Megakaryocytic IGF1 coordinates activation and ferroptosis to safeguard hematopoietic stem cell regeneration after radiation injury.

BACKGROUND: Hematopoietic stem cell (HSC) regeneration underlies hematopoietic recovery from myelosuppression, which is a life-threatening side effect of cytotoxicity. HSC niche is profoundly disrupted after myelosuppressive injury, while if and how the niche is reshaped and regulates HSC regeneration are poorly understood. METHODS: A mouse model of radiation injury-induced myelosuppression was built by exposing mice to a sublethal dose of ionizing radiation. The dynamic changes in the number, distribution and functionality of HSCs and megakaryocytes were determined by flow cytometry, immunofluorescence, colony assay and bone marrow transplantation, in combination with transcriptomic analysis. The communication between HSCs and megakaryocytes was determined using a coculture system and adoptive transfer. The signaling mechanism was investigated both in vivo and in vitro, and was consolidated using megakaryocyte-specific knockout mice and transgenic mice. RESULTS: Megakaryocytes become a predominant component of HSC niche and localize closer to HSCs after radiation injury. Meanwhile, transient insulin-like growth factor 1 (IGF1) hypersecretion is predominantly provoked in megakaryocytes after radiation injury, whereas HSCs regenerate paralleling megakaryocytic IGF1 hypersecretion. Mechanistically, HSCs are particularly susceptible to megakaryocytic IGF1 hypersecretion, and mTOR downstream of IGF1 signaling not only promotes activation including proliferation and mitochondrial oxidative metabolism of HSCs, but also inhibits ferritinophagy to restrict HSC ferroptosis. Consequently, the delicate coordination between proliferation, mitochondrial oxidative metabolism and ferroptosis ensures functional HSC expansion after radiation injury. Importantly, punctual IGF1 administration simultaneously promotes HSC regeneration and hematopoietic recovery after radiation injury, representing a superior therapeutic approach for myelosuppression. CONCLUSIONS: Our study identifies megakaryocytes as a last line of defense against myelosuppressive injury and megakaryocytic IGF1 as a novel niche signal safeguarding HSC regeneration.

Apoptosis-resistant megakaryocytes produce large and hyperreactive platelets in response to radiation injury.

BACKGROUND: The essential roles of platelets in thrombosis have been well recognized. Unexpectedly, thrombosis is prevalent during thrombocytopenia induced by cytotoxicity of biological, physical and chemical origins, which could be suffered by military personnel and civilians during chemical, biological, radioactive, and nuclear events. Especially, thrombosis is considered a major cause of mortality from radiation injury-induced thrombocytopenia, while the underlying pathogenic mechanism remains elusive. METHODS: A mouse model of radiation injury-induced thrombocytopenia was built by exposing mice to a sublethal dose of ionizing radiation (IR). The phenotypic and functional changes of platelets and megakaryocytes (MKs) were determined by a comprehensive set of in vitro and in vivo assays, including flow cytometry, flow chamber, histopathology, Western blotting, and chromatin immunoprecipitation, in combination with transcriptomic analysis. The molecular mechanism was investigated both in vitro and in vivo, and was consolidated using MK-specific knockout mice. The translational potential was evaluated using a human MK cell line and several pharmacological inhibitors. RESULTS: In contrast to primitive MKs, mature MKs (mMKs) are intrinsically programmed to be apoptosis-resistant through reprogramming the Bcl-xL-BAX/BAK axis. Interestingly, mMKs undergo minority mitochondrial outer membrane permeabilization (MOMP) post IR, resulting in the activation of the cyclic GMP-AMP synthase-stimulator of IFN genes (cGAS-STING) pathway via the release of mitochondrial DNA. The subsequent interferon-ß (IFN-ß) response in mMKs upregulates a GTPase guanylate-binding protein 2 (GBP2) to produce large and hyperreactive platelets that favor thrombosis. Further, we unmask that autophagy restrains minority MOMP in mMKs post IR. CONCLUSIONS: Our study identifies that megakaryocytic mitochondria-cGAS/STING-IFN-ß-GBP2 axis serves as a fundamental checkpoint that instructs the size and function of platelets upon radiation injury and can be harnessed to treat platelet pathologies.

Imbalanced lipid homeostasis caused by membrane αKlotho deficiency contributes to the acute kidney injury to chronic kidney disease transition.

After acute kidney injury (AKI), renal tubular epithelial cells (RTECs) are pathologically characterized by intracellular lipid droplet (LD) accumulation, which are involved in RTEC injury and kidney fibrosis. However, its pathogenesis remains incompletely understood. The protein, αKlotho, primarily expressed in RTECs, is well known as an anti-aging hormone wielding versatile functions, and its membrane form predominantly acts as a co-receptor for fibroblast growth factor 23. Here, we discovered a connection between membrane αKlotho and intracellular LDs in RTECs. Fluorescent fatty acid (FA) pulse-chase assays showed that membrane αKlotho deficiency in RTECs, as seen in αKlotho homozygous mutated (kl/kl) mice or in mice with ischemia-reperfusion injury (IRI)-induced AKI, inhibited FA mobilization from LDs by impairing adipose triglyceride lipase (ATGL)-mediated lipolysis and lipophagy. This resulted in LD accumulation and FA underutilization. IRI-induced alterations were more striking in αKlotho deficiency. Mechanistically, membrane αKlotho deficiency promoted E3 ligase peroxin2 binding to ubiquitin-conjugating enzyme E2 D2, resulting in ubiquitin-mediated degradation of ATGL which is a common molecular basis for lipolysis and lipophagy. Overexpression of αKlotho rescued FA mobilization by preventing ATGL ubiquitination, thereby lessening LD accumulation and fibrosis after AKI. This suggests that membrane αKlotho is indispensable for the maintenance of lipid homeostasis in RTECs. Thus, our study identified αKlotho as a critical regulator of lipid turnover and homeostasis in AKI, providing a viable strategy for preventing tubular injury and the AKI-to-chronic kidney disease transition.

Aged hematopoietic stem cells entrap regulatory T cells to create a prosurvival microenvironment.

Although DNA mutation drives stem cell aging, how mutation-accumulated stem cells obtain clonal advantage during aging remains poorly understood. Here, using a mouse model of irradiation-induced premature aging and middle-aged mice, we show that DNA mutation accumulation in hematopoietic stem cells (HSCs) during aging upregulates their surface expression of major histocompatibility complex class II (MHCII). MHCII upregulation increases the chance for recognition by bone marrow (BM)-resident regulatory T cells (Tregs), resulting in their clonal expansion and accumulation in the HSC niche. On the basis of the establishment of connexin 43 (Cx43)-mediated gap junctions, BM Tregs transfer cyclic adenosine monophosphate (cAMP) to aged HSCs to diminish apoptotic priming and promote their survival via activation of protein kinase A (PKA) signaling. Importantly, targeting the HSC-Treg interaction or depleting Tregs effectively prevents the premature/physiological aging of HSCs. These findings show that aged HSCs use an active self-protective mechanism by entrapping local Tregs to construct a prosurvival niche and obtain a clonal advantage.

Cholesterol confers ferroptosis resistance onto myeloid-biased hematopoietic stem cells and prevents irradiation-induced myelosuppression.

There is growing appreciation that hematopoietic alterations underpin the ubiquitous detrimental effects of metabolic disorders. The susceptibility of bone marrow (BM) hematopoiesis to perturbations of cholesterol metabolism is well documented, while the underlying cellular and molecular mechanisms remain poorly understood. Here we reveal a distinct and heterogeneous cholesterol metabolic signature within BM hematopoietic stem cells (HSCs). We further show that cholesterol directly regulates maintenance and lineage differentiation of long-term HSCs (LT-HSCs), with high levels of intracellular cholesterol favoring maintenance and myeloid bias of LT-HSCs. During irradiation-induced myelosuppression, cholesterol also safeguards LT-HSC maintenance and myeloid regeneration. Mechanistically, we unravel that cholesterol directly and distinctively enhances ferroptosis resistance and boosts myeloid but dampens lymphoid lineage differentiation of LT-HSCs. Molecularly, we identify that SLC38A9-mTOR axis mediates cholesterol sensing and signal transduction to instruct lineage differentiation of LT-HSCs as well as to dictate ferroptosis sensitivity of LT-HSCs through orchestrating SLC7A11/GPX4 expression and ferritinophagy. Consequently, myeloid-biased HSCs are endowed with a survival advantage under both hypercholesterolemia and irradiation conditions. Importantly, a mTOR inhibitor rapamycin and a ferroptosis inducer imidazole ketone erastin prevent excess cholesterol-induced HSC expansion and myeloid bias. These findings unveil an unrecognized fundamental role of cholesterol metabolism in HSC survival and fate decisions with valuable clinical implications.

Renal Klotho safeguards platelet lifespan in advanced chronic kidney disease through restraining Bcl-xL ubiquitination and degradation.

BACKGROUND: Thrombosis and hemorrhage as two opposite pathologies are prevalent within the chronic kidney disease (CKD) population. Platelet homeostasis, which positions centrally in their pathogenesis, varies among the CKD population, while the underlying mechanism is poorly understood. OBJECTIVE: To investigate the change character and mechanism of platelet homeostasis in CKD and its association with renal Klotho deficiency. METHODS: The change character of platelet homeostasis and its association with renal Klotho deficiency were determined based on a cohort study as well as CKD mice and Klotho-deficient mice with CKD. The effects on thrombopoiesis and platelet lifespan were examined by flow cytometry and platelet transfer. The underlying mechanism was explored by proteomics, flow cytometry, western blot, and immunoprecipitation. RESULTS: We show that platelet count declines both in patient and mouse models with advanced CKD (Adv-CKD) and is positively associated with circulating Klotho levels. Mechanistically, we identify that ubiquitin ligase UBE2O governs Bcl-xL ubiquitination and degradation in platelets, whereas Adv-CKD-induced oxidative stress in platelets stimulates p38MAPK to promote Bcl-xL phosphorylation, which facilitates UBE2O binding to Bcl-xL and subsequent Bcl-xL degradation. Consequently, platelet lifespan is shortened in Adv-CKD, culminating in platelet count decline. However, kidney-secreted soluble Klotho protein restricts oxidative stress in platelets, thereby preserving Bcl-xL expression and platelet lifespan. CONCLUSIONS: Our findings uncover the mechanism of platelet count decline in Adv-CKD and identify renal Klotho as a long-range regulator of platelet lifespan, which not only provide a molecular mechanism underlying CKD-associated thrombocytopenia and hemorrhage but also offer a promising therapy choice.

Phosphate Metabolic Inhibition Contributes to Irradiation-Induced Myelosuppression through Dampening Hematopoietic Stem Cell Survival.

Myelosuppression is a common and intractable side effect of cancer therapies including radiotherapy and chemotherapy, while the underlying mechanism remains incompletely understood. Here, using a mouse model of radiotherapy-induced myelosuppression, we show that inorganic phosphate (Pi) metabolism is acutely inhibited in hematopoietic stem cells (HSCs) during irradiation-induced myelosuppression, and closely correlated with the severity and prognosis of myelosuppression. Mechanistically, the acute Pi metabolic inhibition in HSCs results from extrinsic Pi loss in the bone marrow niche and the intrinsic transcriptional suppression of soluble carrier family 20 member 1 (SLC20A1)-mediated Pi uptake by p53. Meanwhile, Pi metabolic inhibition blunts irradiation-induced Akt hyperactivation in HSCs, thereby weakening its ability to counteract p53-mediated Pi metabolic inhibition and the apoptosis of HSCs and consequently contributing to myelosuppression progression. Conversely, the modulation of the Pi metabolism in HSCs via a high Pi diet or renal Klotho deficiency protects against irradiation-induced myelosuppression. These findings reveal that Pi metabolism and HSC survival are causally linked by the Akt/p53-SLC20A1 axis during myelosuppression and provide valuable insights into the pathogenesis and management of myelosuppression.

Renal Klotho and inorganic phosphate are extrinsic factors that antagonistically regulate hematopoietic stem cell maintenance.

The composition and origin of extrinsic cues required for hematopoietic stem cell (HSC) maintenance are incompletely understood. Here we identify renal Klotho and inorganic phosphate (Pi) as extrinsic factors that antagonistically regulate HSC maintenance in the bone marrow (BM). Disruption of the Klotho-Pi axis by renal Klotho deficiency or Pi excess causes Pi overload in the BM niche and Pi retention in HSCs, leading to alteration of HSC maintenance. Mechanistically, Pi retention is mediated by soluble carrier family 20 member 1 (SLC20A1) and sensed by diphosphoinositol pentakisphosphate kinase 2 (PPIP5K2) to enhance Akt activation, which then upregulates SLC20A1 to aggravate Pi retention and augments GATA2 activity to drive the expansion and megakaryocyte/myeloid-biased differentiation of HSCs. However, kidney-secreted soluble Klotho directly maintains HSC pool size and differentiation by restraining SLC20A1-mediated Pi absorption of HSCs. These findings uncover a regulatory role of the Klotho-Pi axis orchestrated by the kidneys in BM HSC maintenance.

Caffeic acid attenuates irradiation-induced hematopoietic stem cell apoptosis through inhibiting mitochondrial damage.

Hematopoietic stem cells (HSCs) are sensitive to ionizing radiation (IR) damage, and its injury is the primary cause of bone marrow (BM) hematopoietic failure and even death after exposure to a certain dose of IR. However, the underlying mechanisms remain incompletely understood. Here we show that mitochondrial oxidative damage, which is characterized by mitochondrial reactive oxygen species overproduction, mitochondrial membrane potential reduction and mitochondrial permeability transition pore opening, is rapidly induced in both human and mouse HSCs and directly accelerates HSC apoptosis after IR exposure. Mechanistically, 5-lipoxygenase (5-LOX) is induced by IR exposure and contributes to IR-induced mitochondrial oxidative damage through inducing lipid peroxidation. Intriguingly, a natural antioxidant, caffeic acid (CA), can attenuate IR-induced HSC apoptosis through suppressing 5-LOX-mediated mitochondrial oxidative damage, thus protecting against BM hematopoietic failure after IR exposure. These findings uncover a critical role for mitochondria in IR-induced HSC injury and highlight the therapeutic potential of CA in BM hematopoietic failure induced by IR.

The cGAS-STING Pathway in Hematopoiesis and Its Physiopathological Significance.

Cytosolic DNA sensing is a fundamental mechanism by which organisms handle various stresses, including infection and genotoxicity. The hematopoietic system is sensitive to stresses, and hematopoietic changes are often rapid and the first response to stresses. Based on the transcriptome database, cytosolic DNA sensing pathways are widely expressed in the hematopoietic system, and components of these pathways may be expressed at even higher levels in hematopoietic stem and progenitor cells (HSPCs) than in their certain progeny immune cells. Recent studies have described a previously unrecognized role for cytosolic DNA sensing pathways in the regulation of hematopoiesis under both homeostatic and stress conditions. In particular, the recently discovered cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway is a critical modulator of hematopoiesis. Perturbation of the cGAS-STING pathway in HSPCs may be involved in the pathogenesis of hematopoietic disorders, autoimmune diseases, and inflammation-related diseases and may be candidate therapeutic targets. In this review, we focus on the recent findings of the cGAS-STING pathway in the regulation of hematopoiesis, and its physiopathological significance including its implications in diseases and therapeutic potential.