Haematopoietic stem cell quiescence exposed using mitochondrial membrane potential

PURPOSE OF REVIEW

Quiescence is a fundamental property of haematopoietic stem cells (HSCs). Despite the importance of quiescence in predicting the potency of HSCs, tools that measure routinely the degree of quiescence or select for quiescent HSCs have been lacking. This Commentary discusses recent findings that address this fundamental gap in the HSC toolbox.

RECENT FINDINGS

Highly purified, phenotypically-defined HSCs are heterogeneous in their mitochondrial membrane potential (MMP). The lowest MMP subsets are enriched in greatly quiescent HSCs with the highest potency within the purified HSC population. MMP provides an intrinsic probe to select HSC subsets with unique cell cycle properties and distinct stem cell potential. Using this approach, new and unanticipated metabolic properties of quiescent HSCs' exit have been discovered. This methodology may improve the mechanistic understanding, of HSCs' exit from and entry to, quiescence.

SUMMARY

Selecting HSCs using MMP is likely to lead to discoveries of new HSC properties, may improve the ex vivo maintenance of HSCs and has implications for the clinic, including for improving HSC transplantations.

Heterogeneity within Stratified Epithelial Stem Cell Populations Maintains the Oral Mucosa in Response to Physiological Stress

Stem cells in stratified epithelia are generally believed to adhere to a non-hierarchical single-progenitor model. Using lineage tracing and genetic label-retention assays, we show that the hard palatal epithelium of the oral cavity is unique in displaying marked proliferative heterogeneity. We identify a previously uncharacterized, infrequently-dividing stem cell population that resides within a candidate niche, the junctional zone (JZ). JZ stem cells tend to self-renew by planar symmetric divisions, respond to masticatory stresses, and promote wound healing, whereas frequently-dividing cells reside outside the JZ, preferentially renew through perpendicular asymmetric divisions, and are less responsive to injury. LRIG1 is enriched in the infrequently-dividing population in homeostasis, dynamically changes expression in response to tissue stresses, and promotes quiescence, whereas Igfbp5 preferentially labels a rapidly-growing, differentiation-prone population. These studies establish the oral mucosa as an important model system to study epithelial stem cell populations and how they respond to tissue stresses.

Liang R, Arif T, Kalmykova S, Kasianov A, Lin M, Menon V, Qiu J, Bernitz JM, Moore K, Lin F, Benson DL, Tzavaras N, Mahajan M, Papatsenko D, Ghaffari S. Restraining Lysosomal Activity Preserves Hematopoietic Stem Cell Quiescence and Potency. Cell Stem Cell 2020; 26: 359-376. e7. [PMID: 32109377 DOI: 10.1016/j.stem.2020.01.013]

Quiescence is a fundamental property that maintains hematopoietic stem cell (HSC) potency throughout life. Quiescent HSCs are thought to rely on glycolysis for their energy, but the overall metabolic properties of HSCs remain elusive. Using combined approaches, including single-cell RNA sequencing (RNA-seq), we show that mitochondrial membrane potential (MMP) distinguishes quiescent from cycling-primed HSCs. We found that primed, but not quiescent, HSCs relied readily on glycolysis. Notably, in vivo inhibition of glycolysis enhanced the competitive repopulation ability of primed HSCs. We further show that HSC quiescence is maintained by an abundance of large lysosomes. Repression of lysosomal activation in HSCs led to further enlargement of lysosomes while suppressing glucose uptake. This also induced increased lysosomal sequestration of mitochondria and enhanced the competitive repopulation ability of primed HSCs by over 90-fold in vivo. These findings show that restraining lysosomal activity preserves HSC quiescence and potency and may be therapeutically relevant.

Quantification of IgG monoclonal antibody clearance in tissues

Monoclonal antibodies are an important therapeutic entity, and knowledge of antibody pharmacokinetics has steadily increased over the years. Despite this effort, little is known about the extent of IgG antibody degradation in different tissues of the body. While studies have been published identifying sites of degradation with the use of residualizing and non-residualizing radiolabels, quantitative tissue clearances have not yet been derived. Here, we show that in physiologically-based pharmacokinetic (PBPK) models we can combine mouse data of Indium-111 and Iodine-125 labeled antibodies with prior physiologic knowledge to determine tissue-specific intrinsic clearances. Unspecific total tissue clearance (mL/day) in the mouse was estimated to be: liver = 4.75; brain = 0.02; gut = 0.40; heart = 0.07; kidney = 0.97; lung = 0.20; muscle = 3.02; skin = 3.89; spleen = 0.45; rest of body = 2.16. The highest catabolic activity (per g tissue) was in spleen for an FcRn wild-type antibody, but shifts to the liver for an antibody with reduced FcRn affinity. In the model developed, this shift can be explained by the liver having a greater FcRn-mediated protection capacity than the spleen. The quantification of tissue intrinsic clearances and FcRn salvage capacity increases our understanding of quantitative processes that drive the therapeutic responses of antibodies. This knowledge is critical, for instance to estimate the non-specific cellular uptake and degradation of antibodies used for targeted delivery of payloads.

Sleeping beauty: awakening urothelium from its slumber

The bladder urothelium is essentially quiescent but regenerates readily upon injury. The process of urothelial regeneration harkens back to the process of urothelial development whereby urothelial stem/progenitor cells must proliferate and terminally differentiate to establish all three urothelial layers. How the urothelium regulates the level of proliferation and the timing of differentiation to ensure the precise degree of regeneration is of significant interest in the field. Without a carefully-orchestrated process, urothelial regeneration may be inadequate, thereby exposing the host to toxins or pathogens. Alternatively, regeneration may be excessive, thereby setting the stage for tumor development. This review describes our current understanding of urothelial regeneration. The current controversies surrounding the identity and location of urothelial progenitor cells that mediate urothelial regeneration are discussed and evidence for each model is provided. We emphasize the factors that have been shown to be crucial for urothelial regeneration, including local growth factors that stimulate repair, and epithelial-mesenchymal cross talk, which ensures feedback regulation. Also highlighted is the emerging concept of epigenetic regulation of urothelial regeneration, which additionally fine tunes the process through transcriptional regulation of cell cycle genes and growth and differentiation factors. Finally, we emphasize how several of these pathways and/or programs are often dysregulated during malignant transformation, further corroborating their importance in directing normal urothelial regeneration. Together, evidence in the field suggests that any attempt to exploit regenerative programs for the purposes of enhanced urothelial repair or replacement must take into account this delicate balance.

Comparison of the transcriptomes of long-term label retaining-cells and control cells microdissected from mammary epithelium: an initial study to characterize potential stem/progenitor cells

Background: Previous molecular characterizations of mammary stem cells (MaSC) have utilized fluorescence-activated cell sorting or in vitro cultivation of cells from enzymatically dissociated tissue to enrich for MaSC. These approaches result in the loss of all histological information pertaining to the in vivo locale of MaSC and progenitor cells. Instead, we used laser microdissection to excise putative progenitor cells and control cells from their in situ locations in cryosections and characterized the molecular properties of these cells. MaSC/progenitor cells were identified based on their ability to retain bromodeoxyuridine for an extended period.

Results: We isolated four categories of cells from mammary epithelium of female calves: bromodeoxyuridine label retaining epithelial cells (LREC) from basal (LRECb) and embedded layers (LRECe), and epithelial control cells from basal and embedded layers. Enriched expression of genes in LRECb was associated with stem cell attributes and identified WNT, TGF-β, and MAPK pathways of self renewal and proliferation. Genes expressed in LRECe revealed retention of some stem-like properties along with up-regulation of differentiation factors.

Conclusion: Our data suggest that LREC in the basal epithelial layer are enriched for MaSC, as these cells showed increased expression of genes that reflect stem cell attributes; whereas LREC in suprabasal epithelial layers are enriched for more committed progenitor cells, expressing some genes that are associated with stem cell attributes along with those indicative of cell differentiation. Our results support the use of DNA label retention to identify MaSC and also provide a molecular profile and novel candidate markers for these cells. Insights into the biology of stem cells will be gained by confirmation and characterization of candidate MaSC markers identified in this study.