Custom-engineered hydrogels for delivery of human iPSC-derived neurons into the injured cervical spinal cord

Cervical damage is the most prevalent type of spinal cord injury clinically, although few preclinical research studies focus on this anatomical region of injury. Here we present a combinatorial therapy composed of a custom-engineered, injectable hydrogel and human induced pluripotent stem cell (iPSC)-derived deep cortical neurons. The biomimetic hydrogel has a modular design that includes a protein-engineered component to allow customization of the cell-adhesive peptide sequence and a synthetic polymer component to allow customization of the gel mechanical properties. In vitro studies with encapsulated iPSC-neurons were used to select a bespoke hydrogel formulation that maintains cell viability and promotes neurite extension. Following injection into the injured cervical spinal cord in a rat contusion model, the hydrogel biodegraded over six weeks without causing any adverse reaction. Compared to cell delivery using saline, the hydrogel significantly improved the reproducibility of cell transplantation and integration into the host tissue. Across three metrics of animal behavior, this combinatorial therapy significantly improved sensorimotor function by six weeks post transplantation. Taken together, these findings demonstrate that design of a combinatorial therapy that includes a gel customized for a specific fate-restricted cell type can induce regeneration in the injured cervical spinal cord.

Implantation of Collagen Iv/Poly(2-Hydroxyethyl Methacrylate) Hydrogels Containing Schwann Cells into the Lesioned Rat Optic Tract

Poly (2-hydroxyethylmethacrylate) (PolyHEMA) hydrogels, when combined with extracellular matrix molecules and infiltrated with cultured Schwann cells, have the capability to induce CNS axonal regrowth after injury. We have further investigated these PolyHEMA hydrogels and their potential to bridge CNS injury sites. Collagen IV-impregnated hydrogels containing Schwann cells were implanted into the lesioned optic tract in 14 rats. On examination 2–4 months later, there was good adherence between the implants and CNS tissue, and large numbers of viable Schwann cells (S100+, GFAP+, Laminin+, and LNGFR+) were seen within the hydrogel matrices. Immunohistochemical analysis showed that the collagen IV-impregnated PolyHEMA hydrogels preferentially supported the transplanted Schwann cells and not host glial cells such as astrocytes (GFAP+) or oligodendroglia (CAII+). Macrophages (ED1+) were also seen within the sponge structure. Eighty-three percent of the implanted hydrogels contained RT97+ axons within their trabecular networks. Regrowing axons were associated with the transplanted Schwann cells and not with the small number of infiltrating astrocytes. RT97+ axons were traced up to 510 μm from the nearest host neuropil. These axons were sometimes myelinated by the transplanted Schwann cells and expressed the peripheral myelin marker Po+. WGA/HRP-labeled retinal axons were seen within transplanted hydrogel sponges, with 40% of the cases growing for distances up to 350–450 μm within the polymer network. The data indicate that impregnating PolyHEMA sponges with collagen IV can modify the host glial reaction and support the survival of transplanted Schwann cells. This study thus provides new information on how biomaterials could be used to modify and bridge CNS injury sites.

Scaffolds to promote spinal cord regeneration

Substantial research effort in the spinal cord injury (SCI) field is directed towards reduction of secondary injury changes and enhancement of tissue sparing. However, pathway repair after complete transections, large lesions, or after chronic injury may require the implantation of some form of oriented bridging structure to restore tissue continuity across a trauma zone. These matrices or scaffolds should be biocompatible and create an environment that facilitates tissue growth and vascularization, and allow axons to regenerate through and beyond the implant in order to reconnect with "normal" tissue distal to the injury. The myelination of regrown axons is another important requirement. In this chapter, we describe recent advances in biomaterial technology designed to provide a terrain for regenerating axons to grow across the site of injury and/or create an environment for endogenous repair. Many different types of scaffold are under investigation; they can be biodegradable or nondegradable, natural or synthetic. Scaffolds can be designed to incorporate immobilized signaling molecules and/or used as devices for controlled release of therapeutic agents, including growth factors. These bridging structures can also be infiltrated with specific cell types deemed suitable for spinal cord repair.

Regrowth of axons within Schwann cell-filled polycarbonate tubes implanted into the damaged optic tract and cerebral cortex of rats

The efficacy of Schwann cell-filled polycarbonate tubes as a bridging substrate for regrowing axons following lesions of the rat optic tract or cerebral cortex has been assessed after short (11-31 days) or long (82-119 days) survival times. Tubes were impregnated with Iaminin and poly-L-lysine, soaked in basic FGF and filled with Schwann cells. They were implanted into optic tract lesions in 34 rats aged 15-21 days and into cortical lesion cavities in 3 adult rats. Gelfoam soaked in basic FGF and Schwann cell conditioned medium was placed over the tubes. In one group of rats, axon regrowth into implants was assessed using neurofilament antibody RT97; antibodies to proteolipid protein, Po, Iaminin, the low-affinity nerve growth factor receptor (NGFr), S-100 and EDI were also used to study myelination and the cellular content of the tubes. In a second group of rats, anterograde tracing techniques were used to specifically identify host retinal axons within the implanted polymers. After long survival times, the relationships between regrown axons and cells inside the tubes were also examined ultrastructurally. In all implants examined immunohistochemically at short survival times, large numbers of RT97+ axons were found throughout the tubes, usually in association with Iaminin+, NGFr+ Schwann cells. At longer survival times, viable Schwann cells were still present, but tubes contained fewer axons and less cellular material. This material often formed a cord (200-250 μm thick) which extended the length of the implant. In the second group of rats, labelled retinal axons were found in 11 of the 16 implants that were attached to the dLGN. Axons regrew up to 1 mm but did not reach the distal (tectal) end of the implants. Interestingly, there was no evidence of myelinogenesis by either implanted Schwann cells or by host-derived oligodendroglia which had migrated into the tubes. Oligodendroglia were usually encircled by processes, many of which originated from Schwann cells, suggesting that the grafted cells may have been involved in isolating the central glia. The data show that Schwann cell-filled polycarbonate tubes provided a favourable milieu for axonal regeneration in the short term; however over time there was a decrease in the cellular and fibre content of the tubes. After intracranial implantation, an additional supporting matrix inside the polycarbonate tubes may aid in providing an environment conducive to the long term maintenance of regenerated retinal and other axons.

Culture conditions affect proliferative responsiveness of olfactory ensheathing glia to neuregulins

Olfactory ensheathing glia (OEG) have been used to improve outcome after experimental spinal cord injury and are being trialed clinically. Their rapid proliferation in vitro is essential to optimize clinical application, with neuregulins (NRG) being potential mitogens. We examined the effects of NRG-1beta, NRG-2alpha, and NRG3 on proliferation of p75-immunopurified adult OEG. OEG were grown in serum-containing medium with added bovine pituitary extract and forskolin (added mitogens) or in serum-containing medium (no added mitogens). Cultures were switched to chemically defined medium (no added mitogens or serum), NRG added and OEG proliferation assayed using BrdU. OEG grown initially with added mitogens were not responsive to added NRGs and pre-exposure to forskolin and pituitary extract increased basal proliferation rates so that OEG no longer responded to added NRG. However, NRG promoted proliferation but only if cells were initially grown in mitogen-free medium. Primary OEG express ErbB2, ErbB3, and small levels of ErbB4 receptors; functional blocking indicates that ErbB2 and ErbB3 are the main NRG receptors utilized in the presence of NRG-1beta. The long-term stimulation of OEG proliferation by initial culture conditions raises the possibility of manipulating OEG before therapeutic transplantation.

AAV-mediated expression of CNTF promotes long-term survival and regeneration of adult rat retinal ganglion cells

We compared the effects of intravitreal injection of bi-cistronic adeno-associated viral (AAV-2) vectors encoding enhanced green fluorescent protein (GFP) and either ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF) or growth-associated protein-43 (GAP43) on adult retinal ganglion cell (RGC) survival and regeneration following (i) optic nerve (ON) crush or (ii) after ON cut and attachment of a peripheral nerve (PN). At 7 weeks after ON crush, quantification of betaIII-tubulin immunostaining revealed that, compared to AAV-GFP controls, RGC survival was not enhanced by AAV-GAP43-GFP but was increased in AAV-CNTF-GFP (mean RGCs/retina: 17 450+/-358 s.e.m.) and AAV-BDNF-GFP injected eyes (10 200+/-4064 RGCs/retina). Consistent with increased RGC viability in AAV-CNTF-GFP and AAV-BDNF-GFP injected eyes, these animals possessed many betaIII-tubulin- and GFP-positive fibres proximal to the ON crush. However, only in the AAV-CNTF-GFP group were regenerating RGC axons seen in distal ON (1135+/-367 axons/nerve, 0.5 mm post-crush), some reaching the optic chiasm. RGCs were immunoreactive for CNTF and quantitative RT-PCR revealed a substantial increase in CNTF mRNA expression in retinas transduced with AAV-CNTF-GFP. The combination of AAV-CNTF-GFP transduction of RGCs with autologous PN-ON transplantation resulted in even greater RGC survival and regeneration. At 7 weeks after PN transplantation there were 27 954 (+/-2833) surviving RGCs/retina, about 25% of the adult RGC population. Of these, 13 352 (+/-1868) RGCs/retina were retrogradely labelled after fluorogold injections into PN grafts. In summary, AAV-mediated expression of CNTF promotes long-term survival and regeneration of injured adult RGCs, effects that are substantially enhanced by combining gene and cell-based therapies/interventions.

Adult olfactory ensheathing glia promote the long-distance growth of adult retinal ganglion cell neurites in vitro

In vivo, transplanted adult olfactory ensheathing glia (OEG) and adult Schwann cells (SC) can support the regrowth of at least some transected axons within adult CNS neuropil. In the present study, we developed an in vitro adult rat retinal explant model to explore the influence of primary adult SC and OEG on retinal ganglion cell (RGC) neurite regrowth in the presence of glial cells endogenous to the retina. Retinal quadrants were plated RGC-side down onto aclar hats coated with either pure collagen (type 1), collagen with OEG, collagen with SCs, or collagen coated with both OEG and SCs. Regrowing retinal neurites extended onto the pure collagen substrate, largely in association with astrocytes that migrated out from the explants (mean number of neurites: 144+/-65 SEM). The additional presence of OEG (669+/-122), but not SCs (97+/-41), supported the regrowth of significantly greater numbers of RGC neurites. Furthermore, this OEG-stimulated regeneration was over significantly greater distances; >68% of neurites extended >500 microm from the explant, compared with explants plated onto SCs or collagen alone (15% and 29%, respectively). When OEG and SCs were co-cultured the number of regenerating neurites was reduced (397+/-81) compared with the pure OEG treatment. Analysis of explants on pure collagen substrates fed with media conditioned by purified OEG or SC showed no increase in neurite outgrowth compared with control treatments, suggesting that the enhanced growth in the presence of OEG is a contact-mediated effect. The observed differences between the abilities of OEG and SC to support the growth of CNS-derived fibers in the presence of astrocytes support the suggestion that OEG may be better suited for direct transplantation into CNS neuropil following injury.

Viral vector-mediated gene expression in olfactory ensheathing glia implants in the lesioned rat spinal cord

Implantation of olfactory ensheathing glia (OEG) is a promising strategy to augment long-distance regeneration in the injured spinal cord. In this study, implantation of OEG following unilateral hemisection of the dorsal cervical spinal cord was combined with ex vivo gene transfer techniques. We report, to our knowledge for the first time, that purified cultures of primary OEG are capable of expressing a foreign gene following adenoviral (AdV) and lentiviral (LV) vector-mediated gene transfer. OEG implants subjected to AdV vector-mediated gene transfer expressed high levels of transgenic protein in both intact and lesioned spinal cord at 7 days after implantation. However, the levels of transgene expression gradually declined between 7 and 30 days after implantation in lesioned spinal cord. Infection with LV vectors resulted in stable transduction of primary OEG cultures and transgene expression persisted for at least 4 months after implantation. Genetic engineering of OEG opens the possibility of expressing additional neurotrophic genes and create optimal 'bridging' substrates to support spinal axon regeneration. Furthermore, stable transduction of OEG allows us to reliably study the behaviour of implanted cells and to obtain better understanding of their regeneration supporting properties.

Mitogenic response of adult rat olfactory ensheathing glia to four growth factors

Olfactory ensheathing glia (EG) from adult rat proliferate slowly in vitro without added mitogens. The potential future use of EG in transplantation within the central nervous system to improve neural repair is dependent on identifying mitogens that will effectively expand EG without altering their phenotype. The mitogenic effects of heregulin (HRG), fibroblast growth factor 2 (FGF-2), platelet-derived growth factor BB (PDGF-BB), insulin-like growth factor 1 (IGF-1), and forskolin (FSK) on cultured adult-derived rat EG were monitored by tritiated-thymidine labeling and p75 immunostaining. In serum-containing medium, HRG, FGF-2, PDGF-BB, IGF-1, and FSK were capable of stimulating EG proliferation, and the stimulation by these growth factors was potentiated by FSK. The combinations of HRG + FGF-2, HRG + PDGF-BB, HRG + IGF-1, FGF-2 + PDGF-BB, and FGF-2 + IGF-1 all promoted EG proliferation in an additive manner. In serum-free medium, HRG and FGF-2 were mitogenic, but PDGF-BB, IGF-1 and FSK were not; however, FSK potentiated the stimulation by HRG and FGF-2, and the combination of HRG + FGF-2 promoted EG proliferation in an additive manner. This new information will be useful for the design of protocols to achieve sufficient numbers of adult-derived EG for clinical purposes. This study also further establishes similarities between EG and Schwann cells.

Inhibitory proteoglycan immunoreactivity is higher at the caudal than the rostral Schwann cell graft-transected spinal cord interface

To begin to evaluate the influence that proteoglycans may have on the success of Schwann cell (SC) transplants to induce axonal regrowth across a complete transection lesion and beyond, we determined the pattern of expression of inhibitory chondroitin sulfate proteoglycans (CSPGs) 3 weeks after transplantation into completely transected adult rat thoracic spinal cord. Using immunohistochemistry, we observed that: (1) CSPGs recognized by CS-56 antibody are present on astrocytes, fibroblasts, and SCs in the distal graft, and at lesion and cystic cavity borders; (2) CS-56 immunoreactivity (IR) is greater at the caudal SC graft-host cord interface than the rostral interface; (3) phosphacan-IR, also greater at the caudal interface, is associated with astrocytes, fibroblasts, as yet unidentified cells, and extracellular matrix; (4) neurocan-IR is present on astrocytes and as yet unidentified cells in grey and white matter; and (5) NG2-IR is associated with matrix near SC grafts, unidentified cells mainly in white matter, and lesion borders and cysts. Neither oligodendrocytes nor activated macrophages/microglia were immunostained. In sum, the CSPGs studied are increased at 3 weeks, especially at the caudal SC graft-cord interface, possibly contributing to an inhibitory molecular barrier that precludes regrowing descending axons from entering the caudal host cord.

A new type of biocompatible bridging structure supports axon regrowth after implantation into the lesioned rat optic tract

We have developed a new type of polymer/cell/matrix implant and tested whether it can promote the regrowth of retinal ganglion cell (RGC) and other axons across surgically induced tissue defects in the CNS. The constructs, which consisted of 2-2.5-mm-long polycarbonate tubes filled with lens capsule-derived extracellular matrix coated with cultured neonatal Schwann cells, were implanted into lesion cavities made in the left optic tract (OT) of 18-21-day-old rats. In one group, to promote Schwann cell proliferation and perhaps also to stimulate axon regrowth, basic fibroblast growth factor (bFGF) was added to the lens capsule matrix prior to implantation. In another group, to determine whether application of growth factors to the somata of cells enhances the regrowth of distally injured axons, the neurotrophin NT-4/5 was injected into the eye contralateral to the OT lesion. NT-4/5 and bFGF treatments were combined in some rats. After medium-term (4-10 weeks) or long-term (15-20 weeks) survivals, axon growth into implants was assessed immunohistochemically using a neurofilament (RT97) antibody. RGC axons were visualized after injection of WGA/HRP into the right eye. Viable Schwann cells were present in implants at all times after transplantation. Large numbers of RT97+ axons were consistently found within the bridging implants, often associated with the peripheral glia. Axons were traced up to 1.7 mm from the nearest CNS neuropil and there was immunohistochemical evidence of myelination by Schwann cells and by host oligodendrocytes. There were fewer RGC axons in the implants, fibers growing up to 1.6 mm from the thalamus. Neither NT-4/5 nor bFGF, alone or in combination, significantly increased the extent of RGC axon growth within the implants. A group of OT-lesioned rats was implanted with polymer tubes filled with 2-2.5-mm-long pieces of predegenerate peripheral nerve. Surprisingly, polymer/cell/matrix constructs contained comparatively greater numbers of RGC and other axons and supported more extensive axon elongation. Thus, implants of this type may potentially be useful in bridging large tissue defects in the CNS.

Poly(alpha-hydroxyacids) for application in the spinal cord: resorbability and biocompatibility with adult rat Schwann cells and spinal cord

Future surgical strategies to restore neurological function in the damaged human spinal cord may involve replacement of nerve tissue with cultured Schwann cells using biodegradable guiding implants. We have studied the in vitro and in vivo degradability of various aliphatic polyesters as well as their effects on rat Schwann cells in vitro and on spinal cord tissue in vivo. In vitro, cylinders made of poly(D,L-lactic-co-glycolic acid) 50:50 (PLA25GA50) started to degrade at 7 days, compared with 28 days for cylinders made of poly(D,L-lactic acid) (PLA50). This faster degradation of PLA25GA50 was reflected by a much higher absorption of water. In vivo, after implantation of PLA25GA50 or PLA50 cylinders between the stumps of a completely transected adult rat spinal cord, the decrease in molecular weight of both polymers was similar to that found in vitro. In vitro degradation of poly(L-lactic acid) (PLA100) mixed with increasing amounts of PLA100 oligomers also was determined. The degradation rate of PLA100 mixed with 30% oligomers was found to be similar to that of PLA50. In vitro, PLA25GA50 and the breakdown products had no adverse effect on the morphology, survival, and proliferation of cultured rat Schwann cells. In vivo, PLA25GA50 cylinders were integrated into the spinal tissue 2 weeks after implantation, unlike PLA50 cylinders. At all time points after surgery, the glial and inflammatory response near the lesion site was largely similar in both experimental and control animals. At time points later than 1 week, neurofilament-positive fibers were found within PLA25GA50 cylinders or the remains thereof. Growth-associated protein 43, which is indicative of regenerating axons, was observed in fibers in the vicinity of the injury site and in the remains of PLA25GA50 cylinders. The results suggest that poly(alpha-hydroxyacids) are likely candidates for application in spinal cord regeneration paradigms involving Schwann cells.

Implantation of collagen IV/poly(2-hydroxyethyl methacrylate) hydrogels containing Schwann cells into the lesioned rat optic tract

Poly (2-hydroxyethylmethacrylate) (PolyHEMA) hydrogels, when combined with extracellular matrix molecules and infiltrated with cultured Schwann cells, have the capability to induce CNS axonal regrowth after injury. We have further investigated these PolyHEMA hydrogels and their potential to bridge CNS injury sites. Collagen IV-impregnated hydrogels containing Schwann cells were implanted into the lesioned optic tract in 14 rats. On examination 2-4 months later, there was good adherence between the implants and CNS tissue, and large numbers of viable Schwann cells (S100+, GFAP+, Laminin+, and LNGFR+) were seen within the hydrogel matrices. Immunohistochemical analysis showed that the collagen IV-impregnated PolyHEMA hydrogels preferentially supported the transplanted Schwann cells and not host glial cells such as astrocytes (GFAP+) or oligodendroglia (CAII+). Macrophages (ED1+) were also seen within the sponge structure. Eighty-three percent of the implanted hydrogels contained RT97+ axons within their trabecular networks. Regrowing axons were associated with the transplanted Schwann cells and not with the small number of infiltrating astrocytes. RT97+ axons were traced up to 510 microm from the nearest host neuropil. These axons were sometimes myelinated by the transplanted Schwann cells and expressed the peripheral myelin marker Po+. WGA/HRP-labeled retinal axons were seen within transplanted hydrogel sponges, with 40% of the cases growing for distances up to 350-450 microm within the polymer network. The data indicate that impregnating PolyHEMA sponges with collagen IV can modify the host glial reaction and support the survival of transplanted Schwann cells. This study thus provides new information on how biomaterials could be used to modify and bridge CNS injury sites.

In vitro assessment of the biological activity of basic fibroblast growth factor released from various polymers and biomatrices

The kinetics of controlled release of basic fibroblast growth factor (bFGF) from polymers (sutures, polycarbonate, Hydron, and Elvax), biopolymers (alginate), and biomatrices (lens capsules), and conditions for storage of bFGF (temperature, plastic type, heparin) were evaluated in vitro. Tissue culture proliferation bioassays with 3T3 fibroblasts, showed that only lens capsules with bFGF had a sustained release of bFGF for up to three weeks. The other materials released all of the 'bound' bFGF with two hours or produced an inflammatory response in vivo. Therefore, the lens tissue had the most potential for controlled long-term delivery of bFGF in vivo. These studies emphasise the importance of in vitro analysis of release kinetics of growth factors from a range of materials as a basis for potential in vivo applications.

Hydrogels containing peptide or aminosugar sequences implanted into the rat brain: influence on cellular migration and axonal growth

Biocompatible polymer matrices for implantation into lesion sites in the brain were synthesized by incorporating peptide or aminosugar sequences into N-(2-hydroxypropyl)methacrylamide (HPMA) hydrogels. RGD peptide sequences were chemically linked to the hydrogel backbone via a glycylglycine spacer; aminosugars were glucosamine (NHGlc) or N-acetylglucosamine residues. Unmodified or sequence containing HPMA hydrogels were implanted into the lesioned optic tract or cerebral cortex of juvenile (17- to 19-day-old) or adult rat brains, respectively. After 10-12 months host animals were perfused and the brains were processed for immunohistochemistry using antibodies to neurofilaments (RT97), laminin, glial fibrillary acidic protein (GFAP), carbonic anhydrase II (CAII), S100 protein, macrophages (ED1), and myelin basic protein (MBP). Unmodified (control) HPMA hydrogels contained no cellular infiltration or axonal growth. Peptide (RGD)- and aminosugar-modified hydrogels showed increased adhesion properties with host neural tissue, were vascularized, and were infiltrated by host nonneuronal cells. Astrocytes (GFAP+) and macrophages (ED1(+)) were the major cell types seen within modified HPMA hydrogels, the largest numbers being found in RGD-containing polymers. CAII+ oligodendroglia were not seen within any of the hydrogel matrices. RT97(+)/MBP- axons grew into both the RGD and NHGlc hydrogel matrices for small distances. The number of axons was greatest in hydrogels implanted into cerebral cortex but in both cortex and optic tract implants the highest density of axons was seen in polymers containing RGD. The findings of this study are discussed in the context of CNS tissue replacement and the construction of bioactive scaffolds to promote regenerative axonal growth across areas of injury in the brain and spinal cord.

Cultured rat neuronal and glial cells entrapped within hydrogel polymer matrices: a potential tool for neural tissue replacement

Cultured Schwann cells, neonatal astrocytes or cells dissociated from embryonic cerebral hemispheres were dispersed within poly-[N-(2-hydroxypropyl)-methacrylamide]-based hydrogel matrices by gel entrapment and maintained in vitro for 1-6 days. Glial cells were pre-labelled with Hoechst 33342. Cell differentiation and viability were studied by immunocytochemistry. Up to 15% of Schwann cells initially entrapped within the polymer matrices were immunopositive for the low affinity nerve growth factor receptor, S100, glial fibrillary acidic protein (GFAP) and laminin; up to 10% of pre-labelled astrocytes were immunopositive for GFAP and laminin. Embryonic neurons displayed immunostaining for neurofilaments. Hydrogels containing entrapped Schwann cells were implanted into the rat neocortex. These polymers supported cellular and axonal ingrowth within parts of the polymer matrix.

Neural tissue engineering: from polymer to biohybrid organs

This investigation reports on the immobilization of neuronal and glial cells (Schwann cells and astrocytes) within N-(2-hydroxypropyl) methacrylamide (HPMA) polymer hydrogels for the production of cell-based polymer hybrid devices. Cells were included within HPMA polymer networks by gel-entrapment, and these biogels were maintained in vitro for up to 6 days. Cell viability and differentiation were studied using immunocytochemical methods and image analysis techniques. The polymer structure and its relationships with cells were examined by scanning electron microscopy. A proportion of the cell population was viable, expressing its own antigenic profile throughout the period of gel incubation, as cells do in conventional culture conditions, and some cells exhibited behaviour such as spreading or process outgrowth and secretion of laminin. The result of the present study allows us to envisage tissue replacement in the central nervous system by means of such cell-based polymer constructs.

Schwann cells and the regrowth of axons in the mammalian CNS: a review of transplantation studies in the rat visual system

1. We have used peripheral nerve transplants or cultured Schwann cells grafted in association with different types of polymer to study axonal regrowth in the rat visual system. In some instances the glia were co-grafted with fetal tectal tissue. 2. The studies have two main aims: (i) to determine whether retinal axons can be induced to regrow at a site distant from their cell soma, that is, after damage to the brachial region of the optic tract; (ii) to determine whether retinal axons exposed to Schwann cells retain the ability to recognize their appropriate target neurons in CNS tissue. 3. In brachial lesion studies, Schwann cells were placed in the lesion site in association with nitrocellulose papers, within polycarbonate tubes in the presence or absence of a supporting extracellular matrix (ECM), or within polymer hydrogel scaffolds. Autologous sciatic nerve grafts were also used. Immunohistochemical studies revealed the presence of regenerating axons within all polymer bridges. Regrowth of retinal axons was also seen, however, growth was not extensive and was limited to the proximal 1-1.5 mm of the implants. 4. In target innervation experiments, two surgical paradigms were developed. In one experiment, a segment of sciatic nerve was autografted onto the transected optic nerve in adult rats and the distal end of each graft was placed adjacent to fetal tectal (target) tissue implanted into the frontal cortex. To date, we have not been able to demonstrate selective recognition of target regions within tectal transplants by retinal axons exiting the sciatic nerve implants. 5. In the second experiment, Schwann cells were mixed with fetal tectal cells and co-grafted to the midbrain of newborn host rats. Schwann cells altered the characteristic pattern of host retinal growth into tectal grafts; in some cases axons were induced to grow away from appropriate target areas by nearby co-grafted Schwann cells. 6. In summary, Schwann cell/polymer scaffolds may provide a useful way of promoting the regrowth of damaged axons in the CNS, however: (i) in adults, at least, their effectiveness is reduced if they are located at a distance from the cell bodies giving rise to regenerating axons; (ii) in some circumstances exposure to a peripheral glial environment may affect the capacity of regenerating axons to recognize appropriate target cells in the CNS neuropil.

Schwann cells and fetal tectal tissue cografted to the midbrain of newborn rats: fate of Schwann cells and their influence on host retinal innervation of grafts

Schwann cells transplanted into the adult central nervous system (CNS) can exert powerful growth-promoting effects on damaged axons. An important issue is whether central axons induced to regrow by Schwann cells retain the capacity to recognize and selectively innervate their appropriate target cells. To examine how Schwann cells may influence the specificity of neuron-neuron interactions in CNS neuropil, we cultured neonatal rat Schwann cells and mixed them with dissociated fetal tectal cells. In some instances, Schwann cells were prelabeled with Hoechst dye 33342. Schwann cells comprised between 2.5 and 15% of the combined cell population. After reaggregation, cografts were injected onto the midbrain of newborn rats. One to 6 months later, grafts were examined for the presence of Schwann cells and the pattern and density of host retinal innervation of the cografts was assessed. Immunohistochemical studies showed that areas of the transplants containing large numbers of surviving Hoechst-labeled Schwann cells were strongly immunoreactive for the low-affinity nerve growth factor receptor (p75), S-100, GFAP, and laminin. Very little peripheral (Po positive) myelin was seen. As in pure fetal tectal grafts, host retinal axons were sometimes observed to innervate superficial, localized areas in the cografts known to be homologous to the retinorecipient layers of the superior colliculus. Unlike pure tectal grafts, however, optic axons were not confined to these regions and fibers were often dispersed within the cograft neuropil. Dense growth was seen in association with Hoechst-labeled Schwann cells and, in some cases, optic axons were observed to grow toward Schwann cells and away from nearby target areas. These observations suggest that, under certain circumstances, Schwann cells can stimulate retinal axons to grow into inappropriate (nontarget) regions in the CNS, presumably by producing growth promoting factors which mask or compete with signals released from the target neurons themselves.

Axonal growth within poly (2-hydroxyethyl methacrylate) sponges infiltrated with Schwann cells and implanted into the lesioned rat optic tract

Porous hydrophilic sponges made from 2-hydroxyethyl methacrylate (HEMA) have a number of possible biomedical applications. We have investigated whether these poly(HEMA) hydrogels, when coated with collagen and infiltrated in vitro with cultured Schwann cells, can be implanted into the lesioned optic tract and act as prosthetic bridges to promote axonal regeneration. Nineteen rats (20-21 days old) were given hydrogel/Schwann cell implants. No obvious toxic effects were seen, either to the transplanted glia or in the adjacent host tissue. Schwann cells survived the implantation technique and were immunopositive for the low affinity nerve growth factor receptor, S100 and laminin. Immunohistochemical studies showed that host non-neuronal cells (astrocytes, oligodendroglia and macrophages) migrated into the implanted hydrogels. Astrocytes were the most frequently observed host cell in the polymer bridges. RT97-positive axons were seen in about two thirds of the implants. The axons were closely associated with transplanted Schwann cells and, in some cases, host glia (astrocytes). Individual axons regrowing within the implanted hydrogels could be traced for up to 900 microns, showing that there was continuity in the network of channels within the polymer scaffold. Axons did not appear to be myelinated by either Schwann cells or by migrated host oligodendroglia. In three rats, anterograde tracing with WGA/HRP failed to demonstrate the presence of retinal axons within the hydrogels. The data indicate that poly(HEMA) hydrogels containing Schwann cells have the potential to provide a stable three-dimensional scaffold which is capable of supporting axonal regeneration in the damaged CNS.

The distribution of astrocytes, oligodendroglia and myelin in normal and transplanted rat superior colliculus: an immunohistochemical study

Immunohistochemical methods have been used to determine the distribution of macroglia and myelin in the normal rat superior colliculus (SC) and in grafts of fetal tectal tissue. The fetal tissue was derived from 15 day-old (E15) rat embryos and was transplanted onto the midbrain of newborn host rats of the same (PVG/c) strain. Antibodies to glial fibrillary acidic protein (GFAP) and carbonic anhydrase II (CAII) were used to visualize astrocytes and oligodendroglia respectively. Myelin was immunostained with antibodies to either proteolipid protein (PLP) or myelin basic protein (MBP). In the normal SC, GFAP positive astrocytes were found scattered throughout the SC, particularly in the superficial layers. They were especially prominent at the pial surface, around major blood vessels and at the midline between the two colliculi. CAII immunoreactive oligodendroglia and associated myelin were also found throughout the SC; by far the lowest density was seen in the stratum griseum superficiale (SGS). Both types of macroglia cell were found in abundance in tectal transplants, indicating that the precursors of these glial types were present in the E15 rat mesencephalon. In mature grafts, large numbers of fibrous astrocytes were found throughout the neuropil and the level of GFAP immunoreactivity was consistently greater than in host SC. Astrocytes seemed to be maintained in a reactive, perhaps immature state within the grafted tissue. Tectal transplants possessed large numbers of fully differentiated CAII-positive oligodendroglia and the grafts contained a dense network of myelinated axons. However the distribution of CAII and PLP immunoreactivity was not homogeneous; there were localized, well-defined regions that contained few oligodendroglia and relatively little myelin. These areas stained intensely for acetylcholinesterase (AChE) and were almost certainly homologous to the SGS of normal SC. The relative lack of oligodendroglia in the AChE stained patches in grafts and in SGS in situ suggests that local factors influencing the proliferation and distribution of oligodendroglia in normal SC may have been operating in a similar manner within the tectal transplant neuropil.