Elsevier

Current Opinion in Microbiology

Volume 63, October 2021, Pages 126-132
Current Opinion in Microbiology

Metabolic stringent response in intracellular stages of Leishmania

https://doi.org/10.1016/j.mib.2021.07.007Get rights and content

Highlights

  • Intracellular amastigote stages of Leishmania activate a stringent metabolic response.

  • The stringent response, triggered by H2O2, involves extensive metabolic remodelling.

  • Amastigotes upregulate pathways involved in metabolic homeostasis (e.g. mannogen).

  • The stringent response encompasses a range of physiological states in vivo.

Leishmania are unusual in being able to survive long-term in the mature phagolysosome compartment of macrophages and other phagocytic cells in their mammalian hosts. Key to their survival in this niche, Leishmania amastigotes switch to a slow growth state and activate a stringent metabolic response. The stringent metabolic response may be triggered by multiple stresses and is associated with decreased metabolic fluxes, restricted use of sugars and fatty acids as carbon sources and increased dependence on metabolic homeostasis pathways. Heterogeneity in expression of the Leishmania stringent response occurs in vivo reflects temporal and spatial heterogeneity in lesion tissues and includes non-dividing dormant stages. This response underpins the capacity of these parasites to maintain long-term chronic infections and survive drug treatments.

Introduction

Leishmania are sandfly transmitted protozoan parasites that cause a spectrum of localized and disseminating cutaneous infections, as well as metastasizing visceral infections, in a broad range of mammalian hosts including humans [1]. More than 12 million people have active disease and another 120 million are thought to harbour long-term chronic infections. Transmission to the mammalian host is initiated by flagellated promastigotes which are injected into the skin during a sandfly bite and then rapidly internalized by a range of phagocytic cells including neutrophils, dendritic cells and macrophages [2]. Promastigotes differentiate to non-motile amastigotes following their delivery to the mature phagolysosome of infected host cells, primarily macrophages, and perpetuate infection during the initial innate phase and subsequent acute and/or chronic phases of infection. The ability of Leishmania amastigotes to survive within macrophage phagolysosomes is remarkable, as most other bacterial, fungal and protist pathogens that target macrophages, have evolved mechanisms to avoid or escape lysosomes and are killed if delivered to this niche. There is accumulating evidence that amastigote survival within this niche is dependent on the activation of a stringent metabolic response [3]. This response is linked to amastigote differentiation and involves switching to a slow growth state, extensive reprograming of central carbon metabolism, and downregulation of energy consuming processes (e.g. protein, RNA turnover) (Figure 1). Here we summarize some of the key features of the Leishmania amastigote stringent metabolic response, and the exogenous/endogenous signals involved in activating this response. We also discuss accumulating evidence that Leishmania amastigotes switch between different physiological states in vivo, in order to exploit and/or adapt to temporal or spatial heterogeneity in host cell physiology, polarization state and inflammatory responses. Overall, these studies suggest that the Leishmania stringent response encompasses a range of physiological states and likely plays a key role in allowing these parasites to establish long-term chronic infections, while also conferring resistance to front-line drug treatments.

Section snippets

The Leishmania amastigote stringent metabolic response

Depending on the species, Leishmania amastigotes reside within either large communual or individual tight-fitting parasitophorous vacuoles (PVs) that have all of the characteristics of a mature phagolysosome (low pH, LAMP proteins, cysteine proteases, lipases, glycosidases), as well as markers for endosomes, autophagosomes and the endoplasmic reticulum (ER) [4,5] (Figure 2). While nutrient levels in this compartment have not been explicitly measured, they are likely to be similar to those in

Activation of the stringent response is linked to increased ROS production

Leishmania promastigote to amastigote differentiation can be induced in vitro by elevated temperature (27°C to 33−37°C) and reduced pH, conditions that mimic the intracellular niche in the mammalian host. These signals also induce the stringent response, even when parasites are cultivated in rich medium, indicating that it is a stress response rather than a specific reaction to nutrient limitation [3]. Intriguingly, Mittra and colleagues have shown that amastigote differentiation can also be

Constitutive synthesis and cycling of the carbohydrate reserve, mannogen, is required for amastigote differentiation

The development of quiescent or dormant metabolic states in other prokaryotic and eukaryotic microbes is often associated with the accumulation of carbohydrate reserves [31,32]. While the trypanosomatids have lost the capacity to synthesize canonical carbohydrate reserves such as glycogen, Leishmania and related trypanosomatids (e.g. Crithidia, Phytomonas, Blechomonas) synthesize a unique carbohydrate reserve, termed mannogen [33]. Mannogen comprises a family of linear β1,2-linked mannose

Stringent response encompasses semi-quiescent and dormant metabolic states in vivo

A number of elegant approaches have recently been developed to measure heterogeneity in Leishmania amastigote growth and physiology in infected tissues. These include the development of transgenic Leishmania lines expressing the photoconvertible fluorescent protein, mKikumeGR [30], GFP integrated into the ribosomal DNA (rDNA) locus [9], pre-labeling or in-situ labeling of parasites with fluorescence dyes [9,37], or the use of heavy water labelling combined with high resolution imaging mass

Conclusions

Leishmania amastigotes survive within the phagolysosome compartment of macrophages and other phagocytic cells, by activating a stringent metabolic response. This response is associated with a marked reduction in primary metabolism and energy generation, coupled with a decrease in energy intensive processes such as protein synthesis and RNA turnover, a relative increase in metabolic homeostatic mechanisms (e.g. mannogen turnover), and very slow growth rates. Activation of this response appears

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

This work was supported by an Australian National Health and Medical Research Council research grant (NHMRC APP1183085) to MJM. MJM is a NHMRC Principal Research Fellow (APP1154540). We would like to thank McConville lab members for discussions.

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    Given his role as Guest Editor, Malcolm McConville had no involvement in the peer-review of this article and has no access to information regarding its peer-review. Full responsibility for the editorial process for this article was delegated to Barbara Burleigh.

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