Proximal recolonization by self-renewing microglia re-establishes microglial homeostasis in the adult mouse brain.

Microglia are resident immune cells that play critical roles in maintaining the normal physiology of the central nervous system (CNS). Remarkably, microglia have an intrinsic capacity to repopulate themselves after acute ablation. However, the underlying mechanisms that drive such restoration remain elusive. Here, we characterized microglial repopulation both spatially and temporally following removal via treatment with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. We show that microglia were replenished via self-renewal, with no contribution from nonmicroglial lineages, including Nestin+ progenitors and the circulating myeloid population. Interestingly, spatial analyses with dual-color labeling revealed that newborn microglia recolonized the parenchyma by forming distinctive clusters that maintained stable territorial boundaries over time, indicating the proximal expansive nature of adult microgliogenesis and the stability of microglia tiling. Temporal transcriptome profiling at different repopulation stages revealed that adult newborn microglia gradually regain steady-state maturity from an immature state that is reminiscent of the neonatal stage and follow a series of maturation programs, including nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation, interferon immune activation, and apoptosis. Importantly, we show that the restoration of microglial homeostatic density requires NF-κB signaling as well as apoptotic egress of excessive cells. In summary, our study reports key events that take place from microgliogenesis to homeostasis reestablishment.

SIRT1 deficiency in microglia contributes to cognitive decline in aging and neurodegeneration via epigenetic regulation of IL-1ß.

Aging is the predominant risk factor for neurodegenerative diseases. One key phenotype as the brain ages is an aberrant innate immune response characterized by proinflammation. However, the molecular mechanisms underlying aging-associated proinflammation are poorly defined. Whether chronic inflammation plays a causal role in cognitive decline in aging and neurodegeneration has not been established. Here we report a mechanistic link between chronic inflammation and aging microglia and a causal role of aging microglia in neurodegenerative cognitive deficits. We showed that SIRT1 is reduced with the aging of microglia and that microglial SIRT1 deficiency has a causative role in aging- or tau-mediated memory deficits via IL-1ß upregulation in mice. Interestingly, the selective activation of IL-1ß transcription by SIRT1 deficiency is likely mediated through hypomethylating the specific CpG sites on IL-1ß proximal promoter. In humans, hypomethylation of IL-1ß is strongly associated with chronological age and with elevated IL-1ß transcription. Our findings reveal a novel epigenetic mechanism in aging microglia that contributes to cognitive deficits in aging and neurodegenerative diseases.

Case report: Periventricular heterotopia and early-onset bipolar disorder in adolescent patient with history of childhood attention deficit hyperactivity disorder.

Periventricular heterotopia (PH) is a developmental malformation in the brain. Because the clinical symptoms are heterogeneous, few studies have investigated the psychiatric symptoms associated with PH. We describe the case of a 17-year-old male with bipolar disorder (BD), who had been treated for attention deficit-hyperactivity disorder (ADHD) and developmental delay in childhood. He had experienced depression for 1 year and was admitted to the emergency room following a suicide attempt. He was admitted to the psychiatric ward for further evaluation and treatment for elated mood, decreased need for sleep, increased sexuality, and delusion. The patient was diagnosed with BP-I disorder and PH via brain magnetic resonance imaging. After combined treatment with valproic acid and aripiprazole, his manic symptoms stabilized. To our knowledge, this is the first report of an adolescent PH case with a history of early onset BD and ADHD in childhood.

CX3CR1 protein signaling modulates microglial activation and protects against plaque-independent cognitive deficits in a mouse model of Alzheimer disease.

Aberrant microglial activation has been proposed to contribute to the cognitive decline in Alzheimer disease (AD), but the underlying molecular mechanisms remain enigmatic. Fractalkine signaling, a pathway mediating the communication between microglia and neurons, is deficient in AD brains and down-regulated by amyloid-ß. Although fractalkine receptor (CX3CR1) on microglia was found to regulate plaque load, no functional effects have been reported. Our study demonstrates that CX3CR1 deficiency worsens the AD-related neuronal and behavioral deficits. The effects were associated with cytokine production but not with plaque deposition. Ablation of CX3CR1 in mice overexpressing human amyloid precursor protein enhanced Tau pathology and exacerbated the depletion of calbindin in the dentate gyrus. The levels of calbindin in the dentate gyrus correlated negatively with those of tumor necrosis factor α and interleukin 6, suggesting neurotoxic effects of inflammatory factors. Functionally, removing CX3CR1 in human amyloid precursor protein mice worsened the memory retention in passive avoidance and novel object recognition tests, and their memory loss in the novel object recognition test is associated with high levels of interleukin 6. Our findings identify CX3CR1 as a key microglial pathway in protecting against AD-related cognitive deficits that are associated with aberrant microglial activation and elevated inflammatory cytokines.

Poly(ADP-ribose)polymerase-1 modulates microglial responses to amyloid ß.

BACKGROUND: Amyloid ß (Aß) accumulates in Alzheimer's disease (AD) brain. Microglial activation also occurs in AD, and this inflammatory response may contribute to disease progression. Microglial activation can be induced by Aß, but the mechanisms by which this occurs have not been defined. The nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1) regulates microglial activation in response to several stimuli through its interactions with the transcription factor, NF-κB. The purpose of this study was to evaluate whether PARP-1 activation is involved in Aß-induced microglial activation, and whether PARP-1 inhibition can modify microglial responses to Aß. METHODS: hAPP(J20) mice, which accumulate Aß with ageing, were crossed with PARP-1(-/-) mice to assess the effects of PARP-1 depletion on microglial activation, hippocampal synaptic integrity, and cognitive function. Aß peptide was also injected into brain of wt and PARP-1(-/-) mice to directly determine the effects of PARP-1 on Aß-induced microglial activation. The effect of PARP-1 on Aß-induced microglial cytokine production and neurotoxicity was evaluated in primary microglia cultures and in microglia-neuron co-cultures, utilizing PARP-1(-/-) cells and a PARP-1 inhibitor. NF-κB activation was evaluated in microglia infected with a lentivirus reporter gene. RESULTS: The hAPP(J20) mice developed microglial activation, reduced hippocampal CA1 calbindin expression, and impaired novel object recognition by age 6 months. All of these features were attenuated in hAPP(J20)/PARP-1(-/-) mice. Similarly, Aß(1-42) injected into mouse brain produced a robust microglial response in wild-type mice, and this was blocked in mice lacking PARP-1 expression or activity. Studies using microglial cultures showed that PARP-1 activity was required for Aß-induced NF-κB activation, morphological transformation, NO release, TNFα release, and neurotoxicity. Conversely, PARP-1 inhibition increased release of the neurotrophic factors TGFß and VEGF, and did not impair microglial phagocytosis of Aß peptide. CONCLUSIONS: These results identify PARP-1 as a requisite and previously unrecognized factor in Aß-induced microglial activation, and suggest that the effects of PARP-1 are mediated, at least in part, by its interactions with NF-κB. The suppression of Aß-induced microglial activation and neurotoxicity by PARP-1 inhibition suggests this approach could be useful in AD and other disorders in which microglial neurotoxicity may contribute.

Alpha-synuclein induces migration of BV-2 microglial cells by up-regulation of CD44 and MT1-MMP.

Although there is known to be a marked concentration of reactive microglia in the substantia nigra pars compacta (SNpc) of patients with Parkinson's disease (PD), a disorder in which alpha-synuclein plays a key pathogenic role, the specific roles of alpha-synuclein and microglia remains poorly understood. In this study, we investigated the effects of alpha-synuclein and the mechanisms of invasive microglial migration into the SNpc. We show that alpha-synuclein up-regulates the expressions of the cell adhesion molecule CD44 and the cell surface protease membrane-type 1 matrix metalloproteinase through the extracellular regulated kinases 1/2 pathway. These concurrent inductions increased the generation of soluble CD44 to liberate microglia from the surrounding extracellular matrix for migration. The effects of alpha-synuclein were identical in BV-2 murine microglial cells subjected to cDNA transfection and extracellular treatment. These inductions in primary microglial cultures of C57Bl/6 mice were identical to those in BV-2 cells. alpha-Synuclein-induced microglial migration into the SNpc was confirmed in vivo using a 6-hydroxydopamine mouse model of PD. Our data demonstrate a correlation between alpha-synuclein-induced phenotypic changes and microglial migration. With the recruitment of the microglial population into the SNpc during dopaminergic neurodegeneration, alpha-synuclein may play a role in accelerating the pathogenesis of PD.

Alpha-synuclein induces apoptosis by altered expression in human peripheral lymphocyte in Parkinson's disease.

Though the etiology of Parkinson's disease (PD) remains unclear, alpha-synuclein (alpha-SN) is regarded as a major causative agent of PD. Several lines of evidence indicate that immunological abnormalities are associated with PD for unknown reasons. The present study was performed to assess whether peripheral blood mononuclear cells (PBMCs) show altered alpha-SN expression in PD patients and to identify its functions, which may be related to peripheral immune abnormalities in PD. alpha-SN was found to be expressed more in 151 idiopathic PD (IPD) patients than in 101 healthy controls, who nevertheless showed as age-dependent increases. By in vitro transfection, alpha-SN expression was shown to be correlated with glucocorticoid sensitive apoptosis, possibly caused by the enhanced expression of glucocorticoid receptor (GR), caspase activations (caspase-8, caspase-9), CD95 up-regulation, and reactive oxygen species (ROS) production. An understanding of the correlation between alpha-SN levels and apoptosis in the presence of the coordinated involvement of multiple processes would provide an insight into the molecular basis of the disease. The present study provides a clue that the alpha-SN may be one of the primary causes of the immune abnormalities observed in PD and offers new targets for pharmacotherapeutic intervention.

Sirtuins in neurodegenerative diseases: an update on potential mechanisms.

Silent information regulator 2 proteins (sirtuins or SIRTs) are a group of deacetylases (or deacylases) whose activities are dependent on and regulated by nicotinamide adenine dinucleotide (NAD(+)). Compelling evidence supports that sirtuins play major roles in many aspects of physiology, especially in pathways related to aging - the predominant and unifying risk factor for neurodegenerative diseases. In this review, we highlight the molecular mechanisms underlying the protective effects of sirtuins in neurodegenerative diseases, focusing on protein homeostasis, neural plasticity, mitochondrial function, and sustained chronic inflammation. We will also examine the potential and challenges of targeting sirtuin pathways to block these pathogenic pathways.

Acetylation of tau inhibits its degradation and contributes to tauopathy.

Neurodegenerative tauopathies characterized by hyperphosphorylated tau include frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) and Alzheimer's disease (AD). Reducing tau levels improves cognitive function in mouse models of AD and FTDP-17, but the mechanisms regulating the turnover of pathogenic tau are unknown. We found that tau is acetylated and that tau acetylation prevents degradation of phosphorylated tau (p-tau). We generated two antibodies specific for acetylated tau and showed that tau acetylation is elevated in patients at early and moderate Braak stages of tauopathy. Histone acetyltransferase p300 was involved in tau acetylation and the class III protein deacetylase SIRT1 in deacetylation. Deleting SIRT1 enhanced levels of acetylated-tau and pathogenic forms of p-tau, probably by blocking proteasome-mediated degradation. Inhibiting p300 with a small molecule promoted tau deacetylation and eliminated p-tau associated with tauopathy. Modulating tau acetylation could be a new therapeutic strategy to reduce tau-mediated neurodegeneration.

Cystatin C-cathepsin B axis regulates amyloid beta levels and associated neuronal deficits in an animal model of Alzheimer's disease.

Impaired degradation of amyloid beta (Abeta) peptides could lead to Abeta accumulation, an early trigger of Alzheimer's disease (AD). How Abeta-degrading enzymes are regulated remains largely unknown. Cystatin C (CysC, CST3) is an endogenous inhibitor of cysteine proteases, including cathepsin B (CatB), a recently discovered Abeta-degrading enzyme. A CST3 polymorphism is associated with an increased risk of late-onset sporadic AD. Here, we identified CysC as the key inhibitor of CatB-induced Abeta degradation in vivo. Genetic ablation of CST3 in hAPP-J20 mice significantly lowered soluble Abeta levels, the relative abundance of Abeta1-42, and plaque load. CysC removal also attenuated Abeta-associated cognitive deficits and behavioral abnormalities and restored synaptic plasticity in the hippocampus. Importantly, the beneficial effects of CysC reduction were abolished on a CatB null background, providing direct evidence that CysC regulates soluble Abeta and Abeta-associated neuronal deficits through inhibiting CatB-induced Abeta degradation.

Retinoic acid inhibits chondrogenesis of mesenchymal cells by sustaining expression of N-cadherin and its associated proteins.

Retinoic acid (RA) is a well-known regulator of chondrocyte phenotype. RA inhibits chondrogenic differentiation of mesenchymal cells and also causes loss of differentiated chondrocyte phenotype. The present study investigated the mechanisms underlying RA regulation of chondrogenesis. RA treatment in chondrifying mesenchymal cells did not affect precartilage condensation, but blocked progression from precartilage condensation to cartilage nodule formation. This inhibitory effect of RA was independent of protein kinase C and extracellular signal-regulated protein kinase, which are positive and negative regulators of cartilage nodule formation, respectively. The progression from precartilage condensation to cartilage nodule requires downregulation of N-cadherin expression. However, RA treatment caused sustained expression of N-cadherin and its associated proteins including alpha- and beta-catenin suggesting that modulation of expression of these molecules is associated with RA-induced inhibition of chondrogenesis. This hypothesis was supported by the observation that disruption of the actin cytoskeleton by cytochalasin D (CD) blocks RA-induced sustained expression of cell adhesion molecules and overcomes RA-induced inhibition of chondrogenesis. Taken together, our results suggest RA inhibits chondrogenesis by stabilizing cell-to-cell interactions at the post-precartilage condensation stage.