Spinal cord grafts : interaction with cografts and effects of trophic factors

Abstract

Spinal cord injury is common in the modern society, often afflicts young individuals, and often has devastating consequences. The present work studies possibilities to obtain better conditions for regeneration of the transected spinal cord in rats, conditions under which spinal cord grafts are able to establish structural and functional connections with "natural" targets, and effects of a series of neurotrophic factors on spinal cord graft survival and growth. Recognizing that the spinal cord is normally under tension, leading to separation of the stumps after transection, stump approximation through vertebral shortening was studied.

While complicated by the need for better vertebral column stabilization measures, and while regeneration of descending 5HT fibers was not improved in comparison to spinal cord transection alone, animals in which the spinal cord transection was combined with vertebral shortening showed modest, but significantly improved, hind limb motor function compared to spinal cord transection only. To further elucidate establishment of spinal cord connectivity, grafts of spinal cord tissue to the anterior chamber of the eye were combined with either cortex cerebri or another spinal cord graft. Both cortex cerebri and spinal cord were able to establish functional excitatory inputs to the mature spinal cord graft, suggesting that fetal cerebral cortex or spinal cord tissue grafted to the adult injured spinal cord might establish similar connections. Graft growth was not influenced in these double-graft situations, suggesting that the spinal cord did not exert any major trophic effects on itself or on cortex cerebri. When instead fetal spinal cord and fetal skeletal muscle were combined in the eye chamber, the two tissues exerted reciprocal trophic effects upon each other, and stimulation of the spinal cord graft elicited specific muscle contractions.

Spinal cord grafts were also able to exert a trophic, neuron-rescuing effect on fetal cografts of dorsal root ganglia. To elucidate trophic effects exerted by skeletal muscle on spinal cord grafts, single spinal cord grafts to the eye chamber were treated with two trophic factors abundantly present in skeletal muscle, namely GDNF and NT4. Effects of these factors were compared to the possible effects of NGF, BDNF, NT3 and CNTF. GDNF exerted marked trophic effects on survival and growth of spinal cord grafts both in terms of graft volume and graft morphology, manifest as survival of a larger number of neurons and a less pronounced gliosis. Remarkably, GDNF exerted strong effects also on postnatal grafts obtained from 1-, 8- or 14-day-old donors. All neurotrophins except NGF stimulated spinal cord grafts. BDNF was able to stimulate prenatal grafts while NT4, assumed to act upon the same set of receptors as BDNF, exerted strong stimulatory effects on post-natal grafts only. CNTF also stimulated postnatal grafts.

Taken together, the data demonstrate that several known neurotrophic factors exert potent stimulatory effects of spinal cord grafts. The use of neurotrophic factors extends the time window within which spinal cord tissue can be grafted well into the second postnatal week. It is predicted that treatments with neuro-trophic factors or, particularly combinations of neurotrophic factors such as GDNF and NT4, should also enhance survival of immature spinal cord tissue grafted to the adult injured spinal cord. It is further predicted that treatment with combinations of neurotrophic factors might have neuroprotective effects and/or enhance regeneration in the adult injured spinal cord.