1 Formation and patterning of the nervous system!I. Neural Induction and Neurulation - specification of neural fate and formation of the neural tube. II. Neural Patterning - patterning of neural progenitors along the dorsoventral axis III. Neurogenesis - differentiation of neural progenitors into postmitotic neurons and glia. IV. Understanding neural patterning in the context of neurogenesis Overview of stages of neurogenesis Organization of the spinal cord Fig. 11.12 Progenitors (dividing) in ventricular zone Postmitotic neurons in “mantle” layer Detection of proliferating cells in the CNS Cells in S phase can be detected by their ability to incorporate labeled DNA precursors whose presence can be detected after labeling the cells (i.e BrDU or 3H labeling - detection of proliferating cells in the CNS, birthdating neurons, cell fate). The DNA content of a cell reveals its position in the cell cycle2 Neurons are generated from mitotically active precursors and different types of neurons are generated in an orderly progression VZ = ventricular zone IZ = intermediate zone PP = preplate SVG = subventricular zone SP = subplate CP = cortical plate MZ = marginal zone The DNA analogs 3H-thymidine and Bromodeoxyuridine (BrdU) can be used to “birthdate” neurons Incorporated into DNA during S-phase, but only available for a short time (then metabolized) Cells that exit the cell cycle after labeling will be heavily labeled; cells that divide again dilute out the label Cortical neurons are generated in an inside-first, outside-last order What possible mechanisms might control the fates of cells produced at distinct times during development? Are cortical cells determined with respect to layer as they are generated or is their fate dictated from the position they migrate to? How could one test these possibilities experimentally? In the ferret layer6 cells are born in utero, weeks later cells are fated for layers 2 and 3 and must migrate through layers 6, 5 and 4 which are already formed. Possible outcomes Cells are multipotent; environment determines fate Cells are committed to a specific fate prior to migration The behavior of early progenitors depends on their stage in the cell cycle at the time of transplantation 1. Cells transplanted in S-phase change fates and become normal layer 2/3 neurons3 1. Cells transplanted in S-phase change fates and become normal layer 2/3. 2. Cells transplanted after the S- to-G2 transition are committed layer 6 neurons The behavior of early progenitors depends on their stage in the cell cycle at the time of transplantation Early progenitors are multipotent; what about late progenitors? Late progenitors are intrinsically and heritably committed to generating upper-layer neurons Transplant progenitors from older embryos into younger brains: What molecular mechanisms might control changes in progenitor cells over time? 1. First need to make more progenitors 2. To start to make neurons 6!5!4!3!2!G!G!G!G!G!…and glia! What molecular mechanisms might control changes in progenitor cells over time? Lateral inhibition and proneural genes4 Each neuroblast emerges through a series of cell-cell interactions and changes in gene expression. Identification of “proneural” mutations and Ac-Sc bHLH factors Schematic representation of the structure of a bHLH dimer that is complexed to DNA. The basic region fits in the main groove of the DNA, and many residues in this region make direct contact with the E-box sequence. The two -helices of both partners together form a four-helix bundle Identification of “proneural” mutations and Ac-Sc bHLH factors Models of interactions of proneural proteins with cofactors that confer functional specificity. ! Functional specificities among proneural proteins. The functional specificities of the Drosophila proteins Scute (Sc) and Atonal, which are proneural factors for external sense organs and chordotonal organs, respectively, reside in residues that are located in the basic domain34. Residues that differ between the basic regions of Scute and Atonal are predicted not to directly contact the DNA, but to be involved in interactions with cofactors. In this model, a cofactor interacts with both the basic motif of the proneural protein and the DNA sequence, and provides the proneural protein with specificity for binding to a particular E-box sequence Each neuroblast emerges through a series of cell-cell interactions and changes in gene expression. Neurogenic genes in Drosophila: Notch, Delta, Enhancer of Split Schematic illustration of the Notch and Delta genes products: Notch encodes a large transmembrane protein 300kD,that contains a large cytoplasmic domain, a single membrane-spanning segment and a large amino-terminal extracellular domain ( a receptor). Delta, encodes a smaller homologue of Notch with nine-EGF-like repeats in the extracellular domain. Genetic experiments suggested that Notch and Delta interact with one another and biochemical experiments showed that this interaction was direct. Enhancer of Split, encodes for bHLH transcription factors, transcriptional readout of Notch signaling pathway5 Notch signaling pathway Notch signaling underlies the process of lateral inhibition Vertebrate homologues of Drosophila proneural and neurogenic genes • Structure and properties of neural bHLH proteins. ! a | Dendrogram of the sequence of the basic helix–loop–helix (bHLH) domain of invertebrate (blue) and vertebrate (red) neural bHLH proteins. Proteins have been grouped in distinct families on the basis of closer sequence similarities in the bHLH domain. b | Sequence of the bHLH domain of the mouse proneural protein neurogenin 2 (Ngn2). c | Schematic representation of the structure of a bHLH dimer that is complexed to DNA. The basic region fits in the main groove of the DNA, and many residues in this region make direct contact with the E-box sequence. The two -helices of both partners together form a four-helix bundle. d | Sequences of E-boxes that are present in the promoters of target genes and are specifically recognized by different families of neural bHLH proteins. Although neural bHLH proteins from different families recognize the common hexamer CANNTG, they must recognize different bases in the two central positions, as well as in adjacent positions. Vertebrate