Identification of a functional Tie-Wnt signalling circuit controlling liver homeostasis and regeneration

Single cell RNA sequencing (scRNAseq) analyses of hepatocytes and liver endothelial cells (L-EC) have revolutionized the understanding of the spatial architecture of liver structure and function (Halpern et al., 2018; Halpern et al., 2017). The spatial alignment of L-EC and hepatocytes is pivotal for liver function in health and disease given that L-EC act as instructive gatekeeper of nearby hepatocytes (Ding et al., 2010; Hu et al., 2014; Koch et al., 2017; Lorenz et al., 2018) including the maintenance of liver metabolic zonation in a Wnt-dependent manner(Rocha et al., 2015; Wang et al., 2015). Advancing liver biology beyond the ‘transcript-centric’ view of scRNAseq analyses is presently restricted by the limited resolution of proteomics and genome-wide techniques to analyse post-translational modifications (Marx, 2019; Needham et al., 2019). By combining spatial cell sorting methodology with transcriptomic and quantitative proteomic/phosphoproteomic analyses, the first functionally and spatially-resolved proteome landscape of the liver endothelium was presented in this study, yielding deep mechanistic insight into zonated vascular signalling mechanisms. Phosphorylation of receptor tyrosine kinases (RTK) was detected preferentially in the central vein area resulting in an atypical enrichment of tyrosine phosphorylation. Prototypic biological validation of the identified strong phosphorylation gradient of the vascular RTK Tie1 by antibody blockade resulted in the rapid peri-central dysregulation of the L-EC transcriptome. Notably, the expression of Wnt9b in L-EC was discovered as Tie receptor controlled with reciprocal regulation by FoxO1 and STAT3 transcription factors. Genetic inactivation of Tie1 in L-EC or antibody blockade resulted in reduced liver regeneration following partial hepatectomy with reduced Wnt ligand and Wnt target gene expression, including Axin2, Sox9, Tbx3 and Lgr5. Taken together, the study has yielded unparalleled insight into the spatial organization of L-EC signalling and discovered a vascular Tie-Wnt signalling axis as regulator of liver function. The employed spatial sorting technique followed by phosphoproteomic analysis may be employed as a universally adaptable strategy for the spatial phosphoproteomic analysis of scRNAseq-defined relevant cellular (sub)-populations