Retinas, retinas, and technology

Recently, Apple and LG premiered a new technology called the retina display, which features an ultra-high pixel density [1]. For people with 20/20 vision, the pixel density of the display is actually higher than the sampling density of the viewer's retina. It is a principle similar to fast-flicker fusion, or the perception of coherent motion from a sequence of still frames presented at high-frequency. But what about people with degenerating retinas [2]? Fortunately, there are emerging technologies that can improve their viewing experiences as well (see Figure 1). These innovations are not yet ready for market, but are based on recent advances in BioMEMS and cell therapy.

Figure 1. LEFT: An image of the retina display from a next-generation iPhone. COURTESY: [1]. RIGHT: picture of the highly-complex architecture of the retina, in relation to the rest of the eye. COURTESY: [3].

According to two recent papers [4,5], there are two routes to repairing macular degeneration: stimulating existing cells in a way that re-activates them, or introducing precursor cells that can integrate into the retinal architecture. Using photovoltaic implants (Figure 2) made primarily from silicon [6], Mathieson et.al [4] 
were able to recover vision in the rat eye. There approach relies on the observation that loss of vision in degenerative diseases is primarily due to loss of cells in the outer layer (cells they characterize as "image capturing" photoreceptors), while cells in the inner layer (cells they characterize as "image processing" units) remain well- preserved. Loss of function due to degeneration is thus a blockage of this feed-forward component (e.g. from outer layer to inner layer). Using this model, the inner layer of cells can be stimulated 
in a way that mimics the effects of ambient light being processed by the outer layer cells.

This was done using the system shown in Figure 2. A camera was used to capture images in the environment. This was then converted into pulsed NIR (near-infrared) illumination, projected onto the retina using a pair of goggles (see Figure 2). Each pixel on the micropatterned array converted this signal into stimulation currents, which were delivered locally to inner layer neurons. For a rat retina, an entire micropatterned array is 0.8 x 1.2mm in size. A single pixel on this array is about 70um in size, and can elicit a neuronal response (in this case, neural ganglion cell action potentials fired as spike trains). The number of these activity bursts could be modulated by manpulating properties of the NIR stimulus such as irradiance and pulse width. Overall, among six healthy and five degenerate rats, the prosthetic seemed to recover visual function measured as bursts of retinal ganglion cell spikes. However, there are several technical challenges for implementing such a system in the eye for the long-term. One of these is maintaining a normal physiological temperature during pulsed light stimulation. A more fundamental limit involves the curvature of the eye cup (see Figure 1, right) limiting the maximum size of a single array, as graphene is not a highly compliant material. 

Figure 2: RIGHT: Histology of bionic retina demonstrating the size and placement of the implanted device. Notice the implant geometry with respect to the cell populations of interest. COURTESY: Image at left is from Figure 1 in [4], image at right is from Figure 6 in [4].

Pearson et.al [5] decided to take the cell therapy route to solving the same problem. Cell therapy has had many technical challenges in the course of its development [7], but in the past few years a number of promising studies have been published [8]. The cell therapy approach operates from the premise that a general loss of photoreceptors leads to the degradation of sight. There are no assumptions made about how the architecture functions, there simply needs to be an existing architecture in place in which transplanted cells may take root. In the attempt to regenerate this retinal architecture, 200,000 rod precursor cells were transplanted into knockout mice [9]. Of this number, up to 16% of cells are able to successfully (e.g,. functionally) integrate into target area. Some of these cells include a transgene which allows identifying high expression for the gene Nrl [10] using a GFP reporter. When the entire cell population is sorted for the Nrl+ criterion, the efficiency of cellular intergration is improved 20- to 30-fold. 

But what does it mean to functionally integrate? From a purely descriptive standpoint, integration is the ability of cells to migrate to specific target tissues and fully differentiate into mature cells. To fully assess their functionality, a number of assays are performed. Immunohistochemistry and ultrastructural analysis used to find hallmarks of fully-functioning neural cells. Notably, the transplanted rod precursors form triad synapses with existing bipolar and horizontal cells. Transplanted cells are also light repsonsive and exhibit dim-flash kinetics [11], which suggests normal function (see Figure 3). Finally, a number of behavioral tests for visually-guided behaviors such as spatial navigation and visual tracking [12], suggests that integrated cells drive these behaviors in a manner similar to what is seen in healthy, wild-type mice.

Figure 3. TOP: Measurement of dim-flash kinetics in mice using voltage-sensitive dye imaging techniques. BOTTOM: Results of the dye imaging for several different experimental conditions. For the combined panels (far right), the black patches represent mature rods, while red patches represent regenerated rods, and blue patches represent rods derived from Nrl/GFP+ (transplanted) cells. 
COURTESY: Figure 2 from [5].

While the level of development for retinal-related bionic technologies [13] is not as advanced as for visual display technologies, the promise is much greater. It is hard to say which approach is better or worse. Rather, these approaches might be used in combination, and one approach might be superior to another in select cases. Regardless, this is a relatively new and fast-growing area with much potential for new developments and great innovations. Stay tuned.

References:
[1] For more information on the retina display technology, please see the following links: http://en.wikipedia.org/wiki/Retina_display,    
http://www.apple.com/ipodtouch/features/retina-display.html


[2] Degenerating retinal function can either result from a retinopathy (due to diabetes, inflammation, or hypertension) or normal aging. For more information, please see: http://en.wikipedia.org/wiki/Retinopathy

[3] For more information on function and structure of the retina and vertebrate eye, please see: http://webvision.med.utah.edu/book/part-i-foundations/simple-anatomy-of-the-retina/

[4] Mathieson, K. et.al   Photovoltaic retinal prosthesis with high pixel density. Nature Photonics, 6, 391-397 (2012).

[5] Pearson et.al   Restoration of vision after transplantation of photoreceptors. Nature, 485, 99-103 (2012).

[6] Silicon is not the only material being used for implantation. A range of polymers can be used, provided they have the proper characteristics. For example, graphene is fast becoming a primary candidate material for regenerative medicine applications. Graphene is both biocompatible and conductive. Graphene can also be fabricated at nanoscale dimensions using layered deposition techniques. While modern graphene transistor arrays are primarily used as a sensing technology, technical limitations may be overcome that will enable greater computational ability (which will improve their usefulness as retinal prosthetics).

For more information, please see: Hess et.al   Biocompatible graphene transistor array reads cellular signals.
Advanced Materials, 23, 5045-5049 (2011) AND Schmidt, C.   The Bionic Material. Nature, 483, S37 (2012).

[7] Daley, G.Q.   The Promise and Perils of Stem Cell Therapeutics. Cell Stem Cell, 10(6), 740-749 (2012).

[8] Kim, S.U. and de Vellis, J.   Stem Cell-based Cell Therapy in Neurological Diseases: a review. Journal of Neuroscience Research, 87, 2183-2200 (2009).

[9] The knockout phenotype is Gnat double negative (-/-). This phenotype is naturally degenerate for the formation of a normal retinal phenotype. For more information on the function of Gnat (a family of acetyltransferases), please see: Vetting, M.W. Structure and functions of the GNAT superfamily of acetyltransferases. Archives of Biochemistry and Biophysics, 433, 212-226 (2005).

[10] Nrl is the gene that codes for the neural retina-specific leucine zipper protein. For more information, please see: http://ghr.nlm.nih.gov/gene/NRL.

[11] Dim-flash kinetics should be observed in rods that contain active photopigment, and is assayed in the context of light adaptation. For an example (in Salamanders), please see: Sakurai, K. et.al   Variation in rhodopsin kinase expression alters the dim flash response shut off and the light adaptation in rod photoreceptors. Investigations in Ophthalmology and Visual Science, 52(9), 6793-6800 (2011).

[12] tests include a version of the Morris water maze. For more information, please see Methods of [5].

[13] For more information on the development of prosthetic technologies (articles in Nature-related journals), please see: http://go.nature.com/hxbyrm.