Molecular Characterization and Gene Expression Profiling ... - CUSAT

Chapter 1
surfaces secrete AMPs from both barrier epithelia and glandular structures
(Zasloff, 1987; Diamond et al., 1991; Jones and Bevins, 1992; Ouellette and
Selsted, 1996). Phagocytic cells contain several types of storage organelles
(granules) for microbicidal substances and digestive enzymes (Levy, 1996;
Ganz and Lehrer, 1997). In the process of phagocytosis, granules fuse to
phagocytic vacuoles that contain ingested microbes, thereby exposing the
microbes to very high concentrations of microbicidal and digestive
substances. Other granules are secreted into the extracellular fluid where
their contents kill microbes or inhibit their multiplication. Both types of
granules contain abundant AMPs (Selsted et al., 1984; Ganz et al., 1985;
Cowland et al., 1995). In invertebrates, the fluid portion of blood
(hemolymph) as well as the granules of phagocytic cells (haemocytes)
contains AMPs (Boman et al., 1991; Iwanaga et al., 1994). Secretion of AMPs
from the fat body (equivalent to the liver in vertebrates) into haemolymph
appears to be the dominant mechanism in injured or infected insects, while
haemocytes may be the more important source in horseshoe crabs. Like in
vertebrates, insect epithelia, most prominently the gut, secrete tissue-specific
AMPs (Richman and Kafatos, 1996; Richman et al., 1997), a response which is
likely to be important in insect resistance to intestinal parasites.
1.7.10 Mechanism of action
Many AMPs bind in a similar manner to negatively charged
membranes and permeate them, resulting in the formation of a pathway for
ions and solutes (McElhaney et al., 1999). Before reaching the phospholipid
membrane peptides must transverse the negatively charged outer wall of
Gram-negative bacteria containing LPS or through the outer cell wall of
Gram-positive bacteria containing acidic polysaccharides. Hancock and
coworkers described this process as a ‘self-promoted uptake’ with respect to
Gram-negative microorganisms (Hancock, 1997). In this mechanism, the
peptides initially interact with the surface LPS, competitively displacing the
Molecular Characterization and Gene Expression Profiling of Antimicrobial Peptides in Penaeid Shrimps
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