103 The glial cell line derived neurotrophic factor (GDNF) family has recently been expanded to include four members, and the interactions between these neurotrophic factors and their unique receptor system is now beginning to be understood. Furthermore, analysis of mice lacking the genes for GDNF, neurturin, and their related receptors has confirmed the importance of these factors in neurodevelopment. The results of such analyses reveal numerous similarities and potential overlaps in the way the GDNF and the nerve growth factor (NGF) families regulate development of the peripheral nervous system. Addresses * †§ Departments of Pathology and Internal Medicine, ‡ Department of Neurology and Department of Molecular Biology and Pharmacology, Washington University School of Medicine, 660 South Euclid Avenue, St Louis, MO 63110, USA *e-mail: bbaloh@pathbox.wustl.edu † e-mail: henomoto@pathbox.wustl.edu ‡ e-mail: ejohnson@pcg.wustl.edu § e-mail: jeff@pathbox.wustl.edu Current Opinion in Neurobiology 2000, 10:103–110 0959-4388/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved. Abbreviations ARTN artemin DRG dorsal root ganglion E embryonic day GDNF glial cell line derived neurotrophic factor GFL GDNF family ligand GFRα GDNF family receptor α-component NGF nerve growth factor NRTN neurturin PSPN persephin RET rearranged in transfection (receptor tyrosine kinase) SCG superior cervical ganglion Introduction Since the discovery of nerve growth factor (NGF) and the establishment of its ability to support neuronal survival [1–3], extensive efforts have been made to identify additional neu- rotrophic factors that can influence neurons in primary culture, during normal development, or in experimental models of neuronal injury. This work has resulted in the identification of a large and diverse group of proteins that are capable of promoting neuronal survival in various experi- mental paradigms. Glial cell line derived neurotrophic factor (GDNF) was initially identified as a factor secreted from a glioma cell line capable of supporting embryonic ventral midbrain neuron survival in culture [4]. Our knowledge of the in vitro activities of GDNF expanded rapidly after its dis- covery to now include survival promotion of additional central neurons (including spinal motor neurons) and at least a subpopulation of all peripheral ganglia yet examined [5–8,9 •• ]. The discovery of neurturin (NRTN) three years later, which is ~44% identical to GDNF, established the exis- tence of the GDNF family ligands (GFLs) [8]. Furthermore, shortly after the discovery of NRTN, both GDNF and NRTN were found to signal through a multicomponent receptor system comprising a high-affinity ligand-binding co- receptor GFRα (GDNF family receptor α-component) and the RET receptor tyrosine kinase [10–14]. This review briefly describes the recent expansion of the GFLs to include two additional members, persephin (PSPN) and artemin (ARTN), and summarizes the current understanding of ligand–receptor interactions between the four GFLs and GFRα co-receptors. Furthermore, mice with null mutations in the genes encoding GDNF, NRTN and several GDNF family receptors (GFRα1, GFRα2 and GFRα3) have recently provided insight into the critical importance of the GFLs during development, particularly in the peripheral nervous system and in kidney organo- genesis. Several excellent reviews of the literature describing the structural biology and therapeutic prospects of the GFLs [15 • ,16,17 • ] and the oncogenic role of RET mutations in multiple endocrine neoplasia type 2 (MEN2) can be found elsewhere [18–20]. Expansion of the GDNF family A schematic representation of ligand–receptor interactions of the GFLs characterized by in vitro studies is shown in Figure 1. Shortly after the discovery of the second GFL (NRTN), homology-based PCR screening was used to identify PSPN, and shortly thereafter database searching was used to identify ARTN. As mentioned above, the GFLs signal through a multicomponent receptor complex comprising the RET tyrosine kinase and a high-affinity ligand-binding component (of which there are now GFRα1-GFRα4), that is attached to the cell surface via a glycosyl phosphatidylinositol (GPI) anchor. As RET itself cannot bind the GFLs, both a GFRα and RET are required to form a functional GFL receptor. Extensive receptor activation experiments and receptor binding experiments over the past few years have served to further define the interactions shown in Figure 1. Essentially, each GFL has a preferred co-receptor to which it binds with highest affinity and activates RET most potently. These are GDNF–GFRα1, NRTN–GFRα2, and ARTN–GFRα3 [21–23,24 •• ]. PSPN can bind a protein in the chicken called GFRα4 [25]; however, a mammalian orthologue of this receptor has not yet been identified. The alternative interactions (GDNF–GFRα2, NRTN– GFRα1, ARTN–GFRα1) shown in the figure are clearly functional, The GDNF family ligands and receptors — implications for neural development Robert H Baloh*, Hideki Enomoto † , Eugene M Johnson Jr ‡ and Jeffrey Milbrandt §