Effects of botulinum toxin induced muscle paralysis on endocytosis and lysosomal enzyme activities in mouse skeletal muscle

Injection of high dose botulinum-toxin A leads to impaired skeletal muscle function and damage of the fibrilar and non-fibrilar structures

Botulinum-toxin A (BoNT/A) is used for a wide range of conditions. Intramuscular administration of BoNT/A inhibits the release of acetylcholine at the neuromuscular junction from presynaptic motor neurons causing muscle-paralysis. The aim of the present study was to investigate the effect of high dose intramuscular BoNT/A injections (6 UI = 60 pg) on muscle tissue. The gait pattern of the rats was significantly affected 3 weeks after BoNT/A injection. The ankle joint rotated externally, the rats became flat footed, and the stride length decreased after BoNT/A injection. Additionally, there was clear evidence of microstructural changes on the tissue level by as evidenced by 3D imaging of the muscles by Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM). Both the fibrillar and the non-fibrillar tissues were affected. The volume fraction of fibrillary tissue was reduced significantly and the non-fibrillar tissue increased. This was accompanied by a loss of the linear structure of the muscle tissue. Furthermore, gene expression analysis showed a significant upregulation of COL1A1, MMP-2, TGF-b1, IL-6, MHCIIA and MHCIIx in the BoNT/A injected leg, while MHVIIB was significantly downregulated.

IN CONCLUSION

The present study reveals that high dose intramuscular BoNT/A injections cause microstructural damage of the muscle tissue, which contributes to impaired gait.

Heat shock protein 70 overexpression does not attenuate atrophy in botulinum neurotoxin type A-treated skeletal muscle

Botulinum neurotoxin type A (BoNT/A) is used clinically to induce therapeutic chemical denervation of spastically contracted skeletal muscles. However, BoNT/A administration can also cause atrophy. We sought to determine whether a major proteolytic pathway contributing to atrophy in multiple models of muscle wasting, the ubiquitin proteasome system (UPS), is involved in BoNT/A-induced atrophy. Three and ten days following BoNT/A injection of rat hindlimb, soleus muscle fiber cross-sectional area was reduced 25 and 65%, respectively. The transcriptional activity of NF-κB and Foxo was significantly elevated at 3 days (2- to 4-fold) and 10 days (5- to 6-fold). Muscle RING-finger protein-1 (MuRF1) activity was elevated (2-fold) after 3 days but not 10 days, while atrogin-1 activity was not elevated at any time point. BoNT/A-induced polyubiquitination occurred after 3 days (3-fold increase) but was totally absent after 10 days. Proteasome activity was elevated (1.5- to 2-fold) after 3 and 10 days. We employed the use of heat shock protein 70 (Hsp70) to inhibit NF-κB and Foxo transcriptional activity. Electrotransfer of Hsp70 into rat soleus, before BoNT/A administration, was insufficient to attenuate atrophy. It was also insufficient to decrease BoNT/A-induced Foxo activity at 3 days, although NF-κB activity was abolished. By 10 days both NF-κB and Foxo activation were abolished by Hsp70. Hsp70-overexpression was unable to alter the levels of BoNT/A-induced effects on MuRF1/atrogin-1, polyubiquitination, or proteasome activity. In conclusion, Hsp70 overexpression is insufficient to attenuate BoNT/A-induced atrophy. It remains unclear what proteolytic mechanism/s are contributing to BoNT/A-induced atrophy, although a Foxo-MuRF1-ubiquitin-proteasome contribution may exist, at least in early BoNT/A-induced atrophy. Further clarification of UPS involvement in BoNT/A-induced atrophy is warranted.

Botulinum toxin paralysis of the orbicularis oculi muscle. Types and time course of alterations in muscle structure, physiology and lid kinematics

In chronically prepared guinea pigs, we investigated the time course of botulinum toxin A's (Bot A) effect on the blink reflex by monitoring lid movements and EMG activity prior to and after Bot A injection into the orbicularis oculi muscle (OOemg), or after nerve crush of the zygomatic nerve. We correlated these alterations with the morphological changes of the orbicularis oculi (lid-closing) muscles of the same animals. After Bot A treatment there was a profound reduction of OOemg activity and blink amplitudes as well as a slowing of maximum blink down-phase velocity. Blink up-phases, however, remained unchanged. Gradual recovery of OOemg magnitude and blink amplitude started around day 6; a functioning blink reflex appeared on day 21, and full recovery of blink amplitude occurred by day 42. Crushing the zygomatic branch of the facial nerve produced similar changes in blink parameters, but recovery was much more rapid (15 days) than for Bot A-treated guinea pigs. The morphological analysis demonstrated that Bot A produced a denervation-like atrophy in the orbicularis oculi. No fiber type-specific alterations were noted, and all muscle fiber types ultimately recovered, with no longstanding consequences of the transient denervation. Our findings support the notion that functional recovery was the result of preterminal and terminal axonal sprouting that subsequently re-established functional innervation. Moreover, differences between the present findings and those seen after injection of Bot A into the extraocular muscles strongly support the hypothesis that the composition in terms of muscle fiber type and the properties of the motor control system of a given muscle greatly influence both how the particular muscle responds to toxin injection, and how effective the toxin is in resolution of neuromuscular disorders that affect a particular muscle. The present findings were consistent with clinical observations that Bot A produces only temporary relief in patients with essential blepharospasm. It is likely that the efficacy of Bot A in treatment of blepharospasm could be improved by using agents that suppress terminal sprouting. The close correspondence of the changes in blink physiology between human patients and guinea pigs after Bot A treatment demonstrate that the guinea pig is an excellent model system for testing strategies to prolong the beneficial effects of Bot A treatment in relieving lid spasms in human subjects.