Characterization and expression of three forms of cDNA encoding chicken platelet-derived growth factor-A chain

Platelet-derived growth factor (PDGF) affects cell proliferation and differentiation during mammalian embryogenesis. In a number of avian species, PDGF-alpha receptors and PDGF-A chain (PDGF-A) are present during chicken limb and lens development. However, little is understood about the chicken PDGF-A gene. The present study identified short form type 1 (S1), long form (L) and short form type 2 (S2) cDNA clones encoding chicken PDGF-A chain (PDGF-A). These clones were isolated from a chicken hepatoma cell line (LMH) mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR) and cDNA library cloning. Genomic sequencing and Southern blotting revealed that these forms were generated by alternative splicing. The mRNAs of S1 and L contained two transcription start sites on one exon. At the amino acid level, the mature protein encoded by the L clone showed 90 and 85% homology with the processed coding regions of the long form of human and Xenopus PDGF-A, respectively. The putative mature peptides of all forms of chicken PDGF-A encompassed the eight cysteine residues conserved in all known forms of PDGF. We examined the expression of the three forms in chicken tissues and cells using RT-PCR. Expression of these forms varied among tissues and cells. Levels of PDGF mRNAs were very low in chicken thrombocytes, which are analogous to mammalian platelets. However, the level of PDGF-A chain mRNA expression in chicken thrombocytes peaked 4 h after exposure to type 1 collagen or thrombin, and then decreased gradually with continued incubation. These results suggest that chicken PDGF in thrombocytes plays an important role in the vascular system and in healing damaged tissue.