Date of Award

6-2022

Degree Name

Doctor of Philosophy

Department

Biological Sciences

First Advisor

John M. Spitsbergen, Ph.D.

Second Advisor

Christine Byrd-Jacobs, Ph.D.

Third Advisor

Pamela Hoppe, Ph.D.

Fourth Advisor

Amy Gyorkos, Ph.D.

Keywords

End plates, exercise, GDNF, motor neurons

Abstract

Neuromuscular weakness has been associated with age progression. A possible reason is a disconnection between muscle and neuron, causing components from both the central and peripheral nervous systems to deteriorate. Exercise, however, has beneficial effects for both muscle and nervous tissue, inducing neuroprotection. Neurotrophic factors are powerful regulators of neuronal maintenance and synaptic strength. Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic factor that has been shown to be a potent survival factor for somatic motor neurons that innervate skeletal muscle.

This study aims to evaluate changes in structural plasticity of motor neurons in the lumbar spinal cord and neuromuscular junction structures, and expand our understanding of the regulatory processes controlling GDNF expression with sedentary aging and exercise. We hypothesize that motor neuron area and possibly subgroups decrease with sedentary-aging, but in exercised age-matched animals, the area will increase and subgroup diversity will remain stable. At the site of the skeletal muscle, we hypothesize that NMJ structures get degraded with sedentary-aging, but exercise can maintain those structures. Furthermore, we hypothesize that exercise will increase GDNF protein content and expression in lumbar and muscle tissue.

Lumbar spinal tissue, soleus, plantaris and gastrocnemius were taken from sedentary and exercised Sprague-Dawley rats between 1 month and 24 months of age. Sedentary groups consisted of 1-month-old, 3-month-old, 6-month-old, 12-month-old, 18-month-old, and 24- month-old rats, while exercised groups consisted of 3-month-old rats that had access to running wheels for ten weeks and 12-, 18-, and 24-month-old rats that had access to running wheels for 6 months. Lumbar spinal tissue, soleus and plantaris were used for immunohistochemical analysis. For spinal tissue, antibodies against choline acetyltransferase (ChAT), neuronal nuclei (NeuN), and the fluorescent dye DAPI were used to identify neurons in the lateral motor column of the ventral horn of the spinal cord. In skeletal muscle, a-bungarotoxin was used to identify end plates. Lumbar spinal tissue, soleus and plantaris muscle were used for enzyme-linked immunosorbent assay and gastrocnemius muscle was used for RT-qPCR.

Results suggest a decrease in area of neurons after 6 months of age. Exercised animals had a larger area when compared to their respective sedentary age-matched group. Histogram analysis suggests that larger neurons tend to be less numerous in adulthood. The area of end plates from exercised rats was more prominent than their respective sedentary age-matched counterpart. Our results aid in understanding the structural changes that can occur with aging in the neuromuscular system in a mammalian model of aging. Soleus and plantaris muscle from exercised rats showed incremented levels of total and concentration of GDNF protein content. On the other hand, exercise did not have an effect on GDNF protein concentration in spinal cord. GDNF expression increased in gastrocnemius in the 3-month-old exercised group. The results aid in understanding the activity-dependent regulation of GDNF protein production and expression. Furthermore, this work adds to the pool of knowledge of how exercise can impact at different points in the lifespan of an aging mammalian model and its potential benefits.

Access Setting

Dissertation-Open Access

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