Myotoxic effects of the skeletal muscle-specific immunotoxin, ricin-mAb35, on orbicularis oculi muscle after eyelid injections in rabbits

Transition State Structure of RNA Depurination by Saporin L3

Saporin L3 from the leaves of the common soapwort is a catalyst for hydrolytic depurination of adenine from RNA. Saporin L3 is a type 1 ribosome inactivating protein (RIP) composed only of a catalytic domain. Other RIPs have been used in immunotoxin cancer therapy, but off-target effects have limited their development. In the current study, we use transition state theory to understand the chemical mechanism and transition state structure of saporin L3. In favorable cases, transition state structures guide the design of transition state analogues as inhibitors. Kinetic isotope effects (KIEs) were determined for an A14C mutant of saporin L3. To permit KIE measurements, small stem-loop RNAs that contain an AGGG tetraloop structure were enzymatically synthesized with the single adenylate bearing specific isotopic substitutions. KIEs were measured and corrected for forward commitment to obtain intrinsic values. A model of the transition state structure for depurination of stem-loop RNA (5'-GGGAGGGCCC-3') by saporin L3 was determined by matching KIE values predicted via quantum chemical calculations to a family of intrinsic KIEs. This model indicates saporin L3 displays a late transition state with the N-ribosidic bond to the adenine nearly cleaved, and the attacking water nucleophile weakly bonded to the ribosyl anomeric carbon. The transition state retains partial ribocation character, a feature common to most N-ribosyl transferases. However, the transition state geometry for saporin L3 is distinct from ricin A-chain, the only other RIP whose transition state is known.

Modulating neuromuscular junction density changes in botulinum toxin-treated orbicularis oculi muscle

PURPOSE

Botulinum toxin A is the most commonly used treatment for blepharospasm, hemifacial spasm, and other focal dystonias. Its main drawback is its relatively short duration of effect. The goal of this study was to examine the ability of corticotropin releasing factor (CRF) or antibody to insulin growth factor I-receptor (anti-IGFIR) to reduce the up-regulation of neuromuscular junctions that are associated with return of muscle function after botulinum toxin treatment.

METHODS

Eyelids of adult rabbits were locally injected with either botulinum toxin alone or botulinum toxin treatment followed by injection of either CRF or anti-IGFIR. After one, two, or four weeks, the orbicularis oculi muscles within the treated eyelids were examined for density of neuromuscular junctions histologically.

RESULTS

Injection of botulinum toxin into rabbit eyelids resulted in a significant increase in the density of neuromuscular junctions at one and two weeks, and an even greater increase in neuromuscular junction density by four weeks after treatment. Treatment with either CRF or anti-IGFIR completely prevented this increase in neuromuscular junction density.

CONCLUSIONS

The return of function after botulinum toxin-induced muscle paralysis is due to terminal sprouting and formation of new neuromuscular junctions within the paralyzed muscles. Injection with CRF or anti-IGFIR after botulinum toxin treatment prevents this sprouting, which in turn should increase the duration of effectiveness of single botulinum toxin treatments. Future physiology studies will address this. Prolonging botulinum toxin's clinical efficacy should decrease the number of injections needed for patient muscle spasm relief, decreasing the risk of negative side effects and changes in drug effectiveness that often occurs over a lifetime of botulinum toxin exposure.

Effect of reading on surface electromyogram recordings in patients with blepharospasm

PURPOSE

To compare the surface electromyogram recordings between patients with benign essential blepharospasm and controls while maintaining primary gaze and reading in downgaze.

METHODS

A university-based, prospective case series of 7 benign essential blepharospasm patients and 7 normal patients. Bilateral lower eyelid preseptal orbicularis oculi muscle potentials were recorded via surface electromyogram and video while performing a standardized 60-second task that was divided in 3 equal 20-second subtasks. Specifically, patients were asked to maintain primary gaze for the first 20-second interval, to read quietly for the second interval, and again maintain primary gaze for the third 20-second interval. Fourteen trials capturing bilateral recordings were performed in each group; qualitative and quantitative analysis of the surface electromyogram recordings was performed and compared both between patient groups and between subtasks. Blepharospasm patients were also asked to describe the effect of reading on their spasms. Specifically, they were asked before the study if reading lessened or worsened their spasms and then were asked poststudy if their symptoms lessened or worsened during the reading task.

RESULTS

The mean surface electromyogram potential of the orbicularis muscle was significantly less in normal eyes when compared with blepharospasm eyes for each task. Furthermore, the mean surface electromyogram potential during the reading task was significantly reduced when compared with the primary gaze task in all eyes. Only blepharospasm patients noticed this reduction subjectively.

CONCLUSIONS

Although conventional wisdom regarding blepharospasm suggests that reading is an exacerbating factor, short durations of reading seem to relieve spasms. This may be due to the effect of downgaze on the blink reflex and may offer possible areas of investigation for novel blepharospasm treatments.