Spatio-temporal differences in perineuronal net expression in the mouse hippocampus, with reference to parvalbumin

Highlights

• PNN intensity shows no dorsoventral differences in the young mouse hippocampus.

• Developmental increase in PNN intensity is larger in dorsal than in the ventral part.

• PNN intensity is higher in the dorsal than in the ventral part in adult mice.

• Aging-related reduction in PNN intensity occurs in the CA1 region of the dorsal part.

• Correlation of PNN and PV intensities declines during development and aging.

Abstract

Perineuronal net (PNN) is a specialized aggregate of the extracellular matrix, which is considered to be involved in regulation of structural plasticity of neuronal circuits. Here we examined the spatial and temporal differences in Wisteria floribunda agglutinin-labeled PNN intensity in single cells in the mouse hippocampus, where the neuronal circuits engaged in cognition and emotion are embedded in the dorsal and ventral parts, respectively. In young mice, the intensity of PNN was very low, and there were no significant dorsoventral differences in all hippocampal regions. Developmental increase in PNN intensity was larger in the dorsal part than in the ventral part. As a result, PNN intensity was higher in the dorsal part than in the ventral part in adult mice. Aging dissimilarly affects different regions of the dorsal hippocampus. Namely, PNN intensity in the dorsal part of old mice declined in the CA1 region, remained unchanged in the CA3 region, increased in the dentate gyrus. By contrast, there were no significant aging-related changes in PNN intensity in the ventral hippocampus. We also examined the intensity of parvalbumin (PV), an EF-hand calcium-binding protein, because it has been shown that PNNs are closely related to PV-containing GABAergic inhibitory neurons. Contrary to expectations, developmental and aging-related changes in PV intensity were not comparable to those seen in PNN intensity. The correlation coefficients between PNN and PV intensities in single cells showed gradual decline during development and aging in the CA1 and CA3 regions, while there were little correlations in the dentate gyrus regardless of age. In summary, PNNs are differentially expressed in the dorsal and ventral hippocampal circuits during development and aging, indicating their possible role for cognition and emotion control.

Introduction

The perineuronal net (PNN) was first described by Camillo Golgi in 1882 as a reticular structure that enwrapped the soma and dendrites of neurons. Later studies have shown that the PNN is a specialized organization of the extracellular matrix (ECM), which is molecularly distinct from other classical matrices (Giamanco and Matthews, 2012). PNNs are uniquely enriched with chondroitin sulfate proteoglycans (CSPGs) and hyaluronan (Bandtlow and Zimmermann, 2000). Several techniques have been developed to visualize PNNs: cytochemical labeling using plant lectins that have an affinity for N-acetylgalactosamine (Schweizer et al., 1993), e.g., Vicia villosa agglutinin (VVA) and Wisteria floribunda agglutinin (WFA); and immunohistochemical labeling using monoclonal antibodies against CSPGs, e.g., Cat-301 and 6B4 (McKay and Hockfield, 1982, Maeda et al., 1996). In the central nervous system, PNNs are particularly associated with GABAergic inhibitory neurons containing parvalbumin (PV; Kosaka and Heizmann, 1989), and subpopulations of pyramidal neurons (Hausen et al., 1996, Ojima et al., 1998). Interestingly, research conducted in the last 2 decades has indicated that PNNs play a critical role in regulation of neural plasticity (Wang and Fawcett, 2012). Namely, attenuation of PNNs in the visual system has been shown to reopen the critical window for visual system plasticity (Pizzorusso et al., 2002). Formation of PNNs triggered by neuronal cartilage link protein synthesis is a key event in the diminution of plasticity (Carulli et al., 2010). Genetic or enzymatic disruption of PNNs enhances long-term object recognition memory and facilitates long-term depression (Romberg et al., 2013). Ablation of four dominant ECM components compromises synaptic structure and function (Geissler et al., 2013).

The hippocampus is one of the major limbic nuclei. In the rodent brain, the hippocampus appears grossly as an elongated structure with its longitudinal axis extending in a C-shaped fashion from the septal nuclei of the basal forebrain to the temporal lobe. This longitudinal alignment of the rodent hippocampus is referred to as the “dorsoventral” axis. Recent reports substantiate a structural and functional dissociation between the dorsal and ventral hippocampi in learning, memory and emotion (Bannerman et al., 2004). Lesions of the dorsal hippocampus impair spatial learning in rats (Moser et al., 1993). By contrast, injuries of the ventral hippocampus affect anxiety-related behavior and have no effect on spatial learning in rats (Bannerman et al., 2003). Genes expressed in the dorsal hippocampus are associated with brain regions involved in cognitive information processing, while those in the ventral hippocampus are associated with regions involved in emotional behaviors (Dong et al., 2009). Structural and functional differentiation of the hippocampus along the longitudinal axis is evolutionarily conserved, and has been demonstrated in monkeys (Colombo et al., 1998) and humans (Small et al., 2001).

Despite the recent interest in PNN, few studies have investigated the potential alterations in PNN expression during development and aging. By the same token, it remains unclear whether PNN is involved in functional differentiation of the hippocampus. To address these issues, here we examined the spatial and temporal differences in the expression of PNN in the mouse hippocampus. Using digital image analysis, we comparatively estimated the fluorescence intensity of WFA-labeled (WFA+) PNNs in single cells in the dorsal and ventral parts of the hippocampi of young, adult and old mice. The present results indicate that PNNs may differentially regulate structural plasticity of hippocampal neuronal circuits engaged in cognition and emotion, respectively. Furthermore, neuronal activity might independently affect the intensities of PNN and PV through development and aging.