A proteoglycan that activates fibroblast growth factors during early neuronal development is a perlecan variant

Cells in the early embryonic vertebrate nervous system are dependent on members of the fibroblast growth factor (FGF) family for their proliferation and subsequent differentiation. These growth factors will only bind to their specific high affinity cell surface receptors after formation of a ternary complex with the glycosaminoglycan heparan sulfate. Such specific heparan sulfates are secreted as proteoglycans from neural precursor cells and localise to their surfaces. One such proteoglycan, HSPG-PRM (Perlecan-related molecule), was isolated through its ability to potentiate neural cell responses to either FGF-1 or FGF-2. In this study, we have verified the relative molecular mass of the core protein of PRM as 45,000 and obtained partial amino acid sequence from it. The sequences bore significant homology to native perlecan. A probe generated by reverse transcriptase polymerase chain reaction using oligonucleotides designed from the protein sequence used on northern blots of RNA from a neuroepithelial cell line detected perlecan at 12.6 kilobases, as well as novel transcripts at 6.5 and 3.5 kilobases. The latter species appears by virtue of its size and abundance to be the novel PRM transcript. PRM appears to be encoded by the same gene as perlecan, as genomic Southern blotting only detected a single gene. Polyclonal antibodies raised against the PRM molecule detected a single proteoglycan species at 290x10(3) with a core protein of 45x10(3). Polyclonal anti-perlecan antibodies cross-reacted with PRM confirming their relatedness, although immunohistochemical studies revealed a differential staining pattern for PRM as compared to perlecan within the developing nervous system. The PRM molecule was shown to be localised to several different tissues of the developing embryo, indicating that it plays a broad role. We conclude that PRM is a variant of perlecan that is differentially glycosylated in a manner that confers highly specific functions at critical stages of neural development and tissue growth.