Effects of age and insulin-like growth factor-1 on rat neurotrophin receptor expression after nerve injury

Insulin-Like Growth Factor-1: A Promising Therapeutic Target for Peripheral Nerve Injury

Patients who sustain peripheral nerve injuries (PNIs) are often left with debilitating sensory and motor loss. Presently, there is a lack of clinically available therapeutics that can be given as an adjunct to surgical repair to enhance the regenerative process. Insulin-like growth factor-1 (IGF-1) represents a promising therapeutic target to meet this need, given its well-described trophic and anti-apoptotic effects on neurons, Schwann cells (SCs), and myocytes. Here, we review the literature regarding the therapeutic potential of IGF-1 in PNI. We appraised the literature for the various approaches of IGF-1 administration with the aim of identifying which are the most promising in offering a pathway toward clinical application. We also sought to determine the optimal reported dosage ranges for the various delivery approaches that have been investigated.

Age-Dependent Schwann Cell Phenotype Regulation Following Peripheral Nerve Injury

BACKGROUND

Schwann cells are integral to the regenerative capacity of the peripheral nervous system, which declines after adolescence. The mechanisms underlying this decline are poorly understood. This study sought to compare the protein expression of Notch, c-Jun, and Krox-20 after nerve crush injury in adolescent and young adult rats. We hypothesized that these Schwann cell myelinating regulatory factors are down-regulated after nerve injury in an age-dependent fashion.

METHODS

Adolescent (2 months old) and young adult (12 months old) rats (n = 48) underwent sciatic nerve crush injury. Protein expression of Notch, c-Jun, and Krox-20 was quantified by Western blot analysis at 1, 3, and 7 days post-injury. Functional recovery was assessed in a separate group of animals (n = 8) by gait analysis (sciatic functional index) and electromyography (compound motor action potential) over an 8-week post-injury period.

RESULTS

Young adult rats demonstrated a trend of delayed onset of the dedifferentiating regulatory factors, Notch and c-Jun, corresponding to the delayed functional recovery observed in young adult rats compared to adolescent rats. Compound motor action potential area was significantly greater in adolescent rats relative to young adult rats, while amplitude and velocity trended toward statistical significance.

CONCLUSIONS

The process of Schwann cell dedifferentiation following peripheral nerve injury shows different trends with age. These trends of delayed onset of key regulatory factors responsible for Schwann cell myelination may be one of many possible factors mediating the significant differences in functional recovery between adolescent and young adult rats following peripheral nerve injury.

Molecular Mechanism of the "Babysitter" Procedure for Nerve Regeneration and Muscle Preservation in Peripheral Nerve Repair in a Rat Model

OBJECTIVE

To investigate the molecular mechanism of nerve "babysitter" for nerve regeneration and muscle preservation in peripheral nerve repair.

METHODS

Eighty rats were equalized into 4 groups: peroneal nerve transected, group A received no treatment; group B underwent end-to-end repair; group C underwent end-to-side "babysitter" with donor epineurial window; group D underwent end-to-side "babysitter" with 40% donor neurectomy. During second-stage procedure, end-to-end neurorrhaphies were executed in groups A, C, and D. Expression of Insulin-like growth factor (IGF)-1 in spinal cord and IGF-1, TNF-like weak inducer of apoptosis (TWEAK), and Fn14 in anterior tibial muscles were evaluated by histopathology at 4-, 8-, 12-, and 24-week timepoints postoperatively.

RESULTS

At 4 weeks, group D expressed comparable IGF-1 with group B, and greater value than groups A and C in spinal cord. By 24 weeks, groups B and D showed higher values than groups A and C. Insulin-like growth factor 1 in muscles were greater in groups C and D than in groups A and B at 4 weeks, and comparable in all groups at 24 weeks. At 4 weeks, immunoreactive scores of TWEAK were 9.00 ± 0, 3.00 ± 0, 6.75 ± 0.75, and 6.75 ± 0.75, respectively. No differences were noticed in all groups by 24 weeks. At 4 weeks, Fn14 were similar in groups A, C, and D, but lower in group B. Group D showed comparable Fn14 with groups B and C, but lower value than group A at 24 weeks.

CONCLUSIONS

End-to-side nerve "babysitter" in peripheral nerve could promote fiber regeneration and muscle preservation by regulating expression of IGF-1 and TWEAK-Fn14. End-to-side "babysitter" with partial donor neurectomy could achieve comparable effects with end-to-end repair.

Aging Induces Changes in the Somatic Nerve and Postsynaptic Component without Any Alterations in Skeletal Muscles Morphology and Capacity to Carry Load of Wistar Rats

The present study aimed to analyze the morphology of the peripheral nerve, postsynaptic compartment, skeletal muscles and weight-bearing capacity of Wistar rats at specific ages. Twenty rats were divided into groups: 10 months-old (ADULT) and 24 months-old (OLD). After euthanasia, we prepared and analyzed the tibial nerve using transmission electron microscopy and the soleus and plantaris muscles for cytofluorescence and histochemistry. For the comparison of the results between groups we used dependent and independent Student's t-test with level of significance set at p ≤ 0.05. For the tibial nerve, the OLD group presented the following alterations compared to the ADULT group: larger area and diameter of both myelinated fibers and axons, smaller area occupied by myelinated and unmyelinated axons, lower numerical density of myelinated fibers, and fewer myelinated fibers with normal morphology. Both aged soleus and plantaris end-plate showed greater total perimeter, stained perimeter, total area and stained area compared to ADULT group (p < 0.05). Yet, aged soleus end-plate presented greater dispersion than ADULT samples (p < 0.05). For the morphology of soleus and plantaris muscles, density of the interstitial volume was greater in the OLD group (p < 0.05). No statistical difference was found between groups in the weight-bearing tests. The results of the present study demonstrated that the aging process induces changes in the peripheral nerve and postsynaptic compartment without any change in skeletal muscles and ability to carry load in Wistar rats.

Exenatide promotes regeneration of injured rat sciatic nerve

Damage to peripheral nerves results in partial or complete dysfunction. After peripheral nerve injuries, a full functional recovery usually cannot be achieved despite the standard surgical repairs. Neurotrophic factors and growth factors stimulate axonal growth and support the viability of nerve cells. The objective of this study is to investigate the neurotrophic effect of exenatide (glucagon like peptide-1 analog) in a rat sciatic nerve neurotmesis model. We injected 10 μg/d exenatide for 12 weeks in the experimental group (n = 12) and 0.1 mL/d saline for 12 weeks in the control group (n = 12). We evaluated nerve regeneration by conducting electrophysiological and motor functional tests. Histological changes were evaluated at weeks 1, 3, 6, and 9. Nerve regeneration was monitored using stereomicroscopy. The electrophysiological and motor functions in rats treated with exenatide were improved at 12 weeks after surgery. Histological examination revealed a significant increase in the number of axons in injured sciatic nerve following exenatide treatment confirmed by stereomicroscopy. In an experimentally induced neurotmesis model in rats, exenatide had a positive effect on nerve regeneration evidenced by electromyography, functional motor tests, histological and stereomicroscopic findings.