Non-canonical Hedgehog signaling mediates profibrotic hematopoiesis-stroma crosstalk in myeloproliferative neoplasms

The role of hematopoietic Hedgehog signaling in myeloproliferative neoplasms (MPNs) remains incompletely understood despite data suggesting that Hedgehog (Hh) pathway inhibitors have therapeutic activity in patients. We aim to systematically interrogate the role of canonical vs. non-canonical Hh signaling in MPNs. We show that Gli1 protein levels in patient peripheral blood mononuclear cells (PBMCs) mark fibrotic progression and that, in murine MPN models, absence of hematopoietic Gli1, but not Gli2 or Smo, significantly reduces MPN phenotype and fibrosis, indicating that GLI1 in the MPN clone can be activated in a non-canonical fashion. Additionally, we establish that hematopoietic Gli1 has a significant effect on stromal cells, mediated through a druggable MIF-CD74 axis. These data highlight the complex interplay between alterations in the MPN clone and activation of stromal cells and indicate that Gli1 represents a promising therapeutic target in MPNs, particularly that Hh signaling is dispensable for normal hematopoiesis.

MarrowCellDLD: a microfluidic method for label-free retrieval of fragile bone marrow-derived cells

Functional bone marrow studies have focused primarily on hematopoietic progenitors, leaving limited knowledge about other fragile populations, such as bone marrow adipocytes (BMAds) and megakaryocytes. The isolation of these cells is challenging due to rupture susceptibility and large size. We introduce here a label-free cytometry microsystem, MarrowCellDLD, based on deterministic lateral displacement. MarrowCellDLD enables the isolation of large, fragile BM-derived cells based on intrinsic size properties while preserving their viability and functionality. Bone marrow adipocytes, obtained from mouse and human stromal line differentiation, as well as megakaryocytes, from primary human CD34+ hematopoietic stem and progenitor cells, were used for validation. Precise micrometer-range separation cutoffs were adapted for each cell type. Cells were sorted directly in culture media, without pre-labeling steps, and with real-time imaging for quality control. At least 106 cells were retrieved intact per sorting round. Our method outperformed two FACS instruments in purity and yield, particularly for large cell size fractions. MarrowCellDLD represents a non-destructive sorting tool for large, fragile BM-derived cells, facilitating the separation of pure populations of BMAds and megakaryocytes to further investigate their physiological and pathological roles.

Induction of mitochondrial recycling reverts age-associated decline of the hematopoietic and immune systems

Aging compromises hematopoietic and immune system functions, making older adults especially susceptible to hematopoietic failure, infections and tumor development, and thus representing an important medical target for a broad range of diseases. During aging, hematopoietic stem cells (HSCs) lose their blood reconstitution capability and commit preferentially toward the myeloid lineage (myeloid bias)1,2. These processes are accompanied by an aberrant accumulation of mitochondria in HSCs3. The administration of the mitochondrial modulator urolithin A corrects mitochondrial function in HSCs and completely restores the blood reconstitution capability of 'old' HSCs. Moreover, urolithin A-supplemented food restores lymphoid compartments, boosts HSC function and improves the immune response against viral infection in old mice. Altogether our results demonstrate that boosting mitochondrial recycling reverts the aging phenotype in the hematopoietic and immune systems.

MarrowQuant 2.0: A Digital Pathology Workflow Assisting Bone Marrow Evaluation in Experimental and Clinical Hematology

Bone marrow (BM) cellularity assessment is a crucial step in the evaluation of BM trephine biopsies for hematologic and nonhematologic disorders. Clinical assessment is based on a semiquantitative visual estimation of the hematopoietic and adipocytic components by hematopathologists, which does not provide quantitative information on other stromal compartments. In this study, we developed and validated MarrowQuant 2.0, an efficient, user-friendly digital hematopathology workflow integrated within QuPath software, which serves as BM quantifier for 5 mutually exclusive compartments (bone, hematopoietic, adipocytic, and interstitial/microvasculature areas and other) and derives the cellularity of human BM trephine biopsies. Instance segmentation of individual adipocytes is realized through the adaptation of the machine-learning-based algorithm StarDist. We calculated BM compartments and adipocyte size distributions of hematoxylin and eosin images obtained from 250 bone specimens, from control subjects and patients with acute myeloid leukemia or myelodysplastic syndrome, at diagnosis and follow-up, and measured the agreement of cellularity estimates by MarrowQuant 2.0 against visual scores from 4 hematopathologists. The algorithm was capable of robust BM compartment segmentation with an average mask accuracy of 86%, maximal for bone (99%), hematopoietic (92%), and adipocyte (98%) areas. MarrowQuant 2.0 cellularity score and hematopathologist estimations were highly correlated (R² = 0.92-0.98, intraclass correlation coefficient [ICC] = 0.98; interobserver ICC = 0.96). BM compartment segmentation quantitatively confirmed the reciprocity of the hematopoietic and adipocytic compartments. MarrowQuant 2.0 performance was additionally tested for cellularity assessment of specimens prospectively collected from clinical routine diagnosis. After special consideration for the choice of the cellularity equation in specimens with expanded stroma, performance was similar in this setting (R² = 0.86, n = 42). Thus, we conclude that these validation experiments establish MarrowQuant 2.0 as a reliable tool for BM cellularity assessment. We expect this workflow will serve as a clinical research tool to explore novel biomarkers related to BM stromal components and may contribute to further validation of future digitalized diagnostic hematopathology workstreams.

[Short telomere syndrome in adults: a rare entity that should be evoked]

Short telomere syndrome (STS) is a group of rare, often underrecognized, diseases caused by defects in telomere-maintenance genes, leading to abnormal telomere shortening and associated with diverse multi-organ manifestations. In pediatric patients, STS typically presents with mucocutaneous or gastrointestinal lesions, bone marrow failure and neoplasia. In adulthood, aplastic bone marrow disease, liver disease and pulmonary fibrosis are classic clinical manifestations. At present, medical treatment options for STS remain limited. Danazol, a synthetic androgenic hormone, can slow down telomere shortening and thus limit the progression of the disease. Finally, hematopoietic, hepatic and pulmonary transplantation, sometimes combined, may be discussed in a multidisciplinary setting in certain situations.

Differential blood counts do not consistently predict clinical measurements of bone mineral density and microarchitecture at homeostasis

The hematopoietic stem cell niche constitutes a complex bone marrow (BM) microenvironment. Osteoporosis is characterized by both reduced bone mineral density (BMD) and microarchitectural deterioration, constituting the most frequent alteration of the BM microenvironment. It is unclear to which extent modifications of the BM microenvironment, including in the context of osteoporosis, influence blood cell production. We aimed to describe the association between lumbar spine and total hip BMD and microarchitecture (assessed by trabecular bone score [TBS]) and differential blood counts. Data were collected at two time points from 803 (first assessment) and 901 (second assessment) postmenopausal women participating in the CoLaus/OsteoLaus cohort, a population‐based sample in Lausanne, Switzerland. Participants with other active disease or treatment that could influence hematopoiesis or osteoporosis were excluded. Bivariate and multivariate associations between each peripheral blood cell count and BMD or TBS were performed. Additionally, participants in the highest BMD and TBS tertiles were compared with participants in the lowest BMD and TBS tertiles. At first assessment, only neutrophils were significantly different in the lowest BMD and TBS tertile (3.18 ± 0.09 versus 3.47 ± 0.08 G/L, p = 0.028). At the second assessment, leucocytes (5.90 ± 0.11 versus 5.56 ± 0.10 G/L, p = 0.033), lymphocytes (1.87 ± 0.04 versus 1.72 ± 0.04 G/L p = 0.033), and monocytes (0.49 ± 0.01 versus 0.46 ± 0.1 G/L, p = 0.033) were significantly different. Power analysis did not identify quasi‐significant associations missed due to sample size. Although significant associations between blood counts and BMD or TBS were found, none was consistent across bone measurements or assessments. This study suggests that, at homeostasis and in postmenopausal women, there is no clinically significant association between the osteoporotic microenvironment and blood production output as measured by differential blood counts. In the context of conflicting reports on the relationship between osteoporosis and hematopoiesis, our study represents the first prospective two time‐point analysis of a large, homogenous cohort at steady state. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Raman microspectroscopy reveals unsaturation heterogeneity at the lipid droplet level and validates an in vitro model of bone marrow adipocyte subtypes

Bone marrow adipocytes (BMAds) constitute the most abundant stromal component of adult human bone marrow. Two subtypes of BMAds have been described, the more labile regulated adipocytes (rBMAds) and the more stable constitutive adipocytes (cBMAds), which develop earlier in life and are more resilient to environmental and metabolic disruptions. In vivo, rBMAds are enriched in saturated fatty acids, contain smaller lipid droplets (LDs) and more readily provide hematopoietic support than their cBMAd counterparts. Mouse models have been used for BMAds research, but isolation of primary BMAds presents many challenges, and thus in vitro models remain the current standard to study nuances of adipocyte differentiation. No in vitro model has yet been described for the study of rBMAds/cBMAds. Here, we present an in vitro model of BM adipogenesis with differential rBMAd and cBMAd-like characteristics. We used OP9 BM stromal cells derived from a (C57BL/6xC3H)F2-op/op mouse, which have been extensively characterized as feeder layer for hematopoiesis research. We observed similar canonical adipogenesis transcriptional signatures for spontaneously-differentiated (sOP9) and induced (iOP9) cultures, while fatty acid composition and desaturase expression of Scd1 and Fads2 differed at the population level. To resolve differences at the single adipocyte level we tested Raman microspectroscopy and show it constitutes a high-resolution method for studying adipogenesis in vitro in a label-free manner, with resolution to individual LDs. We found sOP9 adipocytes have lower unsaturation ratios, smaller LDs and higher hematopoietic support than iOP9 adipocytes, thus functionally resembling rBMAds, while iOP9 more closely resembled cBMAds. Validation in human primary samples confirmed a higher unsaturation ratio for lipids extracted from stable cBMAd-rich sites (femoral head upon hip-replacement surgery) versus labile rBMAds (iliac crest after chemotherapy). As a result, the 16:1/16:0 fatty acid unsaturation ratio, which was already shown to discriminate BMAd subtypes in rabbit and rat marrow, was validated to discriminate cBMAds from rBMAd in both the OP9 model in vitro system and in human samples. We expect our model will be useful for cBMAd and rBMAd studies, particularly where isolation of primary BMAds is a limiting step.

Cryogel-based Injectable 3D Microcarrier Co-culture for Support of Hematopoietic Progenitor Niches

Although hematopoietic stem cell (HSC) transplantation can restore functional hematopoiesis upon immune or chemotherapy-induced bone marrow failure, complications often arise during recovery, leading to up to 25% transplant-related mortality in treated patients. In hematopoietic homeostasis and regeneration, HSCs in the bone marrow give rise to the entirety of cellular blood components. One of the challenges in studying hematopoiesis is the ability to successfully mimic the relationship between the stroma and hematopoietic stem and progenitor cells (HSPCs). This study and the described protocols propose an advantageous method for culturing and assessing stromal hematopoietic support in three dimensions, representing a simplified in vitro model of the bone marrow niche that can be transplanted in vivo by injection. By co-culturing OP9 bone marrow-derived stromal cells (BMSCs) and cKit⁺ Sca-1⁺ Lin^- (KLS⁺ ) HSPCs on collagen-coated carboxymethylcellulose scaffolds for 2 weeks in the absence of cytokines, we established a methodology for in vivo subcutaneous transplantation. With this model we were able to detect early signs of extramedullary hematopoiesis. This work can be useful for studying various stromal cell populations in co-culture, as well as simple transfer by injection of these scaffolds in vivo for heterotopic regeneration of the marrow microenvironment. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Isolation of HSPCs from mice Basic Protocol 2: Co-seeding of HSPCs and BMSCs on collagen-coated CCMs Basic Protocol 3: Maintenance, real-time imaging, and analysis of co-seeded scaffolds Basic Protocol 4: End-point analysis of co-seeded scaffolds using flow cytometry and CFU assays Basic Protocol 5: Transplantation of scaffolds by subcutaneous injection Support Protocol: Preparation of custom scaffold drying device.

An Injectable Meta-Biomaterial: From Design and Simulation to In Vivo Shaping and Tissue Induction

A novel type of injectable biomaterial with an elastic softening transition is described. The material enables in vivo shaping, followed by induction of 3D stable vascularized tissue. The synthesis of the injectable meta-biomaterial is instructed by extensive numerical simulation as a suspension of irregularly fragmented, highly porous sponge-like microgels. The irregular particle shape dramatically enhances yield strain for in vivo stability against deformation. Porosity of the particles, along with friction between internal surfaces, provides the elastic softening transition. This emergent metamaterial property enables the material to reversibly change stiffness during deformation, allowing native tissue properties to be matched over a wide range of deformation amplitudes. After subcutaneous injection in mice, predetermined shapes can be sculpted manually. The 3D shape is maintained during excellent host tissue integration, with induction of vascular connective tissue that persists to the end of one-year follow-up. The geometrical design is compatible with many hydrogel materials, including cell-adhesion motives for cell transplantation. The injectable meta-biomaterial therefore provides new perspectives in soft tissue engineering and regenerative medicine.

Bone marrow adiposity and the hematopoietic niche: A historical perspective of reciprocity, heterogeneity, and lineage commitment

PURPOSE

Here we review the current knowledge on bone marrow adipocytes (BMAds) as active contributors to the regulation of the hematopoietic niche, and as potentially pivotal players in the progression of hematological malignancies. We highlight the hierarchical and functional heterogeneity of the adipocyte lineage within the bone marrow, and how potentially different contexts dictate their interactions with hematopoietic populations.

RECENT FINDINGS

Growing evidence associates the adipocyte lineage with important functions in hematopoietic regulation within the BM niche. Initially proposed to serve as negative regulators of the hematopoietic microenvironment, studies have also demonstrated that BMAds positively influence the survival and maintenance of hematopoietic stem cells (HSCs). These seemingly incongruous findings may at least be partially explained by stage-specificity across the adipocytic differentiation axis and by BMAds subtypes, suggesting that the heterogeneity of these populations allows for differential context-based interactions. One such distinction relies on the location of adipocytes. Constitutive bone marrow adipose tissue (cBMAT) historically associates to the "yellow" marrow containing so-called "stable" BMAs larger in size, less responsive to stimuli, and linked to HSC quiescence. On the other hand, regulated bone marrow adipose tissue (rBMAT)-associated adipocytes, also referred to as "labile" are smaller, more responsive to hematopoietic demand and strategically situated in hematopoietically active regions of the skeleton. Here we propose a model where the effect of distinct BM stromal cell populations (BMSC) in hematopoiesis is structured along the BMSC-BMAd differentiation axis, and where the effects on HSC maintenance versus hematopoietic proliferation are segregated. In doing so, it is possible to explain how recently identified, adipocyte-primed leptin receptor-expressing, CXCL12-high adventitial reticular cells (AdipoCARs) and marrow adipose lineage precursor cells (MALPs) best support active hematopoietic cell proliferation, while adipose progenitor cells (APCs) and maturing BMAd gradually lose the capacity to support active hematopoiesis, favoring HSC quiescence. Implicated soluble mediators include MCP-1, PAI-1, NRP1, possibly DPP4 and limiting availability of CXCL12 and SCF. How remodeling occurs within the BMSC-BMAd differentiation axis is yet to be elucidated and will likely unravel a three-way regulation of the hematopoietic, bone, and adipocytic compartments orchestrated by vascular elements. The interaction of malignant hematopoietic cells with BMAds is precisely contributing to unravel specific mechanisms of remodeling.

SUMMARY

BMAds are important operative components of the hematopoietic microenvironment. Their heterogeneity directs their ability to exert a range of regulatory capacities in a manner dependent on their hierarchical, spatial, and biological context. This complexity highlights the importance of (i) developing experimental tools and nomenclature adapted to address stage-specificity and heterogeneity across the BMSC-BMAd differentiation axis when reporting effects in hematopoiesis, (ii) interpreting gene reporter studies within this framework, and (iii) quantifying changes in all three compartments (hematopoiesis, adiposity and bone) when addressing interdependency.

Acquired haemophilia A in the postpartum and risk of relapse in subsequent pregnancies: A systematic literature review

BACKGROUND

About 1%-5% of acquired haemophilia A cases affect mothers in the postpartum setting.

AIMS

This study delineates the characteristics of this disease, specific to the postpartum setting, notably relapse in subsequent pregnancies.

METHODS

Report of two cases and literature study (1946-2019), yielding 73 articles describing 174 cases (total 176 cases).

RESULTS

Patients were aged 29.9 years (17-41) and 69% primigravidae. Diagnosis was made at a median of 60 days after delivery (range 0-308). Bleeding types were obstetrical (43.4%), cutaneous (41.3%), and muscular (36.7%). In >90% of the cases, FVIII at diagnosis was <1% (range 0%-8%). FVIII inhibitor was documented in 75.4% cases (median titre of 20 BU/ml, range 1-621). Haemostatic treatment was necessary in 57.1% using fresh frozen plasma (16%), factor concentrate (27.6%) and/or bypassing agents (37.4%). Immunosuppressive treatment was administered in 90.8%, mostly steroids (85.3%), alone or combined with immunosuppressants (27%). Rituximab was used mostly as a second line treatment. Only 24 patients (13.6%) had documented subsequent pregnancies and 6 (22.2%) suffered haemophilia recurrence during pregnancy.

CONCLUSION

This study allows better definition of: (1) clinical and laboratory characteristics of postpartum acquired haemophilia, (2) response to therapy, and (3) the risk of relapse for subsequent pregnancies.

Guidelines for Biobanking of Bone Marrow Adipose Tissue and Related Cell Types: Report of the Biobanking Working Group of the International Bone Marrow Adiposity Society

Over the last two decades, increased interest of scientists to study bone marrow adiposity (BMA) in relation to bone and adipose tissue physiology has expanded the number of publications using different sources of bone marrow adipose tissue (BMAT). However, each source of BMAT has its limitations in the number of downstream analyses for which it can be used. Based on this increased scientific demand, the International Bone Marrow Adiposity Society (BMAS) established a Biobanking Working Group to identify the challenges of biobanking for human BMA-related samples and to develop guidelines to advance establishment of biobanks for BMA research. BMA is a young, growing field with increased interest among many diverse scientific communities. These bring new perspectives and important biological questions on how to improve and build an international community with biobank databases that can be used and shared all over the world. However, to create internationally accessible biobanks, several practical and legislative issues must be addressed to create a general ethical protocol used in all institutes, to allow for exchange of biological material internationally. In this position paper, the BMAS Biobanking Working Group describes similarities and differences of patient information (PIF) and consent forms from different institutes and addresses a possibility to create uniform documents for BMA biobanking purposes. Further, based on discussion among Working Group members, we report an overview of the current isolation protocols for human bone marrow adipocytes (BMAds) and bone marrow stromal cells (BMSCs, formerly mesenchymal), highlighting the specific points crucial for effective isolation. Although we remain far from a unified BMAd isolation protocol and PIF, we have summarized all of these important aspects, which are needed to build a BMA biobank. In conclusion, we believe that harmonizing isolation protocols and PIF globally will help to build international collaborations and improve the quality and interpretation of BMA research outcomes.

The NAD-Booster Nicotinamide Riboside Potently Stimulates Hematopoiesis through Increased Mitochondrial Clearance

It has been recently shown that increased oxidative phosphorylation, as reflected by increased mitochondrial activity, together with impairment of the mitochondrial stress response, can severely compromise hematopoietic stem cell (HSC) regeneration. Here we show that the NAD⁺-boosting agent nicotinamide riboside (NR) reduces mitochondrial activity within HSCs through increased mitochondrial clearance, leading to increased asymmetric HSC divisions. NR dietary supplementation results in a significantly enlarged pool of progenitors, without concurrent HSC exhaustion, improves survival by 80%, and accelerates blood recovery after murine lethal irradiation and limiting-HSC transplantation. In immune-deficient mice, NR increased the production of human leucocytes from hCD34+ progenitors. Our work demonstrates for the first time a positive effect of NAD⁺-boosting strategies on the most primitive blood stem cells, establishing a link between HSC mitochondrial stress, mitophagy, and stem-cell fate decision, and unveiling the potential of NR to improve recovery of patients suffering from hematological failure including post chemo- and radiotherapy.

Smart Hydrogels for the Augmentation of Bone Regeneration by Endogenous Mesenchymal Progenitor Cell Recruitment

The treatment of bone defects with recombinant bone morphogenetic protein-2 (BMP-2) requires high doses precluding broad clinical application. Here, a bioengineering approach is presented that strongly improves low-dose BMP-2-based bone regeneration by mobilizing healing-associated mesenchymal progenitor cells (MPCs). Smart synthetic hydrogels are used to trap and study endogenous MPCs trafficking to bone defects. Hydrogel-trapped and prospectively isolated MPCs differentiate into multiple lineages in vitro and form bone in vivo. In vitro screenings reveal that platelet-derived growth factor BB (PDGF-BB) strongly recruits prospective MPCs making it a promising candidate for the engineering of hydrogels that enrich endogenous MPCs in vivo. However, PDGF-BB inhibits BMP-2-mediated osteogenesis both in vitro and in vivo. In contrast, smart two-way dynamic release hydrogels with fast-release of PDGF-BB and sustained delivery of BMP-2 beneficially promote the healing of bone defects. Collectively, it is shown that modulating the dynamics of endogenous progenitor cells in vivo by smart synthetic hydrogels significantly improves bone healing and holds great potential for other advanced applications in regenerative medicine.

Reporting Guidelines, Review of Methodological Standards, and Challenges Toward Harmonization in Bone Marrow Adiposity Research. Report of the Methodologies Working Group of the International Bone Marrow Adiposity Society

The interest in bone marrow adiposity (BMA) has increased over the last decade due to its association with, and potential role, in a range of diseases (osteoporosis, diabetes, anorexia, cancer) as well as treatments (corticosteroid, radiation, chemotherapy, thiazolidinediones). However, to advance the field of BMA research, standardization of methods is desirable to increase comparability of study outcomes and foster collaboration. Therefore, at the 2017 annual BMA meeting, the International Bone Marrow Adiposity Society (BMAS) founded a working group to evaluate methodologies in BMA research. All BMAS members could volunteer to participate. The working group members, who are all active preclinical or clinical BMA researchers, searched the literature for articles investigating BMA and discussed the results during personal and telephone conferences. According to the consensus opinion, both based on the review of the literature and on expert opinion, we describe existing methodologies and discuss the challenges and future directions for (1) histomorphometry of bone marrow adipocytes, (2) ex vivo BMA imaging, (3) in vivo BMA imaging, (4) cell isolation, culture, differentiation and in vitro modulation of primary bone marrow adipocytes and bone marrow stromal cell precursors, (5) lineage tracing and in vivo BMA modulation, and (6) BMA biobanking. We identify as accepted standards in BMA research: manual histomorphometry and osmium tetroxide 3D contrast-enhanced μCT for ex vivo quantification, specific MRI sequences (WFI and H-MRS) for in vivo studies, and RT-qPCR with a minimal four gene panel or lipid-based assays for in vitro quantification of bone marrow adipogenesis. Emerging techniques are described which may soon come to complement or substitute these gold standards. Known confounding factors and minimal reporting standards are presented, and their use is encouraged to facilitate comparison across studies. In conclusion, specific BMA methodologies have been developed. However, important challenges remain. In particular, we advocate for the harmonization of methodologies, the precise reporting of known confounding factors, and the identification of methods to modulate BMA independently from other tissues. Wider use of existing animal models with impaired BMA production (e.g., Pfrt^-/-, Kit^W/W-v) and development of specific BMA deletion models would be highly desirable for this purpose.

MarrowQuant Across Aging and Aplasia: A Digital Pathology Workflow for Quantification of Bone Marrow Compartments in Histological Sections

The bone marrow (BM) exists heterogeneously as hematopoietic/red or adipocytic/yellow marrow depending on skeletal location, age, and physiological condition. Mouse models and patients undergoing radio/chemotherapy or suffering acute BM failure endure rapid adipocytic conversion of the marrow microenvironment, the so-called "red-to-yellow" transition. Following hematopoietic recovery, such as upon BM transplantation, a "yellow-to-red" transition occurs and functional hematopoiesis is restored. Gold Standards to estimate BM cellular composition are pathologists' assessment of hematopoietic cellularity in hematoxylin and eosin (H&E) stained histological sections as well as volumetric measurements of marrow adiposity with contrast-enhanced micro-computerized tomography (CE-μCT) upon osmium-tetroxide lipid staining. Due to user-dependent variables, reproducibility in longitudinal studies is a challenge for both methods. Here we report the development of a semi-automated image analysis plug-in, MarrowQuant, which employs the open-source software QuPath, to systematically quantify multiple bone components in H&E sections in an unbiased manner. MarrowQuant discerns and quantifies the areas occupied by bone, adipocyte ghosts, hematopoietic cells, and the interstitial/microvascular compartment. A separate feature, AdipoQuant, fragments adipocyte ghosts in H&E-stained sections of extramedullary adipose tissue to render adipocyte area and size distribution. Quantification of BM hematopoietic cellularity with MarrowQuant lies within the range of scoring by four independent pathologists, while quantification of the total adipocyte area in whole bone sections compares with volumetric measurements. Employing our tool, we were able to develop a standardized map of BM hematopoietic cellularity and adiposity in mid-sections of murine C57BL/6 bones in homeostatic conditions, including quantification of the highly predictable red-to-yellow transitions in the proximal section of the caudal tail and in the proximal-to-distal tibia. Additionally, we present a comparative skeletal map induced by lethal irradiation, with longitudinal quantification of the "red-to-yellow-to-red" transition over 2 months in C57BL/6 femurs and tibiae. We find that, following BM transplantation, BM adiposity inversely correlates with kinetics of hematopoietic recovery and that a proximal to distal gradient is conserved. Analysis of in vivo recovery through magnetic resonance imaging (MRI) reveals comparable kinetics. On human trephine biopsies MarrowQuant successfully recognizes the BM compartments, opening avenues for its application in experimental, or clinical contexts that require standardized human BM evaluation.

Measurement of Mitochondrial Mass and Membrane Potential in Hematopoietic Stem Cells and T-cells by Flow Cytometry

A fine balance of quiescence, self-renewal, and differentiation is key to preserve the hematopoietic stem cell (HSC) pool and maintain lifelong production of all mature blood cells. In recent years cellular metabolism has emerged as a crucial regulator of HSC function and fate. We have previously demonstrated that modulation of mitochondrial metabolism influences HSC fate. Specifically, by chemically uncoupling the electron transport chain we were able to maintain HSC function in culture conditions that normally induce rapid differentiation. However, limiting HSC numbers often precludes the use of standard assays to measure HSC metabolism and therefore predict their function. Here, we report a simple flow cytometry assay that allows reliable measurement of mitochondrial membrane potential and mitochondrial mass in scarce cells such as HSCs. We discuss the isolation of HSCs from mouse bone marrow and measurement of mitochondrial mass and membrane potential post ex vivo culture. As an example, we show the modulation of these parameters in HSCs via treatment with a metabolic modulator. Moreover, we extend the application of this methodology on human peripheral blood-derived T cells and human tumor infiltrating lymphocytes (TILs), showing dramatic differences in their mitochondrial profiles, possibly reflecting different T cell functionality. We believe this assay can be employed in screenings to identify modulators of mitochondrial metabolism in various cell types in different contexts.

Injectable, scalable 3D tissue-engineered model of marrow hematopoiesis

Modeling the interaction between the supportive stroma and the hematopoietic stem and progenitor cells (HSPC) is of high interest in the regeneration of the bone marrow niche in blood disorders. In this work, we present an injectable co-culture system to study this interaction in a coherent in vitro culture and in vivo transplantation model. We assemble a 3D hematopoietic niche in vitro by co-culture of supportive OP9 mesenchymal cells and HSPCs in porous, chemically defined collagen-coated carboxymethylcellulose microscaffolds (CCMs). Flow cytometry and hematopoietic colony forming assays demonstrate the stromal supportive capacity for in vitro hematopoiesis in the absence of exogenous cytokines. After in vitro culture, we recover a paste-like living injectable niche biomaterial from CCM co-cultures by controlled, partial dehydration. Cell viability and the association between stroma and HSPCs are maintained in this process. After subcutaneous injection of this living artificial niche in vivo, we find maintenance of stromal and hematopoietic populations over 12 weeks in immunodeficient mice. Indeed, vascularization is enhanced in the presence of HSPCs. Our approach provides a minimalistic, scalable, biomimetic in vitro model of hematopoiesis in a microcarrier format that preserves the HSPC progenitor function, while being injectable in vivo without disrupting the cell-cell interactions established in vitro.

Brief Report From the 3rd International Meeting on Bone Marrow Adiposity (BMA 2017)

The 3rd International Meeting on Bone Marrow Adiposity (BMA) was held at the Olympic Museum in Lausanne, Switzerland, on August 31st and September 1st, 2017. This brief monograph summarizes the scientific contents of the meeting and highlights the birth of the International Bone Marrow Adiposity Society (BMAS).

High-throughput, nonperturbing quantification of lipid droplets with digital holographic microscopy

In vitro differentiating adipocytes are sensitive to liquid manipulations and have the tendency to float. Assessing adipocyte differentiation using current microscopy techniques involves cell staining and washing, while using flow cytometry involves cell retrieval in suspension. These methods induce biases, are difficult to reproduce, and involve tedious optimizations. In this study, we present digital holographic microscopy (DHM) as a label-free, nonperturbing means to quantify lipid droplets in differentiating adipocytes in a robust medium- to high-throughput manner. Taking advantage of the high refractive index of lipid droplets, DHM can assess the production of intracellular lipid droplets by differences in phase shift in a quantitative manner. Adipocytic differentiation, combined with other morphological features including cell confluence and cell death, was tracked over 6 days in live OP9 mesenchymal stromal cells. We compared DHM with other currently available methods of lipid droplet quantification and demonstrated its robustness with modulators of adipocytic differentiation in a dose-responsive manner. This study suggests DHM as a novel marker-free nonperturbing method to study lipid droplet accumulation and may be envisioned for drug screens and mechanistic studies on adipocytic differentiation.

Correlation study between osteoporosis and hematopoiesis in the context of adjuvant chemotherapy for breast cancer

This retrospective study attempts to establish if a correlation exists between osteoporosis and hematopoiesis before and after adjuvant chemotherapy in the context of non-metastatic breast cancer. Osteoporosis is interpreted both as a direct marker of osteoblastic decline and as an indirect marker of increased bone marrow adiposity within the hematopoietic microenvironment. Patients from the “Centre du Sein” at CHUV (Centre Hospitalier Universitaire Vaudois) undergoing adjuvant chemotherapy were included in this study. Evolution of blood counts was studied in correlation with the osteoporosis status. Toxicity of chemotherapy was coded according to published probability of febrile neutropenia. One hundred forty-three women were included: mean age 52.1 ± 12.5 years, mean BMI (body mass index) 24.4 ± 4.1. BMD (bone mineral density) scored osteoporotic in 32% and osteopenic in 45%. Prior to chemotherapy, BMD was positively correlated with neutrophil (p < 0.001) and thrombocyte (p = 0.01) count; TBS (trabecular bone score) was not correlated with blood count. After the first cycle of chemotherapy, an increase of one point in TBS correlated with a decrease of 57% on the time to reach leucocyte nadir (p = 0.004). There was a positive correlation between BMD and risk of infection (p < 0.001). Our data demonstrates an association between osteoporosis and lower blood counts in a younger cohort than previously published, extending it for the first time to neutrophil counts in females. Our results suggest that the healthier the bone, the earlier the lowest leucocyte count value, prompting further research on this area.

Single-cell analyses identify bioengineered niches for enhanced maintenance of hematopoietic stem cells

The in vitro expansion of long-term hematopoietic stem cells (HSCs) remains a substantial challenge, largely because of our limited understanding of the mechanisms that control HSC fate choices. Using single-cell multigene expression analysis and time-lapse microscopy, here we define gene expression signatures and cell cycle hallmarks of murine HSCs and the earliest multipotent progenitors (MPPs), and analyze systematically single HSC fate choices in culture. Our analysis revealed twelve differentially expressed genes marking the quiescent HSC state, including four genes encoding cell–cell interaction signals in the niche. Under basal culture conditions, most HSCs rapidly commit to become early MPPs. In contrast, when we present ligands of the identified niche components such as JamC or Esam within artificial niches, HSC cycling is reduced and long-term multipotency in vivo is maintained. Our approach to bioengineer artificial niches should be useful in other stem cell systems.

Haematopoietic stem cell (HSC) self-renewal is not sufficiently understood to recapitulate in vitro. Here, the authors generate gene signature and cell cycle hallmarks of single murine HSCs, and use identified endothelial receptors Esam and JamC as substrates to enhance HSC growth in engineered niches.

Biomechanical forces promote blood development through prostaglandin E2 and the cAMP-PKA signaling axis

Blood flow promotes emergence of definitive hematopoietic stem cells (HSCs) in the developing embryo, yet the signals generated by hemodynamic forces that influence hematopoietic potential remain poorly defined. Here we show that fluid shear stress endows long-term multilineage engraftment potential upon early hematopoietic tissues at embryonic day 9.5, an embryonic stage not previously described to harbor HSCs. Effects on hematopoiesis are mediated in part by a cascade downstream of wall shear stress that involves calcium efflux and stimulation of the prostaglandin E2 (PGE2)-cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling axis. Blockade of the PGE2-cAMP-PKA pathway in the aorta-gonad-mesonephros (AGM) abolished enhancement in hematopoietic activity. Furthermore, Ncx1 heartbeat mutants, as well as static cultures of AGM, exhibit lower levels of expression of prostaglandin synthases and reduced phosphorylation of the cAMP response element-binding protein (CREB). Similar to flow-exposed cultures, transient treatment of AGM with the synthetic analogue 16,16-dimethyl-PGE2 stimulates more robust engraftment of adult recipients and greater lymphoid reconstitution. These data provide one mechanism by which biomechanical forces induced by blood flow modulate hematopoietic potential.

The transcriptional landscape of hematopoietic stem cell ontogeny

Transcriptome analysis of adult hematopoietic stem cells (HSCs) and their progeny has revealed mechanisms of blood differentiation and leukemogenesis, but a similar analysis of HSC development is lacking. Here, we acquired the transcriptomes of developing HSCs purified from >2,500 murine embryos and adult mice. We found that embryonic hematopoietic elements clustered into three distinct transcriptional states characteristic of the definitive yolk sac, HSCs undergoing specification, and definitive HSCs. We applied a network-biology-based analysis to reconstruct the gene regulatory networks of sequential stages of HSC development and functionally validated candidate transcriptional regulators of HSC ontogeny by morpholino-mediated knockdown in zebrafish embryos. Moreover, we found that HSCs from in vitro differentiated embryonic stem cells closely resemble definitive HSCs, yet lack a Notch-signaling signature, likely accounting for their defective lymphopoiesis. Our analysis and web resource will enhance efforts to identify regulators of HSC ontogeny and facilitate the engineering of hematopoietic specification.

Identification of in vitro HSC fate regulators by differential lipid raft clustering

Most hematopoietic stem cells (HSC) in the bone marrow reside in a quiescent state and occasionally enter the cell cycle upon cytokine-induced activation. Although the mechanisms regulating HSC quiescence and activation remain poorly defined, recent studies have revealed a role of lipid raft clustering (LRC) in HSC activation. Here, we tested the hypothesis that changes in lipid raft distribution could serve as an indicator of the quiescent and activated state of HSCs in response to putative niche signals. A semi-automated image analysis tool was developed to map the presence or absence of lipid raft clusters in live HSCs cultured for just one hour in serum-free medium supplemented with stem cell factor (SCF). By screening the ability of 19 protein candidates to alter lipid raft dynamics, we identified six factors that induced either a marked decrease (Wnt5a, Wnt3a and Osteopontin) or increase (IL3, IL6 and VEGF) in LRC. Cell cycle kinetics of single HSCs exposed to these factors revealed a correlation of LRC dynamics and proliferation kinetics: factors that decreased LRC slowed down cell cycle kinetics, while factors that increased LRC led to faster and more synchronous cycling. The possibility of identifying, by LRC analysis at very early time points, whether a stem cell is activated and possibly committed upon exposure to a signaling cue of interest could open up new avenues for large-scale screening efforts.

Epigenetic memory in induced pluripotent stem cells

Somatic cell nuclear transfer and transcription-factor-based reprogramming revert adult cells to an embryonic state, and yield pluripotent stem cells that can generate all tissues. Through different mechanisms and kinetics, these two reprogramming methods reset genomic methylation, an epigenetic modification of DNA that influences gene expression, leading us to hypothesize that the resulting pluripotent stem cells might have different properties. Here we observe that low-passage induced pluripotent stem cells (iPSCs) derived by factor-based reprogramming of adult murine tissues harbour residual DNA methylation signatures characteristic of their somatic tissue of origin, which favours their differentiation along lineages related to the donor cell, while restricting alternative cell fates. Such an 'epigenetic memory' of the donor tissue could be reset by differentiation and serial reprogramming, or by treatment of iPSCs with chromatin-modifying drugs. In contrast, the differentiation and methylation of nuclear-transfer-derived pluripotent stem cells were more similar to classical embryonic stem cells than were iPSCs. Our data indicate that nuclear transfer is more effective at establishing the ground state of pluripotency than factor-based reprogramming, which can leave an epigenetic memory of the tissue of origin that may influence efforts at directed differentiation for applications in disease modelling or treatment.

Isolation of hematopoietic stem cells from mouse embryonic stem cells

This unit describes a protocol for the isolation of cells from murine embryonic stem cells with hematopoietic stem cell activity, defined by the ability to reconstitute, long term, multiple lineages of the hematopoietic system of lethally irradiated mice. The protocol subjects hematopoietic progenitors specified in differentiating embryoid bodies to ectopic HoxB4 expression (delivered via retroviral infection), followed by coculture and expansion on OP9 stromal cells in the presence of hematopoietic cytokines for 10 days. The protocol results in the generation of hundreds of millions of cells that can rescue mice from lethal irradiation. Although little is known about the phenotype and frequency of the actual hematopoietic stem cell-like cell within the population of cells generated by this protocol, the protocol establishes a system in which these cells can be further studied and the results ultimately translated to the human system.

The Cdx-Hox Pathway in Hematopoietic Stem Cell Formation from Embryonic Stem Cells

Embryonic stem cells (ESCs) differentiated in vitro will yield a multitude of hematopoietic derivatives, yet progenitors displaying true stem cell activity remain difficult to obtain. Possible causes are a biased differentiation to primitive yolk sac-type hematopoiesis, and a variety of developmental or functional deficiencies. Recent studies in the zebrafish have identified the caudal homeobox transcription factors (cdx1/4) and posterior hox genes (hoxa9a, hoxb7a) as key regulators for blood formation during embryonic development. Activation of Cdx and Hox genes during the in vitro differentiation of mouse ESCs followed by co-culture on supportive stromal cells generates ESC-derived hematopoietic stem cells (HSCs) capable of multilineage repopulation of lethally irradiated adult mice. We show here that brief pulses of ectopic Cdx4 or HoxB4 expression are sufficient to enhance hematopoiesis during ESC differentiation, presumably by acting as developmental switches to activate posterior Hox genes. Insights into the role of the Cdx-Hox gene pathway during embryonic hematopoietic development in the zebrafish have allowed us to improve the derivation of repopulating HSCs from murine ESCs.

The May-Hegglin anomaly gene MYH9 is a negative regulator of platelet biogenesis modulated by the Rho-ROCK pathway

The gene implicated in the May-Hegglin anomaly and related macrothrombocytopenias, MYH9, encodes myosin-IIA, a protein that enables morphogenesis in diverse cell types. Defective myosin-IIA complexes are presumed to perturb megakaryocyte (MK) differentiation or generation of proplatelets. We observed that Myh9(-/-) mouse embryonic stem (ES) cells differentiate into MKs that are fully capable of proplatelet formation (PPF). In contrast, elevation of myosin-IIA activity, by exogenous expression or by mimicking constitutive phosphorylation of its regulatory myosin light chain (MLC), significantly attenuates PPF. This effect occurs only in the presence of myosin-IIA and implies that myosin-IIA influences thrombopoiesis negatively. MLC phosphorylation in MKs is regulated by Rho-associated kinase (ROCK), and consistent with our model, ROCK inhibition enhances PPF. Conversely, expression of AV14, a constitutive form of the ROCK activator Rho, blocks PPF, and this effect is rescued by simultaneous expression of a dominant inhibitory MLC form. Hematopoietic transplantation studies in mice confirm that interference with the putative Rho-ROCK-myosin-IIA pathway selectively decreases the number of circulating platelets. Our studies unveil a key regulatory pathway for platelet biogenesis and hint at Sdf-1/CXCL12 as one possible extracellular mediator. The unexpected mechanism for Myh9-associated thrombocytopenia may lead to new molecular approaches to manipulate thrombopoiesis.

Stem cells and their niche: a matter of fate

Embryonic stem cells provide an in vitro model for developmental biologists to study cell fate decisions during ontogenesis, while somatic stem cells allow physiologists to understand tissue homeostasis in the adult. The behavior of stem cells is dependent on an intimate relationship with a supportive niche. This brief review highlights some of the most important recent trends in stem cell biology, focusing in particular on the supportive microenvironments for both embryonic and adult stem cells. Known intrinsic and extrinsic molecular players from the best-characterized stem cell types are summarized, illuminating a number of shared environmental cues among tissues originating from all three embryonic germ layers.

Embryonic stem cell-derived hematopoietic stem cells

Despite two decades of studies documenting the in vitro blood-forming potential of murine embryonic stem cells (ESCs), achieving stable long-term blood engraftment of ESC-derived hematopoietic stem cells in irradiated mice has proven difficult. We have exploited the Cdx-Hox pathway, a genetic program important for blood development, to enhance the differentiation of ESCs along the hematopoietic lineage. Using an embryonic stem cell line engineered with tetracycline-inducible Cdx4, we demonstrate that ectopic Cdx4 expression promotes hematopoietic mesoderm specification, increases hematopoietic progenitor formation, and, together with HoxB4, enhances multilineage hematopoietic engraftment of lethally irradiated adult mice. Clonal analysis of retroviral integration sites confirms a common stem cell origin of lymphoid and myeloid populations in engrafted primary and secondary mice. These data document the cardinal stem cell features of self-renewal and multilineage differentiation of ESC-derived hematopoietic stem cells.