Neurotrophic activity in the central and peripheral nervous systems of the cat. Effects of injury

Beyond Trophic Factors: Exploiting the Intrinsic Regenerative Properties of Adult Neurons

Injuries and diseases of the peripheral nervous system (PNS) are common but frequently irreversible. It is often but mistakenly assumed that peripheral neuron regeneration is robust without a need to be improved or supported. However, axonal lesions, especially those involving proximal nerves rarely recover fully and injuries generally are complicated by slow and incomplete regeneration. Strategies to enhance the intrinsic growth properties of reluctant adult neurons offer an alternative approach to consider during regeneration. Since axons rarely regrow without an intimately partnered Schwann cell (SC), approaches to enhance SC plasticity carry along benefits to their axon partners. Direct targeting of molecules that inhibit growth cone plasticity can inform important regenerative strategies. A newer approach, a focus of our laboratory, exploits tumor suppressor molecules that normally dampen unconstrained growth. However several are also prominently expressed in stable adult neurons. During regeneration their ongoing expression “brakes” growth, whereas their inhibition and knockdown may enhance regrowth. Examples have included phosphatase and tensin homolog deleted on chromosome ten (PTEN), a tumor suppressor that inhibits PI3K/pAkt signaling, Rb1, the protein involved in retinoblastoma development, and adenomatous polyposis coli (APC), a tumor suppressor that inhibits β-Catenin transcriptional signaling and its translocation to the nucleus. The identification of several new targets to manipulate the plasticity of regenerating adult peripheral neurons is exciting. How they fit with canonical regeneration strategies and their feasibility require additional work. Newer forms of nonviral siRNA delivery may be approaches for molecular manipulation to improve regeneration.

Solid human embryonic spinal cord xenografts in acute and chronic spinal cord cavities: a morphological and functional study

While therapeutic spinal cord grafting procedures are of interest in the chronic spinal cord injury stage, previous experimental grafting studies, including human spinal cord tissue, have mainly focused on the acute stage. Therefore, solid human embryonic spinal cord grafts were implanted in acute or chronic spinal cord aspiration cavities of immunodeficient rats to compare the morphological and locomotor outcome to that of lesion alone cases. Locomotor function was assessed using the Basso, Beattie, and Bresnahan open-field locomotor rating scale up to 6 months, while the morphological evaluation of graft survival, growth, and integration was performed at 6 weeks or 6 months after implantation. Graft survival was 94% in both lesion models, while graft growth was enhanced in the chronic compared to the acute cavity group. Human specific Thy-1 and neurofilament immunoreactive fibers were observed up to 7 mm into host white matter, while aminergic fibers were observed up to 1 mm into the grafts. Abundant calcitonin gene-related peptide immunoreactive fibers in the grafts in the absence both of immunoreactive cell bodies and colocalized human-specific neurofilament immunoreactivity, suggested host fiber ingrowth. At 6 months, the grafted cases presented less central canal deformation and lower glial fibrillary acidic protein immunoreactivity at the host cavity border compared to that of the nongrafted cases. The strong compensatory regain of locomotor function after unilateral spinal cord lesions was not affected by the human spinal cord grafts. In conclusion, solid human embryonic spinal cord tissue transplanted to a cavity in the adult injured spinal cord results in beneficial morphological effects in both the acute and chronic spinal cord lesion.

Decrease in the ciliary neurotrophic factor of the spinal cord in amyotrophic lateral sclerosis

Ciliary neurotrophic factor (CNTF), a potent survival factor in spinal motoneurons of embryonic chick and rat, is currently being investigated in humans as a treatment for amyotrophic lateral sclerosis (ALS). However, its physiological and pathological activities in ALS remain unclear. We measured CNTF contents in the cervical enlargement of the spinal cord from 9 ALS patients and 12 age-matched control subjects using a sensitive enzyme-linked immunoassay. CNTF genotypes were determined by the polymerase chain reaction-restriction fragment length polymorphism method. In control subjects, there were 8 homozygotes and 4 heterozygotes, while in ALS patients there were 6 and 3, respectively. In both homozygotes and heterozygotes, CNTF expression in the spinal cord from ALS patients tended to decrease compared to control subjects. In homozygotes, the decrease was significant (p < 0.05). Concerning the regional concentrations of CNTF, in homozygotes, CNTF contents in the lateral corticospinal tract were markedly lower (p < 0.001) in ALS patients than in controls. The decrease in CNTF expression in the lateral corticospinal tract of the spinal cord from ALS patients may be a feature of ALS and could be related to motor neuron loss.

Synthesis and localization of ciliary neurotrophic factor in the sciatic nerve of the adult rat after lesion and during regeneration

Ciliary neurotrophic factor (CNTF) is expressed in high quantities in Schwann cells of peripheral nerves during postnatal development of the rat. The absence of a hydrophobic leader sequence and the immunohistochemical localization of CNTF within the cytoplasm of these cells indicate that the factor might not be available to responsive neurons under physiological conditions. However, CNTF supports the survival of a variety of embryonic neurons, including spinal motoneurons in culture. Moreover we have recently demonstrated that the exogenous application of CNTF protein to the lesioned facial nerve of the newborn rat rescued these motoneurons from cell death. These results indicate that CNTF might indeed play a major role in assisting the survival of lesioned neurons in the adult peripheral nervous system. Here we demonstrate that the CNTF mRNA and protein levels and the manner in which they are regulated are compatible with such a function in lesioned peripheral neurons. In particular, immunohistochemical analysis showed significant quantities of CNTF at extracellular sites after sciatic nerve lesion. Western blots and determination of CNTF biological activity of the same nerve segments indicate that extracellular CNTF seems to be biologically active. After nerve lesion CNTF mRNA levels were reduced to less than 5% in distal regions of the sciatic nerve whereas CNTF bioactivity decreased to only one third of the original before-lesion levels. A gradual reincrease in Schwann cells occurred concomitant with regeneration.

Effects of lesions of the optic nerve, optic tectum and nervus terminalis on rod precursor proliferation in the goldfish retina

Teleost retinas grow throughout life by proliferation of neuroblasts at the retinal margin and dedicated rod precursors in the outer nuclear layer. Mechanisms regulating this proliferation are largely unknown. Previous investigators observed that rod precursor replication, as detected by incorporation of radioactive thymidine into cells of the outer nuclear layer, is enhanced after optic nerve crush. We attempted to determine whether this was due to severing of the retinopetal (nervus terminalis, n.t.) or retinofugal (retinal ganglion cell) axons in the optic nerve of the goldfish, Carassius auratus. In the first series of experiments, we ablated unilaterally the optic nerve, olfactory bulb (containing n.t. ganglia), or optic tectum (containing retinal ganglion cell axons and n.t. collaterals). Rod precursor proliferation increased dramatically in both retinas as soon as 5 days after surgery; in addition, the numbers of dividing cells were greater in the ipsilateral retina 10-15 days after optic nerve crush or tectal ablation and in the contralateral retina 20-25 days after olfactory bulb ablation. These observations are not accounted for by the known projections of retinal ganglion cells, but are consistent with the projections of the n.t. In the second series of experiments, n.t. projections to the brain and retina were severed bilaterally 7-8 weeks before the unilateral optic nerve crush or hemitectal ablation. Rod precursor proliferation increased as before, but the quantities of dividing cells were always equal in both retinas. We conclude that the n.t. may modulate rod proliferation locally and that injury to (some) brain regions may cause release of mitogens that affect rod precursors in both retinas.

Reinnervation of distal sensory nerve environments by regenerating sensory axons

This study investigated the specificity of sensory nerve regeneration in a primate model. In adult cynomolgus monkeys, the femoral nerve was explored in the groin and two sensory branches identified. A sensory branch was sectioned and introduced into the proximal channel of a Y-shaped silicone chamber. This proximal sensory nerve stump was given distal choices of distal sensory nerve graft or distal sensory nerve which was intact to the distal sensory cutaneous receptors. After eight months, histological analysis confirmed axonal growth directed towards both the distal nerve graft and the distal nerve intact to the distal cutaneous receptors. However, the number of nerve fibres directed towards the distal nerve was significantly greater than the number of nerve fibres directed towards the nerve graft (P less than 0.004). These results suggest that while both distal nerve graft and distal intact nerve act as specific targets to regenerating proximal primate nerve, the presence of an intact distal end-organ positively enhances sensory regeneration.

Structure and biosynthesis of nerve growth factor

Most of our knowledge about NGF comes from extensive study of the mouse submaxillary gland protein. NGF from this source is isolated as a high molecular weight complex consisting of beta-NGF and two subunits, alpha and gamma, belonging to the kallikrein family of serine proteases. There are few other tissues where NGF is found in sufficient quantities for protein purification and study, although new molecular biological techniques have accelerated the study of NGFs from a variety of species and tissues. Mouse submaxillary gland NGF is synthesized as a large precursor that is cleaved at both N- and C-terminals to produce mature NGF. This biologically active molecule can be further cleaved by submaxillary gland proteases. The roles of the alpha and gamma subunits in the processing of the beta-NGF precursor, the modulation of the biological activity of beta-NGF, and the protection of mature beta-NGF from degradation have been well studied in the mouse. However, the apparent lack of alpha and gamma subunits in most other tissues and species and the existence of a large family of murine kallikreins, many of which are expressed in the submaxillary gland, challenge the relevance of murine high molecular weight NGF as a proper model for NGF biosynthesis and regulation. It is important therefore to identify and characterize other NGF complexes and to study their subunit interactions, biosynthesis, processing, and regulation. This review points out a number of other species and tissues in which the study of NGF has just begun. At this time, there exist many more questions than answers regarding the presence and the functions of NGF processing and regulatory proteins. By studying NGF in other species and tissues and comparing the processing and regulation of NGF from several sources, we will discover the unifying concepts governing the expression of NGF biological activity.

The conditioning effect of optic nerve injury upon axonal regrowth from adult rat retinal ganglion cells explanted in vitro

An in vitro assay was used to determine the effects of conditioning nerve lesions on the regeneration of adult rat retinal ganglion cell (RGC) axons from retinal explants. Following the conditioning lesion (CL) of unilateral optic nerve transection, maximal regrowth was seen from RGC explanted from ipsilateral retinae 10 days post-CL. Explants from this group initiated axonal regrowth earlier and a greater percentage regrew axons when compared with explants from normal rats. Axonal regrowth from explants of retinae contralateral to CL was also seen earlier than normal. In further experiments, the effects of both exposure of the optic nerve sheath in the orbit and the incision of the dura without injury to optic nerve axons were studied. The conditioning effect of a dural incision was found to be the same as that of optic nerve transection, whilst exposure of the optic nerve sheath had no conditioning effect on RGC axonal regrowth in vitro.

Structural and functional analysis of the promoter region of the nerve growth factor gene

Molecular clones containing the NGF gene promoter regions and exons I were isolated from mouse and rat genomic libraries with synthetic oligonucleotide hybridization probes that corresponded to the 5' end of mouse submandibular gland NGF mRNA. The nucleotide sequences of the 5' flanking regions and exons I were determined and compared. There was 95% similarity in exons I and the adjacent promoter regions between mouse and rat sequences. Further upstream, the similarity decreased to 76%. Both mouse and rat promoter regions were only 33% similar to the presumptive human NGF gene promoter region. Upstream from the capsite of submandibular gland NGF mRNA as determined by an S1-nuclease protection assay, two promoter-like TATA-boxes were found at positions -28 and -49, resp., and two CAAT-like boxes at -379 and -546, resp. Both promoter regions contained a cluster of conserved CpG dinucleotide sequences in a GC-rich island whereas less conserved upstream regions contained only one CpG sequence. The promoter regions were fused to the human growth hormone gene reporter function. Transient expression in L929 cells yielded appropriate fusion mRNAs and secretion of hGH, demonstrating that the cloned promoters are functional.

Transplantation of fetal spinal cord tissue into the chronically injured adult rat spinal cord

Transplants of fetal central nervous system (CNS) tissue into the acutely injured rat spinal cord have been demonstrated to differentiate and partially integrate with the adjacent host neuropil. In the present study, we examined the potential for applying a transplantation approach to chronic spinal cord lesions. In particular, we were interested in learning whether host-graft fusion would be adversely affected by an advanced histopathology characterized in part by glial scar formation. Hemisection cavities were prepared at lumbar levels of the adult rat spinal cord 2-7 weeks prior to the transplantation of spinal cord tissue obtained from 14-day rat fetuses. Graft survival, differentiation, and integration with the host spinal cord were subsequently evaluated by light microscopic techniques at post-transplantation intervals of 1-6 months. Immunocytochemistry was also employed to examine the extent of astrocytic scar formation at the host-graft interface and serotoninergic innervation of the grafts. In some other cases, anterograde and retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase was used to determine whether axonal projections were formed between the host spinal cords and grafts. By 2 weeks after injury the initial lesion cavities were surrounded by a continuous astrocytic scar which remained intact for at least 7 weeks after injury in nongrafted control animals. In other animals, transplantation into these advanced lesions resulted in well-differentiated grafts with a 90% long-term survival rate. Although dense gliosis was still present along the lesion surfaces of the recipient spinal cord, foci of confluent host-graft neuropil were observed where interruptions in the scar had occurred. Donor tissue integrated most often with the host spinal cord at interfaces with host gray matter; however, some implants also exhibited sites of fusion with damaged host white matter. Thus, some regions of confluent graft and host neuropil could be routinely identified, despite the presence of a dense glial scar along the walls of the chronic lesion site at the time of transplantation. Anterograde and retrograde tract-tracing results suggested that some axonal projections into these grafts had originated from host neurons located immediately adjacent to the donor-recipient interface. In addition, immunocytochemistry revealed some host serotoninergic axons (presumably of supraspinal origin) traversing nongliotic interfaces. The results of this study raise the possibility that grafted fetal CNS tissue has a capacity for stimulating partial regression of an established glial scar.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term support by injured brain extract of a subpopulation of ciliary ganglion neurons purified by differential adhesion

Ciliary ganglion neurons and nonneurons can be separated from each other, based on the greater adhesivity of the nonneurons to untreated tissue culture plastic in the presence of serum. When the separation was carried out in the presence of Serum Plus (a commercially available supplemented serum), two populations of neurons were distinguished. Neurons in the first class (50-60% of total) adhered to plastic within 15 min, tended to aggregate into clumps, and were not well supported in long term culture by brain extracts. Neuronal adhesion to plastic was inhibited by heparin but not by chondroitin sulfate. Neurons in the second class did not attach to plastic for up to 90 min (and could thus be purified), were not as prone to aggregation, and were quantitatively supported for long periods (greater than 2 weeks) by the neurotrophic factor(s) present in extracts of injured brain. Although no direct evidence is provided, these populations may correspond to the well characterized ciliary and choroid neurons.