Haematopoietic stem cell numbers are not solely determined by niche availability.

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 1-5 . 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.

Periosteal skeletal stem cells can migrate into the bone marrow and support hematopoiesis after injury.

Functional stromal cells are known to support bone marrow regeneration after chemotherapy or radiation-induced injury to prevent prolonged myelosuppression. However, it is not known how stromal cells within the bone marrow are regenerated after injury. We have utilized a whole bone transplantation model that mimics the initial bone marrow necrosis and fatty infiltration that is seen after bone marrow injury and subsequent recovery. We demonstrate that periosteal skeletal stem cells (P-SSCs) can migrate into the bone marrow and contribute to stromal regeneration and hematopoietic recovery. Once in the bone marrow, P-SSCs are phenotypically and functionally reprogrammed into bone marrow mesenchymal stem cells (BM-MSCs), expressing high levels of hematopoietic stem cell (HSC) niche factors, such as Cxcl12 and Kitl. Additionally, our results further indicate that P-SSCs are more resistant to acute stress than BM-MSCs. Here, we report a new function of P-SSCs, highlighting their major plasticity and the role of the periosteum as a potential source of BM-MSCs following acute bone marrow injury.

Spatiotemporal reprogramming of differentiated cells underlies regeneration and neoplasia in the intestinal epithelium.

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.

Prevention of cancer dormancy by Fbxw7 ablation eradicates disseminated tumor cells.

Dormant cancer cells known as disseminated tumor cells (DTCs) are often present in bone marrow of breast cancer patients. These DTCs are thought to be responsible for the incurable recurrence of breast cancer. The mechanism underlying the long-term maintenance of DTCs remains unclear, however. Here, we show that Fbxw7 is essential for the maintenance of breast cancer dormancy. Genetic ablation of Fbxw7 in breast cancer cells disrupted the quiescence of DTCs, rendering them proliferative, in mouse xenograft and allograft models. Fbxw7-deficient DTCs were significantly depleted by treatment with paclitaxel, suggesting that cell proliferation induced by Fbxw7 ablation sensitized DTCs to chemotherapy. The combination of Fbxw7 ablation and chemotherapy reduced the number of DTCs even when applied after tumor cell dissemination. Mice injected with Fbxw7-deficient cancer cells survived longer after tumor resection and subsequent chemotherapy than did those injected with wild-type cells. Furthermore, database analysis revealed that breast cancer patients whose tumors expressed FBXW7 at a high level had a poorer prognosis than did those with a low FBXW7 expression level. Our results suggest that a wake-up strategy for DTCs based on Fbxw7 inhibition might be of value in combination with conventional chemotherapy for the treatment of breast cancer.

Neural Regulation of Bone and Bone Marrow.

Bones provide both skeletal scaffolding and space for hematopoiesis in its marrow. Previous work has shown that these functions were tightly regulated by the nervous system. The central and peripheral nervous systems tightly regulate compact bone remodeling, its metabolism, and hematopoietic homeostasis in the bone marrow (BM). Accumulating evidence indicates that the nervous system, which fine-tunes inflammatory responses and alterations in neural functions, may regulate autoimmune diseases. Neural signals also influence the progression of hematological malignancies such as acute and chronic myeloid leukemias. Here, we review the interplay of the nervous system with bone, BM, and immunity, and discuss future challenges to target hematological diseases through modulation of activity of the nervous system.

Complexity of bone marrow hematopoietic stem cell niche.

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.

To wake up cancer stem cells, or to let them sleep, that is the question.

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.

[Cell cycle regulation in cancer stem cells].

In addition to the properties of self-renewal and multipotency, cancer stem cells share the characteristics of their distinct cell cycle status with somatic stem cells. Cancer stem cells (CSCs) are maintained in a quiescent state with this characteristic conferring resistance to anticancer therapies that target dividing cells. Elucidation of the mechanisms of CSC quiescence might therefore be expected to provide further insight into CSC behaviors and lead to the elimination of cancer. This review summarizes several key regulators of the cell cycle in CSCs as well as attempts to define future challenges in this field, especially from the point of view of the application of our current understandings to the clinical medicine.

p57 regulates T-cell development and prevents lymphomagenesis by balancing p53 activity and pre-TCR signaling.

T cells are key components of the immune system, playing a central role in cell-mediated immunity. The sequential differentiation of T cells is associated with strict regulation of the cell cycle at each developmental stage. A balance between p53 activity and pre-T cell receptor (TCR) signaling regulates proliferation and differentiation decisions made by these cells. The relation between maintenance of this balance and the function of cell cycle regulators has remained largely unknown, however. We now show that mice with T cell-specific deficiency of the cyclin-dependent kinase inhibitor p57 manifest a differentiation block at the early stage of T cell maturation. Further genetic analysis showed that this defect is attributable to an imbalance between p53 activity and pre-TCR signaling caused by hyperactivation of the E2F-p53 pathway. Moreover, ablation of both p57 and p53 in T cells led to the development of aggressive thymic lymphomas with a reduced latency compared with that apparent for p53-deficient mice, whereas ablation of p57 alone did not confer susceptibility to this hematologic malignancy. Our results thus show that the p57-E2F-p53 axis plays a pivotal role in the proper development of T cells as well as in the prevention of lymphomagenesis.

Ablation of Fbxw7 eliminates leukemia-initiating cells by preventing quiescence.

Imatinib eradicates dividing progenitor cells of chronic myeloid leukemia (CML) but does not effectively target nondividing leukemia-initiating cells (LICs); thus, the disease often relapse after its discontinuation. We now show that Fbxw7 plays a pivotal role in maintenance of quiescence in LICs of CML by reducing the level of c-Myc. Abrogation of quiescence in LICs by Fbxw7 ablation increased their sensitivity to imatinib, and the combination of Fbxw7 ablation with imatinib treatment resulted in a greater depletion of LICs than of normal hematopoietic stem cells in mice. Purging of LICs by targeting Fbxw7 to interrupt their quiescence and subsequent treatment with imatinib may thus provide the basis for a promising therapeutic approach to CML.

Regulation of c-Myc ubiquitination controls chronic myelogenous leukemia initiation and progression.

The molecular mechanisms regulating leukemia-initiating cell (LIC) function are of important clinical significance. We use chronic myelogenous leukemia (CML) as a model of LIC-dependent malignancy and identify the interaction between the ubiquitin ligase Fbw7 and its substrate c-Myc as a regulator of LIC homeostasis. Deletion of Fbw7 leads to c-Myc overexpression, p53-dependent LIC-specific apoptosis, and the eventual inhibition of tumor progression. A decrease of either c-Myc protein levels or attenuation of the p53 response rescues LIC activity and disease progression. Further experiments showed that Fbw7 expression is required for survival and maintenance of human CML LIC. These studies identify a ubiquitin ligase:substrate pair regulating LIC activity, suggesting that targeting of the Fbw7:c-Myc axis is an attractive therapy target in refractory CML.

p57 is required for quiescence and maintenance of adult hematopoietic stem cells.

Quiescence is required for the maintenance of hematopoietic stem cells (HSCs). Members of the Cip/Kip family of cyclin-dependent kinase (CDK) inhibitors (p21, p27, p57) have been implicated in HSC quiescence, but loss of p21 or p27 in mice affects HSC quiescence or functionality only under conditions of stress. Although p57 is the most abundant family member in quiescent HSCs, its role has remained uncharacterized. Here we show a severe defect in the self-renewal capacity of p57-deficient HSCs and a reduction of the proportion of the cells in G(0) phase. Additional ablation of p21 in a p57-null background resulted in a further decrease in the colony-forming activity of HSCs. Moreover, the HSC abnormalities of p57-deficient mice were corrected by knocking in the p27 gene at the p57 locus. Our results therefore suggest that, among Cip/Kip family CDK inhibitors, p57 plays a predominant role in the quiescence and maintenance of adult HSCs.

Encephalomyelitis mimicking multiple sclerosis associated with chronic graft-versus-host disease after allogeneic bone marrow transplantation.

We describe a case of encephalomyelitis mimicking multiple sclerosis associated with chronic graft-versus-host disease (GVHD) occurring after allogeneic bone marrow transplantation (BMT) for myelodysplastic syndrome. Immunosuppressive therapy, consisting of a therapeutic dose of cyclosporine A and a maintenance dose of methylprednisolone, was effective in treating symptoms. Although central nervous system GVHD is very rare and remains controversial, presentation of neurological symptoms after allogeneic BMT warrants consideration of GVHD in the differential diagnosis.

Toxoplasmosis encephalitis following severe graft-vs.-host disease after allogeneic hematopoietic stem cell transplantation: 17 yr experience in Fukuoka BMT group.

Toxoplasmosis is a rare but rapidly fatal complication that can occur following hematopoietic stem cell transplantation (HSCT). Over a 17-yr period at our institutions, a definite diagnosis of toxoplasmosis was made in only two of 925 allogeneic HSCT recipients (0.22%) and none of 641 autologous HSCT recipients. These two patients received a conventional conditioning regimen followed by transplantation from an HLA-matched donor; however, they developed severe graft-vs.-host disease, which required intensive immunosuppressive therapy. Despite prophylactic treatment with trimethoprim/sulfamethoxazole, their immunosuppressive state, as indicated by a low CD4(+) cell count, might have resulted in toxoplasmosis encephalitis. Rapid and non-invasive methods such as a polymerase chain reaction (PCR) test of their cerebrospinal fluid for Toxoplasma gondii and magnetic resonance imaging of the brain were useful for providing a definitive diagnosis and prompt therapy in these patients: one patient stabilized and survived after responding to treatment with pyrimethamine/sulfodiazine whereas the other died of bacterial infection. In addition, retrospective PCR analyses of the frozen stored peripheral blood samples disclosed that detection of T. gondii preceded the onset of disease, indicating routine PCR testing of peripheral blood specimens may be an early diagnostic tool. It should be noted that when patients receiving HSCT have an unexplained fever and/or neurological complications, PCR tests should be considered to avoid cerebral lesions and improve the outcome of the patients.