Human nonvisual opsin 3 regulates pigmentation of epidermal melanocytes through functional interaction with melanocortin 1 receptor.

Opsins form a family of light-activated, retinal-dependent, G protein-coupled receptors (GPCRs) that serve a multitude of visual and nonvisual functions. Opsin 3 (OPN3 or encephalopsin), initially identified in the brain, remains one of the few members of the mammalian opsin family with unknown function and ambiguous light absorption properties. We recently discovered that OPN3 is highly expressed in human epidermal melanocytes (HEMs)-the skin cells that produce melanin. The melanin pigment is a critical defense against ultraviolet radiation (UVR), and its production is mediated by the Gαs-coupled melanocortin 1 receptor (MC1R). The physiological function and light sensitivity of OPN3 in melanocytes are yet to be determined. Here, we show that in HEMs, OPN3 acts as a negative regulator of melanin production by modulating the signaling of MC1R. OPN3 negatively regulates the cyclic adenosine monophosphate (cAMP) response evoked by MC1R via activation of the Gαi subunit of G proteins, thus decreasing cellular melanin levels. In addition to their functional relationship, OPN3 and MC1R colocalize at both the plasma membrane and in intracellular structures, and can form a physical complex. Remarkably, OPN3 can bind retinal, but does not mediate light-induced signaling in melanocytes. Our results identify a function for OPN3 in the regulation of the melanogenic pathway in epidermal melanocytes; we have revealed a light-independent function for the poorly characterized OPN3 and a pathway that greatly expands our understanding of melanocyte and skin physiology.

MuSK is a BMP co-receptor that shapes BMP responses and calcium signaling in muscle cells.

Bone morphogenetic proteins (BMPs) function in most tissues but have cell type-specific effects. Given the relatively small number of BMP receptors, this exquisite signaling specificity requires additional molecules to regulate this pathway's output. The receptor tyrosine kinase MuSK (muscle-specific kinase) is critical for neuromuscular junction formation and maintenance. Here, we show that MuSK also promotes BMP signaling in muscle cells. MuSK bound to BMP4 and related BMPs with low nanomolar affinity in vitro and to the type I BMP receptors ALK3 and ALK6 in a ligand-independent manner both in vitro and in cultured myotubes. High-affinity binding to BMPs required the third, alternatively spliced MuSK immunoglobulin-like domain. In myoblasts, endogenous MuSK promoted BMP4-dependent phosphorylation of SMADs and transcription of Id1, which encodes a transcription factor involved in muscle differentiation. Gene expression profiling showed that MuSK was required for the BMP4-induced expression of a subset of genes in myoblasts, including regulator of G protein signaling 4 (Rgs4). In myotubes, MuSK enhanced the BMP4-induced expression of a distinct set of genes, including transcripts characteristic of slow muscle. MuSK-mediated stimulation of BMP signaling required type I BMP receptor activity but was independent of MuSK tyrosine kinase activity. MuSK-dependent expression of Rgs4 resulted in the inhibition of Ca(2+) signaling induced by the muscarinic acetylcholine receptor in myoblasts. These findings establish that MuSK has dual roles in muscle cells, acting both as a tyrosine kinase-dependent synaptic organizing molecule and as a BMP co-receptor that shapes BMP transcriptional output and cholinergic signaling.

Opsin expression in human epidermal skin.

Human skin is constantly exposed to solar light containing visible and ultraviolet radiation (UVR), a powerful skin carcinogen. UVR elicits cellular responses in epidermal cells via several mechanisms: direct absorption of short-wavelength UVR photons by DNA, oxidative damage caused by long-wavelength UVR, and, as we recently demonstrated, via a retinal-dependent G protein-coupled signaling pathway. Because the human epidermis is exposed to a wide range of light wavelengths, we investigated whether opsins, light-activated receptors that mediate photoreception in the eye, are expressed in epidermal skin to potentially serve as photosensors. Here we show that four opsins­OPN1-SW, OPN2, OPN3 and OPN5­are expressed in the two major human epidermal cell types, melanocytes and keratinocytes, and the mRNA expression profile of these opsins does not change in response to physiological UVR doses. We detected two OPN3 splice variants present in similar amounts in both cell types and three OPN5 splice isoforms, two of which encode truncated proteins. Notably, OPN2 and OPN3 mRNA were significantly more abundant than other opsins and encoded full-length proteins. Our results demonstrate that opsins are expressed in epidermal skin cells and suggest that they might initiate light-induced signaling pathways, possibly contributing to UVR phototransduction.

Muscle lengthening surgery causes differential acute mechanical effects in both targeted and non-targeted synergistic muscles.

Epimuscular myofascial force transmission (EMFT) is a major determinant of muscle force exerted, as well as length range of force exertion. Therefore, EMFT is of importance in remedial surgery performed, e.g., in spastic paresis. We aimed to test the following hypotheses: (1) muscle lengthening surgery (involving preparatory dissection (PD) and subsequent proximal aponeurotomy (AT)) affects the target muscle force exerted at its distal and proximal tendons differentially, (2) forces of non-operated synergistic muscles are affected as well, (3) PD causes some of these effects. In three conditions (control, post-PD, and post-AT exclusively on m. extensor digitorum longus (EDL)), forces exerted by rat anterior crural muscles were measured simultaneously. Our results confirm hypotheses (1-2), and hypothesis (3) in part: Reduction of EDL maximal force differed by location (i.e. 26.3% when tested distally and 44.5% when tested proximally). EDL length range of active force exertion increased only distally. Force reductions were shown also for non-operated tibialis anterior (by 11.9%), as well as for extensor hallucis longus (by 8.4%) muscles. In tibialis anterior only, part of the force reduction (4.9%) is attributable to PD. Due to EMFT, remedial surgery should be considered to have differential effects for targeted and non-targeted synergistic muscles.