Factors influencing survival of transplanted embryonic motoneurones in the spinal cord of adult rats

The role of embryonic motoneuron transplants to restore the lost motor function of the injured spinal cord

Spinal cord injury or disease result in the loss of critical numbers of spinal motoneurons and consequentially, in severe functional impairment. The most successful way to replace missing motoneurons is the use of embryonic postmitotic motoneuron grafts. This method may also at least partially restore integrity of the injured spinal cord. It has been shown that grafted motoneurons survive, differentiate and integrate into the host cord and many of them are able to reinnervate the denervated muscles. If grafted motoneurons are provided with a conduit (e.g. reimplanted ventral root) the grafted cells are able to extend their axons along the entire length of the peripheral nerves and reach the hind or forelimb muscles and to restore limb locomotion patterns. Grafted motoneurons show excellent survival in motoneuron-depleted adult host cords, but the developing spinal cord appears to provide an unfavourable environment for these motoneurons as they do not survive in immature cords. The long term survival and maturation of the grafted neurons depend on the availability of a nerve conduit and one or more target muscles, independently of whether these are ectopic nerve-muscle implants or limb muscles in their original site. Thus, grafted and host motoneurons induce functional recovery in the denervated limb muscles when their axons can grow into an avulsed and reimplanted ventral root and then reach the limb muscles. Following segmental loss of motoneurons induced by partial spinal cord injury, motoneuron-enriched embryonic grafts can be placed into the gap-like hemisection cavity in the cervical spinal cord. Such transplants induce the regeneration of great numbers of host motoneurons possibly by the bridging effect of the grafts. In this case, the regenerating host motoneurons reinnervate their original target muscles while the small graft plays a minimal role in the reinnervation of muscles. These results suggest that reconstruction of the injured spinal cord using an embryonic motoneuron-enriched spinal cord graft is a feasible way to achieve improvement after severe functional motor deficits of the spinal cord.

Long-term survival and development of fetal ventral spinal grafts into the motoneuron-depleted rat spinal cord: role of donor age

Fetal spinal cord (SC) tissue grafts can survive and develop into the lesioned SC, but no conclusive data are available concerning the long-term fate of transplanted material and the relation between the graft fate and the donor embryo age. Here, pre-labelled suspensions of ventral SC from E12 or E17 rat fetuses were grafted to the lumbar SC of adult rats with motoneuron depletion induced by perinatal injection of volkensin. E12 and E17 are presumably the stages when motoneuron development starts and terminates, respectively. Four or 10months post-grafting, SCs were analyzed to check the graft survival rate and to follow the differentiation and spatial distributions of grafted cells. Neurotoxic lesion produced a 61% motoneuronal loss in the lumbar SC. In transplanted animals, all E12 fetal grafts survived until the observed time-points and developed various mature cell phenotypes. Many motoneuron-like labelled cells were found within the graft area or adjacent to it. Conversely, none of the E17 fetal grafts survived, since no graft-derived elements with neuronal morphology were found either in the site of graft placement or adjacent to it. The present findings indicate that spinal neuroblasts can survive for a long time and develop within the motoneuron-depleted SC, and that the donor embryo age is crucial for successful engraftment.

Acetylcholine release from fetal tissue homotopically grafted to the motoneuron-depleted lumbar spinal cord. An in vivo microdialysis study in the awake rat

Grafts of spinal cord (SC) tissue can survive and develop into the severed SC, but no conclusive data are available concerning the functional activity of transplanted neurons. In the present study, suspensions of prelabeled embryonic ventral SC tissue were grafted to the lumbar SC of rats with motoneuron loss induced by perinatal injection of volkensin. Eight to ten months post-grafting, acetylcholine (ACh) release was measured by microdialysis in awake rats, under either basal or stimulated conditions. In normal animals, baseline ACh output averaged 1.6 pmol/30 microl, it exhibited a 4-fold increase after KCl-induced depolarization or handling, and it was completely inhibited by tetrodotoxin administration. Moreover, ACh levels did not change following acute SC transection performed under anesthesia during ongoing dialysis, suggesting an intrinsic source for spinal ACh. Treatment with volkensin produced a severe (>85%) motoneuronal loss accompanied by a similar reduction in baseline ACh release and almost completely abolished effects of depolarization or handling. In transplanted animals, many motoneuron-like labeled cells were found within and just outside the graft area, but apparently in no case were they able to extend fibers towards the denervated muscle. However, the grafts restored baseline ACh output up to near-normal levels and responded with significantly increased release to depolarization, but not to handling. The present findings indicate that spinal neuroblasts can survive and develop within the motoneuron-depleted SC and release ACh in a near-normal, but apparently non-regulated, manner. This may be of importance for future studies involving intraspinal stem cell grafts.

Lineage-restricted neural precursors survive, migrate, and differentiate following transplantation into the injured adult spinal cord

Fetal spinal cord from embryonic day 14 (E14/FSC) has been used for numerous transplantation studies of injured spinal cord. E14/FSC consists primarily of neuronal (NRP)- and glial (GRP)-restricted precursors. Therefore, we reasoned that comparing the fate of E14/FSC with defined populations of lineage-restricted precursors will test the in vivo properties of these precursors in CNS and allow us to define the sequence of events following their grafting into the injured spinal cord. Using tissue derived from transgenic rats expressing the alkaline phosphatase (AP) marker, we found that E14/FSC exhibited early cell loss at 4 days following acute transplantation into a partial hemisection injury, but the surviving cells expanded to fill the entire injury cavity by 3 weeks. E14/FSC grafts integrated into host tissue, differentiated into neurons, astrocytes, and oligodendrocytes, and demonstrated variability in process extension and migration out of the transplant site. Under similar grafting conditions, defined NRP/GRP cells showed excellent survival, consistent migration out of the injury site and robust differentiation into mature CNS phenotypes, including many neurons. Few immature cells remained at 3 weeks in either grafts. These results suggest that by combining neuronal and glial restricted precursors, it is possible to generate a microenvironmental niche where emerging glial cells, derived from GRPs, support survival and neuronal differentiation of NRPs within the non-neurogenic and non-permissive injured adult spinal cord, even when grafted into acute injury. Furthermore, the NRP/GRP grafts have practical advantages over fetal transplants, making them attractive candidates for neural cell replacement.

Pseudorabies virus-based gene delivery to rat embryonic spinal cord grafts

The construction and application of recombinant pseudorabies viruses (PrVs) for the delivery of beta-galactosidase and/or green fluorescent protein (GFP) genes to rat embryonic spinal cord cells are reported here. These viruses were specifically designed to infect embryonic spinal cord neurons, which can be grafted into a lesioned spinal cord in order to restore the lost functions of the host cord. The recombinant viruses were constructed in two steps. The small subunit of the ribonucleotide reductase (RR) gene was first abolished by a frameshift mutation and an expression cassette containing the lacZ gene alone or together with the GFP gene was then inserted in place of the early protein 0 (EP0) gene of PrV. The reporter gene cassettes were positioned downstream from the PrV latency-associated promoter. Using an ex vivo system, we infected embryonic spinal cord explants with these viruses and found that neither vRREP0lac nor vRREP0lacgfp exerted any cytotoxic effect at all. It was also revealed that these viruses infect embryonic cells with high efficiency, and that infected neurons grafted into the spinal cord express the inserted reporter genes for periods of up to 12 weeks. This system offers a new approach for foreign gene transfer to neurons grafted into the CNS.

Reinnervation of the biceps brachii muscle following cotransplantation of fetal spinal cord and autologous peripheral nerve into the injured cervical spinal cord of the adult rat

In order to compensate the loss of motoneurons resulting from severe spinal cord injury and to reestablish peripheral motor connectivity, solid pieces of fetal spinal cord, taken from embryonic day 14 rat embryos, were transplanted into unilateral aspiration lesions of the cervical spinal cord of adult rats. Concomitantly, one end of a 3.5-cm autologous peripheral nerve graft was put in close contact with the embryonic graft; the other end was sutured to the distal stump of the musculocutaneous nerve which innervate the biceps brachii muscle. The animals were examined 3 and 6 months after surgery. Following intramuscular injection of horseradish peroxidase, retrograde axonal labeling studies indicated that both transplanted and host spinal neurons were able to extend axons all the way through the peripheral nerve graft and nerve stump, up to the reconnected muscles. The labeled cells in the transplant were generally observed close to the intraspinal tip of the peripheral nerve graft. Retrograde axonal tracing, as well as electrophysiological and histological data, demonstrated the sensory and motor reinnervation of the reconnected muscles. This muscular reinnervation was able to reverse the atrophic changes observed in the denervated muscle. In control experiments, the extraspinal end of the peripheral nerve graft was ligatured in order to compare the differentiation of the transplanted neurons and the survival of their growing axons with or without their muscular targets. Six months after both types of surgery, large-size grafted neurons, identified as motoneurons by immunocytochemistry for peripherine and calcitonin gene-related peptide, were only observed in fetal spinal cord transplants which were connected to denervated muscles, thus demonstrating the trophic influence of the muscle target on the survival and differentiation of the transplanted neurons and on the maintenance of the axons they had grown into the peripheral nerve graft.

The effect of riluzole treatment in rats on the survival of injured adult and grafted embryonic motoneurons

The effect of riluzole on the survival of injured motoneurons was studied. The L4 ventral root was avulsed and reimplanted into the spinal cord. Immediately after the operation, 4 animals were treated with riluzole for 3 weeks while another 4 animals received no treatment after the operation. Three months later the fluorescent dyes, Fast Blue and Diamidino Yellow, were applied to the cut ventral ramus of the L4 spinal nerve, for retrograde labelling of neurons. Three days later, the spinal cords were processed to reveal the retrograde-labelled cells. In untreated animals, there were 20 +/- 2.1 labelled neurons (+/- SEM), while in animals treated with riluzole there were 723 +/- 26. Thus, treatment with riluzole dramatically enhanced the survival of injured motoneurons. In another series of experiments, after avulsion of the L4 ventral root and its reinsertion, embryonic spinal cord pieces were grafted into the host cord. Five animals received riluzole treatment and 4 were left untreated. In the untreated animals, 125 +/- 5.1 retrograde-labelled cells of both graft and host origin were detected. In rats treated with riluzole, 645 +/- 35.7 retrograde-labelled cells were seen and almost all of these were of host origin. Thus, treatment with riluzole enhanced the survival of injured host motoneurons, and by doing so, (i) reduced the ability of grafted neurons to extend their axons into the reimplanted L4 ventral root, and (ii) reduced the survival of the grafted cells.

Effects of neurotransplants and BDNF on the survival and regeneration of injured adult spinal motoneurons

We compared the effects of peripheral nerve grafts, embryonic spinal cord transplants and brain-derived neurotrophic factor (BDNF) on the survival and axon regeneration of adult rat spinal motor neurons undergoing retrograde degeneration after ventral root avulsion. Following implantation into the dorsolateral funiculus of the injured spinal cord segment, neither a peripheral nerve graft nor a combination of peripheral nerve graft with embryonic spinal cord transplant could prevent the retrograde motor neuron degeneration induced by ventral root avulsion. However, intrathecal infusion of BDNF promoted long-term survival of the lesioned motor neurons and induced abundant motor axon regeneration from the avulsion zone along the spinal cord surface towards the BDNF source. A combination of ventral root reconstitution and BDNF treatment might therefore be a promising means for the support of both motor neuron survival and guided motor axon regeneration after ventral root lesions.

Reinnervation of denervated muscle by transplantation of fetal spinal cord to transected sciatic nerve in the rat

When motor neurons in the spinal cord are destroyed, regeneration of motor axons and muscle reinnervation cannot be expected. We attempted reinnervation of the denervated muscle, i.e. motor unit reconstruction, using transplantation of the fetal spinal cord to the peripheral nerve. The sciatic nerve of an adult rat was resected for 20 mm, and a cavity was prepared using an autologous femoral vein at the distal stump of the nerve. The fetal spinal cord was then transplanted into the venous cavity. After 3-6 months, no voluntary muscle contraction was observed due to the absence of communication with the central nervous system. However, reinnervation of the muscles via the sciatic nerve by the transplanted spinal neurons was demonstrated electrophysiologically and histochemically. This suggested that a motor unit can be reconstructed by fetal spinal cord transplantation even if the original motor neurons in the spinal cord are not available.

Possible strategies for treatment of SMA patients: a neurobiologist's view

This paper discusses possible strategies that might prevent or alleviate muscle weakness of SMA patients and hence improve their condition. The strategies discussed are as follows. (1) Prevention of motoneurone death. To achieve this two main approaches have been applied. Firstly, trophic factors have been used to prevent motoneurone death after nerve injury and clinically in diseases such as motoneurone disease. The results of these attempts will be described. Secondly, the possibility that injured motoneurones die as a result of the excitotoxic effects of the excitatory transmitter glutamate will be explored. Evidence will be presented which indicates that blocking glutamate receptors can rescue injured motoneurones from death. (2) Replacement of lost motoneurones by embryonic grafts. Motoneurones from grafts of embryonic spinal cord have been shown to survive in the adult spinal cord and are able to reinnervate skeletal muscles. The potential and practical problems of this approach will be discussed. (3) Expansion or motor unit territory of surviving motoneurones. Such an expansion of the territory occupied by individual motor units can be achieved by encouraging sprouting and ensuring that the newly formed connections between the motoneurone and muscle fibres are maintained, so that individual motor units are capable of developing more force. Strategies to achieve such an expansion of motor unit territory will be described. Finally, combinations of some of these approaches are considered.

Immunohistochemical detection of cells labeled with bromodeoxyuridine after neural transplantation

Pregnant rats were treated with 5-bromo-2'-deoxyuridine (BrdU) from embryonic d 12 (ED12) to ED14. BrdU administration allowed timed labeling of dividing embryonic cells in utero, since the drug is incorporated into the DNA in place of thymidine during the S-phase of the cell cycle. ED14 rat cerebral cortex or placenta was grafted into the brain of adult rats. Anti-bromodeoxyuridine immunohistochemistry was used for identifying labeled transplanted cells after different survival periods in paraffin-embedded sections. BrdU labeled cells were observed in both intraventricular and intraparenchymal cortical grafts, even after a 3-mo survival. Although the percentage of positive cells decreased in comparison with ED14 cortex, the level of BrdU (i.e., the intensity of anti-BrdU immunohistochemistry) in labeled nuclei was probably the same. BrdU pretreatment of ED14 cells prior to grafting did not affect the proliferative ability of the grafted tissue. In ED14 placental grafts, all trophoblastic cells were labeled distinctly. This precise labeling technique enabled an examination of individual migrating trophoblastic cells. However, migration of these cells into the host brain was very limited.

Human fetal tissue research: practice, prospects, and policy

Tissue from human fetal cadavers has long been used for medical research, experimental therapies, and various other purposes. Research within the last two decades has led to substantial progress in many of these areas, particularly in the application of fetal tissue transplantation to the treatment of human disease. As a result, clinical trials have now been initiated at centers around the world to evaluate the use of human fetal tissue transplantation for the therapy of Parkinson's disease, insulin-dependent diabetes mellitus, and a number of blood, immunological and, metabolic disorders. Laboratory studies suggest a much wider range of disorders may in the future be treatable by transplantation of various types of human fetal tissue. A combination of characteristics renders fetal tissue uniquely valuable for such transplantation, as well as for basic research, the development of vaccines, and a range of other applications. Although substitutes for human fetal tissue are being actively sought, for many of these applications there are at present no satisfactory alternatives. Important issues remain unresolved concerning the procurement, distribution, and use of human fetal cadaver tissue as well as the effects of such use on abortion procedures and incidence. These issues can be addressed by the introduction of appropriate guidelines or legislation, and need not be an impediment to legitimate research and therapeutic use of fetal tissue.