Intercellular Adhesion Molecule-1-Induced Posttraumatic Brain Injury Neuropathology in the Prefrontal Cortex and Hippocampus Leads to Sensorimotor Function Deficits and Psychological Stress.

Intercellular adhesion molecule-1 (ICAM-1) promotes adhesion and transmigration of circulating leukocytes across the blood-brain barrier (BBB). Traumatic brain injury (TBI) causes transmigrated immunocompetent cells to release mediators [function-associated antigen (LFA)-1 and macrophage-1 antigen (Mac-1)] that stimulate glial and endothelial cells to express ICAM-1 and release cytokines, sustaining neuroinflammation and neurodegeneration. Although a strong correlation exists between TBI-mediated inflammation and impairment in functional outcome following brain trauma, the role of ICAM-1 in impairing functional outcome by inducing neuroinflammation and neurodegeneration after TBI remains inconclusive. The experimental TBI was induced in vivo by fluid percussion injury (FPI; 10 and 20 psi) in wild-type (WT) and ICAM-1-/- mice and in vitro by stretch injury (3 psi) in brain endothelial cells. We manipulate ICAM-1 pharmacologically and genetically and conducted several biochemical analyses to gain insight into the mechanisms underlying ICAM-1-mediated neuroinflammation and performed rotarod, grid-walk, sucrose preference, and light-dark tests to assess functional outcome. TBI-induced ICAM-1-mediated neuroinflammation and cell death occur via LFA-1 or Mac-1 signaling pathways that rely on oxidative stress, matrix metalloproteinase (MMP), and vascular endothelial growth factor (VEGF) pathways. The deletion or blocking of ICAM-1 resulted in a better outcome in attenuating neuroinflammation and cell death as marked by the markers such as NF-kB, IL-1ß, TNF-α, cleaved-caspase-3 (cl-caspase-3), Annexin V, and by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and Trypan blue staining. ICAM-1 deletion in TBI improves sensorimotor, depression, and anxiety-like behavior with significant upregulation of norepinephrine (NE), dopamine (DA) D1 receptor (DAD1R), serotonin (5-HT)1AR, and neuropeptide Y (NPY). This study could establish the significance of ICAM-1 as a novel therapeutic target against the pathophysiology to establish functional recovery after TBI.

Traumatic brain injury-induced downregulation of Nrf2 activates inflammatory response and apoptotic cell death.

Recent studies from our group and others have demonstrated that oxidative stress, Ca2+ signaling, and neuroinflammation are major mechanisms contributing to post-traumatic neurodegeneration. The present study investigated the mechanisms of regulation of nuclear factor E2-related factor 2 (Nrf2) and its role in regulating antioxidant genes and oxidative stress-induced neuroinflammation and neurodegeneration following TBI. Nrf2 transcriptional system is the major regulator of endogenous defense mechanisms operating within the cells. Wild-type (Nrf2+/+) and Nrf2-deficient mice (Nrf2-/-) were subjected to 15 psi fluid percussion injury and demonstrated the regulatory role of Nrf2 in the expression antioxidant genes and oxidative stress, neuroinflammation, and cell death. Immunohistochemistry, q-RT-PCR, and western blotting techniques detected downregulation of Nrf2 and antioxidant proteins such as HO-1, GPx1, GSTm1, and NQO1 in mouse brain samples. Further, our study demonstrated that the downregulation of Nrf2 and antioxidant genes in TBI correlated with the induction of free radical-generating enzyme NADPH oxidase 1 and inducible nitric oxide synthase and their corresponding oxidative/nitrosative stress markers 4-hydroxynonenal and 3-nitrotyrosine. The decrease in Nrf2 with subsequent increase in oxidative stress markers led to the activation of MMP3/9, TGF-ß1, and NF-kB that further led to neuroinflammation and apoptosis. The absence of Nrf2 function in mice resulted in exacerbated brain injury as shown by the increased oxidative stress markers, pro-inflammatory cytokines, and apoptosis markers at 24 h after TBI. In conclusion, this study could establish the significance of Nrf2 in transforming into a novel preventive approach against the pathophysiology of TBI. KEY MESSAGES: • Traumatic brain injury impairs Nrf2 signaling in mouse. • Nrf2-mediated activation of antioxidant genes are altered after TBI. • Impairment of Nrf2 signaling leads to oxidative stress. • TBI-induced downregulation of Nrf2 activates MMPs, TGF-ß1, and NF-kB. • Nrf2 regulates neuroinflammation and apoptotic cell death in TB.

Impairment of pericyte-endothelium crosstalk leads to blood-brain barrier dysfunction following traumatic brain injury.

The blood-brain barrier (BBB) constitutes a neurovascular unit formed by microvascular endothelial cells, pericytes, and astrocytes. Brain pericytes are important regulators of BBB integrity, permeability, and blood flow. Pericyte loss has been implicated in injury; however, how the crosstalk among pericytes, endothelial cells, and astrocytes ultimately leads to BBB dysfunction in traumatic brain injury (TBI) remains elusive. In this study, we demonstrate the importance of pericyte-endothelium interaction in maintaining the BBB function. TBI causes the platelet-derived growth factor-B (PDGF-B)/PDGF receptor-ß signaling impairment that results in loss of interaction with endothelium and leads to neurovascular dysfunction. Using in vivo mild (7 psi) and moderate (15 psi) fluid percussion injury (FPI) in mice, we demonstrate the expression of various pericyte markers including PDGFR-ß, NG2 and CD13 that were significantly reduced with a subsequent reduction in the expression of various integrins; adherent junction protein, N-cadherin; gap junction protein, connexin-43; and tight junction proteins such as occludin, claudin-5, ZO-1, and JAM-a. Impairment of pericyte-endothelium interaction increases the BBB permeability to water that is marked by a significant increase in aquaporin4 expression in injured animals. Similarly, pericyte-endothelium integrity impairment in FPI animals greatly increases the permeability of small-molecular-weight sodium fluorescein and high-molecular-weight-tracer Evans blue across the BBB. In addition, the injury-inflicted animals show significantly higher levels of S100ß and NSE in the blood samples compared with controls. In conclusion, our data provide an insight that brain trauma causes an early impairment of pericyte-endothelium integrity and results in BBB dysregulation that initiates pathological consequences associated with TBI.