A clinically relevant BTX-A injection protocol leads to persistent weakness, contractile material loss, and an altered mRNA expression phenotype in rabbit quadriceps muscles

Abstract

Botulinum toxin type-A (BTX-A) injections have become a common treatment modality for patients suffering from muscle spasticity. Despite its benefits, BTX-A treatments have been associated with adverse effects on target muscles. Currently, application of BTX-A is largely based on clinical experience, and research quantifying muscle structure following BTX-A treatment has not been performed systematically. The purpose of this study was to evaluate strength, muscle mass, and contractile material six months following a single or repeated (2 and 3) BTX-A injections into the quadriceps femoris of New Zealand white rabbits. Twenty three skeletally mature rabbits were divided into four groups: experimental group rabbits received 1, 2, or 3 injections at intervals of 3 months (1-BTX-A, 2-BTX-A, 3-BTX-A, respectively) while control group rabbits received volume-matched saline injections. Knee extensor strength, quadriceps muscle mass, and quadriceps contractile material of the experimental group rabbits were expressed as a percentage change relative to the control group rabbits. One-way ANOVA was used to determine group differences in outcome measures (α=0.05). Muscle strength and contractile material were significantly reduced in experimental compared to control group rabbits but did not differ between experimental groups. Muscle mass was the same in experimental BTX-A and control group rabbits. We concluded from these results that muscle strength and contractile material do not fully recover within six months of BTX-A treatment.

Messenger RNA profiling of rabbit quadriceps femoris after repeat injections of botulinum toxin: Evidence for a dynamic pattern without further structural alterations

“…One question that arises after BoNT‐A injection treatment is whether muscle can fully recover to its former state . In the present studies, the VL muscle of the quadriceps complex did not appear to recover at 6 months post‐injection, irrespective of the injection frequency . One set of reasons for this lack of recovery could be the prolonged expression in the injected muscles of molecules, which impeded or interfered with such recovery.…”

“…In a recent report, it was found that a single BoNT‐A injection into the rabbit quadriceps musculature led to persistent muscle weakness and loss of contractile material, and these alterations did not appear to progress with repeat injections. It was noted that the loss of contractile material was associated with the appearance of fatty infiltration, which would likely lead to the secretion of adipokines, such as adiponectin (APN), adipocyte binding protein 2 (AP2), and leptin …”

“…In the aforementioned BoNT‐A injection model, it was noted that the mRNA levels for a restricted number of anabolic (muscle RING finger, or MuRF) or fibrosis‐related genes [transforming growth factor‐beta (TGF‐β), insulin‐like growth factor‐1 (IGF‐1), collagens I and III (Col I and III)] were significantly elevated after 1, 2, or 3 BoNT‐A injections when compared with control values. This restricted data set indicated that muscle regeneration and/or fibrosis was being attempted at the injection site, but that study was incomplete in evaluating the extent of changes or types of changes induced, and so did not allow for a more complete perspective of the impact of single vs. repeat BoNT‐A injections, or of contralateral effects induced by BoNT‐A injections.…”

“…[8][9][10][11] In a recent review, Hart hypothesized that the appearance of fatty infiltration in muscles subjected to various atrophy-inducing stimuli was due, in part, to a "default" differentiation of mesenchymal stem cells (MSCs) to the adipocyte lineage, 12 a hypothesis consistent with what has been observed in the rabbit model after BoNT-A injections. 7,13 It is known that normal muscle contains MSCs, 14,15 and MSCs are also found in fat. 12 Such MSCs can undergo differentiation toward a number of cell lineages, 16,17 with biomarkers for different lineages evident.…”

“…In the aforementioned BoNT-A injection model, 7 it was noted that the mRNA levels for a restricted number of anabolic (muscle RING finger, or MuRF) or fibrosis-related genes [transforming growth factor-beta (TGF-b), insulin-like growth factor-1 (IGF-1), collagens I and III (Col I and III)] were significantly elevated after 1, 2, or 3…”