A non-canonical lysosome biogenesis pathway generates Golgi-associated lysosomes during epidermal differentiation

Abstract

Keratinocytes maintain epidermis integrity and function including physical and antimicrobial barrier through cellular differentiation. This process is predicted to be controlled by calcium ion gradient and nutritional stress. Keratinocytes undergo proteome changes during differentiation, which enhances the intracellular organelle digestion to sustain the stress conditions. However, the molecular mechanism between epidermal differentiation and organelle homeostasis is poorly understood. Here, we used primary neonatal human epidermal keratinocytes to study the link between cellular differentiation, signaling pathways and organelle turnover. Upon addition of calcium chloride (2 mM) to the culture medium, keratinocytes increased their cell size and the expression of differentiation markers. Moreover, differentiated keratinocytes showed enhanced lysosome biogenesis that was dependent on ATF6-arm of UPR signaling but independent of mTOR-MiT/TFE transcription factors. Furthermore, chemical inhibition of mTOR has increased keratinocyte differentiation and relocalized the MiT/TFE TFs to the lysosome membranes, indicating that autophagy activation promotes the epidermal differentiation. Interestingly, differentiation of keratinocytes resulted in dispersal of fragmented Golgi and lysosomes, and the later organelles showed colocalization with Golgi-tethering proteins, suggesting that these lysosomes possibly originated from Golgi, hence named as Golgi-associated lysosomes (GALs). Consistent to this prediction, inhibition of Golgi function using brefeldin A completely abolished the formation of GALs and the keratinocyte differentiation. Thus, ER stress regulates the biogenesis of GALs, which maintains keratinocyte differentiation and epidermal homeostasis.

Lysosome Alterations in the Human Epithelial Cell Line HaCaT and Skin Specimens: Relevance to Psoriasis

“…There may be a mechanism that moderates or even blocks efficient fusion of numerous lysosomes with autophagosomes in proinflammatory cytokine-treated HaCaT cells, resulting in a reduction in the autolysosome population in these cells. On the other hand, it is known from the literature that lysosomes and lysosome-like compartments are involved in epidermal differentiation, the stage of cell maturation that is impaired in PsO [33,34,35,36]. The role of lysosomes in cell differentiation is supported by several studies performed in various cell types including keratinocytes [36].…”

“…Interestingly, there was both an up-regulation of LAMP1 gene activity and an elevated level of signaling protein in psoriatic skin tissue, which may indicate that the overall number of lysosomes is increased in PP skin, but their biogenesis process dependent on the MiT family transcription factors is inhibited. Therefore, endoplasmic reticulum (ER) stress may be a potential cause of the formation of lysosomes [34].…”

“…Despite this, there is an increased expression of the MCOLN and LAMP1 genes, which may indicate: (1) An increased number of lysosomes; and (2) inhibition of the lysosomal biogenesis process dependent on the MiT family transcription factors. Lysosomes can arise from endoplasmic reticulum stress [34]. A similar expression profile for PP and PN vs. NN group may indicate that, the above changes occur before the appearance of the characteristic phenotype of the disease.…”