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

Area of Science:

• Cardiovascular Physiology
• Molecular Cardiology
• Adipocyte Biology

Background:

• Adipocyte Fatty Acid-Binding Protein 4 (FABP4) is known to negatively impact cardiac muscle function.
• The precise cellular mechanisms by which FABP4 influences cardiac contractility and calcium (Ca²⁺) homeostasis are not fully understood.
• Investigating the N-terminal domain of FABP4 is crucial for understanding its role in cardiac regulation.

Purpose of the Study:

• To elucidate the specific effects of FABP4 on the contractility and Ca²⁺ handling of isolated mouse ventricular myocytes.
• To characterize the functional significance of the N-terminal domain of FABP4 and identify critical amino acid residues.
• To differentiate the mechanisms of action between full-length FABP4 and its N-terminal peptide.

Main Methods:

• Simultaneous measurement of myocyte contractility and intracellular Ca²⁺ transients in isolated adult mouse ventricular myocytes.
• Application of recombinant human FABP4 and its synthetic N-terminal peptide (FABP4_aa1-20).
• Assessment of L-type Ca²⁺ current using whole-cell patch-clamp technique and site-directed mutagenesis (E15K) to probe functional importance.

Main Results:

• Full-length FABP4 inhibited myocyte contraction via biphasic, dose-dependent, Ca²⁺-independent pathways, with high (EC₅₀ = 0.010 pM) and low (EC₅₀ = 0.120 nM) affinity components.
• The FABP4_aa1-20 peptide also inhibited contraction (EC₅₀ = 0.110 nM) but through a Ca²⁺-dependent mechanism, significantly suppressing Ca²⁺ transients.
• Mutation of E15K in the N-terminal peptide attenuated its inhibitory activity, and L-type Ca²⁺ current remained unaffected by full-length FABP4.

Conclusions:

• Full-length FABP4 suppresses cardiomyocyte contractility predominantly via a Ca²⁺-independent pathway, likely by altering myofilament Ca²⁺ sensitivity.
• The isolated N-terminal domain of FABP4 exerts its inhibitory effect through a distinct Ca²⁺-dependent mechanism.
• FABP4 exhibits complex dual-pathway regulation of cardiac function, with its N-terminal region representing a potential therapeutic target for obesity-related cardiac dysfunction.