Team:Utah State/Project

Antimicrobial peptides (AMPs) are known for their ability to kill bacteria that are harmful to its host and are part of the innate immune system of all multicellular organisms (Andreu & Rivas, 1998). AMPs from various organisms have shown the ability to inhibit many types of gram-positive/gram-negative bacteria, protists, fungi, and other organisms. The outstanding diversity of AMPs found in all organisms on the planet contribute to a constantly growing list of peptides with varying properties. The size, amino acid sequence, charge, conformation, hydrophobicity, and amphipathicity all contribute to the activity of an AMP. Most AMPs contain fewer than 100 amino acids and are positively charged (Giuliani et al., 2007).

Organisms across our planet produce antimicrobial peptides that allow them to thrive in their distinct environments. For example, crocodiles by nature are extremely territorial and engage in vicious battles with other competing members of their community, sustaining open wounds in the process. The AMPs that are naturally produced in their blood protect them from infections due to the innumerable bacteria and organisms that could otherwise cause them great harm. King penguins produce unique AMPs in their stomach to allow for extended storage of food to later regurgitate and feed to their young. Marsupials have shown to be unique hosts of AMPs. The tammar wallaby relies on the production of AMPs in their pouches to protect their young who do not have a developed immune system for the first few months of their lives (Wang et al., 2011).

The diversity of AMPs is likely another example of adaptations occurring in organisms due to environmental pressures. It has been determined that most AMPs are derived from precursor sequences that are quite similar. Conserved regions on AMPs even between different species of organisms has led to this belief (Zasloff, 2002). The changes present in the current AMP sequence are probably due to the thriving of organisms containing mutations in the AMP sequence that allowed them to gain an advantage over other individuals lacking the mutation (Zasloff, 2002). These AMPs have been tested over many generations in their host organisms and makes them fascinating for studies in which their activity is tested against a variety of organisms.

With so many AMPs, it is no wonder that a variety of mechanisms are possible for bacterial cell killing (Epand et al., 1999). Most AMPs that have been studied follow the Shai-Matsuzaki-Huang (SMH) model (Zasloff, 2002) (Lai & Gallo, 2009). This model overviews the interaction of a peptide with the outer layer of a cell membrane. The general flow of this model includes the “carpeting” of the membrane with the AMP followed by the displacement of lipids in the membrane, and finally the lysing of the membrane. The SMH model can be seen below.

The SMH model demonstrates that one of two outcomes is achieved to reach cell lysis: (1) the peptide will physically break the membrane to cause cell lysis or (2) the peptide will enter the cell to perform various interior mechanisms of action against the cell (Zasloff, 2002). This model demonstrates a general explanation of the mechanism of how AMPs operate. How exactly they act in killing a cell varies greatly from peptide to peptide. Some examples may be: depolarization of charged cell membranes, pore formation in cell membranes, and degrading of important cell structures. However, it is generally agreed that most AMPs work to achieve cell membrane permeabilization (Andreu & Rivas, 1998). Many organisms use a series of different AMPs from various classes to work more effectively against bacteria (Zasloff, 2002). AMPs are usually not produced in high concentrations until injury or illness occurs to the organism. Other AMPs are stored as inactive compounds and are controlled by cellular receptors (Lai & Gallo, 2009).

AMPs are characterized according to their structure and function. Our team selected AMPs from most of the classes of AMPs. These include anionic peptites (Scygonadin), linear cationic alpha-helical peptides (LL37, Grammistin Pp1, EcAMP1), and anionic and cationic peptides that contain cysteine and form disulphide bonds (Cg-Defh1) (Brogden, 2005). Generally, AMPs are alpha helical or contain beta-sheets. However, some are extended in structure or form loops.

Drug resistance is especially important in health settings in modern society. The use of antibiotics to treat bacterial diseases has led to the emergence of “superbugs”, drug resistant bacteria that infect humans. These superbugs are often untreatable with current antibiotics and are the cause of an increasing number of deaths each year. However, it appears as if resistance is harder to come by for bacteria against AMPs. This is likely due to the mechanism of most AMPs acting on bacterial cell membranes instead of its cell walls (Peters et al., 2010). AMP resistance would require a complete re-design of the bacterial membrane to prevent action of the AMP. While still possible, the likelihood of resistance being developed is much smaller, making AMPs a potential source of much more effective bacterial killers (Zasloff, 2002) (Epand et al., 1999).

An extremely important trait of an antimicrobial agent is that it has selective mechanisms against harmful bacteria. For use in the human healthcare system, AMPs must not target mammalian cells or demonstrate hemolytic activity. Due to the mechanism of most AMPs acting on bacterial membranes, it has been proposed that AMPs could work in tandem with current antimicrobials. By letting AMPs degrade the microbe’s membrane, an antimicrobial could easily gain access to the inside of the cell, making it much more effective (Peters et al., 2010). In vitro, it has been observed that minimum inhibitory concentrations (MIC) of AMPs against bacteria are much higher than it appears to be in vivo. It is thought that AMPs work in conjunction with other AMPs to act as more effective inhibitors of bacteria. LL37, a model peptide used in our study, has been shown to work in tandem with other peptides as well as lysozyme to increase toxicity to bacteria (Lai & Gallo, 2009). It has even been observed that AMPs may possess the potential to increase overall immune response, neutralize endotoxins, and aid in wound healing (Peters et al., 2010). The Figure below outlines many of the processes that AMPs accomplish in vivo.



Figure shows the diversity of AMPs used by Utah State iGEM team 2013