Other neurotrophic factors: Glial cell line‐derived neurotrophic factor (GDNF)

Glial cell line‐derived neurotrophic factor (GDNF) was first discovered as a potent survival factor for midbrain dopaminergic neurons and was then shown to rescue these neurons in animal models of Parkinson's disease. GDNF is a more potent survival factor for dopaminergic neurons and the noradrenergic neurons of the locus coeruleus than other neurotrophic factors, and an almost 100 times more efficient survival factor for spinal motor neurons than the neurotrophins. The members of the GDNF family, GDNF, neurturin (NTN), persephin (PSP), and artemin (ART), have seven conserved cysteine residues with similar spacing, making them distant members of the transforming growth factor‐β (TGF‐β) superfamily. Like the members of the neurotrophin family, the GDNF‐like growth factors belong structurally to the cysteine knot proteins. Like neurotrophins, GDNF family proteins are responsible for the development and maintenance of various sets of sensory and sympathetic neurons but, in addition, GDNF and NTN are also responsible for the development and survival of the enteric neurons, and NTN for parasympathetic neurons. All neurotrophins bind to the p75 low‐affinity receptor, but their ligand specificity is determined by trk receptor tyrosine kinases. GDNF, NTN, PSP, and ART mediate their signals via a common receptor tyrosine kinase, Ret, but their ligand specificity is determined by a novel class of glycosyl‐phosphatidylinositol (GPI)‐anchored proteins called the GDNF family receptor α (GFRα). GDNF binds preferentially to GFRα1, NTN GFRα2, ART GRFα3, and PSP GFRα4 as a co‐receptor to activate Ret. GFRα4 has until now been described only from chicken. Although the GDNF family members signal mainly via Ret receptor tyrosine kinase, there is recent evidence that they can also mediate their signals via GFRα receptors independently of Ret. The GDNF family of growth factors, unlike neurotrophins, has a well‐defined function outside the nervous system. Recent transgenic and organ culture experiments have clearly demonstrated that GDNF is a mesenchyme‐derived signaling molecule for the promotion of ureteric branching in kidney development. NTN, ART, and PSP are also expressed in the developing kidney, and NTN and PSP induce ureteric branching in vitro, but their true in vivo role in kidney morphogenesis is still unclear. Microsc. Res. Tech. 45:292–302, 1999. © 1999 Wiley‐Liss, Inc.