Four-and-a-half Lim Domain Protein-1 Upregulation via an Alternate Start Site Prepares the Left Atrium for Birth

Four-and-a-half-LIM-domain protein-1 (FHL1) participates in the heart’s response to biomechanical stress during pathological states. Normally, the basal isoform, bFhl1 , predominates in non-stressed states. In pathology, a change in start site leads to expression of the induced isoform, iFhl1 , which is 16 amino acid residues longer than bFHL1. We have studied the expression and role of Fhl1 during normal cardiac development. The shift from fetal to neonatal circulation signifies new stress for the heart, and provides a unique opportunity to study the role FHL1 in the setting of physiological stress. We hypothesize that selective iFhl1 upregulation during this transitional period prepares the heart for the stress involved. Using 5’ RNA-seq, we demonstrate that iFhl1 was selectively upregulated by 10-fold in the left atrium by embryonic gestation day 17.5 (E17.5; p=2 x 10 -5 ) but remained unchanged in the other cardiac chambers (see B). The preferential increase in expression in the left atrium was confirmed with X-gal staining of Fhl1 Lacz mice hearts (see D, blue stain), and the selective expression of the induced isoform at the protein level was confirmed by Western blot (see C). Assessment of left atrial volume by magnetic resonance imaging in mice at 2 weeks of age showed that Fhl1-null mice have significantly larger left atria than wild-type littermates (0.63±0.10 μl vs. 0.45±0.04 μl, p=0.004). The demonstrated selective upregulation of iFhl1 prior to birth and the increase in left atrial volume that follows genetic ablation of Fhl1 suggests that Fhl1 is central in the heart’s ability to respond to physiological stress as represented by normal birth.