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Isocitrate dehydrogenase (IDH) mutations are common genetic alterations in myeloid disorders, including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). Epigenetic changes, including abnormal histone and DNA methylation, have been implicated in the pathogenic build-up of hematopoietic progenitors, but it is still unclear whether and how IDH mutations themselves affect hematopoiesis. Here, we show that IDH1-mutant mice develop myeloid dysplasia in that these animals exhibit anemia, ineffective erythropoiesis, and increased immature progenitors and erythroblasts. In erythroid cells of these mice, D-2-hydroxyglutarate, an aberrant metabolite produced by the mutant IDH1 enzyme, inhibits oxoglutarate dehydrogenase activity and diminishes succinyl–coenzyme A (CoA) production. This succinyl-CoA deficiency attenuates heme biosynthesis in IDH1-mutant hematopoietic cells, thus blocking erythroid differentiation at the late erythroblast stage and the erythroid commitment of hematopoietic stem cells, while the exogenous succinyl-CoA or 5-ALA rescues erythropoiesis in IDH1-mutant erythroid cells. Heme deficiency also impairs heme oxygenase-1 expression, which reduces levels of important heme catabolites such as biliverdin and bilirubin. These deficits result in accumulation of excessive reactive oxygen species that induce the cell death of IDH1-mutant erythroid cells. Our results clearly show the essential role of IDH1 in normal erythropoiesis and describe how its mutation leads to myeloid disorders. These data thus have important implications for the devising of new treatments for IDH-mutant tumors.

Isocitrate dehydrogenase (IDH) mutations are common genetic alterations in myeloid disorders, including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). Epigenetic changes, including abnormal histone and DNA methylation, have been implicated in the pathogenic build-up of hematopoietic progenitors, but it is still unclear whether and how IDHmutations themselves affect hematopoiesis. Here, we show that IDH1-mutant mice develop myeloid dysplasia in that these animals exhibit anemia, ineffective erythropoiesis, and increased immature progenitors and erythroblasts. In erythroid cells of these mice, D-2-hydroxyglutarate, an aberrant metabolite produced by the mutant IDH1enzyme, inhibits oxoglutarate dehydrogenase activity and diminishes succinyl–coenzyme A (CoA) production. This succinyl-CoA deficiency attenuates heme biosynthesis in IDH1-mutant hematopoietic cells, thus blocking erythroid differentiation at the late erythroblast stage and the erythroid commitment of hematopoietic stem cells, while the exogenous succinyl-CoA or 5-ALA rescues erythropoiesis in IDH1-mutant erythroid cells. Heme deficiency also impairs heme oxygenase-1 expression, which reduces levels of important heme catabolites such as biliverdin and bilirubin. These deficits result in accumulation of excessive reactive oxygen species that induce the cell death of IDH1-mutant erythroid cells. Our results clearly show the essential role of IDH1in normal erythropoiesis and describe how its mutation leads to myeloid disorders. These data thus have important implications for the devising of new treatments for IDH-mutant tumors.

Introduction: Vaso-occlusive episodes (VOEs) contribute to significant morbidity and mortality in patients with sickle cell disease (SCD) and require timely treatment. Prompt intravenous fluids (IVF) and analgesics are key mainstays of care that may influence outcomes such as the increased disease burden from VOE-associated tissue infarction and the development of chronic pain. VOEs often lead to uncontrolled pain, requiring frequent ED (emergency department) visits and hospitalizations. Timely acute care management is crucial for curbing the significant polymerization of hemoglobin (Hgb) S and dehydration, which increases sickling of Hgb and can precipitate VOEs. In this study, we investigate the relationship between variables such as time to first IVF and IV pain medications in the ED for adult and pediatric patients with SCD presenting with VOEs and rates of admission.

Downstream regulatory element antagonist modulator (DREAM), a transcriptional repressor, is known to modulate pain responses. However, it is unknown whether DREAM is expressed in anucleate platelets and plays a role in thrombogenesis. By using intravital microscopy with DREAM-null mice and their bone marrow chimeras, we demonstrated that both hematopoietic and nonhematopoietic cell DREAMs are required for platelet thrombus formation following laser-induced arteriolar injury. In a FeCl3-induced thrombosis model, we found that compared with wild-type (WT) control and nonhematopoietic DREAM knockout (KO) mice, DREAM KO control and hematopoietic DREAM KO mice showed a significant delay in time to occlusion. Tail bleeding time was prolonged in DREAM KO control mice, but not in WT or DREAM bone marrow chimeric mice. In vivo adoptive transfer experiments further indicated the importance of platelet DREAM in thrombogenesis. We found that DREAM deletion does not alter the ultrastructural features of platelets but significantly impairs platelet aggregation and adenosine triphosphate secretion induced by numerous agonists (collagen-related peptide, adenosine 5′-diphosphate, A23187, thrombin, or U46619). Biochemical studies revealed that platelet DREAM positively regulates phosphoinositide 3-kinase (PI3K) activity during platelet activation. Using DREAM-null platelets and PI3K isoform-specific inhibitors, we observed that platelet DREAM is important for α-granule secretion, Ca2+mobilization, and aggregation through PI3K class Iβ (PI3K-Iβ). Genetic and pharmacological studies in human megakaryoblastic MEG-01 cells showed that DREAM is important for A23187-induced Ca2+mobilization and its regulatory function requires Ca2+binding and PI3K-Iβ activation. These results suggest that platelet DREAM regulates PI3K-Iβ activity and plays an important role during thrombus formation.

Downstream regulatory element antagonist modulator (DREAM), a transcriptional repressor, is known to modulate pain responses. However, it is unknown whether DREAM is expressed in anucleate platelets and plays a role in thrombogenesis. By using intravital microscopy with DREAM-null mice and their bone marrow chimeras, we demonstrated that both hematopoietic and nonhematopoietic cell DREAMs are required for platelet thrombus formation following laser-induced arteriolar injury. In a FeCl3-induced thrombosis model, we found that compared with wild-type (WT) control and nonhematopoietic DREAM knockout (KO) mice, DREAM KO control and hematopoietic DREAM KO mice showed a significant delay in time to occlusion. Tail bleeding time was prolonged in DREAM KO control mice, but not in WT or DREAM bone marrow chimeric mice. In vivo adoptive transfer experiments further indicated the importance of platelet DREAM in thrombogenesis. We found that DREAM deletion does not alter the ultrastructural features of platelets but significantly impairs platelet aggregation and adenosine triphosphate secretion induced by numerous agonists (collagen-related peptide, adenosine 5′-diphosphate, A23187, thrombin, or U46619). Biochemical studies revealed that platelet DREAM positively regulates phosphoinositide 3-kinase (PI3K) activity during platelet activation. Using DREAM-null platelets and PI3K isoform-specific inhibitors, we observed that platelet DREAM is important for α-granule secretion, Ca2+ mobilization, and aggregation through PI3K class Iβ (PI3K-Iβ). Genetic and pharmacological studies in human megakaryoblastic MEG-01 cells showed that DREAM is important for A23187-induced Ca2+ mobilization and its regulatory function requires Ca2+ binding and PI3K-Iβ activation. These results suggest that platelet DREAM regulates PI3K-Iβ activity and plays an important role during thrombus formation.

Advancement of engineered ear in clinical practice is limited by several challenges. The complex, largely unsupported, three-dimensional auricular neocartilage structure is difficult to maintain. Neocartilage formation is challenging in an immunocompetent host due to active inflammatory and immunological responses. The large number of autologous chondrogenic cells required for engineering an adult human-sized ear presents an additional challenge because primary chondrocytes rapidly dedifferentiate during in vitroculture. The objective of this study was to engineer a stable, human ear-shaped cartilage in an immunocompetent animal model using expanded chondrocytes. The impact of basic fibroblast growth factor (bFGF) supplementation on achieving clinically relevant expansion of primary sheep chondrocytes by in vitroculture was determined. Chondrocytes expanded in standard medium were either combined with cryopreserved, primary passage 0 chondrocytes at the time of scaffold seeding or used alone as control. Disk and human ear-shaped scaffolds were made from porous collagen; ear scaffolds had an embedded, supporting titanium wire framework. Autologous chondrocyte-seeded scaffolds were implanted subcutaneously in sheep after 2 weeks of in vitroincubation. The quality of the resulting neocartilage and its stability and retention of the original ear size and shape were evaluated at 6, 12, and 20 weeks postimplantation. Neocartilage produced from chondrocytes that were expanded in the presence of bFGF was superior, and its quality improved with increased implantation time. In addition to characteristic morphological cartilage features, its glycosaminoglycan content was high and marked elastin fiber formation was present. The overall shape of engineered ears was preserved at 20 weeks postimplantation, and the dimensional changes did not exceed 10%. The wire frame within the engineered ear was able to withstand mechanical forces during wound healing and neocartilage maturation and prevented shrinkage and distortion. This is the first demonstration of a stable, ear-shaped elastic cartilage engineered from auricular chondrocytes that underwent clinical-scale expansion in an immunocompetent animal over an extended period of time.

Platelet-leukocyte interactions on activated endothelial cells play an important role during microvascular occlusion under oxidative stress conditions. However, it remains poorly understood how neutrophil-platelet interactions are regulated during vascular inflammation. By using intravital microscopy with mice lacking nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) and their bone marrow chimera, we demonstrated that NOX2 from both hematopoietic and endothelial cells is crucial for neutrophil-platelet interactions during tumor necrosis factor alpha-induced venular inflammation. Platelet NOX2-produced reactive oxygen species (ROS) regulated P-selectin exposure upon agonist stimulation and the ligand-binding function of glycoprotein Ibα. Furthermore, neutrophil NOX2-generated ROS enhanced the activation and ligand-binding activity of αMβ2 integrin following N-formyl-methionyl-leucyl phenylalanine stimulation. Studies with isolated cells and a mouse model of hepatic ischemia/reperfusion injury revealed that NOX2 from both platelets and neutrophils is required for cell-cell interactions, which contribute to the pathology of hepatic ischemia/reperfusion injury. Platelet NOX2 modulated intracellular Ca2+ release but not store-operated Ca2+ entry (SOCE), whereas neutrophil NOX2 was crucial for SOCE but not intracellular Ca2+ release. Different regulation of Ca2+ signaling by platelet and neutrophil NOX2 correlated with differences in the phosphorylation of AKT, ERK, and p38MAPK. Our results indicate that platelet and neutrophil NOX2-produced ROS are critical for the function of surface receptors essential for neutrophil-platelet interactions during vascular inflammation.

Platelet-leukocyte interactions on activated endothelial cells play an important role during microvascular occlusion under oxidative stress conditions. However, it remains poorly understood how neutrophil-platelet interactions are regulated during vascular inflammation. By using intravital microscopy with mice lacking nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) and their bone marrow chimera, we demonstrated that NOX2 from both hematopoietic and endothelial cells is crucial for neutrophil-platelet interactions during tumor necrosis factor alpha-induced venular inflammation. Platelet NOX2-produced reactive oxygen species (ROS) regulated P-selectin exposure upon agonist stimulation and the ligand-binding function of glycoprotein Ibα. Furthermore, neutrophil NOX2-generated ROS enhanced the activation and ligand-binding activity of αMβ2 integrin following N-formyl-methionyl-leucyl phenylalanine stimulation. Studies with isolated cells and a mouse model of hepatic ischemia/reperfusion injury revealed that NOX2 from both platelets and neutrophils is required for cell-cell interactions, which contribute to the pathology of hepatic ischemia/reperfusion injury. Platelet NOX2 modulated intracellular Ca2+release but not store-operated Ca2+entry (SOCE), whereas neutrophil NOX2 was crucial for SOCE but not intracellular Ca2+release. Different regulation of Ca2+signaling by platelet and neutrophil NOX2 correlated with differences in the phosphorylation of AKT, ERK, and p38MAPK. Our results indicate that platelet and neutrophil NOX2-produced ROS are critical for the function of surface receptors essential for neutrophil-platelet interactions during vascular inflammation.

Objective Our goal was to engineer cartilage in vivousing auricular chondrocytes that underwent clinically relevant expansion and using methodologies that could be easily translated into health care practice.Design Sheep and human chondrocytes were isolated from auricular cartilage biopsies and expanded in vitro. To reverse dedifferentiation, expanded cells were either mixed with cryopreserved P0 chondrocytes at the time of seeding onto porous collagen scaffolds or proliferated with basic fibroblast growth factor (bFGF). After 2-week in vitroincubation, seeded scaffolds were implanted subcutaneously in nude mice for 6 weeks. The neocartilage quality was evaluated histologically; DNA and glycosaminoglycans were quantified. Cell proliferation rates and collagen gene expression profiles were assessed.Results Clinically sufficient over 500-fold chondrocyte expansion was achieved at passage 3 (P3); cell dedifferentiation was confirmed by the simultaneous COL1A1/3A1 gene upregulation and COL2A1 downregulation. The chondrogenic phenotype of sheep but not human P3 cells was rescued by addition of cryopreserved P0 chondrocytes. With bFGF supplementation, chondrocytes achieved clinically sufficient expansion at P2; COL2A1 expression was not rescued but COL1A1/3A1genes were downregulated. Although bFGF failed to rescue COL2A1 expression during chondrocyte expansion in vitro, elastic neocartilage with obvious collagen II expression was observed on porous collagen scaffolds after implantation in mice for 6 weeks.Conclusions Both animal and human auricular chondrocytes expanded with low-concentration bFGF supplementation formed high-quality elastic neocartilage on porous collagen scaffolds in vivo.

Tissue-engineered cartilage has historically been an attractive alternative treatment option for auricular reconstruction. However, the ability to reliably generate autologous auricular neocartilage in an immunocompetent preclinical model should first be established. The objectives of this study were to demonstrate engineered autologous auricular cartilage in the immunologically aggressive subcutaneous environment of an immunocompetent animal model, and to determine the impact of in vitroculture duration of chondrocyte-seeded constructs on the quality of neocartilage maturation in vivo. Auricular cartilage was harvested from eight adult sheep; chondrocytes were isolated, expanded in vitro, and seeded onto fibrous collagen scaffolds. Constructs were cultured in vitrofor 2, 6, and 12 weeks, and then implanted autologously in sheep and in control nude mice for 6 and 12 weeks. Explanted tissue was stained with hematoxylin and eosin, safranin O, toluidine blue, collagen type II, and elastin. DNA and glycosaminoglycans (GAGs) were quantified. The quality of cartilage engineered in sheep decreased with prolonged in vitroculture time. Superior cartilage formation was demonstrated after 2 weeks of in vitroculture; the neocartilage quality improved with increased implantation time. In nude mice, neocartilage resembled native sheep auricular cartilage regardless of the in vitroculture length, with the exception of elastin expression. The DNA quantification was similar in all engineered and native cartilage (p>0.1). All cartilage engineered in sheep had significantly less GAG than native cartilage (p<0.02); significantly more GAG was observed with increased implantation time (p<0.02). In mice, the GAG content was similar to that of native cartilage and became significantly higher with increased in vitroor in vivodurations (p<0.02). Autologous auricular cartilage was successfully engineered in the subcutaneous environment of an ovine model using expanded chondrocytes seeded on a fibrous collagen scaffold after a 2-week in vitroculture period.

Oxidative stress plays an important role in cancer development and treatment. Recent data implicate the tumor suppressor BRCA1 in regulating oxidative stress, but the molecular mechanism and the impact in BRCA1-associated tumorigenesis remain unclear. Here, we show that BRCA1 regulates Nrf2-dependent antioxidant signaling by physically interacting with Nrf2 and promoting its stability and activation. BRCA1-deficient mouse primary mammary epithelial cells show low expression of Nrf2-regulated antioxidant enzymes and accumulate reactive oxygen species (ROS) that impair survival in vivo. Increased Nrf2 activation rescues survival and ROS levels in BRCA1-null cells. Interestingly, 53BP1 inactivation, which has been shown to alleviate several defects associated with BRCA1 loss, rescues survival of BRCA1-null cells without restoring ROS levels. We demonstrate that estrogen treatment partially restores Nrf2 levels in the absence of BRCA1. Our data suggest that Nrf2-regulated antioxidant response plays a crucial role in controlling survival downstream of BRCA1 loss. The ability of estrogen to induce Nrf2 posits an involvement of an estrogen-Nrf2 connection in BRCA1 tumor suppression. Lastly, BRCA1-mutated tumors retain a defective antioxidant response that increases the sensitivity to oxidative stress. In conclusion, the role of BRCA1 in regulating Nrf2 activity suggests important implications for both the etiology and treatment of BRCA1-related cancers.

APC/Cdh1 is a major cell cycle regulator and its function has been implicated in DNA damage repair; however, its exact role remains unclear. Using affinity purification coupled with mass spectrometry, we identified Claspin as a novel Cdh1-interacting protein and further demonstrated that Claspin is a novel Cdh1 ubiquitin substrate. As a result, inactivation of Cdh1 leads to activation of the Claspin/Chk1 pathway. Previously, we demonstrated that Rb interacts with Cdh1 to influence its ability to degrade Skp2. Here, we report that Cdh1 reciprocally regulates the Rb pathway through competing with E2F1 to bind the hypophosphorylated form of Rb. Although inactivation of Cdh1 in HeLa cells, with defective p53/Rb pathways, led to premature S phase entry, acute depletion of Cdh1 in primary human fibroblasts resulted in premature senescence. Acute loss of many other major tumor suppressors, including PTEN and VHL, also induces premature senescence in a p53- or Rb-dependent manner. Similarly, we showed that inactivation of the p53/Rb pathways by overexpression of SV40 LT-antigen partially reversed Cdh1 depletion–induced growth arrest. Therefore, loss of Cdh1 is only beneficial to cells with abnormal p53 and Rb pathways, which helps explain why Cdh1 loss is not frequently found in many tumors.

APC/Cdh1 is a major cell cycle regulator and its function has been implicated in DNA damage repair; however, its exact role remains unclear. Using affinity purification coupled with mass spectrometry, we identified Claspin as a novel Cdh1-interacting protein and further demonstrated that Claspin is a novel Cdh1 ubiquitin substrate. As a result, inactivation of Cdh1 leads to activation of the Claspin/Chk1 pathway. Previously, we demonstrated that Rb interacts with Cdh1 to influence its ability to degrade Skp2. Here, we report that Cdh1 reciprocally regulates the Rb pathway through competing with E2F1 to bind the hypophosphorylated form of Rb. Although inactivation of Cdh1 in HeLa cells, with defective p53/Rb pathways, led to premature S phase entry, acute depletion of Cdh1 in primary human fibroblasts resulted in premature senescence. Acute loss of many other major tumor suppressors, including PTEN and VHL, also induces premature senescence in a p53- or Rb-dependent manner. Similarly, we showed that inactivation of the p53/Rb pathways by overexpression of SV40 LT-antigen partially reversed Cdh1 depletion–induced growth arrest. Therefore, loss of Cdh1 is only beneficial to cells with abnormal p53 and Rb pathways, which helps explain why Cdh1 loss is not frequently found in many tumors.

Abbadessa: ArQule, Inc: Employment. Yu:ArQule, Inc: Employment. Schwartz:ArQule, Inc: Employment.

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No relevant conflicts of interest to declare.

DREAM (Downstream Regulatory Element Antagonistic Modulator) is a neuronal calcium sensor and acts as a transcriptional repressor for pain modulation. Recent studies showed that endothelial cell DREAM transcriptionally regulates NF-κB signaling during inflammation (Tiruppathi et al. Nat Immunol. 2014). However, it is unknown whether DREAM plays a role during thrombosis and hemostasis. Using real-time fluorescence intravital microscopy with DREAM knockout (KO) mice, we demonstrated that intravascular cell DREAM is required for platelet thrombus formation and fibrin generation at the site of laser-induced cremaster arteriolar injury. Studies of bone marrow chimeric mice further revealed that both hematopoietic and endothelial cell DREAM regulate thrombus formation following arteriolar injury. Using in vitro flow chamber assays, we found that DREAM is important for platelet thrombus formation on collagen-coated surfaces under shear conditions. Further, tail bleeding time and blood loss significantly increased in DREAM KO mice, compared with wild-type (WT) mice, suggesting the importance of DREAM for hemostasis as well as thrombus formation in vivo. We observed that DREAM KO platelets show markedly decreased aggregation, ATP secretion, P-selectin exposure, and αIIbβ3 integrin activation induced by thrombin, collagen-related peptide, ADP, and A23187 (calcium ionophore). Interestingly, DREAM deletion did not affect phorbol 12-myristate 13-acetate-induced platelet functions. Consistently, DREAM KO platelets exhibited decreased phosphorylation of phosphoinositide-3-kinase, AKT, and ERK, but not protein kinase C (PKC) isoforms during cell activation, suggesting that DREAM does not regulate protein kinase C signaling pathway. Electron microscopic analysis demonstrated that ultrastructural features of DREAM KO platelets are indistinguishable from those of WT platelets. Since DREAM is a transcription factor that binds to calcium in four EF-hand motifs, we investigated whether platelet DREAM regulates calcium signaling. Indeed, DREAM deletion in platelets resulted in significant reduction in both intracellular calcium release and store-operated calcium entry (SOCE) following agonist stimulation. When we transiently knocked down DREAM in a human megakaryoblastic leukemia cell line (MEG-01) using siRNA, we found that A23187-induced calcium release and SOCE are impaired by DREAM knockdown in MEG-01 cells without altering the expression of calcium signaling molecules, such as inositol trisphosphate receptor, stromal interaction molecule 1, and ORAI1. These defects were rescued by overexpression of siRNA-resistant WT but not EF-hand domain mutant DREAM. Therefore, our results provide novel evidence that platelet DREAM regulates calcium signaling independently of its transcriptional factor activity, thereby playing a critical role during thrombus formation and hemostasis in vivo.

Engineered cartilage is a promising option for auricular reconstruction. We have previously demonstrated that a titanium wire framework within a composite collagen ear-shaped scaffold helped to maintain the gross dimensions of the engineered ear after implantation, resisting the deformation forces encountered during neocartilage maturation and wound healing. The ear geometry was redesigned to achieve a more accurate aesthetic result when implanted subcutaneously in a nude rat model. A non-invasive method was developed to assess size and shape changes of the engineered ear in three dimensions. Computer models of the titanium framework were obtained from CT scans before and after implantation.Several parameters were measured including the overall length, width and depth, the minimum intrahelical distance and overall curvature values for each beam section within the framework. Local curvature values were measured to gain understanding of the bending forces experienced by the framework structure in situ. Length and width changed by less than 2%, whereas the depth decreased by approximately 8% and the minimum intrahelical distance changed by approximately 12%. Overall curvature changes identified regions most susceptible to deformation. Eighty-nine per cent of local curvature measurements experienced a bending moment less than 50 µN-m owing to deformation forces during implantation. These quantitative shape analysis results have identified opportunities to improve shape fidelity of engineered ear constructs.

During development, hematopoietic stem cells (HSCs) translocate from the fetal liver to the bone marrow (BM), which remains the site of hematopoiesis throughout adulthood. In the BM the HSCs are located at the endosteal surface, where cells of the osteoblastic lineage comprise a key component of the stem cell niche. While hematopoiesis occurs in many bones, the process of bone formation can actually be split into those bones that develop through endochondral ossification (long bones) and those that form through membranous ossification (flat bones). We examined the role played by the microenvironment in these two distinct bones and whether these microenvironments have differing effects on the HSCs. In vitro analysis of the BM stromal cells isolated from long bones and flat bones has demonstrated that calvaria derived stromal layers can support cobblestone area-forming cells 10-fold greater than stromal layers derived from femurs and tibia. Real-time PCR analysis of gene expression has demonstrated that flat bone stromal cells have 5-fold greater expression of N-cadherin than long bone, while other cadherins such as VE-cadherin show no difference. Correlating with this, we found that calvarial derived HSCs demonstrated increased expression of N-cadherin and also increased expression of other genes associated with cadherin signaling, such as cyclinD1. However, no difference in the cell cycle status of the HSCs derived from long bone and flat bone was noted. Functional assays are being performed in order to assess the function of these distinct BM marrow niches in vivo. It is anticipated that we will be able to begin to define the molecular cues the govern HSC physiology in different locations within the mammalian skeleton and thus provide an understanding not only into the continual migration of HSCs between different HSC niches but also the regression of hematopoiesis that occurs from the appendicular skeleton to the axial skeleton during the adult human lifespan.

During development, hematopoietic stem cells (HSCs) translocate from the fetal liver to the bone marrow (BM), which remains the site of hematopoiesis throughout adulthood. In the BM the HSCs are located at the endosteal surface, where cells of the osteoblastic lineage comprise a key component of the stem cell niche. While hematopoiesis occurs in many bones, the process of bone formation can actually be split into those bones that develop through endochondral ossification (long bones) and those that form through membranous ossification (flat bones). We examined the role played by the microenvironment in these two distinct bones and whether these microenvironments have differing effects on the HSCs. In vitroanalysis of the BM stromal cells isolated from long bones and flat bones has demonstrated that calvaria derived stromal layers can support cobblestone area-forming cells 10-fold greater than stromal layers derived from femurs and tibia. Real-time PCR analysis of gene expression has demonstrated that flat bone stromal cells have 5-fold greater expression of N-cadherinthan long bone, while other cadherins such as VE-cadherin show no difference. Correlating with this, we found that calvarial derived HSCs demonstrated increased expression of N-cadherinand also increased expression of other genes associated with cadherin signaling, such as cyclinD1. However, no difference in the cell cycle status of the HSCs derived from long bone and flat bone was noted. Functional assays are being performed in order to assess the function of these distinct BM marrow niches in vivo. It is anticipated that we will be able to begin to define the molecular cues the govern HSC physiology in different locations within the mammalian skeleton and thus provide an understanding not only into the continual migration of HSCs between different HSC niches but also the regression of hematopoiesis that occurs from the appendicular skeleton to the axial skeleton during the adult human lifespan.