Piwnica-Worms D, Schellingerhout D. Axonal Transport as an In Vivo Biomarker for Retinal Neuropathy

We illuminate a possible explanatory pathophysiologic mechanism for retinal cellular neuropathy by means of a novel diagnostic method using ophthalmoscopic imaging and a molecular imaging agent targeted to fast axonal transport. The retinal neuropathies are a group of diseases with damage to retinal neural elements. Retinopathies lead to blindness but are typically diagnosed late, when substantial neuronal loss and vision loss have already occurred. We devised a fluorescent imaging agent based on the non-toxic C fragment of tetanus toxin (TTc), which is taken up and transported in neurons using the highly conserved fast axonal transport mechanism. TTc serves as an imaging biomarker for normal axonal transport and demonstrates impairment of axonal transport early in the course of an N-methyl-D-aspartic acid (NMDA)-induced excitotoxic retinopathy model in rats. Transport-related imaging findings were dramatically different between normal and retinopathic eyes prior to presumed neuronal cell death. This proof-of-concept study provides justification for future clinical translation.

Tetanus toxin fragments and Bcl-2 fusion proteins: cytoprotection and retrograde axonal migration

Background

Tetanus neurotoxin (TeNT) is taken up at nerve terminals and undergoes retrograde migration. The toxic properties of TeNT reside in the toxin light chain (L), but like complete TeNT, the TeNT heavy chain (TTH) and the C-terminal domain (TTC) alone can bind and enter into neurons. Here, we explored whether atoxic fragments of TeNT could act as drug delivery vehicles in neurons. In this study, we used Bcl-2, a protein known to have anti-apoptotic properties in vivo and in vitro, as a parcel to couple to TeNT fragments.

Results

We expressed Bcl-2 and the TTC fragments alone, and also attempted to express fusion proteins with the Bcl-2 coupled at the N-terminus of TTH (Bcl2-TTH) and the N- and C-terminus of TTC (TTC-Bcl2 and Bcl2-TTC) in mammalian (Cos7 cells) and Escherichia coli systems. TTC and Bcl-2 were efficiently expressed in E. coli and Cos7 cells, respectively, but Bcl-2 and the fusion proteins did not express well in E. coli. The fusion proteins were also not expressed in Cos7 cells. To improve the yield and purity of the fusion protein, we genetically deleted the N-terminal half of TTC from the Bcl2-TTC fusion to yield Bcl2-hTTC. Purified Bcl2-hTTC exhibited neuronal binding and prevented cell death of neuronal PC12 cells induced by serum and NGF deprivation, as evidenced by the inhibition of cytochrome C release from the mitochondria. For in vivo assays, Bcl2-hTTC was injected into the tongues of mice and was seen to selectively migrate to hypoglossal nuclei mouse brain stems via retrograde axonal transport.

Conclusions

These results indicate that Bcl2-hTTC retains both Bcl-2 and TTC functions and therefore could be a potent therapeutic agent for various neurological conditions.

Neuroprotective Effects of C-terminal Domain of Tetanus Toxin on Rat Brain Against Motorneuron Damages After Experimental Spinal Cord Injury

STUDY DESIGN

Experimental animal study investigating the efficacy of C-terminal domain of tetanus toxin application as neuroprotective effects on rat brain in a model of spinal cord injury (SCI).

OBJECTIVE

The aim of the present study was to investigate the possible role of C-terminal domain of tetanus toxin (Hc-TeTx) on cell death mechanisms including apoptosis and autophagy following SCI.

SUMMARY OF BACKGROUND DATA

Traumatic SCI can lead to posttraumatic inflammation, oxidative stress, motor neuron apoptosis, necrosis, and autophagy of tissue. To promote and enhance recovery after SCI, recent development of devices and therapeutic interventions are needed.

METHODS

Twenty-eight adult rats were divided into four groups (n = 7 each) as follows: sham, trauma (SCI), SCI + Hc-TeTx, and SCI + methylprednisolone groups. The functional neurological deficits due to the SCI were assessed by behavioral analysis using the Basso, Beattie and Bresnahan (BBB) open-field locomotor test. The alterations in pro-/anti-apoptotic and autophagy related-protein levels were measured by Western blotting technique.

RESULTS

In this study, Hc-TeTx promotes locomotor recovery and motor neuron survival of SCI rats. Hc-TeTx also decreased expression of bax, bad, bak, cleaved caspase-3, Ask1, and autophagy-related proteins including Atg5 and LC3II in brain. Our study provides an evidence that cell death mechanisms play critical roles in SCI and that the nontoxic peptides including Hc-TeTx may exert protective effect and decrease cell death following SCI.

CONCLUSION

Our preliminary findings suggest a possible therapeutic agent to improve survival after spinal cord trauma, but further analysis are still needed to evaluate the difference between acute and chronic injuries.

LEVEL OF EVIDENCE

N/A.

In vivo targeted gene delivery to peripheral neurons mediated by neurotropic poly(ethylene imine)-based nanoparticles

A major challenge in neuronal gene therapy is to achieve safe, efficient, and minimally invasive transgene delivery to neurons. In this study, we report the use of a nonviral neurotropic poly(ethylene imine)-based nanoparticle that is capable of mediating neuron-specific transfection upon a subcutaneous injection. Nanoparticles were targeted to peripheral neurons by using the nontoxic carboxylic fragment of tetanus toxin (HC), which, besides being neurotropic, is capable of being retrogradely transported from neuron terminals to the cell bodies. Nontargeted particles and naked plasmid DNA were used as control. Five days after treatment by subcutaneous injection in the footpad of Wistar rats, it was observed that 56% and 64% of L4 and L5 dorsal root ganglia neurons, respectively, were expressing the reporter protein. The delivery mediated by HC-functionalized nanoparticles spatially limited the transgene expression, in comparison with the controls. Histological examination revealed no significant adverse effects in the use of the proposed delivery system. These findings demonstrate the feasibility and safety of the developed neurotropic nanoparticles for the minimally invasive delivery of genes to the peripheral nervous system, opening new avenues for the application of gene therapy strategies in the treatment of peripheral neuropathies.

Pathogen-inspired drug delivery to the central nervous system

For as long as the human blood-brain barrier (BBB) has been evolving to exclude bloodborne agents from the central nervous system (CNS), pathogens have adopted a multitude of strategies to bypass it. Some pathogens, notably viruses and certain bacteria, enter the CNS in whole form, achieving direct physical passage through endothelial or neuronal cells to infect the brain. Other pathogens, including bacteria and multicellular eukaryotic organisms, secrete toxins that preferentially interact with specific cell types to exert a broad range of biological effects on peripheral and central neurons. In this review, we will discuss the directed mechanisms that viruses, bacteria, and the toxins secreted by higher order organisms use to enter the CNS. Our goal is to identify ligand-mediated strategies that could be used to improve the brain-specific delivery of engineered nanocarriers, including polymers, lipids, biologically sourced materials, and imaging agents.

Apoptotic cell death and altered calcium homeostasis caused by frataxin depletion in dorsal root ganglia neurons can be prevented by BH4 domain of Bcl-xL protein

Friedreich ataxia (FRDA) is a neurodegenerative disease characterized by a decreased expression of the mitochondrial protein frataxin. Major neurological symptoms of the disease are due to degeneration of dorsal root ganglion (DRG) sensory neurons. In this study we have explored the neurodegenerative events occurring by frataxin depletion on primary cultures of neurons obtained from rat DRGs. Reduction of 80% of frataxin levels in these cells was achieved by transduction with lentivirus containing shRNA silencing sequences. Frataxin depletion caused mitochondrial membrane potential decrease, neurite degeneration and apoptotic cell death. A marked increase of free intracellular Ca(2+) levels and alteration in Ca(2+)-mediated signaling pathways was also observed, thus suggesting that altered calcium homeostasis can play a pivotal role in neurodegeneration caused by frataxin deficiency. These deleterious effects were reverted by the addition of a cell-penetrant TAT peptide coupled to the BH4, the anti-apoptotic domain of Bcl-x(L). Treatment of cultured frataxin-depleted neurons with TAT-BH4 was able to restore the free intracellular Ca(2+) levels and protect the neurons from degeneration. These observations open the possibility of new therapies of FRDA based on modulating the Ca(2+) signaling and prevent apoptotic process to protect DRG neurons from neurodegeneration.

The effectiveness of tetanus toxin on sciatic nerve regeneration: a preliminary experimental study in rats

OBJECT

The purpose was to investigate the effects of local tetanus toxin (TeTx) application on sciatic nerve regeneration following a rat model of transection injury.

METHODS

After both sciatic nerves were transected and repaired with three epineural sutures, 12 male Wistar albino rats were divided into two groups. 0.25 ml (2.5 flocculation units) TeTx was injected into a piece of absorbable gelatin sponge in TeTx group. In controls, 0.25 ml saline injected. Assessments were performed by using climbing degrees, compound muscle action potentials (CMAPs) and histological parameters (axon number and axonal diameter) 12th week.

RESULTS

CMAPs amplitudes were 11.6 ± 4.7 mV and 1.4 ± 1.3 mV in gastrocnemius and interdigital muscles in TeTx group (5.8 ± 2.4 mV and 0.2 ± 0.1 mV, P < 0.05). Climbing degrees were significantly different (61.6 ± 1.7 vs. 38.3 ± 2.6, P < 0.05). Total axon numbers were higher (1341.1 ± 57.3 vs. 877.5 ± 34.9, P < 0.05) and the mean axon diameter was smaller (4.2 ± 2.1 vs. 2.5 ± 1.9, P < 0.05) in the TeTx group.

CONCLUSION

This preliminary study firstly demonstrated the effectiveness of TeTx on nerve repair in experimental sciatic rat model based on functional, electromyographic and histological parameters.

Fragment C of tetanus toxin: new insights into its neuronal signaling pathway

When Clostridium tetani was discovered and identified as a Gram-positive anaerobic bacterium of the genus Clostridium, the possibility of turning its toxin into a valuable biological carrier to ameliorate neurodegenerative processes was inconceivable. However, the non-toxic carboxy-terminal fragment of the tetanus toxin heavy chain (fragment C) can be retrogradely transported to the central nervous system; therefore, fragment C has been used as a valuable biological carrier of neurotrophic factors to ameliorate neurodegenerative processes. More recently, the neuroprotective properties of fragment C have also been described in vitro and in vivo, involving the activation of Akt kinase and extracellular signal-regulated kinase (ERK) signaling cascades through neurotrophin tyrosine kinase (Trk) receptors. Although the precise mechanism of the molecular internalization of fragment C in neuronal cells remains unknown, fragment C could be internalized and translocated into the neuronal cytosol through a clathrin-mediated pathway dependent on proteins, such as dynamin and AP-2. In this review, the origins, molecular properties and possible signaling pathways of fragment C are reviewed to understand the biochemical characteristics of its intracellular and synaptic transport.

Tetanus toxin C-fragment: the courier and the cure?

In many neurological disorders strategies for a specific delivery of a biological activity from the periphery to the central nervous system (CNS) remains a considerable challenge for successful therapy. Reporter assays have established that the non-toxic C-fragment of tetanus toxin (TTC), provided either as protein or encoded by non-viral naked DNA plasmid, binds pre-synaptic motor neuron terminals and can facilitate the retrograde axonal transport of desired therapeutic molecules to the CNS. Alleviated symptoms in animal models of neurological diseases upon delivery of therapeutic molecules offer a hopeful prospect for TTC therapy. This review focuses on what has been learned on TTC-mediated neuronal targeting, and discusses the recent discovery that, instead of being merely a carrier molecule, TTC itself may well harbor neuroprotective properties.

The carboxyl-terminal domain of the heavy chain of tetanus toxin prevents dopaminergic degeneration and improves motor behavior in rats with striatal MPP(+)-lesions

Recently it has been shown that the C-terminus fragment of the tetanus toxin (Hc-TeTx) is transported retrogradely and had shown neuroprotective effects, preventing neuronal death by apoptosis. This could be a new alternative preventing ongoing cell death and restoring the motor function in Parkinson's disease (PD), which is characterized by dopaminergic neurodegeneration. Our aim was to evaluate the effects of local administration of Hc-TeTx on motor behavior and the dopamine (DA) levels in the striatum of MPP(+)-treated rats. In the rotational behavior task, the Hc-TeTx [2 microM]+MPP(+) group had a decreased number of contralateral rotations and the cylinder test improved for both forelimb-use asymmetry compared to the MPP(+) group. The staircase test showed that the Hc-TeTx+MPP(+) group had an improvement of fine motor skills compared to the same limb performance of the MPP(+) group. The group of animals with Hc-TeTx+MPP(+) had higher DA and metabolite levels compared to the MPP(+) group. Our study clearly shows that Hc-TeTx improves different motor behavior strongly, which favors the hypothesis of the Hc-TeTx fragment enhancing survival pathways that result in amelioration of the dopaminergic system of rats with a dopaminergic lesion.