Distribution of actin in cone photoreceptor synapses

The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

The actin cytoskeleton in whole mount preparations and sections

In non-muscle cells, the actin cytoskeleton plays a key role by providing a scaffold contributing to the definition of cell shape, force for driving cell motility, cytokinesis, endocytosis, and propulsion of pathogens, as well as tracks for intracellular transport. A thorough understanding of these processes requires insight into the spatial and temporal organisation of actin filaments into diverse higher-order structures, such as networks, parallel bundles, and contractile arrays. Transmission and scanning electron microscopy can be used to visualise the actin cytoskeleton, but due to the delicate nature of actin filaments, they are easily affected by standard preparation protocols, yielding variable degrees of ultrastructural preservation. In this chapter, we describe different conventional and cryo-approaches to visualise the actin cytoskeleton using transmission electron microscopy and discuss their specific advantages and drawbacks. In the first part, we present three different whole mount techniques, which allow visualisation of actin in the peripheral, thinly spread parts of cells grown in monolayers. In the second part, we describe specific issues concerning the visualisation of actin in thin sections. Techniques for three-dimensional visualisation of actin, protein localisation, and correlative light and electron microscopy are also included.

The expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina

In the present study, we generated a systematic overview of the expression pattern and assembly profile of synaptic membrane proteins in ribbon synapses of the developing mouse retina. Using indirect immunofluorescence microscopy, we analyzed the spatial and temporal distribution of 11 important membrane and membrane-associated synaptic proteins (syntaxin 1/3, SNAP-25, synaptobrevin 2, synaptogyrin, synaptotagmin I, SV2A, SV2B, Rab3A, clathrin light chains, CSP and neuroligin I) during synaptogenesis. The temporospatial distribution of these synaptic proteins was "normalized" by the simultaneous visualization of the synaptic vesicle protein synaptophysin, which served as an internal reference protein. We found that expression of various synaptic membrane proteins started at different time points and changed progressively during development. At early stages of development synaptic vesicle membrane proteins at extrasynaptic locations did not always colocalize with synaptophysin, indicating that these proteins probably do not reside in the same transport vesicles. Despite a non-synchronized onset of protein expression, clustering and colocalization of all synaptic membrane proteins at ribbon synapses roughly occurred in the same time window (between day 4 after birth, P4, and P5). Thus, the basic synaptic membrane machinery is already present in ribbon synapses before the well-known complete morphological maturation of ribbon synapses between P7 and P12. We conclude that ribbon synapse formation is a multistep process in which the concerted recruitment of synaptic membrane proteins is a relatively early event and clearly not the final step.

Synapse formation is arrested in retinal photoreceptors of the zebrafish nrc mutant

We describe the effects of a recessive mutation on visual behavior, the electroretinogram (ERG), and photoreceptor structure in zebrafish. At 6 d post-fertilization (dpf), no optokinetic reflex could be elicited in no optokinetic response c (nrc) mutant animals under any test condition. The animals exhibited ERG responses at 5-7 dpf that were markedly abnormal and could be categorized into two groups. The first showed an initial negative a-wave followed by a delayed positive b-wave of small amplitude. Often a second ERG-like response was recorded after the initial b-wave. The second group showed only a large negative a-wave; an initial b-wave was not evident. In most recordings additional oscillatory waves varying in number, amplitude, and time course were observed. Multiple responses at the cessation of long-duration flashes were also observed. Light and electron microscopy revealed that the cone photoreceptor pedicles of nrc fish were highly abnormal. Although the appropriate number of synaptic ribbons formed in these terminals, they "floated" in the terminal, unassociated with postsynaptic processes or arciform densities. The few processes invaginating the nrc pedicles resembled those of horizontal cells. Invaginating bipolar cell processes were rare, but basal contacts were observed on pedicle surfaces. The severity of the mutation did not change between 6 and 8 dpf, showing that there is neither a delay in development nor a degeneration of the terminals; rather, nrc pedicle development appears arrested. Bipolar cell terminals in the inner plexiform layer made normal ribbon synapses; thus, the mutation appears to affect only the outer retina.

RIBEYE, a component of synaptic ribbons: a protein's journey through evolution provides insight into synaptic ribbon function

Photoreceptor cells utilize ribbon synapses to transmit sensory signals at high resolution. Ribbon synapses release neurotransmitters tonically, with a high release rate made possible by continuous docking of synaptic vesicles on presynaptic ribbons. We have partially purified synaptic ribbons from retina and identified a major protein component called RIBEYE. RIBEYE is composed of a unique A domain specific for ribbons, and a B domain identical with CtBP2, a transcriptional repressor that in turn is related to 2-hydroxyacid dehydrogenases. The A domain mediates assembly of RIBEYE into large structures, whereas the B domain binds NAD(+) with high affinity, similar to 2-hydroxyacid dehydrogenases. Our results define a unique component of synaptic ribbons and suggest that RIBEYE evolved in vertebrates under utilization of a preexisting protein to build a unique scaffold for a specialized synapse.

Dystrophin in the retina

Dystrophin is a plasma membrane-associated cytoskeletal protein of the spectrin superfamily. The dystrophin cytoskeleton has been first characterized in muscle. Muscular 427 kDa dystrophin binds to subplasmalemmal actin filaments via its amino-terminal domain. The carboxy-terminus of dystrophin binds to a plasma membrane anchor, beta-dystroglycan, which is associated on the external side with the extracellular matrix receptor, alpha-dystroglycan, that binds to the basal lamina proteins laminin-1, laminin-2, and agrin. In the muscle, the dystroglycan complex is associated with the sarcoglycan complex that consists of several glycosylated, integral membrane proteins. The absence or functional deficiency of the dystrophin cytoskeleton is the cause of several types of muscular dystrophies including the lethal Duchenne muscular dystrophy (DMD), one of the most severe and most common genetic disorders of man. The dystrophin complex is believed to stabilize the plasma membrane during cycles of contraction and relaxation. Muscular dystrophin and several types of dystrophin variants are also present in extramuscular tissues, e.g. in distinct regions of the central nervous systems including the retina. Absence of dystrophin from these sites is believed to be responsible for some extramuscular symptoms of DMD, e.g. mental retardation and disturbances in retinal electrophysiology (reduced b-wave in electroretinograms). The reduced b-wave in electroretinograms indicated a disturbance of neurotransmission between photoreceptors and ON-bipolar cells. At least two different dystrophin variants are present in photoreceptor synaptic complexes. One of these dystrophins (Dp260) is virtually exclusively expressed in the retina. In the neuroretina, dystrophin is found in significant amounts in the invaginated photoreceptor synaptic complexes. At this location dystrophin colocalizes with dystroglycan. Agrin, an extracellular ligand of alpha-dystroglycan, is also present at this location whereas the proteins of the sarcoglycan complex appear to be absent in photoreceptor synaptic complexes. Dystrophin and dystroglycan are located distal from the ribbon-containing active synaptic zones where both proteins are restricted to the photoreceptor plasma membrane bordering on the lateral sides of the synaptic invagination. In addition, some neuronal profiles of the postsynaptic complex also contain dystrophin and beta-dystroglycan. These profiles appear to belong at least in part to projections of the photoreceptor terminals into the postsynaptic dendritic complex. In view of the abnormal neurotransmission between photoreceptors and ON-bipolar cells in DMD patients the dystrophin/beta-dystroglycan-containing projections of photoreceptor presynaptic terminals into the postsynaptic dendritic plexus might somehow modify the ON-bipolar pathway. Another retinal site associated with dystrophin/beta-dystropglycan is the plasma membrane of Müller cells where dystrophin/beta-dystroglycan appear to be present at particular high concentrations. At this location the dystrophin/dystroglycan complex may play a role in the attachment of the retina to the vitreous, and, under pathological conditions, in traction-induced retinal detachment.

Colocalization of retinal dystrophin and actin in postsynaptic dendrites of rod and cone photoreceptor synapses

In this paper we demonstrate immunostaining specific for dystrophin in photoreceptor synapses of human, bovine and rat retinas. Cryosections of retinas incubated with dystrophin-specific monoclonal antibodies displayed a punctuate staining pattern in the outer plexiform layer. This pattern resulted from binding of the antibodies to synaptic complexes of both rods and cones, shown by double-labelling with antibodies to either synaptophysin or actin. Confocal laser fluorescence microscopy demonstrated that dystrophin staining colocalized predominantly with actin, which is concentrated in the postsynaptic portions of the synaptic complex. No significant dystrophin immunolabel was seen in the presynaptic terminals labelled with antibodies to synaptophysin, a marker of synaptic vesicles. Immunoblot analysis confirmed the presence of approximately 420 kDa and approximately 360 kDa dystrophin-like polypeptide bands associated with membranes of the bovine retina. We speculate that retinal dystrophin is involved in the linkage of actin filaments to the postsynaptic plasma membrane. Such a linkage may be important for the generation of synaptic microdomains and for certain phenomena of synaptic plasticity. The absence of dystrophin in patients suffering from Duchenne's muscular dystrophy is accompanied by visual problems and abnormalities of the electroretinogram. Therefore it is likely that retinal dystrophin plays a role in certain stages of synaptic transmission between photoreceptors and the postsynaptic dendritic complex formed by horizontal and bipolar cells.