W3 Cell

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A W3 cell in connection with a few different types of amacrine cells.

The W3 retinal ganglion cell is one particular class of retinal ganglion cell (RGC) that has been identified in the mouse retina and localized to a narrow region of the inner plexiform layer (IPL) (Kim et al., 2010). It is named after the transgenic line of mice in which it was studied, and is named as such because there is not yet an accepted classification or scheme for the nomenclature of RGC subtypes. Studies conducted using W3 mice have been used to characterize the structure and function of the W3 RGC’s and to address questions regarding the development of their axonal and dendritic arbors. Current studies are investigating other properties of W3 (e.g. distinguishing anatomical and physiological features), but these are not yet in press. It has been proposed that W3 is the equivalent of object motion selective (OMS) cells in salamanders, about which there are some papers in press (Baccus et al., 2008), but several marked differences between these two cells types prevent the generalization that the properties in one can be assumed to be present in the other.


Physiology

OMS and W3 both respond sensitively to differential motion between the receptive field center and surround, as produced by an object moving over the background. However, both are strongly suppressed by global image motion, as produced by the observer’s head or eye movements. While it is clear that W3 is responsive to moving stimuli, there is no evidence of preference for motion in a particular direction.

Anatomy

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Rough visualization of the stratification of W3 cell, occupying approximately 2.5/10 sublayers from the INL side.

Mature W3 Characteristics

W3 cells have small cell body and small arbors that are densely branched. Typically, the diameter of their somatic field is about 120 microns (as compared with other RGC types such as W7, which has an average dendritic field diameter of nearly 300 microns). If the IPL is divided into 10 arbitrary and evenly-spaced strata, dendritic arbors of W3 occupy a thick swath in the middle of the IPL (from sublayer4 (SL4) through SL6)) with minor arborization in SL1. The region that is most populated with W3 dendritic arbors (SL4 to SL6)f lies sandwiched between 2 ChAT (choline acetyltransferase) positive bands.

Development

The dendritic arbors of W3 have been shown to form in a step-wise manner at different times during the development of the mouse visual system. Initially, the arbors are completely restricted between sublayers 4 and 6. However, over the next few days in development, dendritic arbors expand to reach sublayers 1 and 2. Later still, the proximal processes between SL4 and SL6 expand, and the distal ones become isolated to SL1 and expand there as well. In adults, there is therefore a bistratified dendritic arbor distribution.

Connections

Molecules

Until recently, few (if any) molecular markers were available to identify subtypes of RGCs; consequently, most analyses depended on nonselective labeling methods, and many developmental studies have treated RGCs as a single population. This limitation severely compromises analysis of RGC projections and development. For example, it is difficult to learn whether subtypes develop in distinct ways if they can be identified only after they have matured.This problem is now being circumvented in mice by generation of genetically engineered lines in which RGC subsets are marked with reporter genes (Hattar et al., 2002; Kim et al., 2008[1]; Yonehara et al., 2008; Badea et al., 2009; Huberman et al., 2009; Siegert et al., 2009). Recent studies have characterized the structure and function of RGCs marked in four transgenic lines, of which one is the W3 line, and have then used the results of their results to address a set of open questions about patterning and development of axonal and dendritic arbors:

W3 mice were generated from a vector in which Thy1 regulatory elements drive expression of YFP, wheat germ agglutinin (WGA), and Escherichia coli beta-galactosidase. The transgene was intended to express WGA plus LacZ following the removal of YFP by restriction enzyme Cre, but neither WGA nor LacZ were expressed at detectable levels. YFP was expressed in distinct and non-overlapping subsets of RGCs in the W3 line, presumably due to effects of sequences near the site of transgene integration in the genome (for discussion, see Feng et al., 2000). To decrease the number of YFPpositive RGCs in these lines, an adenoassociated virus (AAV, serotype 2) that expressed Cre under the control of a CMVpromoter (Harvard Gene Therapy Core, Children’s Hospital, Boston) was injected into the retina.

History

Open Questions and Relevance to the EyeWire Project

References

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Masland RH (2001) The fundamental plan of the retina. Nat Neurosci 4:877– 886.