Nephrocytes are part of the spectrum of filtration epithelial diversity

Combined transcriptome and proteome profiling reveal cell-type-specific functions of Drosophila garland and pericardial nephrocytes

Drosophila nephrocytes are specialised cells that share critical functional, morphological, and molecular features with mammalian podocytes. Accordingly, nephrocytes represent a preferred invertebrate model for human glomerular disease. Here, we established a method for cell-specific isolation of the two types of Drosophila nephrocytes, garland and pericardial cells, from animals of different developmental stages and ages. Mass spectrometry-based proteomics and RNA-Seq-based transcriptomics were applied to characterise the proteome and transcriptome of the respective cells in an integrated and complementary manner. We observed characteristic changes in the proteome and transcriptome due to cellular ageing. Furthermore, functional enrichment analyses suggested that larval and adult nephrocytes, as well as garland and pericardial nephrocytes, fulfil distinct physiological functions. In addition, the pericardial nephrocytes were characterised by transcriptomic and proteomic profiles suggesting an atypical energy metabolism with very low oxidative phosphorylation rates. Moreover, the nephrocytes displayed typical signatures of extensive immune signalling and showed an active antimicrobial response to an infection. Factor-specific comparisons identified novel candidate proteins either expressed and secreted by the nephrocytes or sequestered by them. The data generated in this study represent a valuable basis for a more specific application of the Drosophila model in analysing renal cell function in health and disease.

Ultrastructural analysis of whole glomeruli using array tomography

The renal glomerulus produces primary urine from blood plasma by ultrafiltration. The ultrastructure of the glomerulus is closely related to filtration function and disease development. The ultrastructure of glomeruli has mainly been evaluated using transmission electron microscopy; however, the volume that can be observed using transmission electron microscopy is extremely limited relative to the total volume of the glomerulus. Consequently, observing structures that exist in only one location in each glomerulus, such as the vascular pole, and evaluating low-density or localized lesions are challenging tasks. Array tomography (AT) is a technique used to analyze the ultrastructure of tissues and cells via scanning electron microscopy of serial sections. In this study, we present an AT workflow that is optimized for observing complete serial sections of the whole glomerulus, and we share several analytical examples that use the optimized AT workflow, demonstrating the usefulness of this approach. Overall, this AT workflow can be a powerful tool for structural and pathological evaluation of the glomerulus. This workflow is also expected to provide new insights into the ultrastructure of the glomerulus and its constituent cells.

Disruption of the Rab7-Dependent Final Common Pathway of Endosomal and Autophagic Processing Results in a Severe Podocytopathy

SIGNIFICANCE STATEMENT

Endocytosis, recycling, and degradation of proteins are essential functions of mammalian cells, especially for terminally differentiated cells with limited regeneration rates and complex morphology, such as podocytes. To improve our understanding on how disturbances of these trafficking pathways are linked to podocyte depletion and slit diaphragm (SD) injury, the authors explored the role of the small GTPase Rab7, which is linked to endosomal, lysosomal, and autophagic pathways, using as model systems mice and Drosophila with podocyte-specific or nephrocyte-specific loss of Rab7, and a human podocyte cell line depleted for Rab7. Their findings point to maturation and fusion events during endolysosomal and autophagic maturation as key processes for podocyte homeostasis and function and identify altered lysosomal pH values as a putative novel mechanism for podocytopathies.

BACKGROUND

Endocytosis, recycling, and degradation of proteins are essential functions of mammalian cells, especially for terminally differentiated cells with limited regeneration rates, such as podocytes. How disturbances within these trafficking pathways may act as factors in proteinuric glomerular diseases is poorly understood.

METHODS

To explore how disturbances in trafficking pathways may act as factors in proteinuric glomerular diseases, we focused on Rab7, a highly conserved GTPase that controls the homeostasis of late endolysosomal and autophagic processes. We generated mouse and Drosophila in vivo models lacking Rab7 exclusively in podocytes or nephrocytes, and performed histologic and ultrastructural analyses. To further investigate Rab7 function on lysosomal and autophagic structures, we used immortalized human cell lines depleted for Rab7.

RESULTS

Depletion of Rab7 in mice, Drosophila , and immortalized human cell lines resulted in an accumulation of diverse vesicular structures resembling multivesicular bodies, autophagosomes, and autoendolysosomes. Mice lacking Rab7 developed a severe and lethal renal phenotype with early-onset proteinuria and global or focal segmental glomerulosclerosis, accompanied by an altered distribution of slit diaphragm proteins. Remarkably, structures resembling multivesicular bodies began forming within 2 weeks after birth, prior to the glomerular injuries. In Drosophila nephrocytes, Rab7 knockdown resulted in the accumulation of vesicles and reduced slit diaphragms. In vitro , Rab7 knockout led to similar enlarged vesicles and altered lysosomal pH values, accompanied by an accumulation of lysosomal marker proteins.

CONCLUSIONS

Disruption within the final common pathway of endocytic and autophagic processes may be a novel and insufficiently understood mechanism regulating podocyte health and disease.

The Pathophysiologic Role of Gelsolin in Chronic Kidney Disease: Focus on Podocytes

Chronic kidney disease (CKD) is normally related to proteinuria, a common finding in a compromised glomerular filtration barrier (GFB). GFB is a structure composed of glomerular endothelial cells, the basement membrane, and the podocytes. CKD with podocyte damage may be associated with actin cytoskeleton reorganization, resulting in podocyte effacement. Gelsolin plays a critical role in several diseases, including cardiovascular diseases and cancer. Our current study aimed to determine the connection between gelsolin and podocyte, and thus the mechanism underlying podocyte injury in CKD. Experiments were carried out on Drosophila to demonstrate whether gelsolin had a physiological role in maintaining podocyte. Furthermore, the survival rate of gelsolin-knocked down Drosophila larvae was extensively reduced after AgNO₃ exposure. Secondly, the in vitro podocytes treated with puromycin aminonucleoside (PAN) enhanced the gelsolin protein expression, as well as small GTPase RhoA and Rac1, which also regulated actin dynamic expression incrementally with the PAN concentrations. Thirdly, we further demonstrated in vivo that GSN was highly expressed inside the glomeruli with mitochondrial dysfunction in a CKD mouse model. Our findings suggest that an excess of gelsolin may contribute to podocytes damage in glomeruli.

Molecular evidence for a single origin of ultrafiltration-based excretory organs

Excretion is an essential physiological process, carried out by all living organisms, regardless of their size or complexity.^1-3 Both protostomes (e.g., flies and flatworms) and deuterostomes (e.g., humans and sea urchins) possess specialized excretory organs serving that purpose. Those organs exhibit an astonishing diversity, ranging from units composed of just few distinct cells (e.g., protonephridia) to complex structures, built by millions of cells of multiple types with divergent morphology and function (e.g., vertebrate kidneys).^4,5 Although some molecular similarities between the development of kidneys of vertebrates and the regeneration of the protonephridia of flatworms have been reported,^6,7 the molecular underpinnings of the development of excretory organs have never been systematically studied in a comparative context.⁴ Here, we show that a set of transcription factors (eya, six1/2, pou3, sall, lhx1/5, and osr) and structural proteins (nephrin, kirre, and zo1) is expressed in the excretory organs of a phoronid, brachiopod, annelid, onychophoran, priapulid, and hemichordate that represent major protostome lineages and non-vertebrate deuterostomes. We demonstrate that the molecular similarity observed in the vertebrate kidney and flatworm protonephridia^6,7 is also seen in the developing excretory organs of those animals. Our results show that all types of ultrafiltration-based excretory organs are patterned by a conserved set of developmental genes, an observation that supports their homology. We propose that the last common ancestor of protostomes and deuterostomes already possessed an ultrafiltration-based organ that later gave rise to the vast diversity of extant excretory organs, including both proto- and metanephridia.

Three-Dimensional Architecture of Glomerular Endothelial Cells Revealed by FIB-SEM Tomography

Focused-ion beam-scanning electron microscopic (FIB-SEM) tomography enables easier acquisition of a series of ultrastructural, sectional images directly from resin-embedded biological samples. In this study, to clarify the three-dimensional (3D) architecture of glomerular endothelial cells (GEnCs) in adult rats, we manually extracted GEnCs from serial FIB-SEM images and reconstructed them on an Amira reconstruction software. The luminal and basal surface structures were clearly visualized in the reconstructed GEnCs, although only the luminal surface structures could be observed by conventional SEM. The luminal surface visualized via the reconstructed GEnCs was quite similar to that observed through conventional SEM, indicating that 3D reconstruction could be performed with high accuracy. Thus, we successfully described the 3D architecture of normal GEnCs in adult rats more clearly and precisely than ever before. The GEnCs were found to consist of three major subcellular compartments, namely, the cell body, cytoplasmic ridges, and sieve plates, in addition to two associated subcellular compartments, namely, the globular protrusions and reticular porous structures. Furthermore, most individual GEnCs made up a “seamless” tubular shape, and some of them formed an autocellular junction to make up a tubular shape. FIB-SEM tomography with reconstruction is a powerful approach to better understand the 3D architecture of GEnCs. Moreover, the morphological information revealed in this study will be valuable for the 3D pathologic evaluation of GEnCs in animal and human glomerular diseases and the structural analysis of developmental processes in the glomerular capillary system.