Ephrin Type-A Receptor
 Ephrin Type-A Receptors (EphAs) represent the largest family of receptor tyrosine kinases in the mammalian genome. They regulate various signaling pathways through a number of downstream effectors.[1] EphAs, which are localized on dendritic spines of neurons, bind Ephrins which are bound to the membranes of astrocytes.[2] There are fourteen Eph receptors and eight ephrin ligands in the human genome.[3] Binding of ephrin ligands triggers Eph receptor clustering, autophosphorylation, and downstream signaling cascades that cause cytoskeletal rearrangements and changes in cell adhesion resulting in dendritic spine retraction.[4] The pathway connecting Eph activation and dendritic growth cone collapse has been well established. Upon Eph activation, ERK activity is inhibited leading to activation of TSC2. TSC2 inhibits RHEB which activates mTOR, a critical regulator of protein synthesis. As can be seen in the image at the left, ephrin stimulation reduces protein synthesis in neurons and results in growth cone collapse.[1] This has been validated in mouse models in which EphA4-knockout mice develop dendritic spines that are significantly longer than normal in the hippocampus.[2] A notable feature of Eph receptors and ephrins is that their interaction triggers bidirectional signals between the engaged cells. Forward signals are mediated by the Eph receptors in dendritic spines and reverse signals by the ephrins in ligand expressing astrocytes.[2] Tuberous sclerosis complex (TSC) is an autosomal dominant disease characterized by the presence of benign tumors called hamartomas, which can affect virtually every organ system in the body.[1] TSC is caused by mutations in the TSC1 or TSC2 genes. The loss of TSC2 and subsequent overexpression of constitutively active Rheb reduces the repulsive responsiveness of growth cones to ephrin signals, resulting in dendritic spine abnormalities.[1] Such abnormalities are often associated with neuropsychiatric symptoms, including intellectual disability, specific neuropsychological deficits, and epilepsy. Additionally, 25-50% of individuals with TSC develop Autism.[5] The extracellular part of Eph receptors includes the N-terminal ephrin (Ligand)-binding domain (LBD), a cysteine-rich domain (CRD), and two fibronectin Type-III Repeats (FN3).[4] EphA binds ephrins with its LBD. Most ephrins have a similar rigid structure which includes four loops, AB, CD, FG, & GH. The LBD of EphA4 is said to be a “structural chameleon,” able bind both A and B class ephrins, explaining the basis for EphA-type cross-class reactivity.[3] The overall structure of the EphA4 LBD includes four important loops, the BC, DE, GH, & JK loops. EphA4 binds the GH loop of the ephrin ligand within a deep pocket between EphA4 loops DE and JK. It is these loops, DE and JK which undergo the greatest conformational shifts when binding either EphrinA2 or EphrinB2. When binding EphrinA2, EphA4-Arg 162 forms a hydrogen bond with EphrinA2-Leu 138, while EphA4-Met 164 and EphA4-Leu 166 participate in hydrophobic interactions with EphrinA2-Leu 138 and EphrinA2-PHe 136. Although EphA4 binds EphrinB2 in the same binding pocket, the local interactions are significantly different. Most notably, the alpha helix present in the EphA4-EphrinA2 JK Loop is abolished in the EphA4-EphrinB2 structure. This is due to the steric clash that would occur between EphrinB2-Trp 122 and EphA4 Met 164. Instead, EphA4-Arg 162 and EphrinB2-Trp 122 form hydrophobic stacking interactions among other interactions which stabilize the receptor-ligand complex.[3] A morph of the movements EphA4 undergoes to bind EphrinA2 and EphrinB2 can be seen here. Eph-Ephrin complexes form two unique heterotetrameric assemblies consisting of distinct EphA2-EphA2 interfaces. The first tetrameric form is generated by Eph-Eph interactions only within the LBD. The second tetrameric form involves complex interactions in the LBD and in the region near the CRD.[6] These two heterotetramers generate a continuous Eph-ephrin assembly when combined (Alternative Coloring). The proximity of correctly oriented kinase domains in an eph-ephrin tetramer, favors transphosphorylation of the cytoplasmic domains on tyrosine residues. Phosphorylation promotes kinase activity by conferring a conformational order on the activation segment of the kinase domain that favors subsrate binding and subsequent signaling, leading to growth cone collapse in dendritic spines, among other effects.[4]
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