CD4+ T cell immunity is dependent on an intrinsic stem-like program

CD4+ T cells are central to various immune responses, but the molecular programs that drive and maintain CD4+ T cell immunity are not entirely clear. Here we identify a stem-like program that governs the CD4+ T cell response in transplantation models. Single-cell-transcriptomic analysis revealed that naive alloantigen-specific CD4+ T cells develop into TCF1hi effector precursor (TEP) cells and TCF1-CXCR6+ effectors in transplant recipients. The TCF1-CXCR6+CD4+ effectors lose proliferation capacity and do not reject allografts upon adoptive transfer into secondary hosts. By contrast, the TCF1hiCD4+ TEP cells have dual features of self-renewal and effector differentiation potential, and allograft rejection depends on continuous replenishment of TCF1-CXCR6+ effectors from TCF1hiCD4+ TEP cells. Mechanistically, TCF1 sustains the CD4+ TEP cell population, whereas the transcription factor IRF4 and the glycolytic enzyme LDHA govern the effector differentiation potential of CD4+ TEP cells. Deletion of IRF4 or LDHA in T cells induces transplant acceptance. These findings unravel a stem-like program that controls the self-renewal capacity and effector differentiation potential of CD4+ TEP cells and have implications for T cell-related immunotherapies.

Gasdermin D-mediated pyroptosis is regulated by AMPK-mediated phosphorylation in tumor cells

Gasdermin D (GSDMD) is a critical mediator of pyroptosis, which consists of a N-terminal pore-forming domain and a C-terminal autoinhibitory domain. Its cytolytic activity is sequestered by the intramolecular autoinhibitory mechanism. Upon caspase-1/11 mediated cleavage of GSDMD, the N-terminal pore-forming domain (GD-NT) is released to mediate pyroptosis. However, it remains unclear how GD-NT is regulated once it is generated. In the current study, we developed a TetOn system in which GD-NT was selectively induced in tumor cells to explore how the cytolytic activity of GD-NT is regulated. We found that the cytolytic activity of GD-NT was negatively regulated by the AMP-activated protein kinase (AMPK) and AMPK activation rendered tumor cells resistant to GD-NT-mediated pyroptosis. Mechanistically, AMPK phosphorylated GD-NT at the serine 46 (pS46-GD), which altered GD-NT oligomerization and subsequently eliminated its pore-forming ability. In our in vivo tumor model, AMPK-mediated phosphorylation abolished GD-NT-induced anti-tumor activity and resulted in an aggressive tumor growth. Thus, our data demonstrate the critical role of AMPK in negatively regulating the cytolytic activity of GD-NT. Our data also highlight an unexpected link between GSDMD-mediated pyroptosis and the AMPK signaling pathway in certain tumor cells.

Pre-transplant Biomarkers of Immune Dysfunction Improve Risk Assessment of Post-transplant Mortality Compared to Conventional Clinical Risk Scores

Introduction

There is a critical need to accurately stratify liver transplant (LT) candidates' risk of post-LT mortality prior to LT to optimize patient selection and avoid futility. Here, we compare previously described pre-LT clinical risk scores with the recently developed Liver Immune Frailty Index (LIFI) for prediction of post-LT mortality. LIFI measures immune dysregulation based on pre-LT plasma HCV IgG, MMP3 and Fractalkine. LIFI accurately predicts post-LT mortality, with LIFI-low corresponding to 1.4% 1-year post-LT mortality compared with 58.3% for LIFI-high (C-statistic=0.85).

Methods

LIFI was compared to MELD, MELD-Na, MELD 3.0, D-MELD, MELD-GRAIL, MELD-GRAIL-Na, UCLA-FRS, BAR, SOFT, P-SOFT, and LDRI scores on 289 LT recipients based on waitlist data at the time of LT. Survival, hazard of early post-LT death, and discrimination power (C-statistic) were assessed.

Results

LIFI showed superior discrimination (highest C-statistic) for post-LT mortality when compared to all other risk scores, irrespective of biologic MELD. On univariate analysis, the LIFI showed a significant correlation with mortality 6-months, as well as 1-, 3-, and 5-years. No other pre-LT scoring system significantly correlated with post-LT mortality. On bivariate adjusted analysis, African American race (p<0.05) and pre-LT cardiovascular disease (p=0.053) were associated with early- and long-term post-LT mortality. Patients who died within 1-yr following LT had a significantly higher incidence of infections, including 30-day and 90-day incidence of any infection, pneumonia, abdominal infections, and UTI (p<0.05).

Conclusions

LIFI, which measures pre-LT biomarkers of immune dysfunction, more accurately predicts risk of post-LT futility compared with current clinical predictive models. Pre-LT assessment of immune dysregulation may be critical in predicting mortality after LT and may optimize selection of candidates with lowest risk of futile outcomes.

Pre-transplant Biomarkers of Immune Dysfunction Improve Risk Assessment of Post-transplant Mortality Compared to Conventional Clinical Risk Scores

Introduction: There is a critical need to accurately stratify liver transplant (LT) candidates' risk of post-LT mortality prior to LT to optimize patient selection and avoid futility. Here, we compare previously described pre -LT clinical risk scores with the recently developed Liver Immune Frailty Index (LIFI) for prediction of post -LT mortality. LIFI measures immune dysregulation based on pre-LT plasma HCV IgG, MMP3 and Fractalkine. LIFI accurately predicts post-LT mortality, with LIFI-low corresponding to 1.4% 1-year post-LT mortality compared with 58.3% for LIFI-high (C-statistic=0.85). Methods : LIFI was compared to MELD, MELD-Na, MELD 3.0, D-MELD, MELD-GRAIL, MELD-GRAIL-Na, UCLA-FRS, BAR, SOFT, P-SOFT, and LDRI scores on 289 LT recipients based on waitlist data at the time of LT. Survival, hazard of early post-LT death, and discrimination power (C-statistic) were assessed. Results : LIFI showed superior discrimination (highest C-statistic) for post-LT mortality when compared to all other risk scores, irrespective of biologic MELD. On univariate analysis, the LIFI showed a significant correlation with mortality 6-months, as well as 1-, 3-, and 5-years. No other pre-LT scoring system significantly correlated with post-LT mortality . On bivariate adjusted analysis, African American race (p<0.05) and pre-LT cardiovascular disease (p=0.053) were associated with early- and long-term post-LT mortality. Patients who died within 1-yr following LT had a significantly higher incidence of infections, including 30-day and 90-day incidence of any infection, pneumonia, abdominal infections, and UTI (p<0.05). Conclusions : LIFI, which measures pre-LT biomarkers of immune dysfunction, more accurately predicts risk of post-LT futility compared with current clinical predictive models. Pre-LT assessment of immune dysregulation may be critical in predicting mortality after LT and may optimize selection of candidates with lowest risk of futile outcomes.

TRIM56 coiled-coil domain structure provides insights into its E3 ligase functions

Protein ubiquitination is a post-translation modification mediated by E3 ubiquitin ligases. The RING domain E3 ligases are the largest family of E3 ubiquitin ligases, they act as a scaffold, bringing the E2-ubiquitin complex and its substrate together to facilitate direct ubiquitin transfer. However, the quaternary structures of RING E3 ligases that perform ubiquitin transfer remain poorly understood. In this study, we solved the crystal structure of TRIM56, a member of the RING E3 ligase. The structure of the coiled-coil domain indicated that the two anti-parallel dimers bound together to form a tetramer at a small crossing angle. This tetramer structure allows two RING domains to exist on each side to form an active homodimer in supporting ubiquitin transfer from E2 to its nearby substrate recruited by the C-terminal domains on the same side. These findings suggest that the coiled-coil domain-mediated tetramer is a feasible scaffold for facilitating the recruitment and transfer of ubiquitin to accomplish E3 ligase activity.

Structural studies of the coiled-coil domain of TRIM75 reveal a tetramer architecture facilitating its E3 ligase complex

Protein ubiquitination plays a vital role in controlling the degradation of intracellular proteins and in regulating cell signaling pathways. Functionally, E3 ubiquitin ligases control the transfer of ubiquitin to the target substrates. As a major family of ubiquitin E3 ligases, the structural assembly of RING E3 ligases required to exert their ubiquitin E3 ligase activity remains poorly defined. Here, we solved the crystal structure of the coiled-coil domain of TRIM75, a member of the RING E3 ligase family, which showed that two disulfide bonds stabilize two antiparallel dimers at a small crossing angle. This tetrameric conformation confers two close RING domains on the same side to form a dimer. Furthermore, this architecture allows the RING dimer to present ubiquitin to a substrate on the same side. Overall, this structure reveals a disulfide bond-mediated unique tetramer architecture and provides a tetrameric structural model through which E3 ligases exert their function.

TRIM18 is a critical regulator of viral myocarditis and organ inflammation

Background

Infections by viruses including severe acute respiratory syndrome coronavirus 2 could cause organ inflammations such as myocarditis, pneumonia and encephalitis. Innate immunity to viral nucleic acids mediates antiviral immunity as well as inflammatory organ injury. However, the innate immune mechanisms that control viral induced organ inflammations are unclear.

Methods

To understand the role of the E3 ligase TRIM18 in controlling viral myocarditis and organ inflammation, wild-type and Trim18 knockout mice were infected with coxsackievirus B3 for inducing viral myocarditis, influenza A virus PR8 strain and human adenovirus for inducing viral pneumonia, and herpes simplex virus type I for inducing herpes simplex encephalitis. Mice survivals were monitored, and heart, lung and brain were harvested for histology and immunohistochemistry analysis. Real-time PCR, co-immunoprecipitation, immunoblot, enzyme-linked immunosorbent assay, luciferase assay, flow cytometry, over-expression and knockdown techniques were used to understand the molecular mechanisms of TRIM18 in regulating type I interferon (IFN) production after virus infection in this study.

Results

We find that knockdown or deletion of TRIM18 in human or mouse macrophages enhances production of type I IFN in response to double strand (ds) RNA and dsDNA or RNA and DNA virus infection. Importantly, deletion of TRIM18 protects mice from viral myocarditis, viral pneumonia, and herpes simplex encephalitis due to enhanced type I IFN production in vivo. Mechanistically, we show that TRIM18 recruits protein phosphatase 1A (PPM1A) to dephosphorylate TANK binding kinase 1 (TBK1), which inactivates TBK1 to block TBK1 from interacting with its upstream adaptors, mitochondrial antiviral signaling (MAVS) and stimulator of interferon genes (STING), thereby dampening antiviral signaling during viral infections. Moreover, TRIM18 stabilizes PPM1A by inducing K63-linked ubiquitination of PPM1A.

Conclusions

Our results indicate that TRIM18 serves as a negative regulator of viral myocarditis, lung inflammation and brain damage by downregulating innate immune activation induced by both RNA and DNA viruses. Our data reveal that TRIM18 is a critical regulator of innate immunity in viral induced diseases, thereby identifying a potential therapeutic target for treatment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00840-z.

DHX15 is required to control RNA virus-induced intestinal inflammation

RNA helicases play critical roles in various biological processes, including serving as viral RNA sensors in innate immunity. Here, we find that RNA helicase DEAH-box helicase 15 (DHX15) is essential for type I interferon (IFN-I, IFN-β), type III IFN (IFN-λ3), and inflammasome-derived cytokine IL-18 production by intestinal epithelial cells (IECs) in response to poly I:C and RNA viruses with preference of enteric RNA viruses, but not DNA virus. Importantly, we generate IEC-specific Dhx15-knockout mice and demonstrate that DHX15 is required for controlling intestinal inflammation induced by enteric RNA virus rotavirus in suckling mice and reovirus in adult mice in vivo, which owes to impaired IFN-β, IFN-λ3, and IL-18 production in IECs from Dhx15-deficient mice. Mechanistically, DHX15 interacts with NLRP6 to trigger NLRP6 inflammasome assembly and activation for inducing IL-18 secretion in IECs. Collectively, our report reveals critical roles for DHX15 in sensing enteric RNA viruses in IECs and controlling intestinal inflammation.

Identification of poly(ADP-ribose) polymerase 9 (PARP9) as a noncanonical sensor for RNA virus in dendritic cells

Innate immune cells are critical in protective immunity against viral infections, involved in sensing foreign viral nucleic acids. Here we report that the poly(ADP-ribose) polymerase 9 (PARP9), a member of PARP family, serves as a non-canonical sensor for RNA virus to initiate and amplify type I interferon (IFN) production. We find knockdown or deletion of PARP9 in human or mouse dendritic cells and macrophages inhibits type I IFN production in response to double strand RNA stimulation or RNA virus infection. Furthermore, mice deficient for PARP9 show enhanced susceptibility to infections with RNA viruses because of the impaired type I IFN production. Mechanistically, we show that PARP9 recognizes and binds viral RNA, with resultant recruitment and activation of the phosphoinositide 3-kinase (PI3K) and AKT3 pathway, independent of mitochondrial antiviral-signaling (MAVS). PI3K/AKT3 then activates the IRF3 and IRF7 by phosphorylating IRF3 at Ser385 and IRF7 at Ser437/438 mediating type I IFN production. Together, we reveal a critical role for PARP9 as a non-canonical RNA sensor that depends on the PI3K/AKT3 pathway to produce type I IFN. These findings may have important clinical implications in controlling viral infections and viral-induced diseases by targeting PARP9.

TRIM29 Negatively Regulates the Type I IFN Production in Response to RNA Virus

The innate immunity is critically important in protection against virus infections, and in the case of RNA viral infections, the signaling mechanisms that initiate robust protective innate immunity without triggering autoimmune inflammation remain incompletely defined. In this study, we found the E3 ligase TRIM29 was specifically expressed in poly I:C-stimulated human myeloid dendritic cells. The induced TRIM29 played a negative role in type I IFN production in response to poly I:C or dsRNA virus reovirus infection. Importantly, the challenge of wild-type mice with reovirus led to lethal infection. In contrast, deletion of TRIM29 protected the mice from this developing lethality. Additionally, TRIM29^-/- mice have lower titers of reovirus in the heart, intestine, spleen, liver, and brain because of elevated production of type I IFN. Mechanistically, TRIM29 was shown to interact with MAVS and subsequently induce its K11-linked ubiquitination and degradation. Taken together, TRIM29 regulates negatively the host innate immune response to RNA virus, which could be employed by RNA viruses for viral pathogenesis.

TRIM29 suppresses the innate immune response to RNA virus

The innate immunity is critically important in protection against virus infections, and in the case of RNA viral infections, the signaling mechanisms that initiate robust protective innate immunity without triggering autoimmune inflammation remain incompletely defined. We have previously found that TRIM29 is constitutively expressed in alveolar macrophages and plays a critical role in the control of innate immunity in respiratory tract. Also, we have shown that TRIM29 is highly expressed in human airway epithelial cells and promotes DNA virus infections by inhibiting innate immune response. Here, we found the E3 ligase TRIM29 was specifically expressed in poly I:C stimulated human myeloid dendritic cells (mDCs). The induced TRIM29 played a negative role in type I interferon (IFN) production in response to poly I:C or dsRNA virus reovirus infection. Importantly, the challenge of wildtype mice with reovirus led to lethal infection. In contrast, deletion of TRIM29 protected the mice from this developing lethality. Additionally, TRIM29−/− mice have lower titers of reovirus in the heart, intestine, spleen, liver, and brain due to elevated production of type I IFN. Mechanistically, TRIM29 was shown to interact with MAVS and subsequently induce its K11-linked ubiquitination and degradation. Taken together, TRIM29 regulates negatively the host innate immune response to RNA virus, which could be employed by RNA viruses for immune evasion.

Longitudinal assessment of T cell inhibitory receptors in liver transplant recipients and their association with posttransplant infections

Current immunosuppression regimens in organ transplantation primarily inhibit T cells. However, T cells are also critical in protective immunity, especially in immune-compromised patients. In this study, we examined the association of T cell dysfunction, as marked by expression of T cell exhaustion molecules, and posttransplant infections in a cohort of liver transplant patients. We focused on Programmed Death 1 (PD-1) and T cell Ig- and mucin-domain molecule 3 (Tim-3), which are potent co-inhibitory receptors, and their persistent expression often leads to T cell dysfunction and compromised protective immunity. We found that patients with the highest expression of PD-1 +Tim-3+ T cells in the memory compartment before transplantation had increased incidence of infections after liver transplantation, especially within the first 90 days. Longitudinal analysis in the first year showed a strong association between variability of PD-1 and Tim-3 expression by T cells and infectious episodes in transplant patients. Furthermore, T cells that expressed PD-1 and Tim-3 had a significantly reduced capacity in producing interferon (IFN)-γ in vitro, and this reduced IFN-γ production could be partially reversed by blocking PD-1 and Tim-3. Interestingly, the percentage of Foxp3+ regulatory T cells in liver transplant patients was stable in the study period. We concluded that the functional status of T cells before and after liver transplantation, as shown by PD-1 and Tim-3 expression, may be valuable in prognosis and management of posttransplant infections.

Role of the NF-κB Family Member RelB in Regulation of Foxp3+ Regulatory T Cells In Vivo

The NF-κB family member RelB is an important transcription factor that is capable of regulating diverse immune and inflammatory responses. However, its role in the regulation of Foxp3⁺ regulatory T cells (Tregs) in vivo is poorly defined. In this study, we demonstrated that germline deletion of Relb resulted in systemic autoimmunity, which is associated with significant accumulation of Foxp3⁺ Tregs in lymphoid and nonlymphoid organs. Foxp3⁺ Tregs from RelB-deficient mice were functional and capable of suppressing T effector cells in vitro and in vivo, but Foxp3^- T effector cells from RelB-deficient mice showed features of hyperactivation and spontaneously produced high levels of IL-2. Surprisingly, mice with conditional deletion of Relb in T cells (Cd4^CreRelb^f/f mice) or specifically in Foxp3⁺ Tregs (Foxp3^CreRelb^f/f mice) did not show signs of autoimmunity and had similar frequencies of Foxp3⁺ Tregs in the periphery as wild-type C57BL/6 controls. Both strains of conditional knockout mice also had a normal conventional T cell compartment. However, reconstituting Rag-1^-/-Relb^-/- hosts with wild-type C57BL/6 bone marrow cells led to hyperactivation of T effector cells, as well as marked expansion of Foxp3⁺ T cells. These data suggest that the autoimmune phenotype in germline RelB-deficient mice is most likely caused by T cell-extrinsic mechanisms, and further studies are warranted to uncover such mechanisms.

Ablation of Transcription Factor IRF4 Promotes Transplant Acceptance by Driving Allogenic CD4+ T Cell Dysfunction

CD4⁺ T cells orchestrate immune responses and destruction of allogeneic organ transplants, but how this process is regulated on a transcriptional level remains unclear. Here, we demonstrated that interferon regulatory factor 4 (IRF4) was a key transcriptional determinant controlling T cell responses during transplantation. IRF4 deletion in mice resulted in progressive establishment of CD4⁺ T cell dysfunction and long-term allograft survival. Mechanistically, IRF4 repressed PD-1, Helios, and other molecules associated with T cell dysfunction. In the absence of IRF4, chromatin accessibility and binding of Helios at PD-1 cis-regulatory elements were increased, resulting in enhanced PD-1 expression and CD4⁺ T cell dysfunction. The dysfunctional state of Irf4-deficient T cells was initially reversible by PD-1 ligand blockade, but it progressively developed into an irreversible state. Hence, IRF4 controls a core regulatory circuit of CD4⁺ T cell dysfunction, and targeting IRF4 represents a potential therapeutic strategy for achieving transplant acceptance.

Estrogen receptor alpha activation enhances mitochondrial function and systemic metabolism in high-fat-fed ovariectomized mice

Estrogen impacts insulin action and cardiac metabolism, and menopause dramatically increases cardiometabolic risk in women. However, the mechanism(s) of cardiometabolic protection by estrogen remain incompletely understood. Here, we tested the effects of selective activation of E2 receptor alpha (ERα) on systemic metabolism, insulin action, and cardiac mitochondrial function in a mouse model of metabolic dysfunction (ovariectomy [OVX], insulin resistance, hyperlipidemia, and advanced age). Middle-aged (12-month-old) female low-density lipoprotein receptor (Ldlr)(-/-) mice were subjected to OVX or sham surgery and fed "western" high-fat diet (WHFD) for 3 months. Selective ERα activation with 4,4',4″-(4-Propyl-[1H]-pyrazole-1,3,5-triyl) (PPT), prevented weight gain, improved insulin action, and reduced visceral fat accumulation in WHFD-fed OVX mice. PPT treatment also elevated systemic metabolism, increasing oxygen consumption and core body temperature, induced expression of several metabolic genes such as peroxisome proliferator-activated receptor gamma, coactivator 1 alpha, and nuclear respiratory factor 1 in heart, liver, skeletal muscle, and adipose tissue, and increased cardiac mitochondrial function. Taken together, selective activation of ERα with PPT enhances metabolic effects including insulin resistance, whole body energy metabolism, and mitochondrial function in OVX mice with metabolic syndrome.

Macrophage/monocyte-specific deletion of Ras homolog gene family member A (RhoA) downregulates fractalkine receptor and inhibits chronic rejection of mouse cardiac allografts

BACKGROUND

The cellular and molecular mechanisms of chronic rejection of transplanted organs remain obscure; however, macrophages are known to play a critical role in the injury and repair of allografts. Among multiple factors influencing macrophage infiltration to allografts, the fractalkine chemokine (C-X3-C motif) ligand 1(CX3CL1)/chemokine (C-X3-C motif) receptor 1 (CX3CR1) signaling pathway and actin cytoskeleton, which is regulated by a small guanosine-5׳-triphosphatase Ras homolog gene family member A (RhoA), are of the utmost importance. To define the role of macrophage/RhoA pathway involvement in chronic rejection, we generated mice with monocyte/macrophage-specific deletion of RhoA.

METHODS

Hearts from BALB/c (H-2d) donors were transplanted into RhoA^flox/flox (no Cre) and heterozygous Lyz2^Cre+/-RhoA^flox/flox recipients treated with cytotoxic T-lymphocyte-associated protein 4 immunoglobulin to inhibit early T-cell response. Allografts were assessed for chronic rejection and monocyte/macrophage functions.

RESULTS

The deletion of RhoA inhibited macrophage infiltration, neointimal hyperplasia of vasculature, and abrogated chronic rejection of the allografts. The RhoA deletion downregulated G protein-coupled fractalkine receptor CX3CR1, which activates the RhoA pathway and controls monocyte/macrophage trafficking into the vascular endothelium. This in turn promotes, through overproliferation and differentiation of smooth muscle cells in the arterial walls, neointimal hyperplasia.

CONCLUSIONS

Our finding of codependence of chronic rejection on monocyte/macrophage CX3CR1/CX3CL1 and RhoA signaling pathways may lead to the development of novel anti-chronic rejection therapies.

Osteocalcin protects against nonalcoholic steatohepatitis in a mouse model of metabolic syndrome

Nonalcoholic fatty liver disease, particularly its more aggressive form, nonalcoholic steatohepatitis (NASH), is associated with hepatic insulin resistance. Osteocalcin, a protein secreted by osteoblast cells in bone, has recently emerged as an important metabolic regulator with insulin-sensitizing properties. In humans, osteocalcin levels are inversely associated with liver disease. We thus hypothesized that osteocalcin may attenuate NASH and examined the effects of osteocalcin treatment in middle-aged (12-mo-old) male Ldlr(-/-) mice, which were fed a Western-style high-fat, high-cholesterol diet for 12 weeks to induce metabolic syndrome and NASH. Mice were treated with osteocalcin (4.5 ng/h) or vehicle for the diet duration. Osteocalcin treatment not only protected against Western-style high-fat, high-cholesterol diet-induced insulin resistance but substantially reduced multiple NASH components, including steatosis, ballooning degeneration, and fibrosis, with an overall reduction in nonalcoholic fatty liver disease activity scores. Further, osteocalcin robustly reduced expression of proinflammatory and profibrotic genes (Cd68, Mcp1, Spp1, and Col1a2) in liver and suppressed inflammatory gene expression in white adipose tissue. In conclusion, these results suggest osteocalcin inhibits NASH development by targeting inflammatory and fibrotic processes.

High-fat feeding-induced hyperinsulinemia increases cardiac glucose uptake and mitochondrial function despite peripheral insulin resistance

In obesity, reduced cardiac glucose uptake and mitochondrial abnormalities are putative causes of cardiac dysfunction. However, high-fat diet (HFD) does not consistently induce cardiac insulin resistance and mitochondrial damage, and recent studies suggest HFD may be cardioprotective. To determine cardiac responses to HFD, we investigated cardiac function, glucose uptake, and mitochondrial respiration in young (3-month-old) and middle-aged (MA) (12-month-old) male Ldlr(-/-) mice fed chow or 3 months HFD to induce obesity, systemic insulin resistance, and hyperinsulinemia. In MA Ldlr(-/-) mice, HFD induced accelerated atherosclerosis and nonalcoholic steatohepatitis, common complications of human obesity. Surprisingly, HFD-fed mice demonstrated increased cardiac glucose uptake, which was most prominent in MA mice, in the absence of cardiac contractile dysfunction or hypertrophy. Moreover, hearts of HFD-fed mice had enhanced mitochondrial oxidation of palmitoyl carnitine, glutamate, and succinate and greater basal insulin signaling compared with those of chow-fed mice, suggesting cardiac insulin sensitivity was maintained, despite systemic insulin resistance. Streptozotocin-induced ablation of insulin production markedly reduced cardiac glucose uptake and mitochondrial dysfunction in HFD-fed, but not in chow-fed, mice. Insulin injection reversed these effects, suggesting that insulin may protect cardiac mitochondria during HFD. These results have implications for cardiac metabolism and preservation of mitochondrial function in obesity.

Functional imaging of brown fat in mice with 18F-FDG micro-PET/CT

Brown adipose tissue (BAT) differs from white adipose tissue (WAT) by its discrete location and a brown-red color due to rich vascularization and high density of mitochondria. BAT plays a major role in energy expenditure and non-shivering thermogenesis in newborn mammals as well as the adults (1). BAT-mediated thermogenesis is highly regulated by the sympathetic nervous system, predominantly via β adrenergic receptor (2, 3). Recent studies have shown that BAT activities in human adults are negatively correlated with body mass index (BMI) and other diabetic parameters (4-6). BAT has thus been proposed as a potential target for anti-obesity/anti-diabetes therapy focusing on modulation of energy balance (6-8). While several cold challenge-based positron emission tomography (PET) methods are established for detecting human BAT (9-13), there is essentially no standardized protocol for imaging and quantification of BAT in small animal models such as mice. Here we describe a robust PET/CT imaging method for functional assessment of BAT in mice. Briefly, adult C57BL/6J mice were cold treated under fasting conditions for a duration of 4 hours before they received one dose of (18)F-Fluorodeoxyglucose (FDG). The mice were remained in the cold for one additional hour post FDG injection, and then scanned with a small animal-dedicated micro-PET/CT system. The acquired PET images were co-registered with the CT images for anatomical references and analyzed for FDG uptake in the interscapular BAT area to present BAT activity. This standardized cold-treatment and imaging protocol has been validated through testing BAT activities during pharmacological interventions, for example, the suppressed BAT activation by the treatment of β-adrenoceptor antagonist propranolol (14, 15), or the enhanced BAT activation by β3 agonist BRL37344 (16). The method described here can be applied to screen for drugs/compounds that modulate BAT activity, or to identify genes/pathways that are involved in BAT development and regulation in various preclinical and basic studies.

Myeloid deletion of nuclear factor erythroid 2-related factor 2 increases atherosclerosis and liver injury

OBJECTIVE

To determine the impact of hematopoietic deletion of nuclear factor- (erythroid-derived 2) like 2 factor (Nrf2) on the development of atherosclerosis and liver injury in an obese, hypercholesterolemic mouse model.

METHODS AND RESULTS

Two-month-old male low-density lipoprotein receptor-deficient mice were lethally irradiated and transplanted with either wild type or Nrf2-deficient (Nrf2(-/-)) bone marrow cells. At 3 months of age, mice were placed on an obesogenic high-fat diet (HFD), high-cholesterol diet for 7 months. Despite no differences in body weight, body fat percentage, liver fat, plasma glucose, lipids, or insulin, the HFD-fed Nrf2(-/-) bone marrow recipients had increased proinflammatory vascular gene expression, a significant increase in atherosclerosis area (18% versus 28%; P=0.018) and lesion complexity, and a marked increase in liver fibrosis. The acceleration of vascular and liver injury may arise from enhanced macrophage migration, inflammation, and oxidative stress resulting from myeloid Nrf2 deficiency.

CONCLUSIONS

Myeloid-derived Nrf2 activity attenuates atherosclerosis development and liver inflammation and fibrosis associated with obesity. Prevention of oxidative stress in macrophage and other myeloid lineage cells may be an important therapeutic target to reduce inflammation-driven complications of obesity.

The clock gene, brain and muscle Arnt-like 1, regulates adipogenesis via Wnt signaling pathway

Circadian clocks in adipose tissue are known to regulate adipocyte biology. Although circadian dysregulation is associated with development of obesity, the underlying mechanism has not been established. Here we report that disruption of the clock gene, brain and muscle Arnt-like 1 (Bmal1), in mice led to increased adipogenesis, adipocyte hypertrophy, and obesity, compared to wild-type (WT) mice. This is due to its cell-autonomous effect, as Bmal1 deficiency in embryonic fibroblasts, as well as stable shRNA knockdown (KD) in 3T3-L1 preadipocyte and C3H10T1/2 mesenchymal stem cells, promoted adipogenic differentiation. We demonstrate that attenuation of Bmal1 function resulted in down-regulation of genes in the canonical Wnt pathway, known to suppress adipogenesis. Promoters of these genes (Wnt10a, β-catenin, Dishevelled2, TCF3) displayed Bmal1 occupancy, indicating direct circadian regulation by Bmal1. As a result, Wnt signaling activity was attenuated by Bmal1 KD and augmented by its overexpression. Furthermore, stabilizing β-catenin through Wnt ligand or GSK-3β inhibition achieved partial restoration of blunted Wnt activity and suppression of increased adipogenesis induced by Bmal1 KD. Taken together, our study demonstrates that Bmal1 is a critical negative regulator of adipocyte development through transcriptional control of components of the canonical Wnt signaling cascade, and provides a mechanistic link between circadian disruption and obesity.

Rosiglitazone attenuates age- and diet-associated nonalcoholic steatohepatitis in male low-density lipoprotein receptor knockout mice

Nonalcoholic fatty liver disease (NAFLD) is a common complication of obesity that can progress to nonalcoholic steatohepatitis (NASH), a serious liver pathology that can advance to cirrhosis. The mechanisms responsible for NAFLD progression to NASH remain unclear. Lack of a suitable animal model that faithfully recapitulates the pathophysiology of human NASH is a major obstacle in delineating mechanisms responsible for progression of NAFLD to NASH and, thus, development of better treatment strategies. We identified and characterized a novel mouse model, middle-aged male low-density lipoprotein receptor (LDLR)(-/-) mice fed a high-fat diet (HFD), which developed NASH associated with four of five metabolic syndrome (MS) components. In these mice, as observed in humans, liver steatosis and oxidative stress promoted NASH development. Aging exacerbated the HFD-induced NASH such that liver steatosis, inflammation, fibrosis, oxidative stress, and liver injury markers were greatly enhanced in middle-aged versus young LDLR(-/-) mice. Although expression of genes mediating fatty acid oxidation and antioxidant responses were up-regulated in young LDLR(-/-) mice fed HFD, they were drastically reduced in MS mice. However, similar to recent human trials, NASH was partially attenuated by an insulin-sensitizing peroxisome proliferator-activated receptor-gamma (PPARγ) ligand, rosiglitazone. In addition to expected improvements in MS, newly identified mechanisms of PPARγ ligand effects included stimulation of antioxidant gene expression and mitochondrial β-oxidation, and suppression of inflammation and fibrosis. LDLR-deficiency promoted NASH, because middle-aged C57BL/6 mice fed HFD did not develop severe inflammation and fibrosis, despite increased steatosis.

CONCLUSION

MS mice represent an ideal model to investigate NASH in the context of MS, as commonly occurs in human disease, and NASH development can be substantially attenuated by PPARγ activation, which enhances β-oxidation.