NEPH2 Is Located at the Glomerular Slit Diaphragm, Interacts with Nephrin and Is Cleaved from Podocytes by Metalloproteinases

A critical role of the podocyte cytoskeleton in the pathogenesis of glomerular proteinuria and autoimmune podocytopathies

Selective glomerular filtration relies on the membrane separating the glomerular arterioles from the Bowman space. As a major component of the glomerular filtration barrier, podocytes form foot processes by the actin cytoskeleton, which dynamically adjusts in response to environmental changes to maintain filtration barrier integrity. The slit diaphragms bridge the filtration slits between neighboring foot processes and act as signaling hubs interacting with the actin cytoskeleton. Focal adhesions relay signals to regulate actin dynamics while allowing podocyte adherence to the basement membrane. Mutations in actin regulatory and signaling proteins may disrupt the actin cytoskeleton, resulting in foot process retraction, effacement, and proteinuria. Large-scale gene expression profiling platforms, transgenic animal models, and other in vivo gene delivery methods now enhance our understanding of the interactions among podocyte focal adhesions, slit diaphragms, and actin dynamics. In addition, our team found that at least 66% of idiopathic nephrotic syndrome (INS) children have podocyte autoantibodies, which was defined as a new disease subgroup-, autoimmune podocytopathies. This review outlines the pathophysiological mechanisms of podocyte cytoskeleton protein interactions in proteinuria and glomerular podocytopathy.

Triptolide Alleviates Podocyte Epithelial-Mesenchymal Transition via Kindlin-2 and EMT-Related TGF-β/Smad Signaling Pathway in Diabetic Kidney Disease

Diabetes-induced chronic kidney diseases are widespread and decrease the quality of life for millions of affected individuals in China. To date, no therapies effectively alleviate these conditions. Triptolide, a traditionally used Chinese medicine, has shown promise in treating renal diseases. Here, the study aimed to decipher the exact mechanism by which it functions. It was hypothesized that triptolide might prevent the epithelial-mesenchymal transition (EMT) of podocytes by activating the kindlin-2 and TGF-β/Smad pathways. Triptolide or telmisartan was intragastrically administered to 9-week-old db/db and dm/dm mice with diabetic nephropathy (DN) for 12 weeks. In addition, biochemical parameters and body weight were detected. WT-1, nephrin, podocin, E-cadherin, and α-SMA were determined by immunohistochemistry in the renal tissues of treated mice. Protein and mRNA expression of podocyte EMT markers, kindlin-2 and TGF-β/Smad, were analyzed to elucidate the underlying mechanism. It was observed that triptolide treatment relieved structural injuries and functional variations in diabetic mice. It also increased the protein and mRNA levels of nephrin, podocin, and E-cadherin and decreased the expression of α-SMA in diabetic mice. The protein and mRNA expressions of TGF-β1, p-SMAD3, and kindlin-2 decreased in diabetic kidneys following triptolide treatment. The findings demonstrated that triptolide might protect podocytes during DN by inhibiting podocyte EMT through inactivation of kindlin-2, combined with the downregulation of P-SMAD3 in the TGF-β/Smad signaling pathway.

Molecular and structural basis of olfactory sensory neuron axon coalescence by Kirrel receptors

Projections from sensory neurons of olfactory systems coalesce into glomeruli in the brain. The Kirrel receptors are believed to homodimerize via their ectodomains and help separate sensory neuron axons into Kirrel2- or Kirrel3-expressing glomeruli. Here, we present the crystal structures of homodimeric Kirrel receptors and show that the closely related Kirrel2 and Kirrel3 have evolved specific sets of polar and hydrophobic interactions, respectively, disallowing heterodimerization while preserving homodimerization, likely resulting in proper segregation and coalescence of Kirrel-expressing axons into glomeruli. We show that the dimerization interface at the N-terminal immunoglobulin (IG) domains is necessary and sufficient to create homodimers and fail to find evidence for a secondary interaction site in Kirrel ectodomains. Furthermore, we show that abolishing dimerization of Kirrel3 in vivo leads to improper formation of glomeruli in the mouse accessory olfactory bulb as observed in Kirrel3-/- animals. Our results provide evidence for Kirrel3 homodimerization controlling axonal coalescence.

Comprehensive expression pattern of kin of irregular chiasm-like 3 in the adult mouse brain

Kin of irregular chiasm-like 3 (Kirrel3), a member of the immunoglobulin superfamily, is expressed in the central nervous system during development and in adulthood. It has been reported that Kirrel3 is involved in the axonal fasciculation in the olfactory bulb, the neuronal migration in the pontine nucleus, and the synapse formation in the hippocampal neurons in mice. Although KIRREL3 mutations have been implicated in autism spectrum disorder and intellectual disability in humans, the comprehensive expression pattern of Kirrel3 in the adult brain is not fully understood. To better visualize Kirrel3 expression pattern and to gain insight into the role of Kirrel3 in the brain, we investigated the expression of Kirrel3 in the adult brain of Kirrel3-heterozygous (Kirrel3^+/-) mice, in which Kirrel3-expressing cells could be identified by the expression of β-galactosidase (β-gal) in the nucleus of cells. The strong expression of β-gal was observed in the hippocampus, cerebral cortex, olfactory bulb, amygdala, thalamus, and cerebellum. In the hippocampus, β-gal was detected in the dentate gyrus and in the ventral parts of CA1 and CA3, which are known to be involved in the social recognition memory. Within the cerebral cortex, many cells with β-gal expression were observed in the olfactory area and auditory area. In the striatum, neurons with β-gal expression were mainly observed in the ventral striatum. Expression of β-gal was observed in all layers in the cerebellum and olfactory bulb, except for the olfactory nerve layer. Double-immunofluorescence staining of β-galactosidase with neuronal markers revealed that β-gal expression was exclusively detected in neurons. These results suggest that Kirrel3 may be involved in the maintenance of neuronal networks, such as the maintenance of synaptic connectivity and plasticity in the motor, sensory, and cognitive circuits of adult brain.

Kirrel3-Mediated Synapse Formation Is Attenuated by Disease-Associated Missense Variants

Missense variants in Kirrel3 are repeatedly identified as risk factors for autism spectrum disorder and intellectual disability, but it has not been reported if or how these variants disrupt Kirrel3 function. Previously, we studied Kirrel3 loss of function using KO mice and showed that Kirrel3 is a synaptic adhesion molecule necessary to form one specific type of hippocampal synapse in vivo Here, we developed an in vitro, gain-of-function assay for Kirrel3 using neuron cultures prepared from male and female mice and rats. We find that WT Kirrel3 induces synapse formation selectively between Kirrel3-expressing neurons via homophilic, transcellular binding. We tested six disease-associated Kirrel3 missense variants and found that five attenuate this synaptogenic function. All variants tested traffic to the cell surface and localize to synapses similar to WT Kirrel3. Two tested variants lack homophilic transcellular binding, which likely accounts for their reduced synaptogenic function. Interestingly, we also identified variants that bind in trans but cannot induce synapses, indicating that Kirrel3 transcellular binding is necessary but not sufficient for its synaptogenic function. Collectively, these results suggest Kirrel3 functions as a synaptogenic, cell-recognition molecule, and this function is attenuated by missense variants associated with autism spectrum disorder and intellectual disability. Thus, we provide critical insight to the mechanism of Kirrel3 function and the consequences of missense variants associated with autism and intellectual disability.SIGNIFICANCE STATEMENT Here, we advance our understanding of mechanisms mediating target-specific synapse formation by providing evidence that Kirrel3 transcellular interactions mediate target recognition and signaling to promote synapse development. Moreover, this study tests the effects of disease-associated Kirrel3 missense variants on synapse formation, and thereby, increases understanding of the complex etiology of neurodevelopmental disorders arising from rare missense variants in synaptic genes.

A molecular mechanism explaining albuminuria in kidney disease

Mammalian kidneys constantly filter large amounts of liquid, with almost complete retention of albumin and other macromolecules in the plasma. Breakdown of the three-layered renal filtration barrier results in loss of albumin into urine (albuminuria) across the wall of small renal capillaries, and is a leading cause of chronic kidney disease. However, exactly how the renal filter works and why its permeability is altered in kidney diseases is poorly understood. Here we show that the permeability of the renal filter is modulated through compression of the capillary wall. We collect morphometric data prior to and after onset of albuminuria in a mouse model equivalent to a human genetic disease affecting the renal filtration barrier. Combining quantitative analyses with mathematical modelling, we demonstrate that morphological alterations of the glomerular filtration barrier lead to reduced compressive forces that counteract filtration pressure, thereby resulting in capillary dilatation, and ultimately albuminuria. Our results reveal distinct functions of the different layers of the filtration barrier and expand the molecular understanding of defective renal filtration in chronic kidney disease.

CD80 and CTLA-4 as diagnostic and prognostic markers in adult-onset minimal change disease: a retrospective study

Background

Minimal change disease (MCD) is a form of idiopathic nephrotic syndrome. Compared to children, adult-onset MCD patients are reported to have delayed responses to glucocorticoid treatment. Several studies of children have suggested detecting urinary CD80 levels to diagnose MCD. There are no effective diagnostic methods to distinguish steroid-sensitive MCD from steroid-resistant MCD unless treatments are used.

Methods

In a total of 55 patients with biopsy-proven MCD and 26 patients with biopsy-proven idiopathic membranous nephropathy, CD80 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) levels in serum, urine and renal tissue were analyzed.

Results

Steroid-sensitive MCD patients in remission had lower urinary CD80 levels and higher CTLA-4 levels than patients in relapse (156.65 ± 24.62 vs 1066.40 ± 176.76 ng/g creatinine; p < 0.0001), (728.73 ± 89.93 vs 151.70 ± 27.01 ng/g creatinine; p < 0.0001). For MCD patients in relapse, mean urinary CD80 level was higher, and CTLA-4 level was lower for those who were steroid-sensitive than those who were steroid-resistant (1066.40 ± 176.76 vs. 203.78 ± 30.65 ng/g creatinine; p < 0.0001), but the mean urinary CTLA-4 level was lower (151.70 ± 27.01 vs. 457.83 ± 99.45 ng/g creatinine; p < 0.0001). CD80 expression in glomeruli was a sensitive marker to diagnose MCD. The absent or minimal expression of CTLA-4 in glomeruli could distinguish steroid-sensitive MCD from steroid-resistant MCD.

Conclusions

Glucocorticoid treatment may result in complete remission for only MCD patients with strongly positive CD80 expression and negative CTLA-4 expression in glomeruli, or higher urinary CD80 level and lower CTLA-4 level.

The long journey through renal filtration: new pieces in the puzzle of slit diaphragm architecture

PURPOSE OF REVIEW

The podocyte slit diaphragm is probably the least understood component of the kidney filtration barrier. In this review, we aim to integrate the most recent findings on the molecular make-up and structural architecture of this specialized cell-cell junction into a current concept of glomerular filtration.

RECENT FINDINGS

Analysis of cryopreserved mammalian tissue revealed a bipartite composition of the slit diaphragm. Single NEPH1 molecules span the lower part of the slit close to the glomerular basement membrane whereas NEPHRIN molecules are positioned in the apical part toward Bowman's space. This molecular arrangement could lead to heterogeneous ellipsoidal and circular pores, which are mainly located in the central region of the slit diaphragm.

SUMMARY

Despite having been first identified in the 1970s, the slit diaphragm's structural architecture has not been fully elucidated to date and remains an area of intense research and scientific debate. The slit diaphragm has been initially described as a rigid 'zipper-like' structure in which periodic, rod-like units extend from a podocyte foot processes to a linear central bar, giving rise to homogeneous 4 × 14 nm pores. Several recent findings have challenged these long-held beliefs and instead pointed to an unanticipated complexity of slit diaphragm structure. High-resolution ultrastructural analysis found evidence that the slit diaphragm is a dynamic and adjustable cell-cell junction that forms a nonclogging barrier within the renal filtration system.

Examining Hippocampal Mossy Fiber Synapses by 3D Electron Microscopy in Wildtype and Kirrel3 Knockout Mice

Neural circuits balance excitatory and inhibitory activity and disruptions in this balance are commonly found in neurodevelopmental disorders. Mice lacking the intellectual disability and autism-associated gene Kirrel3 have an excitation-inhibition imbalance in the hippocampus but the precise synaptic changes underlying this functional defect are unknown. Kirrel3 is a homophilic adhesion molecule expressed in dentate gyrus (DG) and GABA neurons. It was suggested that the excitation-inhibition imbalance of hippocampal neurons in Kirrel3 knockout mice is due to loss of mossy fiber (MF) filopodia, which are DG axon protrusions thought to excite GABA neurons and thereby provide feed-forward inhibition to CA3 pyramidal neurons. Fewer filopodial structures were observed in Kirrel3 knockout mice but neither filopodial synapses nor DG en passant synapses, which also excite GABA neurons, were examined. Here, we used serial block-face scanning electron microscopy (SBEM) with 3D reconstruction to define the precise connectivity of MF filopodia and elucidate synaptic changes induced by Kirrel3 loss. Surprisingly, we discovered wildtype MF filopodia do not synapse exclusively onto GABA neurons as previously thought, but instead synapse with similar frequency onto GABA neurons and CA3 neurons. Moreover, Kirrel3 loss selectively reduces MF filopodial synapses onto GABA neurons but not those made onto CA3 neurons or en passant synapses. In sum, the selective loss of MF filopodial synapses with GABA neurons likely underlies the hippocampal activity imbalance observed in Kirrel3 knockout mice and may impact neural function in patients with Kirrel3-dependent neurodevelopmental disorders.

A functional variant in NEPH3 gene confers high risk of renal failure in primary hematuric glomerulopathies. Evidence for predisposition to microalbuminuria in the general population

Background

Recent data emphasize that thin basement membrane nephropathy (TBMN) should not be viewed as a form of benign familial hematuria since chronic renal failure (CRF) and even end-stage renal disease (ESRD), is a possible development for a subset of patients on long-term follow-up, through the onset of focal and segmental glomerulosclerosis (FSGS). We hypothesize that genetic modifiers may explain this variability of symptoms.

Methods

We looked in silico for potentially deleterious functional SNPs, using very strict criteria, in all the genes significantly expressed in the slit diaphragm (SD). Two variants were genotyped in a cohort of well-studied adult TBMN patients from 19 Greek-Cypriot families, with a homogeneous genetic background. Patients were categorized as “Severe” or “Mild”, based on the presence or not of proteinuria, CRF and ESRD. A larger pooled cohort (HEMATURIA) of 524 patients, including IgA nephropathy patients, was used for verification. Additionally, three large general population cohorts [Framingham Heart Study (FHS), KORAF4 and SAPHIR] were used to investigate if the NEPH3-V353M variant has any renal effect in the general population.

Results and conclusions

Genotyping for two high-scored variants in 103 TBMN adult patients with founder mutations who were classified as mildly or severely affected, pointed to an association with variant NEPH3-V353M (filtrin). This promising result prompted testing in the larger pooled cohort (HEMATURIA), indicating an association of the 353M variant with disease severity under the dominant model (p = 3.0x10^-3, OR = 6.64 adjusting for gender/age; allelic association: p = 4.2x10^-3 adjusting for patients’ kinships). Subsequently, genotyping 6,531 subjects of the Framingham Heart Study (FHS) revealed an association of the homozygous 353M/M genotype with microalbuminuria (p = 1.0x10^-3). Two further general population cohorts, KORAF4 and SAPHIR confirmed the association, and a meta-analysis of all three cohorts (11,258 individuals) was highly significant (p = 1.3x10^-5, OR = 7.46). Functional studies showed that Neph3 homodimerization and Neph3-Nephrin heterodimerization are disturbed by variant 353M. Additionally, 353M was associated with differential activation of the unfolded protein response pathway, when overexpressed in stressed cultured undifferentiated podocyte cells, thus attesting to its functional significance. Genetics and functional studies support a “rare variant-strong effect” role for NEPH3-V353M, by exerting a negative modifier effect on primary glomerular hematuria. Additionally, genetics studies provide evidence for a role in predisposing homozygous subjects of the general population to micro-albuminuria.

Tangzhiqing Granules Alleviate Podocyte Epithelial-Mesenchymal Transition in Kidney of Diabetic Rats

This study discussed the effect of Tangzhiqing granules on podocyte epithelial-mesenchymal transition in kidney of diabetic rats. The diabetic rats were divided randomly into five groups: DM group treated with vehicle, Tangzhiqing granules low-dose treatment group, Tangzhiqing granules middle-dose treatment group, and Tangzhiqing granules high-dose treatment group. Eight Wistar rats used as control group were given saline solution. The intervention was all intragastric administration for 8 weeks. At the end of the 8 weeks, biochemical parameters and kidney weight/body weight ratio were measured. The kidney tissues were observed under light microscope and transmission electron microscopy. To search for the underlying mechanism, we examined the epithelial-to-mesenchymal transition (EMT) related molecular markers and TGF-β/smad signaling pathway key proteins expression. The results showed that Tangzhiqing granules relieved the structural damage and functional changes of diabetic kidneys. Kidney podocyte EMT related molecular markers nephrin and CD2AP expression were increased, when desmin and α-SMA levels were decreased by Tangzhiqing granules in diabetic rats. Further TGF-β/smad signaling pathway key proteins TGF-β1 and p-smad2/3 levels were decreased in diabetic rats after treatment with Tangzhiqing granules. These findings suggest that Tangzhiqing granules may protect the podocytes of diabetic nephropathy rats via alleviating podocyte EMT and likely activating TGFβ/smad signaling pathway.

Store-Operated Ca2+ Channel in Renal Microcirculation and Glomeruli

Store-operated Ca2+ channel (SOC) is defined as a channel that opens in response to depletion of the internal Ca2+ stores. During the last decade, many investigators have made a great effort to identify and characterize SOC, and to evaluate its physiologic function and pathophysiologic relevance in a variety of cell lines, primary cultures, and native tissues. To date, accumulating evidence has demonstrated that SOC is an essential Ca2+ entry mechanism in vascular smooth-muscle cells of renal microvasculature and glomerular mesangial cells, both of which tightly control glomerular hemodynamics and filtration. Store-operated Ca2+, combined with other types of Ca2+ entry channels, constitutes a profile of Ca2+ changes in response to physiologic vasoconstrictors and, thereby, regulates renal microcirculation and mesangial function. In addition, SOC is associated with altered Ca2+ signaling occurring in diseased kidneys, such as diabetic nephropathy. Although the gating mechanism and molecular identity of SOC are still enigmatic and may be cell-type and tissue specific, data from several independent groups suggest that protein kinase C plays an important role in SOC activation and that certain isoforms of canonical transient receptor potential (TRPC) proteins are candidates of SOC in renal mlcrovessels and mesangial cells.

A flexible, multilayered protein scaffold maintains the slit in between glomerular podocytes

Vertebrate life critically depends on renal filtration and excretion of low molecular weight waste products. This process is controlled by a specialized cell-cell contact between podocyte foot processes: the slit diaphragm (SD). Using a comprehensive set of targeted KO mice of key SD molecules, we provided genetic, functional, and high-resolution ultrastructural data highlighting a concept of a flexible, dynamic, and multilayered architecture of the SD. Our data indicate that the mammalian SD is composed of NEPHRIN and NEPH1 molecules, while NEPH2 and NEPH3 do not participate in podocyte intercellular junction formation. Unexpectedly, homo- and heteromeric NEPHRIN/NEPH1 complexes are rarely observed. Instead, single NEPH1 molecules appear to form the lower part of the junction close to the glomerular basement membrane with a width of 23 nm, while single NEPHRIN molecules form an adjacent junction more apically with a width of 45 nm. In both cases, the molecules are quasiperiodically spaced 7 nm apart. These structural findings, in combination with the flexibility inherent to the repetitive Ig folds of NEPHRIN and NEPH1, indicate that the SD likely represents a highly dynamic cell-cell contact that forms an adjustable, nonclogging barrier within the renal filtration apparatus.

Kin of IRRE-like Protein 2 Is a Phosphorylated Glycoprotein That Regulates Basal Insulin Secretion

Direct interactions among pancreatic β-cells via cell surface proteins inhibit basal and enhance stimulated insulin secretion. Here, we functionally and biochemically characterized Kirrel2, an immunoglobulin superfamily protein with β-cell-specific expression in the pancreas. Our results show that Kirrel2 is a phosphorylated glycoprotein that co-localizes and interacts with the adherens junction proteins E-cadherin and β-catenin in MIN6 cells. We further demonstrate that the phosphosites Tyr(595-596) are functionally relevant for the regulation of Kirrel2 stability and localization. Analysis of the extracellular and intracellular domains of Kirrel2 revealed that it is cleaved and shed from MIN6 cells and that the remaining membrane spanning cytoplasmic domain is processed by γ-secretase complex. Kirrel2 knockdown with RNA interference in MIN6 cells and ablation of Kirrel2 from mice with genetic deletion resulted in increased basal insulin secretion from β-cells, with no immediate influence on stimulated insulin secretion, total insulin content, or whole body glucose metabolism. Our results show that in pancreatic β-cells Kirrel2 localizes to adherens junctions, is regulated by multiple post-translational events, including glycosylation, extracellular cleavage, and phosphorylation, and engages in the regulation of basal insulin secretion.

Mice lacking the synaptic adhesion molecule Neph2/Kirrel3 display moderate hyperactivity and defective novel object preference

Synaptic adhesion molecules regulate diverse aspects of neuronal synapse development, including synapse specificity, formation, and maturation. Neph2, also known as Kirrel3, is an immunoglobulin superfamily adhesion molecule implicated in intellectual disability, neurocognitive delay associated with Jacobsen syndrome, and autism spectrum disorders. We here report mice lacking Neph2 (Neph2(-/-) mice) display moderate hyperactivity in a familiar, but not novel, environment and defective novel object recognition with normal performances in Morris water maze spatial learning and memory, contextual fear conditioning and extinction, and pattern separation tests. These mice also show normal levels of anxiety-like behaviors, social interaction, and repetitive behaviors. At the synapse level, Neph2(-/-) dentate gyrus granule cells exhibit unaltered dendritic spine density and spontaneous excitatory synaptic transmission. These results suggest that Neph2 is important for normal locomotor activity and object recognition memory.

Podocytes: recent biomolecular developments

Podocytes are postmitotic renal glomerular cells with multiple ramifications that extend from the cell body. Processes departing from a podocyte interdigitate with corresponding projections from neighboring cells and form an intricate web that enwraps the glomerular capillary completely. Podocyte processes are interconnected by the slit diaphragm, an adhesion junction mostly formed by Ig-like molecules, cadherins/protocadherins, ephrin/eph, and neurexin molecules organized in an assembly that resembles synaptic junctions. Podocyte failure is primarily or secondarily implicated in all forms of proteinuric glomerular diseases, as confirmed by the morphological changes of their elaborate cell architecture detectable by electron microscopy. Importantly, mutations of podocyte proteins are responsible for the most severe forms of congenital nephrotic syndrome. In the last 15 years, progressive technological advances have aided the study of podocyte biology and pathology, confirming the relevance of podocyte molecules and signaling pathways for the function of the glomerular filter. This review will examine the most important and newest discoveries in the field, which is rapidly evolving, hopefully leading to a detailed knowledge of this fascinating cell and to the development of specific therapeutic options for proteinuric diseases.

An update: the role of Nephrin inside and outside the kidney

Nephrin is a key molecule in podocytes to maintain normal slit diaphragm structure. Nephin interacts with many other podocyte and slit diaphragm protein and also mediates important cell signaling pathways in podocytes. Loss of nephrin during the development leads to the congenital nephrotic syndrome in children. Reduction of nephrin expression is often observed in adult kidney diseases including diabetic nephropathy and HIV-associated nephropathy. The critical role of nephrin has been confirmed by different animal models with nephrin knockout and knockdown. Recent studies demonstrate that knockdown of nephrin expression in adult mice aggravates the progression of unilateral nephrectomy and Adriamycin-induced kidney disease. In addition to its critical role in maintaining normal glomerular filtration unit in the kidney, nephrin is also expressed in other organs. However, the exact role of nephrin in kidney and extra-renal organs has not been well characterized. Future studies are required to determine whether nephrin could be developed as a drug target to treat patients with kidney disease.

Identification of novel Kirrel3 gene splice variants in adult human skeletal muscle

Background

Multiple cell types including trophoblasts, osteoclasts and myoblasts require somatic cell fusion events as part of their physiological functions. In Drosophila Melanogaster the paralogus type 1 transmembrane receptors and members of the immunoglobulin superfamily Kin of Irre (Kirre) and roughest (Rst) regulate myoblast fusion during embryonic development. Present within the human genome are three homologs to Kirre termed Kin of Irre like (Kirrel) 1, 2 and 3. Currently it is unknown if Kirrel3 is expressed in adult human skeletal muscle.

Results

We investigated (using PCR and Western blot) Kirrel3 in adult human skeletal muscle samples taken at rest and after mild exercise induced muscle damage. Kirrel3 mRNA expression was verified by sequencing and protein presence via blotting with 2 different anti-Kirrel3 protein antibodies. Evidence for three alternatively spliced Kirrel3 mRNA transcripts in adult human skeletal muscle was obtained. Kirrel3 mRNA in adult human skeletal muscle was detected at low or moderate levels, or not at all. This sporadic expression suggests that Kirrel3 is expressed in a pulsatile manner. Several anti Kirrel3 immunoreactive proteins were detected in all adult human skeletal muscle samples analysed and results suggest the presence of different isoforms or posttranslational modification, or both.

Conclusion

The results presented here demonstrate for the first time that there are at least 3 splice variants of Kirrel3 expressed in adult human skeletal muscle, two of which have never previously been identified in human muscle. Importantly, mRNA of all splice variants was not always present, a finding with potential physiological relevance. These initial discoveries highlight the need for more molecular and functional studies to understand the role of Kirrel3 in human skeletal muscle.

The Rst-Neph family of cell adhesion molecules in Gallus gallus

The Rst-Neph family comprises an evolutionarily conserved group of single-pass transmembrane glycoproteins that belong to the immunoglobulin superfamily and participate in a wide range of cell adhesion and recognition events in both vertebrates and invertebrates. In mammals and fish, three Rst-Neph members, named Neph1-3, are present. Besides being widely expressed in the embryo, particularly in the developing nervous system, they also contribute to the formation and integrity of the urine filtration apparatus in the slit diaphragm of kidney glomerular podocytes, where they form homodimers, as well as heterodimers with Nephrin, another immunoglobulin-like cell adhesion molecule. In mice, absence of Neph1 causes severe proteinuria, podocyte effacement and perinatal death, while in humans, a mutated form of Nephrin leads to congenital nephrotic syndrome of the Finnish type. Intriguingly, neither Nephrin nor Neph3 are present in birds, which nevertheless have typical vertebrate kidneys with mammalian-like slit diaphragms. These characteristics make, in principle, avian systems very helpful for understanding the evolution and functional significance of the complex interactions displayed by Rst-Neph proteins. To this end we have started a systematic study of chicken Neph embryonic and post-embryonic expression, both at mRNA and protein level. RT-qPCR mRNA quantification of the two Neph paralogues in adult tissues showed that both are expressed in heart, brain, and retina. Neph1 is additionally present in kidney, liver, pancreas, lungs, and testicles, while Neph2 mRNA is barely detected in kidney, testicles, pancreas and absent in liver and lungs. In embryos, mRNA from both genes can already be detected at as early as stage HH14, and remain expressed until at least HH28. Finally, we used a specific antibody to examine the spatial dynamics and subcellular distribution of ggNeph2 between stages HH20-28, particularly in the mesonephros, dermomyotomes, developing heart, and retina.

Ex vivo expansion of functional human UCB-HSCs/HPCs by coculture with AFT024-hkirre cells

Kiaa1867 (human Kirre, hKirre) has a critical role in brain development and/or maintenance of the glomerular slit diaphragm in kidneys. Murine homolog of this gene, mKirre expressed in OP9 and AFT024 cells could support hematopoietic stem cells/hematopoietic progenitor cells (HSC/HPC) expansion in vitro. HKirre is also expressed in human FBMOB-hTERT cell line and fetal liver fibroblast-like cells but its function has remained unclear. In this paper, we cloned a hKirre gene from human fetal liver fibroblast-like cells and established a stably overexpressing hKirre-AFT024 cell line. Resultant cells could promote self-renewal and ex vivo expansion of HSCs/HPCs significantly higher than AFT024-control cells transformed with mock plasmid. The Expanded human umbilical cord blood (hUCB) CD34⁺ cells retained the capacity of multipotent differentiation as long as 8 weeks and successfully repopulated the bone marrow of sublethally irradiated NOD/SCID mice, which demonstrated the expansion of long-term primitive transplantable HSCs/HPCs. Importantly, hkirre could upregulate the expressions of Wnt-5A, BMP4, and SDF-1 and downregulate TGF-β with other hematopoietic growth factors. By SDS-PAGE and Western Blot analysis, a ~89 kDa protein in total lysate of AFT024-hKirre was identified. Supernatants from AFT024-hkirre could also support CD34⁺CD38⁻ cells expansion. These results demonstrated that the AFT024-hKirre cells have the ability to efficiently expand HSCs/HPCs.

MeCP2 is required for activity-dependent refinement of olfactory circuits

Methyl CpG binding protein 2 (MeCP2) is a structural chromosomal protein involved in the regulation of gene expression. Alterations in the levels of MeCP2 have been related to neurodevelopmental disorders. Studies in mouse models of MeCP2 deficiency have demonstrated that this protein is important for neuronal maturation, neurite complexity, synaptogenesis, and synaptic plasticity. However, the mechanisms by which MeCP2 dysfunction leads to neurodevelopmental defects, and the role of activity, remain unclear, as most studies examine the adult nervous system, which may obfuscate the primary consequences of MeCP2 mutation. We hypothesize that MeCP2 plays a role during the formation and activity-driven maturation of neural circuits at early postnatal stages. To test this hypothesis, we use the olfactory system as a neurodevelopmental model. This system undergoes postnatal neurogenesis; axons from olfactory neurons form highly stereotyped projections to higher-order neurons, facilitating the detection of possible defects in the establishment of connectivity. In vivo olfactory stimulation paradigms were used to produce physiological synaptic activity in gene-targeted mice in which specific olfactory circuits are visualized. Our results reveal defective postnatal refinement of olfactory circuits in Mecp2 knock out (KO) mice after sensory (odorant) stimulation. This failure in refinement was associated with deficits in the normal responses to odorants, including brain-derived neurotrophic factor (BDNF) production, as well as changes in adhesion molecules known to regulate axonal convergence. The defective refinement observed in Mecp2 KO mice was prevented by daily treatment with ampakine beginning after the first postnatal week. These observations indicate that increasing synaptic activity at early postnatal stage might circumvent the detrimental effect of MeCP2 deficiency on circuitry maturation. The present results provide in vivo evidence in real time for the role of MeCP2 in activity-dependent maturation of olfactory circuitry, with implications for understanding the mechanism of MeCP2 mutations in the development of neural connectivity.

Functions of the podocyte proteins nephrin and Neph3 and the transcriptional regulation of their genes

Nephrin and Neph-family proteins [Neph1–3 (nephrin-like 1–3)] belong to the immunoglobulin superfamily of cell-adhesion receptors and are expressed in the glomerular podocytes. Both nephrin and Neph-family members function in cell adhesion and signalling, and thus regulate the structure and function of podocytes and maintain normal glomerular ultrafiltration. The expression of nephrin and Neph3 is altered in human proteinuric diseases emphasizing the importance of studying the transcriptional regulation of the nephrin and Neph3 genes NPHS1 (nephrosis 1, congenital, Finnish type) and KIRREL2 (kin of IRRE-like 2) respectively. The nephrin and Neph3 genes form a bidirectional gene pair, and they share transcriptional regulatory mechanisms. In the present review, we summarize the current knowledge of the functions of nephrin and Neph-family proteins and transcription factors and agents that control nephrin and Neph3 gene expression.

Identification and characterization of novel Kirrel isoform during myogenesis

Somatic cell fusion is an essential component of skeletal muscle development and growth and repair from injury. Additional cell types such as trophoblasts and osteoclasts also require somatic cell fusion events to perform their physiological functions. Currently we have rudimentary knowledge on molecular mechanisms regulating somatic cell fusion events in mammals. We therefore investigated during in vitro murine myogenesis a mammalian homolog, Kirrel, of the Drosophila Melanogaster genes Roughest (Rst) and Kin of Irre (Kirre) which regulate somatic muscle cell fusion during embryonic development. Our results demonstrate the presence of a novel murine Kirrel isoform containing a truncated cytoplasmic domain which we term Kirrel B. Protein expression levels of Kirrel B are inverse to the occurrence of cell fusion events during in vitro myogenesis which is in stark contrast to the expression profile of Rst and Kirre during myogenesis in Drosophila. Furthermore, chemical inhibition of cell fusion confirmed the inverse expression pattern of Kirrel B protein levels in relation to cell fusion events. The discovery of a novel Kirrel B protein isoform during myogenesis highlights the need for more thorough investigation of the similarities and potential differences between fly and mammals with regards to the muscle cell fusion process.

Signaling from the podocyte intercellular junction to the actin cytoskeleton

Observations of hereditary glomerular disease support the contention that podocyte intercellular junction proteins are essential for junction formation and maintenance. Genetic deletion of most of these podocyte intercellular junction proteins results in foot process effacement and proteinuria. This review focuses on the current understanding of molecular mechanisms by which podocyte intercellular junction proteins such as the nephrin-neph1-podocin-receptor complex coordinate cytoskeletal dynamics and thus intercellular junction formation, maintenance, and injury-dependent remodeling.

Changes in the expression of bone morphogenetic protein 7 and tamm- horsfall protein in the early stages of diabetic nephropathy

BACKGROUND

Bone morphogenetic protein 7 (BMP7) has been suggested to play a protective role against kidney injury in chronic kidney disease.

OBJECTIVES

To identify the critical molecular regulators in the early stage of diabetic nephropathy, we studied the expression of BMP7 and 2 important kidney-specific markers, podocin and Tamm-Horsfall protein (THP).

MATERIALS AND METHODS

A diabetic nephropathy model was established by intraperitoneally injecting streptozotocin (STZ) in male Kunming mice. Kidney weight index was used as an indicator of early renal injury. Kidney tissue from the diabetic model mice was obtained at 4, 8, and 12 weeks, and total protein was extracted to assess the expression of BMP7, podocin, and THP by western blot analysis.

RESULTS

Diabetic model mice were successfully established, and the kidney weight index of the model animals increased significantly. The expression of BMP7 was significantly downregulated, while the expression of THP was increased in the early stage of diabetic nephropathy. However, the expression of podocin did not change.

CONCLUSIONS

Our observations suggested that down-regulation of BMP7 expression and up-regulation of THP expression were early events that occur prior to podocyte injury with the structure protein, podocin spoiled, which further confirmed that BMP7 is a key molecular regulator in the early stage of diabetic nephropathy.

Comparative analysis of Neph gene expression in mouse and chicken development

Neph proteins are evolutionarily conserved members of the immunoglobulin superfamily of adhesion proteins and regulate morphogenesis and patterning of different tissues. They share a common protein structure consisting of extracellular immunoglobulin-like domains, a transmembrane region, and a carboxyl terminal cytoplasmic tail required for signaling. Neph orthologs have been widely characterized in invertebrates where they mediate such diverse processes as neural development, synaptogenesis, or myoblast fusion. Vertebrate Neph proteins have been described first at the glomerular filtration barrier of the kidney. Recently, there has been accumulating evidence suggesting a function of Neph proteins also outside the kidney. Here we demonstrate that Neph1, Neph2, and Neph3 are expressed differentially in various tissues during ontogenesis in mouse and chicken. Neph1 and Neph2 were found to be amply expressed in the central nervous system while Neph3 expression remained localized to the cerebellum anlage and the spinal cord. Outside the nervous system, Neph mRNAs were also differentially expressed in branchial arches, somites, heart, lung bud, and apical ectodermal ridge. Our findings support the concept that vertebrate Neph proteins, similarly to their Drosophila and C. elegans orthologs, provide guidance cues for cell recognition and tissue patterning in various organs which may open interesting perspectives for future research on Neph1-3 controlled morphogenesis.

Canonical Wnt/β-catenin signaling mediates transforming growth factor-β1-driven podocyte injury and proteinuria

Transforming growth factor-β1 (TGF-β1) upregulation occurs in virtually all chronic kidney diseases and is associated with podocyte injury and proteinuria; however, the mechanisms contributing to this in vivo are ambiguous. In vitro, incubation of podocytes with TGF-β1 induced Wnt1 expression, β-catenin activation, and stimulated the expression of Wnt/β-catenin downstream target genes. Ectopic expression of Wnt1 or β-catenin mimicked TGF-β1, induced Snail1, and suppressed nephrin expression. The Wnt antagonist, Dickkopf-1, blocked TGF-β1-induced β-catenin activation, Snail1 induction, and nephrin suppression. In vivo, ectopic expression of TGF-β1 induced Wnt1 expression, activated β-catenin, and upregulated Wnt target genes such as Snail1, MMP-7, MMP-9, desmin, Fsp1, and PAI-1 in mouse glomeruli, leading to podocyte injury and albuminuria. Consistently, concomitant expression of Dickkopf-1 gene abolished β-catenin activation, inhibited TGF-β1-triggered Wnt target gene expression, and mitigated albuminuria. Thus, canonical Wnt/β-catenin signaling mediates TGF-β1-driven podocyte injury and proteinuria. These studies suggest that Wnt/β-catenin signaling may be exploited as a therapeutic target for the treatment of proteinuric kidney diseases.

rst transcriptional activity influences kirre mRNA concentration in the Drosophila pupal retina during the final steps of ommatidial patterning

BACKGROUND

Drosophila retinal architecture is laid down between 24-48 hours after puparium formation, when some of the still uncommitted interommatidial cells (IOCs) are recruited to become secondary and tertiary pigment cells while the remaining ones undergo apoptosis. This choice between survival and death requires the product of the roughest (rst) gene, an immunoglobulin superfamily transmembrane glycoprotein involved in a wide range of developmental processes. Both temporal misexpression of Rst and truncation of the protein intracytoplasmic domain, lead to severe defects in which IOCs either remain mostly undifferentiated and die late and erratically or, instead, differentiate into extra pigment cells. Intriguingly, mutants not expressing wild type protein often have normal or very mild rough eyes.

METHODOLOGY/PRINCIPAL FINDINGS

By using quantitative real time PCR to examine rst transcriptional dynamics in the pupal retina, both in wild type and mutant alleles we showed that tightly regulated temporal changes in rst transcriptional rate underlie its proper function during the final steps of eye patterning. Furthermore we demonstrated that the unexpected wild type eye phenotype of mutants with low or no rst expression correlates with an upregulation in the mRNA levels of the rst paralogue kin-of-irre (kirre), which seems able to substitute for rst function in this process, similarly to their role in myoblast fusion. This compensatory upregulation of kirre mRNA levels could be directly induced in wild type pupa upon RNAi-mediated silencing of rst, indicating that expression of both genes is also coordinately regulated in physiological conditions.

CONCLUSIONS/SIGNIFICANCE

These findings suggest a general mechanism by which rst and kirre expression could be fine tuned to optimize their redundant roles during development and provide a clearer picture of how the specification of survival and apoptotic fates by differential cell adhesion during the final steps of retinal morphogenesis in insects are controlled at the transcriptional level.

Anion exchanger 1 in red blood cells and kidney: Band 3's in a pod

The bicarbonate/chloride exchanger 1 (AE1, Band 3) is abundantly expressed in the red blood cell membrane, where it is involved in gas exchange and functions as a major site of cytoskeletal attachment to the erythrocyte membrane. A truncated kidney isoform (kAE1) is highly expressed in type A intercalated cells of the distal tubules, where it is vital for urinary acidification. Recently, kAE1 has emerged as a novel physiologically significant protein in the kidney glomerulus. This minireview will discuss the known interactions of kAE1 in the podocytes and the possible mechanisms whereby this important multispanning membrane protein may contribute to the function of the glomerular filtration barrier and prevent proteinuria.

Disclosure in 3D of slit-membrane as well as -strands, and en-face basal lamina in situ of renal glomerulus of normal rats in embedment-free section transmission electron microscopy

With higher contrast and transparency due to the absence of epon and stereo-viewing effect due to thicker sections than conventional electron microscopy as methodological advantages, the renal glomerular slits were re-examined in embedment-free section electron microscopy. In addition to clear demonstration of strands bridging the slits in forms of ladders with highly irregular intervals and various extension-directions and length, this study disclosed clearly for the first time in the "section" TEM thin sheets which partially spanned the slit together with the strand-ladders. No strands were found to align in forms of typical zippers in normal kidney. Furthermore, en-face ultrastructure of the basal lamina in situ was clearly demonstrated in superimposed sites of the endothelial fenestrae with the slits.

Maximal 'CD80-uria' with minimal change

Differentiation between minimal-change disease and focal segmental glomerulosclerosis remains challenging, particularly at early stages in children and adults. Garin et al. identify urinary CD80 excretion levels as a potential marker to differentiate the former entity from the latter. Thus, CD80 expression on podocytes, which was initially described in mouse models of foot process effacement and human lupus nephritis, is now brought toward clinical application for use as a diagnostic biomarker.

Nephrinuria associates with multiple renal traits in type 2 diabetes

BACKGROUND

The involvement of nephrin in controlling renal function is unclear with the literature only emphasizing its role in albuminuria. We therefore investigated the potential association between nephrinuria as evidenced by the appearance of urinary immunopositive nephrin fragments, with multiple renal traits.

METHODS

Western blot analysis of the urine samples from a cross-sectional study of 381 Chinese type 2 diabetic patients revealed four distinct protein fragments, indicative of nephrinuria. Albuminuria was measured in random spot urine samples using the albumin/creatinine ratio (ACR), while estimated glomerular filtration rate (eGFR) was calculated using the creatinine-based Modification of Diet in Renal Disease formula.

RESULTS

Each nephrin fragment was associated with a decline in eGFR (smallest P = 0.001). Even with the inclusion of logarithmic form of ACR (ln ACR) in the multivariate model, nephrinuria still remained significantly associated with lower eGFR (smallest P < 0.05). Nephrinuria was also strongly associated with lnACR and this finding was independent of eGFR (smallest P < 0.001). Thus, nephrinuria was independently associated with both renal traits in the form of lnACR and eGFR. Furthermore, nephrinuria was significantly associated with lower eGFR even among normoalbuminuric patients (ACR ≤ 30 mg/g) (smallest P = 0.002), potentially implicating nephrinuria in the development of normoalbuminuric renal insufficiency. Apart from the renal traits under investigation, the presence of nephrinuria did not associate with other patient clinical characteristics.

CONCLUSIONS

Nephrinuria was associated with multiple renal traits in type 2 diabetes even in normoalbuminuric patients who are traditionally perceived as having a low risk of chronic kidney disease.

Anion exchanger 1 interacts with nephrin in podocytes

The central role of the multifunctional protein nephrin within the macromolecular complex forming the glomerular slit diaphragm is well established, but the mechanisms linking the slit diaphragm to the cytoskeleton and to the signaling pathways involved in maintaining the integrity of the glomerular filter remain incompletely understood. Here, we report that nephrin interacts with the bicarbonate/chloride transporter kidney anion exchanger 1 (kAE1), detected by yeast two-hybrid assay and confirmed by immunoprecipitation and co-localization studies. We confirmed low-level glomerular expression of kAE1 in human and mouse kidneys by immunoblotting and immunofluorescence microscopy. We observed less kAE1 in human glomeruli homozygous for the NPHS1(FinMaj) nephrin mutation, whereas kAE1 expression remained unchanged in the collecting duct. We could not detect endogenous kAE1 expression in NPHS1(FinMaj) podocytes in primary culture, but heterologous re-introduction of wild-type nephrin into these podocytes rescued kAE1 expression. In kidneys of Ae1(-/-) mice, nephrin abundance was normal but its distribution was altered along with the reported kAE1-binding protein integrin-linked kinase (ILK). Ae1(-/-) mice had increased albuminuria with glomerular enlargement, mesangial expansion, mesangiosclerosis, and expansion of the glomerular basement membrane. Glomeruli with ILK-deficient podocytes also demonstrated altered AE1 and nephrin expression, further supporting the functional interdependence of these proteins. These data suggest that the podocyte protein kAE1 interacts with nephrin and ILK to maintain the structure and function of the glomerular basement membrane.

beta-Catenin mediates adriamycin-induced albuminuria and podocyte injury in adult mouse kidneys

BACKGROUND

Glomerular slit diaphragm (SD) represents a modified adherens junction composed of molecules belonging to both immunoglobulin and cadherin superfamilies. Cadherins associate with the cytosolic scaffolding protein beta-catenin, but the precise role of beta-catenin in mature or injured podocytes is not known.

METHODS

The conditional podocyte-specific beta-catenin-deficient mouse line was generated using the doxycycline-inducible Cre-loxP system. Expression of the beta-catenin-deficient gene was turned off at the age of 8 weeks by doxycycline treatment and the kidney phenotype was analysed. In addition, beta-catenin-deficient and control mice were treated with adriamycin (ADR) and analysed for albuminuria and morphological alterations.

RESULTS

Deletion of beta-catenin in mature podocytes did not change the morphology of podocytes nor did it lead to albuminuria. However, lack of beta-catenin attenuated albuminuria after ADR treatment. Electron microscopic examination showed increased podocyte foot process effacement associated with SD abnormalities in ADR-treated control mice compared to beta-catenin-deficient mice.

CONCLUSIONS

These results show that beta-catenin in podocytes is dispensable for adult mice, but appears to be important in modulating the SD during ADR-induced perturbation of the filtration barrier.

A model organism approach: defining the role of Neph proteins as regulators of neuron and kidney morphogenesis

Mutations of the immunoglobulin superfamily proteins nephrin and Neph1 lead to congenital nephrotic syndrome in humans or mice. Neph proteins are three closely related molecules that are evolutionarily conserved and mediate cell recognition. Their importance for morphogenetic processes including the formation of the kidney filtration barrier in vertebrates and synaptogenesis in Caenorhabditis elegans has recently been uncovered. However, the individual morphogenetic function of mammalian Neph1-3 isoforms remained elusive. We demonstrate now that the Neph/nephrin family proteins can form cell-cell adhesion modules across species. Expression of all three mammalian Neph isoforms partially rescued mutant C. elegans lacking their Neph homolog syg-1 and restored synapse formation, suggesting a functional redundancy between the three isoforms. Strikingly, the rescue of defective synaptic connectivity was prevented by deletion of the highly conserved cytoplasmic PSD95/Dlg/ZO-1-binding motif of SYG-1/Neph proteins, indicating the critical role of this intracellular signaling motif for SYG-1/Neph-dependent morphogenetic events. To determine the significance of Neph isoform redundancy for vertebrate kidney development, we analyzed the expression pattern and the functional role of Neph proteins in zebrafish. In situ hybridizations identified zNeph1 and zNeph2 as glomerular proteins. Morpholino knockdown of either zNeph1 or zNeph2 resulted in loss of slit diaphragms and leakiness of the glomerular filtration barrier. This is the first report utilizing C. elegans to study mammalian Neph/nephrin protein function and to demonstrate a functional overlap of Neph1-3 proteins. Furthermore, we identify Neph2 as a novel critical regulator of glomerular function, indicating that both Neph1 and Neph2 are required for glomerular maintenance and development.

Ptf1a directly controls expression of immunoglobulin superfamily molecules Nephrin and Neph3 in the developing central nervous system

Ptf1a, a basic helix-loop-helix transcription factor, plays an indispensable role for cell fate specification of subsets of neurons in the developing central nervous system. However, downstream molecules induced by Ptf1a during neural development have not been well characterized. In the present study, we identified immunoglobulin superfamily molecules, Nephrin and Neph3, as direct downstream targets of Ptf1a. First, the expression domains of Nephrin and Neph3 closely resembled those of Ptf1a in the developing retina, hypothalamus, cerebellum, hindbrain, and spinal cord. Second, Ptf1a bound directly to a PTF-binding motif in the 5'-flanking region of Nephrin and Neph3 genes. Third, Ptf1a activated transcription driven by the 5'-flanking region of these genes. Finally, the expression of Nephrin and Neph3 was lost in Ptf1a-null mice, whereas ectopic expression of Nephrin and Neph3 was induced by forced expression of Ptf1a. We provided further evidence that Nephrin and Neph3 could interact homophilically and heterophilically, suggesting that Nephrin and Neph3 might regulate certain developmental aspects of Ptf1a-positive neurons as homo- or heterooligomers.

Regulation of Neph3 gene in podocytes--key roles of transcription factors NF-kappaB and Sp1

BACKGROUND

Neph3 (filtrin) is expressed in the glomerular podocytes where it localizes at the specialized cell adhesion structures of the foot processes called slit diaphragms which form the outermost layer of the glomerular filtration barrier. Neph3 protein shows homology and structural similarity to Neph1, Neph2 and nephrin, which all are crucial for maintaining the normal glomerular ultrafiltration function. The exact function of Neph3 in the kidney is not known but we have previously shown that the level of Neph3 mRNA is decreased in proteinuric diseases. This suggests that Neph3 may play a role in the pathogenesis of kidney damage, and emphasizes the need to analyze the regulatory mechanisms of Neph3 gene. In this study we investigated the transcriptional regulation of Neph3 gene by identifying transcription factors that control Neph3 expression.

RESULTS

We cloned and characterized approximately 5 kb fragment upstream of the Neph3 gene. Neph3 proximal promoter near the transcription start site was found to be devoid of TATA and CAAT boxes, but to contain a highly GC-rich area. Using promoter reporter gene constructs, we localized the main activating regulatory region of Neph3 gene in its proximal promoter region from -105 to -57. Within this region, putative transcription factor binding sites for NF-kappaB and Sp1 were found by computational analysis. Mutational screening indicated that NF-kappaB and Sp1 response elements are essential for the basal transcriptional activity of the Neph3 promoter. Co-transfection studies further showed that NF-kappaB and Sp1 regulate Neph3 promoter activity. In addition, overexpression of NF-kappaB increased endogenous Neph3 gene expression. Chromatin immunoprecipitation assay using cultured human podocytes demonstrated that both NF-kappaB and Sp1 interact with the Neph3 promoter.

CONCLUSION

Our results show that NF-kappaB and Sp1 are key regulators of Neph3 expression at the basal level in podocytes, therefore providing new insight into the molecular mechanisms that contribute to the expression of Neph3 gene.

Expression of galectin-1, a new component of slit diaphragm, is altered in minimal change nephrotic syndrome

Nephrin is an essential structural component of the glomerular slit diaphragm (SD), a highly organized intercellular junction that constitutes the ultrafiltration barrier of the kidney. Recent studies have identified two additional nephrin-interacting SD proteins (NEPH1 and NEPH2), suggesting that the zipper-like pattern of the SD is formed through complex intra- and intermolecular interactions of these proteins. However, the complexity of the SD structure suggests that additional SD components remain to be discovered. In this study, we identified galectin-1 (Gal-1) as a new component of the SD, binding to the ectodomain of nephrin. Using dual-immunofluorescence and confocal microscopy and dual-immunoelectron microscopy, we found Gal-1 co-localizing with the ectodomain of nephrin at the SD in normal human kidney. By immunoprecipitation and surface plasmon resonance, we confirmed a direct molecular interaction between Gal-1 and nephrin. Moreover, recombinant Gal-1 induced tyrosine phosphorylation of the cytoplasmic domain of nephrin and activation of the extracellular signal-regulated kinase 1/2 in podocytes. We also showed that podocytes are a major site of biosynthesis of Gal-1 in the glomerulus and that the normal expression patterns and levels of Gal-1 are altered in patients with minimal change nephrotic syndrome. Finally, in puromycin aminonucleoside-induced rat nephrosis, an apparent reduction in the levels of Gal-1 and nephrin around the onset of heavy proteinuria was also revealed. Our data present Gal-1 as a new extracellular ligand of nephrin localized at the glomerular SD, and provide further insight into the complex molecular organization, interaction, and structure of the SD, which is an active site of intracellular signaling necessary for podocyte function.

The irre cell recognition module (IRM) proteins

One of the most challenging problems in developmental neurosciences is to understand the establishment and maintenance of specific membrane contacts between axonal, dendritic, and glial processes in the neuropils, which eventually secure neuronal connectivity. However, underlying cell recognition events are pivotal in other tissues as well. This brief review focuses on the pleiotropic functions of a small, evolutionarily conserved group of proteins of the immunoglobulin superfamily involved in cell recognition. In Drosophila, this protein family comprises Irregular chiasm C/Roughest (IrreC/Rst), Kin of irre (Kirre), and their interacting protein partners, Sticks and stones (SNS) and Hibris (Hbs). For simplicity, we propose to name this ensemble of proteins the irre cell recognition module (IRM) after the first identified member of this family. Here, we summarize evidence that the IRM proteins function together in various cellular interactions, including myoblast fusion, cell sorting, axonal pathfinding, and target recognition in the optic neuropils of Drosophila. Understanding IRM protein function will help to unravel the epigenetic rules by which the intricate neurite networks in sensory neuropils are formed.

Neph-Nephrin proteins bind the Par3-Par6-atypical protein kinase C (aPKC) complex to regulate podocyte cell polarity

The kidney filter represents a unique assembly of podocyte epithelial cells that tightly enwrap the glomerular capillaries with their foot processes and the interposed slit diaphragm. So far, very little is known about the guidance cues and polarity signals required to regulate proper development and maintenance of the glomerular filtration barrier. We now identify Par3, Par6, and atypical protein kinase C (aPKC) polarity proteins as novel Neph1-Nephrin-associated proteins. The interaction was mediated through the PDZ domain of Par3 and conserved carboxyl terminal residues in Neph1 and Nephrin. Par3, Par6, and aPKC localized to the slit diaphragm as shown in immunofluorescence and immunoelectron microscopy. Consistent with a critical role for aPKC activity in podocytes, inhibition of glomerular aPKC activity with a pseudosubstrate inhibitor resulted in a loss of regular podocyte foot process architecture. These data provide an important link between cell recognition mediated through the Neph1-Nephrin complex and Par-dependent polarity signaling and suggest that this molecular interaction is essential for establishing the three-dimensional architecture of podocytes at the kidney filtration barrier.

Preeclamptic sera induce nephrin shedding from podocytes through endothelin-1 release by endothelial glomerular cells

In preeclampsia (PE), proteinuria has been associated with a reduced expression of nephrin by podocytes. In the present study, we investigated in vitro on human cultured podocytes the mechanism responsible for nephrin loss in PE. Sera from patients with PE did not directly downregulate the expression of nephrin. In contrast, conditioned medium obtained from glomerular endothelial cells incubated with PE sera induced loss of nephrin and synaptopodin, but not of podocin, from podocytes. Nephrin loss was related to a rapid shedding of the protein from the cell surface due to cleavage of its extracellular domain by proteases and to cytoskeleton redistribution. The absence of nephrin mRNA downregulation together with nephrin reexpression within 24 h confirm that the loss of nephrin was not related to a reduced synthesis. Studies with an endothelin-1 (ET-1) receptor antagonist that abrogated the loss of nephrin triggered by glomerular endothelial conditioned medium of PE sera indicated that ET-1 was the main effector of nephrin loss. Indeed, ET-1 was synthesized and released from glomerular endothelial cells when incubated with PE sera, and recombinant ET-1 triggered nephrin shedding from podocytes. Moreover, VEGF blockade induced ET-1 release from endothelial cells, and in turn the conditioned medium obtained triggered nephrin loss. In conclusion, the present study identifies a potential mechanism of nephrin loss in PE that may link endothelial injury with enhanced glomerular permeability.

Epithelial-to-mesenchymal transition is a potential pathway leading to podocyte dysfunction and proteinuria

Podocyte dysfunction plays an essential role in the pathogenesis of proteinuria and glomerulosclerosis. However, the mechanism underlying podocyte dysfunction in many common forms of chronic kidney diseases remains poorly understood. Here we tested the hypothesis that podocytes may undergo epithelial-to-mesenchymal transition after injury. Conditionally immortalized mouse podocytes were incubated with transforming growth factor (TGF)-beta1, a potent fibrogenic cytokine that is up-regulated in the diseased kidney. TGF-beta1 suppressed the slit diaphragm-associated protein P-cadherin, zonula occludens-1, and nephrin, a change consistent with loss of the epithelial feature. Meanwhile, TGF-beta1 induced the expression of the intermediate filament protein desmin and interstitial matrix components fibronectin and collagen I. Furthermore, TGF-beta1 promoted the expression and secretion of matrix metalloproteinase-9 by podocytes. Functionally, TGF-beta1 increased albumin permeability across podocyte monolayers, as demonstrated by a paracellular albumin influx assay. The expression of Snail, a key transcriptional factor that has been implicated in initiating epithelial-to-mesenchymal transition, was induced by TGF-beta1, and ectopic expression of Snail suppressed P-cadherin and nephrin in podocytes. In vivo, in addition to loss of nephrin and zonula occludens-1, mesenchymal markers such as desmin, fibroblast-specific protein-1, and matrix metalloproteinase-9 could be observed in glomerular podocytes of diabetic nephropathy. These results suggest that podocyte dedifferentiation and mesenchymal transition could be a potential pathway leading to their dysfunction, thereby playing a role in the genesis of proteinuria.

Development of the renal glomerulus: good neighbors and good fences

The glomerulus of the mammalian kidney is an intricate structure that contains an unusual filtration barrier that retains higher molecular weight proteins and blood cells in the circulation. Recent studies have changed our conception of the glomerulus from a relatively static structure to a dynamic one, whose integrity depends on signaling between the three major cell lineages: podocytes, endothelial and mesangial cells. Research into the signaling pathways that control glomerular development and then maintain glomerular integrity and function has recently identified several genes, such as the nephrin and Wilms' tumor 1 genes, that are mutated in human kidney disease.