Microglial Inflammatory Mechanisms in Stroke: The Jury Is Still Out.

Microglia represent the main immune cell population in the CNS with unique homeostatic roles and contribution to broad neurological conditions. Stroke is associated with marked changes in microglial phenotypes and induction of inflammatory responses, which emerge as key modulators of brain injury, neurological outcome and regeneration. However, due to the limited availability of functional studies with selective targeting of microglia and microglia-related inflammatory pathways in stroke, the vast majority of observations remain correlative and controversial. Because extensive review articles discussing the role of inflammatory mechanisms in different forms of acute brain injury are available, here we focus on some specific pathways that appear to be important for stroke pathophysiology with assumed contribution by microglia. While the growing toolkit for microglia manipulation increasingly allows targeting inflammatory pathways in a cell-specific manner, reconsideration of some effects devoted to microglia may also be required. This may particularly concern the interpretation of inflammatory mechanisms that emerge in response to stroke as a form of sterile injury and change markedly in chronic inflammation and common stroke comorbidities.

HLA DQ protein changes the cell surface distribution pattern of HLA proteins as monitored by Förster resonance energy transfer and high-resolution electron microscopy.

Peptide presentation by MHC class I and MHC class II molecules plays important roles in the regulation of the immune response. One factor in these displays is the density of antigen, which must exceed a critical threshold for the effective activation of T cells. Nonrandom distribution of MHC class I and class II has already been detected at the nanometer level and at higher hierarchical levels. It is not clear how the absence and reappearance of some protein molecules can influence the nonrandom distribution. Therefore, we performed experiments on HLA II-deficient bare lymphocyte syndrome (BLS1) cells: we created a stable transfected cell line, tDQ6-BLS-1, and were able to detect the effect of the appearance of HLA-DQ6 molecules on the homo and heteroassociation of different cell surface molecules by comparing Förster resonance energy transfer (FRET) efficiency on transfected cells to that on nontransfected BLS-1 and JY human B-cell lines. Our FRET results show a decrease in homoassociation FRET between HLA I chains in HLA-DQ6-transfected tDQ6-BLS-1 cells compared with the parent BLS-1 cell line and an increase in heteroassociation FRET between HLA I and HLA II (compared with JY cells), suggesting a similar pattern of antigen presentation by the HLA-DQ6 allele. Transmission electron microscopy (TEM) revealed that both HLA class I and class II molecules formed clusters at higher hierarchical levels on the tDQ6-BLS-1 cells, and the de novo synthesized HLA DQ molecules did not intersperse with HLA class I islands. These observations could be important in understanding the fine tuning of the immune response.

Early suppression of antiviral host response and protocadherins by SARS-CoV-2 Spike protein in THP-1-derived macrophage-like cells.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of coronavirus disease-19 (COVID-19). The spike protein (S) of SARS-CoV-2 plays a crucial role in mediating viral infectivity; hence, in an extensive effort to curb the pandemic, many urgently approved vaccines rely on the expression of the S protein, aiming to induce a humoral and cellular response to protect against the infection. Given the very limited information about the effects of intracellular expression of the S protein in host cells, we aimed to characterize the early cellular transcriptomic changes induced by expression of the S protein in THP-1-derived macrophage-like cells. Results showed that a wide variety of genes were differentially expressed, products of which are mainly involved in cell adhesion, homeostasis, and most notably, antiviral and immune responses, depicted by significant downregulation of protocadherins and type I alpha interferons (IFNAs). While initially, the levels of IFNAs were higher in the medium of S protein expressing cells, the downregulation observed on the transcriptomic level might have been reflected by no further increase of IFNA cytokines beyond the 5 h time-point, compared to the mock control. Our study highlights the intrinsic pathogenic role of the S protein and sheds some light on the potential drawbacks of its utilization in the context of vaccination strategies.

Whole blood transcriptome characterization of young female triathlon athletes following an endurance exercise: a pilot study.

The vast majority of studies focusing on the effects of endurance exercise on hematological parameters and leukocyte gene expression were performed in adult men, so our aim was to investigate these changes in young females. Four young (age 15.3 ± 1.3 yr) elite female athletes completed an exercise session, in which they accomplished the cycling and running disciplines of a junior triathlon race. Blood samples were taken immediately before the exercise, right after the exercise, and then 1, 2, and 7 days later. Analysis of cell counts and routine biochemical parameters were complemented by RNA sequencing (RNA-seq) to whole blood samples. The applied exercise load did not trigger remarkable changes in either cardiovascular or biochemical parameters; however, it caused a significant increase in the percentage of neutrophils and a significant reduction in the ratio of lymphocytes immediately after exercise. Furthermore, endurance exercise induced a characteristic gene expression pattern change in the blood transcriptome. Gene set enrichment analysis (GSEA) using the Reactome database revealed that the expression of genes involved in immune processes and neutrophil granulocyte activation was upregulated, whereas the expression of genes important in translation and rRNA metabolism was downregulated. Comparison of a set of immune cell gene signatures (ImSig) and our transcriptomic data identified 15 overlapping genes related to T-cell functions and involved in podosome formation and adhesion to the vessel wall. Our results suggest that RNA-seq to whole blood together with ImSig analysis are useful tools for the investigation of systemic responses to endurance exercise.

The NKCC1 ion transporter modulates microglial phenotype and inflammatory response to brain injury in a cell-autonomous manner.

The NKCC1 ion transporter contributes to the pathophysiology of common neurological disorders, but its function in microglia, the main inflammatory cells of the brain, has remained unclear to date. Therefore, we generated a novel transgenic mouse line in which microglial NKCC1 was deleted. We show that microglial NKCC1 shapes both baseline and reactive microglia morphology, process recruitment to the site of injury, and adaptation to changes in cellular volume in a cell-autonomous manner via regulating membrane conductance. In addition, microglial NKCC1 deficiency results in NLRP3 inflammasome priming and increased production of interleukin-1ß (IL-1ß), rendering microglia prone to exaggerated inflammatory responses. In line with this, central (intracortical) administration of the NKCC1 blocker, bumetanide, potentiated intracortical lipopolysaccharide (LPS)-induced cytokine levels. In contrast, systemic bumetanide application decreased inflammation in the brain. Microglial NKCC1 KO animals exposed to experimental stroke showed significantly increased brain injury, inflammation, cerebral edema and worse neurological outcome. Thus, NKCC1 emerges as an important player in controlling microglial ion homeostasis and inflammatory responses through which microglia modulate brain injury. The contribution of microglia to central NKCC1 actions is likely to be relevant for common neurological disorders.

Caffeine Has Different Immunomodulatory Effect on the Cytokine Expression and NLRP3 Inflammasome Function in Various Human Macrophage Subpopulations.

Besides its well-known psychoactive effects, caffeine has a broad range of actions. It regulates several physiological mechanisms as well as modulates both native and adaptive immune responses by various ways. Although caffeine is assumed to be a negative regulator of inflammation, the effect on the secretion of pro- and anti-inflammatory cytokines is highly controversial. Macrophages are major mediators of inflammatory responses; however, the various subpopulations develop different effects ranging from the initiation to the resolution of inflammation. Here we report a comparative analysis of the effect of caffeine on two subpopulations of human monocyte-derived macrophages differentiated in the presence of macrophage colony-stimulating factor (M-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF), resulting in M-MΦs and GM-MΦs, respectively. We showed that although TNF-α secretion was downregulated in both LPS-activated MΦ subtypes by caffeine, the secretion of IL-8, IL-6, and IL-1ß as well as the expression of Nod-like receptors was enhanced in M-MΦs, while it did not change in GM-MΦs. We showed that caffeine (1) altered adenosine receptor expression, (2) changed Akt/AMPK/mTOR signaling pathways, and (3) inhibited STAT1/IL-10 signaling axis in M-MΦs. We hypothesized that these alterations play an important modulatory role in the upregulation of NLRP3 inflammasome-mediated IL-1ß secretion in LPS-activated M-MΦs following caffeine treatment.

Autoinflammation and autoimmunity across rheumatic and musculoskeletal diseases.

Most rheumatic and musculoskeletal diseases (RMDs) can be placed along a spectrum of disorders, with autoinflammatory diseases (including monogenic systemic autoinflammatory diseases) and autoimmune diseases (such as systemic lupus erythematosus and antiphospholipid syndrome) representing the two ends of this spectrum. However, although most autoinflammatory diseases are characterized by the activation of innate immunity and inflammasomes and classical autoimmunity typically involves adaptive immune responses, there is some overlap in the features of autoimmunity and autoinflammation in RMDs. Indeed, some 'mixed-pattern' diseases such as spondyloarthritis and some forms of rheumatoid arthritis can also be delineated. A better understanding of the pathogenic pathways of autoinflammation and autoimmunity in RMDs, as well as the preferential cytokine patterns observed in these diseases, could help us to design targeted treatment strategies.

Nuclear Receptors as Multiple Regulators of NLRP3 Inflammasome Function.

Nuclear receptors are important bridges between lipid signaling molecules and transcription responses. Beside their role in several developmental and physiological processes, many of these receptors have been shown to regulate and determine the fate of immune cells, and the outcome of immune responses under physiological and pathological conditions. While NLRP3 inflammasome is assumed as key regulator for innate and adaptive immune responses, and has been associated with various pathological events, the precise impact of the nuclear receptors on the function of inflammasome is hardly investigated. A wide variety of factors and conditions have been identified as modulators of NLRP3 inflammasome activation, and at the same time, many of the nuclear receptors are known to regulate, and interact with these factors, including cellular metabolism and various signaling pathways. Nuclear receptors are in the focus of many researches, as these receptors are easy to manipulate by lipid soluble molecules. Importantly, nuclear receptors mediate regulatory mechanisms at multiple levels: not only at transcription level, but also in the cytosol via non-genomic effects. Their importance is also reflected by the numerous approved drugs that have been developed in the past decade to specifically target nuclear receptors subtypes. Researches aiming to delineate mechanisms that regulate NLRP3 inflammasome activation draw a wide range of attention due to their unquestionable importance in infectious and sterile inflammatory conditions. In this review, we provide an overview of current reports and knowledge about NLRP3 inflammasome regulation from the perspective of nuclear receptors, in order to bring new insight to the potentially therapeutic aspect in targeting NLRP3 inflammasome and NLRP3 inflammasome-associated diseases.

Focusing on the Cell Type Specific Regulatory Actions of NLRX1.

Cells utilize a diverse repertoire of cell surface and intracellular receptors to detect exogenous or endogenous danger signals and even the changes of their microenvironment. However, some cytosolic NOD-like receptors (NLR), including NLRX1, serve more functions than just being general pattern recognition receptors. The dynamic translocation between the cytosol and the mitochondria allows NLRX1 to interact with many molecules and thereby to control multiple cellular functions. As a regulatory NLR, NLRX1 fine-tunes inflammatory signaling cascades, regulates mitochondria-associated functions, and controls metabolism, autophagy and cell death. Nevertheless, literature data are inconsistent and often contradictory regarding its effects on individual cellular functions. One plausible explanation might be that the regulatory effects of NLRX1 are highly cell type specific and the features of NLRX1 mediated regulation might be determined by the unique functional activity or metabolic profile of the given cell type. Here we review the cell type specific actions of NLRX1 with a special focus on cells of the immune system. NLRX1 has already emerged as a potential therapeutic target in numerous immune-related diseases, thus we aim to highlight which regulatory properties of NLRX1 are manifested in disease-associated dominant immune cells that presumably offer promising therapeutic solutions to treat these disorders.

All-Trans Retinoic Acid Enhances both the Signaling for Priming and the Glycolysis for Activation of NLRP3 Inflammasome in Human Macrophage.

All-trans retinoic acid (ATRA) is a derivative of vitamin A that has many important biological functions, including the modulation of immune responses. ATRA actions are mediated through the retinoic acid receptor that functions as a nuclear receptor, either regulating gene transcription in the nucleus or modulating signal transduction in the cytoplasm. NLRP3 inflammasome is a multiprotein complex that is activated by a huge variety of stimuli, including pathogen- or danger-related molecules. Activation of the inflammasome is required for the production of IL-1ß, which drives the inflammatory responses of infectious or non-infectious sterile inflammation. Here, we showed that ATRA prolongs the expression of IL-6 and IL-1ß following a 2-, 6-, 12-, and 24-h LPS (100ng/mL) activation in human monocyte-derived macrophages. We describe for the first time that ATRA modulates both priming and activation signals required for NLRP3 inflammasome function. ATRA alone induces NLRP3 expression, and enhances LPS-induced expression of NLRP3 and pro-IL-1ß via the regulation of signal transduction pathways, like NF-κB, p38, and ERK. We show that ATRA alleviates the negative feedback loop effect of IL-10 anti-inflammatory cytokine on NLRP3 inflammasome function by inhibiting the Akt-mTOR-STAT3 signaling axis. We also provide evidence that ATRA enhances hexokinase 2 expression, and shifts the metabolism of LPS-activated macrophages toward glycolysis, leading to the activation of NLRP3 inflammasome.

Hematopoietic or Osteoclast-Specific Deletion of Syk Leads to Increased Bone Mass in Experimental Mice.

Syk is a non-receptor tyrosine kinase critically involved in signaling by various immunoreceptors including B-cell-receptors and activating Fc-receptors. We have previously shown that Syk also mediates immunoreceptor-like signals required for the in vitro development and function of osteoclasts. However, the perinatal lethality of Syk-/- mice precluded the analysis of the role of Syk in in vivo bone metabolism. To overcome that problem, we generated mice with osteoclast-specific (SykΔOC ) or hematopoietic (SykΔHaemo ) Syk deficiency by conditional deletion of Syk using Cre recombinase expressed under the control of the Ctsk or Vav1 promoter, respectively. Micro-CT analysis revealed increased bone trabecular density in both SykΔOC and SykΔHaemo mice, although hematopoietic Syk deficiency caused a more severe phenotype than osteoclast-specific Syk deficiency. Osteoclast-specific Syk deficiency reduced, whereas hematopoietic Syk deficiency completely blocked in vitro development of osteoclasts. Both interventions inhibited the resorptive activity of osteoclasts and osteoclast-specific gene expression. Kinetic analysis of Syk protein levels, Cre expression and the genomic deletion of the Sykflox allele revealed complete and early deletion of Syk from SykΔHaemo osteoclasts whereas Syk was incompletely deleted at a later stage of osteoclast development from SykΔOC cultures. Those results provide an explanation for the in vivo and in vitro difference between the SykΔOC and SykΔHaemo mutant strains and suggest late activation of, and incomplete target gene deletion upon, osteoclast-specific Cre expression driven by the Ctsk promoter. Taken together, our results indicate that Syk plays an indispensable role in osteoclast-mediated in vivo bone resorption and suggest that Syk-specific inhibitors may provide therapeutic benefit in inflammatory and other diseases characterized by excessive osteoclast-mediated bone resorption.

The NLRP3 inflammasome - interleukin 1 pathway as a therapeutic target in gout.

The NLRP3 inflammasome is implicated in the processing of the pro-inflammatory cytokine interleukin 1ß. Inflammatory disorders associated with the activation of the NLRP3 inflammasome - IL-1 axis are termed autoinflammatory diseases. Gout is an autoinflammatory disease, which is triggered by the deposition of monosodium urate crystals of precipitated uric acid. It is characterized by recurrent attacks of inflammation due to the activation of phagocytic cells that try to clear the crystals. NLRP3 inflammasome-mediated IL-1ß production plays a key role in the manifestation of the disease. Currently, the best approach to treat gout is to reduce uric acid concentration by targeting xanthine oxidase or uric acid transporters, or to use non-steroidal anti-inflammatory drugs. Nevertheless, most of these treatments are not effective enough and may results in side effects. During the past decades, our knowledge has greatly improved about the molecular mechanisms of NLRP3 activation. This knowledge enables and urges scientists to discover or design drugs that target pathways of NLRP3 inflammasome activation, or more preferentially, NLRP3 inflammasome itself. In this review, we discuss the already available drugs and products, that target the diverse pathways of the NLRP3 - IL-1ß axis, and the future therapeutic perspectives.

[Recent advances in gout: pathogenesis, diagnosis, comorbidities, treatment]. / Újdonságok köszvényben: patogenezis, társbetegségek, diagnosztika és terápia.

After a "silence" period for decades, a great body of new information has become available about the pathogenesis, diagnosis and treatment of gout. New data on purine metabolism and urate transporters have been published. It has become evident that gout is an autoinflammatory disease involving the inflammasome and interleukin-1. With respect to diagnosis, microscopic evaluation of the urate crystal is still the gold standard, however, sensitive imaging techniques (ultrasound, modern computed tomography methods) are able to visualize crystal deposition and tophus formation. Tophus size may also be monitored over time. We see a renaissance of non-pharmacological, lifestyle-related treatment modalities. Pharmacotherapy includes the resolution of attacks and urate-lowering maintenance therapy. In 2016, two recent series of recommendations have been published. Treat-to-target therapy aiming at urate levels ≤360 µmol/l is crucial. Urate-lowering therapy includes xanthine oxidase inhibitors (allopurinol, febuxostat). However, a number of novel compounds (urate transporter inhibitors, recombinant uricase, interleukin-1 inhibitors) are under development or before introduction to gout treatment. Comorbidites should be considered throughout the follow-up of gout patients. Orv Hetil. 2018; 159(40): 1625-1636.

The Transcription Factor STAT6 Mediates Direct Repression of Inflammatory Enhancers and Limits Activation of Alternatively Polarized Macrophages.

The molecular basis of signal-dependent transcriptional activation has been extensively studied in macrophage polarization, but our understanding remains limited regarding the molecular determinants of repression. Here we show that IL-4-activated STAT6 transcription factor is required for the direct transcriptional repression of a large number of genes during in vitro and in vivo alternative macrophage polarization. Repression results in decreased lineage-determining transcription factor, p300, and RNA polymerase II binding followed by reduced enhancer RNA expression, H3K27 acetylation, and chromatin accessibility. The repressor function of STAT6 is HDAC3 dependent on a subset of IL-4-repressed genes. In addition, STAT6-repressed enhancers show extensive overlap with the NF-κB p65 cistrome and exhibit decreased responsiveness to lipopolysaccharide after IL-4 stimulus on a subset of genes. As a consequence, macrophages exhibit diminished inflammasome activation, decreased IL-1ß production, and pyroptosis. Thus, the IL-4-STAT6 signaling pathway establishes an alternative polarization-specific epigenenomic signature resulting in dampened macrophage responsiveness to inflammatory stimuli.

NLRC5 Functions beyond MHC I Regulation-What Do We Know So Far?

NLRC5 is a member of the NLR family that acts as a transcriptional activator of MHC class I genes. In line with the function of several related NLR proteins in innate immune responses, there is, however, also ample evidence that NLRC5 contributes to innate and adaptive immune responses beyond the regulation of MHC class I genes. In human and murine cells, for example, NLRC5 was proposed to contribute to inflammatory and type I interferon responses. The role of NLRC5 in these and other cellular processes is hitherto still not well understood and blurred by discrepancies in the reported data. Here, we provide a detailed and critical discussion of the available experimental data on the emerging biological functions of NLRC5 in innate immune responses in men and mice. Better awareness of the multiple roles of NLRC5 will help to define its overall contribution to immune responses and cancer.

Different dynamics of NLRP3 inflammasome-mediated IL­1ß production in GM-CSF- and M-CSF-differentiated human macrophages.

IL-1ß is a "master" cytokine regulating a wide variety of physiologic and immunologic processes. The most frequently studied models for NLRP3 inflammasome-mediated IL-1ß production are the macrophages; however, depending on their microenvironment, they can develop into functionally different cells. Several protocols have been developed to model the diversity of these cells in vitro. Here, we report for the first time, to our knowledge, a comparative study about the dynamics and molecular mechanisms of NLRP3 inflammasome priming and activation in LPS-stimulated, human, monocyte-derived GM- or M-macrophages, differentiated in the presence of GM-CSF or M-CSF, respectively. Our results show that IL-1ß production by LPS-stimulated M-macrophages is a rapid and short event that requires ATP supplementation and is attenuated, in part, by the presence of IL-10, which reduces Akt signaling. However, IL-1ß production by GM-macrophages develops gradually, and these cells produce IL-1ß, even in the absence of ATP supplementation, because of the constitutively active caspase-1 enzyme. We show that the membrane-bound ectonucleotidases have an important regulatory role on the IL-1ß secretion in GM-macrophages. Furthermore, we provide evidence that adenosine treatment enhances LPS-primed IL-1ß secretion by GM-macrophages, but not by M-macrophages. These results show that, because of the different activation status and expression levels of the NLRP3 inflammasome components, as well as the signaling activity of the pathways, the two subtypes of macrophages respond very differently to the same stimuli. For this reason, the molecular composition of the microenvironment that shapes macrophage development should be considered when research or therapeutic methods are planned to control IL-1ß production.

Immunogenic Dendritic Cell Generation from Pluripotent Stem Cells by Ectopic Expression of Runx3.

Application of dendritic cells (DCs) to prime responses to tumor Ags provides a promising approach to immunotherapy. However, only a limited number of DCs can be manufactured from adult precursors. In contrast, pluripotent embryonic stem (ES) cells represent an inexhaustible source for DC production, although it remains a major challenge to steer directional differentiation because ES cell-derived cells are typically immature with impaired functional capacity. Consistent with this notion, we found that mouse ES cell-derived DCs (ES-DCs) represented less mature cells compared with bone marrow-derived DCs. This finding prompted us to compare the gene expression profile of the ES cell- and adult progenitor-derived, GM-CSF-instructed, nonconventional DC subsets. We quantified the mRNA level of 17 DC-specific transcription factors and observed that 3 transcriptional regulators (Irf4, Spi-B, and Runx3) showed lower expression in ES-DCs than in bone marrow-derived DCs. In light of this altered gene expression, we probed the effects of these transcription factors in developing mouse ES-DCs with an isogenic expression screen. Our analysis revealed that forced expression of Irf4 repressed ES-DC development, whereas, in contrast, Runx3 improved the ES-DC maturation capacity. Moreover, LPS-treated and Runx3-activated ES-DCs exhibited enhanced T cell activation and migratory potential. In summary, we found that ex vivo-generated ES-DCs had a compromised maturation ability and immunogenicity. However, ectopic expression of Runx3 enhances cytokine-driven ES-DC development and acts as an instructive tool for the generation of mature DCs with enhanced immunogenicity from pluripotent stem cells.

Natural Compounds as Regulators of NLRP3 Inflammasome-Mediated IL-1ß Production.

IL-1ß is one of the main proinflammatory cytokines that regulates a broad range of immune responses and also participates in several physiological processes. The canonical production of IL-1ß requires multiprotein complexes called inflammasomes. One of the most intensively studied inflammasome complexes is the NLRP3 inflammasome. Its activation requires two signals: one signal "primes" the cells and induces the expression of NLRP3 and pro-IL-1ß, while the other signal leads to the assembly and activation of the complex. Several stimuli were reported to function as the second signal including reactive oxygen species, lysosomal rupture, or cytosolic ion perturbation. Despite very intensive studies, the precise function and regulation of the NLRP3 inflammasome are still not clear. However, many chronic inflammatory diseases are related to the overproduction of IL-1ß that is mediated via the NLRP3 inflammasome. In this review, we aimed to provide an overview of studies that demonstrated the effect of plant-derived natural compounds on NLRP3 inflammasome-mediated IL-1ß production. Although many of these studies lack the mechanistic explanation of their action, these compounds may be considered as complementary supplements in the treatment of chronic inflammatory diseases, consumed as preventive agents, and may also be considered as molecular tools to study NLRP3 function.

The phosphoinositide 3-kinase isoform PI3Kß regulates osteoclast-mediated bone resorption in humans and mice.

OBJECTIVE: While phosphoinositide 3-kinases (PI3Ks) are involved in various intracellular signal transduction processes, the specific functions of the different PI3K isoforms are poorly understood. We have previously shown that the PI3Kß isoform is required for arthritis development in the K/BxN serum-transfer model. Since osteoclasts play a critical role in pathologic bone loss during inflammatory arthritis and other diseases, we undertook this study to test the role of PI3Kß in osteoclast development and function using a combined genetic and pharmacologic approach. METHODS: The role of PI3Kß in primary human and murine osteoclast cultures was tested with the PI3Kß-selective inhibitor TGX221 and by using PI3Kß(-/-) mice. The trabecular bone architecture of PI3Kß(-/-) mice was evaluated using micro-computed tomography and histomorphometric analyses. RESULTS: The expression of PI3Kß was strongly and specifically up-regulated during in vitro osteoclast differentiation. In vitro development of large multinucleated osteoclasts from human or murine progenitors and their resorption capacity were strongly reduced by the PI3Kß inhibitor TGX221 or by the genetic deficiency of PI3Kß. This was likely due to defective cytoskeletal reorganization and vesicular trafficking, since PI3Kß(-/-) mouse multinucleated cells failed to form actin rings and retained intracellular acidic vesicles and cathepsin K. In contrast, osteoclast-specific gene expression and the survival and apoptosis of osteoclasts were not affected. PI3Kß(-/-) mice had significantly increased trabecular bone volume and showed abnormal osteoclast morphology with defective resorption pit formation. CONCLUSION: PI3Kß plays an important role in osteoclast development and function and is required for in vivo bone homeostasis.