A secretome CRISPR screen identifies SFRP1 as a regulator of epidermal self-renewal [ATAC-seq]

Secreted epidermal proteins play pivotal roles in cell-cell communication, extracellular matrix remodeling, and antimicrobial defense. However, the complete spectrum of secreted proteins in the epidermis has not been experimentally defined. In this study, we performed mass spectrometry on conditioned media from primary human keratinocytes, identifying 406 proteins that constitute the keratinocyte secretome. To assess functional impacts of uncharacterized secreted proteins on epidermal stem cell behavior, we devised a colony formation assay-based CRISPR screen. The screen identified six candidate proteins that promoted proliferation of epidermal progenitors and two proteins that inhibited it. Secreted frizzled related protein-1 (SFRP1), an inhibitor of Wnt signaling, was the most potent inhibitor of progenitor proliferation in the screen. We discovered that in addition to regulating Wnt activity, SFRP1 restrained epidermal stem cell proliferation by inhibiting ectopic expression of leukemia inhibitory factor (LIF). Collectively, our study defines the keratinocyte secretome, demonstrates the application of a CRISPR screen to assess the function of non-cell autonomous factors, and highlights SFRP1’s role in regulating epidermal balance.