An immunofluorescence method to analyze the proliferation status of individual nephron segments in the Xenopus pronephric kidney

CUG-BP, Elav-like family member 1 (CELF1) is required for normal myofibrillogenesis, morphogenesis, and contractile function in the embryonic heart

BACKGROUND

CUG-BP, Elav-like family member 1 (CELF1) is a multifunctional RNA binding protein found in a variety of adult and embryonic tissues. In the heart, CELF1 is found exclusively in the myocardium. However, the roles of CELF1 during cardiac development have not been completely elucidated.

RESULTS

Myofibrillar organization is disrupted and proliferation is reduced following knockdown of CELF1 in cultured chicken primary embryonic cardiomyocytes. In vivo knockdown of Celf1 in developing Xenopus laevis embryos resulted in myofibrillar disorganization and a trend toward reduced proliferation in heart muscle, indicating conserved roles for CELF1 orthologs in embryonic cardiomyocytes. Loss of Celf1 also resulted in morphogenetic abnormalities in the developing heart and gut. Using optical coherence tomography, we showed that cardiac contraction was impaired following depletion of Celf1, while heart rhythm remained unperturbed. In contrast to cardiac muscle, loss of Celf1 did not disrupt myofibril organization in skeletal muscle cells, although it did lead to fragmentation of skeletal muscle bundles.

CONCLUSIONS

CELF1 is required for normal myofibril organization, proliferation, morphogenesis, and contractile performance in the developing myocardium. Developmental Dynamics 245:854-873, 2016. © 2016 Wiley Periodicals, Inc.

Isolating dividing neural and brain tumour cells for gene expression profiling

BACKGROUND

The characterisation of dividing brain cells is fundamental for studies ranging from developmental and stem cell biology, to brain cancers. Whilst there is extensive anatomical data on these dividing cells, limited gene transcription data is available due to technical constraints.

NEW METHOD

We focally isolated dividing cells whilst conserving RNA, from culture, primary neural tissue and xenografted glioma tumours, using a thymidine analogue that enables gene transcription analysis.

RESULTS

5-ethynyl-2-deoxyuridine labels the replicating DNA of dividing cells. Once labelled, cultured cells and tissues were dissociated, fluorescently tagged with a revised click chemistry technique and the dividing cells isolated using fluorescence-assisted cell sorting. RNA was extracted and analysed using real time PCR. Proliferation and maturation related gene expression in neurogenic tissues was demonstrated in acutely and 3 day old labelled cells, respectively. An elevated expression of marker and pathway genes was demonstrated in the dividing cells of xenografted brain tumours, with the non-dividing cells showing relatively low levels of expression.

COMPARISON WITH EXISTING METHOD

BrdU "immune-labelling", the most frequently used protocol for detecting cell proliferation, causes complete denaturation of RNA, precluding gene transcription analysis. This EdU labelling technique, maintained cell integrity during dissociation, minimized copper exposure during labelling and used a cell isolation protocol that avoided cell lysis, thus conserving RNA.

CONCLUSIONS

The technique conserves RNA, enabling the definition of cell proliferation-related changes in gene transcription of neural and pathological brain cells in cells harvested immediately after division, or following a period of maturation.

Sterol carrier protein 2 regulates proximal tubule size in the Xenopus pronephric kidney by modulating lipid rafts

The kidney is a homeostatic organ required for waste excretion and reabsorption of water, salts and other macromolecules. To this end, a complex series of developmental steps ensures the formation of a correctly patterned and properly proportioned organ. While previous studies have mainly focused on the individual signaling pathways, the formation of higher order receptor complexes in lipid rafts is an equally important aspect. These membrane platforms are characterized by differences in local lipid and protein compositions. Indeed, the cells in the Xenopus pronephric kidney were positive for the lipid raft markers ganglioside GM1 and Caveolin-1. To specifically interfere with lipid raft function in vivo, we focused on the Sterol Carrier Protein 2 (scp2), a multifunctional protein that is an important player in remodeling lipid raft composition. In Xenopus, scp2 mRNA was strongly expressed in differentiated epithelial structures of the pronephric kidney. Knockdown of scp2 did not interfere with the patterning of the kidney along its proximo-distal axis, but dramatically decreased the size of the kidney, in particular the proximal tubules. This phenotype was accompanied by a reduction of lipid rafts, but was independent of the peroxisomal or transcriptional activities of scp2. Finally, disrupting lipid micro-domains by inhibiting cholesterol synthesis using Mevinolin phenocopied the defects seen in scp2 morphants. Together these data underscore the importance for localized signaling platforms in the proper formation of the Xenopus kidney.

MicroRNAs are critical regulators of tuberous sclerosis complex and mTORC1 activity in the size control of the Xenopus kidney

MicroRNAs (miRNAs) are major posttranscriptional regulators of a wide variety of biological processes. However, redundancy among most miRNAs has made it difficult to identify their in vivo functions. We previously demonstrated that global inhibition of miRNA biogenesis in Xenopus resulted in a dramatically smaller pronephric kidney. This suggested that microRNAs play a pivotal role in organ size control. Here we now provide a detailed mechanistic explanation for this phenotype. We identified that the activation of the mechanistic target of rapamycin complex 1 (mTORC1) by Insulin and insulin-like growth factor (Igf) 2 is an important regulator in kidney growth, which in turn is modulated by microRNAs. Molecular analyses demonstrate that microRNAs set a threshold for mTORC1 signaling by down-regulating one of its core negative regulators, tuberous sclerosis 1 (Tsc1). Most importantly, this rheostat can be reprogrammed experimentally. Whereas knockdown of miRNAs causes growth arrest, concomitant knockdown of Tsc1 restores mTORC1 activity and proximal tubular size. Together, these data establish a previously unidentified in vivo paradigm for the importance of posttranscriptional regulation in organ size control.

The bigger the better: determining nephron size in kidney

The main functions of the kidney are to excrete metabolic waste products and actively reabsorb essential molecules such as amino acids, ions, glucose and water. In humans, a wide range of genetic disorders exist characterized by wasting of metabolically important compounds. At the cellular level, more than 20 highly specialized renal epithelial cell types located in different segments of the nephron contribute to the reabsorption process. In particular, proximal tubular cells play a crucial role and are uniquely adapted to maximize reabsorption efficiency. They accommodate high numbers of transporters and channels by increasing the apical surface area in contact with the primary filtrate by forming a brush border as well as undergoing hypertrophy and hyperplasia. This adaptation is evolutionarily conserved and is detected in the primitive pronephric kidney of fish and amphibians as well as the metanephric kidney of higher vertebrates. Surprisingly, signaling pathways regulating these three processes have remained largely unknown. Here we summarize recent studies that highlight the early phases of kidney development as a critical juncture in establishing proximal tubule size.

Regulation of G-protein signaling via Gnas is required to regulate proximal tubular growth in the Xenopus pronephros

In the kidney, proximal tubules are very important for the reabsorption of water, ions and organic solutes from the primary urine. They are composed of highly specialized epithelial cells that are characterized by an elaborate apical brush border to increase transport efficiency. Using the pronephric kidney of Xenopus laevis we discovered that the G-protein modulator cholera toxin resulted in a dramatic reduction of the proximal tubular size. This phenotype was accompanied by changes in the cytoarchitecture characterized by ectopic expression of the distal tubular marker 4A6 and an impairment of yolk platelet degradation. In addition, cholera toxin caused edema formation. However, this phenotype was not due to kidney defects, but rather due to impaired vasculature development. Based on experiments with antisense morpholino oligomers as well as pharmacological agonists and antagonists, we could show that the complex phenotype of cholera toxin in the pronephric kidney was caused by the hyperactivation of a single G-protein alpha subunit, Gnas. This-in turn-caused elevated cAMP levels, triggered a Rapgef4-dependent signaling cassette and perturbed exo- and endocytosis. This perturbation of the secretory pathway by Ctx was not only observed in Xenopus embryos. Also, in a human proximal tubular cell line, cholera toxin or a Rapgef4-specific agonist increased uptake and decreased secretion of FITC-labeled Albumin. Based on these data we propose that the Gnas/cAMP/Rapgef4 pathway regulates the signals inducing the proliferation of proximal tubules to acquire their final organ size.