Abstract
CD73 is a bifunctional glycosylphosphatidylinositol (GPI)-anchored membrane protein which functions as ecto-5′-nucleotidase and a membrane receptor for extracellular matrix protein (ECM). A large body of evidence demonstrates a critical involvement of altered purine metabolism and particularly, increased expression of CD73 in a number of human disorders, including cancer and immunodeficiency. Massive up-regulation of CD73 was also found in reactive astrocytes in several experimental models of human neuropathologies. In all the pathological contexts studied so far, the increased expression of CD73 has been associated with the altered ability of cells to adhere and/or migrate. Thus, we hypothesized that increased expression of CD73 in reactive astrocytes has a role in the process of astrocyte adhesion and migration. In the present study, the involvement of CD73 in astrocyte migration was investigated in the scratch wound assay (SW), using primary astrocyte culture prepared from neonatal rat cortex. The cultures were treated with one of the following pharmacological inhibitors which preferentially target individual functions of CD73: (a) α,β-methylene ADP (APCP), which inhibits the catalytic activity of CD73 (b) polyclonal anti-CD73 antibodies, which bind to the internal epitope of CD73 molecule and mask their surface exposure and (c) small interfering CD73-RNA (siCD73), which silences the expression of CD73 gene. It was concluded that approaches that reduce surface expression of CD73 increase migration velocity and promote wound closure in the scratch wound assay, while inhibition of the enzyme activity by APCP induces redistribution of CD73 molecules at the cell surface, thus indirectly affecting cell adhesion and migration. Application of anti-CD73 antibodies induces a decrease in CD73 activity and membrane expression, through CD73 molecules shedding and their release to the culture media. In addition, all applied pharmacological inhibitors differentially affect other aspects of astrocyte function in vitro, including reduced cell proliferation, altered expression of adenosine receptors and increased expression of ERK1/2. Altogether these data imply that CD73 participates in cell adhesion/migration and transmits extracellular signals through interactions with ECM.
Highlights
Ecto-5 -nucleotidase, known as CD73 is Zn2+-binding membrane enzyme with its active site facing the extracellular compartment (Zimmermann, 1992; Zimmermann et al, 2012)
The role of CD73 in astrocyte adhesion and migration was investigated in the scratch wound assay using primary astrocyte culture prepared from neonatal rat cortex
Several experimental models of human neuropathologies studied so far demonstrate that reactive astrocytes express significantly higher level of CD73 compared to their normal counterparts (Braun et al, 1998; Bonan et al, 2000; Nedeljkovic et al, 2006, 2008; Lavrnja et al, 2009, 2015; Gandelman et al, 2010; Bjelobaba et al, 2011; Bonan, 2012)
Summary
Ecto-5 -nucleotidase, known as CD73 (eNT; E.C. 3.1.3.5) is Zn2+-binding membrane enzyme with its active site facing the extracellular compartment (Zimmermann, 1992; Zimmermann et al, 2012). Two 26-amino acid signal peptides in the N-terminal domains coordinate binding of two Zn2+ ions required for the catalytic activity, while two opposing C-terminal domains provide a binding site for AMP (Strater, 2006). While bulk CD73 is membrane-bound glycoprotein attached to the cell surface via GPI anchor, the enzyme may be shed from the membrane by phosphatidylinositol-specific phospholipase or by proteolytic cleavage, to give rise to the soluble variant which retains its catalytic activity (Heuts et al, 2012). The soluble CD73 constitutes an important auxiliary system for maintaining extracellular nucleotide concentrations in blood and body fluids (Yegutkin, 2008; Heuts et al, 2012; Laketa et al, 2015)
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