PolyP (a Procoagulant Molecule) and the Mechanisms of Its Degradation

Basic Science Poster

PolyP (a Procoagulant Molecule) and the Mechanisms of Its Degradation


Alma Mater

Author/Presenter: Carleigh F. F. Hebbard
Co-Author(s): Stephanie A. Smith, Yan Wang, Rachel Breitenfeld, Catherine J. Baker, Mark A. Klein, James H. Morrissey

We investigated the mechanisms by which inorganic polyphosphate (polyP) is degraded in vivo. PolyP is a chain of phosphate moieties linked through high-energy phosphoanhydride bonds, ranging in size from three- to thousands-of-phosphates long. Naturally, these molecular chains are stored within cellular organelles called acidocalcisomes: acidic, metal-filled compartments. Human platelets store polyP with metals in dense granules and, upon activation, release polyP into blood. Our laboratory has shown that polyP is potently procoagulant and plays important roles in hemostasis, thrombosis, and inflammation. PolyP’s half-life ex vivo human serum is ~90’. We therefore hypothesized that native serum endo- and exopolyphosphatases degraded polyP.

To test our hypothesis, we needed to develop high-throughput methods of measuring polyphosphatase activity because there existed a paucity of facile methods for detecting and quantifying different polyP degradation types. Adapting carboxylic acid chemistry, we created chromogenic and fluorogenic polyphosphatase substrates, allowing us to spectrophotometrically monitor the activities of commercially-available polyphosphatases in real time. With these substrates, we identified a new function of the clinically significant enzyme, Nudt2.

Using an assay previously developed in our lab, we found that most polyphosphate degradation in serum was metal-mediated, rather than enzyme-mediated. PolyP degradation in serum had the following characteristics: resistant to canonical phosphatase inhibitors, heat resistant, calcium dependent, and pH dependent. The reigning hypothesis has been that polyphosphate degradation were enzyme-catalyzed. Our findings may have large implications: Could acidocalcisome acidification be an evolutionary cellular adaption preventing calcium-mediated polyP degradation during times of non-stress? Additional work may be needed to investigate this possibility.

Bio: Carleigh Hebbard is an MD/PhD student in the Medical Scholars Program at University of Illinois. She currently is finishing her first year of medical school, having defended her doctoral work in 2015.  Carleigh studied blood clotting in Dr. Jim Morrissey’s lab, expanding our understanding of how inorganic polyphosphates—procoagulant molecules released from cells such as platelets—are regulatedin vivo.  Carleigh has been interested in the convergence of science and medicine since high school.  Carleigh finished her studies at Tulane University in 2009 with a BA in Spanish & Portuguese and a BS in Cell and Molecular Biology, and she earned her MS in Microbiology from UIUC in 2011.

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