Project description:The mammalian telencephalon contains a tremendous diversity of GABAergic projection neuron and interneuron types, that originate in a germinal zone of the embryonic basal ganglia. How genetic information in this transient structure is transformed into different cell types is not yet fully understood. Using a combination of in vivo lineage tracing, CRISPR perturbation and ChIP-seq in mice, we found that the transcription factor MEIS2 favors the development of projection neurons through genomic binding sites in regulatory enhancers of projection neuron specific genes. MEIS2 requires the presence of the homeodomain transcription factor DLX5 to direct its functional activity towards these sites. In interneurons, the activation of projection neuron specific enhancers by MEIS2 and DLX5 is repressed by the transcription factor LHX6. When MEIS2 carries a mutation associated with intellectual disability in humans, it is less effective at activating enhancers involved in projection neuron development. This suggests that GABAergic differentiation may be impaired in patients carrying this mutation. Our research has uncovered a mechanism by which the selective activation of enhancers plays a crucial role in the establishment of neuronal identity, as well as in potential pathological mechanisms
Project description:The mammalian telencephalon contains distinct GABAergic projection neuron and interneuron types, originating in the germinal zone of the embryonic basal ganglia. How genetic information in the germinal zone determines cell types is unclear. Here we use a combination of in vivo CRISPR perturbation, lineage tracing and ChIP-sequencing analyses and show that the transcription factor MEIS2 favors the development of projection neurons by binding enhancer regions in projection-neuron-specific genes during mouse embryonic development. MEIS2 requires the presence of the homeodomain transcription factor DLX5 to direct its functional activity toward the appropriate binding sites. In interneuron precursors, the transcription factor LHX6 represses the MEIS2-DLX5-dependent activation of projection-neuron-specific enhancers. Mutations of Meis2 result in decreased activation of regulatory enhancers, affecting GABAergic differentiation. We propose a differential binding model where the binding of transcription factors at cis-regulatory elements determines differential gene expression programs regulating cell fate specification in the mouse ganglionic eminence.
Project description:Pre–B-cell leukemia homeobox (PBX) and myeloid ecotropic viral integration site (MEIS) proteins control cell fate decisions in many physiological and pathophysiological contexts, but how these proteins function mechanistically remains poorly defined. Focusing on the first hours of neuronal differentiation of adult subventricular zone–derived stem/progenitor cells, we describe a sequence of events by which PBX-MEIS facilitates chromatin accessibility of transcriptionally inactive genes: In undifferentiated cells, PBX1 is bound to the H1-compacted promoter/proximal enhancer of the neuron-specific gene doublecortin (Dcx). Once differentiation is induced, MEIS associates with chromatin-bound PBX1, recruits PARP1/ARTD1, and initiates PARP1-mediated eviction of H1 from the chromatin fiber. These results for the first time link MEIS proteins to PARP-regulated chromatin dynamics and provide a mechanistic basis to explain the profound cellular changes elicited by these proteins.
Project description:Vertebrate axial skeletal patterning is controlled by coordinated collinear expression of Hox genes and axial level-dependent activity of Hox protein combinations. Transcription factors of the Meis family act as cofactors of Hox proteins and profusely bind to Hox complex DNA, however their roles in mammalian axial patterning have not been established. Similarly, retinoic acid (RA) is known to regulate axial skeletal element identity through the transcriptional activity of its receptors, however whether this role is related to Meis/Hox regulation or functions in axial patterning remains unknown. Here we study the role of Meis factors in axial skeleton formation and its relationship to the RA pathway by characterizing Meis1, Meis2 and Raldh2 mutant mice. We report that Meis and Raldh2 regulate each other in a positive feedback regulatory loop that controls axial skeletal identity. Meis elimination produces homeotic transformations similar to those found in Raldh2 and anterior-Hox mutants and disrupts the expression of Hox target genes without changing the transcriptional profiles of Hox complexes. We propose that Meis regulates vertebrate axial skeleton patterning by exclusively affecting Hox protein function, and that alterations in RA levels can produce homeotic transformations without altering Hox transcription through regulating Meis expression.
Project description:The mammalian neocortex comprises an enormous diversity regarding cell types, morphology, and connectivity. In this work, we discover a post-transcriptional mechanism of gene expression regulation, protein translation, as a determinant of cortical neuron identity. We find specific upregulation of protein synthesis in the progenitors of later-born neurons and show that translation rates and concomitantly protein half-lives are inherent features of cortical neuron subtypes. In a small molecule screening, we identify Ire1a as a regulator of Satb2 expression and neuronal polarity. In the developing brain, Ire1a regulates global translation rates, coordinates ribosome traffic, and the expression of eIF4A1. Furthermore, we demonstrate that the Satb2 mRNA translation requires eIF4A1 helicase activity towards its 5’-untranslated region. Altogether, we show that cortical neuron diversity is generated by mechanisms operating beyond gene transcription, with Ire1a-safeguarded proteostasis serving as an essential regulator of brain development.
Project description:Direct neuronal reprogramming provides a powerful strategy to generate neurons from somatic cells without passing through a pluripotent state. However, the post-transcriptional mechanisms that refine neuronal identity downstream of fate induction remain poorly understood. Here, we identify alternative splicing as a critical regulator of neuronal identity during fibroblast-to-neuron reprogramming. Knockdown of the RNA-binding protein MBNL1 establishes a distinct reprogramming state (AMmnp) that enhances neuronal maturation and strongly biases induced neurons toward an inhibitory, GABAergic identity, without markedly altering overall conversion efficiency. Transcriptomic analyses revealed coordinated repression of extracellular matrix-associated gene programs together with widespread remodeling of alternative splicing. Among MBNL1-dependent splicing events, NUMA1 exon 16 emerged as a prominent target. Functional manipulation of NUMA1 isoforms demonstrated that exon 16 inclusion suppresses neuronal marker expression specifically in the AMmnp context, whereas exon skipping is permissive for neuronal differentiation. These findings establish MBNL1-dependent alternative splicing of NUMA1 exon 16 as a post-transcriptional mechanism that refines neuronal identity acquisition during direct reprogramming, highlighting alternative splicing as an active determinant of induced neuron quality and subtype specification.
Project description:Vertebrate limbs develop by integrating signals that control patterning along three main orthogonal axes. Flank-produced retinoic acid (RA) is initially required for limb induction and establishment of the apical ectodermal ridge (AER), a distal signaling center that produces fibroblast growth factors (FGFs), which are essential for limb growth and distalization. Once the AER is established, RA:FGF antagonism determines the restricted expression of a set of genes that control limb proximodistal patterning. Essential for this antagonism is the activation by FGF of the RA-degrading enzyme CYP26B1 in the distal limb bud. In addition, sonic hedgehog produced from the zone of polarizing activity (ZPA) is essential for distal limb anteroposterior patterning and contributes to RA reduction by cooperating in CYP26B1 activation. Meis transcription factors are expressed in the proximal limb bud, are activated by RA and can regulate proximodistal limb development; however, the mechanisms underlying their activity remain unknown. Here we studied Meis function in the mouse limb bud through Meis2 conditional overexpression and elimination of Meis1 and Meis2. We found that Meis activity is first required for limb bud initiation and the proper establishment of the AER and ZPA signaling centers, and subsequently for the development of proximal limb structures. Functional genomic analyses reveal that Meis is an important conveyor of the RA:FGF antagonism through the regulation of components of the RA and FGF signaling pathways, including CYP26B1. In addition, Meis regulates a set of proximal limb genes controlling proximodistal patterning and differentiation. Our work reveals a regulatory module essential for limb patterning and potentially co-opted in other patterning processes involving RA:FGF antagonism.