Xiaohong Li

Indiana University-Purdue University Indianapolis, Indianapolis, Indiana, United States

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Publications (14)103.57 Total impact

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    ABSTRACT: Despite the high prevalence and significant morbidity of spinal anomalies in neurofibromatosis type 1 (NF1), the pathogenesis of these defects remains largely unknown. Here, we present two murine models: Nf1flox/-;PeriCre and Nf1flox/-;Col.2.3Cre mice, which recapitulate spinal deformities seen in the human disease. Dynamic histomorphometry and microtomographic studies show recalcitrant bone remodeling and distorted bone microarchitecture within the vertebral spine of Nf1flox/-;PeriCre and Nf1flox/-;Col2.3Cre mice, with analogous histological features present in a human patient with dystrophic scoliosis. Intriguingly, 36-60% of Nf1flox/-;PeriCre and Nf1flox/-;Col2.3Cre mice exhibit segmental vertebral fusion anomalies with boney obliteration of the intervertebral disc (IVD). While analogous findings have not yet been reported in the NF1 patient population, we herein present two case reports of IVD defects and interarticular vertebral fusion in patients with NF1. Collectively, these data provide novel insights regarding the pathophysiology of dystrophic spinal anomalies in NF1, and provide impetus for future radiographic analyses of larger patient cohorts to determine whether IVD and vertebral fusion defects may have been previously overlooked or underreported in the NF1 patient population.
    PLoS ONE 01/2015; 10(3):e0119093. DOI:10.1371/journal.pone.0119093 · 3.53 Impact Factor
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    ABSTRACT: Neurofibromatosis type 1 (NF1) is a common genetic disorder affecting 1 in 3500 individuals. Patients with NF1 are predisposed to debilitating skeletal manifestations, including osteopenia/osteoporosis and long bone pseudarthrosis (non-union fracture). Hyperactivation of the Ras/mitogen-activated protein kinase (MAPK) pathway in NF1 is known to underlie aberrant proliferation and differentiation in cell lineages, including osteoclast progenitors and mesenchymal stem cells (MSCs) also known as osteoblast progenitors (pro-OBLs). Our current study demonstrates the hyper Ras/MAPK as a critical pathway underlying the pathogenesis of NF1 associated fracture repair deficits. Nf1-deficient pro-OBLs exhibit Ras/MAPK hyperactivation. Introduction of the NF1 GTPase activating related domain (NF1 GAP-related domain) in vitro is sufficient to rescue hyper Ras activity and enhance osteoblast (OBL) differentiation in Nf1(-/-) pro-OBLs and NF1 human (h) MSCs cultured from NF1 patients with skeletal abnormalities including pseudarthrosis or scoliosis. Pharmacologic inhibition of mitogen-activated protein kinase kinase (MEK) signaling with PD98059 partially rescues aberrant Erk activation while enhancing OBL differentiation and expression of OBL markers, osterix and osteocalcin, in Nf1-deficient murine pro-OBLs. Similarly, MEK inhibition enhances OBL differentiation of hMSCs. In addition, PD98059 rescues aberrant osteoclast maturation in Nf1 haploinsufficient bone marrow mononuclear cells. Importantly, MEK inhibitor significantly improves fracture healing in an NF1 murine model, Col2.3Cre;Nf1(flox/-). Collectively, these data indicate the Ras/MAPK cascade as a critical pathway in the pathogenesis of bone loss and pseudarthrosis related to NF1 mutations. These studies provide evidence for targeting the MAPK pathway to improve bone mass and treat pseudarthrosis in NF1.
    Human Molecular Genetics 07/2013; DOI:10.1093/hmg/ddt333 · 6.68 Impact Factor
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    ABSTRACT: Germline mutations in the NF1 tumor suppressor gene cause neurofibromatosis type 1 (NF1), a complex genetic disorder with a high predisposition of numerous skeletal dysplasias including short stature, osteoporosis, kyphoscoliosis, and fracture non-union (pseudoarthrosis). We have developed murine models that phenocopy many of the skeletal dysplasias observed in NF1 patients, including reduced bone mass and fracture non-union. We also show that the development of these skeletal manifestations requires an Nf1 haploinsufficient background in addition to nullizygous loss of Nf1 in mesenchymal stem/progenitor cells (MSCs) and/or their progenies. This is replicated in two animal models of NF1, PeriCre(+);Nf1(flox/-) and Col2.3Cre(+);Nf1(flox/-) mice. Adoptive transfer experiments demonstrate a critical role of the Nf1+/- marrow microenvironment in the impaired fracture healing in both models and adoptive transfer of WT bone marrow cells improves fracture healing in these mice. To our knowledge, this is the first demonstration of a non-cell autonomous mechanism in non-malignant NF1 manifestations. Collectively, these data provide evidence of a combinatory effect between nullizygous loss of Nf1 in osteoblast progenitors and haploinsufficiency in hematopoietic cells in the development of non-malignant NF1 manifestations.
    PLoS ONE 09/2011; 6(9):e24917. DOI:10.1371/journal.pone.0024917 · 3.53 Impact Factor
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    ABSTRACT: The extracellular signal-regulated kinases (ERK1 and 2) are widely-expressed and they modulate proliferation, survival, differentiation, and protein synthesis in multiple cell lineages. Altered ERK1/2 signaling is found in several genetic diseases with skeletal phenotypes, including Noonan syndrome, Neurofibromatosis type 1, and Cardio-facio-cutaneous syndrome, suggesting that MEK-ERK signals regulate human skeletal development. Here, we examine the consequence of Erk1 and Erk2 disruption in multiple functions of osteoclasts, specialized macrophage/monocyte lineage-derived cells that resorb bone. We demonstrate that Erk1 positively regulates osteoclast development and bone resorptive activity, as genetic disruption of Erk1 reduced osteoclast progenitor cell numbers, compromised pit formation, and diminished M-CSF-mediated adhesion and migration. Moreover, WT mice reconstituted long-term with Erk1(-/-) bone marrow mononuclear cells (BMMNCs) demonstrated increased bone mineral density as compared to recipients transplanted with WT and Erk2(-/-) BMMNCs, implicating marrow autonomous, Erk1-dependent osteoclast function. These data demonstrate Erk1 plays an important role in osteoclast functions while providing rationale for the development of Erk1-specific inhibitors for experimental investigation and/or therapeutic modulation of aberrant osteoclast function.
    PLoS ONE 09/2011; 6(9):e24780. DOI:10.1371/journal.pone.0024780 · 3.53 Impact Factor
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    ABSTRACT: The intricately regulated Ras pathway coordinates multiple kit-ligand-induced mast cell functions, including chemotaxis, proliferation, and degranulation. However, the intracellular proteins that modulate the intensity and duration of stem cell factor-induced signals and the consequent cellular response are incompletely understood. Scaffolding proteins coordinate the spatial organization of mitogen-activated protein kinase proteins that may potentiate and/or inhibit cell functions. The kinase suppressor of Ras (KSR1) protein is known to function as a molecular scaffold and coordinates the organization of Raf/Mek/Erk in response to receptor tyrosine kinases. However, the impact of KSR1 in myeloid mast cell functions and in response to stem cell factor remains unknown. In the present study, we investigated the role of KSR1 in regulating cellular functions of bone marrow-derived mast cells of KSR1-deficient ((-/-)) mice. Genetic disruption of KSR1 resulted in both striking reductions in kit-ligand-mediated proliferation and degranulation, which are commonly attributed to mitogen-activated protein kinase signals. Surprisingly, disruption of the KSR1 scaffold also resulted in a decline in migration that is generally not linked to Raf-Erk signals. We found that loss of KSR1 does impact the biochemical activation of p21-activated kinase, a kinase that is known to modulate Raf-Erk signals and also F-actin polymerization key to mast cell migration. Collectively, these studies demonstrate that the scaffolding protein KSR1 has an important role in multiple kit-ligand-mediated mast cell functions. This study elucidates varied mast cell physiological functions for KSR1, including those related to cytoskeletal organization, and it suggests a novel molecular target for attenuating mast cell-mediated inflammation.
    Experimental hematology 07/2011; 39(10):969-76. DOI:10.1016/j.exphem.2011.06.009 · 2.81 Impact Factor
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    ABSTRACT: Skeletal abnormalities including scoliosis, tibial dysplasia, sphenoid wing dysplasia, and decreased bone mineral density (BMD) are associated with neurofibromatosis type 1 (NF1). We report the cellular phenotype of NF1 human-derived osteoclasts and compare the in vitro findings with the clinical phenotype. Functional characteristics (e.g., osteoclast formation, migration, adhesion, resorptive capacity) and cellular mechanistic alterations (e.g., F-actin polymerization, MAPK phosphorylation, RhoGTPase activity) from osteoclasts cultured from peripheral blood of individuals with NF1 (N = 75) were assessed. Osteoclast formation was compared to phenotypic, radiologic, and biochemical data. NF1 osteoprogenitor cells demonstrated increased osteoclast forming capacity. Human NF1-derived osteoclasts demonstrated increased migration, adhesion, and in vitro bone resorption. These activities coincided with increased actin belt formation and hyperactivity in MAPK and RhoGTPase pathways. Although osteoclast formation was increased, no direct correlation of osteoclast formation with BMD, markers of bone resorption, or the clinical skeletal phenotype was observed suggesting that osteoclast formation in vitro cannot directly predict NF1 skeletal phenotypes. While NF1 haploinsufficiency produces a generalized osteoclast gain-in-function and may contribute to increased bone resorption, reduced BMD, and focal skeletal defects associated with NF1, additional and perhaps local modifiers are likely required for the development of skeletal abnormalities in NF1.
    American Journal of Medical Genetics Part A 05/2011; 155A(5):1050-9. DOI:10.1002/ajmg.a.33965 · 2.05 Impact Factor
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    ABSTRACT: Neurofibromatosis type 1 (NF1) is a common autosomal dominant genetic disorder caused by mutation of the NF1 tumor suppressor gene. Spinal deformities are common skeletal manifestations in patients with NF1. To date, the mechanism of vertebral abnormalities remains unclear because of the lack of appropriate animal models for the skeletal manifestations of NF1. In the present study, we report a novel murine NF1 model, Nf1(flox/-);Col2.3Cre(+) mice. These mice display short vertebral segments. In addition, a significant reduction in cortical and trabecular bone mass of the vertebrae was observed in Nf1(flox/-);Col2.3Cre(+) mice as measured by dual-energy X-ray absorptiometry (DEXA) and peripheral quantitative computed tomography (pQCT). Peak stress and peak load were also significantly reduced in Nf1(flox/-);Col2.3Cre(+) mice as compared to controls. Furthermore, the lumbar vertebrae showed enlargement of the inter-vertebral canal, a characteristic feature of lumbar vertebrae in NF1 patients. Finally, histologic analysis demonstrated increased numbers of osteoclasts and decreased numbers of osteoblasts in the vertebrae of Nf1(flox/-);Col2.3Cre(+) mice in comparison to controls. In summary, Nf1(flox/-);Col2.3Cre(+) mice demonstrate multiple structural and functional abnormalities in the lumbar vertebrae which recapitulate the dystrophic vertebral changes in NF1 patients. This novel murine model provides a platform to understand the cellular and molecular mechanisms underlying the pathogenesis of spinal deficits in NF1 patients.
    Bone 03/2011; 48(6):1378-87. DOI:10.1016/j.bone.2011.03.760 · 4.46 Impact Factor
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    ABSTRACT: Neurofibromatosis type 1 (NF1) is a common genetic disorder and is characterized by both malignant and nonmalignant neurofibromas, which are composed of Schwann cells, degranulating mast cells, fibroblasts, and extracellular matrix. We and others have previously shown that hyperactivation of the c-Kit pathway in an Nf1 haploinsufficient microenvironment is required for both tumor formation and progression. Mast cells play a key role in both tumorigenesis and neoangiogenesis via the production of matrix metalloproteinases, heparin, and a range of different growth factors. In the present study, we show that tumorigenic Schwann cells derived from Nf1(-/-) embryos promote increased degranulation of Nf1(+/-) mast cells compared with wild-type mast cells via the secretion of the Kit ligand. Furthermore, we used genetic intercrosses as well as pharmacological agents to link the hyperactivation of the p21(Ras)-phosphatidylinositol 3-kinase (PI3K) pathway to the increased degranulation of Nf1(+/-) mast cells both in vitro and in vivo. These studies identify the p21(Ras)-PI3K pathway as a major regulator of the gain in Nf1(+/-) mast cell degranulation in neurofibromas. Collectively, these studies identify both c-Kit and PI3K as molecular targets that modulate mast cell functions in cases of NF1.
    American Journal Of Pathology 10/2010; 177(6):3125-32. DOI:10.2353/ajpath.2010.100369 · 4.60 Impact Factor
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    ABSTRACT: Interactions between tumorigenic cells and their surrounding microenvironment are critical for tumor progression yet remain incompletely understood. Germline mutations in the NF1 tumor suppressor gene cause neurofibromatosis type 1 (NF1), a common genetic disorder characterized by complex tumors called neurofibromas. Genetic studies indicate that biallelic loss of Nf1 is required in the tumorigenic cell of origin in the embryonic Schwann cell lineage. However, in the physiologic state, Schwann cell loss of heterozygosity is not sufficient for neurofibroma formation and Nf1 haploinsufficiency in at least one additional nonneoplastic lineage is required for tumor progression. Here, we establish that Nf1 heterozygosity of bone marrow-derived cells in the tumor microenvironment is sufficient to allow neurofibroma progression in the context of Schwann cell Nf1 deficiency. Further, genetic or pharmacologic attenuation of c-kit signaling in Nf1+/- hematopoietic cells diminishes neurofibroma initiation and progression. Finally, these studies implicate mast cells as critical mediators of tumor initiation.
    Cell 11/2008; 135(3):437-48. DOI:10.1016/j.cell.2008.08.041 · 33.12 Impact Factor
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    ABSTRACT: Neurofibromatosis type I (NF1) is a congenital disorder resulting from loss-of-function of the tumor suppressor gene, NF1, a GTPase-activating protein for p21ras. Fifty percent of NF1 patients have osseous manifestations including a high incidence of osteoporosis. Osteoclasts are specialized macrophage/monocyte lineage-derived cells that resorb bone and NF1 haploinsufficient osteoclasts have abnormal Ras-dependent bone resorption. Ras-regulated functions are in part mediated via the activation of small Rho family of GTPases including the Rac-GTPases. In the present study, we demonstrate that the Rho-GTPase Rac1 is a crucial Ras-mediated effector in Nf1 haploinsufficient (+/-) osteoclasts. Nf1+/- mice were intercrossed with conditional Rac1(flox/flox)Mxcre+ (Rac1-/-) mice to generate Nf1+/-; Rac1-/- mice. Genetic disruption of Rac1 restored the pathological increase in osteoclast progenitor cells in Nf1+/- mice and was sufficient to correct the increased Nf1+/- osteoclast motility and osteoclast belt formation, an f-actin structure observed in mature osteoclasts critical for bone resorption and lytic activity. Finally, we demonstrate that Nf1+/-; Rac1-/- osteoclasts have normalized Erk activation compared with Nf1+/- osteoclasts, a biochemical function critical for osteoclast formation, actin organization and motility. Collectively, these data demonstrate that Rac1 critically contributes to increased osteoclast function induced by haploinsufficiency of Nf1 and implicate Rac1 as a rational therapeutic target for osteoporosis.
    Human Molecular Genetics 05/2008; 17(7):936-48. DOI:10.1093/hmg/ddm366 · 6.68 Impact Factor
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    ABSTRACT: Neurofibromas are the clinical hallmark of neurofibromatosis Type 1 (NF1), a genetic disorder caused by mutations of the NF1 tumor suppressor gene, which encodes neurofibromin that functions as a GTPase activating protein (GAP) for Ras. During pregnancy, up to 50% of existing neurofibromas enlarge and as many as 60% of new neurofibromas appear for the first time. Lysophosphatidic acid (LPA) is a prototypic lysophospholipid that modulates cell migration and survival of Schwann cells (SCs) and is made in increasing concentrations throughout pregnancy. We addressed the influence of LPA on the biochemical and cellular functions of SCs with a homozygous mutation of the murine homologue of the NF1 gene (Nf1-/-). LPA promoted F-actin polymerization and increased migration and survival of Nf1-/- SCs as compared to wild type (WT) SCs. Furthermore, LPA induced a higher level of Ras-GTP and Akt phosphorylation in Nf1-/- SCs as compared to WT cells. Pharmacologic inhibition or siRNA for the p85beta regulatory subunit of Class I A PI3-K significantly reduced LPA-induced Schwann cell survival and migration. Introduction of NF1-GRD reconstitution was sufficient to normalize the LPA-mediated motility of Nf1-/- SCs. As LPA modulates excessive cell survival and motility of Nf1-/- SCs, which are the tumorigenic cells in NF1, targeting PI3-K may be a potential therapeutic approach in diminishing the development and progression of neurofibromas in pregnant women with NF1.
    Glia 04/2007; 55(5):527-36. DOI:10.1002/glia.20482 · 5.47 Impact Factor
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    ABSTRACT: Individuals with neurofibromatosis type 1 (NF1) have a high incidence of osteoporosis and osteopenia. However, understanding of the cellular and molecular basis of these sequelae is incomplete. Osteoclasts are specialized myeloid cells that are the principal bone-resorbing cells of the skeleton. We found that Nf1(+/-) mice contain elevated numbers of multinucleated osteoclasts. Both osteoclasts and osteoclast progenitors from Nf1(+/-) mice were hyperresponsive to limiting concentrations of M-CSF and receptor activator of NF-kappaB ligand (RANKL) levels. M-CSF-stimulated p21(ras)-GTP and Akt phosphorylation was elevated in Nf1(+/-) osteoclasts associated with gains of function in survival, proliferation, migration, adhesion, and lytic activity. These gains of function are associated with more severe bone loss following ovariectomy as compared with that in syngeneic WT mice. Intercrossing Nf1(+/-) mice and mice deficient in class 1(A) PI3K (p85alpha) restored elevated PI3K activity and Nf1(+/-) osteoclast functions to WT levels. Furthermore, in vitro-differentiated osteoclasts from NF1 patients also displayed elevated Ras/PI3K activity and increased lytic activity analogous to those in murine Nf1(+/-) osteoclasts. Collectively, our results identify a what we believe to be a novel cellular and biochemical NF1-haploinsufficient phenotype in osteoclasts that has potential implications for the pathogenesis of NF1 bone disease.
    Journal of Clinical Investigation 12/2006; 116(11):2880-91. DOI:10.1172/JCI29092 · 13.77 Impact Factor
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    ABSTRACT: Neurofibromas are common tumors found in neurofibromatosis type 1 (NF1) patients. These complex tumors are composed of Schwann cells, mast cells, fibroblasts and perineurial cells embedded in collagen that provide a lattice for tumor invasion. Genetic studies demonstrate that in neurofibromas, nullizygous loss of Nf1 in Schwann cells and haploinsufficiency of Nf1 in non-neuronal cells are required for tumorigenesis. Fibroblasts are a major cellular constituent in neurofibromas and are a source of collagen that constitutes approximately 50% of the dry weight of the tumor. Here, we show that two of the prevalent heterozygous cells found in neurofibromas, mast cells and fibroblasts interact directly to contribute to tumor phenotype. Nf1+/- mast cells secrete elevated concentrations of the profibrotic transforming growth factor-beta (TGF-beta). In response to TGF-beta, both murine Nf1+/- fibroblasts and fibroblasts from human neurofibromas proliferate and synthesize excessive collagen, a hallmark of neurofibromas. We also establish that the TGF-beta response occurs via hyperactivation of a novel Ras-c-abl signaling pathway. Genetic or pharmacological inhibition of c-abl reverses fibroblast proliferation and collagen synthesis to wild-type levels. These studies identify a novel molecular target to inhibit neurofibroma formation.
    Human Molecular Genetics 09/2006; 15(16):2421-37. DOI:10.1093/hmg/ddl165 · 6.68 Impact Factor
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    ABSTRACT: Genetic inactivation of tumor suppressor genes initiates human cancers. However, interaction of accessory cells with the tumor-initiating cell within the microenvironment is often required for tumor progression. This paradigm is relevant to understanding neurofibroma development in neurofibromatosis type I patients. Somatic inactivation of the Nf1 tumor suppressor gene, which encodes neurofibromin, is necessary but not sufficient to initiate neurofibroma development. In contrast, neurofibromas occur with high penetrance in mice in which Nf1 is ablated in Schwann cells in the context of a heterozygous mutant (Nf1+/-) microenvironment. Neurofibromas are highly vascularized, and recent studies suggest that Nf1+/- mice have increased angiogenesis in vivo. However, the function of neurofibromin in human endothelial cells (ECs) and the biochemical mechanism by which neurofibromin regulates neoangiogenesis are not known. Utilizing Nf1+/- mice, primary human ECs and endothelial progenitor cells harvested from NF1 patients, we identified a discrete Ras effector pathway, which alters the proliferation and migration of neurofibromin-deficient ECs in response to neurofibroma-derived growth factors both in vitro and in vivo. Thus, these studies identify a unique biochemical pathway in Nf1+/- ECs as a potential therapeutic target in the neurofibroma microenvironment.
    Human Molecular Genetics 07/2006; 15(11):1858-69. DOI:10.1093/hmg/ddl108 · 6.68 Impact Factor

Publication Stats

385 Citations
103.57 Total Impact Points

Institutions

  • 2006–2015
    • Indiana University-Purdue University Indianapolis
      • • Herman B Wells Center for Pediatric Research
      • • Department of Pediatrics
      Indianapolis, Indiana, United States
  • 2007
    • Indiana University-Purdue University School of Medicine
      • Pediatrics
      Indianapolis, Indiana, United States