Laminins (LM) are extracellular matrix molecules that contribute to and are required for the formation of basement membranes. They participate in the modulation of epithelial/mesenchymal interactions and are implicated in organogenesis and maintenance of organ homeostasis. Among the LM molecules, the LM α5 chain (LMα5) is one of the most widely distributed LM in the developing and mature organism. Its presence in some basement membranes during embryogenesis is absolutely required for maintenance of basement membrane integrity and thus for proper organogenesis. LMα5 also regulates the expression of genes important for major biological processes, in part by repressing or activating signaling pathways, depending upon the physiological context.

The glomerular basement membrane (GBM) is a crucial component of the kidney's filtration barrier that separates the vasculature from the urinary space. During glomerulogenesis, the GBM is formed from fusion of two distinct basement membranes, one synthesized by the glomerular epithelial cell (podocyte) and the other by the glomerular endothelial cell. The main components of the GBM are laminin-521 (α5β2γ1), collagen α3α4α5(IV), nidogen and the heparan sulfate proteoglycan, agrin. By studying mice lacking specific GBM components, we have shown that during glomerulogenesis, laminin is the only one that is required for GBM integrity and in turn, the GBM is required for completion of glomerulogenesis and glomerular vascularization. In addition, our results from laminin β2-null mice suggest that laminin-521, and thus the GBM, contribute to the establishment and maintenance of the glomerular filtration barrier to plasma albumin. In contrast, mutations that affect GBM collagen IV or agrin do not impair glomerular development or cause immediate leakage of plasma proteins. However, collagen IV mutation, which causes Alport syndrome and ESRD in humans, leads to gradual damage to the GBM that eventually leads to albuminuria and renal failure. These results highlight the importance of the GBM for establishing and maintaining a perfectly functioning, highly selective glomerular filter.

The glomerular basement membrane (GBM) is an especially thick basement membrane that contributes importantly to the kidney’s filtration barrier. The GBM derives from the fusion of separate podocyte and endothelial cell basement membranes during glomerulogenesis and consists primarily of laminin-521 (α5β2γ1), collagen α3α4α5(IV), nidogens-1 and -2, and agrin. Of these nine proteins, mutations in the genes encoding four of them (LAMB2, COL4A3, COL4A4, and COL4A5) cause glomerular disease in humans as well as in mice. Furthermore, mutation of a fifth (Lama5) gene in podocytes in mice causes proteinuria, nephrotic syndrome, and progression to renal failure. These results highlight the importance of the GBM for establishing and maintaining a properly functioning glomerular filtration barrier.

Background. For several decades, it has been thought that the glomerular basement membrane (GBM) provides a charge-selective barrier for glomerular filtration. However, recent evidence has presented challenges to this concept: selective removal of heparan sulfate (HS) moieties that impart a negative charge to the GBM causes little if any increase in proteinuria. Removal of agrin, the major GBM HS-proteoglycan (HSPG), from the GBM causes a profound reduction in the glomerular anionic charge without changing the excretion of a negatively charged tracer. Perlecan is another HSPG present in the GBM, as well as in the mesangium and Bowman's capsule, that could potentially contribute to a charge barrier in the absence of agrin.

Methods. Here we studied the nature of the glomerular filtration barrier to albumin in mice lacking the HS chains of perlecan either alone or in combination with podocyte-specific loss of agrin.

Results. The results show significant reductions in anionic sites within the GBM in perlecan-HS and in perlecan-HS/agrin double mutants. Podocyte and overall glomerular architecture were normal, and renal function was normal up to 15 months of age with no measurable proteinuria. Moreover, excretion of a negatively charged Ficoll tracer was unchanged as compared to control mice.

Conclusions. These findings cast further doubt upon a critical role for the GBM in charge selectivity.

Purpose of the review

The nephrology community lacks a unified view of protein sieving through the glomerular capillary wall (GCW). The GCW consists of three distinct but closely interacting layers: the fenestrated endothelium, with its glycocalyx; the podocytes, with their interdigitated foot processes and slit diaphragms; and the intervening glomerular basement membrane (GBM). Proteinuria is associated with abnormalities in any one layer, suggesting that each contributes to the glomerular filtration barrier (GFB). Proteinuria can also be induced in the context of a normal GCW. Here we review some classic studies as well as some newer concepts and present competing hypotheses about the GFB.

Recent findings

Two almost forgotten concepts have recently emerged. One group has challenged the exquisite selectivity of the GFB to albumin and suggested that proteinuria is the result of abnormal tubular uptake. There has also been a reemphasis on diffusion through the GBM as the driving force behind macromolecular filtration. New evidence suggests that the endothelial glycocalyx is an important charge-selective barrier.

Summary

We suggest viewing the GFB as a dynamic rather than as a rigid barrier, requiring three healthy layers and a hemodynamic steady state. Multiple challenges to studying the endothelium, the tubular handling of albumin, and the role of hemodynamic forces will require new tools, new hypotheses, and open minds.

Summary

Pax3-Cre (P3Pro-Cre) transgenic mice have been used for conditional gene deletion and/or lineage tracing in derivatives of neural crest, neural tube, metanephric mesenchyme, and ureteric mesenchyme. However, the extent of its expression in skeletal muscle has not been reported. We investigated the expression of P3Pro-Cre in the skeletal muscle lineage using the R26R reporter and found an unexpected rostrocaudal gradient of expression. By X-gal staining, head, neck, forelimb, diaphragm, and most of the chest wall muscles did not show evidence of Cre expression, whereas all muscle groups posterior of the diaphragm stained blue. Intercostal muscles exhibited a rostrocaudal gradient of staining. The consistency of this expression pattern was demonstrated by using P3Pro-Cre to mutate a conditional dystroglycan allele. The result was loss of dystroglycan from caudal muscles, which exhibited the histological signs of muscle fiber injury and regeneration characteristic of muscular dystrophy. The lack of dystroglycan in regenerating myofibers suggests that the P3Pro-Cre transgene is active in satellite cells and/or in their precursors. In contrast, rostral muscles, including feeding and breathing muscles, maintained dystroglycan expression and were spared from disease. Accordingly, the mutants were viable for over a year. Its unique gradient of activity makes the P3Pro-Cre transgene a previously unappreciated yet powerful tool for manipulating gene expression in skeletal muscle and its precursors.

Summary

The mammalian intestine displays two distinct patterns of mucosal organization. The small intestine contains mucosal epithelial invaginations called crypts of Lieberkühn that are continuous with evaginations into the lumen called villi. The colon also contains crypts, but its epithelial surface is lined by flat surface cuffs. The epithelial cells of both organs communicate with the underlying mesenchyme through a basement membrane that is composed of a variety of extracellular matrix proteins, including members of the laminin family. The basement membranes of the small intestine and colon contain distinct laminin subtypes; notably, the villus basement membrane is rich in laminin α5. Here we show that diminution of laminin α5 in a mouse model led to a compensatory deposition of colonic laminins that resulted in a transformation from a small intestinal to a colonic mucosal architecture. The alteration in mucosal architecture was associated with reduced levels of nuclear p27Kip1, a cell cycle regulator, and altered intestinal epithelial cell proliferation, migration, and differentiation. Our results suggest that laminin α5 plays a crucial role in establishing and maintaining the specific mucosal pattern of the mouse small intestine.

The kidney filter represents a unique assembly of podocyte epithelial cells that tightly enwrap the glomerular capillaries with their complex foot process network. While deficiency of the polarity proteins Crumbs and aPKC result in impaired podocyte foot process architecture, the function of basolateral polarity proteins for podocyte differentiation and maintenance remained unclear. Here we report, that Scribble is expressed in developing podocytes, where it translocates from the lateral aspects of immature podocytes to the basal cell membrane and foot processes of mature podocytes. Immunogold electron microscopy reveals membrane associated localisation of Scribble predominantly at the basolateral site of foot processes. To further study the role of Scribble for podocyte differentiation Scribbleflox/flox mice were generated by introducing loxP-sites into the Scribble introns 1 and 8 and these mice were crossed to NPHS2.Cre mice and Cre deleter mice. Podocyte-specific Scribble knockout mice develop normally and display no histological, ultrastructural or clinical abnormalities up to 12 months of age. In addition, no increased susceptibility to glomerular stress could be detected in these mice. In contrast, constitutive Scribble knockout animals die during embryonic development indicating the fundamental importance of Scribble for embryogenesis. Like in podocyte-specific Scribble knockout mice, the development of podocyte foot processes and the slit diaphragm was unaffected in kidney cultures from constitutive Scribble knockout animals. In summary these results indicate that basolateral polarity signaling via Scribble is dispensable for podocyte function, highlighting the unique feature of podocyte development with its significant apical membrane expansions being dominated by apical polarity complexes rather than by basolateral polarity signaling.

Extracellular matrix abnormalities have been found in both human and animal models of polycystic kidney disease (PKD). We have produced a new mouse PKD model through insertion of a PGKneo cassette in an intron of the gene encoding laminin α5, a major tubular and glomerular basement membrane component important for glomerulogenesis and ureteric bud branching. Lama5neo represents a hypomorphic allele due to aberrant splicing. Lama5neo/neo mice exhibit PKD, proteinuria, and death from renal failure by 4 weeks of age. This contrasts with mice totally lacking laminin α5, which die in utero with multiple developmental defects. At 2 days of age, Lama5neo/neo mice exhibited mild proteinuria and microscopic cystic transformation. By 2 weeks, cysts were grossly apparent in cortex and medulla, involving both nephron and collecting duct segments. Tubular basement membranes appeared to form normally, and early cyst basement membranes showed normal ultrastructure but developed marked thickening as cysts enlarged. Overall, laminin α5 protein levels were severely reduced due to mRNA frameshift caused by exon skipping. This was accompanied by aberrant accumulation of laminin-332 (α3β3γ2; formerly called laminin-5) in some cysts, as also observed in human PKD. This constitutes the first evidence that a primary defect in an extracellular matrix component can cause PKD.

Summary

Pierson syndrome is a recently defined disease usually lethal within the first postnatal months and caused by mutations in the gene encoding laminin β2 (LAMB2). The hallmarks of Pierson syndrome are congenital nephrotic syndrome accompanied by ocular abnormalities, including microcoria (small pupils), with muscular and neurological developmental defects also present. Lamb2−/− mice are a model for Pierson syndrome; they exhibit defects in the kidney glomerular barrier, in the development and organization of the neuromuscular junction, and in the retina. Lamb2−/− mice fail to thrive and die very small at 3 weeks of age, but to what extent the kidney and neuromuscular defects each contribute to this severe phenotype has been obscure, though highly relevant to understanding Pierson syndrome. To investigate this, we generated transgenic mouse lines expressing rat laminin β2 either in muscle or in glomerular epithelial cells (podocytes) and crossed them onto the Lamb2−/− background. Rat β2 was confined in skeletal muscle to synapses and myotendinous junctions, and in kidney to the glomerular basement membrane. In transgenic Lamb2−/− mice, β2 deposition only in glomeruli prevented proteinuria but did not ameliorate the severe phenotype. In contrast, β2 expression only in muscle restored synaptic architecture and led to greatly improved health, but the mice died from kidney disease at 1 month. Rescue of both glomeruli and synapses was associated with normal weight gain, fertility, and lifespan. We conclude that muscle defects in Lamb2−/− mice are responsible for the severe failure to thrive phenotype, and that renal replacement therapy alone will be an inadequate treatment for Pierson syndrome.

Primary defects in either podocytes or the glomerular basement membrane (GBM) cause proteinuria, a fact that complicates defining the barrier to albumin. Laminin β2 (LAMB2) is a GBM component required for proper functioning of the glomerular filtration barrier. To investigate the GBM’s role in glomerular filtration, we characterized GBM and overlying podocyte architecture in relation to development and progression of proteinuria in Lamb2–/– mice, which model Pierson syndrome, a rare congenital nephrotic syndrome. We found ectopic deposition of several laminins and mislocalization of anionic sites in the GBM, which together suggest that the Lamb2–/– GBM is severely disorganized, although it is ultrastructurally intact. Importantly, albuminuria was detectable shortly after birth and preceded podocyte foot process effacement and loss of slit diaphragms by at least 7 days. Expression and localization of slit diaphragm and foot process–associated proteins appeared normal at early stages. GBM permeability to the electron-dense tracer ferritin was dramatically elevated in Lamb2–/– mice, even before widespread foot process effacement. Increased ferritin permeability was not observed in nephrotic CD2-associated protein–null (Cd2ap–/–) mice, which have a primary podocyte defect. Together these data show that the GBM serves as a barrier to protein in vivo and that the glomerular slit diaphragm alone is not sufficient to prevent the passage of albumin into the urinary space.

In developing glomeruli, laminin α5 replaces laminin α1 in the glomerular basement membrane (GBM) at the capillary loop stage, a transition required for glomerulogenesis. To investigate domain-specific functions of laminin α5 during glomerulogenesis, we produced transgenic mice that express a chimeric laminin composed of laminin α5 domains VI through I fused to the human laminin α1 globular (G) domain, designated Mr51. Transgene-derived protein accumulated in many basement membranes, including the developing GBM. When bred onto the Lama5 −/− background, Mr51 supported GBM formation, preventing the breakdown that normally occurs in Lama5 −/− glomeruli. In addition, podocytes exhibited their typical arrangement in a single cell layer epithelium adjacent to the GBM, but convolution of glomerular capillaries did not occur. Instead, capillaries were distended and exhibited a ballooned appearance, a phenotype similar to that observed in the total absence of mesangial cells. However, here the phenotype could be attributed to the lack of mesangial cell adhesion to the GBM, suggesting that the G domain of laminin α5 is essential for this adhesion. Analysis of an additional chimeric transgene allowed us to narrow the region of the α5 G domain essential for mesangial cell adhesion to α5LG3-5. Finally, in vitro studies showed that integrin α3β1 and the Lutheran glycoprotein mediate adhesion of mesangial cells to laminin α5. Our results elucidate a mechanism whereby mesangial cells organize the glomerular capillaries by adhering to the G domain of laminin α5 in the GBM.

Mutant mice lacking the integrin α8 subunit exhibit variable defects in kidney development with most mutants missing both kidneys. Several lines of evidence indicate that the known extracellular matrix ligands for integrin α8β1 are either dispensable for or not involved in α8β1 signaling during kidney development. This suggests the presence of an unknown ligand. A novel α8β1 ligand, nephronectin, has now been identified. Nephronectin is a new extracellular matrix protein associated with the Wolffian duct and the ureteric bud, epithelial structures with well-defined roles in kidney development.

Background

The uptake and trans-placental trafficking of fatty acids from the maternal blood into the fetal circulation are essential for embryonic development, and involve several families of proteins. Fatty acid transport proteins (FATPs) uniquely transport fatty acids into cells. We surmised that placental FATPs are germane for fetal growth, and are regulated during hypoxic stress, which is associated with reduced fat supply to the fetus.

Methodology/Principal Findings

Using cultured primary term human trophoblasts we found that FATP2, FATP4 and FATP6 were highly expressed in trophoblasts. Hypoxia enhanced the expression of trophoblastic FATP2 and reduced the expression of FATP4, with no change in FATP6. We also found that Fatp2 and Fatp4 are expressed in the mouse amnion and placenta, respectively. Mice deficient in Fatp2 or Fatp4 did not deviate from normal Mendelian distribution, with both embryos and placentas exhibiting normal weight and morphology, triglyceride content, and expression of genes related to fatty acid mobilization.

Conclusions/Significance

We conclude that even though hypoxia regulates the expression of FATP2 and FATP4 in human trophoblasts, mouse Fatp2 and Fatp4 are not essential for intrauterine fetal growth.

Laminins are the major noncollagenous glycoproteins of all basal laminae (BLs). They are α/β/γ heterotrimers assembled from 10 known chains, and they subserve both structural and signaling roles. Previously described mutations in laminin chain genes result in diverse disorders that are manifested postnatally and therefore provide little insight into laminin's roles in embryonic development. Here, we show that the laminin α5 chain is required during embryogenesis. The α5 chain is present in virtually all BLs of early somite stage embryos and then becomes restricted to specific BLs as development proceeds, including those of the surface ectoderm and placental vasculature. BLs that lose α5 retain or acquire other α chains. Embryos lacking laminin α5 die late in embryogenesis. They exhibit multiple developmental defects, including failure of anterior neural tube closure (exencephaly), failure of digit septation (syndactyly), and dysmorphogenesis of the placental labyrinth. These defects are all attributable to defects in BLs that are α5 positive in controls and that appear ultrastructurally abnormal in its absence. Other laminin α chains accumulate in these BLs, but this compensation is apparently functionally inadequate. Our results identify new roles for laminins and BLs in diverse developmental processes.

Nephrogenic Systemic Fibrosis (NSF) occurs in renally impaired patients who have undergone contrast enhanced MR examination using intravenous gadolinium (Gd)-based contrast agents. The effect of impaired kidney function on the biodistribution of Gd-based contrast agents was investigated using radiolabeled 153/NatGd-DOTA, 153/NatGd-DTPA and 153/NatGd-DTPA-BMA in a transgenic mouse model of renal impairment. Renally impaired (RI) animals had more activity associated with their tissues than did control mice, and this increase varied according to the radiotracer injected. For example, after 7 days, RI animals that received 153/NatGd-DOTA had 3-fold (p < 0.037) more activity in their bone tissue while RI animals receiving 153/NatGd-DTPA and 153/NatGd-DTPA-BMA had 8-fold (p < 0.0001) and 24-fold (p < 0.0001) more activity in their bone tissue, respectively. These findings demonstrate that renal impairment dramatically alters the tissue distribution of Gd3+ ions in vivo, which are likely a critical factor in the development of NSF.

Fatty acid transport protein (FATP) 4 is one of a family of six FATPs that facilitate long- and very long-chain fatty acid uptake. Mice lacking FATP4 are born with tight, thick skin and a defective epidermal barrier; they die neonatally due to dehydration and restricted movements. Both the skin phenotype and the lethality are rescued by transgene-driven expression of FATP4 solely in suprabasal keratinocytes. Here we show that Fatp4 mutants exhibit epidermal hyperplasia resulting from an increased number of proliferating suprabasal cells. In addition, barrier formation initiates precociously but never progresses to completion. To investigate possible mechanisms whereby Fatp4 influences skin development, we identified misregulated genes in Fatp4 mutants. Remarkably, three members of the epidermal growth factor (EGF) family (Ereg, Areg, and Epgn) showed increased expression that was associated with elevated epidermal activation of the EGF receptor (EGFR) and STAT3, a downstream effector of EGFR signaling. Both Tyrphostin AG1478, an EGFR tyrosine kinase inhibitor, and curcumin, an inhibitor of both STAT3 and EGFR, attenuated STAT3 activation/nuclear translocation, reduced skin thickening, and partially suppressed the barrier abnormalities. These data identify FATP4 activity as negatively influencing EGFR activation and the resulting STAT3 signaling during normal skin development. These findings have important implications for understanding the pathogenesis of ichthyosis prematurity syndrome, a disease recently shown to be caused by FATP4 mutations.

Skin epidermis is an active site of lipid synthesis. The intercellular lipids of human stratum corneum (SC) are unique in composition and quite different from the lipids found in most biological membranes. The three major lipids in the SC are free fatty acids, cholesterol and ceramides. Fatty acids can be synthesized by keratinocytes de novo and, in addition, need to be taken up from the circulation. The latter process has been shown to be protein mediated, and several fatty acid transporters are expressed in skin. Recent studies of transgenic and knockout animal models for fatty acid transporters and the identification of fatty acid transport protein 4 (FATP4 or SLC27A4) mutations as causative for Ichthyosis Prematurity Syndrome highlight the vital roles of fatty acid transport and metabolism in skin homeostasis. This review provides an overview of our current understanding of the role of fatty acids and their transporters in cutaneous biology, including their involvement in epidermal barrier generation and skin inflammation.

We have previously shown that mice with a CNS restricted knock-out of the integrin β1 subunit gene (Itgb1-CNSko mice) have defects in the formation of lamina and folia in the cerebral and cerebellar cortices that are caused by disruption of the cortical marginal zones. Cortical structures in postnatal and adult Itgb1-CNSko animals are also reduced in size, but the mechanism that causes the size defect has remained unclear. We now demonstrate that proliferation of granule cell precursors (GCPs) is severely affected in the developing cerebellum of Itgb1-CNSko mice. In the absence of β1 expression, GCPs lose contact with laminin in the meningeal basement membrane, cease proliferating, and differentiate prematurely. In vitro studies provide evidence thatβ1 integrins act at least in part cell autonomously in GCPs to regulate their proliferation. Previous studies have shown that sonic hedgehog (Shh)-induced GCP proliferation is potentiated by the integrin ligand laminin. We show that Shh directly binds to laminin and that laminin–Shh induced cell proliferation is dependent on β1 integrin expression in GCPs. Taken together, these data are consistent with a model in which β1 integrin expression in GCPs is required to recruit a laminin–Shh complex to the surface of GCPs and to subsequently modulate the activity of signaling pathways that regulate proliferation.

Summary

Integrins are transmembrane heteromeric receptors that mediate interactions between cells and extracellular matrix (ECM). β1, the most abundantly expressed integrin subunit, binds at least 12 α subunits. β1 containing integrins are highly expressed in the glomerulus of the kidney; however their role in glomerular morphogenesis and maintenance of glomerular filtration barrier integrity is poorly understood. To study these questions we selectively deleted β1 integrin in the podocyte by crossing β1flox/flox mice with podocyte specific podocin-cre mice (pod-Cre), which express cre at the time of glomerular capillary formation. We demonstrate that podocyte abnormalities are visualized during glomerulogenesis of the pod-Cre;β1flox/flox mice and proteinuria is present at birth, despite a grossly normal glomerular basement membrane. Following the advent of glomerular filtration there is progressive podocyte loss and the mice develop capillary loop and mesangium degeneration with little evidence of glomerulosclerosis. By three weeks of age the mice develop severe end stage renal failure characterized by both tubulointerstitial and glomerular pathology. Thus, expression of β1 containing integrins by the podocyte is critical for maintaining the structural integrity of the glomerulus.

A prominent feature of synaptic maturation at the neuromuscular junction (NMJ) is the topological transformation of the acetylcholine receptor (AChR)-rich postsynaptic membrane from an ovoid plaque into a complex array of branches. We show here that laminins play an autocrine role in promoting this transformation. Laminins containing the α4, α5, and β2 subunits are synthesized by muscle fibers and concentrated in the small portion of the basal lamina that passes through the synaptic cleft at the NMJ. Topological maturation of AChR clusters was delayed in targeted mutant mice lacking laminin α5 and arrested in mutants lacking both α4 and α5. Analysis of chimeric laminins in vivo and of mutant myotubes cultured aneurally demonstrated that the laminins act directly on muscle cells to promote postsynaptic maturation. Immunohistochemical studies in vivo and in vitro along with analysis of targeted mutants provide evidence that laminin-dependent aggregation of dystroglycan in the postsynaptic membrane is a key step in synaptic maturation. Another synaptically concentrated laminin receptor, Bcam, is dispensable. Together with previous studies implicating laminins as organizers of presynaptic differentiation, these results show that laminins coordinate post- with presynaptic maturation.

Epidermal keratinocytes form a highly organized stratified epithelium and sustain a competent barrier function together with dermal and hematopoietic cells. The Notch signaling pathway is a critical regulator of epidermal integrity. Here, we show that keratinocyte-specific deletion of total Notch signaling triggered a severe systemic B-lymphoproliferative disorder, causing death. RBP-j is the DNA binding partner of Notch, but both RBP-j–dependent and independent Notch signaling were necessary for proper epidermal differentiation and lipid deposition. Loss of both pathways caused a persistent defect in skin differentiation/barrier formation. In response, high levels of thymic stromal lymphopoietin (TSLP) were released into systemic circulation by Notch-deficient keratinocytes that failed to differentiate, starting in utero. Exposure to high TSLP levels during neonatal hematopoiesis resulted in drastic expansion of peripheral pre- and immature B-lymphocytes, causing B-lymphoproliferative disorder associated with major organ infiltration and subsequent death, a previously unappreciated systemic effect of TSLP. These observations demonstrate that local skin perturbations can drive a lethal systemic disease and have important implications for a wide range of humoral and autoimmune diseases with skin manifestations.

Author Summary

Skin is the largest organ of the body, forming an elaborate barrier that prevents water loss and protects the internal environment from outside invaders. When this barrier is compromised, keratinocytes, keratin-producing epidermal cells, alert and recruit the immune cells to the site of the breach as part of an adaptive defense mechanism. However, chronic activation of such an “alarm” could have undesired consequences. Using genetic engineering to progressively remove components of Notch signaling from mouse skin in utero resulted in chronic skin-barrier defects, mimicking a form of human skin disease called atopic dermatitis. Surprisingly, we discovered that a persistent alarm signal in newborns triggered a systemic B-lymphoproliferative disorder, which precisely mirrored the degree of skin defect and was lethal in its extreme form. This alarm signal, in the form of a cytokine called thymic stromal lymphopoietin, was produced by Notch-deficient keratinocytes that failed to form a competent skin barrier. Therefore, we uncovered a long-range proliferative effect on fetal pre-B cells in vivo that is induced by injured skin and mediated by thymic stromal lymphopoietin. These findings highlight the central role that skin-derived factors can play in initiating systemic diseases with skin involvement.

A skin-derived factor plays a lead role in initiating a lethal systemic disease. The current findings point to a novel impact of Notch-deficient skin on hematopoiesis, where unique biology of a keratinocyte-derived cytokine contributes to a severe neonatal blood disease.

Restrictive dermopathy is a lethal human genetic disorder characterized by very tight, thin, easily eroded skin, rocker bottom feet, and joint contractures. This disease was recently reported to be associated with a single heterozygous mutation in ZMPSTE24 and hypothesized to be a digenic disorder (Navarro et al., Lamin A and ZMPSTE24 (FACE-1) defects cause nuclear disorganization and identify restrictive dermopathy as a lethal neonatal laminopathy. Hum Mol Genet 13:2493-2503, 2004). ZMPSTE24 encodes an enzyme necessary for the correct processing and maturation of lamin A, an intermediate filament component of the nuclear envelope. Here we present four unrelated patients with homozygous mutations in ZMPSTE24 and a fifth patient with compound heterozygous mutations in ZMPSTE24. Two of the three different mutations we found are novel, and all are single base insertions that result in mRNA frameshifts. As a consequence of the presumed lack of ZMPSTE24 activity, prelamin A, the unprocessed toxic form of lamin A, was detected in the nuclei of both cultured cells and tissue from restrictive dermopathy patients, but not in control nuclei. Abnormally aggregated lamin A/C was also observed. These results indicate that restrictive dermopathy is an autosomal recessive laminopathy caused by inactivating ZMPSTE24 mutations that result in defective processing and nuclear accumulation of prelamin A.

Focal segmental glomerulosclerosis (FSGS) is the most common primary glomerular diagnosis resulting in end-stage renal disease. Defects in several podocyte proteins have been implicated in the etiology of FSGS, including podocin, α-actinin–4, CD2-associated protein (CD2AP), and TRPC6. Despite our growing understanding of genes involved in the pathogenesis of focal segmental sclerosis, the vast majority of patients with this disease, even those with a familial linkage, lack a clear genetic diagnosis. Here, we tested whether combinations of genetic heterozygosity (bigenic heterozygosity) that alone do not result in clinical kidney disease could function together to enhance susceptibility to glomerular damage and FSGS. Combinations of Cd2ap heterozygosity and heterozygosity of either synaptopodin (Synpo) or Fyn proto-oncogene (Fyn) but not kin of IRRE like 1 (Neph1) resulted in spontaneous proteinuria and in FSGS-like glomerular damage. These genetic interactions were also reflected at a functional level, as we found that CD2AP associates with Fyn and Synpo but not with Neph1. This demonstrates that bigenic heterozygosity can lead to FSGS and suggests that combined mutations in 2 or multiple podocyte genes may be a common etiology for glomerular disease.