Eva S. Anton, Claudia Bagni, Alexandre Benmerah, Oliver Blacque (organisateur), Juan Burrone, Tom Dufor, Peter K. Jackson, Alex L. Kolodkin, Kirk Mykytyn, Maxence Nachury, Carolyn Ott, Timothy A. (Tim) Ryan, Patricia Salinas, Piali Sengupta (organisateur), Nathalie Spassky, Lauren Tereshko, Gina Turrigiano (organisateur), Enza Maria Valente
by Piali Sengupta, Oliver E. Blacque and Gina Turrigiano
11 – 16 March, 2019
Les cils sont des structures assemblées autour d’un tronc de microtubules qui participent à la signalisation cellulaire. Tous les types cellulaires représentés dans le système nerveux central (SNC) possèdent un cil. Malgré l’importance émergente des cils dans le SNC, démontrée en particulier par une série de pathologies cérébrales liées au cil, la communauté scientifique ignore comment les cils modulent les propriétés neuronales. Dix-huit chercheurs venant des Etats-Unis et d’Europe dont deux chercheurs en début de carrière se sont rencontrés pour discuter comment les cils contribuent au développement et à la fonction des neurones du SNC. Le but de ce séminaire était de fournir une introduction à la biologie du cil pour les neuro-scientifiques et, vice-versa, d’éduquer les biologistes du cil à la complexité du système nerveux. Cette synthèse avait donc pour objectif global d’initier de nouvelles directions de recherche portant à l’étude des cils dans le cerveau. Les participants à ce séminaire ont inclus des chercheurs travaillant sur des maladies du développement du système nerveux liées au cil, sur les mécanismes de la signalisation ciliaire dans les neurones et d’autres types cellulaires, sur la visualisation des cils dans le cerveau, sur l’établissement de la connectivité neuronale, et sur les mécanismes de signalisation synaptique et la plasticité neuronale. Un nombre d’approches expérimentales ont été représentées, dont les techniques de biologie cellulaire, de neurosciences du développement, de la génétique clinique, de la neuroscience cellulaire et des systèmes, et de l’imagerie cellulaire. Les intervenants ont présenté des travaux s’étendant sur de nombreux systèmes cellulaires et organismes modèles dont le nématode C. elegans, la souris et les cellules humaines en culture.
Les présentations ont engendré une variété de discussions autour de thèmes importants comme: (i) les rôles du cils aux différents stades de développement neuronal, (ii) la signalisation ciliaire dans les neurones, (iii) les mécanismes en commun entre la compartimentation des cils et des épines dendritiques, (iv) les corrélations génotype-phénotype dans les maladies du cils atteignant le SNC, (v) la transmission des signaux ciliaires vers des sites neuronaux à distance, et (vi) les différents types de cil dans les neurones. Les participants à cette conférence furent unanimement enthousiastes sur le contenu et l’actualité du sujet. Ils ont formulé de nouveaux axes de recherche et la conférence a rendu possible l’établissement de nouvelles collaborations.
Mots-clés : cils, ciliopathies, signalisation cellulaire, neurones, système nerveux central
Compte rendu (en anglais)
Overall goal of the seminar
Primary cilia are sensory organelles found on nearly all mammalian cell types, including mature neurons of the central nervous system (CNS). Defects in these organelles disrupt the development and maintenance of many tissue types, including the brain. However, despite intriguing hints that primary cilia play critical roles in the maintenance and plasticity of neurons and neuronal circuits, the roles of cilia in central neurons are largely unknown and understudied. The goal of this seminar was to bring together cilia biologists and neuroscientists to explore the role of cilia in the CNS, with an emphasis on how ciliary signaling impacts both the developing and mature brain.
General overview of presentations
Participants were divided roughly equally between cilia biologists and neuroscientists. A subset of participating cilia biologists also have expertise in neuroscience. 15 of the 18 participants were Principal Investigators, with the remainder comprising a PhD student, a postdoctoral fellow, and a Senior Scientist. The talks were lively and interactive, and extensive time was allocated during and after the presentations for questions, clarifications, and the discussion of emerging ideas. In particular, since several neuroscientists were unfamiliar with cilia and vice versa, each presentation included detailed background information along with descriptions of recent work in the research area of the presenter. The seminar concluded with a discussion of several key themes that were raised repeatedly throughout the presentations, which lead to suggestions and ideas for future work in this exciting area.
Do cilia play different roles at different stages of neuronal development? Cilia contribution to neuronal functions has typically been assessed using gene knockouts that disrupt cilia throughout all stages of development. However, neuronal development proceeds through multiple stages (eg. neurogenesis, cell fate specification, process outgrowth, synapse formation, circuit refinement), and moreover, neuronal circuits continue to be modified in the adult. Although roles for cilia in the embryonic brain are established, their functions in the adult brain remain mysterious. There is a great need to conditionally disrupt cilia at different time-points to uncover how ciliary signaling contributes to distinct stages of embryonic brain development, and the maintenance and plasticity of the adult brain.
Why is signaling via cilia important in neurons? Neurons are characterized by complex dendritic and axonal morphologies, which mediate neuronal communication via chemical and electrical synapses, as well as via extrasynaptic neuropeptidergic signaling. A major discussed issue was why these cells would also require signaling via a small (typically <10 mm) ciliary structure, and whether such signaling could result in different cellular effects than signaling from elsewhere in the cell. Participants agreed that concentrating signaling molecules within cilia may allow for more effective, localized signaling. Moreover, the compartmentalization of multiple signaling pathways within cilia may permit efficient cross-talk and modulation. A conclusion was that this was a major issue that requires investigation in the nervous system.
What is the protein content of neuronal cilia? The brain is characterized by the presence of many thousands of different cell types. A subset of signaling proteins such as the somatostatin receptor 3 and neuropeptide Y receptors have been shown to be specifically (see below) localized to neuronal cilia in defined brain regions such as in the hippocampus and hypothalamus. However, whether these receptors are present on all or only a subset of neurons in these brain regions is unknown. Moreover, additional components of cilia in these and other brain regions are largely unidentified. There is a great need to systematically catalog the ciliary proteome as a function of neuronal cell type and developmental stage across the brain in order to understand how ciliary signaling may modulate neuronal functions.
Are ciliary proteins cilia-specific? A major point of discussion was whether proteins described as cilia-specific are truly only localized to cilia. This is critical to establish since if proteins are also extra-ciliary, it would be difficult to dissect their contributions from cilia as compared to elsewhere in the plasma membrane. It was also noted that this may be technically challenging to assess due to the larger cell membrane volume compared to the ciliary membrane.
Do cilia and synapses / dendritic spines share similar compartmentalization mechanisms? There was significant discussion as to whether ciliary compartmentalization paradigms operate at synapses/dendritic spines. Are there diffusion barriers or lipid gates at the dendritic spine neck analogous to those found at the ciliary base, and if so, could they employ some of the ciliary gating machinery? Currently such barriers are not known in spines. Also, do components of ciliary transport machinery (eg., IFT-A/B and BBSome complexes) have transport-associated roles in maintaining synapse/spine composition and structure? Indeed, there are reports of IFT and BBS proteins localizing at synaptic structures.
Genotype-phenotype correlations in ciliopathies with CNS disruption? We do not have a good understanding as to why ciliopathies such as Joubert Syndrome (JBTS) can present with large symptomatic variation across different patient cohorts. Mutations in the same gene can give rise to a strikingly different phenotype. Discussion centered around the importance of investigating allele-specific effects on the JBTS phenotype, and how cell-specific mechanisms of JBTS gene function can influence the disease phenotype.
Where are cilia localized on neurons? The location of cilia in different cell types in different parts of the brain are unknown. Initial electron microscopy reconstructions of the brain appear to demonstrate tremendous heterogeneity in the sites and structures of neuronal cilia. Establishing this issue is important since ciliary signaling from different cellular locations may influence neuronal properties via different pathways.
How are signals from cilia transmitted to distant neuronal sites? A hallmark of many neurons is the presence of long processes with synapses present at sites as distant as several meters from the cell body. How are signals from the cilia transmitted to these distant sites? Possibilities are via second messenger molecules generated by ciliary signaling, or longer-term effects mediated by transcriptional mechanisms initiated by signals from the cilia.
What role might ciliary signaling play in the maintenance of neuronal activity? Maintaining the proper balance between excitatory and inhibitory synaptic input is crucial for neuronal circuit function. An intriguing possibility is that cilia integrate disparate signals from many sources in a neuronal circuit, and then use those signals to adjust excitatory or inhibitory synaptic strength. This model is highly speculative but intriguing preliminary results suggest it should be investigated further.
Summary of outcomes from seminar
The seminar successfully interested participating neuroscientists in the roles of cilia in the brain, and informed cilia biologists about brain complexity and function. The seminar also allowed two groups of scientists who may not otherwise communicate to meet and interact extensively. The presentations and discussions highlighted the many fascinating and complex questions in the field, and led to the establishment of at least two collaborations to begin to address some of these issues. We thank the Fondation for their support of this very successful and interesting seminar and look forward to learning more about the functions of cilia in the brain in the future.