Osteopetrosis and thalamic hypomyelinosis with synaptic degeneration in DAP12-deficient mice

Graft-infiltrating PD-L1hi cross-dressed dendritic cells regulate antidonor T cell responses in mouse liver transplant tolerance

Although a key role of cross-dressing has been established in immunity to viral infection and more recently in the instigation of transplant rejection, its role in tolerance is unclear. We investigated the role of intragraft dendritic cells (DCs) and cross-dressing in mouse major histocompatibility complex (MHC)-mismatched liver transplant tolerance that occurs without therapeutic immunosuppression. Although donor interstitial DCs diminished rapidly after transplantation, they were replaced in the liver by host DCs that peaked on postoperative day (POD) 7 and persisted indefinitely. Approximately 60% of these recipient DCs displayed donor MHC class I, indicating cross-dressing. By contrast, only a very minor fraction (0%-2%) of cross-dressed DCs (CD-DCs) was evident in the spleen. CD-DCs sorted from liver grafts expressed much higher levels of T cell inhibitory programed death ligand 1 (PD-L1) and high levels of interleukin-10 compared with non-CD-DCs (nCD-DCs) isolated from the graft. Concomitantly, high incidences of programed death protein 1 (PD-1)^hi T cell immunoglobulin and mucin domain containing 3 (TIM-3)⁺ exhausted graft-infiltrating CD8⁺ T cells were observed. Unlike nCD-DCs, the CD-DCs failed to stimulate proliferation of allogeneic T cells but markedly suppressed antidonor host T cell proliferation. CD-DCs were much less evident in allografts from DNAX-activating protein of 12 kDa (DAP12)^-/- donors that were rejected acutely.

CONCLUSION

These findings suggest that graft-infiltrating PD-L1^hi CD-DCs may play a key role in the regulation of alloimmunity and in the induction of liver transplant tolerance. (Hepatology 2018;67:1499-1515).

Role of microglia-neuron interactions in diabetic encephalopathy

In the central nervous system, the primary immune cells, the microglia, prevent pathogenic invasion as the first line of defense. Microglial energy consumption is dependent on their degree of activity. Microglia express transporters for the three primary energy substrates (glucose, fatty acids, glutamine) and regulate diabetic encephalopathy via microglia-neuron interactions. Microglia may play a sentry role for rapid protection or even ablation of impaired neurons. Neurons exhibit hyperactivity in response to hyperglycemia, hyperlipidemia, and neurotoxic factors and release potential microglial activators. Microglial activation is also regulated by proinflammatory factors, caspase-3 activity, P2X₇ receptor, interferon regulatory factor-8, and glucocorticoids. Modulation of microglia in diabetic encephalopathy may involve CX3CL1, p38 MAPK, purinergic, and CD200/CD200R signaling pathways, and pattern recognition receptors. The microglia-neuron interactions play an important role in diabetic encephalopathy, and modulation of microglial activation may be a therapeutic target for diabetic encephalopathy.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

Microglia and Aging: The Role of the TREM2-DAP12 and CX3CL1-CX3CR1 Axes

Depending on the species, microglial cells represent 5-20% of glial cells in the adult brain. As the innate immune effector of the brain, microglia are involved in several functions: regulation of inflammation, synaptic connectivity, programmed cell death, wiring and circuitry formation, phagocytosis of cell debris, and synaptic pruning and sculpting of postnatal neural circuits. Moreover, microglia contribute to some neurodevelopmental disorders such as Nasu-Hakola disease (NHD), and to aged-associated neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and others. There is evidence that human and rodent microglia may become senescent. This event determines alterations in the microglia activation status, associated with a chronic inflammation phenotype and with the loss of neuroprotective functions that lead to a greater susceptibility to the neurodegenerative diseases of aging. In the central nervous system (CNS), Triggering Receptor Expressed on Myeloid Cells 2-DNAX activation protein 12 (TREM2-DAP12) is a signaling complex expressed exclusively in microglia. As a microglial surface receptor, TREM2 interacts with DAP12 to initiate signal transduction pathways that promote microglial cell activation, phagocytosis, and microglial cell survival. Defective TREM2-DAP12 functions play a central role in the pathogenesis of several diseases. The CX3CL1 (fractalkine)-CX3CR1 signaling represents the most important communication channel between neurons and microglia. The expression of CX3CL1 in neurons and of its receptor CX3CR1 in microglia determines a specific interaction, playing fundamental roles in the regulation of the maturation and function of these cells. Here, we review the role of the TREM2-DAP12 and CX3CL1-CX3CR1 axes in aged microglia and the involvement of these pathways in physiological CNS aging and in age-associated neurodegenerative diseases.

The CD300e molecule in mice is an immune-activating receptor

CD300 molecules (CD300s) belong to paired activating and inhibitory receptor families, which mediate immune responses. Human CD300e (hCD300e) is expressed in monocytes and myeloid dendritic cells and transmits an immune-activating signal by interacting with DNAX-activating protein 12 (DAP12). However, the CD300e ortholog in mice (mCD300e) is poorly characterized. Here, we found that mCD300e is also an immune-activating receptor. We found that mCD300e engagement triggers cytokine production in mCD300e-transduced bone marrow-derived mast cells (BMMCs). Loss of DAP12 and another signaling protein, FcRγ, did not affect surface expression of transduced mCD300e, but abrogated mCD300e-mediated cytokine production in the BMMCs. Co-immunoprecipitation experiments revealed that mCD300e physically interacts with both FcRγ and DAP12, suggesting that mCD300e delivers an activating signal via these two proteins. Binding and reporter assays with the mCD300e extracellular domain identified sphingomyelin as a ligand of both mCD300e and hCD300e. Notably, the binding of sphingomyelin to mCD300e stimulated cytokine production in the transduced BMMCs in an FcRγ- and DAP12-dependent manner. Flow cytometric analysis with an mCD300e-specific Ab disclosed that mCD300e expression is highly restricted to CD115⁺Ly-6C^low/int peripheral blood monocytes, corresponding to CD14^dim/+CD16⁺ human nonclassical and intermediate monocytes. Loss of FcRγ or DAP12 lowered the surface expression of endogenous mCD300e in the CD115⁺Ly-6C^low/int monocytes. Stimulation with sphingomyelin failed to activate the CD115⁺Ly-6C^low/int mouse monocytes, but induced hCD300e-mediated cytokine production in the CD14^dimCD16⁺ human monocytes. Taken together, these observations indicate that mCD300e recognizes sphingomyelin and thereby regulates nonclassical and intermediate monocyte functions through FcRγ and DAP12.

STAC2 negatively regulates osteoclast formation by targeting the RANK signaling complex

The receptor activator of nuclear factor-κB (RANK) protein activates various protein kinase signaling cascades, including those involving NF-κB, mitogen-activated protein kinase (MAPK), and Bruton tyrosine kinase (Btk)/tyrosine-protein kinase Tec. However, the mechanism underlying the negative regulation of RANK by downstream signaling molecules remains unclear. Here, we report that Src homology 3 domain and cysteine-rich domain-containing protein 2 (STAC2) is a novel RANK ligand-inducible protein that negatively regulates RANK-mediated osteoclast formation. STAC2 physically interacts with RANK and inhibits the formation of the RANK signaling complex, which contains Grb-2-associated binder 2 (Gab2) and phospholipase Cγ2 (PLCγ2), thus leading to the suppression of RANK-mediated NF-κB and MAPK activation. Furthermore, STAC2 overexpression limits Btk/Tec-mediated PLCγ2 phosphorylation via the interaction between STAC2 and Btk/Tec. Taken together, our results reveal a novel mechanism whereby RANK signaling is restricted by its physical interaction with STAC2.

Microglia Gone Rogue: Impacts on Psychiatric Disorders across the Lifespan

Microglia are the predominant immune response cells and professional phagocytes of the central nervous system (CNS) that have been shown to be important for brain development and homeostasis. These cells present a broad spectrum of phenotypes across stages of the lifespan and especially in CNS diseases. Their prevalence in all neurological pathologies makes it pertinent to reexamine their distinct roles during steady-state and disease conditions. A major question in the field is determining whether the clustering and phenotypical transformation of microglial cells are leading causes of pathogenesis, or potentially neuroprotective responses to the onset of disease. The recent explosive growth in our understanding of the origin and homeostasis of microglia, uncovering their roles in shaping of the neural circuitry and synaptic plasticity, allows us to discuss their emerging functions in the contexts of cognitive control and psychiatric disorders. The distinct mesodermal origin and genetic signature of microglia in contrast to other neuroglial cells also make them an interesting target for the development of therapeutics. Here, we review the physiological roles of microglia, their contribution to the effects of environmental risk factors (e.g., maternal infection, early-life stress, dietary imbalance), and their impact on psychiatric disorders initiated during development (e.g., Nasu-Hakola disease (NHD), hereditary diffuse leukoencephaly with spheroids, Rett syndrome, autism spectrum disorders (ASDs), and obsessive-compulsive disorder (OCD)) or adulthood (e.g., alcohol and drug abuse, major depressive disorder (MDD), bipolar disorder (BD), schizophrenia, eating disorders and sleep disorders). Furthermore, we discuss the changes in microglial functions in the context of cognitive aging, and review their implication in neurodegenerative diseases of the aged adult (e.g., Alzheimer’s and Parkinson’s). Taking into account the recent identification of microglia-specific markers, and the availability of compounds that target these cells selectively in vivo, we consider the prospect of disease intervention via the microglial route.

Genetic and Pharmacological Dissection of the Role of Spleen Tyrosine Kinase (Syk) in Intestinal Inflammation and Immune Dysfunction in Inflammatory Bowel Diseases

BACKGROUND

The DNAX adaptor protein 12 (DAP12) is a transmembrane adaptor molecule that signals through the activation of Syk (Spleen Tyrosine Kinase) in myeloid cells. The purpose of this study is to investigate the role of DAP12 and Syk pathways in inflammatory bowel diseases (IBDs).

METHODS

DAP12 deficient and DAP12 transgenic, overexpressing an increased amount of DAP12, mice and Syk deficient mice in the C57/BL6 background were used for these studies. Colitis was induced by administering mice with dextran sulfate sodium (DSS), in drinking water, or 2,4,6-trinitrobenzene sulfonic acid (TNBS), by intrarectal enema.

RESULTS

Abundant expression of DAP12 and Syk was detected in colon samples obtained from Crohn's disease patients with expression restricted to immune cells infiltrating the colonic wall. In rodents development of DSS colitis as measured by assessing severity of wasting diseases, global colitis score,and macroscopic and histology scores was robustly attenuated in DAP12-/- and Syk-/- mice. In contrast, DAP12 overexpression resulted in a striking exacerbation of colon damage caused by DSS. Induction of colon expression of proinflammatory cytokines and chemokines in response to DSS administration was attenuated in DAP12-/- and Syk-/- mice, whereas opposite results were observed in DAP12 transgenic mice. Treating wild-type mice with a DAP-12 inhibitor or a Syk inhibitor caused a robust attenuation of colitis induced by DSS and TNBS.

CONCLUSIONS

DAP12 and Syk are essential mediators in inflammation-driven immune dysfunction in murine colitides. Because DAP12 and Syk expression is upregulated in patients with active disease, present findings suggest a beneficial role for DAP12 and Syk inhibitors in IBD.

Macrophages and osteoclasts stem from a bipotent progenitor downstream of a macrophage/osteoclast/dendritic cell progenitor

Monocytes/macrophages (MΦs), osteoclasts (OCs), and dendritic cells (DCs) are closely related cell types of high clinical significance, but the exact steps in their lineage commitment are unclear. In studies on MΦ and DC development, OC development is generally not addressed. Furthermore, findings on DC development are confusing, because monocytes can also differentiate into DC-like cells. To resolve these issues, we have examined the development of monocytes/MΦs, OCs, and DCs from common progenitors, using the homeostatic driver cytokines macrophage colony-stimulating factor, RANK ligand (L), and Flt3L. In mouse bone marrow, B220^-CD11b^low/-c-Kit⁺c-Fms⁺ cells could be dissected into a CD27⁺Flt3⁺ population that proved oligopotent for MΦ/OC/DC development (MODP) and a CD27^low/-Flt3^- population that proved bipotent for MΦ/OC development (MOP). Developmental potential and relationship of MODP and downstream MOP populations are demonstrated by differentiation cultures, functional analysis of MΦ/OC/DC offspring, and genome-wide messenger RNA expression analysis. A common DC progenitor (CDP) has been described as committed to plasmacytoid and conventional DC development. However, the human CDP proved identical to the MODP population, whereas the mouse CDP largely overlapped with the MODP population and was accordingly oligopotent for MΦ, OC, and DC development. The CX₃CR1⁺ MΦ/DC progenitor (MDP) population described in the mouse generated MΦs and OCs but not DCs. Thus, monocytes/MΦs, OCs, and DCs share a common progenitor that gives rise to a bipotent MΦ/OC progenitor, but a dedicated DC progenitor is currently undefined. The definition of these progenitor populations may serve diagnostics and interventions in diseases with pathogenic activity of MΦs, OCs, or DCs.

Osteoimmunology: The Conceptual Framework Unifying the Immune and Skeletal Systems

The immune and skeletal systems share a variety of molecules, including cytokines, chemokines, hormones, receptors, and transcription factors. Bone cells interact with immune cells under physiological and pathological conditions. Osteoimmunology was created as a new interdisciplinary field in large part to highlight the shared molecules and reciprocal interactions between the two systems in both heath and disease. Receptor activator of NF-κB ligand (RANKL) plays an essential role not only in the development of immune organs and bones, but also in autoimmune diseases affecting bone, thus effectively comprising the molecule that links the two systems. Here we review the function, gene regulation, and signal transduction of osteoimmune molecules, including RANKL, in the context of osteoclastogenesis as well as multiple other regulatory functions. Osteoimmunology has become indispensable for understanding the pathogenesis of a number of diseases such as rheumatoid arthritis (RA). We review the various osteoimmune pathologies, including the bone destruction in RA, in which pathogenic helper T cell subsets [such as IL-17-expressing helper T (Th17) cells] induce bone erosion through aberrant RANKL expression. We also focus on cellular interactions and the identification of the communication factors in the bone marrow, discussing the contribution of bone cells to the maintenance and regulation of hematopoietic stem and progenitors cells. Thus the time has come for a basic reappraisal of the framework for understanding both the immune and bone systems. The concept of a unified osteoimmune system will be absolutely indispensable for basic and translational approaches to diseases related to bone and/or the immune system.

TREM2 in Neurodegenerative Diseases

TREM2 variants have been identified as risk factors for Alzheimer's disease (AD) and other neurodegenerative diseases (NDDs). Because TREM2 encodes a receptor exclusively expressed on immune cells, identification of these variants conclusively demonstrates that the immune response can play an active role in the pathogenesis of NDDs. These TREM2 variants also confer the highest risk for developing Alzheimer's disease of any risk factor identified in nearly two decades, suggesting that understanding more about TREM2 function could provide key insights into NDD pathology and provide avenues for novel immune-related NDD biomarkers and therapeutics. The expression, signaling and function of TREM2 in NDDs have been extensively investigated in an effort to understand the role of immune function in disease pathogenesis and progression. We provide a comprehensive review of our current understanding of TREM2 biology, including new insights into the regulation of TREM2 expression, and TREM2 signaling and function across NDDs. While many open questions remain, the current body of literature provides clarity on several issues. While it is still often cited that TREM2 expression is decreased by pro-inflammatory stimuli, it is now clear that this is true in vitro, but inflammatory stimuli in vivo almost universally increase TREM2 expression. Likewise, while TREM2 function is classically described as promoting an anti-inflammatory phenotype, more than half of published studies demonstrate a pro-inflammatory role for TREM2, suggesting that its role in inflammation is much more complex. Finally, these components of TREM2 biology are applied to a discussion of how TREM2 impacts NDD pathologies and the latest assessment of how these findings might be applied to immune-directed clinical biomarkers and therapeutics.

TREM2/DAP12 Signal Elicits Proinflammatory Response in Microglia and Exacerbates Neuropathic Pain

Neuropathic pain afflicts millions of people, and the development of an effective treatment for this intractable pain is an urgent issue. Recent evidence has implicated microglia in neuropathic pain. The present study showed that the DNAX-activating protein of 12 kDa (DAP12) and its associated "triggering receptor expressed on myeloid cells 2" (TREM2) were predominantly expressed by microglia in the dorsal horn after spinal nerve injury, revealing a role for TREM2/DAP12 signaling in neuropathic pain. Nerve injury-induced proinflammatory cytokine expression in microglia and pain behaviors were significantly suppressed in Dap12-deficient mice. Furthermore, intrathecal administration of TREM2 agonistic antibody induced proinflammatory cytokine expression, as well as neuropathic pain, in mice without nerve injury. The agonistic antibody induced proinflammatory responses and neuropathic pain was not observed in Dap12-deficient mice. Together, these results suggest that TREM2/DAP12-mediated signals in microglia exacerbate nerve injury-induced neuropathic pain by inducing proinflammatory cytokine secretion from microglia. Suppression of DAP12-mediated signals could be a therapeutic target for neuropathic pain.

SIGNIFICANCE STATEMENT

Recent studies have revealed that activated microglia in the spinal dorsal horn exacerbate neuropathic pain, which has suggested that suppression of microglial activity should be considered as a therapeutic target. However, only a few molecules have been identified as regulators of microglial activity. In this study, we focused on a receptor complex of TREM2 and DAP12, both of which are expressed by microglia and have been implicated in the pathogenesis of Alzheimer's disease, and demonstrated that TREM2/DAP12 signaling promoted proinflammatory responses in microglia and exacerbates neuropathic pain. The present results revealed the functional significance of TREM2/DAP12 signaling in microglial activation after neuronal injury, and could help in the development of treatments for neuropathic pain and neurodegenerative diseases.

Function and Dysfunction of Microglia during Brain Development: Consequences for Synapses and Neural Circuits

Many diverse factors, ranging from stress to infections, can perturb brain homeostasis and alter the physiological activity of microglia, the immune cells of the central nervous system. Microglia play critical roles in the process of synaptic maturation and brain wiring during development. Any perturbation affecting microglial physiological function during critical developmental periods could result in defective maturation of synaptic circuits. In this review, we critically appraise the recent literature on the alterations of microglial activity induced by environmental and genetic factors occurring at pre- and early post-natal stages. Furthermore, we discuss the long-lasting consequences of early-life microglial perturbation on synaptic function and on vulnerability to neurodevelopmental and psychiatric disorders.

SLAMF7 is critical for phagocytosis of haematopoietic tumour cells via Mac-1 integrin

Cancer cells elude anti-tumour immunity through multiple mechanisms, including upregulated expression of ligands for inhibitory immune checkpoint receptors. Phagocytosis by macrophages plays a critical role in cancer control. Therapeutic blockade of signal regulatory protein (SIRP)-α, an inhibitory receptor on macrophages, or of its ligand CD47 expressed on tumour cells, improves tumour cell elimination in vitro and in vivo, suggesting that blockade of the SIRPα-CD47 checkpoint could be useful in treating human cancer. However, the pro-phagocytic receptor(s) responsible for tumour cell phagocytosis is(are) largely unknown. Here we find that macrophages are much more efficient at phagocytosis of haematopoietic tumour cells, compared with non-haematopoietic tumour cells, in response to SIRPα-CD47 blockade. Using a mouse lacking the signalling lymphocytic activation molecule (SLAM) family of homotypic haematopoietic cell-specific receptors, we determined that phagocytosis of haematopoietic tumour cells during SIRPα-CD47 blockade was strictly dependent on SLAM family receptors in vitro and in vivo. In both mouse and human cells, this function required a single SLAM family member, SLAMF7 (also known as CRACC, CS1, CD319), expressed on macrophages and tumour cell targets. In contrast to most SLAM receptor functions, SLAMF7-mediated phagocytosis was independent of signalling lymphocyte activation molecule-associated protein (SAP) adaptors. Instead, it depended on the ability of SLAMF7 to interact with integrin Mac-1 (refs 18, 19, 20) and utilize signals involving immunoreceptor tyrosine-based activation motifs. These findings elucidate the mechanism by which macrophages engulf and destroy haematopoietic tumour cells. They also reveal a novel SAP adaptor-independent function for a SLAM receptor. Lastly, they suggest that patients with tumours expressing SLAMF7 are more likely to respond to SIRPα-CD47 blockade therapy.

A Comprehensive Review of Immunoreceptor Regulation of Osteoclasts

Osteoclasts require coordinated co-stimulation by several signaling pathways to initiate and regulate their cellular differentiation. Receptor activator for NF-κB ligand (RANKL or TNFSF11), a tumor necrosis factor (TNF) superfamily member, is the master cytokine required for osteoclastogenesis with essential co-stimulatory signals mediated by immunoreceptor tyrosine-based activation motif (ITAM)-signaling adaptors, DNAX-associated protein 12 kDa size (DAP12) and FcεRI gamma chain (FcRγ). The ITAM-signaling adaptors do not have an extracellular ligand-binding domain and, therefore, must pair with ligand-binding immunoreceptors to interact with their extracellular environment. DAP12 pairs with a number of different immunoreceptors including triggering receptor expressed on myeloid cells 2 (TREM2), myeloid DAP12-associated lectin (MDL-1), and sialic acid-binding immunoglobulin-type lectin 15 (Siglec-15); while FcRγ pairs with a different set of receptors including osteoclast-specific activating receptor (OSCAR), paired immunoglobulin receptor A (PIR-A), and Fc receptors. The ligands for many of these receptors in the bone microenvironment remain unknown. Here, we will review immunoreceptors known to pair with either DAP12 or FcRγ that have been shown to regulate osteoclastogenesis. Co-stimulation and the effects of ITAM-signaling have turned out to be complex, and now include paradoxical findings that ITAM-signaling adaptor-associated receptors can inhibit osteoclastogenesis and immunoreceptor tyrosine-based inhibitory motif (ITIM) receptors can promote osteoclastogenesis. Thus, co-stimulation of osteoclastogenesis continues to reveal additional complexities that are important in the regulatory mechanisms that seek to maintain bone homeostasis.

The Alzheimer's Disease-Associated R47H Variant of TREM2 Has an Altered Glycosylation Pattern and Protein Stability

The R47H coding variant of the triggering receptor expressed on myeloid cells-2 (TREM2) increases the risk of Alzheimer's disease (AD) similar to apolipoprotein E4. TREM2 R47H has recently been shown to have impaired binding to damage-associated lipid or apolipoprotein ligands. However, it is not known how this R47H variant affects the biochemical characteristics of TREM2 and alters the pathogenesis of AD. We previously reported that TREM2-R47H has a slightly different glycosylation pattern from wild-type. A more detailed characterization in our present study confirms that TREM2 R47H has an altered glycosylation pattern and reduced stability. TREM2 R47H shows different glycosylation profiles from analysis using monensin or kifunensine treatment which were confirmed by mass spectrometry. The solubility of TREM2 R47H and its cleaved products such as intracellular domain (ICD) is also decreased, increasing its proteasomal and lysosomal degradation. The different biochemical characteristics of TREM2 R47H, including glycosylation, solubility and processing, may offer insights into a future therapeutic strategy for AD.

Pharmacological Tools to Activate Microglia and their Possible use to Study Neural Network Patho-physiology

BACKGROUND

Microglia are the resident immunocompetent cells of the CNS and also constitute a unique cell type that contributes to neural network homeostasis and function. Understanding microglia cell-signaling not only will reveal their diverse functions but also will help to identify pharmacological and non-pharmacological tools to modulate the activity of these cells.

METHODS

We undertook a search of bibliographic databases for peer-reviewed research literature to identify microglial activators and their cell-specificity. We also looked for their effects on neural network function and dysfunction.

RESULTS

We identified several pharmacological targets to modulate microglial function, which are more or less specific (with the proper control experiments). We also identified pharmacological targets that would require the development of new potent and specific modulators. We identified a wealth of evidence about the participation of microglia in neural network function and their alterations in pathological conditions.

CONCLUSION

The identification of specific microglia-activating signals provides experimental tools to modulate the activity of this heterogeneous cell type in order to evaluate its impact on other components of the nervous system, and it also helps to identify therapeutic approaches to ease some pathological conditions related to microglial dysfunction.

Semaphorin signaling in bone

Semaphorin molecules regulate cell adhesion and motility in a wide variety of cell types and are therefore involved in numerous processes including axon guidance, angiogenesis, cardiogenesis, tumor growth, and immune response. Increasing evidence points to a role of transmembrane, membrane-associated and soluble semaphorins during bone development as well as in the control of normal bone homeostasis. Within bone, semaphorins are implicated in the communication between different cell types by relaying signals in an autocrine or paracrine way. Semaphorins are not only involved in bone resorption but also in bone formation. Therefore, targeting semaphorin-induced signaling in bone may constitute an interesting new therapeutic strategy in osteoporosis. However, all the pioneering research on semaphorins is performed in mice and it remains to be established to what extent semaphorin signaling pathways are conserved between mice and men. In addition, knowledge of semaphorin signaling in bone mostly arises from loss/gain of function studies of one single semaphorin and/or receptor. However, different semaphorin molecules are co-expressed in bone and their signaling pathways are likely to interact in a complex and coherent way that needs proper understanding before targeting semaphorin signaling can be therapeutically exploited.

White matter volume change and its correlation with symptom severity in patients with schizophrenia: a VBM-DARTEL study

The aim of this study was to evaluate the white matter (WM) volume change and its correlation with symptom severity in patients with schizophrenia using voxel-based morphometry. A total of 20 patients with schizophrenia and 20 age-matched healthy controls participated in this study. MR image data were processed using SPM8 software with diffeomorphic anatomical registration through an exponentiated Lie algebra (DARTEL) algorithm. The patients with schizophrenia showed significant decreases (P=0.042) in the WM volumes of the temporal lobe and superior frontal gyrus compared with the healthy controls. The WM volumes of the middle temporal gyrus were negatively correlated with the scores of both the Positive Subscale (Pearson's ρ=-0.68, P=0.001) and the Negative Subscale (ρ=-0.71, P=0.0005) in the Positive and Negative Syndrome Scale. In addition, the scores of the General Psychopathology Subscale were negatively correlated with the WM volumes of the superior frontal gyrus (ρ=-0.68, P=0.0009). This study evaluated the WM volume of patients with schizophrenia compared with healthy controls using DARTEI-based voxel-based morphometry and also assessed the correlation of the localized WM volume changes with the Positive and Negative Syndrome Scale. These findings will be useful to understand the neuropathology associated with WM abnormality in schizophrenia.

Chronic Psychosocial Stress Impairs Bone Homeostasis: A Study in the Social Isolation Reared Rat

Chronic psychosocial stress is a key player in the onset and aggravation of mental diseases, including psychosis. Although a strong association between this psychiatric condition and other medical co-morbidities has been recently demonstrated, few data on the link between psychosis and bone homeostasis are actually available. The aim of this study was to investigate whether chronic psychosocial stress induced by 4 or 7 weeks of social isolation in drug-naïve male Wistar rats could alter bone homeostasis in terms of bone thickness, mineral density and content, as well as markers of bone formation and resorption (sclerostin, cathepsin K, and CTX-I). We found that bone mineral density was increased in rats exposed to 7 weeks of social isolation, while no differences were detected in bone mineral content and area. Moreover, 7 weeks of social isolation lead to increase of femur thickness with respect to controls, suggesting the development of a hyperostosis condition. Isolated rats showed no changes in sclerostin levels, a marker of bone formation, compared to grouped animals. Conversely, bone resorption markers were significantly altered after 7 weeks of social isolation in terms of decrease in cathepsin K and increase of CTX-I. No alterations were found after 4 weeks of isolation rearing. Our observations suggest that chronic psychosocial stress might affect bone homeostasis, more likely independently from drug treatment. Thus, the social isolation model might help to identify possible new therapeutic targets to treat the burden of chronic psychosocial stress and to attempt alternative therapy choices.

Syk kinases are required for spinal commissural axon repulsion at the midline via the ephrin/Eph pathway

In the hematopoietic system, Syk family tyrosine kinases are essential components of immunoreceptor ITAM-based signaling. While there is increasing data indicating the involvement of immunoreceptors in neural functions, the contribution of Syk kinases remains obscure. Previously, we identified phosphorylated forms of Syk kinases in specialized populations of migrating neurons or projecting axons. Moreover, we identified ephrin/Eph as guidance molecules utilizing the ITAM-bearing CD3zeta (Cd247) and associated Syk kinases for the growth cone collapse response induced in vitro Here, we show that in the developing spinal cord, Syk is phosphorylated in navigating commissural axons. By analyzing axon trajectories in open-book preparations of Syk(-/-); Zap70(-/-) mouse embryos, we show that Syk kinases are dispensable for attraction towards the midline but confer growth cone responsiveness to repulsive signals that expel commissural axons from the midline. Known to serve a repulsive function at the midline, ephrin B3/EphB2 are obvious candidates for driving the Syk-dependent repulsive response. Indeed, Syk kinases were found to be required for ephrin B3-induced growth cone collapse in cultured commissural neurons. In fragments of commissural neuron-enriched tissues, Syk is in a constitutively phosphorylated state and ephrin B3 decreased its level of phosphorylation. Direct pharmacological inhibition of Syk kinase activity was sufficient to induce growth cone collapse. In conclusion, Syk kinases act as a molecular switch of growth cone adhesive and repulsive responses.

Microglia: Dynamic Mediators of Synapse Development and Plasticity

Neuronal communication underlies all brain activity and the genesis of complex behavior. Emerging research has revealed an unexpected role for immune molecules in the development and plasticity of neuronal synapses. Moreover microglia, the resident immune cells of the brain, express and secrete immune-related signaling molecules that alter synaptic transmission and plasticity in the absence of inflammation. When inflammation does occur, microglia modify synaptic connections and synaptic plasticity required for learning and memory. Here we review recent findings demonstrating how the dynamic interactions between neurons and microglia shape the circuitry of the nervous system in the healthy brain and how altered neuron-microglia signaling could contribute to disease.

The osteoimmunology of alveolar bone loss

The mineralized structure of bone undergoes constant remodeling by the balanced actions of bone-producing osteoblasts and bone-resorbing osteoclasts (OCLs). Physiologic bone remodeling occurs in response to the body's need to respond to changes in electrolyte levels, or mechanical forces on bone. There are many pathological conditions, however, that cause an imbalance between bone production and resorption due to excessive OCL action that results in net bone loss. Situations involving chronic or acute inflammation are often associated with net bone loss, and research into understanding the mechanisms regulating this bone loss has led to the development of the field of osteoimmunology. It is now evident that the skeletal and immune systems are functionally linked and share common cells and signaling molecules. This review discusses the signaling system of immune cells and cytokines regulating aberrant OCL differentiation and activity. The role of these cells and cytokines in the bone loss occurring in periodontal disease (PD) (chronic inflammation) and orthodontic tooth movement (OTM) (acute inflammation) is then described. The review finishes with an exploration of the emerging role of Notch signaling in the development of the immune cells and OCLs that are involved in osteoimmunological bone loss and the research into Notch signaling in OTM and PD.

Role of the Retromer Complex in Neurodegenerative Diseases

The retromer complex is a protein complex that plays a central role in endosomal trafficking. Retromer dysfunction has been linked to a growing number of neurological disorders. The process of intracellular trafficking and recycling is crucial for maintaining normal intracellular homeostasis, which is partly achieved through the activity of the retromer complex. The retromer complex plays a primary role in sorting endosomal cargo back to the cell surface for reuse, to the trans-Golgi network (TGN), or alternatively to specialized endomembrane compartments, in which the cargo is not subjected to lysosomal-mediated degradation. In most cases, the retromer acts as a core that interacts with associated proteins, including sorting nexin family member 27 (SNX27), members of the vacuolar protein sorting 10 (VPS10) receptor family, the major endosomal actin polymerization-promoting complex known as Wiskott-Aldrich syndrome protein and scar homolog (WASH), and other proteins. Some of the molecules carried by the retromer complex are risk factors for neurodegenerative diseases. Defects such as haplo-insufficiency or mutations in one or several units of the retromer complex lead to various pathologies. Here, we summarize the molecular architecture of the retromer complex and the roles of this system in intracellular trafficking related the pathogenesis of neurodegenerative diseases.

Mediators of inflammation and bone remodeling in rheumatic disease

Remodeling of bone is a continuous process that occurs throughout life. Under normal physiologic conditions, bone-resorbing osteoclasts and bone-forming osteoblasts are tightly coupled and regulated to ensure proper balance, such that there is no net change in bone mass. However, inflammation perturbs normal bone homeostasis. The impact of inflammation on bone is dependent upon the anatomic site affected, cell types, factors and cytokines present in the local microenvironment, and local mechanical forces. Cytokines are central to the pathogenesis of inflammation-induced bone loss and contribute to the uncoupling of osteoclast-mediated bone resorption and osteoblast-mediated bone formation, thereby disrupting normal remodeling. In this review, we will discuss the effects of cytokines on bone in two settings, rheumatoid arthritis and spondyloarthritis, a disease category that includes ankylosing spondylitis, psoriatic arthritis, reactive arthritis, inflammatory bowel disease, and juvenile onset spondyloarthropathy. The outcome for bone in these disease settings is quite different, and an understanding of the pathogenic mechanisms leading to the net impact on bone has been essential in developing new therapeutic approaches to bone health in these diseases.

Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) mediates periarticular bone loss, but not joint destruction, in murine antigen-induced arthritis

Osteoclastogenesis requires immunoreceptor tyrosine-based activation motif signaling. Multiple immunoreceptors associated with immunoreceptor tyrosine-based activation motif adaptor proteins, including DNAX-activating protein 12 kDa (DAP12) and Fc receptor common γ (FcRγ), have been identified in osteoclast lineage cells, and some are involved in arthritis-induced bone destruction. Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) is an immunoreceptor that regulates osteoclast development and bone resorption in association with DAP12. Whether Siglec-15 is involved in arthritis-induced bone lesions, however, remains unknown. Here we used a murine antigen-induced arthritis model to examine the role of Siglec-15 in the development of bone lesions induced by joint inflammation. Arthritis was unilaterally induced in the knee joints of 8-week-old female wild-type (WT) and Siglec-15(-/-) mice, and the contralateral knees were used as a control. The degree of joint inflammation, and cartilage and subchondral bone destruction in Siglec-15(-/-) mice was comparable to that in WT mice, indicating that Siglec-15 is not involved in the development of arthritis and concomitant cartilage and subchondral bone destruction. On the other hand, the degree of periarticular bone loss in the proximal tibia of the arthritic knee was significantly lower in Siglec-15(-/-) mice compared to WT mice. Although osteoclast formation in the metaphysis was enhanced in both WT and Siglec-15(-/-) mice after arthritis induction, mature multinucleated osteoclast formation was impaired in Siglec-15(-/-) mice, indicating impaired osteoclast bone resorptive function in the periarticular regions of the arthritic joint in Siglec-15(-/-) mice. Confirming this result, Siglec-15(-/-) primary unfractionated bone marrow cells harvested from arthritic femurs and tibiae failed to develop into mature multinuclear osteoclasts. Our findings suggest that Siglec-15 is a therapeutic target for periarticular bone loss, but not for joint destruction, in inflammatory arthritis, such as rheumatoid arthritis.

TREM2 in CNS homeostasis and neurodegenerative disease

Myeloid-lineage cells accomplish a myriad of homeostatic tasks including the recognition of pathogens, regulation of the inflammatory milieu, and mediation of tissue repair and regeneration. The innate immune receptor and its adaptor protein—triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12)—possess the ability to modulate critical cellular functions via crosstalk with diverse signaling pathways. As such, mutations in TREM2 and DAP12 have been found to be associated with a range of disease phenotypes. In particular, mutations in TREM2 increase the risk for Alzheimer's disease and other neurodegenerative disorders. The leading hypothesis is that microglia, the resident immune cells of the central nervous system, are the major myeloid cells affected by dysregulated TREM2-DAP12 function. Here, we review how impaired signaling by the TREM2-DAP12 pathway leads to altered immune responses in phagocytosis, cytokine production, and microglial proliferation and survival, thus contributing to disease pathogenesis.

TREM2 mRNA Expression in Leukocytes Is Increased in Alzheimer's Disease and Schizophrenia

TREM2 and TYROBP are causal genes for Nasu–Hakola disease (NHD), a rare autosomal recessive disease characterized by bone lesions and early-onset progressive dementia. TREM2 forms a receptor signaling complex with TYROBP, which triggers the activation of immune responses in macrophages and dendritic cells, and the functional polymorphism of TREM2 is reported to be associated with neurodegenerative disorders such as Alzheimer’s disease (AD). The objective of this study was to reveal the involvement of TYROBP and TREM2 in the pathophysiology of AD and schizophrenia. Methods: We investigated the mRNA expression level of the 2 genes in leukocytes of 26 patients with AD and 24 with schizophrenia in comparison with age-matched controls. Moreover, we performed gene association analysis between these 2 genes and schizophrenia. Results: No differences were found in TYROBP mRNA expression in patients with AD and schizophrenia; however, TREM2 mRNA expression was increased in patients with AD and schizophrenia compared with controls (P < 0.001). There were no genetic associations of either gene with schizophrenia in Japanese patients. Conclusion: TREM2 expression in leukocytes is elevated not only in AD but also in schizophrenia. Inflammatory processes involving TREM2 may occur in schizophrenia, as observed in neurocognitive disorders such as AD. TREM2 expression in leukocytes may be a novel biomarker for neurological and psychiatric disorders.

Immunohistochemical characterization of CD33 expression on microglia in Nasu-Hakola disease brains

Nasu-Hakola disease (NHD) is a rare autosomal recessive disorder, characterized by formation of multifocal bone cysts and development of leukoencephalopathy, caused by genetic mutations of either DNAX-activation protein 12 (DAP12) or triggering receptor expressed on myeloid cells 2 (TREM2). Although increasing evidence suggests a defect in microglial TREM2/DAP12 function in NHD, the molecular mechanism underlying leukoencephalopathy with relevance to microglial dysfunction remains unknown. TREM2, by transmitting signals via the immunoreceptor tyrosine-based activation motif (ITAM) of DAP12, stimulates phagocytic activity of microglia, and ITAM signaling is counterbalanced by sialic acid-binding immunoglobulin (Ig)-like lectins (Siglecs)-mediated immunoreceptor tyrosine-based inhibitory motif (ITIM) signaling. To investigate a role of CD33, a member of the Siglecs family acting as a negative regulator of microglia activation, in the pathology of NHD, we studied CD33 expression patterns in five NHD brains and 11 controls by immunohistochemistry. In NHD brains, CD33 was identified exclusively on ramified and amoeboid microglia accumulated in demyelinated white matter lesions but not expressed in astrocytes, oligodendrocytes, or neurons. However, the number of CD33-immunoreactive microglia showed great variability from case to case and from lesion to lesion without significant differences between NHD and control brains. These results do not support the view that CD33-expressing microglia play a central role in the development of leukoencephalopathy in NHD brains.

Impaired Fracture Healing Caused by Deficiency of the Immunoreceptor Adaptor Protein DAP12

Osteoclasts play an important role in bone metabolism, but their exact role in fracture healing remains unclear. DAP12 is an immunoadaptor protein with associated immunoreceptors on myeloid lineage cells, including osteoclasts. Its deficiency causes osteopetrosis due to suppression of osteoclast development and activation. In this report, we assessed the impact of DAP12 on the fracture healing process using C57BL/6 (B6) and DAP12^–/– mice. Healing was evaluated using radiography, micro-CT, histology, immunohistochemistry and real-time RT-PCR. Radiography showed lower callus volume and lower callus radiolucency in DAP12^–/– mice during later stages. Micro-CT images and quantitative structural analysis indicated that DAP12^–/– mice developed calluses of dense trabecular structures and experienced deteriorated cortical shell formation on the surface. Histologically, DAP12^–/– mice showed less cartilaginous resorption and woven bone formation. In addition, prominent cortical shell formation was much less in DAP12^–/– mice. Immunohistochemistry revealed lower invasion of F4/80 positive monocytes and macrophages into the fracture hematoma in DAP12^–/– mice. The expression levels of Col1a1, Col2a1 and Col10a1 in DAP12^–/– mice increased and subsequently became higher than those in B6 mice. There was a decrease in the gene expression of Tnf during the early stages in DAP12^–/– mice. Our results indicate that DAP12 deficiency impairs fracture healing, suggesting a significant role of DAP12 in the initial inflammatory response, bone remodeling and regeneration.

Evidence for TLR4 and FcRγ-CARD9 activation by cholera toxin B subunit and its direct bindings to TREM2 and LMIR5 receptors

Cholera toxin (CTX) is a virulent factor of Vibrio cholerae that causes life-threatening diarrheal disease. Its non-toxic subunit CTB has been extensively studied for vaccine delivery. In immune cells, CTB induces a number of signaling molecules related to cellular activation and cytokine production. The mechanisms by which CTB exerts its immunological effects are not understood. We report here the immunological targets of CTB. The unexpected finding that GM1 ganglioside inhibited NF-κB activation in human monocytes stimulated with CTX and agonists of Toll-like receptors (TLR) suggests the possibility of CTX-TLR interaction. Indeed, CTX-induced IL-6 production was substantially reduced in MyD88(-/-) or TLR4(-/-) macrophages. Ectopic expression of TLR4 was required for CTX-induced NF-κB activation in HEK 293 cells. Furthermore, the inflammatory capacity of CTB was lost in the absence of TLR4, adaptor protein FcRγ, or its downstream signaling molecule CARD9. Attempts have been made to identify CTB-binding targets from various C-type lectin and immunoglobulin-like receptors. CTB targeted not only GM1 and TLR4 but also TREM2 and LMIR5/CD300b. CTB-TREM2 interaction initiated signal transduction through adaptor protein DAP12. The binding of CTB inhibited LMIR5 activation induced by its endogenous ligand 3-O-sulfo-β-d-galactosylceramide C24:1. In summary, CTB targets TLR4, FcRγ-CARD9, TREM2, and LMIR5. These findings provide new insights into the immunobiology of cholera toxin.

Th2-M2 immunity in lesions of muscular sarcoidosis and macrophagic myofasciitis

OBJECTIVE

To analyse the paradox of a lack of giant cell formation and fibrosis in chronic lesions of macrophagic myofasciitis (MMF) in comparison with muscular sarcoidosis (MuS).

METHODS

Inflammatory lesions and contiguous muscle regions from biopsy samples of 10 patients with MuS and 10 patients with MMF were cut out by laser microdissection. Mediators of the T helper cell (Th)1 inducing classical macrophage activation (e.g. STAT1, IFNγ and CXCR3), and Th2 inducing alternative activation of macrophages (e.g. CD206/MRC1, STAT6, SOCS1), molecules involved in development of fibrosis (e.g. TGFβ) and giant cells (e.g. TYROBP), were assessed by immunohistochemistry and real-time polymerase chain reaction (PCR).

RESULTS

STAT6-induced Th2 immunity was associated with up-regulated gene expression of MRC1, SOCS1 and TGFB in inflammatory foci, in comparison with adjacent tissue. TYROBP and TREM2, genes regulating giant cell formation, were more strongly expressed in lesions of MuS patients than in those of MMF. TGFβ co-localized with CD206(+) macrophages in MuS but not in MMF. Conversely, Th1 immunity was illustrated by STAT1 staining both in macrophages and myofibres in MuS, but not in MMF. Also, STAT1-induced IFNG and CXCR3 expression in lesions and the surrounding tissue was elevated compared with normal controls, but without statistically significant differences.

CONCLUSION

Giant cell and typical granuloma formations, including fibrogenesis, is dependent on two main mechanisms, both involving specific macrophage activation: a strong Th2-M2 polarization and a significant expression of TYROBP and TGFβ in macrophages. The low-grade alternative activation of macrophages in MMF lesions and poor TYROBP and TGFβco-expression are obviously insufficient to produce giant cells.

Osteoimmunology: Major and Costimulatory Pathway Expression Associated with Chronic Inflammatory Induced Bone Loss

The field of osteoimmunology has emerged in response to the range of evidences demonstrating the close interrelationship between the immune system and bone metabolism. This is pertinent to immune-mediated diseases, such as rheumatoid arthritis and periodontal disease, where there are chronic inflammation and local bone erosion. Periprosthetic osteolysis is another example of chronic inflammation with associated osteolysis. This may also involve immune mediation when occurring in a patient with rheumatoid arthritis (RA). Similarities in the regulation and mechanisms of bone loss are likely to be related to the inflammatory cytokines expressed in these diseases. This review highlights the role of immune-related factors influencing bone loss particularly in diseases of chronic inflammation where there is associated localized bone loss. The importance of the balance of the RANKL-RANK-OPG axis is discussed as well as the more recently appreciated role that receptors and adaptor proteins involved in the immunoreceptor tyrosine-based activation motif (ITAM) signaling pathway play. Although animal models are briefly discussed, the focus of this review is on the expression of ITAM associated molecules in relation to inflammation induced localized bone loss in RA, chronic periodontitis, and periprosthetic osteolysis, with an emphasis on the soluble and membrane bound factor osteoclast-associated receptor (OSCAR).

TREM-2 promotes macrophage survival and lung disease after respiratory viral infection

Wu et al. use a mouse model to show that active respiratory viral infection triggers TREM-2 expression on the macrophage cell surface and thereby prevents macrophage apoptosis during the acute illness. In addition, long after viral clearance, IL-13 and DAP12 promote TREM-2 cleavage to its soluble form that unexpectedly also enhances macrophage survival and promotes chronic inflammatory disease.

Viral infections and type 2 immune responses are thought to be critical for the development of chronic respiratory disease, but the link between these events needs to be better defined. Here, we study a mouse model in which infection with a mouse parainfluenza virus known as Sendai virus (SeV) leads to long-term activation of innate immune cells that drive IL-13–dependent lung disease. We find that chronic postviral disease (signified by formation of excess airway mucus and accumulation of M2-differentiating lung macrophages) requires macrophage expression of triggering receptor expressed on myeloid cells-2 (TREM-2). Analysis of mechanism shows that viral replication increases lung macrophage levels of intracellular and cell surface TREM-2, and this action prevents macrophage apoptosis that would otherwise occur during the acute illness (5–12 d after inoculation). However, the largest increases in TREM-2 levels are found as the soluble form (sTREM-2) long after clearance of infection (49 d after inoculation). At this time, IL-13 and the adapter protein DAP12 promote TREM-2 cleavage to sTREM-2 that is unexpectedly active in preventing macrophage apoptosis. The results thereby define an unprecedented mechanism for a feed-forward expansion of lung macrophages (with IL-13 production and consequent M2 differentiation) that further explains how acute infection leads to chronic inflammatory disease.

TREM2 sustains microglial expansion during aging and response to demyelination

Microglia contribute to development, homeostasis, and immunity of the CNS. Like other tissue-resident macrophage populations, microglia express the surface receptor triggering receptor expressed on myeloid cells 2 (TREM2), which binds polyanions, such as dextran sulphate and bacterial LPS, and activates downstream signaling cascades through the adapter DAP12. Individuals homozygous for inactivating mutations in TREM2 exhibit demyelination of subcortical white matter and a lethal early onset dementia known as Nasu-Hakola disease. How TREM2 deficiency mediates demyelination and disease is unknown. Here, we addressed the basis for this genetic association using Trem2(-/-) mice. In WT mice, microglia expanded in the corpus callosum with age, whereas aged Trem2(-/-) mice had fewer microglia with an abnormal morphology. In the cuprizone model of oligodendrocyte degeneration and demyelination, Trem2(-/-) microglia failed to amplify transcripts indicative of activation, phagocytosis, and lipid catabolism in response to myelin damage. As a result, Trem2(-/-) mice exhibited impaired myelin debris clearance, axonal dystrophy, oligodendrocyte reduction, and persistent demyelination after prolonged cuprizone treatment. Moreover, myelin-associated lipids robustly triggered TREM2 signaling in vitro, suggesting that TREM2 may directly sense lipid components exposed during myelin damage. We conclude that TREM2 is required for promoting microglial expansion during aging and microglial response to insults of the white matter.

Immune complexes regulate bone metabolism through FcRγ signalling

Autoantibody production and immune complex (IC) formation are frequently observed in autoimmune diseases associated with bone loss. However, it has been poorly understood whether ICs regulate bone metabolism directly. Here we show that the level of osteoclastogenesis is determined by the strength of FcRγ signalling, which is dependent on the relative expression of positive and negative FcγRs (FcγRI/III/IV and IIB, respectively) as well as the availability of their ligands, ICs. Under physiological conditions, unexpectedly, FcγRIII inhibits osteoclastogenesis by depriving other osteoclastogenic Ig-like receptors of FcRγ. Fcgr2b(-/-) mice lose bone upon the onset of a hypergammaglobulinemia or the administration of IgG1 ICs, which act mainly through FcγRIII. The IgG2 IC activates osteoclastogenesis by binding to FcγRI and FcγRIV, which is induced under inflammatory conditions. These results demonstrate a link between the adaptive immunity and bone, suggesting a regulatory role for ICs in bone resorption in general, and not only in inflammatory diseases.

The TREM2-DAP12 signaling pathway in Nasu-Hakola disease: a molecular genetics perspective

Nasu–Hakola disease or polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL) is a rare recessively inherited disease that is associated with early dementia and bone cysts with fractures. Here, we review the genetic causes of PLOSL with loss-of-function mutations or deletions in one of two genes, TYROBP and TREM2, encoding for two proteins DNAX-activating protein 12 (DAP12) and triggering receptor expressed on myeloid cells-2 (TREM2). TREM2 and DAP12 form an immunoreceptor signaling complex that mediates myeloid cell, including microglia and osteoclasts, development, activation, and function. Functionally, TREM2-DAP12 mediates osteoclast multi-nucleation, migration, and resorption. In microglia, TREM2-DAP12 participates in recognition and apoptosis of neuronal debris and amyloid deposits. Review of the complex immunoregulatory roles of TREM2-DAP12 in the innate immune system, where it can both promote and inhibit pro-inflammatory responses, is given. Little is known about the function of TREM2-DAP12 in normal brain homeostasis or in pathological central nervous system diseases. Based on the state of the field, genetic testing now aids in diagnosis of PLOSL, but therapeutics and interventions are still under development.

Disease-Associated Mutations of TREM2 Alter the Processing of N-Linked Oligosaccharides in the Golgi Apparatus

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune-modulatory receptor involved in phagocytosis and inflammation. Mutations of Q33X, Y38C and T66M cause Nasu-Hakola disease (NHD) which is characterized by early onset of dementia and bone cysts. A recent, genome-wide association study also revealed that single nucleotide polymorphism of TREM2, such as R47H, increased the risk of Alzheimer's disease (AD) similar to ApoE4. However, how these mutations affect the trafficking of TREM2, which may affect the normal functions of TREM2, was not known. In this study, we show that TREM2 with NHD mutations are impaired in the glycosylation with complex oligosaccharides in the Golgi apparatus, in the trafficking to plasma membrane and further processing by γ-secretase. Although R47H mutation in AD affected the glycosylation and normal trafficking of TREM2 less, the detailed pattern of glycosylated TREM2 differs from that of the wild type, thus suggesting that precise regulation of TREM2 glycosylation is impaired when arginine at 47 is mutated to histidine. Our results suggest that the impaired glycosylation and trafficking of TREM2 from endoplasmic reticulum/Golgi to plasma membrane by mutations may inhibit its normal functions in the plasma membrane, which may contribute to the disease.

A DAP12-dependent signal promotes pro-inflammatory polarization in microglia following nerve injury and exacerbates degeneration of injured neurons

Under pathological conditions, activated microglia play paradoxical roles and could have neurotoxic or neuroprotective effects. However, the signal determining how activated microglia affects the fate of neuronal cells remains largely unknown. Here we demonstrate that DNAX-activating protein of 12 kDa (DAP12), a transmembrane adaptor protein that contains an immunoreceptor tyrosine-based activation motif, is a critical regulator of microglial function after nerve injury. In a model of mouse hypoglossal nerve injury, the duration of microglial increase after nerve injury became shorter in mice lacking DAP12, although microglial morphology and total cell numbers were not significantly affected during early phase after nerve injury. Intriguingly, expressions of M1-phenotype markers including pro-inflammatory cytokines were suppressed in DAP12-deficient microglia. Furthermore, axotomy-induced motor neuron death was markedly prevented in DAP12-deficient mice. Collectively, DAP12-mediated microglial activation following axotomy promotes pro-inflammatory responses, and thereby accelerates nerve injury-induced neuron death, suggesting that DAP12 is a potential therapeutic target for the protection of neuronal degeneration caused by microglial activation.

Siglec-15 is a potential therapeutic target for postmenopausal osteoporosis

Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) is an immunoreceptor that regulates osteoclast development and bone resorption in association with an immunoreceptor tyrosine-based activation motif (ITAM) adaptor protein, DNAX-activating protein 12kDa (DAP12). Although Siglec-15 has an important role in physiologic bone remodeling by modulating RANKL signaling, it is unclear whether it is involved in pathologic bone loss in which multiple osteoclastogenic factors participate in excessive osteoclastogenesis. Here we demonstrated that Siglec-15 is involved in estrogen deficiency-induced bone loss. WT and Siglec-15(-/-) mice were ovariectomized (Ovx) or sham-operated at 14wk of age and their skeletal phenotype was evaluated at 18 and 22wk of age. Siglec-15(-/-) mice showed resistance to estrogen deficiency-induced bone loss compared to WT mice. Although the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts increased after ovariectomy in both WT and Siglec-15(-/-) mice, the increase was lower in Siglec-15(-/-) mice than in WT mice. Importantly, osteoclasts in Siglec-15(-/-) mice were small and failed to spread on the bone surface, indicating impaired osteoclast differentiation. Because upregulated production of TNF-α as well as RANKL is mainly responsible for estrogen deficiency-induced development of osteoclasts, we examined whether Siglec-15 deficiency affects TNF-α-induced osteoclastogenesis in vitro. The TNF-α mediated induction of TRAP-positive multinucleated cells was impaired in Siglec-15(-/-) cells, suggesting that Siglec-15 is involved in TNF-α induced osteoclastogenesis. We also confirmed that signaling through osteoclast-associated receptor/Fc receptor common γ chain, which is an alternative ITAM adaptor to DAP12, rescues multinucleation but not cytoskeletal organization of TNF-α and RANKL-induced Siglec-15(-/-) osteoclasts, indicating that the Siglec-15/DAP12 pathway is especially important for cytoskeletal organization of osteoclasts in both RANKL and TNF-α induced osteoclastogenesis. The present findings indicate that Siglec-15 is involved in estrogen deficiency-induced differentiation of osteoclasts and is thus a potential therapeutic target for postmenopausal osteoporosis.

DNAX-activating protein of 12 kDa impairs host defense in pneumococcal pneumonia

OBJECTIVES

Streptococcus pneumoniae is the most common causative organism in community-acquired pneumonia responsible for millions of deaths every year. DNAX-activating protein of 12 kDa is an adaptor molecule for different myeloid expressed receptors involved in innate immunity.

DESIGN

Animal study.

SETTING

University research laboratory.

SUBJECTS

DNAX-activating protein of 12 kDa-deficient (dap12) and wild-type mice.

INTERVENTIONS

Mice were intranasally infected with S. pneumoniae. In addition, ex vivo responsiveness of alveolar macrophages was examined.

MEASUREMENTS AND MAIN RESULTS

dap12 alveolar macrophages released more tumor necrosis factor-α upon stimulation with S. pneumoniae and displayed increased phagocytosis of this pathogen compared with wild-type cells. After infection with S. pneumoniae via the airways, dap12 mice demonstrated reduced bacterial outgrowth in the lungs together with delayed dissemination to distant body sites relative to wild-type mice. This favorable response in dap12 mice was accompanied by reduced lung inflammation and an improved survival.

CONCLUSIONS

These data suggest that DNAX-activating protein of 12 kDa impairs host defense during pneumococcal pneumonia at the primary site of infection at least in part by inhibiting phagocytosis by alveolar macrophages.

Absence of Dap12 and the αvβ3 integrin causes severe osteopetrosis

The ITAM protein FcRγ acts via αvβ3 integrin to counteract the high bone mass phenotype in Dap12-deficient mice.

In vitro, ligand occupancy of αvβ3 integrin induces phosphorylation of Dap12, which is essential for osteoclast function. Like mice deleted of only αvβ3, Dap12^−/− mice exhibited a slight increase in bone mass, but Dap12^−/− mice, lacking another ITAM protein, FcRγ, were severely osteopetrotic. The mechanism by which FcRγ compensates for Dap12 deficiency is unknown. We find that co-deletion of FcRγ did not exacerbate the skeletal phenotype of β3^−/− mice. In contrast, β3/Dap12 double-deficient (DAP/β3^−/−) mice (but not β1/Dap12 double-deficient mice) were profoundly osteopetrotic, reflecting severe osteoclast dysfunction relative to those lacking αvβ3 or Dap12 alone. Activation of OSCAR, the FcRγ co-receptor, rescued Dap12^−/− but not DAP/β3^−/−osteoclasts. Thus, the absence of αvβ3 precluded compensation for Dap12 deficiency by FcRγ. In keeping with this, Syk phosphorylation did not occur in OSCAR-activated DAP/β3^−/− osteoclasts. Thus, FcRγ requires the osteoclast αvβ3 integrin to normalize the Dap12-deficient skeleton.

Crosstalk among IL-23 and DNAX activating protein of 12 kDa-dependent pathways promotes osteoclastogenesis

IL-23 has been well studied in the context of T cell differentiation; however, its role in the differentiation of myeloid progenitors is less clear. In this paper, we describe a novel role of IL-23 in myeloid cell differentiation. Specifically, we have identified that in human PBMCs, IL-23 induces the expression of MDL-1, a PU.1 transcriptional target during myeloid differentiation, which orchestrates osteoclast differentiation through activation of DNAX activating protein of 12 kDa and its ITAMs. The molecular events that lead to the differentiation of human macrophages to terminally differentiated osteoclasts are dependent on spleen tyrosine kinase and phospholipase Cγ2 phosphorylation for the induction of intracellular calcium flux and the subsequent activation of master regulator osteoclast transcription factor NFATc1. IL-23-elicited osteoclastogenesis is independent of the receptor activator of NF-κB ligand pathway and uses a unique myeloid DNAX activating protein of 12 kDa-associated lectin-1(+)/DNAX activating protein of 12 kDa(+) cell subset. Our data define a novel pathway that is used by IL-23 in myeloid cells and identify a major mechanism for the stimulation of osteoclastogenesis in inflammatory arthritis.

Regulation of NFATc1 in Osteoclast Differentiation

Osteoclasts are unique cells that degrade the bone matrix. These large multinucleated cells differentiate from the monocyte/macrophage lineage upon stimulation by two essential cytokines, macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-kappa B (NF-κB) ligand (RANKL). Activation of transcription factors such as microphthalmia transcription factor (MITF), c-Fos, NF-κB, and nuclear factor-activated T cells c1 (NFATc1) is required for sufficient osteoclast differentiation. In particular, NFATc1 plays the role of a master transcription regulator of osteoclast differentiation. To date, several mechanisms, including transcription, methylation, ubiquitination, acetylation, and non-coding RNAs, have been shown to regulate expression and activation of NFATc1. In this review, we have summarized the various mechanisms that control NFATc1 regulation during osteoclast differentiation.