Functional and transcriptional analysis revealed that GTP cyclohydrolase 1 is required for cuticle pigmentation and metamorphosis in Tribolium castaneum

GTP cyclohydrolase 1 (GCH1) is the rate-limiting enzyme of tetrahydrobiopterin (BH4), which plays crucial roles in cuticle pigmentation in insect. Whether this enzyme has other functions and its regulating mechanism is not clear. Here, we characterized the functions of GCH1 and analyzed its signal transduction pathway in Tribolium castaneum. TcGCH1 gene has two alternative isoforms derived from different promoters, this is a common phenomenon in holometabolic insect which occurred after diversification of vertebrate and invertebrate. Functional analysis revealed that both TcGCH1_X1 and TcGCH1_X2 are involved in cuticle pigmentation by regulating melanin synthesis. Interestingly, these two isoforms also play critical roles in larvae to pupae metamorphosis by regulating JH and 20E synthesis pathway. Moreover, RNA-seq result revealed that, in addition to the well-known substrates of BH4, a large amount of iron-dependent enzymes were differentially expressed after knockdown TcGCH1, indicating BH4 may also act as the electron donor for these enzymes. Furthermore, different pattern recognition receptors including peptidoglycan recognition protein SA, SC2, SC1a/b were significantly upregulated, indicating TcGCH1 may play important roles in immune defense of T. castaneum. Our study will promote understanding the functions of GCH1 in insect and give new insights into the functions of BH4 in eukaryotes.