Ultrastructural obsevations on synaptic ribbons in the pineal organ of the goldfish

Rabconnectin-3α/DMXL2 Is Locally Enriched at the Synaptic Ribbon of Rod Photoreceptor Synapses

Ribbon synapses reliably transmit synaptic signals over a broad signalling range. Rod photoreceptor ribbon synapses are capable of transmitting signals generated by the absorption of single photons. The high precision of ribbon synapses emphasizes the need for particularly efficient signalling mechanisms. Synaptic ribbons are presynaptic specializations of ribbon synapses and are anchored to the active zone. Synaptic ribbons bind many synaptic vesicles that are delivered to the active zone for continuous and faithful signalling. In the present study we demonstrate with independent antibodies at the light- and electron microscopic level that rabconnectin-3α (RC3α)-alternative name Dmx-like 2 (DMXL2)-is localized to the synaptic ribbons of rod photoreceptor synapses in the mouse retina. In the brain, RC3α-containing complexes are known to interact with important components of synaptic vesicles, including Rab3-activating/inactivating enzymes, priming proteins and the vesicular H⁺-ATPase that acidifies the synaptic vesicle lumen to promote full neurotransmitter loading. The association of RC3α/DMXL2 with rod synaptic ribbons of the mouse retina could enable these structures to deliver only fully signalling-competent synaptic vesicles to the active zone thus contributing to reliable synaptic communication.

Novel subcellular localization for α-synuclein: possible functional consequences

α-synuclein (α-syn) is one of the genes that when mutated or overexpressed causes Parkinson’s Disease (PD). Initially, it was described as a synaptic terminal protein and later was found to be localized at mitochondria. Mitochondria-associated membranes (MAM) have emerged as a central endoplasmic reticulum (ER) subcellular compartments where key functions of the cell occur. These domains, enriched in cholesterol and anionic phospholipids, are where calcium homeostasis, lipid transfer, and cholesterol metabolism are regulated. Some proteins, related to mitochondrial dynamics and function, are also localized to this area. Several neurodegenerative diseases have shown alterations in MAM functions and resident proteins, including Charcot Marie-Tooth and Alzheimer’s disease (AD). We have recently reported that MAM function is downregulated in cell and mouse models of PD expressing pathogenic mutations of α-syn. This review focuses on the possible role of α-syn in these cellular domains and the early pathogenic features of PD that could be explained by α-syn-MAM disturbances.

α-Synuclein is localized to mitochondria-associated ER membranes

Familial Parkinson disease is associated with mutations in α-synuclein (α-syn), a presynaptic protein that has been localized not only to the cytosol, but also to mitochondria. We report here that wild-type α-syn from cell lines, and brain tissue from humans and mice, is present not in mitochondria but rather in mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a structurally and functionally distinct subdomain of the ER. Remarkably, we found that pathogenic point mutations in human α-syn result in its reduced association with MAM, coincident with a lower degree of apposition of ER with mitochondria, a decrease in MAM function, and an increase in mitochondrial fragmentation compared with wild-type. Although overexpression of wild-type α-syn in mutant α-syn-expressing cells reverted the fragmentation phenotype, neither overexpression of the mitochondrial fusion/MAM-tethering protein MFN2 nor inhibition/ablation of the mitochondrial fission protein DRP1 was able to do so, implying that α-syn operates downstream of the mitochondrial fusion/fission machinery. These novel results indicate that wild-type α-syn localizes to the MAM and modulates mitochondrial morphology, and that these behaviors are impaired by pathogenic mutations in α-syn. We believe that our results have far-reaching implications for both our understanding of α-syn biology and the treatment of synucleinopathies.

Pinealocyte synaptic ribbons and neuroendocrine function

A comparative study of pinealocyte synaptic ribbons (SR) revealed two predominant populations exhibiting either a rod/ribbon shape (SRr) or a spherical/punctate shape (SRsp). Species-specific differences were found in the abundance of SR, the ratio of SRr/SRsp, and the occurrence of SR in ribbon fields. The close topographical relationship of SR to the plasma membrane and the numerical changes that occurred with changes in metabolism of the pinealocytes suggest that SR have important vesicle-mediated interactions with the cell membrane. Experiments designed to clarify the relationship between SR and pineal neuroendocrine function revealed a positive correlation between SR numbers and indole intermediates during pineal development in the rat, and increased SR frequency after denervation of the rat pineal gland or administration of the beta-adrenergic agonist, isoproterenol. These data are consistent with the hypothesis that SR function is linked to receptor mechanisms regulating indoleamine production in the pineal gland.

Ultrastructural study of the cellular types in the pineal organ of Gambusia affinis (teleost)

The pineal organ of Gambusia affinis was studied via light and electron microscopy. The cell types studied included photoreceptor cells, supporting cells, and a third cell type. The photoreceptor cells, which appear to form clusters, are divided into four regions: outer segment, inner segment, cell soma, and synaptic pedicle. Synaptic ribbons are commonly observed in the synaptic pedicle. The supporting cells separate the photoreceptor cells from the thick basal lamina that surrounds the entire pineal organ. The supporting cells show highly organized membrane formations, some lipid-like inclusions, and a diplosome. One of the centrioles gives rise to an invaginated cilium. The third cell type is observed infrequently and appears to be located mainly in the vicinity of the outer segments. The morphological characteristics of this cell type are similar to those of phagocytic cells. The ultrastructural features of the pineal organ of G. affinis are compared with those of other teleosts.

Day-night differences in the number and structure of synaptic ribbons in chick pineal

The objective of this study was to evaluate the effect of lighting (day-night) changes on pinealocyte synaptic ribbon shape (conformation) and numbers. Three-dimensional reconstruction analysis of pinealocyte basal processes revealed that 30% (6/20) of all ribbons from dark-adapted animals were either curved or split. Synaptic ribbons from light-adapted animals did not show this variant morphology; all were linear structures. An analysis of each section from each series, containing curved or split ribbons, revealed that 44% (16/36) of all ribbon profiles would yield inflated counts if used in random morphometric sampling protocols. Therefore, split and curved (variant) ribbon morphologies could result in an overestimation of synaptic ribbon populations of approximately 13% (0.44 X 30%) of dark-adapted samples. In spite of this potential sampling error, the fourfold increase in the number of synaptic ribbons observed during the dark phase of a light-dark cycle remains highly significant (P less than 0.0001).

Responses of synaptic ribbons in pineal photoreceptors under normal and experimental lighting conditions

The size of synaptic ribbons (SR) in photoreceptor cells of the goldfish pineal organ was quantified over 24-h light:dark cycles of long (16:8) and short (10:14) photoperiods during summer and winter months, respectively. The amplitude of both rhythms was similar with peak values occurring toward the latter part of the photophase or early dark. When fish were entrained to the long photoperiod and exposed to continual light, SR size continued to increase during the expected dark time. The effect of extending the photoperiod into the expected dark time was diminished when fish were entrained to a short photoperiod and presented with 6 h of darkness at the end of the 24-h period. The size increase in response to environmental lighting is believed to reflect a greater demand for either vesicle attachment sites or neurotransmitter storage sites since vesicles (neurotransmitter) have been hypothesized to accumulate in the synaptic pedicles during inhibition by light. From a comparative standpoint it is noteworthy that synaptic ribbons (vesicle-crowned rods) in mammals react in a similar manner to both normal and experimental lighting conditions.

Inverse correlation between "synaptic" ribbon number and the density of adrenergic nerve endings in the pineal gland of various mammals

The number of "synaptic" ribbons was inversely correlated with the density of the adrenergic nerve endings of the pineal gland compared among a diverse group of species including the fox, cat, rat, cotton rat, white-footed mouse, Djungarian hamster, ground squirrel, and chipmunk. The concentration of norepinephrine paralleled the number of adrenergic nerve terminals in the pineal glands of the cotton rat, rat, and ground squirrel, the only species in which norepinephrine concentrations were measured. The number of ribbon fields paralleled numbers of "synaptic" ribbons in all species examined. Adrenergic nerve endings were observed primarily within the perivascular spaces, although some endings also were found among parenchymal cells. Adrenergic nerve endings forming synaptic junctions with pinealocytes were not observed in any of these species, nor was there any physical association between these nerve endings and "synaptic" ribbons.

The effects of constant light and constant darkness on daily changes in the morphology of the pineal organ in the goldfish, Carassius auratus

The fine structure of photoreceptor cells in the pineal organ of the goldfish was found to vary quantitatively over a 24-hour period. Stereological analysis revealed significant daily changes in the volume of the cell and inner segment, nuclear volume and nucleolar diameter, volume of endoplasmic reticulum and Golgi bodies, area of both rough and smooth endoplasmic reticulum, and number of vesicles associated with each Golgi body. Peak values of these variables occurred either during the dark phase or latter part of the light phase. These findings agree closely with those reported in higher vertebrates, and suggest that metabolic activities, and possible secretory functions, of the pineal organ of fishes are synchronized to the light:dark cycle. Daily changes in these variables generally persisted in fish exposed to constant darkness for seven days, with the peak in these rhythms coinciding closely with those observed in fish exposed to a light:dark cycle. In contrast, the rhythms in all variables were abolished in fish kept in continual light for seven days. Photoreceptor cells from fish exposed to continuous light had larger nucleoli and greater amounts of rough endoplasmic reticulum indicating a further effect of light on pineal metabolism in lower vertebrates.

Age-related changes in pineal "synaptic" ribbon populations in rats exposed to continuous light or darkness

"Synaptic" ribbon (SR) populations in the rat pineal gland were analyzed morphometrically at various ages from birth to early adulthood. The rats were born and raised in continuous light or continuous darkness. SR numbers were high irrespective of the presence or absence of light during the early neonatal period. However, SR numbers decreased substantially during the second neonatal week of continuous light, remaining low at subsequent ages, including early adulthood. In contrast, SR numbers apparently remained elevated during continuous darkness at various ages from the early neonatal period to early adulthood. Stimulation of pineal adrenergic receptors with L-isoproterenol during continuous light resulted in an acute, reversible increase in SR numbers; blockade of these receptors with L-propranolol during continuous darkness resulted in an acute decrease in SR numbers. These results indicate that light-mediated changes in SR populations were 1) age related and 2) related to some component of the pineal beta-adrenergic receptor mechanism. A hypothesis that SR formation may be related to beta-adrenergic receptor-associated membrane turnover is presented.