RESUMO
Semaphorins and Plexins form ligand/receptor pairs that are crucial for a wide range of developmental processes from cell proliferation to axon guidance. The ability of semaphorins to act both as signaling receptors and ligands yields a multitude of responses. Here, we describe a novel role for Semaphorin-6D (Sema6D) and Plexin-A1 in the positioning and targeting of retinogeniculate axons. In Plexin-A1 or Sema6D mutant mice of either sex, the optic tract courses through, rather than along, the border of the dorsal lateral geniculate nucleus (dLGN), and some retinal axons ectopically arborize adjacent and lateral to the optic tract rather than defasciculating and entering the target region. We find that Sema6D and Plexin-A1 act together in a dose-dependent manner, as the number of the ectopic retinal projections is altered in proportion to the level of Sema6D or Plexin-A1 expression. Moreover, using retinal in utero electroporation of Sema6D or Plexin-A1 shRNA, we show that Sema6D and Plexin-A1 are both required in retinal ganglion cells for axon positioning and targeting. Strikingly, nonelectroporated retinal ganglion cell axons also mistarget in the tract region, indicating that Sema6D and Plexin-A1 can act non-cell-autonomously, potentially through axon-axon interactions. These data provide novel evidence for a dose-dependent and non-cell-autonomous role for Sema6D and Plexin-A1 in retinal axon organization in the optic tract and dLGN.SIGNIFICANCE STATEMENT Before innervating their central brain targets, retinal ganglion cell axons fasciculate in the optic tract and then branch and arborize in their target areas. Upon deletion of the guidance molecules Plexin-A1 or Semaphorin-6D, the optic tract becomes disorganized near and extends within the dorsal lateral geniculate nucleus. In addition, some retinal axons form ectopic aggregates within the defasciculated tract. Sema6D and Plexin-A1 act together as a receptor-ligand pair in a dose-dependent manner, and non-cell-autonomously, to produce this developmental aberration. Such a phenotype highlights an underappreciated role for axon guidance molecules in tract cohesion and appropriate defasciculation near, and arborization within, targets.
Assuntos
Células Ganglionares da Retina , Semaforinas , Animais , Camundongos , Axônios/fisiologia , Ligantes , Células Ganglionares da Retina/metabolismo , Semaforinas/genética , Semaforinas/metabolismoRESUMO
A surprising new mechanism that regulates the plasticity of postnatal neurons is reported in this issue by Sugiyama et al. (2008). These authors show in mice that visual experience triggers cell-to-cell transfer of the homeoprotein Otx2 to cortical interneurons, where it promotes maturation of inhibitory neural circuitry and opens the critical period for plasticity in the visual cortex.
Assuntos
Plasticidade Neuronal , Fatores de Transcrição Otx/metabolismo , Córtex Visual/fisiologia , Animais , Humanos , Interneurônios/fisiologia , Camundongos , Privação SensorialRESUMO
Although much is known about the regenerative capacity of retinal ganglion cells, very significant barriers remain in our ability to restore visual function following traumatic injury or disease-induced degeneration. Here we summarize our current understanding of the factors regulating axon guidance and target engagement in regenerating axons, and review the state of the field of neural regeneration, focusing on the visual system and highlighting studies using other model systems that can inform analysis of visual system regeneration. This overview is motivated by a Society for Neuroscience Satellite meeting, "Reconnecting Neurons in the Visual System," held in October 2015 sponsored by the National Eye Institute as part of their "Audacious Goals Initiative" and co-organized by Carol Mason (Columbia University) and Michael Crair (Yale University). The collective wisdom of the conference participants pointed to important gaps in our knowledge and barriers to progress in promoting the restoration of visual system function. This article is thus a summary of our existing understanding of visual system regeneration and provides a blueprint for future progress in the field.
Assuntos
Encéfalo/fisiologia , Fenômenos Fisiológicos Oculares , Vias Visuais/fisiologia , Animais , Axônios/fisiologia , Humanos , Nervo Óptico/fisiologia , Células Ganglionares da Retina/fisiologia , Vias Visuais/citologiaRESUMO
Thalamus is a potent driver of cortical activity even though cortical synapses onto excitatory layer 4 neurons outnumber thalamic synapses 10 to 1. Previous in vitro studies have proposed that thalamocortical (TC) synapses are stronger than corticocortical (CC) synapses. Here, we investigated possible anatomical and physiological differences between these inputs in the rat in vivo. We developed a high-throughput light microscopy method, validated by electron microscopy, to completely map the locations of synapses across an entire dendritic tree. This demonstrated that TC synapses are slightly more proximal to the soma than CC synapses, but detailed compartmental modeling predicted that dendritic filtering does not appreciably favor one synaptic class over another. Measurements of synaptic strength in intact animals confirmed that both TC and CC synapses are weak and approximately equivalent. We conclude that thalamic effectiveness does not rely on enhanced TC strength, but rather on coincident activation of converging inputs.
Assuntos
Córtex Cerebral/fisiologia , Dendritos/fisiologia , Neurônios/fisiologia , Sinapses/fisiologia , Tálamo/fisiologia , Potenciais de Ação/fisiologia , Animais , Espinhas Dendríticas/fisiologia , Modelos Neurológicos , Vias Neurais/fisiologia , Ratos , Ratos Wistar , Transmissão Sináptica/fisiologiaRESUMO
The function of neuronal networks relies on selective assembly of synaptic connections during development. We examined how synaptic specificity emerges in the pontocerebellar projection. Analysis of axon-target interactions with correlated light-electron microscopy revealed that developing pontine mossy fibers elaborate extensive cell-cell contacts and synaptic connections with Purkinje cells, an inappropriate target. Subsequently, mossy fiber-Purkinje cell connections are eliminated resulting in granule cell-specific mossy fiber connectivity as observed in mature cerebellar circuits. Formation of mossy fiber-Purkinje cell contacts is negatively regulated by Purkinje cell-derived BMP4. BMP4 limits mossy fiber growth in vitro and Purkinje cell-specific ablation of BMP4 in mice results in exuberant mossy fiber-Purkinje cell interactions. These findings demonstrate that synaptic specificity in the pontocerebellar projection is achieved through a stepwise mechanism that entails transient innervation of Purkinje cells, followed by synapse elimination. Moreover, this work establishes BMP4 as a retrograde signal that regulates the axon-target interactions during development.
Assuntos
Axônios/fisiologia , Comunicação Celular/fisiologia , Rede Nervosa/fisiologia , Animais , Axônios/ultraestrutura , Proteína Morfogenética Óssea 4/genética , Proteína Morfogenética Óssea 4/metabolismo , Proteína Morfogenética Óssea 4/fisiologia , Cerebelo/embriologia , Cerebelo/fisiologia , Cerebelo/ultraestrutura , Camundongos , Rede Nervosa/embriologia , Células de Purkinje/fisiologia , Células de Purkinje/ultraestrutura , Transmissão Sináptica/fisiologiaRESUMO
Carefully designed animal models of genetic risk factors are likely to aid our understanding of the pathogenesis of schizophrenia. Here, we study a mouse strain with a truncating lesion in the endogenous Disc1 ortholog designed to model the effects of a schizophrenia-predisposing mutation and offer a detailed account of the consequences that this mutation has on the development and function of a hippocampal circuit. We uncover widespread and cumulative cytoarchitectural alterations in the dentate gyrus during neonatal and adult neurogenesis, which include errors in axonal targeting and are accompanied by changes in short-term plasticity at the mossy fiber/CA3 circuit. We also provide evidence that cAMP levels are elevated as a result of the Disc1 mutation, leading to altered axonal targeting and dendritic growth. The identified structural alterations are, for the most part, not consistent with the growth-promoting and premature maturation effects inferred from previous RNAi-based Disc1 knockdown. Our results provide support to the notion that modest disturbances of neuronal connectivity and accompanying deficits in short-term synaptic dynamics is a general feature of schizophrenia-predisposing mutations.
Assuntos
Axônios/metabolismo , Hipocampo/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Plasticidade Neuronal , Potenciais de Ação , Animais , Animais Recém-Nascidos , Proliferação de Células , Células Cultivadas , AMP Cíclico/metabolismo , Nucleotídeo Cíclico Fosfodiesterase do Tipo 4/metabolismo , Dendritos/metabolismo , Dendritos/fisiologia , Giro Denteado/citologia , Giro Denteado/crescimento & desenvolvimento , Giro Denteado/metabolismo , Hipocampo/citologia , Hipocampo/crescimento & desenvolvimento , Imuno-Histoquímica , Potenciação de Longa Duração , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Camundongos Transgênicos , Fibras Musgosas Hipocampais/metabolismo , Proteínas do Tecido Nervoso/genética , Neurogênese , Neurônios/citologia , Neurônios/metabolismo , Neurônios/fisiologia , Técnicas de Patch-ClampRESUMO
The divergence of retinal ganglion cell (RGC) axons into ipsilateral and contralateral projections at the optic chiasm and the subsequent segregation of retinal inputs into eye-specific domains in their target, the dorsal lateral geniculate nucleus (dLGN), are crucial for binocular vision. In albinism, affected individuals exhibit a lack or reduction of pigmentation in the eye and skin, a concomitant reduced ipsilateral projection, and diverse visual defects. Here we investigate how such altered decussation affects eye-specific retinogeniculate targeting in albino mice using the C57BL/6 Tyr(c-2J/c-2J) strain, in which tyrosinase, necessary for melanogenesis, is mutated. In albino mice, fewer RGCs from the ventrotemporal (VT) retina project ipsilaterally, reflected in a decrease in cells expressing ipsilateral markers. In addition, a population of RGCs from the VT retina projects contralaterally and, within the dLGN, their axons cluster into a patch separated from the contralateral termination area. Furthermore, eye-specific segregation is not complete in the albino dLGN and, upon perturbing postnatal retinal activity with epibatidine, the ipsilateral projection fragments and the aberrant contralateral patch disappears. These results suggest that the defects in afferent targeting and activity-dependent refinement in the albino dLGN arise from RGC misspecification together with potential perturbations of early activity patterns in the albino retina.
Assuntos
Albinismo/patologia , Axônios/patologia , Olho/patologia , Corpos Geniculados/patologia , Vias Visuais/patologia , Albinismo/genética , Animais , Animais Recém-Nascidos , Olho/crescimento & desenvolvimento , Olho/inervação , Feminino , Corpos Geniculados/crescimento & desenvolvimento , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Mutantes , Neurogênese/genética , Células Ganglionares da Retina/patologia , Vias Visuais/crescimento & desenvolvimentoRESUMO
Purpose: The Retinal Ganglion Cell (RGC) Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) consortium was founded in 2021 to help address the numerous scientific and clinical obstacles that impede development of vision-restorative treatments for patients with optic neuropathies. The goals of the RReSTORe consortium are: (1) to define and prioritize the most critical challenges and questions related to RGC regeneration; (2) to brainstorm innovative tools and experimental approaches to meet these challenges; and (3) to foster opportunities for collaborative scientific research among diverse investigators. Design and Participants: The RReSTORe consortium currently includes > 220 members spanning all career stages worldwide and is directed by an organizing committee comprised of 15 leading scientists and physician-scientists of diverse backgrounds. Methods: Herein, we describe the structure and organization of the RReSTORe consortium, its activities to date, and the perceived impact that the consortium has had on the field based on a survey of participants. Results: In addition to helping propel the field of regenerative medicine as applied to optic neuropathies, the RReSTORe consortium serves as a framework for developing large collaborative groups aimed at tackling audacious goals that may be expanded beyond ophthalmology and vision science. Conclusions: The development of innovative interventions capable of restoring vision for patients suffering from optic neuropathy would be transformative for the ophthalmology field, and may set the stage for functional restoration in other central nervous system disorders. By coordinating large-scale, international collaborations among scientists with diverse and complementary expertise, we are confident that the RReSTORe consortium will help to accelerate the field toward clinical translation. Financial Disclosures: Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.
RESUMO
Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.
Assuntos
Doenças do Nervo Óptico , Células Ganglionares da Retina , Animais , Humanos , Retina , Encéfalo , Diferenciação Celular , MamíferosRESUMO
Retinal ganglion cell (RGC) axons diverge within the optic chiasm to project to opposite sides of the brain. In mouse, contralateral RGCs are distributed throughout the retina, whereas ipsilateral RGCs are restricted to the ventrotemporal crescent (VTC). While repulsive guidance mechanisms play a major role in the formation of the ipsilateral projection, little is known about the contribution of growth-promoting interactions to the formation of binocular visual projections. Here, we show that the cell adhesion molecule Nr-CAM is expressed by RGCs that project contralaterally and is critical for the guidance of late-born RGCs within the VTC. Blocking Nr-CAM function causes an increase in the size of the ipsilateral projection and reduces neurite outgrowth on chiasm cells in an age- and region-specific manner. Finally, we demonstrate that EphB1/ephrin-B2-mediated repulsion and Nr-CAM-mediated attraction comprise distinct molecular programs that each contributes to the proper formation of binocular visual pathways.
Assuntos
Moléculas de Adesão Celular Neurônio-Glia/metabolismo , Quiasma Óptico/crescimento & desenvolvimento , Células Ganglionares da Retina/metabolismo , Visão Binocular/fisiologia , Vias Visuais/crescimento & desenvolvimento , Animais , Moléculas de Adesão Celular Neurônio-Glia/genética , Lateralidade Funcional , Imuno-Histoquímica , Hibridização In Situ , Camundongos , Camundongos Knockout , Camundongos Transgênicos , Quiasma Óptico/embriologia , Vias Visuais/embriologia , Vias Visuais/metabolismoRESUMO
Partial decussation of sensory pathways allows neural inputs from both sides of the body to project to the same target region where these signals will be integrated. Here, to better understand mechanisms of eye-specific targeting, we studied how retinal ganglion cell (RGC) axons terminate in their thalamic target, the dorsal lateral geniculate nucleus (dLGN), when crossing at the optic chiasm midline is altered. In models with gain- and loss-of-function of EphB1, the receptor that directs the ipsilateral projection at the optic chiasm, misrouted RGCs target the appropriate retinotopic zone in the opposite dLGN. However, in EphB1(-/-) mice, the misrouted axons do not intermingle with normally projecting RGC axons and segregate instead into a distinct patch. We also revisited the role of retinal activity on eye-specific targeting by blocking correlated waves of activity with epibatidine into both eyes. We show that, in wild-type mice, retinal waves are necessary during the first postnatal week for both proper distribution and eye-specific segregation of ipsilateral axons in the mature dLGN. Moreover, in EphB1(-/-) mice, refinement of ipsilateral axons is perturbed in control conditions and is further impaired after epibatidine treatment. Finally, retinal waves are required for the formation of the segregated patch of misrouted axons in EphB1(-/-) mice. These findings implicate molecular determinants for targeting of eye-specific zones that are independent of midline guidance cues and that function in concert with correlated retinal activity to sculpt retinogeniculate projections.
Assuntos
Regulação da Expressão Gênica no Desenvolvimento/genética , Corpos Geniculados/anormalidades , Cones de Crescimento/ultraestrutura , Receptor EphB1/genética , Retina/anormalidades , Vias Visuais/anormalidades , Potenciais de Ação/fisiologia , Animais , Compostos Bicíclicos Heterocíclicos com Pontes/farmacologia , Sinais (Psicologia) , Lateralidade Funcional/genética , Corpos Geniculados/metabolismo , Cones de Crescimento/metabolismo , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Neurogênese/genética , Agonistas Nicotínicos/farmacologia , Piridinas/farmacologia , Retina/metabolismo , Células Ganglionares da Retina/citologia , Células Ganglionares da Retina/metabolismo , Sinapses/metabolismo , Sinapses/ultraestrutura , Transmissão Sináptica/genética , Visão Ocular/fisiologia , Vias Visuais/metabolismoRESUMO
BACKGROUND: Activity-dependent competition that operates on branch stability or formation plays a critical role in shaping the pattern and complexity of axonal terminal arbors. In the mammalian central nervous system (CNS), the effect of activity-dependent competition on axon arborization and on the assembly of sensory maps is well established. However, the molecular pathways that modulate axonal-branch stability or formation in competitive environments remain unknown. RESULTS: We establish an in vivo axonal-competition paradigm in the mouse olfactory system by employing a genetic strategy that permits suppression of neurosecretory activity in random subsets of olfactory sensory neurons (OSNs). Long-term follow up confirmed that this genetic manipulation triggers competition by revealing a bias toward selective stabilization of active arbors and local degeneration of synaptically silent ones. By using a battery of genetically modified mouse models, we demonstrate that a decrease either in the total levels or the levels of activity-dependent secreted BDNF (due to a val66met substitution), rescues silent arbors from withering. We show that this effect may be mediated, at least in part, by p75(NTR). CONCLUSIONS: We establish and experimentally validate a genetic in vivo axonal-competition paradigm in the mammalian CNS. By using this paradigm, we provide evidence for a specific effect of BDNF signaling on terminal-arbor pruning under competition in vivo. Our results have implications for the formation and refinement of the olfactory and other sensory maps, as well as for neuropsychiatric diseases and traits modulated by the BDNF val66met variant.
Assuntos
Axônios/fisiologia , Fator Neurotrófico Derivado do Encéfalo/metabolismo , Transdução de Sinais/genética , Substituição de Aminoácidos , Animais , Fator Neurotrófico Derivado do Encéfalo/química , Fator Neurotrófico Derivado do Encéfalo/genética , Camundongos , Camundongos Transgênicos , Neurônios Aferentes/fisiologia , Receptor de Fator de Crescimento Neural/metabolismo , Receptor de Fator de Crescimento Neural/fisiologia , Receptores de N-Metil-D-Aspartato/metabolismo , Olfato/genética , Olfato/fisiologiaRESUMO
The navigation of retinal axons to ipsilateral and contralateral targets in the brain depends on the decision to cross or avoid the midline at the optic chiasm, a critical guidance maneuver that establishes the binocular visual pathway. Previous work has identified a specific guidance receptor, EphB1, that mediates the repulsion of uncrossed axons away from its ligand, ephrinB2, at the optic chiasm midline (Williams et al., 2003), and a transcription factor Zic2, that, like EphB1, is required for formation of the ipsilateral retinal projection (Herrera et al., 2003). Although the reported similarities in localization implicated that Zic2 regulates EphB1 (Herrera et al., 2003; Williams et al., 2003; Pak et al., 2004), whether Zic2 drives expression of EphB1 protein has not been elucidated. Here we show that EphB1 protein is expressed in the growth cones of axons from ventrotemporal (VT) retina that project ipsilaterally and that repulsion by ephrinB2 is determined by the presence of this receptor on growth cones. Moreover, ectopic delivery of Zic2 into explants from non-VT retina induces expression of EphB1 mRNA and protein. The upregulated EphB1 receptor protein is localized to growth cones and is functional, because it is sufficient to change retinal ganglion cell axon behavior from extension onto, to avoidance of, ephrinB2 substrates. Our results demonstrate that Zic2 upregulates EphB1 expression and define a link between a transcription factor and expression of a guidance receptor protein essential for axon guidance at the vertebrate midline.
Assuntos
Axônios/fisiologia , Movimento Celular/fisiologia , Efrina-B2/metabolismo , Regulação da Expressão Gênica no Desenvolvimento/fisiologia , Receptor EphB1/biossíntese , Células Ganglionares da Retina/fisiologia , Fatores de Transcrição/fisiologia , Dedos de Zinco/fisiologia , Animais , Células Cultivadas , Feminino , Camundongos , Camundongos Mutantes , Gravidez , Receptor EphB1/genética , Receptor EphB1/fisiologia , Células Ganglionares da Retina/citologia , Células Ganglionares da Retina/metabolismoRESUMO
In the developing mouse optic tract, retinal ganglion cell (RGC) axon position is organized by topography and laterality (i.e., eye-specific or ipsi- and contralateral segregation). Our lab previously showed that ipsilaterally projecting RGCs are segregated to the lateral aspect of the developing optic tract and found that ipsilateral axons self-fasciculate to a greater extent than contralaterally projecting RGC axons in vitro. However, the full complement of axon-intrinsic and -extrinsic factors mediating eye-specific segregation in the tract remain poorly understood. Glia, which are known to express several guidance cues in the visual system and regulate the navigation of ipsilateral and contralateral RGC axons at the optic chiasm, are natural candidates for contributing to eye-specific pre-target axon organization. Here, we investigate the spatiotemporal expression patterns of both putative astrocytes (Aldh1l1+ cells) and microglia (Iba1+ cells) in the embryonic and neonatal optic tract. We quantified the localization of ipsilateral RGC axons to the lateral two-thirds of the optic tract and analyzed glia position and distribution relative to eye-specific axon organization. While our results indicate that glial segregation patterns do not strictly align with eye-specific RGC axon segregation in the tract, we identify distinct spatiotemporal organization of both Aldh1l1+ cells and microglia in and around the developing optic tract. These findings inform future research into molecular mechanisms of glial involvement in RGC axon growth and organization in the developing retinogeniculate pathway.
Assuntos
Família Aldeído Desidrogenase 1/metabolismo , Neuroglia/metabolismo , Trato Óptico/embriologia , Trato Óptico/metabolismo , Retinal Desidrogenase/metabolismo , Células Ganglionares da Retina/metabolismo , Fatores Etários , Família Aldeído Desidrogenase 1/análise , Animais , Axônios/metabolismo , Camundongos , Camundongos Endogâmicos C57BL , Trato Óptico/citologia , Retinal Desidrogenase/análise , Vias Visuais/citologia , Vias Visuais/embriologia , Vias Visuais/metabolismoRESUMO
The rapid motility of axonal filopodia and dendritic spines is prevalent throughout the developing CNS, although the function of this motility remains controversial. Using two-photon microscopy, we imaged hippocampal mossy fiber axons in slice cultures and discovered that filopodial extensions are highly motile. Axonal filopodial motility is actin based and is downregulated with development, although it remains in mature cultures. This motility is correlated with free extracellular space yet is inversely correlated with contact with postsynaptic targets, indicating a potential role in synaptogenesis. Filopodial motility is differentially regulated by kainate receptors: synaptic stimulation of kainate receptors enhances motility in younger slices, but it inhibits it in mature slices. We propose that neuronal activity controls filopodial motility in a developmentally regulated manner, in order to establish synaptic contacts in a two-step process. A two-step model of synaptogenesis can also explain the opposite effects of neuronal activity on the motility of dendritic protrusions.
Assuntos
Diferenciação Celular/fisiologia , Movimento Celular/fisiologia , Ácido Glutâmico/metabolismo , Cones de Crescimento/metabolismo , Fibras Musgosas Hipocampais/crescimento & desenvolvimento , Pseudópodes/metabolismo , Receptores de Ácido Caínico/metabolismo , Citoesqueleto de Actina/metabolismo , Animais , Animais Recém-Nascidos , Comunicação Celular/efeitos dos fármacos , Comunicação Celular/fisiologia , Diferenciação Celular/efeitos dos fármacos , Movimento Celular/efeitos dos fármacos , Estimulação Elétrica , Agonistas de Aminoácidos Excitatórios/farmacologia , Espaço Extracelular/metabolismo , Ácido Glutâmico/farmacologia , Cones de Crescimento/ultraestrutura , Camundongos , Microscopia Eletrônica , Fibras Musgosas Hipocampais/metabolismo , Fibras Musgosas Hipocampais/ultraestrutura , Técnicas de Cultura de Órgãos , Pseudópodes/ultraestrutura , Receptores de Ácido Caínico/antagonistas & inibidores , Membranas Sinápticas/efeitos dos fármacos , Membranas Sinápticas/metabolismo , Transmissão Sináptica/efeitos dos fármacos , Transmissão Sináptica/fisiologiaRESUMO
During development, retinal ganglion cell (RGC) axons either cross or avoid the midline at the optic chiasm. In Drosophila, the Slit protein regulates midline axon crossing through repulsion. To determine the role of Slit proteins in RGC axon guidance, we disrupted Slit1 and Slit2, two of three known mouse Slit genes. Mice defective in either gene alone exhibited few RGC axon guidance defects, but in double mutant mice a large additional chiasm developed anterior to the true chiasm, many retinal axons projected into the contralateral optic nerve, and some extended ectopically-dorsal and lateral to the chiasm. Our results indicate that Slit proteins repel retinal axons in vivo and cooperate to establish a corridor through which the axons are channeled, thereby helping define the site in the ventral diencephalon where the optic chiasm forms.
Assuntos
Axônios/fisiologia , Proteínas do Tecido Nervoso/fisiologia , Retina/embriologia , Vias Visuais/embriologia , Animais , Diencéfalo/embriologia , Desenvolvimento Embrionário e Fetal/fisiologia , Peptídeos e Proteínas de Sinalização Intercelular , Camundongos , Camundongos Knockout/genética , Proteínas do Tecido Nervoso/deficiência , Proteínas do Tecido Nervoso/genética , Inibição Neural/fisiologia , Quiasma Óptico/embriologia , Área Pré-Óptica/embriologia , Células Ganglionares da Retina/fisiologia , Transmissão Sináptica/fisiologiaRESUMO
In animals with binocular vision, retinal ganglion cell (RGC) axons either cross or avoid the midline at the optic chiasm. Here, we show that ephrin-Bs in the chiasm region direct the divergence of retinal axons through the selective repulsion of a subset of RGCs that express EphB1. Ephrin-B2 is expressed at the mouse chiasm midline as the ipsilateral projection is generated and is selectively inhibitory to axons from ventrotemporal (VT) retina, where ipsilaterally projecting RGCs reside. Moreover, blocking ephrin-B2 function in vitro rescues the inhibitory effect of chiasm cells and eliminates the ipsilateral projection in the semiintact mouse visual system. A receptor for ephrin-B2, EphB1, is found exclusively in regions of retina that give rise to the ipsilateral projection. EphB1 null mice exhibit a dramatically reduced ipsilateral projection, suggesting that this receptor contributes to the formation of the ipsilateral retinal projection, most likely through its repulsive interaction with ephrin-B2.
Assuntos
Axônios/metabolismo , Efrina-B1/deficiência , Efrina-B2/biossíntese , Quiasma Óptico/metabolismo , Retina/metabolismo , Animais , Células Cultivadas , Efrina-B1/biossíntese , Efrina-B1/genética , Efrina-B2/genética , Feminino , Regulação da Expressão Gênica no Desenvolvimento/fisiologia , Humanos , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Quiasma Óptico/embriologia , Gravidez , Retina/embriologia , Células Ganglionares da Retina/metabolismo , XenopusRESUMO
Prior to forming and refining synaptic connections, axons of projection neurons navigate long distances to their targets. While much is known about guidance cues for axon navigation through intermediate choice points, whether and how axons are organized within tracts is less clear. Here we analyze the organization of retinal ganglion cell (RGC) axons in the developing mouse retinogeniculate pathway. RGC axons are organized by both eye-specificity and topography in the optic nerve and tract: ipsilateral RGC axons are segregated from contralateral axons and are offset laterally in the tract relative to contralateral axon topographic position. To identify potential cell-autonomous factors contributing to the segregation of ipsilateral and contralateral RGC axons in the visual pathway, we assessed their fasciculation behavior in a retinal explant assay. Ipsilateral RGC neurites self-fasciculate more than contralateral neurites in vitro and maintain this difference in the presence of extrinsic chiasm cues. To further probe the role of axon self-association in circuit formation in vivo, we examined RGC axon organization and fasciculation in an EphB1-/- mutant, in which a subset of ipsilateral RGC axons aberrantly crosses the midline but targets the ipsilateral zone in the dorsal lateral geniculate nucleus on the opposite side. Aberrantly crossing axons retain their association with ipsilateral axons in the contralateral tract, indicating that cohort-specific axon affinity is maintained independently of guidance signals present at the midline. Our results provide a comprehensive assessment of RGC axon organization in the retinogeniculate pathway and suggest that axon self-association contributes to pre-target axon organization.
Assuntos
Axônios/fisiologia , Nervo Óptico/fisiologia , Células Ganglionares da Retina/citologia , Vias Visuais , Aminoácidos/metabolismo , Animais , Animais Recém-Nascidos , Embrião de Mamíferos , Olho/citologia , Olho/inervação , Fasciculação , Lateralidade Funcional , Técnicas In Vitro , Filamentos Intermediários/metabolismo , Proteínas Luminescentes/genética , Proteínas Luminescentes/metabolismo , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Nervo Óptico/embriologia , Nervo Óptico/crescimento & desenvolvimento , Receptor EphB1/genética , Receptor EphB1/metabolismo , Proteínas da Membrana Plasmática de Transporte de Serotonina/genética , Proteínas da Membrana Plasmática de Transporte de Serotonina/metabolismo , Vias Visuais/anatomia & histologia , Vias Visuais/embriologia , Vias Visuais/crescimento & desenvolvimentoRESUMO
During the formation of neuronal circuits, afferent axons often enter target regions before their target cells are mature and then make temporary contacts with nonspecific targets before forming synapses on specific target cells. The regulation of these different steps of afferent-target interactions is poorly understood. The cerebellum is a good model for addressing these aspects, because cerebellar development is well defined and identified neurons in the circuitry can be purified and combined in vitro. Previous reports from our laboratory showed that cultured granule neurons specifically arrest the extension of their pontine mossy fiber afferents, leading us to propose that granule cells arrested growth of their afferents as a prelude to synaptogenesis. However, we knew little about the differentiation state of the cultured granule cells that mediate afferent arrest. In this study, we better define the purified granule cell fraction by marker expression and morphology, and demonstrate that only freshly plated granule cells in the precursor and premigratory state arrest mossy fiber outgrowth. Mature granule cells, in contrast, support extension, defasciculation, and synapse formation, as in vivo. In addition, axonal tracing in vivo during the first postnatal week indicates that immature mossy fibers extend into the Purkinje cell layer but never into the external germinal layer (EGL), where precursors of granule cell targets reside. We found that the stop-growing signals are dependent on heparin-binding factors, and we propose that such signals in the EGL restrict the extension of mossy fiber afferents and prevent invasion of proliferative regions.