Axon initial segment ensheathed by extracellular matrix in perineuronal nets

Perineuronal nets of extracellular matrix are associated with distinct types of neurons in the cerebral cortex and many subcortical regions. Large complexes of aggregating proteoglycans form a chemically specified microenvironment around the somata, proximal dendrites and the axon initial segment, including the presynaptic boutons attached to these domains. The subcellular distribution and the temporal course of postnatal formation suggest that perineuronal nets may be involved in the regulation of synaptic plasticity. Here we investigate structural and cytochemical characteristics of the extracellular matrix around axon initial segments virtually devoid of synaptic contacts. Wisteria floribunda agglutinin staining, the immunocytochemical detection of aggrecan and tenascin-R, as well as affinity-labeling of hyaluronan were used to analyze perineuronal nets associated with large motoneurons in the mouse superior colliculus. The molecular composition of perineuronal nets was divergent between neurons but was identical around the different cellular domains of the individual neurons. The axon initial segments largely devoid of synapses were covered by a continuous matrix sheath infiltrating the adjacent neuropil. The periaxonal zone penetrated by matrix components often increased in diameter along the initial segment from the axon hillock toward the myelinated part of the axon. The axonal and somatodendritic domains of perineuronal nets were concomitantly formed during the first three weeks of postnatal development. The common molecular properties and major structural features of subcellular perineuronal net domains were retained in organotypic midbrain slice cultures. The results support the hypothesis that the aggrecan-related extracellular matrix of perineuronal nets provides a continuous micromilieu for different subcellular domains performing integration and generation of the electrical activity of neurons.