In preprints: progress in sebaceous gland homeostasis, regeneration and immunomodulatory functions

Sebaceous glands (SGs) supply the skin and hairs with sebum, a mixture of protective lipids and other rather uncharacterized compounds with antimicrobial and antioxidative functions (Dahlhoff et al., 2016; Zouboulis et al., 2022). Although its involvement in acne, the most common skin disease of adolescence, shaped SG research for decades, recent studies have addressed numerous topics beyond skin diseases, including stem cell adhesion and metabolism, lipid droplet dynamics, innate immunity and the significance of sebum for whole body lipid and energy metabolism. In particular, it has been shown that overabundance of the cytokine thymic stromal lymphopoietin (TSLP) increased sebum secretion in mice to the point of depleting fat depots, with the consequence of avoiding or reverting obesity and improving metabolic parameters (Choa et al., 2021). Here, we highlight two recent preprints that provide important novel insights into SG biology. The first preprint comes from the labs of Sunny Y. Wong and Scott X. Atwood (Veniaminova et al., 2023 preprint) and tackles the long-standing challenging issue of how the SGs self-renew and regenerate. The study pioneers the systematic characterization of the SG lineage transcriptional program and addresses cellular mechanisms governing SG development, homeostasis and regeneration. The second preprint, from Taku Kambayashi's lab (Harris et al., 2023 preprint), follows up on the identified immune-modulated sebum circuit, whereby TSLP-stimulated T cells can regulate sebum secretion, and reports a robust sebum secretion defect in germ-free mice, which persisted across multiple generations despite microbial colonization and breeding with conventional mice.Differentiating sebocytes progressively accumulate lipids, degrade their organelles, rupture and are released en masse into the sebaceous duct towards the skin surface, a process known as holocrine secretion (Schneider and Paus, 2010). Understanding the SG transcriptional program and the role of stem cells in SG homeostasis and regeneration has been challenging because of the inability to genetically target the full sebocyte lineage and the extremely low ratio of sebocytes in skin samples. Having established that the key lipid metabolism regulator PPARγ marks multiple sebocyte differentiation stages but no other skin epithelial cell types, Veniaminova and colleagues next studied SG dynamics by employing a mouse line (PPARγ;YFP) permitting the inducible labeling of PPARγ+ cells and their descendants via the presence/absence of doxycycline in the chow. When labeled SGs are ‘turned off’ for 30 weeks, almost 90% of the glands remained stained, indicating that SGs are largely maintained by their own stem cells at homeostasis. Following skin injury, SG-derived stem cells moved out of the SG and contributed to the epidermal regeneration. Conversely, PPARγ-ablated SGs were shown to regenerate after injury exactly at the site they were lost, fueled by bulge-derived cells that began expressing PPARγ. These findings support the notion that skin compartments operate autonomically at homeostasis, but become highly promiscuous upon injury.Due to their specific labeling of SGs, PPARγ;YFP mice turned out to be a suitable source for sebocytes at different stages. Veniaminova et al. (2023 preprint) therefore submitted sorted SG cells to single cell (sc) RNA-seq and identified seven cell clusters, including three sebocyte clusters expressing established SG markers (SEB1-3), one SG basal cluster with high Krt5/Krt14/Lrig1 expression, one Krt5/Krt14/Pparg-positive transitional basal cell cluster, and a ductal and an epidermal cell cluster. Further biostatistical analysis predicted a trajectory whereby basal cells turn into SEB1 cells either directly or via the transitional state, uncovering both direct and indirect paths by which SG progenitors differentiate into mature sebocytes. Finally, when confirming the spatial localization of selected cluster-specific transcripts by RNAscope in situ staining, the authors identified an SG population close to the sebaceous duct, which, being RNA-low, was likely not represented in the scRNA-seq and appears to correspond to terminally differentiated sebocytes downstream of SEB3. These findings sharpen our picture of the SG lineage transcriptional program, and will be a useful blueprint for interpreting upcoming SG spatial transcriptome analyses.Tissue adaptation to environmental challenges is fundamental for organismal fitness. Just as human evolution has been shaped by environmental pressures, the skin microbiome has also evolved along with our species, adapting to changes in our environment, diet and lifestyle (Callewaert et al., 2020). Among other effects, the skin microbiota stimulates keratinocytes to produce TSLP (Kabashima et al., 2019), now recognized as a key mediator of sebum production, which itself is important for skin barrier function (Choa et al., 2021). Studying germ-free mice, Harris et al. (2023 preprint) observed a defect in sebum secretion as well as an SG-specific transcriptional downregulation of lipid metabolism and cell death pathways important for sebum secretion. This defect cannot be overcome by TSLP overexpression. After breeding with conventionally raised mice, the sebum secretion defect was inherited, even after in vitro fertilization-based breeding, indicating a gamete-driven transmission of the phenotype. Notably, the SG transcriptional fingerprint of the offspring had considerable overlap with that of the parental germ-free mice. How is this phenotype transferred across the generations? Noncoding RNA profiling of F1 germ cells displayed differences that suggest alterations in embryonic gene regulation, including tRNA fragments and miRNAs, in germ-free mice. Embryonic gene expression changes were confirmed by performing RNA-seq at the four-cell embryo stage, and similar experiments performed in T cell-defective mice revealed that parental T cell presence is important for transgenerational phenotype transfer. Gamete noncoding RNA profiling and four-cell embryo RNA-seq showed important alterations, some of them in a similar direction as the germ-free mice.Harris et al. (2023 preprint) shows how both microbiome and immune control contribute to the modulation of epigenetic information in the germline. Interestingly, germ-free mice and T cell-deficient mice displayed similar deviations in non-coding RNAs related to embryo development. At least to some extent, T cells may serve as detectors of the microbial environment, subsequently relaying this information to gametes, resulting in changes in the epigenetic information via non-coding RNAs. This process potentially imparts phenotypic diversity to subsequent generations. Harris et al. suggest that this epigenetic inheritance may be advantageous to adapt faster to environmental conditions than genetic changes. The highlighted preprint substantiates that the commensal microbiota plays a significant role, not just in immediate alterations in organ function, but also in exerting a lasting impact that can influence subsequent generations.These preprints make important steps towards understanding cellular and molecular mechanisms of SG homeostasis and regeneration, as well as its involvement in host-microbe interactions. Also, there are key questions to be addressed in further studies. As SG basal cells are PPARγ-negative, how does the whole gland become labeled in PPARγ;YFP mice? Which factors (or their gradients) specify the precise site of SG regeneration via bulge-derived cells? What mechanisms trigger the multigenerational inheritance of sebum secretion which reaches the gametes from skin? What are the main microbiome components that contribute to these effects? A common limitation of the two preprints is that they address SG processes exclusively in mice. As SG morphology, sebum composition and function are quite species-specific, confirmatory studies will be needed before extrapolating these findings to humans.