Internal structure and remodeling in dystrophin-deficient cardiomyocytes using second harmonic generation

Ryanodine receptor dysfunction causes senescence and fibrosis in Duchenne dilated cardiomyopathy

BACKGROUND

Duchenne muscular dystrophy (DMD) is an X-linked disorder characterized by progressive muscle weakness due to the absence of functional dystrophin. DMD patients also develop dilated cardiomyopathy (DCM). We have previously shown that DMD (mdx) mice and a canine DMD model (GRMD) exhibit abnormal intracellular calcium (Ca2+) cycling related to early-stage pathological remodelling of the ryanodine receptor intracellular calcium release channel (RyR2) on the sarcoplasmic reticulum (SR) contributing to age-dependent DCM.

METHODS

Here, we used hiPSC-CMs from DMD patients selected by Speckle-tracking echocardiography and canine DMD cardiac biopsies to assess key early-stage Duchenne DCM features.

RESULTS

Dystrophin deficiency was associated with RyR2 remodelling and SR Ca2+ leak (RyR2 Po of 0.03 ± 0.01 for HC vs. 0.16 ± 0.01 for DMD, P < 0.01), which led to early-stage defects including senescence. We observed higher levels of senescence markers including p15 (2.03 ± 0.75 for HC vs. 13.67 ± 5.49 for DMD, P < 0.05) and p16 (1.86 ± 0.83 for HC vs. 10.71 ± 3.00 for DMD, P < 0.01) in DMD hiPSC-CMs and in the canine DMD model. The fibrosis was increased in DMD hiPSC-CMs. We observed cardiac hypocontractility in DMD hiPSC-CMs. Stabilizing RyR2 pharmacologically by S107 prevented most of these pathological features, including the rescue of the contraction amplitude (1.65 ± 0.06 μm for DMD vs. 2.26 ± 0.08 μm for DMD + S107, P < 0.01). These data were confirmed by proteomic analyses, in particular ECM remodelling and fibrosis.

CONCLUSIONS

We identified key cellular damages that are established earlier than cardiac clinical pathology in DMD patients, with major perturbation of the cardiac ECC. Our results demonstrated that cardiac fibrosis and premature senescence are induced by RyR2 mediated SR Ca2+ leak in DMD cardiomyocytes. We revealed that RyR2 is an early biomarker of DMD-associated cardiac damages in DMD patients. The progressive and later DCM onset could be linked with the RyR2-mediated increased fibrosis and premature senescence, eventually causing cell death and further cardiac fibrosis in a vicious cycle leading to further hypocontractility as a major feature of DCM. The present study provides a novel understanding of the pathophysiological mechanisms of the DMD-induced DCM. By targeting RyR2 channels, it provides a potential pharmacological treatment.

Cardiomyocyte sarcomere length variability: Membrane fluorescence versus second harmonic generation myosin imaging

Sarcomere length (SL) and its variation along the myofibril strongly regulate integrated coordinated myocyte contraction. It is therefore important to obtain individual SL properties. Optical imaging by confocal fluorescence (for example, using ANEPPS) or transmitted light microscopy is often used for this purpose. However, this allows for the visualization of structures related to Z-disks only. In contrast, second-harmonic generation (SHG) microscopy visualizes A-band sarcomeric structures directly. Here, we compared averaged SL and its variability in isolated relaxed rat cardiomyocytes by imaging with ANEPPS and SHG. We found that SL variability, evaluated by several absolute and relative measures, is two times smaller using SHG vs. ANEPPS, while both optical methods give the same average (median) SL. We conclude that optical methods with similar optical spatial resolution provide valid estimations of average SL, but the use of SHG microscopy for visualization of sarcomeric A-bands may be the "gold standard" for evaluation of SL variability due to the absence of optical interference between the sarcomere center and non-sarcomeric structures. This contrasts with sarcomere edges where t-tubules may not consistently colocalize to Z-disks. The use of SHG microscopy instead of fluorescent imaging can be a prospective tool to map sarcomere variability both in vitro and in vivo conditions and to reveal its role in the functional behavior of living myocardium.

Exploring Macrophage-Dependent Wound Regeneration During Mycobacterial Infection in Zebrafish

The molecular and cellular mechanisms associated with tissue degradation or regeneration in an infectious context are poorly defined. Herein, we explored the role of macrophages in orchestrating either tissue regeneration or degradation in zebrafish embryos pre-infected with the fish pathogen Mycobacterium marinum. Zebrafish were inoculated with different infectious doses of M. marinum prior to fin resection. While mild infection accelerated fin regeneration, moderate or severe infection delayed this process by reducing blastemal cell proliferation and impeding tissue morphogenesis. This was correlated with impaired macrophage recruitment at the wound of the larvae receiving high infectious doses. Macrophage activation characterized, in part, by a high expression level of tnfa was exacerbated in severely infected fish during the early phase of the regeneration process, leading to macrophage necrosis and their complete absence in the later phase. Our results demonstrate how a mycobacterial infection influences the macrophage response and tissue regenerative processes.

Label-free 3D characterization of cardiac fibrosis in muscular dystrophy using SHG imaging of cleared tissue

BACKGROUND INFORMATION

Duchenne muscular dystrophy (DMD) is a neuromuscular disease caused by mutations in the gene encoding dystrophin. It leads to repeated cycles of muscle fiber necrosis and regeneration and progressive replacement of fibers by fibrotic and adipose tissue, with consequent muscle weakness and premature death. Fibrosis and, in particular, collagen accumulation are important pathological features of dystrophic muscle. A better understanding of the development of fibrosis is crucial to enable better management of DMD. Three-dimensional (3D) characterization of collagen organization by second harmonic generation (SHG) microscopy has already proven a highly informative means of studying the fibrotic network in tissue.

RESULTS

Here, we combine for the first-time tissue clearing with SHG microscopy to characterize in depth the 3D cardiac fibrosis network from DMD^mdx rat model. Heart sections (1-mm-thick) from 1-year-old wild-type (WT) and DMD^mdx rats were cleared using the CUBIC protocol. SHG microscopy revealed significantly greater collagen deposition in DMD^mdx versus WT sections. Analyses revealed a specific pattern of SHG⁺ segmented objects in DMD^mdx cardiac muscle, characterized by a less elongated shape and increased density. Compared with the observed alignment of SHG⁺ collagen fibers in WT rats, profound fiber disorganization was observed in DMD^mdx rats, in which we observed two distinct SHG⁺ collagen fiber profiles, which may reflect two distinct stages of the fibrotic process in DMD.

CONCLUSION AND SIGNIFICANCE

The current work highlights the interest to combine multiphoton SHG microscopy and tissue clearing for 3D fibrosis network characterization in label free organ. It could be a relevant tool to characterize the fibrotic tissue remodeling in relation to the disease progression and/or to evaluate the efficacy of therapeutic strategies in preclinical studies in DMD model or others fibrosis-related cardiomyopathies diseases. This article is protected by copyright. All rights reserved.

X-ray diffraction and second harmonic imaging reveal new insights into structural alterations caused by pressure-overload in murine hearts

We demonstrate a label-free imaging approach to study cardiac remodeling of fibrotic and hypertrophic hearts, bridging scales from the whole organ down to the molecular level. To this end, we have used mice subjected to transverse aortic constriction and imaged adjacent cardiac tissue sections by microfocus X-ray diffraction and second harmonic generation (SHG) imaging. In this way, the acto-myosin structure was probed in a spatially resolved manner for entire heart sections. From the recorded diffraction data, spatial maps of diffraction intensity, anisotropy and orientation were obtained, and fully automated analysis depicted the acto-myosin filament spacing and direction. X-ray diffraction presented an overview of entire heart sections and revealed that in regions of severe cardiac remodeling the muscle mass is partly replaced by connective tissue and the acto-myosin lattice spacing is increased at these regions. SHG imaging revealed sub-cellular structure of cardiac tissue and complemented the findings from X-ray diffraction by revealing micro-level distortion of myofibrils, immune cell infiltration at regions of cardiac remodeling and the development of fibrosis down to the scale of a single collagen fibril. Overall, our results show that both X-ray diffraction and SHG imaging can be used for label-free and high-resolution visualization of cardiac remodeling and fibrosis progression at different stages in a cardiac pressure-overload mouse model that cannot be achieved by conventional histology.