Modulation of neurite promoting proteoglycans by neuronal differentiation

Sulfate in fetal development

Sulfate (SO(4)(2-)) is an important nutrient for human growth and development, and is obtained from the diet and the intra-cellular metabolism of sulfur-containing amino acids, including methionine and cysteine. During pregnancy, fetal tissues have a limited capacity to produce sulfate, and rely on sulfate obtained from the maternal circulation. Sulfate enters and exits placental and fetal cells via transporters on the plasma membrane, which maintain a sufficient intracellular supply of sulfate and its universal sulfonate donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) for sulfate conjugation (sulfonation) reactions to function effectively. Sulfotransferases mediate sulfonation of numerous endogenous compounds, including proteins and steroids, which biotransforms their biological activities. In addition, sulfonation of proteoglycans is important for maintaining normal structure and development of tissues, as shown for reduced sulfonation of cartilage proteoglycans that leads to developmental dwarfism disorders and four different osteochondrodysplasias (diastrophic dysplasia, atelosteogenesis type II, achondrogenesis type IB and multiple epiphyseal dysplasia). The removal of sulfate via sulfatases is an important step in proteoglycan degradation, and defects in several sulfatases are linked to perturbed fetal bone development, including mesomelia-synostoses syndrome and chondrodysplasia punctata 1. In recent years, interest in sulfate and its role in developmental biology has expanded following the characterisation of sulfate transporters, sulfotransferases and sulfatases and their involvement in fetal growth. This review will focus on the physiological roles of sulfate in fetal development, with links to human and animal pathophysiologies.

The rat Na+-sulfate cotransporter rNaS2: functional characterization, tissue distribution, and gene (slc13a4) structure

Inorganic sulfate is essential for numerous functions in mammalian physiology. In the present study, we characterized the functional properties of the rat Na+-sulfate cotransporter NaS2 (rNaS2), determined its tissue distribution, and identified its gene (slc13a4) structure. Expression of rNaS2 protein in Xenopus oocytes led to a Na+-dependent transport of sulfate that was inhibited by phosphate, thiosulfate, tungstate, selenate, oxalate, and molybdate, but not by citrate, succinate, or DIDS. Transport kinetics of rNaS2 determined a K(M) for sulfate of 1.26 mM. Na+ kinetics determined a Hill coefficient of n=3.0+/-0.7, suggesting a Na+:SO4 (2-) stoichiometry of 3:1. rNaS2 mRNA was highly expressed in placenta, with lower levels found in the brain and liver. slc13a4 maps to rat chromosome 4 and contains 17 exons, spanning over 46 kb in length. This gene produces two alternatively spliced transcripts, of which the transcript lacking exon 2 is the most abundant form. Its 5' flanking region contains CAAT- and GC-box motifs and a number of putative transcription factor binding sites, including GATA-1, SP1, and AP-2 consensus sequences. This is the first study to characterize rNaS2 transport kinetics, define its tissue distribution, and resolve its gene (slc13a4) structure and 5' flanking region.

Functional characterization and genomic organization of the human Na(+)-sulfate cotransporter hNaS2 gene (SLC13A4)

Sulfate plays an essential role in human growth and development. Here, we characterized the functional properties of the human Na(+)-sulfate cotransporter (hNaS2), determined its tissue distribution, and identified its gene (SLC13A4) structure. Expression of hNaS2 protein in Xenopus oocytes led to a Na(+)-dependent transport of sulfate that was inhibited by thiosulfate, phosphate, molybdate, selenate and tungstate, but not by oxalate, citrate, succinate, phenol red or DIDS. Transport kinetics of hNaS2 determined a K(m) for sulfate of 0.38mM, suggestive of a high affinity sulfate transporter. Na(+) kinetics determined a Hill coefficient of n=1.6+/-0.6, suggesting a Na:SO(4)(2-) stoichiometry of 2:1. hNaS2 mRNA was highly expressed in placenta and testis, with intermediate levels in brain and lower levels found in the heart, thymus, and liver. The SLC13A4 gene contains 16 exons, spanning over 47kb in length. Its 5'-flanking region contains CAAT- and GC-box motifs, and a number of putative transcription factor binding sites, including GATA-1, AP-1, and AP-2 consensus sequences. This is the first study to characterize hNaS2 transport kinetics, define its tissue distribution, and resolve its gene (SLC13A4) structure and 5' flanking region.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Contribution of heparan sulfate to the non-permissive role of the midline glia to the growth of midbrain neurites

Radial glial cells and astrocytes are heterogeneous with respect to morphology, cytoskeletal- and membrane-associated molecules and intercellular interactions. Astrocytes derived from lateral (L) and medial (M) midbrain sectors differ in their abilities to support neuritic growth of midbrain neurons in coculture (Garcia-Abreu et al. J Neurosci Res 40:471, 1995). There is a correlation between these abilities and the differential patterns of laminin (LN) organization that is fibrillar in growth-permissive L astrocytes and punctate in the non-permissive M astroglia (Garcia-Abreu et al. NeuroReport 6:761, 1995). There are also differences in the production of glycosaminoglycans (GAGs) by L and M midbrain astrocytes (Garcia-Abreu et al. Glia 17:339, 1996). We show that the relative amounts of the glycoproteins laminin LN, fibronectin (FN) and tenascin (TN) are virtually identical in L and M glia, thus, confirming that an abundant content of LN is not sufficient to promote neurite growth. To further analyze the role of GAGs in the properties of M and L glia, we employed enzymatic degradation of the GAGs chondroitin sulfate (CS) and heparan sulfate (HS). Treatment with chondroitinase has little effect on the non-permissive properties of M glia but reduces the growth-supporting ability of L glia. By contrast, heparitinase I produces no significant changes on L glia but leads to neurite growth promotion by M glia. Taken together, these results suggest that glial CS helps to promote neurite growth and, more importantly, they indicate that a HS proteoglycan is, at least, partially responsible for the non-permissive role of the midline glia to the growth of midbrain neurites.

Heparan sulfate in the inner limiting membrane of embryonic chicken retina binds basic fibroblast growth factor to promote axonal outgrowth

During neural development retinal ganglion cell axons migrate over the retinal basal lamina (inner limiting membrane, ILM) in directed growth toward the optic nerve. We found that both growth rate and distribution density of the ganglion cell axons on isolated cell-free ILM was greatly inhibited by pretreatment with heparitinase but not with chondroitinase ABC. The persistence of radioactively labeled proteoglycans added to the culture medium eliminated residual heparitinase as an explanation for the inhibition. A cell binding assay showed that heparitinase acted on the ILM to influence axonal behavior without apparent inhibition of cell adhesion. These results indicated that the neurite outgrowth promoting activity of the ILM depended on the heparan sulfate (HS) side chains of its proteoglycans. Basic fibroblast growth factor (bFGF) stimulated additional neuronal sprouting and neurite elongation on the ILM. This neurotropic activity of bFGF was inhibited by heparitinase pretreatment of the ILM, suggesting that bFGF bound to HS on the ILM. The activity of bFGF was enhanced by exogenous heparin added to the culture medium; although heparin alone failed to stimulate either neurite extension or neuronal cell sprouting. These results demonstrate that HS in the ILM possesses neurotropic activity for axons of the ganglion cells by binding bFGF for presentation to cell-surface receptors and may, therefore, play a significant role in stimulating axonal outgrowth during development.

Identification of a mammalian brain sulfate transporter

Sulfate is an essential anion involved in many biosynthetic and pharmacological reactions. Sulfate is an important constituent of myelin membranes in the brain; however, very little is known as to how sulfate enters brain cells. In this study, our aim was to determine whether the mammalian brain possesses a sulfate transporter. Injection of rat brain poly A(+) RNA into Xenopus oocytes led to an induction of Na(+)-independent sulfate transport, which was inhibited by oxalate, probenecid, phenol red, thiosulfate and DIDS. Hybrid depletion using sat-1 antisense oligodeoxyribonucleotides led to a complete inhibition of brain mRNA-induced sulfate transport in Xenopus oocytes, suggesting the presence of a functional sat-1 transcript in the brain. By RT-PCR, sat-1 mRNA was detected throughout the rat brain and in situ hybridisation showed highest sat-1 expression in the hippocampus and cerebellum. This is the first study to identify and characterise a functional mammalian brain sulfate transporter.

Extracellular matrix molecules play diverse roles in the growth and guidance of central nervous system axons

Axon growth and guidance represent complex biological processes in which probably intervene diverse sets of molecular cues that allow for the appropriate wiring of the central nervous system (CNS). The extracellular matrix (ECM) represents a major contributor of molecular signals either diffusible or membrane-bound that may regulate different stages of neural development. Some of the brain ECM molecules form tridimensional structures (tunnels and boundaries) that appear during time- and space-regulated events, possibly playing relevant roles in the control of axon elongation and pathfinding. This short review focuses mainly on the recognized roles played by proteoglycans, laminin, fibronectin and tenascin in axonal development during ontogenesis.

Molecular tunnels and boundaries for growing axons in the anterior commissure of hamster embryos

We have analyzed the immunohistochemical expression of chondroitin sulfate proteoglycan (CSPG), fibronectin (FN), laminin (LN), tenascin (TN), and glial fibrillary acidic protein (GFAP) along the anterior commissure (AC) of hamster embryos (n=175; from embryonic day (E)12 to E16). Frozen sections were cut at different planes from embryonic brains between E12 and E16, treated for immunohistochemistry, and observed under epifluorescence microscopy. During the pre-crossing stage (E12-E13), CSPG was expressed as a sagittal stratum between the interhemispheric fissure and the prospective AC region. TN appeared rostral to the third ventricle and along the medial subventricular zone of the lateral ventricles. LN and FN both presented a faint expression, and GFAP was not detected. Although AC axons started crossing the midline region (E13.5-E14), CSPG, FN, LN, and, much less intensely, GFAP circumscribed the AC bundle, forming a tunnel through which AC fibers elongate. TN was no longer seen at the midplane but remained visible laterally. During the post-crossing stage (E14.5-E16), CSPG and TN were no longer seen at the midline, although both could be observed between the AC limbs, seeming to form boundaries for AC lateral growth. LN and FN were then absent near the AC bundle. During this late stage, GFAP expression became most intense, forming a distinct tunnel around the AC. We have shown that the expression of extracellular matrix molecules and GFAP follow a time- and space-regulated course related to AC development, plausibly representing influential factors for growth and guidance of commissural fibers.

Excitatory amino acid regulation of astrocyte proteoglycans

Activity-dependent enduring change in cellular communication is essential for specific connectivity during development of the nervous system and for adaptive responses of the mature nervous system. Here we report that glutamate activation of excitatory amino acid receptors induces the synthesis and release of proteoglycans (PGs) from fetal hippocampal-astrocytes in dissociated culture. PG synthesis and release are mediated via kainate and metabotropic receptor activation. Glutamate exposure did not regulate the release of a specific family of PG, but glutamate inhibited the synthesis of heparan sulfate (HS) PGs that appeared within the extracellular environment of the astrocyte. Particulate protein kinase C (PKC) activity was increased by glutamate and the PKC activator phorbol 10-myristate 13-acetate produced a dose-dependent increase in PG release. However, glutamate-induced PG release was not blocked by inhibition of PKC activity. These data suggest that PKC activation can lead to PG release, but is not necessary for it. Activity-dependent influences on a class of substrate-bound molecular species with growth-modulatory properties may be involved in spatial regulation of neuronal growth responses produced by excitatory amino acids.