Neurotransmitter release at ribbon synapses in the retina

Transcription

1 Immunology and Cell Biology (2000) 78, Curtin Conference Neurotransmitter release at ribbon synapses in the retina CATHERINE W MORGANS Synaptic Biochemistry Group, Division of Neuroscience, John Curtin School of Medical Research, Canberra, Australian Capital Territory, Australia Summary The synapses of photoreceptors and bipolar cells in the retina are easily identified ultrastructurally by the presence of synaptic ribbons, electron-dense bars perpendicular to the plasma membrane at the active zones, extending about 0.5 µm into the cytoplasm. The neurotransmitter, glutamate, is released continuously (tonically) from these ribbon synapses and the rate of release is modulated in response to graded changes in the membrane potential. This contrasts with action potential-driven bursts of release at conventional synapses. Similar to other synapses, neurotransmitter is released at ribbon synapses by the calcium-dependent exocytosis of synaptic vesicles. Most components of the molecular machinery governing transmitter release are conserved between ribbon and conventional synapses, but a few differences have been identified that may be important determinants of tonic transmitter release. For example, the presynaptic calcium channels of bipolar cells and photoreceptors are different from those elsewhere in the brain. Differences have also been found in the proteins involved in synaptic vesicle recruitment to the active zone and in synaptic vesicle fusion. These differences and others are discussed in terms of their implications for neurotransmitter release from photoreceptors and bipolar cells in the retina. Key words: bipolar cell, calcium, photoreceptor, synapse, synaptic ribbon, synaptic vesicle. Introduction Neurons transmit signals from one to another at specialized contacts called synapses, where a neurotransmitter is secreted by the presynaptic neuron and detected by the post-synaptic neuron. The ribbon synapse is a morphologically and physiologically specialized type of synapse found in certain sensory neurons, including photoreceptors and bipolar cells in the retina and saccular and vestibular hair cells in the inner ear. In electron micrographs, ribbon synapses are easily identified by the presence of an electron-dense bar, the synaptic ribbon, extending approximately µm from the active zone into the cytoplasm. 1,2 This structure is located perpendicular to and immediately above the plasma membrane at the site of synaptic vesicle fusion and is surrounded by a halo of synaptic vesicles that are tethered to the ribbon by short filaments (Fig. 1). What photoreceptors, bipolar cells and hair cells have in common with each other physiologically and what sets them apart from most other neurons is that they are tonically active; that is, they release neurotransmitter continuously. In the brain, transmitter release is triggered by brief (approximately 1 msec) voltage signals called action potentials. The release of transmitter is correspondingly short and the neuron signals in an all or none fashion. In contrast, ribbon synapse-forming neurons do not fire action potentials, but transmit information as graded changes in the membrane potential. Correspondence: Dr Catherine Morgans, Synaptic Biochemistry Group, Division of Neuroscience, John Curtin School of Medical Research, Canberra, ACT 2600, Australia. catherine.morgans@anu.edu.au Received 10 April 2000; accepted 10 April Similar to conventional synapses, neurotransmitter release from ribbon synapses is mediated by the calcium-dependent fusion of transmitter-filled synaptic vesicles with the plasma membrane. Depolarization of the nerve terminal opens voltage-gated calcium channels, allowing calcium ions to flow down their electrochemical gradient into the terminal. Fusion is initiated on the detection of calcium by a synaptic vesicle-associated calcium sensor, synaptotagmin. Following fusion, the synaptic vesicle membrane is retrieved by endocytosis and a transmitter-loaded synaptic vesicle is regenerated within the terminal. The basic molecular machinery governing neurotransmitter release and the synaptic vesicle cycle is conserved between ribbon and conventional synapses. In contrast to action potential-initiated bursts of transmitter release at conventional synapses, transmitter is released continuously at ribbon synapses and the rate of release is modulated in response to graded changes in the membrane potential. Photoreceptors, for example, are depolarized in darkness and experience a continuous influx of calcium into the synaptic terminals through voltage-gated calcium channels, resulting in continuous exocytosis of synaptic vesicles. Detection of photons leads to a hyperpolarization of the plasma membrane, closure of presynaptic calcium channels, reduction of internal calcium and a concomitant pause in neurotransmitter release. 3 Comparative studies of ribbon and conventional synapses in the retina suggest that it is a combination of molecular and structural differences that determines whether a synapse releases neurotransmitter tonically or phasically. Some differences are obvious. For instance, electron micrographs show that the synaptic terminals of photoreceptor and bipolar cells contain a vast number of synaptic vesicles compared with conventional synaptic terminals. Calculations of vesicle numbers based on serial electron micrographs of giant

2 Neurotransmitter release at ribbon synapses 443 Figure 1 An electron micrograph of photoreceptor ribbon synapses in the rat retina. Synaptic ribbons (SR) are indicated with arrowheads. The synaptic ribbons are approximately 0.5 µm long. bipolar cell terminals in the goldfish retina have yielded estimates of between and 1 million. 4 This is far more vesicles than in a conventional terminal and is presumably essential to support tonic transmitter release. The present review will focus on the biochemical differences identified in ribbon synapses compared with conventional synapses in the retina that may be important determinants of tonic transmitter release. The synaptic ribbon Reconstruction of the rod photoreceptor terminal by Rao- Mirotznik et al. has revealed that the synaptic ribbons are horseshoe-shaped, planar structures running above a linear active zone, approximately 2 µm in length. 5 In ultrathin sections, ribbons cut en face are rare; hence, in electron micrographs they usually appear as electron dense bars (Fig. 1). Each face of a ribbon binds 365 synaptic vesicles. The vesicles at the base of the ribbon are docked at the plasma membrane on each side of the active zone. Rod photoreceptors have a single active zone, which is the largest in the central nervous system. Cone photoreceptors and bipolar cells have multiple active zones, but the synaptic ribbons and the length of the active zones are smaller than in the rod. 5 The localization of synaptic ribbons to the active zone and their association with synaptic vesicles suggests that the role of the ribbons may be to capture synaptic vesicles from the cytosol and transport them to the active zone. Perhaps they serve to increase the rate of vesicle docking by functioning analogously to funnels, collecting vesicles from a large volume of the cytosol and targeting them to the linear active zones at the membrane, thereby reducing the occurrence of spurious pauses in the rate of exocytosis. Physiological experiments now provide indirect evidence consistent with this. Capacitance measurements of rates of exocytosis in bipolar cells have indicated the existence of a readily releasable pool of synaptic vesicles equal in size to the total pool of ribbon-bound vesicles in the terminal. 4 Recently, antibody staining of synaptic ribbons in retina sections has led to the identification of some of the protein components of the ribbons. Rim, a putative Rab3-effector protein, is a component of the presynaptic active zone in conventional synapses. 6 Immunoelectron microscopy indicates that, in photoreceptor terminals, Rim is localized exclusively to the synaptic ribbons. 6 The localization of Rim to the synaptic ribbons suggests that some aspects of ribbon function are analogous to those served by the presynaptic density at the active zone of conventional synapses. Rim interacts with synaptic vesicle-associated Rab3-GTP 6 and has been proposed to have a role in the recruitment of vesicles to the presynaptic density in conventional synapses or to the synaptic ribbons in ribbon synapses. KIF3A is a member of the kinesin family of motor proteins that transport vesicles along microtubules, powered by the hydrolysis of ATP. Staining of retina sections with KIF3A antibodies reveals two sites of immunoreactivity within photoreceptors: the connecting cilium and the synaptic ribbons. 7 Both are sites of vesicle transport. The KIF3A antibody stains horseshoe-shaped structures in the photoreceptor terminals typical of synaptic ribbons. 7 Post-embedding immunoelectron microscopy has confirmed that the KIF3A immunoreactivity occurs on the synaptic ribbons. 7 Synaptic vesicles attached to the ribbons or in the close vicinity of the ribbons are also labelled. 7 The localization of KIF3A to synaptic ribbons in the retina suggests that synaptic vesicles may move along the ribbons by active transport. The first specific labelling of synaptic ribbons was produced with the B16 antibody in 1991, but the antigen was undefined. 8 Recently, the B16 antigen has been identified as a retina-specific 54 kda protein that shows homology to the µ chain of an adaptor protein complex of the trans-golgi network. 9 The adaptor protein family (AP-1 to AP-4) mediates the binding of clathrin to vesicles in clathrin-mediated endocytosis at different stages of the secretory pathway. This suggests that the B16 antigen is part of an adaptor complex, possibly mediating the binding of synaptic vesicles to the ribbons. It further raises the possibility that, rather than moving vesicles towards the active zone for exocytosis, the ribbons may be involved in recycling vesicles after exocytosis. Synaptic vesicle-associated proteins Most synaptic vesicle-associated proteins are identical between ribbon and conventional synapses, indicating strict conservation of many stages of the synaptic vesicle cycle. 10,11 Two important differences have been identified, however, that provide clues to understanding the physiological differences between the two types of synapses. Synaptic vesicle recruitment Synapsins I and II are peripheral membrane proteins of synaptic vesicles and are implicated in the recruitment of synaptic vesicles to the active zone in conventional synapses. 12 Synaptic vesicles are clustered at the active zone by crosslinking between the synapsins and the actin-based cytoskeleton, thereby maintaining a reserve pool of vesicles and limiting the releasable pool. It has been proposed that the influx of calcium disrupts the association of synapsin with the actin cytoskeleton, allowing synaptic vesicles to move to the active zone Although present at all conventional synapses, including those in the retina, no immunoreactivity

3 444 CW Morgans with antibodies against either synapsin I or synapsin II has been detected at ribbon synapses of photoreceptor or bipolar cells in the retina. 15,16 Expression of synapsin I as a transgene in the rod photoreceptors of mice does not result in any measurable physiological, developmental or morphological changes compared to wild-type. 17 Physiological studies of synaptic transmission at conventional synapses in synapsin knock-out mice have indicated that synapsin is required for the tight temporal control and reproducibility of action potential-triggered transmitter release. 12,13 Clearly, this function is not necessary for tonic release at ribbon synapses. Instead, the main requirements of the synaptic vesicle recruitment mechanism in photoreceptors and bipolar cells are reliability and efficiency, to ensure that spurious pauses in synaptic vesicle exocytosis do not occur. Physiological and anatomical evidence supports the idea that the synaptic ribbons fulfil this role. Synaptic vesicle fusion Synaptic vesicle fusion with the plasma membrane is catalyzed by a complex of three proteins, termed soluble N- ethylmaleimide-sensitive factor attachment protein receptors (SNARE). The SNARE proteins comprise gene families, different members of which have been implicated in specific membrane fusion events in the secretory pathway ubiquitous to all cells. 18 The neuron-specific SNAREs synaptobrevin, an integral membrane protein of synaptic vesicles, and syntaxin 1 and SNAP-25 (synaptosomal associated protein of 25 kda), both on the presynaptic plasma membrane, are specifically required for synaptic vesicle exocytosis. Despite the absolute requirement for these proteins in the brain, syntaxin 1 is absent from ribbon synapses in the retina. 19 Photoreceptors and bipolar cells in the retina have been shown to express a different member of the syntaxin gene family, syntaxin Syntaxin 3 can be isolated from solubilized retinal membranes in a complex with SNAP-25, synaptobrevin and complexin, proving that it is capable of interacting with the other neuronal SNAREs similarly to syntaxin Immunostaining of retina sections for syntaxin 3 results in strong staining of the photoreceptor terminals as well as staining of the bipolar cell terminals, confirming the presence of syntaxin 3 in retinal ribbon synapses. 20 In conventional synapses, syntaxin 1 is the principal target for regulation of the synaptic vesicle fusion reaction and also appears to be important in setting the calcium sensitivity of transmitter release through its interactions with the synaptic vesicle calcium sensor, synaptotagmin, and the presynaptic calcium channels. The identification of syntaxin 3 rather than syntaxin 1 in ribbon synapses raises interesting questions about the relative calcium sensitivity and regulation of transmitter release in ribbon versus conventional synapses. Measurements of synaptic vesicle exocytosis in goldfish photoreceptors and bipolar cells suggest that much lower calcium concentrations are sufficient to maintain glutamate release at ribbon synapses than are required for transmitter release at conventional synapses. 21,22 It is possible that the calcium sensitivity is in part determined by the type of syntaxin expressed. It will be interesting to determine, therefore, if there is a difference in calcium dependence of binding of synaptotagmin to syntaxin 1 versus syntaxin 3. Calcium influx and extrusion in photoreceptor and bipolar cell terminals Presynaptic calcium channels at retinal ribbon synapses In both ribbon and conventional synapses, calcium ions enter the nerve terminal through voltage-gated calcium channels: electrically sensitive, pore-forming proteins in the plasma membrane that open in response to depolarization. The electrophysiological properties of the channels, their interactions with the synaptic vesicle fusion proteins and their distribution in the nerve terminal strongly influence the time course and probability of transmitter release. The physiological and pharmacological properties of the calcium channels mediating neurotransmitter release from photoreceptors and bipolar cells differ from those in action potential-driven, conventional neurons. Voltage-activated calcium channels are heteromeric protein complexes containing a principal poreforming α 1 subunit in association with a β subunit and a disulfide-linked α2/δ subunit. It has been shown that α 1A and α 1B subunits give rise to N- and P/Q-type channels, respectively, which show differential sensitivity to ω-conotoxins; 23,24 α 1C and α 1D subunits give rise to L-type channels, which are sensitive to dihydropyridines (DHP); 25 and the α 1E subunit is found in toxin-resistant R-type channels. 26 Typically, the presynaptic calcium channels at conventional synapses are N, P, Q or R type and inactivate in response to sustained depolarization. Photoreceptor calcium channels, in contrast, are non-inactivating, a property essential to sustaining tonic transmitter release. 27,28 The calcium currents recorded from photoreceptors and bipolar cells in amphibian and fish retinas are sensitive to DHP and are thus classified as L type Photoreceptor and bipolar cell calcium currents in the mammalian retina exhibit a lower activation threshold and a significantly lower sensitivity to DHP than those in fish and amphibian retinas. 27,28,32 Thus, the mammalian calcium channels are likely to be somewhat different. Recently, the gene for the α 1 subunit of a retina-specific calcium channel, α 1F, was identified. 34,35 Sequence comparisons indicate that α 1F belongs to the L-type calcium channel family. Mutations in this gene cause incomplete X-linked congenital stationary night blindness (CSNB2), the electrophysiological and psychophysical features of which are consistent with a defect in neurotransmission from rod photoreceptors. 34,35 Antibodies raised against a human α 1F peptide give a synaptic staining pattern in the rat retina. 36 In the outer plexiform layer, which contains the photoreceptor synapses, the α 1F antibody labels horseshoe-shaped structures reminiscent of rod active zones as defined by the synaptic ribbons. Thus, an α 1F -like channel may mediate glutamate release from rod photoreceptors in the rat retina. The plasma membrane calcium ATPase Tonic glutamate release from photoreceptors and bipolar cells is sustained by non-inactivating calcium channels that remain open during prolonged depolarizations. 27 A fundamental problem faced by these cells is to maintain a high calcium concentration at the sites of synaptic vesicle fusion and at the same time to keep calcium levels elsewhere in the cell low. The continuous influx of calcium into photoreceptor terminals in darkness implies the existence of a calcium

4 Neurotransmitter release at ribbon synapses 445 localized along the sides and neck of the terminals. 37 In darkness, when photoreceptors are tonically depolarized, this segregation of the sites of calcium influx and extrusion could give rise to a standing calcium gradient within the terminal, as illustrated in Fig. 2. Figure 2 Diagram of the synaptic terminal of a cone photoreceptor showing segregation of the sites of calcium influx (voltage-gated calcium channels) and efflux (plasma membrane calcium ATPase). In darkness, when the photoreceptor is tonically depolarized, a standing calcium gradient may be generated within the terminal. As depicted by a fusing synaptic vesicle, glutamate release occurs from the base of the terminal where the calcium channels are located and the calcium concentration is highest. extrusion mechanism that balances the influx through the channels. The two known mechanisms for the extrusion of intracellular calcium are the plasma membrane calcium ATPase (PMCA) and the Na + /Ca 2+ exchanger. Immunostaining of rat retina sections with antibodies against these two proteins shows intense staining of the photoreceptor terminals in the outer plexiform layer with the PMCA antibody. 37 In addition, weaker PMCA staining is observed in the inner plexiform layer, consistent with expression in bipolar cell terminals. In contrast, very little if any Na + /Ca 2+ exchanger immunoreactivity is observed in photoreceptors, and in the inner retina it appears confined to amacrine cells. Calcium extrusion has been measured by patch-clamp recordings of a calcium-activated chloride tail current in cone photoreceptors in the tree shrew retina. It has been found that inhibition of the PMCA with orthovanadate or ATP depletion potently blocks calcium extrusion, while lithium, a blocker of the Na + /Ca 2+ exchanger, has no effect. 37 Together, the immunohistochemical and electrophysiological data strongly indicate that calcium extrusion from photoreceptors is mediated by the PMCA. Morgans et al. have further shown that the PMCA and the calcium channels are differentially distributed within cone photoreceptor terminals. 37 Presynaptic calcium channels in the tree shrew retina have been labelled with an antibody against the α 1D subunit of brain L-type calcium channels. 27 The calcium channels are distributed at the base of the terminals where the active zones are located and the PMCA is Conclusion The biochemical and structural properties of ribbon synapses allow them to release neurotransmitter continuously and to respond to graded changes in membrane potential with graded changes in the rate of glutamate release. Comparative studies of photoreceptor and bipolar cell ribbon synapses in the retina compared with conventional synapses have led to the identification of a number of differences in the presynaptic proteins of the two types of synapses. In particular, differences have been identified in the presynaptic calcium channels, the synaptic vesicle fusion complex (syntaxin 1 in conventional synapses and syntaxin 3 in ribbon synapses) and the mechanism of synaptic vesicle recruitment to the active zone (absence of synapsins and the presence of synaptic ribbons at ribbon synapses). These differences are likely to be important determinants of graded versus all-or-none neuronal signalling. References 1 Sjöstrand FS. The ultrastructure of the retinal rod synapses of the guinea pig eye. J. Appl. Physiol. 1953; 24: Sjöstrand FS. Ultrastructure of retinal rod synapses of the guinea pig eye as revealed by three-dimensional reconstructions from serial sections. J. Ultrastruct. 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