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Hematopoietic stem cells (HSCs) that produce a variety of hematopoietic lineage cells throughout the life reside in specialized microenvironment called “niche” in the bone marrow (BM) where they are tightly regulated. With the recent advances in experimental technologies enabling the selective deletion of molecules, various types of cells in the BM have been proposed to contribute to HSC niche activity. Among these are stromal cells closely associated with the vasculature. In this review, we provide an overview of recent advances in HSC niche research, and focus on the studies describing the functional roles of perivascular cells for HSC maintenance and mobilization. Not only for physiologic state, we also discuss the recent evidences suggesting the importance of microenvironment for emergence of malignant hematopoietic diseases.

Although the mammalian intestinal epithelium manifests robust regenerative capacity after various cytotoxic injuries, the underlying mechanism has remained unclear. Here we identify the cyclin-dependent kinase inhibitor p57 as a specific marker for a quiescent cell population located around the +4 position of intestinal crypts. Lineage tracing reveals that the p57 + cells serve as enteroendocrine/tuft cell precursors under normal conditions but dedifferentiate and act as facultative stem cells to support regeneration after injury. Single-cell transcriptomics analysis shows that the p57 + cells undergo a dynamic reprogramming process after injury that is characterized by fetal-like conversion and metaplasia-like transformation. Population-level analysis also detects such spatiotemporal reprogramming widely in other differentiated cell types. In intestinal adenoma, p57 + cells manifest homeostatic stem cell activity, in the context of constitutively activated spatiotemporal reprogramming. Our results highlight a pronounced plasticity of the intestinal epithelium that supports maintenance of tissue integrity in normal and neoplastic contexts. Rapid turnover and regeneration of intestinal epithelium requires distinct intestinal stem cell (ISC) populations. Here the authors show p57 marks quiescent ISCs, and that differentiated cells revert to stem cell state after injury, through dynamic reprogramming characterized by fetal- and metaplastic-like changes.

Skeletal stem cells (SSCs) have been isolated from various tissues, including periosteum and bone marrow, where they exhibit key functions in bone biology and hematopoiesis, respectively. The role of periosteal SSCs (P-SSCs) in bone regeneration and healing has been extensively studied, but their ability to contribute to the bone marrow stroma is still under debate. In the present study, we characterized a mouse whole bone transplantation model that mimics the initial bone marrow necrosis and fatty infiltration seen after injury. Using this model and a lineage tracing approach, we observed the migration of P-SSCs into the bone marrow after transplantation. Once in the bone marrow, P-SSCs are phenotypically and functionally reprogrammed into bone marrow mesenchymal stem cells (BM-MSCs) that express high levels of hematopoietic stem cell niche factors such as Cxcl12 and Kitl. In addition, using ex vivo and in vivo approaches, we found that P-SSCs are more resistant to acute stress than BM-MSCs. These results highlight the plasticity of P-SSCs and their potential role in bone marrow regeneration after bone marrow injury. Bone marrow is the soft tissue inside the bones in our bodies. It is the main production facility for new blood cells and makes billions of blood cells daily. However, like any other tissue or organ, the bone marrow can be damaged, for example, by radiation, chemotherapy drugs, or physical injuries like broken bones. Bone marrow mesenchymal stromal cells or BM-MSCs are a key component of the bone marrow and are responsible for regulating self-renewal, and for proliferation and differentiation of a group of blood-cell producing stem cells called hematopoietic stem cells or HSCs. A type of skeletal stem cell, the periosteal skeletal stem cells or P-SSCs, are located in a part of the bone called the periosteum, a thin tissue surrounding long bones. These cells are known to help bones regenerate and heal following a fracture. They are thought to share functional similarities with BM-MSCs . However, it is unclear if P-SSCs also support the recovery of blood cell production after damage to the bone marrow. To find out more, Marchand, Akkinola et al. used bone tissue derived from genetically engineered mice whose P-SSCs produced a fluorescent tag. Transplanting this bone tissue into otherwise healthy mice mimicked the changes usually seen in the bone marrow after an injury. Microscopy imaging of the transplanted bone at different stages revealed that P-SSCs migrated into the bone marrow after the transplantation injury. Once in the bone marrow, the P-SSCs developed BM-MSC-like characteristics, producing proteins known to support HSPCs. In other words, P-SSCs effectively transformed into new BM-MSCs, to the extent that the transplanted bone marrow could begin producing blood cells again. Further genetic analysis of P-SSCs and BM-MSCs showed that genes involved in stress resistance were more active in the P-SSCs. This suggests that P-SSCs are better at responding to stress, which may be helpful immediately after an injury. Marchand, Akkinola et al. have developed a new model to study how the bone marrow repairs itself after it is damaged. These findings may help contribute to a more detailed understanding of the mechanisms behind bone marrow regeneration, as well as treatments to improve recovery following injury.

Cancer stem cells (CSCs) generate transient‐amplifying cells and thereby contribute to cancer propagation. A fuller understanding of the biological features of CSCs is expected to lead to the development of new anticancer therapies capable of eradicating this life‐threatening disease. Cancer stem cells are known to maintain a non‐proliferative state and to enter the cell cycle only infrequently. Given that conventional anticancer therapies preferentially target dividing cells, CSCs are resistant to such treatments, with those remaining after elimination of bulk cancer cells potentially giving rise to disease relapse and metastasis as they re‐enter the cell cycle after a period of latency. Targeting of the switch between quiescence and proliferation in CSCs is therefore a potential strategy for preventing the reinitiation of malignancy, underscoring the importance of elucidation of the mechanisms by which these cells are maintained in the quiescent state. The fundamental properties of CSCs are thought to be governed cooperatively by internal molecules and cues from the external microenvironment (stem cell niche). Several such intrinsic and extrinsic regulators are responsible for the control of cell cycle progression in CSCs. In this review, we address two opposite approaches to the therapeutic targeting of CSCs – wake‐up and hibernation therapies – that either promote or prevent the entry of CSCs into the cell cycle, respectively, and we discuss the potential advantages and risks of each strategy. Intrinsic and extrinsic regulation of CSC behavior. CSC quiescence is regulated by internal molecules as well as by those in the CSC niche.

Aberrant epigenetic modification is one of the characteristics of the cancer genome. DNA hypermethylation of cytosine-phospho-guanine (CpG) islands, a hallmark of cancer cells, is well-studied and contributes to cancer development by silencing tumor suppressor genes. However, the mechanisms and biological significance of global DNA hypomethylation in cancer are still unclear. Here, using the v-Raf murine sarcoma viral oncogene homolog B1 (BRAF) V600E knock-in mouse models, we demonstrate that endogenous expression of oncogenic BRAFV600E in non-transformed cells promotes global DNA hypomethylation by increasing the levels of ten-eleven translocation 3 (TET3), which converts 5-methylcytosine (5-mC) into 5-hydroxymethylcytosine (5-hmC). Furthermore, TET3 is targeted for proteasomal degradation by F-box and WD repeat domain containing 7 (FBXW7). BRAFV600E increases TET3 levels by inhibiting glycogen synthase kinase 3β (GSK3β), which phosphorylates TET3 and leads to its ubiquitination and proteasomal degradation. We further found elevated levels of TET3 and 5-hmC in BRAFV600E-induced mouse lung tumors and show that TET3 enhances the ability of BRAFV600E to induce the formation of lung tumors. Notably, endogenous expression of oncogenic Kirsten rat sarcoma virus (KRAS) G12D also promotes global DNA hypomethylation and induces lung tumors through a similar TET3-mediated mechanism. Our findings elucidate one of the unknown mechanisms of global DNA hypomethylation promoted by oncogenic BRAF and KRAS and establish a role for TET3 to promote transformation in cooperation with BRAF and KRAS at an early stage of tumorigenesis.

Skeletal stem cells (SSCs) have been isolated from various tissues, including periosteum and bone marrow, where they exhibit key functions in bone biology and hematopoiesis, respectively. The role of periosteal SSCs (P-SSCs) in bone regeneration and healing has been extensively studied, but their ability to contribute to the bone marrow stroma is still under debate. In the present study, we characterized a mouse whole bone transplantation model that mimics the initial bone marrow necrosis and fatty infiltration seen after injury. Using this model and a lineage tracing approach, we observed the migration of P-SSCs into the bone marrow after transplantation. Once in the bone marrow, P-SSCs are phenotypically and functionally reprogrammed into bone marrow mesenchymal stem cells (BM-MSCs) that express high levels of hematopoietic stem cell niche factors such as Cxcl12 and Kitl. In addition, using ex vivo and in vivo approaches, we found that P-SSCs are more resistant to acute stress than BM-MSCs. These results highlight the plasticity of P-SSCs and their potential role in bone marrow regeneration after bone marrow injury. Bone marrow is the soft tissue inside the bones in our bodies. It is the main production facility for new blood cells and makes billions of blood cells daily. However, like any other tissue or organ, the bone marrow can be damaged, for example, by radiation, chemotherapy drugs, or physical injuries like broken bones. Bone marrow mesenchymal stromal cells or BM-MSCs are a key component of the bone marrow and are responsible for regulating self-renewal, and for proliferation and differentiation of a group of blood-cell producing stem cells called hematopoietic stem cells or HSCs. A type of skeletal stem cell, the periosteal skeletal stem cells or P-SSCs, are located in a part of the bone called the periosteum, a thin tissue surrounding long bones. These cells are known to help bones regenerate and heal following a fracture. They are thought to share functional similarities with BM-MSCs . However, it is unclear if P-SSCs also support the recovery of blood cell production after damage to the bone marrow. To find out more, Marchand, Akkinola et al. used bone tissue derived from genetically engineered mice whose P-SSCs produced a fluorescent tag. Transplanting this bone tissue into otherwise healthy mice mimicked the changes usually seen in the bone marrow after an injury. Microscopy imaging of the transplanted bone at different stages revealed that P-SSCs migrated into the bone marrow after the transplantation injury. Once in the bone marrow, the P-SSCs developed BM-MSC-like characteristics, producing proteins known to support HSPCs. In other words, P-SSCs effectively transformed into new BM-MSCs, to the extent that the transplanted bone marrow could begin producing blood cells again. Further genetic analysis of P-SSCs and BM-MSCs showed that genes involved in stress resistance were more active in the P-SSCs. This suggests that P-SSCs are better at responding to stress, which may be helpful immediately after an injury. Marchand, Akkinola et al. have developed a new model to study how the bone marrow repairs itself after it is damaged. These findings may help contribute to a more detailed understanding of the mechanisms behind bone marrow regeneration, as well as treatments to improve recovery following injury.

Skeletal stem cells have been isolated from various tissues, including periosteum and bone marrow, where they exhibit key functions in bone biology and hematopoiesis, respectively. The role of periosteal skeletal stem cells in bone regeneration and healing has been extensively studied, but their ability to contribute to the bone marrow stroma is still under debate. In the present study, we characterized a whole bone transplantation model that mimics the initial bone marrow necrosis and fatty infiltration seen after injury. Using this model and a lineage tracing approach, we observed the migration of periosteal skeletal stem cells into the bone marrow after transplantation. Once in the bone marrow, periosteal skeletal stem cells are phenotypically and functionally reprogrammed into bone marrow mesenchymal stem cells that express high levels of hematopoietic stem cell niche factors such as Cxcl12 and Kitl. In addition, using and approaches, we found that periosteal skeletal stem cells are more resistant to acute stress than bone marrow mesenchymal stem cells. These results highlight the plasticity of periosteal skeletal stem cells and their potential role in bone marrow regeneration after bone marrow injury.

Haematopoietic stem cells (HSCs) reside in specialized microenvironments, also referred to as niches, and it has been widely believed that HSC numbers are determined by the niche size alone . However, the vast excess of the number of niche cells over that of HSCs raises questions about this model. We initially established a mathematical model of niche availability and occupancy, which predicted that HSC numbers are restricted at both systemic and local levels. To address this question experimentally, we developed a femoral bone transplantation system, enabling us to increase the number of available HSC niches. We found that the addition of niches does not alter total HSC numbers in the body, regardless of whether the endogenous (host) niche is intact or defective, suggesting that HSC numbers are limited at the systemic level. Additionally, HSC numbers in transplanted wild-type femurs did not increase beyond physiological levels when HSCs were mobilized from defective endogenous niches to the periphery, indicating that HSC numbers are also constrained at the local level. Our study demonstrates that HSC numbers are not solely determined by niche availability, thereby rewriting the long-standing model for the regulation of HSC numbers.

The reverse mortgage is popular in the United States for elderly homeowners to enjoy a fruitful life by receiving an annuity or other financial benefits through leveraging owned houses. The Home Equity Conversion Mortgage (HECM) insured by the Federal Housing Administration is securitized in HECM mortgage-backed securities, or HMBS, guaranteed by Ginnie Mae—both are government agencies in the United States. Reverse mortgage markets exist in other jurisdictions, including the United Kingdom, Australia, and Canada, among others, without direct intervention from the public sector; however, the size of the reverse mortgage markets in those jurisdictions is much smaller than in the United States. Japan is the forerunner of an aging society, and the country has good reason to develop a reverse mortgage market to supplement the spending power of elderly homeowners. The persistent decline of property prices after the collapse of the asset bubble in the early 1990s hindered the development of the reverse mortgage market, because financial institutions were not willing to underwrite credit risk associated with such transactions. This article describes the current status of the reverse mortgage market in Japan and analyzes challenges for the development of the market by comparing foreign cases.