Date Published: February 24, 2016
Publisher: Public Library of Science
Author(s): Mario Dejung, Ines Subota, Ferdinand Bucerius, Gülcin Dindar, Anja Freiwald, Markus Engstler, Michael Boshart, Falk Butter, Christian J. Janzen, Najib M. El-Sayed.
Developmental differentiation is a universal biological process that allows cells to adapt to different environments to perform specific functions. African trypanosomes progress through a tightly regulated life cycle in order to survive in different host environments when they shuttle between an insect vector and a vertebrate host. Transcriptomics has been useful to gain insight into RNA changes during stage transitions; however, RNA levels are only a moderate proxy for protein abundance in trypanosomes. We quantified 4270 protein groups during stage differentiation from the mammalian-infective to the insect form and provide classification for their expression profiles during development. Our label-free quantitative proteomics study revealed previously unknown components of the differentiation machinery that are involved in essential biological processes such as signaling, posttranslational protein modifications, trafficking and nuclear transport. Furthermore, guided by our proteomic survey, we identified the cause of the previously observed differentiation impairment in the histone methyltransferase DOT1B knock-out strain as it is required for accurate karyokinesis in the first cell division during differentiation. This epigenetic regulator is likely involved in essential chromatin restructuring during developmental differentiation, which might also be important for differentiation in higher eukaryotic cells. Our proteome dataset will serve as a resource for detailed investigations of cell differentiation to shed more light on the molecular mechanisms of this process in trypanosomes and other eukaryotes.
Trypanosoma brucei is a protozoan pathogen, which threatens thousands of people and kills millions of farm animals in sub-Saharan Africa . In order to shuttle between different hosts, this parasite had to develop a complex life cycle, which includes two very different host environments: the vascular system and tissue fluids in the mammalian host and the intestinal tract and salivary glands of the vector, the tsetse fly. Many basic biological processes like motility, energy metabolism and morphology have to be adapted during several developmental differentiation events in order to survive and proliferate in these different environments (reviewed in [2,3]). Trypanosomes differentiate from the bloodstream form (BSF) in the mammalian host to the procyclic form (PF), which is adapted to live in the insect vector. Developmental differentiation in the mammalian host can be divided into two steps. First, proliferating bloodstream forms (called long slender, LS) differentiate into cell cycle-arrested bloodstream forms (called short stumpy, SS). Only the SS form is capable of differentiating efficiently to the PF that can resume proliferation in the fly. Interestingly, a form of quorum sensing pathway controls differentiation to the SS form. As parasitemia increases during the proliferation of slender LS, a parasite derived factor, so-called stumpy-induction factor (SIF), accumulates and promotes formation of SS forms, which arrest in G1/G0 phase of the cell cycle . Although in general cell cycle arrest appears to be a prerequisite for differentiation to procyclic form, the link between cell cycle control and the differentiation process remains elusive. For example overexpression of the variant surface glycoprotein (VSG) in the long slender form causes only a G1 dormancy but initiates the slender-to-stumpy pathway in a reversible way . Several components that might be associated with differentiation have been identified recently including kinases, phosphatases and components of a cAMP-signaling pathway ([6–8] reviewed in ). However, the consequences of these signaling events are largely unknown. Every step of the differentiation process involves coordinated changes of the parasite’s gene expression profiles to provide host specific surface proteins or to change metabolism, morphology and organelle activity. There has been substantial progress in understanding the differentiation process of trypanosomes after several groups analyzed changes in transcription profiles during this process ([9–10] reviewed in ). However, it has to be taken into account that trypanosomes regulate steady-state protein levels mainly by posttranscriptional mechanisms . Recently, genome-wide comparative ribosome profiling confirmed the importance of translation efficiency to regulate protein abundance in two different life cycle stages of trypanosomes [13–14]. Because translation efficiency can vary up to 100-fold between individual genes, substantial differences in the level of ribosome-bound mRNAs for the same transcripts were detected in different life cycle stages. These experiments demonstrate that translational control regulates protein abundance to a similar extent as RNA stability. Hence, to understand the developmental differentiation of trypanosomes, it is necessary to analyze the proteome during differentiation. Comparison of steady-state proteomes of SS, LS and PF have already shed more light on the differences between these life cycle stages [15–18]. To fully understand the required dynamics of proteome remodeling during the differentiation process, we here used quantitative label-free proteomics to monitor changes in protein expression during transition from LS to SS form and subsequent synchronous differentiation to the PF form of the parasite. While our analysis suggests previously unknown components of the differentiation machinery, we were also able to clarify the involvement of the histone methyltransferase DOT1B (disrupter of telomeric silencing) in this process. DOT1B was already reported to be essential for the differentiation process . However, the exact step of the differentiation process that was impaired remained enigmatic. Our proteomic survey detected upregulation of the DOT1B protein in the first S-phase during differentiation of short stumpy forms to PF. Careful dissection of the phenotype of DOT1B-depleted parasites suggests a function in chromatin remodeling during developmental differentiation.