Amastigote population heterogeneity
Transcriptomic approaches have been used to assess T. cruzi stage-specific gene expression profiles3,8-10. Here, we focused on amastigote gene expression profile during infection of human fibroblasts, from 4 to 72h post-infection (Figure 1), using a well validated data set based on RNA-sequencing8. We used CDSeq for the deconvolution of gene expression data from bulk RNA-sequencing of these cells to simultaneously estimate amastigote cell-type proportions and their specific gene expression profiles. Deconvolution revealed that amastigotes were not a homogenous cell type, but rather a collection of cells presenting heterogenous transcription profiles. The best posterior distribution indicated six subpopulations of amastigotes (Figure 2A), which was thus selected for further analysis. Five of six of these amastigote subpopulations were rather abundant and expanded over time, accounting for 10-20% of the amastigotes each, while one subpopulation (labeled as B) accounted for only 2-5% of all amastigotes and was constantly detected (Figure 2B). This small population of amastigote was also non-replicative, while all other populations replicated at different rates, the most replicative being populations A and F (Figure 2C).
Functional properties of amastigote populations
We then assessed functional properties of amastigote populations by analyzing their respective gene expression profiles. Expression data from 6,834 genes comprised the expression profile of amastigotes. As shown in Figure 3, there was a good clustering of samples from each population at each time point, based on principal component analysis (PCA) of gene expression data, although the best clustering was observed at 12 and 24h post-infection. The non-replicative population B clustered particularly well apart from the others at all time points. These transcriptomic differences suggested potential functional differences.
To further characterize these amastigote subpopulations, we focused on the expression levels of PUF-9 (TcCLB.506563.10), Histone hairpin RBP (TcCLB.511867.150), and cyclin 6 (TcCLB.507089.260), which are up-regulated in replicating T. cruzi parasites18. As shown in Figure 4A, B, C, most amastigote subpopulations presented elevated expression levels of these replication-associated genes, except subpopulation B, which presented low levels of expression of all three replication markers. These data are consistent with this specific amastigote subpopulation being non-replicative. We further analyzed the expression profile of TcRAD51 in these cells, as its expression is increased in dormant amastigotes 15, and found that it was over-expressed in amastigote subpopulation B compared to the other subpopulations (Figure 4D). Together, these data demonstrate that we were able to detect transcriptomic heterogeneity in T. cruzi amastigote cells, and that a specific subpopulation with dormant characteristics could be identified by deconvolution of bulk RNA-sequencing data.
We then focused on the identification of genes that were differentially expressed among amastigote populations. DESeq2 analysis indicated that there were 91 genes with at least a 1.5-fold difference in expression level among populations (false discovery rate (FDR)-adjusted P<0.05) (Supplementary Table 1). Pathway analysis further indicated that these genes were involved in multiple functional aspects of amastigote biology, highlighting the heterogeneity of these populations. As expected from the differences in replication rates shown above (Figure 2), underlying pathways associated with protein-DNA interactions, chromatin organization and remodeling, chromosome and nucleosome organization, and detection of cell density were identified (Table 1). In addition, multiple catabolic and biosynthesis pathways suggested broad metabolic differences among amastigote populations. Pathways associated with cell differentiation were also identified, including multivesicular body sorting pathway, cellular component assembly, biogenesis, and organization, organelle organization, ribosomal assembly, and macromolecule biosynthesis (Table 1). Finally, differences in signaling pathways including calcium and other secondary messenger signaling suggest different interactions with host cells (Table 1). Together, these data indicate broad structural and metabolic heterogeneity among amastigote populations, in addition to their differences in replication.