Provided herein are methods for promoting differentiation of cardiac progenitor cells toward myocytes and suppressing the conversion of cardiac progenitor cells into fibroblasts and myofibroblasts cells in a subject determined to have cardiac progenitor ceils in or around the heart, by administering a therapeutically effective amount of an NRG-l peptide or functional variant or fragment thereof. The methods disclosed herein can be used to treat, prevent, or delay the progression of cardiac injury, for example heart failure or myocardial infarction.
