The FGF2 mRNA encodes distinct protein isoforms expressed by utilization of several alternative CUG start codons in addition to the classical AUG start codon.
20 In mice, alternative start site selection generates two high molecular weight (HMW) species of 21 and 22 kDa, while activation of the classic AUG produces a single, low molecular weight (LMW) 17.5-kDa isoform.
20 Immunoblotting identified all three isoforms at each age, but the distribution of the HMW and LMW proteins varied with age (
Fig. 5A). In control pups, total FGF2 varied little, but significantly, over the 2-week examination period, with decrements noted at P12 and P13. Age produced a shift in isoform distribution favoring HMW proteins (ANOVA,
P < 0.05). From P8 to P17, the ratio of HMW:LMW increased more than 10-fold in both control and OIR groups (P8: Con [control] = 0.21 ± 0.02; OIR = 0.14 ± 0.06; P17: Con = 2.47 ± 0.21; OIR = 2.60 ± 0.21;
P < 0.01). Although the OIR protocol failed to alter the ratio of HMW-to-LMW FGF2, exposure to 75% O
2 decreased LMW, HMW, and total FGF2 protein expression relative to controls (P12;
Fig. 5B). Upon return to room air at P13, total, LMW, and HMW FGF2 protein expression increased in a time-dependent manner. Relative to controls, total FGF2 protein and both isoforms were greater at P13, P17, and P21. Of note, the expression trends for total FGF2 protein in C57BL/6 animals was similar to that of hybrids at P12, P15, and P17 in the hybrid mice (not shown).