Perineuronal nets affect parvalbumin expression in GABAergic neurons of the mouse hippocampus.

Recent studies have suggested that the perineuronal net (PNN), a specialised extracellular matrix structure, and parvalbumin (PV), an EF-hand calcium-binding protein, are involved in the regulation of plasticity of neural circuits. Here, we aimed to quantitatively estimate the relationship between the two plasticity regulators, PV and PNNs, in the hippocampus of young adult mice. Dual fluorescence staining for PV and Wisteria floribunda agglutinin (a broad PNN marker) showed that a substantial population of PV-expressing (PV(+) ) GABAergic neurons lacked PNNs. Optical disector analysis demonstrated that there were fewer PNN(+) neurons than PV(+) neurons. The ratio of PNN expression in PV(+) neurons was generally lower in the dendritic layers than in the principal cell layers, whereas the ratio of PV expression in PNN(+) neurons was effectively 100%. The mean PV fluorescence was significantly higher in PNN(+) /PV(+) neurons than in PNN(-) /PV(+) neurons. Cumulative frequencies for single-cell PV fluorescence indicated that intensely stained PV(+) neurons tend to be enwrapped by PNNs, whereas weakly stained PV(+) neurons are likely to lack PNNs. We digested the PNNs by a unilateral injection of chondroitinase ABC (chABC) into the dorsal CA1 region. Although the densities of PV(+) neurons remained unchanged, the PV fluorescence declined 7 days after chABC injection. Quantitative real-time polymerase chain reaction analysis demonstrated a reduction in PV mRNA expression following chABC injection. These findings indicate that the presence or absence of PNNs affects the relative PV expression in GABAergic neurons in the hippocampus.

Keratan sulfate expression in microglia is diminished in the spinal cord in experimental autoimmune neuritis.

Experimental autoimmune neuritis (EAN) is an animal model of Guillain-Barré syndrome, an inflammatory demyelination disease of the peripheral nervous system. Although this disease has been extensively studied on peripheral nerves, the pathology of the central nervous system has not been fully understood. Previous studies demonstrate that expression of keratan sulfate (KS), the sugar chain of proteoglycan, is associated with activated microglia/macrophages accumulated after neuronal injuries. Unexpectedly, we found here that KS is rather diminished in rat EAN. KS was restrictively expressed in microglia in the spinal cord of normal rats. KS was positive in 50% microglia in the ventral horn and 20% in the dorsal horn. In EAN, microglia increased in number and expressed the activation marker CD68, but KS expression was abolished. Concomitantly, pro-inflammatory cytokines, i.e., interferon (IFN)-γ, interleukin (IL)-1ß, and tumor necrosis factor (TNF)-α, were increased in the spinal cord of EAN rats, whereas anti-inflammatory cytokines, such as IL-4 and IL-10, were decreased. In addition, silencing of KSGal6ST attenuated KS expression on the primary cultured microglia and upregulated expression of some activation markers (TNF-α, IL-1ß, and iNOS) under the stimulation with lipopolysaccharide and IFN-γ. This study demonstrates for the first time a close association of EAN and disappearance of KS on microglia. KS expression could be a useful marker to evaluate the status of polyneuropathy.

Minocycline selectively inhibits M1 polarization of microglia.

Minocycline is commonly used to inhibit microglial activation. It is widely accepted that activated microglia exert dual functions, that is, pro-inflammatory (M1) and anti-inflammatory (M2) functions. The in vivo status of activated microglia is probably on a continuum between these two extreme states. However, the mechanisms regulating microglial polarity remain elusive. Here, we addressed this question focusing on minocycline. We used SOD1(G93A) mice as a model, which exhibit the motor neuron-specific neurodegenerative disease, amyotrophic lateral sclerosis. Administration of minocycline attenuated the induction of the expression of M1 microglia markers during the progressive phase, whereas it did not affect the transient enhancement of expression of M2 microglia markers during the early pathogenesis phase. This selective inhibitory effect was confirmed using primary cultured microglia stimulated by lipopolysaccharide (LPS) or interleukin (IL)-4, which induced M1 or M2 polarization, respectively. Furthermore, minocycline inhibited the upregulation of NF-κB in the LPS-stimulated primary cultured microglia and in the spinal cord of SOD1(G93A) mice. On the other hand, IL-4 did not induce upregulation of NF-κB. This study indicates that minocycline selectively inhibits the microglia polarization to a proinflammatory state, and provides a basis for understanding pathogeneses of many diseases accompanied by microglial activation.