anesthesia-intubation-preadolescent-mouse-pups-for-cardiothoracic

Anesthesia and Intubation of Preadolescent Mouse Pups for Cardiothoracic Surgery

Murine surgical models play an important role in preclinical research. Mechanistic insights into myocardial regeneration after cardiac injury may be ...

Victor Chang Cardiac Research Institute

Mutations in the LMNA gene, which encodes the nuclear lamina proteins lamin A and lamin C, are the most common cause of familial dilated cardiomyopathy (DCM). Mechanical stress-induced apoptosis has been proposed as the mechanism underpinning DCM in lamin A/C-deficient hearts, but supporting in vivo evidence has been lacking.

Effects of mechanical stress and carvedilol in lamin A/C-deficient dilated cardiomyopathy.

Sympathetic regulation of cardiac contractility is mediated in part by α(1)-adrenergic receptors (ARs), and the α(1A)-subtype has been implicated in the pathogenesis of cardiac hypertrophy. However, little is known about α(1A)-AR signalling pathways in ventricular myocardium. The aim of this study was to determine the signalling pathway that mediates α(1A)-AR-coupled cardiac contractility.

Regulation of murine cardiac contractility by activation of α(1A)-adrenergic receptor-operated Ca(2+) entry.

It is widely believed that perinatal cardiomyocyte terminal differentiation blocks cytokinesis, thereby causing binucleation and limiting regenerative repair after injury. This suggests that heart growth should occur entirely by cardiomyocyte hypertrophy during preadolescence when, in mice, cardiac mass increases many-fold over a few weeks. Here, we show that a thyroid hormone surge activates the IGF-1/IGF-1-R/Akt pathway on postnatal day 15 and initiates a brief but intense proliferative burst of predominantly binuclear cardiomyocytes. This proliferation increases cardiomyocyte numbers by ~40%, causing a major disparity between heart and cardiomyocyte growth. Also, the response to cardiac injury at postnatal day 15 is intermediate between that observed at postnatal days 2 and 21, further suggesting persistence of cardiomyocyte proliferative capacity beyond the perinatal period. If replicated in humans, this may allow novel regenerative therapies for heart diseases.

A proliferative burst during preadolescence establishes the final cardiomyocyte number.

Mutations in the giant sarcomeric protein titin (TTN) are a major cause for inherited forms of dilated cardiomyopathy (DCM). We have previously developed a mouse model that imitates a TTN truncation mutation we found in a large pedigree with DCM. While heterozygous Ttn knock-in mice do not display signs of heart failure under sedentary conditions, they recapitulate the human phenotype when exposed to the pharmacological stressor angiotensin II or isoproterenol. In this study we investigated the effects of pressure overload by transverse aortic constriction (TAC) in heterozygous (Het) Ttn knock-in mice. Two weeks after TAC, Het mice developed marked impairment of left ventricular ejection fraction (p < 0.05), while wild-type (WT) TAC mice did not. Het mice also trended toward increased ventricular end diastolic pressure and volume compared to WT littermates. We found an increase in histologically diffuse cardiac fibrosis in Het compared to WT in TAC mice. This study shows that a pattern of DCM can be induced by TAC-mediated pressure overload in a TTN-truncated mouse model. This model enlarges our arsenal of cardiac disease models, adding a valuable tool to understand cardiac pathophysiological remodeling processes and to develop therapeutic approaches to combat heart failure.

Pressure Overload by Transverse Aortic Constriction Induces Maladaptive Hypertrophy in a Titin-Truncated Mouse Model.

Over 100 mammalian G protein-coupled receptors are yet to be matched with endogenous ligands; these so-called orphans are prospective drug targets for the treatment of disease. GPR37L1 is one such orphan, abundant in the brain and detectable as mRNA in the heart and kidney. GPR37L1 ablation was reported to cause hypertension and left ventricular hypertrophy, and thus, we sought to further define the role of GPR37L1 in blood pressure homeostasis.

Orphan receptor GPR37L1 contributes to the sexual dimorphism of central cardiovascular control.

We have previously demonstrated that adult transgenic C57BL/6J mice with CM-restricted overexpression of the dominant negative W  mutant protein (dn-c-kit-Tg) respond to pressure overload with robust cardiomyocyte (CM) cell cycle entry. Here, we tested if outcomes after myocardial infarction (MI) due to coronary artery ligation are improved in this transgenic model. Compared to non-transgenic littermates (NTLs), adult male dn-c-kit-Tg mice displayed CM hypertrophy and concentric left ventricular (LV) hypertrophy in the absence of an increase in workload. Stroke volume and cardiac output were preserved and LV wall stress was markedly lower than that in NTLs, leading to a more energy-efficient heart. In response to MI, infarct size in adult (16-week old) dn-c-kit-Tg hearts was similar to that of NTL after 24 h but was half that in NTL hearts 12 weeks post-MI. Cumulative CM cell cycle entry was only modestly increased in dn-c-kit-Tg hearts. However, dn-c-kit-Tg mice were more resistant to infarct expansion, adverse LV remodelling and contractile dysfunction, and suffered no early death from LV rupture, relative to NTL mice. Thus, pre-existing cardiac hypertrophy lowers wall stress in dn-c-kit-Tg hearts, limits infarct expansion and prevents death from myocardial rupture.

Cardiac hypertrophy limits infarct expansion after myocardial infarction in mice.

Animal models of pressure overload are valuable for understanding hypertensive heart disease. We characterised a surgical model of pressure overload-induced hypertrophy in C57BL/6J mice produced by suprarenal aortic constriction (SAC). Compared to sham controls, at one week post-SAC systolic blood pressure was significantly elevated and left ventricular (LV) hypertrophy was evident by a 50% increase in the LV weight-to-tibia length ratio due to cardiomyocyte hypertrophy. As a result, LV end-diastolic wall thickness-to-chamber radius (h/R) ratio increased, consistent with the development of concentric hypertrophy. LV wall thickening was not sufficient to normalise LV wall stress, which also increased, resulting in LV systolic dysfunction with reductions in ejection fraction and fractional shortening, but no evidence of heart failure. Pathological LV remodelling was evident by the re-expression of fetal genes and coronary artery perivascular fibrosis, with ischaemia indicated by enhanced cardiomyocyte Hif1a expression. The expression of stem cell factor receptor, c-Kit, was low basally in cardiomyocytes and did not change following the development of robust hypertrophy, suggesting there is no role for cardiomyocyte c-Kit signalling in pathological LV remodelling following pressure overload.

Pressure overload by suprarenal aortic constriction in mice leads to left ventricular hypertrophy without c-Kit expression in cardiomyocytes.

The 'fight or flight' response to physiological stress involves sympathetic nervous system activation, catecholamine release and adrenergic receptor stimulation. In the heart, this induces positive inotropy, previously attributed to the β-adrenergic receptor subtype. However, the role of the α-adrenergic receptor, which has been suggested to be protective in cardiac pathology, has not been investigated in the setting of physiological stress. To explore this, we developed a tamoxifen-inducible, cardiomyocyte-specific α-adrenergic receptor knock-down mouse model, challenged mice to four weeks of endurance swim training and assessed cardiac outcomes. With 4-OH tamoxifen treatment, expression of the α-adrenergic receptor was knocked down by 80-89%, without any compensatory changes in the expression of other adrenergic receptors, or changes to baseline cardiac structure and function. Swim training caused eccentric hypertrophy, regardless of genotype, demonstrated by an increase in heart weight/tibia length ratio (30% and 22% in vehicle- and tamoxifen-treated animals, respectively) and an increase in left ventricular end diastolic volume (30% and 24% in vehicle- and tamoxifen-treated animals, respectively) without any change in the wall thickness/chamber radius ratio. Consistent with physiological hypertrophy, there was no increase in fetal gene program (Myh7, Nppa, Nppb or Acta1) expression. In response to exercise-induced volume overload, stroke volume (39% and 30% in vehicle- and tamoxifen-treated animals, respectively), cardiac output/tibia length ratio (41% in vehicle-treated animals) and stroke work (61% and 33% in vehicle- and tamoxifen-treated animals, respectively) increased, regardless of genotype. These findings demonstrate that cardiomyocyte α-adrenergic receptors are not necessary for cardiac adaptation to endurance exercise stress and their acute ablation is not deleterious.

Adaptation to exercise-induced stress is not dependent on cardiomyocyte α-adrenergic receptors.

Gq-coupled receptors are thought to play a critical role in the induction of left ventricular hypertrophy (LVH) secondary to pressure overload, although mechano-sensitive channel activation by a variety of mechanisms has also been proposed, and the relative importance of calcineurin- and calmodulin kinase II (CaMKII)-dependent hypertrophic pathways remains controversial.

Cardiac Gq Receptors and Calcineurin Activation Are Not Required for the Hypertrophic Response to Mechanical Left Ventricular Pressure Overload.

Pathological left ventricular hypertrophy (LVH) occurs in response to pressure overload and remains the single most important clinical predictor of cardiac mortality. The molecular pathways in the induction of pressure overload LVH are potential targets for therapeutic intervention. Current treatments aim to remove the pressure overload stimulus for LVH, but do not completely reverse adverse cardiac remodelling. Although numerous molecular signalling steps in the induction of LVH have been identified, the initial step by which mechanical stretch associated with cardiac pressure overload is converted into a chemical signal that initiates hypertrophic signalling remains unresolved. In this study, we show that selective deletion of transient receptor potential melastatin 4 (TRPM4) channels in mouse cardiomyocytes results in an approximately 50% reduction in the LVH induced by transverse aortic constriction. Our results suggest that TRPM4 channel is an important component of the mechanosensory signalling pathway that induces LVH in response to pressure overload and represents a potential novel therapeutic target for the prevention of pathological LVH.

The Ca-activated cation channel TRPM4 is a positive regulator of pressure overload-induced cardiac hypertrophy.

Multiple mouse lines lacking the orphan G protein-coupled receptor, GPR37L1, have elicited disparate cardiovascular phenotypes. The first  knockout mice study to be published reported a marked elevation in systolic blood pressure (SBP; ∼60 mmHg), revealing a potential therapeutic opportunity. The phenotype differed from our own independently generated knockout line, where male mice exhibited equivalent baseline blood pressure to wild type. Here, we attempted to reproduce the first study by characterizing the cardiovascular phenotype of both the original knockout and transgenic lines alongside a C57BL/6J control line, using the same method of blood pressure measurement. The present study supports the findings from our independently developed  knockout line, finding that SBP and diastolic blood pressure (DBP) are not different in the original  knockout male mice (SBP: 130.9 ± 5.3 mmHg; DBP: 90.7 ± 3.0 mmHg) compared with C57BL/6J mice (SBP: 123.1 ± 4.1 mmHg; DBP: 87.0 ± 2.7 mmHg). Instead, we attribute the apparent hypertension of the knockout line originally described to comparison with a seemingly hypotensive transgenic line (SBP 103.7 ± 5.0 mmHg; DBP 71.9 ± 3.7 mmHg). Additionally, we quantified myocardial  transcript in humans, which was suggested to be downregulated in cardiovascular disease. We found that  has very low native transcript levels in human myocardium and that expression is not different in tissue samples from patients with heart failure compared with sex-matched healthy control tissue. These findings indicate that cardiac GPR37L1 expression is unlikely to contribute to the pathophysiology of human heart failure. This study characterizes systolic blood pressure (SBP) in a  knockout mouse line, which was previously reported to have ∼60 mmHg higher SBP compared with a transgenic line. We observed only a ∼27 mmHg SBP difference between the lines. However, when compared with C57BL/6J mice, knockout mice showed no difference in SBP. We also investigated  mRNA abundance in human hearts and observed no difference between healthy and failing heart samples.

Article	Year
Effects of mechanical stress and carvedilol in lamin A/C-deficient dilated cardiomyopathy. Circulation research\| PubMed ID: 20019332	2010
Regulation of murine cardiac contractility by activation of α(1A)-adrenergic receptor-operated Ca(2+) entry. Cardiovascular research\| PubMed ID: 21546445	2011
A proliferative burst during preadolescence establishes the final cardiomyocyte number. Cell\| PubMed ID: 24813607	2014
Pressure Overload by Transverse Aortic Constriction Induces Maladaptive Hypertrophy in a Titin-Truncated Mouse Model. BioMed research international\| PubMed ID: 26504781	2015
Orphan receptor GPR37L1 contributes to the sexual dimorphism of central cardiovascular control. Biology of sex differences\| PubMed ID: 29625592	2018
Cardiac hypertrophy limits infarct expansion after myocardial infarction in mice. Scientific reports\| PubMed ID: 29666426	2018
Pressure overload by suprarenal aortic constriction in mice leads to left ventricular hypertrophy without c-Kit expression in cardiomyocytes. Scientific reports\| PubMed ID: 32948799	2020
Adaptation to exercise-induced stress is not dependent on cardiomyocyte α-adrenergic receptors. Journal of molecular and cellular cardiology\| PubMed ID: 33647309	2021
Cardiac Gq Receptors and Calcineurin Activation Are Not Required for the Hypertrophic Response to Mechanical Left Ventricular Pressure Overload. Frontiers in cell and developmental biology\| PubMed ID: 33659256	2021
The Ca-activated cation channel TRPM4 is a positive regulator of pressure overload-induced cardiac hypertrophy. eLife\| PubMed ID: 34190686	2021
Involvement of GPR37L1 in murine blood pressure regulation and human cardiac disease pathophysiology. American journal of physiology. Heart and circulatory physiology\| PubMed ID: 34533400	2021

Jianxin Wu

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