Cellular and Molecular Mechanisms Underlying Merkel Cell Loss and Regeneration

Summary The mechanosensory Merkel cells (MCs) are crucial for encoding the sense of light touch. Recent studies have shown that MCs also significantly regulate alloknesis, the itch sensation caused by light mechanical stimulation. Notably, alloknesis is commonly observed in the elderly, with research indicating a decrease in MC numbers in

aged human and mouse skin. Among many skin abnormalities occurring with aging, dry skin condition appears to be involved in alloknesis. In experiments using a mouse model of experimental dry skin induced by acetone- ether-water (AEW) treatment, a decline in MC numbers and an increase in alloknesis were observed.

Interestingly, upon discontinuing the AEW treatment, MC numbers returned to normal, and the mechanical itch was gradually alleviated. This suggests that uncovering the cellular and molecular mechanisms controlling MC decrease or MC regeneration could pave the way to designing therapies to prevent alloknesis in aging and dry

skin. In Aim 1, we will focus on understanding the mechanisms behind the regeneration of MCs. Pilot studies showed that MC recovery is not due to MC proliferation; however, the exact source of MC regeneration remains unknown. We demonstrated that the Tenascin C (TNC)-expressing epidermal keratinocytes are MC progenitors.

Here, we will use lineage tracing and single-cell RNA sequencing to investigate if TNC-positive cells are responsible for MC regeneration upon AEW treatment and to identify cellular and molecular processes controlling MC regeneration. We will then perform genetic studies to examine the functional significance of identified

processes on MC regeneration. Finally, we will analyze molecular changes in MC progenitors and the MC differentiation program in young vs old mice and uncover the functional significance of the identified changes on the age-related decline of MCs. In Aim 2, we will investigate the interaction between MRGPRA3/TRPV1+ itch-

initiating C fibers and MCs, as our studies showed that the ablation of MRGPRA3/TRPV1+ neurons prevents MC reduction upon AEW treatment. Here, we will perform chemogenetic manipulations of the activity of MRGPRA3+/TRPV1+ sensory neurons to reveal the causal relationship between sensory neuron activities and

MC numbers. We will further identify molecules secreted by MRGPRA3/TRPV1+ neurons post-AEW treatment and receptors of these molecules in MCs or MC progenitors, shedding light on the cellular and molecular dynamics controlling MC numbers. We will test the functional significance of the identified ligand-receptor pairs

for controlling MC numbers and inducing alloknesis by performing functional genetic studies in the settings of aging and dry skin. Overall, these studies aim to offer new insights into preventing age-related MC decline and associated alloknesis, potentially leading to novel therapeutic interventions.