Role of GDNF family receptors in development and MEN2 cancer syndromes

M Saarma

Program in Molecular Neurobiology, Institute of Biotechnology, University of Helsinki, Helsinki, Finland

Neurotrophic factors control the development and maturation of neurons, and have also roles in synaptic plasticity in adults. Glial cell line-derived neurotrophic factor (GDNF) and related ligands neurturin (NRTN), artemin (ARTN) and persephin (PSPN) form a subgroup in TGF-β superfamily. Responses to these ligands are mediated by a receptor complex composed of the transmembrane tyrosine kinase c-Ret and one of the glycosyl-phosphatidyl inositol (GPI-) linked GDNF family α-receptors (GFRα1 – GFRα 4). GFRα1 is the principal co-receptor for GDNF, GFRα2 for NRTN, GFRα3 for ARTN and GFRα4 for PSPN. GDNF, found as a trophic factor for midbrain dopaminergic neurons, has recently received attention as a therapeutic agent for the treatment of Parkinson’s disease. Mice lacking GDNF, GFRα1 or c-Ret all die at birth and lack kidneys and enteric neurons below stomach, consistent with inactivating mutations in RET and GDNF causing the Hirschsprung disease. In addition, GDNF is essential for other sub-populations of PNS and CNS neurons, as well as for spermatogenesis. Following GDNF stimulation, the GPI-anchored GFRα1 recruits c-Ret to lipid rafts, where c-Ret interacts with different proteins than outside the rafts. This localization of c-Ret to lipid rafts is required for effective downstream signalling, differentiation and neuronal survival. The autophosphorylated c-Ret in turn activates several intracellular signalling proteins that regulate cell survival, differentiation, proliferation, migration, neurite growth and synaptic modulation. Activating mutations in RET cause the inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN2), characterized by tumors arising from adrenal chromaffin (pheochromocytoma) and thyroid C-cells (medullary thyroid carcinoma, MTC). In addition, most sporadic cases of MTC have mutations in RET. MEN2 is divided into three subtypes depending on other organs involved. It is not clear why different specific RET mutations associate with the different disease phenotypes. It is believed that the RET mutations cause a partially or completely ligand-independent activation of RET-signalling. A possible role of GFRα co-receptors in MEN2 has not been addressed. An open question in the MEN2 syndromes is the cause for the cell specificity of the cancer, because RET is also expressed in many other cells outside the thyroid and adrenal gland, including central and peripheral nervous system, kidney and testis, but only thyroid and adrenal are affected. We have cloned the mouse and human GFRα4 receptors and showed that the GPI-linked GFRα4 binds PSPN and mediates its biological effects via Ret on transfected cells. The mammalian GFRα4-receptor gene lacks exons coding for domain-1 that is found in other GFRα receptors. The GPI-linked isoform of GFRα4 is predominantly expressed in juvenile thyroid C-cells. These cells co-express Ret, but no Gfra1-Gfra3 mRNAs. Interestingly, thyroid, parathyroid, and adrenal glands also produce a secreted GFR α 4 isoform. GFRα4, which is co-expressed with mutant RET in the thyroid medullary C-cells, may be an “instructive” factor necessary for the MEN2 cancer formation. To address this question we have crossed our GFRα4-deficient mice with Ret-MEN2B “knock-in” mice (Ret2B/2B) that have a point mutation in mouse Ret tyrosine kinase domain corresponding to human MEN2B.

Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in oral session 892 (Genetic predisposition - Part II).