Insight into podocyte differentiation from the study of human genetic disease: nail-patella syndrome and transcriptional regulation in podocytes

LMX1B-Associated Nephropathy With Type III Collagen Deposition in the Glomerular and Tubular Basement Membranes

Variants in the LMX1B gene cause nail-patella syndrome, a rare autosomal dominant disorder characterized by dysplasia of nails, patella and elbow abnormalities, iliac "horns," and glaucoma. We describe an adult man with nephrotic syndrome and no systemic manifestations of nail-patella syndrome at the time of his initial kidney biopsy. His kidney biopsy was initially interpreted as a form of segmental sclerosis with unusual fibrillar deposits. At the time of consideration for kidney transplantation, a family history was notable for end-stage renal disease in 3 generations. Subsequent reanalysis of the initial biopsy showed infiltration of the lamina densa by type III collagen fibrils, and molecular studies identified a pathogenic variant in one allele of LMX1B (a guanine to adenine substitution at nucleoide 737 of the coding sequence [c.737G>A], predicted to result in an arginine to glutamine substitution at amino acid 246 [p.Arg246Gln]). This variant has been described previously in multiple unrelated families who presented with autosomal dominant nephropathy without nail and patellar abnormalities.

Spectrum of LMX1B mutations: from nail-patella syndrome to isolated nephropathy

Nail-patella syndrome (NPS) is an autosomal-dominant disease caused by LMX1B mutations and is characterized by dysplastic nails, absent or hypoplastic patellae, elbow dysplasia, and iliac horns. Renal involvement is the major determinant of the prognosis for NPS. Patients often present with varying degrees of proteinuria or hematuria, and can occasionally progress to chronic renal failure. Recent genetic analysis has found that some mutations in the homeodomain of LMX1B cause isolated nephropathy without nail, patellar or skeletal abnormality (LMX1B-associated nephropathy). The classic term "nail-patella syndrome" would not represent disease conditions in these cases. This review provides an overview of NPS, and highlights the molecular genetics of NPS nephropathy and LMX1B-associated nephropathy. Our current understanding of LMX1B function in the pathogenesis of NPS and LMX1B-associated nephropathy is also presented, and its downstream regulatory networks discussed. This recent progress provides insights that help to define potential targeted therapeutic strategies for LMX1B-associated diseases.

Cell fate determination, neuronal maintenance and disease state: The emerging role of transcription factors Lmx1a and Lmx1b

LIM-homeodomain (LIM-HD) proteins are evolutionary conserved developmental transcription factors. LIM-HD Lmx1a and Lmx1b orchestrate complex temporal and spatial gene expression of the dopaminergic pathway, and evidence shows they are also involved in adult neuronal homeostasis. In this review, the multiple roles played by Lmx1a and Lmx1b will be discussed. Controlled Lmx1a and Lmx1b expression and activities ensure the proper formation of critical signaling centers, including the embryonic ventral mesencephalon floor plate and sharp boundaries between lineage-specific cells. Lmx1a and Lmx1b expression persists in mature dopaminergic neurons of the substantia nigra pars compacta and the ventral tegmental area, and their role in the adult brain is beginning to be revealed. Notably, LMX1B expression was lower in brain tissue affected by Parkinson's disease. Actual and future applications of Lmx1a and Lmx1b transcription factors in stem cell production as well as in direct conversion of fibroblast into dopaminergic neurons are also discussed. A thorough understanding of the role of LMX1A and LMX1B in a number of disease states, including developmental diseases, cancer and neurodegenerative diseases, could lead to significant benefits for human healthcare.

LIM homeobox transcription factor 1B expression affects renal interstitial fibrosis and apoptosis in unilateral ureteral obstructed rats

LIM homeobox transcription factor 1B (LMX1B) is a transcription factor of the LIM homeodomain type and has been implicated in the development of diverse structures such as limbs, kidneys, eyes, and the brain. Furthermore, LMX1B has been implicated in nail-patella syndrome, which is predominantly characterized by malformation of limbs and nails, and in 30% of patients, nephropathy, including renal fibrosis, is observed. Since no reports were available that studied the link between LMX1B expression and renal interstitial fibrosis, we explored if LMX1B affects typical markers of fibrosis, e.g., extracellular matrix components, profibrotic factors, and apoptosis as the final detrimental consequence. We recently showed that LMX1B acts as a negative regulator of transforming growth factor-βl, collagen type III, fibronectin, cleaved caspase-3, and the cell apoptosis rate in a renal tubular epithelial cell system under hypoxic conditions. Here, we confirmed these results in unilateral ureteral obstructed rats. Furthermore, LMX1B was distinctly expressed throughout the glomerulus and tubule lining, including epithelial cells. Knockdown of LMX1B aggravated the expression of fibrosis markers, oxidative stress, and apoptosis compared with the already increased levels due to unilateral ureteral obstruction, whereas overexpression attenuated these effects. In conclusion, reduced LMX1B levels clearly represent a risk factor for renal fibrosis, whereas overexpression affords some level of protection. In general, LMX1B may be considered to be a negative regulator of the fibrosis index, transforming growth factor-βl, collagen type III, fibronectin, cleaved caspase-3, cell apoptosis, ROS, and malondialdehyde (r = -0.756, -0.698, -0.921, -0.923, -0.843, -0.794, -0.883, and -0.825, all P < 0.01).

LIM homeobox transcription factor 1B is associated with pro-fibrotic components and apoptosis in hypoxia/reoxygenation renal tubular epithelial cells

LIM homeobox transcription factor 1B (LMX1B) is a transcription factor of the LIM-homeodomain type, which plays an important role in foetal development during formation of the extremities, kidneys, eyes, and the brain. Furthermore, LMX1B has been implicated in nail-patella syndrome, which is predominantly characterized by malformation of limbs and nails, and in 30 % of patients, nephropathy, including renal fibrosis, is observed. Since no reports were available that studied the link between LMX1B expression and pro-fibrotic components and apoptosis in hypoxic renal tubular epithelial cells (RTEC), we explored if LMX1B was associated with extracellular matrix components, profibrotic factors, and apoptosis induced by hypoxia/reoxygenation (H-R). In this cell system under hypoxic conditions, when the expression of LMX1B was inhibited in H-R RTEC, the expression of transforming growth factor-βl, collagen-III, fibronectin, cleaved caspase-3, and cell apoptosis rate was increased. Consequently, overexpression of LMX1B was associated with reduced cell apoptosis, whilst downregulation of LMX1B was associated with increased cell apoptosis in H-R RTEC.

The signaling pathways of LMX1B and its role in glomerulosclerosis

LMX1B, a developmental LIM-homeodomain transcription factor, is widely expressed in vertebrate embryos, and it takes part in the development of diverse structures such as limbs, kidneys, eyes, brains, etc. LMX1B contributes to transcriptional regulation of glomerular basement membrane (GBM) collagen expression by podocytes. The normal function of podocytes and the normal morphology of GBM are very important to maintain the healthy renal filtration barrier. Recent discoveries find that the LMX1B gene is pivotal in glomus development and it is implicated in the dysfunction of the podocytes. Here, we review the signal transduction pathways of LMX1B and its role in the pathogenesis of glomerulosclerosis.

Kidney podocytes as specific targets for cyclo(RGDfC)-modified nanoparticles

Renal nanoparticle passage opens the door for targeting new cells like podocytes, which constitute the exterior part of the renal filter. When cyclo(RGDfC)-modified Qdots are tested on isolated primary podocytes for selective binding to the αvβ3 integrin receptor a highly cell- and receptor-specific binding can be observed. In displacement experiments with free cyclo(RGDfC) IC(50) values of 150 nM for αvβ3 integrin over-expressing U87-MG cells and 60 nM for podocytes are measured. Confocal microscopy shows a cellular Qdot uptake into vesicle-like structures. Our ex vivo study gives clear evidence that, after renal filtration, nanoparticles can be targeted to podocyte integrin receptors in the future. This could be a highly promising approach for future therapy and diagnostics of podocyte-associated diseases.

Dynamic lineage analysis of embryonic morphogenesis using transgenic quail and 4D multispectral imaging

We describe the development of transgenic quail that express various fluorescent proteins in targeted manners and their use as a model system that integrates advanced imaging approaches with conventional and emerging molecular genetics technologies. We also review the progression and complications of past fate mapping techniques that led us to generate transgenic quail, which permit dynamic imaging of amniote embryogenesis with unprecedented subcellular resolution.

The glomerulus--a view from the outside--the podocyte

In the past decade, podocyte research has been greatly aided by the development of powerful new molecular, cellular and animal tools, leading to elucidation of an increasing number of proteins involved in podocyte function and identification of mutated genes in hereditary glomerulopathies. Accumulating evidence indicates that podocyte disorders may not only underlie these hereditary glomerulopathies but also play crucial role in a broad spectrum of acquired glomerular diseases. Genetic susceptibility, environmental influence and systemic responses are all involved in the mediation of the pathogenesis of podocytopathies. Injured podocytes may predisopose to further injury of other podocytes and other adjacent/distant renal cells in a vicious cycle, leading to inexorable progression of glomerular injury. The classic view is that podocytes have a limited ability to proliferate in the normal mature kidney. However, recent research in rodents has provided suggestive evidence for podocyte regeneration resulting from differentiation of progenitor cells within Bowman's capsule.

Kidney disease in nail-patella syndrome

Nail-patella syndrome (NPS) is a pleiotropic autosomal-dominant disorder due to mutations in the gene LMX1B. It has traditionally been characterized by a tetrad of dermatologic and musculoskeletal abnormalities. However, one of the most serious manifestations of NPS is kidney disease, which may be present in up to 40% of affected individuals. Although LMX1B is a developmental LIM-homeodomain transcription factor, it is expressed in post-natal life in the glomerular podocyte, suggesting a regulatory role in that cell. Kidney disease in NPS seems to occur more often in some families with NPS, but it does not segregate with any particular mutation type or location. Two patterns of NPS nephropathy may be distinguished. Most affected individuals manifest only an accelerated age-related loss of filtration function in comparison with unaffected individuals. Development of symptomatic kidney failure is rare in this group, and proteinuria (present in approximately one-third) does not appear to be progressive. A small minority (5-10%) of individuals with NPS develop nephrotic-range proteinuria as early as childhood or young adulthood and progress to end-stage kidney failure over variable periods of time. It is proposed that this latter group reflects the effects of more global podocyte dysfunction, possibly due to the combination of a mutation in LMX1B along with an otherwise innocuous polymorphism or mutation involving any of several genes expressed in podocytes (e.g. NPHS2, CD2AP), the transription of which is regulated by LMX1B.

Transcriptional regulation of podocyte disease

The podocyte is a highly specialized visceral epithelial cell that forms the outermost layer of the glomerular capillary loop and plays a critical role in the maintenance of the glomerular filtration barrier. Several transcriptional factors regulate the podocyte function under normal and disease conditions. In this review, the role of Wilms tumor 1 (WT1), LIM homeobox transcription factor 1, beta (Lmx1b), pod1, pax-2, kreisler, nuclear factor-kappa B (NF-kappaB), smad7, and zinc fingers and homeoboxes (ZHX) proteins in the development of podocyte disease is outlined. The regulation of several important podocyte genes, including transcriptional factors, by ZHX proteins, their predominant non-nuclear localization in the normal in vivo podocyte, and changes in ZHX expression related to the development of minimal change disease and focal and segmental glomerulosclerosis are discussed. Finally, some future therapeutic strategies for glomerular disease are proposed.

Klinische, radiologische und arthroskopische Aspekte des Nagel-Patella-Syndroms (NPS): Literaturübersicht am Beispiel einer betroffenen Familie

Nail patella syndrome (NPS) is an autosomal dominant hereditary disorder affecting the nails, skeletal system, kidneys, and eyes. Skeletal features include absent or hypoplastic patellae, patella dislocations, elbow abnormalities, talipes and iliac horns on plain films. The existing literature focuses on clinical and radiographic findings in patients with NPS. We also report the case of a 40-year-old male patient and his family affected by NPS and includes clinical, radiographic as well as arthroscopic findings. Arthroscopic findings in this case are characterized by multiple synovial plicae in the knee joint leading to cartilage defects on the corresponding cartilage surfaces. A review of the recent literature suggests that the occurrence of synovial plicae might be related to NPS and might account for at least part of the characteristic symptoms of these patients. Therefore, in cases of unspecific knee pain associated with NPS arthroscopy of the knee joints and plicae resection seems to be recommendable.

Interaction of the LMX1B and PAX2 gene products suggests possible molecular basis of differential phenotypes in Nail-Patella syndrome

The LMX1B gene, encoding a protein involved in limb, kidney and eye development, is mutated in patients affected by Nail-Patella syndrome. Inter- and intrafamilial variability is common in this disorder for skeletal abnormalities, presence and severity of nephropathy and ocular anomalies. Phenotypic variability might depend on interactions of the LMX1B causative gene with other genes during development of both kidney and eye, which might act as modifier genes. Results are presented on the interaction between LMX1B and PAX2 proteins, obtained by both direct yeast two-hybrid assay and coimmunoprecipitation. Such interaction provides support to further studies on pathways underlying important developmental processes.

Complement C2 receptor inhibitor trispanning: a novel human complement inhibitory receptor

The complement system presents a powerful defense against infection and is tightly regulated to prevent damage to self by functionally equivalent soluble and membrane regulators. We describe complement C2 receptor inhibitor trispanning (CRIT), a novel human complement regulatory receptor, expressed on hemopoietic cells and a wide range of tissues throughout the body. CRIT is present in human parasites through horizontal transmission. Serum complement component C2 binds to the N-terminal extracellular domain 1 of CRIT, which, in peptide form, blocks C3 convertase formation and complement-mediated inflammation. Unlike C1 inhibitor, which inhibits the cleavage of C4 and C2, CRIT only blocks C2 cleavage but, in so doing, shares with C1 inhibitor the same functional effect, of preventing classical pathway C3 convertase formation. Ab blockage of cellular CRIT reduces inhibition of cytolysis, indicating that CRIT is a novel complement regulator protecting autologous cells.

Nephrotic syndrome in childhood

Childhood nephrotic syndromes are most commonly caused by one of two idiopathic diseases: minimal-change nephrotic syndrome (MCNS) and focal segmental glomerulosclerosis (FSGS). A third distinct type, membranous nephropathy, is rare in children. Other causes of isolated nephrotic syndrome can be subdivided into two major categories: rare genetic disorders, and secondary diseases associated with drugs, infections, or neoplasia. The cause of idiopathic nephrotic syndrome remains unknown, but evidence suggests it may be a primary T-cell disorder that leads to glomerular podocyte dysfunction. Genetic studies in children with familial nephrotic syndrome have identified mutations in genes that encode important podocyte proteins. Patients with idiopathic nephrotic syndrome are initially treated with corticosteroids. Steroid-responsiveness is of greater prognostic use than renal histology. Several second-line drugs, including alkylating agents, ciclosporin, and levamisole, may be effective for complicated and steroid-unresponsive MCNS and FSGS patients. Nephrotic syndrome is associated with several medical complications, the most severe and potentially fatal being bacterial infections and thromboembolism. Idiopathic nephrotic syndrome is a chronic relapsing disease for most steroid-responsive patients, whereas most children with refractory FSGS ultimately develop end-stage renal disease. Research is being done to further elucidate the disorder's molecular pathogenesis, identify new prognostic indicators, and to develop better approaches to treatment.

Update in podocyte biology: putting one's best foot forward

PURPOSE OF REVIEW

The rapidly developing field of podocyte cell biology is reviewed, focusing on papers published in the last 12 months.

RECENT FINDINGS

Four areas of particular progress can be discerned. First, podocytes proliferate during early metanephric development, are quiescent after the capillary loop stage, and re-enter the cell cycle only in the disease group termed collapsing glomerulopathy. We have learned that control of the podocyte cell cycle involves both expression of cell-cycle regulating proteins and the process of cytokinesis. Second, the podocyte slit diaphragm is the final component of the filtration barrier. The structure and maintenance of the slit diaphragm has been a major focus of research activity, and a multiplicity of relevant molecular interactions have been defined. Significant advances have been made in understanding the complex and interacting role of nephrin and podocin mutations in the genesis of clinical glomerular disease. Third, several proteins essential to controlling discrete podocyte transcriptional programs have been defined. Finally, conditionally-immortalized podocyte cell lines, derived from mouse and human tissue, have proven their worth as models to advance investigations of podocyte biology.

SUMMARY

Podocyte injury occurs as a consequence of genetic mutation, immunological injury, viral infection, or abnormal hemodynamic forces within the glomerulus. As we understand more about the podocyte proteome and cell biology, we gain an increasingly detailed molecular understanding of podocyte structure and function. In this drama we have many molecular players and increasing stretches of molecular dialogue, but the script remains largely to be deciphered. Nevertheless, we do understand the consequences that arise when the podocyte cannot put its best foot (processes) forward.

Identification of a key protein associated with cerebral ischemia

The biochemical effects of permanent focal ischemia following unilateral occlusion of the middle cerebral artery in rats were studied by determining the content of specific proteins of the affected areas in the cerebral hemisphere. Brain proteins were prepared 72 h after the occlusion and analyzed by sodium dodecylsulfate-polyacrylamide gel electrophoresis. A significant increase in 66 and 80 kDa components and a paradoxical decrease in 260 kDa protein occurred in the ischemic brain tissues. The 66 and 80 kDa protein bands were identified as albumin and transferrin, respectively. The 260 kDa protein was analyzed by peptide mass fingerprinting (PMF) and matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS). The isoelectric point of the 260 kDa protein was 4.65 determined by isoelectric focusing. The data obtained from PMF were used in searching the protein database for homologous components. Three proteins with partial homology were identified. They were the microtubule-associated protein 1A, protein-tyrosine phosphatase zeta precursor (phosphacan), and protein kinase A anchoring protein 6. Polyclonal antibodies against the 260 kDa protein were raised and used to immunolocalize the antigen in various tissues. Positive staining occurred with brain neurons and pyramidal cells, islet cells, podocytes of kidney glomeruli, and endothelial cells of the venous sinuses of the spleen. The localization of 260 kDa protein strongly implies its function in these tissues. Its physiological and pathophysiological significances need to be clarified in future.

The genetic basis of FSGS and steroid-resistant nephrosis

Studies of Mendelian forms of focal segmental glomerulosclerosis (FSGS) and nephrotic syndrome have provided new insights into the mechanism of these diseases. Congenital nephrotic syndrome and familial forms of FSGS form a spectrum of podocyte diseases of varying severity and age of onset. Mutations in both nephrin gene (NPHS1) alleles lead to congenital nephrosis, podocyte foot process efacement, and loss of slit-diaphragm structure. Mutations in both podocin gene (NPHS2) alleles lead to a wide range of human disease, from childhood-onset steroid-resistant FSGS and minimal change disease to adult-onset FSGS. Dominantly inherited mutations in ACTN4, the alpha-actinin-4 gene, can lead to a slowly progressive adult-onset form of FSGS. In addition, FSGS is observed as part of several rare multisystem inherited syndromes. Here we review recent progress in understanding the genetic basis of FSGS in humans.

Nail patella syndrome: a review of the phenotype aided by developmental biology

Nail patella syndrome (NPS) is an autosomal dominant condition affecting the nails, skeletal system, kidneys, and eyes. Skeletal features include absent or hypoplastic patellae, patella dislocations, elbow abnormalities, talipes, and iliac horns on x ray. Kidney involvement may lead to renal failure and there is also a risk of glaucoma. There is marked inter- and intrafamilial variability. The results of a British study involving 123 NPS patients are compared with previously published studies and it is suggested that neurological and vasomotor symptoms are also part of the NPS phenotype. In addition, the first data on the incidence of glaucoma and gastrointestinal (GI) symptoms in NPS are presented. NPS is caused by loss of function mutations in the transcription factor LMX1B at 9q34. The expansion of the clinical phenotype is supported by the role of LMX1B during development.