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The ability of an organism to overcome infectious diseases has traditionally been linked to killing invading pathogens. Accumulating evidence, however, indicates that, apart from restricting pathogen loads, organismal survival is coupled to an additional yet poorly understood mechanism called disease tolerance. Here we report that p16High immune cells play a key role in establishing disease tolerance. We found that the FDA-approved BNT162b2 mRNA COVID-19 vaccine is a potent and rapid inducer of p16High immune subsets both in mice and humans. In turn, p16High immune cells were indispensable for counteracting different lethal conditions, including LPS-induced sepsis, acute SARS-CoV-2 infection and ionizing irradiation. Mechanistically, we propose that activation of TLR7 or a low physiological activity of STING is sufficient to induce p16High immune subset that, in turn, establishes a low adenosine environment and disease tolerance. Furthermore, containing these signals within a beneficial range by deleting MDA5 that appeared sufficient to maintain a low activity of STING, induces p16High immune cells and delays organ deterioration upon aging with improved healthspan. Our data highlight the beneficial role of p16High immune subsets in establishing a low adenosine environment and disease tolerance.
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Myelodysplastic/myeloproliferative diseases of childhood cause a relevant disease burden, and many of these diseases may have a fatal course. The use of next-generation sequencing (NGS) has led to the identification of novel genetic variants in patients with these diseases, advancing our understanding of the underlying pathophysiology. However, novel mutations can often only be interpreted as variants of unknown significance (VUS), hindering adequate diagnosis and the use of a targeted therapy. To improve variant interpretation and test targeted therapies in a preclinical setting, we are using a rapid zebrafish embryo model that allows functional evaluation of the novel variant and possible therapeutic approaches within days. Thereby, we accelerate the translation from genetic findings to treatment options. Here, we establish this workflow on a novel in-frame tandem duplication in NRAS (c.192_227dup; p.G75_E76insDS65_G75) identified by Sanger sequencing in a 2.5-year-old patient with an unclassifiable myelodysplastic/myeloproliferative neoplasm (MDS/MPN-U). We show that this variant results in a myeloproliferative phenotype in zebrafish embryos with expansion of immature myeloid cells in the caudal hematopoietic tissue, which can be reversed by MEK inhibition. Thus, we could reclassify the variant from likely pathogenic to pathogenic using the American College of Medical Genetics (ACMG) criteria.
Assuntos
GTP Fosfo-Hidrolases , Proteínas de Membrana , Doenças Mieloproliferativas-Mielodisplásicas , Peixe-Zebra , Animais , Pré-Escolar , Feminino , Humanos , Masculino , Duplicação Gênica , GTP Fosfo-Hidrolases/genética , Sequenciamento de Nucleotídeos em Larga Escala , Proteínas de Membrana/genética , Doenças Mieloproliferativas-Mielodisplásicas/genética , Sequências de Repetição em Tandem , Peixe-Zebra/genéticaRESUMO
Hematopoiesis, the process of generating blood cells, starts during development with the primitive, pro-definitive, and definitive hematopoietic waves. The first two waves will generate erythrocytes and myeloid cells, although the definitive wave will give rise to hematopoietic stem cells (HSCs) that are multipotent and can produce most of the blood cells in an adult. Although HSCs are highly proliferative during development, during adulthood they remain quiescent in the bone marrow. Inflammatory signaling in the form of interferons, interleukins, tumor necrosis factors, and others is well-established to influence both developmental and adult hematopoiesis. Here we discuss the role of specific inflammatory pathways that are induced by sensing nucleic acids. We discuss the role of RNA-sensing members of the Toll-like, Rig-I-like, nucleotide-binding oligomerization domain (NOD)-like, and AIM2-like protein kinase receptors and the DNA-sensing receptors, DEAD-Box helicase 41 (DDX41) and cGAS. The main downstream pathways of these receptors are discussed, as well as their influence on developmental and adult hematopoiesis, including hematopoietic pathologies.
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Ácidos Nucleicos , Humanos , Adulto , Ácidos Nucleicos/metabolismo , Células-Tronco Hematopoéticas/metabolismo , Hematopoese/fisiologia , Medula Óssea , Inflamação/metabolismoRESUMO
Transposable elements (TEs) are dispersed repetitive DNA sequences that can move within a genome. Even though hundreds of years of evolution have led to the accumulation of mutations that render most TEs unable to transpose, they still exert multiple important functions. They play a role in hematopoiesis, especially during periods of high cellular plasticity, such as development, regeneration and aging. This is because TEs can populate functional elements, such as enhancers. Furthermore, TE RNA can be sensed by innate immune sensors that play a role in inflammation and inflammaging. TEs also play an important role in different aspects of leukemia and lymphoma, leading to either beneficial or detrimental outcomes. Further studies into the function of TEs in healthy or diseased hematopoietic systems are necessary to manipulate them for therapeutic benefit.
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Elementos de DNA Transponíveis , Evolução Molecular , Elementos de DNA Transponíveis/genética , MutaçãoRESUMO
Successful elimination of bacteria in phagocytes occurs in the phago-lysosomal system, but also depends on mitochondrial pathways. Yet, how these two organelle systems communicate is largely unknown. Here we identify the lysosomal biogenesis factor transcription factor EB (TFEB) as regulator for phago-lysosome-mitochondria crosstalk in macrophages. By combining cellular imaging and metabolic profiling, we find that TFEB activation, in response to bacterial stimuli, promotes the transcription of aconitate decarboxylase (Acod1, Irg1) and synthesis of its product itaconate, a mitochondrial metabolite with antimicrobial activity. Activation of the TFEB-Irg1-itaconate signalling axis reduces the survival of the intravacuolar pathogen Salmonella enterica serovar Typhimurium. TFEB-driven itaconate is subsequently transferred via the Irg1-Rab32-BLOC3 system into the Salmonella-containing vacuole, thereby exposing the pathogen to elevated itaconate levels. By activating itaconate production, TFEB selectively restricts proliferating Salmonella, a bacterial subpopulation that normally escapes macrophage control, which contrasts TFEB's role in autophagy-mediated pathogen degradation. Together, our data define a TFEB-driven metabolic pathway between phago-lysosomes and mitochondria that restrains Salmonella Typhimurium burden in macrophages in vitro and in vivo.
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Lisossomos , Succinatos , Autofagia/fisiologia , Lisossomos/metabolismo , Macrófagos/metabolismo , Succinatos/metabolismo , Succinatos/farmacologiaRESUMO
Hematopoietic stem cells (HSCs) rely on complex regulatory networks to preserve stemness. Due to the scarcity of HSCs, technical challenges have limited our insights into the interplay between metabolites, transcription, and the epigenome. In this study, we generated low-input metabolomics, transcriptomics, chromatin accessibility, and chromatin immunoprecipitation data, revealing distinct metabolic hubs that are enriched in HSCs and their downstream multipotent progenitors. Mechanistically, we uncover a non-classical retinoic acid (RA) signaling axis that regulates HSC function. We show that HSCs rely on Cyp26b1, an enzyme conventionally considered to limit RA effects in the cell. In contrast to the traditional view, we demonstrate that Cyp26b1 is indispensable for production of the active metabolite 4-oxo-RA. Further, RA receptor beta (Rarb) is required for complete transmission of 4-oxo-RA-mediated signaling to maintain stem cells. Our findings emphasize that a single metabolite controls stem cell fate by instructing epigenetic and transcriptional attributes.
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Células-Tronco Hematopoéticas , Tretinoína , Diferenciação Celular , Ácido Retinoico 4 Hidroxilase/genética , Transdução de Sinais , Tretinoína/farmacologiaRESUMO
PURPOSE OF REVIEW: Hematopoietic stem cells (HSCs) are formed embryonically during a dynamic developmental process and later reside in adult hematopoietic organs in a quiescent state. In response to their changing environment, HSCs have evolved diverse mechanisms to cope with intrinsic and extrinsic challenges. This review intends to discuss how HSCs and other stem cells co-opted DNA and RNA innate immune pathways to fine-tune developmental processes. RECENT FINDINGS: Innate immune receptors for nucleic acids like the RIG-I-like family receptors and members of DNA sensing pathways are expressed in HSCs and other stem cells. Even though the "classic" role of these receptors is recognition of foreign DNA or RNA from pathogens, it was recently shown that cellular transposable element (TE) RNA or R-loops activate such receptors, serving as endogenous triggers of inflammatory signaling that can shape HSC formation during development and regeneration. SUMMARY: Endogenous TEs and R-loops activate RNA and DNA sensors, which trigger distinct inflammatory signals to fine-tune stem cell decisions. This phenomenon could have broad implications for diverse somatic stem cells, for a variety of diseases and during aging.
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Haematopoietic stem cells (HSCs) are normally quiescent, but have evolved mechanisms to respond to stress. Here, we evaluate haematopoietic regeneration induced by chemotherapy. We detect robust chromatin reorganization followed by increased transcription of transposable elements (TEs) during early recovery. TE transcripts bind to and activate the innate immune receptor melanoma differentiation-associated protein 5 (MDA5) that generates an inflammatory response that is necessary for HSCs to exit quiescence. HSCs that lack MDA5 exhibit an impaired inflammatory response after chemotherapy and retain their quiescence, with consequent better long-term repopulation capacity. We show that the overexpression of ERV and LINE superfamily TE copies in wild-type HSCs, but not in Mda5-/- HSCs, results in their cycling. By contrast, after knockdown of LINE1 family copies, HSCs retain their quiescence. Our results show that TE transcripts act as ligands that activate MDA5 during haematopoietic regeneration, thereby enabling HSCs to mount an inflammatory response necessary for their exit from quiescence.
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Proliferação de Células/efeitos dos fármacos , Senescência Celular/efeitos dos fármacos , Elementos de DNA Transponíveis , Hematopoese/efeitos dos fármacos , Células-Tronco Hematopoéticas/efeitos dos fármacos , Helicase IFIH1 Induzida por Interferon/metabolismo , Agonistas Mieloablativos/farmacologia , Animais , Montagem e Desmontagem da Cromatina/efeitos dos fármacos , Retrovirus Endógenos/genética , Ativação Enzimática , Células HEK293 , Células-Tronco Hematopoéticas/enzimologia , Humanos , Helicase IFIH1 Induzida por Interferon/genética , Ligantes , Elementos Nucleotídeos Longos e Dispersos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Transdução de SinaisRESUMO
All vertebrate blood cells descend from multipotent hematopoietic stem cells (HSCs), whose activity and differentiation depend on a complex and incompletely understood relationship with inflammatory signals. Although homeostatic levels of inflammatory signaling play an intricate role in HSC maintenance, activation, proliferation, and differentiation, acute or chronic exposure to inflammation can have deleterious effects on HSC function and self-renewal capacity, and bias their differentiation program. Increased levels of inflammatory signaling are observed during aging, affecting HSCs either directly or indirectly via the bone marrow niche and contributing to their loss of self-renewal capacity, diminished overall functionality, and myeloid differentiation skewing. These changes can have significant pathological consequences. Here, we provide an overview of the current literature on the complex interplay between HSCs and inflammatory signaling, and how this relationship contributes to age-related phenotypes. Understanding the mechanisms and outcomes of this interaction during different life stages will have significant implications in the modulation and restoration of the hematopoietic system in human disease, recovery from cancer and chemotherapeutic treatments, stem cell transplantation, and aging.
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Envelhecimento/metabolismo , Diferenciação Celular , Hematopoese , Células-Tronco Hematopoéticas/metabolismo , Transdução de Sinais , Animais , Transplante de Células-Tronco Hematopoéticas , Humanos , Inflamação/metabolismo , Inflamação/terapia , Neoplasias/metabolismo , Neoplasias/terapiaRESUMO
Zebrafish has been established as a classical model for developmental studies, yet in the past years, with the explosion of novel technological methods, the use of zebrafish as a model has expanded. One of the prominent fields that took advantage of zebrafish as a model organism early on is hematopoiesis, the process of blood cell generation from hematopoietic stem and progenitor cells (HSPCs). In zebrafish, HSPCs are born early during development in the aorta-gonad-mesonephros region and then translocate to the caudal hematopoietic tissue, where they expand and finally take residence in the kidney marrow. This journey is tightly regulated at multiple levels from extracellular signals to chromatin. In order to delineate the mechanistic underpinnings of this process, next-generation sequencing techniques could be an important ally. Here, we describe genome-wide approaches that have been undertaken to delineate zebrafish hematopoiesis.
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Mitochondria constantly adapt to the metabolic needs of a cell. This mitochondrial plasticity is critical to T cells, which modulate metabolism depending on antigen-driven signals and environment. We show here that de novo synthesis of the mitochondrial membrane-specific lipid cardiolipin maintains CD8+ T cell function. T cells deficient for the cardiolipin-synthesizing enzyme PTPMT1 had reduced cardiolipin and responded poorly to antigen because basal cardiolipin levels were required for activation. However, neither de novo cardiolipin synthesis, nor its Tafazzin-dependent remodeling, was needed for T cell activation. In contrast, PTPMT1-dependent cardiolipin synthesis was vital when mitochondrial fitness was required, most notably during memory T cell differentiation or nutrient stress. We also found CD8+ T cell defects in a small cohort of patients with Barth syndrome, where TAFAZZIN is mutated, and in a Tafazzin-deficient mouse model. Thus, the dynamic regulation of a single mitochondrial lipid is crucial for CD8+ T cell immunity.
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Aciltransferases/imunologia , Síndrome de Barth/imunologia , Linfócitos T CD8-Positivos/imunologia , Cardiolipinas/imunologia , Mitocôndrias/imunologia , PTEN Fosfo-Hidrolase/imunologia , Animais , Síndrome de Barth/patologia , Linfócitos T CD8-Positivos/citologia , Células Cultivadas , Feminino , Humanos , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos KnockoutRESUMO
Inflammatory signaling is required for hematopoietic stem and progenitor cell (HSPC) development. Here, we studied the involvement of RIG-I-like receptors (RLRs) in HSPC formation. Rig-I or Mda5 deficiency impaired, while Lgp2 deficiency enhanced, HSPC emergence in zebrafish embryos. Rig-I or Mda5 deficiency reduced HSPC numbers by inhibiting inflammatory signals that were in turn enhanced in Lgp2 deficient embryos. Simultaneous reduction of Lgp2 and either Rig-I or Mda5 rescued inflammatory signals and HSPC numbers. Modulating the expression of the signaling mediator Traf6 in RLR deficient embryos restored HSPC numbers. Repetitive element transcripts could be detected in hemogenic endothelial cells and HSPCs, suggesting a role as RLR ligands. Indeed, ectopic expression of repetitive elements enhanced HSPC formation in wild-type, but not in Rig-I or Mda5 deficient embryos. Manipulation of RLR expression in mouse fetal liver HSPCs indicated functional conservation among species. Thus, repetitive elements transcribed during development drive RLR-mediated inflammatory signals that regulate HSPC formation.
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Regulação da Expressão Gênica no Desenvolvimento , Células-Tronco Hematopoéticas/metabolismo , Sequências Repetitivas de Ácido Nucleico , Transdução de Sinais , Proteínas de Peixe-Zebra/genética , Proteínas de Peixe-Zebra/metabolismo , Animais , Biomarcadores , Montagem e Desmontagem da Cromatina , Elementos de DNA Transponíveis , Suscetibilidade a Doenças , Hematopoese/genética , Células-Tronco Hematopoéticas/citologia , Imunidade Inata , Imuno-Histoquímica , Inflamação/etiologia , Inflamação/metabolismo , Inflamação/patologia , RNA Helicases/deficiência , RNA Helicases/genética , Proteínas de Ligação a RNA/metabolismo , Fator 6 Associado a Receptor de TNF/metabolismo , Ácido Valproico/farmacologia , Peixe-ZebraRESUMO
Hematopoietic stem and progenitor cell (HSPC) formation and lineage differentiation involve gene expression programs orchestrated by transcription factors and epigenetic regulators. Genetic disruption of the chromatin remodeler chromodomain-helicase-DNA-binding protein 7 (CHD7) expanded phenotypic HSPCs, erythroid, and myeloid lineages in zebrafish and mouse embryos. CHD7 acts to suppress hematopoietic differentiation. Binding motifs for RUNX and other hematopoietic transcription factors are enriched at sites occupied by CHD7, and decreased RUNX1 occupancy correlated with loss of CHD7 localization. CHD7 physically interacts with RUNX1 and suppresses RUNX1-induced expansion of HSPCs during development through modulation of RUNX1 activity. Consequently, the RUNX1:CHD7 axis provides proper timing and function of HSPCs as they emerge during hematopoietic development or mature in adults, representing a distinct and evolutionarily conserved control mechanism to ensure accurate hematopoietic lineage differentiation.
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Subunidade alfa 2 de Fator de Ligação ao Core , Proteínas de Ligação a DNA , Hematopoese , Animais , Diferenciação Celular , Linhagem Celular , Subunidade alfa 2 de Fator de Ligação ao Core/química , Subunidade alfa 2 de Fator de Ligação ao Core/genética , Subunidade alfa 2 de Fator de Ligação ao Core/metabolismo , Proteínas de Ligação a DNA/química , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/metabolismo , Feminino , Células-Tronco Hematopoéticas , Humanos , Masculino , Camundongos , Baço/citologia , Peixe-ZebraRESUMO
Deficiency of the transcription factor GATA2 is a highly penetrant genetic disorder predisposing to myelodysplastic syndromes (MDS) and immunodeficiency. It has been recognized as the most common cause underlying primary MDS in children. Triggered by the discovery of a recurrent synonymous GATA2 variant, we systematically investigated 911 patients with phenotype of pediatric MDS or cellular deficiencies for the presence of synonymous alterations in GATA2. In total, we identified nine individuals with five heterozygous synonymous mutations: c.351C>G, p.T117T (N = 4); c.649C>T, p.L217L; c.981G>A, p.G327G; c.1023C>T, p.A341A; and c.1416G>A, p.P472P (N = 2). They accounted for 8.2% (9/110) of cases with GATA2 deficiency in our cohort and resulted in selective loss of mutant RNA. While for the hotspot mutation (c.351C>G) a splicing error leading to RNA and protein reduction was identified, severe, likely late stage RNA loss without splicing disruption was found for other mutations. Finally, the synonymous mutations did not alter protein function or stability. In summary, synonymous GATA2 substitutions are a new common cause of GATA2 deficiency. These findings have broad implications for genetic counseling and pathogenic variant discovery in Mendelian disorders.
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Deficiência de GATA2/genética , Fator de Transcrição GATA2/deficiência , Fator de Transcrição GATA2/genética , RNA/genética , Mutação Silenciosa/genética , Adolescente , Adulto , Criança , Pré-Escolar , Feminino , Estudos de Associação Genética , Predisposição Genética para Doença/genética , Mutação em Linhagem Germinativa/genética , Heterozigoto , Humanos , Síndromes de Imunodeficiência/genética , Masculino , Síndromes Mielodisplásicas/genética , Fenótipo , Adulto JovemRESUMO
PURPOSE OF REVIEW: Since the discovery of master transcription factors that regulate hematopoietic regeneration following different stressors, many more layers of regulation have been discovered. The purpose of this review is to outline the recent discoveries of epigenetic and epitranscriptomic control of hematopoietic regeneration and highlight the novel involvement of transposable elements in this process. RECENT FINDINGS: Over the past 2 years, we have gained additional knowledge in the role of epigenetic regulators in hematopoietic regeneration. Histone modifiers, like SETD1A, JARID2, KDM6B, and classic DNA methylation regulators, like DNMT3A and TET2, govern hematopoietic regeneration. Concomitantly, the significance of RNA modifications and the expanding functions of transposable elements establish novel layers of regulation of hematopoietic regeneration. Capitalizing on this newly acquired knowledge may provide insights on new therapies or drug targets that will improve or accelerate hematopoietic regeneration. SUMMARY: The spectrum of epigenetic and epitranscriptomic modifications that affect hematopoietic regeneration is continually expanding. Transposable elements are also emerging as potent responders of stress stimuli that affect the self-renewal capacity of hematopoietic stem cells. The future challenge is to understand the hierarchy of these control mechanisms and how they integrate and consolidate information from transcription factors and external stimuli.
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Elementos de DNA Transponíveis , Epigênese Genética , Hematopoese , Células-Tronco Hematopoéticas/metabolismo , Regeneração , DNA (Citosina-5-)-Metiltransferases/genética , DNA (Citosina-5-)-Metiltransferases/metabolismo , DNA Metiltransferase 3A , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/metabolismo , Dioxigenases , Histona-Lisina N-Metiltransferase/genética , Histona-Lisina N-Metiltransferase/metabolismo , Humanos , Proteínas Proto-Oncogênicas/genética , Proteínas Proto-Oncogênicas/metabolismoRESUMO
Hematopoiesis is an optimal system for studying stem cell maintenance and lineage differentiation under physiological and pathological conditions. In vertebrate organisms, billions of differentiated hematopoietic cells need to be continuously produced to replenish the blood cell pool. Disruptions in this process have immediate consequences for oxygen transport, responses against pathogens, maintenance of hemostasis and vascular integrity. Zebrafish is a widely used and well-established model for studying the hematopoietic system. Several new hematopoietic regulators were identified in genetic and chemical screens using the zebrafish model. Moreover, zebrafish enables in vivo imaging of hematopoietic stem cell generation and differentiation during embryogenesis, and adulthood. Finally, zebrafish has been used to model hematopoietic diseases. Recent technological advances in single-cell transcriptome analysis, epigenetic regulation, proteomics, metabolomics, and processing of large data sets promise to transform the current understanding of normal, abnormal, and malignant hematopoiesis. In this perspective, we discuss how the zebrafish model has proven beneficial for studying physiological and pathological hematopoiesis and how these novel technologies are transforming the field.
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The H2.0-like homeobox transcription factor (HLX) regulates hematopoietic differentiation and is overexpressed in Acute Myeloid Leukemia (AML), but the mechanisms underlying these functions remain unclear. We demonstrate here that HLX overexpression leads to a myeloid differentiation block both in zebrafish and human hematopoietic stem and progenitor cells (HSPCs). We show that HLX overexpression leads to downregulation of genes encoding electron transport chain (ETC) components and upregulation of PPARδ gene expression in zebrafish and human HSPCs. HLX overexpression also results in AMPK activation. Pharmacological modulation of PPARδ signaling relieves the HLX-induced myeloid differentiation block and rescues HSPC loss upon HLX knockdown but it has no effect on AML cell lines. In contrast, AMPK inhibition results in reduced viability of AML cell lines, but minimally affects myeloid progenitors. This newly described role of HLX in regulating the metabolic state of hematopoietic cells may have important therapeutic implications.
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Regulação da Expressão Gênica , Células-Tronco Hematopoéticas/metabolismo , Proteínas de Homeodomínio/fisiologia , Leucemia Mieloide Aguda/metabolismo , Fatores de Transcrição/fisiologia , Proteínas de Peixe-Zebra/fisiologia , Animais , Autofagia , Diferenciação Celular , Proliferação de Células , Sobrevivência Celular , Regulação Leucêmica da Expressão Gênica , Hematopoese , Proteínas de Homeodomínio/genética , Humanos , Células K562 , Leucemia Mieloide Aguda/genética , Potencial da Membrana Mitocondrial , PPAR gama/metabolismo , Fenótipo , Espécies Reativas de Oxigênio/metabolismo , Transdução de Sinais , Células-Tronco/metabolismo , Fatores de Transcrição/genética , Peixe-Zebra , Proteínas de Peixe-Zebra/genéticaRESUMO
Haematopoietic stem and progenitor cells (HSPCs) require a specific microenvironment, the haematopoietic niche, which regulates HSPC behaviour1,2. The location of this niche varies across species, but the evolutionary pressures that drive HSPCs to different microenvironments remain unknown. The niche is located in the bone marrow in adult mammals, whereas it is found in other locations in non-mammalian vertebrates, for example, in the kidney marrow in teleost fish. Here we show that a melanocyte umbrella above the kidney marrow protects HSPCs against ultraviolet light in zebrafish. Because mutants that lack melanocytes have normal steady-state haematopoiesis under standard laboratory conditions, we hypothesized that melanocytes above the stem cell niche protect HSPCs against ultraviolet-light-induced DNA damage. Indeed, after ultraviolet-light irradiation, unpigmented larvae show higher levels of DNA damage in HSPCs, as indicated by staining of cyclobutane pyrimidine dimers and have reduced numbers of HSPCs, as shown by cmyb (also known as myb) expression. The umbrella of melanocytes associated with the haematopoietic niche is highly evolutionarily conserved in aquatic animals, including the sea lamprey, a basal vertebrate. During the transition from an aquatic to a terrestrial environment, HSPCs relocated into the bone marrow, which is protected from ultraviolet light by the cortical bone around the marrow. Our studies reveal that melanocytes above the haematopoietic niche protect HSPCs from ultraviolet-light-induced DNA damage in aquatic vertebrates and suggest that during the transition to terrestrial life, ultraviolet light was an evolutionary pressure affecting the location of the haematopoietic niche.