Reprogramming of cardiac cell fate as a therapeutic strategy for ischemic heart disease.

Direct reprogramming of resident cardiac fibroblasts to induced cardiomyocytes is an attractive therapeutic strategy to restore function and remuscularize the injured heart. The cardiac transcription factors Gata4, Mef2c, and Tbx5 have been the mainstay of direct cardiac reprogramming strategies for the past decade. Yet, recent discoveries have identified alternative epigenetic factors capable of reprogramming human cells in the absence of these canonical factors. Further, single-cell genomics evaluating cellular maturation and epigenetics in the setting of injury and heart failure models following reprogramming have continued to inform the mechanistic underpinnings of this process and point toward future areas of discovery for the field. These discoveries and others covered in this review have provided complementary approaches that further enhance the effectiveness of reprogramming as a means of promoting cardiac regeneration following myocardial infarction and heart failure.

Direct reprogramming as a route to cardiac repair.

Ischemic heart disease is the leading cause of morbidity, mortality, and healthcare expenditure worldwide due to an inability of the heart to regenerate following injury. Thus, novel heart failure therapies aimed at promoting cardiomyocyte regeneration are desperately needed. In recent years, direct reprogramming of resident cardiac fibroblasts to induced cardiac-like myocytes (iCMs) has emerged as a promising therapeutic strategy to repurpose the fibrotic response of the injured heart toward a functional myocardium. Direct cardiac reprogramming was initially achieved through the overexpression of the transcription factors (TFs) Gata4, Mef2c, and Tbx5 (GMT). However, this combination of TFs and other subsequent cocktails demonstrated limited success in reprogramming adult human and mouse fibroblasts, constraining the clinical translation of this therapy. Over the past decade, significant effort has been dedicated to optimizing reprogramming cocktails comprised of cardiac TFs, epigenetic factors, microRNAs, or small molecules to yield efficient cardiac cell fate conversion. Yet, efficient reprogramming of adult human fibroblasts remains a significant challenge. Underlying mechanisms identified to accelerate this process have been centered on epigenetic remodeling at cardiac gene regulatory regions. Further studies to achieve a refined understanding and directed means of overcoming epigenetic barriers are merited to more rapidly translate these promising therapies to the clinic.

The histone reader PHF7 cooperates with the SWI/SNF complex at cardiac super enhancers to promote direct reprogramming.

Direct cardiac reprogramming of fibroblasts to cardiomyocytes presents an attractive therapeutic strategy to restore cardiac function following injury. Cardiac reprogramming was initially achieved through overexpression of the transcription factors Gata4, Mef2c and Tbx5; later, Hand2 and Akt1 were found to further enhance this process1-5. Yet, staunch epigenetic barriers severely limit the ability of these cocktails to reprogramme adult fibroblasts6,7. We undertook a screen of mammalian gene regulatory factors to discover novel regulators of cardiac reprogramming in adult fibroblasts and identified the histone reader PHF7 as the most potent activating factor8. Mechanistically, PHF7 localizes to cardiac super enhancers in fibroblasts, and through cooperation with the SWI/SNF complex, it increases chromatin accessibility and transcription factor binding at these sites. Furthermore, PHF7 recruits cardiac transcription factors to activate a positive transcriptional autoregulatory circuit in reprogramming. Importantly, PHF7 achieves efficient reprogramming in the absence of Gata4. Here, we highlight the underexplored necessity of cardiac epigenetic readers, such as PHF7, in harnessing chromatin remodelling and transcriptional complexes to overcome critical barriers to direct cardiac reprogramming.

Cardiac Reprogramming Factors Synergistically Activate Genome-wide Cardiogenic Stage-Specific Enhancers.

The cardiogenic transcription factors (TFs) Mef2c, Gata4, and Tbx5 can directly reprogram fibroblasts to induced cardiac-like myocytes (iCLMs), presenting a potential source of cells for cardiac repair. While activity of these TFs is enhanced by Hand2 and Akt1, their genomic targets and interactions during reprogramming are not well studied. We performed genome-wide analyses of cardiogenic TF binding and enhancer profiling during cardiac reprogramming. We found that these TFs synergistically activate enhancers highlighted by Mef2c binding sites and that Hand2 and Akt1 coordinately recruit other TFs to enhancer elements. Intriguingly, these enhancer landscapes collectively resemble patterns of enhancer activation during embryonic cardiogenesis. We further constructed a cardiac reprogramming gene regulatory network and found repression of EGFR signaling pathway genes. Consistently, chemical inhibition of EGFR signaling augmented reprogramming. Thus, by defining epigenetic landscapes these findings reveal synergistic transcriptional activation across a broad landscape of cardiac enhancers and key signaling pathways that govern iCLM reprogramming.

Deficiency in Kelch protein Klhl31 causes congenital myopathy in mice.

Maintenance of muscle structure and function depends on the precise organization of contractile proteins into sarcomeres and coupling of the contractile apparatus to the sarcoplasmic reticulum (SR), which serves as the reservoir for calcium required for contraction. Several members of the Kelch superfamily of proteins, which modulate protein stability as substrate-specific adaptors for ubiquitination, have been implicated in sarcomere formation. The Kelch protein Klhl31 is expressed in a muscle-specific manner under control of the transcription factor MEF2. To explore its functions in vivo, we created a mouse model of Klhl31 loss of function using the CRISPR-Cas9 system. Mice lacking Klhl31 exhibited stunted postnatal skeletal muscle growth, centronuclear myopathy, central cores, Z-disc streaming, and SR dilation. We used proteomics to identify several candidate Klhl31 substrates, including Filamin-C (FlnC). In the Klhl31-knockout mice, FlnC protein levels were highly upregulated with no change in transcription, and we further demonstrated that Klhl31 targets FlnC for ubiquitination and degradation. These findings highlight a role for Klhl31 in the maintenance of skeletal muscle structure and provide insight into the mechanisms underlying congenital myopathies.

A Twist2-dependent progenitor cell contributes to adult skeletal muscle.

Skeletal muscle possesses remarkable regenerative potential due to satellite cells, an injury-responsive stem cell population located beneath the muscle basal lamina that expresses Pax7. By lineage tracing of progenitor cells expressing the Twist2 (Tw2) transcription factor in mice, we discovered a myogenic lineage that resides outside the basal lamina of adult skeletal muscle. Tw2+ progenitors are molecularly and anatomically distinct from satellite cells, are highly myogenic in vitro, and can fuse with themselves and with satellite cells. Tw2+ progenitors contribute specifically to type IIb/x myofibres during adulthood and muscle regeneration, and their genetic ablation causes wasting of type IIb myofibres. We show that Tw2 expression maintains progenitor cells in an undifferentiated state that is poised to initiate myogenesis in response to appropriate cues that extinguish Tw2 expression. Tw2-expressing myogenic progenitors represent a previously unrecognized, fibre-type-specific stem cell involved in postnatal muscle growth and regeneration.

Cardiotoxin Induced Injury and Skeletal Muscle Regeneration.

Skeletal muscles have a tremendous capacity for repair and regeneration in response to injury. This capacity for regeneration is largely due to a myogenic stem cell population, termed satellite cells, which are resident in adult skeletal muscles. In order to decipher the mechanisms that govern myogenic stem cell quiescence, activation, differentiation, and self-renewal, a reproducible injury model is required. Therefore, we have utilized the delivery of the myonecrotic agent, cardiotoxin, to examine the molecular mechanisms of myogenic stem cells in response to injury. Here, we describe our experience using cardiotoxin as a potent myonecrotic agent to study skeletal muscle regeneration. We provide a detailed protocol to examine skeletal muscle injury and regeneration using morphological analyses.

Validation of Spot screening device for amblyopia risk factors.

PURPOSE: To validate the Spot Vision Screener, a handheld digital screening device that evaluates children for amblyopia risk factors as defined by 2013 criteria of the American Association for Pediatric Ophthalmology and Strabismus (AAPOS), in the setting of a controlled pediatric ophthalmology clinic. METHODS: During a 3-month period, children 2-9 years of age were screened using Spot in a pediatric ophthalmology clinic before receiving a gold standard eye examination. Gold standard examinations were evaluated using the 2013 AAPOS Vision Screening Committee guidelines and compared with results from Spot, which were evaluated using two different manufacturer referral criteria: v1.0.3 and v1.1.51. The specificity and sensitivity for each set of referral criteria to detect both amblyopia risk factors and amblyopia were calculated. RESULTS: A total of 233 children were included. Of these, 155 were successfully screened and analyzed according to two different referral criteria. Spot screeing revealed ambyopia risk factors in 109 patients; examination confirmed amblyopia in 64. Using the original manufacturer's criteria (v1.0.3), Spot was 89% sensitive and 71% specific in detecting amblyopia risk factors. The updated referral criteria (v1.1.51) were applied to the same 155 patients, and specificity improved to 88% (P < 0.02); sensitivity remained minimally affected, at 85% (P < 0.05). Spot-v1.0.3 was 92% sensitive and 41% specific in detecting amblyopia, whereas Spot-v1.1.51 was 89% sensitive and 53% specific for detecting amblyopia. CONCLUSIONS: The Spot-v1.0.3 had high sensitivity but overreferred for suspected myopia and strabismus; Spot-v1.1.51 maintained high sensitivity and improved specificity. The original referral criteria has a high sensitivity to detect amblyopia risk factors but low specificty; v1.1.51 criteria increases specificity with minimal impact on sensitivity.