The Transcriptional Response of Neurotrophins and Their Tyrosine Kinase Receptors in Lumbar Sensorimotor Circuits to Spinal Cord Contusion is Affected by Injury Severity and Survival Time

Abstract

Traumatic spinal cord injury (SCI) results in changes to the anatomical, neurochemical, and physiological properties of cells in the central and peripheral nervous system. Neurotrophins, acting by binding to their cognate Trk receptors on target cell membranes, contribute to modulation of anatomical, neurochemical, and physiological properties of neurons in sensorimotor circuits in both the intact and injured spinal cord. Neurotrophin signaling is associated with many post-SCI changes including maladaptive plasticity leading to pain and autonomic dysreflexia, but also therapeutic approaches such as training-induced locomotor improvement. Here we characterize expression of mRNA for neurotrophins and Trk receptors in lumbar dorsal root ganglia (DRG) and spinal cord after two different severities of mid-thoracic injury and at 6 and 12 weeks post-SCI. There was complex regulation that differed with tissue, injury severity, and survival time, including reversals of regulation between 6 and 12 weeks, and the data suggest that natural regulation of neurotrophins in the spinal cord may continue for months after birth. Our assessments determined that a coordination of gene expression emerged at the 12-week post-SCI time point and bioinformatic analyses address possible mechanisms. These data can inform studies meant to determine the role of the neurotrophin signaling system in post-SCI function and plasticity, and studies using this signaling system as a therapeutic approach.

Keywords: contusions; genetic regulation; injury mechanisms; neurotrophin receptors; neurotrophins; sensory neurons; spinal cord injury; transcription.

Figure 1

Injuries were characterized using BBB scores to assess hindlimb locomotor function and white matter sparing (WMS) at epicenter using EC stain. (A) BBB scores of groups that received 12.5 g cm (blue) or 25 g cm (green) NYU injuries. Starting at week 5, a significant difference was observed between the two injury severities. Both groups were significantly different (p < 0.05) from controls (not shown) at all time points. (B) White matter sparing at epicenter (x axis) plotted vs. BBB score. Black dots represent 6 week values. Green dots represent 12 week, 25 g cm injured animals. Blue x represent the two animals from the 12-week, 12.5 g cm group with the lowest BBB scores within the group. Blue dots represent the four animals from the 12-week, 12.5 g cm group with the highest BBB scores within the group. (C) Image taken from a 12.5 g cm contused animal showing a laterally-symmetrical injury pattern at the epicenter. Note the difference from (D), which was taken from an animal that also received a 12.5 g cm spinal cord contusion but which yielded an asymmetrical injury at epicenter. *p < 0.05, ***p < 0.001.

Figure 2

mRNA expression of Trk receptors is altered in L4/5 DRG at 6 and 12 weeks after receiving either 12.5 g cm (blue) or 25 g cm (green) NYU contusion injury to spinal cord at vertebral level T9. Fold-change (FC) is reported as change of 12 week relative to 6 week time points in all figures. Black bar on TrkA reports fold-change (fc) of 25 g cm at 12 weeks relative to 12.5 g cm at 12 weeks. X axis denotes weeks post injury. White lines in box-plots indicate group mean. Dotted gray lines indicate expression level of controls (normalized to 1), with ±SEM indicated by the vertical arrows at right end of the control line. #p < 0.05 vs. control, *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3

mRNA expression of Neurotrophins is altered in L4/5 DRG at 6 and 12 weeks after receiving either 12.5 g cm (blue) or 25 g cm (green) NYU contusion injury to spinal cord at vertebral level T9. Fold-change (fc) is reported as change of 12 week relative to 6 week time points in all figures. X axis denotes weeks post injury. White lines in box-plots indicate group mean. Dotted gray lines indicate expression level of controls (normalized to 1), with ±SEM indicated by the vertical arrows at right end of the control line. #p < 0.05 from control. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 4

mRNA expression of Trk receptors is altered in L4/5 spinal cord at 6 and 12 weeks after receiving either 12.5 g cm (blue) or 25 g cm (green) NYU contusion injury to spinal cord at vertebral level T9. Fold-change (fc) is reported as change of 12 week relative to 6 week time points in all figures. X axis denotes weeks post injury. White lines in box-plots indicate group mean. Dotted gray lines indicate expression level of controls (normalized to 1), with ±SEM indicated by the vertical arrows at right end of the control line. #p < 0.05 from control. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 5

(A) Scatterplots of Neurotrophin mRNA expression in L4/5 SC at 6 and 12 weeks after receiving either 12.5 g cm (blue) or 25 g cm (green) NYU contusion injury to spinal cord at vertebral level T9, relative to the unified control group as was done for the other data. x’s represent mRNA expression of age-matched naive animals. Open circles represent expression of age-matched laminectomy control animals. *p < 0.05, **p < 0.01 (B) mRNA expression of Neurotrophins in L4/5 SC at 6 and 12 weeks after receiving either 12.5 g cm (blue) or 25 g cm (green) NYU contusion injury to spinal cord at vertebral level T9, relative to their respective age-matched control groups. X axis denotes weeks post injury. White lines in box-plots indicate group mean. Dotted gray lines indicate expression level of controls (normalized to 1), with ±SEM indicated by the vertical arrows to the right of each time point pair. #p < 0.05 from control.

Figure 6

Correlated expression of neurotrophins in DRG emerges at chronic time points. Values represent fold-change of the individual animals vs. mean of control group. Blue dots represent animals with 12.5 g cm injuries. Green dots represent animals with 25 g cm injuries.

Figure 7

Correlated expression of Trk receptors in DRG emerges at chronic time points. Values represent fold-change of the individual animals vs. mean of control group. Blue dots represent animals with 12.5 g cm injuries. Green dots represent animals with 25 g cm injuries.