HYALURONIC-ACID AND HYALURONIC ACID-BINDING PROTEINS IN BRAIN EXTRACELLULAR-MATRIX

Hyaluronic acid (HA) plays the main structural role in the formation o f brain extracellular matrix (ECM). The extracellular space appears em pty by electron microscopy because HA is readily dissolved during the preparation of tissues for ultrastructural studies. The HA-binding pro teins so far identified in brain ECM are versican, aggrecan and the gl ial HA-binding protein. Versican is a large fibroblast proteoglycan pr eferentially expressed in embryonic cartilage at the time of mesenchym al condensation. Glial HA-binding protein (GHAP) is probably a proteol ytic product of versican corresponding to its HA-binding amino-termina l domain. It is mainly a white-matter protein, suggesting that the pro teinase responsible for its cleavage from versican is normally activat ed in this location. Versican is found in both white matter and gray m atter, where it forms pericellular coats around large neurons. Aggreca n, the aggregating proteoglycan of mature cartilage, co-localizes with versican in this location. In white matter, the localization of GHAP and versican is identical to that of the glial fibrillary acid protein , suggesting that both proteins are produced by astrocytes. An importa nt difference between GHAP and versican is that GHAP but not versican is released from the tissues by hyaluronidase digestion, which suggest s that versican is anchored to the cell membranes lining the extracell ular space. GHAP was localized at the ultrastructural level in the gra nule cell layer of rat cerebellum, the only region of gray matter that is positive for GHAP in this species. Rats were perfused with aqueous fixatives containing cetylpyridinium chloride or tannic acid to preve nt the solubilization of HA. GHAP is found throughout the extracellula r space, the synaptic clefts being a notable exception. GHAP appears l ate in development, and the same is true for versican, the characteris tic perineuronal coats first becoming apparent in the third postnatal week. It is suggested that a marked change occurs in the structure of brain ECM when HA-binding proteins first appear, and that the change i s similar to that observed in prechondrogenic mesenchyme, i.e., reduct ion of the extracellular space and cell aggregation.