Differential regulation of tyrosine hydroxylase in cuticular melanization and innate immunity in the silkworm Bombyx mori

• During development, BmTH is responsible for cuticular melanization.
• Tyrosine-induced BmTH triggers BmPPAE expression and cuticular melanization.
• Differential regulation of BmTH primes cuticular melanization and innate immunity.
• Cuticular melanization is a defense mechanism that uses the proPO system.

Insect cuticular melanization is a defense mechanism involving the prophenoloxidase (proPO)-activating system. Here, we showed the differential regulation of tyrosine hydroxylase in silkworm (Bombyx mori) pupae. B. mori tyrosine hydroxylase (BmTH) was expressed in the epidermis during development or in the fat body during infection, which indicates that the differential expression of BmTH is responsible for cuticular melanization or innate immunity. The melanin of the silkworm pupal cuticle formed a melanization complex with Beauveria bassiana and this effect showed an antifungal activity, indicating that melanin in silkworm pupal cuticle plays a role in the defense against fungal pathogens. Finally, we found that the differential expression of BmTH is involved in pupal cuticular melanization and adult pigmentation as well as the innate immunity of silkworms, demonstrating a differential regulation of BmTH in a tissue-specific manner.

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