Difference between revisions of "Neuroinflammation"

Latest revision as of 18:20, 11 August 2017

Key concepts in ALS

Wikipedia

• Neuroinflammation is inflammation of the nervous tissue.
• It may be initiated in response to a variety of cues:
  • Toxic metabolites
  • Autoimmunity
  • Aging
  • Infection
    • Microbes
    • Viruses
  • Traumatic brain injury
  • Air pollution
  • Passive smoke
• In the central nervous system (CNS), including the brain and spinal cord, microglia are the resident innate immune cells that are activated in response to these cues.
• The CNS is typically an immunologically privileged site because peripheral immune cells are generally blocked by the blood–brain barrier (BBB), a specialized structure composed of astrocytes and endothelial cells.
• However, circulating peripheral immune cells may surpass a compromised BBB and encounter neurons and glial cells expressing major histocompatibility complex molecules, perpetuating the immune response.
• Although the response is initiated to protect the CNS from the infectious agent, the effect may be toxic and widespread inflammation as well as further migration of leukocytes through the BBB.

Neuroinflammation and ALS[edit]

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, ALS, and frontotemporal lobar dementia are among the most pressing problems of developed societies with aging populations. Neurons carry out essential functions such as signal transmission and network integration in the central nervous system and are the main targets of neurodegenerative disease. In this Review, I address how the neuron's environment also contributes to neurodegeneration. Maintaining an optimal milieu for neuronal function rests with supportive cells termed glia and the blood-brain barrier. Accumulating evidence suggests that neurodegeneration occurs in part because the environment is affected during disease in a cascade of processes collectively termed neuroinflammation. These observations indicate that therapies targeting glial cells might provide benefit for those afflicted by neurodegenerative disorders.^[1]

Dysregulated microglial responses are central in neurodegenerative proteinopathies, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar disease (FTLD). Pathologic TDP-43, which is typically found in intracellular inclusions, is a misfolding protein with emerging roles in ALS and FTLD. Recently, TDP-43 species have been found in extracellular fluids of patients; however, the overall implications of TDP-43-mediated signaling linked to neuroinflammation are poorly understood. Our work-the first, to our knowledge, to focus on innate immunity responses to TDP-43 aggregates-shows that such species are internalized by microglia and cause abnormal mobilization of endogenous TDP-43. Exposure to TDP-43 aggregates elicited not only IL-1β, but also NLRP3-dependent and noncanonical IL-18 processing. Moreover, we report a link between TDP-43 and neuronal loss via the apoptosis-independent emerging roles of caspase-3 in neurotoxic inflammation. Our results further support the view of noncell autonomous neurodegenerative mechanisms in ALS. Remarkably, we demonstrate that TDP-43 aggregates bind to and colocalize with MAPK/MAK/MRK overlapping kinase (MOK) and show that its phosphorylation status is disrupted. Finally, we show that this TDP-43-caused activation state can be altered by exogenous Hsp27 and Hsp70 chaperones. Our study provides new insight into the immune phenotype, mechanisms, and signaling pathways that operate in microglial neurotoxic activation in ALS.-Leal-Lasarte, M. M., Franco, J. M., Labrador-Garrido, A., Pozo, D., Roodveldt, C. Extracellular TDP-43 aggregates target MAPK/MAK/MRK overlapping kinase (MOK) and trigger caspase-3/IL-18 signaling in microglia. ^[2]

Neuroinflammation is a pathological hallmark of ALS in both transgenic rodent models and patients, and is characterized by proinflammatory T lymphocytes and activated macrophages/microglia. In ALS mouse models, decreased regulatory T lymphocytes (Tregs) exacerbate the neuroinflammatory process, leading to accelerated motoneuron death and shortened survival; passive transfer of Tregs suppresses the neuroinflammation and prolongs survival. Treg numbers and FOXP3 expression are also decreased in rapidly progressing ALS patients. A key question is whether the marked neuroinflammation in ALS can be attributed to the impaired suppressive function of ALS Tregs in addition to their decreased numbers. To address this question, T lymphocyte proliferation assays were performed. Compared with control Tregs, ALS Tregs were less effective in suppressing responder T lymphocyte proliferation. Although both slowly and rapidly progressing ALS patients had dysfunctional Tregs, the greater the clinically assessed disease burden or the more rapidly progressing the patient, the greater the Treg dysfunction. Epigenetically, the percentage methylation of the Treg-specific demethylated region was greater in ALS Tregs. After in vitro expansion, ALS Tregs regained suppressive abilities to the levels of control Tregs, suggesting that autologous passive transfer of expanded Tregs might offer a novel cellular therapy to slow disease progression. ^[3]

Despite this, there are no validated biomarkers of neuroinflammation for use in clinical practice or clinical trials. Biomarkers of neuroinflammation could improve patient management, provide new therapeutic targets, and possibly help stratify clinical trial selection and monitoring. However, attempts to identify a singular cause of neuroinflammation have not been successful. Here, we performed multi-parameter flow cytometry to comprehensively assess 116 leukocyte populations and phenotypes from lymphocytes, monocytes, and granulocytes in a cohort of 80 ALS patients. We identified 32 leukocyte phenotypes that were altered in ALS patients compared to age and gender matched healthy volunteers (HV) that included phenotypes of both inflammation and immune suppression. Unsupervised hierarchical clustering and principle component analysis of ALS and HV immunophenotypes revealed two distinct immune profiles of ALS patients. ALS patients were clustered into a profile distinct from HVs primarily due to differences in a multiple T cell phenotypes, CD3+CD56+ T cells and HLA-DR on monocytes. Patients clustered into an abnormal immune profile were younger, more likely to have a familial form of the disease, and survived longer than those patients who clustered similarly with healthy volunteers (344 weeks versus 184 weeks; p = 0.012). The data set generated from this study establishes an extensive accounting of immunophenotypic changes readily suitable for biomarker validation studies. The extensive immune system changes measured in this study indicate that normal immune homeostatic mechanisms are disrupted in ALS patients, and that multiple immune states likely exist within a population of patients with ALS. ^[4]

While it is generally believed that the disease onset takes place inside motor neurons, different cell types mediating neuroinflammatory processes are considered deeply involved in the progression of the disease. On these grounds, many treatments have been tested on ALS animals with the aim of inhibiting or reducing the pro-inflammatory action of microglia and astrocytes and counteract the progression of the disease. Unfortunately, these anti-inflammatory therapies have been only modestly successful. The non-univocal role played by microglia during stress and injuries might explain this failure. Indeed, it is now well recognized that, during ALS, microglia displays different phenotypes, from surveillant in early stages, to activated states, M1 and M2, characterized by the expression of respectively harmful and protective genes in later phases of the disease. Consistently, the inhibition of microglial function seems to be a valid strategy only if the different stages of microglia polarization are taken into account, interfering with the reactivity of microglia specifically targeting only the harmful pathways and/or potentiating the trophic ones. In this review article, we will analyze the features and timing of microglia activation in the light of M1/M2 phenotypes in the main mice models of ALS. Moreover, we will also revise the results obtained by different anti-inflammatory therapies aimed to unbalance the M1/M2 ratio, shifting it towards a protective outcome. ^[5]

In particular, in in vitro settings, lipopolysaccharide (LPS) or IFN-γ stimulate “classically activated” M1 microglia, which release pro-inflammatory mediators. They include pro-inflammatory cytokines (interleukin [IL]-1α, IL-1β, IL-6, IL-12, IL-23, tumor necrosis factor-α [TNF-α]), chemokines, prostaglandin E2, chemokine (C-C motif) ligand 2, ROS and inducible nitric oxide synthase (iNOS; Bagasra et al., 1995; Du et al., 2016; Orihuela et al., 2016). In contrast, “alternatively activated” M2 phenotype, which is induced by anti-inflammatory cytokines IL-4, IL-10 or IL-13, suppresses inflammation, clears cellular debris through phagocytosis, promotes extracellular matrix reconstruction and supports neuron survival through the release of protective/trophic factors (Hu et al., 2015; Du et al., 2016; Tang and Le, 2016). “Acquired deactivation” represents another M2 anti-inflammatory phenotype and it is mainly induced by the uptake of apoptotic cells or exposure to anti-inflammatory cytokines, such as IL-10 and transforming growth factor-β (Tang and Le, 2016). ^[5]

References[edit]

1. ↑ Ransohoff &: How neuroinflammation contributes to neurodegeneration. Science 2016;353:777-83. PMID: 27540165. DOI. Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and frontotemporal lobar dementia are among the most pressing problems of developed societies with aging populations. Neurons carry out essential functions such as signal transmission and network integration in the central nervous system and are the main targets of neurodegenerative disease. In this Review, I address how the neuron's environment also contributes to neurodegeneration. Maintaining an optimal milieu for neuronal function rests with supportive cells termed glia and the blood-brain barrier. Accumulating evidence suggests that neurodegeneration occurs in part because the environment is affected during disease in a cascade of processes collectively termed neuroinflammation. These observations indicate that therapies targeting glial cells might provide benefit for those afflicted by neurodegenerative disorders.
2. ↑ Leal-Lasarte et al.: Extracellular TDP-43 aggregates target MAPK/MAK/MRK overlapping kinase (MOK) and trigger caspase-3/IL-18 signaling in microglia. FASEB J. 2017;31:2797-2816. PMID: 28336525. DOI. Dysregulated microglial responses are central in neurodegenerative proteinopathies, including amyotrophic lateral sclerosis (ALS) and frontotemporal lobar disease (FTLD). Pathologic TDP-43, which is typically found in intracellular inclusions, is a misfolding protein with emerging roles in ALS and FTLD. Recently, TDP-43 species have been found in extracellular fluids of patients; however, the overall implications of TDP-43-mediated signaling linked to neuroinflammation are poorly understood. Our work-the first, to our knowledge, to focus on innate immunity responses to TDP-43 aggregates-shows that such species are internalized by microglia and cause abnormal mobilization of endogenous TDP-43. Exposure to TDP-43 aggregates elicited not only IL-1β, but also NLRP3-dependent and noncanonical IL-18 processing. Moreover, we report a link between TDP-43 and neuronal loss via the apoptosis-independent emerging roles of caspase-3 in neurotoxic inflammation. Our results further support the view of noncell autonomous neurodegenerative mechanisms in ALS. Remarkably, we demonstrate that TDP-43 aggregates bind to and colocalize with MAPK/MAK/MRK overlapping kinase (MOK) and show that its phosphorylation status is disrupted. Finally, we show that this TDP-43-caused activation state can be altered by exogenous Hsp27 and Hsp70 chaperones. Our study provides new insight into the immune phenotype, mechanisms, and signaling pathways that operate in microglial neurotoxic activation in ALS.-Leal-Lasarte, M. M., Franco, J. M., Labrador-Garrido, A., Pozo, D., Roodveldt, C. Extracellular TDP-43 aggregates target MAPK/MAK/MRK overlapping kinase (MOK) and trigger caspase-3/IL-18 signaling in microglia.
3. ↑ Beers et al.: ALS patients' regulatory T lymphocytes are dysfunctional, and correlate with disease progression rate and severity. JCI Insight 2017;2:e89530. PMID: 28289705. DOI. Neuroinflammation is a pathological hallmark of ALS in both transgenic rodent models and patients, and is characterized by proinflammatory T lymphocytes and activated macrophages/microglia. In ALS mouse models, decreased regulatory T lymphocytes (Tregs) exacerbate the neuroinflammatory process, leading to accelerated motoneuron death and shortened survival; passive transfer of Tregs suppresses the neuroinflammation and prolongs survival. Treg numbers and FOXP3 expression are also decreased in rapidly progressing ALS patients. A key question is whether the marked neuroinflammation in ALS can be attributed to the impaired suppressive function of ALS Tregs in addition to their decreased numbers. To address this question, T lymphocyte proliferation assays were performed. Compared with control Tregs, ALS Tregs were less effective in suppressing responder T lymphocyte proliferation. Although both slowly and rapidly progressing ALS patients had dysfunctional Tregs, the greater the clinically assessed disease burden or the more rapidly progressing the patient, the greater the Treg dysfunction. Epigenetically, the percentage methylation of the Treg-specific demethylated region was greater in ALS Tregs. After in vitro expansion, ALS Tregs regained suppressive abilities to the levels of control Tregs, suggesting that autologous passive transfer of expanded Tregs might offer a novel cellular therapy to slow disease progression.
4. ↑ Gustafson et al.: Comprehensive immune profiling reveals substantial immune system alterations in a subset of patients with amyotrophic lateral sclerosis. PLoS ONE 2017;12:e0182002. PMID: 28742871. DOI. Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with a median lifespan of 2-3 years after diagnosis. There are few meaningful treatments that alter progression in this disease. Preclinical and clinical studies have demonstrated that neuroinflammation may play a key role in the progression rate of ALS. Despite this, there are no validated biomarkers of neuroinflammation for use in clinical practice or clinical trials. Biomarkers of neuroinflammation could improve patient management, provide new therapeutic targets, and possibly help stratify clinical trial selection and monitoring. However, attempts to identify a singular cause of neuroinflammation have not been successful. Here, we performed multi-parameter flow cytometry to comprehensively assess 116 leukocyte populations and phenotypes from lymphocytes, monocytes, and granulocytes in a cohort of 80 ALS patients. We identified 32 leukocyte phenotypes that were altered in ALS patients compared to age and gender matched healthy volunteers (HV) that included phenotypes of both inflammation and immune suppression. Unsupervised hierarchical clustering and principle component analysis of ALS and HV immunophenotypes revealed two distinct immune profiles of ALS patients. ALS patients were clustered into a profile distinct from HVs primarily due to differences in a multiple T cell phenotypes, CD3+CD56+ T cells and HLA-DR on monocytes. Patients clustered into an abnormal immune profile were younger, more likely to have a familial form of the disease, and survived longer than those patients who clustered similarly with healthy volunteers (344 weeks versus 184 weeks; p = 0.012). The data set generated from this study establishes an extensive accounting of immunophenotypic changes readily suitable for biomarker validation studies. The extensive immune system changes measured in this study indicate that normal immune homeostatic mechanisms are disrupted in ALS patients, and that multiple immune states likely exist within a population of patients with ALS.
5. ↑ ^{5.0 5.1} Geloso et al.: The Dual Role of Microglia in ALS: Mechanisms and Therapeutic Approaches. Front Aging Neurosci 2017;9:242. PMID: 28790913. DOI. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by a non-cell autonomous motor neuron loss. While it is generally believed that the disease onset takes place inside motor neurons, different cell types mediating neuroinflammatory processes are considered deeply involved in the progression of the disease. On these grounds, many treatments have been tested on ALS animals with the aim of inhibiting or reducing the pro-inflammatory action of microglia and astrocytes and counteract the progression of the disease. Unfortunately, these anti-inflammatory therapies have been only modestly successful. The non-univocal role played by microglia during stress and injuries might explain this failure. Indeed, it is now well recognized that, during ALS, microglia displays different phenotypes, from surveillant in early stages, to activated states, M1 and M2, characterized by the expression of respectively harmful and protective genes in later phases of the disease. Consistently, the inhibition of microglial function seems to be a valid strategy only if the different stages of microglia polarization are taken into account, interfering with the reactivity of microglia specifically targeting only the harmful pathways and/or potentiating the trophic ones. In this review article, we will analyze the features and timing of microglia activation in the light of M1/M2 phenotypes in the main mice models of ALS. Moreover, we will also revise the results obtained by different anti-inflammatory therapies aimed to unbalance the M1/M2 ratio, shifting it towards a protective outcome.