Trem2 promotes anti-inflammatory responses in microglia and is suppressed under pro-inflammatory conditions

Recent genome-wide association studies have reported that, amongst other microglial genes, variants in TREM2 can profoundly increase the incidence of developing Alzheimer's disease (AD). We have investigated the role of TREM2 in primary microglial cultures from wild-type mice by using siRNA to decrease Trem2 expression, and in parallel from knock-in mice heterozygous or homozygous for the Trem2 R47H AD risk variant (from the Jackson laboratories). The prevailing phenotype of Trem2 R47H knock-in mice was decreased expression levels of Trem2 in microglia, which resulted in decreased density of microglia in the hippocampus. Overall, primary microglia with reduced Trem2 expression, either by siRNA or from the R47H knock-in mice, displayed a similar phenotype. Comparison of the effects of decreased Trem2 expression under conditions of LPS pro-inflammatory or IL4 anti-inflammatory stimulation revealed the importance of Trem2 in driving a number of the genes up-regulated in the anti-inflammatory phenotype, whether treated with RNAi or performed with microglia carrying the R47H variant. In particular, Trem2 knockdown decreased levels of the transcription factor STAT6. STAT6 is the key mediator downstream from IL4 and controls expression of genes including Arg1, which also showed decreased IL4-induced expression when Trem2 expression was decreased. LPS-induced pro-inflammatory stimulation suppressed Trem2 expression, thus preventing TREM2's anti-inflammatory drive. The importance of Trem2 activity in regulating the pro- and anti-inflammatory balance of microglia, particularly mediating effects of the IL4-regulated anti-inflammatory pathway, has important implications for fighting neurodegenerative disease

Human induced pluripotent stem cell-derived microglia-like cells harboring TREM2 missense mutations show specific deficits in phagocytosis

Dysfunction of microglia, the brain’s immune cells, is linked to neurodegeneration. Homozygous missense mutations in TREM2 cause Nasu-Hakola disease (NHD), an early-onset dementia. To study the consequences of these TREM2 variants, we generated induced pluripotent stem cell-derived microglia-like cells (iPSC-MGLCs) from patients with NHD caused by homozygous T66M or W50C missense mutations. iPSC-MGLCs expressed microglial markers and secreted higher levels of TREM2 than primary macrophages. TREM2 expression and secretion were reduced in variant lines. LPS-mediated cytokine secretion was comparable between control and TREM2 variant iPSC-MGLCs, whereas survival was markedly reduced in cells harboring missense mutations when compared with controls. Furthermore, TREM2 missense mutations caused a marked impairment in the phagocytosis of apoptotic bodies, but not in Escherichia coli or zymosan substrates. Coupled with changes in apoptotic cell-induced cytokine release and migration, these data identify specific deficits in the ability of iPSC-MGLCs harboring TREM2 missense mutations to respond to specific pathogenic signals

Endogenous alpha-synuclein influences the number of dopaminergic neurons in mouse substantia nigra.

The presynaptic protein α-synuclein is central to the pathogenesis of α-synucleinopathies. We show that the presence of endogenous mouse α-synuclein leads to higher number of dopaminergic neurons in the substantia nigra of wild-type C57Bl/6J mice compared with C57Bl/6S mice with a spontaneous deletion of the α-synuclein gene or C57Bl/6J mice with a targeted deletion of the α-synuclein gene. This effect of α-synuclein on dopaminergic neuron occurs during development between E10.5 and E13.5 and persists in adult life supporting the involvement of α-synuclein in the development of a subset of dopaminergic neurons

Hyperacute neurology at a regional neurosciences centre: a 1-year experience of an innovative service model.

St George's Hospital hyperacute neurology service (HANS) is a comprehensive, consultant-delivered service set in a teaching hospital regional neuroscience centre. The service addresses deficiencies in acute neurological care previously highlighted by the Royal College of Physicians and the Association of British Neurologists. HANS adopts a disease-agnostic approach to acute neurology, prioritising the emergency department (ED) management of both stroke and stroke mimics alike alongside proactive daily support to the acute medical unit and acute medical take. Rapid access clinics provide a means to assess ambulatory patients, providing an outlet to reduce the burden of referral from primary care to acute medicine. This paper reports the results from the first year of the service. Admission was avoided in 25% of the cases reviewed in the ED. Compared to historic data, there was a significant improvement in the length of stay for non-stroke disorders while the occupancy of stroke beds by non-stroke cases reduced by 50%. The configuration of this service is replicable in other neuroscience centres and provides a framework to reduce the barriers facing patients who present with acute neurological symptoms

Trem2 promotes anti-inflammatory responses in microglia and is suppressed under pro-inflammatory conditions

Genome-wide association studies have reported that, amongst other microglial genes, variants in TREM2 can profoundly increase the incidence of developing Alzheimer’s disease (AD). We have investigated the role of TREM2 in primary microglial cultures from wild type mice by using siRNA to decrease Trem2 expression, and in parallel from knock-in mice heterozygous or homozygous for the Trem2 R47H AD risk variant. The prevailing phenotype of Trem2 R47H knock-in mice was decreased expression levels of Trem2 in microglia, which resulted in decreased density of microglia in the hippocampus. Overall, primary microglia with reduced Trem2 expression, either by siRNA or from the R47H knock-in mice, displayed a similar phenotype. Comparison of the effects of decreased Trem2 expression under conditions of LPS pro-inflammatory or IL-4 anti-inflammatory stimulation revealed the importance of Trem2 in driving a number of the genes up-regulated in the anti-inflammatory phenotype. RNA-seq analysis showed that IL-4 induced the expression of a programme of genes including Arg1 and Ap1b1 in microglia, which showed an attenuated response to IL-4 when Trem2 expression was decreased. Genes showing a similar expression profile to Arg1 were enriched for STAT6 transcription factor recognition elements in their promoter, and Trem2 knockdown decreased levels of STAT6. LPS-induced pro-inflammatory stimulation suppressed Trem2 expression, thus preventing TREM2’s anti-inflammatory drive. Given that anti-inflammatory signaling is associated with tissue repair, understanding the signaling mechanisms downstream of Trem2 in coordinating the pro- and anti-inflammatory balance of microglia, particularly mediating effects of the IL-4-regulated anti-inflammatory pathway, has important implications for fighting neurodegenerative disease

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Human studies and preclinical models of Parkinson\u27s disease implicate the involvement of both the innate and adaptive immune systems in disease progression. Further, pro-inflammatory markers are highly enriched near neurons containing pathological forms of alpha synuclein (α-syn), and α-syn overexpression recapitulates neuroinflammatory changes in models of Parkinson\u27s disease. These data suggest that α-syn may initiate a pathological inflammatory response, however the mechanism by which α-syn initiates neuroinflammation is poorly understood. Silencing endogenous α-syn results in a similar pattern of nigral degeneration observed following α-syn overexpression. Here we aimed to test the hypothesis that loss of α-syn function within nigrostriatal neurons results in neuronal dysfunction, which subsequently stimulates neuroinflammation. Adeno-associated virus (AAV) expressing an short hairpin RNA (shRNA) targeting endogenous α-syn was unilaterally injected into the substantia nigra pars compacta (SNc) of adult rats, after which nigrostriatal pathology and indices of neuroinflammation were examined at 7, 10, 14 and 21 days post-surgery. Removing endogenous α-syn from nigrostriatal neurons resulted in a rapid up-regulation of the major histocompatibility complex class 1 (MHC-1) within transduced nigral neurons. Nigral MHC-1 expression occurred prior to any overt cell death and coincided with the recruitment of reactive microglia and T-cells to affected neurons. Following the induction of neuroinflammation, α-syn knockdown resulted in a 50% loss of nigrostriatal neurons in the SNc and a corresponding loss of nigrostriatal terminals and dopamine (DA) concentrations within the striatum. Expression of a control shRNA did not elicit any pathological changes. Silencing α-syn within glutamatergic neurons of the cerebellum did not elicit inflammation or cell death, suggesting that toxicity initiated by α-syn silencing is specific to DA neurons. These data provide evidence that loss of α-syn function within nigrostriatal neurons initiates a neuronal-mediated neuroinflammatory cascade, involving both the innate and adaptive immune systems, which ultimately results in the death of affected neurons