Osteopontin is a hematopoietic stem cell niche component that negatively regulates stem cell pool size

Stem cells reside in a specialized niche that regulates their abundance and fate. Components of the niche have generally been defined in terms of cells and signaling pathways. We define a role for a matrix glycoprotein, osteopontin (OPN), as a constraining factor on hematopoietic stem cells within the bone marrow microenvironment. Osteoblasts that participate in the niche produce varying amounts of OPN in response to stimulation. Using studies that combine OPN-deficient mice and exogenous OPN, we demonstrate that OPN modifies primitive hematopoietic cell number and function in a stem cell–nonautonomous manner. The OPN-null microenvironment was sufficient to increase the number of stem cells associated with increased stromal Jagged1 and Angiopoietin-1 expression and reduced primitive hematopoietic cell apoptosis. The activation of the stem cell microenvironment with parathyroid hormone induced a superphysiologic increase in stem cells in the absence of OPN. Therefore, OPN is a negative regulatory element of the stem cell niche that limits the size of the stem cell pool and may provide a mechanism for restricting excess stem cell expansion under conditions of niche stimulation

Tightening the knot in phytochrome by single molecule atomic force microscopy

A growing number of proteins have been shown to adopt knotted folds. Yet the biological roles and biophysical properties of these knots remain poorly understood. We have used protein engineering and atomic force microscopy to explore single-molecule mechanics of the figure-of-eight knot in the chromophore-binding domain of the red/far red photoreceptor, phytochrome. Under load, apo phytochrome unfolds at forces of ~47 pN, while phytochrome carrying its covalently bound tetrapyrrole chromophore unfolds at ~73 pN. These forces are among the lowest measured in mechanical protein unfolding, hence the presence of the knot does not automatically indicate a super-stable protein. Our experiments reveal a stable intermediate along the mechanical unfolding pathway, reflecting sequential unfolding of two distinct subdomains in phytochrome, potentially the GAF and PAS domains. For the first time, our experiments allow direct determination of knot size under load. In the unfolded chain, the tightened knot is reduced to 17 amino acids, resulting in apparent shortening of the polypeptide chain by 6.2 nm. Steered molecular dynamics simulations corroborate this number. Finally, we found that covalent phytochrome dimers created for these experiments retain characteristic photoreversibility, unexpectedly arguing against dramatic rearrangement of the native GAF dimer interface upon photoconversion.Comment: 12 pages plus five figures; has been submitted to Biophysical J. Replacement on 9/16 is ONLY to correct a typo in the meta data; the uploaded file is identical to first versio

Mixed Chimerism, Lymphocyte Recovery, and Evidence for Early Donor-Specific Unresponsiveness in Patients Receiving Combined Kidney and Bone Marrow Transplantation to Induce Tolerance

Background We have previously reported operational tolerance in patients receiving HLA-mismatched combined kidney and bone marrow transplantation (CKBMT). We now report on transient multilineage hematopoietic chimerism and lymphocyte recovery in five patients receiving a modified CKBMT protocol, and evidence for early donor-specific unresponsiveness in one of these patients. Methods Five patients with end-stage renal disease received CKBMT from HLA-mismatched, haploidentical living related donors following modified non-myeloablative conditioning. Polychromatic flow cytometry (FCM) was used to assess multilineage chimerism where evaluable and lymphocyte recovery post-transplant. Limiting dilution analysis was used to assess helper-T-lymphocyte reactivity to donor antigens. Results Transient multilineage mixed chimerism was observed in all patients but chimerism became undetectable by 2 weeks post-CKBMT. A marked decrease in T and B lymphocyte counts immediately following transplant was followed by gradual recovery. Initially recovering T cells were depleted of CD45RA+/CD45RO− “naïve-like” cells, which have shown strong recovery in two patients and CD4/CD8 ratios increased immediately following transplant but then declined markedly. NK cells were enriched in the peripheral blood of all patients following transplant. For Subject 2, a pre-transplant limiting dilution assay revealed T helper cells recognizing both donor and third-party PBMCs. However, the anti-donor response was completely undetectable by Day 24, while third-party reactivity persisted. Conclusion These results characterize the transient multilineage mixed hematopoietic chimerism and recovery of lymphocyte subsets in patients receiving a modified CKBMT protocol. The observations are relevant to the mechanisms of donor-specific tolerance in this patient group

Differential regulation of myeloid leukemias by the bone marrow microenvironment

Like their normal hematopoietic stem cell counterparts, leukemia stem cells (LSC) in chronic myelogenous leukemia (CML) and acute myeloid leukemia (AML) are presumed to reside in specific niches in the bone marrow microenvironment (BMM)1, and may be the cause of relapse following chemotherapy.2 Targeting the niche is a novel strategy to eliminate persistent and drug-resistant LSC. CD443,4 and IL-65 have been implicated previously in the LSC niche. Transforming growth factor (TGF)-β1 is released during bone remodeling6 and plays a role in maintenance of CML LSCs7, but a role for TGF-β1 from the BMM has not been defined. Here, we show that alteration of the BMM by osteoblastic cell-specific activation of the parathyroid hormone (PTH) receptor8,9 attenuates BCR-ABL1-induced CML-like myeloproliferative neoplasia (MPN)10 but enhances MLL-AF9-induced AML11 in mouse transplantation models, possibly through opposing effects of increased TGF-β1 on the respective LSC. PTH treatment caused a 15-fold decrease in LSCs in wildtype mice with CML-like MPN, and reduced engraftment of immune deficient mice with primary human CML cells. These results demonstrate that LSC niches in chronic and acute myeloid leukemias are distinct, and suggest that modulation of the BMM by PTH may be a feasible strategy to reduce LSC, a prerequisite for the cure of CML

Transcriptional Profiling of Human Liver Identifies Sex-Biased Genes Associated with Polygenic Dyslipidemia and Coronary Artery Disease

Sex-differences in human liver gene expression were characterized on a genome-wide scale using a large liver sample collection, allowing for detection of small expression differences with high statistical power. 1,249 sex-biased genes were identified, 70% showing higher expression in females. Chromosomal bias was apparent, with female-biased genes enriched on chrX and male-biased genes enriched on chrY and chr19, where 11 male-biased zinc-finger KRAB-repressor domain genes are distributed in six clusters. Top biological functions and diseases significantly enriched in sex-biased genes include transcription, chromatin organization and modification, sexual reproduction, lipid metabolism and cardiovascular disease. Notably, sex-biased genes are enriched at loci associated with polygenic dyslipidemia and coronary artery disease in genome-wide association studies. Moreover, of the 8 sex-biased genes at these loci, 4 have been directly linked to monogenic disorders of lipid metabolism and show an expression profile in females (elevated expression of ABCA1, APOA5 and LDLR; reduced expression of LIPC) that is consistent with the lower female risk of coronary artery disease. Female-biased expression was also observed for CYP7A1, which is activated by drugs used to treat hypercholesterolemia. Several sex-biased drug-metabolizing enzyme genes were identified, including members of the CYP, UGT, GPX and ALDH families. Half of 879 mouse orthologs, including many genes of lipid metabolism and homeostasis, show growth hormone-regulated sex-biased expression in mouse liver, suggesting growth hormone might play a similar regulatory role in human liver. Finally, the evolutionary rate of protein coding regions for human-mouse orthologs, revealed by dN/dS ratio, is significantly higher for genes showing the same sex-bias in both species than for non-sex-biased genes. These findings establish that human hepatic sex differences are widespread and affect diverse cell metabolic processes, and may help explain sex differences in lipid profiles associated with sex differential risk of coronary artery disease