Disorders of neurogenesis and cortical development

Abstract

The development of the cerebral cortex requires complex sequential processes that have to be precisely orchestrated. The localization and timing of neuronal progenitor proliferation and of neuronal migration define the identity, laminar positioning, and specific connectivity of each single cortical neuron. Alterations at any step of this organized series of events-due to genetic mutations or environmental factors-lead to defined brain pathologies collectively known as malformations of cortical development (MCDs), which are now recognized as a leading cause of drug-resistant epilepsy and intellectual disability. In this heterogeneous group of disorders, macroscopic alterations of brain structure (eg, heterotopic nodules, small or absent gyri, double cortex) can be recognized and probably subtend a general reorganization of neuronal circuits. In this review, we provide an overview of the molecular mechanisms that are implicated in the generation of genetic MCDs associated with aberrations at various steps of neurogenesis and cortical development.

Keywords: circuit formation; cortical lamination; neuronal connectivity neuronal migration; progenitor proliferation.

Figure 1.. (Opposite). Schematic representation of cortical development in normal and pathological conditions. (A) During development, radial glia cells (in white) divide symmetrically to expand the pool of progenitor cells and asymmetrically to produce postmitotic neurons (1). Neurons acquire a multipolar morphology and pause in the SVZ (2), before becoming bipolar and starting to migrate along radial glia fibers (3). Once they reach their final position in the CP, neurons mature and establish precise input and output connections (4). Genetic or environmental factors can induce defects at each step of this process. (B) Abnormal progenitor proliferation can be caused by a defective timing of symmetric-to-asymmetric division switch or by defects in the orientation of the mitotic spindle. These lead to the production of an aberrant number of neurons (eg, an increase in neuronal production, as depicted in the figure), thus often causing megalencephaly or microcephaly. (C) Alterations in radial glia scaffold (eg, defective radial glia anchoring to the apical membrane) impair neuronal proliferation and/or migration and can lead to the accumulation of neurons in the VZ and to the formation of nodular heterotopia. (D) As opposite, defective formation of the basal membrane leads to neuronal overmigration and accumulation on the pial surface, with the formation of cobblestone malformations. (E) Delayed or aberrant neuronal migration can arise from cell-autonomous defects (ie, neurons are intrinsically unable to migrate properly to the CP) and lead to cortical layering alterations as subcortical band heterotopia. (F) Even when early defects in neurogenesis are caught up and do not lead to macroscopic alterations of brain structure or layering, subtler defects in neurite extension, synaptogenesis, short-term and long-term connectivity with target cells can also be detected. MZ: marginal zone; CP: cortical plate; IZ: intermediate zone; SVZ: subventricular zone; VZ: ventricular zone.

Figure 2.. (Opposite). Novel approaches for measuring input-output connectivity. (A) Input connectivity with the rabies monsynaptic approach. A1) MCD mouse models are crossed with the rabies TVA-G mouse line, which expresses the avian EnvA receptor TVA as well as the rabies virus glycoprotein G necessary for rabies retrograde transport downstream of a floxed stop cassette. A2) In utero electroporation of a Cre-RFP plasmid labels cortical neurons of interest (depending on the mouse age at electroporation) in red and makes them starter cells, responsive to the modified rabies virus coated with the EnvA ligand. A3) In postnatal mice, stereotaxic injection of the modified rabies virus expressing GFP (DG-EGFP-RV) determines the infection of red starter cells only, which thus express both GFP and RFP and appear as yellow. A4) Since starter cells express protein G, the rabies virus can retrogradely pass one synapse and thus label immediate presynaptic partners of the starter cells in green. (B) Output connectivity with anterograde AAV-hSyn-Cre serotype-1. This virus has intrinsic anterograde properties so that it induces Tomato expression exclusively in direct post-synaptic partners of infected neurons. B1) MCD mouse models are crossed with the Ai14 reporter line. B2) In postnatal mice, minute amounts of AAV1-hSyn-CRE virus are co-injected with the Cre-dependent AAV-CAG-FLEX-GFP in the cortical region of interest. B3) Cre-recombined, starter cells turn red (because Cre removes the stop cassette in Ai14 locus) and green (because Cre removes stop cassette in co-injected AAV-CAG-FLEX-GFP). Immediate postsynaptic partners receive AAV1-hSyn-CRE by anterograde transport and thus express tomato only.