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Home | Pregnancy Timeline | News Alerts |News Archive Nov 5, 2013

 

The genes that carry the blueprint for proteins in two of the 10 layers of cells in the mammalian retina.are made up of so-called starburst amacrine cells (SACs). Sema6A might be the key difference that enables the "On" and "Off" SACs to segregate from one another.

Development of direction-selective circuitry. On and Off mouse SACs normally stratify in discrete layers (top left) and exhibit radial dendrite morphology (top right). Sema6A and its PlexA2 receptor are expressed in On SACs, but only PlexA2 is expressed in Off SACs. In Sema6A mutants, SACs fail to stratify (bottom left) and On SACs are misshapen (bottom right), compromising responses to “light on” directional cues.

Image Credit: Alex L. Kolodkin laboratory







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Gene in retinal development and motion sensing

Discovered gene aids in understanding the organization of parts of the eye and brain.

Our vision depends on exquisitely organized layers of cells within the eye's retina, each with a distinct role in perception. Johns Hopkins researchers say they have taken an important step toward understanding how those cells are organized to produce what the brain "sees." Specifically, they report identification of a gene that guides the separation of two types of motion-sensing cells, offering insight into how cellular layering develops in the retina, with possible implications for the brain's cerebral cortex. A report on the discovery is published in the Nov. 1 issue of the journal Science.

"The separation of different types of cells into layers is critical to their ability to form the precise sets of connections with each other — the circuitry — that lets us process visual information," says Alex Kolodkin, Ph.D., a professor in the Johns Hopkins University School of Medicine's Solomon H. Snyder Department of Neuroscience and an investigator at the Howard Hughes Medical Institute. "There is still much to learn about how that separation happens during development, but we've identified for the first time proteins that enable two very similar types of cells to segregate into their own distinct neuronal layers."


Kolodkin's research group specializes in studying how circuitry forms among neurons (brain and nerve cells).


Past experiments revealed that two types of proteins, called semaphorins and plexins, help guide this process. In the current study, Lu Sun, a graduate student in Kolodkin's laboratory, focused on the genes that carry the blueprint for these proteins in two of the 10 layers of cells in the mammalian retina.

Those two layers are made up of so-called starburst amacrine cells (SACs). One type of SAC, known as "Off," detects motion by sensing decreases in the amount of light hitting the retina, while the other type, "On," detects increases in light. Sun examined the amounts of several semaphorin and plexin proteins being made by each type of cell, and found that only the "On" SACs were making a semaphorin called Sema6A. Sema6A can only work in the retina by interacting with its receptor, a plexin called PlexA2, but Sun found both types of SAC were churning out roughly equal amounts of PlexA2.

Reasoning that Sema6A might be the key difference that enabled the "On" and "Off" SACs to segregate from one another, Kolodkin's team analyzed mice in which the genes for either Sema6A, PlexA2 or both could be switched off, and looked at the effects of this manipulation on their retinas. "Knocking out" either gene during development led the "On" and "Off" layers to run together, the team found, and caused abnormalities in the "On" SACs' tree-like extensions. However, the "Off" SACs, which hadn't been using their Sema6A gene in the first place, still looked and functioned normally.

"When signaling between Sema6A and PlexA2 was lost, not only was layering compromised, but the 'On' SACs lost both their distinctive symmetrical appearance, and, importantly, their motion-detecting ability," Sun says. "This is evidence that the beautiful symmetric shape that gives starburst amacrine cells their name is necessary for their function."


"We hope that learning how layering occurs in these very specific cell types will help us begin sorting out how connections are made not just in the retina, but also in neurons throughout the nervous system.

"Layering also occurs in the cerebral cortex, for example, which is responsible for thought and consciousness, and we really want to know how this is organized during neural development."

Alex Kolodkin, Ph.D., professor, the Johns Hopkins University School of Medicine's Solomon H. Snyder Department of Neuroscience, investigator at Howard Hughes Medical Institute


Abstract
Introduction
Direction-selective responses to visual cues depend upon precise connectivity between inhibitory starburst amacrine cells (SACs) and direction-selective ganglion cells (DSGCs). Motion is detected by SAC responses to illumination onset (On) or cessation (Off). On and Off SACs costratify in the inner plexiform layer of the vertebrate retina with distinct DSGC dendritic arborizations that mediate On or Off directional responses. Here, we study the molecular mechanisms that specify On versus Off SACs and the signaling pathways governing the functional assembly of murine retinal direction-selective circuitry. We show that signaling between the transmembrane guidance cue semaphorin 6A (Sema6A) and its receptor plexinA2 (PlexA2) regulates dendritic morphology of On but not Off SACs, thereby controlling direction-selective responses to visual stimuli.

Development of direction-selective circuitry. On and Off mouse SACs normally stratify in discrete layers (top left) and exhibit radial dendrite morphology (top right). Sema6A and its PlexA2 receptor are expressed in On SACs, but only PlexA2 is expressed in Off SACs. In Sema6A mutants, SACs fail to stratify (bottom left) and On SACs are misshapen (bottom right), compromising responses to “light on” directional cues.

Methods
We analyzed SAC stratification in the inner plexiform layer, and dendritic morphology of individual On and Off SACs, throughout retinal development in Sema6A and PlexA2 mutant mice. We used dissociated retinal cultures to determine how neurites from genetically labeled SACs respond to exogenous Sema6A. We determined the light-evoked excitatory and inhibitory responses of Sema6A–/– On SACs. Finally, we analyzed direction-selective responses in isolated retinas by On-Off direction-selective ganglion cells.

Results
Sema6A is expressed in On, but not Off, SACs, whereas PlexA2 is expressed in all SACs. In vitro, exogenous Sema6A repels neurites only from PlexA2+, Sema6A– SACs, the in vivo expression profile of Off SACs. In PlexA2–/– or Sema6A–/– retinas, SAC dendritic stratifications fail to completely segregate from each other; therefore, in vitro and in vivo observations suggest that repulsive interactions between Sema6A and PlexA2 mediate SAC dendritic stratification. Analysis of dendritic morphology in individual SACs in the x-y plane reveals that On SACs in PlexA2–/– and Sema6A–/– mutants are missing extensive portions of their dendritic fields, have asymmetric dendritic arbors, and exhibit self-avoidance defects; Off SACs in these mutants have normal x-y plane dendritic arbors. Specific On-Off bistratified direction-selective ganglion cells in Sema6A–/– mutant retinas exhibit decreased tuning of On-directional motion responses, whereas Off responses in these same cells are unaffected, correlating the elaboration of symmetric SAC dendritic morphology and asymmetric responses to motion.

Discussion
Our findings show that, in addition to contributing to the separation between On and Off SAC dendritic stratifications into distinct inner plexiform layer laminae, Sema6A-PlexA2 signaling selectively regulates the elaboration of symmetric On SAC dendritic fields. Disruption of Sema6A-PlexA2 signaling ultimately results in compromised On, but not Off, directional tuning in a subclass of On-Off direction-selective ganglion cells. Our elucidation of molecular events critical for functional assembly of retinal direction-selective circuitry may have general implications for understanding the establishment of circuitry in which individual neurons participate in multiple distinct pathways.

Other authors on the report are Zheng Jiang, Randal Hand, Colleen M. Brady, Ryota L. Matsuoka and King-Wai Yau of the Johns Hopkins University School of Medicine, and Michal Rivlin-Etzion and Marla B. Feller of the University of California, Berkeley.

This work was supported by the National Institute of Neurological Disorders and Stroke (grant number NS35165), the National Eye Institute (grant numbers EY06837, EY019498 and EY013528), the Human Frontier Science Program, the Weizmann Institute's National Postdoctoral Award Program for Advancing Women in Science, and the Edmond and Lily Safra (ELSC) Fellowship for Postdoctoral Training in Brain Science.

Original press release: http://www.eurekalert.org/pub_releases/2013-10/jhm-cgi103013.php