The Microglial Sensome Revealed by Direct RNA Sequencing

Microglia, the principal neuroimmune sentinels of the brain, continuously sense changes in their environment and respond to invading pathogens, toxins and cellular debris. Microglia exhibit plasticity and can assume neurotoxic or neuroprotective priming states that determine their responses to danger. We used direct RNA sequencing, without amplification or cDNA synthesis, to determine the quantitative transcriptomes of microglia of healthy adult and aged mice. We validated our findings by fluorescent dual in-situ hybridization, unbiased proteomic analysis and quantitative PCR. We report here that microglia have a distinct transcriptomic signature and express a unique cluster of transcripts encoding proteins for sensing endogenous ligands and microbes that we term the “sensome”. With aging, sensome transcripts for endogenous ligand recognition are downregulated, whereas those involved in microbe recognition and host defense are upregulated. In addition, aging is associated with an overall increase in expression of microglial genes involved in neuroprotection

Scara1 deficiency impairs clearance of soluble Amyloid-β by mononuclear phagocytes and accelerates Alzheimer’s-like disease progression

In Alzheimer’s disease soluble amyloid beta (sAβ) causes synaptic dysfunction and neuronal loss. Receptors involved in clearance of sAβ are not known. Here we use shRNA screening and identify the scavenger receptor Scara1 as a receptor for sAβ expressed on myeloid cells. To determine the role of Scara1 in clearance of sAβ in vivo, we cross Scara1 null mice with PS1-APP mice, a mouse model of Alzheimer’s disease and generate PS1-APP- Scara1-deficient mice. Scara1 deficiency markedly accelerates Aβ accumulation leading to increased mortality. In contrast, pharmacological upregulation of Scara1 expression on mononuclear phagocytes increases Aβ clearance. This approach is a potential treatment strategy for Alzheimer’s disease

Scara1 deficiency impairs clearance of soluble Amyloid-β by mononuclear phagocytes and accelerates Alzheimer’s-like disease progression

In Alzheimer’s disease soluble amyloid beta (sAβ) causes synaptic dysfunction and neuronal loss. Receptors involved in clearance of sAβ are not known. Here we use shRNA screening and identify the scavenger receptor Scara1 as a receptor for sAβ expressed on myeloid cells. To determine the role of Scara1 in clearance of sAβ in vivo, we cross Scara1 null mice with PS1-APP mice, a mouse model of Alzheimer’s disease and generate PS1-APP- Scara1-deficient mice. Scara1 deficiency markedly accelerates Aβ accumulation leading to increased mortality. In contrast, pharmacological upregulation of Scara1 expression on mononuclear phagocytes increases Aβ clearance. This approach is a potential treatment strategy for Alzheimer’s disease

CD36 Mediates the Innate Host Response to β-Amyloid

Accumulation of inflammatory microglia in Alzheimer's senile plaques is a hallmark of the innate response to β-amyloid fibrils and can initiate and propagate neurodegeneration characteristic of Alzheimer's disease (AD). The molecular mechanism whereby fibrillar β-amyloid activates the inflammatory response has not been elucidated. CD36, a class B scavenger receptor, is expressed on microglia in normal and AD brains and binds to β-amyloid fibrils in vitro. We report here that microglia and macrophages, isolated from CD36 null mice, had marked reductions in fibrillar β-amyloid–induced secretion of cytokines, chemokines, and reactive oxygen species. Intraperitoneal and stereotaxic intracerebral injection of fibrillar β-amyloid in CD36 null mice induced significantly less macrophage and microglial recruitment into the peritoneum and brain, respectively, than in wild-type mice. Our data reveal that CD36, a major pattern recognition receptor, mediates microglial and macrophage response to β-amyloid, and imply that CD36 plays a key role in the proinflammatory events associated with AD

The scavenger receptor SCARF1 mediates apoptotic cell clearance and prevents autoimmunity

Clearance of apoptotic cells is critical for control of tissue homeostasis however the full range of receptor(s) on phagocytes responsible for recognition of apoptotic cells remains to be identified. Here we show that dendritic cells (DCs), macrophages and endothelial cells use scavenger receptor type F family member 1 (SCARF1) to recognize and engulf apoptotic cells via C1q. Loss of SCARF1 impairs uptake of apoptotic cells. Consequently, in SCARF1-deficient mice, dying cells accumulate in tissues leading to a lupus-like disease with the spontaneous generation of autoantibodies to DNA-containing antigens, immune cell activation, dermatitis and nephritis. The discovery of SCARF1 interactions with C1q and apoptotic cells provides insights into molecular mechanisms involved in maintenance of tolerance and prevention of autoimmune disease

Standardizing Scavenger Receptor Nomenclature

Scavenger receptors constitute a large family of proteins that are structurally diverse and participate in a wide range of biological functions. These receptors are expressed predominantly by myeloid cells and recognize a variety of ligands, including endogenous and modified host-derived molecules and microbial pathogens. There are currently eight classes of scavenger receptors, many of which have multiple names, leading to inconsistencies and confusion in the literature. To address this problem, a workshop was organized by the U.S. National Institute of Allergy and Infectious Diseases, National Institutes of Health to help develop a clear definition of scavenger receptors and a standardized nomenclature based on that definition. Fifteen experts in the scavenger receptor field attended the workshop and, after extensive discussion, reached a consensus regarding the definition of scavenger receptors and a proposed scavenger receptor nomenclature. Scavenger receptors were defined as cell surface receptors that typically bind multiple ligands and promote the removal of non-self or altered-self targets. They often function by mechanisms that include endocytosis, phagocytosis, adhesion, and signaling that ultimately lead to the elimination of degraded or harmful substances. Based on this definition, nomenclature and classification of these receptors into 10 classes were proposed. The discussion and nomenclature recommendations described in this report only refer to mammalian scavenger receptors. The purpose of this article is to describe the proposed mammalian nomenclature and classification developed at the workshop and to solicit additional feedback from the broader research community

Depletion of CD11c+ cells in the CD11c.DTR model drives expansion of unique CD64+ Ly6C+ monocytes that are poised to release TNF-α

Dendritic cells (DCs) play a vital role in innate and adaptive immunities. Inducible depletion of CD11c+ DCs engineered to express a high-affinity diphtheria toxin receptor has been a powerful tool to dissect DC function in vivo. However, despite reports showing that loss of DCs induces transient monocytosis, the monocyte population that emerges and the potential impact of monocytes on studies of DC function have not been investigated. We found that depletion of CD11c+ cells from CD11c.DTR mice induced the expansion of a variant CD64+ Ly6C+ monocyte population in the spleen and blood that was distinct from conventional monocytes. Expansion of CD64+ Ly6C+ monocytes was independent of mobilization from the BM via CCR2 but required the cytokine, G-CSF. Indeed, this population was also expanded upon exposure to exogenous G-CSF in the absence of DC depletion. CD64+ Ly6C+ monocytes were characterized by upregulation of innate signaling apparatus despite the absence of inflammation, and an increased capacity to produce TNF-α following LPS stimulation. Thus, depletion of CD11c+ cells induces expansion of a unique CD64+ Ly6C+ monocyte population poised to synthesize TNF-α. This finding will require consideration in experiments using depletion strategies to test the role of CD11c+ DCs in immunity

The receptor TREML4 amplifies TLR7-mediated signaling during antiviral responses and autoimmunity

The genes and pathways that fine-tune TLR7-mediated innate inflammatory responses remain to be fully elucidated. Using an unbiased genome-scale shRNA screen, we identified the receptor TREML4 as an essential positive regulator of TLR7 signaling. Macrophages from Treml4–/– mice were hyporesponsive to TLR7 agonists and failed to produce type I interferon due to impaired phosphorylation of the transcription factor STAT1 by the MAP kinase p38 and decreased recruitment of MyD88 to TLR7. TREML4 deficiency reduced production of inflammatory cytokines and autoantibodies in SLE-prone MRL/lpr mice and inhibited the antiviral immune response to influenza. Our data identify TREML4 as a positive regulator of TLR7 signaling and provide insight into the molecular mechanisms that control antiviral immunity and the development of autoimmunity

A Consensus Definitive Classification of Scavenger Receptors and Their Roles in Health and Disease

Scavenger receptors constitute a large family of proteins that are structurally diverse and participate in a wide range of biological functions. These receptors are expressed predominantly by myeloid cells and recognize a diverse variety of ligands including endogenous and modified host-derived molecules and microbial pathogens. There are currently eight classes of scavenger receptors, many of which have multiple names, leading to inconsistencies and confusion in the literature. To address this problem, a workshop was organized by theUnited StatesNational Institute of Allergy and Infectious Diseases, National Institutes of Health, to help develop a clear definition of scavenger receptors and a standardized nomenclature based on that definition. Fifteen experts in the scavenger receptor field attended the workshop and, after extensive discussion, reached a consensus regarding the definition of scavenger receptors and a proposed scavenger receptor nomenclature. Scavenger receptors were defined as cell surface receptors that typically bind multiple ligands and promote the removal of nonself or altered-self targets. They often function by mechanisms that include endocytosis, phagocytosis, adhesion, and signaling that ultimately lead to the elimination of degraded or harmful substances. Based on this definition, nomenclature and classification of these receptors into 10 classes were proposed. This classification was discussed at three national meetings and input from participants at these meetings was requested. The following manuscript is a consensus statement that combines the recommendations of the initial workshop and incorporates the input received from the participants at the three national meetings