Impaired neural development in a zebrafish model for Lowe syndrome

Ultrahigh field diffusion magnetic resonance imaging uncovers intriguing microstructural changes in the adult zebrafish brain caused by Toll-like receptor 2 genomic deletion

Toll-like receptor 2 (TLR2) belongs to the TLR protein family that plays an important role in the immune and inflammation response system. While TLR2 is predominantly expressed in immune cells, its expression has also been detected in the brain, specifically in microglia and astrocytes. Recent studies indicate that genomic deletion of TLR2 can result in impaired neurobehavioural function. It is currently not clear if the genomic deletion of TLR2 leads to any alterations in the microstructural features of the brain. In the current study, we noninvasively assess microstructural changes in the brain of TLR2-deficient (tlr2-/-) zebrafish using state-of-the art magnetic resonance imaging (MRI) methods at ultrahigh magnetic field strength (17.6 T). A significant increase in cortical thickness and an overall trend towards increased brain volumes were observed in young tlr2-/- zebrafish. An elevated T2 relaxation time and significantly reduced apparent diffusion coefficient (ADC) unveil brain-wide microstructural alterations, potentially indicative of cytotoxic oedema and astrogliosis in the tlr2-/- zebrafish. Multicomponent analysis of the ADC diffusivity signal by the phasor approach shows an increase in the slow ADC component associated with restricted diffusion. Diffusion tensor imaging and diffusion kurtosis imaging analysis revealed diminished diffusivity and enhanced kurtosis in various white matter tracks in tlr2-/- compared with control zebrafish, identifying the microstructural underpinnings associated with compromised white matter integrity and axonal degeneration. Taken together, our findings demonstrate that the genomic deletion of TLR2 results in severe alterations to the microstructural features of the zebrafish brain. This study also highlights the potential of ultrahigh field diffusion MRI techniques in discerning exceptionally fine microstructural details within the small zebrafish brain, offering potential for investigating microstructural changes in zebrafish models of various brain diseases.

Unveiling the Exquisite Microstructural Details in Zebrafish Brain Non-Invasively Using Magnetic Resonance Imaging at 28.2 T

Zebrafish (Danio rerio) is an important animal model for a wide range of neurodegenerative diseases. However, obtaining the cellular resolution that is essential for studying the zebrafish brain remains challenging as it requires high spatial resolution and signal-to-noise ratios (SNR). In the current study, we present the first MRI results of the zebrafish brain at the state-of-the-art magnetic field strength of 28.2 T. The performance of MRI at 28.2 T was compared to 17.6 T. A 20% improvement in SNR was observed at 28.2 T as compared to 17.6 T. Excellent contrast, resolution, and SNR allowed the identification of several brain structures. The normative T1 and T2 relaxation values were established over different zebrafish brain structures at 28.2 T. To zoom into the white matter structures, we applied diffusion tensor imaging (DTI) and obtained axial, radial, and mean diffusivity, as well as fractional anisotropy, at a very high spatial resolution. Visualisation of white matter structures was achieved by short-track track-density imaging by applying the constrained spherical deconvolution method (stTDI CSD). For the first time, an algorithm for stTDI with multi-shell multi-tissue (msmt) CSD was tested on zebrafish brain data. A significant reduction in false-positive tracks from grey matter signals was observed compared to stTDI with single-shell single-tissue (ssst) CSD. This allowed the non-invasive identification of white matter structures at high resolution and contrast. Our results show that ultra-high field DTI and tractography provide reproducible and quantitative maps of fibre organisation from tiny zebrafish brains, which can be implemented in the future for a mechanistic understanding of disease-related microstructural changes in zebrafish models of various brain diseases.

OCRL1 Deficiency Affects the Intracellular Traffic of ApoER2 and Impairs Reelin-Induced Responses

Lowe Syndrome (LS) is a rare X-linked disorder characterized by renal dysfunction, cataracts, and several central nervous system (CNS) anomalies. The mechanisms underlying the neurological dysfunction in LS remain unclear, albeit they share some phenotypic characteristics similar to the deficiency or dysfunction of the Reelin signaling, a relevant pathway with roles in CNS development and neuronal functions. In this study, we investigated the role of OCRL1, an inositol polyphosphate 5-phosphatase encoded by the OCRL gene, mutated in LS, focusing on its impact on endosomal trafficking and receptor recycling in human neuronal cells. Specifically, we tested the effects of OCRL1 deficiency in the trafficking and signaling of ApoER2/LRP8, a receptor for the ligand Reelin. We found that loss of OCRL1 impairs ApoER2 intracellular trafficking, leading to reduced receptor expression and decreased levels at the plasma membrane. Additionally, human neurons deficient in OCRL1 showed impairments in ApoER2/Reelin-induced responses. Our findings highlight the critical role of OCRL1 in regulating ApoER2 endosomal recycling and its impact on the ApoER2/Reelin signaling pathway, providing insights into potential mechanisms underlying the neurological manifestations of LS.

Dental needs and conditions of individuals with Lowe syndrome: An observational study

INTRODUCTION

Lowe syndrome (LS) is an uncommon condition that affects the brain, kidneys, nervous system, and eyes, predominantly in males. The aim of this study was to examine dental conditions, dental treatments, and access and/or barriers to care for those with LS compared to healthy individuals.

METHODS

Surveys assessing dental conditions, dental treatments, and access and/or barriers to care were administered to families in the Lowe Syndrome Association and families with healthy children who had dental appointments at the Tufts University School of Dental Medicine (TUSDM) pediatric dental clinic. One parent or a guardian of pediatric patients with LS or not at TUSDM was asked to complete an online survey.

RESULTS

One hundred and eight surveys were obtained (n:58 from the LS group and n:50 from the healthy group). The LS group was significantly more likely (p < .05) to report "crooked/misaligned teeth," "difficult time chewing," "bad breath," and "mouth cysts" and was significantly less likely to report 6-month examination, "cleaning," and "filling." The LS group reported significantly greater difficulty locating a dentist.

CONCLUSION

The findings of this study indicate that individuals with LS are more vulnerable to developing severe dental conditions and experiencing difficulties in accessing dental care than healthy individuals. Additionally, those who present with this syndrome may be less likely to receive specific necessary dental treatments. As a result, it is essential to offer appropriate dental care and support to individuals with LS to guarantee they achieve optimal oral health.

Endocrine and behavioural features of Lowe syndrome and their potential molecular mechanisms

BACKGROUND

Lowe syndrome (LS) is an X linked disease caused by pathogenic variants in the OCRL gene that impacts approximately 1 in 500 000 children. Classic features include congenital cataract, cognitive/behavioural impairment and renal tubulopathy.

METHODS

This study is a retrospective review of clinical features reported by family based survey conducted by Lowe Syndrome Association. Frequency of non-ocular clinical feature(s) of LS and their age of onset was summarised. An LS-specific therapy effectiveness scale was used to assess the response to the administered treatment. Expression of OCRL and relevant neuropeptides was measured in postmortem human brain by qPCR. Gene expression in the mouse brain was determined by reanalysis of publicly available bulk and single cell RNA sequencing.

RESULTS

A total of 137 individuals (1 female, 89.1% white, median age 14 years (range 0.8-56)) were included in the study. Short stature (height <3rd percentile) was noted in 81% (n=111) individuals, and 15% (n=20) received growth hormone therapy. Undescended testis was reported in 47% (n=64), and median age of onset of puberty was 15 years. Additional features were dental problems (n=77, 56%), bone fractures (n=63, 46%), hypophosphataemia (n=60, 44%), developmental delay and behavioural issues. OCRL is expressed in human and mouse hypothalami, and in hypothalamic cell clusters expressing Ghrh, Sst, Oxt, Pomc and pituitary cells expressing Gh and Prl.

CONCLUSIONS

There is a wide spectrum of the clinical phenotype of LS. Some of the features may be partly driven by the loss of function of OCRL in the hypothalamus and the pituitary.

A novel nanoluciferase transgenic reporter measures proteinuria in zebrafish

The zebrafish is an important animal system for modeling human diseases. This includes kidney dysfunction as the embryonic kidney (pronephros) shares considerable molecular and morphological homology with the human nephron. A key clinical indicator of kidney disease is proteinuria, but a high-throughput readout of proteinuria in the zebrafish is currently lacking. To remedy this, we used the Tol2 transposon system to generate a transgenic zebrafish line that uses the fabp10a liver-specific promoter to over-express a nanoluciferase molecule fused with the D3 domain of Receptor-Associated Protein (a type of molecular chaperone) which we term NL-D3. Using a luminometer, we quantified proteinuria in NL-D3 zebrafish larvae by measuring the intensity of luminescence in the embryo medium. In the healthy state, NL-D3 is not excreted, but when embryos were treated with chemicals that affected either proximal tubular reabsorption (cisplatin, gentamicin) or glomerular filtration (angiotensin II, Hanks Balanced Salt Solution, Bovine Serum Albumin), NL-D3 is detected in fish medium. Similarly, depletion of several gene products associated with kidney disease (nphs1, nphs2, lrp2a, ocrl, col4a3, and col4a4) also induced NL-D3 proteinuria. Treating col4a4 depleted zebrafish larvae (a model of Alport syndrome) with captopril reduced proteinuria in this system. Thus, our findings validate the use of the NL-D3 transgenic zebrafish as a robust and quantifiable proteinuria reporter. Hence, given the feasibility of high-throughput assays in zebrafish, this novel reporter will permit screening for drugs that ameliorate proteinuria, thereby prioritizing candidates for further translational studies.

A novel nanoluciferase transgenic reporter measures proteinuria in zebrafish

The zebrafish is an important animal system for modeling human diseases. This includes kidney dysfunction as the embryonic kidney (pronephros) shares considerable molecular and morphological homology with the human nephron. A key clinical indicator of kidney disease is proteinuria, but a high-throughput readout of proteinuria in the zebrafish is currently lacking. To remedy this, we used the Tol2 transposon system to generate a transgenic zebrafish line that uses the fabp10a liver-specific promoter to over-express a nanoluciferase molecule fused with the D3 domain of Receptor-Associated Protein (a type of molecular chaperone) which we term NL-D3. Using a luminometer, we quantified proteinuria in NL-D3 zebrafish larvae by measuring the intensity of luminescence in the embryo medium. In the healthy state, NL-D3 is not excreted, but when embryos were treated with chemicals that affected either proximal tubular reabsorption (cisplatin, gentamicin) or glomerular filtration (angiotensin II, Hanks Balanced Salt Solution, Bovine Serum Albumin), NL-D3 is detected in fish medium. Similarly, depletion of several gene products associated with kidney disease (nphs1, nphs2, lrp2a, ocrl, col4a3, and col4a4) also induced NL-D3 proteinuria. Treating col4a4 depleted zebrafish larvae (a model of Alport syndrome) with captopril reduced proteinuria in this system. Thus, our findings validate the use of the NL-D3 transgenic zebrafish as a robust and quantifiable proteinuria reporter. Hence, given the feasibility of high-throughput assays in zebrafish, this novel reporter will permit screening for drugs that ameliorate proteinuria, thereby prioritizing candidates for further translational studies.

Assessment of endocytic traffic and Ocrl function in the developing zebrafish neuroepithelium

Endocytosis allows cells to internalise a wide range of molecules from their environment and to maintain their plasma membrane composition. It is vital during development and for maintenance of tissue homeostasis. The ability to visualise endocytosis in vivo requires suitable assays to monitor the process. Here, we describe imaging-based assays to visualise endocytosis in the neuroepithelium of living zebrafish embryos. Injection of fluorescent tracers into the brain ventricles followed by live imaging was used to study fluid-phase or receptor-mediated endocytosis, for which we used receptor-associated protein (RAP, encoded by Lrpap1) as a ligand for low-density lipoprotein receptor-related protein (LRP) receptors. Using dual-colour imaging combined with expression of endocytic markers, it is possible to track the progression of endocytosed tracers and to monitor trafficking dynamics. Using these assays, we reveal a role for the Lowe syndrome protein Ocrl in endocytic trafficking within the neuroepithelium. We also found that the RAP-binding receptor Lrp2 (encoded by lrp2a) appears to contribute only partially to neuroepithelial RAP endocytosis. Altogether, our results provide a basis to track endocytosis within the neuroepithelium in vivo and support a role for Ocrl in this process.

This article has an associated First Person interview with the first author of the paper.

Pacsin2 is required for endocytosis in the zebrafish pronephric tubule

Endocytosis mediates the cellular uptake of numerous molecules from the extracellular space and is a fundamentally important process. In the renal proximal tubule, the scavenger receptor megalin and its co-receptor cubilin mediate endocytosis of low molecular weight proteins from the renal filtrate. However, the extent to which megalin endocytosis relies on different components of the trafficking machinery remains relatively poorly defined in vivo. In this study, we identify a functional requirement for the F-BAR protein pacsin2 in endocytosis in the renal proximal tubule of zebrafish larvae. Pacsin2 is expressed throughout development and in all zebrafish tissues, similar to the mammalian orthologue. Within renal tubular epithelial cells, pacsin2 is enriched at the apical pole where it is localised to endocytic structures. Loss of pacsin2 results in reduced endocytosis within the proximal tubule, which is accompanied by a reduction in the abundance of megalin and endocytic organelles. Our results indicate that pacsin2 is required for efficient endocytosis in the proximal tubule, where it likely cooperates with other trafficking machinery to maintain endocytic uptake and recycling of megalin.

Summary: We identify a role for the F-BAR protein pacsin2 in endocytosis in the renal tubule of zebrafish larvae.

Albumin Expands Albumin Reabsorption Capacity in Proximal Tubule Epithelial Cells through a Positive Feedback Loop between AKT and Megalin

Renal proximal tubule cells (PTECs) act as urine gatekeepers, constantly and efficiently avoiding urinary protein waste through receptor-mediated endocytosis. Despite its importance, little is known about how this process is modulated in physiologic conditions. Data suggest that the phosphoinositide-3-kinase (PI3K)/protein kinase B (AKT) pathway regulates PTEC protein reabsorption. Here, we worked on the hypothesis that the physiologic albumin concentration and PI3K/AKT pathway form a positive feedback loop to expand endocytic capacity. Using LLC-PK1 cells, a model of PTECs, we showed that the PI3K/AKT pathway is required for megalin recycling and surface expression, affecting albumin uptake. Inhibition of this pathway stalls megalin at EEA1⁺ endosomes. Physiologic albumin concentration (0.01 mg/mL) activated AKT; this depends on megalin-mediated albumin endocytosis and requires previous activation of PI3K/mTORC2. This effect is correlated to the increase in albumin endocytosis, a phenomenon that we refer to as “albumin-induced albumin endocytosis”. Mice treated with L-lysine present decreased albumin endocytosis leading to proteinuria and albuminuria associated with inhibition of AKT activity. Renal cortex explants obtained from control mice treated with MK-2206 decreased albumin uptake and promoted megalin internalization. Our data highlight the mechanism behind the capacity of PTECs to adapt albumin reabsorption to physiologic fluctuations in its filtration, avoiding urinary excretion.

A human stem cell resource to decipher the biochemical and cellular basis of neurodevelopmental defects in Lowe Syndrome

Human brain development is a complex process where multiple cellular and developmental events are coordinated to generate normal structure and function. Alteration in any of these events can impact brain development, manifesting clinically as neurodevelopmental disorders. Human genetic disorders of lipid metabolism often present with features of altered brain function. Lowe syndrome (LS) is an X-linked recessive disease with features of altered brain function. LS results from mutations in OCRL1, which encodes a phosphoinositide 5-phosphatase enzyme. However, the cellular mechanisms by which loss of OCRL1 leads to brain defects remain unknown. Human brain development involves several cellular and developmental features not conserved in other species and understanding such mechanisms remains a challenge. Rodent models of LS have been generated but failed to recapitulate features of the human disease. Here we describe the generation of human stem cell lines from LS patients. Further, we present biochemical characterization of lipid metabolism in patient cell lines and demonstrate their use as a ‘disease-in-a-dish’ model for understanding the mechanism by which loss of OCRL1 leads to altered cellular and physiological brain development.

This article has an associated First Person interview with the first author of the paper.

Summary: A human iPSC resource to study the cellular basis of neurodevelopmental defects in Lowe syndrome.

SdhA blocks disruption of the Legionella-containing vacuole by hijacking the OCRL phosphatase

Legionella pneumophila grows intracellularly within a replication vacuole via action of Icm/Dot-secreted proteins. One such protein, SdhA, maintains the integrity of the vacuolar membrane, thereby preventing cytoplasmic degradation of bacteria. We show here that SdhA binds and blocks the action of OCRL (OculoCerebroRenal syndrome of Lowe), an inositol 5-phosphatase pivotal for controlling endosomal dynamics. OCRL depletion results in enhanced vacuole integrity and intracellular growth of a sdhA mutant, consistent with OCRL participating in vacuole disruption. Overexpressed SdhA alters OCRL function, enlarging endosomes, driving endosomal accumulation of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂), and interfering with endosomal trafficking. SdhA interrupts Rab guanosine triphosphatase (GTPase)-OCRL interactions by binding to the OCRL ASPM-SPD2-Hydin (ASH) domain, without directly altering OCRL 5-phosphatase activity. The Legionella vacuole encompassing the sdhA mutant accumulates OCRL and endosomal antigen EEA1 (Early Endosome Antigen 1), consistent with SdhA blocking accumulation of OCRL-containing endosomal vesicles. Therefore, SdhA hijacking of OCRL is associated with blocking trafficking events that disrupt the pathogen vacuole.

IPIP27A cooperates with OCRL to support endocytic traffic in the zebrafish pronephric tubule

Endocytosis is a fundamentally important process through which material is internalized into cells from the extracellular environment. In the renal proximal tubule, endocytosis of the abundant scavenger receptor megalin and its co-receptor cubilin play a vital role in retrieving low molecular weight proteins from the renal filtrate. Although we know much about megalin and its ligands, the machinery and mechanisms by which the receptor is trafficked through the endosomal system remain poorly defined. In this study, we show that Ipip27A, an interacting partner of the Lowe syndrome protein OCRL, is required for endocytic traffic of megalin within the proximal renal tubule of zebrafish larvae. Knockout of Ipip27A phenocopies the endocytic phenotype seen upon loss of OCRL, with a deficit in uptake of both fluid-phase and protein cargo, which is accompanied by a reduction in megalin abundance and altered endosome morphology. Rescue and co-depletion experiments indicate that Ipip27A functions together with OCRL to support proximal tubule endocytosis. The results therefore identify Ipip27A as a new player in endocytic traffic in the proximal tubule in vivo and support the view that defective endocytosis underlies the renal tubulopathy in Lowe syndrome and Dent-2 disease.

Lowe syndrome patient cells display mTOR- and RhoGTPase-dependent phenotypes alleviated by rapamycin and statins

Lowe syndrome (LS) is an X-linked developmental disease characterized by cognitive deficiencies, bilateral congenital cataracts and renal dysfunction. Unfortunately, this disease leads to the early death of affected children often due to kidney failure. Although this condition was first described in the early 1950s and the affected gene (OCRL1) was identified in the early 1990s, its pathophysiological mechanism is not fully understood and there is no LS-specific cure available to patients. Here we report two important signaling pathways affected in LS patient cells. While RhoGTPase signaling abnormalities led to adhesion and spreading defects as compared to normal controls, PI3K/mTOR hyperactivation interfered with primary cilia assembly (scenario also observed in other ciliopathies with compromised kidney function). Importantly, we identified two FDA-approved drugs able to ameliorate these phenotypes. Specifically, statins mitigated adhesion and spreading abnormalities while rapamycin facilitated ciliogenesis in LS patient cells. However, no single drug was able to alleviate both phenotypes. Based on these and other observations, we speculate that Ocrl1 has dual, independent functions supporting proper RhoGTPase and PI3K/mTOR signaling. Therefore, this study suggest that Ocrl1-deficiency leads to signaling defects likely to require combinatorial drug treatment to suppress patient phenotypes and symptoms.

A zebrafish-centric approach to antiepileptic drug development

Danio rerio (zebrafish) are a powerful experimental model for genetic and developmental studies. Adaptation of zebrafish to study seizures was initially established using the common convulsant agent pentylenetetrazole (PTZ). Larval PTZ-exposed zebrafish exhibit clear behavioral convulsions and abnormal electrographic activity, reminiscent of interictal and ictal epileptiform discharge. By using this model, our laboratory developed simple locomotion-based and electrophysiological assays to monitor and quantify seizures in larval zebrafish. Zebrafish also offer multiple advantages for rapid genetic manipulation and high-throughput phenotype-based drug screening. Combining these seizure assays with genetically modified zebrafish that represent Dravet syndrome, a rare genetic epilepsy, ultimately contributed to a phenotype-based screen of over 3500 drugs. Several drugs identified in these zebrafish screens are currently in clinical or compassionate-use trials. The emergence of this 'aquarium-to-bedside' approach suggests that broader efforts to adapt and improve upon this zebrafish-centric strategy can drive a variety of exciting new discoveries.

A 3D Renal Proximal Tubule on Chip Model Phenocopies Lowe Syndrome and Dent II Disease Tubulopathy

Lowe syndrome and Dent II disease are X-linked monogenetic diseases characterised by a renal reabsorption defect in the proximal tubules and caused by mutations in the OCRL gene, which codes for an inositol-5-phosphatase. The life expectancy of patients suffering from Lowe syndrome is largely reduced because of the development of chronic kidney disease and related complications. There is a need for physiological human in vitro models for Lowe syndrome/Dent II disease to study the underpinning disease mechanisms and to identify and characterise potential drugs and drug targets. Here, we describe a proximal tubule organ on chip model combining a 3D tubule architecture with fluid flow shear stress that phenocopies hallmarks of Lowe syndrome/Dent II disease. We demonstrate the high suitability of our in vitro model for drug target validation. Furthermore, using this model, we demonstrate that proximal tubule cells lacking OCRL expression upregulate markers typical for epithelial–mesenchymal transition (EMT), including the transcription factor SNAI2/Slug, and show increased collagen expression and deposition, which potentially contributes to interstitial fibrosis and disease progression as observed in Lowe syndrome and Dent II disease.

Deficiency in the endocytic adaptor proteins PHETA1/2 impairs renal and craniofacial development

A critical barrier in the treatment of endosomal and lysosomal diseases is the lack of understanding of the in vivo functions of the putative causative genes. We addressed this by investigating a key pair of endocytic adaptor proteins, PH domain-containing endocytic trafficking adaptor 1 and 2 (PHETA1/2; also known as FAM109A/B, Ses1/2, IPIP27A/B), which interact with the protein product of OCRL, the causative gene for Lowe syndrome. Here, we conducted the first study of PHETA1/2 in vivo, utilizing the zebrafish system. We found that impairment of both zebrafish orthologs, pheta1 and pheta2, disrupted endocytosis and ciliogenesis in renal tissues. In addition, pheta1/2 mutant animals exhibited reduced jaw size and delayed chondrocyte differentiation, indicating a role in craniofacial development. Deficiency of pheta1/2 resulted in dysregulation of cathepsin K, which led to an increased abundance of type II collagen in craniofacial cartilages, a marker of immature cartilage extracellular matrix. Cathepsin K inhibition rescued the craniofacial phenotypes in the pheta1/2 double mutants. The abnormal renal and craniofacial phenotypes in the pheta1/2 mutant animals were consistent with the clinical presentation of a patient with a de novo arginine (R) to cysteine (C) variant (R6C) of PHETA1. Expressing the patient-specific variant in zebrafish exacerbated craniofacial deficits, suggesting that the R6C allele acts in a dominant-negative manner. Together, these results provide insights into the in vivo roles of PHETA1/2 and suggest that the R6C variant is contributory to the pathogenesis of disease in the patient.This article has an associated First Person interview with the first author of the paper.

Incomplete cryptic splicing by an intronic mutation of OCRL in patients with partial phenotypes of Lowe syndrome

Mutations of OCRL cause Lowe syndrome, which is characterised by congenital cataracts, infantile hypotonia with mental retardation, and renal tubular dysfunction and Dent-2 disease, which only affects the kidney. While few patients with an intermediate phenotype between these diseases have been reported, the mechanism underlying variability in the phenotype is unclear. We identified an intronic mutation, c.2257-5G>A, in intron 20 of OCRL in an older brother with atypical Lowe syndrome without eye involvement and a younger brother with renal phenotype alone. This mutation created a splice acceptor motif that was accompanied by a cryptic premature termination codon at the junction of exons 20 and 21. The mutation caused incomplete alternative splicing, which created a small amount of wild-type transcript and a relatively large amount of alternatively spliced transcript with a premature termination codon. In the patients' cells, the alternatively spliced transcript was degraded by nonsense-mediated decay and the wild-type transcript was significantly decreased, but not completely depleted. These findings imply that an intronic mutation creating an incomplete alternative splicing acceptor site results in a relatively low level of wild-type OCRL mRNA expression, leading to partial phenotypes of Lowe syndrome.

Transcriptome analysis of neural progenitor cells derived from Lowe syndrome induced pluripotent stem cells: identification of candidate genes for the neurodevelopmental and eye manifestations

BACKGROUND

Lowe syndrome (LS) is caused by loss-of-function mutations in the X-linked gene OCRL, which codes for an inositol polyphosphate 5-phosphatase that plays a key role in endosome recycling, clathrin-coated pit formation, and actin polymerization. It is characterized by congenital cataracts, intellectual and developmental disability, and renal proximal tubular dysfunction. Patients are also at high risk for developing glaucoma and seizures. We recently developed induced pluripotent stem cell (iPSC) lines from three patients with LS who have hypomorphic variants affecting the 3' end of the gene, and their neurotypical brothers to serve as controls.

METHODS

In this study, we used RNA sequencing (RNA-seq) to obtain transcriptome profiles in LS and control neural progenitor cells (NPCs).

RESULTS

In a comparison of the patient and control NPCs (n = 3), we found 16 differentially expressed genes (DEGs) at the multiple test adjusted p value (padj) < 0.1, with nine at padj < 0.05. Using nominal p value < 0.05, 319 DEGs were detected. The relatively small number of DEGs could be due to the fact that OCRL is not a transcription factor per se, although it could have secondary effects on gene expression through several different mechanisms. Although the number of DEGs passing multiple test correction was small, those that were found are quite consistent with some of the known molecular effects of OCRL protein, and the clinical manifestations of LS. Furthermore, using gene set enrichment analysis (GSEA), we found that genes increased expression in the patient NPCs showed enrichments of several gene ontology (GO) terms (false discovery rate < 0.25): telencephalon development, pallium development, NPC proliferation, and cortex development, which are consistent with a condition characterized by intellectual disabilities and psychiatric manifestations. In addition, a significant enrichment among the nominal DEGs for genes implicated in autism spectrum disorder (ASD) was found (e.g., AFF2, DNER, DPP6, DPP10, RELN, CACNA1C), as well as several that are strong candidate genes for the development of eye problems found in LS, including glaucoma. The most notable example is EFEMP1, a well-known candidate gene for glaucoma and other eye pathologies.

CONCLUSION

Overall, the RNA-seq findings present several candidate genes that could help explain the underlying basis for the neurodevelopmental and eye problems seen in boys with LS.

A role for OCRL in glomerular function and disease

BACKGROUND

Lowe syndrome and Dent-2 disease are caused by mutations in the OCRL gene, which encodes for an inositol 5-phosphatase. The renal phenotype associated with OCRL mutations typically comprises a selective proximal tubulopathy, which can manifest as Fanconi syndrome in the most extreme cases.

METHODS

Here, we report a 12-year-old male with nephrotic-range proteinuria and focal segmental glomerulosclerosis on renal biopsy. As a glomerular pathology was suspected, extensive investigation of tubular function was not performed.

RESULTS

Surprisingly, whole exome sequencing identified a genetic variant in OCRL (c1467-2A>G) that introduced a novel splice mutation leading to skipping of exon 15. In situ hybridisation of adult human kidney tissue and zebrafish larvae showed OCRL expression in the glomerulus, supporting a role for OCRL in glomerular function. In cultured podocytes, we found that OCRL associated with the linker protein IPIP27A and CD2AP, a protein that is important for maintenance of the podocyte slit diaphragm.

CONCLUSION

Taken together, this work suggests a previously under-appreciated role for OCRL in glomerular function and highlights the importance of investigating tubular function in patients with persistent proteinuria.

Hypotonia and delayed motor development as an early presentation of Lowe syndrome: case report and literature review

We describe a boy who presented with neonatal hypotonia, followed by delayed motor development and growth impairment. Further evaluation revealed rickets caused by proximal renal tubular dysfunction. At age 3, the boy exhibited dysmorphic features and bilateral cataract. Genetic analysis of the OCRL gene showed a novel variant in exon 13: c.1250T>A, p.Val417Asp; in silico and segregation analysis confirmed the variant to be pathogenic, compatible with the diagnosis of the oculocerebrorenal syndrome of Lowe. Lowe syndrome is a rare multisystemic disorder; the diagnostic triad requires involvement of the eye, central nervous system and the proximal renal tubule. Typical clinical features are congenital cataract, glaucoma, hypotonia, mental and behavioral problems, benign skin lesions, platelet dysfunction and dental abnormalities. Phenotypic features early in life may be nonspecific, which is illustrated by this case with a late manifestation of cataract. Because an early diagnosis can lead to better counseling and treatment, we suggest urinary testing for proteinuria as a part of the evaluation of children with unexplained hypotonia.

Effects of Proximal Tubule Shortening on Protein Excretion in a Lowe Syndrome Model

BACKGROUND

Lowe syndrome (LS) is an X-linked recessive disorder caused by mutations in OCRL, which encodes the enzyme OCRL. Symptoms of LS include proximal tubule (PT) dysfunction typically characterized by low molecular weight proteinuria, renal tubular acidosis (RTA), aminoaciduria, and hypercalciuria. How mutant OCRL causes these symptoms isn't clear.

METHODS

We examined the effect of deleting OCRL on endocytic traffic and cell division in newly created human PT CRISPR/Cas9 OCRL knockout cells, multiple PT cell lines treated with OCRL-targeting siRNA, and in orcl-mutant zebrafish.

RESULTS

OCRL-depleted human cells proliferated more slowly and about 10% of them were multinucleated compared with fewer than 2% of matched control cells. Heterologous expression of wild-type, but not phosphatase-deficient, OCRL prevented the accumulation of multinucleated cells after acute knockdown of OCRL but could not rescue the phenotype in stably edited knockout cell lines. Mathematic modeling confirmed that reduced PT length can account for the urinary excretion profile in LS. Both ocrl mutant zebrafish and zebrafish injected with ocrl morpholino showed truncated expression of megalin along the pronephric kidney, consistent with a shortened S1 segment.

CONCLUSIONS

Our data suggest a unifying model to explain how loss of OCRL results in tubular proteinuria as well as the other commonly observed renal manifestations of LS. We hypothesize that defective cell division during kidney development and/or repair compromises PT length and impairs kidney function in LS patients.

The expanding spectrum of neurological disorders of phosphoinositide metabolism

Phosphoinositides (PIPs) are a ubiquitous group of seven low-abundance phospholipids that play a crucial role in defining localized membrane properties and that regulate myriad cellular processes, including cytoskeletal remodeling, cell signaling cascades, ion channel activity and membrane traffic. PIP homeostasis is tightly regulated by numerous inositol kinases and phosphatases, which phosphorylate and dephosphorylate distinct PIP species. The importance of these phospholipids, and of the enzymes that regulate them, is increasingly being recognized, with the identification of human neurological disorders that are caused by mutations in PIP-modulating enzymes. Genetic disorders of PIP metabolism include forms of epilepsy, neurodegenerative disease, brain malformation syndromes, peripheral neuropathy and congenital myopathy. In this Review, we provide an overview of PIP function and regulation, delineate the disorders associated with mutations in genes that modulate or utilize PIPs, and discuss what is understood about gene function and disease pathogenesis as established through animal models of these diseases.

Summary: This Review highlights the intersection between phosphoinositides and the enzymes that regulate their metabolism, which together are crucial regulators of myriad cellular processes and neurological disorders.

PTEN reduces endosomal PtdIns(4,5)P2 in a phosphatase-independent manner via a PLC pathway

This work reveals that the tumor suppressor PTEN acts through a PLC to reduce levels of endosomal PtdIns(4,5)P₂, its own enzymatic product. This pathway can be chemically activated to rescue OCRL1 depletion in several disease models of the Lowe syndrome, a rare multisystemic genetic disease.

The tumor suppressor PTEN dephosphorylates PtdIns(3,4,5)P₃ into PtdIns(4,5)P₂. Here, we make the unexpected discovery that in Drosophila melanogaster PTEN reduces PtdIns(4,5)P₂ levels on endosomes, independently of its phosphatase activity. This new PTEN function requires the enzymatic action of dPLCXD, an atypical phospholipase C. Importantly, we discovered that this novel PTEN/dPLCXD pathway can compensate for depletion of dOCRL, a PtdIns(4,5)P₂ phosphatase. Mutation of OCRL1, the human orthologue of dOCRL, causes oculocerebrorenal Lowe syndrome, a rare multisystemic genetic disease. Both OCRL1 and dOCRL loss have been shown to promote accumulation of PtdIns(4,5)P₂ on endosomes and cytokinesis defects. Here, we show that PTEN or dPLCXD overexpression prevents these defects. In addition, we found that chemical activation of this pathway restores normal cytokinesis in human Lowe syndrome cells and rescues OCRL phenotypes in a zebrafish Lowe syndrome model. Our findings identify a novel PTEN/dPLCXD pathway that controls PtdIns(4,5)P₂ levels on endosomes. They also point to a potential new strategy for the treatment of Lowe syndrome.

Zebrafish as a model for kidney function and disease

Kidney disease is a global problem with around three million people diagnosed in the UK alone and the incidence is rising. Research is critical to develop better treatments. Animal models can help to better understand the pathophysiology behind the various kidney diseases and to screen for therapeutic compounds, but the use especially of mammalian models should be minimised in the interest of animal welfare. Zebrafish are increasingly used, as they are genetically tractable and have a basic renal anatomy comparable to mammalian kidneys with glomerular filtration and tubular filtration processing. Here, we discuss how zebrafish have advanced the study of nephrology and the mechanisms underlying kidney disease.

Network Properties Revealed during Multi-Scale Calcium Imaging of Seizure Activity in Zebrafish

Seizures are characterized by hypersynchronization of neuronal networks. Understanding these networks could provide a critical window for therapeutic control of recurrent seizure activity, i.e., epilepsy. However, imaging seizure networks has largely been limited to microcircuits in vitro or small “windows” in vivo. Here, we combine fast confocal imaging of genetically encoded calcium indicator (GCaMP)-expressing larval zebrafish with local field potential (LFP) recordings to study epileptiform events at whole-brain and single-neuron levels in vivo. Using an acute seizure model (pentylenetetrazole, PTZ), we reliably observed recurrent electrographic ictal-like events associated with generalized activation of all major brain regions and uncovered a well-preserved anterior-to-posterior seizure propagation pattern. We also examined brain-wide network synchronization and spatiotemporal patterns of neuronal activity in the optic tectum microcircuit. Brain-wide and single-neuronal level analysis of PTZ-exposed and 4-aminopyridine (4-AP)-exposed zebrafish revealed distinct network dynamics associated with seizure and non-seizure hyperexcitable states, respectively. Neuronal ensembles, comprised of coactive neurons, were also uncovered during interictal-like periods. Taken together, these results demonstrate that macro- and micro-network calcium motifs in zebrafish may provide a greater understanding of epilepsy.

Modeling the neuropsychiatric manifestations of Lowe syndrome using induced pluripotent stem cells: defective F-actin polymerization and WAVE-1 expression in neuronal cells

Background

Lowe syndrome (LS) is a rare genetic disorder caused by loss of function mutations in the X-linked gene, OCRL, which codes for inositol polyphosphate 5-phosphatase. LS is characterized by the triad of congenital cataracts, neurodevelopmental impairment (primarily intellectual and developmental disabilities [IDD]), and renal proximal tubular dysfunction. Studies carried out over the years have shown that hypomorphic mutations in OCRL adversely affect endosome recycling and actin polymerization in kidney cells and patient-derived fibroblasts. The renal problem has been traced to an impaired recycling of megalin, a multi-ligand receptor that plays a key role in the reuptake of lipoproteins, amino acids, vitamin-binding proteins, and hormones. However, the neurodevelopmental aspects of the disorder have been difficult to study because the mouse knockout (KO) model does not display LS-related phenotypes. Fortunately, the discovery of induced pluripotent stem (iPS) cells has provided an opportunity to grow patient-specific neurons, which can be used to model neurodevelopmental disorders in vitro, as demonstrated in the many studies that have been published in the past few years in autism spectrum disorders (ASD), schizophrenia (SZ), bipolar disorder (BD), and IDD.

Methods

We now report the first findings in neurons and neural progenitor cells (NPCs) generated from iPS cells derived from patients with LS and their typically developing male siblings, as well as an isogenic line in which the OCRL gene has been incapacitated by a null mutation generated using CRISPR-Cas9 gene editing.

Results

We show that neuronal cells derived from patient-specific iPS cells containing hypomorphic variants are deficient in their capacity to produce F-filamentous actin (F-actin) fibers. Abnormalities were also found in the expression of WAVE-1, a component of the WAVE regulatory complex (WRC) that regulates actin polymerization. Curiously, neuronal cells carrying the engineered OCRL null mutation, in which OCRL protein is not expressed, did not show similar defects in F-actin and WAVE-1 expression. This is similar to the apparent lack of a phenotype in the mouse Ocrl KO model, and suggests that in the complete absence of OCRL protein, as opposed to producing a dysfunctional protein, as seen with the hypomorphic variants, there is partial compensation for the F-actin/WAVE-1 regulating function of OCRL.

Conclusions

Alterations in F-actin polymerization and WRC have been found in a number of genetic subgroups of IDD and ASD. Thus, LS, a very rare genetic condition, is linked to a more expansive family of genes responsible for neurodevelopmental disorders that have shared pathogenic features.

Electronic supplementary material

The online version of this article (10.1186/s13229-018-0227-3) contains supplementary material, which is available to authorized users.

dOCRL maintains immune cell quiescence by regulating endosomal traffic

Lowe Syndrome is a developmental disorder characterized by eye, kidney, and neurological pathologies, and is caused by mutations in the phosphatidylinositol-5-phosphatase OCRL. OCRL plays diverse roles in endocytic and endolysosomal trafficking, cytokinesis, and ciliogenesis, but it is unclear which of these cellular functions underlie specific patient symptoms. Here, we show that mutation of Drosophila OCRL causes cell-autonomous activation of hemocytes, which are macrophage-like cells of the innate immune system. Among many cell biological defects that we identified in docrl mutant hemocytes, we pinpointed the cause of innate immune cell activation to reduced Rab11-dependent recycling traffic and concomitantly increased Rab7-dependent late endosome traffic. Loss of docrl amplifies multiple immune-relevant signals, including Toll, Jun kinase, and STAT, and leads to Rab11-sensitive mis-sorting and excessive secretion of the Toll ligand Spåtzle. Thus, docrl regulation of endosomal traffic maintains hemocytes in a poised, but quiescent state, suggesting mechanisms by which endosomal misregulation of signaling may contribute to symptoms of Lowe syndrome.

Lowe syndrome is a developmental disorder characterized by severe kidney, eye, and neurological symptoms, and is caused by mutations in the gene OCRL. OCRL has been shown to control many steps of packaging and transport of materials within cells, though it remains unclear which of these disrupted transport steps cause each of the many symptoms in Lowe syndrome patients. We found that in fruit flies, loss of OCRL caused transport defects at specific internal compartments in innate immune cells, resulting in amplification of multiple critical inflammatory signals. Similar inflammatory signals have been implicated in forms of epilepsy, which is a primary symptom in Lowe syndrome patients. Thus, our work uncovers a new function for OCRL in animals, and opens an exciting new avenue of investigation into how loss of OCRL causes the symptoms of Lowe syndrome.

Understanding taurine CNS activity using alternative zebrafish models

Taurine is a highly abundant "amino acid" in the brain. Despite the potential neuroactive role of taurine in vertebrates has long been recognized, the underlying molecular mechanisms related to its pleiotropic effects in the brain remain poorly understood. Due to the genetic tractability, rich behavioral repertoire, neurochemical conservation, and small size, the zebrafish (Danio rerio) has emerged as a powerful candidate for neuropsychopharmacology investigation and in vivo drug screening. Here, we summarize the main physiological roles of taurine in mammals, including neuromodulation, osmoregulation, membrane stabilization, and antioxidant action. In this context, we also highlight how zebrafish models of brain disorders may present interesting approaches to assess molecular mechanisms underlying positive effects of taurine in the brain. Finally, we outline recent advances in zebrafish drug screening that significantly improve neuropsychiatric translational researches and small molecule screens.

The 5-phosphatase OCRL in Lowe syndrome and Dent disease 2

Lowe syndrome is an X-linked disease that is characterized by congenital cataracts, central hypotonia, intellectual disability and renal Fanconi syndrome. The disease is caused by mutations in OCRL, which encodes an inositol polyphosphate 5-phosphatase (OCRL) that acts on phosphoinositides - quantitatively minor constituents of cell membranes that are nonetheless pivotal regulators of intracellular trafficking. In this Review we summarize the considerable progress made over the past decade in understanding the cellular roles of OCRL in regulating phosphoinositide balance along the endolysosomal pathway, a fundamental system for the reabsorption of proteins and solutes by proximal tubular cells. We discuss how studies of OCRL have led to important discoveries about the basic mechanisms of membrane trafficking and describe the key features and limitations of the currently available animal models of Lowe syndrome. Mutations in OCRL can also give rise to a milder pathology, Dent disease 2, which is characterized by renal Fanconi syndrome in the absence of extrarenal pathologies. Understanding how mutations in OCRL give rise to two clinical entities with differing extrarenal manifestations represents an opportunity to identify molecular pathways that could be targeted to develop treatments for these conditions.

Ocular Pathology of Oculocerebrorenal Syndrome of Lowe: Novel Mutations and Genotype-Phenotype Analysis

Mutations in the OCRL1 gene result in the oculocerebrorenal syndrome of Lowe, with symptoms including congenital bilateral cataracts, glaucoma, renal failure, and neurological impairments. OCRL1 encodes an inositol polyphosphate 5-phosphatase which preferentially dephosphorylates phosphatidylinositide 4,5 bisphosphate (PI(4,5)P₂). We have identified two novel mutations in two unrelated Lowe syndrome patients with congenital glaucoma. Novel deletion mutations are detected at c.739-742delAAAG in Lowe patient 1 and c.1595-1631del in Lowe patient 2. End stage glaucoma in patient 2 resulted in the enucleation of the eye, which on histology demonstrated corneal keloid, fibrous infiltration of the angle, ectropion uvea, retinal gliosis, and retinal ganglion cell loss. We measured OCRL protein levels in patient keratinocytes and found that Lowe 1 patient cells had significantly reduced OCRL protein as compared to the control keratinocytes. Genotype-phenotype correlation of OCRL1 mutations associated with congenital glaucoma revealed clustering of missense and deletion mutations in the 5-phosphatase domain and the RhoGAP-like domain. In conclusion, we report novel OCRL1 mutations in Lowe syndrome patients and the corresponding histopathologic analysis of one patient’s ocular pathology.

Ion transport in the zebrafish kidney from a human disease angle: possibilities, considerations, and future perspectives

Unique experimental advantages, such as its embryonic/larval transparency, high-throughput nature, and ease of genetic modification, underpin the rapid emergence of the zebrafish ( Danio rerio) as a preeminent model in biomedical research. Particularly in the field of nephrology, the zebrafish provides a promising model for studying the physiological implications of human solute transport processes along consecutive nephron segments. However, although the zebrafish might be considered a valuable model for numerous renal ion transport diseases and functional studies of many channels and transporters, not all human renal electrolyte transport mechanisms and human diseases can be modeled in the zebrafish. With this review, we explore the ontogeny of zebrafish renal ion transport, its nephron structure and function, and thereby demonstrate the clinical translational value of this model. By critical assessment of genomic and amino acid conservation of human proteins involved in renal ion handling (channels, transporters, and claudins), kidney and nephron segment conservation, and renal electrolyte transport physiology in the zebrafish, we provide researchers and nephrologists with an indication of the possibilities and considerations of the zebrafish as a model for human renal ion transport. Combined with advanced techniques envisioned for the future, implementation of the zebrafish might expand beyond unraveling pathophysiological mechanisms that underlie distinct genetic or environmentally, i.e., pharmacological and lifestyle, induced renal transport deficits. Specifically, the ease of drug administration and the exploitation of improved genetic approaches might argue for the adoption of the zebrafish as a model for preclinical personalized medicine for distinct renal diseases and renal electrolyte transport proteins.

The role of the Lowe syndrome protein OCRL in the endocytic pathway

Mutations of the inositol-5-phosphatase OCRL cause Lowe syndrome and Dent-II disease. Both are rare genetic disorders characterized by renal defects. Lowe syndrome is furthermore characterized by defects of the eye (congenital cataracts) and nervous system (mental disabilities, hypotonia). OCRL has been localised to various endocytic compartments suggesting impairments in the endocytic pathway as possible disease mechanism. Recent evidence strongly supports this view and shows essential roles of OCRL at clathrin coated pits, transport of cargo from endosomes to the trans-Golgi network as well as recycling of receptors from endosomes to the plasma membrane. In particular in vitro and in vivo evidence demonstrates an important role of OCRL in recycling of megalin, a multi-ligand receptor crucial for reabsorption of nutrients in the proximal tubulus, a process severely impaired in Lowe syndrome patients. Thus defects in the endocytic pathway are likely to significantly contribute to the kidney phenotype in Lowe syndrome and Dent-II disease.

Autophagosome-lysosome fusion triggers a lysosomal response mediated by TLR9 and controlled by OCRL

Phosphoinositides (PtdIns) control fundamental cell processes, and inherited defects of PtdIns kinases or phosphatases cause severe human diseases, including Lowe syndrome due to mutations in OCRL, which encodes a PtdIns(4,5)P2 5-phosphatase. Here we unveil a lysosomal response to the arrival of autophagosomal cargo in which OCRL plays a key part. We identify mitochondrial DNA and TLR9 as the cargo and the receptor that triggers and mediates, respectively, this response. This lysosome-cargo response is required to sustain the autophagic flux and involves a local increase in PtdIns(4,5)P2 that is confined in space and time by OCRL. Depleting or inhibiting OCRL leads to an accumulation of lysosomal PtdIns(4,5)P2, an inhibitor of the calcium channel mucolipin-1 that controls autophagosome-lysosome fusion. Hence, autophagosomes accumulate in OCRL-depleted cells and in the kidneys of Lowe syndrome patients. Importantly, boosting the activity of mucolipin-1 with selective agonists restores the autophagic flux in cells from Lowe syndrome patients.

Advancing epilepsy treatment through personalized genetic zebrafish models

With an increase in the number of disease causing genetic mutations identified from epilepsy cohorts, zebrafish are proving to be an attractive vertebrate model for functional analysis of these allele variants. Not only do zebrafish have conserved gene functions, but larvae harboring mutations in identified human epileptic genes show spontaneous seizure activity and mimic the convulsive behavioral movements observed in humans. With zebrafish being compatible with medium to high-throughput screening, they are also proving to be a unique and powerful system for early preclinical drug screening, including novel target identification, pharmacology, and toxicology. Additionally, with recent advances in genomic engineering technologies, it is now possible to study the precise pathophysiology of patient-specific gene mutations in zebrafish. The following sections highlight how the unique attributes of zebrafish, in combination with genetic modifications, are continuing to transform our understanding of epilepsy and help identify personalized therapeutics for specific patient cohorts.

Epilepsy, Behavioral Abnormalities, and Physiological Comorbidities in Syntaxin-Binding Protein 1 (STXBP1) Mutant Zebrafish

Mutations in the synaptic machinery gene syntaxin-binding protein 1, STXBP1 (also known as MUNC18-1), are linked to childhood epilepsies and other neurodevelopmental disorders. Zebrafish STXBP1 homologs (stxbp1a and stxbp1b) have highly conserved sequence and are prominently expressed in the larval zebrafish brain. To understand the functions of stxbp1a and stxbp1b, we generated loss-of-function mutations using CRISPR/Cas9 gene editing and studied brain electrical activity, behavior, development, heart physiology, metabolism, and survival in larval zebrafish. Homozygous stxbp1a mutants exhibited a profound lack of movement, low electrical brain activity, low heart rate, decreased glucose and mitochondrial metabolism, and early fatality compared to controls. On the other hand, homozygous stxbp1b mutants had spontaneous electrographic seizures, and reduced locomotor activity response to a movement-inducing “dark-flash” visual stimulus, despite showing normal metabolism, heart rate, survival, and baseline locomotor activity. Our findings in these newly generated mutant lines of zebrafish suggest that zebrafish recapitulate clinical phenotypes associated with human syntaxin-binding protein 1 mutations.

RNA polymerase III component Rpc9 regulates hematopoietic stem and progenitor cell maintenance in zebrafish

Hematopoietic stem and progenitor cells (HSPCs) are capable of self-renewal and replenishing all lineages of blood cells throughout the lifetime and thus critical for tissue homeostasis. However, the mechanism regulating HSPC development is still incompletely understood. Here, we isolate a zebrafish mutant with defective T lymphopoiesis and positional cloning identifies that Rpc9, a component of DNA-directed RNA polymerase III (Pol III) complex, is responsible for the mutant phenotype. Further analysis shows that rpc9-deficiency leads to the impairment of HSPCs and their derivatives in zebrafish embryos. Excessive apoptosis is observed in the caudal hematopoietic tissue (CHT, the equivalent of fetal liver in mammals) of rpc9−/− embryos and the hematopoietic defects in rpc9−/− embryos can be fully rescued by suppression of p53. Thus, our work illustrate that Rpc9, a component of Pol III, plays an important tissue-specific role in HSPC maintenance during zebrafish embryogenesis and that it might be conserved across vertebrates including mammals.

Methods for studying the zebrafish brain: past, present and future

The zebrafish (Danio rerio) is one of the most promising new model organisms. The increasing popularity of this amazing small vertebrate is evident from the exponentially growing numbers of research articles, funded projects and new discoveries associated with the use of zebrafish for studying development, brain function, human diseases and screening for new drugs. Thanks to the development of novel technologies, the range of zebrafish research is constantly expanding with new tools synergistically enhancing traditional techniques. In this review we will highlight the past and present techniques which have made, and continue to make, zebrafish an attractive model organism for various fields of biology, with a specific focus on neuroscience.

The Lowe syndrome protein OCRL1 is required for endocytosis in the zebrafish pronephric tubule

Lowe syndrome and Dent-2 disease are caused by mutation of the inositol 5-phosphatase OCRL1. Despite our increased understanding of the cellular functions of OCRL1, the underlying basis for the renal tubulopathy seen in both human disorders, of which a hallmark is low molecular weight proteinuria, is currently unknown. Here, we show that deficiency in OCRL1 causes a defect in endocytosis in the zebrafish pronephric tubule, a model for the mammalian renal tubule. This coincides with a reduction in levels of the scavenger receptor megalin and its accumulation in endocytic compartments, consistent with reduced recycling within the endocytic pathway. We also observe reduced numbers of early endocytic compartments and enlarged vacuolar endosomes in the sub-apical region of pronephric cells. Cell polarity within the pronephric tubule is unaffected in mutant embryos. The OCRL1-deficient embryos exhibit a mild ciliogenesis defect, but this cannot account for the observed impairment of endocytosis. Catalytic activity of OCRL1 is required for renal tubular endocytosis and the endocytic defect can be rescued by suppression of PIP5K. These results indicate for the first time that OCRL1 is required for endocytic trafficking in vivo, and strongly support the hypothesis that endocytic defects are responsible for the renal tubulopathy in Lowe syndrome and Dent-2 disease. Moreover, our results reveal PIP5K as a potential therapeutic target for Lowe syndrome and Dent-2 disease.

Phosphoinositide lipids are key regulators of cellular physiology and consequently enzymes that generate or remove these lipids are of fundamental importance. Mutation of one such enzyme, called OCRL1, causes two disorders in humans, Lowe syndrome and Dent-2 disease. However, the underlying mechanisms remain poorly defined. Here, we demonstrate that OCRL1 regulates endocytosis, the process by which cells internalize material from their extracellular environment. Importantly, this is demonstrated in a physiologically relevant tissue in vivo, namely the zebrafish renal tubule. Defective endocytosis can explain the renal symptoms seen in Lowe syndrome and Dent-2 patients. We also report that defects in cell polarity or cilia formation cannot explain the renal symptoms. This study not only increases our understanding of the endocytic pathway, it also provides a mechanistic explanation for the renal defects observed in Lowe syndrome and Dent-2 patients.

Cross-species pharmacological characterization of the allylglycine seizure model in mice and larval zebrafish

Treatment-resistant seizures affect about a third of patients suffering from epilepsy. To fulfill the need for new medications targeting treatment-resistant seizures, a number of rodent models offer the opportunity to assess a variety of potential treatment approaches. The use of such models, however, has proven to be time-consuming and labor-intensive. In this study, we performed pharmacological characterization of the allylglycine (AG) seizure model, a simple in vivo model for which we demonstrated a high level of treatment resistance. (d,l)-Allylglycine inhibits glutamic acid decarboxylase (GAD) - the key enzyme in γ-aminobutyric acid (GABA) biosynthesis - leading to GABA depletion, seizures, and neuronal damage. We performed a side-by-side comparison of mouse and zebrafish acute AG treatments including biochemical, electrographic, and behavioral assessments. Interestingly, seizure progression rate and GABA depletion kinetics were comparable in both species. Five mechanistically diverse antiepileptic drugs (AEDs) were used. Three out of the five AEDs (levetiracetam, phenytoin, and topiramate) showed only a limited protective effect (mainly mortality delay) at doses close to the TD50 (dose inducing motor impairment in 50% of animals) in mice. The two remaining AEDs (diazepam and sodium valproate) displayed protective activity against AG-induced seizures. Experiments performed in zebrafish larvae revealed behavioral AED activity profiles highly analogous to those obtained in mice. Having demonstrated cross-species similarities and limited efficacy of tested AEDs, we propose the use of AG in zebrafish as a convenient and high-throughput model of treatment-resistant seizures.

Animal models in epilepsy research: legacies and new directions

Human epilepsies encompass a wide variety of clinical, behavioral and electrical manifestations. Correspondingly, studies of this disease in nonhuman animals have brought forward an equally wide array of animal models; that is, species and acute or chronic seizure induction protocols. Epilepsy research has a long history of comparative anatomical and physiological studies on a range of mostly mammalian species. Nonetheless, a relatively limited number of rodent models have emerged as the primary choices for most investigations. In many cases, these animal models are selected on the basis of convenience or tradition, although technical or experimental rationale does, and should, factor into these decisions. More complex mammalian brains and genetic model organisms including zebrafish have been studied less, but offer substantial advantages that are becoming widely recognized.

Mendelian disorders of PI metabolizing enzymes

More than twenty different genetic diseases have been described that are caused by mutations in phosphoinositide metabolizing enzymes, mostly in phosphoinositide phosphatases. Although generally ubiquitously expressed, mutations in these enzymes, which are mainly loss-of-function, result in tissue-restricted clinical manifestations through mechanisms that are not completely understood. Here we analyze selected disorders of phosphoinositide metabolism grouped according to the principle tissue affected: the nervous system, muscle, kidney, the osteoskeletal system, the eye, and the immune system. We will highlight what has been learnt so far from the study of these disorders about not only the cellular and molecular pathways that are involved or are governed by phosphoinositides, but also the many gaps that remain to be filled to gain a full understanding of the pathophysiological mechanisms underlying the clinical manifestations of this steadily growing class of diseases, most of which still remain orphan in terms of treatment. This article is part of a Special Issue entitled Phosphoinositides.

Epilepsy research methods update: Understanding the causes of epileptic seizures and identifying new treatments using non-mammalian model organisms

This narrative review is intended to introduce clinicians treating epilepsy and researchers familiar with mammalian models of epilepsy to experimentally tractable, non-mammalian research models used in epilepsy research, ranging from unicellular eukaryotes to more complex multicellular organisms. The review focuses on four model organisms: the social amoeba Dictyostelium discoideum, the roundworm Caenorhabditis elegans, the fruit fly Drosophila melanogaster and the zebrafish Danio rerio. We consider recent discoveries made with each model organism and discuss the importance of these advances for the understanding and treatment of epilepsy in humans. The relative ease with which mutations in genes of interest can be produced and studied quickly and cheaply in these organisms, together with their anatomical and physiological simplicity in comparison to mammalian species, are major advantages when researchers are trying to unravel complex disease mechanisms. The short generation times of most of these model organisms also mean that they lend themselves particularly conveniently to the investigation of drug effects or epileptogenic processes across the lifecourse.

OCRL-mutated fibroblasts from patients with Dent-2 disease exhibit INPP5B-independent phenotypic variability relatively to Lowe syndrome cells

OCRL mutations are associated with both Lowe syndrome and Dent-2 disease, two rare X-linked conditions. Lowe syndrome is an oculo-cerebro-renal disorder, whereas Dent-2 patients mainly present renal proximal tubulopathy. Loss of OCRL-1, a phosphoinositide-5-phosphatase, leads in Lowe patients' fibroblasts to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) accumulation, with defects in F-actin network, α-actinin distribution and ciliogenesis, whereas fibroblasts of Dent-2 patients are still uncharacterized. To search for mechanisms linked to clinical variability observed between these two OCRL mutation-associated pathologies, we compared dermal fibroblasts from independent patients, four affected by Dent-2 disease and six with Lowe syndrome. For the first time, we describe that Dent-2 fibroblasts with OCRL loss-of-function (LOF) mutations exhibit decrease in actin stress fibers, appearance of punctate α-actinin signals and alteration in primary cilia formation. Interestingly, we quantified these phenotypes as clearly intermediate between Lowe and control fibroblasts, thus suggesting that levels of these defects correlate with clinical variations observed between patients with OCRL mutations. In addition, we show that Lowe and Dent-2 fibroblasts display similar PI(4,5)P2 accumulation levels. Finally, we analyzed INPP5B, a paralogous gene already reported to exhibit functional redundancy with OCRL, and report neither differences in its expression at RNA or protein levels, nor specific allelic variations between fibroblasts of patients. Altogether, we describe here differential phenotypes between fibroblasts from Lowe and Dent-2 patients, both associated with OCRL LOF mutations, we exclude direct roles of PI(4,5)P2 and INPP5B in this phenotypic variability and we underline potential key alterations leading to ocular and neurological clinical features in Lowe syndrome.