Molecular heterogeneity of aggrecan-based perineuronal nets around five subclasses of parvalbumin-expressing neurons in the mouse hippocampus

Subsets of GABAergic neurons are surrounded by perineuronal nets (PNNs), which play a critical role in the regulation of neural plasticity and neuroprotection. Although the plant lectin Wisteria floribunda agglutinin (WFA) has been commonly used to label PNNs, WFA only detects N-acetyl-d-galactosamine on aggrecan, a member of the lectican family. In this study, we used WFA and the antibody against the core protein of aggrecan (ACAN) to investigate the molecular heterogeneity of aggrecan-based PNNs around five subclasses of parvalbumin-expressing (PV+) γ-aminobutyric acid (GABA)ergic neurons in the CA1 and CA3 regions of the mouse hippocampus. The vast majority of ACAN+ PNNs were colocalized with WFA in the stratum pyramidale, whereas a substantial population of ACAN+ PNNs lacked WFA labeling in the stratum oriens. We then defined the subclasses of PV+ neurons based on their cellular locations, molecular expression, and septal projection. Like the WFA+ PNNs, ACAN+ PNNs surrounded PV+ basket cells and bistratified cells but not axo-axonic cells. Unlike the WFA+ PNNs, ACAN+ PNNs frequently surrounded PV+ oriens-lacunosum moleculare cells and hippocampo-septal cells. Interestingly, the relative densities of GABAergic synapses were higher around PV+ neurons with ACAN+ PNNs than around those without ACAN+ PNNs. Degradation of WFA+ PNNs by chondroitinase ABC did not affect the GABAergic synaptic densities around PV+ neurons. Our findings suggest that the molecular composition of aggrecan-based PNNs around PV+ neurons may differ in a subclass-specific manner, and also might help determine the functional involvement of PNNs in the regulation of GABAergic synapses around PV+ neurons in the hippocampus. J. Comp. Neurol. 525:1234–1249, 2017.