Combined natural killer cell and dendritic cell functional deficiency in KARAP/DAP12 loss-of-function mutant mice

Overactivated MX1 Positive Natural Killer Cells Promote the Progression of Sepsis-Induced Acute Respiratory Distress Syndrome

Background

Natural killer (NK) cells are key regulators of immune defense in sepsis-induced acute respiratory distress syndrome (ARDS), yet the characteristics of NK cell clusters in ARDS remain poorly understood.

Methods

A prospective and observational study enrolled septic patients with ARDS or not was conducted to determine the percentage of NK cells via flow cytometry. The transcriptomes of peripheral blood mononuclear cells (PBMCs) from healthy controls, patients with sepsis only, and patients with sepsis-induced ARDS were profiled. Vitro experiments were performed to confirm the mechanism mediating MX1+NK cell infiltration.

Results

A total of 115 septic patients were analyzed, among whom 63 patients developed ARDS and 52 patients did not. Decreased NK percentages were found in sepsis with ARDS patients (%, 7.46±4.40 vs 11.65±6.88, P=0.0001) compared with sepsis-only patients. A lower percentage of NK cells showed a significant increase in 28-day mortality. Single-cell sequencing analysis revealed distinct characteristics of NK cells in sepsis-induced ARDS, notably the identification of a unique cluster defined as MX1+NK cells. Flow cytometry analysis showed an elevated percentage of MX1+NK cells specifically in individuals with sepsis-induced ARDS, compared with patients with sepsis only. Pseudo-time analysis showed that MX1+NK cells were characterized by upregulation of type I interferon-induced genes and other pro-inflammatory genes. MX1+NK cells can respond to type I interferons and secrete type I interferons themselves. Ligand-receptor interaction analysis also revealed extensive interaction between MX1+NK cells and T/B cells, leading to an uncontrolled inflammatory response in ARDS.

Conclusion

MX1+NK cells can respond to type I interferons and secrete type I interferons themselves, promoting the development of sepsis-induced ARDS. Interfering with the infiltration of MX1+NK cells could be a therapeutic approach for this disease. Due to the limited sample size, a larger sample size was needed for further exploration.

Microglia maintain structural integrity during fetal brain morphogenesis

Microglia (MG), the brain-resident macrophages, play major roles in health and disease via a diversity of cellular states. While embryonic MG display a large heterogeneity of cellular distribution and transcriptomic states, their functions remain poorly characterized. Here, we uncovered a role for MG in the maintenance of structural integrity at two fetal cortical boundaries. At these boundaries between structures that grow in distinct directions, embryonic MG accumulate, display a state resembling post-natal axon-tract-associated microglia (ATM) and prevent the progression of microcavities into large cavitary lesions, in part via a mechanism involving the ATM-factor Spp1. MG and Spp1 furthermore contribute to the rapid repair of lesions, collectively highlighting protective functions that preserve the fetal brain from physiological morphogenetic stress and injury. Our study thus highlights key major roles for embryonic MG and Spp1 in maintaining structural integrity during morphogenesis, with major implications for our understanding of MG functions and brain development.

Human early-onset dementia caused by DAP12 deficiency reveals a unique signature of dysregulated microglia

The TREM2-DAP12 receptor complex sustains microglia functions. Heterozygous hypofunctional TREM2 variants impair microglia, accelerating late-onset Alzheimer's disease. Homozygous inactivating variants of TREM2 or TYROBP-encoding DAP12 cause Nasu-Hakola disease (NHD), an early-onset dementia characterized by cerebral atrophy, myelin loss and gliosis. Mechanisms underpinning NHD are unknown. Here, single-nucleus RNA-sequencing analysis of brain specimens from DAP12-deficient NHD individuals revealed a unique microglia signature indicating heightened RUNX1, STAT3 and transforming growth factor-β signaling pathways that mediate repair responses to injuries. This profile correlated with a wound healing signature in astrocytes and impaired myelination in oligodendrocytes, while pericyte profiles indicated vascular abnormalities. Conversely, single-nuclei signatures in mice lacking DAP12 signaling reflected very mild microglial defects that did not recapitulate NHD. We envision that DAP12 signaling in microglia attenuates wound healing pathways that, if left unchecked, interfere with microglial physiological functions, causing pathology in human. The identification of a dysregulated NHD microglia signature sparks potential therapeutic strategies aimed at resetting microglia signaling pathways.

TREM2 drives microglia response to amyloid-β via SYK-dependent and -independent pathways

Genetic studies have highlighted microglia as pivotal in orchestrating Alzheimer's disease (AD). Microglia that adhere to Aβ plaques acquire a transcriptional signature, "disease-associated microglia" (DAM), which largely emanates from the TREM2-DAP12 receptor complex that transmits intracellular signals through the protein tyrosine kinase SYK. The human TREM2R47H variant associated with high AD risk fails to activate microglia via SYK. We found that SYK-deficient microglia cannot encase Aβ plaques, accelerating brain pathology and behavioral deficits. SYK deficiency impaired the PI3K-AKT-GSK-3β-mTOR pathway, incapacitating anabolic support required for attaining the DAM profile. However, SYK-deficient microglia proliferated and advanced to an Apoe-expressing prodromal stage of DAM; this pathway relied on the adapter DAP10, which also binds TREM2. Thus, microglial responses to Aβ involve non-redundant SYK- and DAP10-pathways. Systemic administration of an antibody against CLEC7A, a receptor that directly activates SYK, rescued microglia activation in mice expressing the TREM2R47H allele, unveiling new options for AD immunotherapy.

Identifying Lung Cancer Cell Markers with Machine Learning Methods and Single-Cell RNA-Seq Data

Non-small cell lung cancer is a major lethal subtype of epithelial lung cancer, with high morbidity and mortality. The single-cell sequencing technique plays a key role in exploring the pathogenesis of non-small cell lung cancer. We proposed a computational method for distinguishing cell subtypes from the different pathological regions of non-small cell lung cancer on the basis of transcriptomic profiles, including a group of qualitative classification criteria (biomarkers) and various rules. The random forest classifier reached a Matthew’s correlation coefficient (MCC) of 0.922 by using 720 features, and the decision tree reached an MCC of 0.786 by using 1880 features. The obtained biomarkers and rules were analyzed in the end of this study.

Role of OSCAR Signaling in Osteoclastogenesis and Bone Disease

Formation of mature bone-resorbing cells through osteoclastogenesis is required for the continuous remodeling and repair of bone tissue. In aging and disease this process may become aberrant, resulting in excessive bone degradation and fragility fractures. Interaction of receptor-activator of nuclear factor-κB (RANK) with its ligand RANKL activates the main signaling pathway for osteoclastogenesis. However, compelling evidence indicates that this pathway may not be sufficient for the production of mature osteoclast cells and that co-stimulatory signals may be required for both the expression of osteoclast-specific genes and the activation of osteoclasts. Osteoclast-associated receptor (OSCAR), a regulator of osteoclast differentiation, provides one such co-stimulatory pathway. This review summarizes our present knowledge of osteoclastogenesis signaling and the role of OSCAR in the normal production of bone-resorbing cells and in bone disease. Understanding the signaling mechanism through this receptor and how it contributes to the production of mature osteoclasts may offer a more specific and targeted approach for pharmacological intervention against pathological bone resorption.

DNAX Activating Protein of 12 kDa/Triggering Receptor Expressed on Myeloid Cells 2 Expression by Mouse and Human Liver Dendritic Cells: Functional Implications and Regulation of Liver Ischemia-Reperfusion Injury

Liver interstitial dendritic cells (DCs) have been implicated in the control of ischemia-reperfusion injury (IRI) and host immune responses following liver transplantation. Mechanisms underlying these regulatory functions of hepatic DCs remain unclear. We have shown recently that the transmembrane immunoadaptor DNAX-activating protein of 12 kDa (DAP12) negatively regulates mouse liver DC maturation and proinflammatory and immune stimulatory functions. Here, we used PCR analysis and flow cytometry to characterize expression of DAP12 and its associated triggering receptor, triggering receptor expressed on myeloid cells 2 (TREM2), by mouse and human liver DCs and other immune cells compared with DCs in other tissues. We also examined the roles of DAP12 and TREM2 and their expression by liver DCs in the regulation of liver IRI. Injury was induced in DAP12^-/- , TREM2^-/- , or wild-type (WT) mice by 1 hour of 70% clamping and quantified following 6 hours of reperfusion. Both DAP12 and TREM2 were coexpressed at comparatively high levels by liver DCs. Mouse liver DCs lacking DAP12 or TREM2 displayed enhanced levels of nuclear factor κB and costimulatory molecule expression. Unlike normal WT liver DCs, DAP12^-/- liver DC failed to inhibit proliferative responses of activated T cells. In vivo, DAP12^-/- and TREM2^-/- mice exhibited enhanced IRI accompanied by augmented liver DC activation. Elevated alanine aminotransferase levels and tissue injury were markedly reduced by infusion of WT but not DAP12^-/- DC. Conclusion: Our data reveal a close association between DAP12 and TREM2 expression by liver DC and suggest that, by negatively regulating liver DC stimulatory function, DAP12 promotes their control of hepatic inflammatory responses; the DAP12/TREM2 signaling complex may represent a therapeutic target for control of acute liver injury/liver inflammatory disorders.

Microglial TREM2/DAP12 Signaling: A Double-Edged Sword in Neural Diseases

Microglia are activated after neuronal injury and in neurodegenerative diseases, and trigger neuroinflammation in the central nervous system (CNS). Microglia-derived neuroinflammation has both beneficial and detrimental effects on neurons. Because the timing and magnitude of microglial activation is thought to be a critical determinant of neuronal fate, understanding the molecular mechanisms underlying microglial activation is required to enable establishment of microglia-targeted therapies for neural diseases. Plasma membrane receptors play primary roles as activators of microglia and in this review, we focus on a receptor complex involving triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12), both of which are causative genes for Nasu-Hakola disease, a dementia with bone cysts. Recent transcriptome approaches demonstrated TREM2/DAP12 signaling as the principal regulator that transforms microglia from a homeostatic to a neural disease-associated state. Furthermore, animal model studies revealed critical roles for TREM2/DAP12 in the regulation of microglial activity, including survival, phagocytosis, and cytokine production, not only in Alzheimer's disease but also in other neural diseases, such as Parkinson's disease, demyelinating disease, ischemia, and peripheral nerve injury. Intriguingly, while TREM2/DAP12-mediated microglial activation is detrimental for some diseases, including peripheral nerve injury, it is beneficial for other diseases. As the role of activated microglia differs among disease models, TREM2/DAP12 signaling may result in different outcomes in different diseases. In this review we discuss recent perspectives on the role of TREM2/DAP12 in microglia and their contribution to neural diseases.

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.

The Indispensable Roles of Microglia and Astrocytes during Brain Development

Glia are essential for brain functioning during development and in the adult brain. Here, we discuss the various roles of both microglia and astrocytes, and their interactions during brain development. Although both cells are fundamentally different in origin and function, they often affect the same developmental processes such as neuro-/gliogenesis, angiogenesis, axonal outgrowth, synaptogenesis and synaptic pruning. Due to their important instructive roles in these processes, dysfunction of microglia or astrocytes during brain development could contribute to neurodevelopmental disorders and potentially even late-onset neuropathology. A better understanding of the origin, differentiation process and developmental functions of microglia and astrocytes will help to fully appreciate their role both in the developing as well as in the adult brain, in health and disease.

Depletion of the triggering receptor expressed on myeloid cells 2 inhibits progression of renal cell carcinoma via regulating related protein expression and PTEN-PI3K/Akt pathway

The triggering receptor expressed on myeloid cells 2 (TREM-2) is suggested to be involved in the development of certain human malignancies. However, the functions of TREM-2 in renal cell carcinoma (RCC) are still less known. To reveal the effects of TREM-2 on the RCC progression, we examined the TREM-2 expression in RCC tumor tissues. Then, we analyzed the cell proliferation, cell apoptosis, cell cycle and expression of the relative factors in two selected RCC cell lines post RNA interference. We also analyzed the functions of TREM-2 in an in vivo nude mouse model. We found that, the expression of TREM-2 was abnormally elevated in RCC tumor tissues. Silencing TREM-2 inhibited cell growth, induced G1 phase arrest of cell cycle and cell apoptosis in RCC cells. In vivo, the results showed that depletion of TREM-2 significantly inhibited the ACHN tumor growth in the nude mouse model. The analysis of relative protein factors suggested that silencing TREM-2 downregulated the expression levels of Bcl2 and PCNA, and upregulated the expression levels of Bax and caspase-3 in RCC cell lines. Depletion of TREM-2 inactivated PI3K/Akt pathway through increasing the expression of PTEN. Taken together, TREM-2 acts as an oncogene in the development of RCC and can be considered as a novel therapeutic factor in the treatment of RCC.

The Stalk Domain of NKp30 Contributes to Ligand Binding and Signaling of a Preassembled NKp30-CD3ζ Complex

The natural cytotoxicity receptor (NCR) NKp30 (CD337) is a key player for NK cell immunosurveillance of infections and cancer. The molecular details of ligand recognition and its connection to CD3ζ signaling remain unsolved. Here, we show that the stalk domain (¹²⁹KEHPQLGAGTVLLLR¹⁴³) of NKp30 is very sensitive to sequence alterations, as mutations lead to impaired ligand binding and/or signaling capacity. Surprisingly, the stalk domains of NKp30 and NKp46, another NCR employing CD3ζ for signaling, were not exchangeable without drastic deficiencies in folding, plasma membrane targeting, and/or ligand-induced receptor signaling. Further mutational studies, N-glycosylation mapping, and plasma membrane targeting studies in the absence and presence of CD3ζ suggest two interconvertible types of NCR-CD3ζ assemblies: 1) a signaling incompetent structural NKp30-CD3ζ complex and 2) a ligand-induced signaling competent NKp30-CD3ζ complex. Moreover, we propose that ligand binding triggers translocation of Arg-143 from the membrane interface into the membrane to enable alignment with oppositely charged aspartate residues within CD3ζ and activation of CD3ζ-signaling.

Microglia Function in Central Nervous System Development and Plasticity

The nervous system comprises a remarkably diverse and complex network of different cell types, which must communicate with one another with speed, reliability, and precision. Thus, the developmental patterning and maintenance of these cell populations and their connections with one another pose a rather formidable task. Emerging data implicate microglia, the resident myeloid-derived cells of the central nervous system (CNS), in the spatial patterning and synaptic wiring throughout the healthy, developing, and adult CNS. Importantly, new tools to specifically manipulate microglia function have revealed that these cellular functions translate, on a systems level, to effects on overall behavior. In this review, we give a historical perspective of work to identify microglia function in the healthy CNS and highlight exciting new work in the field that has identified roles for these cells in CNS development, maintenance, and plasticity.

Genetics and molecular biology of brain calcification

Brain calcification is a common neuroimaging finding in patients with neurological, metabolic, or developmental disorders, mitochondrial diseases, infectious diseases, traumatic or toxic history, as well as in otherwise normal older people. Patients with brain calcification may exhibit movement disorders, seizures, cognitive impairment, and a variety of other neurologic and psychiatric symptoms. Brain calcification may also present as a single, isolated neuroimaging finding. When no specific cause is evident, a genetic etiology should be considered. The aim of the review is to highlight clinical disorders associated with brain calcification and provide summary of current knowledge of diagnosis, genetics, and pathogenesis of brain calcification.

Astrocyte and microglial control of glutamatergic signalling: a primer on understanding the disruptive role of chronic stress

It is now well established that chronic stress can induce significant structural remodelling of astrocytes and microglia. Until recently, however, the full significance of these morphological disturbances has remained unclear. Clues to the significance of astroglial re-organisation following stress are beginning to emerge from a compelling literature describing how astrocytes contribute to glutamatergic neurotransmission. The present review briefly summarises these two fields of research, identifies points of overlap and, in doing so, pin-points future research directions for stress neurobiology. Ultimately, understanding how chronic stress can disrupt the interactions of astrocytes and microglia with neurones has the potential in the future to improve the development of therapeutics designed to treat stress-related illnesses such as depression.

A genetic defect in mice that impairs missing self recognition despite evidence for normal maturation and MHC class I-dependent education of NK cells

In studies of a CD1d1-deficient mouse strain, we unexpectedly observed a severely impaired capacity for NK cell-mediated rejection of MHC class I-deficient (spleen or tumor) cells. Studies of another CD1-defective strain, as well as intercrosses with C57BL/6 mice, indicated that the impaired missing self rejection (IMSR) NK cell defect was a recessive trait, independent from the targeted CD1 locus. Studies with mixed bone marrow chimeras indicated that the defect is intrinsic to NK cells. The IMSR mice had normal proportions of NK cells, displaying a typical cell surface phenotype, as evaluated using a panel of Abs to developmental markers and known receptors. The impaired missing self recognition could not be overcome through cytokine stimulation. There was also an impaired capacity with respect to NKG2D-dependent cytotoxicity, whereas the mice exhibited normal Ly49D/DAP12-dependent responses in vivo and in vitro. The NK cell system of IMSR mice showed two hallmarks of MHC-dependent education: skewing of the Ly49 receptor repertoire and differential in vitro responsiveness between NK cells with and without inhibitory receptors for self-MHC ("licensing"). We conclude that these mice have a recessive trait that perturbs the missing self reaction, as well as NKG2D-dependent responses, whereas other aspects of the NK system, such as development, capacity to sense MHC molecules during education, and Ly49D/DAP12-dependent responses, are largely intact.

Emerging roles for triggering receptor expressed on myeloid cells receptor family signaling in inflammatory diseases

Innate immune receptors represent important therapeutic targets for inflammatory disorders. In particular, the Toll-like receptor (TLR) family has emerged as a promoter of chronic inflammation that contributes to obesity, insulin resistance and atherosclerosis. Importantly, triggering receptor expressed on myeloid cells-1 (TREM-1) has been characterized as an 'amplifier' of TLR2 and TLR4 signaling. TREM-1- and TREM-2-dependent signaling, as opposed to TREM-like transcript-1 (TLT-1 or TREML1), are mediated through association with the transmembrane adaptor DNAX activation protein of 12 kDa (DAP12). Recessive inheritance of rare mutations in DAP12 or TREM-2 results in a disorder called polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, and surprisingly these subjects are not immunocompromised. Recent progress into the roles of TREM/DAP12 signaling is critically reviewed here with a focus on metabolic, cardiovascular and inflammatory diseases. The expanding repertoire of putative ligands for TREM receptors is also discussed.

DAP12 and CD11b contribute to the microglial-induced death of dopaminergic neurons in vitro but not in vivo in the MPTP mouse model of Parkinson's disease

Background

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by a loss of dopaminergic neurons (DN) in the substantia nigra (SN). Several lines of evidence suggest that apoptotic cell death of DN is driven in part by non-cell autonomous mechanisms orchestrated by microglial cell-mediated inflammatory processes. Although the mechanisms and molecular network underlying this deleterious cross-talk between DN and microglial cells remain largely unknown, previous work indicates that, upon DN injury, activation of the β2 integrin subunit CD11b is required for microglia-mediated DN cell death. Interestingly, during brain development, the CD11b integrin is also involved in microglial induction of neuronal apoptosis and has been shown to act in concert with the DAP12 immunoreceptor. Whether such a developmental CD11b/DAP12 pathway could be reactivated in a pathological context such as PD and play a role in microglia-induced DN cell death is a tantalizing hypothesis that we wished to test in this study.

Methods

To test the possibility that DAP12 could be involved in microglia-associated DN injury, we used both in vitro and in vivo toxin-based experimental models of PD recapitulating microglial-mediated non-cell autonomous mechanisms of DN cell death. In vitro, enriched mesencephalic neuronal/microglial co-cultures were exposed to the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) whereas in vivo, mice were administrated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) according to acute or subchronic mode. Mice deficient for DAP12 or CD11b were used to determine the pathological function of the CD11b/DAP12 pathway in our disease models.

Results

Our results show that DAP12 and CD11b partially contribute to microglia-induced DN cell death in vitro. Yet, in vivo, mice deficient for either of these factors develop similar neuropathological alterations as their wild-type counterparts in two different MPTP mouse models of PD.

Conclusion

Overall, our data suggest that DAP12 and CD11b contribute to microglial-induced DN cell death in vitro but not in vivo in the MPTP mouse model of PD. Therefore, the CD11b/DAP12 pathway may not be considered as a promising therapeutic target for PD.

Neuroimmune regulation of homeostatic synaptic plasticity

Homeostatic synaptic plasticity refers to a set of negative-feedback mechanisms that are used by neurons to maintain activity within a functional range. While it is becoming increasingly clear that homeostatic regulation of synapse function is a key principle in the nervous system, the molecular details of this regulation are only beginning to be uncovered. Recent evidence implicates molecules classically associated with the peripheral immune system in the modulation of homeostatic synaptic plasticity. In particular, the pro-inflammatory cytokine TNFα, class I major histocompatibility complex, and neuronal pentraxin 2 are essential in the regulation of the compensatory synaptic response that occurs in response to prolonged neuronal inactivity. This review will present and discuss current evidence implicating neuroimmune molecules in the homeostatic regulation of synapse function. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Markers of nonselective and specific NK cell activation

NK cell activation is controlled by the integration of signals from cytokine receptors and germline-encoded activation and inhibitory receptors. NK cells undergo two distinct phases of activation during murine CMV (MCMV) infection: a nonselective phase mediated by proinflammatory cytokines and a specific phase driven by signaling through Ly49H, an NK cell activation receptor that recognizes infected cells. We sought to delineate cell surface markers that could distinguish NK cells that had been activated nonselectively from those that had been specifically activated through NK cell receptors. We demonstrated that stem cell Ag 1 (Sca-1) is highly upregulated during viral infections (to an even greater extent than CD69) and serves as a novel marker of early, nonselective NK cell activation. Indeed, a greater proportion of Sca-1(+) NK cells produced IFN-γ compared with Sca-1(-) NK cells during MCMV infection. In contrast to the universal upregulation of Sca-1 (as well as KLRG1) on NK cells early during MCMV infection, differential expression of Sca-1, as well as CD27 and KLRG1, was observed on Ly49H(+) and Ly49H(-) NK cells late during MCMV infection. Persistently elevated levels of KLRG1 in the context of downregulation of Sca-1 and CD27 were observed on NK cells that expressed Ly49H. Furthermore, the differential expression patterns of these cell surface markers were dependent on Ly49H recognition of its ligand and did not occur solely as a result of cellular proliferation. These findings demonstrate that a combination of Sca-1, CD27, and KLRG1 can distinguish NK cells nonselectively activated by cytokines from those specifically stimulated through activation receptors.

Global H3K4me3 genome mapping reveals alterations of innate immunity signaling and overexpression of JMJD3 in human myelodysplastic syndrome CD34+ cells

The molecular bases of myelodysplastic syndromes (MDS) are not fully understood. Trimethylated histone 3 lysine 4 (H3K4me3) is present in promoters of actively transcribed genes and has been shown to be involved in hematopoietic differentiation. We performed a genome-wide H3K4me3 CHIP-Seq (chromatin immunoprecipitation coupled with whole genome sequencing) analysis of primary MDS bone marrow (BM) CD34+ cells. This resulted in the identification of 36 genes marked by distinct higher levels of promoter H3K4me3 in MDS. A majority of these genes are involved in nuclear factor (NF)-κB activation and innate immunity signaling. We then analyzed expression of histone demethylases and observed significant overexpression of the JmjC-domain histone demethylase JMJD3 (KDM6b) in MDS CD34+ cells. Furthermore, we demonstrate that JMJD3 has a positive effect on transcription of multiple CHIP-Seq identified genes involved in NF-κB activation. Inhibition of JMJD3 using shRNA in primary BM MDS CD34+ cells resulted in an increased number of erythroid colonies in samples isolated from patients with lower-risk MDS. Taken together, these data indicate the deregulation of H3K4me3 and associated abnormal activation of innate immunity signals have a role in the pathogenesis of MDS and that targeting these signals may have potential therapeutic value in MDS.

Microglial control of neuronal activity

Fine-tuning of neuronal activity was thought to be a neuron-autonomous mechanism until the discovery that astrocytes are active players of synaptic transmission. The involvement of astrocytes has changed our understanding of the roles of non-neuronal cells and shed new light on the regulation of neuronal activity. Microglial cells are the macrophages of the brain and they have been mostly investigated as immune cells. However, recent data discussed in this review support the notion that, similarly to astrocytes, microglia are involved in the regulation of neuronal activity. For instance, in most, if not all, brain pathologies a strong temporal correlation has long been known to exist between the pathological activation of microglia and dysfunction of neuronal activity. Recent studies have convincingly shown that alteration of microglial function is responsible for pathological neuronal activity. This causal relationship has also been demonstrated in mice bearing loss-of-function mutations in genes specifically expressed by microglia. In addition to these long-term regulations of neuronal activity, recent data show that microglia can also rapidly regulate neuronal activity, thereby acting as partners of neurotransmission.

The "quad-partite" synapse: microglia-synapse interactions in the developing and mature CNS

Microglia are the resident immune cells and phagocytes of our central nervous system (CNS). While most work has focused on the rapid and robust responses of microglia during CNS disease and injury, emerging evidence suggests that these mysterious cells have important roles at CNS synapses in the healthy, intact CNS. Groundbreaking live imaging studies in the anesthetized, adult mouse demonstrated that microglia processes dynamically survey their environment and interact with other brain cells including neurons and astrocytes. More recent imaging studies have revealed that microglia dynamically interact with synapses where they appear to serve as "synaptic sensors," responding to changes in neural activity and neurotransmitter release. In the following review, we discuss the most recent work demonstrating that microglia play active roles at developing and mature synapses. We first discuss the important imaging studies that have led us to better understand the physical relationship between microglia and synapses in the healthy brain. Following this discussion, we review known molecular mechanisms and functional consequences of microglia-synapse interactions in the developing and mature CNS. Our current knowledge sheds new light on the critical functions of these mysterious cells in synapse development and function in the healthy CNS, but has also incited several new and interesting questions that remain to be explored. We discuss these open questions, and how the most recent findings in the healthy CNS may be related to pathologies associated with abnormal and/or loss of neural circuits.

Sex differences in murine susceptibility to systemic viral infections

Increased susceptibility to autoimmunity in females is often viewed as the consequence of enhanced immunoreactivity providing superior protection against infections. We paradoxically observed greater mortality in female compared to male mice during systemic viral infections with three large double-stranded DNA viruses (herpes simplex virus type I [HSV], murine cytomegalovirus [MCMV], and vaccinia virus [VV]). Indeed, female mice were 27-fold more susceptible to infection with HSV than male mice. Elimination of estrogen by ovariectomy in female mice or addition of estrogen to castrated male mice only partially eliminated the observed sex differences following HSV infection. However, the differences observed in survival between female and male mice were nearly abrogated in the absence of type I interferon receptor signaling and substantially mitigated in absence of DAP12 signaling. Interestingly, the sex-specific impact of type I interferon receptor and DAP12 signaling differentially influenced survival during systemic viral infections with type I interferon receptor signaling enhancing male survival and DAP12 signaling increasing the susceptibility of female mice. These results have potential implications for the sex disparities observed in human autoimmune disorders.

Individual organelle pH determinations of magnetically enriched endocytic organelles via laser-induced fluorescence detection

The analysis of biotransformations that occur in lysosomes and other endocytic organelles is critical to studies on intracellular degradation, nutrient recycling, and lysosomal storage disorders. Such analyses require bioactive organelle preparations that are devoid of other contaminating organelles. Commonly used differential centrifugation techniques produce impure fractions and may not be compatible with microscale separation platforms. Density gradient centrifugation procedures reduce the level of impurities but may compromise bioactivity. Here we report on simple magnetic setup and a procedure that produce highly enriched bioactive organelles based on their magnetic capture as they traveled through open tubes. Following capture, in-line laser-induced fluorecence detection (LIF) determined for the first time the pH of each magnetically retained individual endocytic organelle. Unlike bulk measurements, this method was suitable to describe the distributions of pH values in endocytic organelles from L6 rat myoblasts treated with dextran-coated iron oxide nanoparticles (for magnetic retention) and fluorescein/TMRM-conjugated dextran (for pH measurements by LIF). Their individual pH values ranged from 4 to 6, which is typical of bioactive endocytic organelles. These analytical procedures are of high relevance to evaluate lysosomal-related degradation pathways in aging, storage disorders, and drug development.

Control of pathogenic CD4 T cells and lethal immunopathology by signaling immunoadaptor DAP12 during influenza infection

Immunopathology is a major cause of influenza-associated morbidity and mortality worldwide. However, the role and regulatory mechanisms of CD4 T cells in severe lung immunopathology following acute influenza infection are poorly understood. In this paper, we report that the emergence of immunopathogenic CD4 T cells is under the control of a transmembrane immunoadaptor DAP12 pathway during influenza infection. We find that the mice lacking DAP12 have unaltered viral clearance but easily succumb to influenza infection as a result of uncontrolled immunopathology. Such immunopathology is associated with markedly increased CD4 T cells displaying markedly increased cytotoxicity and Fas ligand expression. Furthermore, the immunopathogenic property of these CD4 T cells is transferrable. Thus, depletion of CD4 T cells or abrogation of Fas/Fas ligand signaling pathway improves survival and immunopathology. We further find that DAP12 expressed by dendritic cells plays an important role in controlling the immunopathogenic CD4 T cells during influenza infection. Our findings identify a novel pathway that controls the level of immune-pathogenic CD4 T cells during acute influenza infection.

The immunoreceptor adapter protein DAP12 suppresses B lymphocyte-driven adaptive immune responses

DAP12, an immunoreceptor tyrosine-based activation motif-bearing adapter protein, is involved in innate immunity mediated by natural killer cells and myeloid cells. We show that DAP12-deficient mouse B cells and B cells from a patient with Nasu-Hakola disease, a recessive genetic disorder resulting from loss of DAP12, showed enhanced proliferation after stimulation with anti-IgM or CpG. Myeloid-associated immunoglobulin-like receptor (MAIR) II (Cd300d) is a DAP12-associated immune receptor. Like DAP12-deficient B cells, MAIR-II-deficient B cells were hyperresponsive. Expression of a chimeric receptor composed of the MAIR-II extracellular domain directly coupled to DAP12 into the DAP12-deficient or MAIR-II-deficient B cells suppressed B cell receptor (BCR)-mediated proliferation. The chimeric MAIR-II-DAP12 receptor recruited the SH2 domain-containing protein tyrosine phosphatase 1 (SHP-1) after BCR stimulation. DAP12-deficient mice showed elevated serum antibodies against self-antigens and enhanced humoral immune responses against T cell-dependent and T cell-independent antigens. Thus, DAP12-coupled MAIR-II negatively regulates B cell-mediated adaptive immune responses.

Activation receptor-induced tolerance of mature NK cells in vivo requires signaling through the receptor and is reversible

NK cell responses are determined by signals received through activating and inhibitory cell surface receptors. Ly49H is an NK cell-specific activating receptor that accounts for the genetic resistance to murine CMV (MCMV). The Ly49H receptor has been shown to interact with two adaptor proteins (DAP12 and DAP10). In the context of MCMV infection, interaction of m157 (the MCMV-encoded ligand for Ly49H) with Ly49H results in activation of Ly49H-expressing NK cells. Chronic exposure of Ly49H with m157, however, induces tolerance in these same cells. The mechanism of this tolerance remains poorly understood. Using a transgenic mouse model, we demonstrate that induction of tolerance in Ly49H(+) NK cells by chronic exposure to m157, in vivo, requires signaling through the Ly49H adaptor protein DAP12, but not the DAP10 adaptor protein. Furthermore, mature Ly49H-expressing NK cells from wild-type mice can acquire a tolerant phenotype by 24 h posttransfer into a transgenic C57BL/6 mouse that expresses m157. The tolerant phenotype can be reversed, in vivo, if tolerant NK cells are transferred to mice that do not express the m157 protein. Thus, continuous activating receptor engagement can induce a transient tolerance in mature NK cells in vivo. These observations provide new insight into how activating receptor engagement shapes NK cell function and has important implications in how NK cells respond to tumors and during chronic viral infection.

DAP12 promotes IRAK-M expression and IL-10 production by liver myeloid dendritic cells and restrains their T cell allostimulatory ability

Freshly isolated hepatic dendritic cells (DC) are comparatively immature, relatively resistant to maturation, and can downmodulate effector T cell responses. Molecular mechanisms that underlie these properties are ill defined. DNAX-activating protein of 12 kDa (DAP12) is an ITAM-bearing transmembrane adaptor protein that integrates signals through several receptors, including triggering receptor expressed on myeloid cells-1, -2, and CD200R. Notably, DC propagated from DAP12-deficient mice exhibit enhanced maturation in response to TLR ligation. Given the constitutive exposure of liver DC to endotoxin draining from the gut, we hypothesized that DAP12 might regulate liver DC maturation. We show that DAP12 is expressed by freshly isolated liver, spleen, kidney, and lung myeloid DC. Moreover, inhibition of DAP12 expression by liver DC using small interfering RNA promotes their phenotypic and functional maturation, resulting in enhanced TNF-α, IL-6, and IL-12p70 production, reduced secretion of IL-10, and enhanced CD4(+) and CD8(+) T cell proliferation. Furthermore, DAP12 silencing correlates with decreased STAT3 phosphorylation in mature liver DC and with diminished expression of the IL-1R-associated kinase-M, a negative regulator of TLR signaling. These findings highlight a regulatory role for DAP12 in hepatic DC maturation, and suggest a mechanism whereby this function may be induced/maintained.

NKG2D initiates caspase-mediated CD3zeta degradation and lymphocyte receptor impairments associated with human cancer and autoimmune disease

Deficiencies of the T cell and NK cell CD3ζ signaling adapter protein in patients with cancer and autoimmune diseases are well documented, but mechanistic explanations are fragmentary. The stimulatory NKG2D receptor on T and NK cells mediates tumor immunity but can also promote local and systemic immune suppression in conditions of persistent NKG2D ligand induction that include cancer and certain autoimmune diseases. In this paper, we provide evidence that establishes a causative link between CD3ζ impairment and chronic NKG2D stimulation due to pathological ligand expression. We describe a mechanism whereby NKG2D signaling in human T and NK cells initiates Fas ligand/Fas-mediated caspase-3/-7 activation and resultant CD3ζ degradation. As a consequence, the functional capacities of the TCR, the low-affinity Fc receptor for IgG, and the NKp30 and NKp46 natural cytotoxicity receptors, which all signal through CD3ζ, are impaired. These findings are extended to ex vivo phenotypes of T and NK cells among tumor-infiltrating lymphocytes and in peripheral blood from patients with juvenile-onset lupus. Collectively, these results indicate that pathological NKG2D ligand expression leads to simultaneous impairment of multiple CD3ζ-dependent receptor functions, thus offering an explanation that may be applicable to CD3ζ deficiencies associated with diverse disease conditions.

Current perspectives of natural killer cell education by MHC class I molecules

From the early days of natural killer (NK) cell research, it was clear that MHC genes controlled the specificity of mouse NK cell-dependent responses, such as the ability to reject transplanted allogeneic bone marrow and to kill tumour cells. Although several mechanisms that are involved in this 'education' process have been clarified, most of the mechanisms have still to be identified. Here, we review the current understanding of the processes that are involved in NK cell education, including how the host MHC class I molecules regulate responsiveness and receptor repertoire formation in NK cells and the signalling pathways that are involved.

Regulation of immune cell responses by semaphorins and their receptors

Semaphorins were originally identified as axon guidance factors involved in the development of the neuronal system. However, accumulating evidence indicates that several members of semaphorins, so-called 'immune semaphorins', are crucially involved in various phases of immune responses. These semaphorins regulate both immune cell interactions and immune cell trafficking during physiological and pathological immune responses. Here, we review the following two functional aspects of semaphorins and their receptors in immune responses: their functions in cell-cell interactions and their involvement in immune cell trafficking.

Coactivation of Syk kinase and MyD88 adaptor protein pathways by bacteria promotes regulatory properties of neutrophils

Neutrophils are one of the first lines of defense against microbial pathogens and are rapidly recruited at the infection site upon inflammatory conditions. We show here that after bacterial stimulation, and in contrast to monocytes and macrophages, murine neutrophils contributed poorly to inflammatory responses; however, they secreted high amounts of the anti-inflammatory cytokine IL-10 in a DAP12 adaptor-Syk kinase and MyD88 adaptor-dependent manner. Cotriggering of TLR-MyD88- and C-type lectin receptor (CLR)-Syk-dependent pathways led to a quick and sustained phosphorylation of p38 MAP and Akt kinases in neutrophils. In vivo, both Gram-negative bacteria and mycobacteria induced the recruitment of neutrophils secreting IL-10. In acute mycobacterial infection, neutrophil-derived IL-10 controlled the inflammatory response of dendritic cells, monocytes and macrophages in the lung. During a chronic infection, neutrophil depletion promoted inflammation and decreased the mycobacterial burden. Therefore, neutrophils can have a previously unsuspected regulatory role during acute and chronic microbial infections.

Ly49H engagement compensates for the absence of type I interferon signaling in stimulating NK cell proliferation during murine cytomegalovirus infection

NK cells vigorously proliferate during viral infections, resulting in an expanded pool of innate lymphocytes that are able to participate in early host defense. The relative contributions of cytokines and activation receptors in stimulating NK cell proliferation during viral infections are not well characterized. In this study, we demonstrated that signaling through the NK cell activation receptor Ly49H was able to compensate for the absence of cytokine stimulation in the preferential phase of viral-induced proliferation during murine cytomegalovirus infection. In the absence of type I IFN stimulation, NK cell proliferation was strongly biased toward cells expressing the Ly49H receptor, even at early time points when minimal preferential Ly49H-mediated proliferation was observed in wild-type mice. In the absence of effective Ly49H signaling or following infection with virus that did not express the ligand for Ly49H, no difference was observed in the proliferation of subsets of NK cells that either express or lack expression of Ly49H, although the overall proliferation of NK cells in IFNalphabetaR(-/-) mice was substantially reduced. These results highlight the contribution of NK cell activation receptors in stimulating proliferation and subsequent expansion of NK cells that are able to recognize virally infected cells.

Altered NK cell development and enhanced NK cell-mediated resistance to mouse cytomegalovirus in NKG2D-deficient mice

NKG2D is a potent activating receptor on natural killer (NK) cells and acts as a molecular sensor for stressed cells expressing NKG2D ligands such as infected or tumor-transformed cells. Although NKG2D is expressed on NK cell precursors, its role in NK cell development is not known. We have generated NKG2D-deficient mice by targeting the Klrk1 locus. Here we provide evidence for an important regulatory role of NKG2D in the development of NK cells. The absence of NKG2D caused faster division of NK cells, perturbation in size of some NK cell subpopulations, and their augmented sensitivity to apoptosis. As expected, Klrk1(-/-) NK cells are less responsive to tumor targets expressing NKG2D ligands. Klrk1(-/-) mice, however, showed an enhanced NK cell-mediated resistance to mouse cytomegalovirus infection as a consequence of NK cell dysregulation. Altogether, these findings provide evidence for regulatory function of NKG2D in NK cell physiology.

The Role of NK Cells and NK Cell Receptors inAutoimmune Disease

Natural killer (NK) cells are the first line of defense against infection and transformation. Additionally, NK cells can play seemingly opposite roles in autoimmune disease. Here, we summarize the functions of NK cells as both regulators and inducers of autoimmune disease. The role NK cells play depends on which cells become targets for NK cell attack. The activity of NK cells is controlled by inhibitory receptors specific for MHC Class I molecules, and by activating receptors with diverse specificities. The ligands for both activating and inhibitory receptors are present on potential target cells. It is the balance in expression of these different ligands that determines NK cell activation and therefore whether the cell becomes a target for NK cell-mediated killing. We further discuss the roles of NK cell receptors and their ligands in autoimmune disease.

DAP10 associates with Ly49 receptors but contributes minimally to their expression and function in vivo

NK cells recognize target cells through activating receptors, many of which rely on the transmembrane adaptors DAP10, DAP12 and FcR-gamma to deliver intracellular signals. Because these adaptors initiate distinct signaling pathways, they dictate the type of response mediated by receptor engagement. DAP10, for example, primarily triggers cytotoxicity, whereas DAP12 induces both cytotoxicity and IFN-gamma secretion. In mice, NKG2D signals through both DAP10 and DAP12, which broadens and modulates the type of response engendered by encounter with ligand. Although initial studies indicated that Ly49H and Ly49D recruit only DAP12, a recent report suggested that they also associate with DAP10. We asked whether this association occurs and is functionally significant under physiologic conditions. Our data demonstrate that DAP10 does associate with Ly49H and Ly49D in primary NK cells. While this association contributes slightly to cell surface expression of both receptors, it has no significant impact on Ly49H-mediated control of murine cytomegalovirus infection. Thus, while many activating NK-cell receptors are promiscuous in terms of adaptor association, our data indicate that the functional consequences of such promiscuity may vary widely and may not be evident in all cases.

Ly49H signaling through DAP10 is essential for optimal natural killer cell responses to mouse cytomegalovirus infection

The activating natural killer (NK) cell receptor Ly49H recognizes the mouse cytomegalovirus (MCMV) m157 glycoprotein expressed on the surface of infected cells and is required for protection against MCMV. Although Ly49H has previously been shown to signal via DAP12, we now show that Ly49H must also associate with and signal via DAP10 for optimal function. In the absence of DAP12, DAP10 enables Ly49H-mediated killing of m157-bearing target cells, proliferation in response to MCMV infection, and partial protection against MCMV. DAP10-deficient Ly49H⁺ NK cells, expressing only Ly49H–DAP12 receptor complexes, are partially impaired in their ability to proliferate during MCMV infection, display diminished ERK1/2 activation, produce less IFN-γ upon Ly49H engagement, and demonstrate reduced control of MCMV infection. Deletion of both DAP10 and DAP12 completely abrogates Ly49H surface expression and control of MCMV infection. Thus, optimal NK cell–mediated immunity to MCMV depends on Ly49H signaling through both DAP10 and DAP12.

Increased diabetes development and decreased function of CD4+CD25+ Treg in the absence of a functional DAP12 adaptor protein

Prior to the development of type 1 diabetes, T cells are primed in the pancreatic lymph nodes (PLN) where they interact with APC displaying beta cell-derived peptides. The details concerning the regulation of autoreactive T cell responses in the PLN are unclear. BDC2.5/B6g7 TCR transgenic mice represent a simplified model of type 1 diabetes, in which beta cell-specific CD4+ T cells expressing a diabetogenic transgenic TCR are first activated in the PLN and subsequently home to the pancreas where they mediate killing of beta cells. DNAX-activating protein of 12 kDa (DAP12) is an adaptor molecule carrying an ITAM motif. It associates with receptors on lymphoid and myeloid cells, including APC. We here show that introduction of a DAP12 null mutation in BDC2.5/B6g7 mice accelerated diabetes development and promoted an augmented activation state of PLN T cells expressing the transgenic TCR. Transferred BDC2.5 T cells proliferated more efficiently in the PLN of DAP12-deficient B6g7 recipients, which correlated with a decreased impact of co-transferred BDC2.5+CD4+CD25+ T cells. We propose that signaling through a DAP12-associated receptor on APC facilitates activation of Treg in the PLN and by this contributes to the maintenance of peripheral tolerance to beta cell-derived antigens.

TREM-2 mediated signaling induces antigen uptake and retention in mature myeloid dendritic cells

Myeloid dendritic cells (mDC) activated with a B7-DC-specific cross-linking IgM Ab (B7-DC XAb) take up and retain Ag and interact with T cell compartments to affect a number of biologic changes that together cause strong antitumor responses and blockade of inflammatory airway disease in animal models. The molecular events mediating the initial responses in mDC remain unclear. In this study we show that B7-DC XAb caused rapid phosphorylation of the adaptor protein DAP12 and intracellular kinases Syk and phospholipase C-gamma1. Pretreatment of mDC with the Syk inhibitor piceatannol blocked B7-DC XAb-induced Ag uptake with a concomitant loss of tumor protection in mice. Vaccination with tumor lysate-pulsed wild-type B7-DC XAb-activated mDC, but not TREM-2 knockout XAb-activated mDC, protected mice from lethal melanoma challenge. Multimolecular caps appeared within minutes of B7-DC XAb binding to either human or mouse mDC, and FRET analysis showed that class II, CD80, CD86, and TREM-2 are recruited in tight association on the cell surface. When TREM-2 expression was reduced in wild-type mDC using short hairpin RNA or by using mDC from TREM-2 knockout mice, in vitro DC failed to take up Ag after B7-DC XAb stimulation. These results directly link TREM-2 signaling with one change in the mDC phenotype that occurs in response to this unique Ab. The parallel signaling events observed in both human and mouse mDC support the hypothesis that B7-DC cross-linking may be useful as a therapeutic immune modulator in human patients.

Essential role of DAP12 signaling in macrophage programming into a fusion-competent state

Multinucleated giant cells, formed by fusion of macrophages, are a hallmark of granulomatous inflammation. With a genetic approach, we show that signaling through the adaptor protein DAP12 (DNAX activating protein of 12 kD), its associated receptor triggering receptor expressed by myeloid cells 2 (TREM-2), and the downstream protein tyrosine kinase Syk is required for the cytokine-induced formation of giant cells and that overexpression of DAP12 potentiates macrophage fusion. We also present evidence that DAP12 is a general macrophage fusion regulator and is involved in modulating the expression of several macrophage-associated genes, including those encoding known mediators of macrophage fusion, such as DC-STAMP and Cadherin 1. Thus, DAP12 is involved in programming of macrophages through the regulation of gene and protein expression to induce a fusion-competent state.

Mechanisms and consequences of dendritic cell migration

Dendritic cells (DCs) are critical for adaptive immunity and tolerance. Most DCs are strategically positioned as immune sentinels poised to respond to invading pathogens in tissues throughout the body. Differentiated DCs and their precursors also circulate in blood and can get rapidly recruited to sites of challenge. Within peripheral tissues, DCs collect antigenic material and then traffic to secondary lymphoid organs, where they communicate with lymphocytes to orchestrate adaptive immune responses. Hence, the migration and accurate positioning of DCs is indispensable for immune surveillance. Here, we review the molecular traffic signals that govern the migration of DCs throughout their life cycle.

Developmental neuronal death in hippocampus requires the microglial CD11b integrin and DAP12 immunoreceptor

In several brain regions, microglia actively promote neuronal apoptosis during development. However, molecular actors leading microglia to trigger death remain mostly unknown. Here, we show that, in the developing hippocampus, apoptotic neurons are contacted by microglia expressing both the integrin CD11b and the immunoreceptor DAP12. We demonstrate that developmental apoptosis decreases in mice deficient for CD11b or DAP12. In addition, function-blocking antibodies directed against CD11b decrease neuronal death when injected into wild-type neonates, but have no effect when injected into DAP12-deficient littermates. This demonstrates that DAP12 and CD11b act in converging pathways to induce neuronal death. Finally, we show that DAP12 and CD11b control the production of microglial superoxide ions, which kill the neurons. Thus, our data show that the process of developmental neuronal death triggered by microglia is similar to the elimination of pathogenic cells by the innate immune cells.

DAP12 couples c-Fms activation to the osteoclast cytoskeleton by recruitment of Syk

We examined the mechanism by which M-CSF regulates the cytoskeleton and function of the osteoclast, the exclusive bone resorptive cell. We show that binding of M-CSF to its receptor c-Fms generates a signaling complex comprising phosphorylated DAP12, an adaptor containing an immunoreceptor tyrosine-based activation motif (ITAM) and the nonreceptor tyrosine kinase Syk. c-Fms tyrosine 559, the exclusive binding site of c-Src, is necessary for regulation of DAP12/Syk signaling. Deletion of either of these molecules yields osteoclasts that fail to reorganize their cytoskeleton. Retroviral transduction of null precursors with wild-type or mutant DAP12 or Syk reveals that the SH2 domain of Syk and the ITAM tyrosine residues and transmembrane domain of DAP12 mediate M-CSF signaling. Our data provide genetic and biochemical evidence that uncovers an epistatic signaling pathway linking the receptor tyrosine kinase c-Fms to the immune adaptor DAP12 and the cytoskeleton.