Structural basis for the dominant or recessive character of GLIALCAM mutations found in leukodystrophies

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a type of leukodystrophy characterized by white matter edema, and it is caused mainly by recessive mutations in MLC1 and GLIALCAM genes. These variants are called MLC1 and MLC2A with both types of patients sharing the same clinical phenotype. In addition, dominant mutations in GLIALCAM have also been identified in a subtype of MLC patients with a remitting phenotype. This variant has been named MLC2B. GLIALCAM encodes for an adhesion protein containing two immunoglobulin (Ig) domains and it is needed for MLC1 targeting to astrocyte-astrocyte junctions. Most mutations identified in GLIALCAM abolish GlialCAM targeting to junctions. However, it is unclear why some mutations behave as recessive or dominant. Here, we used a combination of biochemistry methods with a new developed anti-GlialCAM nanobody, double-mutants and cysteine cross-links experiments, together with computer docking, to create a structural model of GlialCAM homo-interactions. Using this model, we suggest that dominant mutations affect different GlialCAM-GlialCAM interacting surfaces in the first Ig domain, which can occur between GlialCAM molecules present in the same cell (cis) or present in neighbouring cells (trans). Our results provide a framework that can be used to understand the molecular basis of pathogenesis of all identified GLIALCAM mutations

Unveiling the role of Her9/Notch pathway in the maintenance of stemnes in the hindbrain boundaries

The generation of cell diversity in the embryonic brain relies on the balance between cell proliferation and differentiation. The hindbrain boundary cells specify at the interface between adjacent compartments. Boundary cells display specialized functions and a specific gene profile. When adjacent compartments are engaged in neurogenesis, boundary cells remain non-neurogenic. To unveil how boundary cells maintain their stemness, we combined functional approaches with high-resolution in vivo imaging in zebrafish embryos. At early stages, boundary cells display enriched her9 and no Notch activity. Later, boundary cells lose her9 enrichment and activate Notch3 that triggers the transition from neuroepithelial to radial glia progenitors that undergo asymmetric divisions. Her9 keeps boundary cells as progenitors by promoting symmetric proliferative divisions and preventing asymmetric cell divisions. Moreover, Her9 potentially controls boundary cell proliferation by inhibiting cdkn1ca and promoting YAP/TAZ-TEAD activity. Altogether, Her9 maintains hindbrain boundary cells as a progenitor pool at early embryonic stages.La generació de diversitat cel·lular al cervell embrionari recau en el balanç entre la proliferació i la diferenciació cel·lular. Les cèl·lules de la frontera del cervell posterior s’especifiquen a la interfície entre compartiments adjacents. Les cèl·lules de la frontera constitueixen funcions especialitzades i un perfil genètic específic. Quan els compartiments adjacents es troben implicats en la neurogènesi, les cèl·lules de la frontera es mantenen no neurogèniques. Per tal d’esclarir com les cèl·lules de la frontera mantenen la seva pluripotència, hem combinat estratègies funcionals amb la presa d’imatges d’alta resolució en viu en embrions de peix zebra. A estadis temprans, les cèl·lules de la frontera mostren un enriquiment de her9 i una absència en l’activitat de Notch. Posteriorment, les cèl·lules de la frontera perden l’enriquiment i activen Notch3, fet que dispara la transició de progenitors neuroepitelials a progenitors de la glia radial que pateixen divisions asimètriques. Her9 manté les cèl·lules de la frontera com a progenitors promovent divisions simètriques proliferatives i prevenint les divisions asimètriques. A més, Her9 també controla potencialment la proliferació de les cèl·lules de la frontera a través de la inhibició de cdkn1ca i del manteniment de l’activitat de YAP/TAZ-TEAD. En conjunt, Her9 manté les cèl·lules de la frontera del cervell posterior com progenitores durant els estadis embrionaris temprans.Programa de Doctorat en Biomedicin

Interplay between notch and YAP/TAZ pathways in the regulation of cell fate during embryo development

Cells in growing tissues receive both biochemical and physical cues from their microenvironment. Growing evidence has shown that mechanical signals are fundamental regulators of cell behavior. However, how physical properties of the microenvironment are transduced into critical cell behaviors, such as proliferation, progenitor maintenance, or differentiation during development, is still poorly understood. The transcriptional co-activators YAP/TAZ shuttle between the cytoplasm and the nucleus in response to multiple inputs and have emerged as important regulators of tissue growth and regeneration. YAP/TAZ sense and transduce physical cues, such as those from the extracellular matrix or the actomyosin cytoskeleton, to regulate gene expression, thus allowing them to function as gatekeepers of progenitor behavior in several developmental contexts. The Notch pathway is a key signaling pathway that controls binary cell fate decisions through cell-cell communication in a context-dependent manner. Recent reports now suggest that the crosstalk between these two pathways is critical for maintaining the balance between progenitor maintenance and cell differentiation in different tissues. How this crosstalk integrates with morphogenesis and changes in tissue architecture during development is still an open question. Here, we discuss how progenitor cell proliferation, specification, and differentiation are coordinated with morphogenesis to construct a functional organ. We will pay special attention to the interplay between YAP/TAZ and Notch signaling pathways in determining cell fate decisions and discuss whether this represents a general mechanism of regulating cell fate during development. We will focus on research carried out in vertebrate embryos that demonstrate the important roles of mechanical cues in stem cell biology and discuss future challenges.This work was funded by the Agencia Española de Investigación AEI-PGC2018-095663-B-I00 and RED2018-102553-T grants (MICIU-FEDER) to CP. DCEXS-UPF is a Unidad de Excelencia María de Maeztu funded by the AEI (CEX2018-000792-M). CE-P is a recipient of a predoctoral FPU fellowship from the Spanish Ministry of Universities (FPU17/06034). CP is a recipient of ICREA Academia award (Generalitat de Catalunya)

The neurogenic fate of the hindbrain boundaries relies on Notch3-dependent asymmetric cell divisions

Elucidating the cellular and molecular mechanisms that regulate the balance between progenitor cell proliferation and neuronal differentiation in the construction of the embryonic brain demands the combination of cell lineage and functional approaches. Here, we generate the comprehensive lineage of hindbrain boundary cells by using a CRISPR-based knockin zebrafish transgenic line that specifically labels the boundaries. We unveil that boundary cells asynchronously engage in neurogenesis undergoing a functional transition from neuroepithelial progenitors to radial glia cells, coinciding with the onset of Notch3 signaling that triggers their asymmetrical cell division. Upon notch3 loss of function, boundary cells lose radial glia properties and symmetrically divide undergoing neuronal differentiation. Finally, we show that the fate of boundary cells is to become neurons, the subtype of which relies on their axial position, suggesting that boundary cells contribute to refine the number and proportion of the distinct neuronal populations.This work was funded by grant PGC2018-095663-B-I00 from Spanish Ministry of Science and Innovation (MICIN), Agencia Estatal de Investigación (AEI), and Fondo Europeo de Desarrollo Regional (FEDER) to C.P. The Department of Medicine and Life Sciences (UPF) is an Unidad de Excelencia María de Maeztu funded by the MICIN and the AEI (DOI: 10.13039/501100011033) Ref: CEX2018-000792-M. C.E.P. is a recipient of a predoctoral FPU fellowship from the Spanish Ministry of Universities. F.U.’s work was supported by PGC2018-101643-B-I00 (MICIN/AEI-FEDER). C.P. is a recipient of ICREA Academia award (Generalitat de Catalunya)

Yap/Taz-TEAD activity links mechanical cues to progenitor cell behavior during zebrafish hindbrain segmentation

Cells perceive their microenvironment through chemical and physical cues. However, how the mechanical signals are interpreted during embryonic tissue deformation to result in specific cell behaviors is largely unknown. The Yap/Taz family of transcriptional co-activators has emerged as an important regulator of tissue growth and regeneration, responding to physical cues from the extracellular matrix, and to cell shape and actomyosin cytoskeletal changes. In this study, we demonstrate the role of Yap/Taz-TEAD activity as a sensor of mechanical signals in the regulation of the progenitor behavior of boundary cells during zebrafish hindbrain compartmentalization. Monitoring of in vivo Yap/Taz activity during hindbrain segmentation indicated that boundary cells responded to mechanical cues in a cell-autonomous manner through Yap/Taz-TEAD activity. Cell-lineage analysis revealed that Yap/Taz-TEAD boundary cells decreased their proliferative activity when Yap/Taz-TEAD activity ceased, which preceded changes in their cell fate from proliferating progenitors to differentiated neurons. Functional experiments demonstrated the pivotal role of Yap/Taz-TEAD signaling in maintaining progenitor features in the hindbrain boundary cell population.This work was supported by a La Marató-TV3 grant (345/C/2014) and by a Ministerio de Ciencia, Innovación y Universidades grant (BFU2015-67400-P and BFU2016-81887-REDT/AEI) to C.P.; and by a Unidad de Excelencia María de Maetzu grant (MDM-2014-0370) to the Department of Experimental and Health Sciences of the Pompeu Fabra University (DCEXS-UPF). A.V. was a recipient of a predoctoral fellowship from the Fundació La Caixa, and C.E.-P. holds a predoctoral fellowship from the Ministerio de Ciencia, Innovación y Universidades (FPU). J.T. was a recipient of a postdoctoral Beatriu de Pinos fellowship (AGAUR, Generalitat de Catalunya). C.P. is recipient of an Institució Catalana per la Recerca i Estudis Avançats Academia award (Generalitat de Catalunya)