Microglia contribute to social behavioral adaptation to chronic stress.

Microglial activation has been regarded mainly as an exacerbator of stress response, a common symptom in psychiatric disorders. This study aimed to determine whether microglia contribute to adaptive response of the brain and behavior toward stress using a mild and adaptive stress model - chronic restraint stress (CRS) - with wild type (WT) and CX3CR1-GFP (CX3CR1[G]) mice and human schizophrenia patients' data. Our results revealed that CRS did not exacerbate anxiety and depressive-like behaviors, but instead strengthened social dominance and short-term spatial learning in WT mice. Compared to WT and CX3CR1(+/G) heterozygous mice, CX3CR1(G/G) homozygotes were subordinate in social interaction before and after CRS. Microglia in WT mice underwent a series of region-specific changes involving their phagocytosis of presynaptic vesicular glutamate transporter 2 protein, contacts with synaptic elements, CD206+ microglial proportion, and gene expressions such as Cx3cr1. By contrast, CX3CR1-deficient microglia showed decreased CD206+ while increased MHCII+ subpopulations and hypo-ramification in the hippocampus, as well as sensitized polarization and morphological change in response to CRS. Furthermore, CD206+ microglial abundancy was positively correlated with social dominancy and microglial ramification in CX3CR1-GFP mice. Moreover, CX3CR1 mRNA level was reduced in CRS-treated mouse brains and showed a smaller interactome with other brain genes in the dorsal-lateral prefrontal cortices of patients with schizophrenia. Our findings overall highlight microglia and its receptor CX3CR1 as key contributors in regulation of social behavioral adaptation to chronic stress.

Spinal versus brain microglial and macrophage activation traits determine the differential neuroinflammatory responses and analgesic effect of minocycline in chronic neuropathic pain.

Substantial evidence indicates involvement of microglia/macrophages in chronic neuropathic pain. However, the temporal-spatial features of microglial/macrophage activation and their pain-bound roles remain elusive. Here, we evaluated microglia/macrophages and the subtypes in the lumbar spinal cord (SC) and prefrontal cortex (PFC), and analgesic-anxiolytic effect of minocycline at different stages following spared nerve injury (SNI) in rats. While SNI enhanced the number of spinal microglia/macrophages since post-operative day (POD)3, pro-inflammatory MHCII+ spinal microglia/macrophages were unexpectedly less abundant in SNI rats than shams on POD21. By contrast, less abundant anti-inflammatory CD172a (SIRPα)+ microglia/macrophages were found in the PFC of SNI rats. Interestingly in naïve rats, microglial/macrophage expression of CD11b/c, MHCII and MHCII+/CD172a+ ratio were higher in the SC than the cortex. Consistently, multiple immune genes involved in anti-inflammation, phagocytosis, complement activation and M2 microglial/macrophage polarization were upregulated in the spinal dorsal horn and dorsal root ganglia but downregulated in the PFC of SNI rats. Furthermore, daily intrathecal minocycline treatment starting from POD0 for two weeks alleviated mechanical allodynia most robustly before POD3 and attenuated anxiety on POD9. Although minocycline dampened spinal MHCII+ microglia/macrophages until POD13, it failed to do so on cortical microglia/macrophages, indicating that dampening only spinal inflammation may not be enough to alleviate centralized pain at the chronic stage. Taken together, our data provide the first evidence that basal microglial/macrophage traits underlie differential region-specific responses to SNI and minocycline treatment, and suggest that drug treatment efficiently targeting not only spinal but also brain inflammation may be more effective in treating chronic neuropathic pain.

Association of brain immune genes with social behavior of inbred mouse strains.

BACKGROUND: Social deficit is one of the core symptoms of neuropsychiatric diseases, in which immune genes play an important role. Although a few immune genes have been shown to regulate social and emotional behaviors, how immune gene network(s) may jointly regulate sociability has not been investigated so far. METHODS: To decipher the potential immune-mediated mechanisms underlying social behavior, we first studied the brain microarray data of eight inbred mouse strains with known variations in social behavior and retrieved the differentially expressed immune genes. We then made a protein-protein interaction analysis of them to find the major networks and explored the potential association of these genes with the behavior and brain morphology in the mouse phenome database. To validate the expression and function of the candidate immune genes, we selected the C57BL/6 J and DBA/2 J strains among the eight inbred strains, compared their social behaviors in resident-intruder and 3-chambered social tests and the mRNA levels of these genes, and analyzed the correlations of these genes with the social behaviors. RESULTS: A group of immune genes were differentially expressed in the brains of these mouse strains. The representative C57BL/6 J and DBA/2 J strains displayed significant differences in social behaviors, DBA/2 J mice being less active in social dominance and social interaction than C57BL/6 J mice. The mRNA levels of H2-d1 in the prefrontal cortex, hippocampus, and hypothalamus and C1qb in the hippocampus of the DBA/2 J strain were significantly down-regulated as compared to those in the C57BL/6 J strain. In contrast, Polr3b in the hippocampus and Tnfsf13b in the prefrontal cortex of the DBA/2 J strain were up-regulated. Furthermore, C1qb, Cx3cl1, H2-d1, H2-k1, Polr3b, and Tnfsf13b were predicted to be associated with various behavioral and brain morphological features across the eight inbred strains. Importantly, the C1qb mRNA level was confirmed to be significantly correlated with the sociability in DBA/2 J but not in C57BL/6 J mice. CONCLUSIONS: Our study provided evidence on the association of immune gene network(s) with the brain development and behavior in animals and revealed neurobiological functions of novel brain immune genes that may contribute to social deficiency in animal models of neuropsychiatric disorders.

Low-Molecular Weight Protamine Overcomes Chondroitin Sulfate Inhibition of Neural Regeneration.

Protamine is an arginine-rich peptide that replaces histones in the DNA-protein complex during spermatogenesis. Protamine is clinically used in cardiopulmonary bypass surgery to neutralize the effects of heparin that is required during the treatment. Here we demonstrate that protamine and its 14-22 amino acid long fragments overcome the neurite outgrowth inhibition by chondroitin sulfate proteoglycans (CSPGs) that are generally regarded as major inhibitors of regenerative neurite growth after injuries of the adult central nervous system (CNS). Since the full-length protamine was found to have toxic effects on neuronal cells we used the in vitro neurite outgrowth assay to select a protamine fragment that retains the activity to overcome the neurite outgrowth inhibition on CSPG substrate and ended up in the 14 amino acid fragment, low-molecular weight protamine (LMWP). In contrast to the full-length protamine, LMWP displays very low or no toxicity in our assays in vitro and in vivo. We therefore started studies on LMWP as a possible drug lead in treatment of CNS injuries, such as the spinal cord injury (SCI). LMWP mimicks HB-GAM (heparin-binding growth-associated molecule; pleiotrophin) in that it overcomes the CSPG inhibition on neurite outgrowth in primary CNS neurons in vitro and inhibits binding of protein tyrosine phosphatase (PTP) sigma, an inhibitory receptor in neurite outgrowth, to its CSPG ligand. Furthermore, the chondroitin sulfate (CS) chains of the cell matrix even enhance the LMWP-induced neurite outgrowth on CSPG substrate. In vivo studies using the hemisection and hemicontusion SCI models in mice at the cervical level C5 revealed that LMWP enhances recovery when administered through intracerebroventricular or systemic route. We suggest that LMWP is a promising drug lead to develop therapies for CNS injuries.

Psychosocial stress on neuroinflammation and cognitive dysfunctions in Alzheimer's disease: the emerging role for microglia?

Chronic psychosocial stress is increasingly recognized as a risk factor for late-onset Alzheimer's disease (LOAD) and associated cognitive deficits. Chronic stress also primes microglia and induces inflammatory responses in the adult brain, thereby compromising synapse-supportive roles of microglia and deteriorating cognitive functions during aging. Substantial evidence demonstrates that failure of microglia to clear abnormally accumulating amyloid-beta (Aß) peptide contributes to neuroinflammation and neurodegeneration in AD. Moreover, genome-wide association studies have linked variants in several immune genes, such as TREM2 and CD33, the expression of which in the brain is restricted to microglia, with cognitive dysfunctions in LOAD. Thus, inflammation-promoting chronic stress may create a vicious cycle of aggravated microglial dysfunction accompanied by increased Aß accumulation, collectively exacerbating neurodegeneration. Surprisingly, however, little is known about whether and how chronic stress contributes to microglia-mediated neuroinflammation that may underlie cognitive impairments in AD. This review aims to summarize the currently available clinical and preclinical data and outline potential molecular mechanisms linking stress, microglia and neurodegeneration, to foster future research in this field.