For the May 25, 2012 Circulation Research Journal Club, we examine a groundbreaking article by Jayawardena et al that reports on the direct conversion of fibroblasts to cardiomyocytes by transfection with microRNAs (miRNAs).
If cardiologists could transform the scar tissue formed after myocardial infarction into functional muscle, patients might regain better heart function and avoid heart failure. To that end, researchers have shown that mouse fibroblasts can be directly converted into cardiomyocytes by transfection with three particular transcription factors. However, it had not been shown whether such conversion could take place in vivo. Jayawardena et al have now achieved just that, but they did not use the three transcription factors. Their approach was to use miRNAs, on the basis that these small noncoding RNAs can downregulate the expression of multiple genes at once, and that a number of specific miRNAs have been found to control cardiomyocyte development. In summary, the miRNAs might be more effective. The team transfected individual candidate miRNAs, and combinations thereof, into mouse cardiac fibroblasts. They came up with a combination of four miRNAs that could convert fibroblasts to cardiomyoctyes in vitro and in the hearts of mice after myocardial infarction. The researchers have yet to show whether such in vivo conversion confers functional improvement; nevertheless, they provide proof of principle that in vivo transformation is a possibility. [More] [Novelty & Significance]
Complete PDF: MicroRNA-Mediated In Vitro and In Vivo Direct Reprogramming of Cardiac Fibroblasts to Cardiomyocytes
Journal Club Pack [Abstract, Novelty & Significance section, and all figures]
Related Editorial by Benoit G. Bruneau [PDF]: Direct Reprogramming for Cardiac Regeneration: From Dream to Reality
This article by Jayawardena et al arrives just as Srivastava and colleagues have shown in Nature similar success with direct reprogramming of cardiomyoctyes using gene transcription factors. As discussed by Benoit Bruneau in his editorial, “these landmark studies will hopefully pave the way for effective approaches to restoring cardiac function after cardiac injury.”
Some areas to consider when discussing this article include the strengths/weaknesses of this methodology versus the approach outlined by Srivastava’s group; alternate interpretations of the data; suggestions for where the research should go from here and what will be needed to carry these direct reprogramming breakthroughs over to human trials; and the significance for not only the broader community of cardiovascular scientists but other areas of therapeutic tissue regeneration research.
Download the Journal Club Packs for offline discussion or start/join the discussion in the Comments section.
In a recent issue of Nature, He et al demonstrate that autophagy is required for optimal physical endurance as well as for the beneficial effects of exercise on glucose and lipid metabolism. These data not only shed new insights into the mechanisms whereby exercise is healthy, but also indirectly strengthen the notion that autophagy exerts lifespan-extending effects. [
Cells isolated from embryonic and newborn mouse hearts that express the transcription factor Islet-1 are able to give rise to different types of heart cells, suggesting that Islet-1 might be a marker of cardiac progenitor cells. It has been reported that Islet-1 cells are present in the adult heart. This has led to the hope that activating these cells might help repair myocardial injuries. Weinberger et al examined genetically engineered mice in which expression of the Islet-1 gene made cells turn blue. In the hearts of these mice, blue cells were restricted to 3 particular locations—the interatrial septum, the walls of the aorta and pulmonary artery, and between the right atrium and superior vena cava. This latter location is where the sinoatrial node (SAN) lies, and staining the hearts with a SAN marker confirmed that some SAN cells expressed Islet-1. SAN tissue from humans and wild-type mice was also found to express abundant Islet-1 mRNA. Furthermore, for the most part, the number and localization of blue cells was unaltered after myocardial infarction. Taken together, the results provide no evidence to support the view that Islet-1 marks progenitor cells in adult hearts but that it might be a good marker for SAN cells. [
Multiple sclerosis (MS) is a disease in which the body’s own immune cells attack the myelin sheaths that surround and protect neurons in the central nervous system. As a consequence, neuronal function is impaired causing an array of debilitating physical and cognitive disabilities. A recent expression profiling study of postmortem MS patient brain lesions revealed an upregulation of a platelet adhesion receptor transcript. This led Langer et al to investigate whether platelets might play a role in MS disease pathology. They found that platelets were indeed abundant in the brain lesions of MS patients and of mice with experimental autoimmune encephalomyelitis (EAE)—an animal model of MS. They also showed that depleting platelets in EAE mice reduced the recruitment of other inflammatory cell types to the inflamed CNS. And, importantly, that it considerably improved the disease symptoms in the mice. Blocking the adhesion receptors on the platelet cells with antibodies produced similar results. Together the data show that platelets are important perpetrators of MS pathology and that targeting these cells might be a novel therapeutic approach to consider. [
The complexity of mitochondrial diseases has made their treatment problematic. However, in a recent study from PNAS in 2011, researchers show how diet may be the key to better understanding and possibly fighting the expression of these diseases. [
Prolyl hydroxylase domain (PHD) molecules sense oxygen availability in mammalian cells and can serve as drug targets. Recent work from Takeda et al suggest a previously unappreciated role for PHD2—the main HIF prolyl hydroxylase—in arteriogenesis via macrophage skewing. Deletion of PHD2 in macrophages associated with activation of the canonical NF-κB pathway and arteriogenesis. [
Atherosclerosis is a chronic inflammatory disorder associated with lipid accumulation in the vessel wall. Although the inflammation is known to be promoted, at least in part, by neutrophils, the precise mechanism by which these cells contribute to atherogenesis remains unclear. When activated, neutrophils release secretory vesicles called granules, and certain granule proteins, such as CRAMP (or LL37 in humans), are able to recruit other inflammatory cell types. To determine whether this happens during atherosclerotic lesion formation, Döring et al examined atherosclerosis-prone mice lacking CRAMP. Sure enough, these mice had fewer inflammatory cells adhering to their blood vessel walls, which resulted in smaller atherosclerotic plaques containing a reduced proportion of macrophages. CRAMP previously has been detected in endothelial cells and macrophages, but the team showed that in atherosclerotic vessels, CRAMP was specifically upregulated in neutrophils. The team also found that CRAMP promotes inflammatory cell recruitment by interacting with their formyl-peptide membrane receptors. Blocking these receptors, or indeed CRAMP activity, may be an avenue toward atherosclerosis therapies, as suggested by the authors. [
Commentary: “Why Don’t Macrophages Leave Atherosclerotic Lesions?”
Commentary on:
The Neuroimmune Guidance Cue Netrin-1 Promotes Atherosclerosis by Inhibiting the Emigration of Macrophages From Plaques
Van Gils et al
Nature Immunology. 2012;13:136–143.
By Gabriel K. Griffin & Andrew H. Lichtman
A recent study proposes a novel role for inhibitory guidance cues in regulating macrophage trafficking during atherosclerosis. The study authors demonstrate that Netrin-1, a laminin-related protein with a previously established role in axon migration and tumorigenesis, contributes to atherosclerosis by preventing the emigration of macrophages from plaque. [More] [PDF]
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