Mechanosensitive molecular mechanisms of myocardial fibrosis in living myocardial slices

AIMS

Altered mechanical load in response to injury is a main driver of myocardial interstitial fibrosis. No current in vitro model can precisely modulate mechanical load in a multicellular environment while maintaining physiological behaviour. Living myocardial slices (LMS) are a 300 μm-thick cardiac preparation with preserved physiological structure and function. Here we apply varying degrees of mechanical preload to rat and human LMS to evaluate early cellular, molecular, and functionality changes related to myocardial fibrosis.

METHODS AND RESULTS

Left ventricular LMS were obtained from Sprague Dawley rat hearts and human cardiac samples from healthy and failing (dilated cardiomyopathy) hearts. LMS were mounted on custom stretchers and two degrees of diastolic load were applied: physiological sarcomere length (SL) (SL = 2.2 μm) and overload (SL = 2.4 μm). LMS were maintained for 48 h under electrical stimulation in circulating, oxygenated media at 37°C. In overloaded conditions, LMS displayed an increase in nucleus translocation of Yes-associated protein (YAP) and an up-regulation of mechanotransduction markers without loss in cell viability. Expression of fibrotic and inflammatory markers, as well as Collagen I deposition were also observed. Functionally, overloaded LMS displayed lower contractility (7.48 ± 3.07 mN mm^-2 at 2.2 SL vs. 3.53 ± 1.80 mN mm^-2 at 2.4 SL). The addition of the profibrotic protein interleukin-11 (IL-11) showed similar results to the application of overload with enhanced fibrosis (8% more of collagen surface coverage) and reduced LMS contractility at physiological load. Conversely, treatment with the Transforming growth factor β receptor (TGF-βR) blocker SB-431542, showed down-regulation of genes associated with mechanical stress, prevention of fibrotic response and improvement in cardiac function despite overload (from 2.40 ± 0.8 mN mm^-2 to 4.60 ± 1.08 mN mm^-2 ).

CONCLUSIONS

The LMS have a consistent fibrotic remodelling response to pathological load, which can be modulated by a TGF-βR blocker. The LMS platform allows the study of mechanosensitive molecular mechanisms of myocardial fibrosis and can lead to the development of novel therapeutic strategies.

CaMKII inhibition reduces arrhythmogenic Ca2+ events in subendocardial cryoinjured rat living myocardial slices

Spontaneous Ca²⁺ release (SCR) can cause triggered activity and initiate arrhythmias. Intrinsic transmural heterogeneities in Ca²⁺ handling and their propensity to disease remodeling may differentially modulate SCR throughout the left ventricular (LV) wall and cause transmural differences in arrhythmia susceptibility. Here, we aimed to dissect the effect of cardiac injury on SCR in different regions in the intact LV myocardium using cryoinjury on rat living myocardial slices (LMS). We studied SCR under proarrhythmic conditions using a fluorescent Ca²⁺ indicator and high-resolution imaging in LMS from the subendocardium (ENDO) and subepicardium (EPI). Cryoinjury caused structural remodeling, with loss in T-tubule density and an increased time of Ca²⁺ transients to peak after injury. In ENDO LMS, the Ca²⁺ transient amplitude and decay phase were reduced, while these were not affected in EPI LMS after cryoinjury. The frequency of spontaneous whole-slice contractions increased in ENDO LMS without affecting EPI LMS after injury. Cryoinjury caused an increase in foci that generates SCR in both ENDO and EPI LMS. In ENDO LMS, SCRs were more closely distributed and had reduced latencies after cryoinjury, whereas this was not affected in EPI LMS. Inhibition of CaMKII reduced the number, distribution, and latencies of SCR, as well as whole-slice contractions in ENDO LMS, but not in EPI LMS after cryoinjury. Furthermore, CaMKII inhibition did not affect the excitation–contraction coupling in cryoinjured ENDO or EPI LMS. In conclusion, we demonstrate increased arrhythmogenic susceptibility in the injured ENDO. Our findings show involvement of CaMKII and highlight the need for region-specific targeting in cardiac therapies.

P6300Endothelial cell activation by pro-inflammatory cytokines exerts novel paracrine effects on co-cultured cardiomyocytes

Background

Anti-inflammatory therapies have failed to meet expectations in recent clinical trials for heart failure, despite the number of studies demonstrating pro-fibrotic, arrhythmogenic and hypertrophic effects of inflammation. To enlighten this, we sought to examine the contribution of non-myocytes in the cardiac inflammatory response. Endothelial cells (EC) can regulate cardiomyocyte (CM) function with multiple soluble factors. While many studies have shown the effects of pro-inflammatory cytokines on EC or CM separately, it is still unclear how the activation of EC can affect CM function.

Purpose

We studied the effect of pro-inflammatory pre-conditioning of EC on CM in indirect co-culture systems and in an ex vivo model of cardiac tissue. We hypothesised that pro-inflammatory activation of EC would alter the contractility of co-cultured CM.

Methods

Human cardiac microvascular EC were first pre-conditioned for 24h with Cytomix (1 ng/ml TNF-α, 1 ng/ml IL-1β, 25 ng/ml IL-6 Rα/IL-6 chimera) and co-cultured in a transwell system (pore size: 0.4μm) with adult rat ventricular CM. Co-culture supernatants were screened using a Cytokine Profiler Array. In vitro analysis of calcium handling in CM utilised the optical mapping technique and Fluo-4. Contractility of cardiac tissue was measured ex vivo using myocardial slices of 300 μm, prepared from left ventricles of adult rat and cultured for 24h with field stimulation and a fixed stretch producing a sarcomeric length of 2.2 μm.

Results

Treatment of EC by Cytomix prior to co-cultures induced a release of CC and CXC chemokines, G-/GM-CSF, ST2 and the adhesion molecule ICAM-1. Using published RNAseq datasets we noticed that adult CM do not constitutively express the receptors for the chemokines identified with the Profiler Array. However, duration of calcium transients in CM was significantly reduced from 454 to 322 ms in co-cultures with EC pre-conditioned by Cytomix, compared to untreated EC. Amplitude and time to peak were unchanged. In contrast, myocardial slices treated with Cytomix demonstrated a significant increase in contractility compared to control (from 3.3 to 5.3 mN/mm2) but no significant change in the duration of contraction (from 467 to 434 ms) or the rate of relaxation.

Conclusions

Pre-conditioned EC exert paracrine effects on the calcium handling of isolated CM, suggesting pro-inflammatory activation of proximal EC can affect CM function. In myocardial slices, pro-inflammatory stimulation provoked an inotropic response. The divergence of effect with findings in isolated cells may reflect differences in experimental design and multicellularity. Future work will aim to characterise the soluble mediators involved, and selectively target EC in slices to contextualise the effects described in co-cultures. Ultimately, this may inform the development of novel anti-inflammatory strategies for clinical use.

Acknowledgement/Funding

British Heart Foundation

Investigation of cardiac fibroblasts using myocardial slices

Aims

Cardiac fibroblasts (CFs) are considered the principal regulators of cardiac fibrosis. Factors that influence CF activity are difficult to determine. When isolated and cultured in vitro, CFs undergo rapid phenotypic changes including increased expression of α-SMA. Here we describe a new model to study CFs and their response to pharmacological and mechanical stimuli using in vitro cultured mouse, dog and human myocardial slices.

Methods and results

Unloading of myocardial slices induced CF proliferation without α-SMA expression up to 7 days in culture. CFs migrating onto the culture plastic support or cultured on glass expressed αSMA within 3 days. The cells on the slice remained αSMA(−) despite transforming growth factor-β (20 ng/ml) or angiotensin II (200 µM) stimulation. When diastolic load was applied to myocardial slices using A-shaped stretchers, CF proliferation was significantly prevented at Days 3 and 7 (P < 0.001).

Conclusions

Myocardial slices allow the study of CFs in a multicellular environment and may be used to effectively study mechanisms of cardiac fibrosis and potential targets.

Biomimetic electromechanical stimulation to maintain adult myocardial slices in vitro

Adult cardiac tissue undergoes a rapid process of dedifferentiation when cultured outside the body. The in vivo environment, particularly constant electromechanical stimulation, is fundamental to the regulation of cardiac structure and function. We investigated the role of electromechanical stimulation in preventing culture-induced dedifferentiation of adult cardiac tissue using rat, rabbit and human heart failure myocardial slices. Here we report that the application of a preload equivalent to sarcomere length (SL) = 2.2 μm is optimal for the maintenance of rat myocardial slice structural, functional and transcriptional properties at 24 h. Gene sets associated with the preservation of structure and function are activated, while gene sets involved in dedifferentiation are suppressed. The maximum contractility of human heart failure myocardial slices at 24 h is also optimally maintained at SL = 2.2 μm. Rabbit myocardial slices cultured at SL = 2.2 μm remain stable for 5 days. This approach substantially prolongs the culture of adult cardiac tissue in vitro.

Heterocellularity and Cellular Cross-Talk in the Cardiovascular System

Cellular specialization and interactions with other cell types are the essence of complex multicellular life. The orchestrated function of different cell populations in the heart, in combination with a complex network of intercellular circuits of communication, is essential to maintain a healthy heart and its disruption gives rise to pathological conditions. Over the past few years, the development of new biological research tools has facilitated more accurate identification of the cardiac cell populations and their specific roles. This review aims to provide an overview on the significance and contributions of the various cellular components: cardiomyocytes, fibroblasts, endothelial cells, vascular smooth muscle cells, pericytes, and inflammatory cells. It also aims to describe their role in cardiac development, physiology and pathology with a particular focus on the importance of heterocellularity and cellular interaction between these different cell types.

Auxetic Cardiac Patches with Tunable Mechanical and Conductive Properties toward Treating Myocardial Infarction

An auxetic conductive cardiac patch (AuxCP) for the treatment of myocardial infarction (MI) is introduced. The auxetic design gives the patch a negative Poisson's ratio, providing it with the ability to conform to the demanding mechanics of the heart. The conductivity allows the patch to interface with electroresponsive tissues such as the heart. Excimer laser microablation is used to micropattern a re-entrant honeycomb (bow-tie) design into a chitosan-polyaniline composite. It is shown that the bow-tie design can produce patches with a wide range in mechanical strength and anisotropy, which can be tuned to match native heart tissue. Further, the auxetic patches are conductive and cytocompatible with murine neonatal cardiomyocytes in vitro. Ex vivo studies demonstrate that the auxetic patches have no detrimental effect on the electrophysiology of both healthy and MI rat hearts and conform better to native heart movements than unpatterned patches of the same material. Finally, the AuxCP applied in a rat MI model results in no detrimental effect on cardiac function and negligible fibrotic response after two weeks in vivo. This approach represents a versatile and robust platform for cardiac biomaterial design and could therefore lead to a promising treatment for MI.

Preparation of viable adult ventricular myocardial slices from large and small mammals

This protocol describes the preparation of highly viable adult ventricular myocardial slices from the hearts of small and large mammals, including rodents, pigs, dogs and humans. Adult ventricular myocardial slices are 100- to 400-μm-thick slices of living myocardium that retain the native multicellularity, architecture and physiology of the heart. This protocol provides a list of the equipment and reagents required alongside a detailed description of the methodology for heart explantation, tissue preparation, slicing with a vibratome and handling of myocardial slices. Supplementary videos are included to visually demonstrate these steps. A number of critical steps are addressed that must be followed in order to prepare highly viable myocardial slices. These include identification of myocardial fiber direction and fiber alignment within the tissue block, careful temperature control, use of an excitation-contraction uncoupler, optimal vibratome settings and correct handling of myocardial slices. Many aspects of cardiac structure and function can be studied using myocardial slices in vitro. Typical results obtained with hearts from a small mammal (rat) and a large mammal (human) with heart failure are shown, demonstrating myocardial slice viability, maximum contractility, Ca²⁺ handling and structure. This protocol can be completed in ∼4 h.

[Mycotic aneurysm of the iliac artery disclosed by an abdominal mass]

BACKGROUND

Mycotic aneurysms are classic complications of infective endocarditis. Their diagnosis can be difficult when they are located in deep vessels.

CASE REPORT

A 8 year-old boy was admitted for psoitis, tenderness of the left iliac fossa and a palpable mass. He was treated with antibiotics for 2 months for infective endocarditis, but vomiting and fever began after 3 weeks of therapy. Blood cultures showed Staphylococcus epidermidis. Ultrasonography of the mass showed findings compatible with an abscess but sampling of its contents showed blood. Immediate Doppler echography showed that the mass was an aneurysm of the left iliac artery. This diagnosis was confirmed by CT scan and aortography. The aneurysm was excised and a satisfactory repair was made using a Gore-tex graft.

CONCLUSION

Although this mass had no clinical vascular characteristics, its origin should have been recognized because of the history of infective endocarditis and a better analysis of the ultrasonographic findings.

[Tumoral calcinosis. Apropos of a further case in a child]

Tumor-like calcinosis is an infrequent condition whose etiology is poorly understood. Calcifications develop in the subcutaneous tissue neighboring the large joints. All age groups can be affected, although the disease is more common before the age of twenty years. A new case of tumor-like calcinosis in an eight-year-old girl is reported here. A huge mass in the right hip region and a smaller mass around the right elbow were found. Biologic studies revealed increased serum phosphorus levels with normal serum calcium levels. Roentgenograms confirmed the diagnosis of tumor-like calcinosis by showing calcified masses independent from the bones of the neighboring joint. Management consisted in complete removal of both masses. Outcome was favorable. In this patient's family, the disease appears to be inherited according to a dominant pattern, although recessive autosomal transmission is believed to be more common.

[Pneumoblastoma]

Pneumoblastoma is a rare malignant pulmonary tumor. A new case is reported in a 2 years 5 month-old girl. Because the lack of locoregional extension an isolated surgical excision was performed. However, 6 months later, a local relapse occurred, with inframediastinal extension and cerebral metastases. On the occasion of this case, the authors review the problems set by the histogenesis of this tumor, its therapeutic indications, poorly codified due to its rarity and, overall, its often unfavourable outcome.