Syndecans have attracted interest because of their potential roles in development and disease, including vascular diseases, inflammation and various cancers. Recent structural data is providing important insights into their functions, which are complex, involving both intrinsic signaling through cytoplasmic binding partners and co-operative mechanisms where syndecans form a signaling nexus with other receptors such as integrins and tyrosine kinase growth factor receptors. While the cytoplasmic domain of syndecan-4 has a well-defined dimeric structure, the syndecan ectodomains are intrinsically disordered, which is linked to a capacity to interact with multiple partners. However, it remains to fully establish the impact of glycanation and partner proteins on syndecan core protein conformations. Genetic models indicate that a conserved property of syndecans links the cytoskeleton to calcium channels of the transient receptor potential class, compatible with roles as mechanosensors. In turn, syndecans influence actin cytoskeleton organization to impact motility, adhesion and the extracellular matrix environment. Syndecan clustering with other cell surface receptors into signaling microdomains has relevance to tissue differentiation in development, for example in stem cells, but also in disease where syndecan expression can be markedly up-regulated. Since syndecans have potential as diagnostic and prognostic markers as well as possible targets in some forms of cancer, it remains important to unravel structure/function relationships in the four mammalian syndecans.

Heparan sulfate proteoglycans in cancer: Pathogenesis and therapeutic potential

The heparan sulfate proteoglycans (HSPGs) are glycoproteins that consist of a proteoglycan "core" protein and covalently attached heparan sulfate (HS) chain. HSPGs are ubiquitously expressed in mammalian cells on the cell surface and in the extracellular matrix (ECM) and secretory vesicles. Within HSPGs, the protein cores determine when and where HSPG expression takes place, and the HS chains mediate most of HSPG's biological roles through binding various protein ligands, including cytokines, chemokines, growth factors and receptors, morphogens, proteases, protease inhibitors, and ECM proteins. Through these interactions, HSPGs modulate cell proliferation, adhesion, migration, invasion, and angiogenesis to display essential functions in physiology and pathology. Under physiological conditions, the expression and localization of HSPGs are finely regulated to orchestrate their physiological functions, and this is disrupted in cancer. The HSPG dysregulation elicits multiple oncogenic signaling, including growth factor signaling, ECM and Integrin signaling, chemokine and immune signaling, cancer stem cell, cell differentiation, apoptosis, and senescence, to prompt cell transformation, proliferation, tumor invasion and metastasis, tumor angiogenesis and inflammation, and immunotolerance. These oncogenic roles make HSPGs an attractive pharmacological target for anti-cancer therapy. Several therapeutic strategies have been under development, including anti-HSPG antibodies, peptides and HS mimetics, synthetic xylosides, and heparinase inhibitors, and shown promising anti-cancer efficacy. Therefore, much progress has been made in this line of study. However, it needs to bear in mind that the roles of HSPGs in cancer can be either oncogenic or tumor-suppressive, depending on the HSPG and the cancer cell type with the underlying mechanisms that remain obscure. Further studies need to address these to fill the knowledge gap and rationalize more efficient therapeutic targeting.

Plasma membrane proteoglycans syndecan-2 and syndecan-4 engage with EGFR and RON kinase to sustain carcinoma cell cycle progression

Epidermal growth factor receptor (EGFR) is a causal factor in carcinoma, yet many carcinoma patients are resistant to EGFR inhibitors. Potential insight into this resistance stems from prior work that showed EGFR in normal epithelial cells docks to the extracellular domain of the plasma membrane proteoglycan syndecan-4 (Sdc4) engaged with α3β1 and α6β4 integrins. We now report that this receptor complex is modified by the recruitment of syndecan-2 (Sdc2), the Recepteur d'Origine Nantais (RON) tyrosine kinase, and the cellular signaling mediator Abelson murine leukemia viral oncogene homolog 1 (ABL1) in triple-negative breast carcinoma and head and neck squamous cell carcinoma, where it contributes to EGFR kinase-independent proliferation. Treatment with a peptide mimetic of the EGFR docking site in the extracellular domain of Sdc4 (called SSTNEGFR) disrupts the entire complex and causes a rapid, global arrest of the cell cycle. Normal epithelial cells do not recruit these additional receptors to the adhesion mechanism and are not arrested by SSTNEGFR. Although EGFR docking with Sdc4 in the tumor cells is required, cell cycle progression does not depend on EGFR kinase. Instead, progression depends on RON kinase, activated by its incorporation into the complex. RON activates ABL1, which suppresses p38 mitogen-activated protein kinase and prevents a p38-mediated signal that would otherwise arrest the cell cycle. These findings add to the growing list of receptor tyrosine kinases that support tumorigenesis when activated by their association with syndecans at sites of matrix adhesion and identify new potential targets for cancer therapy.

Extended disorder at the cell surface: The conformational landscape of the ectodomains of syndecans

Syndecans are membrane proteoglycans regulating extracellular matrix assembly, cell adhesion and signaling. Their ectodomains can be shed from the cell surface, and act as paracrine and autocrine effectors or as competitors of full-length syndecans. We report the first biophysical characterization of the recombinant ectodomains of the four human syndecans using biophysical techniques, and show that they behave like flexible random-coil intrinsically disordered proteins, and adopt several conformation ensembles in solution. We have characterized their conformational landscapes using native mass spectrometry (MS) and ion-mobility MS, and demonstrated that the syndecan ectodomains explore the majority of their conformational landscape, from minor compact, globular-like, conformations to extended ones. We also report that the ectodomain of syndecan-4, corresponding to a natural isoform, is able to dimerize via a disulfide bond. We have generated a three-dimensional model of the C-terminus of this dimer, which supports the dimerization via a disulfide bond. Furthermore, we have mapped the NXIP adhesion motif of syndecans and their sequences involved in the formation of ternary complexes with integrins and growth factor receptors on the major conformations of their ectodomains, and shown that these sequences are not accessible in all the conformations, suggesting that only some of them are biologically active. Lastly, although the syndecan ectodomains have a far lower number of amino acid residues than their membrane partners, their intrinsic disorder and flexibility allow them to adopt extended conformations, which have roughly the same size as the cell surface receptors (e.g., integrins and growth factor receptors) they bind to.

Syndecan Transmembrane Domain Specifically Regulates Downstream Signaling Events of the Transmembrane Receptor Cytoplasmic Domain

Despite the known importance of the transmembrane domain (TMD) of syndecan receptors in cell adhesion and signaling, the molecular basis for syndecan TMD function remains unknown. Using in vivo invertebrate models, we found that mammalian syndecan-2 rescued both the guidance defects in C. elegans hermaphrodite-specific neurons and the impaired development of the midline axons of Drosophila caused by the loss of endogenous syndecan. These compensatory effects, however, were reduced significantly when syndecan-2 dimerization-defective TMD mutants were introduced. To further investigate the role of the TMD, we generated a chimera, 2eTPC, comprising the TMD of syndecan-2 linked to the cytoplasmic domain of platelet-derived growth factor receptor (PDGFR). This chimera exhibited SDS-resistant dimer formation that was lost in the corresponding dimerization-defective syndecan-2 TMD mutant, 2eT(GL)PC. Moreover, 2eTPC specifically enhanced Tyr 579 and Tyr 857 phosphorylation in the PDGFR cytoplasmic domain, while the TMD mutant failed to support such phosphorylation. Finally, 2eTPC, but not 2eT(GL)PC, induced phosphorylation of Src and PI3 kinase (known downstream effectors of Tyr 579 phosphorylation) and promoted Src-mediated migration of NIH3T3 cells. Taken together, these data suggest that the TMD of a syndecan-2 specifically regulates receptor cytoplasmic domain function and subsequent downstream signaling events controlling cell behavior.

Syndecan-1 (CD138), Carcinomas and EMT

Cell surface proteoglycans are known to be important regulators of many aspects of cell behavior. The principal family of transmembrane proteoglycans is the syndecans, of which there are four in mammals. Syndecan-1 is mostly restricted to epithelia, and bears heparan sulfate chains that are capable of interacting with a large array of polypeptides, including extracellular matrix components and potent mediators of proliferation, adhesion and migration. For this reason, it has been studied extensively with respect to carcinomas and tumor progression. Frequently, but not always, syndecan-1 levels decrease as tumor grade, stage and invasiveness and dedifferentiation increase. This parallels experiments that show depletion of syndecan-1 can be accompanied by loss of cadherin-mediated adhesion. However, in some tumors, levels of syndecan-1 increase, but the characterization of its distribution is relevant. There can be loss of membrane staining, but acquisition of cytoplasmic and/or nuclear staining that is abnormal. Moreover, the appearance of syndecan-1 in the tumor stroma, either associated with its cellular component or the collagenous matrix, is nearly always a sign of poor prognosis. Given its relevance to myeloma progression, syndecan-1-directed antibody—toxin conjugates are being tested in clinical and preclinical trials, and may have future relevance to some carcinomas.

Syndecan-4 as a Pathogenesis Factor and Therapeutic Target in Cancer

Cancer is an important cause of morbidity and mortality worldwide. Advances in research on the biology of cancer revealed alterations in several key pathways underlying tumorigenesis and provided molecular targets for developing new and improved existing therapies. Syndecan-4, a transmembrane heparan sulfate proteoglycan, is a central mediator of cell adhesion, migration and proliferation. Although several studies have demonstrated important roles of syndecan-4 in cell behavior and its interactions with growth factors, extracellular matrix (ECM) molecules and cytoskeletal signaling proteins, less is known about its role and expression in multiple cancer. The data summarized in this review demonstrate that high expression of syndecan-4 is an unfavorable biomarker for estrogen receptor-negative breast cancer, glioma, liver cancer, melanoma, osteosarcoma, papillary thyroid carcinoma and testicular, kidney and bladder cancer. In contrast, in neuroblastoma and colorectal cancer, syndecan-4 is downregulated. Interestingly, syndecan-4 expression is modulated by anticancer drugs. It is upregulated upon treatment with zoledronate and this effect reduces invasion of breast cancer cells. In our recent work, we demonstrated that the syndecan-4 level was reduced after trastuzumab treatment. Similarly, syndecan-4 levels are also reduced after panitumumab treatment. Together, the data found suggest that syndecan-4 level is crucial for understanding the changes involving in malignant transformation, and also demonstrate that syndecan-4 emerges as an important target for cancer therapy and diagnosis.

3-O-sulfated heparan sulfate interactors target synaptic adhesion molecules from neonatal mouse brain and inhibit neural activity and synaptogenesis in vitro

Heparan sulfate (HS) chains, covalently linked to heparan sulfate proteoglycans (HSPG), promote synaptic development and functions by connecting various synaptic adhesion proteins (AP). HS binding to AP could vary according to modifications of HS chains by different sulfotransferases. 3-O-sulfotransferases (Hs3sts) produce rare 3-O-sulfated HSs (3S-HSs), of poorly known functions in the nervous system. Here, we showed that a peptide known to block herpes simplex virus by interfering with 3S-HSs in vitro and in vivo (i.e. G2 peptide), specifically inhibited neural activity, reduced evoked glutamate release, and impaired synaptic assembly in hippocampal cell cultures. A role for 3S-HSs in promoting synaptic assembly and neural activity is consistent with the synaptic interactome of G2 peptide, and with the detection of Hs3sts and their products in synapses of cultured neurons and in synaptosomes prepared from developing brains. Our study suggests that 3S-HSs acting as receptors for herpesviruses might be important regulators of neuronal and synaptic development in vertebrates.

Cell-surface heparan sulfate proteoglycans as multifunctional integrators of signaling in cancer

Proteoglycans (PGs) represent a large proportion of the components that constitute the extracellular matrix (ECM). They are a diverse group of glycoproteins characterized by a covalent link to a specific glycosaminoglycan type. As part of the ECM, heparan sulfate (HS)PGs participate in both physiological and pathological processes including cell recruitment during inflammation and the promotion of cell proliferation, adhesion and motility during development, angiogenesis, wound repair and tumor progression. A key function of HSPGs is their ability to modulate the expression and function of cytokines, chemokines, growth factors, morphogens, and adhesion molecules. This is due to their capacity to act as ligands or co-receptors for various signal-transducing receptors, affecting pathways such as FGF, VEGF, chemokines, integrins, Wnt, notch, IL-6/JAK-STAT3, and NF-κB. The activation of those pathways has been implicated in the induction, progression, and malignancy of a tumor. For many years, the study of signaling has allowed for designing specific drugs targeting these pathways for cancer treatment, with very positive results. Likewise, HSPGs have become the subject of cancer research and are increasingly recognized as important therapeutic targets. Although they have been studied in a variety of preclinical and experimental models, their mechanism of action in malignancy still needs to be more clearly defined. In this review, we discuss the role of cell-surface HSPGs as pleiotropic modulators of signaling in cancer and identify them as promising markers and targets for cancer treatment.

The Cardiac Syndecan-2 Interactome

The extracellular matrix (ECM) is important in cardiac remodeling and syndecans have gained increased interest in this process due to their ability to convert changes in the ECM to cell signaling. In particular, syndecan-4 has been shown to be important for cardiac remodeling, whereas the role of its close relative syndecan-2 is largely unknown in the heart. To get more insight into the role of syndecan-2, we here sought to identify interaction partners of syndecan-2 in rat left ventricle. By using three different affinity purification methods combined with mass spectrometry (MS) analysis, we identified 30 novel partners and 9 partners previously described in the literature, which together make up the first cardiac syndecan-2 interactome. Eleven of the novel partners were also verified in HEK293 cells (i.e., AP2A2, CAVIN2, DDX19A, EIF4E, JPH2, MYL12A, NSF, PFDN2, PSMC5, PSMD11, and RRAD). The cardiac syndecan-2 interactome partners formed connections to each other and grouped into clusters mainly involved in cytoskeletal remodeling and protein metabolism, but also into a cluster consisting of a family of novel syndecan-2 interaction partners, the CAVINs. MS analyses revealed that although syndecan-2 was significantly enriched in fibroblast fractions, most of its partners were present in both cardiomyocytes and fibroblasts. Finally, a comparison of the cardiac syndecan-2 and -4 interactomes revealed surprisingly few protein partners in common.

Cell Density-Dependent Fibroblast Growth Factor-2 Signaling Regulates Syndecan-4 Expression in Cultured Vascular Endothelial Cells

Syndecan-4 is a member of the syndecan family of transmembrane heparan sulfate proteoglycans, and is involved in cell protection, proliferation, and the blood coagulation-fibrinolytic system in vascular endothelial cells. Heparan sulfate chains enable fibroblast growth factor-2 (FGF-2) to form a complex with its receptor and to transduce the cell growth signal. In the present study, bovine aortic endothelial cells were cultured, and the intracellular signal pathways that mediate the regulation of syndecan-4 expression in dense and sparse cultures by FGF-2 were analyzed. We demonstrated the cell density-dependent differential regulation of syndecan-4 expression. Specifically, we found that FGF-2 upregulated the synthesis of syndecan-4 in vascular endothelial cells via the MEK1/2-ERK1/2 pathway in dense cell cultures, with only a transcriptional induction of syndecan-4 at a low cell density via the Akt pathway. This study highlights a critical mechanism underlying the regulation of endothelial cell functions by proteoglycans.

The oligomerization mediated by the alanine 397 residue in the transmembrane domain is crucial to sydecan-3 functions

Syndecans are single-pass transmembrane proteins on the cell surface that are involved in various cellular functions. Previously, we reported that both homo- and hetero-form of syndecan dimers affected their functionality. However, little is known about the structural role of the transmembrane domain of syndecan-3. A series of glutathione-S-transferase syndecan-3 proteins showed that syndecan-3 formed SDS-resistant dimers and oligomers. SDS-resistant oligomer formation was barely observed in the syndecan deletion mutants lacking the transmembrane domain. Interestingly, the presence of an alanine 397 residue in the transmembrane domain correlated with SDS-resistant oligomer, and its replacement by phenylalanine (AF mutant) significantly reduced SDS-resistant oligomer formation. Beside the AF mutant significantly reduced syndecan-3 mediated cellular processes such as cell adhesion, migration and neurite outgrowth of SH-SY5Y neuroblastoma. Furthermore, the alanine residue regulated hetero-oligomer formation of syndecan-3, and hetero-oligomer formation significantly reduced syndecan-3-mediated neurite outgrowth of SH-SY5Y cells. Taken together, all these data suggest that syndecan-3 has a specific feature of oligomerization by the transmembrane domain and this oligomerization tendency is crucial for the function of syndecan-3.

Contribution of syndecans to cellular uptake and fibrillation of α-synuclein and tau

Scientific evidence suggests that α-synuclein and tau have prion-like properties and that prion-like spreading and seeding of misfolded protein aggregates constitutes a central mechanism for neurodegeneration. Heparan sulfate proteoglycans (HSPGs) in the plasma membrane support this process by attaching misfolded protein fibrils. Despite of intense studies, contribution of specific HSPGs to seeding and spreading of α-synuclein and tau has not been explored yet. Here we report that members of the syndecan family of HSPGs mediate cellular uptake of α-synuclein and tau fibrils via a lipid-raft dependent and clathrin-independent endocytic route. Among syndecans, the neuron predominant syndecan-3 exhibits the highest affinity for both α-synuclein and tau. Syndecan-mediated internalization of α-synuclein and tau depends heavily on conformation as uptake via syndecans start to dominate once fibrils are formed. Overexpression of syndecans, on the other hand, reduces cellular uptake of monomeric α-synuclein and tau, yet exerts a fibril forming effect on both proteins. Data obtained from syndecan overexpressing cellular models presents syndecans, especially the neuron predominant syndecan-3, as important mediators of seeding and spreading of α-synuclein and tau and reveal how syndecans contribute to fundamental molecular events of α-synuclein and tau pathology.

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.

A cytoplasmic C-terminal fragment of Syndecan-1 is generated by sequential proteolysis and antagonizes Syndecan-1 dependent lung tumor cell migration

Syndecan-1 is a surface expressed heparan sulphate proteoglycan, which is upregulated by several tumor types and involved in tumor cell migration and metastasis. Syndecan-1 is shed from the cell surface and the remaining transmembrane fragment undergoes intramembrane proteolysis by γ-secretase. We here show that this generates a cytoplasmic C-terminal fragment (cCTF). In epithelial lung tumor A549 cells the endogenously produced cCTF accumulated when its proteasomal degradation was blocked with bortezomib and this accumulation was prevented by γ-secretase inhibition. Overexpression of the cCTF suppressed migration and invasion of A549 cells. This inhibitory effect was only seen when endogenous syndecan-1 was present, but not in syndecan-1 deficient cells. Further, overexpression of syndecan-1 cCTF increased the basal activation of Src kinase, focal adhesion kinase (FAK) and Rho GTPase. This was associated with increased adhesion to fibronectin and collagen G and an increased recruitment of paxillin to focal adhesions. Moreover, lung tumor formation of A549 cells in mice was reduced by overexpression of syndecan-1 cCTF. Finally, delivery of a synthetic peptide corresponding to the syndecan-1 cCTF suppressed A549 cell migration and increased basal phosphorylation of Src and FAK. Our data indicate that the syndecan-1 cCTF antagonizes syndecan-1 dependent tumor cell migration in vitro and in vivo by dysregulating proadhesive signaling pathways and suggest that the cCTF can be used as an inhibitory peptide.

Minireview: Syndecans and their crucial roles during tissue regeneration

Syndecans are transmembrane heparan sulfate proteoglycans, with roles in development, tumorigenesis and inflammation, and growing evidence for involvement in tissue regeneration. This is a fast developing field with the prospect of utilizing tissue engineering and biomaterials in novel therapies. Syndecan receptors are not only ubiquitous in mammalian tissues, regulating cell adhesion, migration, proliferation, and differentiation through independent signaling but also working alongside other receptors. Their importance is highlighted by an ability to interact with a diverse array of ligands, including extracellular matrix glycoproteins, growth factors, morphogens, and cytokines that are important regulators of regeneration. We also discuss the potential for syndecans to regulate stem cell properties, and suggest that understanding these proteoglycans is relevant to exploiting cell, tissue, and materials technologies.

Syndecans as Cell Surface Receptors in Cancer Biology. A Focus on their Interaction with PDZ Domain Proteins

Syndecans are transmembrane receptors with ectodomains that are modified by glycosaminoglycan chains. The ectodomains can interact with a wide variety of molecules, including growth factors, cytokines, proteinases, adhesion receptors, and extracellular matrix (ECM) components. The four syndecans in mammals are expressed in a development-, cell-type-, and tissue-specific manner and can function either as co-receptors with other cell surface receptors or as independent adhesion receptors that mediate cell signaling. They help regulate cell proliferation and migration, angiogenesis, cell/cell and cell/ECM adhesion, and they may participate in several key tumorigenesis processes. In some cancers, syndecan expression regulates tumor cell proliferation, adhesion, motility, and other functions, and may be a prognostic marker for tumor progression and patient survival. The short cytoplasmic tail is likely to be involved in these events through recruitment of signaling partners. In particular, the conserved carboxyl-terminal EFYA tetrapeptide sequence that is present in all syndecans binds to some PDZ domain-containing proteins that may function as scaffold proteins that recruit signaling and cytoskeletal proteins to the plasma membrane. There is growing interest in understanding these interactions at both the structural and biological levels, and recent findings show their high degree of complexity. Parameters that influence the recruitment of PDZ domain proteins by syndecans, such as binding specificity and affinity, are the focus of active investigations and are important for understanding regulatory mechanisms. Recent studies show that binding may be affected by post-translational events that influence regulatory mechanisms, such as phosphorylation within the syndecan cytoplasmic tail.

The Conserved Phenylalanine in the Transmembrane Domain Enhances Heteromeric Interactions of Syndecans

The transmembrane domain (TMD) of the syndecans, a family of transmembrane heparin sulfate proteoglycans, is involved in forming homo- and heterodimers and oligomers that transmit signaling events. Recently, we reported that the unique phenylalanine in TMD positively regulates intramolecular interactions of syndecan-2. Besides the unique phenylalanine, syndecan-2 contains a conserved phenylalanine (SDC2-Phe-169) that is present in all syndecan TMDs, but its function has not been determined. We therefore investigated the structural role of SDC2-Phe-169 in syndecan TMDs. Replacement of SDC2-Phe-169 by tyrosine (S2F169Y) did not affect SDS-resistant homodimer formation but significantly reduced SDS-resistant heterodimer formation between syndecan-2 and -4, suggesting that SDC2-Phe-169 is involved in the heterodimerization/oligomerization of syndecans. Similarly, in an in vitro binding assay, a syndecan-2 mutant (S2(F169Y)) showed a significantly reduced interaction with syndecan-4. FRET assays showed that heteromolecular interactions between syndecan-2 and -4 were reduced in HEK293T cells transfected with S2(F169Y) compared with syndecan-2. Moreover, S2(F169Y) reduced downstream reactions mediated by the heterodimerization of syndecan-2 and -4, including Rac activity, cell migration, membrane localization of PKCα, and focal adhesion formation. The conserved phenylalanine in syndecan-1 and -3 also showed heterodimeric interaction with syndecan-2 and -4. Taken together, these findings suggest that the conserved phenylalanine in the TMD of syndecans is crucial in regulating heteromeric interactions of syndecans.