Muscle protection following motor nerve repair in combination with leukemia inhibitory factor

Key changes in denervated muscles and their impact on regeneration and reinnervation

The neuromuscular junction becomes progressively less receptive to regenerating axons if nerve repair is delayed for a long period of time. It is difficult to ascertain the denervated muscle's residual receptivity by time alone. Other sensitive markers that closely correlate with the extent of denervation should be found. After a denervated muscle develops a fibrillation potential, muscle fiber conduction velocity, muscle fiber diameter, muscle wet weight, and maximal isometric force all decrease; remodeling increases neuromuscular junction fragmentation and plantar area, and expression of myogenesis-related genes is initially up-regulated and then down-regulated. All these changes correlate with both the time course and degree of denervation. The nature and time course of these denervation changes in muscle are reviewed from the literature to explore their roles in assessing both the degree of detrimental changes and the potential success of a nerve repair. Fibrillation potential amplitude, muscle fiber conduction velocity, muscle fiber diameter, mRNA expression levels of myogenic regulatory factors and nicotinic acetylcholine receptor could all reflect the severity and length of denervation and the receptiveness of denervated muscle to regenerating axons, which could possibly offer an important clue for surgical choices and predict the outcomes of delayed nerve repair.

Embryonic stem cell-derived motor neurons preserve muscle after peripheral nerve injury

BACKGROUND

The potential of motor neuron progenitor cell transplants to preserve muscle tissue after denervation was studied in in vivo and in vitro adult mammalian model of peripheral nerve injury.

METHODS

Embryonic stem cells were differentiated to induce cholinergic motor neuron progenitors. Flourescent-labeled progenitor cells were injected into the gastrocnemius muscle of Sprague-Dawley rats (n = 10) after denervation by ipilateral sciatic nerve transection. Control rats received injections of either a phosphate-buffered saline solution only (n = 12), murine embryonic fibroblast (STO) cells (n= 6), or undifferentiated embryonic stem cells (n= 6). Muscles were weighed and analyzed at 7 and 21 days using histology, histomorphometry, and immunostaining.

RESULTS

Seven days after progenitor cell transplant, both muscle mass and myocyte cross-sectional area were preserved, compared with control muscles, which demonstrated muscle mass reduction to 70 percent and reduction of cross-sectional area to 72 percent of normal. Fluorescent microscopy of transplanted muscles confirmed the presence of motor neuron progenitors. Presynaptic neuronal staining of the transplants overlapped with alpha-bungarotoxin-labeled muscle fibers, revealing the presence of new neuromuscular junctions. By 21 days, muscle atrophy in the experimental muscles was equal to that of controls and no transplanted cells were observed. Co-culture of the motor neuron progenitor cells and myocytes also demonstrated new neuromuscular junctions by immunofluorescence.

CONCLUSIONS

Transplanted motor neuron progenitors prevent muscle atrophy after denervation for a brief time. These progenitor cell transplants appear to form new neuromuscular junctions with denervated muscle fibers in vivo and with myocytes in vitro.

Evolving therapeutic strategies for Duchenne muscular dystrophy: targeting downstream events

Duchenne muscular dystrophy (DMD) is a progressive, lethal, muscle wasting disease that affects 1 of 3500 boys born worldwide. The disease results from mutation of the dystrophin gene that encodes a cytoskeletal protein associated with the muscle cell membrane. Although gene therapy will likely provide the cure for DMD, it remains on the distant horizon, emphasizing the need for more rapid development of palliative treatments that build on improved understanding of the complex pathology of dystrophin deficiency. In this review, we have focused on therapeutic strategies that target downstream events in the pathologic progression of DMD. Much of this work has been developed initially using the dystrophin-deficient mdx mouse to explore basic features of the pathophysiology of dystrophin deficiency and to test potential therapeutic interventions to slow, reverse, or compensate for functional losses that occur in muscular dystrophy. In some cases, the initial findings in the mdx model have led to clinical treatments for DMD boys that have produced improvements in muscle function and quality of life. Many of these investigations have concerned interventions that can affect protein balance in muscle, by inhibiting specific proteases implicated in the DMD pathology, or by providing anabolic factors or depleting catabolic factors that can contribute to muscle wasting. Other investigations have exploited the use of anti-inflammatory agents that can reduce the contribution of leukocytes to promoting secondary damage to dystrophic muscle. A third general strategy is designed to increase the regenerative capacity of dystrophic muscle and thereby help retain functional muscle mass. Each of these general approaches to slowing the pathology of dystrophin deficiency has yielded encouragement and suggests that targeting downstream events in dystrophinopathy can yield worthwhile, functional improvements in DMD.

An injectable nerve regeneration chamber for studies of unstable soluble growth factors

Modern surgical techniques cannot guarantee functional recovery following peripheral nerve injuries. Research into factors that may influence nerve regeneration has therefore assumed a prominent potential therapeutic role. We report here on the development of an approach to allow for direct manipulation of the microenvironment of regenerating peripheral nerve axons. We show that solutions can be delivered directly to this local milieu in vivo and that such a delivery can be performed multiple times over an extended period, potentially facilitating studies of multiple molecular players that act locally. We also demonstrate that the bundle of regenerated axons are amenable to morphological analysis by 21 days and that the injection system remains patent for at least 21 days.

Insulin growth factor-1 decreases muscle atrophy following denervation

Despite modern microsurgical techniques for nerve repair, functional outcome following proximal injury is often unsatisfactory because irreversible muscle atrophy may develop before reinnervation occurs. Because insulin growth factor-1 (IGF-1) has been shown to improve muscle regeneration after injury, and may have a role in muscle preservation following denervation, the purpose of this investigation was to evaluate the histological, immunohistochemical, and electrophysiological differences between normal, denervated, and IGF-1-injected denervated muscle over an 8-week period. Denervated mice gastrocnemius muscles demonstrated a decrease in muscle weight, a decrease in myofiber diameter, an absence of muscle regeneration, an early increase in the number of neuromuscular junctions (NMJs), and a decrease in fast-twitch and maximum tetanic strength as compared to normal muscle up to 8 weeks following denervation. IGF-1-injected denervated muscle, on the other hand, sustained muscle diameter and muscle weight, maintained a smaller number of NMJs, and relatively sustained fast-twitch and maximum tetanic strength as compared to normal muscle over 8 weeks. These data suggest that IGF-1 may help prevent muscle atrophy and secondary functional compromise after denervation.

Effects of LIF dose and laminin plus fibronectin on axotomized sciatic nerves

Leukemia inhibitory factor (LIF), a cytokine which has neurotrophic and myotrophic activities, has been shown to enhance nerve regeneration and consequent return of muscle function in the entubulation model of sciatic nerve repair. Fibronectin (FN) and laminin (LN) are two extracellular matrix (ECM) components that, when combined, promote axon growth in the entubulation model. The aim of this study was to determine the optimal LIF dose and the efficacy of FN plus LN administered either alone or simultaneously with the optimal LIF dose. We found that at 12 weeks following nerve repair, a single 10 ng LIF dose produced the largest medial gastrocnemius (MG) muscle mass (P < 0.0001) and maximum force contraction (P < 0.001). The diameter of the axons in the FN plus LN group were significantly greater than for saline (P < 0.001) and the LIF dose groups (P < 0.01). When 10 ng LIF was combined with FN plus LN, the MG muscle mass was significantly greater than the optimal LIF dose (P < 0.05), suggesting an additive effect. Our findings support the view that combinations of factors, which perhaps act on complementary mechanisms for nerve regeneration, will be required to maximally potentiate nerve regeneration and return of muscle function after nerve injury.