Co-expression and interaction of cubilin and megalin in the adult male rat reproductive system

Severe kidney dysfunction in sialidosis mice reveals an essential role for neuraminidase 1 in reabsorption

Sialidosis is an ultra-rare multisystemic lysosomal disease caused by mutations in the neuraminidase 1 (NEU1) gene. The severe type II form of the disease manifests with a prenatal/infantile or juvenile onset, bone abnormalities, severe neuropathology, and visceromegaly. A subset of these patients present with nephrosialidosis, characterized by abrupt onset of fulminant glomerular nephropathy. We studied the pathophysiological mechanism of the disease in 2 NEU1-deficient mouse models, a constitutive Neu1-knockout, Neu1ΔEx3, and a conditional phagocyte-specific knockout, Neu1Cx3cr1ΔEx3. Mice of both strains exhibited terminal urinary retention and severe kidney damage with elevated urinary albumin levels, loss of nephrons, renal fibrosis, presence of storage vacuoles, and dysmorphic mitochondria in the intraglomerular and tubular cells. Glycoprotein sialylation in glomeruli, proximal distal tubules, and distal tubules was drastically increased, including that of an endocytic reabsorption receptor megalin. The pool of megalin bearing O-linked glycans with terminal galactose residues, essential for protein targeting and activity, was reduced to below detection levels. Megalin levels were severely reduced, and the protein was directed to lysosomes instead of the apical membrane. Together, our results demonstrated that desialylation by NEU1 plays a crucial role in processing and cellular trafficking of megalin and that NEU1 deficiency in sialidosis impairs megalin-mediated protein reabsorption.

Transcriptome analysis of turkey (Meleagris gallopavo) reproductive tract revealed key pathways regulating spermatogenesis and post-testicular sperm maturation

The application of transcriptomics to the study of the reproductive tract in male turkeys can significantly increase our current knowledge regarding the specifics of bird reproduction. To characterize the complex transcriptomic changes that occur in the testis, epididymis, and ductus deferens, deep sequencing of male turkey RNA samples (n = 6) was performed, using Illumina RNA-Seq. The obtained sequence reads were mapped to the turkey genome, and relative expression values were calculated to analyze differentially expressed genes (DEGs). Statistical analysis revealed 1,682; 2,150; and 340 DEGs in testis/epididymis, testis/ductus deferens, and epididymis/ductus deferens comparisons, respectively. The expression of selected genes was validated using quantitative real-time reverse transcriptase-polymerase chain reaction. Bioinformatics analysis revealed several potential candidate genes involved in spermatogenesis, spermiogenesis and flagellum formation in the testis, and in post-testicular sperm maturation in the epididymis and ductus deferens. In the testis, genes were linked with the mitotic proliferation of spermatogonia and the meiotic division of spermatocytes. Histone ubiquitination and protamine phosphorylation were shown to be regulatory mechanisms for nuclear condensation during spermiogenesis. The characterization of testicular transcripts allowed a better understanding of acrosome formation and development and flagellum formation, including axoneme structures and functions. Spermatozoa motility during post-testicular maturation was linked to the development of flagellar actin filaments and biochemical processes, including Ca2+ influx and protein phosphorylation/dephosphorylation. Spermatozoa quality appeared to be controlled by apoptosis and antioxidant systems in the epididymis and ductus deferens. Finally, genes associated with reproductive system development and morphogenesis were identified. To the best of our knowledge, this is the first genome-wide functional investigation of genes associated with tissue-specific processes in turkey reproductive tract. A catalog of genes worthy of further studies to understand the avian reproductive physiology and regulation was provided.

Vitamin D and male reproduction

Vitamin D is a versatile signalling molecule with a well-established role in the regulation of calcium homeostasis and bone health. The spectrum of vitamin D target organs has expanded and the reproductive role of vitamin D is highlighted by expression of the vitamin D receptor (VDR) and enzymes that metabolize vitamin D in testis, male reproductive tract and human spermatozoa. The expression levels of VDR and CYP24A1 in human spermatozoa serve as positive predictive markers of semen quality, and VDR mediates a nongenomic increase in intracellular calcium concentration that induces sperm motility. Interestingly, functional animal models show that vitamin D is important for estrogen signalling and sperm motility, while cross-sectional studies support the positive association between serum 25-hydroxyvitamin D level and sperm motility in both fertile and infertile men. Expression of VDR and enzymes that metabolize vitamin D in fetal testis indicates a yet unknown role during development, which may be extrapolated from invasive testicular germ cell tumours where 1α,25-dihydroxyvitamin D induces a mesodermal differentiation of the pluripotent testicular cancer cells. Taken together, vitamin D signalling has a positive effect on semen quality, increases estrogen responsiveness and differentiates germ cell tumours. Future studies are needed to determine when 1α,25-dihydroxyvitamin D acts in a paracrine manner and whether systemic changes, which are subject to pharmacological modulation, could influence male reproductive function.

Progesterone induces expression of Lrp2 in the murine uterus

Progesterone (P4) and progesterone receptor (PR) have important functions in uterine environment. In previous studies, using high density DNA microarray analysis, we identified low density lipoprotein receptor-related protein 2 (Lrp2) is one of the genes upregulated by P4 and PR. In present studies, we examined the expression of Lrp2 through real-time PCR, in situ hybridization and immunohistochemistry by P4-PR response. Lrp2 mRNA transcript was significantly increased after P4 treatment in the luminal and glandular epithelium of the wild-type mice. However, Lrp2 expression was not observed in the progesterone receptor knock out (PRKO) mice treated with P4. The expression of Lrp2 expression is not regulated by estrogen. During early pregnancy, the expression of Lrp2 was detected at 2.5 dpc and then significantly increased at 3.5 dpc in luminal and glandular epithelium. These results suggest that Lrp2 is a novel target gene by P4 and PR.

Cubilin expression is monoallelic and epigenetically augmented via PPARs

Background

Cubilin is an endocytic receptor that is necessary for renal and intestinal absorption of a range of ligands. Endocytosis mediated by cubilin and its co-receptor megalin is the principal mechanism for proximal tubule reabsorption of proteins from the glomerular filtrate. Cubilin is also required for intestinal endocytosis of intrinsic factor-vitamin B₁₂ complex. Despite its importance, little is known about the regulation of cubilin expression.

Results

Here we show that cubilin expression is under epigenetic regulation by at least two processes. The first process involves inactivation of expression of one of the cubilin alleles. This monoallelic expression state could not be transformed to biallelic by inhibiting DNA methylation or histone deacetylation. The second process involves transcriptional regulation of cubilin by peroxisome proliferator-activated receptor (PPAR) transcription factors that are themselves regulated by DNA methylation and histone deacetylation. This is supported by findings that inhibitors of DNA methylation and histone deacetylation, 5Aza and TSA, increase cubilin mRNA and protein in renal and intestinal cell lines. Not only was the expression of PPARα and γ inducible by 5Aza and TSA, but the positive effects of TSA and 5Aza on cubilin expression were also dependent on both increased PPAR transcription and activation. Additionally, 5Aza and TSA had similar effects on the expression of the cubilin co-receptor, megalin.

Conclusions

Together, these findings reveal that cubilin and megalin mRNA expression is under epigenetic control and thus point to new avenues for overcoming pathological suppression of these genes through targeting of epigenetic regulatory processes.

Analysis of the expression pattern of the carrier protein transthyretin and its receptor megalin in the human scalp skin and hair follicles: hair cycle-associated changes

Transthyretin is a serum and cerebrospinal fluid protein synthesized early in development by the liver, choroid plexus and several other tissues. It is a carrier protein for the antioxidant vitamins, retinol, and thyroid hormones. Transthyretin helps internalize thyroxine and retinol-binding protein into cells by binding to megalin, which is a multi-ligand receptor expressed on the luminal surface of various epithelia. We investigated the expression of transthyretin and its receptor megalin in the human skin; however, their expression pattern in the hair follicle is still to be elucidated. This study addresses this issue and tests the hypothesis that "the expression of transthyretin and megalin undergoes hair follicle cycle-dependent changes." A total of 50 normal human scalp skin biopsies were examined (healthy females, 53-62 years) using immunofluorescence staining methods and real-time PCR. In each case, 50 hair follicles were analyzed (35, 10, and 5 follicles in anagen, catagen, and telogen, respectively). Transthyretin and megalin were prominently expressed in the human scalp skin and hair follicles, on both gene and protein levels. The concentrations of transthyretin and megalin were 0.12 and 0.03 Ul/ml, respectively, as indicated by PCR. The expression showed hair follicle cycle-associated changes i.e., strong expression during early and mature anagen, very weak expression during catagen and moderate expression during telogen. The expression values of these proteins in the anagen were statistically significantly higher than those of either catagen or telogen hair follicles (P ≤ 0.001). This study provides the first morphologic indication that transthyretin and megalin are variably expressed in the human scalp skin and hair follicles. It also reports variations in the expression of these proteins during hair follicle cycling. The clinical ramifications of these findings are open for further investigations.

Megalin and cubilin in the human gallbladder epithelium

Although the role of cholesterol absorption by the gallbladder epithelium in gallstone formation is well established, the exact process is poorly understood. Potential candidates for regulation of transepithelial cholesterol transport are suggested to be two large membrane multiple ligand receptors, megalin and cubilin. We studied the expression of these two proteins in both acalculous and calculous human gallbladder epithelia. Adult human gallbladder tissues were received from 21 patients (9 men, 12 women) who had undergone cholecystectomy. The patients were divided into two groups: group A (calculous gallbladder group; 5 men, 6 women; mean age 64.4 +/- 11.1 years) with cholelithiasis, and group B (acalculous gallbladder group; 4 men, 6 women; mean age 55.3 +/- 16.1 years). In the gallbladder tissues megalin and cubilin expression was studied by immunohistochemistry and conventional RT-PCR, and gene expression levels were estimated by real-time RT-PCR. Both megalin and cubilin gene transcripts were found in total RNA preparations from acalculous gallbladder. In contrast, in preparations from calculous gallbladder, none or only one of the proteins was detected. Immunoreactive proteins were detected in the simple columnar acalculous gallbladder epithelium but not in the calculous gallbladder epithelium. Our results show different expression patterns of the two proteins in calculous gallbladders and acalculous gallbladders. In the latter both proteins are expressed, suggesting an association with gallstone formation and implying a putative role of the two proteins in cholesterol endocytosis. In other words, the presence of both proteins may be essential for the prevention of stone formation.

ATP-binding cassette transporters ABCA1, ABCA7, and ABCG1 in mouse spermatozoa

Mammalian spermatozoa lose plasma membrane cholesterol during their maturation in the epididymis and during their capacitation in the female reproductive tract. While acceptors such as high-density lipoproteins (HDL) and apolipoproteins A-I (apoA-I) and J have been found in male and female reproductive tracts, transporters that mediate cholesterol efflux from plasma membranes of spermatozoa to such acceptors have not yet been defined. Candidate transporters are members of the ATP-binding cassette (ABC) transporter superfamily including ABCA1, ABCA7, ABCG1 and ABCG4, which have all been implicated in the transport of sterols and phospholipids to apolipoproteins and HDL. Here we show that mouse spermatozoa in the seminiferous tubules and epididymis express ABCA1, ABCA7 and ABCG1, but not ABCG4. Moreover, we show that ABCA1, ABCA7, and ABCG1 antibodies decrease cholesterol efflux from spermatozoa to lipid acceptors apoA-I and albumin and inhibit in vitro fertilization.

Retinoids modulate expression of the endocytic partners megalin, cubilin, and disabled-2 and uptake of vitamin D-binding protein in human mammary cells

The major circulating form of vitamin D, 25-hydroxycholecalciferol (25D3), circulates bound to vitamin D-binding protein (DBP). Prior to activation to 1,25-dihydroxycholecalciferol in the kidney, the 25D3-DBP complex is internalized via receptor-mediated endocytosis, which is absolutely dependent on the membrane receptors megalin and cubilin and the adaptor protein disabled-2 (Dab2). We recently reported that mammary epithelial cells (T-47D) expressing megalin, cubilin, and Dab2 rapidly internalize DBP via endocytosis, whereas cells that do not express all 3 proteins (MCF-7) do not. The objectives of this study were to characterize megalin, cubilin, and Dab2 expression and transport of DBP in human mammary epithelial cells. Using immunoblotting and real-time PCR, we found that megalin, cubilin, and Dab2 were expressed and dose dependently induced by all-trans-retinoic acid (RA) in T-47D human breast cancer cells and that RA-treated T-47D cells exhibited enhanced DBP internalization. These are the first studies to our knowledge to demonstrate that mammary epithelial cells express megalin, cubilin, and Dab2, which are enhanced during differentiation and may explain, at least in part, our finding that receptor-mediated endocytosis of DBP is upregulated in differentiated mammary epithelial cells.

A cytoplasmic PPPSP motif determines megalin's phosphorylation and regulates receptor's recycling and surface expression

Megalin is a large endocytic receptor expressed at the apical surface of several absorptive epithelia. It binds multiple ligands including apolipoproteins, vitamin and hormone carrier proteins and signaling molecules such as parathyroid hormone and the morphogen sonic hedgehog. An important characteristic of megalin is its high endocytic activity, which is mediated by tyrosine-based endocytic motifs within the receptor's cytoplasmic tail. This domain also harbors several putative consensus phosphorylation motifs for protein kinase (PK) C and casein kinase-II and one consensus motif for PKA and glycogen synthase kinase-3 (GSK3). Here we report that the cytoplasmic domain of megalin is constitutively phosphorylated depending on the integrity of a PPPSP motif, a putative GSK3 site, with a minor participation of the other phosphorylation motifs. Mutation of the serine residue within the PPPSP motif as well as blocking GSK3 activity, with two different inhibitors, significantly decreased the phosphorylation levels of the receptor. Both the megalin PPPAP mutant and the underphosphorylated wild-type receptor, by inhibition of GSK3 activity, were more expressed at the cell surface and more efficiently recycled, but they were not inhibited in their initial endocytosis rates. Altogether, these results show that the PPPSP motif and the GSK3 activity are critical to allow megalin phosphorylation and also negatively regulate the receptor's recycling.

The role of megalin (LRP-2/Gp330) during development

Megalin (LRP-2/GP330), a member of the LDL receptor family, is an endocytic receptor expressed mainly in polarised epithelial cells. Identified as the pathogenic autoantigen of Heymann nephritis in rats, its functions have been studied in greatest detail in adult mammalian kidney, but there is increasing recognition of its involvement in embryonic development. The megalin homologue LRP-1 is essential for growth and development in Caenorhabditis elegans and megalin plays a role in CNS development in zebrafish. There is now also evidence for a homologue in Drosophila. However, most research concerns mammalian embryogenesis; it is widely accepted to be important during forebrain development and the developing renal proximal tubule. Megalin is also expressed in lung, eye, intestine, uterus, oviduct, and male reproductive tract. It is found in yolk sacs and the outer cells of pre-implantation mouse embryos, where interactions with cubilin result in nutrient endocytosis, and it may be important during implantation. Models for megalin interaction(s) with Sonic Hedgehog (Shh) have been proposed. The importance of Shh signalling during embryogenesis is well established; how and when megalin interacts with Shh is becoming a pertinent question in developmental biology.

Megalin-Dependent Internalization of Cadmium-Metallothionein and Cytotoxicity in Cultured Renal Proximal Tubule Cells

Chronic cadmium (Cd2+) exposure results in renal proximal tubular cell damage. Delivery of Cd2+ to the kidney occurs mainly as complexes with metallothionein-1 (molecular mass approximately 7 kDa), freely filtered at the glomerulus. For Cd2+ to gain access to the proximal tubule cells, these complexes are thought to be internalized via receptors for small protein ligands, such as megalin and cubilin, followed by release of Cd2+ from metallothionein-1 in endosomal/lysosomal compartments. To investigate the role of megalin in renal cadmium-metallothionein-1 reabsorption, megalin expression and dependence of cadmium-metallothionein-1 internalization and cytotoxicity on megalin were studied in a renal proximal tubular cell model (WKPT-0293 Cl.2 cells). Expression of megalin was detected by reverse transcriptase-polymerase chain reaction and visualized by immunofluorescence both at the cell surface (live staining) and intracellularly (permeabilized cells). Internalization of Alexa Fluor 488-coupled metallothionein-1 was concentration-dependent, saturating at approximately 15 microM. At 14.3 microM, metallothionein-1 uptake could be significantly attenuated by 30.9 +/- 6.6% (n = 4) by 1 muM of the receptor-associated protein (RAP) used as a competitive inhibitor of cadmium-metallothionein-1 binding to megalin and cubilin. Consistently, cytotoxicity of a 24-h treatment with 7.14 muM cadmium-metallothionein-1 was significantly reduced by 41.0 +/- 7.6%, 61.6 +/- 3.4%, and 26.2 +/- 1.8% (n = 4-5 each) by the presence of 1 microM RAP, 400 microg/ml anti-megalin antibody, or 5 microM of the cubilin-specific ligand, apo-transferrin, respectively. Cubilin expression in proximal tubule cells was also confirmed at the mRNA and protein level. The data indicate that renal proximal tubular cadmium-metallothionein-1 uptake and cell death are mediated at least in part by megalin.

Immunolocalization of cubilin, megalin, apolipoprotein J, and apolipoprotein A-I in the uterus and oviduct

Spermatozoa maturation and capacitation occurring in the male and female reproductive tracts, respectively, involves the remodeling of the spermatozoa plasma membrane. Apolipoprotein J (apoJ) and apolipoprotein A-I (apoA-I) have been implicated in the process of lipid exchange from the spermatozoa plasma membrane to epithelial cells lining the male reproductive tract. Evidence suggests that this process is mediated by the cooperative action of the endocytic lipoprotein receptors megalin and cubilin, which are expressed at the apical surface of absorptive epithelia in various tissues, including the efferent ducts and epididymis. Here, we investigated the possibility that these receptors and their lipid-binding ligands, apoJ and apoA-I, might function similarly in the female reproductive tract. We show that megalin and cubilin are expressed in the uterine epithelium at all stages of the estrous cycle, maximally during estrous and metestrous stages. In the oviduct, there is pronounced expression of both megalin and cubilin in the nonciliated cells of the proximal oviduct and epithelial cells of the distal oviduct, particularly during estrous and metestrous stages. In both uterine and oviduct epithelial cells, megalin and cubilin were located on the apical regions of the cells, consistent with a distribution at the cell surface and in endosomes. ApoJ and apoA-I were both detected in apical regions of uterine and oviduct epithelial cells. Secretory cells of the uterine glands were found to express apoJ and apoA-I suggesting that the glands are a site of synthesis for both proteins. In summary, our findings indicate that megalin and cubilin function within the female reproductive tract, possibly mediating uterine and oviduct epithelial cell endocytosis of apoJ/apoA-I-lipid complexes and thus playing a role in lipid efflux from the sperm plasma membrane, a major initiator of capacitation.

Megalin and cubilin expression in gallbladder epithelium and regulation by bile acids

Cholesterol crystal formation in the gallbladder is a key step in gallstone pathogenesis. Gallbladder epithelial cells might prevent luminal gallstone formation through a poorly understood cholesterol absorption process. Genetic studies in mice have highlighted potential gallstone susceptibility alleles, Lith genes, which include the gene for megalin. Megalin, in conjunction with the large peripheral membrane protein cubilin, mediates the endocytosis of numerous ligands, including HDL/apolipoprotein A-I (apoA-I). Although the bile contains apoA-I and several cholesterol-binding megalin ligands, the expression of megalin and cubilin in the gallbladder has not been investigated. Here, we show that both proteins are expressed by human and mouse gallbladder epithelia. In vitro studies using a megalin-expressing cell line showed that lithocholic acid strongly inhibits and cholic and chenodeoxycholic acids increase megalin expression. The effects of bile acids (BAs) were also demonstrated in vivo, analyzing gallbladder levels of megalin and cubilin from mice fed with different BAs. The BA effects could be mediated by the farnesoid X receptor, expressed in the gallbladder. Megalin protein was also strongly increased after feeding a lithogenic diet. These results indicate a physiological role for megalin and cubilin in the gallbladder and provide support for a role for megalin in gallstone pathogenesis.

Differential distribution of cubilin and megalin expression in the mouse embryo

Cubilin and megalin are cell surface proteins that work cooperatively in many absorptive epithelia to mediate endocytosis of lipoproteins, vitamin carriers, and other proteins. Here we have investigated the coordinate expression of these receptors during mouse development. Our findings indicate that while there are sites where the receptors are co-expressed, there are other tissues where expression is not overlapping. Apical cubilin expression is pronounced in the extraembryonic visceral endoderm (VE) of 6-9.5 days postcoitum (dpc) embryos. By contrast, little megalin expression is evident in the VE at 6 dpc. However, megalin expression in the VE increases as development progresses (7.5-9.5 dpc), although it is not as uniformly distributed as cubilin. Punctate expression of megalin is also apparent in the region of the ectoplacental cone associated with decidual cells, whereas cubilin expression is not seen in association with the ectoplacenta. Strong expression of megalin is observed in the neural ectoderm, neural plate and neural tube (6-8.5 dpc), but cubilin expression is not apparent in any of these tissues. At 8.5 dpc, megalin is expressed in the developing endothelial cells of blood islands, whereas cubilin is absent from these cells. Finally, cubilin, but not megalin, is expressed by a subpopulation of cells dispersed within the 7.5 dpc embryonic endoderm and having a migratory morphology. In summary, the co-expression of cubilin and megalin in the VE is consistent with the two proteins functioning jointly in this tissue. However, the differential distribution pattern indicates that the proteins also function independent of one another. Furthermore, the finding of megalin expression in blood island endothelial cells and cubilin expression in embryonic endoderm highlight potential new developmental roles for these proteins.

Cytokines activate genes of the endocytotic pathway in insulin-producing RINm5F cells

AIMS/HYPOTHESIS

Cytokines are important humoral mediators of beta cell destruction in autoimmune diabetes. The aim of this study was to identify novel cytokine-induced genes in insulin-producing RINm5F cells, which may contribute to beta cell death or survival.

METHODS

A global gene expression profile in cytokine-exposed insulin-producing RINm5F cells was achieved by automated restriction fragment differential display PCR. The expression of selected candidate genes was confirmed by real-time RT-PCR analysis.

RESULTS

Exposure of RINm5F cells to IL-1beta or to a cytokine mixture (IL-1beta, TNF-alpha, IFN-gamma) for 6 h resulted in the differential expression of a functional gene cluster. Apart from the well-known up-regulation of the cytokine-responsive genes iNOS, NF-kappaB, MnSOD and Hsp70, several genes that belong to the functional cluster of the endocytotic pathway were identified. These endocytotic genes comprised: clathrin, megalin, synaptotagmin and calcineurin, which were up-regulated by IL-1beta or the cytokine mixture. In contrast, the expression of the calcineurin inhibitor CAIN and of the GDP/GTP exchange protein Rab3 was down-regulated by cytokines. Other up-regulated cytokine-responsive genes were: agrin, murine adherent macrophage protein mRNA ( MAMA) and transport-associated protein ( TAP1/MTP), whereas the plasma membrane calcium ATPase ( PMCA) 2 and PMCA 3 genes were down-regulated by cytokines.

CONCLUSIONS/INTERPRETATION

Our results indicate that genes of the endocytotic pathway are regulated by pro-inflammatory cytokines. This might affect the density of cytokine receptors at the beta cell surface and concomitantly the sensitivity of the cells to cytokine toxicity. A better understanding of the functional cross-talk between endocytotic and cytokine signalling pathways could further the development of novel strategies to protect pancreatic beta cells against toxic effects of pro-inflammatory cytokines.

The functional cobalamin (vitamin B12)–intrinsic factor receptor is a novel complex of cubilin and amnionless

Imerslund-Gräsbeck syndrome (I-GS, megaloblastic anemia 1) is an autosomal recessive disorder characterized by intestinal cobalamin (vitamin B12) malabsorption and proteinuria. I-GS–causing mutations are found in either of 2 genes encoding the epithelial proteins: cubilin and amnionless (AMN). Cubilin recognizes intrinsic factor (IF)–cobalamin and various other proteins to be endocytosed in the intestine and kidney, respectively, whereas the function of AMN is unknown. Here we show that cubilin and AMN colocalize in the endocytic apparatus of polarized epithelial cells and copurify as a tight complex during IF-cobalamin affinity and nondenaturing gel filtration chromatography. In transfected cells expressing either AMN or a truncated IF-cobalamin–binding cubilin construct, neither protein alone conferred ligand endocytosis. In cubilin transfectants, cubilin accumulated in early biosynthetic compartments. However, in cells cotransfected with AMN and the cubilin construct, cubilin trafficked to the cell surface and endosomes, and the cells exhibited IF-cobalamin endocytosis and lysosomal degradation of IF. These data indicate that cubilin and AMN are subunits of a novel cubilin/AMN (cubam) complex, where AMN binds to the amino-terminal third of cubilin and directs subcellular localization and endocytosis of cubilin with its ligand. Therefore, mutations affecting either of the 2 proteins may abrogate function of the cubam complex and cause IG-S.

Essential role of the apolipoprotein E receptor-2 in sperm development

The apolipoprotein (apo) E receptor-2 (apoER2) is a member of the low density lipoprotein receptor gene family and an important regulator of neuronal migration. It acts as a receptor for the signaling factor Reelin and provides positional cues to neurons that migrate to their proper position in the developing brain. Besides brain formation defects, apoER2-deficient mice also exhibit male infertility. The role of the receptor in male reproduction, however, remained unclear. Here we demonstrate that apoER2 is highly expressed in the initial segment of the epididymis, where it affects the functional expression of clusterin and phospholipid hydroperoxide glutathione peroxidase (PHGPx), two proteins required for sperm maturation. Reduced PHGPx expression in apoER2 knockout mice results in the inability of the sperm to regulate the cell volume and in abnormal sperm morphology and immotility. Because insufficient expression of PHGPx is a major cause of infertility in men, these findings not only highlight an important new function for apoER2 that is unrelated to neuronal migration, but they also suggest a possible role for apoER2 in human infertility.