Defect of the
purine salvage
enzyme,
hypoxanthine phosphoribosyl
transferase (
HPRT), results in
Lesch-Nyhan disease (LND). It is unknown how the metabolic defect translates into the severe neuropsychiatric phenotype characterized by self-injurious behavior,
dystonia and
mental retardation. There are abnormalities in
GTP,
UTP and
CTP concentrations in
HPRT-deficient cells. Moreover,
GTP,
ITP,
XTP,
UTP and
CTP differentially support Gs-
protein-mediated
adenylyl cyclase (AC) activation. Based on these findings we hypothesized that abnormal AC regulation may constitute the missing link between
HPRT deficiency and the neuropsychiatric symptoms in LND. To test this hypothesis, we studied AC activity in membranes from primary human skin and immortalized mouse skin fibroblasts, mouse Neuro-2a
neuroblastoma cells and rat B103
neuroblastoma cells. In B103 control membranes,
GTP,
ITP,
XTP and
UTP exhibited profound stimulatory effects on basal AC activity that approached the effects of hydrolysis-resistant
nucleotide analogs. In
HPRT- membranes, the stimulatory effects of
GTP,
ITP,
XTP and
UTP were strongly reduced. Similarly, in human and mouse skin fibroblast membranes we also observed a decrease in
GTP-stimulated AC activity in
HPRT-deficient cells compared with the respective controls. In mouse Neuro-2a
neuroblastoma membranes, AC activity in the presence of
GTP was below the detection limit of the assay. We discuss several possibilities to explain the abnormalities in AC regulation in
HPRT deficiency that encompass various species and cell types.