RESUMO
Distinct classes of serotonergic (5-HT) neurons develop along the ventral midline of the vertebrate hindbrain. Here, we identify a Sonic hedgehog (Shh)-regulated cascade of transcription factors that acts to generate a specific subset of 5-HT neurons. This transcriptional cascade is sufficient for the induction of rostral 5-HT neurons within rhombomere 1 (r1), which project to the forebrain, but not for the induction of caudal 5-HT neurons, which largely terminate in the spinal cord. Within the rostral hindbrain, the Shh-activated homeodomain proteins Nkx2.2 and Nkx6.1 cooperate to induce the closely related zinc-finger transcription factors Gata2 and Gata3. Gata2 in turn is necessary and sufficient to activate the transcription factors Lmx1b and Pet1, and to induce 5-HT neurons within r1. In contrast to Gata2, Gata3 is not required for the specification of rostral 5-HT neurons and appears unable to substitute for the loss of Gata2. Our findings reveal that the identity of closely related 5-HT subclasses occurs through distinct responses of adjacent rostrocaudal progenitor domains to broad ventral inducers.
Assuntos
Proteínas de Ligação a DNA/metabolismo , Neurônios/metabolismo , Rombencéfalo/embriologia , Serotonina/metabolismo , Transativadores/metabolismo , Fatores de Transcrição/metabolismo , Animais , Sequência de Bases , Embrião de Galinha , DNA Complementar/genética , Proteínas de Ligação a DNA/genética , Fator de Transcrição GATA2 , Regulação da Expressão Gênica no Desenvolvimento , Proteínas Hedgehog , Proteína Homeobox Nkx-2.2 , Proteínas de Homeodomínio/genética , Proteínas de Homeodomínio/metabolismo , Hibridização In Situ , Rombencéfalo/citologia , Rombencéfalo/metabolismo , Transativadores/genética , Fatores de Transcrição/genética , Proteínas de Peixe-ZebraRESUMO
Netrin-G1 is a lipid-anchored protein that is structurally related to the netrin family of axon guidance molecules. Netrin-G1 does not bind any of the known netrin receptors and its function is not known. Here we identify human netrin-G1 ligand (NGL-1), a transmembrane protein containing leucine-rich repeat (LRR) and immunoglobulin (Ig) domains that specifically interacts with netrin-G1 through its LRR region. Whereas netrin-G1 is expressed highly in mouse thalamic axons, NGL-1 is most abundant in the striatum and the cerebral cortex--the intermediate and final targets, respectively, of thalamocortical axons (TCAs). Surface-bound NGL-1 stimulates, but soluble NGL-1 disrupts, the growth of embryonic thalamic axons, and in vitro data indicate that NGL-1 activity may be mediated at least partially by netrin-G1. Our findings provide evidence that netrin-G1 functions as an important component of the NGL-1 receptor to promote TCA outgrowth and that membrane-bound netrins can participate in receiving axonal signaling pathways.