Nonproteoglycans

Nonproteoglycans are extracellular matrix components that are not proteoglycans. In connective tissues, the ECM consists of proteoglycans and a diverse set of nonproteoglycan macromolecules, principally structural proteins and adhesive glycoproteins. The nonproteoglycan fraction includes collagens (especially fibrillar types I, II, III), elastin, and a variety of matricellular and adhesion proteins such as fibronectin, laminin, tenascin, thrombospondin, and nidogen. These molecules are primarily protein-based and typically lack the long sulfated glycosaminoglycan (GAG) chains that characterize proteoglycans.

Functions of nonproteoglycans include providing mechanical strength, elasticity, and a scaffold for tissue organization. Collagens form fibrils that confer tensile strength; elastin provides elasticity and recoil; fibronectin and laminin mediate cell adhesion, migration, and basement membrane assembly. Matricellular proteins modulate cell signaling and matrix remodeling, coordinating interactions between cells and their surrounding matrix.

Synthesis and localization: Nonproteoglycans are secreted by fibroblasts, chondrocytes, osteoblasts, epithelial cells, and other cell types, and accumulate in interstitial matrices and basement membranes. They interact with cells via integrins and other receptors to influence attachment, shape, and signaling.

Clinical relevance: Alterations in nonproteoglycan ECM components can affect tissue integrity, wound healing, and pathological remodeling. Defects in collagens cause connective tissue disorders such as osteogenesis imperfecta and Ehlers-Danlos syndrome, while abnormal expression of laminin, fibronectin, or tenascin is linked to developmental defects and cancer progression.