Constitutive secretion of β‐trace protein by cultivated porcine choroid plexus epithelial cells: Elucidation of its complete amino acid and cDNA sequences

Abstract

Primary porcine choroid plexus epithelial cells cultivated in chemically defined medium maintain their epithelial characteristics and form confluent monolayers. They produce a fluid the composition of which resembles cerebrospinal fluid. The present study demonstrates constitutive secretion of large amounts of beta-trace protein. This intrathecally synthesized protein is a prominent polypeptide constituent of natural cerebrospinal fluid. According to the identity of amino acid sequences it has previously been tentatively identified as a prostaglandin-D synthase and as a member of the lipocalin protein family. beta-Trace was purified from cell culture supernatants and was subjected to tryptic digestion and amino acid sequencing of the resulting peptides. The complete primary structure of the protein was obtained by additional isolation of the cDNA from cultured epithelial cells. The porcine 163-amino acid polypeptide showed 69% identity with the human beta-trace and contained two N-glycosylation sites occupied by complex-type oligosaccharides as is the case for the human protein. The amino acid sequences around the N-glycosylation sites of mammalian beta-trace proteins (porcine, human, murine, and rat) were highly conserved. The nucleotide sequence was found to be less conserved; the porcine cDNA had a strikingly high GC-content (67%). The constitutive secretion of beta-trace protein from the in vitro cultivated porcine choroid plexus epithelial cells demonstrates that the cells have retained their major in vivo physiological properties: secretion of cerebrospinal fluid proteins. Therefore, this in vitro culture system may be used as a versatile tool for studying the regulation of the formation of cerebrospinal fluid.

Expression of the beta‐trace protein in human pachymeninx as revealed by in situ hybridization and immunocytochemistry

“…As beta-trace is found in high concentrations in human CSF and since it lacks a signal peptide (Zahn et al, 1993), in situ hybridization and immunocytochemical staining lend support to the idea that beta-trace is synthesized and secreted in the human CNS by the epithelial layer of the choroid plexus (Blödorn et al, 1996). This hypothesis is further supported by the results obtained by Hoffmann et al (1996b) who demonstrated that cultured epithelial cells of porcine choroid plexus constitutively secrete beta-trace.…”

Cytology of the Normal and Abnormal Choroid Plexi in Selected Domestic Mammals, Wildlife Species, and Man

“…24 This supports the currently held concept that in addition to producing CSF, the ChPCs have an endocrine function because they produce an insulin-like growth factor II 19 and calcyphosine. 9 Also, the ChPCs produce several active substances, among which are the transport protein transthyretin, 23 a ganglioside GD3, 12 a prostaglandin-D synthase (''beta-trace protein''), 11 and a retinaldehyde dehydrogenase. 26 Recently, synaptophysin has been found to be present in both normal and neoplastic ChPCs.…”