A protein synthesis brake for hematopoietic stem cell maintenance

In this Outlook, Lv and Tong discuss a paper in this issue of Genes & Development from Burgess et al., who demonstrate a novel role of polycomb complex (PRC1) component Bmi1 in controlling HSC regeneration capacity by regulating ribosome biogenesis and protein synthesis.

Abstract

Bmi1 is essential for normal and leukemic hematopoiesis, but its target genes in hematopoietic stem cells (HSCs) are incompletely understood. In this issue of Genes & Development, Burgess et al. (pp. 887–900) demonstrate a novel role of Bmi1 in regulating ribosome biogenesis and protein synthesis. Bmi1-deficient HSCs exhibited reduced transplantability, with the up-regulation of ARX and genes involved in ribosome biogenesis. However, depletion of ARX or its known targets, p16^Ink4a/p19^Arf, only partially rescues Bmi1 loss-induced hematopoietic defects. They further demonstrate an increased protein synthesis rate and resultant proteostatic stress in Bmi1^−/− HSCs, indicating a novel mechanism by which Bmi1 controls HSC maintenance.

Proteostasis is now recognized to play a crucial role in hematopoietic stem cell maintenance and function (Tahmasebi et al. 2019; Chua and Signer 2020). Hematopoietic stem cells (HSCs) exhibit a low protein synthesis rate to maintain their stemness (Signer et al. 2014). Multiple intrinsic mechanisms orchestrate this elaborate process within stem cells, including ribosome biogenesis and assembly, RNA modifications, the ubiquitin–proteasome system, and other post-translational modifications. Among these, ribosome biogenesis and assembly have been underscored by their etiological connection to human diseases, as exemplified by bone marrow failure syndromes (Kang et al. 2021). Moreover, several studies suggest that targeting ribosome biogenesis and protein homeostasis shows promising therapeutic efficacy in improving HSC function (Kruta et al. 2021; Lv et al. 2021).

In addition to the aforementioned mechanisms, transcription factors and epigenetic regulators are found to control HSC function by targeting ribosome-mediated protein translation (Cai et al. 2015; Nachmani et al. 2019). Bmi1 is an essential component of Polycomb repressive complex 1 (PRC1). This complex acts predominantly as a transcription repressor, but also possesses transcription activation function under specific circumstances (Geng and Gao 2020). Bmi1 is crucial for HSC self-renewal and balanced lineage differentiation (Park et al. 2003; Oguro et al. 2010). However, the downstream effectors of Bmi1 in regulating HSC function remain incompletely understood.

In this issue of Genes & Development, Burgess et al. (2022) did extensive analyses to define the underlying mechanism (Fig. 1). Due to the early death of Bmi1 germline knockout mice, the investigators started their study by using a Vav-Cre-mediated conditional Bmi1 depletion in all hematopoietic cells. The Bmi1-depleted mice exhibit decreased total cellularity in multiple hematopoietic organs, lineage cell production in peripheral blood, and blockade of B cell development. Moreover, these mice have significant decreases in multipotential progenitors (MPPs) and a decrease in HSCs during aging. Importantly, competitive bone marrow transplantation assays showed that Bmi1-deficient HSCs do not reconstitute lethally irradiated recipient mice. The above hematologic defects recapitulated observations in the Mx1-Cre-mediated Bmi1 knockout mouse model by the investigators and others (Yu et al. 2021), suggesting a cell-intrinsic role of BMI1.

Figure 1.

A schematic model summarizing how Bmi1 regulates HSC function through different mechanisms. Bmi1 deficiency leads to the increase of p16, p19, ARX, and ribosome-related genes. Loss of p16^Ink4a and p19^Arf only partially rescues Bmi1^−/− HSC regeneration ability and colony-forming capacity, whereas loss of ARX partially blunts the abnormal increase of cell cycle, rRNA, and ribosome protein levels, with negligible effects on HSC function. The study identified a novel role for Bmi1 in controlling ribosome biogenesis and global protein synthesis. The resultant proteostatic stress provides a new molecular explanation of compromised HSC function and overall hematopoietic failures in Bmi1-deficient mice.

Having confirmed the impact of Bmi1 on both steady-state and stress hematopoiesis, the investigators performed singe-cell RNA-seq analysis of Bmi1-deficient HSCs. However, they did not identify any transcriptionally unique population upon Bmi1 loss despite the important role of the Bmi1-containing PRC1 complex in transcriptional regulation. Interestingly, Bmi1-deficient HSCs tend to exit the stemness state more readily than control HSCs, as they display more of a short-term HSC transcriptional signature. Next, the investigators determined whether p16^Ink4a and p19^Arf are the downstream mediators of Bmi1, since they are known Bmi1 targets and are among the top up-regulated hits in their RNA-seq data. However, their elegant and extensive genetic studies with various mouse models suggest that loss of either gene alone or both genes together restores Bmi1-induced hematopoietic defects, corroborating the conclusion drawn by Oguro et al. (2006, 2010). They then switched their focus to another promising target gene, ARX, as it shows preferential expression in HSPCs of old Bmi1-deficient mice when HSCs start to be depleted. The data suggest that ARX loss only rescues the increased cell cycle but not other hematopoietic defects induced by Bmi1 loss, suggesting that ARX plays a minor role in this process.

Gene ontology analysis of differentially expressed genes points to several pathways related to protein synthesis. The investigators discovered a striking increase of rRNA (25 out of 87 ribosome protein-encoding genes) and, as a result, an elevation of global protein synthesis in Bmi1^−/− HSCs. This accelerated protein synthesis adversely augments translation infidelity and protein folding burden, as Bmi1^−/− HSPCs display more protein aggregates, a higher unfolded protein response, and more K48 polyubiquitinated proteins. These data therefore provide a direct link between Bmi1 and proteostasis that is independent of cell division. While this work provides significant insights into the contribution of current Bmi1 targets and mechanism(s) to HSC biology, it will be interesting to determine some questions in depth in the future. Does Bmi1 directly repress rRNA and RP gene expression at the transcription level? Is this repression specific to the Bmi1-containing PRC1.4 complex, instead of other PRC1 complexes? To what extent could inhibition of the protein synthesis rate (or rates) rescue the hematopoietic defects in Bmi1-deficient mice? Are there any specific genes with skewed expression at the translational level by the increased ribosome pools? Is there any correlation between Bmi1 expression and bone marrow failure syndromes clinically? The answers to these questions will facilitate our understanding toward the molecular mechanism underlying Bmi1-mediated HSC function as well as the regulation of protein synthesis rates in HSCs.