cDNA Cloning, Genomic Organization, and in Vivo Expression of Rat N-syndecan

Abstract

The amino acid sequence of rat N-syndecan core protein was deduced from the cloned cDNA sequence. The sequence predicts a core protein of 442 amino acids with six structural domains: an NH 2 -terminal signal peptide, a membrane distal glycosaminoglycan attachment domain, a mucin homology domain, a membrane proximal glycosaminoglycan attachment domain, a single transmembrane domain, and a noncatalytic COOH-terminal cytoplasmic domain. Transfection of human 293 cells resulted in the expression of N-syndecan that was modified by heparan sulfate chain addition. Heparitinase digestion of the expressed proteoglycan produced a core protein that migrated on SDS-polyacrylamide gels at an apparent molecular weight of 120,000, identical to Nsyndecan synthesized by neonatal rat brain or Schwann cells. Rat genomic DNA coding for N-syndecan was isolated by hybridization screening. The rat N-syndecan gene is comprised of five exons. Each exon corresponds to a specific core protein structural domain, with the exception of the fifth exon, which contains the coding information for both the transmembrane and cytoplasmic domains as well as the 3-untranslated region of the mRNA. The first intron is large, with a length of 22 kilobases. The expression of N-syndecan was investigated in late embryonic, neonatal, and adult rats by immunoblotting and Northern blotting analysis. Among the tissues and developmental stages studied, high levels of N-syndecan expression were restricted to the early postnatal nervous system. N-syndecan was expressed in all regions of the nervous system, including cortex, midbrain, spinal cord, and peripheral nerve. Immunohistochemical staining revealed high levels of N-syndecan expression in all brain regions and fiber tract areas.

Syndecans: multifunctional cell-surface co-receptors

“…Genes coding for syndecans 1, 3 and 4 have been cloned [12,[17][18][19]. They show a strikingly similar exon-intron organization, which supports the idea that the syndecans arose by gene duplication from a single ancestral gene.…”

“…There is evidence that syndecan shedding also occurs in i o. Extraction of neonatal rat brain tissue with buffer lacking detergent results in the solubilization of a large fraction of the syndecan-3 that is present, suggesting that these syndecan-3 molecules had lost their membrane attachment [12]. These syndecan-3 molecules are not stained on immunoblots by an antibody directed against the C-terminal cytoplasmic domain, consistently with the loss of the C-terminal domain by proteolysis.…”

“…Most of our knowledge of syndecan structure is based on analysis of the deduced amino acid sequences of syndecan cDNAs. cDNAs coding for four distinct but homologous syndecan core proteins have been cloned from vertebrate cells [1][2][3][4][5][6][7][8][9][10][11][12]. A proposal for a common nomenclature designates the syndecans numerically, based on the order of cloning of their cDNAs [13].…”

“…Syndecan expression is often associated with morphological transitions or major changes in tissue organization. For example, in rodents there is a period of high-level syndecan-3 expression in the early postnatal central nervous system [12]. Syndecan-3 levels rise rapidly at birth, peak on postnatal day 7, and then decline to low levels in the adult nervous system.…”

“…There is a burst of high-level syndecan-3 expression in the central nervous system during the early postnatal period [12]. Heparin-binding growth-associated molecule (HB-GAM, also called pleiotrophin) is an 18 kDa secreted protein that is expressed in the central nervous system with a developmental time course that is essentially identical with that of syndecan-3 [120].…”