The W3 cell is one particular subtype 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). Retinal ganglion cells (RGCs) are the cells that are responsible for transferring information from the eye to the brain. These cells come in a range of subtypes (of which W3 is one), each exhibiting distinct functional signature, size, and morphology; further, the dendritic arbors of different RGC subtypes have been shown to be confined to specific sublayers of the inner plexiform layer (IPL) of the retina. However, despite this heterogeneity in structure, connectivity and function across different RGC subtypes, to this point, many studies on the development of the visual system and its pathways have regarded them all as being part of a single group. Studies are now emerging that show that different subclasses of RGC's are maximally responsive to particular visual features, and that they arborize selectively within particular layers of the IPL. The number of RGC subtypes and the extent to which they differ from one another remains to be seen, but based upon studies of dendritic morphology(Masland, xxx) it is estimated that most mammals have approximately 20 subtypes.
The W3 RGC subtype 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. inputs to W3s and adhesion molecule mutants that seem to affect W3 dendritic morphology), 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.
Responsiveness and Physiology
In studies investigating the physiological properties of W3, a W3 mouse cell line was generated that used Thy1 regulatory elements to drive expression of yellow fluorescent protein (YFP) in the RGC’s of interest (Kay et al, 2011; Kim et al., 2010). Dark-adapted retinas from these mice were isolated in Ringer’s solution and targeted for cell-attached recording with patch microelectrodes, and light stimuli from a computer-driven video projector were used to determine receptive field centers and direction selectivity in the marked RGC’s. From such studies, it has been found that W3 cells have dendritic arbors in both ON and OFF regions of the inner plexiform layer, and their physiological response to light stimuli confirms this: W3-RGCs responded to a flashing spot of light both at the onset and at the end of the delivery of the stimulus. In generating this response, it was found that the optimal spot size of light had an average radius of 60 μm for the OFF response and 80 μm for the ON response. These studies also showed that W3-RGCs respond to moving stimuli, but show no preference for the direction of motion of a moving stimulus (Kim et al, 2010). Studies on the visual response properties of OMS cells have found that these cells exhibit a sensitive response to differential motion between the receptive field center and its surround (e.g. an object moving in the background of the cell’s receptive filed), but that they are suppressed by the motion of a global image, such as that produced by the a movement of the mouse’s head. Findings on the response of W3 cells to differential and global motion have not yet been reported.
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.
It was recently found that restriction of dendritic arbors to appropriate IPL sublaminae occurs differently in different subtypes of RGC's: while some initially have arbors in multiple sublaminae and then undergo a pruning process in which they achieve laminar specificity, others are confined to a single layer from the outset. The dendritic arbors of W3 have been shown to form in a step-wise manner: initially, the arbors are completely restricted between IPL sublayers 4 and 6 (SL4 and SL6). However, over the next few days in development, dendritic arbors expand to reach SL1 and SL2. 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. (Kim et al., 2010)
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; 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.
Open Questions and Relevance to the EyeWire Project
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