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Background/Purpose: LMBD1 protein, a type IV-B plasma membrane protein possessing nine putative trans-membrane domains, was previously demonstrated at cellular level to play a critical part in the signaling cascade of insulin receptor through its involvement in regulating clathrin-mediated endocytosis. However, at physiological level, the significance of LMBD1 protein in cardiac development remains unclear. Methods: To understand the role of Lmbrd1 gene involved in the cardiac function, heterozygous knockout mice were used as an animal model system. The pathological outcomes were analyzed by micro-positron emission tomography, ECG acquisition, cardiac ultrasound, and immunohistochemistry. Results: By studying the heterozygous knockout of Lmbrd1 (Lmbrd1+/−), we discovered that lack of Lmbrd1 not only resulted in the increase of cardiac-glucose uptake, pathological consequences were also observed. Here, we have distinguished that Lmbrd1+/− is sufficient in causing cardiac diseases through a pathway independent of the recessive vitamin B12 cblF cobalamin transport defect. Lmbrd1+/− mice exhibited an increase in myocardial glucose uptake and insulin receptor signaling that is insensitive to the administration of additional insulin. Pathological symptoms such as cardiac hypertrophy, ventricular tissue fibrosis, along with the increase of heart rate and cardiac muscle contractility were observed. As Lmbrd1+/− mice aged, the decrease in ejection fraction and fraction shortening showed signs of ventricular function deterioration. Conclusion: The results suggested that Lmbrd1 gene not only plays a significant role in mediating the energy homeostasis in cardiac tissue, it may also be a key factor in the regulation of cardiac function in mice. Keywords: Cardiac hypertrophy, Disease animal model, Gene knockout, Mouse Lmbrd1 gene, Human LMBRD1 gene, LMBD1 protein

Energy homeostasis is crucial for maintaining normally functioning cells; disturbances in this balance often cause various diseases. The limb region 1 (LMBR1) domain containing 1 gene (lmbrd1) encodes the LMBD1 protein that possesses 9 putative transmembrane domains. LMBD1 has been suggested to be involved in the lysosome in aiding the export of cobalamin. In this study, we determined that LMBD1 plays a regulatory role in the plasma membrane. A micro-positron emission tomography analysis showed that a single-allele knock-out of lmbrd1 increased the 18F-fluorodeoxyglucose uptake in murine hearts. In addition, the knockdown of lmbrd1 resulted in an up-regulated signaling of the insulin receptor (IR) and its downstream signaling molecule, Akt. Confocal and live total internal reflection fluorescence microscopy showed that LMBD1 co-localized and co-internalized with clathrin and the IR, but not with the transferrin receptor. The results of the mutation analysis and phenotypic rescue experiments indicate that LMBD1 interacts with adaptor protein-2 and is involved in the unique clathrin-mediated endocytosis of the IR. LMBD1 selectively interacts with the IR. The knockdown of lmbrd1 attenuated IR endocytosis, resulting in the perturbation of the IR recycling pathway and consequential enhancement of the IR signaling cascade. In summary, LMBD1 plays an imperative role in mediating and regulating the endocytosis of the IR. Background: LMBD1 is a transmembrane protein encoded by the lmbrd1 gene. Its biological function remains unclear. Results: lmbrd1 knockdown hinders the internalization of the insulin receptor, allowing it and its downstream signaling molecules to remain activated. Conclusion: LMBD1 functions as an insulin receptor-specific adaptor for endocytosis. Significance: LMBD1 is the first identified protein that participates in the regulation of insulin receptor endocytosis.