A few animals, notably Cnidarians, including Hydra and
some species of jellyfish, and Planaria possess unlimited
regenerative capacity and effectively elude both senescence
and biological aging (1). However, humans, and most
other vertebrates, exhibit limited regenerative capability
for a few tissues that declines with age. Given the isolation
of human embryonic stem cells (ESCs), the creation of
induced pluripotent stem cells (iPSCs), and the discovery
of many adult stem cell types, the drive to harness these
cells to reverse the ravages of disease and age captivates the
imagination of the public and medical scientists alike. The
enthusiasm for unlocking human regenerative potential for
clinical use currently drives both private and public research
initiatives, giving hope to millions of sight impaired
and blind people. For example, the Audacious Goals
Initiative, sponsored by The National Eye Institute of the
United States, specifically seeks to fund cross-disciplinary
research leading to the restoration of vision through retina
regeneration.
Although most efforts for regenerative medicine
in ophthalmology focus on the retina, lens disease
resulting in cataracts remains the leading cause of human
blindness (2). Fortunately, surgical removal of the cataract
with replacement by a synthetic intraocular lens (IOL)
provides excellent potential for vision restoration in adults
with access to the procedure. However, sight threatening
complications of cataract surgery remain a problem,
particularly for pediatric patients (3). The recent article
by Lin and colleagues provides an alternative strategy for
treating pediatric cataracts that relies on the inherent ability
of lens epithelial cells to regenerate new lens fiber cells to
replace the excised, cataractous fiber cell mass (4).
Some species of frogs and newts are the only terrestrial
vertebrates exhibiting lens regeneration following
complete lens loss or surgical removal (5). In both types of
amphibians, lens regeneration results from the conversion
of a previously differentiated tissue into lens tissue through
a process known as transdifferentiation. In frogs, lens
regeneration occurs through transdifferentiation of corneal
epithelial cells and this process only occurs in larval tadpoles
prior to metamorphosis. In contrast, adult newts regenerate
the lens from transdifferentiation of pigmented epithelial
cells of the dorsal iris. Regeneration by transdifferentiation
involves widespread genetic and epigenetic reprogramming
and requires dedifferentiation, proliferation and subsequent
differentiation of the missing cell type.
In contrast to newts and frogs, mammals do not
spontaneously regenerate lens by transdifferentiation of
other existing tissues. However, as early as 1827, surgeons
in France noted the regeneration of rabbit lenses in eyes
following surgical removal of the lens fiber cell mass while
leaving the lens capsule intact (6). Lens regeneration from
epithelial cells remaining adherent to the lens capsule
following removal of the fiber cell mass has since been
demonstrated in dogs, cats, sheep, guinea pigs, rats, mice
and non-human primates (7-9). In these experimental
animals, the quality of lens regeneration depended on
the state of the lens capsule and freedom from excessive
inflammation. In particular, filling the evacuated capsule
to prevent wrinkling and adhesion of the capsule surfaces,
followed by sealing the capsule wound, resulted in
accelerated regeneration and improved lens size (10). Also,
the speed and efficiency with which mammals regenerate
their lenses declines with age.
The current standard treatment for cataracts involves
creating a reasonably large hole in the anterior lens capsule followed by the removal of the fiber cell mass. After cataract
removal, a synthetic IOL, placed into the remaining lens
capsular bag, replaces the function of the natural lens.
The major complication of cataract surgery results from
proliferation and fibrosis of lens epithelial cells that remain
following cataract extraction. These cells often migrate
onto the posterior lens capsule or onto the IOL causing
capsular wrinkling and creating opacities within the visual
axis. This requires additional procedures to remove the
posterior capsule and/or adherent lens cells to clear the
visual axis. The overall frequency of visual axis opacification
(VAO) has declined in recent years as the result of both
better IOL design and more complete removal of lens
epithelial cells during surgery. However, in contrast to the
adult population, infant cataracts present unique challenges
and these patients continue to experience high rates of
complications, including VAO, amblyopia and glaucoma,
following the initial cataract extraction.
Lin and colleagues revisited previous evidence
demonstrating the potential of mammalian lens
regeneration. In so doing, they used BrdU incorporation
to reaffirm that the proliferative potential of human lens
epithelial cells declines with age. However, human central
lens epithelial cells, where cell division is normally rare,
ramped up their BrdU incorporation rate 11-fold following
removal of the fiber cell contents from the capsular bag.
Also, as previously shown, the authors demonstrated that
neonatal rabbit lens epithelial cells could be maintained in
culture and form lentoid bodies that express elevated levels
of αA-, β- and γ-crystallins, proteins characteristic of lens
fiber cells.
The continuous postnatal growth of mammalian lenses
depends on lens epithelial cell proliferation, mostly in
the germinative zone slightly anterior to the lens equator,
followed by differentiation of equatorial epithelial cells
into secondary fiber cells. Investigators in the Lin et al.,
study sought to investigate the role of Bmi-1, a member
of the Polycomb Repressor Complex 1 required for selfrenewal
of stem cells, in lens epithelial cell proliferation.
To do this, they deleted the Bmi-1 gene using the NestinCre
deleter strain where Cre is active in the developing lens
epithelium (11). Bmi-1 deficient mouse lenses exhibited
normal prenatal development and remained clear through
2 months of age. However, by 7 months of age, Bmi-1 deficient lenses developed cataracts and most lens epithelial
cells lost expression of both Pax6 and Sox2. Progressive
reduction of lens epithelial cell proliferation in the NestinCre/Bmi-1 floxed mice and reduced proliferation in human lens epithelial cells following RNAi-mediated knockdown Bmi-1 expression supported the notion that Bmi-1 plays an
essential role in long term maintenance of lens epithelial
cell self-renewal required for post-natal lens growth.
However, the authors failed to note any overall decrease in
lens size or reduction in static lens epithelial cell number
with age in the Bmi-1 deficient mouse lenses.
The existence of a distinct stem cell population within
the lens epithelium remains a controversial topic. Lens
epithelial cells express telomerase, a protein typically
restricted to stem cells and cancer (12). Stem cells generally
require a specialized location or niche which maintains
their stem cell properties. No clear consensus has emerged
as to specifically where lens stem cells might reside or the
location of their niche. Clearly, both central and equatorial
lens epithelial cells retain the ability to both proliferate
and differentiate into lens fiber cells. Although a number
of investigations suggested the possibility of specific
mammalian lens stem cells within different locations of the
epithelium (13,14), or even outside the lens (15), a recent
paper suggested a mathematical alternative—the penny
pusher model—explaining continuous lens growth without
the need for a specific lens stem cell population (16). In this
model, all lens epithelial cells are equivalent and differential
proliferative behaviors depend only on the relative positions
of cells within the epithelial layer.
Armed with repeated previous demonstrations of
the regenerative capacity of mammalian lens epithelial
cells, Lin and colleagues developed a modified cataract
extraction (capsulorhexis) method with the goal of
maximally preserving the anterior lens capsule with intact
lens epithelial cells. In this respect, the authors sought to
exploit the regenerative capacity of these cells to recreate
an optically clear lens, rather than treat these epithelial
cells as complications leading to VAO. The key here
involved making a small (1–2 mm) incision peripheral to
the visual axis from which phacoemulsification extracted
the fiber cell mass. They initially demonstrated the
potential of this “minimally invasive” anterior continuous
curvilinear capsulorhexis (ACCC) technique in rabbits
where they followed the fate of the remaining capsular bag
for up to 5 months post-surgery. Despite initial adherence
of the posterior and anterior capsules, within 4–5 weeks
lens fiber cells began regenerating from the periphery
toward the center of the lens, such that a biconvex, clear
regenerated lens with an average of 15.6 dioptres had
formed by 5 months post-surgery.
Following the success of their surgical procedure in rabbits, Lin et al., tested their technique in juvenile
long-tailed macaques as a proxy for human infants
4–12 months old. As seen in the rabbits, the surviving
macaque lens epithelial cells fueled the curvilinear pattern of
lens regeneration resulting in clear, biconvex lenses within
5 months of surgery. Furthermore, in the six macaques
undergoing this procedure, no post-surgical complications
common to pediatric cataract surgery in humans occurred.
Given this outcome, the authors took the bold step of
testing their minimally invasive ACCC technique in human
infants with bilateral cataracts.
The human trial consisted of 12 infants who received
the modified ACCC and 25 control infants that received
standard cataract treatment with ACCC through a
large anterior capsule opening. The standard treatment
was subsequently followed, in the majority of cases, by
laser capsulotomy or posterior continuous curvilinear
capsulorhexis and anterior vitrectomy with or without the
insertion of an IOL. All infants were followed by slit-lamp
microscopy to follow process of lens regeneration in vivo.
Amazingly, the capsular opening healed and a transparent
biconvex lens regenerated in all 24 eyes receiving the
minimally invasive ACCC treatment within 3 months of the
surgery. These regenerated lenses reached sizes comparable
to native lenses by 8 months following surgery and, in
contrast to the standard treated eyes, achieved a mean
accommodative response of 2.5 dioptres. Furthermore,
the mean visual acuity achieved in eyes treated with the
modified ACCC was indistinguishable from that of eyes
receiving the standard treatment.
The large reduction in post-operative complications
using the minimally invasive ACCC technique argues in
favor of clinical superiority to standard ACCC treatment.
As expected, the majority (92%) of eyes receiving standard
treatment developed complications including 84% that
developed VAO that required additional surgery. In
contrast, only 17% of the modified ACCC treated eyes
developed complications and only one eye (4.2%) developed
VAO. Although all of the lenses treated with the modified
ACCC developed a scar resulting from localized fibrosis
at the site of capsular opening, the peripheral scar failed to
interfere with transparency of the visual axis. As noted in
mice (8), an initial fibrotic response to fiber cell removal
by the lens epithelium apparently preceded a more normal
fiber cell differentiation response in human infants.
Lin and colleagues provide a compelling case to reexamine
current standard treatment for pediatric cataracts.
Exploiting the regenerative capacity inherent in the native lens represents a desirable, less invasive and by all
measures a seemingly superior strategy that works with
rather than against lens biology. However, some important
considerations remain. First among these likely rests in
the selection of appropriate candidates for the procedure.
While this technique likely holds promise for patients with
cataracts resulting from traumatic or infectious causes, this
route may prove less successful in patients with cataracts
resulting from genetic causes. There seems little reason to
suggest that abnormalities in lens fiber cell clarity resulting
from a genetic deficiency or gain of function mutation
would improve upon regeneration without correction of
the underlying genetic lesion. As a significant portion of
bilateral congenital and pediatric cataracts result from
genetic abnormalities (17,18), this remains an important
consideration when selecting patients for this procedure.
Since age-related cataract removal remains among
the most commonly performed surgical procedures the
potential of minimally invasive ACCC to treat elderly
patients merits serious consideration. However, as pointed
out by Lin and colleagues age related cataracts present a
number of different challenges than pediatric cataracts.
Among these are the hardness of the senile cataracts, and
the reduced proliferative capacity of aged lens epithelial
cells relative to those of pediatric lenses. Also, the standard
treatment for age-related cataracts enjoys higher rates of
success and fewer complications than for pediatric cataracts.
Even under the best of circumstances, restoration of sight
by the modified ACCC will require several months as the
lens undergoes the process of regeneration. Patients and
clinicians will need to carefully weigh the benefits and
risks of this newer treatment in light of the effectiveness of
standard treatments.
Further basic research may also reveal ways to improve
the potential of lens regeneration for the treatment of age-related
cataract. The differentiation of lens progenitor
cells from both human ESCs and iPSCs (19,20) suggests
the possibility that “new” lens epithelial cells might be
generated from age-related cataract patients for use in
seeding the capsular bag with lens cells with regenerative
potential more akin to those of infants. These approaches
might also result in the correction of genetic anomalies
leading to cataracts in iPSCs from pediatric cataract patients
before differentiation of lens progenitor cells for the same
purpose. Lin and colleagues effectively demonstrate how
basic research in animal models can fuel paradigm-shifting
advances in clinical practice. These advances provide an
example and set the stage for new advances to achieve the goal of rapidly growing new, natural lenses to replace
synthetic lenses for the treatment of both pediatric and age-related
cataracts.
The regeneration of retinal neurons in human patients
remains an important goal in the treatment of human
blindness and sight impairment. However, at present, many
challenges remain to achieve clinically significant retina
regeneration in human patients. In contrast, limbal stem cell
transplantation already shows widespread clinical success
in regenerating corneal epithelium in patients with limbal
stem cell deficiency (21). Lin and colleagues have now
elegantly demonstrated that endogenous lens epithelial cells
provide the basis for clinically relevant vision restoration
in pediatric cataract patients with fewer complications than
current surgical practice.
