Human central nerve system (CNS) is an extremely complex
and delicate structure. While regeneration is possible in
some reptiles and fish CNS, the regeneration capacity
seems completely lost in adult mammals. Therefore, the
classic concept is that once neurons in mammal CNS
are damaged in injury or disease, they cannot regenerate
themselves anymore. Although we have known this feature
of mammals CNS for quite some time, many scientists have
never given up their dreams in finding the “Elysium” for
CNS regeneration. In newborn children and in very rare
cases of adult humans, we do see, amazingly, some reports
that show successful regeneration of CNS neurons or their
axons(1). From a clinical perspective, however, regeneration
of neuron or its axon is still not good enough. The key issue
to address is to reestablish neural circuit connections with
functional neural electrical activities.
We know that neural retina is an extension of CNS
brain. The model of eye-to-brain visual pathway, consisting
of retinal ganglion cell (RGC) and subcortical targets is a
quite popular model investigating neural axon regeneration.
Previous study demonstrates that electrical stimulation
can shape corticospinal (CS) axon outgrowth and augment
connections after injury(2). Similarly, another study
using RGCs shows that electrical stimulating accelerates
axonal outgrowth in vitro(3). Interestingly, electrical
stimulates works through the electrical activity of RGCs.
Consequently, Lim et al. translate these findings and uses
electrical stimulating into visual signal in their study, which
is a potent stimulus that closely mimics genuine visual
signals of the eye(4). After the optic nerve is crushed, the
adult mice are exposed to high-contrast visual stimulation
daily for 3 weeks. Amazingly, RGCs axons are observed to
regenerate over long distance into the brain. To confirm
whether RGCs axon regeneration is initiated by electrical
activity from visual stimulation, RGCs are being either
chemo genetically activated or silenced. The distance of
regeneration is proportional to the RGC activity level; the
effect of visual stimulation on RGC axon regeneration is
abolished by RGC silence, while increased activity leads to
better regeneration.
Optic nerve axon regeneration has been investigated
for many years. The following factors are shown to
be related to the capacity of regeneration, including
cell-intrinsic signals, transcription factors and their
inhibitors, receptors to cell-extrinsic inhibitors and
intraocular inffammation(5). Among these factors, PTEN
and SOCS3, as cell-intrinsic suppressors, are the most
promising. PTEN deletion with SOCS3 deletion, in the
presence of CNTF, successful induces a longer distance
axonal regeneration(6,7). Recently, more attention is
directed at mTOR, a downstream molecule within the
PI3K/PTEN/mTOR pathway. The mTOR signaling
pathway has a pivotal role in numerous cellular processes,
including axonal regeneration. Strengthening mTOR
signaling shows increased axons regeneration in optic nerve
lesion(8). Therefore, cRheb1, which is a positive regulator of mTOR signaling, is introduced into Lim’s study. In the
present of visual stimulation in addition to Rheb1, axon
regeneration reaches a longer distance. Unfortunately, the
stimulation of mTOR signaling in RGCs is not the “cure”
for regeneration. RGC axons still fail to pass beyond the
mid-optic nerve and optic chiasm.
Following the thinking that optic nerve is an extended
part from CNS, Lim and his colleagues were enlightened
by the rehabilitation treatment of limb paralysis caused by
spinal cord injury. It has been reported that forced use of
an impaired limb promotes sprouting of CS axons(9). By
suturing shut the non-lesioned eye, the lesioned eye was
forced to be biased used. Surprisingly, the cocktail scheme
combining visual stimulation with cRheb1 plus biased use
led to axonal regeneration extended through optic chiasm,
down the optic nerve and back to the brain.
As a milestone that long-distance regeneration of RGC
axons to the brain is possible, the next critical question
“where would the axon go” is on the table. A new transgenic
mouse line is adopted in the study, in which Cochlin-GFP
(CoCH-GFP) is used to label a specific subtype of RGC
that densely innervate the vLGN, dLGN, OPN and SC.
These RGCs avoid the SCN, MTN and intergeniculate
leaflet (IGL). After crushing the optic nerves in these
transgenic mice, the regenerated CoCH-GFP + RGC
axons are found to reach the vLGN, dLGN, OPN and
SC, bypassing the SCN, the nucleus of the optic tract
(NOT) and the MTN. This result indicates that RGCs are
remarkably capable of navigating the axons back to and reinnervate
their original targets in the brain. Furthermore, the
rebuilding of visual function is also detected in these mice.
The remarkable advantages in this study bring the
scientists closer to goal of curing nerve injuries. Especially,
it is a sparkling strategy to apply unilateral lid suture in
order to force the biased use of the other eye, reminding
us the developmental connections between CNS and optic
nerve. This work is promising in the entire ffeld of neural
regeneration in the CNS. Forcing the use of an impaired
limb promotes CS axonal regeneration, not only by activity
stimulation, but also by biased use. Considering these
results in spinal injury, in terms of optic nerve, the activity
stimulation is translated into visual signal and the biased use
is achieved by unilateral lid suture, similar to the eye patch
in amblyopia.
However, since the optic nerve crushing model is used,
the result probably means more to patients with axon
injury. As to glaucoma, it is still unknown whether the
transplanted RGCs can integrate into the host retina or
regenerate axons. In the optic nerve-crushing model, the
connection between photoreceptors and RGC cell bodies
are still intact. This is the structural foundation of “visual
stimulation”. However, in glaucoma, the RGCs are dead. If
the transplanted RGCs cannot reconnect to photoreceptors,
it is meaningless to provide the visual stimulus. On the
other hand, the previous in vitro study shows that the
regeneration capacity of RGCs is lost when dendritic grows
and synaptic inputs expand (3,10). Moreover, during the
course of development, RGC axons were guided by the
recognitions of receptors expressed on growth cones and
their ligand molecules, such as Cadherin 6 (11) and Eph (12,13). Another study in spinal cord injuries showed that
the axonal re-innervation is guided chemotropically (14). As
the ganglion cell bodies are already ruined in glaucomatous
retina, it is in doubt that whether the new RGCs still have
the memory to differentiate into the speciffc missing RGC
subtypes and then recognize the guided signals and go back
to the right targets. The current study also suggests that the
time window critically affects the capacity of RGCs axon
regeneration after crush injury. We do have the concerns in
this study whether the axons are completely crushed and the
debris disappeared before regenerating axon finding their
paths across the abandoned field through the optic nerve.
Are there any essential guiding structures or molecules
presented within optic nerve which help the regenerating
axons ffnding home to their brain targets? Or otherwise, are
there any possibilities that some axons are partially damaged
and survive the crush, while they are the ones regain their
anatomical and functional integrity afterwards.
Nevertheless, the current study provides us with
an exciting new strategy investigating RGCs exon
regeneration. It is our genuine wish that more and more
studies are completed based on this model, which will
unequivocally confirm the regenerating capacity that we
have been dreaming for in decades. If the injured axons
in optic nerve are truly capable of re-growing back to the
brain, this could be promising for some clinical patients,
though there is a long way to go. We wish that strategy for
axon regeneration in the current study can be translated
into numerous studies using stem cell transplantation.
Hopefully, neural activity stimulation and mTOR pathway
modulation can be helpful to regenerating axons from
RGCs derived from transplanted stem cells.
