PTBP1 promotes hematopoietic stem cell maintenance and red blood cell development by ensuring sufficient availability of ribosomal constituents.

Ribosomopathies constitute a range of disorders associated with defective protein synthesis mainly affecting hematopoietic stem cells (HSCs) and erythroid development. Here, we demonstrate that deletion of poly-pyrimidine-tract-binding protein 1 (PTBP1) in the hematopoietic compartment leads to the development of a ribosomopathy-like condition. Specifically, loss of PTBP1 is associated with decreases in HSC self-renewal, erythroid differentiation, and protein synthesis. Consistent with its function as a splicing regulator, PTBP1 deficiency results in splicing defects in hundreds of genes, and we demonstrate that the up-regulation of a specific isoform of CDC42 partly mimics the protein-synthesis defect associated with loss of PTBP1. Furthermore, PTBP1 deficiency is associated with a marked defect in ribosome biogenesis and a selective reduction in the translation of mRNAs encoding ribosomal proteins. Collectively, this work identifies PTBP1 as a key integrator of ribosomal functions and highlights the broad functional repertoire of RNA-binding proteins.

Multiplex secretome engineering enhances recombinant protein production and purity.

Host cell proteins (HCPs) are process-related impurities generated during biotherapeutic protein production. HCPs can be problematic if they pose a significant metabolic demand, degrade product quality, or contaminate the final product. Here, we present an effort to create a "clean" Chinese hamster ovary (CHO) cell by disrupting multiple genes to eliminate HCPs. Using a model of CHO cell protein secretion, we predict that the elimination of unnecessary HCPs could have a non-negligible impact on protein production. We analyze the HCP content of 6-protein, 11-protein, and 14-protein knockout clones. These cell lines exhibit a substantial reduction in total HCP content (40%-70%). We also observe higher productivity and improved growth characteristics in specific clones. The reduced HCP content facilitates purification of a monoclonal antibody. Thus, substantial improvements can be made in protein titer and purity through large-scale HCP deletion, providing an avenue to increased quality and affordability of high-value biopharmaceuticals.

Differences in Cell Cycle Status Underlie Transcriptional Heterogeneity in the HSC Compartment.

Hematopoietic stem cells (HSCs) are considered a heterogeneous cell population. To further resolve the HSC compartment, we characterized a retinoic acid (RA) reporter mouse line. Sub-fractionation of the HSC compartment in RA-CFP reporter mice demonstrated that RA-CFP-dim HSCs were largely non-proliferative and displayed superior engraftment potential in comparison with RA-CFP-bright HSCs. Gene expression analysis demonstrated higher expression of RA-target genes in RA-CFP-dim HSCs, in contrast to the RA-CFP reporter expression, but both RA-CFP-dim and RA-CFP-bright HSCs responded efficiently to RA in vitro. Single-cell RNA sequencing (RNA-seq) of >1,200 HSCs showed that differences in cell cycle activity constituted the main driver of transcriptional heterogeneity in HSCs. Moreover, further analysis of the single-cell RNA-seq data revealed that stochastic low-level expression of distinct lineage-affiliated transcriptional programs is a common feature of HSCs. Collectively, this work demonstrates the utility of the RA-CFP reporter line as a tool for the isolation of superior HSCs.