Regulated Extracellular Nucleotide Metabolism and Function at the Mucosa - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 122

their metabolites have been long identified as playing a crucial role in cell-cell
signaling in a number of different tissues, ranging from early observations in the
context of neurotransmission [53] to more recent observations in mucosal tissues
[20, 40, 49]. Circulating nucleotides and nucleosides may be metabolized under
the control of a variety of cell types and the end products of these metabolic pro-
cesses may signal in an autocrine or paracrine manner. Nucleotide regulation takes
place via a cascade of hydrolyzing enzymes that serve to sequentially cleave phos-
phate groups from adenine nucleotides (adenosine triphosphate [ATP], adenosine
diphosphate [ADP] and adenosine monophosphate [AMP]), ultimately producing
nucleoside molecules such as adenosine. The nucleotide intermediates themselves
may signal through a subset of purinergic receptors of the P2 family (further subdi-
vided into P2Y and P2X receptors). The end product of these hydrolysis reactions,
adenosine may then elicit its functions by binding one of its cognate G-protein cou-
pled receptor (GPCR), namely the A1, A2A, A2B and A3 receptors. It is at this
level that adenine nucleotides and adenosine may act to modulate cellular processes
such as enhancing barrier function, inducing chloride secretion and dampening of
the immune response, to name but a few.

8.2 Role of NTPDases (CD39 and CD39-Like)/
5 -Ectonucleotidase (CD73) Enzymes

One of the levels at which extracellular nucleotides may be regulated is under
the control of a number of cell-surface molecules called ectonucleotidases, whose
enzymatic activity is to cleave phosphate groups from circulating nucleotides
with varying degrees of specificity for their substrates. CD39 is one such ectonu-
cleotidase initially observed as an activation marker in paracortical lymphocytes,
macrophages, and dendritic cells resident within lymphoid tissue. Cloning of CD39
revealed the presence of apyrase conserved regions (ACR) and striking sequence
homology with yeast guanosine diphosphatase, an enzyme involved in catalyzing
the removal of a phosphate from GDP after sugar transfer within the Golgi appa-
ratus [43]. Further evidence for the ATP-hydrolyzing capabilities of CD39 came
with the cloning and sequencing of a potato tuber ATP apyrase, which shared
sequence homology with both mouse and human CD39 [24]. The ecto-pyrase
activity of CD39 was demonstrated in EBV transformed B-cells and CD39 over-
expressing COS-7 cells, assessed by the ability of CD39 to liberate free [ 32 P]P i
from [

32 P]ATP [70].
Further identification of a number of CD39-like nucleoside triphosphate diphos-
phohydrolases (NTPDases) have since revealed a family of 8 related proteins,
denoted NTPdase1-8 (NTPDase1 representing CD39 under this nomenclature).
NTPDase1,2,3 and 8 are transmembrane proteins with 5 ACRs situated on the
extracellular region, conferring nucleotidase activity to the enzyme and allowing for
hydrolysis of extracellular nucleotides [60]. Differential regulation of hydrolysis is
evident between each NTPDase subtype, with NTPDase1 having similar hydrolyz-
ing activity on both ATP and ADP, and NTPDases3 & 8 preferentially hydrolyzing

γ