Cellular remodeling during differentiation is essential for life-cycle progression of many unicellular eukaryotic pathogens such as Leishmania, but the mechanisms involved are largely uncharacterized. The role of endosomal sorting in differentiation was analyzed in Leishmania major by overexpression of a dominant-negative ATPase, VPS4. VPS4(E235Q) accumulated in vesicles from the endocytic pathway, and the mutant L. major was deficient in endosome sorting. Mutant parasites failed to differentiate to the obligate infective metacyclic promastigote form. Furthermore, the autophagy pathway, monitored via the expression of autophagosome marker GFP-ATG8, and shown to normally peak during initiation of metacyclogenesis, was disrupted in the mutants. The defect in late endosome-autophagosome function in the VPS4(E235Q) parasites made them less able to withstand starvation than wild-type L. major. In addition, a L. major ATG4-deficient mutant was found also to be defective in the ability to differentiate. This finding, that transformation to the infective metacyclic form is dependent on late endosome function and, more directly, autophagy, makes L. major a good model for studying the roles of these processes in differentiation.

Leishmania occurs in several developmental forms and thus undergoes complex cell differentiation events during its life-cycle. Those are required to allow the parasite to adapt to the different environmental conditions. The sequencing of the genome of L. major has facilitated the identification of the parasite's vast arsenal of proteolytic enzymes, a few of which have already been carefully studied and found to be important for the development and virulence of the parasite. This review focuses on these peptidases and their role in the cellular differentiation of Leishmania through their key involvement in a variety of degradative pathways in the lysosomal and autophagy networks.

Summary: In the past, ultrastructural investigations of Leishmania mexicana amastigotes revealed structures that were tentatively identified as autophagosomes. This study has now provided definitive data that autophagy occurs in the parasite during differentiation both to metacyclic promastigotes and to amastigotes, autophagosomes being particularly numerous during metacyclic to amastigote form transformation. Moreover, the results demonstrate that inhibiting two major lysosomal cysteine peptidases (CPA and CPB) or removing their genes not only interferes with the autophagy pathway but also prevents metacyclogenesis and transformation to amastigotes, thus adding support to the hypothesis that autophagy is required for cell differentiation. The study suggests that L. mexicana CPA and CPB perform similar roles to the aspartic peptidase PEP4 and the serine peptidase PRB1 in Saccharomyces cerevisiae. The results also provide an explanation for why L. mexicana CPA/CPB‐deficient mutants transform to amastigotes very poorly and lack virulence in macrophages and mice.

The protozoan Leishmania mexicana parasite causes chronic non-healing cutaneous lesions in humans and mice with poor parasite control. The mechanisms preventing the development of a protective immune response against this parasite are unclear. Here we provide data demonstrating that parasite sequestration by neutrophils is responsible for disease progression in mice. Within hours of infection L. mexicana induced the local recruitment of neutrophils, which ingested parasites and formed extracellular traps without markedly impairing parasite survival. We further showed that the L. mexicana-induced recruitment of neutrophils impaired the early recruitment of dendritic cells at the site of infection as observed by intravital 2-photon microscopy and flow cytometry analysis. Indeed, infection of neutropenic Genista mice and of mice depleted of neutrophils at the onset of infection demonstrated a prominent role for neutrophils in this process. Furthermore, an increase in monocyte-derived dendritic cells was also observed in draining lymph nodes of neutropenic mice, correlating with subsequent increased frequency of IFNγ-secreting T helper cells, and better parasite control leading ultimately to complete healing of the lesion. Altogether, these findings show that L. mexicana exploits neutrophils to block the induction of a protective immune response and impairs the control of lesion development. Our data thus demonstrate an unanticipated negative role for these innate immune cells in host defense, suggesting that in certain forms of cutaneous leishmaniasis, regulating neutrophil recruitment could be a strategy to promote lesion healing. Infection with the protozoan Leishmania parasites causes a spectrum of diseases ranging from cutaneous to visceral forms that are fatal if left untreated. Among the different Leishmania species, Leishmania mexicana causes chronic cutaneous lesions in humans. To study this disease, we used a murine model. Following infection with Leishmania mexicana, most mouse species including C57BL/6 develop chronic non-healing lesion. Within hours of infection, neutrophils are recruited locally and they ingest the parasites. Although neutrophils are leukocytes that are able to rapidly kill pathogens using an arsenal of molecules, several microorganisms including some, but not all, Leishmania species are able to survive within these cells. Here, we show that L. mexicana elicits the rapid recruitment of neutrophils at the site of infection, survives within these cells and uses them to its advantage. Furthermore, transient parasite sequestration by neutrophils delays recruitment of other leukocytes such as monocytes, contributing to the impaired development of a protective immune response against the parasite and chronic lesion development. Thus, we describe a previously unanticipated pathogenic role for neutrophils in chronic lesion development. More importantly, our data suggest that in certain forms of cutaneous leishmaniasis, regulating neutrophil recruitment could be a strategy to promote lesion healing.

Leishmania major possesses, apparently uniquely, four families of ATG8-like genes, designated ATG8, ATG8A, ATG8B and ATG8C, and 25 genes in total. L. major ATG8 and examples from the ATG8A, ATG8B ...

Macroautophagy has been shown to be important for the cellular remodelling required for Leishmania differentiation. We now demonstrate that L. major contains a functional ATG12-ATG5 conjugation system, which is required for ATG8-dependent autophagosome formation. Nascent autophagosomes were found commonly associated with the mitochondrion. L. major mutants lacking ATG5 (Δatg5) were viable as promastigotes but were unable to form autophagosomes, had morphological abnormalities including a much reduced flagellum, were less able to differentiate and had greatly reduced virulence to macrophages and mice. Analyses of the lipid metabolome of Δatg5 revealed marked elevation of phosphatidylethanolamines (PE) in comparison to wild type parasites. The Δatg5 mutants also had increased mitochondrial mass but reduced mitochondrial membrane potential and higher levels of reactive oxygen species. These findings indicate that the lack of ATG5 and autophagy leads to perturbation of the phospholipid balance in the mitochondrion, possibly through ablation of membrane use and conjugation of mitochondrial PE to ATG8 for autophagosome biogenesis, resulting in a dysfunctional mitochondrion with impaired oxidative ability and energy generation. The overall result of this is reduced virulence. Leishmaniasis is a disease of humans that is of major significance throughout many parts of the world. It is caused by the protozoan parasite Leishmania and mammals are infected through the bite of a sand fly in which the parasite develops. Parasite remodelling crucial for generation of the human-infective forms is aided by the catabolic process known as autophagy in which cell material is packaged within organelles called autophagosomes and subsequently broken down in the digestive lysosomal compartment. Here we show that autophagy in Leishmania requires the coordinated actions of two pathways, one of which involves a protein called ATG5. We have generated parasite mutants lacking this protein and shown that ATG5 is required for both autophagosome formation and also maintenance of a fully functional mitochondrion. The mutants lacking ATG5 have increased mitochondrial mass and phospholipid content, high levels of oxidants and reduced membrane potential, all being hallmarks of a dysfunctional mitochondrion with impaired ability for energy generation. Our results have thus revealed that a functional autophagic pathway is crucial for phospholipid homeostasis and mitochondrial function in the parasite and important for the parasite's differentiation, infectivity and virulence to its mammalian host.