Antimicrobial Peptides For Tuberculosis Treatment

Tuberculosis

Tuberculosis (TB) is caused by disease causing pathogen called Mycobacterium tuberculosis. Tuberculosis generally affects lungs; however, it can also affect other parts of the body. Most common symptoms of TB include chronic cough with blood-containing sputum, fever, night sweats, and weight loss. TB usually spreads through air when active TB patient cough, spit, speak, or sneeze (Venketaraman et al., 2015).

Duration and complexity of treatment; and adverse events associated with TB treatment leads to nonadherence to treatment. It results in the suboptimal response and emergence of resistance. Increased incidence of multidrug-resistance and drug-resistant TB are the serious problems associated with TB. Prophylactic treatment of latent TB with drugs like isoniazid is associated with nonadherence to the treatment. Efforts to shorten treatment duration with alternative drugs resulted in the severe adverse events (Mandal et al., 2014).

Even though antimicrobial peptides (AMPs) have low level of amino acid sequence, these are associated with similar structural scaffolds. Hence, AMPs have potential antimicrobial action. It is difficult for the antimicrobial agents to cross the microbial cell-wall scaffold because it is composed of complex grid of macromolecules like peptidoglycan, arabinogalactan, and mycolic acids (MAgP complex) (Arranz-Trullén et al., 2017).

AMPs are small, cationic and amphipathic peptides which make part of the innate immune system; hence, considered as host defence peptides (HDPs). Expression of endogenous AMPs is an effective host defence strategy of living organisms. Characteristics of AMPs like multifunctional model of action, natural origin and effectiveness at low concentration made them potential candidates for anti-tubercular treatment (Arranz-Trullén et al., 2017).

AMPs exhibit its action through three different mechanisms like membrane disruption, metabolic inhibitor and immunomodulator. AMPs exhibit its action on the bacterial membranes. AMPs possess positive charge and these positive charges get attracted towards the negative charges of bacterial membrane.  Immune response develops following infection with mycobacterium. AMPs get engaged in the area of infection in the form of granuloma. AMPs disrupt cell was and plasmatic membrane disruption which results in the membrane pore formation. Membrane disruption mainly occur through three different mechanisms like toroidal pore formation, carpet formation and barrel stave formation. It leads to cytoplasmic leakage and death of bacteria. AMPs inhibit ATPase (Arranz-Trullén et al., 2017). AMP also responsible for protein degradation by exhibiting intracellular actions like nucleic acid binding, inhibition of replication, transcription and inhibition of translocation. Thus, AMPs exhibits its antimicrobial activity through functioning as metabolic inhibitors. AMPs also exhibit its action through functioning as immunomodulators. Through immunomodulation, AMPs doesn’t inhibit bacterial growth; however, it alters immune system of host through mechanisms like chemokine induction, histamine release, and angiogenesis modulation (Gutsmann, 2016).

Human derived AMPs are mainly responsible for the immune host defense against mycobacteria. Human AMPs include cathelicidin, defensins, hepcidins, lactoferrin, azurocidin, elastases, antimicrobial RNases, eosinophil peroxidase, cathepsins, granulysin, calgranulin/calprotectin, ubiquitinated peptides and lipocalin2 (Arranz-Trullén et al., 2017).

Synthetic AMPs are considered as the next generation antibiotics and these are useful to combat drug-resistant strains. Most widely used strategy for synthetic AMPs is to engineer stabilized amphipathic α-helix with selected antimicrobial prone amino acids. Synthetic AMP include 1-C134mer, A18G5, A24C1ac, A29C5FA, A38A1guan, CAMP/PL-D, CP26, d-LAK 120, d-LL37, E2 and E6, HHC-10, hLFcin1-11/ hLFcin17-30, Innate defense regulators like (IDR)1002, -HH2, and IDR-1018, LLAP, LLKKK18, MU1140, MIAP, Pin2 variants, RN3(1-45) RN6(1-45) RN7(1-45), SAMPs-Dma and X(LLKK) 2X: II-D, II-Orn, IIDab, and IIDa (Arranz-Trullén et al., 2017).

Name	Source	Mode of action	Activity
Cathelicidin (hCAP18/LL-37) (Torres-Juarez et al., 2015; Yu et al., 2013; Rekha et al., 2015)	Neutrophils,  Monocytes,   Epithelial cells,  Mast cells, Macrophages, Dendritic cells, Natural killer cells.	Monocytes,  Epithelial cells,  Mast cells,  Macrophages,  Dendritic cells, Natural killer cells, Mycobacterial cell wall lysis,  Immunomodulation,  Pro-inflammatory action,  Autophagy activation,  Chemotaxis,  Neutrophil extracellular traps (NETs) promotion, Bind with Mycobacterium tuberculosis within  the macrophage phagosome.	In vitro, In vivo
Defensins (Sharma et al., 2000; Sharma et al., 2001; Rivas-Santiago et al., 2011)	Eosinophils, Macrophages, Epithelial cells, Dendritic cells, Neutrophils	Mycobacterial cell membrane lysis (HBD), Membrane pore formation (HNPs),  Mycobacterial growth inhibition,  Dendritic and macrophage cells chemotaxis (HBD/HNPs), Inflammation regulation (HBD), zHNP1),  Intracellular DNA target (HNPs).	In vitro, In vivo, ex vivo
Hepcidin (Gutsmann, 2016; Yamaji, 2004)	Hepatocytes,  Macrophages,  Dendritic cells,  Lung epithelial cells, Lymphocytes.	Mycobacterial cell wall lysis,  Inhibition of mycobacterial infection, Iron homeostasis regulation, Pro-inflammatory activity.	In vitro, In vivo
Lactoferrin (Hwang et al., 2007)	Epithelial cells,  Neutrophils,  Polymorphonuclear (PMN) leukocytes	Bacterial cell permeation, Iron kidnapping, Anti-inflammatory activity.	In vitro, In vivo
Azurocidin (Jena et al., 2012)	PMN leukocytes,   Neutrophils	Mycobacterial cell wall lysis,  Promotion of phagolysosomal fusion	In vitro
Elastases (Wong and Jacobs, 2013)	Neutrophil azurophilic granules, bone marrow cells, Macrophages	Bacterial cell membrane lysis,  Serine protease activity, Cell chemotaxis induction,  Immunomodulation, NETs formation,  Macrophage extracellular traps (METs) formation.	In vitro, In vivo
Antimicrobial RNases (Becknell et al., 2015)	Eosinophils (RNase3/ECP), Neutrophils and monocytes,  Epithelial cells and leukocytes	Mycobacterial cell agglutination, Mycobacteria cell wall and membrane lysis.	In vitroI, In vivo, Clinical
Eosinophil peroxidase (Pulido et al., 2013)	Eosinophils	Bacterial cell wall lysis.	In vitro
Cathepsins (Walter et al., 2015)	Neutrophils, Monocytes	Mediation of apoptosis pathway, Immunomodualtion.	In vitro, In vivo
Granulysin (Stenger et al., 1998)	Lymphocytes	Mycobacterial cell lysis.	In vitro
Calgranulin/calprotectin (Dhiman et al., 2014)	Neutrophils, Monocytes,  Keratinocytes,  Leukocytes	Phagolysosomal fusion, Pro-inflammatory action.	In vitro, In vivo
Ubiquitinated peptides (Gutsmann, 2016)	Macrophages	Mycobacterial cell lysis.	In vitro
Lipocalin2 ((Gutsmann, 2016))	Neutrophils	Mycobacterial growth inhibition, Immunoregulation.	In vitro, In vivo

Issues with current therapy for TB

Synthetic AMPs:

Name	Source	Mode of action	Activity
1-C134mer (Kapoor et al., 2011)	De novo design by oligo N-substituted glycines (peptoid) and alkylation	Pore formation	In vitro
A18G5, A24C1ac, A29C5FA, and A38A1guan (Hoffmann  and Czihal, 2009)	Derived from the insect proline-rich peptide Apidaecin. Steps involved are alkylation, tetramethyl guanidinilation, and polyethylene glycol conjugation.	Bacterial membrane permeation and inhibition of protein synthesis	In vitro
CAMP/PL-D (Ramón-García et al., 2013)	Short cationic peptides (10 AA) rich in W and R selected from peptide libraries	Pore formation.	In vitro
CP26 (Rivas-Santiago et al., 2013)	Derived from cecropin A: mellitin	Bacterial cell wall disruption.	In vitro
d-LAK 120 (Lan et al., 2014)	Synthetic α-helical peptides	Pore formation and inhibition of protein synthesis.	In vitro, Ex vivo
d-LL37 (Jiang et al., 2011)	Derived from LL-37	Pore formation and immunomodulatory activity.	In vitro
E2 and E6 (Rivas-Santiago et al., 2013)	Derived from bactenecin (bovine cathelicidin) Bac8c (8 AA)	Bacterial cell wall disruption.	In vitro
HHC-10 (Llamas-González et al., 2013)	Derived from bactenecin	Bacteria membrane lysis.	In vitro, In vivo
hLFcin1-11/ hLFcin17-30 (Silva et al., 2014)	Derived from lactoferricin (All-R and All-K substitutions)	Bacterial cell wall and membrane lysis.	In vivo
Innate defense regulators [innate defense regulator (IDR)1002, -HH2, IDR-1018] (Rivas-Santiago et al., 2013)	Derived from macrophage chemotactic protein-1 (MCP-1)	Immunomodulatory and anti-inflammatory activity.	In vitro, In vivo
LLAP (Chingaté et al., 2015)	Derived from LL-37	Inhibition of ATPase.	In vitro
LLKKK18 (Silva et al., 2016)	Derived from LL-37 through Hyaluronic acid nanogel conjugation.	Pore formation and immunomodulatory activity.	In vivo
MU1140 (Ghobrial et al., 2010)	Derived from Streptococcus mutans lantibiotics	Inhibition of cell wall synthesis.	In vivo, In vivo
MIAP (Santos et al., 2012)	Derived from Magainin-I	Inhibition of ATPase.	In vitro
Pin2 variants (Rodríguez et al., 2014)	Derived from short helical peptides like pandinin2	Membrane disruption.	In vitro
RN3(1-45) RN6(1-45) RN7(1-45) (Pulido et al., 2013)	Derived from human RNases N-terminus	Bacterial cell wall disruption and cell agglutination and intracellular macrophage killing.	In vitro, ex-vivo
Synthetic AMPs (SAMPs-Dma) (Sharma et al., 2015)	De novo design through Dimethylamination and imidazolation.	Cell penetration and DNA binding.	In vitro
X(LLKK) 2X: II-D, II-Orn, IIDab, and IIDap (Khara et al., 2014).	Short stabilized α-helix amphipatic peptides	Pore formation.	In vitro

Conclusion:

In recent past, number of AMPs are discovered to combat resistance in TB. These AMPs exhibit its action through direct killing of bacteria and immunomodulation; hence, there is more potential to evade resistance problem. In future, AMPs with low-cost synthesis method should be prepared because its cost of synthesis is more. AMPs are susceptible to proteolytic cleavage after systemic administration; hence, these should be produced using novel drug delivery systems. It is essential to produce sufficient evidence to address the issues related to peptide-based therapy for TB.

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Bacterial unique structure

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