Fyn tyrosine kinase is a critical regulator of disabled-1 during brain development.
BACKGROUND
Disabled-1 (Dab1) is an intracellular adaptor protein that regulates migrations of various classes of neurons during mammalian brain development. Dab1 function depends on its tyrosine phosphorylation, which is stimulated by Reelin, an extracellular signaling molecule. Reelin increases the stoichiometry of Dab1 phosphorylation and downregulates Dab1 protein levels. Reelin binds to various cell surface receptors, including two members of the low-density lipoprotein receptor family that also bind to Dab1. Mutations in Dab1, its phosphorylation sites, Reelin, or the Reelin receptors cause a common phenotype. However, the molecular mechanism whereby Reelin regulates Dab1 tyrosine phosphorylation is poorly understood.


RESULTS
We found that Reelin-induced Dab1 tyrosine phosphorylation in neuron cultures is inhibited by acute treatment with pharmacological inhibitors of Src family, but not Abl family, kinases. In addition, Reelin stimulates Src family kinases by a mechanism involving Dab1. We analyzed the Dab1 protein level and tyrosine phosphorylation stoichiometry by using brain samples and cultured neurons that were obtained from mouse embryos carrying mutations in Src family tyrosine kinases. We found that fyn is required for proper Dab1 levels and phosphorylation in vivo and in vitro. When fyn copy number is reduced, src, but not yes, becomes important, reflecting a partial redundancy between fyn and src.


CONCLUSIONS
Reelin activates Fyn to phosphorylate and downregulate Dab1 during brain development. The results were unexpected because Fyn deficiency does not cause the same developmental phenotype as Dab1 or Reelin deficiency. This suggests additional complexity in the Reelin signaling pathway.
Arnaud
Lionel
L
Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, 98109, Seattle, WA, USA.
Ballif
Bryan A
BA
Förster
Eckart
E
Cooper
Jonathan A
JA
D000818:Animals; D001921:Brain / Q000196:embryology / Q000254:growth & development* / Q000378:metabolism; D015816:Cell Adhesion Molecules, Neuronal / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016326:Extracellular Matrix Proteins / Q000187:drug effects / Q000502:physiology; D018507:Gene Expression Regulation, Developmental; D006801:Humans; D051379:Mice; D008817:Mice, Mutant Strains; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011505:Protein-Tyrosine Kinases / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology*; D016315:Proto-Oncogene Proteins c-abl / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D051076:Proto-Oncogene Proteins c-fyn; D051575:Proto-Oncogene Proteins c-yes; D000091042:Reelin Protein; D029589:Seminal Plasma Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D012697:Serine Endopeptidases; D015398:Signal Transduction; D014443:Tyrosine / Q000378:metabolism; D019061:src-Family Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism
D015816:Cell Adhesion Molecules, Neuronal; C103773:Dab1 protein, mouse; D004791:Enzyme Inhibitors; D016326:Extracellular Matrix Proteins; D009419:Nerve Tissue Proteins; D010750:Phosphoproteins; D011518:Proto-Oncogene Proteins; D000091042:Reelin Protein; D029589:Seminal Plasma Proteins; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; C494957:FYN protein, human; C494960:Fyn protein, mouse; D016315:Proto-Oncogene Proteins c-abl; D051076:Proto-Oncogene Proteins c-fyn; D051575:Proto-Oncogene Proteins c-yes; C489834:YES1 protein, human; D019061:src-Family Kinases; C000717127:RELN protein, human; C000717128:Reln protein, mouse; D012697:Serine Endopeptidases