Members of the Glial Cell Line-Derived Neurotrophic Factor (GDNF) family include GDNF, Neurturin (NTN), Artemin (ART), and Persephin (PSP). These neurotrophic factors exhibit an array of biological activities ranging from the promotion of cell survival to the enhancement of neurite outgrowth.1,2 The GDNF family signals through a multi-molecular protein complex that includes receptors of the GFR alpha family and the RET receptor tyrosine kinase.3 Although there is some crosstalk between them, GDNF, NTN, ART, and PSP associate primarily with GFR alpha-1 through GFR alpha-4 respectively, leading to tyrosine phosphorylation and activation of RET.1 The observation that there are regions in the nervous system where GFR alpha family members and RET are independently expressed, suggests that the GDNF family might also signal through an as yet undescribed mechanism.4,5
Neural Cell Adhesion Molecule (NCAM) is widely expressed in the nervous system and plays important roles in neurodevelopment and synaptic plasticity.6 NCAM function is thought to be mediated via homophilic or heterophilic interactions with membrane-associated proteins.7-9 However, a recent study by Paratcha et al. suggests that some activities attributed to NCAM may be mediated through activation by members of the GDNF family.2 Alternative splicing leads to the generation of three principle NCAM isoforms including 120, 140, and 180 kDa proteins that differ in the length of their C-terminal domains and exhibit varied expression patterns and functions.10-13 Homophilic binding of the 140 kDa isoform (p140NCAM) leads to activation of the Src-related Fyn kinase and recruitment and activation of Focal Adhesion Kinase (FAK).14 Homophilic interactions also lead to the activation of Mitogen-Activated Protein (MAP) kinase signaling cascades including phosphorylation of ERK1 and ERK2.15 Paratcha et al. demonstrate that GDNF treatment of RN33B cells (immortalized neuronal precursors) or cultured Schwann cells from rat sciatic nerve results in NCAM-dependent activation of these same signaling cascades (Figure 1A).2 The ability of GDNF to stimulate NCAM signaling also appears to require the presence of the GFR alpha-1 receptor. Whether other members of the GDNF family activate similar signaling is unknown. However, binding assays suggest that NTN/GFR alpha-2 and PSP/GFR alpha-4 can directly interact with p140NCAM as well.2
|Figure 1A. GDNF/GFR alpha-1 stimulates NCAM leading to the activation of Fyn kinase, FAK, and ERK signaling cascades.
Figure 1B. Transfection of Jurkat cells with p140NCAM results in cell aggregation via homophilic binding. p140NCAM homophilic binding and aggregation are inhibited by co-transfection with GFR alpha-1.
GDNF also exhibits the ability to modulate NCAM adhesive activity. Transfection of Jurkat cells with p140NCAM leads to cell aggregation, an effect that is inhibited by co-transfection with GFR alpha-1. This is presumably due to GFR alpha-1 interference with p140NCAM homophilic binding (Figure 1B).2 GDNF stimulates Schwann cells to migrate and enhances the outgrowth of hippocampal and cortical neurons in vitro. This is blocked either by pharmacological inhibition of Fyn kinase or function-blocking antibodies to NCAM.2 GDNF has no affect on these cell types in cultures from NCAM-/- mice but is still effective on cells obtained from RET-/- mice, suggesting that GDNF is indeed signaling through a novel NCAM-dependent, RET-independent pathway.2
NCAM is known for mediating short-range signaling via homophilic binding or interactions with other membrane-associated proteins including proteoglycans and NCAM-L1.7-9 The demonstration that a diffusible molecule, such as GDNF, has the ability to activate NCAM could have important implications with regard to understanding the mechanisms that underlie NCAM roles in developmental processes and regeneration. Events generally attributed to short-range, NCAM-mediated cell/cell interactions might instead be due to long-range signals carried by secreted GDNF family members.