Spinal plasticity in stroke patients after botulinum neurotoxin A injection in ankle plantar flexors

Intramuscular Botulinum Neurotoxin Serotypes E and A Elicit Distinct Effects on SNAP25 Protein Fragments, Muscular Histology, Spread and Neuronal Transport: An Integrated Histology-Based Study in the Rat

Botulinum neurotoxins E (BoNT/E) and A (BoNT/A) act by cleaving Synaptosome-Associated Protein 25 (SNAP25) at two different C-terminal sites, but they display very distinct durations of action, BoNT/E being short acting and BoNT/A long acting. We investigated the duration of action, spread and neuronal transport of BoNT/E (6.5 ng/kg) and BoNT/A (125 pg/kg) after single intramuscular administrations of high equivalent efficacious doses, in rats, over a 30- or 75-day periods, respectively. To achieve this, we used (i) digit abduction score assay, (ii) immunohistochemistry for SNAP25 (N-ter part; SNAP25N-ter and C-ter part; SNAP25C-ter) and its cleavage sites (cleaved SNAP25; c-SNAP25E and c-SNAP25A) and (iii) muscular changes in histopathology evaluation. Combined in vivo observation and immunohistochemistry analysis revealed that, compared to BoNT/A, BoNT/E induces minimal muscular changes, possesses a lower duration of action, a reduced ability to spread and a decreased capacity to be transported to the lumbar spinal cord. Interestingly, SNAP25C-ter completely disappeared for both toxins during the peak of efficacy, suggesting that the persistence of toxin effects is driven by the persistence of proteases in tissues. These data unveil some new molecular mechanisms of action of the short-acting BoNT/E and long-acting BoNT/A, and reinforce their overall safety profiles.

Robotic gait training and botulinum toxin injection improve gait in the chronic post-stroke phase: A randomized controlled trial

BACKGROUND

Improving walking ability is one of the main goals of rehabilitation after stroke. When lower limb spasticity increases walking difficulty, botulinum toxin type A (BTx-A) injections can be combined with non-pharmacologic interventions such as intensive rehabilitation using a robotic approach. To the best of our knowledge, no comparisons have been made between the efficacy of robotic gait training and conventional physical therapy in combination with BTx-A injections.

OBJECTIVE

To conduct a randomized controlled trial to compare the efficacy on gait of robotic gait training versus conventional physiotherapy after BTx-A injection into the spastic triceps surae in people after stroke.

METHOD

Thirty-three participants in the chronic stroke phase with triceps surae spasticity inducing gait impairment were included. After BTx-A injection, participants were randomized into 2 groups. Group A underwent robotic gait training (Lokomat®) for 2 weeks, followed by conventional physiotherapy for 2 weeks (n = 15) and Group B underwent the same treatment in reverse order (n = 18). The efficacy of these methods was tested using the 6-minute walk test (6MWT), comparing post-test 1 and post-test 2 with the pre-test.

RESULTS

After the first period, the 6MWT increased significantly more in Group A than in Group B: the mean difference between the interventions was 33 m (95%CI 9; 58 p = 0.007; g = 0.95), in favor of Group A; after the second period, the 6MWT increased in both groups, but the 30 m difference between the groups still remained (95%CI 5; 55 p = 0.019; g = 0.73).

CONCLUSION

Two weeks of robotic gait training performed 2 weeks after BTx-A injections improved walking performance more than conventional physiotherapy. Large-scale studies are now required on the timing of robotic rehabilitation after BTx-A injection.

Pathogenicity and virulence of Clostridium botulinum

Clostridium botulinum, a polyphyletic Gram-positive taxon of bacteria, is classified purely by their ability to produce botulinum neurotoxin (BoNT). BoNT is the primary virulence factor and the causative agent of botulism. A potentially fatal disease, botulism is classically characterized by a symmetrical descending flaccid paralysis, which is left untreated can lead to respiratory failure and death. Botulism cases are classified into three main forms dependent on the nature of intoxication; foodborne, wound and infant. The BoNT, regarded as the most potent biological substance known, is a zinc metalloprotease that specifically cleaves SNARE proteins at neuromuscular junctions, preventing exocytosis of neurotransmitters, leading to muscle paralysis. The BoNT is now used to treat numerous medical conditions caused by overactive or spastic muscles and is extensively used in the cosmetic industry due to its high specificity and the exceedingly small doses needed to exert long-lasting pharmacological effects. Additionally, the ability to form endospores is critical to the pathogenicity of the bacteria. Disease transmission is often facilitated via the metabolically dormant spores that are highly resistant to environment stresses, allowing persistence in the environment in unfavourable conditions. Infant and wound botulism infections are initiated upon germination of the spores into neurotoxin producing vegetative cells, whereas foodborne botulism is attributed to ingestion of preformed BoNT. C. botulinum is a saprophytic bacterium, thought to have evolved its potent neurotoxin to establish a source of nutrients by killing its host.

Improved Gait and Radiological Measurements After injection of Botulinum Toxin Into Peroneus Longus in Young Children With USCP and Equinovalgus Gait

BACKGROUND

Children with cerebral palsy develop foot deformities due to a combination of factors including muscle shortening, hypertonia, weakness, and cocontraction of muscles acting at the ankle joint resulting in an altered gait pattern. We hypothesized these factors affect the peroneus longus (PL) and tibialis anterior (TA) muscles couple in children who develop equinovalgus gait first followed by planovalgus foot deformities. Our aim was to evaluate the effects of abobotulinum toxin A injection to the PL muscle, in a cohort of children with unilateral spastic cerebral palsy and equinovalgus gait.

METHODS

This was a prospective cohort study. The children were examined within 12 months before and after injection to their PL muscle. Twenty-five children of mean age 3.4 (S.D.: 1.1) years were recruited.

RESULTS

We found significant improvement in foot radiology measures. Passive extensibility of the triceps surae did not change, whereas active dorsiflexion increased significantly. Nondimensional walking speed increased by 0.1 (95% confidence interval [CI], [0.07, 0.16]; P < 0.001), and the Edinburgh visual gait score improved by 2.8 (95% CI, [-4.06, -1.46]; P < 0.001). Electromyography showed increased recruitment for gastrocnemius medialis (GM) and TA but not for PL during the reference exercises (standing on tip toes for GM/PL, active dorsiflexion for TA) and decreased activation percentages for PL/GM and TA across sub-phases of gait.

CONCLUSIONS

One key advantage of treating the PL muscle only might be to address foot deformities without interfering with the main plantar flexors that are instrumental to support body weight during gait.

Lasting Peripheral and Central Effects of Botulinum Toxin Type A on Experimental Muscle Hypertonia in Rats

Recent animal experiments suggested that centrally transported botulinum toxin type A (BoNT-A) might reduce an abnormal muscle tone, though with an unknown contribution to the dominant peripheral muscular effect observed clinically. Herein, we examined if late BoNT-A antispastic actions persist due to possible central toxin actions in rats. The early effect of intramuscular (i.m.) BoNT-A (5, 2 and 1 U/kg) on a reversible tetanus toxin (TeNT)-induced calf muscle spasm was examined 7 d post-TeNT and later during recovery from flaccid paralysis (TeNT reinjected on day 49 post-BoNT-A). Lumbar intrathecal (i.t.) BoNT-A-neutralizing antiserum was used to discriminate the transcytosis-dependent central toxin action of 5 U/kg BoNT-A. BoNT-A-truncated synaptosomal-associated protein 25 immunoreactivity was examined in the muscles and spinal cord at day 71 post-BoNT-A. All doses (5, 2 and 1 U/kg) induced similar antispastic actions in the early period (days 1-14) post-BoNT-A. After repeated TeNT, only the higher two doses prevented the muscle spasm and associated locomotor deficit. Central trans-synaptic activity contributed to the late antispastic effect of 5 U/kg BoNT-A. Ongoing BoNT-A enzymatic activity was present in both injected muscle and the spinal cord. These observations suggest that the treatment duration in sustained or intermittent muscular hyperactivity might be maintained by higher doses and combined peripheral and central BoNT-A action.

Botulinum Neurotoxins in Central Nervous System: An Overview from Animal Models to Human Therapy

Botulinum neurotoxins (BoNTs) are potent inhibitors of synaptic vesicle fusion and transmitter release. The natural target of BoNTs is the peripheral neuromuscular junction (NMJ) where, by blocking the release of acetylcholine (ACh), they functionally denervate muscles and alter muscle tone. This leads them to be an excellent drug for the therapy of muscle hyperactivity disorders, such as dystonia, spasticity, and many other movement disorders. BoNTs are also effective in inhibiting both the release of ACh at sites other than NMJ and the release of neurotransmitters other than ACh. Furthermore, much evidence shows that BoNTs can act not only on the peripheral nervous system (PNS), but also on the central nervous system (CNS). Under this view, central changes may result either from sensory input from the PNS, from retrograde transport of BoNTs, or from direct injection of BoNTs into the CNS. The aim of this review is to give an update on available data, both from animal models or human studies, which suggest or confirm central alterations induced by peripheral or central BoNTs treatment. The data will be discussed with particular attention to the possible therapeutic applications to pathological conditions and degenerative diseases of the CNS.

Dramatic neurological and biological effects by botulinum neurotoxin type A on SH-SY5Y neuroblastoma cells, beyond the blockade of neurotransmitter release

BACKGROUND

Gene expression profile analysis on mammalian cell lines and animal models after exposure to botulinum neurotoxin (BoNT) has been investigated in several studies in recent years. Microarray analysis provides a powerful tool for identifying critical signaling pathways involved in the biological and inflammatory responses to BoNT and helps determine the mechanism of the function of botulinum toxins. One of the pivotal clinical characteristics of BoNT is its prolonged on-site effects. The role of BoNT on the blockage of neurotransmitter acetylcholine release in the neuromuscular junction has been well established. However, the effects of the treatment time of BoNT on the human cellular model and its potential mechanism remain to be defined.

METHODS

This study aimed to use gene microarray technology to compare the two physiological critical time points of BoNT type A (BoNT/A) treatment of human neuroblastoma cells and to advance our understanding of the profound biological influences that toxin molecules play in the neuronal cellular system. SH-SY5Y neuroblastoma cells were treated with BoNT/A for 4 and 48 h, which represent the time needed for the entrance of toxin into the cells and the time necessary for the initial appearance of the on-site effects after BoNT application, respectively.

RESULTS

A comparison of the two time points identified 122 functional groups that are significantly changed. The top five groups are alternative splicing, phosphoprotein, nucleus, cytoplasm, and acetylation. Furthermore, after 48 h, there were 744 genes significantly up-regulated, and 624 genes significantly down-regulated (p‹ 0.01). These genes fell into the following neurological and biological annotation groups: Nervous system development, proteinaceous extracellular matrix, signaling pathways regulating pluripotency of stem cells, cellular function and signal transduction, and apoptosis. We have also noticed that the up-regulated groups contained neuronal cell development, nervous system development, and metabolic processes. In contrast, the down-regulated groups contained many chromosomes and cell cycle categories.

CONCLUSIONS

The effects of BoNT/A on neuronal cells extend beyond blocking the neurotransmitter release, and that BoNT/A is a multifunctional molecule that can evoke profound cellular responses which warrant a more in-depth understanding of the mechanism of the toxin's effects after administration.

Efficacy of incobotulinumtoxinA for the treatment of adult lower-limb post-stroke spasticity, including pes equinovarus

BACKGROUND

Lower-limb spasticity can impair ambulation and gait, impacting quality of life.

OBJECTIVES

This ancillary analysis of the TOWER study (NCT01603459) assessed the efficacy of incobotulinumtoxinA for lower-limb post-stroke spasticity including pes equinovarus.

METHODS

Participants received escalating incobotulinumtoxinA doses (400-800U) across 3 injection cycles. Changes were compared for those treated in the lower limb (with/without upper-limb treatment) or the upper limb only or for participants treated or untreated for pes equinovarus. Outcome measures were those used in the seminal study: resistance to passive movement scale (REPAS), Ashworth Scale (AS), functional ambulation and lower-limb goal attainment.

RESULTS

Among 132/155 (85%) participants with post-stroke spasticity, in cycles 1, 2 and 3, 99, 119 and 121 participants received lower-limb treatment with mean (SD) total limb incobotulinumtoxinA doses of 189.2 (99.2), 257.1 (115.0) and 321.3 (129.2) U, respectively. Of these, 80, 105 and 107, respectively, were treated for pes equinovarus. The mean (SD) improvement in REPAS lower-limb score was greater with treatment in the lower limb versus the upper limb only: -1.6 (2.1) versus-0.4 (1.4); -1.9 (1.9) versus -0.6 (1.6); -2.2 (2.2) versus -1.0 (0.0) (P=0.0005, P=0.0133 and P=0.3581; analysis of covariance [ANCOVA], between-group differences) in cycles 1, 2 and 3, respectively. For all cycles, the mean improvement in ankle joint AS score from injection to 4 weeks post-treatment was greater for participants treated versus not treated for pes equinovarus, with a significant between-group difference in cycle 1 (P=0.0099; ANCOVA). At the end of cycle 3, 42% of participants walked independently and 63% achieved 2 of 2 lower-limb treatment goals (baseline 23% and 34%, respectively).

CONCLUSIONS

This study supports the efficacy of incobotulinumtoxinA for treatment of pes equinovarus and other patterns of lower-limb post-stroke spasticity.

Evidence for central antispastic effect of botulinum toxin type A

BACKGROUND AND PURPOSE

Botulinum toxin type A (BoNT/A) injections into hyperactive muscles provide effective treatment for spasticity and dystonias, presumably due to its local effects on extrafusal and intrafusal motor fibres. A recent discovery of toxin's retrograde axonal transport to CNS might suggest additional action sites. However, in comparison to cholinergic peripheral terminals, functional consequences of BoNT/A direct central action on abnormally increased muscle tone are presently unknown. To address this question, the central effects of BoNT/A were assessed in experimental local spastic paralysis.

EXPERIMENTAL APPROACH

Local spastic paralysis was induced by injection of tetanus toxin (1.5 ng) into rat gastrocnemius. Subsequently, BoNT/A (5 U·kg^-1 ) was applied i.m. into the spastic muscle or intraneurally (i.n.) into the sciatic nerve to mimic the action of axonally transported toxin. Functional role of BoNT/A transcytosis in spinal cord was evaluated by lumbar i.t. application of BoNT/A-neutralizing antitoxin. BoNT/A effects were studied by behavioural motor assessment and cleaved synaptosomal-associated protein 25 (SNAP-25) immunohistochemistry.

KEY RESULTS

Tetanus toxin evoked muscular spasm (sustained rigid hind paw extension and resistance to passive ankle flexion). Subsequent injections of BoNT/A, i.m. or i.n, reduced tetanus toxin-evoked spastic paralysis. Beneficial effects of i.n. BoNT/A and occurrence of cleaved SNAP-25 in ventral horn were prevented by i.t. antitoxin.

CONCLUSIONS AND IMPLICATIONS

Axonally transported BoNT/A relieves muscle hypertonia induced by tetanus toxin, following the trans-synaptic movement of BoNT/A in the CNS. These results suggest that such direct, centrally mediated reduction of abnormal muscle tone might contribute to the effectiveness of BoNT/A in spasticity and hyperkinetic movement disorders.

Contribution of muscle activity at different gait phases for improving walking performance in chronic stroke patients with hemiparesis

[Purpose] The aim of this study was to clarify the optimal timing for increasing muscle activity in the paralyzed lower limb of stroke survivors by evaluating the relationship between gait muscle activity patterns and gait parameters. [Participants and Methods] Electromyography of the tibialis anterior, soleus, rectus femoris, and biceps femoris on the paralyzed side and spatiotemporal gait parameters were evaluated in 40 chronic post-stroke patients as they walked at a comfortable speed. The normalized average amplitude and asymmetry indexes of each gait phase were calculated. The correlations between gait velocity or asymmetry indexes and the activity amplitudes of various muscles during each gait phase were analyzed. Multiple regression analysis was performed with gait velocity or asymmetry indexes as the response variable and the muscle activity amplitudes in the various gait phases as explanatory variables. [Results] The major determinants of gait velocity were the tibialis anterior activity (β=−0.35) and biceps femoris activity (β=0.45) during the swing phase. In addition, the biceps femoris activity during the swing phase was the major determinant of the asymmetry index for the swing phase duration (β=−0.41). [Conclusion] For patients with hemiparesis, increasing the biceps femoris activity during the swing phase is considered optimal, which may lead to improvement in walking performance.

Transynaptic Action of Botulinum Neurotoxin Type A at Central Cholinergic Boutons

Botulinum neurotoxin Type A (BoNT/A) is an effective treatment for several movement disorders, including spasticity and dystonia. BoNT/A acts by cleaving synaptosomal-associated protein of 25 kDa (SNAP-25) at the neuromuscular junction, thus blocking synaptic transmission and weakening overactive muscles. However, not all the therapeutic benefits of the neurotoxin are explained by peripheral neuroparalysis, suggesting an action of BoNT/A on central circuits. Currently, the specific targets of BoNT/A central activity remain unclear. Here, we show that catalytically active BoNT/A is transported to the facial nucleus (FN) after injection into the nasolabial musculature of rats and mice. BoNT/A-mediated cleavage of SNAP-25 in the FN is prevented by intracerebroventricular delivery of antitoxin antibodies, demonstrating that BoNT/A physically leaves the motoneurons to enter second-order neurons. Analysis of intoxicated terminals within the FN shows that BoNT/A is transcytosed preferentially into cholinergic synapses. The cholinergic boutons containing cleaved SNAP-25 are associated with a larger size, suggesting impaired neuroexocytosis. Together, the present findings indicate a previously unrecognized source of reduced motoneuron drive after BoNT/A via blockade of central, excitatory cholinergic inputs. These data highlight the ability of BoNT/A to selectively target and modulate specific central circuits, with consequent impact on its therapeutic effectiveness in movement disorders.SIGNIFICANCE STATEMENT Botulinum neurotoxins are among the most potent toxins known. Despite this, their specific and reversible action prompted their use in clinical practice to treat several neuromuscular pathologies (dystonia, spasticity, muscle spasms) characterized by hyperexcitability of peripheral nerve terminals or even in nonpathological applications (i.e., cosmetic use). Substantial experimental and clinical evidence indicates that not all botulinum neurotoxin Type A (BoNT/A) effects can be explained solely by the local action (i.e., silencing of the neuromuscular junction). In particular, there are cases in which the clinical benefit exceeds the duration of peripheral neurotransmission blockade. In this study, we demonstrate that BoNT/A is transported to facial motoneurons, released, and internalized preferentially into cholinergic terminals impinging onto the motoneurons. Our data demonstrate a direct central action of BoNT/A.

The Expanding Therapeutic Utility of Botulinum Neurotoxins

Botulinum neurotoxin (BoNT) is a major therapeutic agent that is licensed in neurological indications, such as dystonia and spasticity. The BoNT family, which is produced in nature by clostridial bacteria, comprises several pharmacologically distinct proteins with distinct properties. In this review, we present an overview of the current therapeutic landscape and explore the diversity of BoNT proteins as future therapeutics. In recent years, novel indications have emerged in the fields of pain, migraine, overactive bladder, osteoarthritis, and wound healing. The study of biological effects distal to the injection site could provide future opportunities for disease-tailored BoNT therapies. However, there are some challenges in the pharmaceutical development of BoNTs, such as liquid and slow-release BoNT formulations; and, transdermal, transurothelial, and transepithelial delivery. Innovative approaches in the areas of formulation and delivery, together with highly sensitive analytical tools, will be key for the success of next generation BoNT clinical products.

Factors affecting premature plantarflexor muscle activity during hemiparetic gait

In hemiparetic stroke survivors, premature plantarflexor muscle activity (PPF) often appears as a gait abnormality from terminal swing to the loading response on the paretic side. This study aimed to discern factors giving rise to PPF. Lower extremity function, spasticity magnitude, and gait electromyograms were assessed in 31 hemiparetic stroke survivors. Mean amplitudes during gait phases were determined for the paretic soleus, tibialis anterior, rectus femoris, and biceps femoris. The subjects were classified into PPF and non-PPF groups based on their relative soleus amplitude at different phases of gait, and group differences in each measurement were calculated and subjected to logistic regression. The PPF group showed less activity of the tibialis anterior during the swing phase but greater activity of the rectus femoris during the swing phase and of the biceps femoris, both prematurely and during the loading response. Logistic regression revealed premature activity of the biceps femoris to be a significant variable related to presence of PPF (odds ratio = 1.054). PPF in hemiparetic gait may work with the biceps femoris to supplement compromised lower extremity extension strength. PPF might be reduced by attaining enhanced strength of the hip and knee extensors at the time of initial contact during gait.

Direct central nervous system effects of botulinum neurotoxin

Local intramuscular injections of botulinum neurotoxin type A (BoNT/A) are effective in the treatment of focal dystonias, muscle spasms, and spasticity. However, not all clinical effects of BoNT/A may be explained by its action at peripheral nerve terminals. For example, the therapeutic benefit may exceed the duration of the peripheral neuroparalysis induced by the neurotoxin. In cellular and animal models, evidence demonstrates retrograde transport of catalytically active BoNT/A in projection neurons. This process of long-range trafficking is followed by transcytosis and action at second-order synapses. In humans, several physiological changes have been described following intramuscular delivery of BoNT/A. In particular, clinical studies have documented a decrease in Renshaw cell-mediated inhibition (i.e., recurrent inhibition), which may be important therapeutically for normalizing uncoordinated movements and overflow of muscle activity. In this review, we present data obtained in animal and experimental models that support direct central actions of BoNT/A mediated via retrograde axonal trafficking. We also discuss the reorganization of central circuitry induced by BoNT/A in patients, and the potential contribution of these effects to the therapeutic efficacy of the neurotoxin.

Botulinum neurotoxin type A-cleaved SNAP25 is confined to primary motor neurons and localized on the plasma membrane following intramuscular toxin injection

The mechanism of action of botulinum neurotoxin type A (BoNT/A) is well characterized, but some published evidence suggests the potential for neuronal retrograde transport and cell-to-cell transfer (transcytosis) under certain experimental conditions. The present study evaluated the potential for these processes using a highly selective antibody for the BoNT/A-cleaved substrate (SNAP25₁₉₇) combined with 3-dimensional imaging. SNAP25₁₉₇ was characterized in a rat motor neuron (MN) pathway following toxin intramuscular injections at various doses to determine whether SNAP25₁₉₇ is confined to MNs or also found in neighboring cells or nerve fibers within spinal cord (SC). Results demonstrated that SNAP25₁₉₇ immuno-reactive staining was colocalized with biomarkers for MNs, but not with markers for neighboring neurons, nerve fibers or glial cells. Additionally, a high dose of BoNT/A, but not a lower dose, resulted in sporadic SNAP25₁₉₇ signal in distal muscles and associated SC regions without evidence for transcytosis, suggesting that the staining was due to systemic spread of the toxin. Despite this spread, functional effects were not detected in the distal muscles. Therefore, under the present experimental conditions, our results suggest that BoNT/A is confined to MNs and any evidence of distal activity is due to limited systemic spread of the toxin at higher doses and not through transcytosis within SC. Lastly, at higher doses of BoNT/A, SNAP25₁₉₇ was expressed throughout MNs and colocalized with synaptic markers on the plasma membrane at 6 days post-treatment. These data support previous studies suggesting that SNAP25₁₉₇ may be incorporated into SNARE-protein complexes within the affected MNs.

Botulinum toxin injection causes hyper-reflexia and increased muscle stiffness of the triceps surae muscle in the rat

Botulinum toxin is used with the intention of diminishing spasticity and reducing the risk of development of contractures. Here, we investigated changes in muscle stiffness caused by reflex activity or elastic muscle properties following botulinum toxin injection in the triceps surae muscle in rats. Forty-four rats received injection of botulinum toxin in the left triceps surae muscle. Control measurements were performed on the noninjected contralateral side in all rats. Acute experiments were performed, 1, 2, 4, and 8 wk following injection. The triceps surae muscle was dissected free, and the Achilles tendon was cut and attached to a muscle puller. The resistance of the muscle to stretches of different amplitudes and velocities was systematically investigated. Reflex-mediated torque was normalized to the maximal muscle force evoked by supramaximal stimulation of the tibial nerve. Botulinum toxin injection caused severe atrophy of the triceps surae muscle at all time points. The force generated by stretch reflex activity was also strongly diminished but not to the same extent as the maximal muscle force at 2 and 4 wk, signifying a relative reflex hyperexcitability. Passive muscle stiffness was unaltered at 1 wk but increased at 2, 4, and 8 wk (P < 0.01). These data demonstrate that botulinum toxin causes a relative increase in reflex stiffness, which is likely caused by compensatory neuroplastic changes. The stiffness of elastic elements in the muscles also increased. The data are not consistent with the ideas that botulinum toxin is an efficient antispastic medication or that it may prevent development of contractures.

Interneuronal Transfer and Distal Action of Tetanus Toxin and Botulinum Neurotoxins A and D in Central Neurons

Recent reports suggest that botulinum neurotoxin (BoNT) A, which is widely used clinically to inhibit neurotransmission, can spread within networks of neurons to have distal effects, but this remains controversial. Moreover, it is not known whether other members of this toxin family are transferred between neurons. Here, we investigate the potential distal effects of BoNT/A, BoNT/D, and tetanus toxin (TeNT), using central neurons grown in microfluidic devices. Toxins acted upon the neurons that mediated initial entry, but all three toxins were also taken up, via an alternative pathway, into non-acidified organelles that mediated retrograde transport to the somato-dendritic compartment. Toxins were then released into the media, where they entered and exerted their effects upon upstream neurons. These findings directly demonstrate that these agents undergo transcytosis and interneuronal transfer in an active form, resulting in long-distance effects.

More than at the neuromuscular synapse: actions of botulinum neurotoxin A in the central nervous system

Botulinum neurotoxin type A (BoNT/A) is a metalloprotease that produces a sustained yet transitory blockade of transmitter release from peripheral nerve terminals. Local delivery of this neurotoxin is successfully employed in clinical practice to reduce muscle hyperactivity such as in spasticity and dystonia, and to relieve pain with long-lasting therapeutic effects. However, not all BoNT/A effects can be explained by an action at peripheral nerve terminals. Indeed, it appears that BoNT/A is endowed with trafficking properties similar to the parental tetanus neurotoxin and thus be able to directly affect the CNS. In this review, we present and discuss novel compelling evidence for a direct central effect of BoNT/A in both dorsal and ventral horns of the animal and human spinal cord after peripheral injection of the neurotoxin, with important consequences on pain and motor control. This new knowledge is expected to radically change the approach to the use of BoNT/A in the future. As BoNT/A central action appears to also contribute to functional improvement, for instance in human spastic gait, the challenge will be to develop new subtypes or BoNT derivatives with deliberate, cell-specific central effects in order to fully exploit the spectrum of BoNT/A therapeutic activity.