Defining the role of TrkB signalling in CCK-expressing neurons

The neurotrophin receptor TrkB has been shown to regulate neuronal survival, migration, differentiation and innervation in the peripheral and central nervous system. In the mature nervous system, it can modulate synaptic plasticity and excitatory neuron-specific deletion of Trkb results in compromised learning ability and impaired long-term potentiation. TrkB is also expressed in interneurons and was reported to regulate interneuron differentiation, synapse assembly, maintenance and development of inhibitory networks. Given the heterogeneity of interneurons, cell-specific approaches are required to study the function of TrkB in different interneuronal subtypes. This study investigates the role of TrkB in a specific subset of interneurons that express cholecystokinin (CCK). Trkb was specifically deleted from CCK-neurons by crossing a transgenic BAC-Cre mouse line that expresses Cre under the CCK-promotor (BAC-CCK-Cre line) to a Trkb-floxed mouse line. CCK-Cre specific Trkb-knockout mice (TrkbCCK-KO mice) develop mature-onset central obesity and show hyperactivity of the HPA axis with peripheral signs of hypercortisolism. Analysis of food intake revealed that central obesity is not associated with hyperphagia but is a cause of hypercortisolism. Furthermore, we show that hypercortisolism-induced obesity is associated with increased leptin and insulin levels. Hyperactivity of the HPA axis in TrkbCCK-KO mice is associated with increased activity of the central HPA axis regulator, the paraventricular nucleus of the hypothalamus (PVN). PVN activity is strictly regulated by surrounding inhibitory interneurons and glucocorticoid feedback inhibition. We show that GABAergic interneurons in the vicinity of the PVN are recombined in the BAC-CCK-Cre line and colocalize with the glucocorticoid receptor GR. Furthermore, we present data indicating an impaired glucocorticoid feedback inhibition in TrkbCCK-KO mice. TrkB was previously shown to interact directly with the GR leading to enhanced phosphorylation of PLC-gamma1 by TrkB. Analysis of mice with a mutation in either the PLC or SHC adaptor site of TrkB reveals that the phenotype observed here is dependent on PLC-gamma1 signaling. Therefore we conclude that TrkB signaling in hypothalamic CCK-interneurons integrates glucocorticoid feedback inhibition and is required for inhibitory control of PVN activity

Deviations from temporal scaling support a stage-specific regulation for C. elegans postembryonic development

Background After embryonic development, Caenorhabditis elegans progress through for larval stages, each of them finishing with molting. The repetitive nature of C. elegans postembryonic development is considered an oscillatory process, a concept that has gained traction from regulation by a circadian clock gene homologue. Nevertheless, each larval stage has a defined duration and entails specific events. Since the overall duration of development is controlled by numerous factors, we have asked whether different rate-limiting interventions impact all stages equally. Results We have measured the duration of each stage of development for over 2500 larvae, under varied environmental conditions known to alter overall developmental rate. We applied changes in temperature and in the quantity and quality of nutrition and analysed the effect of genetically reduced insulin signalling. Our results show that the distinct developmental stages respond differently to these perturbations. The changes in the duration of specific larval stages seem to depend on stage-specific events. Furthermore, our high-resolution measurement of the effect of temperature on the stage-specific duration of development has unveiled novel features of temperature dependence in C. elegans postembryonic development. Conclusions Altogether, our results show that multiple factors fine tune developmental timing, impacting larval stages independently. Further understanding of the regulation of this process will allow modelling the mechanisms that control developmental timing.España Ministerio de Economía y Competitividad, (RYC-2014-15551)Junta de Andalucía FEDER Andalucía 2014-2020 y Consejería de Transformación Económica, Industria, Conocimiento y Universidades grants PID2019-104632GB-I00 (Ayuda PID2019-104632GBI0

Loss of NGF-TrkA signaling from the CNS is not sufficient to induce cognitive impairments in young adult or intermediate-aged mice

Many molecules expressed in the CNS contribute to cognitive functions either by modulating neuronal activity or by mediating neuronal trophic support and/or connectivity. An ongoing discussion is whether signaling of nerve growth factor (NGF) through its high-affinity receptor TrkA contributes to attention behavior and/or learning and memory, based on its expression in relevant regions of the CNS such as the hippocampus, cerebral cortex, amygdala and basal forebrain. Previous animal models carrying either a null allele or transgenic manipulation of Ngf or Trka have proved difficult in addressing this question. To overcome this problem, we conditionally deleted Ngf or Trka from the CNS. Our findings confirm that NGF-TrkA signaling supports survival of only a small proportion of cholinergic neurons during development; however, this signaling is not required for trophic support or connectivity of the remaining basal forebrain cholinergic neurons. Moreover, comprehensive behavioral analysis of young adult and intermediate-aged mice lacking NGF-TrkA signaling demonstrates that this signaling is dispensable for both attention behavior and various aspects of learning and memory

A high-throughput method for the analysis of larval developmental phenotypes in Caenorhabditis elegans

Caenorhabditis elegans postembryonic development consists of four discrete larval stages separated by molts. Typically, the speed of progression through these larval stages is investigated by visual inspection of the molting process. Here, we describe an automated method to monitor the timing of these discrete phases of C. elegans maturation, from the first larval stage through adulthood, using bioluminescence. The method was validated with a lin-42 mutant strain that shows delayed development relative to wild-type animals and with a daf-2 mutant that shows an extended second larval stage. This new method is inherently high throughput and will finally allow dissecting the molecular machinery governing the speed of the developmental clock, which has so far been hampered by the lack of a method suitable for genetic screens.Our work is supported by the Ludwig-Maximilians-University Munich and the Friederich-Bauer-Stiftung (M.M., M.O., and M.G.); the European Research Council (ERC-2011-StG-281691) and the Spanish Ministerio de Economía y Competitividad (BFU2012-35509) (M.A.-S.); and a Marie-Curie Intra-European Fellowship (FP7-PEOPLE-2013-IEF/GA Nr: 627263) (to M.O. and M.A.-S.).Peer Reviewe

Ablation of TrkB signalling in CCK neurons results in hypercortisolism and obesity

Dysregulation of hypothalamic-pituitary-adrenal (HPA) axis activity leads to debilitating neuroendocrine or metabolic disorders such as Cushing's syndrome (CS). Glucocorticoids control HPA axis activity through negative feedback to the pituitary gland and the central nervous system (CNS). However, the cellular mechanisms involved are poorly understood, particularly in the CNS. Here we show that, in mice, selective loss of TrkB signalling in cholecystokinin (CCK)-GABAergic neurons induces glucocorticoid resistance, resulting in increased corticotrophin-releasing hormone expression, chronic hypercortisolism, adrenocortical hyperplasia, glucose intolerance and mature-onset obesity, reminiscent of the human CS phenotype. Interestingly, obesity is not due to hyperphagia or decreased energy expenditure, but is associated with increased de novo lipogenesis in the liver. Our study therefore identifies CCK neurons as a novel and critical cellular component of the HPA axis, and demonstrates the requirement of TrkB for the transmission of glucocorticoid signalling