Analysis of soluble protein entry into primary cilia using semipermeabilized cells

Rab35 Is Required for Embryonic Development and Kidney and Ureter Homeostasis through Regulation of Epithelial Cell Junctions

Key Points

Loss of Rab35 leads to nonobstructive hydronephrosis because of loss of ureter epithelium. Rab35 regulates kidney and ureter epithelial cell adhesion and polarity. Rab35 is required for embryonic development.

Background

Rab35 is a member of a GTPase family of endocytic trafficking proteins. Studies in cell lines have indicated that Rab35 participates in cell adhesion, polarity, cytokinesis, and primary cilia length and composition. In addition, sea urchin Rab35 regulates actin organization and is required for gastrulation. In mice, loss of Rab35 in the central nervous system disrupts hippocampal development and neuronal organization. Outside of the central nervous system, the functions of mammalian Rab35 in vivo are unknown.

Methods

We generated and analyzed the consequences of both congenital and conditional null Rab35 mutations in mice. Using a LacZ reporter allele, we assessed Rab35 expression during development and postnatally. We assessed Rab35 loss in the kidney and ureter using histology, immunofluorescence microscopy, and western blotting.

Results

Congenital Rab35 loss of function caused embryonic lethality: homozygous mutants arrested at E7.5 with cardiac edema. Conditional loss of Rab35, either during gestation or postnatally, caused hydronephrosis. The kidney and ureter phenotype were associated with disrupted actin cytoskeletal architecture, altered Arf6 epithelial polarity, reduced adherens junctions, loss of tight junction formation, defects in epithelial growth factor receptor expression and localization, disrupted cell differentiation, and shortened primary cilia.

Conclusions

Rab35 may be essential for mammalian development and the maintenance of kidney and ureter architecture. Loss of Rab35 leads to nonobstructive hydronephrosis, making the Rab35 mutant mouse a novel mammalian model to study mechanisms underlying this disease.

Rab35 is required for embryonic development and kidney and ureter homeostasis through regulation of epithelial cell junctions

Background

Rab35 is a member of a GTPase family of endocytic trafficking proteins. Studies in cell lines have indicated that Rab35 participates in cell adhesion, polarity, cytokinesis, and primary cilia length and composition. Additionally, sea urchin Rab35 regulates actin organization and is required for gastrulation. In mice, loss of Rab35 in the CNS disrupts hippocampal development and neuronal organization. Outside of the CNS, the functions of mammalian Rab35 in vivo are unknown.

Methods

We generated and analyzed the consequences of both congenital and conditional null Rab35 mutations in mice. Using a LacZ reporter allele, we assessed Rab35 expression during development and postnatally. We assessed Rab35 loss in the kidney and ureter using histology, immunofluorescence microscopy, and western blotting.

Results

Congenital Rab35 loss of function caused embryonic lethality: homozygous mutants arrested at E7.5 with cardiac edema. Conditional loss of Rab35, either during gestation or postnatally, caused hydronephrosis. The kidney and ureter phenotype were associated with disrupted actin cytoskeletal architecture, altered Arf6 epithelial polarity, reduced adherens junctions, loss of tight junction formation, defects in EGFR expression and localization, disrupted cell differentiation, and shortened primary cilia.

Conclusion

Rab35 is essential for mammalian development and the maintenance of kidney and ureter architecture. Loss of Rab35 leads to non-obstructive hydronephrosis, making the Rab35 mutant mouse a novel mammalian model to study mechanisms underlying this disease.

Imaging the cAMP Signaling Microdomain of the Primary Cilium Using Targeted FRET-Based Biosensors

Optical approaches have revolutionized our view of second messenger signaling in organelles, allowing precise time-resolved assessment of soluble signaling molecules in situ. Among the most challenging of subcellular signaling microdomains to assay is the primary cilium. A petite but visually arresting organelle, the primary cilium extends from the cell surface of most non-dividing cells. Recently, the concept of the primary cilium as an independent cAMP signaling organelle has attracted substantial interest. The cilium sequesters a very specific subset of ciliary cAMP-linked GPCRs in its membrane (e.g., 5-HT6, D1R, MCR4, FFAR4, TGR5), as well as other key components of the cAMP signaling machinery that include adenylyl cyclases, GNAS, phosphodiesterases, PKA holoenzyme, and biologically important PKA targets. Here we provide a practical guide to assessing ciliary cAMP signals in live cells using targeted genetically encoded FRET biosensors. Key experimental difficulties include gathering sufficient signal from such a small, photon-limited volume, and the susceptibility of cilia to movement artifacts. Other challenges are associated with the fidelity of sensor targeting and the difficulties in distinguishing between cAMP signals produced exclusively within the cilium vs. those that emanate from the cell body. Here we describe ratio imaging approaches used in our lab for time-resolved visualization of ciliary cAMP in cultured renal cells. These methods can be readily adapted to other cell types and microscopy platforms according to the needs of the user.

ATXN10 Is Required for Embryonic Heart Development and Maintenance of Epithelial Cell Phenotypes in the Adult Kidney and Pancreas

Atxn10 is a gene known for its role in cytokinesis and is associated with spinocerebellar ataxia (SCA10), a slowly progressing cerebellar syndrome caused by an intragenic pentanucleotide repeat expansion. Atxn10 is also implicated in the ciliopathy syndromes nephronophthisis (NPHP) and Joubert syndrome (JBTS), which are caused by the disruption of cilia function leading to nephron loss, impaired renal function, and cerebellar hypoplasia. How Atxn10 disruption contributes to these disorders remains unknown. Here, we generated Atxn10 congenital and conditional mutant mouse models. Our data indicate that while ATXN10 protein can be detected around the base of the cilium as well as in the cytosol, its loss does not cause overt changes in cilia formation or morphology. Congenital loss of Atxn10 results in embryonic lethality around E10.5 associated with pericardial effusion and loss of trabeculation. Similarly, tissue-specific loss of ATXN10 in the developing endothelium (Tie2-Cre) and myocardium (cTnT-Cre) also results in embryonic lethality with severe cardiac malformations occurring in the latter. Using an inducible Cagg-CreER to disrupt ATXN10 systemically at postnatal stages, we show that ATXN10 is also required for survival in adult mice. Loss of ATXN10 results in severe pancreatic and renal abnormalities leading to lethality within a few weeks post ATXN10 deletion in adult mice. Evaluation of these phenotypes further identified rapid epithelial-to-mesenchymal transition (EMT) in these tissues. In the pancreas, the phenotype includes signs of both acinar to ductal metaplasia and EMT with aberrant cilia formation and severe defects in glucose homeostasis related to pancreatic insufficiency or defects in feeding or nutrient intake. Collectively, this study identifies ATXN10 as an essential protein for survival.

Translational readthrough of ciliopathy genes BBS2 and ALMS1 restores protein, ciliogenesis and function in patient fibroblasts

BACKGROUND

Ciliary dysfunction underlies a range of genetic disorders collectively termed ciliopathies, for which there are no treatments available. Bardet-Biedl syndrome (BBS) is characterised by multisystemic involvement, including rod-cone dystrophy and renal abnormalities. Together with Alström syndrome (AS), they are known as the 'obesity ciliopathies' due to their common phenotype. Nonsense mutations are responsible for approximately 11% and 40% of BBS and AS cases, respectively. Translational readthrough inducing drugs (TRIDs) can restore full-length protein bypassing in-frame premature termination codons, and are a potential therapeutic approach for nonsense-mediated ciliopathies.

METHODS

Patient fibroblasts harbouring nonsense mutations from two different ciliopathies (Bardet-Biedl Syndrome and Alström Syndrome) were treated with PTC124 (ataluren) or amlexanox. Following treatment, gene expression, protein levels and ciliogenesis were evaluated. The expression of intraflagellar transport protein IFT88 and G-protein coupled receptor SSTR3 was investigated as a readout of ciliary function.

FINDINGS

mRNA expression was significantly increased in amlexanox-treated patient fibroblasts, and full-length BBS2 or ALMS1 protein expression was restored in PTC124- and amlexanox-treated fibroblasts. Treatment with TRIDs significantly improved ciliogenesis defects in BBS2^Y24*/R275* fibroblasts. Treatment recovered IFT88 expression and corrected SSTR3 mislocalisation in BBS2^Y24*/R275* and ALMS1^{S1645*/S1645*} fibroblasts, suggesting rescue of ciliary function.

INTERPRETATION

The recovery of full-length BBS2 and ALMS1 expression and correction of anatomical and functional ciliary defects in BBS2^Y24*/R275* and ALMS1^{S1645*/S1645*} fibroblasts suggest TRIDs are a potential therapeutic option for the treatment of nonsense-mediated ciliopathies.

FUNDING

Wellcome Trust 205174/Z/16/Z, National Centre for the Replacement, Refinement & Reduction of Animals in Research. Deutsche Forschungsgemeinschaft SPP2127 (DFG Grant MA 6139/3-1).

Time-resolved proteomics profiling of the ciliary Hedgehog response

The primary cilium is a signaling compartment that interprets Hedgehog signals through changes of its protein, lipid, and second messenger compositions. Here, we combine proximity labeling of cilia with quantitative mass spectrometry to unbiasedly profile the time-dependent alterations of the ciliary proteome in response to Hedgehog. This approach correctly identifies the three factors known to undergo Hedgehog-regulated ciliary redistribution and reveals two such additional proteins. First, we find that a regulatory subunit of the cAMP-dependent protein kinase (PKA) rapidly exits cilia together with the G protein-coupled receptor GPR161 in response to Hedgehog, and we propose that the GPR161/PKA module senses and amplifies cAMP signals to modulate ciliary PKA activity. Second, we identify the phosphatase Paladin as a cell type-specific regulator of Hedgehog signaling that enters primary cilia upon pathway activation. The broad applicability of quantitative ciliary proteome profiling promises a rapid characterization of ciliopathies and their underlying signaling malfunctions.

A mouse model of BBS identifies developmental and homeostatic effects of BBS5 mutation and identifies novel pituitary abnormalities

Primary cilia are critical sensory and signaling compartments present on most mammalian cell types. These specialized structures require a unique signaling protein composition relative to the rest of the cell to carry out their functions. Defects in ciliary structure and signaling result in a broad group of disorders collectively known as ciliopathies. One ciliopathy, Bardet-Biedl syndrome (BBS; OMIM 209900), presents with diverse clinical features, many of which are attributed to defects in ciliary signaling during both embryonic development and postnatal life. For example, patients exhibit obesity, polydactyly, hypogonadism, developmental delay and skeletal abnormalities along with sensory and cognitive deficits, but for many of these phenotypes it is uncertain, which are developmental in origin. A subset of BBS proteins assembles into the core BBSome complex, which is responsible for mediating transport of membrane proteins into and out of the cilium, establishing it as a sensory and signaling hub. Here, we describe two new mouse models for BBS resulting from a targeted LacZ gene trap allele (Bbs5-/-) that is a predicted congenital null mutation and conditional (Bbs5flox/flox) allele of Bbs5. Bbs5-/- mice develop a complex phenotype consisting of increased pre-weaning lethality craniofacial and skeletal defects, ventriculomegaly, infertility and pituitary anomalies. Utilizing the conditional allele, we show that the male fertility defects, ventriculomegaly and pituitary abnormalities are only present when Bbs5 is disrupted prior to postnatal day 7, indicating a developmental origin. In contrast, mutation of Bbs5 results in obesity, independent of the age of Bbs5 loss.

Direct visualization of cAMP signaling in primary cilia reveals up-regulation of ciliary GPCR activity following Hedgehog activation

The primary cilium permits compartmentalization of specific signaling pathways, including elements of the Hedgehog (Hh) pathway. Hh transcriptional activity is thought to be negatively regulated by constitutively high ciliary cAMP maintained by the Gα(s)-coupled GPCR, GPR161. However, cilia also sequester many other Gα(s)-coupled GPCRs with unknown potential to regulate Hh. Here we used biosensors optimized for ciliary cAMP and strategies to isolate signals in the cilium from the cell body and neighboring cells. We found that ciliary cAMP was not elevated relative to cellular cAMP, inconsistent with constitutive cAMP production. Gα(s)-coupled GPCRs (e.g., the 5-HT₆ serotonin and D1R dopamine receptor) had reduced ability to generate cAMP upon trafficking to the ciliary membrane. However, activation of the Hh pathway restored or amplified GPCR function to permit cAMP elevation selectively in the cilium. Hh therefore enables its own local GPCR-dependent cAMP regulatory circuit. Considering that GPCRs comprise much of the druggable genome, these data suggest alternative strategies to modify Hh signaling.

Cilia in autophagy and cancer

Cancer cells are distinguished from normal cells by increased proliferation and metabolism, loss of polarity control, and the potential to invade other tissues of the body. As hubs of signaling transduction, primary cilia have been linked to diverse developmental and degenerative disorders. Interestingly, loss of cilia has been observed in multiple malignant tumors, suggesting a potential suppressive role of cilia in cancer development. More recently, emerging studies began to unveil the bidirectional interaction of cilia and autophagy, a basic cellular clearance and recycling mechanism to regulate cell homeostasis. Here, we summarize the interplay between cilia and autophagy and discuss the roles of cilia in both autophagy and cancer.