MULTIPLE ASTROCYTE TRANSCRIPTS ENCODE NIGRAL TROPHIC FACTORS IN RAT AND HUMAN

The recent discovery of glial cell line-derived neurotrophic factor (G DNF) identified a novel trophin that selectively increases survival of substantia nigra dopaminergic neurons, which degenerate in Parkinson' s disease. Our previous studies indicated that GDNF RNA can be amplifi ed from cultured rat nigral type 1 astrocytes and from rat striatum in vivo, implying local as well as target trophic support. The current s tudy establishes the regional pattern of GDNF RNA expression in adult human brain. Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed the highest expression of GDNF mRNA in the human cau date, with low levels in the putamen and no detectable message in the nigra, suggesting that GDNF is a target-derived factor in humans. We a lso report the isolation of two additional GDNF-related cDNAs, termed astrocyte-derived trophic factors (ATF), which apparently result from differential RNA processing. Sequence analysis of rat ATF-1 revealed a 78-bp deletion corresponding to a loss of 26 amino acids within the p repro region of the predicted GDNF protein. The RNA processing events responsible for ATF-1 formation in rat brain are conserved in humans; we report the isolation of a full-length human ATF-1 homologue. We ide ntified a second alternative transcript, human ATF-2; the transcript e ncodes a protein which differs in its first 18 amino acids from the pr edicted mature GDNF and ATF-1 proteins and shares the terminal 115 res idues with the other two forms. To begin assessing the biologic signif icance of multiple transcript expression we characterized the actions of COS-expressed GDNF and ATF-1 cDNAs. Both exerted trophic effects on cultured nigral dopaminergic neurons, but did not affect basal forebr ain cholinergic neurons, thereby exhibiting both selectivity and commo nalities of action. The expression of multiple transcripts in a region -specific fashion implies complex regulation of nigral trophic factor gene expression and gene product processing. (C) 1994 Academic Press, Inc.