YAP/TAZ Promote Hematopoietic Regeneration Via Accelerating the Recovery of Bone Marrow Niche

Adult hematopoietic stem cells (HSCs) reside and are protected in a unique bone marrow (BM) microenvironment, termed the HSC niche, which consists mainly of vascular endothelial cells (EC) and EC-associated mesenchymal stromal cells (MSC). Myeloablative stresses, such as ionizing radiation (IR) and chemotherapy, induce not only depletion of hematopoietic cells but also disruption of HSC niche components, as exemplified by dilation and leakiness of BM vasculature and depletion and dysfunction of BM MSCs. These structural and functional changes in the HSC niche restrain efficient hematopoietic recovery, which often compromises the efficacy of HSC transplantation (HSCT) and chemotherapy. YAP/TAZ are the two transcriptional coactivators normally repressed by LATS kinases downstream of the Hippo pathway. Although cumulative evidence has established a critical role of YAP/TAZ activation in tissue regeneration of various solid organs, their role in BM regeneration remains poorly understood. Our quantitive RT-PCR revealed that YAP/TAZ are abundantly expressed in steady-state mouse ECs (CD45 -Ter119 -CD31 +Sca1 +CD105 hi) and MSCs (CD45 -Ter119 -CD31 -PDGFRα +CD51 +LepR +) but scarcely in hematopoietic cells including HSCs (Lin -cKit +Sca1 +Flk2 -CD150 +CD48 -CD34 lo), which was confirmed by reanalysis of the published single cell RNA-seq datasets (GSE128423). Immunofluorescent imaging of BM sections revealed that YAP/TAZ are distributed mainly in the cytoplasm of ECs but evenly in the cytoplasm and nuclei of MSCs, indicating their differential basal activity in these two HSC niche components. Kinetic transcriptome analysis revealed that YAP/TAZ activity is transiently activated in ECs at 24 hours and returns to a basal repressive state by day 3 after sublethal IR. This transient activation of endothelial YAP/TAZ was critical for vascular integrity, as conditional deletion of YAP/TAZ in ECs (Cdh5-Cre ERT2Yap1 f/fTaz f/f) caused 100% lethality of mice within 10 days following sublethal IR. In sharp contrast, the kinetic expression analysis of a YAP/TAZ target gene CTGF indicated their transient inhibition in MSCs after sublethal IR, and the conditional YAP/TAZ deletion in BM MSCs (Ebf3-Cre ERT2Yap1 f/fTaz f/f) led to their reduced colony forming ability when assessed by colony forming unit fibroblast (CFU-F) assay. Recently, we discovered a novel and potent LATS inhibitor GA-003 that selectively induces mouse and human YAP/TAZ activation in vitro (IC 50 against LATS1 = 1.06 ± 0.08 nM). To analyze the effect of pharmacological YAP/TAZ activation on BM regeneration in vivo, we treated mice with intraperitoneal injection of GA-003 (50 mg/kg per day, for 8 days) following sublethal IR. Remarkably, we observed an accelerated recovery of hematopoiesis, with the absolute numbers of BM cellularity, GMP (Lin -cKit +Sca1 -FcγR +CD34 +) and HSC EPCR (Lin -cKit +Sca1 +CD150 +EPCR +) on day 14 increased by 3.50-fold (p=0.0002), 6.49-fold (p=0.0022) and 11.41-fold (p=0.022), respectively in the GA-003-treated group compared to vehicle-treated group. In addition, GA-003 also promoted hematopoietic recovery after 5-FU injection (150 mg/kg) and HSCT. Nonetheless, consistent with the scarce expression of YAP/TAZ in hematopoietic stem and progenitor cells (HSPC), in vitro GA-003 treatment did not enhance HSPC growth, suggesting niche-mediated effects by GA-003. Indeed, in vitro tube formation assay indicated accelerated angiogenesis by GA-003-treated human umbilical vein ECs, and CFU-F assays revealed significant enhancement of colony formation by mouse BM-derived MSCs upon GA-003 treatment. To reveal the effect of GA-003 on the HSC niche components in vivo, we performed whole BM immunofluorescent imaging at various time points following sublethal IR and GA-003 treatment. We observed alleviated vascular dilation and leakiness and earlier restoration of vascular damage in GA-003-treated group compared to vehicle-treated group, which was associated with increased VE-Cadherin expression in ECs. These results suggest that reinforcing YAP/TAZ activity upon myelosuppression promotes HSC niche integrity and recovery and accelerates hematopoietic regeneration. Taken together, our results establish YAP/TAZ as novel regulators of HSC niche and highlight YAP/TAZ as promising therapeutic targets to boost hematopoietic recovery after myeloablative interventions such as chemotherapy and HSCT. Disclosures Aihara: Nissan Chemical Corporation: Current Employment. Iwawaki: Nissan Chemical Corporation: Current Employment. Nishino: Nissan Chemical Corporation: Current Employment. Iwama: Nissan Chemical Corporation: Research Funding.

An Intrinsic Oscillation of Gene Networks Inside Hair Follicle Stem Cells: An Additional Layer That Can Modulate Hair Stem Cell Activities

This article explores and summarizes recent progress in and the characterization of main players in the regulation and cyclic regeneration of hair follicles. The review discusses current views and discoveries on the molecular mechanisms that allow hair follicle stem cells (hfSCs) to synergistically integrate homeostasis during quiescence and activation. Discussion elaborates on a model that shows how different populations of skin stem cells coalesce intrinsic and extrinsic mechanisms, resulting in the maintenance of stemness and hair regenerative potential during an organism’s lifespan. Primarily, we focus on the question of how the intrinsic oscillation of gene networks in hfSCs sense and respond to the surrounding niche environment. The review also investigates the existence of a cell-autonomous mechanism and the reciprocal interactions between molecular signaling axes in hfSCs and niche components, which demonstrates its critical driving force in either the activation of whole mini-organ regeneration or quiescent homeostasis maintenance. These exciting novel discoveries in skin stem cells and the surrounding niche components propose a model of the intrinsic stem cell oscillator which is potentially instructive for translational regenerative medicine. Further studies, deciphering of the distribution of molecular signals coupled with the nature of their oscillation within the stem cells and niche environments, may impact the speed and efficiency of various approaches that could stimulate the development of self-renewal and cell-based therapies for hair follicle stem cell regeneration.

Stem cell–driven lymphatic remodeling coordinates tissue regeneration

Tissues rely on stem cells (SCs) for homeostasis and wound repair. SCs reside in specialized microenvironments (niches) whose complexities and roles in orchestrating tissue growth are still unfolding. Here, we identify lymphatic capillaries as critical SC-niche components. In skin, lymphatics form intimate networks around hair follicle (HF) SCs. When HFs regenerate, lymphatic–SC connections become dynamic. Using a mouse model, we unravel a secretome switch in SCs that controls lymphatic behavior. Resting SCs express angiopoietin-like protein 7 (Angptl7), promoting lymphatic drainage. Activated SCs switch to Angptl4, triggering transient lymphatic dissociation and reduced drainage. When lymphatics are perturbed or the secretome switch is disrupted, HFs cycle precociously and tissue regeneration becomes asynchronous. In unearthing lymphatic capillaries as a critical SC-niche element, we have learned how SCs coordinate their activity across a tissue.