Successful transmission of Leishmania parasites requires rapid adaptation to distinct environments encountered in the sand fly vector and the mammalian host. One conserved environmental cue in the sand fly is acidic pH, which acts as a developmental trigger. Prior to this work, the molecular mechanisms by which Leishmania senses and responds to acid stress to direct stage differentiation were poorly defined. The study summarized here used an unbiased genetic screen to identify protein kinases and other regulators that determine parasite survival and developmental fate at low pH.
The investigators deployed a barcoded protein kinase loss-of-function library in Leishmania mexicana to screen for regulators of acid adaptation. The screen identified multiple kinases affecting survival at low pH. Specific experimental parameters, such as the size of the library, screening thresholds, and readout metrics, are described in the original preprint but are not reproduced in this summary.
From the screen, nine protein kinases were reported to influence parasite survival when exposed to acidic conditions. Among these were kinases involved in differentiation and lipid signalling. The lipid kinases PI4K and PI4P5K were highlighted as required for adaptation to low pH, implicating phosphoinositide balance in the acid-response machinery. Additionally, two STE-family transmembrane kinases were nominated as candidate pH sensors based on the screen hits.
A central hit from the screen was a kinase termed haptomonad differentiation regulator kinase 1 (HDRK1). Genetic deletion of HDRK1 (Δhdrk1) predisposed parasites to adopt haptomonad-like morphologies when subjected to low pH. This indicates HDRK1 functions to suppress or redirect differentiation towards haptomonad development under acid stress in wild-type parasites.
Functional assays in the sand fly vector showed that Δhdrk1 mutants are capable of infecting the midgut but fail to colonise the stomodeal valve. Because stomodeal valve colonisation is critical for effective transmission to mammalian hosts, loss of HDRK1 compromises the parasite's transmission potential. The preprint reports these colonisation phenotypes, but detailed quantitative infection data and experimental replicates should be consulted in the source document.
Integrated transcriptomic and proteomic analyses of Δhdrk1 parasites exposed to low pH revealed a shift toward a low energy state. The molecular profile resembled cells in which the energy sensor AMPK is activated, suggesting that HDRK1 loss leads to metabolic adjustments under acid stress. The preprint provides the omics datasets and analyses; precise gene and protein changes are not restated here.
A second kinase, HDRK2, was identified and characterized as part of a pH-dependent signalling cascade. The authors propose that HDRK1 and HDRK2 form a pathway that governs divergent differentiation: directing parasites either towards mammalian-infective metacyclic stages or towards vector-attached haptomonad stages depending on pH-driven signals. The mechanistic relationships, phosphorylation targets, and biochemical interactions between HDRK1 and HDRK2 are presented in the preprint.
The screen implicated lipid kinases PI4K and PI4P5K in acid adaptation, indicating that maintenance of phosphoinositide balance is important for survival at low pH. Two STE transmembrane kinases were proposed as potential pH sensors based on their screen phenotypes. Details on how phosphoinositide dynamics and transmembrane STE kinases integrate with the HDRK1/HDRK2 cascade are discussed by the authors, and full experimental support is available in the source material.
Together, the findings provide an organizing framework for how Leishmania detects acidic conditions in the sand fly and channels that information through a kinase cascade and lipid signalling nodes to make developmental decisions. By biasing differentiation toward either metacyclic or haptomonad fates, this pH-dependent signalling network directly links environmental sensing to stages that determine transmission success.
This report is a preprint and has not undergone peer review; readers should interpret the findings accordingly. The summary above is restricted to the results and claims presented in the preprint. Some experimental specifics, quantitative outcomes, and mechanistic details are available only in the full manuscript and supplementary materials. Further work to validate kinase substrates, define the biochemical sensing mechanism of the candidate transmembrane kinases, and test the pathway in additional Leishmania species and natural vector contexts would strengthen the model.
Using a barcoded protein kinase screen, the authors identify a pH-dependent kinase cascade involving HDRK1 and HDRK2, lipid kinases PI4K and PI4P5K, and candidate STE transmembrane sensors that together regulate how Leishmania adapts to acidic conditions in the sand fly. Loss of HDRK1 biases parasites toward haptomonad differentiation, induces a low-energy, AMPK-like state at low pH, and prevents stomodeal valve colonisation, thereby impairing transmission. These results offer a testable model for how environmental pH is transduced into developmental outcomes in Leishmania.