Denny L. Cottle

Primary cilia are cell signalling and environment sensing organelles and have important roles during embryogenesis and homeostasis. We demonstrate glucocorticoid signalling is essential for normal cilia formation in mouse and human renal tubules. RNA sequencing of E18.5 kidneys from glucocorticoid receptor (GR) null mice identified significant reductions in key ciliogenesis-related genes including Ccp110, Cep97, Cep290 and Kif3a. Confocal microscopy reveals abnormal, stunted cilia on proximal tubules, podocytes, and collecting duct cells in mice with global or conditional deletion of GR. In contrast, activation of GR signalling with dexamethasone in human kidney organoids or mouse IMCD3 cells increases cilia length, an effect blocked by the GR antagonist RU486. Analysis of GR-null kidney extracts demonstrates reduced levels of pERK and SUFU identifying potential cell pathway crosstalk with GR signalling that coordinately regulate ciliogenesis in the renal tubule. Finally, dexamethasone reduces Aurora kinase A levels, a factor driving cilia disassembly and implicated in the pathogenesis of polycystic kidney disease.

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.

Ciliated bronchial epithelium 1 (CBE1) is a microtubule-associated protein localized to the manchette and developing flagellum during spermiogenesis, and associated with sperm maturation arrest in humans. It was hypothesized that CBE1 functions in microtubule-mediated transport mechanisms and sperm tail formation. To test this hypothesis, we analysed Cbe1 expression and localization during spermiogenesis, and in mouse IMCD3 cells as a model of ciliogenesis. Further, we generated and analysed the fertility of a Cbe1 mutant mouse line. Mice containing a homozygous deletion in the long forms of Cbe1 were born at a lower frequency than predicted by Mendelian inheritance, however, adult male mice were fertile. An in-depth analysis of the Cbe1 gene revealed alternative transcript variants, which were not affected by the exon 2 mutation. To assess whether short variants compensate for the loss of long variants, exons 2 and 4 (which affect all variants) were individually mutated in IMCD3 cells and the effect on cell proliferation and ciliogenesis analysed. In wild type IMCD3 cells, both variants were upregulated during cilia assembly. CBE1 protein was not a structural component of cilia, rather, CBE1 localised to the mitochondria and the contractile ring of dividing IMCD3 cells. While IMCD3 cells carrying the mutation in long variants showed no phenotypic alterations, the mutation in exon 4 resulted in a significantly decreased proliferation rate. This study reveals that long isoforms of CBE1 are not essential for male fertility. Data, however, suggests that CBE1 is associated to intra-manchette transport and mid-piece formation of the sperm tail.

Identifying soluble factors that influence epidermal integrity is critical for the development of preventative and therapeutic strategies for disorders such as ichthyosis, psoriasis, dermatitis and epidermal cancers. The transcription factor Grainyhead-like 3 (GRHL3) is essential for maintaining barrier integrity and preventing development of cutaneous squamous cell carcinoma (SCC); however, how loss of this factor, which in the skin is expressed exclusively within suprabasal epidermal layers triggers proliferation of basal keratinocytes, had thus far remained elusive. Our present study identifies thymus and activation-regulated chemokine (TARC) as a novel soluble chemokine mediator of keratinocyte proliferation following loss of GRHL3. Knockdown of GRHL3 in human keratinocytes showed that of 42 cytokines examined, TARC was the only significantly upregulated chemokine. Mouse skin lacking Grhl3 presented an inflammatory response with hallmarks of TARC activation, including heightened induction of blood clotting, increased infiltration of mast cells and pro-inflammatory T cells, increased expression of the pro-proliferative/pro-inflammatory markers CD3 and pSTAT3, and significantly elevated basal keratinocyte proliferation. Treatment of skin cultures lacking Grhl3 with the broad spectrum anti-inflammatory 5-aminosalicylic acid (5ASA) partially restored epidermal differentiation, indicating that abnormal keratinocyte proliferation/differentiation balance is a key driver of barrier dysfunction following loss of Grhl3, and providing a promising therapeutic avenue in the treatment of GRHL3-mediated epidermal disorders.

Keratins are cytoskeletal intermediate filament proteins that are increasingly being recognised for their diverse cellular functions. Here we report the consequences of germ line inactivation of Keratin 76 (Krt76) in mice. Homozygous disruption of this epidermally expressed gene causes neonatal skin flaking, hyperpigmentation, inflammation, impaired wound healing, and death prior to 12 weeks of age. We show that this phenotype is associated with functionally defective tight junctions that are characterised by mislocalization of the integral protein CLDN1. We further demonstrate that KRT76 interacts with CLDN1 and propose that this interaction is necessary to correctly position CLDN1 in tight junctions. The mislocalization of CLDN1 has been associated in various dermopathies, including the inflammatory disease, psoriasis. These observations establish a previously unknown connection between the intermediate filament cytoskeleton network and tight junctions and showcase Krt76 null mice as a possible model to study aberrant tight junction driven skin diseases. The generation of knockout mice is a central approach to studying gene function. We have examined the consequences of the germ line inactivation of Keratin 76 in mice and in doing so we reveal a previously undescribed mechanism by which keratin intermediate filaments regulate cellular interactions and tissue homeostasis. Our study supports an emerging body of evidence which challenges the classical view of the keratin intermediate filaments as simple structural proteins, highlighting Krt76 as a gene whose function is indispensable for barrier function and skin wound repair as a result of its novel interaction with tight junction complexes. This study identifies a previously unknown and critical link between intermediate filaments and tight junctions where intermediate filament dysfunction influences skin disease.