Th1 effector T cells selectively orchestrate cardiac fibrosis in nonischemic heart failure

Despite emerging data indicating a role for T cells in profibrotic cardiac repair and healing after ischemia, little is known about whether T cells directly impact cardiac fibroblasts (CFBs) to promote cardiac fibrosis (CF) in nonischemic heart failure (HF). Recently, we reported increased T cell infiltration in the fibrotic myocardium of nonischemic HF patients, as well as the protection from CF and HF in TCR-α^-/- mice. Here, we report that T cells activated in such a context are mainly IFN-γ⁺, adhere to CFB, and induce their transition into myofibroblasts. Th1 effector cells selectively drive CF both in vitro and in vivo, whereas adoptive transfer of Th1 cells, opposite to activated IFN-γ^-/- Th cells, partially reconstituted CF and HF in TCR-α^-/- recipient mice. Mechanistically, Th1 cells use integrin α4 to adhere to and induce TGF-β in CFB in an IFN-γ-dependent manner. Our findings identify a previously unrecognized role for Th1 cells as integrators of perivascular CF and cardiac dysfunction in nonischemic HF.

Abstract 154: CCDC80 Functions as a Protein Kinase GI Substrate and is Secreted by Cardiac Myocytes

Background: Protein kinase G I alpha (PKGIa) inhibits cardiac hypertrophy, remodeling, and dysfunction. Downstream PKGI substrates remain incompletely understood and represent potential novel therapeutic targets for myocardial disease. We previously identified through a molecular screen that PKGIa binds and phosphorylates the protein coiled-coiled domain containing 80 (Ccdc80; also termed SSG1 and URB) in vascular smooth muscle cells. Previous work also identified that Ccdc80 is secreted from adipocytes. However, the expression and secretion of Ccdc80 from the cardiac myocyte has not been investigated. The current study tested the hypothesis that Ccdc80 is expressed in and secreted from the cardiac myocyte.

Results: In cultured rat cardiac myocytes (CM), we detected Ccdc80 by western blot. Western blot for Ccdc80 also detected a band of the predicted Ccdc80 molecular weight present in media from these cells, but not in uncultured media. Ccdc80 could be detected in the human left ventricle (LV), though expression did not differ between hearts of normal controls and patients with hypertrophic cardiomyopathy. In the setting of LV pressure overload induced by transaortic constriction (TAC), we observed an increase in Ccdc80 expression in 1 week TAC LVs, compared with sham LVs (5.0 +/- 0.3 arbitrary densitometric units in sham versus 9.6 +/- 0.9 in TAC; n=4 per group).

Conclusion: Taken together, our findings identify that the PKGIa substrate Ccdc80 expresses in cardiac myocytes, becomes secreted from CMs, resides in the human heart, and increases in expression in the mouse LV in response to pressure overload. Given the anti-remodeling role of PKGIa, these findings support future studies to understand the in vivo role of Ccdc80 in the cardiovascular system. Future studies will also explore the significance of Ccdc80 secretion from the CM and its potential regulation by PKG.

Reduced activin receptor-like kinase 1 activity promotes cardiac fibrosis in heart failure

INTRODUCTION

Activin receptor-like kinase 1 (ALK1) mediates signaling via the transforming growth factor beta-1 (TGFβ1), a pro-fibrogenic cytokine. No studies have defined a role for ALK1 in heart failure.

HYPOTHESIS

We tested the hypothesis that reduced ALK1 expression promotes maladaptive cardiac remodeling in heart failure.

METHODS AND RESULTS

In patients with advanced heart failure referred for left ventricular (LV) assist device implantation, LV Alk1 mRNA and protein levels were lower than control LV obtained from patients without heart failure. To investigate the role of ALK1 in heart failure, Alk1 haploinsufficient (Alk1^+/-) and wild-type (WT) mice were studied 2 weeks after severe transverse aortic constriction (TAC). LV and lung weights were higher in Alk1^+/- mice after TAC. Cardiomyocyte area and LV mRNA levels of brain natriuretic peptide and β-myosin heavy chain were increased similarly in Alk1^+/- and WT mice after TAC. Alk-1 mice exhibited reduced Smad 1 phosphorylation and signaling compared to WT mice after TAC. Compared to WT, LV fibrosis and Type 1 collagen mRNA and protein levels were higher in Alk1^+/- mice. LV fractional shortening was lower in Alk1^+/- mice after TAC.

CONCLUSIONS

Reduced expression of ALK1 promotes cardiac fibrosis and impaired LV function in a murine model of heart failure. Further studies examining the role of ALK1 and ALK1 inhibitors on cardiac remodeling are required.

Short-Term Administration of Serelaxin Produces Predominantly Vascular Benefits in the Angiotensin II/L-NAME Chronic Heart Failure Model

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Temporary administration of recombinant relaxin-2 (serelaxin) in patients hospitalized with HF was associated with improved mortality 6 months after discharge.

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The specific effects of serelaxin on vascular and myocardial structure and function in HF have not been studied.

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In mice subjected to continuous 28-day heart failure stimulus of AngII and L-NAME, serelaxin was administered for 3 days (days 7 to 9), and both the acute effects during serelaxin infusion and the delayed effects after termination of serelaxin on cardiovascular structure and function were studied.

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Temporary serelaxin improved vascular fibrosis and myocardial capillary density and reduced resistance vessel constriction to potassium chloride during administration. These effects unexpectedly persisted 19 days after discontinuation of serelaxin, despite continued exposure to AngII/L-NAME. Serelaxin did not alter cardiac hypertrophy, geometry, or dysfunction at either time point.

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These findings support that serelaxin predominantly affects vascular structure and function in the setting of HF.

In patients hospitalized with acute heart failure, temporary serelaxin infusion reduced 6-month mortality through unknown mechanisms. This study therefore explored the cardiovascular effects of temporary serelaxin administration in mice subjected to the angiotensin II (AngII)/L-NG-nitroarginine methyl ester (L-NAME) heart failure model, both during serelaxin infusion and 19 days post–serelaxin infusion. Serelaxin administration did not alter AngII/L-NAME-induced cardiac hypertrophy, geometry, or dysfunction. However, serelaxin-treated mice had reduced perivascular left ventricular fibrosis and preserved left ventricular capillary density at both time points. Furthermore, resistance vessels from serelaxin-treated mice displayed decreased potassium chloride–induced constriction and reduced aortic fibrosis. These findings suggest that serelaxin improves outcomes in patients through vascular-protective effects.

Abstract 251: Mixed Lineage Kinase 3 Functions as a Protein Kinase G I Alpha Antiremodeling Substrate in the Heart

Background: cGMP-dependent protein kinase G I α (PKGIα) via its leucine zipper (LZ) domain prevents adverse cardiac remodeling. Identifying and characterizing PKGIα-LZ dependent substrates may reveal novel therapeutic targets in the myocardium. We previously identified the LZ-containing Mixed Lineage Kinase 3 (MLK3) as a potential PKGIα substrate. Further, MLK3 whole body knockout mice had increased LV hypertrophy and dysfunction after pressure overload. In this study we sought to further explore the PKGIα-MLK3 interaction, and to investigate MLK3 in human cardiomyopathy.

Results: We first tested for a direct interaction between PKGIα and MLK3. Using affinity purified recombinant proteins we observed co-precipitation of PKGIα and MLK3 that was disrupted by mutation of the PKGIα LZ domain (LZ mutation: MLK3 binding decreased by 61.61% ±13.6, n=4). In mouse heart the interaction between native MLK3 and PKGIα was observed by co-immunoprecipitation (n=3). PKGIα phosphorylated MLK3 at the activation loop in vitro, which was attenuated by inhibiting PKGIα kinase function (n=3).

We next tested if MLK3 regulates hypertrophy of cultured cardiomyocytes. Adult rat ventricular cardiomyocytes treated with the MLK3 inhibitor URMC-099 (100 nM, 48 hrs) exhibited increased cell size compared to vehicle treated cells (23.3% increase ± 3.64 SEM vs DMSO vehicle, n=3, 50 cells per treatment).

Finally, we examined MLK3 expression in hearts from human patients with non-ischemic or hypertrophic cardiomyopathy. Compared to normal LV samples (NDRI, n=4), MLK3 expression was markedly elevated in both non-ischemic and hypertrophic cardiomyopathy LV samples (MLK3/GAPDH: non-ischemic: 6.26 ADU ± 0.85, n=9, hypertrophic: 6.97 ADU ± 1.42, n=8).

Conclusion: These data support a model in which PKGIa directly binds and activates MLK3, leading in the cardiomyocyte to repression of cellular hypertrophy. Our findings in tissue from human failing hearts further suggest that MLK3 upregulation may act as a compensatory anti-remodeling signal in the setting of cardiac dysfunction, which ultimately becomes overwhelmed by pro-remodeling signals. More broadly our findings support that identifying PKGIa LZ-dependent substrates can reveal novel anti-remodeling molecules.

Intercellular Adhesion Molecule 1 Regulates Left Ventricular Leukocyte Infiltration, Cardiac Remodeling, and Function in Pressure Overload-Induced Heart Failure

Background

Left ventricular dysfunction and heart failure are strongly associated in humans with increased circulating levels of proinflammatory cytokines, T cells, and soluble intercellular cell adhesion molecule 1 (ICAM1). In mice, infiltration of T cells into the left ventricle contributes to pathological cardiac remodeling, but the mechanisms regulating their recruitment to the heart are unclear. We hypothesized that ICAM1 regulates cardiac inflammation and pathological cardiac remodeling by mediating left ventricular T‐cell recruitment and thus contributing to cardiac dysfunction and heart failure.

Methods and Results

In a mouse model of pressure overload–induced heart failure, intramyocardial endothelial ICAM1 increased within 48 hours in response to thoracic aortic constriction and remained upregulated as heart failure progressed. ICAM1‐deficient mice had decreased T‐cell and proinflammatory monocyte infiltration in the left ventricle in response to thoracic aortic constriction, despite having numbers of circulating T cells and activated T cells in the heart‐draining lymph nodes that were similar to those of wild‐type mice. ICAM1‐deficient mice did not develop cardiac fibrosis or systolic and diastolic dysfunction in response to thoracic aortic constriction. Exploration of the mechanisms regulating ICAM1 expression revealed that endothelial ICAM1 upregulation and T‐cell infiltration were not mediated by endothelial mineralocorticoid receptor signaling, as demonstrated in thoracic aortic constriction studies in mice with endothelial mineralocorticoid receptor deficiency, but rather were induced by the cardiac cytokines interleukin 1β and 6.

Conclusions

ICAM1 regulates pathological cardiac remodeling by mediating proinflammatory leukocyte infiltration in the left ventricle and cardiac fibrosis and dysfunction and thus represents a novel target for treatment of heart failure.

Molecular Screen Identifies Cardiac Myosin-Binding Protein-C as a Protein Kinase G-Iα Substrate

BACKGROUND

Pharmacological activation of cGMP-dependent protein kinase G I (PKGI) has emerged as a therapeutic strategy for humans with heart failure. However, PKG-activating drugs have been limited by hypotension arising from PKG-induced vasodilation. PKGIα antiremodeling substrates specific to the myocardium might provide targets to circumvent this limitation, but currently remain poorly understood.

METHODS AND RESULTS

We performed a screen for myocardial proteins interacting with the PKGIα leucine zipper (LZ)-binding domain to identify myocardial-specific PKGI antiremodeling substrates. Our screen identified cardiac myosin-binding protein-C (cMyBP-C), a cardiac myocyte-specific protein, which has been demonstrated to inhibit cardiac remodeling in the phosphorylated state, and when mutated leads to hypertrophic cardiomyopathy in humans. GST pulldowns and precipitations with cGMP-conjugated beads confirmed the PKGIα-cMyBP-C interaction in myocardial lysates. In vitro studies demonstrated that purified PKGIα phosphorylates the cMyBP-C M-domain at Ser-273, Ser-282, and Ser-302. cGMP induced cMyBP-C phosphorylation at these residues in COS cells transfected with PKGIα, but not in cells transfected with LZ mutant PKGIα, containing mutations to disrupt LZ substrate binding. In mice subjected to left ventricular pressure overload, PKGI activation with sildenafil increased cMyBP-C phosphorylation at Ser-273 compared with untreated mice. cGMP also induced cMyBP-C phosphorylation in isolated cardiac myocytes.

CONCLUSIONS

Taken together, these data support that PKGIα and cMyBP-C interact in the heart and that cMyBP-C is an anti remodeling PKGIα kinase substrate. This study provides the first identification of a myocardial-specific PKGIα LZ-dependent antiremodeling substrate and supports further exploration of PKGIα myocardial LZ substrates as potential therapeutic targets for heart failure.

Left Ventricular T-Cell Recruitment Contributes to the Pathogenesis of Heart Failure

BACKGROUND

Despite the emerging association between heart failure (HF) and inflammation, the role of T cells, major players in chronic inflammation, has only recently begun to be explored. Whether T-cell recruitment to the left ventricle (LV) participates in the development of HF requires further investigation to identify novel mechanisms that may serve for the design of alternative therapeutic interventions.

METHODS AND RESULTS

Real-time videomicroscopy of T cells from nonischemic HF patients or from mice with HF induced by transverse aortic constriction revealed enhanced adhesion to activated vascular endothelial cells under flow conditions in vitro compared with T cells from healthy subjects or sham mice. T cells in the mediastinal lymph nodes and the intramyocardial endothelium were both activated in response to transverse aortic constriction and the kinetics of LV T-cell infiltration was directly associated with the development of systolic dysfunction. In response to transverse aortic constriction, T cell-deficient mice (T-cell receptor, TCRα(-/-)) had preserved LV systolic and diastolic function, reduced LV fibrosis, hypertrophy and inflammation, and improved survival compared with wild-type mice. Furthermore, T-cell depletion in wild-type mice after transverse aortic constriction prevented HF.

CONCLUSIONS

T cells are major contributors to nonischemic HF. Their activation combined with the activation of the LV endothelium results in LV T-cell infiltration negatively contributing to HF progression through mechanisms involving cytokine release and induction of cardiac fibrosis and hypertrophy. Reduction of T-cell infiltration is thus identified as a novel translational target in HF.

Signaling Pathways and Gene Regulatory Networks in Cardiomyocyte Differentiation

Strategies for harnessing stem cells as a source to treat cell loss in heart disease are the subject of intense research. Human pluripotent stem cells (hPSCs) can be expanded extensively in vitro and therefore can potentially provide sufficient quantities of patient-specific differentiated cardiomyocytes. Although multiple stimuli direct heart development, the differentiation process is driven in large part by signaling activity. The engineering of hPSCs to heart cell progeny has extensively relied on establishing proper combinations of soluble signals, which target genetic programs thereby inducing cardiomyocyte specification. Pertinent differentiation strategies have relied as a template on the development of embryonic heart in multiple model organisms. Here, information on the regulation of cardiomyocyte development from in vivo genetic and embryological studies is critically reviewed. A fresh interpretation is provided of in vivo and in vitro data on signaling pathways and gene regulatory networks (GRNs) underlying cardiopoiesis. The state-of-the-art understanding of signaling pathways and GRNs presented here can inform the design and optimization of methods for the engineering of tissues for heart therapies.

Exposure to experimental preeclampsia in mice enhances the vascular response to future injury

Cardiovascular disease (CVD) remains the leading killer of women in developed nations. One sex-specific risk factor is preeclampsia, a syndrome of hypertension and proteinuria that complicates 5% of pregnancies. Although preeclampsia resolves after delivery, exposed women are at increased long-term risk of premature CVD and mortality. Pre-existing CVD risk factors are associated with increased risk of developing preeclampsia but whether preeclampsia merely uncovers risk or contributes directly to future CVD remains a critical unanswered question. A mouse preeclampsia model was used to test the hypothesis that preeclampsia causes an enhanced vascular response to future vessel injury. A preeclampsia-like state was induced in pregnant CD1 mice by overexpressing soluble fms-like tyrosine kinase-1, a circulating antiangiogenic protein that induces hypertension and glomerular disease resembling human preeclampsia. Two months postpartum, soluble fms-like tyrosine kinase-1 levels and blood pressure normalized and cardiac size and function by echocardiography and renal histology were indistinguishable in preeclampsia-exposed compared with control mice. Mice were then challenged with unilateral carotid injury. Preeclampsia-exposed mice had significantly enhanced vascular remodeling with increased vascular smooth muscle cell proliferation (180% increase; P<0.01) and vessel fibrosis (216% increase; P<0.001) compared with control pregnancy. In the contralateral uninjured vessel, there was no difference in remodeling after exposure to preeclampsia. These data support a new model in which vessels exposed to preeclampsia retain a persistently enhanced vascular response to injury despite resolution of preeclampsia after delivery. This new paradigm may contribute to the substantially increased risk of CVD in woman exposed to preeclampsia.

Abstract 186: Identification of Novel Protein Kinase G I Alpha Antiremodeling Substrates in the Myocardium

Protein kinase G I α (PKGIα) inhibits cardiac remodeling, and this effect requires the PKGIα leucine zipper (LZ) binding domain. However, PKGIα LZ-dependent cardiac substrates remain poorly understood. Clinical trials of PKGI activating drugs have been limited to date by hypotension arising from vascular PKGI activation. Therefore, we explored downstream PKGIα substrates in the heart which may inhibit remodeling, yet circumvent the hypotensive effects of systemic PKGI activation. A screen for PKGIα LZ-interacting proteins identified: 1)cardiac myosin binding protein-C (cMyBP-C) and 2) mixed lineage kinase 3 (MLK3).

cMyBP-C is a cardiac myocyte protein known to inhibit remodeling when phosphorylated. Co-precipitations with cGMP-conjugated beads confirmed the PKGIα-cMyBP-C interaction. Purified PKGIα phosphorylated cMyBP-C in vitro at Ser-273, Ser-282, and Ser-302. cGMP induced cMyBP-C phosphorylation at these sites in COS cells transfected with WT PKGIα, but not in cells transfected with either LZ mutant PKGIα or kinase-inactive PKGIα. In hearts of 9 month old PKGIα Leucine Zipper mutant mice, which have LV hypertrophy (LVH) and diastolic dysfunction, we observed decreased phosphorylated cMyBP-C as well as decreased total cMyBP-C, compared with WT littermate hearts.

We next tested the effect of MLK3, which interacts with PKGIα in the heart, on remodeling in vivo. We performed 7 day Transaortic Constriction (TAC) on MLK3 KO mice and WT littermates (n=5 shams, 8 TAC per genotype). MLK3 KO TAC mice had increased LVH (LV mass/tibia length 71.1 ± 2.7 g/cm KO TAC vs 62.1 ± 2.7 WT TAC; p<0.05). Further, MLK3 KO mice developed overt CHF compared with WT littermates (LV end diastolic pressure 14.8 ± 1.9 mmHg KO TAC vs 7.7 ± 2.1 WT TAC, p <0.05), as well as accelerated decrements in LV preload recruitable stroke work (36.6 ± 11.9 mmHg/ul KO TAC vs 94.6 ± 12.9 WT TAC, p<0.05) and min dP/dt (-6292 ± 519 mmHg/s KO TAC vs −8157 ± 554 WT TAC , p <0.05). We observed no differences in LV structure or function between sham genotypes.

These studies reveal 2 novel PKGIα anti-remodeling substrates, and they support that exploring PKGIα substrates in the heart may identify novel therapeutic targets to inhibit cardiac remodeling but avoid excessive PKGI induced vasodilation.

Glycogen synthase kinase-3β inhibition ameliorates cardiac parasympathetic dysfunction in type 1 diabetic Akita mice

Decreased heart rate variability (HRV) is a major risk factor for sudden death and cardiovascular disease. We previously demonstrated that parasympathetic dysfunction in the heart of the Akita type 1 diabetic mouse was due to a decrease in the level of the sterol response element-binding protein (SREBP-1). Here we demonstrate that hyperactivity of glycogen synthase kinase-3β (GSK3β) in the atrium of the Akita mouse results in decreased SREBP-1, attenuation of parasympathetic modulation of heart rate, measured as a decrease in the high-frequency (HF) fraction of HRV in the presence of propranolol, and a decrease in expression of the G-protein coupled inward rectifying K(+) (GIRK4) subunit of the acetylcholine (ACh)-activated inward-rectifying K(+) channel (IKACh), the ion channel that mediates the heart rate response to parasympathetic stimulation. Treatment of atrial myocytes with the GSK3β inhibitor Kenpaullone increased levels of SREBP-1 and expression of GIRK4 and IKACh, whereas a dominant-active GSK3β mutant decreased SREBP-1 and GIRK4 expression. In Akita mice treated with GSK3β inhibitors Li(+) and/or CHIR-99021, Li(+) increased IKACh, and Li(+) and CHIR-99021 both partially reversed the decrease in HF fraction while increasing GIRK4 and SREBP-1 expression. These data support the conclusion that increased GSK3β activity in the type 1 diabetic heart plays a critical role in parasympathetic dysfunction through an effect on SREBP-1, supporting GSK3β as a new therapeutic target for diabetic autonomic neuropathy.

PDE5 inhibitor efficacy is estrogen dependent in female heart disease

Inhibition of cGMP-specific phosphodiesterase 5 (PDE5) ameliorates pathological cardiac remodeling and has been gaining attention as a potential therapy for heart failure. Despite promising results in males, the efficacy of the PDE5 inhibitor sildenafil in female cardiac pathologies has not been determined and might be affected by estrogen levels, given the hormone's involvement in cGMP synthesis. Here, we determined that the heart-protective effect of sildenafil in female mice depends on the presence of estrogen via a mechanism that involves myocyte eNOS-dependent cGMP synthesis and the cGMP-dependent protein kinase Iα (PKGIα). Sildenafil treatment failed to exert antiremodeling properties in female pathological hearts from Gαq-overexpressing or pressure-overloaded mice after ovary removal; however, estrogen replacement restored the effectiveness of sildenafil in these animals. In females, sildenafil-elicited myocardial PKG activity required estrogen, which stimulated tonic cardiomyocyte cGMP synthesis via an eNOS/soluble guanylate cyclase pathway. In contrast, eNOS activation, cGMP synthesis, and sildenafil efficacy were not estrogen dependent in male hearts. Estrogen and sildenafil had no impact on pressure-overloaded hearts from animals expressing dysfunctional PKGIα, indicating that PKGIα mediates antiremodeling effects. These results support the importance of sex differences in the use of PDE5 inhibitors for treating heart disease and the critical role of estrogen status when these agents are used in females.

Abstract 187: Identification of Cardiac-specific Downstream Substrates of Protein Kinase G I as Potential Novel Anti Cardiac Remodeling Targets

We recently reported that mutation of the cGMP-dependent Protein Kinase G I alpha (PKGIα) N-terminal leucine zipper (LZ) domain (in the PKGIα LZ mutant, or LZM, mouse) accelerates LV remodeling and heart failure after TAC, and prevents the anti-remodeling effect of sildenafil. We therefore hypothesized that PKGIα attenuates remodeling by regulating cardiac signaling pathways that are dependent on substrate interactions mediated by its LZ domain. As a first step to identifying cardiac proteins downstream of PKGIα, we screened myocardial lysates for PKGIα LZ domain-interacting proteins.

Our previous work revealed a requirement for the PKGIα LZ domain for the activation of anti-remodeling myocardial JNK activity after LV pressure overload. MLK3 is an MAPKKK that contains an LZ domain and activates JNK. We now demonstrate, by immunoprecipitation, that MLK 3 interacts with the PKGIα LZ domain in myocardial lysates. We show further that 8-Br-cGMP induces MLK3 phosphorylation on Threonine 277 and Serine 281 in WT, but not LZM myocardial lysates. And, in 293 cells transfected with FLAG-MLK3, 8Br-cGMP induced PKGIα-MLK3 co-precipitation, and increased phosphorylation of MLK3 on Thr277/Ser281. Co-transfection of MLK3 and PKGIα also induced MLK3 phosphorylation at the same sites. We next examined the cardiovascular effect of MLK3 deletion in vivo. Male 8 week old MLK3 -/- mice display basal bi-ventricular hypertrophy compared with littermate controls (LV/Tibia length 42.8 + 0.6 mg/cm in WT, 52.9 + 1.8 in MLK3 -/-; P <0.01; RV/TL 10.8 + 0.1 mg/cm in WT, 13.3 + 0.3 in MLK3 -/-; P <0.01; n= 7 WT, 5 MLK3 -/-). By 14-16 weeks of age, LVH progressed in the MLK3 -/- mice (LV/TL 47.7 + 1.3 mg/cm in WT, 59.8 + 7.5 in MLK3-/-; n= 6 WT, 9 MLK3-.-; P <0.01). Arterial blood pressure was modestly increased, though still normal, in the MLK3 -/- mice (SBP 93 + 1 in WT, 113 + 1 in MLK3 -/-). And, 14-16 week MLK3 -/- mice have impaired LV diastolic function (tau 3.2 + 0.1 ms WT, 3.7 + 0.1 MLK3-/-; P 0.06).

Our studies reveal a previously unknown function of MLK3 as a myocardial PKGIα effector and inhibitor of LVH. Together these results support the strategy of exploring LZ-dependent PKGIα substrates in the myocardium to identify novel therapeutic targets for cardiac remodeling.

Steroid-sensitive gene 1 is a novel cyclic GMP-dependent protein kinase I substrate in vascular smooth muscle cells

NO, via its second messenger cGMP, activates protein kinase GI (PKGI) to induce vascular smooth muscle cell relaxation. The mechanisms by which PKGI kinase activity regulates cardiovascular function remain incompletely understood. Therefore, to identify novel protein kinase G substrates in vascular cells, a λ phage coronary artery smooth muscle cell library was constructed and screened for phosphorylation by PKGI. The screen identified steroid-sensitive gene 1 (SSG1), which harbors several predicted PKGI phosphorylation sites. We observed direct and cGMP-regulated interaction between PKGI and SSG1. In cultured vascular smooth muscle cells, both the NO donor S-nitrosocysteine and atrial natriuretic peptide induced SSG1 phosphorylation, and mutation of SSG1 at each of the two predicted PKGI phosphorylation sites completely abolished its basal phosphorylation by PKGI. We detected high SSG1 expression in cardiovascular tissues. Finally, we found that activation of PKGI with cGMP regulated SSG1 intracellular distribution.

Mutation of the protein kinase I alpha leucine zipper domain produces hypertension and progressive left ventricular hypertrophy: a novel mouse model of age-dependent hypertensive heart disease

Hypertensive heart disease causes significant mortality in older patients, yet there is an incomplete understanding of molecular mechanisms that regulate age-dependent hypertensive left ventricular hypertrophy (LVH). Therefore, we tested the hypothesis that the cGMP-dependent protein kinase G I alpha (PKGIα) attenuates hypertensive LVH by evaluating the cardiac phenotype in mice with selective mutations of the PKGIα leucine zipper domain. These leucine zipper mutant (LZM) mice develop basal hypertension. Compared with wild-type controls, 8-month-old adult LZM mice developed increased left ventricular end-diastolic pressure but without frank LVH. In advanced age (15 months), the LZM mice developed overt pathological LVH. These findings reveal a role of PKGIα in normally attenuating hypertensive LVH. Therefore, mutation of the PKGIα LZ domain produces a clinically relevant model for hypertensive heart disease of aging.

Protein kinase g iα inhibits pressure overload-induced cardiac remodeling and is required for the cardioprotective effect of sildenafil in vivo

BACKGROUND

Cyclic GMP (cGMP) signaling attenuates cardiac remodeling, but it is unclear which cGMP effectors mediate these effects and thus might serve as novel therapeutic targets. Therefore, we tested whether the cGMP downstream effector, cGMP-dependent protein kinase G Iα (PKGIα), attenuates pressure overload-induced remodeling in vivo.

METHODS AND RESULTS

The effect of transaortic constriction (TAC)-induced left ventricular (LV) pressure overload was examined in mice with selective mutations in the PKGIα leucine zipper interaction domain. Compared with wild-type littermate controls, in response to TAC, these Leucine Zipper Mutant (LZM) mice developed significant LV systolic and diastolic dysfunction by 48 hours (n=6 WT sham, 6 WT TAC, 5 LZM sham, 9 LZM TAC). In response to 7-day TAC, the LZM mice developed increased pathologic hypertrophy compared with controls (n=5 WT sham, 4 LZM sham, 8 WT TAC, 11 LZM TAC). In WT mice, but not in LZM mice, phosphodiesterase 5 (PDE5) inhibition with sildenafil (Sil) significantly inhibited TAC-induced cardiac hypertrophy and LV systolic dysfunction in WT mice, but this was abolished in the LZM mice (n=3 WT sham, 4 LZM sham, 3 WT TAC vehicle, 6 LZM TAC vehicle, 4 WT TAC Sil, 6 LZM TAC Sil). And in response to prolonged, 21-day TAC (n=8 WT sham, 7 LZM sham, 21 WT TAC, 15 LZM TAC), the LZM mice developed markedly accelerated mortality and congestive heart failure. TAC induced activation of JNK, which inhibits cardiac remodeling in vivo, in WT, but not in LZM, hearts, identifying a novel signaling pathway activated by PKGIα in the heart in response to LV pressure overload.

CONCLUSIONS

These findings reveal direct roles for PKGIα in attenuating pressure overload-induced remodeling in vivo and as a required effector for the cardioprotective effects of sildenafil.