Serine 1283 in extracellular matrix glycoprotein Reelin is crucial for Reelin's function in brain development

Deficiency in the extracellular matrix glycoprotein Reelin severely affects migration of neurons during development. The function of serine at position 1283 in Reelin has remained uncertain. To explore its relevance we generated rlnA/A mice that carry alanine instead of serine at position 1283, thereby disrupting the putative casein kinase 2 (CK2) phosphorylation site S1283DGD. Mutated mice displayed reeler-like locomotor behavior, abnormal brain anatomy and decrease of Reelin RNA and protein levels during development and in adulthood. Since serine 1283 was previously proposed to mediate proteolysis of adhesion molecules, we investigated proteolysis of cell adhesion molecule L1 and found it normal in rlnA/A mice. Neuronal migration in the embryonic rlnA/A cerebral cortex was impaired, but rescued by in utero electroporation of the Reelin fragment N-R6 containing the putative CK2 phosphorylation site. In rlnA/A mice migration of cerebellar granule cells in vitro was promoted by application of wild-type but not by mutated Reelin. In cerebellar neuron cultures, Reelin expression was decreased upon inhibition of ecto-phosphorylation by CK2. Biochemically purified wild-type, but not mutated Reelin was found phosphorylated. Altogether, the results indicate that ecto-phosphorylation at serine 1283 rather than proteolytic processing of adhesion molecules by Reelin plays an important role in Reelin functions.