Zachary B. Gaber

Coordinated Actions of the Forkhead Protein Foxp1 and Hox Proteins in the Columnar Organization of Spinal Motor Neurons

The formation of locomotor circuits depends on the spatially organized generation of motor columns that innervate distinct muscle and autonomic nervous system targets along the body axis. Within each spinal segment, multiple motor neuron classes arise from a common progenitor population; however, the mechanisms underlying their diversification remain poorly understood. Here, we show that the Forkhead domain transcription factor Foxp1 plays a critical role in defining the columnar identity of motor neurons at each axial position. Using genetic manipulations, we demonstrate that Foxp1 establishes the pattern of LIM-HD protein expression and accordingly organizes motor axon projections, their connectivity with peripheral targets, and the establishment of motor pools. These functions of Foxp1 act in accordance with the rostrocaudal pattern provided by Hox proteins along the length of the spinal cord, suggesting a model by which motor neuron diversity is achieved through the coordinated actions of Foxp1 and Hox proteins.

Sox9 and NFIA Coordinate a Transcriptional Regulatory Cascade during the Initiation of Gliogenesis

Transcriptional cascades that operate over the course of lineage development are fundamental mechanisms that control cellular differentiation. In the developing central nervous system (CNS), these mechanisms are well characterized during neurogenesis, but remain poorly defined during neural stem cell commitment to the glial lineage. NFIA is a transcription factor that plays a crucial role in the onset of gliogenesis; we found that its induction is regulated by the transcription factor Sox9 and that this relationship mediates the initiation of gliogenesis. Subsequently, Sox9 and NFIA form a complex and coregulate a set of genes induced after glial initiation. Functional studies revealed that a subset of these genes, Apcdd1 and Mmd2, perform key migratory and metabolic roles during astro-gliogenesis, respectively. In sum, these studies delineate a transcriptional regulatory cascade that operates during the initiation of gliogenesis and identifies a unique set of genes that regulate key aspects of astro-glial precursor physiology during development.

Foxp-Mediated Suppression of N-Cadherin Regulates Neuroepithelial Character and Progenitor Maintenance in the CNS

Neuroepithelial attachments at adherens junctions are essential for the self-renewal of neural stem and progenitor cells and the polarized organization of the developing central nervous system. The balance between stem cell maintenance and differentiation depends on the precise assembly and disassembly of these adhesive contacts, but the gene regulatory mechanisms orchestrating this process are not known. Here, we demonstrate that two Forkhead transcription factors, Foxp2 and Foxp4, are progressively expressed upon neural differentiation in the spinal cord. Elevated expression of either Foxp represses the expression of a key component of adherens junctions, N-cadherin, and promotes the detachment of differentiating neurons from the neuroepithelium. Conversely, inactivation of Foxp2 and Foxp4 function in both chick and mouse results in a spectrum of neural tube defects associated with neuroepithelial disorganization and enhanced progenitor maintenance. Together, these data reveal a Foxp-based transcriptional mechanism that regulates the integrity and cytoarchitecture of neuroepithelial progenitors.

Hox5 interacts with Plzf to restrict Shh expression in the developing forelimb

Significance Mammalian Hox genes are important for limb development. Posterior abdominal B (AbdB) Hox groups (Hox9–Hox13) are required for establishment of the limb proximodistal axis. In addition, Hox9 genes control the onset of Hand2 expression in the posterior forelimb, and HoxA/D AbdB genes are responsible for the initiation and maintenances of Sonic Hedgehog (Shh). In this study, we generated Hox5 triple mutants, resulting in embryos with severe forelimb anterior patterning defects. We found that Hox5 proteins interact with promyelocytic leukemia zinc finger to restrict Shh expression in the forelimb bud. The hindlimb in Hox5 mutants develops normally, revealing distinct differences in anteroposterior field establishment in the forelimb and hindlimb and unanticipated roles for non-AbdB Hox genes, including HoxB and HoxC group genes, in limb development. To date, only the five most posterior groups of Hox genes, Hox9–Hox13, have demonstrated loss-of-function roles in limb patterning. Individual paralog groups control proximodistal patterning of the limb skeletal elements. Hox9 genes also initiate the onset of Hand2 expression in the posterior forelimb compartment, and collectively, the posterior HoxA/D genes maintain posterior Sonic Hedgehog (Shh) expression. Here we show that an anterior Hox paralog group, Hox5, is required for forelimb anterior patterning. Deletion of all three Hox5 genes (Hoxa5, Hoxb5, and Hoxc5) leads to anterior forelimb defects resulting from derepression of Shh expression. The phenotype requires the loss of all three Hox5 genes, demonstrating the high level of redundancy in this Hox paralogous group. Further analyses reveal that Hox5 interacts with promyelocytic leukemia zinc finger biochemically and genetically to restrict Shh expression. These findings, along with previous reports showing that point mutations in the Shh limb enhancer lead to similar anterior limb defects, highlight the importance of Shh repression for proper patterning of the vertebrate limb.

PLZF regulates fibroblast growth factor responsiveness and maintenance of neural progenitors.

A transcription factor called Promyelocytic Leukemia Zinc Finger (PLZF) calibrates the balance between spinal cord progenitor maintenance and differentiation by enhancing their sensitivity to mitogens that are present in developing embryos.

All the Embryo's a Stage, and Olig2 in Its Time Plays Many Parts

Olig2 is essential for the selection of motor neuron and oligodendrocyte fates and the choice of neural progenitors to either proliferate or differentiate. Three new studies demonstrate that these diverse actions of Olig2 are gated by phosphorylation at two distinct motifs and that Olig2s proliferative function acts in opposition to the p53 tumor suppressor pathway.

Olig2 and Hes regulatory dynamics during motor neuron differentiation revealed by single cell transcriptomics

During tissue development, multipotent progenitors differentiate into specific cell types in characteristic spatial and temporal patterns. We addressed the mechanism linking progenitor identity and differentiation rate in the neural tube, where motor neuron (MN) progenitors differentiate more rapidly than other progenitors. Using single cell transcriptomics, we defined the transcriptional changes associated with the transition of neural progenitors into MNs. Reconstruction of gene expression dynamics from these data indicate a pivotal role for the MN determinant Olig2 just prior to MN differentiation. Olig2 represses expression of the Notch signaling pathway effectors Hes1 and Hes5. Olig2 repression of Hes5 appears to be direct, via a conserved regulatory element within the Hes5 locus that restricts expression from MN progenitors. These findings reveal a tight coupling between the regulatory networks that control patterning and neuronal differentiation and demonstrate how Olig2 acts as the developmental pacemaker coordinating the spatial and temporal pattern of MN generation.

A requirement for the forkhead protein foxp1 and hox proteins in the columnar organization of spinal motor neurons

cyclin EmRNA level, increased cyclin/CDKactivitymight account for the increased cell proliferation. To characterize effects on cortex structure, we performed stereological analysis at E18.5 and found a 15% increase in total volumeand26% increase in total cells.Whileno change in cell density was observed, the depth of layers I and VIwas increased (39% and 19%, respectively). Furthermore, using the layer VI specific marker Tbr1, we found a 40% increase in the mutants at E18.5 and a 2-fold increase in protein by western blotting. Our observations suggest that p57 is a critical regulator of cortical precursor proliferation, andmay be determinant of laminar-specific cell production during cortical neurogenesis.

Sox9 induces oligodendrocyte formation in the developing spinal cord

Tiam1 gene. These SNPs are rs7280029, rs8131958, rs2833423, and rs8133912 and had p-values of 0.03, 0.04, 0.03, and 0.05 respectively, corrected for multiple testing. Haplotype analysis within Cdc42 showed suggestive results for three overtransmitted two-SNP haplotypes: CT (rs2056974-rs10917145), CT (rs2056975rs2056974), and GT (rs10917145-rs2268177) with nominal pvalues of 0.003, 0.002, and 0.002 respectively. Our results suggest that Tiam1, a critical mediator of NMDA receptors and Cdc42, an actin cytoskeletal regulator, may be implicated in the etiology of autism.