Fluorescence imaging of beta cell primary cilia

ARL13B-Cerulean rescues Arl13b-null mouse from embryonic lethality and reveals a role for ARL13B in spermatogenesis

ARL13B is a regulatory GTPase enriched in cilia, making it a popular marker for this organelle. Arl13b hnn/hnn mice lack ARL13B expression, die during midgestation, and exhibit defects in ciliogenesis. The R26Arl13b-Fucci2aR biosensor mouse line directs the expression of fluorescently tagged full-length Arl13b cDNA upon Cre recombination. To determine whether constitutive, ubiquitous expression of ARL13B-Cerulean can replace endogenous gene expression, we generated Arl13b hnn/hnn animals expressing ARL13B-Cerulean. We show that Arl13b hnn/hnn ;Arl13b-Cerulean mice survive to adulthood with no obvious physical or behavioral defects, indicating that the fluorescently tagged protein can functionally replace the endogenous protein during development. However, we observed that rescued males failed to sire offspring, revealing a role for ARL13B in spermatogenesis. This work shows that the R26Arl13b-Fucci2aR mouse contains an inducible allele of Arl13b capable of functioning in most tissues and biological processes.

Intravital imaging reveals glucose-dependent cilia movement in pancreatic islets in vivo

Pancreatic islet cells harbor primary cilia, small sensory organelles that detect environmental changes to regulate hormone secretion and intercellular communication. While the sensory and signaling capacity of primary cilia are well-appreciated, it is less recognized that these organelles also possess active motility, including in dense multicellular tissues such as the pancreatic islet. In this manuscript, we use transgenic cilia reporter mice and an intravital imaging approach to quantitate primary cilia dynamics as it occurs in live mouse pancreatic islets. We validate this imaging workflow as suitable for studying islet cilia motion in real time in vivo and demonstrate that glucose stimulation corresponds to a change in cilia motility, which may be a physiologic measure of nutrient-dependent fluxes in islet cell function. Complementary ex vivo analysis of isolated islets further demonstrates that metabolic stress in the form of lipotoxicity impairs cilia motility and these effects can be reversed by glucose elevation. These findings suggest that cilia motility is sensitive to metabolic stress and highlight its potential functional role in beta cell adaptation.

Ultrastructure expansion microscopy of axonemal dynein in islet primary cilia

Primary cilia are vital sensory organelles whose structures are challenging to study due to their solitary nature and intricate cytoskeleton. Current imaging modalities are limited in their ability to visualize structural details that are important for understanding primary cilia function. Ultrastructure expansion microscopy (U-ExM) is a recent superresolution imaging technique that physically expands biological specimens using a swellable hydrogel, allowing structural interrogation of small cellular components such as cilia. In this study, we apply U-ExM to mouse and human pancreatic islets to visualize the axonemal cytoskeleton and associated proteins in primary cilia. Our study reveals the expression of axonemal dynein in islet primary cilia and centrioles, with DNAI1 being a principal subunit which we validate using targeted shRNA knockdown. We conclude that U-ExM is suitable for localizing protein expression in pancreatic islet cilia which contain axonemal dynein.

Immuno-scanning electron microscopy of islet primary cilia

The definitive demonstration of protein localization on primary cilia has been a challenge for cilia biologists. Primary cilia are solitary thread-like projections that have a specialized protein composition, but as the ciliary structure overlays the cell membrane and other cell parts, the identity of ciliary proteins are difficult to ascertain by conventional imaging approaches like immunofluorescence microscopy. Surface scanning electron microscopy combined with immunolabeling (immuno-SEM) bypasses some of these indeterminacies by unambiguously showing protein expression in the context of the three-dimensional ultrastructure of the cilium. Here, we apply immuno-SEM to specifically identify proteins on the primary cilia of mouse and human pancreatic islets, including post-translationally modified tubulin, intraflagellar transport (IFT)88, the small GTPase Arl13b, as well as subunits of axonemal dynein. Key parameters in sample preparation, immunolabeling and imaging acquisition are discussed to facilitate similar studies by others in the cilia research community.

Retinal primary cilia and their dysfunction in retinal neurodegenerative diseases: beyond ciliopathies

Primary cilia are sensory organelles that extend from the cellular membrane and are found in a wide range of cell types. Cilia possess a plethora of vital components that enable the detection and transmission of several signaling pathways, including Wnt and Shh. In turn, the regulation of ciliogenesis and cilium length is influenced by various factors, including autophagy, organization of the actin cytoskeleton, and signaling inside the cilium. Irregularities in the development, maintenance, and function of this cellular component lead to a range of clinical manifestations known as ciliopathies. The majority of people with ciliopathies have a high prevalence of retinal degeneration. The most common theory is that retinal degeneration is primarily caused by functional and developmental problems within retinal photoreceptors. The contribution of other ciliated retinal cell types to retinal degeneration has not been explored to date. In this review, we examine the occurrence of primary cilia in various retinal cell types and their significance in pathology. Additionally, we explore potential therapeutic approaches targeting ciliopathies. By engaging in this endeavor, we present new ideas that elucidate innovative concepts for the future investigation and treatment of retinal ciliopathies.

In-section Click-iT detection and super-resolution CLEM analysis of nucleolar ultrastructure and replication in plants

Correlative light and electron microscopy (CLEM) is an important tool for the localisation of target molecule(s) and their spatial correlation with the ultrastructural map of subcellular features at the nanometre scale. Adoption of these advanced imaging methods has been limited in plant biology, due to challenges with plant tissue permeability, fluorescence labelling efficiency, indexing of features of interest throughout the complex 3D volume and their re-localization on micrographs of ultrathin cross-sections. Here, we demonstrate an imaging approach based on tissue processing and embedding into methacrylate resin followed by imaging of sections by both, single-molecule localization microscopy and transmission electron microscopy using consecutive CLEM and same-section CLEM correlative workflow. Importantly, we demonstrate that the use of a particular type of embedding resin is not only compatible with single-molecule localization microscopy but shows improvements in the fluorophore blinking behavior relative to the whole-mount approaches. Here, we use a commercially available Click-iT ethynyl-deoxyuridine cell proliferation kit to visualize the DNA replication sites of wild-type Arabidopsis thaliana seedlings, as well as fasciata1 and nucleolin1 plants and apply our in-section CLEM imaging workflow for the analysis of S-phase progression and nucleolar organization in mutant plants with aberrant nucleolar phenotypes.

Immuno-Scanning Electron Microscopy of Islet Primary Cilia

The definitive demonstration of protein localization on primary cilia has been a challenge for cilia biologists. Primary cilia are solitary thread-like projections that contain specialized protein composition, but as the ciliary structure overlays the cell membrane and other cell parts, the identity of ciliary proteins are difficult to ascertain by conventional imaging approaches like immunofluorescence microscopy. Surface scanning electron microscopy combined with immuno-labeling (immuno-SEM) bypasses some of these indeterminacies by unambiguously showing protein expression in the context of the 3D ultrastructure of the cilium. Here we apply immuno-SEM to specifically identify proteins on the primary cilia of mouse and human pancreatic islets, including post-translationally modified tubulin, intraflagellar transport (IFT) 88, the small GTPase Arl13b, as well as subunits of axonemal dynein. Key parameters in sample preparation, immuno-labeling, and imaging acquisition are discussed to facilitate similar studies by others in the cilia research community.

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

Rediscovering Primary Cilia in Pancreatic Islets

Primary cilia are microtubule-based sensory and signaling organelles on the surfaces of most eukaryotic cells. Despite their early description by microscopy studies, islet cilia had not been examined in the functional context until recent decades. In pancreatic islets as in other tissues, primary cilia facilitate crucial developmental and signaling pathways in response to extracellular stimuli. Many human developmental and genetic disorders are associated with ciliary dysfunction, some manifesting as obesity and diabetes. Understanding the basis for metabolic diseases in human ciliopathies has been aided by close examination of cilia action in pancreatic islets at cellular and molecular levels. In this article, we review the evidence for ciliary expression on islet cells, known roles of cilia in pancreas development and islet hormone secretion, and summarize metabolic manifestations of human ciliopathy syndromes. We discuss emerging data on primary cilia regulation of islet cell signaling and the structural basis of cilia-mediated cell crosstalk, and offer our interpretation on the role of cilia in glucose homeostasis and human diseases.

Scanning electron microscopy of human islet cilia

Human islet primary cilia are vital glucose-regulating organelles whose structure remains uncharacterized. Scanning electron microscopy (SEM) is a useful technique for studying the surface morphology of membrane projections like cilia, but conventional sample preparation does not reveal the submembrane axonemal structure, which holds key implications for ciliary function. To overcome this challenge, we combined SEM with membrane-extraction techniques to examine primary cilia in native human islets. Our data show well-preserved cilia subdomains which demonstrate both expected and unexpected ultrastructural motifs. Morphometric features were quantified when possible, including axonemal length and diameter, microtubule conformations, and chirality. We further describe a ciliary ring, a structure that may be a specialization in human islets. Key findings are correlated with fluorescence microscopy and interpreted in the context of cilia function as a cellular sensor and communications locus in pancreatic islets.

Scanning electron microscopy of human islet cilia

Human islet primary cilia are vital glucose-regulating organelles whose structure remains uncharacterized. Scanning electron microscopy (SEM) is a useful technique for studying the surface morphology of membrane projections like primary cilia, but conventional sample preparation does not reveal the sub-membrane axonemal structure which holds key implications for cilia function. To overcome this challenge, we combined SEM with membrane-extraction techniques to examine cilia in native human islets. Our data show well-preserved cilia subdomains which demonstrate both expected and unexpected ultrastructural motifs. Morphometric features were quantified when possible, including axonemal length and diameter, microtubule conformations and chirality. We further describe a novel ciliary ring, a structure that may be a specialization in human islets. Key findings are correlated with fluorescence microscopy and interpreted in the context of cilia function as a cellular sensor and communications locus in pancreatic islets.