Moreover, its emission area and spectra are unbiased of phospholipid mind groupings and notably, indirectly survey about the comparative packing order simply by sensing the amount of drinking water penetration in its vicinity (Parasassi et al., 1990, 1994, 1997; Bagatolli et al., 1998, 1999); the research demonstrated that Laurdan could feeling the well-known stage transition from liquid to gel at 41C in in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. of the stiff and strengthened level mechanically. To time, the assessment from the development of keratinocyte differentiation is possible through perseverance of particular differentiation markers, e.g., through the use of proteomics-based approaches. However, this involves cell or fixation lysis, and currently there is no strong methodology available to study keratinocyte differentiation in living cells in real-time. Here, we explore new live-cell based approaches for screening differentiation advancement in keratinocytes, in a calcium switch model. We employ a polarity-sensitive dye, Laurdan, and Laurdan general polarization function (GP) as a reporter of the degree of membrane lateral packing order or condensation, as an adequate marker of differentiation. We show that this assay is straightforward and can be 6-O-2-Propyn-1-yl-D-galactose conducted either on a single cell level using confocal spectral imaging or around the ensemble level using a fluorescence plate reader. Such systematic quantification may become useful for understanding mechanisms of keratinocyte differentiation, such as the role of membrane in homogeneities in stiffness, and for future therapeutic development. are reflected by multiple morphological changes (i.e., related to the cell shape and adherence) and distinct organellar transformations). For instance, the appearance and maturation of keratohyalin granules (KHGs) (Gutowska-Owsiak et al., 2018), 6-O-2-Propyn-1-yl-D-galactose nuclear condensation 6-O-2-Propyn-1-yl-D-galactose and extrusion (Gdula et al., 2013; Rogerson et al., 2018), cytoskeleton collapse (Gutowska-Owsiak et al., 2018), and alterations in mitochondria (Ipponjima et al., 2020); in some of those events, autophagy has been shown to be involved (Akinduro et al., 2016). At the molecular level, shifts in cytokeratin expression have been documented (keratin 5, keratin 14 are progressively replaced with keratin 1 and keratin 10) while additional proteins are upregulated, e.g., involucrin and transglutaminase as well as the markers of late differentiation, defined by the expression of late envelope proteins (e.g., involucrin, filaggrin, loricrin) (Marshall et al., 2001; Candi et al., 2005, 2016; Sandilands et al., 2009; Eckhart et al., 2013). These differentiation-dependent proteins contribute to barrier strengthening by incorporation into the insoluble cornified envelope (CE; also known as cornified cell envelope, CCE) or by supporting the hydration of the stratum corneum (Kezic et al., 2009). Open in a separate window Physique 1 Organellar and molecular changes in keratinocytes during the differentiation process. Stratified epidermis consists of multiple layers of epidermal keratinocytes characterized by increasing advancement of cell differentiation. Cells undergo extensive organellar and molecular alterations as they gradually move to the upper layers. Where SC, Stratum Corneum; SL, Stratum Lucidum; SG, Stratum Granulosum; SS, Stratum Spinosum; SB, Stratum Basale; FLG, filaggrin; KRT5, keratin 5; KRT14, keratin 14; KRT1, keratin 1; KRT10, keratin 10; LOR, loricrin; INV, involucrin; TG1, transglutaminase 1. Along with the expression of protein markers and organellar changes, an overall increase in cell stiffness can be observed (Plasencia et al., 2007; Janssens et al., 2012; Wennberg et al., 2018). This stiffness is a consequence of two main processes; the formation of a rigid, protein-based CE and extensive lipid remodeling leading to specialized lipid profile adaptation (Ponec et al., 1987; Schmidt et al., 1991; Lpez et al., 2007). Specifically, the high phospholipid and cholesterol content is gradually replaced by a predominance of ceramides and saturated triglycerides (Ponec et al., 1988), especially in conditions where stratification is usually supported. These lipid composition variations lead to more ordered membrane packing (Janssens et al., 2012), higher stiffness and consequently, changes in the relative lateral heterogeneity properties of the plasma membrane (Bernardino de la Serna et al., 2016). This selection has specific functional properties; the ceramides and free fatty acids provide attachment points for the CE proteins (Behne et al., 2000; Lpez et al., 2007; Akiyama, 2017) and a scaffold to replace the plasma membrane by a corneocyte lipid envelope (CLE), which links CE with the lipids of the intercellular matrix (Elias et al., 2014). CD164 It is believed that ceramide enrichment displaces the balance between ordered and disordered regions to support membrane heterogeneity, where higher order regions work as docking molecular anchors supporting the CLE function and ultimately, stabilizing the CE structure. Regulation of ceramide synthesis is usually mediated via peroxisome proliferator-activated receptor (PPAR) pathways (Chon et al., 2015) associated with keratinocyte differentiation (Mao-Qiang et al., 2004); disturbances in these pathways result in clinical manifestations (Akiyama, 2017). To date, no strong experimental readout for the assessment of differentiation advancement in live primary keratinocytes in real-time is usually available. Protein-targeted 6-O-2-Propyn-1-yl-D-galactose detection methods, such as the expression quantification by proteomics-based approaches, as well as other commonly used, e.g., immunostaining or western blot,.