Gerstmann K. et al. Medical Research Archives, vol. 6, issue 3, March 2018 issue Page 1 of 26 RESEARCH ARTICLE The role of the Eph/ephrin family during cortical development and cerebral malformations Katrin Gerstmann1 and Geraldine Zimmer2* Authors’ affiliations: 1 Institute NeuroMyoGène, CNRS UMR 5310, INSERM U1217, Université Lyon 1, Villeurbanne, France, E-mail: ([email protected]) 2 University Hospital Jena, Institute of Human Genetics, Am Klinikum 1, 07747 Jena, Germany, E-mail: ([email protected]) * Corresponding Author Abstract Neuronal numbers and the associated size of the cerebral cortex, surface folding and laminar organisation are determined by precise developmental mechanisms that are orchestrated by several intrinsic and extrinsic molecules. Abnormalities during development can cause manifold microscopic and macroscopic cortical malformations, mostly accompanied by clinical consequences such as mental disorders, intellectual disabilities, or epileptic seizures. Most cortical malformations and associated neurological disorders result from genetic defects, however the cellular mechanisms remain complex and poorly understood. Eph receptor tyrosine kinases and their ligands, the ephrins, are abundantly expressed in the developing brain where they regulate several developmental processes that are crucial for correct brain formation. Ephrin family members represent membrane-bound proteins that are key players in complex short-range cell-cell communication. In addition, mechanisms for long-range interactions have been described recently. Several ephrins have already been shown to control cell cycle dynamics of cortical stem cells during corticogenesis and the positioning of postmitotic neurons. In addition, mutations in genes encoding for members of the Eph/ephrin family are implicated in mental disorders, although the underlying mechanisms remain to be elucidated. A deeper understanding of Eph/ephrin interactions during cerebral cortex development will be beneficial to shed light on developmental disabilities. Here, we discuss the function of Eph/ephrin system during the different processes of corticogenesis and the impact on cerebral malformations. Keywords: ephrins, corticogenesis, malformation Copyright 2018 KEI Journals. All Rights Reserved http://journals.ke-i.org/index.php/mra Gerstmann K. et al. Medical Research Archives, vol. 6, issue 3, March 2018 issue Page 2 of 26 Introduction amplifying progenitor cells. These intermediate progenitors translocate their The formation of the cerebral cortex, the cell bodies more basally, forming the seat of higher cognitive functions in the subventricular zone and dividing mammalian brain, is a highly sophisticated symmetrically to indirectly generate the process requiring the precise interplay of majority of neurons (1, 4, 5). The transient several developmental steps. Perturbation amplifying progenitors are already present of neural development can result in at early stages of neurogenesis and are manifold cortical malformations, often suggested to contribute to the neuronal associated with psychological disorders and production of all cortical layers (1, 4, 6). mental disabilities. The human cerebral cortex is generated during the first two The precise regulation of the progenitor trimesters of gestation. Within this period, pool is crucial for the correct development neuronal stem cells residing in the of the cerebral cortex. Overproduction of epithelium of the neural tube generate stem cells can lead to megalencephaly, diverse subtypes of progenitor, neuronal whereas the loss of neuronal stem cells and glial cells. These stem cells display a caused by precocious differentiation or polarized morphology with a basal process increased apoptosis results in anchored to the pial surface and an apical microencephaly (7). The cerebral cortex is process that is in contact with the formed in a temporally regulated inside-out cerebrospinal fluid. During mitotic cell fashion. Neurons destined for deep layers division, they perform characteristic are generated first, whereas those born later interkinetic nuclear migration synchronized migrate through the already existing deeper with cell cycle progression. The S-phase layers to form the superficial ones (8). takes place in the basal part of the Thereby, the radial processes of the ventricular zone, whereas G2 nuclei progenitor cells serve as a scaffold guiding translocate apically towards the ventricular the migrating post-mitotic neurons to their surface, where the M-phase occurs (1). target layers. Impaired neuronal migration However, the relevance of this nuclear is implicated in cortical malformations like translocation along the vertical axis is not lissencephaly, polymicrogyria, or hetero- clear and until now no human cortical topia (9). Interestingly, defects in cellular malformations are associated with defects adhesion result in equal phenotypes (9), as in these nuclear movements. For this adhesion is critical for neuronal migration. reason, we will not discuss it further. Eph (Erythropoietin-producing hepato- Before the first neurons are generated, cellular) receptor tyrosine kinases, and their neuronal stem cells divide symmetrically to membrane-bound ligands, the ephrins (Eph expand the pool of progenitor cells (2, 3). receptor interacting proteins), are critically At the onset of neurogenesis, stem cells involved in the regulation of developmental divide asymmetrically to generate post- processes underlying the formation of the mitotic neurons or intermediate, transient cerebral cortex including proliferation, Copyright 2018 KEI Journals. All Rights Reserved http://journals.ke-i.org/index.php/mra Gerstmann K. et al. Medical Research Archives, vol. 6, issue 3, March 2018 issue Page 3 of 26 apoptosis, cellular adhesion, division interact with ephrinA1 (14, 19, 20) and orientation, and cell fate determination. EphrinB2 (21), respectively. Furthermore, Here we discuss the Eph/ephrin-dependent ephrins display different affinities to regulation of developmental processes and distinct receptors. For example, ephrinA5 potential implications in cortical has a stronger affinity to EphA4 and EphA5 malformations and neurological disorders. than to EphA3 (22). A special feature of the Eph/ephrin family Structure, interactions and signalling of is the potential of bidirectional signalling the Eph/ephrin family (Figure 1). Forward signalling describes the induction of intracellular signalling in Eph receptor tyrosine kinases and their the receptor-bearing cell upon ligand ephrin ligands are widely expressed in the binding inducing receptor auto- developing brain (10-13) and numerous phosphorylation and subsequent activation studies have demonstrated their function in of downstream targets. Although ephrins do various developmental processes. The 15 not exhibit a catalytic activity, they can Eph receptor tyrosine kinases and 9 ephrin trigger a reverse signalling in ligand- ligands are classified into two groups, expressing cells after binding to cognate according to their structural similarities and receptors (17, 23-25). For class B-ephrins, binding affinities (Figure 1; 14). Class-A the reverse signalling results in the ephrins are tethered to the membrane by a phosphorylation of their cytoplasmic glycosylphosphatidyl-inositol anchor, domain, which triggers signal transduction. whereas class-B ephrins are trans- Although class A-ephrins do not possess an membrane proteins with a cytoplasmic intracellular domain, there is some evidence domain and a PDZ binding motif (15, 16). for the initiation of reverse signalling Eph receptor tyrosine kinases are through interactions with adapter proteins monomeric receptors that homodimerize (26, 27). and phosphorylate each other upon ligand binding. Furthermore, the receptors contain Another unique feature of Eph receptors a C-terminal PDZ-domain that potentially among tyrosine kinases is the capability to modulates intracellular signalling (15, 17, form higher-order clusters (28, 29). Such 18). EphA receptors bind promiscuously to protein assemblies can include more than A-ligands and EphB receptors to B-ephrins one Eph receptor, and the size as well as the (Figure 2). However, class-crossing composition of the cluster determines the interactions as well as exclusive binding cellular response. Various factors regulate affinities have also been described (14, 19, cluster size and several Eph receptors have 20). For instance, EphA4 shows class- different capacities to cluster (30). Thus, the crossing interactions with ephrinB2 and nature and specificity of a biological ephrinB3, whereas EphB2 binds ephrinA5. response arises from the particular In turn, EphA1 and EphB4 exclusively clustering of a receptor. Copyright 2018 KEI Journals. All Rights Reserved http://journals.ke-i.org/index.php/mra Gerstmann K. et al. Medical Research Archives, vol. 6, issue 3, March 2018 issue Page 4 of 26 Figure 1: Schematic structure of Eph-receptors and their membrane-bound ephrin ligands. Class A-ephrins are tethered to the membrane by a glycosylphosphatidyl-inositol anchor, whereas class B-ephrins are trans-membrane proteins with a cytoplasmic domain and a PDZ binding motif. Upon ligand binding, forward signalling is induced in the receptor-bearing cell. Although ephrins do not exhibit a catalytic activity, they can trigger a reverse signalling after receptor binding. Copyright 2018 KEI Journals. All Rights Reserved http://journals.ke-i.org/index.php/mra Gerstmann K. et al. Medical Research Archives,