Abstract. Skin fulfils a plethora of eminent physiological functions ranging from physical barrier over immunity shield to the interface mediating social interaction. Prone to several acquired and inherited diseases, skin is therefore a major target of pharmaceutical and cosmetic research. The lack of similarity between human and animal skin and rising ethical concerns in the use of animal models have driven the search for novel realistic three-dimensional skin models. This review provides a survey of contemporary skin models and compares them in terms of applicability, reliability, cost and complexity.Keywords: Skin, phenotypic screening, 3D models, pharmaceutical research Skin -composition and functionHuman skin covers an area of almost 2 m 2 in the adult and consists of the three major layers, subcutis, dermis, and epidermis. The subcutis is composed of adipose and epithelial cells. It harbours blood vessels, neurites of peripheral neurons, Vater-Pacini mechanosensors, and, partially, also sweat glands and hair follicles. It connects the skin to periosteum and fascia, absorbs forces, and mediates thermal insulation. The dermis supplies the epidermis with mechanical support and nutrients. It is stratified into an inner, reticular, and an outer, papillary, zone. The dermis houses most sebaceous glands, sweat glands, hair follicles, smooth muscle cells, and capillary beds and, thus, regulates skin moisture, body temperature, and performs the secretory function of skin. The papillary layer is characterized by relatively loose connective tissue, where Meissner corpuscles sense touch. Immune cells, particularly mast cells and dendritic cells, are patrolling in the papillary layer and mediate local inflammatory reactions and immune surveillance. Finally, dermal fibroblasts secrete extracellular matrix (ECM) and basement membrane components. These are primarily collagens I and III, and a proteoglycan-rich ground substance [1]. The resulting ECM mediates tensile strength of the dermis. The dermo-epidermal junction is centered around a special basement membrane. This is composed of a laminin/collagen IV scaffold and further typical basement membrane components such as perlecans and nidogens [1].The epidermis is a squamous epithelium of 50-100 m thickness. It is devoid of blood vessels but contains keratinocytes, Merkel cell mechanosensors, Langerhans immune cells, and melanocytes. The 1 These authors contributed equally.

Three-dimensional cell cultures, such as spheroids and organoids, serve as increasingly important models in fundamental and applied research and start to be used for drug screening purposes. Optical tissue clearing procedures are employed to enhance visualization of fluorescence-stained organs, tissues, and three-dimensional cell cultures. To get a more systematic overview about the effects and applicability of optical tissue clearing on three-dimensional cell cultures, we compared six different clearing/embedding protocols on seven types of spheroid-and chip-based threedimensional cell cultures of approximately 300 µm in size that were stained with nuclear dyes, immunofluorescence, cell trackers, and cyan fluorescent protein. Subsequent whole mount confocal microscopy and semi-automated image analysis were performed to quantify the effects. Quantitative analysis included fluorescence signal intensity and signal-to-noise ratio as a function of z-depth as well as segmentation and counting of nuclei and immunopositive cells. In general, these analyses revealed five key points, which largely confirmed current knowledge and were quantified in this study. First, there was a massive variability of effects of different clearing protocols on sample transparency and shrinkage as well as on dye quenching. Second, all tested clearing protocols worked more efficiently on samples prepared with one cell type than on co-cultures. Third, z-compensation was imperative to minimize variations in signal-tonoise ratio. Fourth, a combination of sample-inherent cell density, sample shrinkage, uniformity of signal-to-noise ratio, and image resolution had a strong impact on data segmentation, cell counts, and relative numbers of immunofluorescence-positive cells. Finally, considering all mentioned aspects and including a wish for simplicity and speed of protocols-in particular, for screening purposes-clearing with 88% Glycerol appeared to be the most promising option amongst the ones tested.

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca2+ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G proteincoupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K ATP channels. Their activation may induce depolarization-dependent Ca 2+ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion. Keywords Taste-bud cells . Sweet-taste receptor . TAS1R2 . TAS1R3 . Artificial sweeteners . Cephalic-phase insulin release . Glucagon-like peptide-1 . Glucose transporters Abbreviations CPIR Cephalic-phase insulin release DMNX Dorsal motor nucleus of vagus nerve GLP-1 Glucagon-like peptide-1 GLUT Glucose -transporter IP3 Inositol triphosphate K ATP ATP-sensitive K + channels NTS Nucleus of the solitary tract PLCβ2 Phospholipase-Cβ2 SGLT Sodium/glucose cotransporter T1R1 Taste receptor type 1 member 1 T1R2 Taste receptor type 1 member 2 T1R3 Taste receptor type 1 member 3 TRPM5 Membrane-associated transient receptor potential channel subfamily M member 5 VDCCs Voltage-dependent calcium channels VRAC Volume-regulated anion channel

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The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca2+ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca2+, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

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