My master’s thesis work involves studying the stability of alternative forms of IGF-I mRNA transcripts in chondrocytes.

Four classes of IGF-I mRNA transcripts resulting from different transcription start sites, alternative splicing of exon 5 and different polyadenylation sites on exon 6 have been identified in growth plate chondrocytes. These classes are characterized according to the presence of exon 1 (class 1) or exon 2 (class 2) and the presence of exon 5 (Eb) or absence of exon 5 (Ea). These alternative mRNA transcripts are expressed in a tissue specific manner, display variations in developmental regulation, and are differentially regulated by growth hormone (Lowe, 1987 and Adamo, 1989).

Alternative mRNA transcripts are differentially expressed in the rat growth plate at various stages of differentiation. IGF-I class 1 Ea mRNA is the most abundant transcript expressed in the growth plate. Proliferative chondrocytes express ten fold higher levels of IGF-I class 1 Ea mRNA than the resting and hypertrophic chondrocytes. IGF-I class 1Eb and class 2Ea transcripts are constitutively expressed at low levels in the chondrocytes at all stages of differentiation (Lin, W. and Oberbauer, A.M., 1996).

Individual mRNA species differ widely with respect to mRNA stability. Rates of decay can be altered in response to physiological signals, such as hormones (Brawerman, G., 1987). In addition, there are structural features of mRNA that determine its susceptibility to decay. The potential to form a stem-loop structure increases the stability of the mRNA. The intact stem-loop structure acts as a barrier against 3’ exonucleolytic cleavage (Guarneros et al., 1982). Insertion of a 3’ terminal AU-rich sequence acts to destabilize the mRNA . However, the manner in which the AU-rich sequence promotes mRNA decay is not known (Shaw and Kamen, 1986). In addition, sequences at the 5’ end of the mRNA may also influence the decay process (Brawerman, G., 1987).

The nature of the structural differences among IGF-I mRNA variants gives reason to propose that the transcripts have varying stabilities in chondrocytes. Recognizing the varying stabilities may provide insights as to the importance of the alternative mRNA transcripts at various stages of differentiation in chondrocytes and could provide a more comprehensive understanding of the control of IGF-I gene expression in chondrocytes.