The retina of the eye is part of the central nervous system, making the retina really an extension of the brain. Many types of cells make up the central nervous system, including the well-known neurons. Astrocytes are glial cells, another type of cell in the central nervous system that is responsible for a variety of functions, including playing a role in the response to injury. After an injury, astrocytes change (reactive gliosis) by synthesizing Glial Fibrillary Acidic Protein (GFAP), which helps cells maintain their mechanical strength and results in glial scarring. Another feature of reactive gliosis is the production of molecules that break down proteins (proteases), which result in the death of retinal ganglion cells (RGCs), an important type of neuron in the retina.
In a recent paper published in the PLoS ONE, CBR member Shravan Chintala, of the Eye Research Institute, examines the process of reactive gliosis in more detail. He and his coauthor hypothesize that “inhibition of reactive gliosis reduces protease levels in the retina and prevents protease-mediated death of RGCs.” To test this hypothesis, they use a special in-bred type of laboratory mouse, called C57BL/6, mimic injury by applying kainic acid, apply two known glial poisoins--alpha-aminoadipic acid (AAA) and Neurostatin--and measure protease levels and retinal ganglion cell death. Interestingly, they found that AAA does not decrease protease levels and thereby protect retinal ganglion cells, but Neurostatin does. The discussion section of the paper summarizes these results, and speculates on the potential role of Neurostatin in treating glaucoma, an eye disorder that can lead to blindness through the loss of retinal ganglion cells.
“Since our previous studies indicated that reactive astrocytes play
a major role in protease-mediated death of RGCs, we hypothesized
that inhibition of reactive gliosis down-regulates the
expression of detrimental proteases and prevents RGC loss. To
investigate this hypothesis, we induced RGC loss in C57BL/6
mice by intravitreal injections of a non-NMDA-type glutamate
receptor agonist, kainic acid (KA), along with two glial toxins,
AAA and Neurostatin, which are known to prevent reactive gliosis.
We noticed that these two glial toxins exert different inhibitory
activities on astrocytes and [Muller] cells. First, AAA inhibited the
activation of [Muller] cells, but not astrocytes. Second, since
astrocytes are responsible for the expression of proteases and since AAA did not decrease activation of astrocytes, protease levels
were not reduced. Third, since protease levels were not reduced in
the retinas, RGC death was also not inhibited.
In contrast, Neurostatin inhibited the activation of not only
Muller cells, but also inhibited the activation of astrocytes.
Interestingly, since Neurostatin inhibited the activation of
astrocytes, which are responsible for proteases, protease levels
were reduced significantly in Neurostatin-treated retinas. In
addition, due to low levels of proteases, RGC death was inhibited
significantly in Neurostatin-treated retinas. These results, for the
first time, suggest that Neurostatin may be useful as an adjuvant
therapeutic agent to inhibit activation of glial cells and to prevent
RGC death not only under excitotoxic conditions, but also in
glaucoma in which excitotoxicity has been implicated.
Until now, it was unclear whether reactive gliosis can be
reversed in retinal degenerative conditions, including glaucoma.
Results presented in this study, at least in an excitotoxicity model
of retinal degeneration, suggest that reactive gliosis is reversible,
and reversal of reactive gliosis is neuroprotective. In a broader
picture, use of Neurostatin seems to be advantageous because one
can down-regulate three proteases by inhibiting reactive gliosis
selectively, rather than by using three different protease inhibitors
to prevent RGC loss as we have shown in previous studies....
In the future, it would be worthwhile to investigate whether a
topical application rather than invasive application of Neurostatin
would prevent reactive gliosis to prevent RGC loss in retinal
neurodegenerative conditions, including glaucoma. In addition, it
would be worthwhile to investigate the effect of Neurostatin in a
slow retinal degenerative model such as glaucoma.”