Transcriptional profiling of the zebrafish proximal tubule

(Zebra)fishing for nephrogenesis genes

Kidney disease is a devastating condition affecting millions of people worldwide, where over 100,000 patients in the United States alone remain waiting for a lifesaving organ transplant. Concomitant with a surge in personalized medicine, single-gene mutations, and polygenic risk alleles have been brought to the forefront as core causes of a spectrum of renal disorders. With the increasing prevalence of kidney disease, it is imperative to make substantial strides in the field of kidney genetics. Nephrons, the core functional units of the kidney, are epithelial tubules that act as gatekeepers of body homeostasis by absorbing and secreting ions, water, and small molecules to filter the blood. Each nephron contains a series of proximal and distal segments with explicit metabolic functions. The embryonic zebrafish provides an ideal platform to systematically dissect the genetic cues governing kidney development. Here, we review the use of zebrafish to discover nephrogenesis genes.

odd skipped-related 2 as a novel mark for labeling the proximal convoluted tubule within the zebrafish kidney

The proximal convoluted tubule (PCT) of the kidney is a crucial functional segment responsible for reabsorption, secretion, and the maintenance of electrolyte and water balance within the renal tubule. However, there is a lack of a well-defined endogenous transgenic line for studying PCT morphogenesis. By analyzing single-cell transcriptome data from the adult zebrafish kidney, we have identified the expression of odd-skipped-related 2 (osr2, which encodes an odd-skipped zinc-finger transcription factor) in the PCT. To gain insight into the role of osr2 in PCT morphogenesis, we have generated a transgenic zebrafish line Tg(osr2:EGFP), expressing enhanced green fluorescent protein (EGFP). The EGFP expression pattern closely mirrors that of endogenous Osr2, faithfully recapitulating its native expression profile. During kidney development, we can use EGFP to track PCT development, which is also preserved in adult zebrafish. Additionally, osr2:EGFP-labeled zebrafish PCT fragments displayed short lengths with infrequent overlap, rendering them conducive for nephrons counting. The generation of Tg(osr2:EGFP) transgenic line is accompanied by simultaneous disruption of osr2 activity. Importantly, our findings demonstrate that osr2 inactivation had no discernible impact on the development and regeneration of Tg(osr2:EGFP) zebrafish nephrons. Overall, the establishment of this transgenic zebrafish line offers a valuable tool for both genetic and chemical analysis of PCT.

Mapping the cellular landscape of Atlantic salmon head kidney by single cell and single nucleus transcriptomics

Single-cell transcriptomics is the current gold standard for global gene expression profiling, not only in mammals and model species, but also in non-model fish species. This is a rapidly expanding field, creating a deeper understanding of tissue heterogeneity and the distinct functions of individual cells, making it possible to explore the complexities of immunology and gene expression on a highly resolved level. In this study, we compared two single cell transcriptomic approaches to investigate cellular heterogeneity within the head kidney of healthy farmed Atlantic salmon (Salmo salar). We compared 14,149 cell transcriptomes assayed by single cell RNA-seq (scRNA-seq) with 18,067 nuclei transcriptomes captured by single nucleus RNA-Seq (snRNA-seq). Both approaches detected eight major cell populations in common: granulocytes, heamatopoietic stem cells, erythrocytes, mononuclear phagocytes, thrombocytes, B cells, NK-like cells, and T cells. Four additional cell types, endothelial, epithelial, interrenal, and mesenchymal cells, were detected in the snRNA-seq dataset, but appeared to be lost during preparation of the single cell suspension submitted for scRNA-seq library generation. We identified additional heterogeneity and subpopulations within the B cells, T cells, and endothelial cells, and revealed developmental trajectories of heamatopoietic stem cells into differentiated granulocyte and mononuclear phagocyte populations. Gene expression profiles of B cell subtypes revealed distinct IgM and IgT-skewed resting B cell lineages and provided insights into the regulation of B cell lymphopoiesis. The analysis revealed eleven T cell sub-populations, displaying a level of T cell heterogeneity in salmon head kidney comparable to that observed in mammals, including distinct subsets of cd4/cd8-negative T cells, such as tcrγ positive, progenitor-like, and cytotoxic cells. Although snRNA-seq and scRNA-seq were both useful to resolve cell type-specific expression in the Atlantic salmon head kidney, the snRNA-seq pipeline was overall more robust in identifying several cell types and subpopulations. While scRNA-seq displayed higher levels of ribosomal and mitochondrial genes, snRNA-seq captured more transcription factor genes. However, only scRNA-seq-generated data was useful for cell trajectory inference within the myeloid lineage. In conclusion, this study systematically outlines the relative merits of scRNA-seq and snRNA-seq in Atlantic salmon, enhances understanding of teleost immune cell lineages, and provides a comprehensive list of markers for identifying major cell populations in the head kidney with significant immune relevance.

Zebrafish as a model for podocyte research

Podocytes, specialized postmitotic cells, are central players in various kidney-related diseases. Zebrafish have become a valuable model system for studying podocyte biology because they are genetically easy to manipulate, transparent, and their glomerular structure is similar to that of mammals. This review provides an overview of the knowledge of podocyte biology in zebrafish larvae, with particular focus on their essential contribution to understanding the mechanisms that underlie kidney diseases as well as supporting drug development. In addition, special attention is given to advances in live-imaging techniques allowing the observation of dynamic processes, including podocyte motility, podocyte process behavior, and glomerulus maturation. The review further addresses the functional aspects of podocytes in zebrafish larvae. This includes topics such as glomerular filtration, ultrastructural analyses, and evaluation of podocyte response to nephrotoxic insults. Studies presented in this context have provided important insights into the maintenance and resistance of the glomerular filtration barrier in zebrafish larvae and explored the potential transferability of these findings to mammals such as mice, rats, and most importantly, humans. The recent ability to identify potential therapeutic targets represents a promising new way to identify drugs that could effectively treat podocyte-associated glomerulopathies in humans. In summary, this review gives an overview about the importance of zebrafish as a model for podocyte-related disease and targeted drug development. It also highlights the key role of advanced imaging techniques in transparent zebrafish larvae, improving our understanding of glomerular diseases and the significant potential for translation of these findings to humans.

Adverse effects of plastic leachate and its component 2,4-DTBP on the early development of zebrafish embryos

Plastic waste has become a global environmental problem threatening the health of aquatic organisms especially via leachate. In this study, the test of zebrafish embryo showed adverse effects of leachate from some agricultural mulching films after UV light aging for 60 h. A typical phenolic antioxidant 2,4-di-tert-butylphenol (2,4-DTBP) was detected in the leachate and tested further for the zebrafish embryo biotoxicity. The microplastic leachate (6, 8 g/L, mass concentration measured by weight of plastic) increased the death and malformation rates, and reduced the hatching rate, heart rate, and body length of zebrafish larvae in the 96-hour early development period. Similar adverse effects were also caused by the 2,4-DTBP (0.01, 0.1, 1.0 mg/L, corresponding to 0.049, 0.49, and 4.85 μM) to some degree but could not completely explain the significant influences caused by the plastic leachate. Transcriptome analysis of zebrafish embryos exposed to the 2,4-DTBP for 96 h showed that the protein, fat, and carbohydrate digestion and absorption pathways, pancreatic secretion, PPAR signaling pathway, tryptophan metabolism, and adipocytokine signaling pathway were considerably down-regulated, but the cholesterol metabolism pathway was up-regulated in larval zebrafish. The altered transcriptional expression of mRNA at early development stage (96 h post fertilization) of zebrafish suggested that the 2,4-DTBP caused reduction of digestive capacity and pancreatic secretory function, and adversely affected processes associated with energy metabolism and glycolipid metabolism of larval zebrafish. This study helps us further understanding the effects of plastic leachate on the early development of fishes.

Proenkephalin-A secreted by renal proximal tubules functions as a brake in kidney regeneration

Organ regeneration necessitates precise coordination of accelerators and brakes to restore organ function. However, the mechanisms underlying this intricate molecular crosstalk remain elusive. In this study, the level of proenkephalin-A (PENK-A), expressed by renal proximal tubular epithelial cells, decreases significantly with the loss of renal proximal tubules and increased at the termination phase of zebrafish kidney regeneration. Notably, this change contrasts with the role of hydrogen peroxide (H2O2), which acts as an accelerator in kidney regeneration. Through experiments with penka mutants and pharmaceutical treatments, we demonstrate that PENK-A inhibits H2O2 production in a dose-dependent manner, suggesting its involvement in regulating the rate and termination of regeneration. Furthermore, H2O2 influences the expression of tcf21, a vital factor in the formation of renal progenitor cell aggregates, by remodeling H3K4me3 in renal cells. Overall, our findings highlight the regulatory role of PENK-A as a brake in kidney regeneration.

OCT4 induces long-lived dedifferentiated kidney progenitors poised to redifferentiate in 3D kidney spheroids

Upscaling of kidney epithelial cells is crucial for renal regenerative medicine. Nonetheless, the adult kidney lacks a distinct stem cell hierarchy, limiting the ability to long-term propagate clonal populations of primary cells that retain renal identity. Toward this goal, we tested the paradigm of shifting the balance between differentiation and stemness in the kidney by introducing a single pluripotency factor, OCT4. Here we show that ectopic expression of OCT4 in human adult kidney epithelial cells (hKEpC) induces the cells to dedifferentiate, stably proliferate, and clonally emerge over many generations. Control hKEpC dedifferentiate, assume fibroblastic morphology, and completely lose clonogenic capacity. Analysis of gene expression and histone methylation patterns revealed that OCT4 represses the HNF1B gene module, which is critical for kidney epithelial differentiation, and concomitantly activates stemness-related pathways. OCT4-hKEpC can be long-term expanded in the dedifferentiated state that is primed for renal differentiation. Thus, when expanded OCT4-hKEpC are grown as kidney spheroids (OCT4-kSPH), they reactivate the HNF1B gene signature, redifferentiate, and efficiently generate renal structures in vivo. Hence, changes occurring in the cellular state of hKEpC following OCT4 induction, long-term propagation, and 3D aggregation afford rapid scale-up technology of primary renal tissue-forming cells.

Kinase Activity Is Not Required for G Protein-Coupled Receptor Kinase 4 Restraining mTOR Signaling during Cilia and Kidney Development

SIGNIFICANCE STATEMENT

G protein-coupled receptor kinase 4 (GRK4) regulates renal sodium and water reabsorption. Although GRK4 variants with elevated kinase activity have been associated with salt-sensitive or essential hypertension, this association has been inconsistent among different study populations. In addition, studies elucidating how GRK4 may modulate cellular signaling are sparse. In an analysis of how GRK4 affects the developing kidney, the authors found that GRK4 modulates mammalian target of rapamycin (mTOR) signaling. Loss of GRK4 in embryonic zebrafish causes kidney dysfunction and glomerular cysts. Moreover, GRK4 depletion in zebrafish and cellular mammalian models results in elongated cilia. Rescue experiments suggest that hypertension in carriers of GRK4 variants may not be explained solely by kinase hyperactivity; instead, elevated mTOR signaling may be the underlying cause.

BACKGROUND

G protein-coupled receptor kinase 4 (GRK4) is considered a central regulator of blood pressure through phosphorylation of renal dopaminergic receptors and subsequent modulation of sodium excretion. Several nonsynonymous genetic variants of GRK4 have been only partially linked to hypertension, although these variants demonstrate elevated kinase activity. However, some evidence suggests that function of GRK4 variants may involve more than regulation of dopaminergic receptors alone. Little is known about the effects of GRK4 on cellular signaling, and it is also unclear whether or how altered GRK4 function might affect kidney development.

METHODS

To better understand the effect of GRK4 variants on the functionality of GRK4 and GRK4's actions in cellular signaling during kidney development, we studied zebrafish, human cells, and a murine kidney spheroid model.

RESULTS

Zebrafish depleted of Grk4 develop impaired glomerular filtration, generalized edema, glomerular cysts, pronephric dilatation, and expansion of kidney cilia. In human fibroblasts and in a kidney spheroid model, GRK4 knockdown produced elongated primary cilia. Reconstitution with human wild-type GRK4 partially rescues these phenotypes. We found that kinase activity is dispensable because kinase-dead GRK4 (altered GRK4 that cannot result in phosphorylation of the targeted protein) prevented cyst formation and restored normal ciliogenesis in all tested models. Hypertension-associated genetic variants of GRK4 fail to rescue any of the observed phenotypes, suggesting a receptor-independent mechanism. Instead, we discovered unrestrained mammalian target of rapamycin signaling as an underlying cause.

CONCLUSIONS

These findings identify GRK4 as novel regulator of cilia and of kidney development independent of GRK4's kinase function and provide evidence that the GRK4 variants believed to act as hyperactive kinases are dysfunctional for normal ciliogenesis.

Principles of Zebrafish Nephron Segment Development

Nephrons are the functional units which comprise the kidney. Each nephron contains a number of physiologically unique populations of specialized epithelial cells that are organized into discrete domains known as segments. The principles of nephron segment development have been the subject of many studies in recent years. Understanding the mechanisms of nephrogenesis has enormous potential to expand our knowledge about the basis of congenital anomalies of the kidney and urinary tract (CAKUT), and to contribute to ongoing regenerative medicine efforts aimed at identifying renal repair mechanisms and generating replacement kidney tissue. The study of the zebrafish embryonic kidney, or pronephros, provides many opportunities to identify the genes and signaling pathways that control nephron segment development. Here, we describe recent advances of nephron segment patterning and differentiation in the zebrafish, with a focus on distal segment formation.

HNF1B Alters an Evolutionarily Conserved Nephrogenic Program of Target Genes

SIGNIFICANCE STATEMENT

Mutations in hepatocyte nuclear factor-1 β ( HNF1B ) are the most common monogenic causes of congenital renal malformations. HNF1B is necessary to directly reprogram fibroblasts to induced renal tubule epithelial cells (iRECs) and, as we demonstrate, can induce ectopic pronephric tissue in Xenopus ectodermal organoids. Using these two systems, we analyzed the effect of HNF1B mutations found in patients with cystic dysplastic kidney disease. We found cross-species conserved targets of HNF1B, identified transcripts that are differentially regulated by the patient-specific mutant protein, and functionally validated novel HNF1B targets in vivo . These results highlight evolutionarily conserved transcriptional mechanisms and provide insights into the genetic circuitry of nephrogenesis.

BACKGROUND

Hepatocyte nuclear factor-1 β (HNF1B) is an essential transcription factor during embryogenesis. Mutations in HNF1B are the most common monogenic causes of congenital cystic dysplastic renal malformations. The direct functional consequences of mutations in HNF1B on its transcriptional activity are unknown.

METHODS

Direct reprogramming of mouse fibroblasts to induced renal tubular epithelial cells was conducted both with wild-type HNF1B and with patient mutations. HNF1B was expressed in Xenopus ectodermal explants. Transcriptomic analysis by bulk RNA-Seq identified conserved targets with differentially regulated expression by the wild-type or R295C mutant. CRISPR/Cas9 genome editing in Xenopus embryos evaluated transcriptional targets in vivo .

RESULTS

HNF1B is essential for reprogramming mouse fibroblasts to induced renal tubular epithelial cells and induces development of ectopic renal organoids from pluripotent Xenopus cells. The mutation R295C retains reprogramming and inductive capacity but alters the expression of specific sets of downstream target genes instead of diminishing overall transcriptional activity of HNF1B. Surprisingly, targets associated with polycystic kidney disease were less affected than genes affected in congenital renal anomalies. Cross-species-conserved transcriptional targets were dysregulated in hnf1b CRISPR-depleted Xenopus embryos, confirming their dependence on hnf1b .

CONCLUSIONS

HNF1B activates an evolutionarily conserved program of target genes that disease-causing mutations selectively disrupt. These findings provide insights into the renal transcriptional network that controls nephrogenesis.

Nrf2a dependent and independent effects of early life exposure to 3,3'-dichlorobiphenyl (PCB-11) in zebrafish (Danio rerio)

The environmental pollutant 3,3'-dichlorobiphenyl (PCB-11) is a lower-chlorinated polychlorinated biphenyl (PCB) congener present in air and water samples. Both PCB-11 and its metabolite, 4-PCB-11-Sulfate, are detected in humans, including in pregnant women. Previous research in zebrafish (Danio rerio) has shown that 0.2 μM exposures to 4-PCB-11-Sulfate starting at 1 day post fertilization (dpf) increase hepatic neutral lipid accumulation in larvae at 15 dpf. Here, we explored whether nuclear factor erythroid 2-related factor 2 (Nrf2), known as the master-regulator of the adaptive response to oxidative stress, contributes to metabolic impacts of 4-PCB-11-Sulfate. For this work, embryos were collected from homozygous wildtype or Nrf2a mutant adult zebrafish that also express GFP in pancreatic β-cells, rendering Tg(ins:GFP;nrf2afh318+/+) and Tg(ins:GFP;nrf2afh318-/-) lines. Exposures were conducted from 1-15 dpf to either 0.05% DMSO or DMSO-matched 0.2 µM 4-PCB-11-Sulfate, and at 15 dpf subsets of larvae were imaged for overall morphology, primary pancreatic islet area, and collected for fatty acid profiling and RNAseq. At 15 dpf, independent of genotype, fish exposed to 4-PCB-11-Sulfate survived significantly more at 80-85% compared to 65-73% survival for unexposed fish, and had primary pancreatic islets 8% larger compared to unexposed fish. Fish growth at 15 dpf was dependent on genotype, with Nrf2a mutant fish a significant 3-5% shorter than wildtype fish, and an interaction effect was observed where Nrf2a mutant fish exposed to 4-PCB-11-Sulfate experienced a significant 29% decrease in the omega-3 fatty acid DHA compared to unexposed mutant fish. RNAseq revealed 308 differentially expressed genes, most of which were dependent on genotype. These findings suggest that Nrf2a plays an important role in growth as well as for DHA production in the presence of 4-PCB-11-Sulfate. Further research would be beneficial to understand the importance of Nrf2a throughout the lifecourse, especially in the context of toxicant exposures.

Single-cell mRNA profiling reveals changes in solute carrier expression and suggests a metabolic switch during zebrafish pronephros development

Developing organisms need to adapt to environmental variations as well as to rapid changes in substrate availability and energy demands imposed by fast-growing tissues and organs. Little is known about the adjustments that kidneys undergo in response to these challenges. We performed single-cell RNA sequencing of zebrafish pronephric duct cells to understand how the developing kidney responds to changes in filtered substrates and intrinsic energy requirements. We found high levels of glucose transporters early in development and increased expression of monocarboxylate transporters at later times. This indicates that the zebrafish embryonic kidney displays a high glucose transporting capacity during early development, which is replaced by the ability to absorb monocarboxylates and amino acids at later stages. This change in transport capacity was accompanied by the upregulation of mitochondrial carriers, indicating a switch to increased oxidative phosphorylation to meet the increasing energy demand of a developing kidney.NEW & NOTEWORTHY The zebrafish embryonic kidney has high levels of glucose transporters during early development, which are replaced by monocarboxylate and amino acid transporters later on. Inhibition of Na⁺-glucose cotransporter-dependent glucose transport by sotagliflozin also increased slc2a1a expression, supporting the idea that the glucose transport capacity is dynamically adjusted during zebrafish pronephros development. Concurrent upregulation of mitochondrial SCL25 transporters at later stages supports the idea that the pronephros adjusts to changing substrate supplies and/or energy demands during embryonic development.

New zebrafish model for monitoring proximal tubule physiology in genetic and acquired renal Fanconi syndromes

Inherited and acquired disorders that affect proximal tubule endocytosis and lysosomal processing manifest with improper loss of solutes and proteins. The zebrafish pronephros is conserved with humans and is used to model numerous renal conditions, but has few quantitative measures for proximal tubule function. Here, Chen et al. developed a high-throughput assay to quantify proteinuria and lysosomal processing in transgenic zebrafish by labeling vitamin D protein, allowing for precise reporting of proximal tubule function.