Biphasic control of WNT signaling is essential during cardiogenesis, but how the pathway switches from promoting cardiac mesoderm
to restricting cardiomyocyte progenitor fate is unknown. We identified genes expressed in lateral mesoderm that are dysregulated
in zebrafish when both gata5 and gata6 are depleted, causing a block to cardiomyocyte specification. This screen identified tmem88a, which is expressed in the early cardiac progenitor field and was previously implicated in WNT modulation by overexpression
studies. Depletion of tmem88a results in a profound cardiomyopathy, secondary to impaired cardiomyocyte specification. In tmem88a morphants, activation of the WNT pathway exacerbates the cardiomyocyte deficiency, whereas WNT inhibition rescues progenitor
cells and cardiogenesis. We conclude that specification of cardiac fate downstream of gata5/6 involves activation of the tmem88a gene to constrain WNT signaling and expand the number of cardiac progenitors. Tmem88a is a novel component of the regulatory
mechanism controlling the second phase of biphasic WNT activity essential for embryonic cardiogenesis.