Publications by authors named "Gregory Simchick"

10 Publications

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Detecting functional connectivity disruptions in a translational pediatric traumatic brain injury porcine model using resting-state and task-based fMRI.

Sci Rep 2021 Jun 11;11(1):12406. Epub 2021 Jun 11.

Department of Physics and Astronomy, Franklin College of Arts and Sciences, University of Georgia, 500 D.W. Brooks Drive Rm 119, Athens, GA, 30602, USA.

Functional magnetic resonance imaging (fMRI) has significant potential to evaluate changes in brain network activity after traumatic brain injury (TBI) and enable early prognosis of potential functional (e.g., motor, cognitive, behavior) deficits. In this study, resting-state and task-based fMRI (rs- and tb-fMRI) were utilized to examine network changes in a pediatric porcine TBI model that has increased predictive potential in the development of novel therapies. rs- and tb-fMRI were performed one day post-TBI in piglets. Activation maps were generated using group independent component analysis (ICA) and sparse dictionary learning (sDL). Activation maps were compared to pig reference functional connectivity atlases and evaluated using Pearson spatial correlation coefficients and mean ratios. Nonparametric permutation analyses were used to determine significantly different activation areas between the TBI and healthy control groups. Significantly lower Pearson values and mean ratios were observed in the visual, executive control, and sensorimotor networks for TBI piglets compared to controls. Significant differences were also observed within several specific individual anatomical structures within each network. In conclusion, both rs- and tb-fMRI demonstrate the ability to detect functional connectivity disruptions in a translational TBI piglet model, and these disruptions can be traced to specific affected anatomical structures.
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http://dx.doi.org/10.1038/s41598-021-91853-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8196021PMC
June 2021

Engineered glycomaterial implants orchestrate large-scale functional repair of brain tissue chronically after severe traumatic brain injury.

Sci Adv 2021 Mar 5;7(10). Epub 2021 Mar 5.

Regenerative Bioscience Center, University of Georgia, Athens, GA 30602, USA.

Severe traumatic brain injury (sTBI) survivors experience permanent functional disabilities due to significant volume loss and the brain's poor capacity to regenerate. Chondroitin sulfate glycosaminoglycans (CS-GAGs) are key regulators of growth factor signaling and neural stem cell homeostasis in the brain. However, the efficacy of engineered CS (eCS) matrices in mediating structural and functional recovery chronically after sTBI has not been investigated. We report that neurotrophic factor functionalized acellular eCS matrices implanted into the rat M1 region acutely after sTBI significantly enhanced cellular repair and gross motor function recovery when compared to controls 20 weeks after sTBI. Animals subjected to M2 region injuries followed by eCS matrix implantations demonstrated the significant recovery of "reach-to-grasp" function. This was attributed to enhanced volumetric vascularization, activity-regulated cytoskeleton (Arc) protein expression, and perilesional sensorimotor connectivity. These findings indicate that eCS matrices implanted acutely after sTBI can support complex cellular, vascular, and neuronal circuit repair chronically after sTBI.
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http://dx.doi.org/10.1126/sciadv.abe0207DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7935369PMC
March 2021

Exploring the predictive value of lesion topology on motor function outcomes in a porcine ischemic stroke model.

Sci Rep 2021 Feb 15;11(1):3814. Epub 2021 Feb 15.

Regenerative Bioscience Center, University of Georgia, Athens, GA, USA.

Harnessing the maximum diagnostic potential of magnetic resonance imaging (MRI) by including stroke lesion location in relation to specific structures that are associated with particular functions will likely increase the potential to predict functional deficit type, severity, and recovery in stroke patients. This exploratory study aims to identify key structures lesioned by a middle cerebral artery occlusion (MCAO) that impact stroke recovery and to strengthen the predictive capacity of neuroimaging techniques that characterize stroke outcomes in a translational porcine model. Clinically relevant MRI measures showed significant lesion volumes, midline shifts, and decreased white matter integrity post-MCAO. Using a pig brain atlas, damaged brain structures included the insular cortex, somatosensory cortices, temporal gyri, claustrum, and visual cortices, among others. MCAO resulted in severely impaired spatiotemporal gait parameters, decreased voluntary movement in open field testing, and higher modified Rankin Scale scores at acute timepoints. Pearson correlation analyses at acute timepoints between standard MRI metrics (e.g., lesion volume) and functional outcomes displayed moderate R values to functional gait outcomes. Moreover, Pearson correlation analyses showed higher R values between functional gait deficits and increased lesioning of structures associated with motor function, such as the putamen, globus pallidus, and primary somatosensory cortex. This correlation analysis approach helped identify neuroanatomical structures predictive of stroke outcomes and may lead to the translation of this topological analysis approach from preclinical stroke assessment to a clinical biomarker.
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http://dx.doi.org/10.1038/s41598-021-83432-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7884696PMC
February 2021

Reactive Oxygen Species-Triggered Dissociation of a Polyrotaxane-Based Nanochelator for Enhanced Clearance of Systemic and Hepatic Iron.

ACS Nano 2021 01 30;15(1):419-433. Epub 2020 Dec 30.

Department of Pharmaceutical & Biomedical Sciences, College of Pharmacy, University of Georgia, Athens, Georgia 30602, United States.

Chronic blood transfusions are used to alleviate anemic symptoms in thalassemia and sickle cell anemia patients but can eventually result in iron overload (IO) and subsequently lead to severe oxidative stress in cells and tissues. Deferoxamine (DFO) is clinically approved to treat transfusional IO, but the use of the iron chelator is hindered by nonspecific toxicity and poor pharmacokinetic (PK) properties in humans, resulting in the need to administer the drug long-term infusion regimens that can often lead to poor patient compliance. Herein, a nanochelator system that uses the characteristic IO physiological environment to dissociate was prepared through the incorporation of DFO and reactive oxygen species (ROS)-sensitive thioketal groups into an α-cyclodextrin-based polyrotaxane platform (rPR-DFO). ROS-induced dissociation of this nanochelator (. 10 nm) into constructs averaging 2 nm in diameter significantly increased urine and fecal elimination of excess iron . In addition to significantly improved PK properties, rPR-DFO was well-tolerated in mice and no adverse side effects were noted in single high dose or multiple dose acute toxicity studies. The overall features of rPR-DFO as a promising system for iron chelation therapy can be attributed to a combination of the nanochelator's improved PK, favorable distribution to the liver, and ROS-induced dissociation properties into constructs <6 nm for faster renal elimination. This ROS-responsive nanochelator design may serve as a promising alternative for safely prolonging the circulation of DFO and more rapidly eliminating iron chelates from the body in iron chelation therapy regimens requiring repeated dosing of nanochelators.
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http://dx.doi.org/10.1021/acsnano.0c01083DOI Listing
January 2021

Surfen-mediated blockade of extratumoral chondroitin sulfate glycosaminoglycans inhibits glioblastoma invasion.

FASEB J 2019 11 9;33(11):11973-11992. Epub 2019 Aug 9.

Regenerative Bioscience Center, University of Georgia, Athens, Georgia, USA.

Invasive spread of glioblastoma (GBM) is linked to changes in chondroitin sulfate (CS) proteoglycan (CSPG)-associated sulfated glycosaminoglycans (GAGs) that are selectively up-regulated in the tumor microenvironment (TME). We hypothesized that inhibiting CS-GAG signaling in the TME would stem GBM invasion. Rat F98 GBM cells demonstrated enhanced preferential cell invasion into oversulfated 3-dimensional composite of CS-A and CS-E [4- and 4,6-sulfated CS-GAG (COMP)] matrices compared with monosulfated (4-sulfated) and unsulfated hyaluronic acid matrices in microfluidics-based choice assays, which is likely influenced by differential GAG receptor binding specificities. Both F98 and human patient-derived glioma stem cells (GSCs) demonstrated a high degree of colocalization of the GSC marker CD133 and CSPGs. The small molecule sulfated GAG antagonist bis-2-methyl-4-amino-quinolyl-6-carbamide (surfen) reduced invasion and focal adhesions in F98 cells encapsulated in COMP matrices and blocked CD133 and antichondroitin sulfate antibody (CS-56) detection of respective antigens in F98 cells and human GSCs. Surfen-treated F98 cells down-regulated CSPG-binding receptor transcripts and protein, as well as total and activated ERK and protein kinase B. Lastly, rats induced with frontal lobe tumors and treated with a single intratumoral dose of surfen demonstrated reduced tumor burden and spread compared with untreated controls. These results present a first demonstration of surfen as an inhibitor of sulfated GAG signaling to stem GBM invasion.-Logun, M. T., Wynens, K. E., Simchick, G., Zhao, W., Mao, L., Zhao, Q., Mukherjee, S., Brat, D. J., Karumbaiah, L. Surfen-mediated blockade of extratumoral chondroitin sulfate glycosaminoglycans inhibits glioblastoma invasion.
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http://dx.doi.org/10.1096/fj.201802610RRDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6902699PMC
November 2019

Pig Brains Have Homologous Resting-State Networks with Human Brains.

Brain Connect 2019 09 24;9(7):566-579. Epub 2019 Jun 24.

Bio-Imaging Research Center, University of Georgia, Athens, Georgia.

Many neurological and psychiatric diseases in humans are caused by disruptions to large-scale functional properties of the brain, including functional connectivity. There has been growing interest in discovering the functional organization of brain networks in larger animal models. As a result, the use of translational pig models in neuroscience has significantly increased in the past decades. The gyrencephalic pig brain resembles the human brain more in anatomy, growth, and development than the brains of commonly used small laboratory animals such as rodents. In this work, resting-state functional magnetic resonance imaging (rs-fMRI) and diffusion tensor imaging (DTI) data were acquired from a group of pigs ( = 12). rs-fMRI data were analyzed for resting-state networks (RSNs) by using independent component analysis and sparse dictionary learning. Six RSNs (executive control, cerebellar, sensorimotor, visual, auditory, and default mode) were detected that resemble their counterparts in human brains, as measured by Pearson spatial correlations and mean ratios. Supporting evidence of the validity of these RSNs was provided through the evaluation and quantification of structural connectivity measures (mean diffusivity, fractional anisotropy, fiber length, and fiber density) estimated from the DTI data. This study shows that as a translational, large animal model, pigs demonstrate great potential for mapping connectome-scale functional connectivity in experimental modeling of human brain disorders.
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http://dx.doi.org/10.1089/brain.2019.0673DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6727477PMC
September 2019

Fat spectral modeling on triglyceride composition quantification using chemical shift encoded magnetic resonance imaging.

Magn Reson Imaging 2018 10 19;52:84-93. Epub 2018 Jun 19.

Physics and Astronomy, University of Georgia, Athens, GA, United States; Bio-Imaging Research Center, University of Georgia, Athens, GA, United States. Electronic address:

Purpose: To explore, at a high field strength of 7T, the performance of various fat spectral models on the quantification of triglyceride composition and proton density fat fraction (PDFF) using chemical-shift encoded MRI (CSE-MRI).

Methods: MR data was acquired from CSE-MRI experiments for various fatty materials, including oil and butter samples and in vivo brown and white adipose mouse tissues. Triglyceride composition and PDFF were estimated using various a priori 6- or 9-peak fat spectral models. To serve as references, NMR spectroscopy experiments were conducted to obtain material specific fat spectral models and triglyceride composition estimates for the same fatty materials. Results obtained using the spectroscopy derived material specific models were compared to results obtained using various published fat spectral models.

Results: Using a 6-peak fat spectral model to quantify triglyceride composition may lead to large biases at high field strengths. When using a 9-peak model, triglyceride composition estimations vary greatly depending on the relative amplitudes of the chosen a priori spectral model, while PDFF estimations show small variations across spectral models. Material specific spectroscopy derived spectral models produce estimations that better correlate with NMR spectroscopy estimations in comparison to those obtained using non-material specific models.

Conclusion: At a high field strength of 7T, a material specific 9-peak fat spectral model, opposed to a widely accepted or generic human liver model, is necessary to accurately quantify triglyceride composition when using CSE-MRI estimation methods that assume the spectral model to be known as a priori information. CSE-MRI allows for the quantification of the spatial distribution of triglyceride composition for certain in vivo applications. Additionally, PDFF quantification is shown to be independent of the chosen a priori spectral model, which agrees with previously reported results obtained at lower field strengths (e.g. 3T).
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http://dx.doi.org/10.1016/j.mri.2018.06.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6537901PMC
October 2018

Assessment of MR-based R2* and quantitative susceptibility mapping for the quantification of liver iron concentration in a mouse model at 7T.

Magn Reson Med 2018 11 25;80(5):2081-2093. Epub 2018 Mar 25.

Physics and Astronomy, University of Georgia, Athens, Georgia.

Purpose: To assess the feasibility of quantifying liver iron concentration (LIC) using R2* and quantitative susceptibility mapping (QSM) at a high field strength of 7 Tesla (T).

Methods: Five different concentrations of Fe-dextran were injected into 12 mice to produce various degrees of liver iron overload. After mice were sacrificed, blood and liver samples were harvested. Ferritin enzyme-linked immunosorbent assay (ELISA) and inductively coupled plasma mass spectrometry were performed to quantify serum ferritin concentration and LIC. Multiecho gradient echo MRI was conducted to estimate R2* and the magnetic susceptibility of each liver sample through complex nonlinear least squares fitting and a morphology enabled dipole inversion method, respectively.

Results: Average estimates of serum ferritin concentration, LIC, R2*, and susceptibility all show good linear correlations with injected Fe-dextran concentration; however, the standard deviations in the estimates of R2* and susceptibility increase with injected Fe-dextran concentration. Both R2* and susceptibility measurements also show good linear correlations with LIC (R  = 0.78 and R  = 0.91, respectively), and a susceptibility-to-LIC conversion factor of 0.829 ppm/(mg/g wet) is derived.

Conclusion: The feasibility of quantifying LIC using MR-based  R2* and QSM at a high field strength of 7T is demonstrated. Susceptibility quantification, which is an intrinsic property of tissues and benefits from being field-strength independent, is more robust than R2* quantification in this ex vivo study. A susceptibility-to-LIC conversion factor is presented that agrees relatively well with previously published QSM derived results obtained at 1.5T and 3T.
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http://dx.doi.org/10.1002/mrm.27173DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6107404PMC
November 2018

Tracking and Quantification of Magnetically Labeled Stem Cells using Magnetic Resonance Imaging.

Adv Funct Mater 2016 Jun 17;26(22):3899-3915. Epub 2016 Feb 17.

Bioimaging Research Center, Regenerative Bioscience Center, and Department of Physics University of Georgia, Athens, GA. 30602, USA.

Stem cell based therapies have critical impacts on treatments and cures of diseases such as neurodegenerative or cardiovascular disease. In vivo tracking of stem cells labeled with magnetic contrast agents is of particular interest and importance as it allows for monitoring of the cells' bio-distribution, viability, and physiological responses. Herein, recent advances are introduced in tracking and quantification of super-paramagnetic iron oxide (SPIO) nanoparticles-labeled cells with magnetic resonance imaging, a noninvasive approach that can longitudinally monitor transplanted cells. This is followed by recent translational research on human stem cells that are dual-labeled with green fluorescence protein (GFP) and SPIO nanoparticles, then transplanted and tracked in a chicken embryo model. Cell labeling efficiency, viability, and cell differentiation are also presented.
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http://dx.doi.org/10.1002/adfm.201504444DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5526633PMC
June 2016

Zika Virus Induced Mortality and Microcephaly in Chicken Embryos.

Stem Cells Dev 2016 11 17;25(22):1691-1697. Epub 2016 Oct 17.

5 Department of Animal and Dairy Science, Regenerative Bioscience Center, College of Agriculture and Environmental Science, University of Georgia , Athens, Georgia .

The explosive spread of the Zika virus (ZIKV) through South and Central America has been linked to an increase in congenital birth defects, specifically microcephaly. Representative rodent models for investigating infections include direct central nervous system (CNS) injections late in pregnancy and transplacental transmission in immunodeficient mice. Microcephaly in humans may be the result of infection occurring early in pregnancy, therefore recapitulating that the human course of ZIKV infection should include normal embryo exposed to ZIKV during the first trimester. In ovo development of the chicken embryo closely mirrors human fetal neurodevelopment and, as a comparative model, could provide key insights into both temporal and pathophysiological effects of ZIKV. Chick embryos were directly infected early and throughout incubation with ZIKV isolated from a Mexican mosquito in January 2016. High doses of virus caused embryonic lethality. In a subset of lower dosed embryos, replicating ZIKV was present in various organs, including the CNS, throughout development. Surviving ZIKV-infected embryos presented a microcephaly-like phenotype. Chick embryos were longitudinally monitored by magnetic resonance imaging that documented CNS structural malformations, including enlarged ventricles (30% increase) and stunted cortical growth (decreased telencephalon by 18%, brain stem by 32%, and total brain volume by 18%), on both embryonic day 15 (E15) and E20 of development. ZIKV-induced microcephaly was observed with inoculations of as few as 2-20 viral particles. The chick embryo model presented ZIKV embryonic lethal effects and progressive CNS damage similar to microcephaly.
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http://dx.doi.org/10.1089/scd.2016.0231DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6453490PMC
November 2016
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