Glenn A. Walter, Ph.D.
Phone: (352) 294-5996
Office: CTRB 2213
Education and Training/Previous Appointments
My lab focuses on the pathophysiology of muscle damage and the development of novel molecular and cellular imaging techniques. In addition, we develop stem-cell therapies and utilize viral delivery of therapeutic genes to mitigate muscle damage and restore the regenerative potential of dystrophic and atrophied muscle. The lab also studies the physiological effects of muscle atrophy and therapeutic gene transfer to both skeletal and cardiac muscle. Our research is funded by grants from the National Institutes of Health, National Science Foundation, the Howard Hughes Medical Institute, and the Muscular Dystrophy Association.
Bioengineering of Muscle Structure and Metabolism
Thermodynamic bioengineering with invertebrate genes. We have shown that arginine kinase (AK) can serve as a noninvasive monitoring system for viral mediated gene delivery. The general implication of these results are twofold. First arginine kinase can be used as a noninvasive marker for gene transfer in vertebrate skeletal muscles. Secondly, skeletal muscles that express a combination of creatine kinase and arginine kinase provide an extended thermodynamic buffering range. Thus the coexpression of AK and CK should prove beneficial to skeletal cells under conditions of prolonged ischemia or fatiguing conditions. Following the introduction of AK into the mammalian muscle cytoplasm a large PArg pool is expected to be formed without disturbing the normal levels of ATP or PCr. However, upon depletion of PCr which would occur in ischemia, creatine kinase can no longer buffer changes in ATP. At this point, the PArg pool will continue to buffer changes in ATP levels. In addition, the AK reaction will tend to slow the fall of pH.
Restoration of membrane integrity and sarcolemmal integrity. We use a combination of unsuppressed 1H-MRS, multi-echo and dynamic, macromolecule contrast enhanced MRI, to investigate the intrinsic properties that lead to changes in muscle T2 in dystrophy and following damage, ischemia, and exercise. Contraction-induced injury can be characterized using a variety of methods. However, many of the histological methods have the disadvantage they are invasive and require the animal to be sacrificed. MRI/MRS due to its noninvasive nature and short acquisition periods, can be used to follow both primary (0-3 day) and secondary (3-21 day) injury processes within the same mouse providing a tool for longitudinal studies in both control and dystrophic animals. Differences in T2 provide sensitive markers of muscle damage producing differences in image contrast with a spatial resolution on the order of that of a single muscle fiber. MRI for detection of muscle injury has the added advantage that large muscle volumes can be sampled instead of a small number of muscle fibers or muscles. This is particular important due to monitoring of gene transfer efficacy and expression in clinical settings currently requires invasive techniques. This can be problematic in subjects with extensive muscle damage and necrosis such as children with Duchene muscular dystrophy and LGMD.
Augmenting angiogenesis and perfusion in skeletal muscle. We are using MRI and MRS methods to measure the effectiveness of gene mediated collateral formation as a potential therapy for chronically ischemic muscle. We have found that sufficient signal-to-noise and spatial resolution is achievable at field strengths greater than 4T to examine heterogeneity in muscle perfusion. This work has been extended to study whether the lack of NOS in dystrophic muscle is associated with poor muscle perfusion and exercise ischemia in the murine and human muscular dystrophies. Animal models are used to test different molecule weight contrast agents as to the appropriateness for blood perfusion measures as confirmed by ex vivo microsphere methods.
Monitoring of Transgene Delivery and Cell Migration
Common techniques to monitor muscle stem-cell transplants typically rely on ex vivo genetic modification to allow expression of reporter genes. The statement of specific reporter genes allows for graft identification during post-mortem analysis. Using these conventional techniques, however, even simple and practical questions are difficult and labor-intensive to answer. For example, to determine the distribution of the graft, the entire organ must be harvested and sectioned, followed by identification of individual cells by conventional microscopy. More complex questions, such as cell homing, identification of migration events, and engraftment rates, may be impossible to accurately and quantitatively address using conventional microscopy. Novel techniques that allow non-invasive, continuous imaging of stem cell transplants have recently been proposed and evaluated in a limited number of cell delivery models. We have previously found that magnetic resonance (MR) methods can be used to noninvasively monitor the widespread expression of a MR marker gene (arginine kinase) and therapeutic genes for the muscular dystrophies and cardiomyopathies. On the other hand, cell-based therapies represent a greater challenge for noninvasive monitoring due to the variability and limited stem cell incorporation. We found that MR imaging (MRI) has the ability to provide extremely sensitive, high-resolution images of magnetically labeled cells in both skeletal and cardiac muscle. As such, the application of MRI of stem cell investigations is of great importance to enhance the development of stem cell therapies. We have evaluated the application of magnetically labeled stem cells for the noninvasive monitoring of therapeutic stem cell transplants in murine dystrophies, senescent muscle, and cardiac dysfunction. Additional studies have revealed that MRI can be implemented to track the migration of a small number of labeled cells following arterial delivery to regions of targeted gene expression and tissue damage. These MR labeling strategies are not limited to muscle applications but can be readily extended to the noninvasive cell tracking in the brain, liver, and retina in both small animal models and humans.
Diagnostic Optical Imaging for Duchenne Muscular Dystrophy
Related Research: http://cdmrp.army.mil/dmdrp/pbks/dmdrppbk2012.pdf
Currently offering post-doctoral positions in MR cell and molecular imaging. If interested please send a current biosketch and research statement to email@example.com
Sites Related to Our Research
- National High Field Magnet Lab (NHFML) Home Page: http://www.magnet.fsu.edu
- MBI IMAGING FACILITY (AMRIS): http://amris.mbi.ufl.edu/
- Powell Gene Therapy Center: http://www.gtc.ufl.edu
- Southeast Center for Integrated Metabolomics: http://www.secim.ufl.edu
Teaching (Including Courses)
- Cardiovascular and Muscle Physiology (GMS 6411)
- Fundamentals of Physiology and Functional Genomics 1 (GMS 6471)
- Physiology Journal Club (GMS 6491)
- Dental Physiology (DEN 5120C)
- Chrzanowski, S. M., Baligand, C., Willcocks, R. J., Deol, J., Schmalfuss, I., Lott, D. J., Daniels, M. J., Senesac, C., Walter, G. A., … Vandenborne, K. (2017). Multi-slice MRI reveals heterogeneity in disease distribution along the length of muscle in Duchenne muscular dystrophy. Acta myologica : myopathies and cardiomyopathies : official journal of the Mediterranean Society of Myology, 36(3), 151-162. (PMID: 29774305; PMCID: PMC5953226)
- Arora, H., Willcocks, R. J., Lott, D. J., Harrington, A. T., Senesac, C. R., Zilke, K. L., Daniels, M. J., Xu, D., Tennekoon, G. I., Finager, E. L., Russman, B. S., Finkel, R. S., Triplett, W. T., Byrne, B. J., Walter, G. A., Sweeney, H. L., … Vandenborne, K. (2018). Longitudinal timed function tests in Duchenne muscular dystrophy: Imagingdmd cohort natural history. Muscle & nerve, May 9. doi: 10.1002/mus.26161. (PMID: 29742798)
- Barnard, A. M., Willcocks, R. J., Finanger, E. L., Daniels, M. J., Triplett, W. T., Rooney, W. D., Lott, D. J., Forbes, S. C., Wang, D. J., Senesac, C. R., Harrington, A. T., Finkel, R. S., Russman, B. S., Byrne, B. J., Tennekoon, G. I., Walter, G. A., Sweeney, H. L., … Vandenborne, K. (2018). Skeletal muscle magnetic resonance biomarkers correlate with function and sentinel events in Duchenne muscular dystrophy. PloS one, 13(3), e0194283. doi:10.1371/journal.pone.0194283. (PMID: 29554116; PMCID: PMC5858773)
- Willcocks, R. J., Triplett, W. T., Lott, D. J., Forbes, S. C., Batra, A., Sweeney, H. L., Mendell, J. R., Vandenborne, K., … Walter, G. A. (2018). Leg muscle MRI in identical twin boys with duchenne muscular dystrophy. Muscle & nerve, Jan 24. doi: 10.1002/mus.26081. (PMID: 29365354; PMCID: PMC6057851)
- Chrzanowski, S. M., Vohra, R. S., Lee-McMullen, B. A., Batra, A., Spradlin, R. A., Morales, J., Forbes, S., Vandenborne, K., Barton, E. R., … Walter, G. A. (2017). Contrast-Enhanced Near-Infrared Optical Imaging Detects Exacerbation and Amelioration of Murine Muscular Dystrophy. Molecular imaging, 16, 1536012117732439. (PMID: 29271299; PMCID: PMC5985549)
- Costford, S. R., Brouwers, B., Hopf, M. E., Sparks, L. M., Dispagna, M., Gomes, A. P., Cornnell, H. H., Petucci, C., Phelan, P., Xie, H., Yi, F., Walter, G. A., Osborne, T. F., Sinclair, D. A., Mynatt, R. L., Ayala, J. E., Gardell, S. J., … Smith, S. R. (2017). Skeletal muscle overexpression of nicotinamide phosphoribosyl transferase in mice coupled with voluntary exercise augments exercise endurance. Molecular metabolism, 7, 1-11. (PMID: 29146412; PMCID: PMC5784330)
- Baligand, C., Todd, A. G., Lee-McMullen, B., Vohra, R. S., Byrne, B. J., Falk, D. J., & Walter, G. A. (2017). 13C/31P MRS Metabolic Biomarkers of Disease Progression and Response to AAV Delivery of hGAA in a Mouse Model of Pompe Disease. Molecular therapy. Methods & clinical development, 7, 42-49. doi:10.1016/j.omtm.2017.09.002. (PMID: 29018835; PMCID: PMC5626920)
- Vohra, R., Batra, A., Forbes, S. C., Vandenborne, K., & Walter, G. A. (2017). Magnetic Resonance Monitoring of Disease Progression in mdx Mice on Different Genetic Backgrounds. The American journal of pathology, 187(9), 2060-2070. (PMID: 28826559; PMCID: PMC5809503)
- Burakiewicz, J., Sinclair, C., Fischer, D., Walter, G. A., Kan, H. E., & Hollingsworth, K. G. (2017). Quantifying fat replacement of muscle by quantitative MRI in muscular dystrophy. Journal of neurology, 264(10), 2053-2067. (PMID: 28669118; PMCID: PMC5617883)
- Lee, C. H., Bengtsson, N., Chrzanowski, S. M., Flint, J. J., Walter, G. A., & Blackband, S. J. (2017). Magnetic Resonance Microscopy (MRM) of Single Mammalian Myofibers and Myonuclei. Scientific reports, 7, 39496. doi:10.1038/srep39496. (PMID: 28045071; PMCID: PMC5206738)