Diverse viscerotropic isolates of Leishmania all express a highly conserved secretory nuclease during human infections

Stage-Specific Class I Nucleases of Leishmania Play Important Roles in Parasite Infection and Survival

Protozoans of the genus Leishmania are the causative agents of an important neglected tropical disease referred to as leishmaniasis. During their lifecycle, the parasites can colonize the alimentary tract of the sand fly vector and the parasitophorous vacuole of the mammalian host, differentiating into distinct stages. Motile promastigotes are found in the sand fly vector and are transmitted to the mammalian host during the insect blood meal. Once in the vertebrate host, the parasites differentiate into amastigotes and multiply inside macrophages. To successfully establish infection in mammalian hosts, Leishmania parasites exhibit various strategies to impair the microbicidal power of the host immune system. In this context, stage-specific class I nucleases play different and important roles related to parasite growth, survival and development. Promastigotes express 3’-nucleotidase/nuclease (3’-NT/NU), an ectoenzyme that can promote parasite escape from neutrophil extracellular traps (NET)-mediated death through extracellular DNA hydrolysis and increase Leishmania-macrophage interactions due to extracellular adenosine generation. Amastigotes express secreted nuclease activity during the course of human infection that may be involved in the purine salvage pathway and can mobilize extracellular nucleic acids available far from the parasite. Another nuclease expressed in amastigotes (P4/LmC1N) is located in the endoplasmic reticulum of the parasite and may be involved in mRNA stability and DNA repair. Homologs of this class I nuclease can induce protection against infection by eliciting a T helper 1-like immune response. These immunogenic properties render these nucleases good targets for the development of vaccines against leishmaniasis, mainly because amastigotes are the form responsible for the development and progression of the disease. The present review aims to present and discuss the roles played by different class I nucleases during the Leishmania lifecycle, especially regarding the establishment of mammalian host infection.

A unique, highly conserved secretory invertase is differentially expressed by promastigote developmental forms of all species of the human pathogen, Leishmania

Leishmania are protozoan pathogens of humans that exist as extracellular promastigotes in the gut of their sand fly vectors and as obligate intracellular amastigotes within phagolysosomes of infected macrophages. Between infectious blood meal feeds, sand flies take plant juice meals that contain sucrose and store these sugars in their crop. Such sugars are regurgitated into the sand fly anterior midgut where they impact the developing promastigote parasite population. In this report we showed that promastigotes of all Leishmania species secreted an invertase/sucrase enzyme during their growth in vitro. In contrast, neither L. donovani nor L. mexicana amastigotes possessed any detectable invertase activity. Importantly, no released/secreted invertase activity was detected in culture supernatants from either Trypanosoma brucei or Trypanosoma cruzi. Using HPLC, the L. donovani secretory invertase was isolated and subjected to amino acid sequencing. Subsequently, we used a molecular approach to identify the LdINV and LmexINV genes encoding the ~72 kDa invertases produced by these organisms. Interestingly, we identified high fidelity LdINV-like homologs in the genomes of all Leishmania sp. but none were present in either T. brucei or T. cruzi. Northern blot and RT-PCR analyses showed that these genes were developmentally/differentially expressed in promastigotes but not amastigotes of these parasites. Homologous transfection studies demonstrated that these genes in fact encoded the functional secretory invertases produced by these parasites. Cumulatively, our results suggest that these secretory enzymes play critical roles in the survival/growth/development and transmission of all Leishmania parasites within their sand fly vector hosts.

Helicobacter pylori salvages purines from extracellular host cell DNA utilizing the outer membrane-associated nuclease NucT

Helicobacter pylori is a bacterial pathogen that establishes life-long infections in humans, and its presence in the gastric epithelium is strongly associated with gastritis, peptic ulcer disease, and gastric cancer. Having evolved in this specific gastric niche for hundreds of thousands of years, this microbe has become dependent on its human host. Bioinformatic analysis reveals that H. pylori has lost several genes involved in the de novo synthesis of purine nucleotides, and without this pathway present, H. pylori must salvage purines from its environment in order to grow. While the presence and abundance of free purines in various mammalian tissues has been loosely quantified, the concentration of purines present within the gastric mucosa remains unknown. There is evidence, however, that a significant amount of extracellular DNA is present in the human gastric mucosal layer as a result of epithelial cell turnover, and this DNA has the potential to serve as an adequate purine source for gastric purine auxotrophs. In this study, we characterize the ability of H. pylori to grow utilizing only DNA as a purine source. We show that this ability is independent of the ComB DNA uptake system, and that H. pylori utilization of DNA as a purine source is largely influenced by the presence of an outer membrane-associated nuclease (NucT). A ΔnucT mutant exhibits significantly reduced extracellular nuclease activity and is deficient in growth when DNA is provided as the sole purine source in laboratory growth media. These growth defects are also evident when this nuclease mutant is grown in the presence of AGS cells or in purine-free tissue culture medium that has been conditioned by AGS cells in the absence of fetal bovine serum. Taken together, these results indicate that the salvage of purines from exogenous host cell DNA plays an important role in allowing H. pylori to meet its purine requirements for growth.