The neuroendocrine control of animal size, body proportion and symmetry

• Autores: Sergio Juárez Carreño
• Directores de la Tesis: María Domínguez Castellano (dir. tes.), Javier Morante Oria (codir. tes.)
• Lectura: En la Universidad Miguel Hernández de Elche ( España ) en 2018
• Idioma: español
• Tribunal Calificador de la Tesis: Marco Milán Kalbfleisch (presid.), Eduardo de Puelles Martínez de la Torre (secret.), Frédérique Peronnet (voc.), Ana Carmena de la Cruz (voc.), Francisco Antonio Martín Castro (voc.)
• Programa de doctorado: Programa de Doctorado en Neurociencias por la Universidad Miguel Hernández de Elche
• Enlaces
  • Tesis en acceso abierto en: RediUMH (pdf)
• Resumen

  • Understanding how animals control their size is of fundamental importance in biology and clinical research. It is known that juvenile organisms can adjust their size in response to changes in their environment (plastic response), therefore they produce adults with correct size by counteracting growth anomalies. It is currently unclear exactly how immature animals (including children) compensate these potentially substantial variations in their size. Such compensatory mechanism delays the onset of the reproductive stage of adulthood until a correct size has been reached. This process slows down the growth of normal tissues in order to maintain body and organ proportions within the normal range.

    However, the neural mechanism of such homeostatic size regulation has yet to be fully defined in any species. In Drosophila, body size is controlled by two prominent neuronal populations: the prothoracicotropic hormone (PTTH)-expressing neurons, which are analogous to the gonadotropin-releasing hormone (GnRH) neurons in mammals; and the neurons located in the pars intercerebralis (called insulin-producing cells, IPCs) which produce insulin-like peptides and regulate tissue growth, metabolism and developmental timing. Experiments in which the activity of each of these neurons is altered have shown that these neurons operate independently, albeit both regulate maturation time resulting in larger or smaller adults. Thus, it is now apparent that the activity of these neuronal populations operating independently might not be sufficient to explain the reliable size control, which in turn may require more complex or synchronized regulatory circuits.

    Previous studies have established that the insulin/relaxin-like peptide 8 (Dilp8) controls homeostasis size in Drosophila, although its receptor and site of action remained uncharacterized.

    In the present thesis proyect, I employed a candidate approach to demonstrate that the orphan relaxin receptor Lgr3 acts as a Dilp8 receptor. Lgr3 receptor is activated by nanomolar concentrations of Dilp8 hormone and results in a robust production of cyclic AMP. Furthemore, using a biochemical readout of Lgr3 response to Dilp8 in vivo, I identified that a pair of neurons acutely respond to Dilp8 signal. I unveiled that these neurons have extensive axonal arborizations (hub-like structure) and connect with both PTTH-producing neurons and the IPCs.

    Functional relevance of connectivity between Lgr3/PTTH-producing neurons and Lgr3/IPCs were evaluated using several genetical approaches as perturbing neural activity, and/or assessing changes in transcription of genes in postsynaptic targets. Regarding to this, I identified Dilp3 and Dilp5 and the Juvenile hormone signalling as output pathways of this circuit for growth compensation through IPCs. Moreover, I demostrated the ecdysone inhibition through PTTHproducing neurons as output pathway of this circuit for developmental timing regulation.

    Acordingly with previous studies, the circadian clock regulates the onset of maturation in animals. To clarify the role of circadian clock in Dilp8/lgr3 neural circuit, I characterized the role of the master clock neurons (PDF neurons) and the synaptical connections with Lgr3-positive neurons, and PTTH-producing neurons (where PDF receptor or PDFR is expressed to mediate the function of PDF neuropeptide). I demostrated the Dilp8-Lgr3 homeostatic growth control circuit in collaboration with circadian clock during development has an impact in the lipogenic larval metabolism and adult fitness, providing a better performance upon inanition condictions.

    In adult female flies, Lgr3-positive neurons are connected synaptically with IPCs, controlling the expression levels of insulin-like peptides 2 and 5 (dilp2 and dilp5). Previous studies have postulated that Dilp2, Dilp3 and Dilp5 could be involve in courtship behaviour and metabolism. Furthemore, the Lgr3-positive neurons have been involved in courtship behaviour.

    Nevertheless, the activation of Lgr3-positive neurons by Dilp8 do not show impact neither in mating behaviour, nor in the offspring generated.

    On the other hand, I colaborated with Javier Morante PhD in an independent project to clarify the potential role of Sema1a as a sensor of fat content during development, since this receptor is necessary to detect the critical weight and surpase the juvenile stages to puberty.

    sema1a depletion in the prothoracic gland allows the larvae to extent the growth period, followed by the inhibition of the ecdysis. The extension of this growth period in sema1a mutants generates as consequences larvae with aberrant lipid content, desproportionate weight, and bigger sizes.

    Finally, sema1a depletion in the prothoracic gland shows higher insulin and juvenile hormone signalling, disrupting the critical weight detection necessary to promote the ecdysone synthesis.