RdgB2 is required for dim-light input into intrinsically photosensitive retinal ganglion cells

Mammalian START-like phosphatidylinositol transfer proteins - Physiological perspectives and roles in cancer biology

PtdIns and its phosphorylated derivatives, the phosphoinositides, are the biochemical components of a major pathway of intracellular signaling in all eukaryotic cells. These lipids are few in terms of cohort of unique positional isomers, and are quantitatively minor species of the bulk cellular lipidome. Nevertheless, phosphoinositides regulate an impressively diverse set of biological processes. It is from that perspective that perturbations in phosphoinositide-dependent signaling pathways are increasingly being recognized as causal foundations of many human diseases - including cancer. Although phosphatidylinositol transfer proteins (PITPs) are not enzymes, these proteins are physiologically significant regulators of phosphoinositide signaling. As such, PITPs are conserved throughout the eukaryotic kingdom. Their biological importance notwithstanding, PITPs remain understudied. Herein, we review current information regarding PITP biology primarily focusing on how derangements in PITP function disrupt key signaling/developmental pathways and are associated with a growing list of pathologies in mammals.

Trpm1: Novel function at the intersection of light and pain response in the iris

In mammals, the retina is the photosensitive tissue that is responsible for the capture of light and the transduction of the light-initiated signals to the brain. These visual signals help to drive image and non-image forming behaviors. The pupillary light reflex (PLR) is an involuntary non-image forming behavior which involves the constriction of the iris muscle tissue in response to ambient light intensity. A subset of photosensitive retinal ganglion cells provides the principal pathway for all light input to the olivary pretectal nucleus which directs the neuronal input to drive iris constriction. Transient receptor potential melastatin 1 (Trpm1) knockout mice have a severe defect in PLR, but it remains unclear how the Trpm1 channel contributes to this behavior. We have demonstrated that the reduced PLR in Trpm1^-/- mice at scotopic and photopic intensities is due to a functional loss of Trpm1 in the retina as well as the iris sphincter muscle. We have also tested constriction in isolated eyes and have shown that light-driven constriction independent of signaling from the brain also requires Trpm1 expression. In both the in vivo PLR and the iris photomechanical response, melanopsin is required for the light-dependent activation. Finally, pharmacological experiments using capsaicin to activate pain afferents in the eye demonstrate that Trpm1 expression is required for all sensory driven iris constriction. Our results demonstrate for the first time that Trpm1 has a novel and necessary role in iridial cells and is required for all sensory-driven constriction in the iris.

Emerging perspectives on multidomain phosphatidylinositol transfer proteins

The phosphatidylinositol transfer protein domain (PITPd) is an evolutionarily conserved protein that is able to transfer phosphatidylinositol between membranes in vitro and in vivo. However some animal genomes also include genes that encode proteins where the PITPd is found in cis with a number of additional domains and recent large scale genome sequencing efforts indicate that this type of multidomain architecture is widespread in the animal kingdom. In Drosophila photoreceptors, the multidomain phosphatidylinositol transfer protein RDGB is required to regulate phosphoinositide turnover during G-protein activated phospholipase C signalling. Recent studies in flies and mammalian cell culture models have begun to elucidate functions for the non-PITPd of RDGB and its vertebrate orthologs. We review emerging evidence on the genomics, functional and cell biological perspectives of these multi-domain PITPd containing proteins.

Contribution of M-opsin-based color vision to refractive development in mice

M-opsin, encoded by opn1mw gene, is involved in green-light perception of mice. The role of M-opsin in emmetropization of mice remains uncertain. To answer the above question, 4-week-old wild-type (WT) mice were exposed to white light or green light (460-600 nm, a peak at 510 nm) for 12 weeks. Refractive development was estimated biweekly. After treatment, retinal function was assessed using electroretinogram (ERG). Dopamine (DA) in the retina was evaluated by high-performance liquid chromatography, M-opsin and S-opsin protein levels by Western blot and ELISA, and mRNA expressions of opn1mw and opn1sw by RT-PCR. Effects of M-opsin were further verified in Opn1mw^-/- and WT mice raised in white light for 4 weeks. Refractive development was examined at 4, 6, and 8 weeks after birth. The retinal structure was estimated through hematoxylin and eosin staining (H&E) and transmission electron microscopy (TEM). Retinal wholemounts from WT and Opn1mw^-/- mice were co-immunolabeled with M-opsin and S-opsin, their distribution and quantity were then assayed by immunofluorescence staining (IF). Expression of S-opsin protein and opn1sw mRNA were determined by Western blot, ELISA, or RT-PCR. Retinal function and DA content were analyzed by ERG and liquid chromatography tandem-mass spectrometry (LC-MS/MS), respectively. Lastly, visual cliff test was used to evaluate the depth perception of the Opn1mw^-/- mice. We found that green light-treated WT mice were more myopic with increased M-opsin expression and decreased DA content than white light-treated WT mice after 12-week illumination. No electrophysiologic abnormalities were recorded in mice exposed to green light compared to those exposed to white light. A more hyperopic shift was further observed in 8-week-old Opn1mw^-/- mice in white light with lower DA level and weakened cone function than the WT mice under white light. Neither obvious structural disruption of the retina nor abnormal depth perception was found in Opn1mw^-/- mice. Together, these results suggested that the M-opsin-based color vision participated in the refractive development of mice. Overexposure to green light caused myopia, but less perception of the middle-wavelength components in white light promoted hyperopia in mice. Furthermore, possible dopaminergic signaling pathway was suggested in myopia induced by green light.

How lateral inhibition and fast retinogeniculo-cortical oscillations create vision: A new hypothesis

The role of the physiological processes involved in human vision escapes clarification in current literature. Many unanswered questions about vision include: 1) whether there is more to lateral inhibition than previously proposed, 2) the role of the discs in rods and cones, 3) how inverted images on the retina are converted to erect images for visual perception, 4) what portion of the image formed on the retina is actually processed in the brain, 5) the reason we have an after-image with antagonistic colors, and 6) how we remember space. This theoretical article attempts to clarify some of the physiological processes involved with human vision. The global integration of visual information is conceptual; therefore, we include illustrations to present our theory. Universally, the eyeball is 2.4cm and works together with membrane potential, correspondingly representing the retinal layers, photoreceptors, and cortex. Images formed within the photoreceptors must first be converted into chemical signals on the photoreceptors' individual discs and the signals at each disc are transduced from light photons into electrical signals. We contend that the discs code the electrical signals into accurate distances and are shown in our figures. The pre-existing oscillations among the various cortices including the striate and parietal cortex, and the retina work in unison to create an infrastructure of visual space that functionally "places" the objects within this "neural" space. The horizontal layers integrate all discs accurately to create a retina that is pre-coded for distance. Our theory suggests image inversion never takes place on the retina, but rather images fall onto the retina as compressed and coiled, then amplified through lateral inhibition through intensification and amplification on the OFF-center cones. The intensified and amplified images are decompressed and expanded in the brain, which become the images we perceive as external vision.

SUMMARY

This is a theoretical article presenting a novel hypothesis about the physiological processes in vision, and expounds upon the visual aspect of two of our previously published articles, "A unified 3D default space consciousness model combining neurological and physiological processes that underlie conscious experience", and "Functional representation of vision within the mind: A visual consciousness model based in 3D default space." Currently, neuroscience teaches that visual images are initially inverted on the retina, processed in the brain, and then conscious perception of vision happens in the visual cortex. Here, we propose that inversion of visual images never takes place because images enter the retina as coiled and compressed graded potentials that are intensified and amplified in OFF-center photoreceptors. Once they reach the brain, they are decompressed and expanded to the original size of the image, which is perceived by the brain as the external image. We adduce that pre-existing oscillations (alpha, beta, and gamma) among the various cortices in the brain (including the striate and parietal cortex) and the retina, work together in unison to create an infrastructure of visual space thatfunctionally "places" the objects within a "neural" space. These fast oscillations "bring" the faculties of the cortical activity to the retina, creating the infrastructure of the space within the eye where visual information can be immediately recognized by the brain. By this we mean that the visual (striate) cortex synchronizes the information with the photoreceptors in the retina, and the brain instantaneously receives the already processed visual image, thereby relinquishing the eye from being required to send the information to the brain to be interpreted before it can rise to consciousness. The visual system is a heavily studied area of neuroscience yet very little is known about how vision occurs. We believe that our novel hypothesis provides new insights into how vision becomes part of consciousness, helps to reconcile various previously proposed models, and further elucidates current questions in vision based on our unified 3D default space model. Illustrations are provided to aid in explaining our theory.

RdgBα reciprocally transfers PA and PI at ER–PM contact sites to maintain PI(4,5)P2 homoeostasis during phospholipase C signalling in Drosophila photoreceptors

Phosphatidylinositol (PI) is the precursor lipid for the synthesis of PI 4,5-bisphosphate [PI(4,5)P2] at the plasma membrane (PM) and is sequentially phosphorylated by the lipid kinases, PI 4-kinase and phosphatidylinositol 4-phosphate (PI4P)-5-kinase. Receptor-mediated hydrolysis of PI(4,5)P2 takes place at the PM but PI resynthesis occurs at the endoplasmic reticulum (ER). Thus PI(4,5)P2 resynthesis requires the reciprocal transport of two key intermediates, phosphatidic acid (PA) and PI between the ER and the PM. PI transfer proteins (PITPs), defined by the presence of the PITP domain, can facilitate lipid transfer between membranes; the PITP domain comprises a hydrophobic cavity with dual specificity but accommodates a single phospholipid molecule. The class II PITP, retinal degeneration type B (RdgB)α is a multi-domain protein and its PITP domain can bind and transfer PI and PA. In Drosophila photoreceptors, a well-defined G-protein-coupled phospholipase Cβ (PLCβ) signalling pathway, phototransduction defects resulting from loss of RdgBα can be rescued by expression of the PITP domain provided it is competent for both PI and PA transfer. We propose that RdgBα proteins maintain PI(4,5)P2 homoeostasis after PLC activation by facilitating the reciprocal transport of PA and PI at ER–PM membrane contact sites.