Summary During eukaryotic evolution, ribosomes have considerably increased in size forming a surface exposed ribosomal RNA (rRNA) shell of unknown function, which may create an interface for yet uncharacterized interacting proteins. To investigate such protein interactions, we establish a ribosome affinity purification method that unexpectedly identified hundreds of ribosome associated proteins (RAPs) from categories including metabolism, cell cycle, as well as RNA and protein modifying enzymes that functionally diversify mammalian ribosomes. By further characterizing RAPs, we discover the presence of ufmylation, a metazoan-specific posttranslational modification, on ribosomes and define its direct substrates. Moreover, we show that the metabolic enzyme, pyruvate kinase muscle (PKM), interacts with sub-pools of endoplasmic reticulum (ER)-associated ribosomes, exerting a non-canonical function as an RNA binding protein in the translation of ER-destined mRNAs. Therefore, RAPs interconnect one of life’s most ancient molecular machines with diverse cellular processes, providing an additional layer of regulatory potential to protein expression.

Summary Embryonic gene expression intricately reflects anatomical context, developmental stage, and cell type. To address whether the precise spatial origins of cardiac cells can be deduced solely from their transcriptional profiles, we established a genome-wide expression database from 118, 949, and 1166 single murine heart cells at embryonic days (e)8.5, 9.5, and 10.5, respectively. We segregated these cells by type using unsupervised bioinformatic analysis and identified chamber-specific genes. Using a random forest algorithm, we reconstructed the spatial origin of single e9.5 and e10.5 cardiomyocytes with 92.0+/-3.2% and 91.2+/-2.8% accuracy respectively (99.4+/-1.0% and 99.1+/-1.1% if a +/-1 zone margin is permitted) and predicted the second heart field distribution of Isl1-lineage descendants. When applied to Nkx2-5-/- cardiomyocytes from murine e9.5 hearts, we showed their transcriptional alteration and lack of ventricular phenotype. Our database and zone classification algorithm will enable the discovery of novel mechanisms in early cardiac development and disease.

The heart is the first organ to form during embryonic development. Given the complex nature of cardiac differentiation and morphogenesis, it is not surprising that some form of congenital heart disease is present in approximately one percent of newborns. The molecular determinants of heart development have received much attention over the past several decades. This has been driven in large part by an interest in understanding the etiology of congenital heart disease coupled with the potential of utilizing knowledge from developmental biology to generate functional cells and tissues that could be used for regenerative medicine purposes. In this review, we highlight the critical signaling pathways and transcription factor networks that regulate cardiomyocyte lineage specification in both in vivo and in vitro models. Special focus will be given to epigenetic regulators that drive the commitment of cardiomyogenic cells from nascent mesoderm and their differentiation into chamber-specific myocytes as well as regulation of myocardial trabeculation.

Therapeutic mRNAs and vaccines are being developed for a broad range of human diseases, including COVID-19. However, their optimization is hindered by mRNA instability and inefficient protein expression. Here, we describe design principles that overcome these barriers. We develop an RNA sequencing-based platform called PERSIST-seq to systematically delineate in-cell mRNA stability, ribosome load, as well as in-solution stability of a library of diverse mRNAs. We find that, surprisingly, in-cell stability is a greater driver of protein output than high ribosome load. We further introduce a method called In-line-seq, applied to thousands of diverse RNAs, that reveals sequence and structure-based rules for mitigating hydrolytic degradation. Our findings show that highly structured “superfolder” mRNAs can be designed to improve both stability and expression with further enhancement through pseudouridine nucleoside modification. Together, our study demonstrates simultaneous improvement of mRNA stability and protein expression and provides a computational-experimental platform for the enhancement of mRNA medicines.