CLEM Methods for Studying Primary Cilia

Fluorescence imaging of beta cell primary cilia

Primary cilia are slender cell-surface organelles that project into the intercellular space. In pancreatic beta cells, primary cilia coordinate a variety of cell responses including GPCR signaling, calcium influx, and insulin secretion, along with likely many underappreciated roles in islet development and differentiation. To study cilia function in islet biology, direct visualization of primary cilia by microscopic methods is often a necessary first step. Ciliary abundance, distribution, and morphology are heterogeneous among islet cells and are best visualized by fluorescence microscopy, the tools for which are readily accessible to most researchers. Here we present a collection of fluorescence imaging methods that we have adopted and optimized for the observation of primary cilia in mouse and human islets. These include conventional confocal microscopy using fixed islets and pancreas sections, live-cell imaging with cilia-targeted biosensors and probes, cilia motion recordings, and quantitative analysis of primary cilia waveform in the ex vivo environment. We discuss practical considerations and limitations of our approaches as well as new tools on the horizon to facilitate the observation of primary cilia in pancreatic islets.

Super-resolution microscopy reveals photoreceptor-specific subciliary location and function of ciliopathy-associated protein CEP290

Mutations in the cilium-associated protein CEP290 cause retinal degeneration as part of multiorgan ciliopathies or as retina-specific diseases. The precise location and the functional roles of CEP290 within cilia and, specifically, the connecting cilia (CC) of photoreceptors, remain unclear. We used super-resolution fluorescence microscopy and electron microscopy to localize CEP290 in the CC and in the primary cilia of cultured cells with subdiffraction resolution and to determine effects of CEP290 deficiency in 3 mutant models. Radially, CEP290 localizes in close proximity to the microtubule doublets in the region between the doublets and the ciliary membrane. Longitudinally, it is distributed throughout the length of the CC whereas it is confined to the very base of primary cilia in human retinal pigment epithelium-1 cells. We found Y-shaped links, ciliary substructures between microtubules and membrane, throughout the length of the CC. Severe CEP290 deficiencies in mouse models did not prevent assembly of cilia or cause obvious mislocalization of ciliary components in early stages of degeneration. There were fewer cilia and no normal outer segments in the mutants, but the Y-shaped links were clearly present. These results point to photoreceptor-specific functions of CEP290 essential for CC maturation and stability following the earliest stages of ciliogenesis.

The tubulin code specializes neuronal cilia for extracellular vesicle release

Cilia are microtubule-based organelles that display diversity in morphology, ultrastructure, protein composition, and function. The ciliary microtubules of C. elegans sensory neurons exemplify this diversity and provide a paradigm to understand mechanisms driving ciliary specialization. Only a subset of ciliated neurons in C. elegans are specialized to make and release bioactive extracellular vesicles (EVs) into the environment. The cilia of extracellular vesicle releasing neurons have distinct axonemal features and specialized intraflagellar transport that are important for releasing EVs. In this review, we discuss the role of the tubulin code in the specialization of microtubules in cilia of EV releasing neurons.

Ciliary gene RPGRIP1L is required for hypothalamic arcuate neuron development

Intronic polymorphisms in the α-ketoglutarate-dependent dioxygenase gene (FTO) that are highly associated with increased body weight have been implicated in the transcriptional control of a nearby ciliary gene, retinitis pigmentosa GTPase regulator-interacting protein-1 like (RPGRIP1L). Previous studies have shown that congenital Rpgrip1l hypomorphism in murine proopiomelanocortin (Pomc) neurons causes obesity by increasing food intake. Here, we show by congenital and adult-onset Rpgrip1l deletion in Pomc-expressing neurons that the hyperphagia and obesity are likely due to neurodevelopmental effects that are characterized by a reduction in the Pomc/Neuropeptide Y (Npy) neuronal number ratio and marked increases in arcuate hypothalamic-paraventricular hypothalamic (ARH-PVH) axonal projections. Biallelic RPGRIP1L mutations result in fewer cilia-positive human induced pluripotent stem cell-derived (iPSC-derived) neurons and blunted responses to Sonic Hedgehog (SHH). Isogenic human ARH-like embryonic stem cell-derived (ESc-derived) neurons homozygous for the obesity-risk alleles at rs8050136 or rs1421085 have decreased RPGRIP1L expression and have lower numbers of POMC neurons. RPGRIP1L overexpression increases POMC cell number. These findings suggest that apparently functional intronic polymorphisms affect hypothalamic RPGRIP1L expression and impact development of POMC neurons and their derivatives, leading to hyperphagia and increased adiposity.