Rationale: Ca 2+ -Calmodulin dependent protein kinase (CaMKII) δC activation is implicated in pathological progression of heart failure (HF) and CaMKIIδC transgenic mice rapidly develop HF and arrhythmias. However, little is known about early spatio-temporal Ca 2+ handling and CaMKII activation in hypertrophy and HF. Objective: To measure time- and location-dependent activation of CaMKIIδC signaling in adult ventricular cardiomyocytes, during trans-aortic constriction (TAC) and in CaMKIIδC transgenic mice. Methods and Results: We used human tissue from nonfailing and HF hearts, four mouse lines: wild type (WT), CaMKIIδ-knockout (KO), CaMKIIδC transgenic in WT (TG) or KO background and WT mice exposed to TAC. Confocal imaging and biochemistry revealed disproportional CaMKIIδC activation and accumulation in nuclear and perinuclear vs. cytosolic regions at 5 days post-TAC. This CaMKIIδ activation caused a compensatory increase in sarcoplasmic reticulum Ca 2+ content, Ca 2+ transient amplitude and [Ca2+] decline rates, with reduced phospholamban expression, all of which were most prominent near and in the nucleus. These early adaptive effects in TAC were entirely mimicked in young CaMKIIδ TG mice (6-8 wk) where no overt cardiac dysfunction was present. The (peri)nuclear CaMKII accumulation also correlated with enhanced HDAC4 nuclear export, creating a microdomain for transcriptional regulation. At longer times both TAC and TG mice progressed to overt HF (at 45 days and 11-13 wk, respectively), during which time the compensatory Ca 2+ transient effects reversed, but further increases in nuclear and time-averaged [Ca 2+ ] and CaMKII activation occurred. CaMKIIδ TG mice lacking δB exhibited more severe HF, eccentric myocyte growth and nuclear changes. Patient HF samples also showed greatly increased CaMKIIδ expression, especially for CaMKIIδC in nuclear fractions. Conclusions: We conclude that in early TAC perinuclear CaMKIIδC activation promotes adaptive increases in myocyte Ca 2+ transients and nuclear transcriptional responses but that chronic progression of this nuclear Ca 2+ -CaMKIIδC axis contributes to eccentric hypertrophy and HF.

Cardiac remodelling describes the gradual functional and structural changes in which the heart undergoes under pathological conditions. 1 Excitation-transcription coupling (ETC) is a very early key feature of this remodelling process and consists of the activation of ion-handling molecules at the sarcolemma followed by altered fluxes of Na + and Ca 2+ , which trigger downstream signalling cascades that eventually target transcription with subsequent structural changes. 2,3 Hypertrophy is one of the earliest structural

Mitochondrial dysfunction in cardiomyocytes is a hallmark of heart failure development. Although initial studies recognized the importance of different mitochondrial subpopulations, there is a striking lack of direct comparison of intrafibrillar (IF) versus perinuclear (PN) mitochondria during the development of HF. Here, we use multiple approaches to examine the morphology and functional properties of IF versus PN mitochondria in pressure overload-induced cardiac remodelling in mice, and in non-failing and failing human cardiomyocytes. We demonstrate that PN mitochondria from failing cardiomyocytes are more susceptible to depolarization of mitochondrial membrane potential, reactive oxygen species generation and impairment in Ca 2+ uptake compared with IF mitochondria at baseline and under physiological stress protocol. We also demonstrate, for the first time to our knowledge, that under normal conditions PN mitochondrial Ca 2+ uptake shapes nucleoplasmic Ca 2+ transients (CaTs) and limits nucleoplasmic Ca 2+ loading. The loss of PN mitochondrial Ca 2+ buffering capacity translates into increased nucleoplasmic CaTs and may explain disproportionate rise in nucleoplasmic [Ca 2+ ] in failing cardiomyocytes at increased stimulation frequencies. Therefore, a previously unidentified benefit of restoring the mitochondrial Ca 2+ uptake may be normalization of nuclear Ca 2+ signalling and alleviation of altered excitation–transcription, which could be an important therapeutic approach to prevent adverse cardiac remodelling. This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

Funding Acknowledgements Type of funding sources: Foundation. Main funding source(s): BioTechMed-Graz Mitochondrial dysfunction in cardiomyocytes is a hallmark of heart failure (HF) development. Although initial studies recognized the importance of different mitochondrial subpopulations, there is a striking lack of direct comparison of intrafibrillar (IF) vs. perinuclear (PN) mitochondria during the development of HF. Here, we use multiple approaches to examine the morphology and functional properties of IF vs. PN mitochondria in pressure overload-induced cardiac remodeling in mice, and in non-failing and failing human cardiomyocytes. We could demonstrate that PN mitochondria from failing cardiomyocytes are more susceptible to changes in mitochondrial membrane potential (ΔΨm), ROS generation and impairment in Ca2+ uptake compared to IF mitochondria at baseline and under physiological stress protocol. We also demonstrated, for the first time, that under normal conditions PN mitochondrial Ca2+ uptake shapes nucleoplasmic Ca2+ transients (CaTs) and prevents nucleoplasmic Ca2+ overload. Loss of PN mitochondria Ca2+ buffering capacity translates into increased nucleoplasmic CaTs and may explain disproportionate rise in nucleoplasmic [Ca2+] in failing cardiomyocytes at increased stimulation frequencies. Therefore, a previously unidentified benefit of restoring the mitochondrial Ca2+ uptake may be normalization of nuclear Ca2+ signaling and alleviation of altered excitation-transcription, which could be an important therapeutic approach to prevent adverse cardiac remodeling.

Excessive β-adrenergic stimulation and tachycardia are potent triggers of cardiac remodeling; however, their exact cellular effects remain elusive. Here, we sought to determine the potency of β-adrenergic stimulation and tachycardia to modulate gene expression profiles of cardiomyocytes. Using neonatal rat ventricular cardiomyocytes, we showed that tachycardia caused a significant upregulation of sodium–calcium exchanger (NCX) and the activation of calcium/calmodulin-dependent kinase II (CaMKII) in the nuclear region. Acute isoprenaline treatment ameliorated NCX-upregulation and potentiated CaMKII activity, specifically on the sarcoplasmic reticulum and the nuclear envelope, while preincubation with the β-blocker propranolol abolished both isoprenaline-mediated effects. On a transcriptional level, screening for hypertrophy-related genes revealed tachycardia-induced upregulation of interleukin-6 receptor (IL6R). While isoprenaline prevented this effect, pharmacological intervention with propranolol or NCX inhibitor ORM-10962 demonstrated that simultaneous CaMKII activation on the subcellular Ca2+ stores and prevention of NCX upregulation are needed for keeping IL6R activation low. Finally, using hypertensive Dahl salt-sensitive rats, we showed that blunted β-adrenergic signaling is associated with NCX upregulation and enhanced IL6R signaling. We therefore propose a previously unrecognized protective role of β-adrenergic signaling, which is compromised in cardiac pathologies, in preventing IL6R overactivation under increased workload. A better understanding of these processes may contribute to refinement of therapeutic options for patients receiving β-blockers.