Cholinergic system conducts the signal transmission in cholinergic synapse, playing an important role in the brain and the neuromuscular junction (NMJ). Besides nervous system, the non-neuronal functions of cholinergic system have been proposed in various tissues, such as inflammation, stress and bone development. The expression of acetylcholinesterase (AChE) and the release of acetylcholine (ACh) have been reported in human skin. However, the physiological function of cholinergic system in skin is not well studied yet. In this thesis, the role of cholinergic system in skin melanogenesis is elucidated.

Melanogenesis in skin is a process of melanin production in melanocyte, which is a physiological response to protect cells from DNA damage and apoptosis in exposing to sunlight. Melanin,...[ Read more ]

Cholinergic system conducts the signal transmission in cholinergic synapse, playing an important role in the brain and the neuromuscular junction (NMJ). Besides nervous system, the non-neuronal functions of cholinergic system have been proposed in various tissues, such as inflammation, stress and bone development. The expression of acetylcholinesterase (AChE) and the release of acetylcholine (ACh) have been reported in human skin. However, the physiological function of cholinergic system in skin is not well studied yet. In this thesis, the role of cholinergic system in skin melanogenesis is elucidated.

Melanogenesis in skin is a process of melanin production in melanocyte, which is a physiological response to protect cells from DNA damage and apoptosis in exposing to sunlight. Melanin, induced by α-melanocyte stimulating hormone (α-MSH) under stimulation of sunlight, is synthesized in melanocyte as a melanosome. Here, we reported that AChE was expressed in melanocytes and melanoma cells, the tetrameric (G4) form was the major form of AChE. During melanogenesis of murine melanoma cells, increase in the levels of intracellular cyclic adenosine monophosphate (cAMP) and tyrosinase (TYR) were revealed. In contrast, the expression of AChE showed a markedly decrease, and the expression of microphthalmia-associated transcription factor (MITF), a key transcription factor in regulating melanogenesis, was down-regulated. In viewing of transcriptional regulation of AChE in melanocyte, two transcription factors cAMP response element-binding protein (CREB) and MITF were shown to regulate AChE during melanogenesis.

To reveal the mechanism of ACh release in skin, the expression profile of cholinergic markers was investigated in various skin cells. In differentiating keratinocytes, the expression levels of choline acetyltransferase (ChAT), AChE, vesicular acetylcholine transporter (VAChT) and synaptophysin (SYP), were upregulated during differentiation of keratinocyte: these increases of cholinergic molecules were in parallel to involucrin (IVL), a marker of differentiated keratinocyte. In cultured keratinocyte, a transient exposure of simulated sunlight induced the release of ACh, which was mediated by intracellular Ca²⁺ mobilization. The Ca²⁺ chelator BAPTA-AM blocked this release of ACh. The sunlight-induced ACh release was mediated by the presence of opsin; because the knock-down of opsin in keratinocyte reduced this ACh release.

AChE and muscarinic acetylcholine receptor (mAChR) were highly expressed in melanocytes, while ChAT was highly expressed in keratinocytes. The interplay of keratinocyte and melanocyte in regulating melanin production could be similar to a typical cholinergic synapse in the brain, i.e. forming a “skin synapse” by using ACh as a transmitter. ACh is released, triggered by sunlight, from keratinocyte (pre-synaptic) binds to AChRs on melanocyte (post-synaptic), and ACh hydrolysis is triggered by AChE. Our result indicated that cholinergic system played roles in melanin production both in melanocytes in vitro and ex vivo mouse skin. An AChE inhibitor BW284c51 greatly enhanced ACh-mediated inhibition on the transcription of melanogenic genes and the production of melanin. Moreover, cholinergic system regulated a transient melanin transportation from melanocytes to keratinocytes. ACh accelerated the uptake of melanosomes by keratinocytes through α7 nAChR mediated calcium signaling. Thus, the cholinergic signaling in skin plays roles in melanogenesis, particularly in regulating the effect of sunlight on skin color.