Ciliated renal tubular cells in crescentic glomerulonephritis

mtor Haploinsufficiency Ameliorates Renal Cysts and Cilia Abnormality in Adult Zebrafish tmem67 Mutants

BACKGROUND

Although zebrafish embryos have been used to study ciliogenesis and model polycystic kidney disease (PKD), adult zebrafish remain unexplored.

METHODS

Transcription activator-like effector nucleases (TALEN) technology was used to generate mutant for tmem67, the homolog of the mammalian causative gene for Meckel syndrome type 3 (MKS3). Classic 2D and optical-clearing 3D imaging of an isolated adult zebrafish kidney were used to examine cystic and ciliary phenotypes. A hypomorphic mtor strain or rapamycin was used to inhibit mTOR activity.

RESULTS

Adult tmem67 zebrafish developed progressive mesonephric cysts that share conserved features of mammalian cystogenesis, including a switch of cyst origin with age and an increase in proliferation of cyst-lining epithelial cells. The mutants had shorter and fewer distal single cilia and greater numbers of multiciliated cells (MCCs). Absence of a single cilium preceded cystogenesis, and expansion of MCCs occurred after pronephric cyst formation and was inversely correlated with the severity of renal cysts in young adult zebrafish, suggesting a primary defect and an adaptive action, respectively. Finally, the mutants exhibited hyperactive mTOR signaling. mTOR inhibition ameliorated renal cysts in both the embryonic and adult zebrafish models; however, it only rescued ciliary abnormalities in the adult mutants.

CONCLUSIONS

Adult zebrafish tmem67 mutants offer a new vertebrate model for renal cystic diseases, in which cilia morphology can be analyzed at a single-nephron resolution and mTOR inhibition proves to be a candidate therapeutic strategy.

Visualizing Multiciliated Cells in the Zebrafish Through a Combined Protocol of Whole Mount Fluorescent In Situ Hybridization and Immunofluorescence

In recent years, the zebrafish embryo has emerged as a popular model to study developmental biology due to traits such as ex utero embryo development and optical transparency. In particular, the zebrafish embryo has become an important organism to study vertebrate kidney organogenesis as well as multiciliated cell (MCC) development. To visualize MCCs in the embryonic zebrafish kidney, we have developed a combined protocol of whole-mount fluorescent in situ hybridization (FISH) and whole mount immunofluorescence (IF) that enables high resolution imaging. This manuscript describes our technique for co-localizing RNA transcripts and protein as a tool to better understand the regulation of developmental programs through the expression of various lineage factors.

Antennas of organ morphogenesis: the roles of cilia in vertebrate kidney development

Cilia arose early during eukaryotic evolution, and their structural components are highly conserved from the simplest protists to complex metazoan species. In recent years, the role of cilia in the ontogeny of vertebrate organs has received increasing attention due to a staggering correlation between human disease and dysfunctional cilia. In particular, the presence of cilia in both the developing and mature kidney has become a deep area of research due to ciliopathies common to the kidney, such as polycystic kidney disease (PKD). Interestingly, mutations in genes encoding proteins that localize to the cilia cause similar cystic phenotypes in kidneys of various vertebrates, suggesting an essential role for cilia in kidney organogenesis and homeostasis as well. Importantly, the genes so far identified in kidney disease have conserved functions across species, whose kidneys include both primary and motile cilia. Here, we aim to provide a comprehensive description of cilia and their role in kidney development, as well as highlight the usefulness of the zebrafish embryonic kidney as a model to further understand the function of cilia in kidney health.

Epithelial cell fate in the nephron tubule is mediated by the ETS transcription factors etv5a and etv4 during zebrafish kidney development

Kidney development requires the differentiation and organization of discrete nephron epithelial lineages, yet the genetic and molecular pathways involved in these events remain poorly understood. The embryonic zebrafish kidney, or pronephros, provides a simple and useful model to study nephrogenesis. The pronephros is primarily comprised of two types of epithelial cells: transportive and multiciliated cells (MCCs). Transportive cells occupy distinct tubule segments and are characterized by the expression of various solute transporters, while MCCs function in fluid propulsion and are dispersed in a "salt-and-pepper" fashion within the tubule. Epithelial cell identity is reliant on interplay between the Notch signaling pathway and retinoic acid (RA) signaling, where RA promotes MCC fate by inhibiting Notch activity in renal progenitors, while Notch acts downstream to trigger transportive cell formation and block adoption of an MCC identity. Previous research has shown that the transcription factor ets variant 5a (etv5a), and its closely related ETS family members, are required for ciliogenesis in other zebrafish tissues. Here, we mapped etv5a expression to renal progenitors that occupy domains where MCCs later emerge. Thus, we hypothesized that etv5a is required for normal development of MCCs in the nephron. etv5a loss of function caused a decline of MCC number as indicated by the reduced frequency of cells that expressed the MCC-specific markers outer dense fiber of sperm tails 3b (odf3b) and centrin 4 (cetn4), where rescue experiments partially restored MCC incidence. Interestingly, deficiency of ets variant 4 (etv4), a related gene that is broadly expressed in the posterior mesoderm during somitogenesis stages, also led to reduced MCC numbers, which were further reduced by dual etv5a/4 deficiency, suggesting that both of these ETS factors are essential for MCC formation and that they also might have redundant activities. In epistatic studies, exogenous RA treatment expanded the etv5a domain within the renal progenitor field and RA inhibition blocked etv5a in this populace, indicating that etv5a acts downstream of RA. Additionally, treatment with exogenous RA partially rescued the reduced MCC phenotype after loss of etv5a. Further, abrogation of Notch with the small molecule inhibitor DAPT increased the renal progenitor etv5a expression domain as well as MCC density in etv5a deficient embryos, suggesting Notch acts upstream to inhibit etv5a. In contrast, etv4 levels in renal progenitors were unaffected by changes in RA or Notch signaling levels, suggesting a possible non-cell autonomous role during pronephros formation. Taken together, these findings have revealed new insights about the genetic mechanisms of epithelial cell development during nephrogenesis.

Stem cells and fluid flow drive cyst formation in an invertebrate excretory organ

Cystic kidney diseases (CKDs) affect millions of people worldwide. The defining pathological features are fluid-filled cysts developing from nephric tubules due to defective flow sensing, cell proliferation and differentiation. The underlying molecular mechanisms, however, remain poorly understood, and the derived excretory systems of established invertebrate models (Caenorhabditis elegans and Drosophila melanogaster) are unsuitable to model CKDs. Systematic structure/function comparisons revealed that the combination of ultrafiltration and flow-associated filtrate modification that is central to CKD etiology is remarkably conserved between the planarian excretory system and the vertebrate nephron. Consistently, both RNA-mediated genetic interference (RNAi) of planarian orthologues of human CKD genes and inhibition of tubule flow led to tubular cystogenesis that share many features with vertebrate CKDs, suggesting deep mechanistic conservation. Our results demonstrate a common evolutionary origin of animal excretory systems and establish planarians as a novel and experimentally accessible invertebrate model for the study of human kidney pathologies.

The zebrafish foxj1a transcription factor regulates cilia function in response to injury and epithelial stretch

Cilia are essential for normal organ function and developmental patterning, but their role in injury and regeneration responses is unknown. To probe the role of cilia in injury, we analyzed the function of foxj1, a transcriptional regulator of cilia genes, in response to tissue damage and renal cyst formation. Zebrafish foxj1a, but not foxj1b, was rapidly induced in response to epithelial distension and stretch, kidney cyst formation, acute kidney injury by gentamicin, and crush injury in spinal cord cells. Obstruction-induced up-regulation of foxj1a was not inhibited by cycloheximide, identifying foxj1a as a primary response gene to epithelial injury. Foxj1 was also dramatically up-regulated in murine cystic kidney disease epithelia [jck/jck (nek8) and Ift88Tg737Rpw(-/-)] as well as in response to kidney ischemia-reperfusion injury. Obstruction of the zebrafish pronephric tubule caused a rapid increase in cilia beat rate that correlated tightly with expanded tubule diameter and epithelial stretch. Zebrafish foxj1a was specifically required for cilia motility. Enhanced foxj1a expression in obstructed tubules induced cilia motility target genes efhc1, tektin-1, and dnahc9. foxj1a-deficient embryos failed to up-regulate efhc1, tektin-1, and dnahc9 and could not maintain enhanced cilia beat rates after obstruction, identifying an essential role for foxj1 in modulating cilia function after injury. These studies reveal that activation of a Foxj1 transcriptional network of ciliogenic genes is an evolutionarily conserved response to multiple forms of tissue damage and highlight enhanced cilia function as a previously uncharacterized component of organ homeostasis.

Ciliary and centrosomal defects associated with mutation and depletion of the Meckel syndrome genes MKS1 and MKS3

Meckel syndrome (MKS) is a lethal disorder characterized by renal cystic dysplasia, encephalocele, polydactyly and biliary dysgenesis. It is highly genetically heterogeneous with nine different genes implicated in this disorder. MKS is thought to be a ciliopathy because of the range of phenotypes and localization of some of the implicated proteins. However, limited data are available about the phenotypes associated with MKS1 and MKS3, and the published ciliary data are conflicting. Analysis of the wpk rat model of MKS3 revealed functional defects of the connecting cilium in the eye that resulted in lack of formation of the outer segment, whereas infertile wpk males developed spermatids with very short flagella that did not extend beyond the cell body. In wpk renal collecting duct cysts, cilia were generally longer than normal, with additional evidence of cells with multiple primary cilia and centrosome over-duplication. Kidney tissue and cells from MKS1 and MKS3 patients showed defects in centrosome and cilia number, including multi-ciliated respiratory-like epithelia, and longer cilia. Stable shRNA knockdown of Mks1 and Mks3 in IMCD3 cells induced multi-ciliated and multi-centrosomal phenotypes. These studies demonstrate that MKS1 and MKS3 are ciliopathies, with new cilia-related eye and sperm phenotypes defined. MKS1 and MKS3 functions are required for ciliary structure and function, including a role in regulating length and appropriate number through modulating centrosome duplication.

Fish and frogs: models for vertebrate cilia signaling

The presence of cilia in many vertebrate cell types and its function has been ignored for many years. Only in the past few years has its importance been rediscovered. In part, this was triggered by the realization that many gene products mutated in polycystic kidney diseases are localized to cilia and dysfunctional cilia result in kidney disease. Another breakthrough was the observation that the establishment of the left-right body axis is dependent on cilia function. Since then, many other developmental paradigms have been shown to rely on cilia-dependent signaling. In addition to mouse and Chlamydomonas, lower vertebrate model systems such as zebrafish, medaka and Xenopus have provided important new insights into cilia signaling and its role during embryonic development. This review will summarize those studies. We will also illustrate how these lower vertebrates are promising model systems for future studies defining the physiological function of cilia during organogenesis and disease pathophysiology.

Notch signaling controls the differentiation of transporting epithelia and multiciliated cells in the zebrafish pronephros

Epithelial tubules consist of multiple cell types that are specialized for specific aspects of organ function. In the zebrafish pronephros, multiciliated cells (MCCs) are specialized for fluid propulsion, whereas transporting epithelial cells recover filtered-blood solutes. These cell types are distributed in a ;salt-and-pepper' fashion in the pronephros, suggesting that a lateral inhibition mechanism may play a role in their differentiation. We find that the Notch ligand Jagged 2 is expressed in MCCs and that notch3 is expressed in pronephric epithelial cells. Morpholino knockdown of either jagged 2 or notch3, or mutation in mind bomb (in which Notch signaling is impaired), dramatically expands ciliogenic gene expression, whereas ion transporter expression is lost, indicating that pronephric cells are transfated to MCCs. Conversely, ectopic expression of the Notch1a intracellular domain represses MCC differentiation. Gamma-secretase inhibition using DAPT demonstrated a requirement for Notch signaling early in pronephric development, before the pattern of MCC differentiation is apparent. Strikingly, we find that jagged 2 knockdown generates extra cilia and is sufficient to rescue the kidney cilia mutant double bubble. Our results indicate that Jagged 2/Notch signaling modulates the number of multiciliated versus transporting epithelial cells in the pronephros by way of a genetic pathway involving repression of rfx2, a key transcriptional regulator of the ciliogenesis program.

Detection of proximal tubular motile cilia in a patient with renal sarcoidosis associated with hypercalcemia

In a normal human kidney, almost every cell type gives rise to a solitary primary cilium, an apical hair like nonmotile organelle that recently was linked to the pathogenesis of polycystic kidney disease. Motile cilia normally are not expressed by renal tubular cells in the mammalian kidney. Here, we report the unexpected detection of motile cilia arising from proximal tubular cells in a patient with sarcoidosis associated with hypercalcemia and renal impairment. The possible significance of this unusual finding is discussed.

Cilia-driven fluid flow in the zebrafish pronephros, brain and Kupffer's vesicle is required for normal organogenesis

Cilia, as motile and sensory organelles, have been implicated in normal development, as well as diseases including cystic kidney disease, hydrocephalus and situs inversus. In kidney epithelia, cilia are proposed to be non-motile sensory organelles, while in the mouse node, two cilia populations, motile and non-motile have been proposed to regulate situs. We show that cilia in the zebrafish larval kidney, the spinal cord and Kupffer's vesicle are motile, suggesting that fluid flow is a common feature of each of these organs. Disruption of cilia structure or motility resulted in pronephric cyst formation, hydrocephalus and left-right asymmetry defects. The data show that loss of fluid flow leads to fluid accumulation, which can account for organ distension pathologies in the kidney and brain. In Kupffer's vesicle, loss of flow is associated with loss of left-right patterning, indicating that the 'nodal flow' mechanism of generating situs is conserved in non-mammalian vertebrates.

Renal epithelial neoplasms: the diagnostic implications of electron microscopic study in 55 cases

Several unsettled histogenetic, nosologic and diagnostic considerations for renal epithelial tumors may have ultrastructural ramifications. Yet, a comprehensive electron microscopic study of renal epithelial neoplasms, in light of the recent classification, is not available. The ultrastructural findings from fifty-five renal epithelial neoplasms [31 clear cell renal cell carcinomas (RCC), 11 papillary RCC, 5 chromophobe RCC, 3 sarcomatoid RCC and 5 oncocytomas] were correlated with their light microscopic appearance. Clear cell RCC showed long microvilli similar to the brush border of the normal proximal tubules, with abundant cytoplasmic lipid and glycogen. Papillary RCC showed variably sized microvilli, and small amounts of cytoplasmic lipid, but no glycogen. Chromophobe RCC showed many cytoplasmic vesicles and abnormal mitochondria, with rare short and stubby microvilli. Renal oncocytoma showed many mitochondria with a few vesicles in the apical portion of the cytoplasm and rare short and stubby microvilli. The eosinophilic cell variants of clear cell RCC, papillary RCC and chromophobe RCC showed ultrastructural features similar to those of their respective prototypes, except for an increased numbers of mitochondria in the cytoplasm. One sarcomatoid clear cell RCC showed skeletal muscle differentiation. Two types of cytoplasmic inclusions, i.e. hyaline globules and granules similar to those in the Paneth cells (PC-like granules) were identified only in clear cell RCC, which displayed distinctive ultrastructural features. The current EM study demonstrates distinctive ultrastructural features of renal epithelial neoplasms. The findings lend additional support to the current classification of the pertinent tumor types, facilitate the differential diagnoses, and provide insights into the possible histogenesis of renal epithelial neoplasms.