Adult hematopoietic Stem and Progenitor Cells (HSPCs) reside in the bone marrow hematopoietic niche, which regulates HSPC quiescence, self-renewal, and commitment in a demand-adapted manner. While the complex bone marrow niche is responsible for adult hematopoiesis, evidence exists for simpler, albeit functional and more accessible, extramedullary hematopoietic niches. Inspired by the anecdotal description of retroperitoneal hematopoietic masses occurring at higher frequency upon hormonal dysregulation within the adrenal gland, we hypothesized that the adult adrenal gland could be induced into a hematopoietic supportive environment in a systematic manner, thus revealing mechanisms underlying de novo niche formation in the adult. Here we show that upon splenectomy and hormonal stimulation, the adult adrenal gland of mice can be induced to recruit and host functional HSPCs, capable of serial transplantation, and that this phenomenon is associated with de novo formation of platelet-derived growth factor receptor α (PDGFRα) expressing stromal nodules. We further show in CXCL12-GFP reporter mice that adrenal glands contain a stromal population reminiscent of the CXCL12-Abundant Reticular (CAR) cells which compose the bone marrow HSPC niche. Mechanistically, HSPC homing to hormonally-induced adrenal glands was found dependent on the CXCR4/CXCL12 axis. Mirroring our findings in mice, we found reticular CXCL12+ cells co-expressing master niche-regulator FOXC1 in primary samples from human adrenal myelolipomas, a benign tumor composed of adipose and hematopoietic tissue. Our findings reignite long-standing questions regarding hormonal regulation of hematopoiesis and provide a novel model to facilitate the study of adult-specific inducible hematopoietic niches which may pave the way to therapeutic applications.

Engineering protein biosensors that sensitively respond to specific biomolecules by triggering precise cellular responses is a major goal of diagnostics and synthetic cell biology. Previous biosensor designs have largely relied on binding structurally well-defined molecules. In contrast, approaches that couple the sensing of flexible compounds to intended cellular responses would greatly expand potential biosensor applications. Here, to address these challenges, we develop a computational strategy for designing signaling complexes between conformationally dynamic proteins and peptides. To demonstrate the power of the approach, we create ultrasensitive chemotactic receptor—peptide pairs capable of eliciting potent signaling responses and strong chemotaxis in primary human T cells. Unlike traditional approaches that engineer static binding complexes, our dynamic structure design strategy optimizes contacts with multiple binding and allosteric sites accessible through dynamic conformational ensembles to achieve strongly enhanced signaling efficacy and potency. Our study suggests that a conformationally adaptable binding interface coupled to a robust allosteric transmission region is a key evolutionary determinant of peptidergic GPCR signaling systems. The approach lays a foundation for designing peptide-sensing receptors and signaling peptide ligands for basic and therapeutic applications.

Hematopoietic Stem and Progenitor Cells (HSPCs) reside in the hematopoietic niche, a structure that regulates the balance of cellular quiescence, self-renewal and commitment in a demand-adapted manner. The bone marrow (BM) hematopoietic niche is formed by several cellular players, mainly endothelial cells, osteoblasts, adipocytes, and stromal cells. While the BM niche forms a complex structure, evidence exists for simpler, albeit functional, extramedullary hematopoietic niches. However, the composition of what constitutes the simplest unit of an HSPC supportive microenvironment remains largely unknown. Here, we show that the adult adrenal gland can be transformed into a hematopoietic supportive environment. Upon splenectomy and hormonal stimulation, the adult adrenal gland can be induced to recruit and host HSPC function, including serial transplantation. Furthermore, the adrenal stroma contains a CXCL12+ population, reminiscent of BM CXCL12-Abundant Reticular (CAR) cells. Mirroring this, we found CXCL12+ cells in patient samples obtained from a local cohort of myelolipoma, a benign adrenal tumor composed of adipose and hematopoietic tissue that constitutes the most common site of extramedullary hematopoiesis specific to the adult. We present our model as a novel tool to increase our understanding of the physiology of hematopoietic support and to facilitate the development of a boneless niche model.

Bone marrow (BM) preadipocytes and adipocytes are part of the same differentiation axis and co-exist within this organ with hematopoietic cells. Mature adipocytes can slow down hematopoietic progenitor proliferation whereas pre-adipocytes efficiently support their growth, suggesting that modulating the adipocytic differentiation axis could steer hematopoietic function. We used digital holographic microscopy to perform a label-free, high-throughput in vitro phenotypic screen of more than 4000 compounds to identify modulators of adipocytic differentiation. Candidate compounds were counter-screened for niche-mediated effects on hematopoietic proliferation. The screen identified calcipotriol, a vitamin D analogue. Treatment with calcipotriol remodeled adipocytes in a dose-dependent manner and increased hematopoietic progenitors in mice undergoing lethal irradiation followed by BM transplantation. Furthermore, treatment with calcipotriol rescued mortality in CXCL12 haplo-insufficient mice undergoing limiting-dose BM transplantation. Our approach provides a useful tool to study adipocyte biology, demonstrates the feasibility of modulating the adipocytic differentiation axis in the BM and indicates its potential role in clinically relevant conditions of high hematopoietic demand.

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.

During the last 50 years, with over a million transplants, hematopoietic stem cell (HSC) transplantation has been the first and most extensively exploited stem cell therapy and the only curative regime for most acute leukemias. Nevertheless, the success of HSC transplantation and all other intensive ablative chemotherapy regimes is still overshadowed by a procedure-associated mortality of ~25%, due to both graft versus host disease and to the infectious complications associated with the severe leucopenia following bone marrow (BM) ablation, even in spite of standard support with HSC-stimulating factor G-CSF. 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 HSC regeneration. Here we show that the NAD+-boosting agents Nicotinamide Riboside (NR) and Nicotinamide MonoNucleotide (NMN) reduce mitochondrial activity within HSCs through increased mitochondrial clearance via autophagy and possibly the Unfolded Protein Response mitochondria (UPRmt), leading to increased asymmetric HSC divisions as measured by asymmetric mitochondrial distribution in single cell pair-daughter analysis. This process was abrogated in Nrk1-/-;Nrk2-/- double knock out mice, which cannot incorporate NR into the NAD+ salvage pathway. Contrary to controls, purified murine HSC underwent self-renewal in minimal culture conditions in presence of NR, and human CD34+ hematopoietic progenitors (hCD34+) cultured in vitro in presence of NR could repopulated NSG mice in primary and secondary transplant recipients. In vivo, NR dietary supplementation resulted in a significantly enlarged pool of progenitors, without concurrent HSC exhaustion, improved survival by 80%, and accelerated blood recovery after murine lethal irradiation and limiting-HSC transplantation. In human xenotransplanted immune-deficient NSG mice, NR increased the production of human leucocyte progeny from hCD34+ progenitors. Our work demonstrates for the first time a positive effect of NAD+ boosting strategies on the most primitive blood stem cells, establishes a novel link between mitochondrial stress, mitophagy and stem cell fate decision, and unveils the potential of NR in vivo supplementation to improve recovery of patients suffering from hematological failure including post-chemo/radiotherapy. No relevant conflicts of interest to declare.