De novo phytosterol synthesis in animals

Abstract

Sterols are lipids that regulate multiple processes in eukaryotic cells, and are essential components of cellular membranes. Sterols are currently assumed to be kingdom specific, with phytosterol synthesis restricted to plants while animals are only able to synthesize cholesterol. Here, we challenge this assumption by demonstrating that the marine annelids Olavius and Inanidrilus synthesize the phytosterol sitosterol de novo. Using multi-omics, high-resolution metabolite imaging, heterologous gene expression and enzyme assays, we characterized the biosynthetic pathway of sitosterol and showed that it is the most abundant sterol in these worms (60%). We show that phytosterol synthesis partially overlaps with cholesterol synthesis and involves a non-canonical sterol methyltransferase (SMT), C24-SMT, an essential enzyme for sitosterol synthesis in plants, but not known from bilaterians. Our comparative phylogenetic analyses of C24-SMT homologs revealed that these are widely distributed across annelids and other animal phyla, including sponges and rotifers. Our findings show that phytosterol synthesis and use is not restricted to the plant kingdom, and indicate that the evolution of sterols in animals is more complex than previously assumed.

Bacterial sterol methylation confounds eukaryotic biomarker interpretations

“…The results presented here experimentally demonstrate that sponge genomes and transcriptomes do indeed contain functional SMTs capable of sequentially methylating desmosterol to 24-methylenecholesterol to 24-ethyl sterols, similarly to recently reported SMTs found in annelid worms (49). However, as with annelid SMTs (50), we did not observe this activity across all sponge SMTs analyzed in this study.…”

“…The results presented here experimentally verify that sponges do indeed encode functional SMTs capable of sequentially methylating desmosterol to 24-methylenecholesterol to 24-ethyl sterols, similarly to SMTs recently identified in annelid worms 37 . However, as in Annelida 38 , this activity was inconsistent among Porifera as the SMTs from Oscarella carmela (Homoscleromorpha) and Sycon ciliatum (Calcarea) did not sequentially methylate sterols, contrary to previous hypotheses 28 .…”

Sterol Methyltransferases in Annelid Worms Rewrite the Molecular Fossil Record

“…The C. teleta SMT and one of the two P. dumerilii proteins are more structurally similar to the bifunctional SMT2 than ERG6, as demonstrated by their lower RMSD scores, and is particularly noticeable in the alpha helices in the C-terminal domain. This supports recent results by Michellod et al demonstrating that SMTs of the annelids Olavius and Inanidrilus can bifunctionally generate C 28 and C 29 sterols 17 , suggesting this ability is commonplace across annelids.…”

“…A second candidate SMT from P. australiensis did not map to the genome, and shows high sequence similarity to the coral symbiont Symbiodinium, demonstrating that we can distinguish between host and microbial sequences (see Methods). Similar to Michellod et al 17 our putative animal sequences are scattered across the SMT gene tree (Figure S1), which, if taken literally, would support multiple horizontal gene transfer events. But after removing probable contaminants and performing species tree reconciliation (see Methods), the most parsimonious scenario is that all animal SMTs are monophyletic.…”

“…We found that ERG6 -S. cerevisiae are capable of producing ergosterol with the addition of the C. teleta gene (Figure 4). This work complements Michellod et al, demonstrating that multiple annelid species can synthesize phytosterols de novo 17 .…”