Mesencephalic astrocyte-derived neurotrophic factor (MANF) protects neurons and repairs the Parkinson diseaselike symptoms in a rat 6-hydroxydopamine model. We show a three-dimensional solution structure of human MANF that differs drastically from other neurotrophic factors. Remarkably, the C-terminal domain of MANF (C-MANF) is homologous to the SAP domain of Ku70, a well known inhibitor of proapoptotic Bax (Bcl-2-associated X protein). Cellular studies confirm that MANF and C-MANF protect neurons intracellularly as efficiently as Ku70.

The TrkC/NT-3 receptor/ligand pair is believed to be part of the classic neurotrophic theory claiming that neuronal death occurs by default when neurotrophic factors become limited, through loss of survival signals. Here, we show that TrkC is a dependence receptor and, as such, induces caspase-dependent apoptotic death in the absence of NT-3 in immortalized cells, a proapoptotic activity inhibited by the presence of NT-3. This proapoptotic activity of TrkC relies on the caspase-mediated cleavage of the intracellular domain of TrkC, which permits the release of a proapoptotic fragment. This fragment induces apoptosis through a caspase-9-dependent mechanism. Finally, we show that the death of dorsal root ganglion (DRG) neurons provoked by NT-3 withdrawal is inhibited when TrkC-proapoptotic activity is antagonized. Thus, the death of neurons upon disappearance of NT-3 is not only due to a loss of survival signals but also to the active proapoptotic activity of the unbound TrkC dependence receptor.neurotrophin-3 ͉ sensory neurons ͉ programmed cell death ͉ tyrosine kinase

We have identified and characterized N-Bak, a neuron-specific isoform of the pro-apoptotic Bcl-2 family member Bak. N-Bak is generated by neuron-specific splicing of a novel 20-base pair exon, which changes the previously described Bak, containing Bcl-2 homology (BH) domains BH1, BH2, and BH3, into a shorter BH3-only protein. As demonstrated by reverse transcriptionpolymerase chain reaction and RNase protection assay, N-Bak transcripts are expressed only in central and peripheral neurons, but not in other cells, whereas the previously described Bak is expressed ubiquitously, but not in neurons. Neonatal sympathetic neurons microinjected with N-Bak resisted apoptotic death caused by nerve growth factor (NGF) removal, whereas microinjected Bak accelerated NGF deprivation-induced death. Overexpressed Bak killed sympathetic neurons in the presence of NGF, whereas N-Bak did not. N-Bak was, however, still death-promoting when overexpressed in non-neuronal cells. Thus, N-Bak is an anti-apoptotic BH3-only protein, but only in the appropriate cellular environment. This is the first example of a neuron-specific Bcl-2 family member.

Glial cell line-derived neurotrophic factor (GDNF) family ligands signal through receptor complex consisting of a glycosylphosphatidylinositol-linked GDNF family receptor (GFR) ␣ subunit and the transmembrane receptor tyrosine kinase RET. The inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN2), associated with different mutations in RET, is characterized by medullary thyroid carcinoma. GDNF signals via GFR␣1, neurturin via GFR␣2, artemin via GFR␣3, whereas the mammalian GFR␣ receptor for persephin (PSPN) is unknown. Here we characterize the human GFR␣4 as the ligand-binding subunit required together with RET for PSPN signaling. Human and mouse GFR␣4 lack the first Cys-rich domain characteristic of other GFR␣ receptors. Unlabeled PSPN displaces 125 I-PSPN from GFRA4-transfected cells, which express endogenous Ret. PSPN can be specifically cross-linked to mammalian GFR␣4 and Ret, and is able to promote autophosphorylation of Ret in GFRA4-transfected cells. PSPN, but not other GDNF family ligands, promotes the survival of cultured sympathetic neurons microinjected with GFRA4. We identified different splice forms of human GFRA4 mRNA encoding for two glycosylphosphatidylinositol-linked and one putative soluble isoform that were predominantly expressed in the thyroid gland. Overlapping expression of RET and GFRA4 but not other GFRA mRNAs in normal and malignant thyroid medullary cells suggests that GFR␣4 may restrict the MEN2 syndrome to these cells.The glial cell line-derived neurotrophic factor (GDNF) 1 family ligands GDNF, neurturin (NRTN), artemin (ARTN), and persephin (PSPN) are structurally related neurotrophic factors that signal through a multicomponent receptor composed of the transmembrane receptor tyrosine kinase RET and high affinity glycosylphosphatidylinositol (GPI)-anchored proteins, the GDNF family ␣ receptors 1-4 (GFR␣1-4, reviewed in Refs. 1 and 2). GFR␣4 was first described from chicken and shown to be the preferential receptor for PSPN (3, 4). Recently, we characterized a mouse GFR␣4 receptor (5). It differs from all other GFR␣ receptors, including chicken GFR␣4, being smaller in size and lacking the first Cys-rich domain. Mouse Gfra4 transcripts are expressed in many embryonic and adult tissues but efficient splicing leading to a functional GPI-linked isoform, as well as putative transmembrane and soluble isoforms, occurs only in thyroid and adrenal medulla and in pituitary intermediate lobe (5). In mouse, Gfra4 and Ret are coexpressed only in the thyroid C-cells and adrenal chromaffin cells. In chicken, Gfra4 mRNA is broadly expressed during embryonic development, including the spinal motoneurons and kidney (4). Chicken GFR␣4 also binds mouse PSPN and confers survival response to PSPN in the presence of Ret (3). However, due to different structures of chicken and mammalian GFR␣4, as well as the lack of information about the existence of chicken GFR␣3, ligand specificity of mammalian GFR␣4 cannot be directly extrapolated from experiments with chicken GFR␣4. PSPN mRNA is expressed ...