Abstract
Hyaluronan (HA) is essentially secreted by every cell and plays a critical role in maintaining normal cell physiology. While the structure and function of HA have been extensively investigated, questions regarding the sizes and conformation of HA under physiological and inflamed conditions, in relevance to its functions, remain elusive. In this article, we update our knowledge of the HA functional properties, including binding proteins and their signaling networks, as well as matrix formation, which can potentially induce phase separation and affect the mobility and behavior of small molecules, proteins, and cells. We detail the striking differences regarding the biological outcomes of signaling pathways for HA and membrane receptors versus HA and GPI-linked hyaluronidase Hyal-2. We describe: (1) the native, large-sized HA is not proapoptotic but signals with an overexpressed HYAL-2/WWOX/SMAD4 complex to induce apoptosis, which is likely to occur in an inflamed microenvironment; (2) HA-binding proteins are connected via signal pathway networks. The competitive binding of HA and TGF-β to the membrane HYAL-2 and the downstream HYAL-2/WWOX/SMAD4 signaling is addressed; (3) the phase-separated proteins or small molecules in the HA matrices may contribute to the aberrant interactions, leading to inflammation and disease progression; (4) the role of HA and complement C1q in Alzheimer's disease via connection with a risk factor for Alzheimer's disease WWOX is also discussed; (5) a hidden function is the inducible HA conformational changes that confer cancer suppression and, probably, retardation of neurodegeneration.