NRSF-GNAO1-CaMK2 axis exacerbates cardiac remodeling and progresses heart failure by impairing Ca2+ homeostasis

Background

In the development of heart failure, pathological intracellular signaling reactivates fetal cardiac genes, which leads to maladaptive remodeling and cardiac dysfunction. We previously reported that a transcriptional repressor, neuron restrictive silencer factor (NRSF) represses fetal cardiac genes and maintains normal cardiac function under normal conditions, while hypertrophic stimuli de-repress this NRSF mediated repression via activation of CaMKII. Molecular mechanisms by which NRSF maintains cardiac systolic function remains to be determined, however.

Purpose

To elucidate how NRSF maintains normal cardiac homeostasis and identify the novel therapeutic targets for heart failure.

Methods and results

We generated cardiac-specific NRSF knockout mice (NRSF cKO), and found that these NRSF cKO showed cardiac dysfunction and premature deaths accompanied with lethal arrhythmias, as was observed in our previously reported cardiac-specific dominant-negative mutant of NRSF transgenic mice (dnNRSF-Tg).

By cDNA microarray analysis of dnNRSF-Tg and NRSF-cKO, we identified that expression of Gnao1 gene encoding Gαo, a member of inhibitory G proteins, was commonly increased in ventricles of both types of mice.

ChIP-seq analysis, reporter assay and electrophoretic mobility shift assay identified that NRSF transcriptionally regulates Gnao1 gene expression.

Genetic Knockdown of Gαo in dnNRSF-Tg and NRSF-cKO by crossing these mice with Gnao1 knockout mice ameliorated the reduced systolic function, increased arrhythmogenicity and reduced survival rates.

Transgenic mice expressing a human GNAO1 in their hearts (GNAO1-Tg) showed progressive cardiac dysfunction with cardiac dilation. Ventricles obtained from GNAO1-Tg have increased phosphorylation level of CaMKII and increased expression level of endogenous mouse Gnao1 gene. These data suggest that increased cardiac expression of Gαo is sufficient to induce pathological Ca2+-dependent signaling and cardiac dysfunction, and that Gαo forms a positive regulatory circuit with CaMKII and NRSF.

Electrophysiological analysis in ventricular myocytes of dnNRSF-Tg revealed that impaired Ca2+ handling via alterations in localized L-type calcium channel (LTCC) activities; decreased T-tubular and increased surface sarcolemmal LTCC activities, underlies Gαo-mediated cardiac dysfunction.

Furthermore, we also identified increased expression of Gαo in ventricles of two different heart failure mice models, mice with transverse aortic constriction and mice carrying a mutant cardiac troponin T, and confirmed that genetic reduction of Gαo prevented the progression of cardiac dysfunction in both types of mice.

Conclusions

Increased expression of Gαo, induced by attenuation of NRSF-mediated repression forms a pathological circuit via activation of CaMKII. This circuit exacerbates cardiac remodeling and progresses heart failure by impairing Ca2+ homeostasis. Gαo is a potential therapeutic target for heart failure.

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Funding Acknowledgement

Type of funding source: Public grant(s) – National budget only. Main funding source(s): Grants-in –Aid for Scientific Research from the Japan Society for the Promotion of Science

4968Increased Gao expression underlies cardiac dysfunction and lethal arrhythmias accompanied with abnormal Ca2+ handling

Background

We previously demonstrated that a transcriptional repressor, neuron restrictive silencer factor (NRSF), maintains normal cardiac function and electrical stability. Transgenic mice expressing a dominant-negative mutant of NRSF in their hearts (dnNRSF-Tg) exhibit systolic dysfunction with cardiac dilation and premature death due to lethal arrhythmias like human dilated cardiomyopathy (DCM). Underlining mechanisms remain to be elucidated, however.

Purpose

We studied underling mechanisms by which NRSF maintains normal cardiac function to identify novel therapeutic targets for heart failure.

Methods and results

We generated cardiac-specific NRSF knockout mice (NRSFcKO) and confirmed that cardiac phenotypes of NRSFcKO are similar to those of dnNRSF-Tg.

cDNA microarray analysis revealed that cardiac gene expression of GNAO1 that encodes Gαo, a member of inhibitory G protein Gαi family, is increased in both dnNRSF-Tg and NRSFcKO ventricles.

We confirmed that GNAO1 is a direct target of NRSF through ChIP-seq analysis, reporter assay and electrophoretic mobility shift assay.

In dnNRSF-Tg, pharmacological inhibition of Gαo with pertussis toxin improved systolic dysfunction and knockdown of Gαo by crossing with GNAO1 knockout mice improved not only systolic function but also frequency of ventricular arrhythmias and survival rates.

Electrophysiological and biochemical analysis in ventricular myocytes obtained from dnNRSF-Tg demonstrated that genetic reduction of Gαo ameliorated abnormalities in Ca2+ handling, which include increased current density in surface sarcolemmal L-type Ca2+ channel, reduced content of sarcoplasmic reticulum Ca2+ and lowered peak of Ca2+ transient. Furthermore, genetic reduction of Gαo attenuated increased phosphorylation levels of CAMKII in dnNRSF-Tg ventricles, which presumably underlies the improvement in Ca2+ handling. In addition, we identified increased Gαo expression in ventricles of heart failure model mice induced by transverse aortic constriction and cardiac troponin T mutant DCM model mice, in both of which, genetic reduction of Gαo ameliorated cardiac dysfunction.

Figure 1

Conclusions

We found that increased expression of Gαo, induced by attenuation of NRSF-mediated repression, plays a crucial role in the progression of cardiac dysfunction and lethal arrhythmias by evoking Ca2+ handling abnormality. These data demonstrate that Gαo is a potential therapeutic target for heart failure.

[The role of lymphocytes and histiocytes as mechanism of action of BCG for bladder cancer]

Antitumor activity of BCG for transitional cell carcinoma of the urinary bladder was evaluated in 103 patients from October, 1983 to October, 1989 at Hirosaki University Hospital. At the same time, the role of various inflammatory cells infiltrating in the bladder wall after intravesical BCG instillation as mechanism of action of BCG was studied. Of 72 patients with superficial bladder cancer, 19 showed a complete response and 28 showed a partial response. The effective rate was 65.3 percent. Significant efficacy was obtained in patients received more than 5 times instillation of BCG. Before intravesical BCG instillation, a small number of lymphocytes or plasma cells infiltrated in the stroma of the tumor. But histiocytes, neutrophils or eosinophils were hardly found. After BCG instillation, the degree of infiltration of each of them tended to increase. The mean degree of infiltration of those inflammatory cells almost all significantly increased in the stroma of the tumor, while only lymphocyte and histiocyte counts significantly increased in both the basal areas of the advancing margin of tumor cells and the tumor free submucosa after BCG instillation. Epithelioid granulomas were recognized almost in the basal areas of the advancing margin of tumor cells and the tumor free submucosa after BCG therapy. Langhans' giant cells were occasionally found within these lesions. In 2 patients marked central necrosis (caseation) was noticed in epithelioid granulomas. In BCG effective cases with superficial bladder cancer lymphocytes and histiocytes increased significantly in all of the stroma of the tumor, the basal areas of the advancing margin of tumor cells and the tumor free submucosa. On the other hand, in BCG noneffective cases they increased significantly only in the stoma of the tumor. Subsets of lymphocytes which increases after intravesical BCG instillation were mainly T pan cells with the predominance of T helper/inducer cells (Th/Ts ratio always > 1).

[Bone formation in the bilateral kidneys of a chronic renal failure patient. A case report]

A 28-year-old man with back pain visited a near hospital on October, 1987. Afterwards he was referred to our hospital on November 11 for detailed examination of the kidneys. The blood chemical analysis showed marked renal hypofunction. The abdominal USG and CT showed bilateral calcification of the cortices. Biopsy specimens of the kidneys revealed bone formation containing bone marrow tissue. Afterwards he had been treated as an outpatients, but he admitted again on June 3, 1988. He is now being hemodialyzed.