# Adipocyte fatty acid-binding protein 4 suppresses contraction of mouse ventricular myocytes via a calcium-independent pathway

## TL;DR

This study shows how a protein called FABP4 reduces heart muscle contraction in mice through two different mechanisms, one of which does not involve calcium.

## Contribution

The paper identifies a calcium-independent pathway for FABP4's effect on heart muscle contraction and highlights the N-terminal domain's distinct role.

## Key findings

- FABP4 inhibits myocyte contraction via a calcium-independent pathway with high and low affinity components.
- The N-terminal peptide of FABP4 inhibits contraction through a calcium-dependent mechanism.
- Mutation of glutamic acid at position 15 reduces the inhibitory activity of the N-terminal peptide.

## Abstract

Adipocyte Fatty Acid-Binding Protein 4 (FABP4) exerts a direct negative inotropic effect on cardiac muscle, but the underlying cellular mechanisms remain elusive. This study aimed to dissect the specific effects of FABP4 on the contractility and calcium (Ca2+) homeostasis of isolated mouse ventricular myocytes and to characterize the functional role and critical residues of its N-terminal domain.

Contractility and intracellular Ca2+ transients were simultaneously measured in isolated adult mouse ventricular myocytes using an IonOptix system following acute application of recombinant human FABP4 or its synthetic N-terminal peptide (FABP4aa1-20). L-type Ca2+ current was assessed via the whole-cell patch-clamp technique. Dose-response curves were analyzed using non-linear regression, and site-directed mutagenesis (E15K) was performed to evaluate the functional importance of a key amino acid residue.

FABP4 inhibited myocyte contraction in a biphasic, dose-dependent manner, with a high-affinity (EC50 = 0.010 pM) and a low-affinity (EC50 = 0.120 nM) component. This inhibition was largely independent of Ca2+ handling, as Ca2+ transient amplitude was only weakly attenuated at higher concentrations (EC50 = 0.412 nM), and L-type Ca2+ current was unaffected. In stark contrast, the FABP4aa1-20 peptide also inhibited contraction (EC50 = 0.110 nM) but did so via a Ca2+-dependent pathway, robustly suppressing Ca2+ transients. Mutation of glutamic acid at position 15 (E15K) significantly attenuated the peptide’s inhibitory activity.

Full-length FABP4 suppresses cardiomyocyte contractility primarily through a Ca2+-independent pathway, likely by reducing myofilament Ca2+ sensitivity. Conversely, its isolated N-terminal domain operates via a distinct, Ca2+-dependent mechanism. These findings reveal a complex dual-pathway regulation of cardiac function by FABP4 and identify its N-terminal region as a potential therapeutic target for mitigating obesity-related cardiac dysfunction.