Acute and prolonged hindlimb exercise elicits different gene expression in motoneurons than sensory neurons after spinal cord injury

Abstract

We examined gene expression in the lumbar spinal cord and the specific response of motoneurons, intermediate gray and proprioceptive sensory neurons after spinal cord injury and exercise of hindlimbs to identify potential molecular processes involved in activity dependent plasticity. Adult female rats received a low thoracic transection and passive cycling exercise for 1 or 4weeks. Gene expression analysis focused on the neurotrophic factors: brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), and their receptors because of their potential roles in neural plasticity. We also examined expression of genes involved in the cellular response to injury: heat shock proteins (HSP) -27 and -70, glial fibrillary acidic protein (GFAP) and caspases -3, -7, and -9. In lumbar cord samples, injury increased the expression of mRNA for TrkB, all three caspases and the HSPs. Acute and prolonged exercise increased expression of mRNA for the neurotrophic factors BDNF and GDNF, but not their receptors. It also increased HSP expression and decreased caspase-7 expression, with changes in protein levels complimentary to these changes in mRNA expression. Motoneurons and intermediate gray displayed little change in mRNA expression following injury, but acute and prolonged exercise increased levels of mRNA for BDNF, GDNF and NT-4. In large DRG neurons, mRNA for neurotrophic factors and their receptors were largely unaffected by either injury or exercise. However, caspase mRNA expression was increased by injury and decreased by exercise. Our results demonstrate that exercise affects expression of genes involved in plasticity and apoptosis in a cell specific manner and that these change with increased post-injury intervals and/or prolonged periods of exercise.

Figure 1

Experimental design. A) Timeline of injury, exercise (Ex) and tissue collection. B) Schematic showing how spinal cord and DRG tissue was collected for later analysis. C) Diagram of regions used for laser micro dissected sections from spinal cord. Shaded area indicates intermediate grey matter. Motoneurons were isolated from the boxed area. C’) Spinal cord ventral horn. Arrows indicate motoneurons before laser capture. C’’) Spinal cord ventral horn after laser micro dissection. Arrows indicate position of isolated motoneurons.

Figure 2

Whole spinal cord mRNA expression from intact, transected (Tx 10, Tx 31), and transected plus exercise (Tx+Ex 10, Tx+Ex 31) rats (n=6 for all groups). A) mRNA expression of neurotrophic factors in whole cord tissue. B) mRNA expression of neurotrophic factor receptors in whole cord tissue. C) mRNA expression of HSPs in whole cord tissue. D) mRNA expression of Caspases in whole cord tissue. Error bars are indicative of standard deviation of groups. * indicates significantly different from intact; # indicates significantly different from Tx 10; † indicates significantly different from Tx 31.

Figure 3

Whole spinal cord protein expression of intact, transected (Tx 10, Tx 31), and transected plus exercise (Tx+Ex 10, Tx+Ex 31) rats (n=6 for all groups). A) Protein expression of neurotrophic factors in whole cord tissue. B) Protein expression of neurotrophic factor receptors in whole cord tissue. C) Protein expression of HSPs in whole cord tissue. D) Protein expression of Caspases in whole cord tissue. Error bars are indicative of standard deviation of groups. * indicates significantly different from intact; # indicates significantly different from Tx 10; † indicates significantly different from Tx 31; ‡ indicates significantly different from Tx+Ex 10.

Figure 4

Gene expression in motoneurons of intact, transected (Tx 10, Tx 31), and transected plus exercise (Tx+Ex 10, Tx+Ex 31) rats (n=6 for all groups). + error is indicative of standard deviation. Grey background indicates expression of the gene is significantly altered in one of the injury-treatment groups. * indicates significantly different from intact; # indicates significantly different from Tx 10; † indicates significantly different from Tx 31; ‡ indicates significantly different from Tx+Ex 10.

Figure 5

Gene expression in intermediate grey matter of intact, transected (Tx 10, Tx 31), and transected plus exercise (Tx+Ex 10, Tx+Ex 31) rats(n=6 for all groups). + error is indicative of standard deviation. Grey background indicates expression of the gene is significantly altered in one of the injury-treatment groups. * indicates significantly different from intact; # indicates significantly different from Tx 10; † indicates significantly different from Tx 31.

Figure 6

Gene expression in large DRG neurons of intact, transected (Tx 10, Tx 31), and transected plus exercise (Tx+Ex 10, Tx+Ex 31) rats (n=6 for all groups). ± error is indicative of standard deviation. Grey background indicates expression of the gene is significantly altered in one of the injury-treatment groups. * indicates significantly different from intact; # indicates significantly different from Tx 10; † indicates significantly different from Tx 31; ‡ indicates significantly different from Tx+Ex 10.