Single-cell and spatial transcriptomics approaches of the bone marrow microenvironment.

PURPOSE OF REVIEW: The bone marrow is home to hematopoietic stem cells responsible for lifelong blood production, alongside mesenchymal stem cells required for skeletal regeneration. In the bone marrow, a unique combination of signals derived from a multitude of cell types results in the establishment of so-called niches that regulate stem-cell maintenance and differentiation. Recently, single-cell and spatially resolved transcriptomics technologies have been utilized to characterize the murine bone marrow microenvironment during homeostasis, stress and upon cancer-induced remodeling. In this review, we summarize the major findings of these studies. RECENT FINDINGS: Single-cell technologies applied to bone marrow provided the first systematic and label-free identification of bone marrow cell types, enabled their molecular and spatial characterization, and clarified the cellular sources of key prohematopoietic factors. Large transcriptional heterogeneity and novel subpopulations were observed in compartments previously thought to be homogenous. For example, Lepr Cxcl12-abundant reticular cells were shown to constitute the major source of prohematopoietic factors, but consist of subpopulations differing in their adipogenic versus osteogenic priming, morphology and localization. These subpopulations were suggested to act as professional cytokine secreting cells, thereby establishing distinct bone marrow niches. SUMMARY: Single-cell and spatially resolved transcriptomics approaches have clarified the molecular identity and localization of bone marrow-resident cell types, paving the road for a deeper exploration of bone marrow niches in the mouse and humans.

Combined single-cell and spatial transcriptomics reveal the molecular, cellular and spatial bone marrow niche organization.

The bone marrow constitutes the primary site for life-long blood production and skeletal regeneration. However, its cellular and spatial organization remains controversial. Here, we combine single-cell and spatially resolved transcriptomics to systematically map the molecular, cellular and spatial composition of distinct bone marrow niches. This allowed us to transcriptionally profile all major bone-marrow-resident cell types, determine their localization and clarify sources of pro-haematopoietic factors. Our data demonstrate that Cxcl12-abundant-reticular (CAR) cell subsets (Adipo-CAR and Osteo-CAR) differentially localize to sinusoidal and arteriolar surfaces, act locally as 'professional cytokine-secreting cells' and thereby establish peri-vascular micro-niches. Importantly, the three-dimensional bone-marrow organization can be accurately inferred from single-cell transcriptome data using the RNA-Magnet algorithm described here. Together, our study reveals the cellular and spatial organization of bone marrow niches and offers a systematic approach to dissect the complex organization of whole organs.

Loss of Angiotensin-Converting Enzyme 2 Exacerbates Diabetic Retinopathy by Promoting Bone Marrow Dysfunction.

Angiotensin-converting enzyme 2 (ACE2) is the primary enzyme of the vasoprotective axis of the renin angiotensin system (RAS). We tested the hypothesis that loss of ACE2 would exacerbate diabetic retinopathy by promoting bone marrow dysfunction. ACE2-/y were crossed with Akita mice, a model of type 1 diabetes. When comparing the bone marrow of the ACE2-/y -Akita mice to that of Akita mice, we observed a reduction of both short-term and long-term repopulating hematopoietic stem cells, a shift of hematopoiesis toward myelopoiesis, and an impairment of lineage- c-kit+ hematopoietic stem/progenitor cell (HS/PC) migration and proliferation. Migratory and proliferative dysfunction of these cells was corrected by exposure to angiotensin-1-7 (Ang-1-7), the protective peptide generated by ACE2. Over the duration of diabetes examined, ACE2 deficiency led to progressive reduction in electrical responses assessed by electroretinography and to increases in neural infarcts observed by fundus photography. Compared with Akita mice, ACE2-/y -Akita at 9-months of diabetes showed an increased number of acellular capillaries indicative of more severe diabetic retinopathy. In diabetic and control human subjects, CD34+ cells, a key bone marrow HS/PC population, were assessed for changes in mRNA levels for MAS, the receptor for Ang-1-7. Levels were highest in CD34+ cells from diabetics without retinopathy. Higher serum Ang-1-7 levels predicted protection from development of retinopathy in diabetics. Treatment with Ang-1-7 or alamandine restored the impaired migration function of CD34+ cells from subjects with retinopathy. These data support that activation of the protective RAS within HS/PCs may represents a therapeutic strategy for prevention of diabetic retinopathy. Stem Cells 2018;36:1430-1440.

Per2-Mediated Vascular Dysfunction Is Caused by the Upregulation of the Connective Tissue Growth Factor (CTGF).

Period 2-mutant mice (Per2m/m), which possess a circadian dysfunction, recapitulate the retinal vascular phenotype similar to diabetic retinopathy (DR). The vascular dysfunction in Per2m/m is associated with an increase in connective tissue growth factor (CTGF/CCN2). At the molecular level, CTGF gene expression is dependent on the canonical Wnt/ß-catenin pathway. The nuclear binding of ß-catenin to a transcription factor, lymphoid enhancer binding protein (Lef)/ T-cell factor (TCF/LEF), leads to downstream activation of CTGF. For this study, we hypothesized that the silencing of Per2 results in nuclear translocation and subsequent transactivation of the CTGF gene. To test this hypothesis, we performed immunofluorescence labeling for CTGF in retinal sections from wild-type (WT) and Per2m/m mice. Human retinal endothelial cells (HRECs) were transfected with siRNA for Per2, and the protein expression of CTGF and ß-catenin was evaluated. The TCF/LEF luciferase reporter (TOPflash) assay was performed to validate the involvement of ß-catenin in the activation of CTGF. Per2m/m retinas exhibited an increased CTGF immunostaining in ganglion cell layer and retinal endothelium. Silencing of Per2 using siRNA resulted in an upregulation of CTGF and ß-catenin. The TOPflash assay revealed an increase in luminescence for HRECs transfected with Per2 siRNA. Our studies show that loss of Per2 results in an activation of CTGF via nuclear entry of ß-catenin. Our study provides novel insight into the understanding of microvascular dysfunction in Per2m/m mice.

Clinical characteristics and PTPN22 1858C/T variant analysis in Jordanian Arab vitiligo patients.

BACKGROUND AND OBJECTIVE: Vitiligo is an autoimmune polygenic disorder, characterized by loss of pigmentation due to melanocyte destruction. Multiple genes and environmental triggers are thought to play a role in inducing vitiligo. These genes and environmental factors differ across different populations. In this study, we investigated vitiligo patients in Jordan for patient characteristics and analyzed the association of the 1858C/T (rs2476601, R620W) variant of the PTPN22 gene with vitiligo in our patients. METHODS: Fifty-five patients with generalized vitiligo and 85 matched normal control subjects who did not have vitiligo or any apparent autoimmune disorder were recruited and interviewed for clinical and demographic characteristics. DNA samples were obtained from patients and controls and genotyped by restriction fragment length polymorphism for the 1858C/T variant. Fifty-three percent of our patients (29 of 55 overall) were female, the average age at onset was 19.2 years, 84.3% of patients (43 of 51 reported) had changing size of depigmented patches, 3.8% (2 of 53 reported) had other autoimmune disease, and 19.2% (5 of 26 reported) had a family history of vitiligo. RESULTS: The allelic frequency of 1858T (620W) was 1.9% in patients as opposed to 2.9% in controls (p = 0.5). No PTPN22 1858 TT homozygotes were observed among patients or controls; 3.8% of vitiligo patients were 1858 CT heterozygotes compared with 5.9% of controls (p = 0.7). Consequently, no significant association was observed between the 1858C/T functional variant and vitiligo patients. CONCLUSION: Although the PTPN22 1858C/T variant has been reported to play a role in increasing the risk of vitiligo in Caucasian patients, it does not appear to play a similar role in the Jordanian population, though a larger cohort of patients might be needed to confirm such a conclusion.