hnRNP U protein is required for normal pre-mRNA splicing and postnatal heart development and function

Significance We studied the physiological function of the heterogeneous nuclear ribonucleoprotein U (hnRNP U) by generating a conditional knockout mouse in which the Hnrnpu gene is deleted in the heart. We found that hnRNP U is required for normal pre-mRNA splicing and postnatal heart development and function. Mutant mice develop severe dilated cardiomyopathy and die 2 wk after birth. Phenotypic characterization of mutant hearts coupled with RNA-seq data analyses revealed that mutant hearts display multiple cardiac defects as a result of misregulated gene expression and abnormal pre-mRNA splicing. We also identified the sarcoplasmic reticulum membrane protein Junctin as a splicing target of hnRNP U and provide an interesting example of alternative splicing in controlling the modification and function of proteins. We report that mice lacking the heterogeneous nuclear ribonucleoprotein U (hnRNP U) in the heart develop lethal dilated cardiomyopathy and display numerous defects in cardiac pre-mRNA splicing. Mutant hearts have disorganized cardiomyocytes, impaired contractility, and abnormal excitation–contraction coupling activities. RNA-seq analyses of Hnrnpu mutant hearts revealed extensive defects in alternative splicing of pre-mRNAs encoding proteins known to be critical for normal heart development and function, including Titin and calcium/calmodulin-dependent protein kinase II delta (Camk2d). Loss of hnRNP U expression in cardiomyocytes also leads to aberrant splicing of the pre-mRNA encoding the excitation–contraction coupling component Junctin. We found that the protein product of an alternatively spliced Junctin isoform is N-glycosylated at a specific asparagine site that is required for interactions with specific protein partners. Our findings provide conclusive evidence for the essential role of hnRNP U in heart development and function and in the regulation of alternative splicing.

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