A 3D Tissue-wide Digital Imaging Pipeline for Quantitation of Secreted Molecules Shows Absence of CXCL12 Gradients in Bone Marrow

IL-1β promotes MPN disease initiation by favoring early clonal expansion of JAK2-mutant hematopoietic stem cells

ABSTRACT

JAK 2-V617F is the most frequent somatic mutation causing myeloproliferative neoplasm (MPN). JAK2-V617F can be found in healthy individuals with clonal hematopoiesis of indeterminate potential (CHIP) with a frequency much higher than the prevalence of MPNs. The factors controlling the conversion of JAK2-V617F CHIP to MPN are largely unknown. We hypothesized that interleukin-1β (IL-1β)-mediated inflammation can favor this progression. We established an experimental system using bone marrow (BM) transplantations from JAK2-V617F and GFP transgenic (VF;GFP) mice that were further crossed with IL-1β-/- or IL-1R1-/- mice. To study the role of IL-1β and its receptor on monoclonal evolution of MPN, we performed competitive BM transplantations at high dilutions with only 1 to 3 hematopoietic stem cells (HSCs) per recipient. Loss of IL-1β in JAK2-mutant HSCs reduced engraftment, restricted clonal expansion, lowered the total numbers of functional HSCs, and decreased the rate of conversion to MPN. Loss of IL-1R1 in the recipients also lowered the conversion to MPN but did not reduce the frequency of engraftment of JAK2-mutant HSCs. Wild-type (WT) recipients transplanted with VF;GFP BM that developed MPNs had elevated IL-1β levels and reduced frequencies of mesenchymal stromal cells (MSCs). Interestingly, frequencies of MSCs were also reduced in recipients that did not develop MPNs, had only marginally elevated IL-1β levels, and displayed low GFP-chimerism resembling CHIP. Anti-IL-1β antibody preserved high frequencies of MSCs in VF;GFP recipients and reduced the rate of engraftment and the conversion to MPN. Our results identify IL-1β as a potential therapeutic target for preventing the transition from JAK2-V617F CHIP to MPNs.

The hematopoietic microenvironment: a network of niches for the development of all blood cell lineages

Blood cell formation (hematopoiesis) takes place mainly in the bone marrow, within the hematopoietic microenvironment, composed of a number of different cell types and their molecular products that together shape spatially organized and highly specialized microstructures called hematopoietic niches. From the earliest developmental stages and throughout the myeloid and lymphoid lineage differentiation pathways, hematopoietic niches play a crucial role in the preservation of cellular integrity and the regulation of proliferation and differentiation rates. Current evidence suggests that each blood cell lineage develops under specific, discrete niches that support committed progenitor and precursor cells and potentially cooperate with transcriptional programs determining the gradual lineage commitment and specification. This review aims to discuss recent advances on the cellular identity and structural organization of lymphoid, granulocytic, monocytic, megakaryocytic, and erythroid niches throughout the hematopoietic microenvironment and the mechanisms by which they interconnect and regulate viability, maintenance, maturation, and function of the developing blood cells.

Glia Cells Control Olfactory Neurogenesis by Fine-Tuning CXCL12

Olfaction depends on lifelong production of sensory neurons from CXCR4 expressing neurogenic stem cells. Signaling by CXCR4 depends on the concentration of CXCL12, CXCR4's principal ligand. Here, we use several genetic models to investigate how regulation of CXCL12 in the olfactory stem cell niche adjusts neurogenesis. We identify subepithelial tissue and sustentacular cells, the olfactory glia, as main CXCL12 sources. Lamina propria-derived CXCL12 accumulates on quiescent gliogenic stem cells via heparan sulfate. Additionally, CXCL12 is secreted within the olfactory epithelium by sustentacular cells. Both sustentacular-cell-derived and lamina propria-derived CXCL12 are required for CXCR4 activation. ACKR3, a high-affinity CXCL12 scavenger, is expressed by mature glial cells and titrates CXCL12. The accurate adjustment of CXCL12 by ACKR3 is critical for CXCR4-dependent proliferation of neuronal stem cells and for proper lineage progression. Overall, these findings establish precise regulation of CXCL12 by glia cells as a prerequisite for CXCR4-dependent neurogenesis and identify ACKR3 as a scavenger influencing tissue homeostasis beyond embryonic development.

Optimizing In Situ Proximity Ligation Assays for Mitochondria, ER, or MERC Markers in Skeletal Muscle Tissue and Cells

Proximity ligation assays (PLA) use specific antibodies to detect endogenous protein-protein interactions. PLA is a highly useful biochemical technique that allows two proteins within close proximity to be visualized with fluorescent probes amplified by PCR. While this technique has gained prominence, the use of PLA in mouse skeletal muscle (SkM) is novel. In this article, we discuss how the PLA method can be used in SkM to study the protein-protein interactions within mitochondria-endoplasmic reticulum contact sites (MERCs).

Proteolytic inactivation of CXCL12 in the lungs and circulation of COVID-19 patients

The human chemokine stromal cell-derived factor-1 (SDF-1) or CXCL12 is involved in several homeostatic processes and pathologies through interaction with its cognate G protein-coupled receptor CXCR4. Recent research has shown that CXCL12 is present in the lungs and circulation of patients with coronavirus disease 2019 (COVID-19). However, the question whether the detected CXCL12 is bioactive was not addressed. Indeed, the activity of CXCL12 is regulated by NH2- and COOH-terminal post-translational proteolysis, which significantly impairs its biological activity. The aim of the present study was to characterize proteolytic processing of CXCL12 in broncho-alveolar lavage (BAL) fluid and blood plasma samples from critically ill COVID-19 patients. Therefore, we optimized immunosorbent tandem mass spectrometry proteoform analysis (ISTAMPA) for detection of CXCL12 proteoforms. In patient samples, this approach uncovered that CXCL12 is rapidly processed by site-specific NH2- and COOH-terminal proteolysis and ultimately degraded. This proteolytic inactivation occurred more rapidly in COVID-19 plasma than in COVID-19 BAL fluids, whereas BAL fluid samples from stable lung transplantation patients and the non-affected lung of lung cancer patients (control groups) hardly induced any processing of CXCL12. In COVID-19 BAL fluids with high proteolytic activity, processing occurred exclusively NH2-terminally and was predominantly mediated by neutrophil elastase. In low proteolytic activity BAL fluid and plasma samples, NH2- and COOH-terminal proteolysis by CD26 and carboxypeptidases were observed. Finally, protease inhibitors already approved for clinical use such as sitagliptin and sivelestat prevented CXCL12 processing and may therefore be of pharmacological interest to prolong CXCL12 half-life and biological activity in vivo.

The chemokines CXCL8 and CXCL12: molecular and functional properties, role in disease and efforts towards pharmacological intervention

Chemokines are an indispensable component of our immune system through the regulation of directional migration and activation of leukocytes. CXCL8 is the most potent human neutrophil-attracting chemokine and plays crucial roles in the response to infection and tissue injury. CXCL8 activity inherently depends on interaction with the human CXC chemokine receptors CXCR1 and CXCR2, the atypical chemokine receptor ACKR1, and glycosaminoglycans. Furthermore, (hetero)dimerization and tight regulation of transcription and translation, as well as post-translational modifications further fine-tune the spatial and temporal activity of CXCL8 in the context of inflammatory diseases and cancer. The CXCL8 interaction with receptors and glycosaminoglycans is therefore a promising target for therapy, as illustrated by multiple ongoing clinical trials. CXCL8-mediated neutrophil mobilization to blood is directly opposed by CXCL12, which retains leukocytes in bone marrow. CXCL12 is primarily a homeostatic chemokine that induces migration and activation of hematopoietic progenitor cells, endothelial cells, and several leukocytes through interaction with CXCR4, ACKR1, and ACKR3. Thereby, it is an essential player in the regulation of embryogenesis, hematopoiesis, and angiogenesis. However, CXCL12 can also exert inflammatory functions, as illustrated by its pivotal role in a growing list of pathologies and its synergy with CXCL8 and other chemokines to induce leukocyte chemotaxis. Here, we review the plethora of information on the CXCL8 structure, interaction with receptors and glycosaminoglycans, different levels of activity regulation, role in homeostasis and disease, and therapeutic prospects. Finally, we discuss recent research on CXCL12 biochemistry and biology and its role in pathology and pharmacology.

Dynamic organization of the bone marrow plasma cell niche

Prophylactic, serological memory relies on maintaining stable reservoirs of plasma cells, capable of constitutively-secreting high-affinity, anti-pathogen antibody for a lifetime. Although antibody titers generated by some vaccines (e.g. measles) can last a lifetime, other vaccinations (e.g. tetanus) need repeated boosting because long-lived plasma cells are not produced or maintained. Moreover, in old age, the ability to generate long-lived humoral responses diminishes. Despite their importance to health, it is unknown how long-lived plasma cells survive over years, whereas most antibody secreting cells die off within weeks after vaccination. In this review, we focus on how known factors regulate the longevity of plasma cell fitness and survival, and how that landscape is shaped by environmental influences, such as inflammation, infection and aging. In addition, we highlight newly discovered cellular dynamics in the bone marrow that may reframe the mechanisms supporting long-lived plasma cell survival and function.

NfκB signaling dynamics and their target genes differ between mouse blood cell types and induce distinct cell behavior

Cells can use signaling pathway activity over time (i.e., dynamics) to control cell fates. However, little is known about the potential existence and function of signaling dynamics in primary hematopoietic stem and progenitor cells (HSPCs). Here, we use time-lapse imaging and tracking of single murine HSPCs from GFP-p65/H2BmCherry reporter mice to quantify their nuclear factor κB (NfκB) activity dynamics in response to TNFα and IL1β. We find response dynamics to be heterogeneous between individual cells, with cell type specific dynamics distributions. Transcriptome sequencing of single cells physically isolated after live dynamics quantification shows activation of different target gene programs in cells with different dynamics. Finally, artificial induction of oscillatory NfκB activity causes changes in GMP behavior. Thus, HSPC behavior can be influenced by signaling dynamics, which are tightly regulated during hematopoietic differentiation and enable cell type specific responses to the same signaling inputs.

A cholinergic neuroskeletal interface promotes bone formation during postnatal growth and exercise

The autonomic nervous system is a master regulator of homeostatic processes and stress responses. Sympathetic noradrenergic nerve fibers decrease bone mass, but the role of cholinergic signaling in bone has remained largely unknown. Here, we describe that early postnatally, a subset of sympathetic nerve fibers undergoes an interleukin-6 (IL-6)-induced cholinergic switch upon contacting the bone. A neurotrophic dependency mediated through GDNF-family receptor-α2 (GFRα2) and its ligand, neurturin (NRTN), is established between sympathetic cholinergic fibers and bone-embedded osteocytes, which require cholinergic innervation for their survival and connectivity. Bone-lining osteoprogenitors amplify and propagate cholinergic signals in the bone marrow (BM). Moderate exercise augments trabecular bone partly through an IL-6-dependent expansion of sympathetic cholinergic nerve fibers. Consequently, loss of cholinergic skeletal innervation reduces osteocyte survival and function, causing osteopenia and impaired skeletal adaptation to moderate exercise. These results uncover a cholinergic neuro-osteocyte interface that regulates skeletogenesis and skeletal turnover through bone-anabolic effects.

Spatial determinates of effector and memory CD8+ T cell fates

The lymph node plays a critical role in mounting an adaptive immune response to infection, clearance of foreign pathogens, and cancer immunosurveillance. Within this complex structure, intranodal migration is vital for CD8⁺ T cell activation and differentiation. Combining tissue clearing and volumetric light sheet fluorescent microscopy of intact lymph nodes has allowed us to explore the spatial regulation of T cell fates. This has determined that short-lived effector (T_SLEC ) are imprinted in peripheral lymph node interfollicular regions, due to CXCR3 migration. In contrast, stem-like memory cell (T_SCM ) differentiation is determined in the T cell paracortex. Here, we detail the inflammatory and chemokine regulators of spatially restricted T cell differentiation, with a focus on how to promote T_SCM . We propose a default pathway for T_SCM differentiation due to CCR7-directed segregation of precursors away from the inflammatory effector niche. Although volumetric imaging has revealed the consequences of intranodal migration, we still lack knowledge of how this is orchestrated within a complex chemokine environment. Toward this goal, we highlight the potential of combining microfluidic chambers with pre-determined complexity and subcellular resolution microscopy.

Intercrypt sentinel macrophages tune antibacterial NF-κB responses in gut epithelial cells via TNF

Intestinal epithelial cell (IEC) NF-κB signaling regulates the balance between mucosal homeostasis and inflammation. It is not fully understood which signals tune this balance and how bacterial exposure elicits the process. Pure LPS induces epithelial NF-κB activation in vivo. However, we found that in mice, IECs do not respond directly to LPS. Instead, tissue-resident lamina propria intercrypt macrophages sense LPS via TLR4 and rapidly secrete TNF to elicit epithelial NF-κB signaling in their immediate neighborhood. This response pattern is relevant also during oral enteropathogen infection. The macrophage-TNF-IEC axis avoids responses to luminal microbiota LPS but enables crypt- or tissue-scale epithelial NF-κB responses in proportion to the microbial threat. Thereby, intercrypt macrophages fulfill important sentinel functions as first responders to Gram-negative microbes breaching the epithelial barrier. The tunability of this crypt response allows the induction of defense mechanisms at an appropriate scale according to the localization and intensity of microbial triggers.

Rebuilding the hematopoietic stem cell niche: Recent developments and future prospects

Hematopoietic stem cells (HSCs) have proven their clinical relevance in stem cell transplantation to cure patients with hematological disorders. Key to their regenerative potential is their natural microenvironment - their niche - in the bone marrow (BM). Developments in the field of biomaterials enable the recreation of such environments with increasing preciseness in the laboratory. Such artificial niches help to gain a fundamental understanding of the biophysical and biochemical processes underlying the interaction of HSCs with the materials in their environment and the disturbance of this interplay during diseases affecting the BM. Artificial niches also have the potential to multiply HSCs in vitro, to enable the targeted differentiation of HSCs into mature blood cells or to serve as drug-testing platforms. In this review, we will introduce the importance of artificial niches followed by the biology and biophysics of the natural archetype. We will outline how 2D biomaterials can be used to dissect the complexity of the natural niche into individual parameters for fundamental research and how 3D systems evolved from them. We will present commonly used biomaterials for HSC research and their applications. Finally, we will highlight two areas in the field of HSC research, which just started to unlock the possibilities provided by novel biomaterials, in vitro blood production and studying the pathophysiology of the niche in vitro. With these contents, the review aims to give a broad overview of the different biomaterials applied for HSC research and to discuss their potentials, challenges and future directions in the field. STATEMENT OF SIGNIFICANCE: Hematopoietic stem cells (HSCs) are multipotent cells responsible for maintaining the turnover of all blood cells. They are routinely applied to treat patients with hematological diseases. This high clinical relevance explains the necessity of multiplication or differentiation of HSCs in the laboratory, which is hampered by the missing natural microenvironment - the so called niche. Biomaterials offer the possibility to mimic the niche and thus overcome this hurdle. The review introduces the HSC niche in the bone marrow and discusses the utility of biomaterials in creating artificial niches. It outlines how 2D systems evolved into sophisticated 3D platforms, which opened the gateway to applications such as, expansion of clinically relevant HSCs, in vitro blood production, studying niche pathologies and drug testing.

Asymmetric organelle inheritance predicts human blood stem cell fate

Understanding human hematopoietic stem cell fate control is important for their improved therapeutic manipulation. Asymmetric cell division, the asymmetric inheritance of factors during division instructing future daughter cell fates, was recently described in mouse blood stem cells. In human blood stem cells, the possible existence of asymmetric cell division remained unclear due to technical challenges in its direct observation. Here, we use long-term quantitative single-cell imaging to show that lysosomes and active mitochondria are asymmetrically inherited in human blood stem cells and that their inheritance is a coordinated, non-random process. Furthermore, multiple additional organelles, including autophagosomes, mitophagosomes, autolysosomes and recycling endosomes show preferential asymmetric co-segregation with lysosomes. Importantly, asymmetric lysosomal inheritance predicts future asymmetric daughter cell cycle length, differentiation and stem cell marker expression, while asymmetric inheritance of active mitochondria correlates with daughter metabolic activity. Hence, human hematopoietic stem cell fates are regulated by asymmetric cell division, with both mechanistic evolutionary conservation and differences to the mouse system.

Analyzing signaling activity and function in hematopoietic cells

Cells constantly sense their environment, allowing the adaption of cell behavior to changing needs. Fine-tuned responses to complex inputs are computed by signaling pathways, which are wired in complex connected networks. Their activity is highly context-dependent, dynamic, and heterogeneous even between closely related individual cells. Despite lots of progress, our understanding of the precise implementation, relevance, and possible manipulation of cellular signaling in health and disease therefore remains limited. Here, we discuss the requirements, potential, and limitations of the different current technologies for the analysis of hematopoietic stem and progenitor cell signaling and its effect on cell fates.

A spatiotemporal microenvironment model to improve design of a 3D bioreactor for red cell production

Cellular microenvironments provide stimuli including paracrine and autocrine growth factors and physico-chemical cues, which support efficient in vivo cell production unmatched by current in vitro biomanufacturing platforms. While three-dimensional (3D) culture systems aim to recapitulate niche architecture and function of the target tissue/organ, they are limited in accessing spatiotemporal information to evaluate and optimize in situ cell/tissue process development. Herein, a mathematical modelling framework is parameterized by single-cell phenotypic imaging and multiplexed biochemical assays to simulate the non-uniform tissue distribution of nutrients/metabolites and growth factors in cell niche environments. This model is applied to a bone marrow mimicry 3D perfusion bioreactor containing dense stromal and hematopoietic tissue with limited red blood cell (RBC) egress. The model characterized an imbalance between endogenous cytokine production and nutrient starvation within the microenvironmental niches, and recommended increased cell inoculum density and enhanced medium exchange, guiding the development of a miniaturized prototype bioreactor. The second-generation prototype improved the distribution of nutrients and growth factors and supported a 50-fold increase in RBC production efficiency. This image-informed bioprocess modelling framework leverages spatiotemporal niche information to enhance biochemical factor utilization and improve cell manufacturing in 3D systems.

Cytokine combinations for human blood stem cell expansion induce cell type- and cytokine-specific signaling dynamics

How hematopoietic stem cells (HSCs) integrate signals from their environment to make fate decisions remains incompletely understood. Current knowledge is based on either averages of heterogeneous populations or snapshot analyses, both missing important information about the dynamics of intracellular signaling activity. By combining fluorescent biosensors with time-lapse imaging and microfluidics, we measured the activity of the extracellular signal-regulated kinase (ERK) pathway over time (i.e. dynamics) in live single human umbilical cord blood HSCs and multipotent progenitor cells (MPPs). In single cells, ERK signaling dynamics were highly heterogeneous and depended on the cytokines, their combinations, and cell types. ERK signaling was activated by SCF and FLT3L in HSCs, but by SCF, IL3 and GCSF in MPPs. Different cytokines and their combinations led to distinct ERK signaling dynamics frequencies, and ERK dynamics in HSCs were more transient than those in MPPs. A combination of 5 cytokines recently shown to maintain HSCs in long-term culture, had a more-than-additive effect in eliciting sustained ERK dynamics in HSCs. ERK signaling dynamics also predicted future cell fates. E.g. CD45RA expression increased more in HSC daughters with intermediate than with transient or sustained ERK signaling. We demonstrate heterogeneous, cytokine- and cell type- specific ERK signaling dynamics, illustrating their relevance in regulating HSPC fates.

GPR182 is an endothelium-specific atypical chemokine receptor that maintains hematopoietic stem cell homeostasis

G protein–coupled receptors (GPCRs) are important regulators of cellular and biological functions and are primary targets of therapeutic drugs. About 100 mammalian GPCRs are still considered orphan receptors because they lack a known endogenous ligand. We report the deorphanization of GPR182, which is expressed in endothelial cells of the microvasculature. We show that GPR182 is an atypical chemokine receptor, which binds CXCL10, 12, and 13. However, binding does not induce downstream signaling. Consistent with a scavenging function of GPR182, mice lacking GPR182 have increased plasma levels of chemokines. In line with the crucial role of CXCL12 in hematopoietic stem cell homeostasis, we found that loss of GPR182 results in increased egress of hematopoietic stem cells from the bone marrow.

G protein–coupled receptor 182 (GPR182) has been shown to be expressed in endothelial cells; however, its ligand and physiological role has remained elusive. We found GPR182 to be expressed in microvascular and lymphatic endothelial cells of most organs and to bind with nanomolar affinity the chemokines CXCL10, CXCL12, and CXCL13. In contrast to conventional chemokine receptors, binding of chemokines to GPR182 did not induce typical downstream signaling processes, including G_q- and G_i-mediated signaling or β-arrestin recruitment. GPR182 showed relatively high constitutive activity in regard to β-arrestin recruitment and rapidly internalized in a ligand-independent manner. In constitutive GPR182-deficient mice, as well as after induced endothelium-specific loss of GPR182, we found significant increases in the plasma levels of CXCL10, CXCL12, and CXCL13. Global and induced endothelium-specific GPR182-deficient mice showed a significant decrease in hematopoietic stem cells in the bone marrow as well as increased colony-forming units of hematopoietic progenitors in the blood and the spleen. Our data show that GPR182 is a new atypical chemokine receptor for CXCL10, CXCL12, and CXCL13, which is involved in the regulation of hematopoietic stem cell homeostasis.

Structural organization of the bone marrow and its role in hematopoiesis

PURPOSE OF REVIEW

The bone marrow is the main site for hematopoiesis. It contains a unique microenvironment that provides niches that support self-renewal and differentiation of hematopoietic stem cells (HSC), multipotent progenitors (MPP), and lineage committed progenitors to produce the large number of blood cells required to sustain life. The bone marrow is notoriously difficult to image; because of this the anatomy of blood cell production -- and how local signals spatially organize hematopoiesis -- are not well defined. Here we review our current understanding of the spatial organization of the mouse bone marrow with a special focus in recent advances that are transforming our understanding of this tissue.

RECENT FINDINGS

Imaging studies of HSC and their interaction with candidate niches have relied on ex-vivo imaging of fixed tissue. Two recent manuscripts demonstrating live imaging of subsets of HSC in unperturbed bone marrow have revealed unexpected HSC behavior and open the door to examine HSC regulation, in situ, over time. We also discuss recent findings showing that the bone marrow contains distinct microenvironments, spatially organized, that regulate unique aspects of hematopoiesis.

SUMMARY

Defining the spatial architecture of hematopoiesis in the bone marrow is indispensable to understand how this tissue ensures stepwise, balanced, differentiation to meet organism demand; for deciphering alterations to hematopoiesis during disease; and for designing organ systems for blood cell production ex vivo.

Trabecular bone organoid model for studying the regulation of localized bone remodeling

Trabecular bone maintains physiological homeostasis and consistent structure and mass through repeated cycles of bone remodeling by means of tightly localized regulation. The molecular and cellular processes that regulate localized bone remodeling are poorly understood because of a lack of relevant experimental models. A tissue-engineered model is described here that reproduces bone tissue complexity and bone remodeling processes with high fidelity and control. An osteoid-inspired biomaterial-demineralized bone paper-directs osteoblasts to deposit structural mineralized bone tissue and subsequently acquire the resting-state bone lining cell phenotype. These cells activate and shift their secretory profile to induce osteoclastogenesis in response to chemical stimulation. Quantitative spatial mapping of cellular activities in resting and activated bone surface coculture showed that the resting-state bone lining cell network actively directs localized bone remodeling by means of paracrine signaling and cell-to-cell contact. This model may facilitate further investigation of trabecular bone niche biology.

Ubiquitous overexpression of CXCL12 confers radiation protection and enhances mobilization of hematopoietic stem and progenitor cells

C-X-C motif chemokine ligand 12 (CXCL12; aka SDF1α) is a major regulator of a number of cellular systems, including hematopoiesis, where it influences hematopoietic cell trafficking, proliferation, and survival during homeostasis and upon stress and disease. A variety of constitutive, temporal, ubiquitous, and cell-specific loss-of-function models have documented the functional consequences on hematopoiesis upon deletion of Cxcl12. Here, in contrast to loss-of-function experiments, we implemented a gain-of-function approach by generating a doxycycline-inducible transgenic mouse model that enables spatial and temporal overexpression of Cxcl12. We demonstrated that ubiquitous CXCL12 overexpression led to an increase in multipotent progenitors in the bone marrow and spleen. The CXCL12+ mice displayed reduced reconstitution potential as either donors or recipients in transplantation experiments. Additionally, we discovered that Cxcl12 overexpression improved hematopoietic stem and progenitor cell mobilization into the blood, and conferred radioprotection by promoting quiescence. Thus, this new CXCL12+ mouse model provided new insights into major facets of hematopoiesis and serves as a versatile resource for studying CXCL12 function in a variety of contexts.