Pathological and Transcriptome Changes in the ReninAAV db/db uNx Model of Advanced Diabetic Kidney Disease Exhibit Features of Human Disease

The ReninAAV db/db uNx model of diabetic kidney disease (DKD) exhibits hallmarks of advanced human disease, including progressive elevations in albuminuria and serum creatinine, loss of glomerular filtration rate, and pathological changes. Microarray analysis of renal transcriptome changes were more similar to human DKD when compared to db/db eNOS−/− model. The model responds to treatment with arterial pressure lowering (lisinopril) or glycemic control (rosiglitazone) at early stages of disease. We hypothesized the ReninAAV db/db uNx model with advanced disease would have residual disease after treatment with lisinopril, rosiglitazone, or combination of both. To test this, ReninAAV db/db uNx mice with advanced disease were treated with lisinopril, rosiglitazone, or combination of both for 10 weeks. All treatment groups showed significant lowering of urinary albumin to creatinine ratio compared to baseline; however, only combination group exhibited lowering of serum creatinine. Treatment improved renal pathological scores compared to baseline values with residual disease evident in all treatment groups when compared to db/m controls. Gene expression analysis by TaqMan supported pathological changes with increased fibrotic and inflammatory markers. The results further validate this model of DKD in which residual disease is present when treated with agents to lower arterial pressure and glycemic control.

Synopsis of Sweet! Mouse Models of Diabetic Kidney Disease

Diabetes mellitus (types 1 and 2) is the leading cause of glomerular disease and end-stage renal disease in most developed countries, with estimates that one-third of people living with diabetes will develop diabetic kidney disease (DKD). The current standard of care medications slow but do not arrest progression of kidney disease, and therefore, therapy for DKD is a highly unmet medical need for patients. To discover and test novel and durable new therapies, it is necessary to develop animal models of human DKD, which authentically recapitulate the human disease state and provide translatable efficacy to human patients. Here, we review selected mouse models of human DKD, which demonstrate many of the features of type 2 human DKD.

Progressive Renal Disease Established by Renin-Coding Adeno-Associated Virus-Driven Hypertension in Diverse Diabetic Models

Progress in research and developing therapeutics to prevent diabetic kidney disease (DKD) is limited by a lack of animal models exhibiting progressive kidney disease. Chronic hypertension, a driving factor of disease progression in human patients, is lacking in most available models of diabetes. We hypothesized that superimposition of hypertension on diabetic mouse models would accelerate DKD. To test this possibility, we induced persistent hypertension in three mouse models of type 1 diabetes and two models of type 2 diabetes by adeno-associated virus delivery of renin (ReninAAV). Compared with LacZAAV-treated counterparts, ReninAAV-treated type 1 diabetic Akita/129 mice exhibited a substantial increase in albumin-to-creatinine ratio (ACR) and serum creatinine level and more severe renal lesions. In type 2 models of diabetes (C57BKLS db/db and BTBR ob/ob mice), compared with LacZAAV, ReninAAV induced significant elevations in ACR and increased the incidence and severity of histopathologic findings, with increased serum creatinine detected only in the ReninAAV-treated db/db mice. The uninephrectomized ReninAAV db/db model was the most progressive model examined and further characterized. In this model, separate treatment of hyperglycemia with rosiglitazone or hypertension with lisinopril partially reduced ACR, consistent with independent contributions of these disorders to renal disease. Microarray analysis and comparison with human DKD showed common pathways affected in human disease and this model. These results identify novel models of progressive DKD that provide researchers with a facile and reliable method to study disease pathogenesis and support the development of therapeutics.

Role of TGF-alpha in the progression of diabetic kidney disease

Transforming growth factor-alpha (TGFA) has been shown to play a role in experimental chronic kidney disease associated with nephron reduction, while its role in diabetic kidney disease (DKD) is unknown. We show here that intrarenal TGFA mRNA expression, as well as urine and serum TGFA, are increased in human DKD. We used a TGFA neutralizing antibody to determine the role of TGFA in two models of renal disease, the remnant surgical reduction model and the uninephrectomized (uniNx) db/db DKD model. In addition, the contribution of TGFA to DKD progression was examined using an adeno-associated virus approach to increase circulating TGFA in experimental DKD. In vivo blockade of TGFA attenuated kidney disease progression in both nondiabetic 129S6 nephron reduction and Type 2 diabetic uniNx db/db models, whereas overexpression of TGFA in uniNx db/db model accelerated renal disease. Therapeutic activity of the TGFA antibody was enhanced with renin angiotensin system inhibition with further improvement in renal parameters. These findings suggest a pathologic contribution of TGFA in DKD and support the possibility that therapeutic administration of neutralizing antibodies could provide a novel treatment for the disease.

Viral transduction of renin rapidly establishes persistent hypertension in diverse murine strains

Mice provide a unique platform to dissect disease pathogenesis, with the availability of recombinant inbred strains and diverse genetically modified strains. Leveraging these reagents to elucidate the mechanisms of hypertensive tissue injury has been hindered by difficulty establishing persistent hypertension in these inbred lines. ANG II infusion provides relatively short-term activation of the renin-angiotensinogen system (RAS) with concomitant elevated arterial pressure. Longer-duration studies using renin transgenic mice are powerful models of chronic hypertension, yet are limited by the genetic background on which the transgene exists and the exposure throughout development. The present studies characterized hypertension produced by transduction with a renin-coding adeno-associated virus (ReninAAV). ReninAAV mice experienced elevated circulating renin with concurrent elevations in arterial pressure. Following a single injection of ReninAAV, arterial pressure increased on average +56 mmHg, an increase that persisted for at least 12 wk in three distinct and widely used strains of adult mice: 129/S6, C56BL/6, and DBA/2J. This was accomplished without surgical implantation of pumps or complex breeding and backcrossing. In addition, ReninAAV mice developed pathophysiological changes associated with chronic hypertension, including increased heart weight and albuminuria. Thus ReninAAV provides a unique tool to study the onset of and effects of persistent hypertension in diverse murine models. This model should facilitate our understanding of the pathogenesis of hypertensive injury.

Abstract 036: Pomc Neurons, But Not Agrp Neurons, Are Required For Leptin-induced Sympathetic Activation

Leptin acts in the brain to decrease food intake and promote energy expenditure by increasing sympathetic nerve activity (SNA) to thermogenic brown adipose tissue. Leptin also increases SNA to other beds including kidney with implications for obesity-induced hypertension. We previously demonstrated the importance the arcuate nucleus (Arc) of the hypothalamus in mediating leptin-induced increases in regional SNA, but the specific neuronal population within the Arc that mediates these responses is unknown. We hypothesized that agouti-related peptide (AgRP) and/or proopiomelanocortin (POMC) neurons of the Arc are critical for the increases in SNA in response to leptin. To test this, we generated mice lacking the leptin receptor specifically in AgRP or POMC neurons by crossing the LepR flox/flox mice with AgRP Cre or POMC Cre mice. Consistent with previous reports, both AgRP Cre/ LepR flox/flox mice and POMC Cre /LepR flox/flox mice have a slightly elevated body weight relative to littermate controls. Next, we used multifiber sympathetic nerve recording to assess the SNA effects of leptin. Intracerebroventricular (ICV) injection of leptin (2 μg) led to a comparable increase in renal SNA in control mice (210±93%) and AgRP Cre /LepR flox/flox mice (191±53%). AgRP Cre/ LepR flox/flox mice also displayed a normal lumbar SNA response to ICV leptin (384±86%) relative to controls (325±46%). In contrast, POMC Cre /LepR flox/flox mice exhibited a significantly reduced renal SNA response to ICV leptin (5±17%) as compared to controls (174±45%). Lumbar SNA response to leptin was also blunted in POMC Cre /LepR flox/flox mice (24±21%) as compared to controls (358±53%). Thus, deletion of the leptin receptor from POMC neurons, but not AgRP neurons, interferes with the ability of leptin to increase sympathetic traffic. These results demonstrate that the sympathetic nerve responses evoked by leptin emanate from leptin action on POMC neurons.

Hypothalamic mTORC1 signaling controls sympathetic nerve activity and arterial pressure and mediates leptin effects

The fundamental importance of the hypothalamus in the regulation of autonomic and cardiovascular functions is well established. However, the molecular processes involved are not well understood. Here, we show that the mammalian (or mechanistic) target of rapamycin (mTOR) signaling in the hypothalamus is tied to the activity of the sympathetic nervous system and to cardiovascular function. Modulation of mTOR complex 1 (mTORC1) signaling caused dramatic changes in sympathetic traffic, blood flow, and arterial pressure. Our data also demonstrate the importance of hypothalamic mTORC1 signaling in transducing the sympathetic and cardiovascular actions of leptin. Moreover, we show that the PI3K pathway links the leptin receptor to mTORC1 signaling and that changes in its activity impact sympathetic traffic and arterial pressure. These findings establish mTORC1 activity in the hypothalamus as a key determinant of sympathetic and cardiovascular regulation and suggest that dysregulated hypothalamic mTORC1 activity may influence the development of cardiovascular diseases.

PI3K signaling: A key pathway in the control of sympathetic traffic and arterial pressure by leptin

The adipocyte-derived hormone, leptin, is a master regulator of energy homeostasis. Leptin action in the central nervous system also contributes to arterial pressure regulation through its capacity to increase renal sympathetic outflow. The accumulating evidence pointing to a key role for leptin in the adverse sympathetic and cardiovascular consequences of excessive adiposity highlight the importance of understanding the mechanisms underlying the sympathetic and cardiovascular effects of leptin. The ability of the leptin receptor to stimulate various intracellular pathways allows leptin to regulate physiological processes in a specific manner. In this review, we examine the role of the PI3K pathway emanating from the leptin receptor in mediating the sympathetic and arterial pressure effects of leptin. We also discuss the relevance of PI3K signaling for obesity-induced hypertension through its role in mediating selective leptin resistance.

Abstract 109: Leptin Acts in POMC Neurons to Increase Sympathetic Nerve Activity

Leptin acts in the brain to decrease food intake and promote energy expenditure by increasing sympathetic nerve activity (SNA) to thermogenic brown adipose tissue (BAT). Leptin also increases SNA to other beds including kidney with implications for obesity-associated hypertension. We previously demonstrated the importance the arcuate nucleus (Arc) of the hypothalamus in mediating leptin-induced increases in regional SNA, but the specific neuronal population within the Arc that mediates these responses is unknown. We hypothesized that proopiomelanocortin (POMC) neurons of the Arc are critical for the increases in SNA in response to leptin. To test this, we generated mice lacking the leptin receptor (ObR) specifically in POMC neurons (ObR flox/flox / POMC Cre ). We used multifiber sympathetic nerve recording to assess the SNA effects of leptin. ObR flox/flox /POMC Cre mice exhibited a significantly (P<0.01) blunted renal SNA response to intravenous administration of 120 μg of leptin (4±18%) as compared to wild type controls (161±35%). Intracerebroventricular (ICV) injection of leptin (2 μg) also increased renal SNA in wild type mice (220±77%), but not in ObR flox/flox /POMC Cre mice (-1±13%). In contrast, the renal SNA response to ICV insulin (100 μU) was not different (P=0.27) in ObR flox/flox /POMC Cre mice (150±24%) as compared to littermate controls (195±68%) demonstrating that the loss of renal SNA response to leptin in mice lacking ObR in POMC neurons is a specific effect. Next, we investigated the requirement of ObR in POMC neurons for the sympathoexcitatory effects of leptin in other beds. Strikingly, ObR flox/flox /POMC Cre mice had a significantly (P<0.05) blunted lumbar SNA response to ICV leptin (14±22%) as compared to controls (226±38%). In addition, the BAT SNA response to ICV leptin was attenuated (P<0.01) in ObR flox/flox /POMC Cre mice (126±28%) relative to controls (309±80%). These results demonstrate the leptin receptor signaling in POMC neurons in the control of regional sympathetic nerve activity.

Abstract 21: Leptin-Induced Arterial Pressure Increase: Key Role of the PI3K Pathway

Leptin is an adipocyte-derived hormone that acts in the brain to decrease food intake and increases energy expenditure. Leptin also raises arterial pressure by increasing sympathetic nerve activity (SNA). We previously implicated PI3K as a downstream pathway in leptin receptor (ObRb) signaling that mediates the renal SNA effects of leptin. We used genetic approaches to test the hypothesis that bidirectional changes in PI3K activity (gain vs. loss of function) will be associated with contrasting changes in renal SNA and arterial pressure responses to leptin. PI3K signaling was enhanced in leptin-sensitive neurons in Pten ΔObRb mice in which the lipid phosphatase Pten was ablated in cells that express ObRb, leading to accumulating PIP3 in these cells. For loss of function, we used p110α D933A/WT mice which carry a heterozygous mutation in the p110α subunit of PI3K specifically disrupting this pathway. Consistent with our previous findings, the increase in renal SNA caused by intracerebralventricular (ICV) injection of leptin (2 μg) was exaggerated and blunted in Pten ΔObRb mice (286±41%) and p110α D933A/WT mice (-40±6%), respectively, relative to littermate controls (70±4%). These effects were specific to leptin, as the renal SNA response to ICV MTII (melanocortin receptors agonist) was comparable (P=0.3) between controls (196±36%), Pten ΔObRb mice (159±98%) and p110α D933A/WT mice (194±36%). Interestingly, the Pten ΔObRb mice had significantly (P<0.01) higher baseline radiotelemetric mean arterial pressure (MAP, 137±4 mmHg) as compared to littermate controls (107±2 mmHg). In contrast, despite being hyperleptinemic the p110α D933A/WT mice had normal (P=0.34) baseline MAP (112±1 mmHg) as compared to controls (113±2 mmHg). We further measured the arterial pressure response to leptin treatment. Intraperitoneal administration of leptin (60 μg) raised MAP in control mice by 13±5 mmHg and this response was significantly (P<0.05) enhanced in the Pten ΔObRb mice (+35±14 mmHg). Conversely, the p110α D933A/WT mice had a significantly blunted (P<0.05) MAP response to leptin (-3±4 mmHg). Our data demonstrate the importance of PI3K signaling in leptin receptor-containing neurons for the regulation of arterial pressure and in mediating leptin effects.

Ablation of the leptin receptor in the hypothalamic arcuate nucleus abrogates leptin-induced sympathetic activation

RATIONALE

The hypothalamic arcuate nucleus (ARC) is considered a major site for leptin signaling that regulates several physiological processes.

OBJECTIVE

To test the hypothesis that leptin receptor in the ARC is required to mediate leptin-induced sympathetic activation.

METHODS AND RESULTS

First, we used the ROSA Cre-reporter mice to establish the feasibility of driving Cre expression in the ARC in a controlled manner with bilateral microinjection of adenovirus-expressing Cre-recombinase (Ad-Cre). Ad-Cre microinjection into the ARC of ObR(flox/flox) mice robustly reduced ObR expression and leptin-induced Stat3 activation in the ARC but not in the adjacent nuclei, confirming the efficacy and selectivity of the ARC deletion of ObR. Critically, deletion of ObR in the ARC attenuated brown adipose tissue and renal sympathetic nerve responses to leptin. We also examined whether ObR in the ARC is required for the preserved leptin-induced increase in renal sympathetic activity in dietary obesity. We found that deletion of ARC ObR abrogated leptin-induced increases in renal sympathetic discharge and resolved arterial pressure elevation in diet-induced obese ObR(flox/flox) mice.

CONCLUSIONS

These data demonstrate a critical role for ObR in the ARC in mediating the sympathetic nerve responses to leptin and in the adverse sympathoexcitatory effects of leptin in obesity.

Cardiovascular and sympathetic effects of disrupting tyrosine 985 of the leptin receptor

Leptin acts in the brain to regulate food intake and energy expenditure. Leptin also increases renal sympathetic nerve activity and arterial pressure. The divergent signaling capacities of the leptin receptor (ObRb) mediate the stimulation of various intracellular pathways that are important for leptin control of physiological processes. We evaluated the cardiovascular and sympathetic consequences of disrupting the signal emanating from tyrosine985 of ObRb. For this, we used Lepr(L985) (l/l) mice, which carry a loss of function mutation replacing tyrosine985 of ObRb with leucine. Body weight of l/l mice was not significantly different from wild-type controls. In contrast, radiotelemetry measurements revealed that the l/l mice had higher arterial pressure and heart rate as compared with controls. Ganglionic blockade caused a greater arterial pressure fall in the l/l mice relative to controls. In addition, leptin treatment induced a larger increase in arterial pressure and heart rate in the l/l versus wild-type mice. Finally, we compared the response of renal and brown adipose tissue sympathetic nerve activity to intracerebroventricular injection of leptin (2 μg) between l/l and control mice. Leptin-induced increase in renal sympathetic nerve activity was greater in l/l mice relative to controls. In contrast, the brown adipose tissue sympathetic nerve activity response to leptin was attenuated in the l/l mice relative to controls. These data indicate that selective loss of leptin receptor signaling emanating from tyrosine985 enhances the cardiovascular and renal sympathetic effects of leptin. These findings provide important insight into the molecular mechanisms underlying leptin's effects on the sympathetic cardiovascular function and arterial pressure.

Requirement of TCTG(G/C) Direct Repeats and Overlapping GATA Site for Maintaining the Cardiac-Specific Expression of Cardiac troponin T in Developing and Adult Mice

The cardiac-specific -497 bp promoter of rat cardiac troponin T (cTnT) contains two similar modules, D and F, each of which possesses TCTG(G/C) direct repeats and A/T-rich sites. To identify cis-elements critical for cardiac specificity, a -249 bp promoter containing only module F and its site-directed mutations were used to generate transgenic mice. Transgene expression of the -249 bp promoter remained cardiac-specific, despite low and nonuniform expression. The nonuniform expression pattern of the transgene coincided with differential expression of HMGB1, which appeared to be the predominant form of HMGB family proteins in the heart. The HMGB1 binds to the A/T-rich/MEF2-like sites of the cTnT promoter, as determined by chromatin immunoprecipitation assays. Mice carrying the -249 bp promoter with point mutations disrupting the direct repeats expressed transgene at lower levels in the heart and ectopically in the brain. Ectopic expression of transgene was also observed in developing limbs and head. These results suggest an important role for the direct repeat in determining the cardiac specificity. Furthermore, mice carrying a mutant promoter simultaneously disrupting the direct repeats and overlapping GATA site failed to express the transgene in any tissues tested. Therefore, the direct repeat and overlapping GATA site are critical for the expression level and cardiac specificity. The F module controls one level of cardiac specificity. For a uniform and high level of cardiac-specific expression, the upstream element (-497 to -250 bp) is further required, possibly through the D enhancer module and the combination of Nkx2.5 and GATA sites.

The intercalated disk protein, mXinalpha, is capable of interacting with beta-catenin and bundling actin filaments [corrected]

Targeted deletion of mXinalpha results in cardiac hypertrophy and cardiomyopathy with conduction defects (Gustafson-Wagner, E., Sinn, H. W., Chen, Y.-L., Wang, D.-Z., Reiter, R. S., Lin, J. L.-C., Yang, B., Williamson, R. A., Chen, J. N., Lin, C.-I., and Lin, J. J.-C. (2007) Am. J. Physiol. 293, H2680-H2692). To understand the underlying mechanisms leading to such cardiac defects, the functional domains of mXinalpha and its interacting proteins were investigated. Interaction studies using co-immunoprecipitation, pull-down, and yeast two-hybrid assays revealed that mXinalpha directly interacts with beta-catenin. The beta-catenin-binding site on mXinalpha was mapped to amino acids 535-636, which overlaps with the known actin-binding domains composed of the Xin repeats. The overlapping nature of these domains provides insight into the molecular mechanism for mXinalpha localization and function. Purified recombinant glutathione S-transferase- or His-tagged mXinalpha proteins are capable of binding and bundling actin filaments, as determined by co-sedimentation and electron microscopic studies. The binding to actin was saturated at an approximate stoichiometry of nine actin monomers to one mXinalpha. A stronger interaction was observed between mXinalpha C-terminal deletion and actin as compared with the interaction between full-length mXinalpha and actin. Furthermore, force expression of green fluorescent protein fused to an mXinalpha C-terminal deletion in cultured cells showed greater stress fiber localization compared with force-expressed GFP-mXinalpha. These results suggest a model whereby the C terminus of mXinalpha may prevent the full-length molecule from binding to actin, until the beta-catenin-binding domain is occupied by beta-catenin. The binding of mXinalpha to beta-catenin at the adherens junction would then facilitate actin binding. In support of this model, we found that the actin binding and bundling activity of mXinalpha was enhanced in the presence of beta-catenin.

Characterization of cis-regulatory elements and transcription factor binding: gel mobility shift assay

To understand how cardiac gene expression is regulated, the identification and characterization of cis-regulatory elements and their trans-acting factors by gel mobility shift assay (GMSA) or gel retardation assay are essential and common steps. In addition to providing a general protocol for GMSA, this chapter describes some applications of this assay to characterize cardiac-specific and ubiquitous trans-acting factors bound to regulatory elements [novel TCTG(G/C) direct repeat and A/T-rich region] of the rat cardiac troponin T promoter. In GMSA, the specificity of the binding of trans-acting factor to labeled DNA probe should be verified by the addition of unlabeled probe in the reaction mixture. The migratory property of DNA-protein complexes formed by protein extracts prepared from different tissues can be compared to determine the tissue specificity of trans-acting factors. GMSA, coupled with specific antibody to trans-acting factor (antibody supershift assay), is used to identify proteins present in the DNA-protein complex. The gel-shift competition assay with an unlabeled probe containing a slightly different sequence is a powerful technique used to assess the sequence specificity and relative binding affinity of a DNA-protein interaction. GMSA with SDS-PAGE fractionated proteins allows for the determination of the apparent molecular mass of bound trans-acting factor.