An Optogenetic Strategy to Determine if Merkel Cells Are Excitatory in the Skin

Columbia University Health Sciences, New York NY

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Abstract

DESCRIPTION (provided by applicant): The sense of touch is an integral component of countless essential behaviors such as feeding, social exchange and avoiding bodily harm. In mammals, different tactile qualities are encoded by touch receptors with distinct physiological properties and morphological end organs; however, the cellular mechanisms underlying this diversity is unknown. The long-term goal of this research is to determine how mammalian Merkel cell-neurite complexes transduce touch stimuli into neural signals that inform the brain about objects in our dynamic environment. The objective of this application is to determine whether epidermal Merkel cells play an excitatory role in cutaneous touch reception. In complex with myelinated cutaneous afferents, they form gentle-touch receptors that mediate slowly adapting type I (SAI) responses. This project is highly relevant to human health because 1) SAI responses in the skin are thought to underlie high tactile acuity in humans and other mammals that rely on discriminative touch for recognizing and manipulating objects; and 2) Merkel cells are one of only four fundamental cell types in the mammalian epidermis and yet their biological function in the skin remains unknown 135 years after their discovery. Based on morphology, Merkel cells have long been proposed to be mechanosensory cells, analogous to inner-ear hair cells. Alternatively, Merkel cells might be accessory cells that shape sensory output of mechanosensitive SAI afferents. This exploratory project will use innovative technologies to directly distinguish between these two models. This application's central hypothesis is that Merkel cells release excitatory neurotransmitters to evoke action potentials in SAI afferents. To test this hypothesis, Merkel cells must be selectively excited without activating intrinsic mechanosensory mechanisms that might be present in SAI afferents. Since Merkel cells and SAI afferents are tightly juxtaposed in the skin, touch stimuli cannot be used to independently activate each cell type in situ. To break this barrier, the approach will employ a combination of optogenetics, mouse models and ex vivo skin-nerve recordings. Because mouse Merkel cells reside in the transparent epidermis, the light-gated ion channel channelrhodopsin-2 (ChR2) provides an ideal tool to excite Merkel cells in the intact skin. Aims are to 1) achieve selective and robust expression of ChR2 in Merkel cells in vivo, and 2) determine whether exciting Merkel cells in the absence of touch stimuli drives action potential firing in SAI afferents. The project is innovative because it represents first use of a light-activated protein to dissect the function of any skin cell type or to address mechanisms of touch reception. Furthermore, a novel Rosa26ChR2 knock-in mouse model has been generated that, when validated, can be broadly used by the scientific community.