HOLFORD, KC; LI, S; BORST, DW; Purdue Univ. North Central; Illinois State Univ.; Illinois State Univ.: Regulation of the methyl farnesoate biosynthetic pathway in the lobster mandibular organ.

Methyl farnesoate (MF) is a major product of the crustacean mandibular organ (MO). The synthesis of this isoprenoid is regulated by neuropeptides from the sinus gland (SG) of the eyestalk. To understand how SG neuropeptides regulate MF levels in the lobster (Homarus americanus), we studied two enzymes in the MF biosynthetic pathway: 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR, one of the first steps in isoprenoid synthesis) and farnesoic acid O-methyltransferase (FAOMeT, the last step in MF synthesis). 14d after eyestalk ablation, MO weight increased 1.7-fold above initial levels, while FAOMeT and hemolymph levels of MF increased 19.6 and 15.9 -fold, respectively. The level of FAOMeT-mRNA (measured by real time PCR) increased 2.5-fold during this period, indicating that FAOMeT activity is regulated in part at the transcription level. HMGR activity rose only 2.4-fold during this period, suggesting that the amount of this enzyme in intact animals is sufficient to support a considerable increase in MF synthesis. The role of SG peptides was further studied by treating 14d eyestalk ablated lobsters with 0.05 SG equivalents of MO-inhibiting hormone (MOIH; purified by reversed phase HPLC) or the saline vehicle. Five hours later, the levels of MF, FAOMeT activity, and FAOMeT-mRNA had fallen to 17%, 11%, and 29% of those observed in saline treated animals, respectively, indicating that the final step in MF synthesis is rapidly regulated by MOIH. In contrast, HMGR activity was not changed by this treatment, though treatment of animals with whole SG extract (0.01-2 equivalents) decreased HMGR activity by 70-80%. This observation suggests that HMGR is regulated by another SG peptide. (Supported by NSF grant IBN – 0240903 to DWB).