Differential Expression of Glutamic Acid Decarboxylase in Rat and Human Islets

The GABA synthesizing enzyme GAD is a prominent islet cell autoantigen in type I diabetes. The two forms of GAD (GAD₆₄ and GAD₆₇) are encoded by different genes in both rats and humans. By in situ hybridization analysis of rat and human pancreases, expression of both genes was detected in rat islets, whereas only GAD₆₄ mRNA was detected in human islets. Immunocytochemical analysis of rat and human pancreatic sections or isolated islets with antibodies to GAD₆₄ and GAD₆₇ in combination with antibodies to insulin, glucagon, or SRIF confirmed that a GAD₆₄ and GAD₆₇ expression were β-cell specific in rat islets. In contrast, only GAD₆₄ was detected in human islets and was, in addition to β-cells, also surprisingly localized to some α-cells, δ-cells, and PP-cells. In long-term (4 wk) monolayer cultures of newborn rat islet cells, GAD₆₄ expression remained β-cell specific as observed in vivo, whereas GAD₆₇ was localized not only to the β-cells but also in the α-cells and δ-cells. A small but distinct fraction of GAD positive cells in these monolayer cultures did not accumulate GABA immunoreactivity, which may indicate cellular heterogeneity with respect to GABA catabolism or GAD enzyme activity. In a rat insulinoma cell line (NHI-6F) producing both glucagon and insulin depending on the culture conditions, GAD₆₄ expression was detected only in cultures in which the insulin producing phenotype dominated. In conclusion, these data demonstrate that the two GAD isoforms are differentially expressed in rat and human islets but also that the expression differs according to culture conditions. These findings emphasize the need to consider both the species and culture conditions of islets.