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. 2018 May 18;13(5):e0197486.
doi: 10.1371/journal.pone.0197486. eCollection 2018.

Acute intermittent hypoxia and rehabilitative training following cervical spinal injury alters neuronal hypoxia- and plasticity-associated protein expression

Affiliations

Acute intermittent hypoxia and rehabilitative training following cervical spinal injury alters neuronal hypoxia- and plasticity-associated protein expression

Atiq Hassan et al. PLoS One. .

Abstract

One of the most promising approaches to improve recovery after spinal cord injury (SCI) is the augmentation of spontaneously occurring plasticity in uninjured neural pathways. Acute intermittent hypoxia (AIH, brief exposures to reduced O2 levels alternating with normal O2 levels) initiates plasticity in respiratory systems and has been shown to improve recovery in respiratory and non-respiratory spinal systems after SCI in experimental animals and humans. Although the mechanism by which AIH elicits its effects after SCI are not well understood, AIH is known to alter protein expression in spinal neurons in uninjured animals. Here, we examine hypoxia- and plasticity-related protein expression using immunofluorescence in spinal neurons in SCI rats that were treated with AIH combined with motor training, a protocol which has been demonstrated to improve recovery of forelimb function in this lesion model. Specifically, we assessed protein expression in spinal neurons from animals with incomplete cervical SCI which were exposed to AIH treatment + motor training either for 1 or 7 days. AIH treatment consisted of 10 episodes of AIH: (5 min 11% O2: 5 min 21% O2) for 7 days beginning at 4 weeks post-SCI. Both 1 or 7 days of AIH treatment + motor training resulted in significantly increased expression of the transcription factor hypoxia-inducible factor-1α (HIF-1α) relative to normoxia-treated controls, in neurons both proximal (cervical) and remote (lumbar) to the SCI. All other markers examined were significantly elevated in the 7 day AIH + motor training group only, at both cervical and lumbar levels. These markers included vascular endothelial growth factor (VEGF), brain-derived neurotrophic factor (BDNF), and phosphorylated and nonphosphorylated forms of the BDNF receptor tropomyosin-related kinase B (TrkB). In summary, AIH induces plasticity at the cellular level after SCI by altering the expression of major plasticity- and hypoxia-related proteins at spinal regions proximal and remote to the SCI. These changes occur under the same AIH protocol which resulted in recovery of limb function in this animal model. Thus AIH, which induces plasticity in spinal circuitry, could also be an effective therapy to restore motor function after nervous system injury.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Experimental design.
All animals were initially conditioned on the ladder task for 3–8 weeks, underwent spinal surgery and were assessed on the ladder task 4 weeks after surgery but before treatment (Pre-tr). Animals then received AIH or normoxia treatment plus ladder training for one day (Day 1) or every day for 7 days (Day 7). Spinal cord tissue was taken from half of the animals (n = 6/treatment group), which were euthanized and perfused after 1 day of treatment and training (Day 1), and from the remaining animals (n = 6/treatment group) after 7 days of treatment and training (Day 7).
Fig 2
Fig 2. Spinal neurons identified as motoneurons with choline acetyl-transferase (ChAT).
Photomicrograph of the spinal cord in C6-7 region (A) processed for ChAT, HIF-1α and DAPI immunofluorescence from a 7 day AIH-treated spinal-injured rat. Scale bar = 250 μm. White box shows location of representative images in (B) processed for each of ChAT, HIF-1α and DAPI. Merged image shows HIF-1α co-localization with ChAT and DAPI in spinal neurons (e.g. arrows). Scale bar = 100 μm.
Fig 3
Fig 3. AIH treatment for either 1 or 7 days increased HIF-1α protein levels in multiple spinal segments.
Representative photomicrographs of the ventral grey matter in C6-7 (A) or L4-5 (B) spinal segments sections processed for HIF-1α immunofluorescence from 1 day (Day-1) or 7 day (Day-7) in normoxia-treated versus AIH-treated spinal injured rats. Arrows indicate representative neurons. Scale bar = 50 μm. Histograms summarize the mean immunofluorescence signal intensity detected ± SEM as measured in gray values over individual neurons within the ventral horn from normoxia- and AIH-treated spinal injured rats [n = 3 rats per experimental group analysed; 60–65 neurons analysed/time point/experimental condition at C6-7 (A); 80–85 neurons analysed/time point/experimental condition at L4-5 (B)]. Asterisks indicate significant differences between experimental groups; * p < 0.05.
Fig 4
Fig 4. AIH treatment for 7 days increased neuronal VEGF protein levels in multiple spinal segments.
Representative photomicrographs of the ventral grey matter in C6-7 (A) or L4-5 (B) spinal segments sections processed for VEGF immunofluorescence from 1 day (Day-1) or 7 day (Day-7) in normoxia-treated versus AIH-treated spinal injured rats. Arrows indicate representative neurons. Scale bar = 50 μm. Histograms summarize the mean immunofluorescence signal intensity detected ± SEM as measured in gray values over individual neurons within the ventral horn from normoxia- and AIH-treated spinal injured rats [n = 3 rats per experimental group analysed; 60–65 neurons analysed/time point/experimental condition at C6-7 (A); 80–85 neurons analysed/time point/experimental condition at L4-5 (B)]. Asterisks indicate significant differences between experimental groups; * p < 0.05.
Fig 5
Fig 5. AIH treatment for 7 days increased BDNF protein levels in multiple spinal segments.
Representative photomicrographs of the ventral grey matter in C6-7 (A) or L4-5 (B) spinal segments sections processed for BDNF immunofluorescence from 1 day (Day-1) or 7 day (Day-7) in normoxia-treated versus AIH-treated spinal injured rats. Arrows indicate representative neurons. Scale bar = 50 μm. Histograms summarize the mean immunofluorescence signal intensity detected ± SEM as measured in gray values over individual neurons within the ventral horn from normoxia- and AIH-treated spinal injured rats [n = 3 rats per experimental group analysed; 60–65 neurons analysed/time point/experimental condition at C6-7 (A); 80–85 neurons analysed/time point/experimental condition at L4-5 (B)]. Asterisks indicate significant differences between experimental groups; * p < 0.05.
Fig 6
Fig 6. AIH treatment for 7 days increased TrkB protein levels in multiple spinal segments.
Representative photomicrographs of the ventral grey matter in C6-7 (A) or L4-5 (B) spinal segments sections processed for TrkB immunofluorescence from 1 day (Day-1) or 7 day (Day-7) in normoxia-treated versus AIH-treated spinal injured rats. Arrows indicate representative neurons. Scale bar = 50 μm. Histograms summarize the mean immunofluorescence signal intensity detected ± SEM as measured in gray values over individual neurons within the ventral horn from normoxia- and AIH-treated spinal injured rats [n = 3 rats per experimental group analysed; 60–65 neurons analysed/time point/experimental condition at C6-7 (A); 80–85 neurons analysed/time point/experimental condition at L4-5 (B)]. Asterisks indicate significant differences between experimental groups; * p < 0.05.
Fig 7
Fig 7. AIH treatment for 7 days increases phospho-TrkB (p-TrkB) protein levels in multiple spinal segments.
Representative photomicrographs of the ventral grey matter in C6-7 (A) or L4-5 (B) spinal segments sections processed for p-TrkB immunofluorescence from 1 day (Day-1) or 7 day (Day-7) in normoxia-treated versus AIH-treated spinal injured rats. Arrows indicate representative neurons. Scale bar = 50 μm. Histograms summarize the mean immunofluorescence signal intensity detected ± SEM as measured in gray values over individual neurons within the ventral horn from normoxia- and AIH-treated spinal injured rats [n = 3 rats per experimental group analysed; 60–65 neurons analysed/time point/experimental condition at C6-7 (A); 80–85 neurons analysed/time point/experimental condition at L4–5 (B) ]. Asterisks indicate significant differences between experimental groups; * p < 0.05.

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This work was supported by the US Department of Defense SC090355P1 (GDM), Saskatchewan Health Research Fund 2668 (VMV, GDM), Saskatchewan Health Research Fund 3695 (GDM, VMV), and Saskatchewan Health Research Fund 6362 (GDM, VMV). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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