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Fibrosis is a component of all forms of heart disease regardless of etiology, and while much progress has been made in the field of cardiac matrix biology, there are still major gaps related to how the matrix is formed, how physiological and pathological remodeling differ, and most importantly how matrix dynamics might be manipulated to promote healing and inhibit fibrosis. There is currently no treatment option for controlling, preventing, or reversing cardiac fibrosis. Part of the reason is likely the sheer complexity of cardiac scar formation, such as occurs after myocardial infarction to immediately replace dead or dying cardiomyocytes. The extracellular matrix itself participates in remodeling by activating resident cells and also by helping to guide infiltrating cells to the defunct lesion. The matrix is also a storage locker of sorts for matricellular proteins that are crucial to normal matrix turnover, as well as fibrotic signaling. The matrix has additionally been demonstrated to play an electromechanical role in cardiac tissue. Most techniques for assessing fibrosis are not qualitative in nature, but rather provide quantitative results that are useful for comparing two groups but that do not provide information related to the underlying matrix structure. Highlighted here is a technique for visualizing cardiac matrix ultrastructure. Scanning electron microscopy of decellularized heart tissue reveals striking differences in structure that might otherwise be missed using traditional quantitative research methods.

Background Neuregulin‐1β ( NRG ‐1β) is a growth factor critical for cardiac development and repair with therapeutic potential for heart failure. We previously showed that the glial growth factor 2 ( GGF 2) isoform of NRG ‐1β improves cardiac function in rodents after myocardial infarction ( MI ), but its efficacy in a large animal model of cardiac injury has not been examined. We therefore sought to examine the effects of GGF 2 on ventricular remodeling, cardiac function, and global transcription in post‐ MI swine, as well as potential mechanisms for anti‐remodeling effects. Methods and Results MI was induced in anesthetized swine (n=23) by intracoronary balloon occlusion. At 1 week post‐ MI , survivors (n=13) received GGF 2 treatment (intravenous, biweekly for 4 weeks; n=8) or were untreated (n=5). At 5 weeks post‐ MI , fractional shortening was higher (32.8% versus 25.3%, P =0.019), and left ventricular ( LV ) end‐diastolic dimension lower (4.5 versus 5.3 cm, P =0.003) in GGF 2‐treated animals. Treatment altered expression of 528 genes, as measured by microarrays, including collagens, basal lamina components, and matricellular proteins. GGF 2‐treated pigs exhibited improvements in LV cardiomyocyte mitochondria and intercalated disk structures and showed less fibrosis, altered matrix structure, and fewer myofibroblasts (myoFbs), based on trichrome staining, electron microscopy, and immunostaining. In vitro experiments with isolated murine and rat cardiac fibroblasts demonstrate that NRG ‐1β reduces myoFbs, and suppresses TGF β‐induced phospho‐ SMAD 3 as well as α SMA expression. Conclusions These results suggest that GGF 2/ NRG ‐1β prevents adverse remodeling after injury in part via anti‐fibrotic effects in the heart.