Polycystins 1 and 2 mediate mechanosensation in the primary cilium of kidney cells

Mechanosensitive ion channels in glaucoma pathophysiology

Force sensing is a fundamental ability that allows cells and organisms to interact with their physical environment. The eye is constantly subjected to mechanical forces such as blinking and eye movements. Furthermore, elevated intraocular pressure (IOP) can cause mechanical strain at the optic nerve head, resulting in retinal ganglion cell death (RGC) in glaucoma. How mechanical stimuli are sensed and affect cellular physiology in the eye is unclear. Recent studies have shown that mechanosensitive ion channels are expressed in many ocular tissues relevant to glaucoma and may influence IOP regulation and RGC survival. Furthermore, variants in mechanosensitive ion channel genes may be associated with risk for primary open angle glaucoma. These findings suggest that mechanosensitive channels may be important mechanosensors mediating cellular responses to pressure signals in the eye. In this review, we focus on mechanosensitive ion channels from three major channel families-PIEZO, two-pore potassium and transient receptor potential channels. We review the key properties of these channels, their effects on cell function and physiology, and discuss their possible roles in glaucoma pathophysiology.

Identification and Properties of TRPV4 Mutant Channels Present in Polycystic Kidney Disease Patients

Polycystic kidney disease (PKD), a disease characterized by the enlargement of the kidney through cystic growth is the fourth leading cause of end-stage kidney disease world-wide. Transient receptor potential Vanilloid 4 (TRPV4), a calcium-permeable TRP, channel participates in kidney cell physiology and since TRPV4 forms complexes with another channel whose malfunction is associated to PKD, TRPP2 (or PKD2), we sought to determine whether patients with PKD, exhibit previously unknown mutations in TRPV4. Here, we report the presence of mutations in the TRPV4 gene in patients diagnosed with PKD and determine that they produce gain-of-function (GOF). Mutations in the sequence of the TRPV4 gene have been associated to a broad spectrum of neuropathies and skeletal dysplasias but not PKD, and their biophysical effects on channel function have not been elucidated. We identified and examined the functional behavior of a novel E6K mutant and of the previously known S94L and A217S mutant TRVP4 channels. The A217S mutation has been associated to mixed neuropathy and/or skeletal dysplasia phenotypes, however, the PKD carriers of these variants had not been diagnosed with these reported clinical manifestations. The presence of certain mutations in TRPV4 may influence the progression and severity of PKD through GOF mechanisms. PKD patients carrying TRVP4 mutations are putatively more likely to require dialysis or renal transplant as compared to those without these mutations.

Genome-wide identification, characterization, and expression analysis of the transient receptor potential gene family in mandarin fish Siniperca chuatsi

BACKGROUND

Temperature is a crucial environmental determinant for the vitality and development of teleost fish, yet the underlying mechanisms by which they sense temperature fluctuations remain largely unexplored. Transient receptor potential (TRP) proteins, renowned for their involvement in temperature sensing, have not been characterized in teleost fish, especially regarding their temperature-sensing capabilities.

RESULTS

In this study, a genome-wide analysis was conducted, identifying a total of 28 TRP genes in the mandarin fish Siniperca chuatsi. These genes were categorized into the families of TRPA, TRPC, TRPP, TRPM, TRPML, and TRPV. Despite notable variations in conserved motifs across different subfamilies, TRP family members shared common structural features, including ankyrin repeats and the TRP domain. Tissue expression analysis showed that each of these TRP genes exhibited a unique expression pattern. Furthermore, examination of the tissue expression patterns of ten selected TRP genes following exposure to both high and low temperature stress indicated the expression of TRP genes were responsive to temperatures changes. Moreover, the expression profiles of TRP genes in response to mandarin fish virus infections showed significant upregulation for most genes after Siniperca chuatsi rhabdovirus, mandarin fish iridovirus and infectious spleen and kidney necrosis virus infection.

CONCLUSIONS

This study characterized the TRP family genes in mandarin fish genome-wide, and explored their expression patterns in response to temperature stress and virus infections. Our work will enhance the overall understanding of fish TRP channels and their possible functions.

Liver-specific DICER1 syndrome model mice develop cystic liver tumors with defective primary cilia

DICER1 syndrome is a tumor predisposition syndrome caused by familial genetic mutations in DICER1. Pathogenic variants of DICER1 have been discovered in many rare cancers, including cystic liver tumors. However, the molecular mechanisms underlying liver lesions induced by these variants remain unclear. In the present study, we sought to gain a better understanding of the pathogenesis of these variants by generating a mouse model of liver-specific DICER1 syndrome. The mouse model developed bile duct hyperplasia with fibrosis, similar to congenital hepatic fibrosis, as well as cystic liver tumors resembling those in Caroli's syndrome, intrahepatic cholangiocarcinoma, and hepatocellular carcinoma. Interestingly, the mouse model of DICER1 syndrome showed abnormal formation of primary cilia in the bile duct epithelium, which is a known cause of bile duct hyperplasia and cyst formation. These results indicated that DICER1 mutations contribute to cystic liver tumors by inducing defective primary cilia. The mouse model generated in this study will be useful for elucidating the potential mechanisms of tumorigenesis induced by DICER1 variants and for obtaining a comprehensive understanding of DICER1 syndrome. © 2024 The Pathological Society of Great Britain and Ireland.

Human efferent ductules and epididymis display unique cell lineages with motile and primary cilia

BACKGROUND

Previous research has illustrated the role of cilia as mechanical and sensory antennae in various organs within the mammalian male reproductive system across different developmental stages. Despite their significance in both organ development and homeostasis, primary cilia in the human male reproductive excurrent duct have been overlooked due to limited access to human specimens.

OBJECTIVE

This study aimed to characterize the unique cellular composition of human efferent and epididymal ducts, with a focus on their association with primary cilia.

MATERIALS AND METHODS

Human efferent ductules/epididymides from five donors aged 32-47 years, were obtained through our local organ transplant program. Cell lineage specificity and primary cilia features were examined by immunofluorescent staining and confocal microscopy in the efferent ductules and the distinct segments of the epididymis.

RESULTS

The epithelium of the human efferent duct exhibited estrogen receptor-positive cells with primary cilia, FoxJ1-positive multiciliated cells with numerous motile cilia, and non-ciliated intraepithelial immune cells. Notably, intraluminal macrophages, identified by CD163/CD68 positivity, were observed to engage in sperm phagocytosis. In all three segments of the human epididymis, primary cilia were found on the surface of principal and basal cells.

DISCUSSION AND CONCLUSIONS

Our research indicates that the human efferent ductules create a distinct environment, characterized by the presence of two types of ciliated cells that are in contact with immune cells. The discovery of sensory primary cilia exposed on the surface of reabsorptive cells in the efferent ductules, as well as on basal and principal cells in the epididymis, lays the foundation for complementary functional studies. This research uncovers novel characteristics exclusive to human efferent ductules and epididymides, providing a basis for exploring innovative approaches to male contraception and infertility treatment.

Excess microtubule and F-actin formation mediates shortening and loss of primary cilia in response to a hyperosmotic milieu

The primary cilium is a small organelle protruding from the cell surface that receives signals from the extracellular milieu. Although dozens of studies have reported that several genetic factors can impair the structure of primary cilia, evidence for environmental stimuli affecting primary cilia structures is limited. Here, we investigated an extracellular stress that affected primary cilia morphology and its underlying mechanisms. Hyperosmotic shock induced reversible shortening and disassembly of the primary cilia of murine intramedullary collecting duct cells. The shortening of primary cilia caused by hyperosmotic shock followed delocalization of the pericentriolar material (PCM). Excessive microtubule and F-actin formation in the cytoplasm coincided with the hyperosmotic shock-induced changes to primary cilia and the PCM. Treatment with a microtubule-disrupting agent, nocodazole, partially prevented the hyperosmotic shock-induced disassembly of primary cilia and almost completely prevented delocalization of the PCM. An actin polymerization inhibitor, latrunculin A, also partially prevented the hyperosmotic shock-induced shortening and disassembly of primary cilia and almost completely prevented delocalization of the PCM. We demonstrate that hyperosmotic shock induces reversible morphological changes in primary cilia and the PCM in a manner dependent on excessive formation of microtubule and F-actin.

Re-evaluating TRP channel mechanosensitivity

Transient receptor potential (TRP) channels are implicated in a wide array of mechanotransduction processes. However, a question remains whether TRP channels directly sense mechanical force, thus acting as primary mechanotransducers. We use several recent examples to demonstrate the difficulty in definitively ascribing mechanosensitivity to TRP channel subfamilies. Ultimately, despite being implicated in an ever-growing list of mechanosignalling events in most cases limited robust or reproducible evidence supports the contention that TRP channels act as primary transducers of mechanical forces. They either (i) possess unique and as yet unspecified structural or local requirements for mechanosensitivity; or (ii) act as mechanoamplifiers responding downstream of the activation of a primary mechanotransducer that could include Ca2+-permeable mechanosensitive (MS) channels or other potentially unidentified mechanosensors.

Disease-associated missense mutations in the pore loop of polycystin-2 alter its ion channel function in a heterologous expression system

Polycystin-2 (PC2) is mutated in ∼15% of patients with autosomal dominant polycystic kidney disease (ADPKD). PC2 belongs to the family of transient receptor potential (TRP) channels and can function as a homotetramer. We investigated whether three disease-associated mutations (F629S, C632R, or R638C) localized in the channel's pore loop alter ion channel properties of human PC2 expressed in Xenopus laevis oocytes. Expression of wild-type (WT) PC2 typically resulted in small but measurable Na+ inward currents in the absence of extracellular divalent cations. These currents were no longer observed when individual pore mutations were introduced in WT PC2. Similarly, Na+ inward currents mediated by the F604P gain-of-function (GOF) PC2 construct (PC2 F604P) were abolished by each of the three pore mutations. In contrast, when the mutations were introduced in another GOF construct, PC2 L677A N681A, only C632R had a complete loss-of-function effect, whereas significant residual Na+ inward currents were observed with F629S (∼15%) and R638C (∼30%). Importantly, the R638C mutation also abolished the Ca2+ permeability of PC2 L677A N681A and altered its monovalent cation selectivity. To elucidate the molecular mechanisms by which the R638C mutation affects channel function, molecular dynamics (MD) simulations were used in combination with functional experiments and site-directed mutagenesis. Our findings suggest that R638C stabilizes ionic interactions between Na+ ions and the selectivity filter residue D643. This probably explains the reduced monovalent cation conductance of the mutant channel. In summary, our data support the concept that altered ion channel properties of PC2 contribute to the pathogenesis of ADPKD.

Calcium signalling and transport in the kidney

The kidney plays a pivotal role in regulating calcium levels within the body. Approximately 98% of the filtered calcium is reabsorbed in the nephron, and this process is tightly controlled to maintain calcium homeostasis, which is required to facilitate optimal bone mineralization, preserve serum calcium levels within a narrow range, and support intracellular signalling mechanisms. The maintenance of these functions is attributed to a delicate balance achieved by various calcium channels, transporters, and calcium-binding proteins in renal cells. Perturbation of this balance due to deficiency or dysfunction of calcium channels and calcium-binding proteins can lead to severe complications. For example, polycystic kidney disease is linked to aberrant calcium transport and signalling. Furthermore, dysregulation of calcium levels can promote the formation of kidney stones. This Review provides an updated description of the key aspects of calcium handling in the kidney, focusing on the function of various calcium channels and the physiological stimuli that control these channels or are communicated through them. A discussion of the role of calcium as an intracellular second messenger and the pathophysiology of renal calcium dysregulation, as well as a summary of gaps in knowledge and future prospects, are also included.

Beyond stiffness: deciphering the role of viscoelasticity in cancer evolution and treatment response

There is increasing evidence that cancer progression is linked to tissue viscoelasticity, which challenges the commonly accepted notion that stiffness is the main mechanical hallmark of cancer. However, this new insight has not reached widespread clinical use, as most clinical trials focus on the application of tissue elasticity and stiffness in diagnostic, therapeutic, and surgical planning. Therefore, there is a need to advance the fundamental understanding of the effect of viscoelasticity on cancer progression, to develop novel mechanical biomarkers of clinical significance. Tissue viscoelasticity is largely determined by the extracellular matrix (ECM), which can be simulatedin vitrousing hydrogel-based platforms. Since the mechanical properties of hydrogels can be easily adjusted by changing parameters such as molecular weight and crosslinking type, they provide a platform to systematically study the relationship between ECM viscoelasticity and cancer progression. This review begins with an overview of cancer viscoelasticity, describing how tumor cells interact with biophysical signals in their environment, how they contribute to tumor viscoelasticity, and how this translates into cancer progression. Next, an overview of clinical trials focused on measuring biomechanical properties of tumors is presented, highlighting the biomechanical properties utilized for cancer diagnosis and monitoring. Finally, this review examines the use of biofabricated tumor models for studying the impact of ECM viscoelasticity on cancer behavior and progression and it explores potential avenues for future research on the production of more sophisticated and biomimetic tumor models, as well as their mechanical evaluation.

Nephronophthisis: a pathological and genetic perspective

Nephronophthisis (NPHP) is an autosomal recessive cystic kidney disease and is one of the most frequent genetic causes for kidney failure (KF) in children and adolescents. Over 20 genes cause NPHP and over 90 genes contribute to renal ciliopathies often involving multiple organs. About 15-20% of NPHP patients have additional extrarenal symptoms affecting other organs than the kidneys. The involvement of additional organ systems in syndromic forms of NPHP is explained by shared expression of most NPHP gene products in centrosomes and primary cilia, a sensory organelle present in most mammalian cells. This finding resulted in the classification of NPHP as a ciliopathy. If extrarenal symptoms are present in addition to NPHP, these disorders are defined as NPHP-related ciliopathies (NPHP-RC) and can involve the retina (e.g., with Senior-Løken syndrome), CNS (central nervous system) (e.g., with Joubert syndrome), liver (e.g., Boichis and Arima syndromes), or bone (e.g., Mainzer-Saldino and Sensenbrenner syndromes). This review focuses on the pathological findings and the recent genetic advances in NPHP and NPHP-RC. Different mechanisms and signaling pathways are involved in NPHP ranging from planar cell polarity, sonic hedgehog signaling (Shh), DNA damage response pathway, Hippo, mTOR, and cAMP signaling. A number of therapeutic interventions appear to be promising, ranging from vasopressin receptor 2 antagonists such as tolvaptan, cyclin-dependent kinase inhibitors such as roscovitine, Hh agonists such as purmorphamine, and mTOR inhibitors such as rapamycin.

Emerging mechanistic understanding of cilia function in cellular signalling

Primary cilia are solitary, immotile sensory organelles present on most cells in the body that participate broadly in human health, physiology and disease. Cilia generate a unique environment for signal transduction with tight control of protein, lipid and second messenger concentrations within a relatively small compartment, enabling reception, transmission and integration of biological information. In this Review, we discuss how cilia function as signalling hubs in cell-cell communication using three signalling pathways as examples: ciliary G-protein-coupled receptors (GPCRs), the Hedgehog (Hh) pathway and polycystin ion channels. We review how defects in these ciliary signalling pathways lead to a heterogeneous group of conditions known as 'ciliopathies', including metabolic syndromes, birth defects and polycystic kidney disease. Emerging understanding of these pathways' transduction mechanisms reveals common themes between these cilia-based signalling pathways that may apply to other pathways as well. These mechanistic insights reveal how cilia orchestrate normal and pathophysiological signalling outputs broadly throughout human biology.

Collective cell dynamics and luminal fluid flow in the epididymis: A mechanobiological perspective

BACKGROUND

The mammalian epididymis is a specialized duct system that serves a critical role in sperm maturation and storage. Its distinctive, highly coiled tissue morphology provides a unique opportunity to investigate the link between form and function in reproductive biology. Although recent genetic studies have identified key genes and signaling pathways involved in the development and physiological functions of the epididymis, there has been limited discussion about the underlying dynamic and mechanical processes that govern these phenomena.

AIMS

In this review, we aim to address this gap by examining two key aspects of the epididymis across its developmental and physiological phases.

RESULTS AND DISCUSSION

First, we discuss how the complex morphology of the Wolffian/epididymal duct emerges through collective cell dynamics, including duct elongation, cell proliferation, and arrangement during embryonic development. Second, we highlight dynamic aspects of luminal fluid flow in the epididymis, essential for regulating the microenvironment for sperm maturation and motility, and discuss how this phenomenon emerges and interplays with epididymal epithelial cells.

CONCLUSION

This review not only aims to summarize current knowledge but also to provide a starting point for further exploration of mechanobiological aspects related to the cellular and extracellular fluid dynamics in the epididymis.

Decoupling shear stress and pressure effects in the biomechanics of autosomal dominant polycystic kidney disease using a perfused kidney-on-chip

Kidney tubular cells are submitted to two distinct mechanical forces generated by the urine flow: shear stress and hydrostatic pressure. In addition, the mechanical properties of the surrounding extracellular matrix modulate tubule deformation under constraints. These mechanical factors likely play a role in the pathophysiology of kidney diseases as exemplified by autosomal dominant polycystic kidney disease, in which pressure, flow and matrix stiffness have been proposed to modulate the cystic dilation of tubules with PKD1 mutations. The lack of in vitro systems recapitulating the mechanical environment of kidney tubules impedes our ability to dissect the role of these mechanical factors. Here we describe a perfused kidney-on-chip with tunable extracellular matrix mechanical properties and hydrodynamic constraints, that allows a decoupling of shear stress and flow. We used this system to dissect how these mechanical cues affect Pkd1 -/- tubule dilation. Our results show two distinct mechanisms leading to tubular dilation. For PCT cells (proximal tubule), overproliferation mechanically leads to tubular dilation, regardless of the mechanical context. For mIMCD-3 cells (collecting duct), tube dilation is associated with a squamous cell morphology but not with overproliferation and is highly sensitive to extracellular matrix properties and hydrodynamic constraints. Surprisingly, flow alone suppressed Pkd1 -/- mIMCD-3 tubule dilation observed in static conditions, while the addition of luminal pressure restored it. Our in vitro model emulating nephron geometrical and mechanical organization sheds light on the roles of mechanical constraints in ADPKD and demonstrates the importance of controlling intraluminal pressure in kidney tubule models.

Regulatory role of primary cilia in oral and maxillofacial development and disease

Primary cilia have versatile functions, such as receiving signals from the extracellular microenvironment, mediating signaling transduction, and transporting ciliary substances, in tissue and organ development and clinical disease pathogenesis. During early development (embryos within 10 weeks) in the oral and maxillofacial region, defects in the structure and function of primary cilia can result in severe craniofacial malformations. For example, mice with mutations in the cilia-related genes Kif3a and IFT88 exhibit midline expansion and cleft lip/palate, which occur due to abnormalities in the fusion of the single frontonasal prominence and maxillary prominences. In the subsequent development of the oral and maxillofacial region, we discussed the regulatory role of primary cilia in the development of the maxilla, mandible, Meckel cartilage, condylar cartilage, lip, tongue, and tooth, among others. Moreover, primary cilia are promising regulators in some oral and maxillofacial diseases, such as tumors and malocclusion. We also summarize the regulatory mechanisms of primary cilia in oral and maxillofacial development and related diseases, including their role in various signaling transduction pathways. For example, aplasia of submandibular glands in the Kif3a mutant mice is associated with a decrease in SHH signaling within the glands. This review summarizes the similarities and specificities of the role of primary cilia in tissue and organ development and disease progression in the oral and maxillofacial region, which is expected to contribute several ideas for the treatment of primary cilia-related diseases.

Mechanosensing by Piezo1 and its implications in the kidney

Piezo1 is an essential mechanosensitive transduction ion channel in mammals. Its unique structure makes it capable of converting mechanical cues into electrical and biological signals, modulating biological and (patho)physiological processes in a wide variety of cells. There is increasing evidence demonstrating that the piezo1 channel plays a vital role in renal physiology and disease conditions. This review summarizes the current evidence on the structure and properties of Piezo1, gating modulation, and pharmacological characteristics, with special focus on the distribution and (patho)physiological significance of Piezo1 in the kidney, which may provide insights into potential treatment targets for renal diseases involving this ion channel.

Vascular Access Outcomes in Patients with Autosomal Dominant Polycystic Kidney Disease

Key Points

More patients with autosomal dominant polycystic kidney disease received their first intervention to re-establish vascular access patency. Patients with autosomal dominant polycystic kidney disease do not require differential monitoring and treatment of hemodialysis vascular access.

Background

Autosomal dominant polycystic kidney disease (ADPKD) is a leading hereditary cause of ESKD, often using hemodialysis as a form of RRT. Patients with ADPKD may also present with extrarenal manifestations, including arterial aneurysms. The gold standard for hemodialysis access is an arteriovenous vascular access (VA), such as arteriovenous fistulas (AVFs) or arteriovenous grafts (AVGs). However, limitations, such as low VA flow and inadequate AVF outward remodeling, affect VA utilization. This study aimed to explore whether ADPKD affects patency rates of AVFs/AVGs in comparison with other underlying ESKD causes.

Methods

We conducted a retrospective cohort study using data from the Swedish Renal Registry from 2011 to 2020, with follow-up until 2022. We included 496 patients with ADPKD and 4321 propensity score–matched controls. VA patency rates of patients with ADPKD were compared with those of non-ADPKD patients using Kaplan–Meier survival curves and Mantel–Cox log-rank test. Interventions to maintain or restore patency were also analyzed.

Results

Patients with ADPKD constituted 8.0% of all patients, with a higher proportion in the pre-ESKD phase during VA creation (51.6% versus 40.6%). No significant differences were observed in primary, postcannulation primary, secondary, or functional patency between patients with ADPKD and non-ADPKD patients. However, more VAs were ligated in patients with ADPKD (10.5% versus 7.7%, P = 0.03), and they underwent more first interventions to re-establish flow (49.4% versus 41.9%, P = 0.02).

Conclusions

These findings suggest that AVF/AVG patency remains comparable in patients with ESKD with or without ADPKD, and VA monitoring and treatment strategies for patients with ADPKD should align with those for individuals with other ESKD causes.

Mechanistic complement of autosomal dominant polycystic kidney disease: the role of aquaporins

Autosomal dominant polycystic kidney disease is a genetic kidney disease caused by mutations in the genes PKD1 or PKD2. Its course is characterized by the formation of progressively enlarged cysts in the renal tubules bilaterally. The basic genetic explanation for autosomal dominant polycystic kidney disease is the double-hit theory, and many of its mechanistic issues can be explained by the cilia doctrine. However, the precise molecular mechanisms underpinning this condition's occurrence are still not completely understood. Experimental evidence suggests that aquaporins, a class of transmembrane channel proteins, including aquaporin-1, aquaporin-2, aquaporin-3, and aquaporin-11, are involved in the mechanism of autosomal dominant polycystic kidney disease. Aquaporins are either a potential new target for the treatment of autosomal dominant polycystic kidney disease, and further study into the physiopathological role of aquaporins in autosomal dominant polycystic kidney disease will assist to clarify the disease's pathophysiology and increase the pool of potential treatment options. We primarily cover pertinent findings on aquaporins in autosomal dominant polycystic kidney disease in this review.

Deciphering vesicle-assisted transport mechanisms in cytoplasm to cilium trafficking

The cilium, a pivotal organelle crucial for cell signaling and proper cell function, relies on meticulous macromolecular transport from the cytoplasm for its formation and maintenance. While the intraflagellar transport (IFT) pathway has traditionally been the focus of extensive study concerning ciliogenesis and ciliary maintenance, recent research highlights a complementary and alternative mechanism-vesicle-assisted transport (VAT) in cytoplasm to cilium trafficking. Despite its potential significance, the VAT pathway remains largely uncharacterized. This review explores recent studies providing evidence for the dynamics of vesicle-related diffusion and transport within the live primary cilium, employing high-speed super-resolution light microscopy. Additionally, we analyze the spatial distribution of vesicles in the cilium, mainly relying on electron microscopy data. By scrutinizing the VAT pathways that facilitate cargo transport into the cilium, with a specific emphasis on recent advancements and imaging data, our objective is to synthesize a comprehensive model of ciliary transport through the integration of IFT-VAT mechanisms.

Activation of Piezo1 Inhibits Kidney Cystogenesis

The disruption of calcium signaling associated with polycystin deficiency has been proposed as the primary event underlying the increased abnormally patterned epithelial cell growth characteristic of Polycystic Kidney Disease. Calcium can be regulated through mechanotransduction, and the mechanosensitive cation channel Piezo1 has been implicated in sensing of intrarenal pressure and in urinary osmoregulation. However, a possible role for PIEZO1 in kidney cystogenesis remains undefined. We hypothesized that cystogenesis in ADPKD reflects altered mechanotransduction, suggesting activation of mechanosensitive cation channels as a therapeutic strategy for ADPKD. Here, we show that Yoda-1 activation of PIEZO1 increases intracellular Ca 2+ and reduces forskolin-induced cAMP levels in mIMCD3 cells. Yoda-1 reduced forskolin-induced IMCD cyst surface area in vitro and in mouse metanephros ex vivo in a dose-dependent manner. Knockout of polycystin-2 dampened the efficacy of PIEZO1 activation in reducing both cAMP levels and cyst surface area in IMCD3 cells. However, collecting duct-specific Piezo1 knockout neither induced cystogenesis in wild-type mice nor affected cystogenesis in the Pkd1 RC/RC model of ADPKD. Our study suggests that polycystin-2 and PIEZO1 play a role in mechanotransduction during cystogenesis in vitro , and ex vivo , but that in vivo cyst expansion may require inactivation or repression of additional suppressors of cystogenesis and/or growth. Our study provides a preliminary proof of concept for PIEZO1 activation as a possible component of combination chemotherapy to retard or halt cystogenesis and/or cyst growth.

Vascular polycystin proteins in health and disease

PKD1 (polycystin 1) and PKD2 (polycystin 2) are expressed in a variety of different cell types, including arterial smooth muscle and endothelial cells. PKD1 is a transmembrane domain protein with a large extracellular N-terminus that is proposed to act as a mechanosensor and receptor. PKD2 is a member of the transient receptor potential (TRP) channel superfamily which is also termed TRPP1. Mutations in the genes which encode PKD1 and PKD2 lead to autosomal dominant polycystic kidney disease (ADPKD). ADPKD is one of the most prevalent monogenic disorders in humans and is associated with extrarenal and vascular complications, including hypertension. Recent studies have uncovered mechanisms of activation and physiological functions of PKD1 and PKD2 in arterial smooth muscle and endothelial cells. It has also been found that PKD function is altered in the vasculature during ADPKD and hypertension. We will summarize this work and discuss future possibilities for this area of research.

Cardiovascular perspectives of the TRP channel polycystin 2

Calcium release from the endoplasmic reticulum (ER) is predominantly driven by two key ion channel receptors, inositol 1, 4, 5-triphosphate receptor (InsP3R) in non-excitable cells and ryanodine receptor (RyR) in excitable and muscle-based cells. These calcium transients can be modified by other less-studied ion channels, including polycystin 2 (PC2), a member of the transient receptor potential (TRP) family. PC2 is found in various cell types and is evolutionarily conserved with paralogues ranging from single-cell organisms to yeasts and mammals. Interest in the mammalian form of PC2 stems from its disease relevance, as mutations in the PKD2 gene, which encodes PC2, result in autosomal dominant polycystic kidney disease (ADPKD). This disease is characterized by renal and liver cysts, and cardiovascular extrarenal manifestations. However, in contrast to the well-defined roles of many TRP channels, the role of PC2 remains unknown, as it has different subcellular locations, and the functional understanding of the channel in each location is still unclear. Recent structural and functional studies have shed light on this channel. Moreover, studies on cardiovascular tissues have demonstrated a diverse role of PC2 in these tissues compared to that in the kidney. We highlight recent advances in understanding the role of this channel in the cardiovascular system and discuss the functional relevance of PC2 in non-renal cells.

Pkd2 Deficiency in Embryonic Aqp2 + Progenitor Cells Is Sufficient to Cause Severe Polycystic Kidney Disease

SIGNIFICANCE STATEMENT

Autosomal dominant polycystic kidney disease (ADPKD) is a devastating disorder caused by mutations in polycystin 1 ( PKD1 ) and polycystin 2 ( PKD2 ). Currently, the mechanism for renal cyst formation remains unclear. Here, we provide convincing and conclusive data in mice demonstrating that Pkd2 deletion in embryonic Aqp2 + progenitor cells (AP), but not in neonate or adult Aqp2 + cells, is sufficient to cause severe polycystic kidney disease (PKD) with progressive loss of intercalated cells and complete elimination of α -intercalated cells, accurately recapitulating a newly identified cellular phenotype of patients with ADPKD. Hence, Pkd2 is a new potential regulator critical for balanced AP differentiation into, proliferation, and/or maintenance of various cell types, particularly α -intercalated cells. The Pkd2 conditional knockout mice developed in this study are valuable tools for further studies on collecting duct development and early steps in cyst formation. The finding that Pkd2 loss triggers the loss of intercalated cells is a suitable topic for further mechanistic studies.

BACKGROUND

Most cases of autosomal dominant polycystic kidney disease (ADPKD) are caused by mutations in PKD1 or PKD2. Currently, the mechanism for renal cyst formation remains unclear. Aqp2 + progenitor cells (AP) (re)generate ≥5 cell types, including principal cells and intercalated cells in the late distal convoluted tubules (DCT2), connecting tubules, and collecting ducts.

METHODS

Here, we tested whether Pkd2 deletion in AP and their derivatives at different developmental stages is sufficient to induce PKD. Aqp2Cre Pkd2f/f ( Pkd2AC ) mice were generated to disrupt Pkd2 in embryonic AP. Aqp2ECE/+Pkd2f/f ( Pkd2ECE ) mice were tamoxifen-inducted at P1 or P60 to inactivate Pkd2 in neonate or adult AP and their derivatives, respectively. All induced mice were sacrificed at P300. Immunofluorescence staining was performed to categorize and quantify cyst-lining cell types. Four other PKD mouse models and patients with ADPKD were similarly analyzed.

RESULTS

Pkd2 was highly expressed in all connecting tubules/collecting duct cell types and weakly in all other tubular segments. Pkd2AC mice had obvious cysts by P6 and developed severe PKD and died by P17. The kidneys had reduced intercalated cells and increased transitional cells. Transitional cells were negative for principal cell and intercalated cell markers examined. A complete loss of α -intercalated cells occurred by P12. Cysts extended from the distal renal segments to DCT1 and possibly to the loop of Henle, but not to the proximal tubules. The induced Pkd2ECE mice developed mild PKD. Cystic α -intercalated cells were found in the other PKD models. AQP2 + cells were found in cysts of only 13/27 ADPKD samples, which had the same cellular phenotype as Pkd2AC mice.

CONCLUSIONS

Hence, Pkd2 deletion in embryonic AP, but unlikely in neonate or adult Aqp2 + cells (principal cells and AP), was sufficient to cause severe PKD with progressive elimination of α -intercalated cells, recapitulating a newly identified cellular phenotype of patients with ADPKD. We proposed that Pkd2 is critical for balanced AP differentiation into, proliferation, and/or maintenance of cystic intercalated cells, particularly α -intercalated cells.

Polycystin-2 Mediated Calcium Signalling in the Dictyostelium Model for Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) occurs when the proteins Polycystin-1 (PC1, PKD1) and Polycystin-2 (PC2, PKD2) contain mutations. PC1 is a large membrane receptor that can interact and form a complex with the calcium-permeable cation channel PC2. This complex localizes to the plasma membrane, primary cilia and ER. Dysregulated calcium signalling and consequential alterations in downstream signalling pathways in ADPKD are linked to cyst formation and expansion; however, it is not completely understood how PC1 and PC2 regulate calcium signalling. We have studied Polycystin-2 mediated calcium signalling in the model organism Dictyostelium discoideum by overexpressing and knocking down the expression of the endogenous Polycystin-2 homologue, Polycystin-2. Chemoattractant-stimulated cytosolic calcium response magnitudes increased and decreased in overexpression and knockdown strains, respectively, and analysis of the response kinetics indicates that Polycystin-2 is a significant contributor to the control of Ca2+ responses. Furthermore, basal cytosolic calcium levels were reduced in Polycystin-2 knockdown transformants. These alterations in Ca2+ signalling also impacted other downstream Ca2+-sensitive processes including growth rates, endocytosis, stalk cell differentiation and spore viability, indicating that Dictyostelium is a useful model to study Polycystin-2 mediated calcium signalling.

Store-operated calcium entry inhibits primary ciliogenesis via the activation of Aurora A

The primary cilium is an antenna-like organelle protruding from the cell surface that can detect physical and chemical stimuli in the extracellular space to activate specific signaling pathways and downstream gene expressions. Calcium ion (Ca2+ ) signaling regulates a wide spectrum of cellular processes, including fertilization, proliferation, differentiation, muscle contraction, migration, and death. This study investigated the effects of the regulation of cytosolic Ca2+ levels on ciliogenesis using chemical, genetic, and optogenetic approaches. We found that ionomycin-induced Ca2+ influx inhibited ciliogenesis and Ca2+ chelator BATPA-AM-induced Ca2+ depletion promoted ciliogenesis. In addition, store-operated Ca2+ entry and the endoplasmic reticulum Ca2+ sensor stromal interaction molecule 1 (STIM1) negatively regulated ciliogenesis. Moreover, an optogenetic platform was used to create different Ca2+ oscillation patterns by manipulating lighting parameters, including density, frequency, exposure time, and duration. Light-activated Ca2+ -translocating channelrhodopsin (CatCh) is activated by 470-nm blue light to induce Ca2+ influx. Our results show that high-frequency Ca2+ oscillations decrease ciliogenesis. Furthermore, the inhibition of cilia formation induced by Ca2+ may occur via the activation of Aurora kinase A. Cilia not only induce Ca2+ signaling but also regulate cilia formation by Ca2+ signaling.

PIEZO1 is a distal nephron mechanosensor and is required for flow-induced K+ secretion

Ca2+-activated BK channels in renal intercalated cells (ICs) mediate luminal flow-induced K+ secretion (FIKS), but how ICs sense increased flow remains uncertain. We examined whether PIEZO1, a mechanosensitive Ca2+-permeable channel expressed in the basolateral membranes of ICs, is required for FIKS. In isolated cortical collecting ducts (CCDs), the mechanosensitive cation-selective channel inhibitor GsMTx4 dampened flow-induced increases in intracellular Ca2+ concentration ([Ca2+]i), whereas the PIEZO1 activator Yoda1 increased [Ca2+]i and BK channel activity. CCDs from mice fed a high-K+ (HK) diet exhibited a greater Yoda1-dependent increase in [Ca2+]i than CCDs from mice fed a control K+ diet. ICs in CCDs isolated from mice with a targeted gene deletion of Piezo1 in ICs (IC-Piezo1-KO) exhibited a blunted [Ca2+]i response to Yoda1 or increased flow, with an associated loss of FIKS in CCDs. Male IC-Piezo1-KO mice selectively exhibited an increased blood [K+] in response to an oral K+ bolus and blunted urinary K+ excretion following a volume challenge. Whole-cell expression of BKα subunit was reduced in ICs of IC-Piezo1-KO mice fed an HK diet. We conclude that PIEZO1 mediates flow-induced basolateral Ca2+ entry into ICs, is upregulated in the CCD in response to an HK diet, and is necessary for FIKS.

Autosomal Dominant Polycystic Kidney Disease: Extrarenal Involvement

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disorder, but kidneys are not the only organs involved in this systemic disorder. Individuals with the condition may display additional manifestations beyond the renal system, involving the liver, pancreas, and brain in the context of cystic manifestations, while involving the vascular system, gastrointestinal tract, bones, and cardiac valves in the context of non-cystic manifestations. Despite kidney involvement remaining the main feature of the disease, thanks to longer survival, early diagnosis, and better management of kidney-related problems, a new wave of complications must be faced by clinicians who treated patients with ADPKD. Involvement of the liver represents the most prevalent extrarenal manifestation and has growing importance in the symptom burden and quality of life. Vascular abnormalities are a key factor for patients' life expectancy and there is still debate whether to screen or not to screen all patients. Arterial hypertension is often the earliest onset symptom among ADPKD patients, leading to frequent cardiovascular complications. Although cardiac valvular abnormalities are a frequent complication, they rarely lead to relevant problems in the clinical history of polycystic patients. One of the newest relevant aspects concerns bone disorders that can exert a considerable influence on the clinical course of these patients. This review aims to provide the "state of the art" among the extrarenal manifestation of ADPKD.

CRP inhibits the osteoblastic differentiation of OPCs via the up-regulation of primary cilia and repression of the Hedgehog signaling pathway

Inflammation disrupts bone metabolism and leads to bone damage. C-reactive protein (CRP) is a typical inflammation marker. Although CRP measurement has been conducted for many decades, how osteoblastic differentiation influences molecular mechanisms remains largely unknown. The present study attempted to investigate the effects of CRP on primary cultured osteoblast precursor cells (OPCs) while elucidating the underlying molecular mechanisms. OPCs were isolated from suckling Sprague-Dawleyrats. Fewer OPCs were observed after recombinant C-reactive protein treatment. In a series of experiments, CRP inhibited OPC proliferation, osteoblastic differentiation, and the OPC gene expression of the hedgehog (Hh) signaling pathway. The inhibitory effect of CRP on OPC proliferation occurred via blockade of the G1-S transition of the cell cycle. In addition, the regulation effect of proto cilium on osteoblastic differentiation was analyzed using the bioinformatics p. This revealed the primary cilia activation of recombinant CRP effect on OPCs through in vitro experiments. A specific Sonic Hedgehog signaling agonist (SAG) rescued osteoblastic differentiation inhibited by recombinant CRP. Moreover, chloral hydrate, which removes primary cilia, inhibited the Suppressor of Fused (SUFU) formation and blocked Gli2 degradation. This counteracted osteogenesis inhibition caused by CRP. Therefore, these data depict that CRP can inhibit the proliferation and osteoblastic differentiation of OPCs. The underlying mechanism could be associated with primary cilia activation and Hh pathway repression.

Stretch response of the mechano-gated channel TMEM63A in membrane patches and single cells

The recently discovered ion channel TMEM63A has biophysical features distinctive for mechano-gated cation channels, activating at high pressures with slow kinetics while not inactivating. However, some biophysical properties are less clear, including no information on its function in whole cells. The aim of this study is to expand the TMEM63A biophysical characterization and examine the function in whole cells. Piezo1-knockout HEK293T cells were cotransfected with human TMEM63A and green fluorescent protein (GFP), and macroscopic currents in cell-attached patches were recorded by high-speed pressure clamp at holding voltages from -120 to -20 mV with 0-100 mmHg patch suction for 1 s. HEK293 cells cotransfected with TMEM63A and GCaMP5 were seeded onto polydimethylsiloxane (PDMS) membrane, and the response to 3-12 s of 1%-15% whole cell isotropic (equi-biaxial) stretch induced by an IsoStretcher was measured by the change in intracellular calcium ([Ca2+]i) and presented as (ΔF/F0 > 1). Increasing patch pressures activated TMEM63A currents with accelerating activation kinetics and current amplitudes that were pressure dependent but voltage independent. TMEM63A currents were plateaued within 2 s, recovered quickly, and were sensitive to Gd3+. In whole cells stretched on flexible membranes, radial stretch increased the [Ca2+]i responses in a larger proportion of cells cotransfected with TMEM63A and GCaMP5 than GCaMP5-only controls. TMEM63A currents are force activated and voltage insensitive, have a high threshold for pressure activation with slow activation and deactivation, and lack inactivation over 5 s. TMEM63A has the net polarity and kinetics that would depolarize plasma membranes and increase inward currents, contributing to a sustained [Ca2+]i increase in response to high stretch.NEW & NOTEWORTHY TMEM63A has biophysical features distinctive for mechano-gated cation channels, but some properties are less clear, including no functional information in whole cells. We report that pressure-dependent yet voltage-independent TMEM63A currents in cell membrane patches correlated with cell size. In addition, radial stretch of whole cells on flexible membranes increased the [Ca2+]i responses more in TMEM63A-transfected cells. Inward TMEM63A currents in response to high stretch can depolarize plasma membranes and contribute to a sustained [Ca2+]i increase.

Rediscovering the Rete Ovarii: a secreting auxiliary structure to the ovary

The rete ovarii (RO) is an appendage of the ovary that has been given little attention. Although the RO appears in drawings of the ovary in early versions of Gray's Anatomy, it disappeared from recent textbooks, and is often dismissed as a functionless vestige in the adult ovary. Using PAX8 immunostaining and confocal microscopy, we characterized the fetal development of the RO in the context of the ovary. The RO consists of three distinct regions that persist in adult life, the intraovarian rete (IOR), the extraovarian rete (EOR), and the connecting rete (CR). While the cells of the IOR appear to form solid cords within the ovary, the EOR rapidly develops into a convoluted tubular epithelium ending in a distal dilated tip. Cells of the EOR are ciliated and exhibit cellular trafficking capabilities. The CR, connecting the EOR to the IOR, gradually acquires tubular epithelial characteristics by birth. Using microinjections into the distal dilated tip of the EOR, we found that luminal contents flow towards the ovary. Mass spectrometry revealed that the EOR lumen contains secreted proteins potentially important for ovarian function. We show that the cells of the EOR are closely associated with vasculature and macrophages, and are contacted by neuronal projections, consistent with a role as a sensory appendage of the ovary. The direct proximity of the RO to the ovary and its integration with the extraovarian landscape suggest that it plays an important role in ovary development and homeostasis.

Primary cilia and actin regulatory pathways in renal ciliopathies

Ciliopathies are a group of rare genetic disorders caused by defects to the structure or function of the primary cilium. They often affect multiple organs, leading to brain malformations, congenital heart defects, and anomalies of the retina or skeletal system. Kidney abnormalities are among the most frequent ciliopathic phenotypes manifesting as smaller, dysplastic, and cystic kidneys that are often accompanied by renal fibrosis. Many renal ciliopathies cause chronic kidney disease and often progress to end-stage renal disease, necessitating replacing therapies. There are more than 35 known ciliopathies; each is a rare hereditary condition, yet collectively they account for a significant proportion of chronic kidney disease worldwide. The primary cilium is a tiny microtubule-based organelle at the apex of almost all vertebrate cells. It serves as a "cellular antenna" surveying environment outside the cell and transducing this information inside the cell to trigger multiple signaling responses crucial for tissue morphogenesis and homeostasis. Hundreds of proteins and unique cellular mechanisms are involved in cilia formation. Recent evidence suggests that actin remodeling and regulation at the base of the primary cilium strongly impacts ciliogenesis. In this review, we provide an overview of the structure and function of the primary cilium, focusing on the role of actin cytoskeleton and its regulators in ciliogenesis. We then describe the key clinical, genetic, and molecular aspects of renal ciliopathies. We highlight what is known about actin regulation in the pathogenesis of these diseases with the aim to consider these recent molecular findings as potential therapeutic targets for renal ciliopathies.

PKD1L1 Is Involved in Congenital Chylothorax

Besides visceral heterotaxia, Pkd1l1 null mouse embryos exhibit general edema and perinatal lethality. In humans, congenital chylothorax (CCT) is a frequent cause of fetal hydrops. In 2021, Correa and colleagues reported ultrarare compound heterozygous variants in PKD1L1 exhibiting in two consecutive fetuses with severe hydrops, implicating a direct role of PKD1L1 in fetal hydrops formation. Here, we performed an exome survey and identified ultrarare compound heterozygous variants in PKD1L1 in two of the five case-parent trios with CCT. In one family, the affected carried the ultrarare missense variants c.1543G>A(p.Gly515Arg) and c.3845T>A(p.Val1282Glu). In the other family, the affected carried the ultrarare loss-of-function variant (LoF) c.863delA(p.Asn288Thrfs*3) and the ultrarare missense variant c.6549G>T(p.Gln2183His). Investigation of the variants' impact on PKD1L1 protein localization suggests the missense variants cause protein dysfunction and the LoF variant causes protein mislocalization. Further analysis of Pkd1l1 mutant mouse embryos revealed about 20% of Pkd1l1-/- embryos display general edema and pleural effusion at 14.5 dpc. Immunofluorescence staining at 14.5 dpc in Pkd1l1-/- embryos displayed both normal and massively altered lymphatic vessel morphologies. Together, our studies suggest the implication of PKD1L1 in congenital lymphatic anomalies, including CCTs.

Deletion of Aurora kinase A prevents the development of polycystic kidney disease in mice

Aurora Kinase A (AURKA) promotes cell proliferation and is overexpressed in different types of polycystic kidney disease (PKD). To understand AURKA's role in regulating renal cyst development we conditionally deleted the gene in mouse models of Autosomal Dominant PKD (ADPKD) and Joubert Syndrome, caused by Polycystin 1 (Pkd1) and Inositol polyphosphate-5-phosphatase E (Inpp5e) mutations respectively. We show that while Aurka is dispensable for collecting duct development and homeostasis, its deletion prevents cyst formation in both disease models. Cross-comparison of transcriptional changes implicated AKT signaling in cyst prevention and we show that (i) AURKA and AKT physically interact, (ii) AURKA regulates AKT activity in a kinase-independent manner and (iii) inhibition of AKT can reduce disease severity. AKT activation also regulates Aurka expression, creating a feed-forward loop driving renal cystogenesis. We find that the AURKA kinase inhibitor Alisertib stabilises the AURKA protein, agonizing its cystogenic functions. These studies identify AURKA as a master regulator of renal cyst development in different types of PKD, functioning in-part via AKT.

TRPP2 is located in the primary cilia of human non-pigmented ciliary epithelial cells

PURPOSE

Mechanosensitive channels (MSCs) and primary cilium possess a possible relevance for the sensation of intraocular pressure (IOP). However, there is only limited data on their expression and localization in the ciliary body epithelium (CBE). The purpose of this study was to characterize the expression and localization of TRPP2 in a human non-pigmented ciliary epithelial cell (HNPCE) line.

METHODS

The expression of the TRPP2 was studied by quantitative (q)RT-PCR and in situ hybridization in rat and human tissue. Protein expression and distribution were studied by western blot analysis, immunohistochemistry, and immunoelectron microscopy. Cellular location of TRPP2 was determined in rat and human CBE by immunofluorescence and immunoblot analysis. Electron microscopy studies were conducted to evaluate where and with substructure TRPP2 is localized in the HNPCE cell line.

RESULTS

The expression of TRPP2 in rat and human non-pigmented ciliary epithelium was detected. TRPP2 was mainly located in nuclei, but also showed a punctate distribution pattern in the cytoplasm of HNPCE of the tissue and the cell line. In HNPCE cell culture, primary cilia did exhibit different length following serum starvation and hydrostatic pressure. TRPP2 was found to be colocalized with these cilia in HNPCE cells.

CONCLUSION

The expression of TRPP2 and the primary cilium in the CB may indicate a possible role, such as the sensing of hydrostatic pressure, for the regulation of IOP. Functional studies via patch clamp or pharmacological intervention have yet to clarify the relevance for the physiological situation or aqueous humor regulation.

Identification and Analysis of Primary Cilia in the Corneal Endothelial Cells of Patients with Bullous Keratopathy

PURPOSE

To identify primary cilia in human corneal endothelial cells (CECs) obtained from patients with bullous keratopathy (BK).

METHODS

This study involved CEC specimens obtained from 10 eyes of 10 consecutive patients (three males and seven females; mean age: 74.5 years, range: 68-90 years) with BK who underwent Descemet's stripping automated endothelial keratoplasty at Baptist Eye Institute, Kyoto, Japan between August 2019 and September 2020. Three corneal buttons obtained from 3 patients who underwent penetrating keratoplasty for keratoconus were used as 'non-BK' controls. All specimens were evaluated with immunofluorescence staining using an antibody against acetylated α-tubulin.

RESULTS

Ciliary expression was observed in six of the 10 CEC specimens; i.e. in two specimens obtained from BK patients after glaucoma surgery (trabeculectomy), in two specimens obtained from patients with Fuchs endothelial corneal dystrophy, and in two specimens obtained from a patient with BK after laser iridotomy for primary angle closure. There was acetylated α-tubulin staining but no hair-like structures in two specimens, and ciliary expression was unknown in two specimens due to the absence of cells. The length of the primary cilia varied between all specimens. In contrast, no primary cilia were observed in the corneal buttons obtained from the three keratoconus patients.

CONCLUSION

The findings in this study clearly demonstrate the expression of primary cilia in the CECs of patients afflicted with BK.

TMEM120A/TACAN: A putative regulator of ion channels, mechanosensation, and lipid metabolism

TMEM120A (TACAN) is an enigmatic protein with several seemingly unconnected functions. It was proposed to be an ion channel involved in sensing mechanical stimuli, and knockdown/knockout experiments have implicated that TMEM120A may be necessary for sensing mechanical pain. TMEM120A's ion channel function has subsequently been challenged, as attempts to replicate electrophysiological experiments have largely been unsuccessful. Several cryo-EM structures revealed TMEM120A is structurally homologous to a lipid modifying enzyme called Elongation of Very Long Chain Fatty Acids 7 (ELOVL7). Although TMEM120A's channel function is debated, it still seems to affect mechanosensation by inhibiting PIEZO2 channels and by modifying tactile pain responses in animal models. TMEM120A was also shown to inhibit polycystin-2 (PKD2) channels through direct physical interaction. Additionally, TMEM120A has been implicated in adipocyte regulation and in innate immune response against Zika virus. The way TMEM120A is proposed to alter each of these processes ranges from regulating gene expression, acting as a lipid modifying enzyme, and controlling subcellular localization of other proteins through direct binding. Here, we examine TMEM120A's structure and proposed functions in diverse physiological contexts.

Osteocytes and Primary Cilia

PURPOSE OF REVIEW

The purpose of this review is to provide a background on osteocytes and the primary cilium, discussing the role it plays in osteocyte mechanosensing.

RECENT FINDINGS

Osteocytes are thought to be the primary mechanosensing cells in bone tissue, regulating bone adaptation in response to exercise, with the primary cilium suggested to be a key mechanosensing mechanism in bone. More recent work has suggested that, rather than being direct mechanosensors themselves, primary cilia in bone may instead form a key chemo-signalling nexus for processing mechanoregulated signalling pathways. Recent evidence suggests that pharmacologically induced lengthening of the primary cilium in osteocytes may potentiate greater mechanotransduction, rather than greater mechanosensing. While more research is required to delineate the specific osteocyte mechanobiological molecular mechanisms governed by the primary cilium, it is clear from the literature that the primary cilium has significant potential as a therapeutic target to treat mechanoregulated bone diseases, such as osteoporosis.

"Soap bubble" sign as an imaging marker for posterior fossa ependymoma Group B

PURPOSE

To investigate the predictive value of the "soap bubble" sign on molecular subtypes (Group A [PFA] and Group B [PFB]) of posterior fossa ependymomas (PF-EPNs).

METHODS

MRI scans of 227 PF-EPNs (internal retrospective discovery set) were evaluated by two independent neuroradiologists to assess the "soap bubble" sign, which was defined as clusters of cysts of various sizes that look like "soap bubbles" on T2-weighted images. Two independent cohorts (external validation set [n = 31] and prospective validation set [n = 27]) were collected to validate the "soap bubble" sign.

RESULTS

Across three datasets, the "soap bubble" sign was observed in 21 PFB cases (7.4% [21/285] of PF-EPNs and 12.9% [21/163] of PFB); none in PFA. Analysis of the internal retrospective discovery set demonstrated substantial interrater agreement (1st Rating: κ = 0.71 [0.53-0.90], 2nd Rating: κ = 0.83 [0.68-0.98]) and intrarater agreement (Rater 1: κ = 0.73 [0.55-0.91], Rater 2: κ = 0.74 [0.55-0.92]) for the "soap bubble" sign; all 13 cases positive for the "soap bubble" sign were PFB (p = 0.002; positive predictive value [PPV] = 100%, negative predictive value [NPV] = 44%, sensitivity = 10%, specificity = 100%). The findings from the external validation set and the prospective validation set were similar, all cases positive for the "soap bubble" sign were PFB (p < 0.001; PPV = 100%).

CONCLUSION

The "soap bubble" sign represents a highly specific imaging marker for the PFB molecular subtype of PF-EPNs.

Paralog-specific TTC30 regulation of Sonic hedgehog signaling

The intraflagellar transport (IFT) machinery is essential for cilia assembly, maintenance, and trans-localization of signaling proteins. The IFT machinery consists of two large multiprotein complexes, one of which is the IFT-B. TTC30A and TTC30B are integral components of this complex and were previously shown to have redundant functions in the context of IFT, preventing the disruption of IFT-B and, thus, having a severe ciliogenesis defect upon loss of one paralog. In this study, we re-analyzed the paralog-specific protein complexes and discovered a potential involvement of TTC30A or TTC30B in ciliary signaling. Specifically, we investigated a TTC30A-specific interaction with protein kinase A catalytic subunit α, a negative regulator of Sonic hedgehog (Shh) signaling. Defects in this ciliary signaling pathway are often correlated to synpolydactyly, which, intriguingly, is also linked to a rare TTC30 variant. For an in-depth analysis of this unique interaction and the influence on Shh, TTC30A or B single- and double-knockout hTERT-RPE1 were employed, as well as rescue cells harboring wildtype TTC30 or the corresponding mutation. We could show that mutant TTC30A inhibits the ciliary localization of Smoothened. This observed effect is independent of Patched1 but associated with a distinct phosphorylated PKA substrate accumulation upon treatment with forskolin. This rather prominent phenotype was attenuated in mutant TTC30B. Mass spectrometry analysis of wildtype versus mutated TTC30A or TTC30B uncovered differences in protein complex patterns and identified an impaired TTC30A-IFT57 interaction as the possible link leading to synpolydactyly. We could observe no impact on cilia assembly, leading to the hypothesis that a slight decrease in IFT-B binding can be compensated, but mild phenotypes, like synpolydactyly, can be induced by subtle signaling changes. Our systematic approach revealed the paralog-specific influence of TTC30A KO and mutated TTC30A on the activity of PRKACA and the uptake of Smoothened into the cilium, resulting in a downregulation of Shh. This downregulation, combined with interactome alterations, suggests a potential mechanism of how mutant TTC30A is linked to synpolydactyly.

Emerging principles of primary cilia dynamics in controlling tissue organization and function

Primary cilia project from the surface of most vertebrate cells and are key in sensing extracellular signals and locally transducing this information into a cellular response. Recent findings show that primary cilia are not merely static organelles with a distinct lipid and protein composition. Instead, the function of primary cilia relies on the dynamic composition of molecules within the cilium, the context-dependent sensing and processing of extracellular stimuli, and cycles of assembly and disassembly in a cell- and tissue-specific manner. Thereby, primary cilia dynamically integrate different cellular inputs and control cell fate and function during tissue development. Here, we review the recently emerging concept of primary cilia dynamics in tissue development, organization, remodeling, and function.

Fluid shear stress stimulates ATP release without regulating purinergic gene expression in the renal inner medullary collecting duct

In the kidney, the flow rate of the pro-urine through the renal tubules is highly variable. The tubular epithelial cells sense these variations in pro-urinary flow rate in order to regulate various physiological processes, including electrolyte reabsorption. One of the mechanosensitive pathways activated by flow is the release of ATP, which can then act as a autocrine or paracrine factor. Increased ATP release is observed in various kidney diseases, among others autosomal dominant polycystic kidney disease (ADPKD). However, the mechanisms underlying flow-induced ATP release in the collecting duct, especially in the inner medullary collecting duct, remain understudied. Using inner medullary collecting duct 3 (IMCD3) cells in a microfluidic setup, we show here that administration of a high flow rate for 1 min results in an increased ATP release compared to a lower flow rate. Although the ATP release channel pannexin-1 contributed to flow-induced ATP release in Pkd1-/- IMCD3 cells, it did not in wildtype IMCD3 cells. In addition, flow application increased the expression of the putative ATP release channel connexin-30.3 (CX30.3) in wildtype and Pkd1-/- IMCD3 cells. However, CX30.3 knockout IMCD3 cells exhibited a similar flow-induced ATP release as wildtype IMCD3 cells, suggesting that CX30.3 does not drive flow-induced ATP release in wildtype IMDC3 cells. Collectively, our results show differential mechanisms underlying flow-induced ATP release in wildtype and Pkd1-/- IMCD3 cells and further strengthen the link between ADPKD and pannexin-1-dependent ATP release.

Folate conjugated nanomedicines for selective inhibition of mTOR signaling in polycystic kidneys at clinically relevant doses

Although rapamycin is a very effective drug for rodents with polycystic kidney disease (PKD), it is not encouraging in the clinical trials due to the suboptimal dosages compelled by the off-target side effects. We here report the generation, characterization, specificity, functionality, pharmacokinetic, pharmacodynamic and toxicology profiles of novel polycystic kidney-specific-targeting nanoparticles (NPs). We formulated folate-conjugated PLGA-PEG NPs, which can be loaded with multiple drugs, including rapamycin (an mTOR inhibitor) and antioxidant 4-hydroxy-TEMPO (a nephroprotective agent). The NPs increased the efficacy, potency and tolerability of rapamycin resulting in an increased survival rate and improved kidney function by decreasing side effects and reducing biodistribution to other organs in PKD mice. The daily administration of rapamycin-alone (1 mg/kg/day) could now be achieved with a weekly injection of NPs containing rapamycin (379 μg/kg/week). This polycystic kidney-targeting nanotechnology, for the first time, integrated advances in the use of 1) nanoparticles as a delivery cargo, 2) folate for targeting, 3) near-infrared Cy5-fluorophore for in vitro and in vivo live imaging, 4) rapamycin as a pharmacological therapy, and 5) TEMPO as a combinational therapy. The slow sustained-release of rapamycin by polycystic kidney-targeting NPs demonstrates a new era of nanomedicine in treatment for chronic kidney diseases at clinically relevant doses.

Mechanoresponses mediated by the TRP11 channel in cilia of Chlamydomonas reinhardtii

Cilia are organelles involved in motility and sensory transduction, but how these two functions coexist has not been elucidated in depth. Here, the involvement of the ciliary transient receptor potential (TRP) channel TRP11 in mechanoresponses is studied in Chlamydomonas reinhardtii using a TRP11-knockout mutant. The mutant has defects in the conversion of the bending mode of the cilium from forward to reverse when tapped with a glass rod, the detachment of cilia when shear is applied, the increase in ciliary beat frequency upon application of mechanical agitation by vortex mixing, and the initiation of gliding while both cilia are attached in opposite directions to a glass surface. These observations indicate that TRP11 can perceive mechanical stimuli with distinct intensities and durations and induce various types of ciliary responses.

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

Mechanosensitive ion channel gene survey suggests potential roles in primary open angle glaucoma

Although glaucoma is a disease modulated by eye pressure, the mechanisms of pressure sensing in the eye are not well understood. Here, we investigated associations between mechanosensitive ion channel gene variants and primary open-angle glaucoma (POAG). Common (minor allele frequency > 5%) single nucleotide polymorphisms located within the genomic regions of 20 mechanosensitive ion channel genes in the K2P, TMEM63, PIEZO and TRP channel families were assessed using genotype data from the NEIGHBORHOOD consortium of 3853 cases and 33,480 controls. Rare (minor allele frequency < 1%) coding variants were assessed using exome array genotyping data for 2606 cases and 2606 controls. Association with POAG was analyzed using logistic regression adjusting for age and sex. Two rare PIEZO1 coding variants with protective effects were identified in the NEIGHBOR dataset: R1527H, (OR 0.17, P = 0.0018) and a variant that alters a canonical splice donor site, g.16-88737727-C-G Hg38 (OR 0.38, P = 0.02). Both variants showed similar effects in the UK Biobank and the R1527H also in the FinnGen database. Several common variants also reached study-specific thresholds for association in the NEIGHBORHOOD dataset. These results identify novel variants in several mechanosensitive channel genes that show associations with POAG, suggesting that these channels may be potential therapeutic targets.

Recent advances in the understanding of cilia mechanisms and their applications as therapeutic targets

The primary cilium is a single immotile microtubule-based organelle that protrudes into the extracellular space. Malformations and dysfunctions of the cilia have been associated with various forms of syndromic and non-syndromic diseases, termed ciliopathies. The primary cilium is therefore gaining attention due to its potential as a therapeutic target. In this review, we examine ciliary receptors, ciliogenesis, and ciliary trafficking as possible therapeutic targets. We first discuss the mechanisms of selective distribution, signal transduction, and physiological roles of ciliary receptors. Next, pathways that regulate ciliogenesis, specifically the Aurora A kinase, mammalian target of rapamycin, and ubiquitin-proteasome pathways are examined as therapeutic targets to regulate ciliogenesis. Then, in the photoreceptors, the mechanism of ciliary trafficking which takes place at the transition zone involving the ciliary membrane proteins is reviewed. Finally, some of the current therapeutic advancements highlighting the role of large animal models of photoreceptor ciliopathy are discussed.

CFTR and PC2, partners in the primary cilia in autosomal dominant polycystic kidney disease

Defects in the primary cilium are associated with autosomal dominant polycystic kidney disease (ADPKD). We used a combination of animal models, Western blotting, and confocal microscopy and discovered that CFTR and polycystin 2 (PC2) are both colocalized to the cilium in normal kidneys, with the levels of both being decreased in cystic epithelia. Cilia were longer in CFTR-null mice and in cystic cells in our ADPKD animal models. We examined septin 2, known to play a role in cilia length, to act as a diffusion barrier and to serve as an enhancer of proliferation. We found that septin 2 protein levels were upregulated and colocalized strongly with CFTR in cystic cells. Application of VX-809, the CFTR corrector, restored CFTR and PC2 toward normal in the cilia, decreased the protein levels of septin 2, and drastically reduced septin 2 colocalization with CFTR. Our data suggest that CFTR is present in the cilia and plays a role there, perhaps through its conductance of Cl-. We also postulate that septin 2 is important for localizing CFTR to the apical membrane in cystic epithelia.NEW & NOTEWORTHY CFTR is present in the primary cilia together with polycystin 2 (PC2). Ablation of CFTR makes cilia longer suggesting that CFTR plays a role there, perhaps through its conductance of Cl.

Polycystin-1 Interacting Protein-1 (CU062) Interacts with the Ectodomain of Polycystin-1 (PC1)

The PKD1 gene, encoding protein polycystin-1 (PC1), is responsible for 85% of cases of autosomal dominant polycystic kidney disease (ADPKD). PC1 has been shown to be present in urinary exosome-like vesicles (PKD-ELVs) and lowered in individuals with germline PKD1 mutations. A label-free mass spectrometry comparison of urinary PKD-ELVs from normal individuals and those with PKD1 mutations showed that several proteins were reduced to a degree that matched the decrease observed in PC1 levels. Some of these proteins, such as polycystin-2 (PC2), may be present in a higher-order multi-protein assembly with PC1-the polycystin complex (PCC). CU062 (Q9NYP8) is decreased in ADPKD PKD-ELVs and, thus, is a candidate PCC component. CU062 is a small glycoprotein with a signal peptide but no transmembrane domain and can oligomerize with itself and interact with PC1. We investigated the localization of CU062 together with PC1 and PC2 using immunofluorescence (IF). In nonconfluent cells, all three proteins were localized in close proximity to focal adhesions (FAs), retraction fibers (RFs), and RF-associated extracellular vesicles (migrasomes). In confluent cells, primary cilia had PC1/PC2/CU062 + extracellular vesicles adherent to their plasma membrane. In cells exposed to mitochondrion-decoupling agents, we detected the development of novel PC1/CU062 + ring-like structures that entrained swollen mitochondria. In contact-inhibited cells under mitochondrial stress, PC1, PC2, and CU062 were observed on large, apically budding extracellular vesicles, where the proteins formed a reticular network on the membrane. CU062 interacts with PC1 and may have a role in the identification of senescent mitochondria and their extrusion in extracellular vesicles.

Functions of the primary cilium in the kidney and its connection with renal diseases

The nonmotile primary cilium is a sensory structure found on most mammalian cell types that integrates multiple signaling pathways involved in tissue development and postnatal function. As such, mutations disrupting cilia activities cause a group of disorders referred to as ciliopathies. These disorders exhibit a wide spectrum of phenotypes impacting nearly every tissue. In the kidney, primary cilia dysfunction caused by mutations in polycystin 1 (Pkd1), polycystin 2 (Pkd2), or polycystic kidney and hepatic disease 1 (Pkhd1), result in polycystic kidney disease (PKD), a progressive disorder causing renal functional decline and end-stage renal disease. PKD affects nearly 1 in 1000 individuals and as there is no cure for PKD, patients frequently require dialysis or renal transplantation. Pkd1, Pkd2, and Pkhd1 encode membrane proteins that all localize in the cilium. Pkd1 and Pkd2 function as a nonselective cation channel complex while Pkhd1 protein function remains uncertain. Data indicate that the cilium may act as a mechanosensor to detect fluid movement through renal tubules. Other functions proposed for the cilium and PKD proteins in cyst development involve regulation of cell cycle and oriented division, regulation of renal inflammation and repair processes, maintenance of epithelial cell differentiation, and regulation of mitochondrial structure and metabolism. However, how loss of cilia or cilia function leads to cyst development remains elusive. Studies directed at understanding the roles of Pkd1, Pkd2, and Pkhd1 in the cilium and other locations within the cell will be important for developing therapeutic strategies to slow cyst progression.