DAP12 signaling: from immune cells to bone modeling and brain myelination

DAP12 interacts with RER1 and is retained in the secretory pathway before assembly with TREM2

DNAX-activating protein of 12 kDa (DAP12) is a transmembrane adapter protein expressed in lymphoid and myeloid lineage cells. It interacts with several immunoreceptors forming functional complexes that trigger intracellular signaling pathways. One of the DAP12 associated receptors is the triggering receptor expressed on myeloid cells 2 (TREM2). Mutations in both DAP12 and TREM2 have been linked to neurodegenerative diseases. However, mechanisms involved in the regulation of subcellular trafficking and turnover of these proteins are not well understood. Here, we demonstrate that proteasomal degradation of DAP12 is increased in the absence of TREM2. Interestingly, unassembled DAP12 is also retained in early secretory compartments, including the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC), thereby preventing its transport to the plasma membrane. We also show that unassembled DAP12 interacts with the retention in ER sorting receptor 1 (RER1). The deletion of endogenous RER1 decreases expression of functional TREM2-DAP12 complexes and membrane proximal signaling, and resulted in almost complete inhibition of phagocytic activity in THP-1 differentiated macrophage-like cells. These results indicate that RER1 acts as an important regulator of DAP12 containing immunoreceptor complexes and immune cell function.

Current understanding on TREM-2 molecular biology and physiopathological functions

Triggering receptor expressed on myeloid cells 2 (TREM-2), a glycosylated receptor belonging to the immunoglobin superfamily and especially expressed in the myeloid cell lineage, is frequently explained as a reminiscent receptor for both adaptive and innate immunity regulation. TREM-2 is also acknowledged to influence NK cell differentiation via the PI3K and PLCγ signaling pathways, as well as the partial activation or direct inhibition of T cells. Additionally, TREM-2 overexpression is substantially linked to cell-specific functions, such as enhanced phagocytosis, reduced toll-like receptor (TLR)-mediated inflammatory cytokine production, increased transcription of anti-inflammatory cytokines, and reshaped T cell function. Whereas TREM-2-deficient cells exhibit diminished phagocytic function and enhanced proinflammatory cytokines production, proceeding to inflammatory injuries and an immunosuppressive environment for disease progression. Despite the growing literature supporting TREM-2+ cells in various diseases, such as neurodegenerative disorders and cancer, substantial facets of TREM-2-mediated signaling remain inadequately understood relevant to pathophysiology conditions. In this direction, herein, we have summarized the current knowledge on TREM-2 biology and cell-specific TREM-2 expression, particularly in the modulation of pivotal TREM-2-dependent functions under physiopathological conditions. Furthermore, molecular regulation and generic biological relevance of TREM-2 are also discussed, which might provide an alternative approach for preventing or reducing TREM-2-associated deformities. At last, we discussed the TREM-2 function in supporting an immunosuppressive cancer environment and as a potential drug target for cancer immunotherapy. Hence, summarized knowledge of TREM-2 might provide a window to overcome challenges in clinically effective therapies for TREM-2-induced diseases in humans.

Role of neuroinflammation in neurodegeneration development

Studies in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and Amyotrophic lateral sclerosis, Huntington's disease, and so on, have suggested that inflammation is not only a result of neurodegeneration but also a crucial player in this process. Protein aggregates which are very common pathological phenomenon in neurodegeneration can induce neuroinflammation which further aggravates protein aggregation and neurodegeneration. Actually, inflammation even happens earlier than protein aggregation. Neuroinflammation induced by genetic variations in CNS cells or by peripheral immune cells may induce protein deposition in some susceptible population. Numerous signaling pathways and a range of CNS cells have been suggested to be involved in the pathogenesis of neurodegeneration, although they are still far from being completely understood. Due to the limited success of traditional treatment methods, blocking or enhancing inflammatory signaling pathways involved in neurodegeneration are considered to be promising strategies for the therapy of neurodegenerative diseases, and many of them have got exciting results in animal models or clinical trials. Some of them, although very few, have been approved by FDA for clinical usage. Here we comprehensively review the factors affecting neuroinflammation and the major inflammatory signaling pathways involved in the pathogenicity of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. We also summarize the current strategies, both in animal models and in the clinic, for the treatment of neurodegenerative diseases.

Therapeutic radiation exposure of the abdomen during childhood induces chronic adipose tissue dysfunction

BACKGROUNDChildhood cancer survivors who received abdominal radiotherapy (RT) or total body irradiation (TBI) are at increased risk for cardiometabolic disease, but the underlying mechanisms are unknown. We hypothesize that RT-induced adipose tissue dysfunction contributes to the development of cardiometabolic disease in the expanding population of childhood cancer survivors.METHODSWe performed clinical metabolic profiling of adult childhood cancer survivors previously exposed to TBI, abdominal RT, or chemotherapy alone, alongside a group of healthy controls. Study participants underwent abdominal s.c. adipose biopsies to obtain tissue for bulk RNA sequencing. Transcriptional signatures were analyzed using pathway and network analyses and cellular deconvolution.RESULTSIrradiated adipose tissue is characterized by a gene expression signature indicative of a complex macrophage expansion. This signature includes activation of the TREM2-TYROBP network, a pathway described in diseases of chronic tissue injury. Radiation exposure of adipose is further associated with dysregulated adipokine secretion, specifically a decrease in insulin-sensitizing adiponectin and an increase in insulin resistance-promoting plasminogen activator inhibitor-1. Accordingly, survivors exhibiting these changes have early signs of clinical metabolic derangement, such as increased fasting glucose and hemoglobin A1c.CONCLUSIONChildhood cancer survivors exposed to abdominal RT or TBI during treatment exhibit signs of chronic s.c. adipose tissue dysfunction, manifested as dysregulated adipokine secretion that may negatively impact their systemic metabolic health.FUNDINGThis study was supported by Rockefeller University Hospital; National Institute of General Medical Sciences (T32GM007739); National Center for Advancing Translational Sciences (UL1 TR001866); National Cancer Institute (P30CA008748); American Cancer Society (133831-CSDG-19-117-01-CPHPS); American Diabetes Association (1-17-ACE-17); and an anonymous donor (MSKCC).

Serine/threonine phosphatases in osteoclastogenesis and bone resorption

Maintenance of optimal bone mass is controlled through the concerted functions of several cell types, including bone resorbing osteoclasts. Osteoclasts function to remove calcified tissue during developmental bone modeling, and degrade bone at sites of damage during bone remodeling. Changes to bone homeostasis can arise with alterations in osteoclastogenesis and/or catabolic activity that are not offset by anabolic activity; thus, factors that regulate osteoclastogenesis and bone resorption are of interest to further our understanding of basic bone biology, and as potential targets for therapeutic intervention. Several key cytokines, including RANKL and M-CSF, as well as co-stimulatory factors elicit kinase signaling cascades that promote osteoclastogenesis. These kinase cascades are offset by the action of protein phosphatases, including members of the serine/threonine phosphatase family. Here we review the functions of serine/threonine phosphatases and their control of osteoclast differentiation and function, while highlighting deficiencies in our understanding of this understudied class of proteins within the field.

A reporter cell system for the triggering receptor expressed on myeloid cells 2 reveals differential effects of disease-associated variants on receptor signaling and activation by antibodies against the stalk region

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor expressed on myeloid-derived cell types. The extracellular immunoglobulin-like domain of TREM2 binds anionic ligands including Apolipoprotein E and Amyloid-β. The transmembrane domain interacts with its adaptor protein DAP12/TYROBP that is responsible for propagation of downstream signaling upon ligand interaction. Several sequence variants of TREM2 have been linked to different neurodegenerative diseases including Alzheimer's disease. Here, we generated HEK 293 Flp-In cell lines stably expressing human TREM2 and DAP12 using a bicistronic construct with a T2A linker sequence allowing initial expression of both proteins in stoichiometric amounts. Cell biological and biochemical analyses revealed transport of TREM2 to the cell surface, and canonical sequential proteolytic processing and shedding of TREM2 (sTREM2). The functionality of this cell system was demonstrated by detection of phosphorylated spleen tyrosine kinase (SYK) upon stimulation of TREM2 with the anionic membrane lipid phosphatidylserine or anti-TREM2 antibodies. Using this cell model, we demonstrated impaired signaling of disease associated TREM2 variants. We also identified a monoclonal antibody against the stalk region of TREM2 with agonistic activity. Activation of TREM2-DAP12 signaling with the monoclonal antibody and the partial loss of function of disease associated variants were recapitulated in induced pluripotent stem cell derived microglia. Thus, this reporter cell model represents a suitable experimental system to investigate signaling of TREM2 variants, and for the identification of ligands and compounds that modulate TREM2-DAP12 signaling. MAIN POINTS: Disease associated variants impair the signaling activity of TREM2 by distinct mechanisms. Targeting the stalk region of TREM2 with bivalent antibodies activates TREM2 signaling.

Single Cell Transcriptome Analysis of Niemann-Pick Disease, Type C1 Cerebella

Niemann-Pick disease, type C1 (NPC1) is a lysosomal disease characterized by endolysosomal storage of unesterified cholesterol and decreased cellular cholesterol bioavailability. A cardinal symptom of NPC1 is cerebellar ataxia due to Purkinje neuron loss. To gain an understanding of the cerebellar neuropathology we obtained single cell transcriptome data from control (Npc1^+/+) and both three-week-old presymptomatic and seven-week-old symptomatic mutant (Npc1^-/-) mice. In seven-week-old Npc1^-/- mice, differential expression data was obtained for neuronal, glial, vascular, and myeloid cells. As anticipated, we observed microglial activation and increased expression of innate immunity genes. We also observed increased expression of innate immunity genes by other cerebellar cell types, including Purkinje neurons. Whereas neuroinflammation mediated by microglia may have both neuroprotective and neurotoxic components, the contribution of increased expression of these genes by non-immune cells to NPC1 pathology is not known. It is possible that dysregulated expression of innate immunity genes by non-immune cells is neurotoxic. We did not anticipate a general lack of transcriptomic changes in cells other than microglia from presymptomatic three-week-old Npc1^-/- mice. This observation suggests that microglia activation precedes neuronal dysfunction. The data presented in this paper will be useful for generating testable hypotheses related to disease progression and Purkinje neurons loss as well as providing insight into potential novel therapeutic interventions.

Neurodegenerative disease mutations in TREM2 reveal a functional surface and distinct loss-of-function mechanisms

Genetic variations in the myeloid immune receptor TREM2 are linked to several neurodegenerative diseases. To determine how TREM2 variants contribute to these diseases, we performed structural and functional studies of wild-type and variant proteins. Our 3.1 Å TREM2 crystal structure revealed that mutations found in Nasu-Hakola disease are buried whereas Alzheimer's disease risk variants are found on the surface, suggesting that these mutations have distinct effects on TREM2 function. Biophysical and cellular methods indicate that Nasu-Hakola mutations impact protein stability and decrease folded TREM2 surface expression, whereas Alzheimer's risk variants impact binding to a TREM2 ligand. Additionally, the Alzheimer's risk variants appear to epitope map a functional surface on TREM2 that is unique within the larger TREM family. These findings provide a guide to structural and functional differences among genetic variants of TREM2, indicating that therapies targeting the TREM2 pathway should be tailored to these genetic and functional differences with patient-specific medicine approaches for neurodegenerative disorders.

Double-stranded RNA analog and type I interferon regulate expression of Trem paired receptors in murine myeloid cells

Background

Triggering receptors expressed on myeloid cells (Trem) proteins are a family of cell surface receptors used to control innate immune responses such as proinflammatory cytokine production in mice. Trem genes belong to a rapidly expanding family of receptors that include activating and inhibitory paired-isoforms.

Results

By comparative genomic analysis, we found that Trem4, Trem5 and Trem-like transcript-6 (Treml6) genes typically paired receptors. These paired Trem genes were murine-specific and originated from an immunoreceptor tyrosine-based inhibition motif (ITIM)-containing gene. Treml6 encoded ITIM, whereas Trem4 and Trem5 lacked the ITIM but possessed positively-charged residues to associate with DNAX activating protein of 12 kDa (DAP12). DAP12 was directly associated with Trem4 and Trem5, and DAP12 coupling was mandatory for their expression on the cell surface. In bone marrow-derived dendritic cells (BMDCs) and macrophages (BMDMs), and splenic DC subsets, polyinosinic-polycytidylic acid (polyI:C) followed by type I interferon (IFN) production induced Trem4 and Treml6 whereas polyI:C or other TLR agonists failed to induce the expression of Trem5. PolyI:C induced Treml6 and Trem4 more efficiently in BMDMs than BMDCs. Treml6 was more potentially up-regulated in conventional DC (cDCs) and plasmacytoid DC (pDCs) than Trem4 in mice upon in vivo stimulation with polyI:C.

Discussion

Treml6-dependent inhibitory signal would be dominant in viral infection compared to resting state. Though no direct ligands of these Trem receptors have been determined, the results infer that a set of Trem receptors are up-regulated in response to viral RNA to regulate myeloid cell activation through modulation of DAP12-associated Trem4 and ITIM-containing Treml6.

Electronic supplementary material

The online version of this article (doi:10.1186/s12865-016-0147-y) contains supplementary material, which is available to authorized users.

The Triggering Receptor Expressed on Myeloid Cells 2: A Molecular Link of Neuroinflammation and Neurodegenerative Diseases

The triggering receptor expressed on myeloid cells (TREM) 2 is a member of the immunoglobulin superfamily of receptors and mediates signaling in immune cells via engagement of its co-receptor DNAX-activating protein of 12 kDa (DAP12). Homozygous mutations in TREM2 or DAP12 cause Nasu-Hakola disease, which is characterized by bone abnormalities and dementia. Recently, a variant of TREM2 has also been associated with an increased risk for Alzheimer disease. The selective expression of TREM2 on immune cells and its association with different forms of dementia indicate a contribution of this receptor in common pathways of neurodegeneration.

Retention of duplicated ITAM-containing transmembrane signaling subunits in the tetraploid amphibian species Xenopus laevis

The ITAM-bearing transmembrane signaling subunits (TSS) are indispensable components of activating leukocyte receptor complexes. The TSS-encoding genes map to paralogous chromosomal regions, which are thought to arise from ancient genome tetraploidization(s). To assess a possible role of tetraploidization in the TSS evolution, we studied TSS and other functionally linked genes in the amphibian species Xenopus laevis whose genome was duplicated about 40 MYR ago. We found that X. laevis has retained a duplicated set of sixteen TSS genes, all except one being transcribed. Furthermore, duplicated TCRα loci and genes encoding TSS-coupling protein kinases have also been retained. No clear evidence for functional divergence of the TSS paralogs was obtained from gene expression and sequence analyses. We suggest that the main factor of maintenance of duplicated TSS genes in X. laevis was a protein dosage effect and that this effect might have facilitated the TSS set expansion in early vertebrates.

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.

Brain innate immunity in the regulation of neuroinflammation: therapeutic strategies by modulating CD200-CD200R interaction involve the cannabinoid system

The central nervous system (CNS) innate immune response includes an arsenal of molecules and receptors expressed by professional phagocytes, glial cells and neurons that is involved in host defence and clearance of toxic and dangerous cell debris. However, any uncontrolled innate immune responses within the CNS are widely recognized as playing a major role in the development of autoimmune disorders and neurodegeneration, with multiple sclerosis (MS) Alzheimer's disease (AD) being primary examples. Hence, it is important to identify the key regulatory mechanisms involved in the control of CNS innate immunity and which could be harnessed to explore novel therapeutic avenues. Neuroimmune regulatory proteins (NIReg) such as CD95L, CD200, CD47, sialic acid, complement regulatory proteins (CD55, CD46, fH, C3a), HMGB1, may control the adverse immune responses in health and diseases. In the absence of these regulators, when neurons die by apoptosis, become infected or damaged, microglia and infiltrating immune cells are free to cause injury as well as an adverse inflammatory response in acute and chronic settings. We will herein provide new emphasis on the role of the pair CD200-CD200R in MS and its experimental models: experimental autoimmune encephalomyelitis (EAE) and Theiler’s virus induced demyelinating disease (TMEV-IDD). The interest of the cannabinoid system as inhibitor of inflammation prompt us to introduce our findings about the role of endocannabinoids (eCBs) in promoting CD200-CD200 receptor (CD200R) interaction and the benefits caused in TMEV-IDD. Finally, we also review the current data on CD200-CD200R interaction in AD, as well as, in the aging brain.

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.

What is microglia neurotoxicity (Not)?

Microglia most likely appeared early in evolution as they are not only present in vertebrates, but are also found in nervous systems of various nonvertebrate organisms. Mammalian microglia are derived from a specific embryonic, self-renewable myeloid cell population that is throughout lifetime not replaced by peripheral myeloid cells. These phylogenic and ontogenic features suggest that microglia serve vital functions. Yet, microglia often are described as neurotoxic cells, that actively kill (healthy) neurons. Since it is from an evolutionary point of view difficult to understand why an important and vulnerable organ like the brain should host numerous potential killers, we here review the concept of microglia neurotoxicity. On one hand it is discussed that most of our understanding about how microglia kill neurons is based on in vitro experiments or correlative staining studies that suffer from the difficulty to discriminate microglia and peripheral myeloid cells in the diseased brain. On the other hand it is described that a more functional approach by mutating, inactivating or deleting microglia is seldom associated with a beneficial outcome in an acute injury situation, suggesting that microglia are normally important protective elements in the brain. This might change in chronic disease or the aged brain, where; however, it remains to be established whether microglia simply lose their protective capacities or whether microglia become truly neurotoxic cells.

The brain's best friend: microglial neurotoxicity revisited

One long standing aspect of microglia biology was never questioned; their involvement in brain disease. Based on morphological changes (retracted processes and amoeboid shape) that inevitably occur in these cells in case of damage in the central nervous system, microglia in the diseased brain were called “activated.” Because “activated” microglia were always found in direct neighborhood to dead or dying neuron, and since it is known now for more than 20 years that cultured microglia release numerous factors that are able to kill neurons, microglia “activation” was often seen as a neurotoxic process. From an evolutionary point of view, however, it is difficult to understand why an important, mostly post-mitotic and highly vulnerable organ like the brain would host numerous potential killers. This review is aimed to critically reconsider the term microglia neurotoxicity and to discuss experimental problems around microglia biology, that often have led to the conclusion that microglia are neurotoxic cells.

Understanding microglia-neuron cross talk: relevance of the microglia-neuron cocultures

Microglia-neuron interaction is a complex process involving a plethora of ligands and receptors. The outcome of this intricate process will depend on the prevailing signals (i.e., whether the microglial cells will produce pro-inflammatory cytokines and/or phagocyte a dying neuron or whether it will produce neurotrophic factors and support neuronal growth, among other possible scenarios). In order to study this complex process, several tools have been developed, ranging from in vivo models (knockout and knock-in mice, conditional transgenic mice, imaging techniques) to in vitro models (microglia-neuron cocultures, transwell cell cultures). Here we describe a protocol for primary microglia-neuron coculture. this coculture allows to combine neurons and microglial cells coming from wild-type and KO mice, making this coculture a useful method to study in vitro the interaction of different sets of ligand-receptor.

RNA-Seq analysis implicates dysregulation of the immune system in schizophrenia

BACKGROUND

While genome-wide association studies identified some promising candidates for schizophrenia, the majority of risk genes remained unknown. We were interested in testing whether integration gene expression and other functional information could facilitate the identification of susceptibility genes and related biological pathways.

RESULTS

We conducted high throughput sequencing analyses to evaluate mRNA expression in blood samples isolated from 3 schizophrenia patients and 3 healthy controls. We also conducted pooled sequencing of 10 schizophrenic patients and matched controls. Differentially expressed genes were identified by t-test. In the individually sequenced dataset, we identified 198 genes differentially expressed between cases and controls, of them 19 had been verified by the pooled sequencing dataset and 21 reached nominal significance in gene-based association analyses of a genome wide association dataset. Pathway analysis of these differentially expressed genes revealed that they were highly enriched in the immune related pathways. Two genes, S100A8 and TYROBP, had consistent changes in expression in both individual and pooled sequencing datasets and were nominally significant in gene-based association analysis.

CONCLUSIONS

Integration of gene expression and pathway analyses with genome-wide association may be an efficient approach to identify risk genes for schizophrenia.

Microglial immunoreceptor tyrosine-based activation and inhibition motif signaling in neuroinflammation

Elimination of extracellular aggregates and apoptotic neural membranes without inflammation is crucial for brain tissue homeostasis. In the mammalian central nervous system, essential molecules in this process are the Fc receptors and the DAP12-associated receptors which both trigger the microglial immunoreceptor tyrosine-based activation motif- (ITAM-) Syk-signaling cascade. Microglial triggering receptor expressed on myeloid cells-2 (TREM2), signal regulatory protein-β1, and complement receptor-3 (CD11b/CD18) signal via the adaptor protein DAP12 and activate phagocytic activity of microglia. Microglial ITAM-signaling receptors are counter-regulated by immunoreceptor tyrosine-based inhibition motif- (ITIM-) signaling molecules such as sialic acid-binding immunoglobulin superfamily lectins (Siglecs). Siglecs can suppress the proinflammatory and phagocytic activity of microglia via ITIM signaling. Moreover, microglial neurotoxicity is alleviated via interaction of Siglec-11 with sialic acids on the neuronal glycocalyx. Thus, ITAM- and ITIM-signaling receptors modulate microglial phagocytosis and cytokine expression during neuroinflammatory processes. Their dysfunction could lead to impaired phagocytic clearance and neurodegeneration triggered by chronic inflammation.

SIGLEC16 encodes a DAP12-associated receptor expressed in macrophages that evolved from its inhibitory counterpart SIGLEC11 and has functional and non-functional alleles in humans

Sialic acid binding immunoglobulin-like lectins (Siglec) are important components of immune recognition. The organization of Siglec genes in different species is consistent with rapid selection imposed by pathogens. We studied SIGLEC11 genes in human, rodent, dog, cow and non-human primates. The lineages of SIGLEC11 genes in these species have undergone dynamic gene duplication and conversion, forming a potential inhibitory (SIGLEC11)/activating (SIGLEC16) receptor pair in chimpanzee and humans. A cDNA encoding human Siglec-16, currently classed as a pseudogene in the databases (SIGLECP16), is expressed in various cell lines and tissues. A polymorphism screen for the two alleles (wild type and four-base pair deletion, 4bpDelta) of SIGLEC16 found their frequencies to be 50% amongst the UK population. A search for donor sequences for SIGLEC16 revealed a subfamily of activating Siglec with charged transmembrane domains predicted to associate with ITAM-encoding adaptor proteins. In support of this, Siglec-16 was expressed at the cell surface in the presence of DAP12, but not the FcRgamma chain. Using antisera specific to the cytoplasmic tail of Siglec-16, we identified Siglec-16 expression in CD14(+) tissue macrophages and in normal human brain, cancerous oesophagus and lung. This is the first activating human Siglec receptor found to have functional and non-functional alleles within the population.

Osteoimmunology: interactions of the bone and immune system

Bone and the immune system are both complex tissues that respectively regulate the skeleton and the body's response to invading pathogens. It has now become clear that these organ systems often interact in their function. This is particularly true for the development of immune cells in the bone marrow and for the function of bone cells in health and disease. Because these two disciplines developed independently, investigators in each don't always fully appreciate the significance that the other system has on the function of the tissue they are studying. This review is meant to provide a broad overview of the many ways that bone and immune cells interact so that a better understanding of the role that each plays in the development and function of the other can develop. It is hoped that an appreciation of the interactions of these two organ systems will lead to better therapeutics for diseases that affect either or both.

Osteopetroses and immunodeficiencies in humans

PURPOSE OF REVIEW

This review focuses on human and murine pathologies involving both osteoclasts and immune cells. These diseases have been relevant to the discovery of novel interactions and pathways shared between these two types of cells.

RECENT FINDINGS

Interactions between immune cells and osteoclasts were originally shown in murine models by gene targeting of molecules involved in the early steps of osteoclast differentiation, since receptor activator of nuclear factor kappa-B ligand (RANKL), RANK and TNFR-associated factor 6 knockout mice bore abnormalities of both bone resorption and immune system. Subsequently, osteoclast stimulation by RANKL secreted by lymphocytes in autoimmune diseases, such as rheumatoid arthritis, was found. More recently, the identification of immunoreceptor tyrosine-based activation motif receptors and adaptors important for both dendritic cells and osteoclast function has established a link between innate and adaptive immunity and bone. Finally, osteoclasts are also important for hematopoietic stem-cell mobilization, providing a further level of regulation of lymphoid cells.

SUMMARY

These findings open up a new field of research, osteoimmunology, which will unravel previously unsuspected links between bone remodelling and the immune response.

The facial motor nucleus transcriptional program in response to peripheral nerve injury identifies Hn1 as a regeneration-associated gene

Facial nerve axotomy (FNA) is a well-established experimental model of motoneuron regeneration. After peripheral nerve axotomy, a sequence of events including glial activation and axonal regrowth leads to functional recovery of the afflicted pool of motoneurons. Using microarray analysis we identified an increase in the expression of 60 genes (at a false discovery rate of 0.1, genes were significant P < 0.004) within the facial nucleus as a consequence of nerve injury. In situ hybridization analysis validated the increased expression of many of these axotomy-induced genes. One specific gene, encoding a unique primary amino acid sequence, termed hemopoietic- and neurologic-expressed sequence-1 (Hn1), was evaluated more extensively using several additional nerve injury paradigms. Hn1 mRNA was upregulated in injured facial motoneurons in both rats and mice. Sustained upregulation of Hn1 mRNA was evident after nerve resection whereas levels of Hn1 mRNA returned to baseline in animals subjected to nerve crush or nerve transection. Hn1 was also increased in the dorsal motor nucleus and the nucleus ambiguous after vagus nerve axotomy, another regeneration model. No upregulation of Hn1 expression was observed, however, in two nonregeneration models: FNA in newborn rats and rubrospinal tractotomy. Hn1 mRNA was ubiquitous in the developing central nervous system whereas its expression in adult brain was confined to neurons of the hippocampus, cortex and cerebellum. These findings identify Hn1 as a gene associated with nervous system development and nerve regeneration.

Macrophages expressing triggering receptor expressed on myeloid cells-1 are underrepresented in the human intestine

Triggering receptor expressed on myeloid cells (TREM)-1 is a cell surface molecule on neutrophils and monocytes/macrophages implicated in the amplification of inflammatory responses by enhancing degranulation and secretion of proinflammatory mediators. Macrophages play an important role in the intestinal mucosal immune system, because they are preferentially localized in the subepithelial region. Despite the presence of enormous numbers of bacteria in the colonic mucosa and the close proximity between mucosal macrophages and luminal bacteria, the intestinal mucosa normally displays minimal signs of inflammation. In this study, we show that the resident macrophage population in normal human small and large intestine contains only few TREM-1-expressing macrophages (<10%), whereas the overwhelming majority of monocytes (>90%) and macrophages from lymph nodes or tonsils (>80%) express TREM-1 on the cell surface. These findings were confirmed by FACS analysis and immunostainings of frozen tissue sections. The differential expression of TREM-1 greatly affects the functional capacities of monocytes and tissue macrophages. Although monocytes and macrophages from spleen, lymph nodes, or tonsils show a substantial increase in oxidative burst after TREM-1 cross-linking, no effect is seen in intestinal macrophages. Intriguingly, in contrast to monocytes, intestinal macrophages fail to up-regulate TREM-1 in response to TNF. This refractory state may be induced in intestinal macrophages by the local presence of IL-10 and TGF-beta, because these two immunoregulatory cytokines synergistically down-regulate TREM-1 expression on monocytes in vitro. The absence of TREM-1 expression on lamina propria macrophages is likely to prevent excessive inflammatory reactions, and thus, excessive tissue damage in the intestine.

Fugu rubripes possesses genes for the entire set of the ITAM-bearing transmembrane signal subunits

The transmembrane signaling subunits (TSSs) bearing the immunoreceptor tyrosine-based activation motif (ITAM) play a crucial role in triggering the effector functions of mammalian leukocytes. The involvement in key immune reactions and obvious extension through duplication events make TSSs valuable markers of the evolution of the immune system. We surveyed the genomic sequences of the teleostean fish Fugu rubripes for the presence of genes encoding these accessory molecules. Automatic gene prediction was not efficient because of the poor ability of the programs used to recognize the short exons encoding the intracellular regions of TSSs. However, the unique compactness of the Fugu genome and the conservation of the exon/intron arrangements of the TSS genes facilitated their recognition by visual inspection of the candidate genomic sequences. Evidence for the presence of the CD3epsilon, CD3gamma/delta, CD79a, CD79b, TCRzeta, FcRgamma, DAP12 and DAP10 genes in the Fugu genome was obtained. Furthermore, conserved synteny for the short regions including the TSS genes was revealed by comparison of the Fugu and human genomes. The data demonstrate that the set of TSSs arose before the teleost-tetrapod split and provide a starting point for experimental investigation of the molecular evolution of the leukocyte-activating receptor complexes from fish species to mammals.