Sepsis hits 49 million people per year, killing 11 million of them (1200 people per hour). Our lab has found that lethality in sepsis is strongly linked to loss of function, in the liver, of the transcription factor PPARa, which is the major coordinator of the transformation of free fatty acids (FFAs) into ketone bodies (KBs). These FFAs are massively released from fat and hence in sepsis they accumulate in tissues and fail to transform to KBs. These latter are essential to prevent coma and heart failure. RNA sequencing has shown that the key PPARa-driving factor HNF4a is also losing activity in sepsis liver and is potentially the culprit of the PPARa problem. Using HNF4a knockout mice, we will investigate the role that HNF4a plays in sepsis, and we will focus our attention on the mechanism of the loss of function of HNF4a. We will study basic aspects of the transcription factor biology of HNF4a, but we will also focus on the role of FFAs in the inhibition of HNF4a function by spatial RNASEQ transcriptomics, and we will apply a pig sepsis model, but above all a humanized mouse model (mice containing 80% human hepatocytes in the liver) to investigate changes in HNF4a biology in the human cells in sepsis. This latter part is both necessary (since human HNF4a biology is far better understood than rodent HNF4a biology) and essential to translate our sepsis findings to human sepsis. With our project, we hope to elucidate how the HNF4a-PPARa axis fails and how to rescue it.