Losing the sugar coating: potential impact of perineuronal net abnormalities on interneurons in schizophrenia

Abstract

Perineuronal nets (PNNs) were shown to be markedly altered in subjects with schizophrenia. In particular, decreases of PNNs have been detected in the amygdala, entorhinal cortex and prefrontal cortex. The formation of these specialized extracellular matrix (ECM) aggregates during postnatal development, their functions, and association with distinct populations of GABAergic interneurons, bear great relevance to the pathophysiology of schizophrenia. PNNs gradually mature in an experience-dependent manner during late stages of postnatal development, overlapping with the prodromal period/age of onset of schizophrenia. Throughout adulthood, PNNs regulate neuronal properties, including synaptic remodeling, cell membrane compartmentalization and subsequent regulation of glutamate receptors and calcium channels, and susceptibility to oxidative stress. With the present paper, we discuss evidence for PNN abnormalities in schizophrenia, the potential functional impact of such abnormalities on inhibitory circuits and, in turn, cognitive and emotion processing. We integrate these considerations with results from recent genetic studies showing genetic susceptibility for schizophrenia associated with genes encoding for PNN components, matrix-regulating molecules and immune system factors. Notably, the composition of PNNs is regulated dynamically in response to factors such as fear, reward, stress, and immune response. This regulation occurs through families of matrix metalloproteinases that cleave ECM components, altering their functions and affecting plasticity. Several metalloproteinases have been proposed as vulnerability factors for schizophrenia. We speculate that the physiological process of PNN remodeling may be disrupted in schizophrenia as a result of interactions between matrix remodeling processes and immune system dysregulation. In turn, these mechanisms may contribute to the dysfunction of GABAergic neurons.

Keywords: Amygdala; Entorhinal cortex; Extracellular matrix; Perineuronal nets; Prefrontal cortex; Schizophrenia.

Figure 1. PNNs in the Human Amygdala

Photomicrographs of PNNs labeled using the CS-6 antibody 3B3 (A), and the lectin WFA (B; counterstained with Nissl cresyl violet) in the lateral nucleus of the normal human amygdala. Scale bar = 50 μm.

Figure 2. CSPG Structure and Cleavage Sites

Schematic diagram depicting the structure of CSPGs, aggrecan in this specific example. CSPGs are composed by a protein core protein that includes distinct domains (e.g. G1, G2). To the protein core are attached numerous chondroitin sulfated glycosaminoglycan chains, creating a CS rich region (A). Each glycosaminoglycan chain consists of repeated pairs of glucuronic acid (GlcA) and N-acetyl-galactosamine (GalNAc). This latter can be sulfated in position 6 (CS-6) or 4 (CS-4) (B). The antibody 3B3 detects a non-reducing terminal end saturated CS disaccharide consisting of glucuronic acid Nacetyl- galactosamine-6-sulfate (CS-6; in A) (Caterson, 2012; Sorrell et al., 1988), whereas the lectin WFA detects N-actetyl-D-galactosamine on the terminal ends of CS chains, with a preference for beta glycosidic linkage (A) (Kurokawa et al., 1976; Young and Williams, 1985). CSPGs are cleaved by several metalloproteases, including ADAMTS (triangles) and MMPs (squares) on various sites of the protein core; indicated in A are those known for aggrecan (Nakamura et al., 2000; Porter et al., 2005).

Figure 3. PNN numbers in the central nucleus of the amygdala: region specificity of PNN abnormalities in SZ

(A) In the normal human amygdala, PNNs immunolabeled with an antibody against the v0/v1 isoform of the CSPG versican were exclusively found in the central nucleus (A). Inset in A shows numerous versican-immunolabeled PNNs in this nucleus. Scale bars = 50 μm. (B) Numbers PNN containing versican V0/V1 in the central nucleus were normal in subjects with (SZ) or bipolar disorder (BD). Similarly, numbers of WFA-labeled PNNs were not altered in this nucleus in either disorder. Black circles indicate values for each subject, black lines indicate 95% confidence intervals (unpublished data).

Figure 4. Neurons expressing somatostatin are ensheathed by PNNs

Dual antigen immunofluorescence shows that a subgroup of neurons expressing somatostatin bear PNNs. These neurons have not been previously shown to bear PNNs. Similar to neurons expressing parvalbumin, to a large extent associated with PNNs, somatostatin-positive neurons express voltage-gated potassium channel subunit Kv3 and have been shown to be altered in psychiatric disorders (Fung et al., 2014; Lin and Sibille, 2013; McDonald and Mascagni, 2006; Volk and Lewis, 2014; Volk et al., 2012). A subpopulation of these neurons has been shown to Photomicrographs in A-C show a neuron expressing somatostatin (green) ensheathed by a PNN containing CSPG CS-6 (3B3). In D-F, a neuron expressing somatostatin (red) is ensheathed by a WFA-labeled PNN. Scale bars = 50 μm.

Figure 5. Numbers of PNNs containing CS-6 (3B3) are decreased in the amygdala of subjects with SZ

Low magnification photomicrographs showing PNNs containing CS-6 (antibody 3B3) in the lateral nucleus of the human amygdala. Note the heterogeneous distribution within the dorsal medial and ventral portions of this nucleus, perhaps reflecting topographic association with distinct inputs. In healthy control subjects, 3B3-immunoreactive PNNs are particularly numerous in the lateral nucleus (A). In subjects with SZ, 3B3-immunoreactive PNNs were sharply decreased (B) (Pantazopolous et al., 2014). Scale bar = 1000 μm.