Neurons from mammalian central nerve system (CNS)
cannot be repaired or replaced after injury or disease.
Scientists and physicians turn to regenerative medicine,
by transplanting stem cells or progenitor cells into the
injured nervous system to promote functional recovery.
Optic nerve, the second of twelve paired cranial nerves,
transmits visual information from the retina to the
brain. As part of the CNS, diseases afflicted the optic
nerve, such as glaucoma and other optic neuropathies,
lead to permanent loss of vision or blindness due to the
degeneration of retinal ganglion cells (RGCs). Stem cell
and cell replacement therapies present potential treatments
for retinal neurodegeneration; however, transplantation
of neural progenitors or differentiated neurons has long
been a challenge because transplanted RGCs not only
must survive, but also must integrate into the mature
retina and extend long axons that can reach the brain to
contribute to the restoration of vision (1). In a recent
report, Venugopalan et al.(2) provide intriguing evidence
that transplanted RGCs derived from early neonatal mice
migrated, integrated, and developed functional connections
with the host retina of adult animals. This finding lights up
new hopes for RGC replacement therapy.
In this paper, the authors investigated whether GFPlabeled
RGCs of early neonatal mice (P0–P5) could
integrate into the mature retina following intravitreal
transplantation in normal, uninjured recipient rats. They
found that not only GFP+ RGCs survived in the host retina,
with morphologic features close to endogenous RGCs, but
a significant number of transplanted RGCs extended long
neurites. Transplanted GFP+ RGCs displayed a polarity
similar to endogenous RGCs, with axons growing toward
the optic nerve and dendrites extending into the inner
plexiform layer of the retina. Interestingly, injection of
fewer GFP+ RGCs resulted in higher retention of donor
cells. Judging by nuclei stain, 70% of GFP+ RGCs were
retained, and more than 60% of surviving GFP+ RGCs
extended neurites, with many reaching the optic nerve
head (ONH) 1–4 weeks after transplantation. Moreover,
about 80% of the retained GFP+ RGCs had dendrite-likeprocesses, displaying a variety of subtype cell morphologies.
The data suggest that differentiated RGCs are capable of
undergoing morphological change and integration in the
host retina following transplantation.
Intriguingly, the group found that at least in one animal,
GFP+ RGCs extended axons into the host optic nerves,
crossing the contralateral optic tract at the optic chiasm,
and reached their central targets in the brain, including the
lateral geniculate nucleus and superior colliculus. Albeit
preliminary, the data is the first to show that differentiated
RGCs could potentially be used for transplantation and
are capable of extending long axons that may reach the
usual RGC targets in the brain. Importantly, transplanted
cells developed synapses with physiological functions,
as demonstrated by electrophysiological recordings.
Furthermore, transplanted RGCs exhibited light responses
similar to host RGCs with ON-, OFF- and ON–OFFsubtypes,
although these synapses appeared to be immature,
showing slower responses and greater adaptation. The
observed spontaneous action potential firings and synaptic
activities from the transplanted cells, however, indicate
that they were functionally connected with the host retina
and able to communicate with surrounding neurons after
transplantation.
This study reports a surprising finding suggesting that
postnatal RGCs may be used in cell replacement therapy for
treatment of vision loss as a result of optic nerve damage. In
an ideal therapy, differentiated RGCs would be produced
from induced pluripotent stem cells, which can be derived
from the skin or blood cells of affected patients, removing
the need for embryo-derived cells and post-transplant
immunosuppression. Some studies have demonstrated
successful replacement of degenerating photoreceptors
with transplanted stem cells or progenitor cells. Little is
reported, however, about the RGC replacement. The study
by Venugopalan et al.(2) offers promising evidence, and
their data showing the large variability of stratification of
the dendritic structures by transplanted cells, as imaged
under confocal microscopy, suggests either the survival, or
the development/differentiation of morphologically distinct
RGCs following transplantation. These data implicate a
potential of transplanted primary RGCs to survive, migrate
and develop functional connections with host neurons in
the adult retina.
The findings presented in this paper were significant and
exciting; nevertheless, it has also raised several important
questions which require further detailed studies. First, the
source and differentiation status of donor cells are thought
to have critical impacts on the efficiency of cell replacement
therapy. Primary fetal RGCs not only may be difficult
to obtain clinically, but likely comprise heterogeneous
populations of neurons at various differentiation stages.
This paper used RGC spurified from mouse pups
between postnatal day 1 and 5, leaving it unknown which
subpopulation of transplanted RGCs survived, integrated
and developed synaptic connections. The observed limited
number of animals (n=15 of total 152) that migrated
through the nerve fiber layer and the small number of GFP+
RGCs detected in the host retina all have implicated that
these may be the behavior of a subpopulation of neonatal
RGCs. While RGCs derived from inducible pluripotent
stem cells (iPSCs) are likely represent a better source of
donor cells than primary RGCs, future attempts to define
the molecular properties of the RGC subpopulations that
integrate would be most helpful.
Second, as in this paper, RGC transplantation was
carried out in recipient rats with a normal retina, the
therapeutic potential of RGC transplantation would need
to be tested in diseased animal models with degenerating
RGCs. The diseased retinal environment is thought not to
be as receptive to transplanted primary neurons as in the
undamaged eyes, due to induction of reactive gliosis and
tissue scarring (3). Would primary RGCs repopulate the
diseased retina as they did in the normal retina and would
the transplantation lead to improvement of visual function?
These are key questions that demand further investigations
in the future.
Recent advancement in electroretinography (ERG) and
mouse behavior assessment, such as optokinetic response
(OKR) tests, has also made it possible to quantitatively
assess visual function before and after cell transplantation in
rodents. The immune-privilege nature of the retina offers
a better opportunity for grafted cells to survival as well
as reduces inflammation and immunorejection following
transplantation, even when the recipient is from a different
species, as it was shown by the authors.
To conclude, this paper offers promising evidence
supporting the likelihood of a cell replacement strategy
in treating diseased retinas involving RGC degeneration.
Transplanted neonatal RGCs, at least a subpopulation of
them, appear to be capable of surviving and integrating
into host retinas, while maintaining their physiological
properties. Whether the proposed therapy can be advanced
to the clinic remains to be seen. In any case, the results
of this study offer a premise for regenerating or replacing
RGCs in adult mammals; these findings may also lay a foundation for future development of transplantation
therapy for neurodegenerative conditions in the brain.
