Role for Gli3 in the formation of the major axonal tracts in the telencephalon

In the adult brain, the thalamocortical tract conveys sensory information from the external environment to the cortex. The cortex analyzes and integrates this information and sends neural responses back to the thalamus through the corticothalamic tract. To reach their final target both thalamocortical and corticothalamic axons have to cover long distances during embryogenesis, changing direction several times and passing through different brain territories. The ventral telencephalon plays a major role in the early development of these tracts. At least three main axon guidance mechanisms act in the ventral telencephalon. First, two different populations of pioneer neurons in the lateral ganglionic eminence (LGE) (LGE pioneer neurons) and medial ganglionic eminence (MGE) (MGE pioneer neurons) provide scaffolds which allow growing corticothalamic and thalamocortical axons to cross the pallium sub pallium boundary (PSPB) and the diencephalic telencephalic boundary (DTB), respectively. Second, the ventral telencephalon forms a permissive corridor for thalamic axons by tangential migration of Isl1 and Ebf1 expressing cells from the LGE into the MGE. Finally, thalamortical and corticothalamic axons guide each other once they have met in the ventral telencephalon (“handshake hypothesis”). The Gli3 transcription factor has been shown to be essential for normal early embryonic regionalization of the mammalian forebrain, although roles of Gli3 in later aspects of forebrain development, like the formation of axonal connections, have not been investigated previously. Here, I present the analysis of axonal tract development in the forebrain of the Gli3 hypomorphic mutant mouse Polydactyly Nagoja (Pdn). These animals lack the major axonal commissures of the forebrain: the corpus callosum, the hippocampal commissure, the anterior commissure and the fimbria. In addition, DiI injections and neurofilament (NF) staining showed defects in the formation of the corticothalamic and thalamocortical tracts. Although the Pdn/Pdn cortex forms early coticofugal neurons and their axons, these axons do not penetrate the LGE and instead run along the PSPB. Later in development, although a thick bundle of Pdn/Pdn cortical axons is still observed to project along the PSPB, some Pdn/Pdn cortical axons eventually enter the ventral telencephalon navigating along several abnormal routes until they reach thalamic regions. In contrast, Pdn/Pdn thalamic axons penetrate into the ventral telencephalon at early stages of thalamic tract development. However, rostrally they deviate from their normal trajectory, leaving the internal capsule prematurely and only few of them reach the developing cortex. Caudally, an ectopic Pdn/Pdn dorsal thalamic axon tract projects ventrally in the ventral telencephalon not entering the internal capsule at all. These defects are still observed in newborn Pdn/Pdn mutant mice. Next, I investigated the developmental mechanisms causing these pathfindings defects. No obvious defects are present in Pdn/Pdn cortical laminae formation and in the patterning of the Pdn/Pdn dorsal thalamus. In addition, Pdn/Pdn thalamocortical axons are able to respond to ventral telencephalic guidance cues when transplanted into wild type brain sections. However, these axonal pathfinding defects correlate with patterning defects of the Pdn/Pdn LGE. This region is partially ventralized and displays a reduction in the number of postmitotic neurons in the mantle zone due to an elongated cell cycle length of LGE progenitor cells. Finally, Pdn/Pdn mutant display an upregulation of Shh expression and Shh signalling in the ventral telencephalon. Interestingly, these patterning defects lead to the absence of DiI back-labelled LGE pioneer neurons, which correlates with the failure of corticothalamic axons to penetrate the ventral telencephalon. In addition, ventral telencephalic thalamocortical guidance mistakes happen at the same time of abnormal formation of the corridor cells. Taken together these data reveal a novel role for Gli3 in the formation of ventral telencephalic intermediate cues important for the development of the thalamocortical and corticothalamic connections. Indeed, Pdn animals are the first known mutants with defective development of the LGE pioneer neurons, and their study provides a link between early patterning defects and axon pathfinding in the developing telencephalon

Identification of ä-Spectrin Domains Susceptible to Ubiquitination

Previously, we demonstrated that alpha-spectrin is a substrate for the ubiquitin system and that this conjugation is a dynamic process (Corsi, D., Galluzzi, L., Crinelli, R., and Magnani, M. (1995) J. Biol. Chem. 270, 8928-8935). In this study, we mapped the sites of ubiquitination on erythrocyte alpha-spectrin. A peptide map of digested alpha-spectrin, previously submitted to in vitro 125I-ubiquitin conjugation, revealed the presence of four distinct labeled bands with Mr 40,000, 36,000, 29,000, and 25,500. Western blotting experiments using antibodies against each alpha-spectrin domain revealed that only IgG anti-alphaIII domain recognized the 125I-labeled ubiquitin peptide of 29 kDa, whereas the IgG anti-alphaV domain recognized the Mr 40,000 125I-ubiquitin-labeled peptide. The other two labeled bands of Mr 36,000 and Mr 25,500 were identified as tetra and tri multiubiquitin chains. Ubiquitination of the alphaIII and alphaV domains was further confirmed by anti-alpha-spectrin domain immunoaffinity chromatography. Endoprotease Lys C-digested spectrin conjugated previously to 125I-ubiquitin was incubated with antibodies against each trypsin-resistant domain of alpha-spectrin. Gamma counting of the radiolabeled antigen-antibody complexes purified by protein A chromatography showed labeling in the IgG anti-alphaIII and anti-alphaV complexes alone. Domain alphaIII is not associated with any known function, whereas domain alphaV contains the nucleation site for the association of the alpha and beta chains. Ubiquitination of the latter domain suggests a role for ubiquitin in the modulation of the stability, deformability, and viscoelastic properties of the erythrocyte membrane

Arretium or Arezzo? A Neural Approach to the Identification of Place Names in Historical Texts

This paper presents the application of a neural architecture to the identification of place names in English historical texts. We test the impact of different word embeddings and we compare the results to the ones obtained with the Stanford NER module of CoreNLP before and after the retraining using a novel corpus of manually annotated historical travel writings

Functional properties of in vitro excitatory cortical neurons derived from human pluripotent stem cells

The in vitro derivation of regionally defined human neuron types from patient‐derived stem cells is now established as a resource to investigate human development and disease. Characterization of such neurons initially focused on the expression of developmentally regulated transcription factors and neural markers, in conjunction with the development of protocols to direct and chart the fate of differentiated neurons. However, crucial to the understanding and exploitation of this technology is to determine the degree to which neurons recapitulate the key functional features exhibited by their native counterparts, essential for determining their usefulness in modelling human physiology and disease in vitro. Here, we review the emerging data concerning functional properties of human pluripotent stem cell‐derived excitatory cortical neurons, in the context of both maturation and regional specificity. [Image: see text

Gli3 is required in Emx1+ progenitors for the development of the corpus callosum

AbstractThe corpus callosum (CC) is the largest commissure in the forebrain and mediates the transfer of sensory, motor and cognitive information between the cerebral hemispheres. During CC development, a number of strategically located glial and neuronal guidepost structures serve to guide callosal axons across the midline at the corticoseptal boundary (CSB). Correct positioning of these guideposts requires the Gli3 gene, mutations of which result in callosal defects in humans and mice. However, as Gli3 is widely expressed during critical stages of forebrain development, the precise temporal and spatial requirements for Gli3 function in callosal development remain unclear. Here, we used a conditional mouse mutant approach to inactivate Gli3 in specific regions of the developing telencephalon in order to delineate the domain(s) in which Gli3 is required for normal development of the corpus callosum. Inactivation of Gli3 in the septum or in the medial ganglionic eminence had no effect on CC formation, however Gli3 inactivation in the developing cerebral cortex led to the formation of a severely hypoplastic CC at E18.5 due to a severe disorganization of midline guideposts. Glial wedge cells translocate prematurely and Slit1/2 are ectopically expressed in the septum. These changes coincide with altered Fgf and Wnt/β-catenin signalling during CSB formation. Collectively, these data demonstrate a crucial role for Gli3 in cortical progenitors to control CC formation and indicate how defects in CSB formation affect the positioning of callosal guidepost cells

Letter

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