pH regulates hematopoietic stem cell potential via polyamines.

Upon ex vivo culture, hematopoietic stem cells (HSCs) quickly lose potential and differentiate into progenitors. The identification of culture conditions that maintain the potential of HSCs ex vivo is therefore of high clinical interest. Here, we demonstrate that the potential of murine and human HSCs is maintained when cultivated for 2 days ex vivo at a pH of 6.9, in contrast to cultivation at the commonly used pH of 7.4. When cultivated at a pH of 6.9, HSCs remain smaller, less metabolically active, less proliferative and show enhanced reconstitution ability upon transplantation compared to HSC cultivated at pH 7.4. HSCs kept at pH 6.9 show an attenuated polyamine pathway. Pharmacological inhibition of the polyamine pathway in HSCs cultivated at pH 7.4 with DFMO mimics phenotypes and potential of HSCs cultivated at pH 6.9. Ex vivo exposure to a pH of 6.9 is therefore a positive regulator of HSC function by reducing polyamines. These findings might improve HSC short-term cultivation protocols for transplantation and gene therapy interventions.

Cdc42-Borg4-Septin7 axis regulates HSC polarity and function.

Aging of hematopoietic stem cells (HSCs) is caused by the elevated activity of the small RhoGTPase Cdc42 and an apolar distribution of proteins. Mechanisms by which Cdc42 activity controls polarity of HSCs are not known. Binder of RhoGTPases proteins (Borgs) are known effector proteins of Cdc42 that are able to regulate the cytoskeletal Septin network. Here, we show that Cdc42 interacts with Borg4, which in turn interacts with Septin7 to regulate the polar distribution of Cdc42, Borg4, and Septin7 within HSCs. Genetic deletion of either Borg4 or Septin7 results in a reduced frequency of HSCs polar for Cdc42 or Borg4 or Septin7, a reduced engraftment potential and decreased lymphoid-primed multipotent progenitor (LMPP) frequency in the bone marrow. Taken together, our data identify a Cdc42-Borg4-Septin7 axis essential for the maintenance of polarity within HSCs and for HSC function and provide a rationale for further investigating the role of Borgs and Septins in the regulation of compartmentalization within stem cells.

An aged bone marrow niche restrains rejuvenated hematopoietic stem cells.

Aging-associated leukemia and aging-associated immune remodeling are in part caused by aging of hematopoietic stem cells (HSCs). An increase in the activity of the small RhoGTPase cell division control protein 42 (Cdc42) within HSCs causes aging of HSCs. Old HSCs, treated ex vivo with a specific inhibitor of Cdc42 activity termed CASIN, stay rejuvenated upon transplantation into young recipients. We determined in this study the influence of an aged niche on the function of ex vivo rejuvenated old HSCs, as the relative contribution of HSCs intrinsic mechanisms vs extrinsic mechanisms (niche) for aging of HSCs still remain unknown. Our results show that an aged niche restrains the function of ex vivo rejuvenated HSCs, which is at least in part linked to a low level of the cytokine osteopontin found in aged niches. The data imply that sustainable rejuvenation of the function of aged HSCs in vivo will need to address the influence of an aged niche on rejuvenated HSCs.

Aging of human hematopoietic stem cells is linked to changes in Cdc42 activity.

In this study, we characterize age-related phenotypes of human hematopoietic stem cells (HSC). We report increased frequencies of HSC, hematopoietic progenitor cells and lineage negative cells in the elderly but a decreased frequency of multi-lymphoid progenitors. Aged human HSC further exhibited a delay in initiating division ex vivo though without changes in their division kinetics. The activity of the small RhoGTPase Cdc42 was elevated in aged human hematopoietic cells and we identified a positive correlation between Cdc42 activity and the frequency of HSC upon aging. The frequency of human HSC polar for polarity proteins was, similar to the mouse, decreased upon aging, while inhibition of Cdc42 activity via the specific pharmacological inhibitor of Cdc42 activity, CASIN, resulted in re-polarization of aged human HSC with respect to Cdc42. Elevated activity of Cdc42 in aged HSC thus contributed to age-related changes in HSC. Xenotransplant, using NBSGW mice as recipients, showed elevated chimerism in recipients of aged compared to young HSC. Aged HSC treated with CASIN ex vivo displayed an engraftment profile similar to recipients of young HSC. Taken together, our work reveals strong evidence for a role of elevated Cdc42 activity in driving aging of human HSC, and similar to mice, this presents a likely possibility for attenuation of aging in human HSC.

Aging alters the epigenetic asymmetry of HSC division.

Hematopoietic stem cells (HSCs) balance self-renewal and differentiation to maintain homeostasis. With aging, the frequency of polar HSCs decreases. Cell polarity in HSCs is controlled by the activity of the small RhoGTPase cell division control protein 42 (Cdc42). Here we demonstrate-using a comprehensive set of paired daughter cell analyses that include single-cell 3D confocal imaging, single-cell transplants, single-cell RNA-seq, and single-cell transposase-accessible chromatin sequencing (ATAC-seq)-that the outcome of HSC divisions is strongly linked to the polarity status before mitosis, which is in turn determined by the level of the activity Cdc42 in stem cells. Aged apolar HSCs undergo preferentially self-renewing symmetric divisions, resulting in daughter stem cells with reduced regenerative capacity and lymphoid potential, while young polar HSCs undergo preferentially asymmetric divisions. Mathematical modeling in combination with experimental data implies a mechanistic role of the asymmetric sorting of Cdc42 in determining the potential of daughter cells via epigenetic mechanisms. Therefore, molecules that control HSC polarity might serve as modulators of the mode of stem cell division regulating the potential of daughter cells.

Aged murine hematopoietic stem cells drive aging-associated immune remodeling.

Aging-associated remodeling of the immune system impairs its functional integrity and contributes to increased morbidity and mortality in the elderly. Aging of hematopoietic stem cells (HSCs), from which all cells of the adaptive immune system ultimately originate, might play a crucial role in the remodeling of the aged immune system. We recently reported that aging of HSCs is, in part, driven by elevated activity of the small RhoGTPase Cdc42 and that aged HSCs can be rejuvenated in vitro by inhibition of the elevated Cdc42 activity in aged HSCs with the pharmacological compound CASIN. To study the quality of immune systems stemming selectively from young or aged HSCs, we established a HSC transplantation model in T- and B-cell-deficient young RAG1-/- hosts. We report that both phenotypic and functional changes in the immune system on aging are primarily a consequence of changes in the function of HSCs on aging and, to a large extent, independent of the thymus, as young and aged HSCs reconstituted distinct T- and B-cell subsets in RAG1-/- hosts that mirrored young and aged immune systems. Importantly, aged HSCs treated with CASIN reestablished an immune system similar to that of young animals, and thus capable of mounting a strong immune response to vaccination. Our studies further imply that epigenetic signatures already imprinted in aged HSCs determine the transcriptional profile and function of HSC-derived T and B cells.

HPRT and Purine Salvaging Are Critical for Hematopoietic Stem Cell Function.

Adult hematopoietic stem cells (HSCs) maintain tissue homeostasis and regenerative capacity of the hematopoietic system through self-renewal and differentiation. Metabolism is recognized as an important regulatory entity controlling stem cells. As purine nucleotides are essential for metabolic functions, we analyzed the role of hypoxanthine guanine phosphoribosyl transferase (HPRT)-associated purine salvaging in HSCs. Here, we demonstrate that hematopoietic stem and progenitor cells (HSPCs) show a strong dependence on HPRT-associated purine salvaging. HSPCs with lower HPRT activity had a severely reduced competitive repopulation ability upon transplantation. Strikingly, HPRT deficiency resulted in altered cell-cycle progression, proliferation kinetics and mitochondrial membrane potential primarily in the HSC compartment, whereas more committed progenitors were less affected. Our data thus imply a unique and important role of HPRT and the purine salvage pathway for HSC function. Stem Cells 2019;37:1606-1614.

Wnt activity and basal niche position sensitize intestinal stem and progenitor cells to DNA damage.

Aging and carcinogenesis coincide with the accumulation of DNA damage and mutations in stem and progenitor cells. Molecular mechanisms that influence responses of stem and progenitor cells to DNA damage remain to be delineated. Here, we show that niche positioning and Wnt signaling activity modulate the sensitivity of intestinal stem and progenitor cells (ISPCs) to DNA damage. ISPCs at the crypt bottom with high Wnt/ß-catenin activity are more sensitive to DNA damage compared to ISPCs in position 4 with low Wnt activity. These differences are not induced by differences in cell cycle activity but relate to DNA damage-dependent activation of Wnt signaling, which in turn amplifies DNA damage checkpoint activation. The study shows that instructed enhancement of Wnt signaling increases radio-sensitivity of ISPCs, while inhibition of Wnt signaling decreases it. These results provide a proof of concept that cell intrinsic levels of Wnt signaling modulate the sensitivity of ISPCs to DNA damage and heterogeneity in Wnt activation in the stem cell niche contributes to the selection of ISPCs in the context of DNA damage.

Senescence and apoptosis block hematopoietic activation of quiescent hematopoietic stem cells with short telomeres.

Telomere shortening limits the proliferative capacity of human cells, and age-dependent shortening of telomeres occurs in somatic tissues including hematopoietic stem cells (HSCs). It is currently unknown whether genomic and molecular damage that occurs in HSCs induced by telomere shortening is transmitted to the progenitor cells. Here we show that telomere shortening results in DNA damage accumulation and gene expression changes in quiescent HSCs of aged mice. Upon activation, a subset of HSCs with elevated levels of DNA damage and p16 expression are blocked from cell cycle entry, and apoptosis is induced in HSCs entering the cell cycle. Activation of both checkpoints associates with normalization of DNA damage and gene expression profiles at early progenitor stages. These findings indicate that quiescent HSCs have an elevated tolerance to accumulate genomic alterations in response to telomere shortening, but the transmission of these aberrations to the progenitor cell level is prevented by senescence and apoptosis.

The promoter of human telomerase reverse transcriptase is activated during liver regeneration and hepatocyte proliferation.

BACKGROUND & AIMS: Telomerase activity has not been detected in healthy human liver biopsy samples, but it is up-regulated in most human liver tumors. It is not clear whether telomerase is activated in response to acute or chronic liver injury. Telomerase activity is closely associated with expression of its catalytic subunit, telomerase reverse transcriptase (TERT). We analyzed the activity of the human TERT (hTERT) promoter during liver regeneration in vivo and hepatocyte proliferation in vitro. METHODS: We used hTERTp-lacZ transgenic mice, which contain an 8.0-kilobase pair fragment of the hTERT gene promoter, to study the role of TERT in liver regeneration following partial hepatectomy. As an in vitro model, we used the HepaRG cell line as a new model system for human hepatocyte proliferation and differentiation. RESULTS: Activity of the hTERT promoter increased significantly after partial hepatectomy; it was also induced in hepatocytes, based on immunohistologic analysis. Similar to the in vivo results, telomerase activity and hTERT expression were up-regulated in proliferating HepaRG cells and repressed in response to growth arrest and differentiation. Promoter mapping revealed that a proximal 0.3-kilobase pair fragment contains all elements necessary for regulation of hTERT in HepaRG cells. We identified E2F2 and E2F7 as transcription factors that control the differential expression of hTERT in proliferating hepatocytes, in vitro and in vivo. CONCLUSIONS: hTERT is induced in hepatocytes during liver regeneration, indicating a functional role for telomerase in human liver.

A Limited Role for AMD3100 Induced Stem Cell Mobilization for Modulation of Thoracic Trauma Outcome.

ABSTRACT: Thoracic trauma is a major cause of mortality due to the associated inflammatory acute respiratory distress syndrome and morbidity due to impaired tissue regeneration. Trauma-induced lung inflammation is characterized by the early recruitment of cells with pro- or anti-inflammatory activity to the lung. Therapeutic interventions reducing the level of tissue inflammation may result in decreased tissue damage and improved healing and recovery. Stem cells might be able to improve trauma outcome via immunomodulation or by enhancing tissue regeneration.Here, we describe the migratory dynamics of murine mesenchymal, hematopoietic and endothelial stem and progenitor cells (SPCs) as well as mature inflammatory cells (monocytes, neutrophils, lymphocytes) to peripheral blood (PB) and lung tissue between 0.2 and 48 h post-blunt chest trauma (TXT). We demonstrate that the kinetics of immune cell and SPC distribution upon trauma are both cell-type and tissue-dependent. We identified a transient, early increase in the number of inflammatory cells in PB and lung at 2 h post-TXT and a second wave of infiltrating SPCs in lungs by 48 h after TXT induction, suggesting a role for SPCs in tissue remodeling after the initial inflammatory phase. Cxcl12/Cxcr4 blockade by AMD3100 within the first 6 h after TXT, while inducing a strong and coordinated mobilization of SPCs and leukocytes to PB and lung tissue, did not significantly affect TXT associated inflammation or tissue damage as determined by inflammatory cytokine levels, plasma markers for organ function, lung cell proliferation and survival, and myofibroblast/fibroblast ratio in the lung. Further understanding the dynamics of the distribution of endogenous SPCs and inflammatory cells will therefore be indispensable for stem cell-based or immunomodulation therapies in trauma.

Reduced adhesion of aged intestinal stem cells contributes to an accelerated clonal drift.

Upon aging, the function of the intestinal epithelium declines with a concomitant increase in aging-related diseases. ISCs play an important role in this process. It is known that ISC clonal dynamics follow a neutral drift model. However, it is not clear whether the drift model is still valid in aged ISCs. Tracking of clonal dynamics by clonal tracing revealed that aged crypts drift into monoclonality substantially faster than young ones. However, ISC tracing experiments, in vivo and ex vivo, implied a similar clonal expansion ability of both young and aged ISCs. Single-cell RNA sequencing for 1,920 high Lgr5 ISCs from young and aged mice revealed increased heterogeneity among subgroups of aged ISCs. Genes associated with cell adhesion were down-regulated in aged ISCs. ISCs of aged mice indeed show weaker adhesion to the matrix. Simulations applying a single cell-based model of the small intestinal crypt demonstrated an accelerated clonal drift at reduced adhesion strength, implying a central role for reduced adhesion for affecting clonal dynamics upon aging.

Fast and high-fidelity in situ 3D imaging protocol for stem cells and niche components for mouse organs and tissues.

Quantitative 3D imaging of organ-wide cellular and subcellular components is central for revealing and understanding complex interactions between stem cells and their microenvironment. Here, we present a gentle but fast whole-mount immunofluorescence staining protocol for 3D confocal microscopy (iFAST3D) that preserves the 3D structure of the entire tissue and that of subcellular structures with high fidelity. The iFAST3D protocol enables reproducible and high-resolution 3D imaging of stem cells and various niche components for many mouse organs and tissues. For complete details on the use and execution of this protocol, please refer to Saçma et al. (2019).

Transplanting rejuvenated blood stem cells extends lifespan of aged immunocompromised mice.

One goal of regenerative medicine is to rejuvenate tissues and extend lifespan by restoring the function of endogenous aged stem cells. However, evidence that somatic stem cells can be targeted in vivo to extend lifespan is still lacking. Here, we demonstrate that after a short systemic treatment with a specific inhibitor of the small RhoGTPase Cdc42 (CASIN), transplanting aged hematopoietic stem cells (HSCs) from treated mice is sufficient to extend the healthspan and lifespan of aged immunocompromised mice without additional treatment. In detail, we show that systemic CASIN treatment improves strength and endurance of aged mice by increasing the myogenic regenerative potential of aged skeletal muscle stem cells. Further, we show that CASIN modifies niche localization and H4K16ac polarity of HSCs in vivo. Single-cell profiling reveals changes in HSC transcriptome, which underlie enhanced lymphoid and regenerative capacity in serial transplantation assays. Overall, we provide proof-of-concept evidence that a short systemic treatment to decrease Cdc42 activity improves the regenerative capacity of different endogenous aged stem cells in vivo, and that rejuvenated HSCs exert a broad systemic effect sufficient to extend murine health- and lifespan.

A Wnt5a-Cdc42 axis controls aging and rejuvenation of hair-follicle stem cells.

Normal hair growth occurs in cycles, comprising growth (anagen), cessation (catagen) and rest (telogen). Upon aging, the initiation of anagen is significantly delayed, which results in impaired hair regeneration. Hair regeneration is driven by hair follicle stem cells (HFSCs). We show here that aged HFSCs present with a decrease in canonical Wnt signaling and a shift towards non-canonical Wnt5a driven signaling which antagonizes canonical Wnt signaling. Elevated expression of Wnt5a in HFSCs upon aging results in elevated activity of the small RhoGTPase Cdc42 as well as a change in the spatial distribution of Cdc42 within HFSCs. Treatment of aged HFSC with a specific pharmacological inhibitor of Cdc42 activity termed CASIN to suppress the aging-associated elevated activity of Cdc42 restored canonical Wnt signaling in aged HFSCs. Treatment of aged mice in vivo with CASIN induced anagen onset and increased the percentage of anagen skin areas. Aging-associated functional deficits of HFSCs are at least in part intrinsic to HFSCs and can be restored by rational pharmacological approaches.

The sphingosine-1-phosphate (S1P) lysophospholipid receptor S1P3 regulates MAdCAM-1+ endothelial cells in splenic marginal sinus organization.

Marginal zones (MZs) are microdomains in the spleen that contain various types of immune cells, including MZ B cells, MOMA1(+) metallophilic macrophages, and mucosal addressin cell adhesion molecule 1 (MAdCAM-1)(+) endothelial cells. MAdCAM-1(+) and MOMA1(+) cells line the sinus, that separates MZs from splenic follicles. Here we show that a receptor for the lysophospholipid sphingosine-1-phosphate (S1P), S1P(3), is required for normal numbers of splenic immature and MZ B cells, and for S1P-induced chemotaxis of MZ B cells. S1P(3) is also essential for proper alignment of MOMA1(+) macrophages and MAdCAM-1(+) endothelial cells along the marginal sinus. The lack of cohesion of the marginal sinus in S1P(3)(-/-) mice affects MZ B cell functions, as wild-type (WT) MZ B cells migrate more into S1P(3)(-/-) follicles than into WT follicles after treatment with lipopolysaccharide. Additionally, short-term homing experiments demonstrate that WT MZ B cells home to the S1P(3)(-/-) spleen in increased numbers, suggesting a role for the marginal sinus in regulating MZ B cells numbers. Moreover, S1P(3)(-/-) mice are defective in mounting immune responses to thymus-independent antigen type 2 due to defects in radiation-resistant cells in the spleen. These data identify lysophospholipids and the S1P(3) receptor as essential regulators of the MZ sinus and its role as a barrier to the follicle.

Inhibition of Cdc42 activity extends lifespan and decreases circulating inflammatory cytokines in aged female C57BL/6 mice.

Cdc42 is a small RhoGTPase regulating multiple functions in eukaryotic cells. The activity of Cdc42 is significantly elevated in several tissues of aged mice, while the Cdc42 gain-of-activity mouse model presents with a premature aging-like phenotype and with decreased lifespan. These data suggest a causal connection between elevated activity of Cdc42, aging, and reduced lifespan. Here, we demonstrate that systemic treatment of aged (75-week-old) female C57BL/6 mice with a Cdc42 activity-specific inhibitor (CASIN) for 4 consecutive days significantly extends average and maximum lifespan. Moreover, aged CASIN-treated animals displayed a youthful level of the aging-associated cytokines IL-1ß, IL-1α, and INFγ in serum and a significantly younger epigenetic clock as based on DNA methylation levels in blood cells. Overall, our data show that systemic administration of CASIN to reduce Cdc42 activity in aged mice extends murine lifespan.

Haematopoietic stem cells in perisinusoidal niches are protected from ageing.

With ageing, intrinsic haematopoietic stem cell (HSC) activity decreases, resulting in impaired tissue homeostasis, reduced engraftment following transplantation and increased susceptibility to diseases. However, whether ageing also affects the HSC niche, and thereby impairs its capacity to support HSC function, is still widely debated. Here, by using in-vivo long-term label-retention assays we demonstrate that aged label-retaining HSCs, which are, in old mice, the most quiescent HSC subpopulation with the highest regenerative capacity and cellular polarity, reside predominantly in perisinusoidal niches. Furthermore, we demonstrate that sinusoidal niches are uniquely preserved in shape, morphology and number on ageing. Finally, we show that myeloablative chemotherapy can selectively disrupt aged sinusoidal niches in the long term, which is linked to the lack of recovery of endothelial Jag2 at sinusoids. Overall, our data characterize the functional alterations of the aged HSC niche and unveil that perisinusoidal niches are uniquely preserved and thereby protect HSCs from ageing.