Biomedical Engineering Reference
In-Depth Information
Ca
2+
accumulation (Ni et al.
2005
) and the inhibition of regulated endocytosis
(Malarkey et al.
2008
) both showed concentration dependence on SWCNTs, which
corresponded well to SWCNT-PEG concentrations affecting neurite numbers and
outgrowth (Ni et al.
2005
). Taken together, these results suggest the exciting possi-
bility that water-soluble SWCNTs could be delivered locally to the site of CNS
injury to enhance neurite outgrowth which might increase the probability to over-
pass the site of injury and aid in the process of regeneration.
A possible therapeutic intervention using water-soluble SWCNTs has been ini-
tiated for treatment of spinal cord injury (SCI) (Roman et al.
2009a
; Roman et al.
2009b;
Roman et al.
2011
). Traumatic SCI causes tissue damage resulting in the
formation of a cavity that inhibits axonal regrowth. Filling this cavity with a
growth-inducing agent, such as SWCNT-PEG, could promote regeneration and
repair. To assess this issue, SCI was induced by complete transection of the spinal
cord at the thoracic 9 vertebrae of adult female Sprague-Dawley rats. Immediately
after transection, the epicenter of the lesion was injected with either SWCNT-
PEG solution or the vehicle. At 28 days post SCI, the rats were euthanized and
spinal cord tissue was examined using immunocytochemistry. The addition of
SWCNT-PEG was found to promote neurite outgrowth and reduce reactive glio-
sis. These data suggest that injectable water-soluble SWCNT-PEG materials hold
promise as a neurite outgrowth-promoting agent in treatment after SCI.
The above studies suggest that CNTs are an excellent source material for generat-
ing injectable materials that can be used to modulate neurite outgrowth in culture and
in vivo. While mechanisms underlying such actions have been investigated, it appears
that they are mediated simply by physical-chemical characteristics of functional
groups and/or of CNTs. One caveat of such an approach is that it may lack specifi city
that neural cells utilize in intercellular interactions. Consequently, it should be impor-
tant to engineer CNTs that are functionalized with biological molecules that possess
ligand-receptor specifi city. Matsumoto et al. (Matsumoto et al.
2007
) have accom-
plished a fi rst step toward such a goal. They functionalized CNTs using endogenous
ligands in the CNS, neurotrophins, to assess the retention of ligand activity when
conjugated to CNTs. Matsumoto et al. covalently bonded nerve growth factor (NGF)
or brain-derived neurotrophic factor (BDNF) to MWCNTs, and studied the effect of
these graft copolymers, delivered by dispersing them in culturing media of neurons
isolated from DRGs of 8-day-old chick embryos. To generate neurotrophin-bearing
MWCNTs, CNTs were fi rst functionalized with amino groups that were then used to
attach NGF or BDNF. The resulting neurotrophin-MWCNT materials were dispersed
in phosphate-buffered saline by sonication and the success of this functionalization
procedure was assessed using an enzyme-linked immunoabsorbent assay and anti-
bodies against NGF or BDNF. DRGs from chick embryos were dissociated to estab-
lish neuronal cultures that were grown in the standard laminin-coated well plates.
The numbers of DRG neurons with neurite outgrowth longer than the cell body were
counted. The addition of a dispersion of either NGF-MWCNT or BDNF-MWCNT
in cell culture media prompted neurite outgrowth, which was comparable to that caused
by the soluble NGF or BDNF. This indicated that neurotrophins covalently attached
to MWCNTs retained their bioactivity, including the engagement of extracellular
