These animals also have defects in the motor, sensory, and sympathetic nervous system [1-3]. In developing and analysing neurturin (NRTN) receptor GFRα2 knock-out mice, we found that NRTN-GFRα2 signalling plays an important role in the development of enteric and parasympathetic innervation . We then developed mice overexpressing GDNF in the testes and in collaboration with Prof. Hannu Sariola's group (Faculty of Medicine, University of Helsinki), observed that GDNF is an important regulator of spermatogenesis .
GDNF is a potent survival factor for midbrain dopamine neurons that can promote axon growth and hypertrophy of these neurons. GDNF and its receptors are expressed in developing and mature dopamine neurons, but analysis of knock-out mice has shown that GFRα1-RET signalling is not required for the embryonic development of dopamine neurons in vivo. GDNF is strongly expressed in the striatum during the first few postnatal weeks, when dopamine neuron target innervation is taking place, indicating that GDNF could still be required for postnatal development, maintenance and plasticity of dopamine neurons. To study the role of GDNF-GFRα1-RET signalling in the postnatal and adult brain dopaminergic system, we analysed the nigrostriatal system of the Ret MEN2B knock-in mice, and of GDNF and GFRα1 conditional knock-out mice, that postdoctoral fellow Dr. Jaan-Olle Andressoo developed. Furthermore, he also developed a unique mouse model where GDNF was overexpressed in cells natively-expressing GDNFunder normal developmental and physiological control. These mice are called GDNF hypermorph mice. In collaboration with Dr. Petteri Piepponen’s group (Faculty of Pharmacy, University of Helsinki), we analysed knock-in MEN2B mice with constitutive Ret activity in the brain dopaminergic system and found robustly increased concentrations of dopamine and its metabolites in the striatum, cortex, and hypothalamus . Somewhat unexpectedly, we found that constitutive Ret signaling is protective for the dopaminergic neuronal somas, but not their axonal terminals . In a large collaborative effort with Dr. Jaan-Olle Andressoo’s current research group (Faculty of Medicine, University of Helsinki), we then characterized GDNF hypermorph mice and found that about a 2-fold increase in the level of GDNF in the central nervous system increases both the number of adult dopamine neurons in the substantia nigra pars compacta, and the number of dopaminergic terminals in the dorsal striatum, by about 15%. Elevated GDNF levels also increased striatal tissue dopamine levels by about 25% and enhanced striatal dopamine release and re-uptake . Characterization of GDNF conditional knock-out mice, again in a close collaboration with Dr. Andressoo group using three complementary approaches, revealed that GDNF reduction, independent of the time of reduction, does not lead to any substantial changes in the number of midbrain dopamine neurons . As GDNF is one of the four ligands activating RET signalling in the brain, our results are in line with those of Ret deletion and allow us to conclude that GDNF reduction is not critical for maintenance of midbrain dopamine neurons in mice . Further analysis of the conditional GDNF knock-out mice revealed that endogenous GDNF regulates the level and function of dopamine transporters in the nigrostriatal dopamine neurons, thereby affecting striatal dopamine homeostasis and amphetamine-induced behaviours . Dissecting the in vivo role of GDNF binding GFRα1 receptor, by analysing the midbrain dopamine system of GFRα1 conditional knock-out mice, is of particular interest to our group. GFRα1 serves as the co-receptor for GDNF-NCAM signalling and can possibly be involved in the signalling of other still unknown alternative GDNF receptors.