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Mer tyrosine kinase (MerTK) is a receptor tyrosine kinase that mediates non-inflammatory, homeostatic phagocytosis of diverse types of cellular debris. Highly expressed on the surface of microglial cells, MerTK is of importance in brain development, homeostasis, plasticity and disease. Yet, involvement of this receptor in the clearance of protein aggregates that accumulate with ageing and in neurodegenerative diseases has yet to be defined. The current study explored the function of MerTK in the microglial uptake of alpha-synuclein fibrils which play a causative role in the pathobiology of synucleinopathies. Using human primary and induced pluripotent stem cell-derived microglia, the MerTK-dependence of alpha-synuclein fibril internalization was investigated in vitro. Relevance of this pathway in synucleinopathies was assessed through burden analysis of MERTK variants and analysis of MerTK expression in patient-derived cells and tissues. Pharmacological inhibition of MerTK and siRNA-mediated MERTK knockdown both caused a decreased rate of alpha-synuclein fibril internalization by human microglia. Consistent with the non-inflammatory nature of MerTK-mediated phagocytosis, alpha-synuclein fibril internalization was not observed to induce secretion of pro-inflammatory cytokines such as IL-6 or TNF, and downmodulated IL-1β secretion from microglia. Burden analysis in two independent patient cohorts revealed a significant association between rare functionally deleterious MERTK variants and Parkinson's disease in one of the cohorts (P = 0.002). Despite a small upregulation in MERTK mRNA expression in nigral microglia from Parkinson's disease/Lewy body dementia patients compared to those from non-neurological control donors in a single-nuclei RNA-sequencing dataset (P = 5.08 × 10−21), no significant upregulation in MerTK protein expression was observed in human cortex and substantia nigra lysates from Lewy body dementia patients compared to controls. Taken together, our findings define a novel role for MerTK in mediating the uptake of alpha-synuclein fibrils by human microglia, with possible involvement in limiting alpha-synuclein spread in synucleinopathies such as Parkinson's disease. Upregulation of this pathway in synucleinopathies could have therapeutic values in enhancing alpha-synuclein fibril clearance in the brain. [ABSTRACT FROM AUTHOR]

Early multiple sclerosis lesions feature relative preservation of oligodendrocyte cell bodies with dying back retraction of their myelinating processes. Cell loss occurs with disease progression. Putative injury mediators include metabolic stress (low glucose/nutrient), pro-inflammatory mediators (interferon γ and tumor necrosis factor α), and excitotoxins (glutamate). Our objective was to compare the impact of these disease relevant mediators on the injury responses of human mature oligodendrocytes. In the current study, we determined the effects of these mediators on process extension and survival of human brain derived mature oligodendrocytes in vitro and used bulk RNA sequencing to identify distinct effector mechanisms that underlie the responses. All mediators induced significant process retraction of the oligodendrocytes in dissociated cell culture. Only metabolic stress (low glucose/nutrient) conditions resulted in delayed (4-6 days) non-apoptotic cell death. Metabolic effects were associated with induction of the integrated stress response, which can be protective or contribute to cell injury dependent on its level and duration of activation. Addition of Sephin1, an agonist of the integrated stress response induced process retraction under control conditions and further enhanced retraction under metabolic stress conditions. The antagonist ISRIB restored process outgrowth under stress conditions, and if added to already stressed cells, reduced delayed cell death and prolonged the period in which recovery could occur. Inflammatory cytokine functional effects were associated with activation of multiple signaling pathways (including Jak/Stat-1) that regulate process outgrowth, without integrated stress response induction. Glutamate application produced limited transcriptional changes suggesting a contribution of effects directly on cell processes. Our comparative studies indicate the need to consider both the specific injury mediators and the distinct cellular mechanisms of responses to them by human oligodendrocytes to identify effective neuroprotective therapies for multiple sclerosis. [ABSTRACT FROM AUTHOR]

Blood-brain barrier function is driven by the influence of astrocyte-secreted factors. During neuroinflammatory responses the blood-brain barrier is compromised resulting in central nervous system damage and exacerbated pathology. Here, we identified endothelial netrin 1 induction as a vascular response to astrocyte-derived sonic hedgehog that promotes autocrine barrier properties during homeostasis and increases with inflammation. Netrin 1 supports blood-brain barrier integrity by upregulating endothelial junctional protein expression, while netrin 1 knockout mice display disorganized tight junction protein expression and barrier breakdown. Upon inflammatory conditions, blood-brain barrier endothelial cells significantly upregulated netrin 1 levels in vitro and in situ, which prevented junctional breach and endothelial cell activation. Finally, netrin 1 treatment during experimental autoimmune encephalomyelitis significantly reduced blood-brain barrier disruption and decreased clinical and pathological indices of disease severity. Our results demonstrate that netrin 1 is an important regulator of blood-brain barrier maintenance that protects the central nervous system against inflammatory conditions such as multiple sclerosis and experimental autoimmune encephalomyelitis. [ABSTRACT FROM AUTHOR]

The emerging roles of microglia are currently being investigated in the healthy and diseased brain with a growing interest in their diverse functions. In recent years, it has been demonstrated that microglia are not only immunocentric, but also neurobiological and can impact neural development and the maintenance of neuronal cell function in both healthy and pathological contexts. In the disease context, there is widespread consensus that microglia are dynamic cells with a potential to contribute to both central nervous system damage and repair. Indeed, a number of studies have found that microenvironmental conditions can selectively modify unique microglia phenotypes and functions. One novel mechanism that has garnered interest involves the regulation of microglial function by microRNAs, which has therapeutic implications such as enhancing microglia-mediated suppression of brain injury and promoting repair following inflammatory injury. Furthermore, recently published articles have identified molecular signatures of myeloid cells, suggesting that microglia are a distinct cell population compared to other cells of myeloid lineage that access the central nervous system under pathological conditions. Thus, new opportunities exist to help distinguish microglia in the brain and permit the study of their unique functions in health and disease. [ABSTRACT FROM AUTHOR]

Antigen-antibody complexes (Ag Ab) prepared from 125-radiolabelled bovine serum albumin (BSA) and guinea-pig antibody, were (1) pre-incubated at 37 C for 30 min with serum or cerebrospinal fluid (CSF) in different proportions and then reacted with cells, (2) incubated at 37 C directly with cells suspended in serum or CSF for different time periods or (3) bound to cells (following incubation with serum in optimal proportions) and the cell-bound immune complexes incubated with serum or CSF at 37 C for different time periods. When Ag Ab were pre-incubated with serum or CSF and reacted with unfractionated blood cells or mononuclear cells, binding decreased as serum to Ag Ab proportion was increased above 1: 1 6, but increased us CSF to Ag Ab proportion was increased. When serum diluted 200-fold (to approximate the protein concentration of CSF) was used in place of undiluted serum. serum-mediated binding paralleled CSF-mediated binding, inactivation of serum. CSF and 1: 200 serum in different ways and substitution of human red blood cells (RBC) (known to possess C3b receptors) or sheep RBC (known not to possess C3b receptors) demonstrated that binding was to C3b receptors. Addition of CSF to serum did not alter serum-mediated binding. When Ag Ab were incubated directly with unfractionated blood cells suspended in serum or CSF, binding increased rapidly in serum, reaching a maximum within 2-4 min, and IC then rapidly dissociated, whereas binding increased gradually in CSF and IC remained associated with cells. When serum diluted approximately 100-fold was used in place of undiluted serum, kinetics of serum-mediated interaction approached that of CSF- mediated interaction. When IC were bound to Raji cells or human RBC and the cell-bound IC incubated in serum or CSF. > 85%, of IC dissociated in serum after 30 min, but no dissociation occurred in (CSF. Dilution of serum > 1: 16. and > 1:64 abolished dissociation from the two cell types respectively. These results indicate that CSF mediates binding of IC to complement receptors on cells but lacks the activities of strum which convert IC into a non-binding state. [ABSTRACT FROM AUTHOR]