Background: New treatments are needed to reduce myocardial infarct size (MI) and prevent heart failure (HF) following acute myocardial infarction (AMI), which are the leading causes of death and disability worldwide. Studies in rodent AMI models showed that genetic and pharmacological inhibition of mitochondrial fission, induced by acute ischemia and reperfusion, reduced MI size. Whether targeting mitochondrial fission at the onset of reperfusion is also cardioprotective in a clinically-relevant large animal AMI model remains to be determined. Methods: Adult pigs (30–40 kg) were subjected to closed-chest 90-min left anterior descending artery ischemia followed by 72 h of reperfusion and were randomized to receive an intracoronary bolus of either mdivi-1 (1.2 mg/kg, a small molecule inhibitor of the mitochondrial fission protein, Drp1) or vehicle control, 10-min prior to reperfusion. The left ventricular (LV) size and function were both assessed by transthoracic echocardiography prior to AMI and after 72 h of reperfusion. MI size and the area-at-risk (AAR) were determined using dual staining with Tetrazolium and Evans blue. Heart samples were collected for histological determination of fibrosis and for electron microscopic analysis of mitochondrial morphology. Results: A total of 14 pigs underwent the treatment protocols (eight control and six mdivi-1). Administration of mdivi-1 immediately prior to the onset of reperfusion did not reduce MI size (MI size as % of AAR: Control 49.2 ± 8.6 vs. mdivi-1 50.5 ± 11.4; p = 0.815) or preserve LV systolic function (LV ejection fraction %: Control 67.5 ± 0.4 vs. mdivi-1 59.6 ± 0.6; p = 0.420), when compared to vehicle control. Similarly, there were no differences in mitochondrial morphology or myocardial fibrosis between mdivi-1 and vehicle control groups. Conclusion: Our pilot study has shown that treatment with mdivi-1 (1.2 mg/kg) at the onset of reperfusion did not reduce MI size or preserve LV function in the clinically-relevant closed-chest pig AMI model. A larger study, testing different doses of mdivi-1 or using a more specific Drp1 inhibitor are required to confirm these findings.

Background. Mitochondria fuse to form elongated networks which are more tolerable to stress and injury. Ischemic pre‐ and postconditioning (IPC and IPost, respectively) are established cardioprotective strategies in the preclinical setting. Whether IPC and IPost modulates mitochondrial morphology is unknown. We hypothesize that the protective effects of IPC and IPost may be conferred via preservation of mitochondrial network. Methods. IPC and IPost were applied to the H9c2 rat myoblast cells, isolated adult primary murine cardiomyocytes, and the Langendorff‐isolated perfused rat hearts. The effects of IPC and IPost on cardiac cell death following ischemia‐reperfusion injury (IRI), mitochondrial morphology, and gene expression of mitochondrial‐shaping proteins were investigated. Results. IPC and IPost successfully reduced cardiac cell death and myocardial infarct size. IPC and IPost maintained the mitochondrial network in both H9c2 and isolated adult primary murine cardiomyocytes. 2D‐length measurement of the 3 mitochondrial subpopulations showed that IPC and IPost significantly increased the length of interfibrillar mitochondria (IFM). Gene expression of the pro‐fusion protein, Mfn1, was significantly increased by IPC, while the pro‐fission protein, Drp1, was significantly reduced by IPost in the H9c2 cells. In the primary cardiomyocytes, gene expression of both Mfn1 and Mfn2 were significantly upregulated by IPC and IPost, while Drp1 was significantly downregulated by IPost. In the Langendorff‐isolated perfused heart, gene expression of Drp1 was significantly downregulated by both IPC and IPost. Conclusion. IPC and IPost‐mediated upregulation of pro‐fusion proteins (Mfn1 and Mfn2) and downregulation of pro‐fission (Drp1) promote maintenance of the interconnected mitochondrial network, ultimately conferring cardioprotection against IRI.

The combination of single-cell RNA sequencing with CRISPR inhibition/activation provides a high-throughput approach to simultaneously study the effects of hundreds if not thousands of gene perturbations in a single experiment. One recent development in CRISPR-based single-cell techniques introduces a feature barcoding technology that allows for the simultaneous capture of mRNA and guide RNA (gRNA) from the same cell. This is achieved by introducing a capture sequence, whose complement can be incorporated into each gRNA and that can be used to amplify these features prior to sequencing. However, because the technology is in its infancy, there is little information available on how such experimental parameters can be optimized. To overcome this, we varied the capture sequence, capture sequence position, and gRNA backbone to identify an optimal gRNA scaffold for CRISPR activation gene perturbation studies. We provide a report on our screening approach along with our observations and recommendations for future use.

Novel therapeutic strategies are required to protect the heart from acute ischaemia-reperfusion injury (IRI) and improve outcomes in patients with acute myocardial infarction (AMI). Mitochondria play a critical role in determining cardiomyocyte fate following acute IRI, with genetic and pharmacological inhibition of Drp1-mediated mitochondrial fission limiting cardiomyocyte death. We investigated the role of the mitochondrial Drp1 receptors, MiD49 and MiD51, as novel targets for cardioprotection. In cardiac cell lines subjected to simulated IRI, dual genetic knockdown of both MiD49 and MiD51 reduced cell death, inhibited mitochondrial fission, prevented mitochondrial permeability transition pore opening, and attenuated mitochondrial calcium overload compared with wild-type cells. However, individual knockdown of either MiD49 or MiD51 did not induce mitochondrial elongation or inhibit MPTP opening. Whole-body genetic ablation of MiD49 in adult mice modestly altered mitochondrial morphology but did not affect myocardial infarct size or cardiac function following AMI. Together with the in vitro protection seen with dual MiD49/51 knockdown, these findings suggest that MiD49 deficiency alone is insufficient and that coordinated inhibition of MiD49 and MiD51 may be required for cardioprotection.