ARGININE SYNTHESIS AND UTILIZATION IN BRAIN

精氨酸琥珀酸合成酶（AS）和精氨酸琥珀酸裂解酶（AL）

Argininosuccinate synthetase (AS) and argininosuccinate lyase (AL) are enzymes of the urea cycle whose heritable deficiencies result in severe disease including mental retardation. The combined actions of these two enzymes leads to synthesis of arginine from citrulline + aspartate. While the complete urea cycle is not present in the brain, AS and AL are expressed in brain tissue. The role(s) of arginine in brain metabolism and neurological function are not clear. Nitric oxide synthase (NOS), catalyzes the conversion of arginine to nitric oxide + citrulline. Recent evidence indicates that nitric oxide (NO) is a novel second messenger molecule which is important in signal transduction processes in vascular endothelium and neural tissue. Regulation of nitric oxide production in vascular endothelium is pertinent to understanding cerebrovascular diseases such as stroke and migraine. Glutamate receptor stimulation is coupled to NO production. The synthesis of NO in response to stimulation of N-methyl-D-aspartate receptors may play a role in synaptic plasticity and long term potentiation. It Is our hypothesis that neural AS and AL play critical roles In nitric oxide-dependent processes by converting citrulline back to arginine for further nitric oxide synthesis. The long term goals of this research are to understand the role(s) of arginine metabolism in the brain and the mechanisms by which it is regulated during development and maturation of the nervous system. In the proposed studies we will determine the distribution of AS, AL and NOS in different regions and cell types of mouse brain using a combination of enzyme assays and immunocytochemical localization techniques. These studies will provide correlations of arginine metabolism with brain functions and will indicate whether the three enzymes are present in distinct compartments. AL expression is detected earlier in fetal brain development than is AS. In contrast, AS and AL are coordinately induced in developing liver and kidney. We will use molecular genetic approaches to characterize the molecular mechanisms which lead to discrete localization of AS and AL in different cell types at different times during development. These studies will compare and contrast the chromatin structure for the AS and AL genes in brain, liver and kidney. We will also determine whether AL is transcribed from an alternative, brain-specific promoter in the central nervous system. Glucocorticoids, cAMP and insulin are known to modulate the transcription and expression of AS and AL in liver and kidney. We will determine the effects of these effectors on AL expression in cultured brain cells. These studies will correlate enzyme activity with steady state mRNA levels and will determine whether regulation is exerted at the level of transcription. Ultimately, we will use recombinant minigenes transfected into cultured brain cells to characterize the sequences required for establishing the temporal and spatial patterns of AL expression observed in the brain.