Photobiomodulation attenuates oligodendrocyte dysfunction and prevents adverse neurological consequences in a rat model of early life adversity

Abstract

Rationale: Adverse experiences in early life including abuse, trauma and neglect, have been linked to poor physical and mental health outcomes. Emerging evidence implies that those who experienced early life adversity (ELA) are more likely to develop cognitive dysfunction and depressive-like symptoms in adulthood. The molecular mechanisms responsible for the negative consequences of ELA, however, remain unclear. In the absence of effective management options, anticipatory guidance is the mainstay of ELA prevention. Furthermore, there is no available treatment that prevents or alleviates the neurologic sequelae of ELA, especially traumatic stress. Hence, the present study aims to investigate the mechanisms for these associations and evaluate whether photobiomodulation (PBM), a non-invasive therapeutic procedure, can prevent the negative cognitive and behavioral manifestations of ELA in later life. Methods: ELA was induced by repeated inescapable electric foot shock of rats from postnatal day 21 to 26. On the day immediately following the last foot shock, 2-min daily PBM treatment was applied transcranially for 7 consecutive days. Cognitive dysfunction and depression-like behaviors were measured by a battery of behavioral tests in adulthood. Subsequently, oligodendrocyte progenitor cells (OPCs) differentiation, the proliferation and apoptosis of oligodendrocyte lineage cells (OLs), mature oligodendrocyte, myelinating oligodendrocyte, the level of oxidative damage, reactive oxygen species (ROS) and total antioxidant capacity were measured and analyzed using immunofluorescence staining, capillary-based immunoassay (ProteinSimple®) and antioxidant assay kit. Results: The rats exposed to ELA exhibited obvious oligodendrocyte dysfunction, including a reduction in OPCs differentiation, diminished generation and survival of OLs, decreased OLs, and decreased matured oligodendrocyte. Furthermore, a deficit in myelinating oligodendrocytes was observed, in conjunction with an imbalance in redox homeostasis and accumulated oxidative damage. These alternations were concomitant with cognitive dysfunction and depression-like behaviors. Importantly, we found that early PBM treatment largely prevented these pathologies and reversed the neurologic sequelae resulting from ELA. Conclusions: Collectively, these findings provide new insights into the mechanism by which ELA affects neurological outcomes. Moreover, our findings support that PBM may be a promising strategy to prevent ELA-induced neurologic sequelae that develops later in life.

Keywords: Cognition; Depression; Early life adversity (ELA); Oligodendrocyte; Photobiomodulation (PBM).

Figure 1

Schematic diagram of the experimental protocol and device Information. A Animals were randomly divided into 3 groups: (1) healthy animals without ELA (Control); (2) animals that received foot shock and sham PBM treatment (ELA); (3) animals that received foot shock with PBM treatment (ELA + PBM). PBM intervention was performed on the 1st day after the last foot shock and lasted for 7 days. Behavioral tests were performed from P75 to P81, followed by brain tissue collection. B Device information. C Red circle shadow denotes the area of PBM application on the rat skull.

Figure 2

Early PBM treatment prevents alternations in body weight and ELA-induced depressive-like behaviors. A No significant difference in BL (baseline) body weight among all groups following ELA. B (a) Schematic diagram displaying the time course of behavioral testing. B (b, c) The forced swimming test and tail suspension test were conducted to measure depressive-like behaviors, in the forced swimming test, ELA-exposured rats exhibited dramatically longer immobility time than the control group, which can be reversed by early PBM treatment. C (a) Schematic illustrating the forced swimming test and tail suspension test. C (b, c) PBM-treated ELA rats also exhibited less immobility time in tail suspension test. All data are presented as mean ± SE (n = 5-7). * P < 0.05 versus Control-group; ^# P < 0.05 versus ELA-group.

Figure 3

Early PBM treatment prevents ELA-induced cognitive deficits and comorbidity. A (a) Schematic diagram of the novel object recognition test. The novel object recognition test was conducted to measure the recognition memory. A (b, c) The elevated plus maze text was performed to measure anxious-like behavior, PBM-treated ELA rats spent significantly more time in the open arms than their untreated counterparts. B (a, d) The representative tracking plots on the novel object recognition test. Between all groups, the time spent on each object and discrimination index was calculated and statistically compared (b-c, e-f). ELA rats spent significantly less time exploring the novel object, while early PBM treatment reserved these deficits. All data are presented as mean ± SE (n = 5-7). * P < 0.05 versus Control-group; ^# P < 0.05 versus ELA-group.

Figure 4

ELA diminishes the differentiation of OPCs into OLs and can be reversed by early PBM treatment. A (a-c) and B (a-c) Representative immunofluorescence staining for - OLs (Olig2⁺, red) - labeled PDGFRα⁺ (blue) in CA1 region of hippocampus. A (d) and B (d) Representative synthetic protein bands of PDGFRα in the hippocampus, corresponding peak areas were normalized with the level of total protein. A (e) The depicted area was imaged in the hippocampal CA1 region, graphical representation of the different oligodendrocyte lineage stages and respective markers used in this study. C (a, b) Results of quantitative analysis of PDGFRα protein levels. C (c, d) Percentage of Olig2+- labeled OPCs (Olig2+ PDGFRα⁺ cells) in the total PDGFRα⁺ cells. ELA-exposured rats exhibited a dramatic reduction in the differentiation of OPCs into OLs, which was reversed by early PBM treatment. Scale bar = 10 µm. All data are presented as mean ± SE (n = 5-7). * P < 0.05 versus Control-group; ^# P < 0.05 versus ELA-group.

Figure 5

Early PBM treatment prevents ELA-induced OLs apoptosis and reduction in mature oligodendrocytes. A (a-c) and C (a-c) Representative immunofluorescence staining for TUNEL⁺ (green) - labeled OLs (Olig2⁺, red) in CA1 region of the hippocampus, small images exhibit a representative single cell from each group. B (a) and D (a) Representative synthetic protein bands of Olig2 in the hippocampus, corresponding peak areas were normalized with the level of total protein. B (b) and D (b) Results of quantitative analysis of Olig2 protein levels. B (c) and D (c) Percentage of TUNEL⁺ - labeled OLs (TUNEL⁺ Olig2⁺ cells) in the total Olig2⁺ population. A (d-f) and C (d-f) Representative immunofluorescence staining for mature oligodendrocyte (CC1, green) with DAPI. B (d) and D (d) Quantification of CC1⁺ cells. ELA-exposured rats exhibited a marked increase in the proportion of oligodendrocyte apoptosis, accompanied by reductions in Olig2 protein content and mature oligodendrocyte (CC1), which was reversed by early PBM treatment. Scale bar = 40 µm. All data are presented as mean ± SE (n = 5-7). * P < 0.05 versus Control-group; ^# P < 0.05 versus ELA-group.

Figure 6

ELA leads to diminished proliferation of OLs, which can be rescued by early PBM treatment. A (a-c) and B (a-c) Representative immunofluorescence staining for Ki67⁺ (green) - labeled OLs (Olig2⁺, red) in CA1 region of the hippocampus, small images exhibit a representative single cell from each group. (A) (d) and B (d) Percentage of Ki67⁺ - labeled OLs (Ki67⁺ Olig2⁺ cells) in the total Olig2⁺ cells. ELA-exposured rats exhibited a dramatic decrease in the proportion of newly differentiated OLs, while PBM-treated ELA rats showed normal differentiation capacity. Scale bar = 40 µm. All data are presented as mean ± SE (n = 5-7). * P < 0.05 versus Control-group; ^# P < 0.05 versus ELA-group.

Figure 7

ELA leads to reduction in myelinating oligodendrocyte, which can be partially mitigateded by early PBM treatment. A (a-c) and C (a-c) Representative immunofluorescence staining for MBP in CA1 region of the hippocampus. A (d-f) and C (d-f) Representative images of MBP staining intensities were presented for a better view. B (a) and D (a) Representative synthetic protein bands of MBP in the hippocampus, corresponding peak areas were normalized with the level of total protein. ELA-exposed rats exhibited a marked reduction in MBP protein content, which was partially ameliorated by early PBM treatment. Scale bar = 40 µm. All data are presented as mean ± SE (n = 5-7). * P < 0.05 versus Control-group; ^# P < 0.05 versus ELA-group.

Figure 8

Early PBM treatment mitigates ELA-induced chronic oxidative damage in oligodendrocyte. A (a-f) and B (a-f) Representative immunofluorescence staining for 4HNE with PDGFRαand Olig2 in CA1 region of the hippocampus. A (g) and B (g) Fluorescent intensity of 4HNE was calculated by ImageJ analysis software and expressed as percentage changes versus respective control group. A (h) and B (h) Detection and quantification of relative levels of ROS in protein samples by fluorescence spectrophotometry. ROS and oxidative damage levels were significantly increased in animals exposed to ELA, which was attenuated by PBM treatment. C (a, b) Quantitative analysis of total antioxidant capacity by an antioxidant assay kit. ELA leads to compromised antioxidant capacity, and early PBM treatment largely reverses this deficiency. Scale bar = 20 µm. All data are presented as mean ± SE (n = 5-7). * P < 0.05 versus Control-group; ^# P < 0.05 versus ELA-group.

Figure 9

Graphical abstract. Repeated unpredictable electric foot shocks early in life lead to compromised antioxidant defenses and accumulation of ROS in the brain, which in turn causes oligodendrocyte oxidative damage and dysfunction as well as neurologic syndromes in adulthood. Conversely, early transcranial PBM treatment improves redox homeostasis in the brain, therefore helps maintain oligodendroglial homeostasis, resulting in better outcomes in animals.