Cationic protein 8 plays multiple roles in Galleria mellonella immunity

Abstract

Galleria mellonella cationic protein 8 (GmCP8) is a hemolymph protein previously identified as an opsonin and an inhibitor of fungal proteases. In this work, we showed its bactericidal activity toward Pseudomonas entomophila, Pseudomonas aeruginosa, Bacillus thuringiensis, Staphylococcus aureus, and Escherichia coli and against yeast-like fungi Candida albicans. The activity against E. coli was correlated with bacterial membrane permeabilization. In turn, in the case of P. entomophila, B. thuringiensis, and C. albicans, the atomic force microscopy analysis of the microbial surface showed changes in the topography of cells and changes in their nanomechanical properties. GmCP8 also showed the inhibitory activity toward the serine protease trypsin and the metalloproteinase thermolysin. The expression of the gene encoding the GmCP8 protein did not increase either in the gut or in the fat body of G. mellonella after oral infection with P. entomophila. Similarly, the amount of GmCP8 in the hemolymph of G. mellonella did not change in immune-challenged insects. However, when GmCP8 was injected into the G. mellonella hemocel, a change in the survival curve was observed in the infected larvae. Our results shed new light on the function of GmCP8 protein in insect immunity, indicating its role in humoral defence mechanisms.

The Virtuous Galleria mellonella Model for Scientific Experimentation

“…These receptors are known as pathogen recognition receptors (PRRs), and are able to recognize pathogen-associated molecular patterns (PAMPs). PAMPs are conserved microbial components, including lipopolysaccharide (LPS), peptidoglycan, lipoteichoic acids (LTA), and β-1,3 glucan [ 9 , 51 ].…”

“…It is secreted in the hemolymph and binds firmly to LPS, LTA, and β-1,3-glucan [ 59 ]. GmCP8 has activity against bacteria and yeast-like fungi, and this activity is correlated with changes in the microbial cell surface due to the disruption of the membrane integrity [ 51 ].…”

“…Gm proline-rich peptides 1 and 2 also appear to increase the membrane permeability of bacteria [ 73 ]. Gm anionic peptides 1 and 2 bind to positively charged regions of the bacterial membrane and are active against Gram-positive bacteria and fungi [ 51 , 73 ]. Lysozymes are muramidases, able to digest bacterial peptidoglycan and act against Gram-positive bacteria, some Gram-negative bacteria, and some fungi [ 51 , 72 ].…”

“…Gm anionic peptides 1 and 2 bind to positively charged regions of the bacterial membrane and are active against Gram-positive bacteria and fungi [ 51 , 73 ]. Lysozymes are muramidases, able to digest bacterial peptidoglycan and act against Gram-positive bacteria, some Gram-negative bacteria, and some fungi [ 51 , 72 ]. These enzymes are also able to induce apoptotic changes in Candida albicans cells [ 75 ].…”

“…IMPI is the first discovered specific inhibitor of microbial metalloproteinases [ 77 ], having zinc in its active center [ 51 , 77 ]. Metalloproteinases are secreted as virulence factors by different human pathogens, being able to destroy insect defense proteins and peptides [ 78 ].…”