Cell-type-specific localization of protocadherin β16 at AMPA and AMPA/Kainate receptor-containing synapses in the primate retina

Identification of rare genetic variants in the PCDH genetic family in a cohort of transgender women

OBJECTIVE

To study the identification of rare genetic variants in the PCDH genetic family in a cohort of transgender women (TGW) and their potential role in gender identity.

DESIGN

Exome sequencing and functional ontology analysis.

SETTING

Outpatient gender health and reproductive endocrinology clinics.

PATIENT(S)

A total of 24 TGW and 22 cisgender men (CM).

INTERVENTION(S)

Exome sequencing followed by variant confirmation through Sanger sequencing and functional classification analysis using the Database for Annotation, Visualization, and Integrated Discovery tool.

MAIN OUTCOME MEASURE(S)

Identification of rare, functionally significant genetic variants in the PCDH gene family and their prevalence in TGW compared with CM.

RESULT(S)

Exome sequencing revealed 38,524 genetic variants, of which 2,441 were rare and predicted to be functionally significant. The Database for Annotation, Visualization, and Integrated Discovery analysis demonstrated a statistically enriched functional group, "homophilic cell adhesion via plasma membrane adhesion molecules," containing 55 genes, including 18 PCDH gene family members. A total of 37 rare variants in 21 PCDH genes were identified, with 36 confirmed using Sanger sequencing. A statistically significant increase in these variants was observed in TGW compared with CM (Z = 2.08905).

CONCLUSION(S)

Transgender women exhibited a greater than threefold increase in functionally significant PCDH gene variants compared with CM. These findings suggest that the PCDH family may play a role in the genetic pathways associated with gender identity in TGW.

Neurochemistry and circuit organization of the lateral spiriform nucleus of birds: A uniquely nonmammalian direct pathway component of the basal ganglia

We used diverse methods to characterize the role of avian lateral spiriform nucleus (SpL) in basal ganglia motor function. Connectivity analysis showed that SpL receives input from globus pallidus (GP), and the intrapeduncular nucleus (INP) located ventromedial to GP, whose neurons express numerous striatal markers. SpL-projecting GP neurons were large and aspiny, while SpL-projecting INP neurons were medium sized and spiny. Connectivity analysis further showed that SpL receives inputs from subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr), and that the SNr also receives inputs from GP, INP, and STN. Neurochemical analysis showed that SpL neurons express ENK, GAD, and a variety of pallidal neuron markers, and receive GABAergic terminals, some of which also contain DARPP32, consistent with GP pallidal and INP striatal inputs. Connectivity and neurochemical analysis showed that the SpL input to tectum prominently ends on GABAA receptor-enriched tectobulbar neurons. Behavioral studies showed that lesions of SpL impair visuomotor behaviors involving tracking and pecking moving targets. Our results suggest that SpL modulates brainstem-projecting tectobulbar neurons in a manner comparable to the demonstrated influence of GP internus on motor thalamus and of SNr on tectobulbar neurons in mammals. Given published data in amphibians and reptiles, it seems likely the SpL circuit represents a major direct pathway-type circuit by which the basal ganglia exerts its motor influence in nonmammalian tetrapods. The present studies also show that avian striatum is divided into three spatially segregated territories with differing connectivity, a medial striato-nigral territory, a dorsolateral striato-GP territory, and the ventrolateral INP motor territory.

Clustered Protocadherins Emerge as Novel Susceptibility Loci for Mental Disorders

The clustered protocadherins (cPcdhs) are a subfamily of type I single-pass transmembrane cell adhesion molecules predominantly expressed in the brain. Their stochastic and combinatorial expression patterns encode highly diverse neural identity codes which are central for neuronal self-avoidance and non-self discrimination in brain circuit formation. In this review, we first briefly outline mechanisms for generating a tremendous diversity of cPcdh cell-surface assemblies. We then summarize the biological functions of cPcdhs in a wide variety of neurodevelopmental processes, such as neuronal migration and survival, dendritic arborization and self-avoidance, axonal tiling and even spacing, and synaptogenesis. We focus on genetic, epigenetic, and 3D genomic dysregulations of cPcdhs that are associated with various neuropsychiatric and neurodevelopmental diseases. A deeper understanding of regulatory mechanisms and physiological functions of cPcdhs should provide significant insights into the pathogenesis of mental disorders and facilitate development of novel diagnostic and therapeutic strategies.

Protocadherins at the Crossroad of Signaling Pathways

Protocadherins (Pcdhs) are cell adhesion molecules that belong to the cadherin superfamily, and are subdivided into clustered (cPcdhs) and non-clustered Pcdhs (ncPcdhs) in vertebrates. In this review, we summarize their discovery, expression mechanisms, and roles in neuronal development and cancer, thereby highlighting the context-dependent nature of their actions. We furthermore provide an extensive overview of current structural knowledge, and its implications concerning extracellular interactions between cPcdhs, ncPcdhs, and classical cadherins. Next, we survey the known molecular action mechanisms of Pcdhs, emphasizing the regulatory functions of proteolytic processing and domain shedding. In addition, we outline the importance of Pcdh intracellular domains in the regulation of downstream signaling cascades, and we describe putative Pcdh interactions with intracellular molecules including components of the WAVE complex, the Wnt pathway, and apoptotic cascades. Our overview combines molecular interaction data from different contexts, such as neural development and cancer. This comprehensive approach reveals potential common Pcdh signaling hubs, and points out future directions for research. Functional studies of such key factors within the context of neural development might yield innovative insights into the molecular etiology of Pcdh-related neurodevelopmental disorders.

Co-expression of glutamatergic and autism-related genes in the hippocampus of male mice with disturbances of social behavior

В настоящее время существует представление о вовлеченности глутаматергической системы (ГГ) в механизмы развития аутизма. В предыдущих исследованиях нами было показано, что негативный социальный опыт, приобретенный в ежедневных межсамцовых конфронтациях, приводит к нарушениям в социальном поведении: снижению коммуникативности, нарушению социализации, появлению стереотипных форм поведения, которые могут рассматриваться как симптомы аутистического спектра. В связи с этим целью нашей работы было изучение с помощью транскриптомного анализа изменений экспрессии генов, кодирующих белки, вовлеченные в функционирование глутаматергической системы, и генов, связанных с патологией аутизма (ГА), в гиппокампе. В эксперименте использовали животных с нарушениями социального поведения, вызванными повторным опытом социальных побед или поражений в ежедневных агонистических взаимодействиях. Для формирования групп животных с контрастными типами поведения использовали модель сенсорного контакта (хронического социального стресса). Полученные образцы мозга были секвенированы в ЗАО «Геноаналитика» (http://genoanalytica.ru/, Россия, Москва). Транскриптомный анализ показал, что у агрессивных животных снижается экспрессия генов Shank3, Auts2, Ctnnd2, Nrxn2, для которых показано участие в развитии аутизма, а также глутаматергического гена Grm4. В то же время у животных с негативным социальным опытом экспрессия ГА Shank2, Nlgn2, Ptcdh10, Reln, Arx возрастает. При этом ГГ (Grik3, Grm2, Grm4, Slc17a7, Slc1a4, Slc25a22), за исключением гена Grin2a, повышают свою экспрессию. Корреляционный анализ выявил статистически значимую взаимосвязь из- мененной экспрессии ГГ и ГА. Полученные результаты, с одной стороны, могут служить подтверждением участия ГГ в патофизиологии развития симптомов аутистического спектра, с другой – свидетельствовать о коэкспрессии ГГ и ГА в гиппокампе, развивающейся под влиянием социальной среды. Так как большинство ГА, изменивших экспрессию в настоящем исследовании, являются генами, связанными с клеточным скелетом и внеклеточным матриксом, в частности участвующими в формировании синапсов, а ГГ, изменившие свою экспрессию, – генами, кодирующими субъединицы рецепторов, то можно предположить, что вовлечение ГГ в патофизиологию аутизма происходит на уровне рецепторов.

Regulation of neural circuit formation by protocadherins

The protocadherins (Pcdhs), which make up the most diverse group within the cadherin superfamily, were first discovered in the early 1990s. Data implicating the Pcdhs, including ~60 proteins encoded by the tandem Pcdha, Pcdhb, and Pcdhg gene clusters and another ~10 non-clustered Pcdhs, in the regulation of neural development have continually accumulated, with a significant expansion of the field over the past decade. Here, we review the many roles played by clustered and non-clustered Pcdhs in multiple steps important for the formation and function of neural circuits, including dendrite arborization, axon outgrowth and targeting, synaptogenesis, and synapse elimination. We further discuss studies implicating mutation or epigenetic dysregulation of Pcdh genes in a variety of human neurodevelopmental and neurological disorders. With recent structural modeling of Pcdh proteins, the prospects for uncovering molecular mechanisms of Pcdh extracellular and intracellular interactions, and their role in normal and disrupted neural circuit formation, are bright.

Regulation of Wnt signaling by protocadherins

The ∼70 protocadherins comprise the largest group within the cadherin superfamily. Their diversity, the complexity of the mechanisms through which their genes are regulated, and their many critical functions in nervous system development have engendered a growing interest in elucidating the intracellular signaling pathways through which they act. Recently, multiple protocadherins across several subfamilies have been implicated as modulators of Wnt signaling pathways, and through this as potential tumor suppressors. Here, we review the extant data on the regulation by protocadherins of Wnt signaling pathways and components, and highlight some key unanswered questions that could shape future research.

Horizontal Cells, the Odd Ones Out in the Retina, Give Insights into Development and Disease

Thorough investigation of a neuronal population can help reveal key aspects regarding the nervous system and its development. The retinal horizontal cells have several extraordinary features making them particularly interesting for addressing questions regarding fate assignment and subtype specification. In this review we discuss and summarize data concerning the formation and diversity of horizontal cells, how morphology is correlated to molecular markers, and how fate assignment separates the horizontal lineage from the lineages of other retinal cell types. We discuss the novel and unique features of the final cell cycle of horizontal cell progenitors and how they may relate to retinoblastoma carcinogenesis.

Functional test of PCDHB11, the most human-specific neuronal surface protein

Background

Brain-expressed proteins that have undergone functional change during human evolution may contribute to human cognitive capacities, and may also leave us vulnerable to specifically human diseases, such as schizophrenia, autism or Alzheimer’s disease. In order to search systematically for those proteins that have changed the most during human evolution and that might contribute to brain function and pathology, all proteins with orthologs in chimpanzee, orangutan and rhesus macaque and annotated as being expressed on the surface of cells in the human central nervous system were ordered by the number of human-specific amino acid differences that are fixed in modern populations.

Results

PCDHB11, a beta-protocadherin homologous to murine cell adhesion proteins, stood out with 12 substitutions and maintained its lead after normalizing for protein size and applying weights for amino acid exchange probabilities. Human PCDHB11 was found to cause homophilic cell adhesion, but at lower levels than shown for other clustered protocadherins. Homophilic adhesion caused by a PCDHB11 with reversion of human-specific changes was as low as for modern human PCDHB11; while neither human nor reverted PCDHB11 adhered to controls, they did adhere to each other. A loss of function in PCDHB11 is unlikely because intra-human variability did not increase relative to the other human beta-protocadherins.

Conclusions

The brain-expressed protein with the highest number of human-specific substitutions is PCDHB11. In spite of its fast evolution and low intra-human variability, cell-based tests on the only proposed function for PCDHB11 did not indicate a functional change.

Protocadherins branch out: Multiple roles in dendrite development

The proper formation of dendritic arbors is a critical step in neural circuit formation, and as such defects in arborization are associated with a variety of neurodevelopmental disorders. Among the best gene candidates are those encoding cell adhesion molecules, including members of the diverse cadherin superfamily characterized by distinctive, repeated adhesive domains in their extracellular regions. Protocadherins (Pcdhs) make up the largest group within this superfamily, encompassing over 80 genes, including the ∼60 genes of the α-, β-, and γ-Pcdh gene clusters and the non-clustered δ-Pcdh genes. An additional group includes the atypical cadherin genes encoding the giant Fat and Dachsous proteins and the 7-transmembrane cadherins. In this review we highlight the many roles that Pcdhs and atypical cadherins have been demonstrated to play in dendritogenesis, dendrite arborization, and dendritic spine regulation. Together, the published studies we discuss implicate these members of the cadherin superfamily as key regulators of dendrite development and function, and as potential therapeutic targets for future interventions in neurodevelopmental disorders.

DSCAM localization and function at the mouse cone synapse

The Down syndrome cell adhesion molecule (DSCAM) is required for regulation of cell number, soma spacing, and cell type-specific dendrite avoidance in many types of retinal ganglion and amacrine cells. In this study we assay the organization of cells making up the outer plexiform layer of the retina in the absence of Dscam. Some types of OFF bipolar cells, type 3b and type 4 bipolar cells, had defects in dendrite arborization in the Dscam mutant retina, whereas other cell types appeared similar to wild type. The cone synapses that these cells project their dendrites to were intact, as visualized by electron microscopy, and had a distribution and density that was not significantly different from that of wild type. The spacing of type 3b bipolar cell dendrites was further analyzed by Voronoi domain analysis, density recovery profiling (DRP) analysis, and nearest neighbor analysis. Spacing was found to be significantly different when wild-type and mutant type 3b bipolar cell dendrites were compared. Defects in arborization of these bipolar cells could not be attributed to the disorganization of inner plexiform layer cells that occurs in the Dscam mutant retina or an increase in cell number, as they arborized when Dscam was targeted in retinal ganglion cells only or in the bax null retina. Localization of DSCAM was assayed and the protein was localized near to cone synapses in mouse, macaque, and ground squirrel retinas. DSCAM protein was detected in several types of bipolar cells, including type 3b and type 4 bipolar cells.

Tomographic reconstruction reveals the morphology of a unique cellular organelle, the aggregated macrotubules (Macrotubuli aggregati) of human retinal horizontal cells

Horizontal cells of the human retina contain unique tubular organelles that have a diameter which is about 10 times larger than that of microtubules (~230 nm). These macrotubuli in most cases form regular aggregates. Therefore we propose to introduce them as Macrotubuli aggregati in the Terminologia histologica. Tomographic investigation of the structures revealed that the walls of the tubules most probably consist of intermediate filaments running nearly parallel to each other and show somewhat regularly attached ribosomes on their inner and also outer surface. About 2% of the organelles exhibit double- to multiple layered walls and less than 1% resemble large scrolls. The tubules may extend 10 to over 20 μm in the cytoplasm and are also encountered in soma-near processes extending into the outer plexiform layer. It remains unclear why these structures are only present in humans and few other species and why almost only in horizontal cells. Speculations on possible functions are discussed.

PCDHB14- and GABRB1-like nervous system developmental genes are altered during early neuronal differentiation of NCCIT cells treated with ethanol

Ethanol (EtOH) exposure during embryonic development causes dysfunction of the central nervous system (CNS). Here, we examined the effects of chronic EtOH on gene expression during early stages of neuronal differentiation. Human embryonic carcinoma (NCCIT) cells were differentiated into neuronal precursors/lineages in the presence or absence of EtOH and folic acid. Gene expression profiling and pathway analysis demonstrated that EtOH deregulates many genes and pathways that are involved in early brain development. EtOH exposure downregulated several important genes, such as PCDHB14, GABRB1, CTNND2, NAV3, RALDH1, and OPN5, which are involved in CNS development, synapse assembly, synaptic transmission, and neurotransmitter receptor activity. GeneGo pathway analysis revealed that the deregulated genes mapped to disease pathways that were relevant to fetal alcohol spectrum disorders (FASD, such as neurotic disorders, epilepsy, and alcohol-related disorders). In conclusion, these findings suggest that the impairment of the neurological system or suboptimal synapse formation resulting from EtOH exposure could underlie the neurodevelopmental disorders in individuals with FASD.

Localization of kainate receptors in inner and outer hair cell synapses

Glutamate plays a role in hair cell afferent transmission, but the receptors that mediate neurotransmission between outer hair cells (OHCs) and type II ganglion neurons are not well defined. A previous study using in situ hybridization showed that several kainate-type glutamate receptor (KAR) subunits are expressed in cochlear ganglion neurons. To determine whether KARs are expressed in hair cell synapses, we performed X-gal staining on mice expressing lacZ driven by the GluK5 promoter, and immunolabeling of glutamate receptors in whole-mount mammalian cochleae. X-gal staining revealed GluK5 expression in both type I and type II ganglion neurons and OHCs in adults. OHCs showed X-gal reactivity throughout maturation from postnatal day 4 (P4) to 1.5 months. Immunoreactivity for GluK5 in IHC afferent synapses appeared to be postsynaptic, similar to GluA2 (GluR2; AMPA-type glutamate receptor (AMPAR) subunit), while GluK2 may be on both sides of the synapses. In OHC afferent synapses, immunoreactivity for GluK2 and GluK5 was found, although GluK2 was only in those synapses bearing ribbons. GluA2 was not detected in adult OHC afferent synapses. Interestingly, GluK1, GluK2 and GluK5 were also detected in OHC efferent synapses, forming several active zones in each synaptic area. At P8, GluA2 and all KAR subunits except GluK4 were detected in OHC afferent synapses in the apical turn, and GluA2, GluK1, GluK3 decreased dramatically in the basal turn. These results indicate that AMPARs and KARs (GluK2/GluK5) are localized to IHC afferent synapses, while only KARs (GluK2/GluK5) are localized to OHC afferent synapses in adults. Glutamate spillover near OHCs may act on KARs in OHC efferent terminals to modulate transmission of acoustic information and OHC electromotility.

Specialized synaptic pathway for chromatic signals beneath S-cone photoreceptors is common to human, Old and New World primates

The distribution of the soluble NSF-attachment protein receptor protein syntaxin-4 and the Na-K-Cl cotransporter (NKCC) were investigated in the outer plexiform layer of human retina using immunohistochemistry. Both proteins, which are proposed to be components of a gamma-aminobutyric acid mediated feed-forward circuit from horizontal cells directly to bipolar cells, were enriched beneath S-cones. The expression pattern of syntaxin-4 was further analyzed in baboon and marmoset to determine if the synaptic specialization is common to primates. Syntaxin-4 was enriched beneath S-cones in both species, which together with the human results indicates that this specialization may have evolved for the purpose of mediating unique color vision capacities that are exclusive to primates.

Synaptic elements for GABAergic feed-forward signaling between HII horizontal cells and blue cone bipolar cells are enriched beneath primate S-cones

The functional roles and synaptic features of horizontal cells in the mammalian retina are still controversial. Evidence exists for feedback signaling from horizontal cells to cones and feed-forward signaling from horizontal cells to bipolar cells, but the details of the latter remain elusive. Here, immunohistochemistry and confocal microscopy were used to analyze the expression patterns of the SNARE protein syntaxin-4, the GABA receptor subunits α1 and ρ, and the cation-chloride cotransporters NKCC and KCC2 in the outer plexiform layer of primate retina. In macaque retina, as observed previously in other species, syntaxin-4 was expressed on dendrites and axon terminals of horizontal cells at cone pedicles and rod spherules. At cones, syntaxin-4 appeared densely clustered in two bands, at horizontal cell dendritic tips and at the level of desmosome-like junctions. Interestingly, in the lower band where horizontal cells may synapse directly onto bipolar cells, syntaxin-4 was highly enriched beneath short-wavelength sensitive (S) cones and colocalized with calbindin, a marker for HII horizontal cells. The enrichment at S-cones was not observed in either mouse or ground squirrel. Furthermore, high amounts of both GABA receptor and cation-chloride cotransporter subunits were found beneath primate S-cones. Finally, while syntaxin-4 was expressed by both HI and HII horizontal cell types, the intense clustering and colocalization with calbindin at S-cones indicated an enhanced expression in HII cells. Taken together, GABA receptors beneath cone pedicles, chloride transporters, and syntaxin-4 are putative constituents of a synaptic set of proteins which would be required for a GABA-mediated feed-forward pathway via horizontal cells carrying signals directly from cones to bipolar cells.

Calcium channel-dependent molecular maturation of photoreceptor synapses

Several studies have shown the importance of calcium channels in the development and/or maturation of synapses. The Ca_V1.4(α_1F) knockout mouse is a unique model to study the role of calcium channels in photoreceptor synapse formation. It features abnormal ribbon synapses and aberrant cone morphology. We investigated the expression and targeting of several key elements of ribbon synapses and analyzed the cone morphology in the Ca_V1.4(α_1F) knockout retina. Our data demonstrate that most abnormalities occur after eye opening. Indeed, scaffolding proteins such as Bassoon and RIM2 are properly targeted at first, but their expression and localization are not maintained in adulthood. This indicates that either calcium or the Ca_V1.4 channel, or both are necessary for the maintenance of their normal expression and distribution in photoreceptors. Other proteins, such as Veli3 and PSD-95, also display abnormal expression in rods prior to eye opening. Conversely, vesicle related proteins appear normal. Our data demonstrate that the Ca_V1.4 channel is important for maintaining scaffolding proteins in the ribbon synapse but less vital for proteins related to vesicular release. This study also confirms that in adult retinae, cones show developmental features such as sprouting and synaptogenesis. Overall we present evidence that in the absence of the Ca_V1.4 channel, photoreceptor synapses remain immature and are unable to stabilize.

OFF bipolar cells express distinct types of dendritic glutamate receptors in the mouse retina

Parallel representations of the visual world are already established at the very first synapse of the visual system. Cone photoreceptors, which hyperpolarize in response to light, forward the visual signal onto distinct types of ON and OFF cone bipolar cells (BCs). In the case of OFF BCs, the glutamatergic cone input is integrated by ionotropic glutamate receptors, giving rise to a sign-preserving mode of synaptic transmission. The combination of glutamate receptor (GluR) subunits, i.e. AMPA or kainate subunits, importantly contributes to shaping the OFF bipolar cells' distinct response properties. The mouse is one of the few mammals in which the (most likely) complete set of (five) retinal OFF BC types is identified. However, it is not clear which GluR subtypes are expressed by the different mouse OFF BC types. We addressed this question by combining immunolabeling, electrical whole-cell recordings and pharmacology, and present evidence that the different types of OFF BCs express distinct types of glutamate receptors: Type 1 BCs exclusively expressed AMPA receptors, whereas type 2 and type 3a BCs expressed kainate receptors of different subunit compositions. Additionally, we found that two OFF BC types (3b and 4) very likely express both AMPA and kainate receptors but, interestingly, the two receptor subunits were not co-localized at the same dendritic site. The complex, BC type-specific expression pattern of GluRs we describe here supports their essential role in establishing parallel pathways at the first synapse of the mouse visual system.

Single-neuron diversity generated by Protocadherin-β cluster in mouse central and peripheral nervous systems

The generation of complex neural circuits depends on the correct wiring of neurons with diverse individual characteristics. To understand the complexity of the nervous system, the molecular mechanisms for specifying the identity and diversity of individual neurons must be elucidated. The clustered protocadherins (Pcdh) in mammals consist of approximately 50 Pcdh genes (Pcdh-α, Pcdh-β, and Pcdh-γ) that encode cadherin-family cell surface adhesion proteins. Individual neurons express a random combination of Pcdh-α and Pcdh-γ, whereas the expression patterns for the Pcdh-β genes, 22 one-exon genes in mouse, are not fully understood. Here we show that the Pcdh-β genes are expressed in a 3'-polyadenylated form in mouse brain. In situ hybridization using a pan-Pcdh-β probe against a conserved Pcdh-β sequence showed widespread labeling in the brain, with prominent signals in the olfactory bulb, hippocampus, and cerebellum. In situ hybridization with specific probes for individual Pcdh-β genes showed their expression to be scattered in Purkinje cells from P10 to P150. The scattered expression patterns were confirmed by performing a newly developed single-cell 3'-RACE analysis of Purkinje cells, which clearly demonstrated that the Pcdh-β genes are expressed monoallelically and combinatorially in individual Purkinje cells. Scattered expression patterns of individual Pcdh-β genes were also observed in pyramidal neurons in the hippocampus and cerebral cortex, neurons in the trigeminal and dorsal root ganglion, GABAergic interneurons, and cholinergic neurons. Our results extend previous observations of diversity at the single-neuron level generated by Pcdh expression and suggest that the Pcdh-β cluster genes contribute to specifying the identity and diversity of individual neurons.

Amacrine and bipolar inputs to midget and parasol ganglion cells in marmoset retina

Retinal ganglion cells receive excitatory synapses from bipolar cells and inhibitory synapses from amacrine cells. Previous studies in primate suggest that the strength of inhibitory amacrine input is greater to cells in peripheral retina than to foveal (central) cells. A comprehensive study of a large number of ganglion cells at different eccentricities, however, is still lacking. Here, we compared the amacrine and bipolar input to midget and parasol ganglion cells in central and peripheral retina of marmosets (Callithrix jacchus). Ganglion cells were labeled by retrograde filling from the lateral geniculate nucleus or by intracellular injection. Presumed amacrine input was identified with antibodies against gephyrin; presumed bipolar input was identified with antibodies against the GluR4 subunit of the AMPA receptor. In vertical sections, about 40% of gephyrin immunoreactive (IR) puncta were colocalized with GABAA receptor subunits, whereas immunoreactivity for gephyrin and GluR4 was found at distinct sets of puncta. The density of gephyrin IR puncta associated with ganglion cell dendrites was comparable for midget and parasol cells at all eccentricities studied (up to 2 mm or about 16 degrees of visual angle for midget cells and up to 10 mm or >80 degrees of visual angle for parasol cells). In central retina, the densities of gephyrin IR and GluR4 IR puncta associated with the dendrites of midget and parasol cells are comparable, but the average density of GluR4 IR puncta decreased slightly in peripheral parasol cells. These anatomical results indicate that the ratio of amacrine to bipolar input does not account for the distinct functional properties of parasol and midget cells or for functional differences between cells of the same type in central and peripheral retina.

Rearrangement of the cone mosaic in the retina of the rat model of retinitis pigmentosa

In retinitis pigmentosa (RP), the death of cones normally follows some time after the degeneration of rods. Recently, surviving cones in RP have been studied and reported in detail. These cones undergo extensive remodeling in their morphology. Here we report an extension of the remodeling study to consider possible modifications of spatial-distribution patterns. For this purpose we used S334ter-line-3 transgenic rats, a transgenic model developed to express a rhodopsin mutation causing RP. In this study, retinas were collected at postnatal (P) days P5-30, 90, 180, and P600. We then immunostained the retinas to examine the morphology and distribution of cones and to quantify the total cone numbers. Our results indicate that cones undergo extensive changes in their spatial distribution to give rise to a mosaic comprising an orderly array of rings. These rings first begin to appear at P15 at random regions of the retina and become ubiquitous throughout the entire tissue by P90. Such distribution pattern loses its clarity by P180 and mostly disappears at P600, at which time the cones are almost all dead. In contrast, the numbers of cones in RP and normal conditions do not show significant differences at stages as late as P180. Therefore, rings do not form by cell death at their centers, but by cone migration. We discuss its possible mechanisms and suggest a role for hot spots of rod death and the remodeling of Müller cell process into zones of low density of photoreceptors.

Immunohistochemical identification and synaptic inputs to the diffuse bipolar cell type DB1 in macaque retina

Detailed analysis of the synaptic inputs to the primate DB1 bipolar cell has been precluded by the absence of a suitable immunohistochemical marker. Here we demonstrate that antibodies for the EF-hand calcium-binding protein, secretagogin, strongly label the DB1 bipolar cell as well as a mixed population of GABAergic amacrine cells in the macaque retina. Using secretagogin as a marker, we show that the DB1 bipolar makes synaptic contact with both L/M as well as S-cone photoreceptors and only minimal contact with rod photoreceptors. Electron microscopy showed that the DB1 bipolar makes flat contacts at both triad-associated and nontriad-associated positions on the cone pedicle. Double labeling with various glutamate receptor subunit antibodies failed to conclusively determine the subunit composition of the glutamate receptors on DB1 bipolar cells. In the IPL, DB1 bipolar cell axon terminals expressed the glycine receptor, GlyRα1, at sites of contact with AII amacrine cells, suggesting that these cells receive input from the rod pathway.