Expression of the small peptide GLP-1 in transgenic plants

Glucagon-like peptide 1 (GLP-1) has great potential in diabetes therapy. In order to accumulate GLP-1 in endosperm tissue of rice, a codon-optimized GLP-1 (mGLP-1) synthetic gene was directly expressed under the control of rice storage protein glutelin GluB-1 promoter in transgenic rice plants. Unexpectedly, neither the transcripts nor the transgene products were detected in the seeds of regenerated plants. Furthermore, transcripts of GluB-1 gene in these transgenic plants were not detected, and small interfering RNAs (siRNAs) corresponding to the transgene were detected. These results indicated that the expression of mGLP-1 was silenced by co-suppression in rice transgenic seeds. To avoid silencing, mGLP-1 was fused to GFP with or without self-processing 2A sequence, and introduced into rice plants. Both chimeric genes were highly expressed in these transgenic rice seeds, indicating that gene silencing could be avoided by changing the transgene components. Furthermore, the fusion protein containing the 2A sequence were processed into GFP-2A and mGLP-1 peptides with the efficiency of more than 80%, but the processed mGLP-1 peptides were not detected. Lack of accumulation of mGLP-1 may be explained by proteolytic digestion in the cytoplasm.