Perineuronal Nets in the Central Nervous System

Abstract

One hundred and twenty years ago, Camillo Golgi described reticular or fenestrated sheaths of extracellular matrix (ECM), enwrapping the cell bodies, axon initial segments and proximal dendrites of certain pyramidal and non‐pyramidal neurons in the adult mammalian central nervous system. Such structures, currently termed perineuronal nets (PNs), are preferentially associated with GABAergic, parvalbumin‐containing fast‐spiking inhibitory types of cells, and mostly expressing the Kv3.1 subunit of voltage‐gated potassium channels. Although some neuroanatomists validated the existence of the PNs on selected neurons in the adult brain, there was a lack of agreement concerning their origin, composition and function. Recent data suggest that PNs result from the visualization of ECM molecules that are confined to the space interposed between glial processes and the nerve cells that they outline. The substance confined to these spaces could be visualized selectively by antibodies directed to glycoproteins, proteoglycans, markers for hyaluronan, some plant lectins recognizing N‐acetylgalactosamine or by monoclonal antibodies directed to epitopes on unknown molecules. The PNs appear only after birth, which they could serve as recognition molecules between certain neurons and their surrounding cells. Other putative roles include stabilization of synapses, maintenance of cellular relationships in the adult brain, concentration of growth factors around certain neurons, generation of a polyanionic ion‐buffering microenvironment, as well as prevention of extracellular‐space occlusion and the formation of certain link with the intracellular cytoskeleton.