Early-onset dementia, leukoencephalopathy and brain calcifications: a clinical, imaging and pathological comparison of ALSP and PLOSL/Nasu Hakola disease

Adult-onset leukoencephalopathy with axonal spheroids and pigmented glia, and Nasu Hakola disease or polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy are both underrecognized progressive degenerative white matter diseases that can present with young dementia, leukoencephalopathy and brain calcifications. We report and compare three cases in terms of clinical phenotype, imaging and neuropathological findings. Both cases have led to the identification of two novel causal mutations.

Syndecan-syntenin-ALIX regulates the biogenesis of exosomes

The biogenesis of exosomes, small secreted vesicles involved in signalling processes, remains incompletely understood. Here, we report evidence that the syndecan heparan sulphate proteoglycans and their cytoplasmic adaptor syntenin control the formation of exosomes. Syntenin interacts directly with ALIX through LYPX(n)L motifs, similarly to retroviral proteins, and supports the intraluminal budding of endosomal membranes. Syntenin exosomes depend on the availability of heparan sulphate, syndecans, ALIX and ESCRTs, and impact on the trafficking and confinement of FGF signals. This study identifies a key role for syndecan-syntenin-ALIX in membrane transport and signalling processes.

Syndecan recycling [corrected] is controlled by syntenin-PIP2 interaction and Arf6

Syndecans are heparan sulfate proteoglycans that modulate the activity of several growth factors and cell adhesion molecules. PDZ domains in the adaptor protein syntenin interact with syndecans and with the phosphoinositide PIP(2), which is involved in the regulation of the actin cytoskeleton and membrane trafficking. Here, we show that the syntenin PDZ domain-PIP(2) interaction controls Arf6-mediated syndecan recycling through endosomal compartments. FGF receptor accompanies syndecan along the syntenin-mediated recycling pathway, in a heparan sulfate- and FGF-dependent manner. Syndecans that cannot recycle via this pathway become trapped intracellularly and inhibit cell spreading. This syntenin-mediated syndecan recycling pathway may regulate the surface availability of a number of cell adhesion and signaling molecules.

Cellular uptake of mammalian heparanase precursor involves low density lipoprotein receptor-related proteins, mannose 6-phosphate receptors, and heparan sulfate proteoglycans

Mammalian heparanase, strongly implicated in the regulation of cell growth, migration, and differentiation, plays a crucial role in inflammation, angiogenesis, and metastasis. There is thus a clear need for understanding how heparanase activity is regulated. Cells can generate an active form of the enzyme from a larger inactive precursor protein by a process of secretion-recapture, internalization, and proteolytic processing in late endosomes/lysosomes. Cell surface heparan sulfate proteoglycans are the sole known components with a role in this trafficking of the heparanase precursor. Here, we provide evidence that heparan sulfate proteoglycans are not strictly required for this process. More importantly, by heparanase transfection, binding, and uptake experiments and by using a combination of specific inhibitors and receptor-defective cells, we have identified low density lipoprotein receptor-related proteins and mannose 6-phosphate receptors as key elements of the receptor system that mediates the capture of secreted heparanase precursor and its trafficking to the intracellular site of processing/activation.

Processing by proprotein convertases is required for glypican-3 modulation of cell survival, Wnt signaling, and gastrulation movements

Glypican (GPC)-3 inhibits cell proliferation and regulates cell survival during development. This action is demonstrated by GPC3 loss-of-function mutations in humans and mice. Here, we show that the GPC3 core protein is processed by a furinlike convertase. This processing is essential for GPC3 modulating Wnt signaling and cell survival in vitro and for supporting embryonic cell movements in zebrafish. The processed GPC3 core protein is necessary and sufficient for the cell-specific induction of apoptosis, but in vitro effects on canonical and noncanonical Wnt signaling additionally require substitution of the core protein with heparan sulfate. Wnt 5A physically associates only with processed GPC3, and only a form of GPC3 that can be processed by a convertase is able to rescue epiboly and convergence/extension movements in GPC3 morphant embryos. Our data imply that the Simpson–Golabi–Behmel syndrome may in part result from a loss of GPC3 controls on Wnt signaling, and suggest that this function requires the cooperation of both the protein and the heparan sulfate moieties of the proteoglycan.

Membrane heparan sulfate proteoglycan-supported FGF2-FGFR1 signaling: evidence in support of the "cooperative end structures" model

Fibroblast growth factor 2 (FGF2)-initiated FGF receptor (FGFR)-signaling requires the assistance of heparin/heparan sulfate. Here, we evaluated the effects of different heparan sulfate proteoglycan (HSPG)-expressing cell lines and HSPGs derived from these cells on FGF2-induced FGFR1-phosphorylation in heparan sulfate-negative BaF3 cells. HSPGs supplied in membrane-associated form, by presenting cells, were all effective promotors of FGF2-initiated FGFR1 phosphorylation, independently of their nature (syndecan/glypican) or cellular origin (human lung fibroblasts, transfected Namalwa cells, or transfected K562 cells). A treatment with heparitinase initially stimulated, but finally completely inhibited, the activity of these presenting cells. In comparison, equivalent amounts of soluble HSPGs, obtained by trypsinization of these cells or by immunopurification from cell extracts, did not promote FGF2-induced FGFR1-phosphorylation, yet removal of the less anionic species or a further treatment with heparitinase converted these soluble fractions into potent activators of FGF2/FGFR1 signaling. Extrapolating from current structural models, we suggest that FGFR dimerization and autophosphorylation is supported by cooperative "heparin-like end structures," and that cell surface association and concentration compensate for the relative scarcity of such end structures in native HSPGs. In this model, "proteolytic" shedding of heparan sulfate would act as a diluting, down-regulatory mechanism, while "heparanolytic" shedding might act as an up-regulatory mechanism, by increasing the concentration of these end structures.

Characterization of syntenin, a syndecan-binding PDZ protein, as a component of cell adhesion sites and microfilaments

Syntenin is a PDZ protein that binds the cytoplasmic C-terminal FYA motif of the syndecans. Syntenin is widely expressed. In cell fractionation experiments, syntenin partitions between the cytosol and microsomes. Immunofluorescence microscopy localizes endogenous and epitope-tagged syntenin to cell adhesion sites, microfilaments, and the nucleus. Syntenin is composed of at least three domains. Both PDZ domains of syntenin are necessary to target reporter tags to the plasma membrane. The addition of a segment of 10 amino acids from the N-terminal domain of syntenin to these PDZ domains increases the localization of the tags to stress fibers and induces the formation of long, branching plasma membrane extensions. The addition of the complete N-terminal region, in contrast, reduces the localization of the tags to plasma membrane/adhesion sites and stress fibers, and reduces the morphotypical effects. Recombinant domains of syntenin with the highest plasma membrane localization display the lowest nuclear localization. Syndecan-1, E-cadherin, beta-catenin, and alpha-catenin colocalize with syntenin at cell-cell contacts in epithelial cells, and coimmunoprecipitate with syntenin from extracts of these cells. These results suggest a role for syntenin in the composition of adherens junctions and the regulation of plasma membrane dynamics, and imply a potential role for syntenin in nuclear processes.

Glypican-6, a new member of the glypican family of cell surface heparan sulfate proteoglycans

The glypicans compose a family of glycosylphosphatidylinositol-anchored heparan sulfate proteoglycans. Mutations in dally, a gene encoding a Drosophila glypican, and in GPC3, the gene for human glypican-3, implicate glypicans in the control of cell growth and division. So far, five members of the glypican family have been identified in vertebrates. By sequencing expressed sequence tag clones and products of rapid amplifications of cDNA ends, we identified a sixth member of the glypican family. The glypican-6 mRNA encodes a protein of 555 amino acids that is most homologous to glypican-4 (identity of 63%). Expression of this protein in Namalwa cells shows a core protein of approximately 60 kDa that is substituted with heparan sulfate only. GPC6, the gene encoding human glypican-6, contains nine exons. Like GPC5, the gene encoding glypican-5, GPC6 maps to chromosome 13q32. Clustering of the GPC5/GPC6 genes on chromosome 13q32 is strongly reminiscent of the clustering of the GPC3/GPC4 genes on chromosome Xq26 and suggests GPCs arose from a series of gene and genome duplications. Based on similarities in sequence and gene organization, glypican-1, glypican-2, glypican-4, and glypican-6 appear to define a subfamily of glypicans, differing from the subfamily comprising so far glypican-3 and glypican-5. Northern blottings indicate that glypican-6 mRNA is widespread, with prominent expressions in human fetal kidney and adult ovary. In situ hybridization studies localize glypican-6 to mesenchymal tissues in the developing mouse embryo. High expressions occur in smooth muscle cells lining the aorta and other major blood vessels and in mesenchymal cells of the intestine, kidney, lung, tooth, and gonad. Growth factor signaling in these tissues might in part be regulated by the presence of glypican-6 on the cell surface.

Molecular polymorphism of the syndecans. Identification of a hypo-glycanated murine syndecan-1 splice variant

We have identified a cDNA that encodes a variant form of murine syndecan-1. The variant cDNA lacks the sequence corresponding to the first 132 nucleotides of the third exon of the syndecan-1 gene. The corresponding message is rare. The alternative splice respects the reading frame and deletes 44 amino acids from the protein, joining the S45GS47GT sequence to a variant immediate downstream context. This sequence context initiates with alanine instead of glycine as residue 50, reducing the number of SGXG sequence motifs in the protein from two to one. Expression of this variant syndecan-1 in Madin-Darby canine kidney or MOLT-4 cells yielded a recombinant proteoglycan with a reduced number and clustering of the heparan sulfate chains. Both the conversions of Ala50 and of Lys53 into glycine enhanced the heparan sulfate substitution of the variant protein. These findings support the concept that serine-glycine dipeptide signals for glycosaminoglycan/heparan sulfate synthesis depend on sequence context (Zhang, L., David, G., and Esko, J. D. (1995) J. Biol. Chem. 270, 27127-27135) and imply that alternative splicing mechanisms may in part control the molecular polymorphism of syndecan-1 and, therefore, the efficiency and versatility of this protein in its co-receptor functions.