Ultrastructural differences impact cilia shape and external exposure across cell classes in the visual cortex

Light-microscopy-based connectomic reconstruction of mammalian brain tissue

The information-processing capability of the brain's cellular network depends on the physical wiring pattern between neurons and their molecular and functional characteristics. Mapping neurons and resolving their individual synaptic connections can be achieved by volumetric imaging at nanoscale resolution1,2 with dense cellular labelling. Light microscopy is uniquely positioned to visualize specific molecules, but dense, synapse-level circuit reconstruction by light microscopy has been out of reach, owing to limitations in resolution, contrast and volumetric imaging capability. Here we describe light-microscopy-based connectomics (LICONN). We integrated specifically engineered hydrogel embedding and expansion with comprehensive deep-learning-based segmentation and analysis of connectivity, thereby directly incorporating molecular information into synapse-level reconstructions of brain tissue. LICONN will allow synapse-level phenotyping of brain tissue in biological experiments in a readily adoptable manner.

Advances in Imaging Techniques for Mammalian/Human Ciliated Cell's Cilia: Insights into Structure, Function, and Dynamics

Cilia are evolutionarily conserved, microtubule-based organelles characterized by their ultrastructures and diverse functional roles, including developmental signaling, mechanosensation, and fluid propulsion. They are widely distributed across cell surfaces and play crucial roles in cell cycle regulation and tissue homeostasis. Despite advances in studying their molecular regulation and functions, demonstrating the precise ultrastructure of cilia remains a challenge. Recent novel microscopy techniques, such as super-resolution microscopy and volume electron microscopy, are revolutionizing our understanding of their architecture and mechanochemical signaling. By integrating findings from different methodologies, this review highlights how these advances bridge basic research and clinical applications and provide a comprehensive understanding of the structural organization, functional mechanisms, and dynamic changes of cilia.

Differences in neuronal ciliation rate and ciliary content revealed by systematic imaging-based analysis of hiPSC-derived models across protocols

Introduction

Ciliopathies are a group of human Mendelian disorders caused by dysfunction of primary cilia, small quasi-ubiquitous sensory organelles. Patients suffering from ciliopathies often display prominent neurodevelopmental phenotypes, underscoring the importance of primary cilia during development and for function of the central nervous system (CNS). Human tissues, in particular from the CNS, are very hard to obtain for research. Patient derived- or genetically engineered human induced pluripotent stem cells (hiPSCs) are therefore a precious resource for investigating the role of cilia in human neurons.

Methods

In this study we used a variety of 2D and 3D neuronal differentiation protocols in multiple hiPSC lines and systematically analyzed ciliation rates and ciliary length in hiPSCs, neural stem cells (NSCs), immature and different types of mature neurons using immunofluorescence.

Results

We found that ciliation rate varied substantially between cell lines and differentiation protocols. Moreover, ciliation rate depended on differentiation stage, being maximal in NSCs and decreasing with neuronal maturation. In various types of mature neurons obtained with different protocols, we found ciliation rates to be as low as ∼10%. Neuronal density also played an important role, with higher ciliation in denser cultures. We further investigated the ciliary protein content in these cells at different differentiation stages using commonly used antibodies against ARL13B, INPP5E, AC3 and GPR161. Cilia in hiPSCs, NSCs and neurons were all positive for ARL13B, with a decreasing trend in intensity in more mature neurons. Likewise, INPP5E was present in all cilia analyzed, while AC3 positivity increased as maturation proceeded. Interestingly, we found that while GPR161 signal almost completely disappeared from cilia upon Sonic hedgehog (SHH) stimulation in NSCs and immature neurons, this was not the case in more mature neurons, suggesting a possible developmental time window for cilia-dependent SHH signaling.

Conclusion

Taken together, our results provide a systematic description of cilia in hiPSC-derived neuronal cells generated with different protocols, underscoring the importance of selecting the optimal model system and controls for investigating primary cilia in hiPSC-derived neuronal cells.

Methods to assess neuronal primary cilia electrochemical signaling

Primary cilia are polymodal sensory organelles which project from the apical side of polarized cells. They are found in all brain hemispheres but are most pronounced in neurons which comprise the granular layers of the hippocampus and cerebellum. Pathogenic variants in genes which encode primary cilia components are responsible for neuronal ciliopathies- a group of central nervous system disorders characterized by neurodevelopmental conditions such as intellectual disability, seizure, ataxia, and sensory deficits. In the hippocampus, neuronal primary cilia form chemical synapses with axons and their membranes are populated with unique sets of ion channels and G protein-coupled receptors (GPCRs). Primary cilia are small and privileged compartments that are challenging organelles to study. In detail, we describe cilia electrophysiology methods and the use of cilia-specific fluorescent sensors to assay neuronal polycystin channel function and serotonergic receptor signaling, respectively. These tools allow researchers to assay calcium, cAMP and channel-related signaling pathways in isolated neurons in real time and in semi-quantitative terms, while enhancing our understanding of this understudied organelle and its dysregulation in ciliopathy disease states.

Perisomatic ultrastructure efficiently classifies cells in mouse cortex

Mammalian neocortex contains a highly diverse set of cell types. These cell types have been mapped systematically using a variety of molecular, electrophysiological and morphological approaches1-4. Each modality offers new perspectives on the variation of biological processes underlying cell-type specialization. Cellular-scale electron microscopy provides dense ultrastructural examination and an unbiased perspective on the subcellular organization of brain cells, including their synaptic connectivity and nanometre-scale morphology. In data that contain tens of thousands of neurons, most of which have incomplete reconstructions, identifying cell types becomes a clear challenge for analysis5. Here, to address this challenge, we present a systematic survey of the somatic region of all cells in a cubic millimetre of cortex using quantitative features obtained from electron microscopy. This analysis demonstrates that the perisomatic region is sufficient to identify cell types, including types defined primarily on the basis of their connectivity patterns. We then describe how this classification facilitates cell-type-specific connectivity characterization and locating cells with rare connectivity patterns in the dataset.

Methods to Assess Neuronal Primary Cilia Electrochemical Signaling

Primary cilia are polymodal sensory organelles which project from the apical side of polarized cells. They are found in all brain hemispheres but are most pronounced in neurons, which comprise the granular layers of the hippocampus and cerebellum. Pathogenic variants in genes which encode primary cilia components are responsible for neuronal ciliopathies-a group of central nervous system disorders characterized by neurodevelopmental conditions such as intellectual disability, seizure, ataxia, and sensory deficits. In the hippocampus, neuronal primary cilia form chemical synapses with axons and their membranes are populated with unique sets of ion channels and G protein-coupled receptors (GPCRs). Primary cilia are small and privileged compartments that are challenging organelles to study. In detail, we describe cilia electrophysiology methods and the use of cilia-specific fluorescent sensors to assay neuronal polycystin channel function and serotonergic receptor signaling, respectively. These tools allow researchers to assay calcium, cAMP and channel-related signaling pathways in isolated neurons in real-time and in semi-quantitative terms, while enhancing our understanding of this understudied organelle and its dysregulation in ciliopathy disease states.

Roles for primary cilia in synapses and neurological disorders

The role of primary cilia has recently garnered significant attention in the field of neurodegeneration. This review explores the diversity of primary cilia in the mature brain and their interrelationships with a multitude of cellular structures, including axons and synapses. Importantly, an overview of the growing prominence of ciliary-related dysfunctions in neurodegenerative diseases is summarized, with a special emphasis on Parkinson's disease (PD) and neuropsychiatric disorders.

Permanent cilia loss during cerebellar granule cell neurogenesis involves withdrawal of cilia maintenance and centriole capping

Brain neurons utilize the primary cilium as a privileged compartment to detect and respond to extracellular ligands such as Sonic hedgehog (SHH). However, cilia in cerebellar granule cell (GC) neurons disassemble during differentiation through ultrastructurally unique intermediates, a process we refer to as cilia deconstruction. In addition, mature neurons do not reciliate despite having docked centrioles. Here, we identify molecular changes that accompany cilia deconstruction and centriole docking in GC neurons. We used single cell transcriptomic and immunocytological analyses to compare the transcript levels and subcellular localization of proteins between progenitor, differentiating, and mature GCs. Differentiating GCs lacked transcripts for key activators of premitotic cilia resorption, indicating that cilia disassembly in differentiating cells is distinct from premitotic cilia resorption. Instead, during differentiation, transcripts of many genes required for cilia maintenance-specifically those encoding components of intraflagellar transport, pericentrosomal material, and centriolar satellites-decreased. The abundance of several corresponding proteins in and around cilia and centrosomes also decreased. These changes coincided with downregulation of SHH signaling prior to differentiation, even in a mutant with excessive SHH activation. Finally, mother centrioles in maturing granule neurons recruited the cap complex protein, CEP97. These data suggest that a global, developmentally programmed decrease in cilium maintenance in differentiating GCs mediates cilia deconstruction, while capping of docked mother centrioles prevents cilia regrowth and dysregulated SHH signaling. Our study provides mechanistic insights expanding our understanding of permanent cilia loss in multiple tissue-specific contexts.

Structure, interaction and nervous connectivity of beta cell primary cilia

Primary cilia are sensory organelles present in many cell types, partaking in various signaling processes. Primary cilia of pancreatic beta cells play pivotal roles in paracrine signaling and their dysfunction is linked to diabetes. Yet, the structural basis for their functions is unclear. We present three-dimensional reconstructions of beta cell primary cilia by electron and expansion microscopy. These cilia are spatially confined within deep ciliary pockets or narrow spaces between cells, lack motility components and display an unstructured axoneme organization. Furthermore, we observe a plethora of beta cell cilia-cilia and cilia-cell interactions with other islet and non-islet cells. Most remarkably, we have identified and characterized axo-ciliary synapses between beta cell cilia and the cholinergic islet innervation. These findings highlight the beta cell cilia's role in islet connectivity, pointing at their function in integrating islet intrinsic and extrinsic signals and contribute to understanding their significance in health and diabetes.

Permanent deconstruction of intracellular primary cilia in differentiating granule cell neurons

Primary cilia on granule cell neuron progenitors in the developing cerebellum detect sonic hedgehog to facilitate proliferation. Following differentiation, cerebellar granule cells become the most abundant neuronal cell type in the brain. While granule cell cilia are essential during early developmental stages, they become infrequent upon maturation. Here, we provide nanoscopic resolution of cilia in situ using large-scale electron microscopy volumes and immunostaining of mouse cerebella. In many granule cells, we found intracellular cilia, concealed from the external environment. Cilia were disassembled in differentiating granule cell neurons-in a process we call cilia deconstruction-distinct from premitotic cilia resorption in proliferating progenitors. In differentiating granule cells, cilia deconstruction involved unique disassembly intermediates, and, as maturation progressed, mother centriolar docking at the plasma membrane. Unlike ciliated neurons in other brain regions, our results show the deconstruction of concealed cilia in differentiating granule cells, which might prevent mitogenic hedgehog responsiveness. Ciliary deconstruction could be paradigmatic of cilia removal during differentiation in other tissues.