Reelin is a large extracellular glycoprotein involved in the development of architectonic patterns, particularly in the cerebral cortex and hippocampus, where primarily Cajal-Retzius cells synthesize it. In the hippocampus, Reelin also regulates the growth and/or distribution of afferent entorhinal and commissural axons (Ref.1). Reelin encodes an mRNA of approximately 12 kb. Reelin mRNA encodes a large extracellular glyco-protein containing eight repeated sequences that include EGF (Epidermal Growth Factor)-like cysteine motifs (Ref.2).
At the surface of target cells, Reelin binds to two lipoprotein receptors, VLDLR (Very-Low-Density Lipoprotein Receptor) and ApoER2 (Apolipoprotein-E Receptor-2), which relay the signal into the cell via the adapter Dab1 (Disabled-1). Binding requires calcium, and it is inhibited in the presence of ApoE (Apolipoprotein-E). Dab1 is an intracellular adapter protein, which encodes a cytoplasmic protein containing a motif known as PI/PTB (Protein Interaction/Phosphotyrosine-Binding) domain (Ref.3). This domain was identified in the adapter protein SHC (Src Homology-2 Domain Containing) transforming protein as a region required for binding to the EGF receptor, the Insulin Receptor, and other tyrosine-phosphorylated proteins. The PTB domain of Dab1 associates with an NPXY motif in the cytoplasmic region of VLDLR and ApoER2. Reelin binds to these receptors on the surface of neurons, triggering tyrosine phosphorylation of Dab1, which binds to the intracellular domains of both receptors. Tyrosine phosphorylation of Dab1 promotes an interaction with several nonreceptor tyrosine kinases, including Src, Fyn, and Abl (v-abl Abelson Murine Leukemia Viral Oncogene Homolog-1) through their SH2 domains, implying Dab1 functions in kinase signaling cascades during development. In the absence of Reelin or the receptors, Dab1 accumulates in a hypo-phosphorylated form. Tyrosine phosphorylated Dab1 may couple Reelin signaling to downstream molecular machinery involved in cell positioning. Dab1 could serve as a nexus for numerous signaling pathways (Ref.4). PTB domain of Dab1 also binds to the transmembrane glycoproteins of the APP (Amyloid Precursor Protein) and low-density lipoprotein receptor families and the cytoplasmic signaling protein SHIP (SH2-Containing Inositol Phosphatase). PTB domain permits Dab1 to bind specifically to transmembrane proteins containing an NPXY internalization signal. The function of Dab1 binding to LRP (LDL receptor-related protein)-Alpha-2 macroglobulin receptor, APP, and their relatives could be to regulate trafficking or processing.
CDK5 (Cyclin-Dependent Kinase-5) mediated serine phosphorylation could potentially modulate the function of the Dab1-associated signaling complex. CDK5 is also implicated in the regulation of numerous other cellular events, including neurite extension, cell adhesion, and axonal transport. CDK5 is a serine-threonine kinase that is ubiquitously expressed, but its catalytic activity is dependent on the neuronal regulators CDK5R (Cyclin-Dependent Kinase-5, Regulatory Subunit-1). CDK5 phosphorylates a variety of substrates, including proteins known to play a role in cell adhesion and migration. One possible way in which CDK5 can influence Reelin signaling is through phosphorylation of Dab1. Dab1 is phosphorylated predominantly on serine-threonine residues in a transient transfection system. Indeed, Dab1 contains numerous potential phosphorylation sites for CDK5. CDK5 phosphorylates serine 491 of Dab1. Unlike tyrosine phosphorylation, serine phosphorylation of Dab1 occurs independently of Reelin signaling (Ref.5).
CDK5, in conjunction with another cytoplasmic serine/threonine kinase, GSK3Beta (Glycogen Synthase Kinase-3-Beta), and PP2A (Protein Phosphatase-2A) regulates the phosphorylation state of the microtubule-associated protein Tau. Phosphorylation of Tau affects its affinity to the microtubules and its ability to form paired helical filaments from which the neurofibrillary tangles in AD (Alzheimer’s disease) develop. Tau is heavily hyperphosphorylated in mice with defects in the Reelin signaling. WNT proteins constitute a large family of secreted glycoproteins with important signaling functions during embryogenesis including establishment of polarity and cell specification. They are also involved in cell transformation and cancer development. Phosphorylation of Ctnn-Beta (Beta-Catenin) by GSK3Beta destabilizes the protein and reduces transcription of target genes. Conversely, inhibition of GSK3Beta stabilizes Ctnn-Beta and increases transcription. WNT-mediated regulation of GSK3Beta activity involves the Frizzled-family of seven-transmembrane receptors and the cytoplasmic proteins axin, APC (Adenomatous Polyposis Coli), and Dsh (Dishevelled) (Ref.6). CDK5 is also known to interact with other known pathways regulating neuronal migration such as Lis1/NUDEL. In addition to its role in neuronal migration, CDK5 is also implicated in neurodegeneration through dysregulation of its kinase activity. Dysregulation is attributed to the Calpain-mediated cleavage of p35 to a C-terminal truncated fragment known as p25. p25 causes mislocalization and prolonged activation of CDK5. CDK5 also phosphorylates APP. Phosphorylation of APP by CDK5 regulates the trafficking and/or processing of APP which in turn contributes to the pathogenesis of Alzheimers disease.
ApoER2 contains a domain in its cytoplasmic tail that associates with the JNK family of interacting proteins, JIP1 (c-Jun N-Terminal Kinase Interacting Proteins-1) and JIP2 (c-Jun N-Terminal Kinase Interacting Proteins-2), which are highly expressed in the developing CNS (Central Nervous System). JIP are kinase-scaffolding proteins important in the JNK signaling , and JIP1 associates with the RhoA GTPase exchange factor, RhoGEF. Thus, activation of MLK3 (Mixed Lineage Kinase-3), MKK7, and JNK (c-Jun N-Terminal Kinase), which assemble on JIPs, could be a downstream consequence of Reelin signaling. ApoER2 is a component of the transport vesicles that are linked to kinesin by JIPs, suggesting that JIPs serve essential functions in the subcellular distribution of this Reelin receptor (Ref.7). In addition, biochemical evidence suggests that the Integrins Alpha3/Beta1 and protocadherins of the CNR (Cadherin-Related Neuronal Receptors) family may also modulate the Reelin signal (Ref.8). Other genes have been implicated in the Reelin-signaling, although their role is not fully understood. Collectively, these genes provide a molecular starting point for biological, biochemical, and cellular studies that are providing important and exciting insights into the molecular mechanisms that control brain development and, potentially, the pathogenesis of neurodegenerative disorders (Ref.9).