18/ During ageing, following YAP activation, BMAL1-YAP complex is directed to enhancers of inflammation genes, many of which are regulated by p65. All this leads to an increase in expression of these genes, contributing to the transcriptional rewiring of epid cells during ageing.

16/ Now, a more detailed p65 genomic occupancy analysis revealed the presence of p65 in promoters and TSSs of up-regulated inflammation-related genes, some of which present BMAL1-YAP bound enhancers.

14/ What’s more, some of the up-regulated inflammation-related genes that were BMAL1 or YAP targets in adult epidermis, gain BMAL1-YAP binding in their enhancers with age.

13/ What about BMAL1-YAP complex? Indeed, when we compare both TFs ChIP-seq data, we observe a stat significant genomic overlap between the two TFs.
+ Notably, during ageing, this happens in active enhancers of inflammation genes, some of which expression is up-regulated.

11/ Yes! We show that a non-canonical YAP activation through phosphorylation of Tyr357 is increased in the aged epidermis, and that this activation is reduced upon anti-inflammatory treatment with anti-IL17 blocking antibodies.

10/ Using a Col14KO mouse to model skin ageing mechanics, we unveiled that YAP activation through mechanotransduction signalling alone is not enough to recapitulate YAP aged binding to inflammation genes.

So, are there other triggers that can boost YAP activation during ageing?

8/ However, during ageing, YAP gains binding to inflammation-related genes, similar to what we see for BMAL1. In fact, BMAL1 binding sites are enriched exclusively in aged YAP peaks located in active enhancers.😮

7/ In vivo ChIP-seq analysis of YAP in the epidermis confirmed its well-known role as a mechanotransduction TF, as its genomic occupancy is enriched in mechanics- and Hippo-related genes.🦛

6/ Other groups have underscored that changes in tissue mechanics occur during ageing. In the IFE, we observed an increased stiffness of the BM during ageing, and this correlates with an increased proportion of epidermal cells positive for the Hippo-dependent active YAP.❗️

5/ TF motif enrichment analysis of BMAL1 peaks in active enhancers revealed TEAD binding sites.

PLA assay on skin sections shows BMAL1-YAP interactions in the nucleus of epidermal cells, suggesting a possible BMAL1 and YAP functional interaction in the epidermis chromatin.🧐

4/ In addition, we also see that BMAL1 binding is not restricted to rhythmically expressed genes, supporting a non-canonical BMAL1 function as a transcription factor beyond its role in the circadian clock.😈

3/ In vivo ChIP-seq revealed BMAL1 binding to be enriched in inflammation genes in the aged epidermis, while during adulthood, this binding is more restricted to homeostatic genes.

Moreover, this binding occurs in enhancers, emphasising BMAL1 role in enhancer regulation.

2/ By performing a comprehensive analysis of DEGs throughout 24 hours in adult and aged murine epidermis, we reveal a list of more than 100 up-regulated genes during ageing associated with inflammation functions.

But who promotes such an expression program?