The central serotonin (5-HT) neurotransmitter system is an important modulator of diverse physiological processes and behaviors; however, the transcriptional mechanisms controlling its development are largely unknown. The Pet-1 ETS factor is a precise marker of developing and adult 5-HT neurons and is expressed shortly before 5-HT appears in the hindbrain. Here we show that in mice lacking Pet-1, the majority of 5-HT neurons fail to differentiate. Remaining ones show deficient expression of genes required for 5-HT synthesis, uptake, and storage. Significantly, defective development of the 5-HT system is followed by heightened anxiety-like and aggressive behavior in adults. These findings indicate that Pet-1 is a critical determinant of 5-HT neuron identity and implicate a Pet-1-dependent program in serotonergic modulation of behavior.

The specification and differentiation of serotonergic (5-HT) neurons require both extrinsic signaling molecules and intrinsic transcription factors to work in concert or in cascade. Here we identify the genetic cascades that control the specification and differentiation of 5-HT neurons in mice. A major determinant in the cascades is an LIM homeodomain-containing gene, Lmx1b, which is required for the development of all 5-HT neurons in the central nervous system. Our results suggest that, during development of 5-HT neurons, Lmx1b is a critical intermediate factor that couples Nkx2-2-mediated early specification with Pet1-mediated terminal differentiation. Moreover, our data indicate that genetic cascades controlling the caudal and rostral 5-HT neurons are distinct, despite their shared components.

Growth of multicellular organisms proceeds through a series of precisely timed developmental events requiring coordination between gene expression, behavioral changes, and environmental conditions. In Drosophila melanogaster larvae, the essential midthird instar transition from foraging (feeding) to wandering (non-feeding) behavior occurs prior to pupariation and metamorphosis. The timing of this key transition is coordinated with larval growth and size, but physiological mechanisms regulating this process are poorly understood. Results presented here show that Drosophila larvae associate specific environmental conditions, such as temperature, with food in order to enact appropriate foraging strategies. The transition from foraging to wandering behavior is associated with a striking reversal in the behavioral responses to food-associated stimuli that begins early in the third instar, well before food exit. Genetic manipulations disrupting expression of the Degenerin/Epithelial Sodium Channel subunit, Pickpocket1(PPK1) or function of PPK1 peripheral sensory neurons caused defects in the timing of these behavioral transitions. Transient inactivation experiments demonstrated that sensory input from PPK1 neurons is required during a critical period early in the third instar to influence this developmental transition. Results demonstrate a key role for the PPK1 sensory neurons in regulation of important behavioral transitions associated with developmental progression of larvae from foraging to wandering stage.

An outstanding problem in the study of vertebrate development is the identification of the genes that direct neural crest precursor cells to adopt and maintain specific differentiated cell fates. In an effort to identify such genes, we have carried out a mutagenesis screen in zebrafish and isolated mutants that lack neural crest-derived melanophores. In this manuscript we describe the phenotype of one such mutant, touchtone(b722) (tct), and the map position of the gene it defines. Analysis of expression of dopachrome tautomerase (dct) and microphthalmia (mitfa) suggests that melanophore precursors are specified normally in homozygous tct mutants. However, differentiated melanophores are pale, small, and about half of them have disappeared by 48 h of development, apparently by cell death. We show that melanophores require Tct function cell autonomously. Signals from the receptor tyrosine kinase receptor C-kit are essential for survival of melanophores in zebrafish and mammals. However, differences in the phenotypes of tct and c-kit homozygous mutants, and an absence of interaction between c-kit and tct heterozygotes, suggest that Tct functions independently of the C-kit pathway. Other neural crest derivatives, including other pigment cell types, appear normal in tct mutants. Interestingly, tct mutant embryos undergo a temporary period of near complete paralyzis during the second day of development, although markers of axons of motor and sensory neurons look normal in this period. A fraction of tct(b722) mutants survive to adulthood, but mutant adults are small, indicating a role for Tct in post-larval growth. The tct gene maps to a small interval near a telomere of chromosome 18. Thus, we have identified a zebrafish gene that when mutated produces semi-viable offspring and that may serve as a model of human diseases that have both pigmentation and neurological symptoms.

Controlled organismal growth to an appropriate adult size requires a regulated balance between nutrient resources, feeding behavior and growth rate. Defects can result in decreased survival and/or reproductive capability. Since Drosophila adults do not grow larger after eclosion, timing of feeding cessation during the third and final larval instar is critical to final size. We demonstrate that larval food exit is preceded by a period of increased larval surfacing behavior termed the Intermediate Surfacing Transition(IST) that correlates with the end of larval feeding. This behavioral transition occurred during the larval Terminal Growth Period (TGP), a period of constant feeding and exponential growth of the animal. IST behavior was dependent upon function of a subset of peripheral sensory neurons expressing the Degenerin/Epithelial sodium channel(DEG/ENaC) subunit, Pickpocket1(PPK1). PPK1 neuron inactivation or loss of PPK1 function caused an absence of IST behavior. Transgenic PPK1 neuron hyperactivation caused premature IST behavior with no significant change in timing of larval food exit resulting in decreased final adult size. These results suggest a peripheral sensory mechanism functioning to alter the relationship between the animal and its environment thereby contributing to the length of the larval TGP and determination of final adult size.