Naïve Huntington's disease microglia mount a normal response to inflammatory stimuli but display a partially impaired development of innate immune tolerance that can be counteracted by ganglioside GM1.

Chronic activation and dysfunction of microglia have been implicated in the pathogenesis and progression of many neurodegenerative disorders, including Huntington's disease (HD). HD is a genetic condition caused by a mutation that affects the folding and function of huntingtin (HTT). Signs of microglia activation have been observed in HD patients even before the onset of symptoms. It is unclear, however, whether pro-inflammatory microglia activation in HD results from cell-autonomous expression of mutant HTT, is the response of microglia to a diseased brain environment, or both. In this study, we used primary microglia isolated from HD knock-in (Q140) and wild-type (Q7) mice to investigate their response to inflammatory conditions in vitro in the absence of confounding effects arising from brain pathology. We show that naïve Q140 microglia do not undergo spontaneous pro-inflammatory activation and respond to inflammatory triggers, including stimulation of TLR4 and TLR2 and exposure to necrotic cells, with similar kinetics of pro-inflammatory gene expression as wild-type microglia. Upon termination of the inflammatory insult, the transcription of pro-inflammatory cytokines is tapered off in Q140 and wild-type microglia with similar kinetics. However, the ability of Q140 microglia to develop tolerance in response to repeated inflammatory stimulations is partially impaired in vitro and in vivo, potentially contributing to the establishment of chronic neuroinflammation in HD. We further show that ganglioside GM1, a glycosphingolipid with anti-inflammatory effects on wild-type microglia, not only decreases the production of pro-inflammatory cytokines and nitric oxide in activated Q140 microglia, but also dramatically dampen microglia response to re-stimulation with LPS in an experimental model of tolerance. These effects are independent from the expression of interleukin 1 receptor associated kinase 3 (Irak-3), a strong modulator of LPS signaling involved in the development of innate immune tolerance and previously shown to be upregulated by immune cell treatment with gangliosides. Altogether, our data suggest that external triggers are required for HD microglia activation, but a cell-autonomous dysfunction that affects the ability of HD microglia to acquire tolerance might contribute to the establishment of neuroinflammation in HD. Administration of GM1 might be beneficial to attenuate chronic microglia activation and neuroinflammation.

The Effects of NLY01, a Novel Glucagon-Like Peptide-1 Receptor Agonist, on Cuprizone-Induced Demyelination and Remyelination: Challenges and Future Perspectives.

Recent evidence suggests that the glucagon-like peptide-1 receptor (GLP-1R) agonists have neuroprotective activities in the CNS in animal models of Parkinson's disease, Alzheimer's disease, and multiple sclerosis (MS). This study aimed to investigate whether a novel long-acting GLP-1R agonist, NLY01, could limit demyelination or improve remyelination as occurs in MS using the cuprizone (CPZ) mouse model. Herein, we assessed the expression of GLP-1R on oligodendrocytes in vitro and found that mature oligodendrocytes (Olig2+PDGFRa-) express GLP-1R. We further confirmed this observation in the brain by immunohistochemistry and found that Olig2+CC1+ cells express GLP-1R. We next administered NLY01 twice per week to C57B6 mice while on CPZ chow diet and found that NLY01 significantly reduced demyelination with greater weight loss than vehicle-treated controls. Because GLP-1R agonists are known to have anorexigenic effect, we then administered CPZ by oral gavage and treated the mice with NLY01 or vehicle to ensure the dose consistency of CPZ ingestion among mice. Using this modified approach, NLY01 was no longer effective in reducing demyelination of the corpus callosum (CC). We next sought to examine the effects of NLY01 treatment on remyelination after CPZ intoxication and during the recovery period using an adoptive transfer-CPZ (AT-CPZ) model. We found no significant differences between the NLY01 and vehicle groups in the amount of myelin or the number of mature oligodendrocytes in the CC. In summary, despite the promising anti-inflammatory and neuroprotective effects of GLP-1R agonists that have been previously described, our experiments provided no evidence to support a beneficial effect of NLY01 on limiting demyelination or enhancing remyelination. This information may be useful in selecting proper outcome measures in clinical trials of this promising class of drugs in MS.

Fractalkine enhances oligodendrocyte regeneration and remyelination in a demyelination mouse model.

Demyelinating disorders of the central nervous system (CNS) occur when myelin and oligodendrocytes are damaged or lost. Remyelination and regeneration of oligodendrocytes can be achieved from endogenous oligodendrocyte precursor cells (OPCs) that reside in the adult CNS tissue. Using a cuprizone mouse model of demyelination, we show that infusion of fractalkine (CX3CL1) into the demyelinated murine brain increases de novo oligodendrocyte formation and enhances remyelination in the corpus callosum and cortical gray matter. This is achieved by increased OPC proliferation in the cortical gray matter as well as OPC differentiation and attenuation of microglia/macrophage activation both in corpus callosum and cortical gray matter. Finally, we show that activated OPCs and microglia/macrophages express fractalkine receptor CX3CR1 in vivo, and that in OPC-microglia co-cultures fractalkine increases in vitro oligodendrocyte differentiation by modulating both OPC and microglia biology. Our results demonstrate a novel pro-regenerative role of fractalkine in a demyelinating mouse model.

Anti-inflammatory role of GM1 and other gangliosides on microglia.

BACKGROUND: Gangliosides are glycosphingolipids highly enriched in the brain, with important roles in cell signaling, cell-to-cell communication, and immunomodulation. Genetic defects in the ganglioside biosynthetic pathway result in severe neurodegenerative diseases, while a partial decrease in the levels of specific gangliosides was reported in Parkinson's disease and Huntington's disease. In models of both diseases and other conditions, administration of GM1-one of the most abundant gangliosides in the brain-provides neuroprotection. Most studies have focused on the direct neuroprotective effects of gangliosides on neurons, but their role in other brain cells, in particular microglia, is not known. In this study we investigated the effects of exogenous ganglioside administration and modulation of endogenous ganglioside levels on the response of microglia to inflammatory stimuli, which often contributes to initiation or exacerbation of neurodegeneration. METHODS: In vitro studies were performed using BV2 cells, mouse, rat, and human primary microglia cultures. Modulation of microglial ganglioside levels was achieved by administration of exogenous gangliosides, or by treatment with GENZ-123346 and L-t-PDMP, an inhibitor and an activator of glycolipid biosynthesis, respectively. Response of microglia to inflammatory stimuli (LPS, IL-1ß, phagocytosis of latex beads) was measured by analysis of gene expression and/or secretion of pro-inflammatory cytokines. The effects of GM1 administration on microglia activation were also assessed in vivo in C57Bl/6 mice, following intraperitoneal injection of LPS. RESULTS: GM1 decreased inflammatory microglia responses in vitro and in vivo, even when administered after microglia activation. These anti-inflammatory effects depended on the presence of the sialic acid residue in the GM1 glycan headgroup and the presence of a lipid tail. Other gangliosides shared similar anti-inflammatory effects in in vitro models, including GD3, GD1a, GD1b, and GT1b. Conversely, GM3 and GQ1b displayed pro-inflammatory activity. The anti-inflammatory effects of GM1 and other gangliosides were partially reproduced by increasing endogenous ganglioside levels with L-t-PDMP, whereas inhibition of glycolipid biosynthesis exacerbated microglial activation in response to LPS stimulation. CONCLUSIONS: Our data suggest that gangliosides are important modulators of microglia inflammatory responses and reveal that administration of GM1 and other complex gangliosides exerts anti-inflammatory effects on microglia that could be exploited therapeutically.

Fractalkine signaling regulates oligodendroglial cell genesis from SVZ precursor cells.

Neural and oligodendrocyte precursor cells (NPCs and OPCs) in the subventricular zone (SVZ) of the brain contribute to oligodendrogenesis throughout life, in part due to direct regulation by chemokines. The role of the chemokine fractalkine is well established in microglia; however, the effect of fractalkine on SVZ precursor cells is unknown. We show that murine SVZ NPCs and OPCs express the fractalkine receptor (CX3CR1) and bind fractalkine. Exogenous fractalkine directly enhances OPC and oligodendrocyte genesis from SVZ NPCs in vitro. Infusion of fractalkine into the lateral ventricle of adult NPC lineage-tracing mice leads to increased newborn OPC and oligodendrocyte formation in vivo. We also show that OPCs secrete fractalkine and that inhibition of endogenous fractalkine signaling reduces oligodendrocyte formation in vitro. Finally, we show that fractalkine signaling regulates oligodendrogenesis in cerebellar slices ex vivo. In summary, we demonstrate a novel role for fractalkine signaling in regulating oligodendrocyte genesis from postnatal CNS precursor cells.

Gangliosides in the Brain: Physiology, Pathophysiology and Therapeutic Applications.

Gangliosides are glycosphingolipids highly abundant in the nervous system, and carry most of the sialic acid residues in the brain. Gangliosides are enriched in cell membrane microdomains ("lipid rafts") and play important roles in the modulation of membrane proteins and ion channels, in cell signaling and in the communication among cells. The importance of gangliosides in the brain is highlighted by the fact that loss of function mutations in ganglioside biosynthetic enzymes result in severe neurodegenerative disorders, often characterized by very early or childhood onset. In addition, changes in the ganglioside profile (i.e., in the relative abundance of specific gangliosides) were reported in healthy aging and in common neurological conditions, including Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), stroke, multiple sclerosis and epilepsy. At least in HD, PD and in some forms of epilepsy, experimental evidence strongly suggests a potential role of gangliosides in disease pathogenesis and potential treatment. In this review, we will summarize ganglioside functions that are crucial to maintain brain health, we will review changes in ganglioside levels that occur in major neurological conditions and we will discuss their contribution to cellular dysfunctions and disease pathogenesis. Finally, we will review evidence of the beneficial roles exerted by gangliosides, GM1 in particular, in disease models and in clinical trials.

Erratum: Author Correction: Repression of phagocytosis by human CD33 is not conserved with mouse CD33.

[This corrects the article DOI: 10.1038/s42003-019-0698-6.].

Repression of phagocytosis by human CD33 is not conserved with mouse CD33.

CD33 is an immunomodulatory receptor linked to Alzheimer's disease (AD) susceptibility via regulation of phagocytosis in microglia. Divergent features between human CD33 (hCD33) and murine CD33 (mCD33) include a unique transmembrane lysine in mCD33 and cytoplasmic tyrosine in hCD33. The functional consequences of these differences in restraining phagocytosis remains poorly understood. Using a new αmCD33 monoclonal antibody, we show that mCD33 is expressed at high levels on neutrophils and low levels on microglia. Notably, cell surface expression of mCD33 is entirely dependent on Dap12 due to an interaction with the transmembrane lysine in mCD33. In RAW264.7 cultured macrophages, BV-2 cultured microglia, primary neonatal and adult microglia, uptake of cargo - including aggregated Aß1-42 - is not altered upon genetic ablation of mCD33. Alternatively, deletion of hCD33 in monocytic cell lines increased cargo uptake. Moreover, transgenic mice expressing hCD33 in the microglial cell lineage showed repressed cargo uptake in primary microglia. Therefore, mCD33 and hCD33 have divergent roles in regulating phagocytosis, highlighting the importance of studying hCD33 in AD susceptibility.

A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling.

Phosphatidylethanolamine N-methyltransferase (PEMT) is an important enzyme in hepatic phosphatidylcholine (PC) biosynthesis. Pemt-/- mice fed a high-fat diet are protected from obesity and whole-body insulin resistance. However, Pemt-/- mice develop severe nonalcoholic steatohepatitis (NASH). Because NASH is often associated with hepatic insulin resistance, we investigated whether the increased insulin sensitivity in Pemt-/- mice was restricted to nonhepatic tissues or whether the liver was also insulin sensitive. Strikingly, the livers of Pemt-/- mice compared with those of Pemt+/+ mice were not insulin resistant, despite elevated levels of hepatic triacylglycerols and diacylglycerols, as well as increased hepatic inflammation and fibrosis. Endogenous glucose production was lower in Pemt-/- mice under both basal and hyperinsulinemic conditions. Experiments in primary hepatocytes and hepatoma cells revealed improved insulin signaling in the absence of PEMT, which was not due to changes in diacylglycerols, ceramides, or gangliosides. On the other hand, the phospholipid composition in hepatocytes seems critically important for insulin signaling such that lowering the PC:phosphatidylethanolamine (PE) ratio improves insulin signaling. Thus, treatments to reduce the PC:PE ratio in liver may protect against the development of hepatic insulin resistance.-Van der Veen, J. N., Lingrell, S., McCloskey, N., LeBlond, N. D., Galleguillos, D., Zhao, Y. Y., Curtis, J. M., Sipione, S., Fullerton, M. D., Vance, D. E., Jacobs, R. L. A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling.

N6-Furfuryladenine is protective in Huntington's disease models by signaling huntingtin phosphorylation.

The huntingtin N17 domain is a modulator of mutant huntingtin toxicity and is hypophosphorylated in Huntington's disease (HD). We conducted high-content analysis to find compounds that could restore N17 phosphorylation. One lead compound from this screen was N6-furfuryladenine (N6FFA). N6FFA was protective in HD model neurons, and N6FFA treatment of an HD mouse model corrects HD phenotypes and eliminates cortical mutant huntingtin inclusions. We show that N6FFA restores N17 phosphorylation levels by being salvaged to a triphosphate form by adenine phosphoribosyltransferase (APRT) and used as a phosphate donor by casein kinase 2 (CK2). N6FFA is a naturally occurring product of oxidative DNA damage. Phosphorylated huntingtin functionally redistributes and colocalizes with CK2, APRT, and N6FFA DNA adducts at sites of induced DNA damage. We present a model in which this natural product compound is salvaged to provide a triphosphate substrate to signal huntingtin phosphorylation via CK2 during low-ATP stress under conditions of DNA damage, with protective effects in HD model systems.

Disease-modifying effects of ganglioside GM1 in Huntington's disease models.

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by motor, cognitive and psychiatric problems. Previous studies indicated that levels of brain gangliosides are lower than normal in HD models and that administration of exogenous ganglioside GM1 corrects motor dysfunction in the YAC128 mouse model of HD In this study, we provide evidence that intraventricular administration of GM1 has profound disease-modifying effects across HD mouse models with different genetic background. GM1 administration results in decreased levels of mutant huntingtin, the protein that causes HD, and in a wide array of beneficial effects that include changes in levels of DARPP32, ferritin, Iba1 and GFAP, modulation of dopamine and serotonin metabolism, and restoration of normal levels of glutamate, GABA, L-Ser and D-Ser. Treatment with GM1 slows down neurodegeneration, white matter atrophy and body weight loss in R6/2 mice. Motor functions are significantly improved in R6/2 mice and restored to normal in Q140 mice, including gait abnormalities that are often resistant to treatments. Psychiatric-like and cognitive dysfunctions are also ameliorated by GM1 administration in Q140 and YAC128 mice. The widespread benefits of GM1 administration, at molecular, cellular and behavioural levels, indicate that this ganglioside has strong therapeutic and disease-modifying potential in HD.

Inhibiting cortical protein kinase A in spinal cord injured rats enhances efficacy of rehabilitative training.

Elevated levels of the second messenger molecule cyclic adenosine monophosphate (cAMP) are often associated with neuron sprouting and neurite extension (i.e., neuroplasticity). Phosphokinase A (PKA) is a prominent downstream target of cAMP that has been associated with neurite outgrowth. We hypothesized that rehabilitative motor training following spinal cord injuries promotes neuroplasticity via PKA activation. However, in two independent experiments, inhibition of cortical PKA using Rp-cAMPS throughout rehabilitative training robustly increased functional recovery and collateral sprouting of injured corticospinal tract axons, an indicator of neuroplasticity. Consistent with these in vivo findings, using cultured STHdh neurons, we found that Rp-cAMPS had no effect on the phosphorylation of CREB (cAMP response element-binding protein), a prominent downstream target of PKA, even with the concomitant application of the adenylate cyclase agonist forskolin to increase cAMP levels. Conversely, when cAMP levels were increased using the phosphodiesterase inhibitor IBMX, Rp-cAMPS potently inhibited CREB phosphorylation. Taken together, our results suggest that an alternate cAMP dependent pathway was involved in increasing CREB phosphorylation and neuroplasticity. This idea was supported by an in vitro neurite outgrowth assay, where inhibiting PKA did enhance neurite outgrowth. However, when PKA inhibition was combined with inhibition of EPAC2 (exchange protein directly activated by cAMP), another downstream target of cAMP in neurons, neurite outgrowth was significantly reduced. In conclusion, blocking PKA in cortical neurons of spinal cord injured rats increases neurite outgrowth of the lesioned corticospinal tract fibres and the efficacy of rehabilitative training, likely via EPAC.

Control of dopaminergic neuron survival by the unfolded protein response transcription factor XBP1.

Parkinson disease (PD) is characterized by the selective loss of dopaminergic neurons of the substantia nigra pars compacta (SNpc). Although growing evidence indicates that endoplasmic reticulum (ER) stress is a hallmark of PD, its exact contribution to the disease process is not well understood. Here we report that developmental ablation of X-Box binding protein 1 (XBP1) in the nervous system, a key regulator of the unfolded protein response (UPR), protects dopaminergic neurons against a PD-inducing neurotoxin. This survival effect was associated with a preconditioning condition that resulted from induction of an adaptive ER stress response in dopaminergic neurons of the SNpc, but not in other brain regions. In contrast, silencing XBP1 in adult animals triggered chronic ER stress and dopaminergic neuron degeneration. Supporting this finding, gene therapy to deliver an active form of XBP1 provided neuroprotection and reduced striatal denervation in animals injected with 6-hydroxydopamine. Our results reveal a physiological role of the UPR in the maintenance of protein homeostasis in dopaminergic neurons that may help explain the differential neuronal vulnerability observed in PD.

Nur transcription factors in stress and addiction.

The Nur transcription factors Nur77 (NGFI-B, NR4A1), Nurr1 (NR4A2), and Nor-1 (NR4A3) are a sub-family of orphan members of the nuclear receptor superfamily. These transcription factors are products of immediate early genes, whose expression is rapidly and transiently induced in the central nervous system by several types of stimuli. Nur factors are present throughout the hypothalamus-pituitary-adrenal (HPA) axis where are prominently induced in response to stress. Drugs of abuse and stress also induce the expression of Nur factors in nuclei of the motivation/reward circuit of the brain, indicating their participation in the process of drug addiction and in non-hypothalamic responses to stress. Repeated use of addictive drugs and chronic stress induce long-lasting dysregulation of the brain motivation/reward circuit due to reprogramming of gene expression and enduring alterations in neuronal function. Here, we review the data supporting that Nur transcription factors are key players in the molecular basis of the dysregulation of neuronal circuits involved in chronic stress and addiction.

Nurr1 regulates RET expression in dopamine neurons of adult rat midbrain.

Genesis of midbrain dopamine (DA) neurons depends on Nurr1, a nuclear receptor expressed during development and adulthood in these neurons. Nurr1 is required for the expression of genes of dopaminergic phenotype such as tyrosine hydroxylase and DA transporter. The expression of the tyrosine kinase receptor RET also depends on Nurr1 during development. However, it is unknown whether RET expression is regulated by Nurr1 during adulthood, and the mechanism by which Nurr1 regulates RET expression. Using an adeno-associated vector-delivered anti-Nurr1 ribozyme, we knocked-down Nurr1 expression unilaterally in the substantia nigra (SN) of adult rats. Animals injected with the ribozyme displayed a 57.3% decrease in Nurr1 mRNA in the SN accompanied by decreased DA extracellular levels in the striatum. RET mRNA in the injected SN and RET protein in the ipsilateral striatum decreased 76.9% and 47%, respectively. Tyrosine hydroxylase and DA transporter mRNA did not change in Nurr1 knocked-down SN. Nurr1 induced the transcription of the human RET promoter in cell type and concentration-dependent manner. Nurr1 induction of RET promoter is independent of NBRE elements. These results show that the expression of RET in rat adult SN is regulated by Nurr1 and suggest that RET is a transcriptional target of this nuclear receptor.

CoREST represses the heat shock response mediated by HSF1.

The stress response in cells involves a rapid and transient transcriptional activation of stress genes. It has been shown that Hsp70 limits its own transcriptional activation functioning as a corepressor of heat shock factor 1 (HSF1) during the attenuation of the stress response. Here we show that the transcriptional corepressor CoREST interacts with Hsp70. Through this interaction, CoREST represses both HSF1-dependent and heat shock-dependent transcriptional activation of the hsp70 promoter. In cells expressing short hairpin RNAs directed against CoREST, Hsp70 cannot repress HSF1-dependent transcription. A reduction of CoREST levels also provoked a significant increase of Hsp70 protein levels and an increase of HSF1-dependent transactivation of hsp70 promoter. Via chromatin immunoprecipitation assays we show that CoREST is bound to the hsp70 gene promoter under basal conditions and that its binding increases during heat shock response. In conclusion, we demonstrated that CoREST is a key regulator of the heat shock stress response.

PIASgamma represses the transcriptional activation induced by the nuclear receptor Nurr1.

Nurr1 is a transcription factor essential for the development of ventral dopaminergic neurons. In search for regulatory mechanisms of Nurr1 function, we identified the SUMO (small ubiquitin-like modifier)-E3 ubiquitin-protein isopeptide ligase, PIASgamma, as an interaction partner of Nurr1. Overexpressed PIASgamma and Nurr1 co-localize in the nuclei of transfected cells, and their interaction is demonstrated through co-immunoprecipitation and glutathione S-transferase pulldown assays. Co-expression of PIASgamma with Nurr1 results in a potent repression of Nurr1-dependent transcriptional activation of an artificial NGFI-B response element (NBRE) reporter as well as of a reporter driven by the native tyrosine hydroxylase promoter. We identified two consensus sumoylation sites in Nurr1. The substitution of lysine 91 by arginine in one SUMO site enhanced the transcriptional activity of Nurr1, whereas the substitution of lysine 577 by arginine in the second SUMO site decreased transcriptional activity of Nurr1. Interestingly, PIASgamma-induced repression of Nurr1 activity does not require the two sumoylation sites, because each mutant is repressed as efficiently as the wild type Nurr1. In addition, the mutations do not alter Nurr1 nuclear localization. Finally, we provide evidence that Nurr1 and PIASgamma co-exist in several nuclei of the rodent central nervous system by demonstrating the co-expression of Nurr1 protein and PIASgamma mRNA in the same cells. In conclusion, our studies identified PIASgamma as a transcriptional co-regulator of Nurr1 and suggest that this interaction may have a physiological role in regulating the expression of Nurr1 target genes.

Rapid increase of Nurr1 expression in the substantia nigra after 6-hydroxydopamine lesion in the striatum of the rat.

Nurr1 is a transcription factor essential for the genesis of ventral dopaminergic neurons. In this study, we investigated the expression of Nurr1 protein and mRNA in the adult rat brain by using immunohistochemistry and in situ hybridization, respectively. Another aim of our study was to investigate Nurr1 expression in substantia nigra after dopamine depletion induced by the injection of 6-hydroxydopamine in the striatum. We observed that Nurr1 mRNA and protein are expressed in several brain regions, including cortex, hippocampus, substantia nigra, and ventral tegmental area, in agreement with previous reports using in situ hybridization. Additionally, we found that Nurr1 is expressed in brain regions that have not been previously reported, such as striatum, septum, and superior colliculus. Highest levels of expression were found in cortex, medial septum, dentate gyrus, some hypothalamic nuclei, and substantia nigra. Interestingly, we observed that, in the superior colliculus, Nurr1 protein is localized in the cytoplasm of cells, whereas, in other regions, it was localized mainly in the nuclei, suggesting that Nurr1 subcellular localization is regulated and may have functional implications. Dopamine depletion induced by an injection of 6-hydroxydopamine into the striatum produced an increase in the number of cells expressing Nurr1 mRNA and protein in both substantia nigra compacta and substantia nigra reticulata, ipsilateral and contralateral to the lesioned side, measured 24 hr after the 6-hydroxydopamine injection. These results suggest that Nurr1 may be involved in many neuronal functions in the adult central nervous system and, in particular, might be related to the compensation processes that take place in dopaminergic cells in order to normalize extracellular dopamine levels in the striatum.