Biomedical subjects

Fred Rieke

Publications and source records attributed to Fred Rieke.

18 recordsLinked to original sources

PubMed2006

Network variability limits stimulus-evoked spike timing precision in retinal ganglion cells.

Visual, auditory, somatosensory, and olfactory stimuli generate temporally precise patterns of action potentials (spikes). It is unclear, however, how the precision of spike generation relates to the pattern and variability of synaptic input elicited by physiological stimuli. We determined how synaptic conductances evoked by light stimuli that activate the rod bipolar pathway control spike generation in three identified types of mouse retinal ganglion cells (RGCs). The relative amplitude, timing, and impact of excitatory and inhibitory input differed dramatically between On and Off RGCs. Spikes evoked by repeated somatic injection of identical light-evoked synaptic conductances were more temporally precise than those evoked by light. However, the precision of spikes evoked by conductances that varied from trial to trial was similar to that of light-evoked spikes. Thus, the rod bipolar pathway modulates different RGCs via unique combinations of synaptic input, and RGC temporal variability reflects variability in the input this circuit provides.

Action Potentials↗

PubMed2006

Multiple phosphorylation sites confer reproducibility of the rod's single-photon responses.

Although signals controlled by single molecules are expected to be inherently variable, rod photoreceptors generate reproducible responses to single absorbed photons. We show that this unexpected reproducibility-the consistency of amplitude and duration of rhodopsin activity-varies in a graded and systematic manner with the number but not the identity of phosphorylation sites on rhodopsin's C terminus. These results indicate that each phosphorylation site provides an independent step in rhodopsin deactivation and that collectively these steps tightly control rhodopsin's active lifetime. Other G protein cascades may exploit a similar mechanism to encode accurately the timing and number of receptor activation.

Animals↗

PubMed2006

The impact of photoreceptor noise on retinal gain controls.

Multiple retinal mechanisms preserve visual sensitivity as the properties of the light inputs change. Rapid gain controls match the effective signaling range of retinal neurons to the local image statistics. Such gain controls trade an increased sensitivity for some aspects of the inputs for a decreased sensitivity to others. Rapid, local gain control comes at another cost: noise in the signal controlling gain (e.g. from the photoreceptors) will cause gain itself to vary even when the statistics of the light input are constant. Recent advances in identifying retinal pathways and the sites and mechanisms of mean and contrast adaptation have begun to clarify the tradeoffs associated with different gain control locations and how these tradeoffs differ for rod and cone vision.

Adaptation, Physiological↗

PubMed2006

Scotopic visual signaling in the mouse retina is modulated by high-affinity plasma membrane calcium extrusion.

Transmission of visual signals at the first retinal synapse is associated with changes in calcium concentration in photoreceptors and bipolar cells. We investigated how loss of plasma membrane Ca2+ ATPase isoform 2 (PMCA2), the calcium transporter isoform with the highest affinity for Ca2+/calmodulin, affects transmission of rod- and cone-mediated responses. PMCA2 expression in the neuroblast layer was observed soon after birth; in the adult, PMCA2 was expressed in inner segments and synaptic terminals of rod photoreceptors, in rod bipolar cells, and in most inner retinal neurons but was absent from cones. To determine the role of PMCA2 in retinal signaling, we compared morphology and light responses of retinas from control mice and deafwaddler dfw2J mice, which lack functional PMCA2 protein. The cytoarchitecture of retinas from control and dfw2J mice was indistinguishable at the light microscope level. Suction electrode recordings revealed no difference in the sensitivity or amplitude of outer segment light responses of control and dfw2J rods. However, rod-mediated ERG b-wave responses in dfw2J mice were approximately 45% smaller and significantly slower than those of control mice. Furthermore, recordings from individual rod bipolar cells showed that the sensitivity of transmission at the rod output synapse was reduced by approximately 50%. No changes in the amplitude or timing of cone-mediated ERG responses were observed. These results suggest that PMCA2-mediated Ca2+ extrusion modulates the amplitude and timing of the high-sensitivity rod pathway to a much greater extent than that of the cone pathway.

Animals↗

PubMed2006

Controlling the gain of rod-mediated signals in the Mammalian retina.

Effective sensory processing requires matching the gain of neural responses to the range of signals encountered. For rod vision, gain controls operate at light levels at which photons arrive rarely at individual rods, light levels too low to cause adaptation in rod phototransduction. Under these conditions, adaptation within a conserved pathway in mammalian retina maintains sensitivity as light levels change. To relate retinal signals to behavioral work on detection at low light levels, we measured how background light affects the gain and noise of primate ganglion cells. To determine where and how gain is controlled, we tracked rod-mediated signals across the mouse retina. These experiments led to three main conclusions: (1) the primary site of adaptation at low light levels is the synapse between rod bipolar and AII amacrine cells; (2) cellular noise after the gain control is nearly independent of background intensity; and (3) at low backgrounds, noise in the circuitry, rather than rod noise or fluctuations in arriving photons, limits ganglion cell sensitivity. This work provides physiological insights into the rich history of experiments characterizing how rod vision avoids saturation as light levels increase.

Adaptation, Ocular↗

PubMed2005

Pharmacological and rAAV gene therapy rescue of visual functions in a blind mouse model of Leber congenital amaurosis.

BACKGROUND: Leber congenital amaurosis (LCA), a heterogeneous early-onset retinal dystrophy, accounts for approximately 15% of inherited congenital blindness. One cause of LCA is loss of the enzyme lecithin:retinol acyl transferase (LRAT), which is required for regeneration of the visual photopigment in the retina. METHODS AND FINDINGS: An animal model of LCA, the Lrat-/- mouse, recapitulates clinical features of the human disease. Here, we report that two interventions--intraocular gene therapy and oral pharmacologic treatment with novel retinoid compounds--each restore retinal function to Lrat-/- mice. Gene therapy using intraocular injection of recombinant adeno-associated virus carrying the Lrat gene successfully restored electroretinographic responses to approximately 50% of wild-type levels (p < 0.05 versus wild-type and knockout controls), and pupillary light responses (PLRs) of Lrat-/- mice increased approximately 2.5 log units (p < 0.05). Pharmacological intervention with orally administered pro-drugs 9-cis-retinyl acetate and 9-cis-retinyl succinate (which chemically bypass the LRAT-catalyzed step in chromophore regeneration) also caused long-lasting restoration of retinal function in LRAT-deficient mice and increased ERG response from approximately 5% of wild-type levels in Lrat-/- mice to approximately 50% of wild-type levels in treated Lrat-/- mice (p < 0.05 versus wild-type and knockout controls). The interventions produced markedly increased levels of visual pigment from undetectable levels to 600 pmoles per eye in retinoid treated mice, and approximately 1,000-fold improvements in PLR and electroretinogram sensitivity. The techniques were complementary when combined. CONCLUSION: Intraocular gene therapy and pharmacologic bypass provide highly effective and complementary means for restoring retinal function in this animal model of human hereditary blindness. These complementary methods offer hope of developing treatment to restore vision in humans with certain forms of hereditary congenital blindness.

Acyltransferases↗

PubMed2005

Recoverin improves rod-mediated vision by enhancing signal transmission in the mouse retina.

Vision in dim light requires that photons absorbed by rod photoreceptors evoke signals that reliably propagate through the retina. We investigated how a perturbation in rod physiology affects propagation of those signals in the retina and ultimately visual sensitivity. Recoverin is a protein in rods that prolongs phototransduction and enhances visual sensitivity. It is not present in neurons postsynaptic to rods, yet we found that light-evoked responses of rod bipolar and ganglion cells were shortened when measured in recoverin-deficient retinas. Unexpectedly, the effect of recoverin on postsynaptic signals could not be explained by its effect on phototransduction. Instead, it is an effect of recoverin downstream of phototransduction in rods that prolongs signal transmission and enhances visual sensitivity. An important implication of our findings is that the recovery phase of the rod photoresponse does not contribute significantly to visual sensitivity near absolute threshold.

Animals↗

PubMed2005

Role of photoreceptor-specific retinol dehydrogenase in the retinoid cycle in vivo.

The retinoid cycle is a recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. Photoreceptor-specific retinol dehydrogenase (prRDH) catalyzes reduction of all-trans-retinal to all-trans-retinol and is thought to be a key enzyme in the retinoid cycle. We disrupted mouse prRDH (human gene symbol RDH8) gene expression by targeted recombination and generated a homozygous prRDH knock-out (prRDH-/-) mouse. Histological analysis and electron microscopy of retinas from 6- to 8-week-old prRDH-/- mice revealed no structural differences of the photoreceptors or inner retina. For brief light exposure, absence of prRDH did not affect the rate of 11-cis-retinal regeneration or the decay of Meta II, the activated form of rhodopsin. Absence of prRDH, however, caused significant accumulation of all-trans-retinal following exposure to bright lights and delayed recovery of rod function as measured by electroretinograms and single cell recordings. Retention of all-trans-retinal resulted in slight overproduction of A2E, a condensation product of all-trans-retinal and phosphatidylethanolamine. We conclude that prRDH is an enzyme that catalyzes reduction of all-trans-retinal in the rod outer segment, most noticeably at higher light intensities and prolonged illumination, but is not an essential enzyme of the retinoid cycle.

Alcohol Oxidoreductases↗

PubMed2005

Retinal processing near absolute threshold: from behavior to mechanism.

Vision at absolute threshold is based on signals produced in a tiny fraction of the rod photoreceptors. This requires that the rods signal the absorption of single photons, and that the resulting signals are transmitted across the retina and encoded in the activity sent from the retina to the brain. Behavioral and ganglion cell sensitivity has often been interpreted to indicate that these biophysical events occur noiselessly, i.e., that vision reaches limits to sensitivity imposed by the division of light into discrete photons and occasional photon-like noise events generated in the rod photoreceptors. We argue that this interpretation is not unique and provide a more conservative view of the constraints behavior and ganglion cell experiments impose on phototransduction and retinal processing. We summarize what is known about how these constraints are met and identify some of the outstanding open issues.

Animals↗

PubMed2004

Essential role of Ca2+-binding protein 4, a Cav1.4 channel regulator, in photoreceptor synaptic function.

CaBP1-8 are neuronal Ca(2+)-binding proteins with similarity to calmodulin (CaM). Here we show that CaBP4 is specifically expressed in photoreceptors, where it is localized to synaptic terminals. The outer plexiform layer, which contains the photoreceptor synapses with secondary neurons, was thinner in the Cabp4(-/-) mice than in control mice. Cabp4(-/-) retinas also had ectopic synapses originating from rod bipolar and horizontal cells tha HJt extended into the outer nuclear layer. Responses of Cabp4(-/-) rod bipolars were reduced in sensitivity about 100-fold. Electroretinograms (ERGs) indicated a reduction in cone and rod synaptic function. The phenotype of Cabp4(-/-) mice shares similarities with that of incomplete congenital stationary night blindness (CSNB2) patients. CaBP4 directly associated with the C-terminal domain of the Ca(v)1.4 alpha(1)-subunit and shifted the activation of Ca(v)1.4 to hyperpolarized voltages in transfected cells. These observations indicate that CaBP4 is important for normal synaptic function, probably through regulation of Ca(2+) influx and neurotransmitter release in photoreceptor synaptic terminals.

Amino Acid Sequence↗

PubMed2004

Selective transmission of single photon responses by saturation at the rod-to-rod bipolar synapse.

A threshold-like nonlinearity in signal transfer from mouse rod photoreceptors to rod bipolar cells dramatically improves the absolute sensitivity of the rod signals. The work described here reaches three conclusions about the mechanisms generating this nonlinearity. (1) The nonlinearity is caused primarily by saturation of the feedforward rod-to-rod bipolar synapse and not by feedback from horizontal or amacrine cells. This saturation renders the rod bipolar current insensitive to small changes in transmitter release from the rod. (2) Saturation occurs within the G protein cascade that couples receptors to channels in the rod bipolar dendrites, with little or no contribution from presynaptic mechanisms or saturation of the postsynaptic receptors. (3) Between 0.5 and 2 bipolar transduction channels are open in darkness at each synapse, compared to the approximately 30 channels open at the peak of the single photon response.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2003

Bandpass filtering at the rod to second-order cell synapse in salamander (Ambystoma tigrinum) retina.

The ability to see at night relies on the transduction of single photons by the rod photoreceptors and transmission of the resulting signals through the retina. Using paired patch-clamp recordings, we investigated the properties of the first stage of neural processing of the rod light responses: signal transfer from rods to bipolar and horizontal cells. Bypassing the relatively slow phototransduction process and directly modulating the rod voltage or current allowed us to characterize signal transfer over a wide range of temporal frequencies. We found that the rod to second-order cell synapse acts as a bandpass filter, preferentially transmitting signals with frequencies between 1.5 and 4 Hz while attenuating higher and lower frequency inputs. The similarity of the responses in different types of postsynaptic cell and the properties of miniature EPSCs (mEPSCs) recorded in OFF bipolar cells suggest that most of the bandpass filtering is mediated presynaptically. Modeling of the network of electrically coupled rod photoreceptors suggests that spread of the signal through the network contributed to the observed high-pass filtering but not to the low-pass filtering. Attenuation of low temporal frequencies at the first retinal synapse sharpens the temporal resolution of the light response; attenuation of high temporal frequencies removes voltage noise in the rod that threatens to swamp the light response.

Animals↗

PubMed2003

Slow Na+ inactivation and variance adaptation in salamander retinal ganglion cells.

The retina adapts to the temporal contrast of the light inputs. One component of contrast adaptation is intrinsic to retinal ganglion cells: temporal contrast affects the variance of the synaptic inputs to ganglion cells, which alters the gain of spike generation. Here we show that slow Na+ inactivation is sufficient to produce the observed variance adaptation. Slow inactivation caused the Na+ current available for spike generation to depend on the past history of activity, both action potentials and subthreshold voltage variations. Recovery from slow inactivation required several hundred milliseconds. Increased current variance caused the threshold for spike generation to increase, presumably because of the decrease in available Na+ current. Simulations indicated that slow Na+ inactivation could account for the observed decrease in excitability. This suggests a simple picture of how ganglion cells contribute to contrast adaptation: (1) increasing contrast causes an increase in input current variance that raises the spike rate, and (2) the increased spike rate reduces the available Na+ current through slow inactivation, which feeds back to reduce excitability. Cells throughout the nervous system face similar problems of accommodating a large range of input signals; furthermore, the Na+ currents of many cells exhibit slow inactivation. Thus, adaptation mediated by feedback modulation of the Na+ current through slow inactivation could serve as a general mechanism to control excitability in spiking neurons.

Action Potentials↗

PubMed2003

Light stimulates a transducin-independent increase of cytoplasmic Ca2+ and suppression of current in cones from the zebrafish mutant nof.

Transducins couple visual pigments to cGMP hydrolysis, the only recognized phototransduction pathway in vertebrate photoreceptors. Here we describe a zebrafish mutant, no optokinetic response f(w21) (nof), with a nonsense mutation in the gene encoding the alpha subunit of cone transducin. Retinal morphology and levels of phototransduction enzymes are normal in nof retinas, but cone transducin is undetectable. Dark current in nof cones is also normal, but it is insensitive to moderate intensity light. The nof cones do respond, however, to bright light. These responses are produced by a light-stimulated, but transducin-independent, release of Ca2+ into the cone cytoplasm. Thus, in addition to stimulating transducin, light also independently induces release of Ca2+ into the photoreceptor cytoplasm.

Adaptation, Ocular↗

PubMed2002

Mechanisms regulating variability of the single photon responses of mammalian rod photoreceptors.

Variability in the single photon responses of rod photoreceptors limits the accuracy with which the number and timing of photon absorptions are encoded. We investigated how much single photon responses of mammalian rods fluctuate and what mechanisms control these fluctuations. Mammalian rods, like those of toads, generated responses to single photons with trial-to-trial fluctuations 3-4 times smaller than other familiar signals produced by single molecules. We used the properties of the measured fluctuations to constrain models for how the single photon responses are regulated. Neither feedback control of rhodopsin's activity nor saturation within the transduction cascade were consistent with experiment. The measured responses, however, could be explained by multistep shutoff of rhodopsin or a combination of multistep shutoff and saturation.

Animals↗

PubMed2002

Nonlinear signal transfer from mouse rods to bipolar cells and implications for visual sensitivity.

We investigated the impact of rod-bipolar signal transfer on visual sensitivity. Two observations indicate that rod-rod bipolar signal transfer is nonlinear. First, responses of rods increased linearly with flash strength, while those of rod bipolars increased supralinearly. Second, fluctuations in the responses of rod bipolars were larger than expected from linear summation of the rod inputs. Rod-OFF bipolar signal transfer did not share this strong nonlinearity. Surprisingly, nonlinear rod-rod bipolar signal transfer eliminated many of the rod's single-photon responses. The impact on sensitivity, however, was more than compensated for by rejection of noise from rods that did not absorb photons. As a consequence, rod bipolars provide a near-optimal readout of rod signals at light levels near visual threshold.

Animals↗

PubMed2002

GCAP1 rescues rod photoreceptor response in GCAP1/GCAP2 knockout mice.

Visual transduction in retinal photoreceptors operates through a dynamic interplay of two second messengers, Ca(2+) and cGMP. Ca(2+) regulates the activity of guanylate cyclase (GC) and the synthesis of cGMP by acting on a GC-activating protein (GCAP). While this action is critical for rapid termination of the light response, the GCAP responsible has not been identified. To test if GCAP1, one of two GCAPs present in mouse rods, supports the generation of normal flash responses, transgenic mice were generated that express only GCAP1 under the control of the endogenous promoter. Paired flash responses revealed a correlation between the degree of recovery of the rod a-wave and expression levels of GCAP1. In single cell recordings, the majority of the rods generated flash responses that were indistinguishable from wild type. These results demonstrate that GCAP1 at near normal levels supports the generation of wild-type flash responses in the absence of GCAP2.

Animals↗

PubMed2002

Recovery of visual functions in a mouse model of Leber congenital amaurosis.

The visual process is initiated by the photoisomerization of 11-cis-retinal to all-trans-retinal. For sustained vision the 11-cis-chromophore must be regenerated from all-trans-retinal. This requires RPE65, a dominant retinal pigment epithelium protein. Disruption of the RPE65 gene results in massive accumulation of all-trans-retinyl esters in the retinal pigment epithelium, lack of 11-cis-retinal and therefore rhodopsin, and ultimately blindness. We reported previously (Van Hooser, J. P., Aleman, T. S., He, Y. G., Cideciyan, A. V., Kuksa, V., Pittler, S. J., Stone, E. M., Jacobson, S. G., and Palczewski, K. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 8623-8628) that in Rpe65-/- mice, oral administration of 9-cis-retinal generated isorhodopsin, a rod photopigment, and restored light sensitivity to the electroretinogram. Here, we provide evidence that early intervention by 9-cis-retinal administration significantly attenuated retinal ester accumulation and supported rod retinal function for more than 6 months post-treatment. In single cell recordings rod light sensitivity was shown to be a function of the amount of regenerated isorhodopsin; high doses restored rod responses with normal sensitivity and kinetics. Highly attenuated residual rod function was observed in untreated Rpe65-/- mice. This rod function is likely a consequence of low efficiency production of 11-cis-retinal by photo-conversion of all-trans-retinal in the retina as demonstrated by retinoid analysis. These studies show that pharmacological intervention produces long lasting preservation of visual function in dark-reared Rpe65-/- mice and may be a useful therapeutic strategy in recovering vision in humans diagnosed with Leber congenital amaurosis caused by mutations in the RPE65 gene, an inherited group of early onset blinding and retinal degenerations.

Animals↗