Autocrine and intracrine signaling for cardiogenesis in embryonic stem cells: a clue for the development of novel differentiating agents.

Cardiogenesis, one of the earliest and most complex morphogenetic events in the embryo, is not fully understood at the molecular level and is typically a low-yield process. Affording a high throughput of cardiogenesis from a suitable population of pluripotent cells is therefore a major assignment in the perspective of a stem cell therapy for heart failure. Analysis of cardiac lineage commitment in mouse embryonic stem cells and in vivo models of cardiac differentiation revealed that a number of crucial growth factors are released from precursor cells, acting in an autocrine fashion on specific plasma membrane receptors to prime a cardiogenic decision. Nevertheless, it is increasingly becoming evident that cell nuclei harbor the potential for intrinsic signal transduction pathways. The term "intracrine" has been proposed for growth regulatory peptides that have been shown to act within their cell of synthesis at the level of the nuclear envelope, chromatin, or other subnuclear components. Considerable evidence links known intracrines with transcriptional responses and self-sustaining loops that behave as long-lived signals and impart features characteristic of differentiation, growth regulation and cell memory. This review focuses on a number of autocrine and intracrine systems within the context of cardiac differentiation and emphasizes the identification of cardiogenic mechanisms as a clue for the development of unprecedented differentiating strategies. In this regard, recently synthesized mixed esters of hyaluronan with butyric and retinoic acid primed the expression of cardiogenic genes and elicited a remarkable increase in cardiomyocyte yield in mouse embryonic stem cells. This demonstrates the potential for chemically modifying the gene program of cardiac differentiation without the aid of gene transfer technologies and sets the basis for the design of a novel generation of chemicals suited for the organization of targeted lineage patterning in stem cells.