SAM68 RNA-Binding Protein: a Conductor of Endothelial Cell Adhesion

Angiogenesis, the formation of new blood vessels from pre-existing ones, hinges on the modified adhesive properties of vascular endothelial cells and the production and assembly of their specialized extracellular matrix (ECM), known as the basement membrane. Endothelial cell adhesion is facilitated by interactions between ECM components and transmembrane receptors (integrins). These interactions lead to the formation of dynamic multimolecular adhesion complexes that control cytoskeleton organization and force transmission (mechanotransduction) essential for cell migration and function. Recent studies indicate that local protein production at adhesion sites plays a role in strengthening and developing these sites.

Proteomic screenings at adhesion sites have identified RNA-binding proteins (RBPs) with roles in gene expression regulation. SAM68 (Src associated in mitosis, of 68 kDa), a member of the STAR (signal transduction and activation of RNA metabolism) family, is known for both RNA biogenesis and signal transduction regulation, acting as a scaffold after cell surface receptor activation. It has been demonstrated that SAM68’s association with Src kinase aids the alternative splicing of genes encoding ECM and ECM-associated proteins like tenascin-C and cell surface glycoprotein CD44, which are instrumental in adhesion/migration.

IIn the context of angiogenesis and endothelial cell adhesion, it was hypothesized that SAM68 could serve as a molecular relay during integrin-mediated mechanotransduction. It might contribute to focal adhesion formation and downstream transcriptional/post-transcriptional responses governing the angiogenic phenotype. Using primary cultures of human endothelial cells in 2D and 3D contexts, the researchers demonstrated SAM68’s role in establishing focal adhesion by assisting with the localized supply of mRNA (including β-actin transcript) necessary for focal adhesion maturation. SAM68 also regulates the expression of genes encoding basal membrane proteins, notably through regulation of the FN1 gene promoter, thereby conditioning the sub-endothelial matrix and the cells' angiogenic phenotype. In a 3D context, SAM68 depletion hinders endothelial cell budding and pseudo-capillary organization.

This research paves the way for investigating other RNA-binding proteins that serve as regulators of adhesion signaling and for assessing their role in physiological and pathological angiogenesis processes.

For more information : Zeinab Rekad, Michaël Ruff, Agata Radwanska, Dominique Grall, Delphine Ciais, Ellen Van Obberghen-Schilling. Coalescent RNA-localizing and transcriptional activities of SAM68 modulate adhesion and subendothelial basement membrane assembly. eLIFE (2023)

Illustration: SAM68 regulates cellular adhesion maturation and matrix conditioning beneath endothelial cells. Upon integrin activation, a cytoplasmic fraction of SAM68 temporarily contributes to stabilize the adhesion complex (adhesome) by regulating integrin signaling and the localization of β-actin mRNA. In the nucleus, SAM68 controls the expression of key sub-endothelial matrix genes, thereby facilitating the assembly and conditioning of the basement membrane. These coalescent functions of SAM68 are instrumental in adapting endothelial cells to their microenvironment and play a crucial role in angiogenesis.