Myosin 5b Deficiency Alters Liver Proliferation, Zonation and Bile Acid Composition
Background: Myosin 5b (Myo5b) is a motor protein critical for trafficking proteins to the apical surface of intestinal epithelial cells. Inactivating mutations in MYO5B cause Microvillus Inclusion Disease (MVID), a congenital diarrhea disorder that often leads to liver cholestasis. While Myo5b's role in the intestine is well characterized, its function in the liver remains unclear. Methods & Results: To define the hepatic consequences of Myo5b loss, we analyzed germline Myo5b knockout (KO) mice. Bulk RNA-seq of KO livers revealed significant transcriptomic alterations, notably downregulation of genes linked to cell proliferation. Immunostaining confirmed reduced Ki67, phospho-histone H3, and cyclin D1 expression, along with impaired growth of liver organoids in Myo5b deficient mice. Histology and lipid staining showed steatosis and enlarged lipid droplets, with gene signatures favoring lipogenesis and ketogenesis in mice lacking Myo5b. Myo5b KO livers also displayed disrupted zonated gene expression and loss of zone 1 and zone 3 markers. Bile acid profiling revealed reduced hepatic bile acid levels, decreased expression of classical pathway genes (Cyp7a1, Cyp7b1), and compensatory upregulation of Cyp27a1. In the ileum, we observed mislocalization of the apical bile acid transporter ASBT and decreased levels of basolateral OSTß, leading to impaired enterohepatic recycling and increased luminal bile acids. Conclusions: These findings reveal a previously unrecognized role for Myo5b in liver proliferation, metabolic zonation, and bile acid homeostasis, highlighting its importance in maintaining hepatobiliary function. Overall design: Neonatal CD1 mice harboring a germline deletion of Myosin 5b and control (wildtype or heterozygous) mice were euthanized between the ages of 3-8 days old. For RNAseq, the entire liver was collected in Trizol and stored at -80 ºC prior to RNA extraction. mRNA was isolated using Trizol extraction and QIAGEN RNA easy kit according to the manufacturer's instructions. mRNA was extracted from the entire liver of 4 Myo5b KO and 4 control littermates and RNAseq was performed by MedGenome. RNAseq data was used for differential gene expression analysis, pathway analysis, and principal component analysis. Differential gene expression and pathway analysis were performed with RStudio (2024.04.2+764) using the DESeq2 R package (1.46.0)