Calcium regulation of phototransduction in vertebrate rod outer segments

Analysis of dim-light responses in rod and cone photoreceptors with altered calcium kinetics

Rod and cone photoreceptors in the retina of vertebrates are the primary sensory neurons underlying vision. They convert light into an electrical current using a signal transduction pathway that depends on Ca[Formula: see text] feedback. It is known that manipulating the Ca[Formula: see text] kinetics affects the response shape and the photoreceptor sensitivity, but a precise quantification of these effects remains unclear. We have approached this task in mouse retina by combining numerical simulations with mathematical analysis. We consider a parsimonious phototransduction model that incorporates negative Ca[Formula: see text] feedback onto the synthesis of cyclic GMP, and fast buffering reactions to alter the Ca[Formula: see text] kinetics. We derive analytic results for the photoreceptor functioning in sufficiently dim light conditions depending on the photoreceptor type. We exploit these results to obtain conceptual and quantitative insight into how response waveform and amplitude depend on the underlying biophysical processes and the Ca[Formula: see text] feedback. With a low amount of buffering, the Ca[Formula: see text] concentration changes in proportion to the current, and responses to flashes of light are monophasic. With more buffering, the change in the Ca[Formula: see text] concentration becomes delayed with respect to the current, which gives rise to a damped oscillation and a biphasic waveform. This shows that biphasic responses are not necessarily a manifestation of slow buffering reactions. We obtain analytic approximations for the peak flash amplitude as a function of the light intensity, which shows how the photoreceptor sensitivity depends on the biophysical parameters. Finally, we study how changing the extracellular Ca[Formula: see text] concentration affects the response.

Advanced real-time recordings of neuronal activity with tailored patch pipettes, diamond multi-electrode arrays and electrochromic voltage-sensitive dyes

To understand the working principles of the nervous system is key to figure out its electrical activity and how this activity spreads along the neuronal network. It is therefore crucial to develop advanced techniques aimed to record in real time the electrical activity, from compartments of single neurons to populations of neurons, to understand how higher functions emerge from coordinated activity. To record from single neurons, a technique will be presented to fabricate patch pipettes able to seal on any membrane with a single glass type and whose shanks can be widened as desired. This dramatically reduces access resistance during whole-cell recording allowing fast intracellular and, if required, extracellular perfusion. To simultaneously record from many neurons, biocompatible probes will be described employing multi-electrodes made with novel technologies, based on diamond substrates. These probes also allow to synchronously record exocytosis and neuronal excitability and to stimulate neurons. Finally, to achieve even higher spatial resolution, it will be shown how voltage imaging, employing fast voltage-sensitive dyes and two-photon microscopy, is able to sample voltage oscillations in the brain spatially resolved and voltage changes in dendrites of single neurons at millisecond and micrometre resolution in awake animals.

Characterization of Zebrafish Green Cone Photoresponse Recorded with Pressure-Polished Patch Pipettes, Yielding Efficient Intracellular Dialysis

The phototransduction enzymatic cascade in cones is less understood than in rods, and the zebrafish is an ideal model with which to investigate vertebrate and human vision. Therefore, here, for the first time, the zebrafish green cone photoresponse is characterized also to obtain a firm basis for evaluating how it is modulated by exogenous molecules. To this aim, a powerful method was developed to obtain long-lasting recordings with low access resistance, employing pressure-polished patch pipettes. This method also enabled fast, efficient delivery of molecules via a perfusion system coupled with pulled quartz or plastic perfusion tubes, inserted very close to the enlarged pipette tip. Sub-saturating flashes elicited responses in different cells with similar rising phase kinetics but with very different recovery kinetics, suggesting the existence of physiologically distinct cones having different Ca2+ dynamics. Theoretical considerations demonstrate that the different recovery kinetics can be modelled by simulating changes in the Ca2+-buffering capacity of the outer segment. Importantly, the Ca2+-buffer action preserves the fast response rising phase, when the Ca2+-dependent negative feedback is activated by the light-induced decline in intracellular Ca2+.

Pressure-polished borosilicate pipettes are "universal sealer" yielding low access resistance and efficient intracellular perfusion

Whole-cell recording is the most widely used configuration of the patch recording technique, mainly because it allows to manipulate the intracellular environment while recording membrane current. However, the patch pipette tapered shank and the small tip opening give high access resistances and preclude efficient exchange between pipette solution and cell cytosol. Independently by the recording configuration, another problem of this technique is to gain consistently tight seals.Here we describe a method to enlarge the pipette shank without affecting the tip opening diameter, through the calibrated combination of heat and air pressure, with a custom-made inexpensive setup. These pressure-polished pipettes give small access resistances and allow for the accommodation of pulled quartz or plastic perfusion tubes very close to the pipette tip (to deliver exogenous molecules into the cytosol with a controlled timing). Finally, we describe a method to consistently attain seals with pipettes made from just one glass type, for a wide variety of cell types, isolated from different amphibian, reptilian, fish, and mammalian tissues, and on artificial membranes composed of many different lipid mixtures.

Regulation of the methylation status of G protein-coupled receptor kinase 1 (rhodopsin kinase)

Rhodopsin kinase (GRK1) is a member of G protein-coupled receptor kinase family and a key enzyme in the quenching of photolysed rhodopsin activity and desensitisation of the rod photoreceptor neurons. Like some other rod proteins involved in phototransduction, GRK1 is posttranslationally modified at the C terminus by isoprenylation (farnesylation), endoproteolysis and α-carboxymethylation. In this study, we examined the potential mechanisms of regulation of GRK1 methylation status, which have remained unexplored so far. We found that considerable fraction of GRK1 is endogenously methylated. In isolated rod outer segments, its methylation is inhibited and demethylation stimulated by low-affinity nucleotide binding. This effect is not specific for ATP and was observed in the presence of a non-hydrolysable ATP analogue AMP-PNP, GTP and other nucleotides, and thus may involve a site distinct from the active site of the kinase. GRK1 demethylation is inhibited in the presence of Ca(2+) by recoverin. This inhibition requires recoverin myristoylation and the presence of the membranes, and may be due to changes in GRK1 availability for processing enzymes upon its redistribution to the membranes induced by recoverin/Ca(2+). We hypothesise that increased GRK1 methylation in dark-adapted rods due to elevated cytoplasmic Ca(2+) levels would further increase its association with the membranes and recoverin, providing a positive feedback to efficiently suppress spurious phosphorylation of non-activated rhodopsin molecules and thus maximise senstivity of the photoreceptor. This study provides the first evidence for dynamic regulation of GRK1 α-carboxymethylation, which might play a role in the regulation of light sensitivity and adaptation in the rod photoreceptors.

A pressure-polishing set-up to fabricate patch pipettes that seal on virtually any membrane, yielding low access resistance and efficient intracellular perfusion

When performing whole-cell configuration recordings, it is important to minimize series resistance to reduce the time constant of charging the cell membrane capacitance and to reduce error in membrane potential control. To this end, an existing method was improved by widening the patch pipette shank through the calibrated combination of heat and air pressure. The heat was produced by passing current through a filament that was shaped appropriately to ensure a homogeneous heating of the pipette shank. Pressurized air was applied to the lumen of a pipette, pulled from a borosilicate glass microcap, via the pressure port of a modified commercial holder. The pipette reshaping was viewed on an LCD monitor connected to a contrast-intensified CCD camera and coupled to a modified bright-field stereomicroscope. By appropriately regulating the timing of air pressure and the application of heating, the pipette shank and, independently, the tip opening diameter were widened as desired. The methods illustrated here to fabricate and use the patch pipettes, using just one glass type, allowed the sealing of a wide variety of cell types isolated from different amphibian, reptilian, fish, and mammalian tissues as well as a variety of artificial membranes made with many different lipid mixtures. The access resistance yielded by pressure-polished pipettes was approximately one-fourth the size of the one attained with conventional pipettes; besides improving the electrical recordings, this minimized intracellular ion accumulation or depletion as well. Enlarged shank geometry allowed for fast intracellular perfusion as shown by fluorescence imaging, also via pulled quartz or plastic tubes, which could be inserted very close to the pipette tip.

Pore forming properties of cecropin-melittin hybrid peptide in a natural membrane

The pore forming properties of synthetic cecropin-melittin hybrid peptide (Acetyl-KWKLFKKIGAVLKVL-CONH₂; CM15) were investigated by using photoreceptor rod outer segments (OS) isolated from frog retinae obtained by using the whole-cell configuration of the patch-clamp technique. CM15 was applied (and removed) to (from) the OS in ~50 ms with a computer-controlled microperfusion system. Once the main OS endogenous conductance was blocked with light, the OS membrane resistance was ≥1 GΩ, allowing high resolution, low-noise recordings. Different to alamethicines, CM15 produced voltage-independent membrane permeabilisation, repetitive peptide application caused a progressive permeabilisation increase, and no single-channel events were detected at low peptide concentrations. Collectively, these results indicate a toroidal mechanism of pore formation by CM15.

The contribution of cationic conductances to the potential of rod photoreceptors

The contribution of cationic conductances in shaping the rod photovoltage was studied in light adapted cells recorded under whole-cell voltage- or current-clamp conditions. Depolarising current steps (of size comparable to the light-regulated current) produced monotonic responses when the prepulse holding potential (V (h)) was -40 mV (i.e. corresponding to the membrane potential in the dark). At V (h) = -60 mV (simulating the steady-state response to an intense background of light) current injections <35 pA (mimicking a light decrement) produced instead an initial depolarisation that declined to a plateau, and voltage transiently overshot V (h) at the stimulus offset. Current steps >40 pA produced a steady depolarisation to approximately -16 mV at both V (h). The difference between the responses at the two V (h) was primarily generated by the slow delayed-rectifier-like K(+) current (I (Kx)), which therefore strongly affects both the photoresponse rising and falling phase. The steady voltage observed at both V (h) in response to large current injections was instead generated by Ca-activated K(+) channels (I (KCa)), as previously found. Both I (Kx) and I (KCa) oppose the cation influx, occurring at the light stimulus offset through the cGMP-gated channels and the voltage-activated Ca(2+) channels (I (Ca)). This avoids that the cation influx could erratically depolarise the rod past its normal resting value, thus allowing a reliable dim stimuli detection, without slowing down the photovoltage recovery kinetics. The latter kinetics was instead accelerated by the hyperpolarisation-activated, non-selective current (I (h)) and I (Ca). Blockade of all K(+) currents with external TEA unmasked a I (Ca)-dependent regenerative behaviour.

Plasma membrane-porating domain in poliovirus 2B protein. A short peptide mimics viroporin activity

Picornavirus 2B, a non-structural protein required for effective viral replication, has been implicated in cell membrane permeabilization during the late phases of infection. Here, we have approached the molecular mechanism of this process by assessing the pore-forming activity of an overlapping peptide library that spanned the complete 2B sequence. At non-cytopathic concentrations, only the P3 peptide, spanning 2B residues 35-55, effectively assembled hydrophilic pores that allowed diffusion of low molecular mass solutes across the cell plasma membrane (IC(50) approximately 4x10(-7) M) and boundary liposome bilayers (starting at peptide to lipid molar ratios>1:10(4)). Circular dichroism data were consistent with its capacity to fold as a helix in a membrane-like environment. Furthermore, addition of this peptide to a sealed plasma-membrane model, consisting of retinal rod outer segments patch-clamped in a whole-cell configuration, induced ion channel activity within seconds at concentrations as low as 10(-8) M. Thus, we have established a "one-helix" 2B version that possesses the intrinsic pore-forming activity required to directly and effectively permeabilize the cell plasma membrane. We conclude that 2B viroporin can be classified as a genuine pore-forming toxin of viral origin, which is produced intracellularly at certain times post infection.

Pore-forming properties of alamethicin F50/5 inserted in a biological membrane

The pore-forming properties of native and synthetic alamethicins were investigated in photoreceptor rod outer segments (OS) isolated from frog retina, and recorded in whole-cell configuration. The peptaibols were applied (and removed) to (from) the OS within less than 50 ms by means of a computer-controlled micro-perfusion system. Once blocked with light, the main OS endogenous conductance, the OS membrane resistance was >1 GOmega, allowing low-noise and high-resolution recordings. Currents of ca. 700 pA were recorded in symmetric K(+) (100 mM) and Ca(2+) (1 mM), upon applying 1 microM of alamethicin F50/5 or its [L-Glu(OMe)(7,18,19)] analogue to the OS membrane (clamped at -20 mV). In the latter peptide, the Gln residues at positions 7, 18, and 19 were substituted with side-chain esterified Glu residues. For both peptides, the current activated exponentially, with a delay from peptide application, and exponentially returned to zero without any delay, upon removing the peptide from the external solution. The delay as well as the activation (tau(a)) and deactivation (tau(d)) time constants of the current produced by the modified alamethicin were much slower, and the current noise was much larger, with respect to the corresponding values for alamethicin F50/5. Therefore, the above three Gln residues are not a key factor for pore formation, but the [L-Glu(OMe)(7,18,19)] analogue produces larger pores with a lower probability of formation.

Modulation of the reaction cycle of the Na+:Ca2+, K+ exchanger

Ca(2+) concentration in retinal photoreceptor rod outer segment (OS) strongly affects the generator potential kinetics and the receptor light adaptation. The response to intense light stimuli delivered in the dark produce potential changes exceeding 40 mV: since the Ca(2+) extrusion in the OS is entirely controlled by the Na(+):Ca(2+), K(+) exchanger, it is important to assess how the exchanger ion transport rate is affected by the voltage and, in general, by intracellular factors. It is indeed known that the cardiac Na(+):Ca(2+) exchanger is regulated by Mg-ATP via a still unknown metabolic pathway. In the present work, the Na(+):Ca(2+), K(+) exchanger regulation was investigated in isolated OS, recorded in whole-cell configuration, using ionic conditions that activated maximally the exchanger in both forward and reverse mode. In all species examined (amphibia: Rana esculenta and Ambystoma mexicanum; reptilia: Gecko gecko), the forward (reverse) exchange current increased about linearly for negative (positive) voltages and exhibited outward (inward) rectification for positive (negative) voltages. Since hyperpolarisation increases Ca(2+) extrusion rate, the recovery of the dark level of Ca(2+) (and, in turn, of the generator potential) after intense light stimuli results accelerated. Mg-ATP increased the size of forward and reverse exchange current by a factor of approximately 2.3 and approximately 2.6, respectively, without modifying their voltage dependence. This indicates that Mg-ATP regulates the number of active exchanger sites and/or the exchanger turnover number, although via an unknown mechanism.

A novel technique to study pore-forming peptides in a natural membrane

The biophysical characteristics and the pore formation dynamics of synthetic or naturally occurring peptides forming membrane-spanning channels were investigated by using isolated photoreceptor rod outer segments (OS) recorded in whole-cell configuration. Once blocking the two OS endogenous conductances (the cGMP channels by light and the Na(+):Ca(2+),K(+) exchanger by removing one of the transported ion species from both sides of the membrane, i.e. K(+), Na(+) or Ca(2+)), the OS membrane resistance (R ( m )) was typically larger than 1 GOmega in the presence of 1 mM external Ca(2+). Therefore, any exogenous current could be studied down to the single channel level. The peptides were applied to (and removed from) the extracellular OS side in approximately 50 ms with a computer-controlled microperfusion system, in which every perfusion parameter, as the rate of solution flow, the temporal sequence of solution changes or the number of automatic, self-washing cycles were controlled by a user-friendly interface. This technique was then used to determine the biophysical properties and the pore formation dynamics of antibiotic peptaibols, as the native alamethicin mixture, the synthesized major component of the neutral fraction (F50/5) of alamethicin, and the synthetic trichogin GA IV.

No evidence for calcium electrogenic exchanger in frog semicircular canal hair cells

We investigated the possibility that, in hair cells mechanically isolated from frog semicircular canals, Ca2+ extrusion occurs via a Na+ : Ca2+ (cardiac type) or a Na+ : Ca2+,K+ (retinal type) exchanger. Cells concurrently imaged during whole-cell patch-clamp recordings using the Ca2+ sensitive fluorescent dye Oregon Green 488 BAPTA-1 (100 micro m) showed no voltage dependence of Ca2+ clearance dynamics following a Ca2+ load through voltage-gated Ca2+ channels. Reverse exchange was probed in hair cells dialyzed with a Ca2+- and K+-free solution, containing a Na+ concentration that saturates the exchanger, after zeroing the contribution to the whole-cell current from Ca2+ and K+ conductances. In these conditions, no reverse exchange current was detected upon switching from a Ca2+-free external solution to a solution containing concentrations of Ca2+ alone, or Ca2+ + K+ that saturated the exchanger. By contrast, the same experimental protocol elicited peak exchange currents exceeding 100 pA in gecko rod photoreceptors, used as positive controls. In both cell types, we also probed the forward mode of the exchanger by rapidly increasing the intracellular Ca2+ concentration using flash photolysis of two novel caged Ca2+ complexes, calcium 2,2'-([1-(2-nitrophenyl)ethane-1,2-diyl]bis(oxy))bis(acetate) and calcium 2,2'-([1-(4,5-dimethoxy-2-nitrophenyl)ethane-1,2-diyl]bis(oxy)) bis(acetate), in the presence of internal K+ and external Na+. No currents were evoked by UV-triggered Ca2+ jumps in hair cells, whereas exchanger conformational currents up to 400 pA, followed by saturating forward exchange currents up to 40 pA, were recorded in rod photoreceptors subjected to the same experimental conditions. We conclude that no functional electrogenic exchanger is present in this hair cell population, which leaves the abundant plasma membrane Ca2+-ATPases as the primary contributors to Ca2+ extrusion.

Calcium-activated potassium current clamps the dark potential of vertebrate rods

Vertebrate photoreceptors respond to light with a graded hyperpolarization from a membrane potential in the dark of approximately -35 mV. The present work investigates the physiological role of the Ca2+-activated K+ current in the photovoltage generation in mechanically isolated rods from salamander retina. Membrane current or voltage in isolated rods was recorded from light- and dark-adapted rods under voltage- or current-clamp conditions, respectively. In light-adapted rods of the salamander, selective blockade of Ca2+-activated K+ channels by means of charybdotoxin depolarized the plasma membrane of current-clamped rods by approximately 30 mV, from a resting potential of approximately -35 mV. A similar depolarization was observed if external Ca2+ (1 mM) was substituted with Ba2+ or Sr2+. Under control conditions, the injection of currents of increasing amplitude (up to -100 pA, to mimic the current entering the rod outer segment) could not depolarize the membrane potential beyond a saturating value of approximately -20 mV. However, in the presence of charybdotoxin, rods depolarized up to +20 mV. In experiments with dark-adapted current-clamped rods, charybdotoxin perfusion lead to transient depolarizations up to 0 mV and steady-state depolarizations of approximately 5 mV above the dark resting potential. Finally, the recovery phase of the voltage response to a flash of light in the presence of charybdotoxin showed a transient overshoot of the membrane potential. It was concluded that Ca2+-activated K+ current is necessary for clamping the rod photovoltage to values close to the dark potential, thus allowing faithful single photon detection and correct synaptic transmission.

Molecular mechanisms of vertebrate photoreceptor light adaptation

An important recent advance in the understanding of vertebrate photoreceptor light adaptation has come from the discovery that as many as eight distinct molecular mechanisms may be involved, and the realization that one of the principal mechanisms is not dependent on calcium. Quantitative analysis of these mechanisms is providing new insights into the nature of rod photoreceptor light adaptation.