Metabolic regulation of skin homeostasis – METABOSKIN

To investigate the molecular mechanisms by which E4f1-deficiency impacts on keratinocyte differentiation and ESC maintenance and because PDH-derived AcCoA is also important for de novo fatty acid synthesis, we investigated whether the epidermal barrier defects previously observed in vivo in E4f1cKO keratinocytes result from defective lipid metabolism. Using metabolomic and fluxomic approaches, this part of our project linked pyruvate metabolism to keratinocyte differentiation and functions. The second axis of our project corresponds to the identification of new metabolic regulators of skin homeostasis. We combined whole-genome gene expression and metabolic profiling of purified basal (undifferentiated), suprabasal (differentiated) keratinocytes to generate the first genome-scale metabolic network related to normal skin homeostasis. Using this computational modeling of metabolic networks, we highlighted metabolic pathways that play important roles in epidermal cell communication.

Our ambitious project led to the discovery of important findings:
- E4F1 regulates epidermal differentiation in a cell-autonomous manner. This regulation is due to its implication in the control of the pyruvate dehydrogenase complex (PDC). This process also implies a link between cellular metabolism to the epigenome of epidermal cells during keratinocyte differentiation.
- E4F1 regulates lipid synthesis in keratinocytes through the transcriptional control of SCD1 enzyme. This regulation drive liver function during fasting.
- a new metabolic pathway in keratinocytes controls melanocyte function to produce melanin but also the cell fate of melanocytes.

One major perspective of our project is to highlight new molecular mechanisms linking pyruvate metabolism to the epigenome of keratinocytes. There is growing interest of the scientific community to understand how changes in metabolism influence cell fate and contribute to the establishment of cell type specific transcriptional programs. Optimal metabolic conditions need not only to be permissive for the differentiation of cells, but could also serve as instructive regulatory signals. At mid term, this ambitious project shed light on the metabolic requirements of progenitor cells and their differentiated progeny and could have an important impact on strategies aiming at improving aged-related diseases and in regenerative medicine. We also expect our results to improve our understanding of unexplained skin alterations observed in some metabolic diseases and skin disorders. Finally, because metabolic enzymes have historically proven to be one of the most druggable class of proteins, pharmacological manipulation of the metabolic status of ESC may prove to be a very suitable strategy to improve their in vitro culture or their use in regenerative medicine.

3 main publications are currently in preparation and should be submitted very soon.

Growing evidences indicate that metabolism plays a pivotal role in regulating directly proliferation, differentiation or stemness. Mainly studied in the hematopoietic and central nervous system, these concepts remain poorly investigated in other tissues. My project aims at addressing this question in the context of the epidermis. I recently identified the multifunctional protein E4F1, an important component of the p53 pathway, as a new essential regulator of pyruvate metabolism implicated in skin homeostasis. Using several genetically engineered mouse models (GEMM) that display genetic inactivation of E4f1 in the epidermis, I previously showed that E4f1-associated metabolic functions regulate epidermal stem cell (ESC) maintenance and keratinocyte differentiation (Lacroix et al., PNAS 2010; Lacroix et al., PNAS 2016; Goguet-Rubio et al., PNAS 2016). At the molecular level, we highlighted that E4F1 controls at the transcriptional level the pyruvate dehydrogenase (PDH) complex (PDC), a mitochondrial complex that catalyzes the oxidation of pyruvate into Acetyl-CoenzymeA (AcCoA). We showed that E4F1-mediated control of PDH is important for ESC maintenance. However, our data obtained with other GEMM suggest that E4F1 functions in metabolism extend beyond the regulation of the PDC and also impacted lipid metabolism in a p53-dependent manner (Lacroix et al., in revision at Nat Comm).
Based on this solid set of data, I propose to further investigate the molecular mechanisms by which E4f1-deficiency impact on keratinocyte differentiation and ESC maintenance. I will explore whether perturbation of AcCoA production impinges on histone acetylation and epigenetic regulation of genes implicated in epidermal differentiation and ESC functions. Because PDH-derived AcCoA and citrate production is also important for de novo fatty acid synthesis, I will also investigate whether the epidermal barrier defects previously observed in vivo in E4f1cKO keratinocytes result form defective lipid metabolism. This part of my project should highlight new molecular mechanisms linking pyruvate metabolism to keratinocyte differentiation and functions. The second axis of my project corresponds to a unbiased screen aiming at identifying new metabolic regulators of skin homeostasis. I will combine whole-genome gene expression and metabolic profiling of purified basal (undifferentiated), suprabasal (differentiated) keratinocytes, and follicular stem cells to generate the first genome-scale metabolic network related to normal skin homeostasis. Using computational modeling of these metabolic networks, I wish to highlight metabolic pathways that play important roles in ESC function and keratinocyte differentiation. Their role will then be functionally validated upon shRNA-mediated depletion of key components of these metabolic pathways in primary keratinocytes that I will use to perform clonogenic assays, 3D in vitro skin reconstruction and in vitro keratinocyte differentiation assays. On the long term, this ambitious project should shed light on some of the unexplained skin defects that commonly occur in some metabolic diseases, may lead to the development of new therapeutic strategies for skin diseases, and could have important perspectives in regenerative medecine.