ReviewStringent response protein as a potential target to intervene persistent bacterial infection
Graphical abstract
Introduction
When pathogenic bacteria infect host, most of them are cleared by the efficient immune system and specific antimicrobial therapy. However, some of them survive and remain in the host for prolong time and leads to persistent infection [1,2]. Persistence help pathogens to establish in the host by tolerating the immune defense and antimicrobial drug therapy [3]. However, most of these infections are clinically asymptomatic, they thus silently affected more than half of the world's population [4,5]. Consequently, they serve as a reservoir and in the long run poses a threat of becoming active infections. For example, the reoccurrence of tuberculosis by reactivation of persister Mycobacterium tuberculosis [6,7]; Helicobacter pylori can persist lifelong in human gastric mucosa [2] and Salmonella enterica serovar typhi causative agent of typhoid fever can remain in the host system for a very long period [8]. Some of these pathogenic bacteria and persistent infections are listed in Table 1. Though, persister pathogens are difficult to eradicate with standard antimicrobial therapy as persister bacteria often establish a protected environment to survive. For example biofilm formation in Salmonella typhi [9], macrophage colonization of Mycobacterium tuberculosis [10] and similar mechanism in other pathogens maintain their survival in the intracellular environment. This physical protection provides an additional barrier to hostile factors such as antibodies, complementary factors, and antibiotics. Once, these pathogens are fully reactivated, standard antimicrobial therapy often fails to clear these clinically persistent pathogens, hence the extended duration of antibiotic therapy is thus required to successfully eradicate them. Additionally, long term persistence has been confirmed to be one of the reasons for the development of antimicrobial resistance in bacterial pathogens [11]; henceforth it is considered as one of the major public health issues. Persister pathogens enforce various approaches to ensure survival for long duration including stringent response, toxin-antitoxin system, oxidative stress response, and SOS response [[12], [13], [14], [15], [16], [17], [18], [19]]. Among these, the stringent response has been shown to be a core regulatory process in persister formation.
Section snippets
Stringent response
It has been proven by several studies that stringent response plays a vital role in bacterial pathogenesis and virulence, therefore, it could be a potential target to check persister formation. The stringent response assist pathogens to survive at various stages of infection [[21], [22], [23], [24]] like host invasion [25], drug tolerance [26,27], virulence [28,29], antibiotic resistance [11,30], and long term survival [12]. The imperativeness of stringent response in bacterial pathogenesis has
RelA/SpoT homologous (RSH) proteins
There are two (p)ppGpp synthetase identified in E. coli; they include a monofunctional (p)ppGpp synthetase (RelA) and bifunctional SpoT having both hydrolase and weak synthetase activity. The distribution of (p)ppGpp synthetases and (p)ppGpp hydrolases among prokaryotes have been classified into a family known as RelA/SpoT homologous (RSH) protein family [73]. This family includes long multi-domain Rel proteins, small alarmone synthetases (SAS), and small alarmone hydrolase (SAH). The
Structures of Rel proteins
Molecular mechanism of (p)ppGpp synthesis was first explained by the crystal structure of a bifunctional (p)ppGpp synthetase/hydrolase enzyme from (RelSeq) Streptococcus dysgalactiae (PDB ID 1VJ7) [75]. The investigators delineated the structural basis of the catalytic mechanism at the molecular level and proposed a mechanism of action that it is governed by antagonistic conformational plasticity of protein. Almost a decade later, the structure of full-length RelA protein was determined by
Stringent response inhibitors
The ubiquitous stringent response is a core process in persistence infection, therefore, several efforts have been successfully endeavored to intervene in the process. The first compound to be used was ppGpp analog; 2′-deoxyguanosine-3′-5′-di(methylene bisphosphonate) which has been reported to be a competitive inhibitor of (p)ppGpp synthetase RelA [82]. To design ppGpp analog, the investigators replaced susceptible pyrophosphate moieties with non-hydrolyzable methylene-bisphosphonate groups.
Conclusion
The ubiquitous stringent response is essential for establishing persistent bacterial infections. It aids and improves pathogens survival propensity in the host during hostile conditions. Stringent response mechanism is regulated by the synthesis of messenger nucleotide, (p)ppGpp, by Rel enzymes. Subsequently, this (p)ppGpp binds to several proteins ranging from DNA replication, protein transcription to various metabolic pathway proteins. (p)ppGpp synthetases are found both as a long
Funding
This work is funded by Science and Engineering Research Board (SERB), Government of India under Young Scientist Start-up grant (SB/YS/LS-176/2014) and ICGEB core funds. Bolaji Fatai Oyeyemi is supported by ICGEB Arturo Falaschi pre-doctoral fellowship.
Conflicts of interest
The authors declare that they have no competing interests.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Author contributions
GSK and NSB conceived, designed, and wrote the paper. BFO edited, modified and revised the manuscript. All authors read and approved the final version of the manuscript.
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2021, Cell Chemical BiologyCitation Excerpt :Therefore, it seems likely that exposure of Rel protein to DMNP would interfere with its binding to GTP, thereby decreasing the (p)ppGpp-synthesizing activity. Further, it is also reasonable to assume that RelZ, the only SAS identified thus far in M. smegmatis, could be targeted by DMNP in a similar fashion because, like other SASs, RelZ is analogous to the SYN domain of the multidomain Rel protein (Kushwaha et al., 2019) (Figures 6A and 6E). M. smegmatis RelZ crystal structure also is not available yet, but there are structural data for several other small alarmone synthetases belonging to different species (Kushwaha et al., 2019).