Entry receptor LDLRAD3 is required for Venezuelan equine encephalitis virus peripheral infection and neurotropism leading to pathogenesis in mice.

Venezuelan equine encephalitis virus (VEEV) is an encephalitic alphavirus responsible for epidemics of neurological disease across the Americas. Low-density lipoprotein receptor class A domain-containing 3 (LDLRAD3) is a recently reported entry receptor for VEEV. Here, using wild-type and Ldlrad3-deficient mice, we define a critical role for LDLRAD3 in controlling steps in VEEV infection, pathogenesis, and neurotropism. Our analysis shows that LDLRAD3 is required for efficient VEEV infection and pathogenesis prior to and after central nervous system invasion. Ldlrad3-deficient mice survive intranasal and intracranial VEEV inoculation and show reduced infection of neurons in different brain regions. As LDLRAD3 is a determinant of pathogenesis and an entry receptor required for VEEV infection of neurons of the brain, receptor-targeted therapies may hold promise as countermeasures.

Zika virus spreads through infection of lymph node-resident macrophages.

To disseminate through the body, Zika virus (ZIKV) is thought to exploit the mobility of myeloid cells, in particular monocytes and dendritic cells. However, the timing and mechanisms underlying shuttling of the virus by immune cells remains unclear. To understand the early steps in ZIKV transit from the skin, at different time points, we spatially mapped ZIKV infection in lymph nodes (LNs), an intermediary site en route to the blood. Contrary to prevailing hypotheses, migratory immune cells are not required for the virus to reach the LNs or blood. Instead, ZIKV rapidly infects a subset of sessile CD169+ macrophages in the LNs, which release the virus to infect downstream LNs. Infection of CD169+ macrophages alone is sufficient to initiate viremia. Overall, our experiments indicate that macrophages that reside in the LNs contribute to initial ZIKV spread. These studies enhance our understanding of ZIKV dissemination and identify another anatomical site for potential antiviral intervention.

Fracture healing is delayed in the absence of gasdermin-interleukin-1 signaling.

Amino-terminal fragments from proteolytically cleaved gasdermins (GSDMs) form plasma membrane pores that enable the secretion of interleukin-1ß (IL-1ß) and IL-18. Excessive GSDM-mediated pore formation can compromise the integrity of the plasma membrane thereby causing the lytic inflammatory cell death, pyroptosis. We found that GSDMD and GSDME were the only GSDMs that were readily expressed in bone microenvironment. Therefore, we tested the hypothesis that GSDMD and GSDME are implicated in fracture healing owing to their role in the obligatory inflammatory response following injury. We found that bone callus volume and biomechanical properties of injured bones were significantly reduced in mice lacking either GSDM compared with wild-type (WT) mice, indicating that fracture healing was compromised in mutant mice. However, compound loss of GSDMD and GSDME did not exacerbate the outcomes, suggesting shared actions of both GSDMs in fracture healing. Mechanistically, bone injury induced IL-1ß and IL-18 secretion in vivo, a response that was mimicked in vitro by bone debris and ATP, which function as inflammatory danger signals. Importantly, the secretion of these cytokines was attenuated in conditions of GSDMD deficiency. Finally, deletion of IL-1 receptor reproduced the phenotype of Gsdmd or Gsdme deficient mice, implying that inflammatory responses induced by the GSDM-IL-1 axis promote bone healing after fracture.

IκB-ζ signaling promotes chondrocyte inflammatory phenotype, senescence, and erosive joint pathology.

Osteoarthritis is a joint disease characterized by a poorly-defined inflammatory response that does not encompass a massive immune cell infiltration yet contributes to cartilage degradation and loss of joint mobility, suggesting a chondrocyte intrinsic inflammatory response. Using primary chondrocytes from joints of osteoarthritic mice and patients, we first show that these cells express ample pro-inflammatory markers and RANKL in an NF-κB dependent manner. The inflammatory phenotype of chondrocytes was recapitulated by exposure of chondrocytes to IL-1ß and bone particles, which were used to model bone matrix breakdown products revealed to be present in synovial fluid of OA patients, albeit their role was not defined. We further show that bone particles and IL-1ß can promote senescent and apoptotic changes in primary chondrocytes due to oxidative stress from various cellular sources such as the mitochondria. Finally, we provide evidence that inflammation, oxidative stress and senescence converge upon IκB-ζ, the principal mediator downstream of NF-κB, which regulates expression of RANKL, inflammatory, catabolic, and SASP genes. Overall, this work highlights the capacity and mechanisms by which inflammatory cues, primarily joint degradation products, i.e., bone matrix particles in concert with IL-1ß in the joint microenvironment, program chondrocytes into an "inflammatory phenotype" which inflects local tissue damage.

Gasdermin D deficiency attenuates arthritis induced by traumatic injury but not autoantibody-assembled immune complexes.

BACKGROUND: Gasdermin D (GSDMD) is cleaved by several proteases including by caspase-1, a component of intracellular protein complexes called inflammasomes. Caspase-1 also converts pro-interleukin-1ß (pro-IL-1ß) and pro-IL-18 into bioactive IL-1ß and IL-18, respectively. GSDMD amino-terminal fragments form plasma membrane pores, which mediate the secretion of IL-1ß and IL-18 and cause the inflammatory form of cell death pyroptosis. Here, we tested the hypothesis that GSDMD contributes to joint degeneration in the K/BxN serum transfer-induced arthritis (STIA) model in which autoantibodies against glucose-6-phosphate isomerase promote the formation of pathogenic immune complexes on the surface of myeloid cells, which highly express the inflammasomes. The unexpected outcomes with the STIA model prompted us to determine the role of GSDMD in the post-traumatic osteoarthritis (PTOA) model caused by meniscus ligamentous injury (MLI) based on the hypothesis that this pore-forming protein is activated by signals released from damaged joint tissues. METHODS: Gsdmd +/+ and Gsdmd-/- mice were injected with K/BxN mouse serum or subjected to MLI to cause STIA or PTOA, respectively. Paw and ankle swelling and DXA scanning were used to assess the outcomes in the STIA model whereas histopathology and micro-computed tomography (µCT) were utilized to monitor joints in the PTOA model. Murine and human joint tissues were also examined for GSDMD, IL-1ß, and IL-18 expression by qPCR, immunohistochemistry, or immunoblotting. RESULTS: GSDMD levels were higher in serum-inoculated paws compared to PBS-injected paws. Unexpectedly, ablation of GSDMD failed to reduce joint swelling and osteolysis, suggesting that GSDMD was dispensable for the pathogenesis of STIA. GSDMD levels were also higher in MLI compared to sham-operated joints. Importantly, ablation of GSDMD attenuated MLI-associated cartilage degradation (p = 0.0097), synovitis (p = 0.014), subchondral bone sclerosis (p = 0.0006), and subchondral bone plate thickness (p = 0.0174) based on histopathological and µCT analyses. CONCLUSION: GSDMD plays a key role in the pathogenesis of PTOA, but not STIA, suggesting that its actions in experimental arthropathy are tissue context-specific.

NLRP3 inflammasome activation triggers gasdermin D-independent inflammation.

NOD-like receptor (NLR), family pyrin domain containing 3 (NLRP3) assembles a protein complex known as the NLRP3 inflammasome upon sensing certain pathogen products or sterile danger signals. Gain-of-function mutations such as the D301N substitution in NLRP3, which cause its constitutive activation (NLRP3CA) also results in inflammasome assembly. This inflammasome processes pro­interleukin-1 ß (pro­IL-1ß) and pro­IL-18 into bioactive IL-1ß and IL-18, respectively, and cleaves gasdermin D (GSDMD). GSDMD amino-terminal fragments form plasma membrane pores that facilitate the secretion of IL-1ß and IL-18 and lead to the inflammatory cell death pyroptosis. Accordingly, GSDMD inactivation results in negligible spontaneous inflammation in various experimental models such as in Nlrp3CA/+ mice lacking GSDMD (Nlrp3CA/+;Gsdmd−/− mice). Here, we found that Nlrp3CA/+;Gsdmd−/− mice, when challenged with LPS or TNF-α, still secreted IL-1ß and IL-18, indicating inflammasome activation independent of GSDMD. Accordingly, Gsdmd−/− macrophages failed to secrete IL-1ß and undergo pyroptosis when briefly exposed to NLRP3 inflammasome activators but released these cytokines when persistently activated. Sustained NLRP3 inflammasome induced caspase-8/-3 and GSDME cleavage and IL-1ß maturation in vitro in Gsdmd−/− macrophages. Thus, a salvage inflammatory pathway involving caspase-8/-3­GSDME was activated after NLRP3 activation when the canonical NLRP3-GSDMD signaling was blocked. Consistent with genetic data, the active metabolite of FDA-approved disulfiram CuET, which inhibited GSDMD and GSDME cleavage in macrophages, reduced the severe inflammation and tissue damage that occurred in the Nlrp3CA/+ mice. Thus, NLRP3 inflammasome activation overwhelms the protection afforded by GSDMD deficiency, rewiring signaling cascades through mechanisms that include GSDME to propagate inflammation.

Actions of the NLRP3 and NLRC4 inflammasomes overlap in bone resorption.

Overwhelming evidence indicates that excessive stimulation of innate immune receptors of the NOD-like receptor (NLR) family causes significant damage to multiple tissues, yet the role of these proteins in bone metabolism is not well known. Here, we studied the interaction between the NLRP3 and NLRC4 inflammasomes in bone homeostasis and disease. We found that loss of NLRP3 or NLRC4 inflammasome attenuated osteoclast differentiation in vitro. At the tissue level, lack of NLRP3, or NLRC4 to a lesser extent, resulted in higher baseline bone mass compared to wild-type (WT) mice, and conferred protection against LPS-induced inflammatory osteolysis. Bone mass accrual in mutant mice correlated with lower serum IL-1ß levels in vivo. Unexpectedly, the phenotype of Nlrp3-deficient mice was reversed upon loss of NLRC4 as bone mass was comparable between WT mice and Nlrp3;Nlrc4 knockout mice. Thus, although bone homeostasis is perturbed to various degrees by the lack of NLRP3 or NLRC4, this tissue appears to function normally upon compound loss of the inflammasomes assembled by these receptors.

Gain-of-Function Lrp5 Mutation Improves Bone Mass and Strength and Delays Hyperglycemia in a Mouse Model of Insulin-Deficient Diabetes.

High fracture rate and high circulating levels of the Wnt inhibitor, sclerostin, have been reported in diabetic patients. We studied the effects of Wnt signaling activation on bone health in a mouse model of insulin-deficient diabetes. We introduced the sclerostin-resistant Lrp5A214V mutation, associated with high bone mass, in mice carrying the Ins2Akita mutation (Akita), which results in loss of beta cells, insulin deficiency, and diabetes in males. Akita mice accrue less trabecular bone mass with age relative to wild type (WT). Double heterozygous Lrp5A214V /Akita mutants have high trabecular bone mass and cortical thickness relative to WT animals, as do Lrp5A214V single mutants. Likewise, the Lrp5A214V mutation prevents deterioration of biomechanical properties occurring in Akita mice. Notably, Lrp5A214V /Akita mice develop fasting hyperglycemia and glucose intolerance with a delay relative to Akita mice (7 to 8 vs. 5 to 6 weeks, respectively), despite lack of insulin production in both groups by 6 weeks of age. Although insulin sensitivity is partially preserved in double heterozygous Lrp5A214V /Akita relative to Akita mutants up to 30 weeks of age, insulin-dependent phosphorylated protein kinase B (pAKT) activation in vitro is not altered by the Lrp5A214V mutation. Although white adipose tissue depots are equally reduced in both compound and Akita mice, the Lrp5A214V mutation prevents brown adipose tissue whitening that occurs in Akita mice. Thus, hyperactivation of Lrp5-dependent signaling fully protects bone mass and strength in prolonged hyperglycemia and improves peripheral glucose metabolism in an insulin independent manner. Wnt signaling activation represents an ideal therapeutic approach for diabetic patients at high risk of fracture. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Radiation causes tissue damage by dysregulating inflammasome-gasdermin D signaling in both host and transplanted cells.

Radiotherapy is a commonly used conditioning regimen for bone marrow transplantation (BMT). Cytotoxicity limits the use of this life-saving therapy, but the underlying mechanisms remain poorly defined. Here, we use the syngeneic mouse BMT model to test the hypothesis that lethal radiation damages tissues, thereby unleashing signals that indiscriminately activate the inflammasome pathways in host and transplanted cells. We find that a clinically relevant high dose of radiation causes severe damage to bones and the spleen through mechanisms involving the NLRP3 and AIM2 inflammasomes but not the NLRC4 inflammasome. Downstream, we demonstrate that gasdermin D (GSDMD), the common effector of the inflammasomes, is also activated by radiation. Remarkably, protection against the injury induced by deadly ionizing radiation occurs only when NLRP3, AIM2, or GSDMD is lost simultaneously in both the donor and host cell compartments. Thus, this study reveals a continuum of the actions of lethal radiation relayed by the inflammasome-GSDMD axis, initially affecting recipient cells and ultimately harming transplanted cells as they grow in the severely injured and toxic environment. This study also suggests that therapeutic targeting of inflammasome-GSDMD signaling has the potential to prevent the collateral effects of intense radiation regimens.

Myeloid-specific Asxl2 deletion limits diet-induced obesity by regulating energy expenditure.

We previously established that global deletion of the enhancer of trithorax and polycomb (ETP) gene, Asxl2, prevents weight gain. Because proinflammatory macrophages recruited to adipose tissue are central to the metabolic complications of obesity, we explored the role of ASXL2 in myeloid lineage cells. Unexpectedly, mice without Asxl2 only in myeloid cells (Asxl2ΔLysM) were completely resistant to diet-induced weight gain and metabolically normal despite increased food intake, comparable activity, and equivalent fecal fat. Asxl2ΔLysM mice resisted HFD-induced adipose tissue macrophage infiltration and inflammatory cytokine gene expression. Energy expenditure and brown adipose tissue metabolism in Asxl2ΔLysM mice were protected from the suppressive effects of HFD, a phenomenon associated with relatively increased catecholamines likely due to their suppressed degradation by macrophages. White adipose tissue of HFD-fed Asxl2ΔLysM mice also exhibited none of the pathological remodeling extant in their control counterparts. Suppression of macrophage Asxl2 expression, via nanoparticle-based siRNA delivery, prevented HFD-induced obesity. Thus, ASXL2 controlled the response of macrophages to dietary factors to regulate metabolic homeostasis, suggesting modulation of the cells' inflammatory phenotype may impact obesity and its complications.

PARP1 Hinders Histone H2B Occupancy at the NFATc1 Promoter to Restrain Osteoclast Differentiation.

Induction of nuclear factor of activated T cell cytoplasmic 1 (NFATc1) by macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) is essential for macrophage differentiation into osteoclasts (OCs), but the underlying mechanisms remain unclear. The ability of poly(ADP-ribose) polymerase 1 (PARP1) to poly-ADP-ribosylate NFATc1 in T cells prompted us to investigate the PARP1 and NFATc1 interaction during osteoclastogenesis. However, extensive studies failed to directly link PARP1 to NFATc1. A combination of transcriptomics and proteomics studies was then used to identify PARP1 targets under these conditions. These unbiased approaches in conjunction with site-directed mutagenesis studies revealed that PARP1 inhibited NFATc1 expression and OC formation by ADP-ribosylating histone H2B at serine 7 and decreasing the occupancy of this histone variant at the NFATc1 promoter. The anti-osteoclastogenic function of PARP1 was confirmed in vivo in several mouse models of PARP1 loss-of-function or gain-of-function, including a novel model in which PARP1 was conditionally ablated in myeloid cells. Thus, PARP1 ADP-ribosylates H2B to negatively regulate NFATc1 expression and OC differentiation. © 2019 American Society for Bone and Mineral Research.

Omnipresence of inflammasome activities in inflammatory bone diseases.

The inflammasomes are intracellular protein complexes that are assembled in response to a variety of perturbations including infections and injuries. Failure of the inflammasomes to rapidly clear the insults or restore tissue homeostasis can result in chronic inflammation. Recurring inflammation is also provoked by mutations that cause the constitutive assembly of the components of these protein platforms. Evidence suggests that chronic inflammation is a shared mechanism in bone loss associated with aging, dysregulated metabolism, autoinflammatory, and autoimmune diseases. Mechanistically, inflammatory mediators promote bone resorption while suppressing bone formation, an imbalance which over time leads to bone loss and increased fracture risk. Thus, while acute inflammation is important for the maintenance of bone integrity, its chronic state damages this tissue. In this review, we discuss the role of the inflammasomes in inflammation-induced osteolysis.

Gasdermin D mediates the pathogenesis of neonatal-onset multisystem inflammatory disease in mice.

Mutated NLRP3 assembles a hyperactive inflammasome, which causes excessive secretion of interleukin (IL)-1ß and IL-18 and, ultimately, a spectrum of autoinflammatory disorders known as cryopyrinopathies of which neonatal-onset multisystem inflammatory disease (NOMID) is the most severe phenotype. NOMID mice phenocopy several features of the human disease as they develop severe systemic inflammation driven by IL-1ß and IL-18 overproduction associated with damage to multiple organs, including spleen, skin, liver, and skeleton. Secretion of IL-1ß and IL-18 requires gasdermin D (GSDMD), which-upon activation by the inflammasomes-translocates to the plasma membrane where it forms pores through which these cytokines are released. However, excessive pore formation resulting from sustained activation of GSDMD compromises membrane integrity and ultimately causes a pro-inflammatory form of cell death, termed pyroptosis. In this study, we first established a strong correlation between NLRP3 inflammasome activation and GSDMD processing and pyroptosis in vitro. Next, we used NOMID mice to determine the extent to which GSDMD-driven pyroptosis influences the pathogenesis of this disorder. Remarkably, all NOMID-associated inflammatory symptoms are prevented upon ablation of GSDMD. Thus, GSDMD-dependent actions are required for the pathogenesis of NOMID in mice.

p66Shc Inactivation Modifies RNS Production, Regulates Sirt3 Activity, and Improves Mitochondrial Homeostasis, Delaying the Aging Process in Mouse Brain.

Programmed and damage aging theories have traditionally been conceived as stand-alone schools of thought. However, the p66Shc adaptor protein has demonstrated that aging-regulating genes and reactive oxygen species (ROS) are closely interconnected, since its absence modifies metabolic homeostasis by providing oxidative stress resistance and promoting longevity. p66Shc(-/-) mice are a unique opportunity to further comprehend the bidirectional relationship between redox homeostasis and the imbalance of mitochondrial biogenesis and dynamics during aging. This study shows that brain mitochondria of p66Shc(-/-) aged mice exhibit a reduced alteration of redox balance with a decrease in both ROS generation and its detoxification activity. We also demonstrate a strong link between reactive nitrogen species (RNS) and mitochondrial function, morphology, and biogenesis, where low levels of ONOO- formation present in aged p66Shc(-/-) mouse brain prevent protein nitration, delaying the loss of biological functions characteristic of the aging process. Sirt3 modulates age-associated mitochondrial biology and function via lysine deacetylation of target proteins, and we show that its regulation depends on its nitration status and is benefited by the improved NAD+/NADH ratio in aged p66Shc(-/-) brain mitochondria. Low levels of protein nitration and acetylation could cause the metabolic homeostasis maintenance observed during aging in this group, thus increasing its lifespan.

Selective inhibition of the p38α MAPK-MK2 axis inhibits inflammatory cues including inflammasome priming signals.

p38α activation of multiple effectors may underlie the failure of global p38α inhibitors in clinical trials. A unique inhibitor (CDD-450) was developed that selectively blocked p38α activation of the proinflammatory kinase MK2 while sparing p38α activation of PRAK and ATF2. Next, the hypothesis that the p38α-MK2 complex mediates inflammasome priming cues was tested. CDD-450 had no effect on NLRP3 expression, but it decreased IL-1ß expression by promoting IL-1ß mRNA degradation. Thus, IL-1ß is regulated not only transcriptionally by NF-κB and posttranslationally by the inflammasomes but also posttranscriptionally by p38α-MK2. CDD-450 also accelerated TNF-α and IL-6 mRNA decay, inhibited inflammation in mice with cryopyrinopathy, and was as efficacious as global p38α inhibitors in attenuating arthritis in rats and cytokine expression by cells from patients with cryopyrinopathy and rheumatoid arthritis. These findings have clinical translation implications as CDD-450 offers the potential to avoid tachyphylaxis associated with global p38α inhibitors that may result from their inhibition of non-MK2 substrates involved in antiinflammatory and housekeeping responses.

Bone matrix components activate the NLRP3 inflammasome and promote osteoclast differentiation.

The NLRP3 inflammasome senses a variety of signals referred to as danger associated molecular patterns (DAMPs), including those triggered by crystalline particulates or degradation products of extracellular matrix. Since some DAMPs confer tissue-specific activation of the inflammasomes, we tested the hypothesis that bone matrix components function as DAMPs for the NLRP3 inflammasome and regulate osteoclast differentiation. Indeed, bone particles cause exuberant osteoclastogenesis in the presence of RANKL, a response that correlates with NLRP3 abundance and the state of inflammasome activation. To determine the relevance of these findings to bone homeostasis, we studied the impact of Nlrp3 deficiency on bone using pre-clinical mouse models of high bone turnover, including estrogen deficiency and sustained exposure to parathyroid hormone or RANKL. Despite comparable baseline indices of bone mass, bone loss caused by hormonal or RANKL perturbations is significantly reduced in Nlrp3 deficient than in wild type mice. Consistent with the notion that osteolysis releases DAMPs from bone matrix, pharmacologic inhibition of bone resorption by zoledronate attenuates inflammasome activation in mice. Thus, signals originating from bone matrix activate the NLRP3 inflammasome in the osteoclast lineage, and may represent a bone-restricted positive feedback mechanism that amplifies bone resorption in pathologic conditions of accelerated bone turnover.

Chronic inflammation triggered by the NLRP3 inflammasome in myeloid cells promotes growth plate dysplasia by mesenchymal cells.

Skeletal complications are common features of neonatal-onset multisystem inflammatory disease (NOMID), a disorder caused by NLRP3-activating mutations. NOMID mice in which NLRP3 is activated globally exhibit several characteristics of the human disease, including systemic inflammation and cartilage dysplasia, but the mechanisms of skeletal manifestations remain unknown. In this study, we find that activation of NLRP3 in myeloid cells, but not mesenchymal cells triggers chronic inflammation, which ultimately, causes growth plate and epiphyseal dysplasia in mice. These responses are IL-1 signaling-dependent, but independent of PARP1, which also functions downstream of NLRP3 and regulates skeletal homeostasis. Mechanistically, inflammation causes severe anemia and hypoxia in the bone environment, yet down-regulates the HIF-1α pathway in chondrocytes, thereby promoting the demise of these cells. Thus, activation of NLRP3 in hematopoietic cells initiates IL-1ß-driven paracrine cascades, which promote abnormal growth plate development in NOMID mice.

N-cadherin Regulation of Bone Growth and Homeostasis Is Osteolineage Stage-Specific.

N-cadherin inhibits osteogenic cell differentiation and canonical Wnt/ß-catenin signaling in vitro. However, in vivo both conditional Cdh2 ablation and overexpression in osteoblasts lead to low bone mass. We tested the hypothesis that N-cadherin has different effects on osteolineage cells depending upon their differentiation stage. Embryonic conditional osteolineage Cdh2 deletion in mice results in defective growth, low bone mass, and reduced osteoprogenitor number. These abnormalities are prevented by delaying Cdh2 ablation until 1 month of age, thus targeting only committed and mature osteoblasts, suggesting they are the consequence of N-cadherin deficiency in osteoprogenitors. Indeed, diaphyseal trabecularization actually increases when Cdh2 is ablated postnatally. The sclerostin-insensitive Lrp5A214V mutant, associated with high bone mass, does not rescue the growth defect, but it overrides the low bone mass of embryonically Cdh2-deleted mice, suggesting N-cadherin interacts with Wnt signaling to control bone mass. Finally, bone accrual and ß-catenin accumulation after administration of an anti-Dkk1 antibody are enhanced in N-cadherin-deficient mice. Thus, although lack of N-cadherin in embryonic and perinatal age is detrimental to bone growth and bone accrual, in adult mice loss of N-cadherin in osteolineage cells favors bone formation. Hence, N-cadherin inhibition may widen the therapeutic window of osteoanabolic agents. © 2017 American Society for Bone and Mineral Research.

Defective leptin-AMP-dependent kinase pathway induces nitric oxide release and contributes to mitochondrial dysfunction and obesity in ob/ob mice.

AIMS: Obesity arises on defective neuroendocrine pathways that increase energy intake and reduce mitochondrial metabolism. In the metabolic syndrome, mitochondrial dysfunction accomplishes defects in fatty acid oxidation and reciprocal increase in triglyceride content with insulin resistance and hyperglycemia. Mitochondrial inhibition is attributed to reduced biogenesis, excessive fission, and low adipokine-AMP-activated protein kinase (AMPK) level, but lateness of the respiratory chain contributes to perturbations. Considering that nitric oxide (NO) binds cytochrome oxidase and inhibits respiration, we explored NO as a direct effector of mitochondrial dysfunction in the leptin-deficient ob/ob mice. RESULTS: A remarkable three- to fourfold increase in neuronal nitric oxide synthase (nNOS) expression and activity was detected by western blot, citrulline assay, electronic and confocal microscopy, flow cytometry, and NO electrode sensor in mitochondria from ob/ob mice. High NO reduced oxygen uptake in ob/ob mitochondria by inhibition of complex IV and nitration of complex I. Low metabolic status restricted ß-oxidation in obese mitochondria and displaced acetyl-CoA to fat synthesis; instead, small interference RNA nNOS caused a phenotype change with fat reduction in ob/ob adipocytes. INNOVATION: We evidenced that leptin increases mitochondrial respiration and fat utilization by potentially inhibiting NO release. Accordingly, leptin administration to ob/ob mice prevented nNOS overexpression and mitochondrial dysfunction in vivo and rescued leptin-dependent effects by matrix NO reduction, whereas leptin-Ob-Rb disruption increased the formation of mitochondrial NO in control adipocytes. We demonstrated that in ob/ob, hypoleptinemia is associated with critically low mitochondrial p-AMPK and that, oppositely to p-Akt2, p-AMPK is a negative modulator of nNOS. CONCLUSION: Thereby, defective leptin-AMPK pathway links mitochondrial NO to obesity with complex I syndrome and dysfunctional mitochondria.

Mitochondrial kinases in cell signaling: Facts and perspectives.

Phylogenetic studies had shown that evolution of mitochondria occurred in parallel with the maturation of kinases implicated in growth and final size of modern organisms. In the last years, different reports confirmed that MAPKs, Akt, PKA and PKC are present in mitochondria, particularly in the intermembrane space and inner membrane where they meet mitochondrial constitutive upstream activators. Although a priori phosphorylation is the apparent aim of translocation, new perspectives indicate that kinase activation depends on redox status as determined by the mitochondrial production of oxygen species. We observed that the degree of mitochondrial oxidation of ERK Cys(38) and Cys(214) discriminates the kinase to be phosphorylated and determines translocation to the nuclear compartment and proliferation, or accumulation in mitochondria and arrest. Otherwise, transcriptional gene regulation by Akt depends on Cys(60) and Cys(310) oxidation to sulfenic and sulfonic acids. It is concluded that the interactions between kinases and mitochondria control cell signaling pathways and participate in the modulation of cell proliferation and arrest, tissue protection, tumorigenesis and cancer progression.