The length of secondary chromosomal constrictions in normal individuals and in a nucleolar mutant of Xenopus laevis

Renal ciliopathies: promising drug targets and prospects for clinical trials

INTRODUCTION

Renal ciliopathies represent a collection of genetic disorders characterized by deficiencies in the biogenesis, maintenance, or functioning of the ciliary complex. These disorders, which encompass autosomal dominant polycystic kidney disease (ADPKD), autosomal recessive polycystic kidney disease (ARPKD), and nephronophthisis (NPHP), typically result in cystic kidney disease, renal fibrosis, and a gradual deterioration of kidney function, culminating in kidney failure.

AREAS COVERED

Here we review the advances in basic science and clinical research into renal ciliopathies which have yielded promising small compounds and drug targets, within both preclinical studies and clinical trials.

EXPERT OPINION

Tolvaptan is currently the sole approved treatment option available for ADPKD patients, while no approved treatment alternatives exist for ARPKD or NPHP patients. Clinical trials are presently underway to evaluate additional medications in ADPKD and ARPKD patients. Based on preclinical models, other potential therapeutic targets for ADPKD, ARPKD, and NPHP look promising. These include molecules targeting fluid transport, cellular metabolism, ciliary signaling and cell-cycle regulation. There is a real and urgent clinical need for translational research to bring novel treatments to clinical use for all forms of renal ciliopathies to reduce kidney disease progression and prevent kidney failure.

Opportunities and Challenges for Molecular Understanding of Ciliopathies-The 100,000 Genomes Project

Cilia are highly specialized cellular organelles that serve multiple functions in human development and health. Their central importance in the body is demonstrated by the occurrence of a diverse range of developmental disorders that arise from defects of cilia structure and function, caused by a range of different inherited mutations found in more than 150 different genes. Genetic analysis has rapidly advanced our understanding of the cell biological basis of ciliopathies over the past two decades, with more recent technological advances in genomics rapidly accelerating this progress. The 100,000 Genomes Project was launched in 2012 in the UK to improve diagnosis and future care for individuals affected by rare diseases like ciliopathies, through whole genome sequencing (WGS). In this review we discuss the potential promise and medical impact of WGS for ciliopathies and report on current progress of the 100,000 Genomes Project, reviewing the medical, technical and ethical challenges and opportunities that new, large scale initiatives such as this can offer.

Fibroblast growth factor receptor 1 signaling transcriptionally regulates the axon guidance cue slit1

Axons sense molecular cues in their environment to arrive at their post-synaptic targets. While many of the molecular cues have been identified, the mechanisms that regulate their spatiotemporal expression remain elusive. We examined here the transcriptional regulation of the guidance gene slit1 both in vitro and in vivo by specific fibroblast growth factor receptors (Fgfrs). We identified an Fgf-responsive 2.3 kb slit1 promoter sequence that recapitulates spatiotemporal endogenous expression in the neural tube and eye of Xenopus embryos. We found that signaling through Fgfr1 is the main regulator of slit1 expression both in vitro in A6 kidney epithelial cells, and in the Xenopus forebrain, even when other Fgfr subtypes are present in cells. These data argue that a specific signaling pathway downstream of Fgfr1 controls in a cell-autonomous manner slit1 forebrain expression and are novel in identifying a specific growth factor receptor for in vivo control of the expression of a key embryonic axon guidance cue.

Perfluoroalkylated Substance Effects in Xenopus laevis A6 Kidney Epithelial Cells Determined by ATR-FTIR Spectroscopy and Chemometric Analysis

The effects of four perfluoroalkylated substances (PFASs), namely, perfluorobutanesulfonate (PFBS), perfluorooctanoic acid (PFOA), perfluorooctanesulfonate (PFOS), and perfluorononanoic acid (PFNA) were assessed in Xenopus laevis A6 kidney epithelial cells by attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy and chemometric analysis. Principal component analysis–linear discriminant analysis (PCA-LDA) was used to visualize wavenumber-related alterations and ANOVA-simultaneous component analysis (ASCA) allowed data processing considering the underlying experimental design. Both analyses evidenced a higher impact of low-dose PFAS-treatments (10^–9 M) on A6 cells forming monolayers, while there was a larger influence of high-dose PFAS-treatments (10^–5 M) on A6 cells differentiated into dome structures. The observed dose–response PFAS-induced effects were to some extent related to their cytotoxicity: the EC₅₀-values of most influential PFAS-treatments increased (PFOS < PFNA < PFOA ≪ PFBS), and higher-doses of these chemicals induced a larger impact. Major spectral alterations were mainly attributed to DNA/RNA, secondary protein structure, lipids, and fatty acids. Finally, PFOS and PFOA caused a decrease in A6 cell numbers compared to controls, whereas PFBS and PFNA did not significantly change cell population levels. Overall, this work highlights the ability of PFASs to alter A6 cells, whether forming monolayers or differentiated into dome structures, and the potential of PFOS and PFOA to induce cell death.

Activation of endogenous FAK via expression of its amino terminal domain in Xenopus embryos

Background

The Focal Adhesion Kinase is a well studied tyrosine kinase involved in a wide number of cellular processes including cell adhesion and migration. It has also been shown to play important roles during embryonic development and targeted disruption of the FAK gene in mice results in embryonic lethality by day 8.5.

Principal Findings

Here we examined the pattern of phosphorylation of FAK during Xenopus development and found that FAK is phosphorylated on all major tyrosine residues examined from early blastula stages well before any morphogenetic movements take place. We go on to show that FRNK fails to act as a dominant negative in the context of the early embryo and that the FERM domain has a major role in determining FAK’s localization at the plasma membrane. Finally, we show that autonomous expression of the FERM domain leads to the activation of endogenous FAK in a tyrosine 397 dependent fashion.

Conclusions

Overall, our data suggest an important role for the FERM domain in the activation of FAK and indicate that integrin signalling plays a limited role in the in vivo activation of FAK at least during the early stages of development.

Identification and expression analysis of GPAT family genes during early development of Xenopus laevis

Production of lysophosphatidic acid (LPA) is the first step in the de novo pathway for glycerolipid biosynthesis, which is mainly catalyzed by the glycerol-3-phosphate acyltransferases (GPATs; EC2.3.1.15). DHAPAT (EC2.3.1.42) also contributes in a minor way, using dihydroxyacetone phosphate as substrate. Final products and intermediates of the glycerolipid synthesis pathway are the main structural components of cellular membranes, and provide signalling molecules that regulate diverse biological processes, including cell proliferation, differentiation and growth. Here we identified the four orthologs of the mammalian GPATs (1-4) and DHAPAT in Xenopus, including a novel, short variant of GPAT2, and analyzed their expression pattern during embryonic development. Xenopus GPAT1/2 localized to mitochondria, while GPAT3/4 associated with the endoplasmic reticulum. All are similarly expressed in the early embryonic nervous system. A more tissue specific pattern emerges during organogenesis, including liver expression for GPAT1/4, and testis expression for GPAT2. All acyltransferases were expressed in kidney, though GPAT3 was excluded from the pronephric ducts. Our results suggest important roles of GPATs and DHAPAT during early organogenesis.

Two promoters with distinct activities in different tissues drive the expression of heparanase in Xenopus

In Xenopus laevis embryos, heparanase, the enzyme that degrades heparan sulfate, is synthesized as a preproheparanase (XHpaL) and processed to become enzymatically active (XHpa active). A short nonenzymatic heparanase splice variant (XHpaS) is also expressed. Using immunohistochemistry, Western blot, and heparanase promoter analysis, we studied the dynamic developmental expression of the three heparanases. Our results indicate that (1) all three isoforms are maternally expressed; (2) XHpaS is a developmental variant; (3) in the early embryo, heparanase is localized to both the plasma membrane and the nucleus; (4) several tissues express heparanase, but expression in the developing nervous system is most evident; (5) two promoters with distinct activities in different tissues drive heparanase expression; (6) Oct binding transcription factors may modulate heparanase promoter activity in the early embryo. These data argue that heparanase is expressed widely during development, but localization and levels are finely regulated.

Scanning ion conductance microscopy reveals how a functional renal epithelial monolayer maintains its integrity

BACKGROUND

To function as a transport barrier a renal tubule epithelial monolayer needs to maintain its integrity when, sudden hypertonic stress causes cell shrinkage, new cells are added, or cells in the monolayer die. However, the mechanism used to achieve this is largely unknown. Scanning ion conductance microscopy (SICM) has been shown to be suitable for imaging the surface of live renal cells with high topographic resolution, and can be used to elucidate how a functional renal epithelial monolayer maintains its integrity.

METHODS

SICM was used for high spatial resolution topographic imaging of Xenopuslaevis renal epithelial A6 cells cultured on membrane filter inserts.

RESULTS

The SICM images of A6 cells showed that the epithelial monolayer maintains its integrity under hypertonic stress, and during cell division and death. Sequential SICM topographic images revealed detailed structural changes and their time course for these protective processes, which involve highly cooperative cell movement. Some "balloon-like" structures were observed at susceptible tight junction regions, which were proposed to help cell maintaining the monolayer permeability integrity.

CONCLUSION

SICM is a powerful tool for research on living renal epithelial cells, and has been used to elucidate how a functional epithelial monolayer maintains its integrity. Using this technique we have observed that during hypertonic stress and regeneration, an organized sequence of events protect the loss of integrity of monolayer so that tight junctions and cell-cell contact are maintained and disruption to the function of whole monolayer is prevented.

Proliferation and differentiation of Xenopus A6 cells under hypergravity as revealed by time-lapse imaging

Xenopus laevis A6 cells, which are cloned epithelial cells from the Xenopus kidney, differentiate into a dome structure when the cells reach confluence. We investigated the gravitational responses of A6 cellular motility during normal differentiation and differentiation under hypergravity conditions using centrifugation (1-100 x g). Progression to dome formation was analyzed by time-lapse micrography. Dome formation and increased expression of Na(+)/K(+)-adenosine triphosphatase were used as markers of differentiation. Interestingly, a high rate of cellular proliferation was observed at a low level of hypergravity (5 x g). Despite this, there was no difference in the time to dome formation between the control cells at primary cell density and those that differentiated under hyper- or hypogravity conditions. In conclusion, this experiment on amphibian cells revealed that the proliferation of A6 cells was strongly affected by gravity conditions, but the differentiation step appears to be controlled by an intra- or intercellular clock.

Expression and partial characterization of kinesin-related proteins in differentiating and adult skeletal muscle

Using pan-kinesin antibodies to screen a differentiating C2C12 cell library, we identified the kinesin proteins KIF3A, KIF3B, and conventional kinesin heavy chain to be present in differentiating skeletal muscle. We compared the expression and subcellular localization characteristics of these kinesins in myogenic cells to others previously identified in muscle, neuronal, and mitotic systems (KIF1C, KIF3C, and mitotic-centromere-associated kinesin). Because members of the KIF3 subfamily of kinesin-related proteins showed altered subcellular fractionation characteristics in differentiating cells, we focused our study of kinesins in muscle on the function of kinesin-II. Kinesin-II is a motor complex comprised of dimerized KIF3A and KIF3B proteins and a tail-associated protein, KAP. The Xenopus homologue of KIF3B, Xklp3, is predominantly localized to the region of the Golgi apparatus, and overexpression of motorless-Xklp3 in Xenopus A6 cells causes mislocalization of Golgi components (). In C2C12 myoblasts and myotubes, KIF3B is diffuse and punctate, and not primarily associated with the Golgi. Overexpression of motorless-KIF3B does not perturb localization of Golgi components in myogenic cells, and myofibrillogenesis is normal. In adult skeletal muscle, KIF3B colocalizes with the excitation-contraction-coupling membranes. We propose that these membranes, consisting of the transverse-tubules and sarcoplasmic reticulum, are dynamic structures in which kinesin-II may function to actively assemble and maintain in myogenic cells.

Xkid, a chromokinesin required for chromosome alignment on the metaphase plate

Metaphase chromosome alignment is a key step of animal cell mitosis. The molecular mechanism leading to this equatorial positioning is still not fully understood. Forces exerted at kinetochores and on chromosome arms drive chromosome movements that culminate in their alignment on the metaphase plate. In this paper, we show that Xkid, a kinesin-like protein localized on chromosome arms, plays an essential role in metaphase chromosome alignment and in its maintenance. We propose that Xkid is responsible for the polar ejection forces acting on chromosome arms. Our results show that these forces are essential to ensure that kinetochores and chromosome arms align on a narrow equatorial plate during metaphase, a prerequisite for proper chromosome segregation.

Role of xklp3, a subunit of the Xenopus kinesin II heterotrimeric complex, in membrane transport between the endoplasmic reticulum and the Golgi apparatus

The function of the Golgi apparatus is to modify proteins and lipids synthesized in the ER and sort them to their final destination. The steady-state size and function of the Golgi apparatus is maintained through the recycling of some components back to the ER. Several lines of evidence indicate that the spatial segregation between the ER and the Golgi apparatus as well as trafficking between these two compartments require both microtubules and motors. We have cloned and characterized a new Xenopus kinesin like protein, Xklp3, a subunit of the heterotrimeric Kinesin II. By immunofluorescence it is found in the Golgi region. A more detailed analysis by EM shows that it is associated with a subset of membranes that contain the KDEL receptor and are localized between the ER and Golgi apparatus. An association of Xklp3 with the recycling compartment is further supported by a biochemical analysis and the behavior of Xklp3 in BFA-treated cells. The function of Xklp3 was analyzed by transfecting cells with a dominant-negative form lacking the motor domain. In these cells, the normal delivery of newly synthesized proteins to the Golgi apparatus is blocked. Taken together, these results indicate that Xklp3 is involved in the transport of tubular-vesicular elements between the ER and the Golgi apparatus.

Isolation of cDNAs from maternal mRNAs specifically present during early development

In many animals the regulation of early embryonic development is under the control of mRNAs of maternal origin. We describe here the construction and characterization of a subtractive cDNA library to isolate genes whose expression is restricted to early development. Such genes might be potentially involved in developmental diseases, if accidentally expressed during adult life. Seven yet unknown genes preferentially expressed in the oocyte and present during early development were isolated from this library by differential screening. Their transcription patterns provide evidence for a selective expression during oogenesis and the existence of a gene family specifically implicated in functions linked to early embryogenesis.

Chromosomal localization of 18S + 28S and 5S Ribosomal RNA genes in evolutionarily diverse anuran amphibians

The chromosomal locations of the 18S + 28S and 5S ribosomal RNA genes have been analyzed by in situ hybridization in ten anuran species of different taxonomic positions. The chosen species belong to both primitive and evolved families of the present day Anura. Each examined species has 18s + 28S rRNA genes clustered in one locus per haploid chromosome set: this locus is placed either in an intercalary position or proximal to the centromere, or close to the telomere. The 5S rRNA genes are arranged in clusters which vary in number from one to six per haploid set. The 5S rDNA sites are found in intercalary positions, at the telomeres, and at, or close to, the centromeres. Microchromosomes and small chromosomes in primitive karyotypes have been found to carry 5S rDNA sequences. The results are discussed in relation to ideas on the karyological evolution of Amphibia.

Transplantation of nuclei from lymphocytes of adult frogs into enucleated eggs: special focus on technical parameters

The technique of transplantation of nuclei from adult lymphocytes into enucleated eggs from Xenopus laevis (South African clawed toad) is described. The splenic lymphocytes from the one-nucleolus mutant were bound via their immunoglobulin receptors to nylon fibers, derivatized with the antigen used for immunization. A technique for coupling other cell types with Woodward reagent is also described. The cells were broken by aspiration into a micropipette and injected into enucleated eggs. The egg pronucleus was eliminated by UV treatment followed by surgical removal. The origin of the genome of developing embryos was determined on karyotype preparations by looking for the nucleolar organizer on the chromosome pair No. 12. Participation of egg pronucleus in development was frequent as judged by the incidence of gynogenetic diploid individuals and of tetraploid animals exhibiting characteristics of both recipient egg and somatic cell donor karyotypes. Apparently normal, lymphocyte-dervived tadpoles were obtained, but they died at 12 days. This shows that differentiated cells from adults, such as lymphocytes, can re-express the genes necessary for ontogeny.

Dimorphic nucleolar organizer regions in the frog Rana blairi

The nucleolar organizer-specific staining procedure, ammoniacal silver (Ag-AS), has been used to study the distribution and size of the nucleolar organizer regions (NORs) in chromosomes of the frog Rana blairi (Mecham, Littlejohn, Oldham, Brown and Brown). The somatic metaphase karyotype of this frog is similar to that of other frogs of the Rana pipiens species complex, numerically (2n=26) and morphologically. Secondary constrictions are detectable in untreated Giemsa-stained metaphase preparations as achromatic gaps in the long arms of a pair of submetacentric chromosomes (no. 10). These constrictions are the only regions which are deeply stained with the Ag-AS method and are thus identified as the nucleolar organizer regions (Ag-NORs). In each of the three individuals, the Ag-NORs as visualized on the homologues are of unequal length.

Biochemical and cytogenetic studies on the nucleolus organizing regions (NOR) of man. II. A family with the 15/21 translocation

The amount of ribosomal DNA (rDNA) was determined quantitatively by RNA-DNA hybridization in the genomes of a mother and her daughter, both with the karyotype 45,XX,t(15q21q). The saturation values found were 0.030% (mother), 0.023% (daughter), and 0.022% for the husband and father of the daughter. A detailed cytogenetic analysis of the short arms of the acrocentric chromosomes of these probands allowed the biochemical results to be interpreted in terms of the size of the individual set of nucleolus organizing regions (NORs) present in each proband. The correlation existing between the biochemical and the cytogenetic findings shows that the amount of rDNA in the human genome is not primarily a function of the number of acrocentric chromosomes, but depends on the individual combination of variant NORs occurring in the human genome.