Dr. Johnson received her A.B. degree from Harvard University and her M.D. degree from the University of California, San Francisco. After residency training and a year of chief residency the University of Washington in Seattle, she returned to UCSF for her Pulmonary/Critical Care Medicine Fellowship and joined the faculty in 2004. Her major academic activities include basic science research, clinical teaching, and patient care. She is an attending physician in the Intensive Care Unit and on the Pulmonary Consult Service and Outpatient Chest Clinic at the San Francisco VA Medical Center.
Lung fluid balance is critical for efficient gas exchange. Alveolar flooding, which may occur in a variety of different pathological states, disturbs normal lung fluid balance, impairs gas exchange, and can lead to significant morbidity and mortality. Fluid balance is governed by ion transport across the alveolar epithelium, which lines more than 99% of the internal surface area of the lung, and is composed of alveolar type I and type II cells. Type II cells, which cover 2-5% of the surface area, have been extensively studied and are known to contain ion transport proteins. Type I cells, which cover the remaining 95% of the internal surface area, have been less well characterized, but were long thought to not be involved in ion transport. Thus the general theory of ion and fluid movement in the lung was that type II cells govern ion transport while type I cells only provide a route for passive water movement. My research focuses on the study of ion and fluid transport in the lung, with an emphasis on ion transport in alveolar type I cells. We have demonstrated that type I cells transport ions and contain the molecular machinery for active ion transport, necessitating modification of the current paradigm of ion and fluid transport in the lung to now state that ion transport occurs across the entire alveolar surface, rather than being limited to type II cells.
Our research goals are to continue the characterization of Na+ and Cl- transport in type I cells, determine what factors modulate ion transport in type I cells, and attempt to elucidate the specific contribution of type I (and type II) cells to total ion and fluid transport in the lung. Better understanding of how the lung maintains and regulates fluid balance will help delineate the mechanisms that regulate alveolar fluid balance and may ultimately provide the basis for developing strategies to prevent or treat the respiratory compromise associated with alveolar flooding.
Dobbs LG, Johnson MD. Alveolar epithelial transport in the adult lung.
Respir Physiol Neurobiol. 2007 Jun 29; [Epub ahead of print].
Johnson MD. Ion transport in alveolar type I cells. Mol Biosyst. 2007 Mar 3 (3): 178-86.
Adenosine regulation of alveolar fluid clearance. Factor P, Mutlu GM, Chen L, Mohameed J, Akhmedov AT, Meng FJ, Jilling T, Lewis ER, Johnson MD, Xu A, Kass D, Martino JM, Bellmeyer A, Albazi JS, Emala C, Lee HT, Dobbs LG, Matalon S.Proc Natl Acad Sci U S A. 2007 Mar 6; 104(10):4083-8.
Johnson M, Bao H, Helms M, Chen X, Tigue Z, Jain L, Dobbs L, Eaton D. Functional ion channels in pulmonary alveolar type I cells support a role for type I cells in lung ion transport. Proc. Natl. Acad. Sci. USA 2006 Mar 28; 103(13):4964-9.
Johnson MD, Brunetta PG. Signs, symptoms and laboratory abnormalities in pulmonary medicine. In: Wachter RM, Goldman L, Hollander H (Eds.). Hospital Medicine. Lippincott-Williams & Wilkins 2005.
Johnson M, Widdicombe J, Allen L, Barbry P, Dobbs L. Alveolar epithelial type I cells actively transport sodium and are likely to play a role in mediating lung liquid homeostasis. Proc. Natl. Acad. Sci. USA 2002 Feb 19; 99(4):1966-71
Johnson M. Pneumothorax. In: Lazarus SC (Ed.). PocketMedicine/IM-Pulmonary Medicine, PocketMedicine.com 2002.
Andress DL, Ozuna J, Tirschwell D, Grande L, Johnson M, Jacobson AF, Spain W. Antiepileptic drug-induced bone loss in young male patients who have seizures. Arch. Neurol.; 59(5):781-6, 2002.
Johnson MD, Popowski J, Cao G-J, Shen P, Sarkar N. Bacteriophage T7 mRNA is polyadenylated. Mol. Micro. 27, 23-30, 1998.
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