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???displayArticle.abstract??? Choroid plexus lipocalin 1 (Cpl1) has been isolated from the African clawed toad (Xenopus laevis) and the cane toad (Bufo marinus). Xcpl1 has been used as a marker for studying early neural development. Due to its retinoid binding properties and the fact that it causes dysmorphogenesis when overexpressed in the early embryo, the protein product is considered to be part of the retinoic acid signalling pathway. Later in development and during adulthood, the epithelial cell sheet of the choroid plexus which forms the blood-cerebrospinal fluid barrier expresses cpl1 as the predominant secretory protein. These data, the similarity of Cpl1 to prostaglandin D(2) synthase and its functional homology to transthyretin will be discussed.
Fig. 1. Multiple sequence alignment. Homologous lipocalin protein sequences have been aligned. To maximize identities, deletions
marked as dashes have been introduced. Residues identical in Xcpl1 and aligned lipocalins have been labelled by black boxes. Xcpl1
(GenBank accession No. S52354), Xenopus choroid plexus lipocalin; Bmlip (Q01584), B. marinus lipocalin; Hspgds (AAA36494),
Homo sapiens prostaglandin D2 synthase; Ambp (BAA13453), X. laevis K1-microglobulin/bikunin precursor; Rngal (P30152), Rattus
norvegicus neutrophil gelatinase-associated lipocalin; Mup5 (P11591), Mus musculus major urinary protein 5.
Fig. 2. In situ hybridization of Xenopus neural plate stage embryos detecting Cpl1 gene expression by digoxigenin labelled antisense
cRNA. (A) Dorsal view of a wholemount specimen, anterior to the right, dashed line marks the embryo's right border of the neural
plate (np), blastoporus (bp) to the right. (B) Histological mid-sagittal section. ar, archenteron; bc, blastocoel; pm, precordal mesoderm.
Fig. 3. Model of retinoid delivery by Cpl1 to the intercellular
space. Cpl1 (in blue) is constitutively secreted and thereby traverses
the ER, Golgi apparatus and the trans-Golgi network. In
the ER, retinoids (in red) are bound to binding proteins such
as Cpl1 or CRBP (drawn as black V-like structures). After exocytosis,
bound retinoids are released from Cpl1 and readily diffuse
into cytoplasm of the cell (green arrow).
Fig. 4. Molecular structures of prostaglandin D2 (upper part of
the panel), all-trans and 9-cis retinoids. The proposed conversion
of all-trans RA to the 9-cis form by Cpl1 is indicated by a
round arrow.
