Human Müller cells express VEGF183, a novel spliced variant of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor expressed as multiple RNA transcripts due to alternative splicing. During an investigation of the expression of VEGF mRNA in human Müller cells cultured under hypoxic conditions, a cDNA species was isolated whose size was incompatible with known VEGF transcripts. This study was performed to determine the nucleotide sequence of the candidate VEGF species and examine the effects of hypoxia on its expression.

Cultured human Müller cells were exposed to normoxic (20% O2, 5% CO2, 75% N2) or hypoxic (2% O2, 5% CO2, 93% N2) conditions at 37 degrees C for 4 hours and processed for reverse transcription-polymerase chain reaction (RT-PCR), molecular cloning, Southern hybridization, nucleotide sequencing, semiquantitative RT-PCR, and ribonuclease protection assay.

The nucleotide sequence of the novel VEGF species isolated from human Müller cells had a short exon 6-encoded sequence without 18-bp nucleotides immediately upstream of the exon 7-encoded sequence in VEGF189. The 18-bp deletion (corresponding to the six amino acids Tyr-Lys-Ser-Trp-Ser-Val) was compatible with a polypeptide containing 183 amino acids (VEGF183). Although VEGF183 mRNA was found in all tissues studied, its expression seemed to be higher than that of VEGF 189 in the brain and spleen; lower in the kidney, retina, skeletal muscle, and liver; and at similar level in the heart. Exposure to hypoxic conditions for 4 hours promoted increased levels of VEGF mRNA including that of VEGF183.

The expression of the novel isoform VEGF 183 in human Müller cells, its variable tissue expression, and its modulation by hypoxia may provide another pathway for VEGF induction of angiogenesis in the retina.