Reelin is a secreted protein that plays versatile roles in neuronal development and function. The strength of Reelin signaling is regulated by proteolytic processing, but its importance in vivo is not yet fully understood. Here, we generated Reelin knock-in (PA-DV KI) mice in which the key cleavage site of Reelin was abolished by mutation. As expected, the cleavage of Reelin was severely abrogated in the cerebral cortex and hippocampus of PA-DV KI mice. The amount of Dab1, whose degradation is induced by Reelin signaling, decreased in these tissues, indicating that the signaling strength of Reelin was augmented. The brains of PA-DV KI mice were largely structurally normal, but unexpectedly, the hippocampal layer was disturbed. This phenotype was ameliorated in hemizygote PA-DV KI mice, indicating that excess Reelin signaling is detrimental to hippocampal layer formation. The neuronal dendrites of PA-DV KI mice had more branches and were elongated compared to wild-type mice. These results present the first direct evidence of the physiological importance of Reelin cleavage.Reelin is a large secreted glycoprotein that regulates many important events in mammalian brain development 1-3 . It is also involved in synaptic plasticity and the modulation of higher brain functions 1,4-8 . At the cellular level, Reelin induces the clustering of apolipoprotein E receptor 2 (ApoER2) and very-low-density lipoprotein receptor (VLDLR), which is the prerequisite for the phosphorylation of the cytosolic adaptor protein Dab1 9-11 . Phosphorylated Dab1 is quickly degraded 1,2,5 . Thus, the amount of Dab1 is indicative of the strength of Reelin signaling [12][13][14][15] . Several pathways act downstream of Dab1 phosphorylation and regulate the cytoskeleton, adhesion molecules, ion channels, and neurotransmitter release 1-3,5,16 . Accordingly, insufficient Reelin signaling could contribute to the pathogenesis of neuropsychiatric disorders, such as epilepsy 17-19 , schizophrenia 6,16,20 , and autism 3,8,16 . Furthermore, recent studies showed that a decrease or insufficiency in Reelin signaling could aggravate Alzheimer's disease (AD) 13,21-26 . Therefore, it is important to understand the molecular mechanisms by which Reelin is regulated. Of particular importance is understanding how Reelin signaling is turned off because inhibition of such a mechanism would strengthen Reelin signaling and may help improve brain disorders caused by diminished Reelin function.Secreted signaling proteins can be inactivated or disabled by a variety of mechanisms, including internalization by its receptor, sequestration by an inactive (decoy) binding partner, and degradation. Internalization via ApoER2 and VLDLR 27,28 and a dominant-negative effect of the secreted extracellular domain of ApoER2 29 play a role in limiting Reelin signaling. Reelin protein (approximately 430 kDa as a monomer) contains three specific cleavage sites (N-t, C-t, and WC; reviewed in 7 ). N-t cleavage occurs between Pro1244 and Ala1245 to generate two fragments with molecular mas...

Reelin is a secreted protein that plays versatile roles in neuronal development and function, and hypoactivity of Reelin is implicated in many neuropsychiatric disorders. The strength of Reelin signaling is regulated by proteolytic processing, but its importance in vivo is not yet fully understood. Here, we generated Reelin knock-in (PA-DV KI) mice in which the key cleavage site of Reelin was abolished by mutation. As expected, the cleavage of Reelin was severely abrogated in the cerebral cortex and hippocampus of PA-DV KI mice. The amount of Dab1, whose degradation is induced by Reelin signaling, decreased in these tissues, indicating that the signaling strength of Reelin was augmented. The brains of PA-DV KI mice were largely structurally normal, but unexpectedly, the hippocampal layer was disturbed. This phenotype was ameliorated in hemizygote PA-DV KI mice, indicating that excess Reelin signaling is detrimental to hippocampal layer formation. The neuronal dendrites of PA-DV KI mice had more branches and were elongated compared to wild-type mice. These results present the first direct evidence of the physiological importance of Reelin cleavage and suggest that inhibition of Reelin cleavage would counteract neuropsychiatric disorders without causing severe systemic side effects.