In-vivo differentiation of adult hematopoietic stem cells from a single-cell point of view.

PURPOSE OF REVIEW: Although hematopoietic stem cell (HSC) function has long been studied by transplantation assays, this does not reflect what HSCs actually do in their native context. Here, we review recent technologic advances that facilitate the study of HSCs in their native context focusing on inducible HSC-specific lineage tracing and inference of hematopoietic trajectories through single-cell RNA sequencing (scRNA-Seq). RECENT FINDINGS: Lineage tracing of HSCs at the population level using multiple systems has suggested that HSCs make a major contribution to steady-state hematopoiesis. Although several genetic systems and novel methods for lineage tracing individual hematopoietic clones have been described, the technology for tracking these cellular barcodes (in particular mutations or insertion sites) is still in its infancy. Thus, lineage tracing of HSC clones in the adult bone marrow remains elusive. Static snapshots of scRNA-Seq of hematopoietic populations have captured the heterogeneity of transcriptional profiles of HSCs and progenitors, with some cells displaying a unilineage signature as well as others with bi or multipotent lineage profiles. Kinetic analysis using HSC-specific lineage tracing combined with scRNA-Seq confirmed this heterogeneity of progenitor populations and revealed a rapid and early emergence of megakaryocytic progeny, followed by erythroid and myeloid lineages, whereas lymphoid differentiation emerged last. SUMMARY: New approaches to study HSCs both in vivo through lineage tracing and at a high-resolution molecular level through scRNA-Seq are providing key insight into HSC differentiation in the absence of transplantation. Recent studies using these approaches are discussed here. These studies pave the way for integration of in-vivo clonal analysis of HSC behavior over time with single-cell sequencing data, including but not limited to transcriptomic, proteomic, and epigenomic, to establish a comprehensive molecular and cellular map of hematopoiesis.

The role of protein disulphide isomerase AGR2 in the tumour niche.

In recent years, the discovery of 'tumour niche', a microenvironment that favours tumour development has changed our perspective of cancer. This microenvironment generated by the tumour cells itself and surrounding cells is capable of providing essential elements for its growth. Consequently, the homoeostasis of the secretory pathway (SP) has become an essential player in cancer development. The SP not only promotes cellular adaptation to protein misfolding due to oncogenic transformation or challenging tumour niche but also allows tumour cells to produce specific secretomes. This impacts tumour cells in cis- or trans- as well as stromal cells in the tumour niche. In this context, the Anterior GRadient 2 (AGR2) protein has been identified as a key player. AGR2 is a protein disulphide isomerase that resides in the endoplasmic reticulum (ER) and mediates the formation of disulphide bonds, catalyses the cysteine-based redox reactions and assists the quality control of proteins. AGR2 not only plays an essential role in the homoeostasis of the SP but also exerts pro-oncogenic gain-of-function due to its reported mislocalisation in the tumour niche microenvironment. In this review, we summarise the dual role of AGR2, inside and outside the ER, on the tumour niche and its microenvironment.