The primary cilium - once a "rudimentary" organelle that is now a ubiquitous sensory cellular structure involved in many pathological disorders

Development and primary characterization of a human thyroid organoid in vitro model for thyroid metabolism investigation

A 3D thyroid model was developed to address the limitations of 2D cultures and study the effects of compounds like 3-MNT on dehalogenase 1 (IYD) and metabolic activity. Morphology was assessed by TEM, and the expression of tissue-specific genes (TPO, TSHR, PAX8, TTF-1, NIS, IYD, TG) and metabolic features were analyzed using qRT-PCR, immunofluorescence, western blotting, ELISA, and LC-MS/MS, with and without TSH stimulus and 3-MNT treatment. Confocal and TEM analyses confirmed a follicle-like 3D structure. Expression of TPO, NIS, TG, TSH, and PAX markers was significantly higher (p < 0.05) in 3D versus 2D cultures, and ELISA showed increased TG protein production. 3-MNT treatment inhibited IYD activity, indicated by increased MIT and DIT in the media, and significantly altered (p < 0.05) NIS, TG, IYD, TSHR, and TPO expression. These findings suggest 3D thyroid cultures closely replicate tissue traits and functionality, providing a valuable tool for thyroid research.

Hedgehog-Related Mutation Causes Bone Malformations with or without Hereditary Gene Mutations

The hedgehog (Hh) family consists of numerous signaling mediators that play important roles at various stages of development. Thus, the Hh pathway is essential for bone tissue development and tumorigenesis. Gorlin syndrome is a skeletal and tumorigenic disorder caused by gain-of-function mutations in Hh signaling. In this review, we first present the phenotype of Gorlin syndrome and the relationship between genotype and phenotype in bone and craniofacial tissues, including the causative gene as well as other Hh-related genes. Next, the importance of new diagnostic methods using next-generation sequencing and multiple gene panels will be discussed. We summarize Hh-related genetic disorders, including cilia disease, and the genetics of Hh-related bone diseases.

Investigations on Primary Cilia of Nthy-ori 3-1 Cells upon Cysteine Cathepsin Inhibition or Thyrotropin Stimulation

In the thyroid gland, cysteine cathepsins are secreted upon thyrotropin stimulation for thyroglobulin processing, and they are present at the primary cilia of thyroid epithelial cells. Treatment with protease inhibitors resulted in the loss of cilia from rodent thyrocytes and caused redistribution of the thyroid co-regulating G protein-coupled receptor Taar1 to the endoplasmic reticulum. These findings suggest that ciliary cysteine cathepsins are important to maintain sensory and signaling properties for the proper regulation and homeostasis of thyroid follicles. Therefore, it is important to better understand how cilia structure and frequencies are maintained in human thyroid epithelial cells. Hence, we aimed to investigate the potential role of cysteine cathepsins for the maintenance of primary cilia in the normal human Nthy-ori 3-1 thyroid cell line. This was approached by determining cilia lengths and frequencies in cysteine peptidase inhibition conditions in Nthy-ori 3-1 cell cultures. Cilia lengths were shortened upon 5 h of cysteine peptidase inhibition with cell-impermeable E64. Likewise, cilia lengths and frequencies were decreased upon additional overnight treatment with the cysteine peptidase-targeting, activity-based probe DCG-04. The results suggest that cysteine cathepsin activity is required for the maintenance of the cellular protrusions not only in rodents, but also in human thyrocytes. Hence, thyrotropin stimulation was used to simulate physiological conditions that eventually lead to cathepsin-mediated thyroglobulin proteolysis, which is initiated in the thyroid follicle lumen. Immunoblotting revealed that thyrotropin stimulation conditions result in the secretion of little procathepsin L and some pro- and mature cathepsin S but no cathepsin B from the human Nthy-ori 3-1 cells. Unexpectedly, however, 24 h incubation periods with thyrotropin shortened the cilia although higher amounts of cysteine cathepsins were present in the conditioned media. These data point to the necessity of further studies to delineate which of the cysteine cathepsins plays the most prominent role in cilia shortening and/or elongation. Collectively, the results of our study provide corroboration for the hypothesis of thyroid autoregulation by local mechanisms that our group previously proposed.

Primary Cilium Identifies a Quiescent Cell Population in the Human Intestinal Crypt

Primary cilia are sensory antennae located at the cell surface which mediate a variety of extracellular signals involved in development, tissue homeostasis, stem cells and cancer. Primary cilia are found in an extensive array of vertebrae cells but can only be generated when cells become quiescent. The small intestinal epithelium is a rapidly self-renewing tissue organized into a functional unit called the crypt–villus axis, containing progenitor and differentiated cells, respectively. Terminally differentiated villus cells are notoriously devoid of primary cilia. We sought to determine if intestinal crypts contain a quiescent cell population that could be identified by the presence of primary cilia. Here we show that primary cilia are detected in a subset of cells located deep in the crypts slightly above a Paneth cell population. Using a normal epithelial proliferative crypt cell model, we show that primary cilia assembly and activity correlate with a quiescent state. These results provide further evidence for the existence of a quiescent cell population in the human small intestine and suggest the potential for new modes of regulation in stem cell dynamics.

Transcriptomic analysis identifies novel potential biomarkers and highlights cilium-related biological processes in the early stages of prion disease in mice

Prion diseases are fatal and irreversible neurodegenerative diseases induced by the pathogenic form of the prion protein (PrP^Sc), which is converted from the benign form of the prion protein (PrP^C). These diseases are characterized by an extended asymptomatic incubation period accompanied by continuous conversion of PrP^C to PrP^Sc. However, to date, the mechanism governing the conversion to PrP^Sc in the initial stages of prion disease has not been fully elucidated. We collected transcriptome data from the hippocampus of wild-type mice and prion-infected mice at 8 weeks post injection from the Gene Expression Omnibus and analysed differentially expressed genes and related signalling biological process using bioinformatic tools. We identified a total of 36 differentially expressed genes, including 22 upregulated genes and 14 downregulated genes. In addition, we identified that the cilium-related biological process was enriched in the early stages of prion disease. Furthermore, up- and down-regulated genes were associated with cilium-related cellular components and synapse-related cellular components, respectively. To the best of our knowledge, our study was the first to observe the upregulation of cilium-related genes in the early stages of prion disease.

Role of Primary Cilia in Bone and Cartilage

The primary cilium is a nonmotile microtubule-based organelle in most vertebrate cell types. The primary cilium plays a critical role in tissue development and homeostasis by sensing and transducing various signaling pathways. Ciliary proteins such as intraflagellar transport (IFT) proteins as well as ciliary motor proteins, kinesin and dynein, comprise a bidirectional intraflagellar transport system needed for cilia formation and function. Mutations in ciliary proteins that lead to loss or dysfunction of primary cilia cause ciliopathies such as Jeune syndrome and Ellis-van Creveld syndrome and cause abnormalities in tooth development. These diseases exhibit severe skeletal and craniofacial dysplasia, highlighting the significance of primary cilia in skeletal development. Cilia are necessary for the propagation of hedgehog, transforming growth factor β, platelet-derived growth factor, and fibroblast growth factor signaling during osteogenesis and chondrogenesis. Ablation of ciliary proteins such as IFT80 or IFT20 blocks cilia formation, which inhibits osteoblast differentiation, osteoblast polarity, and alignment and reduces bone formation. Similarly, cilia facilitate chondrocyte differentiation and production of a cartilage matrix. Cilia also play a key role in mechanosensing and are needed for increased bone formation in response to mechanical forces.

The Cilioprotist Cytoskeleton , a Model for Understanding How Cell Architecture and Pattern Are Specified: Recent Discoveries from Ciliates and Comparable Model Systems

The cytoskeletons of eukaryotic, cilioprotist microorganisms are complex, highly patterned, and diverse, reflecting the varied and elaborate swimming, feeding, reproductive, and sensory behaviors of the multitude of cilioprotist species that inhabit the aquatic environment. In the past 10-20 years, many new discoveries and technologies have helped to advance our understanding of how cytoskeletal organelles are assembled in many different eukaryotic model systems, in relation to the construction and modification of overall cellular architecture and function. Microtubule organizing centers, particularly basal bodies and centrioles, have continued to reveal their central roles in architectural engineering of the eukaryotic cell, including in the cilioprotists. This review calls attention to (1) published resources that illuminate what is known of the cilioprotist cytoskeleton; (2) recent studies on cilioprotists and other model organisms that raise specific questions regarding whether basal body- and centriole-associated nucleic acids, both DNA and RNA, should continue to be considered when seeking to employ cilioprotists as model systems for cytoskeletal research; and (3) new, mainly imaging, technologies that have already proven useful for, but also promise to enhance, future cytoskeletal research on cilioprotists.

Primary cilia: turning points in establishing their ubiquity, sensory role and the pathological consequences of dysfunction

For over 20 years it has finally become accepted that primary cilia are without doubt important cellular organelles, involved in signalling both intrinsically and extrinsically. The consequences of their agenesis, incorrect assembly and dysfunction only began to be fully appreciated after 2000, although this had been demonstrable over the previous two decades. Before 1980, biologists at large thought the organelle rudimentary or vestigial; how a well-developed cilium could be so slated beggars belief. Many pathological conditions have implicated the primary cilium as either a major or contributing factor, ranging from kidney malfunction (e.g. polycystic kidney disease) to mental aberrations. However, the questions of how the recognition of their prevalence, their sensory function, and their pathological involvement finally emerged as substantiated and verifiable facts needs to be addressed because what happened before the 1980s, and then notably between 1980 and 2000, can help guide research towards answering further questions on these issues. Here the intention is to focus on the salient findings (the turning points) that brought about changes in our knowledge of primary cilia. The literature on them is growing fast, with the total moving towards 20,000 reports, of which > 60% have been published in the last decade. PubMed indicates that nearly 1000 papers were published in 2020 alone. We also have to appreciate that the primary cilium can assume many different forms, each of which means that there must be many genes responsible for their development and final structure. This also suggests that there are many more functions than are currently known in both their sensory reception and signalling properties, probably for many highly specialised purposes. Malfunctioning in any of these roles will undoubtedly uncover further pathological conditions.

Centriole is the pivot coordinating dynamic signaling for cell proliferation and organization during early development in the vertebrates

Vertebrates have an elaborate and functionally segmented body. It evolves from a single cell by systematic cell proliferation but attains a complex body structure with exquisite precision. This development requires two cellular events: cell cycle and ciliogenesis. For these events, the dynamic molecular signaling is converged at the centriole. The cell cycle helps in cell proliferation and growth of the body and is a highly regulated and integrated process. Its errors cause malignancies and developmental disorders. The cells newly proliferated are organized during organogenesis. For a cellular organization, dedicated signaling hubs are developed in the cells, and most often cilia are utilized. The cilium is generated from one of the centrioles involved in cell proliferation. The developmental signaling pathways hosted in cilia are essential for the elaboration of the body plan. The cilium's compartmental seclusion is ideal for noise-free molecular signaling and is essential for the precision of the body layout. The dysfunctional centrioles and primary cilia distort the development of body layout that manifest as serious developmental disorders. Thus, centriole has a dual role in the growth and cellular organization. It organizes dynamically expressed molecules of cell cycle and ciliogenesis and plays a balancing act to generate new cells and organize them during development. A putative master molecule may regulate and coordinate the dynamic gene expression at the centrioles. The convergence of many critical signaling components at the centriole reiterates the idea that centriole is a major molecular workstation involved in elaborating the structural design and complexity in vertebrates. This article is protected by copyright. All rights reserved.

Physiological and Pathophysiological Aspects of Primary Cilia-A Literature Review with View on Functional and Structural Relationships in Cartilage

Cilia are cellular organelles that project from the cell. They occur in nearly all non-hematopoietic tissues and have different functions in different tissues. In mesenchymal tissues primary cilia play a crucial role in the adequate morphogenesis during embryological development. In mature articular cartilage, primary cilia fulfil chemo- and mechanosensitive functions to adapt the cellular mechanisms on extracellular changes and thus, maintain tissue homeostasis and morphometry. Ciliary abnormalities in osteoarthritic cartilage could represent pathophysiological relationships between ciliary dysfunction and tissue deformation. Nevertheless, the molecular and pathophysiological relationships of ‘Primary Cilia’ (PC) in the context of osteoarthritis is not yet fully understood. The present review focuses on the current knowledge about PC and provide a short but not exhaustive overview of their role in cartilage.

The Centrosome and the Primary Cilium: The Yin and Yang of a Hybrid Organelle

Centrosomes and primary cilia are usually considered as distinct organelles, although both are assembled with the same evolutionary conserved, microtubule-based templates, the centrioles. Centrosomes serve as major microtubule- and actin cytoskeleton-organizing centers and are involved in a variety of intracellular processes, whereas primary cilia receive and transduce environmental signals to elicit cellular and organismal responses. Understanding the functional relationship between centrosomes and primary cilia is important because defects in both structures have been implicated in various diseases, including cancer. Here, we discuss evidence that the animal centrosome evolved, with the transition to complex multicellularity, as a hybrid organelle comprised of the two distinct, but intertwined, structural-functional modules: the centriole/primary cilium module and the pericentriolar material/centrosome module. The evolution of the former module may have been caused by the expanding cellular diversification and intercommunication, whereas that of the latter module may have been driven by the increasing complexity of mitosis and the requirement for maintaining cell polarity, individuation, and adhesion. Through its unique ability to serve both as a plasma membrane-associated primary cilium organizer and a juxtanuclear microtubule-organizing center, the animal centrosome has become an ideal integrator of extracellular and intracellular signals with the cytoskeleton and a switch between the non-cell autonomous and the cell-autonomous signaling modes. In light of this hypothesis, we discuss centrosome dynamics during cell proliferation, migration, and differentiation and propose a model of centrosome-driven microtubule assembly in mitotic and interphase cells. In addition, we outline the evolutionary benefits of the animal centrosome and highlight the hierarchy and modularity of the centrosome biogenesis networks.

Primary Cilium in the Human Thyrocyte: Changes in Frequency and Length in Relation to the Functional Pathology of the Thyroid Gland

BACKGROUND

Primary cilia (PC) are conserved structures in the adult thyroid gland of different mammals. It was recently described that in humans, PC are usually present as a single copy per follicular cell emerging from the follicular cell apex into the follicular lumen.

METHODS

To understand the role developed by PC in thyroid hormonogenesis better, their changes in different human functional thyroid diseases (diffuse toxic hyperplasia/Graves' disease [GD] and nodular hyperplasia [NH]/nodular goiter), in comparison to normal thyroid tissue, were investigated using immunofluorescence, morphometry, and electron microscopy analyses.

RESULTS

Significantly decreased ciliary frequencies were found in both NH (51.16 ± 11.69%) and GD (44.43 ± 23.70%) compared to normal thyroid tissue (76.09 ± 7.31%). Similarly, PC lengths were also significantly decreased in both NH (2.02 ± 0.35 μm) and GD (2.4 ± 0.48 μm) compared to normal glands (3.93 ± 0.90 μm). Moreover, in GD patients, hyperactive-follicle foci always showed diminished ciliary frequency and length compared to any other thyroid follicle pattern, independent of their thyroid status. Finally, in GD, the percentage of thyrocytes exhibiting PC in the "normal-appearance areas" was significantly lower in correspondence with the subsistence of signs of thyroid biosynthetic hyperactivity after long-term antithyroid drug treatment.

CONCLUSIONS

The results suggest a direct relationship between ciliogenesis and both follicle activity and tissue heterogeneity in the functional pathology of the thyroid gland.

Cilia Distal Domain: Diversity in Evolutionarily Conserved Structures

Eukaryotic cilia are microtubule-based organelles that protrude from the cell surface to fulfill sensory and motility functions. Their basic structure consists of an axoneme templated by a centriole/basal body. Striking differences in ciliary ultra-structures can be found at the ciliary base, the axoneme and the tip, not only throughout the eukaryotic tree of life, but within a single organism. Defects in cilia biogenesis and function are at the origin of human ciliopathies. This structural/functional diversity and its relationship with the etiology of these diseases is poorly understood. Some of the important events in cilia function occur at their distal domain, including cilia assembly/disassembly, IFT (intraflagellar transport) complexes' remodeling, and signal detection/transduction. How axonemal microtubules end at this domain varies with distinct cilia types, originating different tip architectures. Additionally, they show a high degree of dynamic behavior and are able to respond to different stimuli. The existence of microtubule-capping structures (caps) in certain types of cilia contributes to this diversity. It has been proposed that caps play a role in axoneme length control and stabilization, but their roles are still poorly understood. Here, we review the current knowledge on cilia structure diversity with a focus on the cilia distal domain and caps and discuss how they affect cilia structure and function.