Structural cuticular proteins (CPs) and the liner polysaccharide, chitin, are the primary components of insect cuticle or exoskeleton. A large number of insect CP family proteins are divided into several distinct subfamilies defined by the presence of specific amino acid sequence motifs. One of these subfamilies is composed of Cuticular Protein Analogous to Peritrophins (CPAPs), containing one (CPAP1s) or three (CPAP3s) type-2 chitin-binding domains. In this study, we report a novel function of TcCPAP1-C from Tribolium castaneum in movement of legs. RNAi for TcCPAP1-C at larval stage has no effect on insect molting, growth and development. However, the resulting adults exhibit impaired leg movement, in which internal tendon cuticles are ruptured near the femur-tibia joint. The exoskeletal cuticle, hemiadherens junctions, microtubule array, myotendinous junctions and muscle fibers exhibit normal morphology before the tendon breakage. These results indicate functional specialization of TcCPAP1-C in structural integrity of the internal tendon cuticle, and loss of function of TcCPAP1-C caused breakage of the tendon cuticle, resulting in defective limb movement and locomotion.

Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds in crystalline polysaccharides including chitin and cellulose. The recent discovery of LPMO family proteins in many insect species suggests that they presumably play a role in chitin degradation in the cuticle/exoskeleton, tracheae and peritrophic matrix during insect development. Insect LPMOs belong to auxiliary activity family 15 (AA15/LPMO15) and have been classified into at least four groups based on phylogenetic analysis. In this study, we identified, characterized and investigated the physiological functions of group I LPMO15 (MaLPMO15-1 and PhLPMO15-1) in two longhorn beetle species, Monochamus alternatus and Psacothea hilaris. In both species, depletion of LPMO15-1 transcripts in last instar larvae by RNAi had no effect on subsequent larval-pupal molting and the resulting pupae developed normally. However, adverse effects on their development were observed during the pupal-adult molting period. The pharate adults were unable to shed their old pupal cuticle and died entrapped in their exuviae probably due to a failure of degradation of the chitin in their old cuticle, which is critical for completion of the insect molting and continuous growth.

Insect eggshell and cuticle/exoskeleton play vital roles in protecting them from natural environmental stresses. However, these chitinous cuticular extracellular matrices must be degraded at least in part during embryo hatching and molting/ecdysis periods to accommodate continuous growth all the way to the adult stage. In this study, we investigated the functional importance of groups I and II chitinases, TcCHT5 and TcCHT10, in the turnover of chitinous cuticle during both embryonic and post-embryonic development in Tribolium castaneum. RNAi and TEM analyses revealed that TcCHT10 is required for digestion of chitin in the serosal cuticle for embryo hatching as well as in the old cuticle during post-embryonic molts including larval-pupal and pupal-adult metamorphosis. TcCHT10 appears to be able to substitute for TcCHT5 in all these vital physiological events except for the pupal-adult molting in which TcCHT5 is indispensable for complete digestion of chitin in the old pupal cuticle.

Insect cuticle consists of numerous structural proteins, which could interact with polysaccharide, chitin, and alter properly mechanical property of the cuticle. Cuticular Protein Analogous to Peritrophins (CPAPs) are characterized by presence of one (CPAP1s) or three (CPAP3s) chitin-binding domain belong to CBM14/ChtBD2 family. In this study, we investigated physiological functions of TcCPAP1-H and TcCPAP3-C in Tribolium castaneum. RNAi for either TcCPAP1-H or TcCPAP3-C at late instar larvae had no effect on larval-pupal molt nor pupal development. However, the resulting pharate adults failed to shed their old pupal cuticle and died entrapped in it without undergoing eclosion. TEM analysis, in addition, revealed disorganized chitinous horizontal laminae and/or vertical pore canals of rigid cuticle from TcCPAP1-H- and TcCPAP3-C-deficient adults. Desiccation-induced death produced by injection of dsTcCPAP1-H into young instar larvae is also discussed.