Primary Cilia Influence Progenitor Function during Cortical Development

Corticogenesis is an intricate process controlled temporally and spatially by many intrinsic and extrinsic factors. Alterations during this important process can lead to severe cortical malformations. Apical neuronal progenitors are essential cells able to self-amplify and also generate basal progenitors and/or neurons. Apical radial glia (aRG) are neuronal progenitors with a unique morphology. They have a long basal process acting as a support for neuronal migration to the cortical plate and a short apical process directed towards the ventricle from which protrudes a primary cilium. This antenna-like structure allows aRG to sense cues from the embryonic cerebrospinal fluid (eCSF) helping to maintain cell shape and to influence several key functions of aRG such as proliferation and differentiation. Centrosomes, major microtubule organising centres, are crucial for cilia formation. In this review, we focus on how primary cilia influence aRG function during cortical development and pathologies which may arise due to defects in this structure. Reporting and cataloguing a number of ciliary mutant models, we discuss the importance of primary cilia for aRG function and cortical development.

The essential role of primary cilia in cerebral cortical development and disorders

Primary cilium, first described in the 19th century in different cell types and organisms by Alexander Ecker, Albert Kolliker, Aleksandr Kowalevsky, Paul Langerhans, and Karl Zimmermann (Ecker, 1844; Kolliker, 1854; Kowalevsky, 1867; Langerhans, 1876; Zimmermann, 1898), play an essential modulatory role in diverse aspects of nervous system development and function. The primary cilium, sometimes referred to as the cell's 'antennae', can receive wide ranging inputs from cellular milieu, including morphogens, growth factors, neuromodulators, and neurotransmitters. Its unique structural and functional organization bequeaths it the capacity to hyper-concentrate signaling machinery in a restricted cellular domain approximately one-thousandth the volume of cell soma. Thus enabling it to act as a signaling hub that integrates diverse developmental and homestatic information from cellular milieu to regulate the development and function of neural cells. Dysfunction of primary cilia contributes to the pathophysiology of several brain malformations, intellectual disabilities, epilepsy, and psychiatric disorders. This review focuses on the most essential contributions of primary cilia to cerebral cortical development and function, in the context of neurodevelopmental disorders and malformations. It highlights the recent progress made in identifying the mechanisms underlying primary cilia's role in cortical progenitors, neurons and glia, in health and disease. A future challenge will be to translate these insights and advances into effective clinical treatments for ciliopathies.

[Microlissencephaly due to pathogenic variants of NDE1: from pathology to normal brain development]

Pathogenic variants of the gene NDE1 (Nuclear Distribution Element 1) in humans lead to microlissencephaly which associates a reduced head circumference and a simplified gyration. Microlissencephaly is the most severe deficit of neurogenesis described to date but its precise physiopathological mechanism is not yet well known. The NDE1 gene encodes a phosphoprotein that is essential to neurogenesis and that is expressed in various cell compartments of neuroblasts. More than 60 interaction partners with NDE1 have been reported, notably various proteins involved in formation of the mitotic spindle, in ciliation, in genome protection of dividing neuroblasts or even in apoptosis (like LIS1, dynein or cohesin), which are all avenues that we explore in this review.

[Primary cilia control different steps of brain development]

The role of primary cilia in adult neurons remains elusive, however their developmental functions during brain morphogenesis have been recently highlighted thanks to mouse models. Unmistakably, they are needed for Hedgehog (Hh)-dependent patterning in the forebrain. Not only for Hh reception itself, but most importantly for a downstream event in the Hh transduction pathway, independent of Hh ligand: the Gli3 processing. Indeed, phenotypes due to cilia disruption in the developing brain, such as early patterning, olfactory bulb or corpus callosum formation, can be rescued by reintroducing Gli3-R (the short truncated form of Gli3 working as a transcriptional repressor of Hh target gene). In addition, primary cilia control the proliferation rate in different neural progenitors in the cortex, the hippocampus and the cerebellum; they are required for proper migration of interneurons. And cilia dysfunction is correlated with hydrocephaly, synaptogenesis defects and aberrant axonal tract projections. Most of these neurodevelopmental defects can be related to the various neurological features frequently observed across the ciliopathy spectrum. And thus, understanding the underlying mechanisms of these diverse functions of primary cilia in the brain is a new fundamental challenge.

[Non-ciliary functions of cilia proteins]

Cilia proteins have long been characterized for their role in cilia formation and function, and their implications in ciliopathies. However, several cellular defects induced by cilia proteins deregulation suggest that they could have non-ciliary roles. Indeed, several non-ciliary functions have been recently characterized for cilia proteins including roles in intra-cellular and in vesicular transport, in spindle orientation or in the maintenance of genomic stability. These observations thus raise the crucial question of the contribution of non-ciliary functions of cilia proteins to the pathological manifestations associated with ciliopathies such as polycystic kidney disease.

[Bardet-Biedl syndrome: cilia and obesity - from genes to integrative approaches]

The primary cilium is a specialized organelle, present at the surface of most eukaryotic cells, whose main function is to detect, integrate and transmit intra- and extra-cellular signals. Its dysfunction usually results in a group of severe clinical manifestations nowadays termed ciliopathies. The latter can be of syndromic nature with multi-organ dysfunctions and can also be associated with a morbid obese phenotype, like it is the case in the iconic ciliopathy, the Bardet Biedl syndrome (BBS). This review will discuss the contribution of the unique context offered by the emblematic BBS for understanding the mechanisms leading to obesity via the involvement of the primary cilium together with identification of novel molecular players and signaling pathways it has helped to highlight. In the current context of translational medicine and system biology, this article will also discuss the potential benefits and challenges posed by these techniques via multi-level approaches to better dissect the underlying mechanisms leading to the complex condition of obesity.