Deciphering the signalling pathways and regulatory networks of cardiac development has led to therapeutic strategies to trigger cardiac regeneration and repair. Although progress made in promoting cardiomyocyte proliferation and in direct reprogramming of non-myocytes into cardiomyocytes and other cell types of the heart has offered new perspectives in the quest to enhance repair and regeneration of the injured adult heart, many challenges remain to be addressed.
During a heart attack, millions to billions of cardiomyocytes are lost. Generating sufficient numbers of new cardiomyocytes to replace the dead cells is especially challenging. Currently, generating new cardiomyocytes by inducing proliferation of existing cardiomyocytes or by reprogramming of non-myocytes is possible but vastly inefficient. This issue is particularly challenging, given that the cells that result from some reprogramming approaches do not proliferate, perhaps necessitating combined strategies to simultaneously promote reprogramming and proliferation. Thus, to enhance heart regeneration, it will be important to develop procedures that increase the yield and efficiency of generating new cardiomyocytes. To date, strategies to promote cardiac repair by inducing cardiomyocyte proliferation and cellular reprogramming have been tested primarily in rodents, with only a single study being reported in pigs109
. Whether these approaches can be scaled up sufficiently to recreate a functional heart in humans remains to be demonstrated.
Inflammatory signals also play a key part in the response of the heart to injury. Whereas the recruitment of macrophages to sites of injury represents an essential early step to clear dead cells and extracellular debris, later aspects of the immune response can impede cardiac repair. Moreover, precise timing is necessary to therapeutically modulate the inflammatory response, as complete blockade to this process can result in cardiac rupture post-myocardial infarction105
. Stimulation of inflammatory pathways, such as the Toll-like receptor pathway, may improve regeneration efficiency by modulating the expression of epigenetic regulators and thus increasing nuclear reprogramming efficiency108
Efficient cardiac repair will probably also necessitate the delivery of reprogramming factors specifically to cardiac fibroblasts, and not to other fibroblast populations throughout the body, and molecules that promote cardiomyocyte proliferation will need to be delivered selectively to cardiomyocytes, and not to fibroblasts that develop into fibrotic scar tissue. Developing molecules that specifically affect cardiomyocytes could be technically challenging due to the complexity of the cell types in the heart.
It has been reported that cardiac arrhythmias can be induced by the delivery of immature and heterogeneous cardiomyocytes that were derived from progenitor or stem cells110,111
. Currently, reprogrammed iCLMs and dedifferentiated dividing cardiomyocytes are immature and phenotypically heterogeneous, which may contribute to arrhythmogenesis. Therefore, it is crucial to promote complete cardiomyocyte differentiation after proliferation and to reprogramme cardiac fibroblasts into homogeneous, mature cardiomyocytes that can functionally integrate with existing cardiomyocytes in the adult heart to maintain contractility and conductivity.
Another concern is the safety issue associated with the viral-based delivery of factors to the injured heart. As integration of retroviruses into the genome can cause cancer, developing a strategy to enable the delivery of transcription factors without viral vectors would be important. Such methods also need to be non-invasive and yet efficient in promoting cardiomyocyte proliferation and the reprogramming of non-myocytes into cardiomyocytes and possible reprogramming of other cardiac cell types. Cardiogenic small molecules and synthetic oligonucleotides will be attractive alternatives to virus-mediated approaches in the future. Although discoveries in mouse models have provided important insights into human biology, they may not be directly applicable to human disease therapies. Preclinical trials in large animals are necessary to evaluate safety and efficacy issues.
Although beyond the scope of this Review, substantial progress has been made in the field of tissue engineering and the development of artificial matrices, such as algisyl112
and decellularized tissue scaffolds, which provide structural integrity and signals to promote the formation of heart tissue113
. Combining such methods with the activation of developmental regulatory mechanisms and cellular transplantation represents an especially promising approach for successful adult heart regeneration and repair.
In summary, the regeneration of the adult human heart represents a major challenge of fundamental importance, given the magnitude of heart disease and the absence of effective long-term therapies for heart failure. Considering the complexity of the heart, which requires second to second uninterrupted function and seamless integration of cardiomyocytes with other cell types within the organ, the creation of functional heart tissue in vivo will undoubtedly necessitate a combination of approaches. The foundation of knowledge of cardiac developmental regulatory mechanisms provides a strong platform for achieving this goal.