This lecture details the elucidation of cGMP phosphodiesterase (PDEδ), discovered 25 years ago by Joe Beavo at the University of Washington. PDEδ, once identified as a fourth PDE6 subunit, is now ...regarded as a promiscuous prenyl-binding protein and important chaperone of prenylated small G proteins of the Ras superfamily and prenylated proteins of phototransduction. Alfred Wittinghofer's group in Germany showed that PDEδ forms an immunoglobulin-like β-sandwich fold that is closely related in structure to other lipid-binding proteins, for example, Uncoordinated 119 (UNC119) and RhoGDI. His group cocrystallized PDEδ with ARL (Arf-like) 2
GTP
, and later with farnesylated Rheb (ras homolog expressed in brain). PDEδ specifically accommodates farnesyl and geranylgeranyl moieties in the absence of bound protein. Germline deletion of the
Pde6d
gene encoding PDEδ impeded transport of rhodopsin kinase (GRK1) and PDE6 to outer segments, causing slowly progressing, recessive retinitis pigmentosa. A rare
PDE6D
null allele in human patients, discovered by Tania Attié-Bitach in France, specifically impeded trafficking of farnesylated phosphatidylinositol 3,4,5-trisphosphate (PIP3) 5-phosphatase (INPP5E) to cilia, causing severe syndromic ciliopathy (Joubert syndrome). Binding of cargo to PDEδ is controlled by Arf-like proteins, ARL2 and ARL3, charged with guanosine-5′-triphosphate (GTP). Arf-like proteins 2 and 3 are unprenylated small GTPases that serve as cargo displacement factors. The lifetime of ARL3
GTP
is controlled by its GTPase-activating protein, retinitis pigmentosa protein 2 (RP2), which accelerates GTPase activity up to 90,000-fold.
RP2
null alleles in human patients are associated with severe X-linked retinitis pigmentosa (XLRP). Germline deletion of RP2 in mouse, however, causes only a mild form of XLRP. Absence of RP2 prolongs the activity of ARL3
GTP
that, in turn, impedes PDE6δ–cargo interactions and trafficking of prenylated protein to the outer segments. Hyperactive ARL3
GTP
, acting as a hyperactive cargo displacement factor, is predicted to be key in the pathobiology of RP2-XLRP.
Non-image-forming vision in mammals is mediated primarily by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs). In mouse M1-ipRGCs, by far the best-studied subtype, ...melanopsin activates PLCβ4 (phospholipase C-β4) to open TRPC6,7 channels, mechanistically similar to phototransduction in fly rhabdomeric (microvillous) photoreceptors. We report here that, surprisingly, mouse M4-ipRGCs rely on a different and hitherto undescribed melanopsin-driven, ciliary phototransduction mechanism involving cyclic nucleotide as the second messenger and HCN channels rather than CNG channels as the ion channel for phototransduction. Even more surprisingly, within an individual mouse M2-ipRGC, this HCN-channel-dependent, ciliary phototransduction pathway operates in parallel with the TRPC6,7-dependent rhabdomeric pathway. These findings reveal a complex heterogeneity in phototransduction among ipRGCs and, more importantly, break a general dogma about segregation of the two phototransduction motifs, likely with strong evolutionary implications.
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•Rhabdomeric and ciliary phototransductions coexist in the same cell for some ipRGCs•Unique among photoreceptor types, basic transduction traits vary in ipRGC subtypes•IpRGCs use HCN instead of CNG channels for ciliary phototransduction•The self-regulating property of HCN channels may be important for ipRGC signaling
Discovery in retinal ganglion cells of a ciliary phototransduction pathway that uses cyclic nucleotide as the second messenger and HCN as the effector ion channel has evolutionary implications.
Diabetic retinopathy (DR) is a leading cause of blindness. It has long been regarded as vascular disease, but work in the past years has shown abnormalities also in the neural retina. Unfortunately, ...research on the vascular and neural abnormalities have remained largely separate, instead of being integrated into a comprehensive view of DR that includes both the neural and vascular components. Recent evidence suggests that the most predominant neural cell in the retina (photoreceptors) and the adjacent retinal pigment epithelium (RPE) play an important role in the development of vascular lesions characteristic of DR. This review summarizes evidence that the outer retina is altered in diabetes, and that photoreceptors and RPE contribute to retinal vascular alterations in the early stages of the retinopathy. The possible molecular mechanisms by which cells of the outer retina might contribute to retinal vascular damage in diabetes also are discussed. Diabetes-induced alterations in the outer retina represent a novel therapeutic target to inhibit DR.
Intrinsically photosensitive retinal ganglion cells (ipRGCs) serve as primary photoceptors by expressing the photopigment, melanopsin, and also as retinal relay neurons for rod and cone signals en ...route to the brain, in both cases for the purpose of non-image vision as well as aspects of image vision. So far, six subtypes of ipRGCs (M1 through M6) have been characterized. Regarding their phototransduction mechanisms, we have previously found that, unconventionally, rhabdomeric (microvillous) and ciliary signaling motifs co-exist within a given M1-, M2-, and M4-ipRGC, with the first mechanism involving PLCβ4 and TRPC6,7 channels and the second involving cAMP and HCN channels. We have now examined M3-, M5-, and M6-cells and found that each cell likewise uses both signaling pathways for phototransduction, despite differences in the percentage representation by each pathway in a given ipRGC subtype for bright-flash responses (and saturated except for M6-cells). Generally, M3- and M5-cells show responses quite similar in kinetics to M2-responses, and M6-cell responses resemble broadly those of M1-cells although much lower in absolute sensitivity and amplitude. Therefore, similar to rod and cone subtypes in image vision, ipRGC subtypes possess the same phototransduction mechanism(s) even though they do not show microvilli or cilia morphologically.
Autophagy is a conserved pathway that delivers cytoplasmic contents to the lysosome for degradation. Here we consider its roles in neuronal health and disease. We review evidence from mouse knockout ...studies demonstrating the normal functions of autophagy as a protective factor against neurodegeneration associated with intracytoplasmic aggregate-prone protein accumulation as well as other roles, including in neuronal stem cell differentiation. We then describe how autophagy may be affected in a range of neurodegenerative diseases. Finally, we describe how autophagy upregulation may be a therapeutic strategy in a wide range of neurodegenerative conditions and consider possible pathways and druggable targets that may be suitable for this objective.
In this review, Menzies et al. discuss the importance of autophagy function for brain health, outlining connections between autophagy dysfunction and neurodegenerative disorders. The potential for autophagy as a therapeutic strategy for neurodegenerative disease is discussed, along with how this may be achieved.
Noninvasive functional imaging of molecular and cellular processes of vision may have immense impact on research and clinical diagnostics. Although suitable intrinsic optical signals (IOSs) have been ...observed ex vivo and in immobilized animals in vivo, detecting IOSs of photoreceptor activity in living humans was cumbersome and time consuming. Here, we observed clear spatially and temporally resolved changes in the optical path length of the photoreceptor outer segment as a response to an optical stimulus in the living human eye. To witness these changes, we evaluated phase data obtained with a parallelized and computationally aberration-corrected optical coherence tomography system. The noninvasive detection of optical path length changes shows neuronal photoreceptor activity of single cones in living human retina, and therefore, it may provide diagnostic options in ophthalmology and neurology and could provide insights into visual phototransduction in humans.
In the vertebrate retina, rods and cones both detect light, but they differ in functional aspects such as light sensitivity and temporal resolution, and in some cell biological aspects. For ...functional aspects, both types of photoreceptors use a phototransduction cascade, consisting of a series of enzymatic reactions, to convert photon capture to an electrical signal. To understand the mechanisms underlying the functional differences between rods and cones at the molecular level, we compared biochemically, each of the reactions in the phototransduction cascades of rods and cones using the cells isolated and purified from carp retina.
Although the cascade proteins are identical or are functionally similar between rods and cones, their activities together with their expression levels are mostly different. In general, reactions that generate a response are somewhat less effective in cones than in rods, but each of the reactions for termination and recovery of a response are much more effective in cones. These findings explain lower light sensitivity and briefer light responses in cones than in rods. In addition, our considerations suggest that a Ca2+-binding protein, S-modulin or recoverin, has a currently unnoticed role in shaping light responses.
Upon comparison of the expression levels of proteins and/or mRNAs using purified cells, several proteins were found to be specifically or predominantly expressed in cones. These proteins will be of interest in future studies aimed at characterizing the differences between rods and cones.
•Rods/cones are purified from carp to compare the phototransduction cascade reactions.•ON reactions are less and OFF reactions are highly effective in cones than in rods.•The results explain lower light sensitivity and faster response kinetics in cones.•Recoverin is suggested to require further studies to understand its role correctly.•Using purified cones, proteins and reactions specific to cones are studied.
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