Conifolds with Discrete Torsion and Noncommutativity

We study D3 branes at orbifolded conifold singularities in the presence of discrete torsion. The vacuum moduli space of open strings becomes non-commutative due to a deformation of the superpotential and is studied via the representation theory of the moduli algebra. It is also shown that the center of the moduli algebra correctly describes the underlying orbifolded conifolds. The field theory can be obtained by a marginal deformation of the ${\cal N} = 1$ gauge theory on D3 branes at conifold singularity, the global symmetry being broken from $SU(2) \times SU(2)$ to $U(1) \times U(1)$. By using the AdS/CFT correspondence we argue that the marginal deformation is related to massless KK modes of NSNS and RR two form reduced on the compact space $T^{1,1}$. We build a $T^2$ fibration of $T^{1,1}$ and show that a D3 brane in the bulk correspond to a D5 brane on the $T^2$ fibre. We also discuss the possible brane construction of the system.Comment: Latex, 24 pages, v2: small change

Essays on Financial Crisis.

My dissertation examines the effects of economic shocks on acquisition outcomes and the sources of housing market bubble. The first essay investigates how the combined effects of target firm- and industry-level distress affect acquisition outcomes through the fire-sale channel. I show that distressed targets are sold at discounts when the target industry is in distress. Consistent with the Shleifer and Vishny model, the fire-sale effects cause distressed targets to be sold to industry outsiders and acquirers to gain higher return by exploiting target's weakened bargaining power. I further demonstrate the fire-sale effects in acquisitions by showing that these findings are stronger for targets with acquirers that are in different industries or where targets have high industry asset-specificity. I then examine the contagion effects of fire-sale acquisitions on target rivals in the same industry. I find that rivals earn negative abnormal returns at the announcement due to negative information from fire-sale acquisitions. Overall, the results show that the fire-sale discount in distressed target acquisitions is an important determinant of financial distress costs of a firm and contributes to industry-specific contagion of economic shocks. In the second essay, I explore (with a coauthor) whether state-level variation in recourse mortgage laws affects housing prices and mortgage lending. In a state with non-recourse mortgage law, borrowers have limited liability on their mortgage loan. We find that non-recourse law results in larger bubbles in housing prices, and identify the causal effects by comparing housing prices in contiguous border county-pairs in the United States and examine discontinuities at state borders. We also explore whether mortgage lending behavior in non-recourse states reflects anticipation of additional risk. We find that loan-to-value ratio is lower and mortgage interest rate and loan denial rate are higher in non-recourse states, which suggest that lenders are aware of additional risk in non-recourse loans. However, we find that because the emergence of the originate-to-distribute (OTD) model in the housing markets enables lenders to effectively shift the risks to other investors, mortgage lending behavior does not fully reflect the higher risk.PhDBusiness AdministrationUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108833/1/sjoonoh_1.pd

Open-Closed String Duality from Orbifolded Conifolds

We study large N dualities for a class of ${\cal N} = 1$ theories realized on type IIB D5 branes wrapping 2-cycles of local Calabi-Yau threefolds which is obtained from resolving orbifolded conifolds or as effective field theories on D4 branes in type IIA brane configurations. The field theory is $\mathcal{N} =1$ supersymmetric $\prod U(N_{ij})$ Yang-Mills gauge theory. Strong coupling effects are analyzed in the deformed geometry. We propose open-closed string duality via a geometric transition in toric geometry. The T-dual type IIA picture and M-theory lifting are also considered.Comment: arXiv admin note: text overlap with arXiv:hep-th/0105066 by other author

Formation of Five-Dimensional String Solutions from the Gravitational Collapse

We study the formation of five-dimensional string solutions including the Gregory-Laflamme (GL) black string, the Kaluza-Klein (KK) bubble, and the geometry with a naked singularity from the gravitational collapse. The interior solutions of five-dimensional Einstein equations describe collapsing non-isotropic matter clouds. It is shown that the matter cloud always forms the GL black string solution while the KK bubble solution cannot be formed. The numerical study seems to suggest that the collapsing matter forms the geometries with timelike naked curvature singularities, which should be taken cautiously as the general relativity is not reliable in the strong curvature regime.Comment: 17 pages, 10 figures, LaTeX, to appear in Class. Quant. Grav., a appendix and some discussions added, title change

Investigation of Responsiveness to Thyrotropin-Releasing Hormone in Growth Hormone-Producing Pituitary Adenomas

Objective. The aim of this study was to investigate how the paradoxical response of GH secretion to TRH changes according to tumor volumes. Methods. Patients with newly diagnosed acromegaly were classified as either TRH responders or nonresponders according to the results of a TRH stimulation test (TST), and their clinical characteristics were compared according to responsiveness to TRH and tumor volumes. Results. A total of 41 acromegalic patients who underwent the TST were included in this study. Between TRH responders and nonresponders, basal GH, IGF-I levels, peak GH levels, and tumor volume were not significantly different, but the between-group difference of GH levels remained near significant over the entire TST time. during the TST were significantly different according to the responsiveness to TRH. Peak GH levels and during the TST showed significantly positive correlations with tumor volume with higher levels in macroadenomas than in microadenomas. GH levels over the entire TST time also remained significantly higher in macroadenomas than in microadenomas. Conclusion. Our data demonstrated that the paradoxical response of GH secretion to TRH in GH-producing pituitary adenomas was not inversely correlated with tumor volumes

MDGA1 negatively regulates amyloid precursor protein-mediated synapse inhibition in the hippocampus

Abstract Balanced synaptic inhibition, controlled by multiple synaptic adhesion proteins, is critical for proper brain function. MDGA1 (meprin, A-5 protein, and receptor protein-tyrosine phosphatase mu [MAM] domain-containing glycosylphosphatidylinositol anchor protein 1) suppresses synaptic inhibition in mammalian neurons, yet the molecular mechanisms underlying MDGA1-mediated negative regulation of GABAergic synapses remain unresolved. Here, we show that the MDGA1 MAM domain directly interacts with the extension domain of amyloid precursor protein (APP). Strikingly, MDGA1-mediated synaptic disinhibition requires the MDGA1 MAM domain and is prominent at distal dendrites of hippocampal CA1 pyramidal neurons. Down-regulation of APP in presynaptic GABAergic interneurons specifically suppressed GABAergic, but not glutamatergic, synaptic transmission strength and inputs onto both the somatic and dendritic compartments of hippocampal CA1 pyramidal neurons. Moreover, APP deletion manifested differential effects in somatostatin- and parvalbumin-positive interneurons in the hippocampal CA1, resulting in distinct alterations in inhibitory synapse numbers, transmission, and excitability. The infusion of MDGA1 MAM protein mimicked postsynaptic MDGA1 gain-of-function phenotypes that involve the presence of presynaptic APP. The overexpression of MDGA1 wild type or MAM, but not MAM-deleted MDGA1, in the hippocampal CA1 impaired novel object-recognition memory in mice. Thus, our results establish unique roles of APP-MDGA1 complexes in hippocampal neural circuits, providing unprecedented insight into trans-synaptic mechanisms underlying differential tuning of neuronal compartment-specific synaptic inhibition.Peer reviewe

Autoimmune Hypoglycemia in a Patient with Characterization of Insulin Receptor Autoantibodies

BackgroundType B insulin resistance syndrome is a manifestation of autoantibodies to the insulin receptor that results in severe hyperglycemia and acanthosis nigricans. However, the mechanisms by which these autoantibodies induce hypoglycemia are largely unknown. In this paper, we report the case of patient with type B insulin resistance syndrome who presented with frequent severe fasting hypoglycemia and acanthosis nigricans.MethodsTo evaluate the mechanism of hypoglycemia, we measured the inhibition of insulin binding to erythrocytes and IM9 lymphocytes in a sample of the patient's dialyzed serum before and after immunosuppressive therapy.ResultsIn the patient's pre-treatment serum IgG, the binding of 125I-insulin to erythrocytes was markedly inhibited in a dose-dependent manner until the cold insulin level reached 10-9 mol/L. We also observed dose-dependent inhibition of insulin binding to IM9 lymphocytes, which reached approximately 82% inhibition and persisted even when diluted 1:20. After treatment with glucocorticoids, insulin-erythrocyte binding activity returned to between 70% and 80% of normal, while the inhibition of insulin-lymphocyte binding was reduced by 17%.ConclusionWe treated a patient with type B insulin resistance syndrome showing recurrent fasting hypoglycemia with steroids and azathioprine. We characterized the patient's insulin receptor antibodies by measuring the inhibition of insulin binding

Clinical Experience of an Iontophoresis Based Glucose Measuring System

Currently finger pricking is the common method of blood glucose measurement in patients with diabetes mellitus. However, diabetes patients have proven to be reluctant to check their glucose profiles regularly because of the discomfort associated with this technique. Recently, a non-invasive and continuous Reverse Iontophoresis based Glucose Monitoring Device (RIGMD) was developed in Korea. The study was conducted during the period November 2003-January 2004 on 19 in-patients. Glucose measurements were performed using RIGMD between 10 a.m. and 4 p.m. Concurrent plasma glucose levels were checked hourly and subsequently compared with RIGMD data. The mean error of RIGMD measurements was -3.45±52.99 mg/dL with a mean absolute relative error of 20±15.16%. Measurements obtained by RIGMD were correlated with plasma glucose levels (correlation coefficient; 0.784 (p<0.05)) and this correlation was independent of time of data collection. However, after excluding confounding variables this correlation coefficient exhibited a tendency to increase. 98.9% of the results were clinically acceptable by Clarke error grid analysis. We concluded that RIGMD does not have the reliability and accuracy required to wholly replace conventional methods. However, further technical advancements that reduce its shortcomings would make this device useful for the management of diabetes