The molecular basis of canavan disease : aspartoacylase enzyme characteristics /

by Hershfield, Jeremy Ray

Title of Dissertation: “The Molecular Basis of Canavan Disease: Aspartoacylase Enzyme Characteristics” Author: Jeremy Ray Hershfield, Ph.D., 2006 Dissertation Directed By: Aryan M.A. Namboodiri, Ph.D. Associate Professor Department of Anatomy, Physiology, and Genetics Mutations in the gene for aspartoacylase (EC 3.5.1.15; ASPA), which catalyzes deacetylation of N-acetyl-L-aspartate (NAA), correlate with Canavan Disease (CD), a neurodegenerative disorder usually fatal during childhood. Defective ASPA activity has been linked to characteristically elevated NAA levels in the urine of CD patients, and ASPA knockout mice and ASPA deletion rats display CD-like symptoms. While efforts have focused on treating CD, there is limited evidence to support ASPA protein regulation. The ASPA enzyme is thought to be cytoplasmic. In this dissertation, we used immunohistochemistry to show ASPA within nuclei of rat brain oligodendrocytes, in rat kidney proximal tubule cells, and in cultured mouse and rat oligodendrocytes. Subcellular fractionation analysis from wild-type rats revealed low enzyme activity against NAA in nuclear fractions. While two recent reports have indicated that ASPA is a dimer, size-exclusion chromatography of both nuclear and cytoplasmic fractions showed ASPA is an active monomer. Since ASPA is small enough to passively diffuse through the nuclear pore complex, we constructed, expressed, and detected in COS-7 cells a green fluorescent protein-human ASPA fusion protein. The mixed nucleariii cytoplasmic localization of GFP-hASPA demonstrated that the subcellular localization of ASPA is regulated. We then investigated regulation of the ASPA protein at the structural level. A recent alignment study identified ASPA as a member of the carboxypeptidase A (CPA) family. Therefore, we developed and tested a three-dimensional homology model of ASPA based on CPA. Mutations of the putative zinc-binding residues (H21G, E24D, and H116G), the general proton donor (E178A), and mutants designed to switch the order of the zinc-binding residues (H21E/E24H and E24H/H116E) were created and expressed in COS-7 cells. Each mutation yielded wild-type ASPA protein levels, but undetectable ASPA activity. Finally, the analysis of several CD-associated ASPA missense mutations provided a molecular basis for how mutations result in ASPA deficiency. In summary, we have shown that the ASPA enzyme is regulated by its subcellular localization, multimeric state, and innate structure. These studies provide insight into the physiology of the ASPA reaction and the molecular basis of CD. iv The Molecular Basis of Canavan Disease: Aspartoacylase Enzyme Characteristics By Jeremy Ray Hershfield Dissertation submitted to the Faculty of the Molecular and Cell Biology Program Uniformed Services University of the Health Sciences In partial fulfillment of the requirements for the degree of Doctor of Philosophy, 2006 v