| The Reelin-Disabled-1 (Dab1) signaling pathway plays a key role in regulating neuronal positioning and synaptic plasticity. Binding of Reelin to its receptors induces tyrosine phosphorylation of the intracellular adaptor protein Dab1. Tyrosine-phosphorylated Dab1 not only rapidly transmits the Reelin signal to downstream effectors but also terminates Reelin-mediated signaling by targeting itself for degradation. Multiple alternatively-spliced Dab1 isoforms have been reported; however, the functions of Dab1 isoforms, other than the commonly studied Dab1 form, remain unknown.;Here, we show that an alternatively-spliced chicken Dab1 isoform, chDab1-E, is missing two critical tyrosine sites implicated in Reelin signaling, and is not tyrosine phosphorylated upon Reelin stimulation. Knockdown of Dab1-E in chick retina results in a significant reduction in the number of proliferating cells and promotes ganglion cell differentiation, suggesting that chDab1-E is involved in the maintenance of the retinal progenitor pool and retinogenesis. Furthermore, we show that chDab1-E is serine/threonine phosphorylated by cyclin-dependent kinase 2 (Cdk2) independent of Reelin. ChDab1-E phosphorylation destabilizes the protein through proteasome degradation, indicating that Dab1 turnover can be regulated by both Reelin-independent serine/threonine phosphorylation and Reelin-dependent tyrosine phosphorylation. Finally, we demonstrate that Dab1 alternative splicing is highly complex in mouse, with the potential of generating 16 isoforms that differ primarily in the tyrosine-rich region of Dab1. We have identified 11 murine Dab1 isoforms that are differentially phosphorylated on tyrosine residues, suggesting that different Dab1 isoforms may differentially respond to Reelin stimulation.;We propose that Dab1 alternative splicing provides an exquisitely-regulated mechanism to fine-tune the activity of Reelin signaling in a temporal and spatial manner, allowing cells that express different Dab1 isoforms to differentially respond to the Reelin signal during development. Our studies support diverse roles for alternatively-spliced Dab1 isoforms during central nervous system development. We propose a model whereby Dab1 alternative splicing tightly regulates neurogenesis, neuronal migration and synaptic plasticity through both Reelin-independent and Reelin-dependent signaling events. Our findings provide new insight into the roles of developmentally-regulated alternative splicing in controlling gene function and coordinating complex processes at different developmental stages. |
