HAPLN1

HAPLN1, or hyaluronan and proteoglycan link protein 1, is a secreted extracellular matrix protein that facilitates the assembly of hyaluronan-proteoglycan aggregates in vertebrate tissues. It is a member of the hyaluronan and proteoglycan link protein (HAPLNs) family, which also includes HAPLN2, HAPLN3, and HAPLN4.

In connective tissues such as cartilage, HAPLN1 binds to hyaluronan and to proteoglycans like aggrecan, stabilizing the aggregates that confer the matrix its characteristic resilience. This stabilization supports tissue hydration and the compressive strength of cartilage and other extracellular matrix–rich tissues. In the central nervous system, HAPLN1 is a component of perineuronal nets, where it helps link hyaluronan to chondroitin sulfate proteoglycans and other matrix molecules around neurons.

Expression of HAPLN1 has been reported in cartilage and brain, with broader distribution across developing tissues. The protein is secreted into the extracellular matrix and acts in coordination with other ECM components to maintain matrix integrity during development and maintenance.

Clinical and research notes indicate that HAPLN1 and related proteins contribute to skeletogenesis and joint biology, including osteoarthritis, as well as brain ECM organization. While specific pathogenic mutations are not as well defined as for some other ECM components, HAPLN1 is considered important for proper ECM assembly, and alterations in its expression or interactions could impact tissue integrity.