Science is sprouting adventurously as a chemical that could potentially be used in eye drops to reverse
cataracts, the leading cause of blindness, has been identified by a team
of scientists from UC San Francisco (UCSF), the University of Michigan
(U-M), and Washington University in St. Louis (WUSTL).
Identified as a "priority eye disease" by the World Health
Organization, cataracts -- caused when the lenses of the eyes lose their
transparency -- affect more than 30 million people worldwide. Although
cataracts can be successfully removed with surgery, this approach is
expensive, and most individuals blinded by severe cataracts in
developing countries go untreated.
Reported November 5, 2015 in Science, the newly identified
compound is the first that is soluble enough to potentially form the
basis of a practical eye-drop medication for cataracts.
Cataracts are primarily a disease of ageing. As is seen in
neurodegenerative conditions such as Alzheimer's disease and Parkinson's
disease, a hallmark of the condition is the misfolding and clumping
together of crucial proteins. In the case of cataracts, the affected
proteins are known as crystallins.
Crystallins are the major component of fibre cells, which form the
eyes' lenses, and the unique properties of these cells make them
particularly susceptible to damage, said Jason Gestwicki, PhD, associate
professor of pharmaceutical chemistry at UCSF and co-senior author of a
paper on the new research, most of which was undertaken in Gestwicki's
laboratory at the U-M Life Sciences Institute.
"Shortly after you are born, all the fibre cells in the eye lose the
ability to make new proteins, or to discard old proteins," said
Gestwicki, who has continued his work on cataracts at UCSF, where he
joined the faculty about two years ago. "So the crystallins you have in
your eye as an adult are the same as those you're born with."
In order for our lenses to function well, this permanent, finite
reservoir of crystallins must maintain both the transparency of fibre
cells and their flexibility, as the eyes' muscles constantly stretch and
relax the lens to allow us to focus on objects at different distances.
The crystallins accomplish these duties with the help of aptly named
proteins known as chaperones, which act "kind of like antifreeze,"
Gestwicki said, "keeping crystallins soluble in a delicate equilibrium
that's in place for decades and decades."
This state-of-affairs is "delicate" because pathological,
clumped-together configurations of crystallins are far more stable than
properly folded, healthy forms, and fiber-cell chaperones must
continually resist the strong tendency of crystallins to clump. A
similar process underlies other disorders related to ageing, such as
Alzheimer's disease, but in each of these diseases the specific protein
that clumps together and the place in the body that clumping occurs is
different. In all cases, these clumped-together proteins are called
amyloids.
In the new study, led by Leah N. Makley, PhD, and Kathryn McMenimen,
PhD, the scientific team exploited a crucial difference between properly
folded crystallins and their amyloid forms: put simply, amyloids are
harder to melt.
The research group used a method known as high-throughput
differential scanning fluorimetry, or HT-DSF, in which proteins emit
light when they reach their melting point. At the U-M Life Sciences
Institute's Center for Chemical Genomics, the team used HT-DSF to apply
heat to amyloids while applying thousands of chemical compounds.
Because the melting point of amyloids is higher than that of normal
crystallins, the team focused on finding chemicals that that lowered the
melting point of crystallin amyloids to the normal, healthy range.
The group began with 2,450 compounds, eventually zeroing in on 12
that are members of a chemical class known as sterols. One of these,
known as lanosterol, was shown to reverse cataracts in a June, 2015
paper in Nature, but because lanosterol has limited solubility the group
who published that study had to inject the compound into the eye for it
to exert its effects.
Using lanosterol and other sterols as a clue, Gestwicki and his group
assembled and tested 32 additional sterols, and eventually settled on
one, which they call "compound 29," as the most likely candidate that
would be sufficiently soluble to be used in cataract-dissolving eye
drops.
In laboratory dish tests, the team confirmed that compound 29
significantly stabilized crystallins and prevented them from forming
amyloids. They also found that compound 29 dissolved amyloids that had
already formed. Through these experiments, said Gestwicki, "we are
starting to understand the mechanism in detail. We know where compound
29 binds, and we are beginning to know exactly what it's doing."
The team next tested compound 29 in an eye-drop formulation in mice
carrying mutations that make them predisposed to cataracts. In
experiments conducted with Usha P. Andley, PhD, professor of
ophthalmology and visual sciences at WUSTL School of Medicine, they
found that the drops partially restored transparency to mouse lenses
affected by cataracts, as measured by a slit-lamp test of the sort used
by ophthalmologists to measure cataracts in humans.
Similar results were seen when compound 29 eye drops were applied in
mice that naturally developed age-related cataracts, and also when the
compound was applied to human lens tissue affected by cataracts that had
been removed during surgery.
Gestwicki cautions that slit-lamp measures of lens transparency used
in the research are not a direct measure of visual acuity, and that only
clinical trials in humans can establish the value of compound 29 as a
cataract treatment. He has licensed the compound from U-M, however, and
Makley, a former graduate student and postdoctoral fellow in the
Gestwicki laboratory, is founder and chief scientific officer of
ViewPoint Therapeutics, a company that is actively developing compound
29 for human use.
Dogs are also prone to developing cataracts. Half of all dogs have
cataracts by nine years of age, and virtually all dogs develop them
later in life. An effective eye-drop medication could potentially
benefit about 70 million affected pet dogs in the United States.
ViewPoint was formed through the "incubator" program of the
California Institute of Quantitative Biosciences (QB3) at UCSF, and is
located adjacent to the UCSF campus in QB3's Janssen Labs.
McMenimen, also a former graduate student in the Gestwicki
laboratory, is now the Claire Boothe Luce Assistant Professor of
Chemistry at Mt. Holyoke College. The research was supported by the
National Institutes of Health, the American Foundation for
Pharmaceutical Education, and Research to Prevent Blindness.
In addition to compound 29's potential for cataract treatment, the
insights gained through the research could have broader applications,
said Gestwicki, a member of UCSF's Institute for Neurodegenerative
Diseases whose main research interest is dementia and related disorders.
"If you look at an electron micrograph at the protein aggregates that
cause cataracts, you'd be hard-pressed to tell them apart from those
that cause Alzheimer's, Parkinson's, or Huntington's diseases,"
Gestwicki said. "By studying cataracts we've been able to benchmark our
technologies and to show by proof-of-concept that these technologies
could also be used in nervous system diseases, to lead us all the way
from the first idea to a drug we can test in clinical trials."
Story Source:
The above post is reprinted from materials provided by University of California, San Francisco (UCSF).
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