In mammals, a small population of intrinsically photosensitive retinal ganglion cells (ipRGCs) plays a key role in the regulation of nonvisual photic responses, such as behavioral responses to light, pineal melatonin synthesis, pupillary light reflex, and sleep latency. These ipRGCs also express melanopsin (Opn4), a putative opsin-family photopigment that has been shown to play a role in mediating these nonvisual photic responses. Melanopsin is required for the function of this inner retinal pathway, but its precise role in generating photic responses has not yet been determined. We found that expression of melanopsin in Xenopus oocytes results in light-dependent activation of membrane currents through the Galpha(q)/Galpha(11) G protein pathway, with an action spectrum closely matching that of melanopsin-expressing ipRGCs and of behavioral responses to light in mice lacking rods and cones. When coexpressed with arrestins, melanopsin could use all-trans-retinaldehyde as a chromophore, which suggests that it may function as a bireactive opsin. We also found that melanopsin could activate the cation channel TRPC3, a mammalian homolog of the Drosophila phototransduction channels TRP and TRPL. Melanopsin therefore signals more like an invertebrate opsin than like a classical vertebrate rod-and-cone opsin.

Chronic sleep deprivation perturbs the circadian clock and increases susceptibility to diseases such as diabetes, obesity, and cancer. Increased inflammation is one of the common underlying mechanisms of these diseases, thus raising a hypothesis that circadianoscillator components may regulate immune response. Here we show that absence of the core clock component protein cryptochrome (CRY) leads to constitutive elevation of proinflammatory cytokines in a cell-autonomous manner. We observed a constitutive NF-κB and protein kinase A (PKA) signaling activation in Cry1 −/− ; Cry2 −/− cells. We further demonstrate that increased phosphorylation of p65 at S276 residue in Cry1 −/− ;Cry2 −/− cells is due to increased PKA signaling activity, likely induced by a significantly high basal level of cAMP, which we detected in these cells. In addition, we report that CRY1 binds to adenylyl cyclase and limits cAMP production. Based on these data, we propose that absence of CRY protein(s) might release its (their) inhibition on cAMP production, resulting in elevated cAMP and increased PKA activation, subsequently leading to NF-κB activation through phosphorylation of p65 at S276. These results offer a mechanistic framework for understanding the link between circadian rhythm disruption and increased susceptibility to chronic inflammatory diseases.biological clock | immune system I n response to the rotation of the earth, many of the mammalian physiological processes such as sleep-wake cycle and feeding pattern undergo an ∼24-h oscillation referred as the circadian clock. Circadian oscillation helps organisms anticipate and adapt to predictable daily changes in the environment. The hypothalamic suprachiasmatic nucleus (SCN) functions as a master circadian oscillator, which controls behaviors. At molecular level, the circadian oscillator is based on a cell-autonomous transcription-translation feedback loop, in which transcriptional activators circadian locomotor output cycles kaput (CLOCK)/brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like (BMAL)1 activate the expression of Cryptochrome (Cry) and Period (Per), in turn their protein products repress BMAL1 and CLOCK activity, thus producing Cry and Per expression rhythms (1, 2). Reverse orientation c-erb (REV-ERB)s and RAR-related orphan receptor (ROR)s also participate in the rhythmic transcriptional activity of the molecular oscillator (3). Molecular components of the circadian clock network are conserved in the SCN and also in peripheral cells. The oscillator uses direct and indirect mechanisms to impose rhythms and regulate several physiological processes, such as rest-activity cycle, endocrine system, and metabolism (4).Sleep loss or chronic sleep deprivation disrupts the circadian rhythm. It has been reported that people exposed to constant circadian disruption, such as shift-workers, show increased incidence of chronic diseases such as diabetes, obesity, and also cancer (5-7). Chronic inflammation is one of the important pathogenic features of these di...

Homers are scaffolding proteins that bind G protein–coupled receptors (GPCRs), inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs), ryanodine receptors, and TRP channels. However, their role in Ca2+ signaling in vivo is not known. Characterization of Ca2+ signaling in pancreatic acinar cells from Homer2−/− and Homer3−/− mice showed that Homer 3 has no discernible role in Ca2+ signaling in these cells. In contrast, we found that Homer 2 tunes intensity of Ca2+ signaling by GPCRs to regulate the frequency of [Ca2+]i oscillations. Thus, deletion of Homer 2 increased stimulus intensity by increasing the potency for agonists acting on various GPCRs to activate PLCβ and evoke Ca2+ release and oscillations. This was not due to aberrant localization of IP3Rs in cellular microdomains or IP3R channel activity. Rather, deletion of Homer 2 reduced the effectiveness of exogenous regulators of G proteins signaling proteins (RGS) to inhibit Ca2+ signaling in vivo. Moreover, Homer 2 preferentially bound to PLCβ in pancreatic acini and brain extracts and stimulated GAP activity of RGS4 and of PLCβ in an in vitro reconstitution system, with minimal effect on PLCβ-mediated PIP2 hydrolysis. These findings describe a novel, unexpected function of Homer proteins, demonstrate that RGS proteins and PLCβ GAP activities are regulated functions, and provide a molecular mechanism for tuning signal intensity generated by GPCRs and, thus, the characteristics of [Ca2+]i oscillations.

SUMMARY Melanopsin photopigment expressed in intrinsically photosensitive retinal ganglion cells (ipRGCs) plays a crucial role in the adaptation of mammals to their ambient light environment through both image-forming and non-image-forming (NIF) visual responses. ipRGCs are structurally and functionally distinct from classical rod/cone photoreceptors and have unique properties including single-photon response, long response latency, photon integration over time, and slow deactivation. Here we discovered that amino-acid sequence features of melanopsin protein contribute to the functional properties of the ipRGCs. Phosphorylation of a cluster of Ser/Thr residues in the C-terminal cytoplasmic region of melanopsin contributes to deactivation, which in turn determines response latency and threshold sensitivity of the ipRGCs. The poorly conserved region distal to phosphorylation cluster inhibits functional role of phosphorylation, thereby constituting a unique delayed-deactivation mechanism. Concerted action of both regions sustains responses to dim light, allows for the integration of light over time, and results in precise signal duration.

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP), acting through the osteoblast PTH1 receptor (PTH1R), play important roles in bone remodeling. Intermittent administration of PTH(1-34) (teriparatide) leads to bone formation, whereas continuous administration paradoxically leads to bone resorption. Activation of PTH1R promotes regulation of multiple signaling pathways, including G s /cAMP/protein kinase A, G q /calcium/protein kinase C, b-arrestin recruitment, and extracellular signal-related kinase (ERK)1/2 phosphorylation, as well as receptor internalization, but their role in promoting anabolic and catabolic actions of PTH(1-34) are unclear. In the present investigation, a collection of PTH(1-34) and PTHrP(1-34) peptide analogs were evaluated in orthogonal human PTH1R (hPTH1R) functional assays capturing G s -and G q -signaling, b-arrestin recruitment, ERK1/2 phosphorylation, and receptor internalization to further define the patterns of PTH1R signaling that they stimulate and further establish peptide domains contributing to agonist activity. Results indicate that both Nand C-terminal domains of PTH and PTHrP are critical for activation of signaling pathways. However, modifications of both regions lead to more substantial decreases in agonist potency and efficacy to stimulate G q -signaling, b-arrestin recruitment, ERK1/2 phosphorylation, and receptor internalization than to stimulate G s -signaling. The substantial contribution of the peptide C-terminal domain in activation of hPTH1R signaling suggests a role in positioning of the peptide N-terminal region into the receptor J-domain. Several PTH and PTHrP peptides evaluated in this study promote different patterns of biased agonist signaling and may serve as useful tools to further elucidate therapeutically relevant PTH1R signaling in osteoblasts. With a better understanding of therapeutically relevant signaling, novel biased peptides with desired signaling could be designed for safer and more effective treatment of osteoporosis.