Peptidergic Drugs for the Treatment of Traumatic Brain Injury

X Antón Álvarez; Jesús Figueroa; Dafin Muresanu

Disclosures

Future Neurology. 2013;8(2):175-192. 

In This Article

Peptidergic Mechanisms, Neurodegeneration & Neurorepair After TBI

Neuroinflammation: Microglia Activation & Cytokines

Neuroinflammation is one of the main components of TBI pathophysiology, contributing to both secondary damage and recovery mechanisms. TBI elicits an immediate inflammatory response characterized by an increased production of proinflammatory cytokines and activation of microglial cells. Recent studies demonstrated a temporal pattern of upregulation of proinflammatory cytokines (i.e., TNF-α, IL-1β, IL-6 and IL-8) in the brain extracellular fluid 1–2 days after TBI, with later elevations or no changes in the levels of the anti-inflammatory cytokine IL-10.[17,65] Similar elevations of proinflammatory cytokines without upregulation of anti-inflammatory cytokines were also found in post-mortem brain tissue, cerebrospinal fluid (CSF) and blood of TBI patients.[66,67] Activated microglia have been observed in human TBI brains from 3 days after injury[18] and, as assessed by PET, showed a widespread increase in the brain consistent with diffuse neuronal damage 6 months after TBI,[53] and were enhanced in subcortical regions, but not at the original site of focal brain lesion, up to 17 years after TBI.[19]

Acute microglia activation might contribute to neuroprotection after injury through the release of anti-inflammatory cytokines such as IL-10 and IL-1 receptor antagonist, and of growth factors such as NGF.[18,37] A subpopulation of activated microglia that are immunoreactive for galectin-3/Mac-2 and NGF was recently reported to be upregulated shortly after diffuse axonal injury in mice.[68] In humans, higher levels of NGF in the CSF and increased IL-1 receptor antagonist concentrations in brain microdialysates were found to be associated with improved TBI outcomes.[51,69,70] On the other hand, the chronic production of proinflammatory cytokines, nitric oxide and superoxide species by activated microglia seems to contribute to the maintenance of the secondary brain damage mechanism induced by TBI.[18,37] In fact, lower CSF levels of IL-1β were associated with a better outcome in TBI children,[69] whereas increased serum IL-6 levels were associated with unfavorable outcomes[71] and a higher risk of developing elevated intracranial pressure.[54] High circulating levels of soluble TNF receptors showed a predictive capacity for the development of late-onset multiple organ failure after traumatic injury.[72] Inflammatory mediators have been involved in the induction of cerebral edema, vascular permeability, BBB disruption and excitotoxic and apoptotic cell death after brain injury.[6,31,35,73,74] Recently, it was shown that neutralization of IL-1β attenuates TBI-induced edema, tissue loss and learning impairment in mice.[36] TNF may produce excitotoxic and apoptotic damage after TBI by altering the expression of AMPA ionotropic glutamate receptors[31] and by enhancing caspase-dependent apoptosis through TNF receptor-1)/Fas signaling.[75] However, the combination of TNF receptor-2/Fas receptors showed a beneficial role for the recovery of motor and cognitive function after TBI.[75]

Hormones: Progesterone, Growth Hormone & IGF-1

Endocrine dysfunctions are very common in TBI patients during the first 7–10 days postinjury,[24,76] and increased estradiol and testosterone levels over this time period were found to be associated with increased mortality and a worse outcome for both men and women with severe TBI.[24] Deficits of single or multiple pituitary hormones were detected in more than 20% of the TBI patients 1 year after injury, with hypogonadism, growth hormone (GH) deficiency and low circulating levels of IGF-1 being the most frequent findings.[23,77] Enduring low levels of gonadal steroids, GH and IGF-1 may have a negative impact on the mechanisms of neurorepair following TBI, given the putative neuroprotective functions of these hormones.[44,78–80] In fact, post-traumatic hypopituitarism was found to be associated with an unfavorable lipid profile and a worse quality of life at 1 year after TBI.[81] However, cognitive impairment was not found to be associated with GH and IGF-1 deficits in adult patients tested more than 1 year post-TBI.[82]

Recent experimental studies indicate that the neurosteroid progesterone may exert neuroprotective effects in TBI by reducing apoptotic proneurotrophin signaling, cellular apoptosis, inflammation and cerebral edema, and by enhancing prosurvival neurotrophin signaling, vascular remodeling and BBB protection.[79,80,83–85] Treatment with progesterone after TBI reduced the brain levels of the proapoptotic precursor proteins of NGF (pro-NGF) and BDNF (pro-BDNF), increased the levels of the anti-apoptotic mature NGF, reduced the expression of apoptosis markers and improved behavioral parameters in rats with bilateral frontal cortical contusions.[83] However, in the same study, progesterone also reduced the expression of mature BDNF and its TrkB receptor, which are essential for neuroplasticity during brain repair.[83] In rats with unilateral parietal cortical contusion, progesterone increased the circulating levels of endothelial progenitor cells, facilitated vascular remodeling and improved neurological outcome.[84] In another animal model of TBI, progesterone influenced the brain levels of inflammatory cytokines (i.e., IL-1β, IL-6, TNF-α and TGF-β) in a differential dose- and time-related manner, thus reducing IL-6 and TNF-α, but enhancing TGF-β and IL-1β after injury.[85]

Based on experimental data, it has been suggested that women compared with men, and pregnant versus nonpregnant women, may have more favorable outcomes after TBI owing to the neuroprotective effects of higher estrogen and progesterone levels; however, recent clinical studies did not confirm this assumption.[86–88] Pregnant patients with moderate-to-severe TBI showed no differences in mortality compared with their nonpregnant counterparts, but rather a trend toward increased mortality.[86] In a retrospective study comparing hormonally active women with men, no differences in sex-related mortality were found, and brain edema tended to be more frequent in females.[88] An even lower rate of survival was observed in females compared with males with severe TBI.[87]

GH and IGF-1 also have neuroprotective actions, promoting neuroplasticity and influencing the genesis and regeneration of cells in the adult brain.[44,78] IGF-1 acts on its receptor activating PI3K/Akt or pMAPK/ERK intracellular signaling pathways. Upregulated levels of IGF-1, the receptor for IGF-1 and its downstream signaling mediators were found in the hippocampus, cortex, subcortical white matter and cortical vessels in different rodent TBI models.[89–92] This IGF-1 upregulation seems to reflect an endogenous neuroprotection/neurorepair response as indicated by its protective affect on the survival of hippocampal CA1 neurons through Akt activation.[92] On the contrary, decreased serum levels of GH and IGF-1 were found after TBI in adult and inmature rats, respectively.[93,94] Low circulating IGF-1 levels were associated with hippocampal neuron loss and spatial memory deficits,[94] whereas peripheral GH depletion was associated with persistent inflammatory changes in the brain.[93] These data, together with findings that the peripheral administration of IGF-1 analogs improves somatosensory–motor function and long-term histological outcome after brain injury,[44] support the contribution of GH and IGF-1 to the TBI mechanisms of degeneration and repair.

Trophic Factors: BDNF, NGF & VEGF

BDNF and NGF are neurotrophins that promote neuronal survival and plasticity through binding to their specific high-affinity tyrosine kinase receptors (TrkB and TrkA, respectively) and the common low-affinity p75 neurotrophin receptor as well as the activation of downstream PI3K/Akt and MAPK/ERK signaling pathways. However, their precursor proteins (pro-BDNF and pro-NGF) induce apoptosis by coupling to the sortilin and the p75 neurotrophin receptor death-signaling receptor complex.[45]

Human TBI studies found that serum BDNF and NGF levels decreased in young adults as the severity of the injury increased,[27] and that elevated CSF levels of NGF, but not BDNF, correlated with TBI severity and were associated with a better outcome in children.[51] Recent genetic investigations demonstrated associations of some BDNF gene polymorphisms with clinical aspects of TBI outcome. The Val66Met BDNF polymorphism influenced the recovery of executive functioning after penetrating TBI[95] and the response to treatment with citalopram in depression secondary to TBI,[96] but not the cognitive performance 1 month after mild TBI.[97] Two single-nucleotide BDNF polymorphisms were significantly associated with postinjury recovery of general cognitive intelligence,[98] and another four were associated with memory measures 1 month after mild TBI.[97]

The reductions of BNDF, TrkB receptor and the downstream effectors on synaptic plasticity, learning and memory (synapsin 1, CREB and α-CAMKII) observed in injured animal brains suggest an inhibition of the endogenous trophic activity after TBI, particularly in the peritraumatic area.[39,99–101] Interestingly enough, increased BDNF and synapsin 1 within the cortex contralateral to the lesion may reflect compensatory restorative processes in areas homotypical to the injury,[99] which is in agreement with the recent demonstration that endogenous BDNF is essential for the recovery of motor function following unilateral brain injury by inducing reorganization of the corticospinal tract through contralateral sprouting fibers.[102] The potential contribution of BDNF to processes of neurorepair after TBI is highlighted by experimental studies demonstrating that its administration upregulates neuroprotective genes in CA3 hippocampal neurons similar to those genes induced by injury,[103] whereas the blockade of its TrkB receptors blunts the increases of BDNF, synapsin 1 and CREB induced by exercise in rats.[104] Further support for the protective effects of BDNF in TBI arises from pharmacological studies demonstrating that: BDNF mimetics activate TrkB receptors and improve learning after TBI in rats;[46] human mesenchymal stem cells seem to improve TBI functional recovery by increasing the secretion of BDNF and other neurotrophic factors and reducing apoptosis;[105] and that S-nitrosoglutathione protects axonal integrity and promotes synaptic plasticity at the same time that it enhances BDNF and TrkB expression in the TBI brain.[39]

VEGF is upregulated after TBI and participates in several processes of brain repair:[25,43,57,106–110] it has trophic and protective effects on neurons; it stimulates astroglial mitosis, scar formation and production of growth factors; it promotes endothelial cell survival, angiogenesis, vascular remodeling and BBB repair helping to re-establish metabolic and trophic support to the injured tissue.

Elevated levels of VEGF in the brain and serum were found in severe TBI patients during the first 1–3 weeks postinjury.[25,43,109] The peripheral upregulation of VEGF was associated with an increase of circulating endothelial progenitor cells, which suggests its involvement in angiogenesis and vascular repair after TBI.[25,43] In TBI animal models, serum and brain VEGF levels were also found to be upregulated.[106,108]

VEGF acts through its two tyrosine kinase receptors; flt-1 (VEGF-R1) expressed by vascular endothelial cells and activated astrocytes, and flk-1 (VEGF-R2) expressed on vascular endothelium and some neurons.[107] After penetrating TBI, VEGF-R1 was upregulated in reactive astrocytes and its neutralization reduced astroglial mitogenicity, scar formation and expression of growth factors (ciliary neurotrophic factor and FGF), and caused some increase in endothelial degeneration; conversely, VEGF-R2 was upregulated in neurons and its blockade reduced vascular proliferation around the wound and increased endothelial cell degeneration, without effects on astrogliosis and growth factor expression.[107] These data suggest that upregulation of VEGF after TBI may induce angiogenesis and astroglial proliferation, expression of growth factors and scar formation through activation of VEGF-R1. Finally, the involvement of VEGF in brain protection and repair after TBI is also supported by the recent finding that its exogenous (intracerebroventricular) administration resulted in increased neurogenesis and angiogenesis, reduced lesion volume and improved functional outcome in TBI mice.[110]

Alzheimer's Disease-related Proteins: Aβ & Tau

TBI and Alzheimer's disease (AD) share several pathologic mechanisms including the abnormal accumulation of Aβ and tau proteins. Severe TBI is considered a major risk factor for the later development of AD.

Brain Aβ is elevated acutely after TBI and its intra-axonal and extracellular (Aβ plaques) accumulation may contribute to secondary injury mechanisms including neuronal apoptosis, axonal damage and microglia activation, among others. Increased levels of tau and the axonal and intracytoplasmic accumulation of its aggregates are also involved in TBI pathophysiology.

Autopsy studies found Aβ plaques in approximately 30% of TBI victims[111] and these deposits developed rapidly after injury,[111,112] were associated with higher levels of soluble Aβ42[113] and were influenced by a genetic polymorphism of neprilysin, a major Aβ-degrading enzyme.[114] Acutely after TBI, Aβ also showed a remarkable intra-axonal co-accumulation with amyloid precursor protein and enzymes necessary for Aβ genesis, including BACE1 protein (β-secretase) and the γ-secretase complex protein, presenilin-1.[112,115]42 levels were also found to be increased in ventricular CSF,[116] but decreased in lumbar CSF after TBI.[117,118] Interestingly, a rise of interstitial Aβ after acute TBI was found to correlate positively with improvements in the neurological status, as assessed by the Glasgow Coma Score (GCS), and with brain glucose levels, and negatively with elevated intracranial pressure and with tissue hypoxia markers.[28] These findings suggest that a reduced release of soluble Aβ may reflect an increased accumulation of Aβ in patients with more severe brain damage and a worse neurological functioning. In fact, reduced levels of interstitial Aβ were associated with low EEG activity after experimental TBI,[119] and with a progressive deposition of Aβ in the brain parenchyma during aging.[120] Experimental pharmacological studies demonstrated that reductions in Aβ accumulation were associated with improved brain pathology, decreased microglia activation and better outcome after TBI.[41,121,122]

Tau-related pathology is present in the brains of patients with chronic traumatic encephalopathy or acute TBI,[55,115] and increased levels of the tau protein were found in the brain, CSF and serum after TBI in humans.[29,123–125] The levels of tau in the brain extracellular fluid rose shortly after TBI, and this tau increase was associated with low Aβ levels and worse clinical outcome in patients with severe TBI.[29] Initial elevations of CSF and serum tau levels were also found to be predictors of poor clinical outcome in severe TBI.[123–125]

Other Peptide-related Mechanisms: The PI3K/Akt/GSK-3β Signaling Pathway

TBI produces neuronal damage and deficits in neuroplasticity, and the capacity of injured neurons to regenerate is modulated to some extent by changes in the expression of intracellular signaling molecules such as Akt and GSK-3β.[47,48,126] GSK-3β is a constitutively active kinase involved in neuronal apoptosis, tau hyperphosphorylation and other pathologic processes relevant for TBI. Neurotrophic factors are major regulators of GSK-3β activity. Growth factors act on their receptors, inducing PI3K to phosphorylate Akt by activating it, which in turn leads to GSK-3β inactivation through direct phosphorylation. The PI3K/Akt/GSK-3β intracellular signaling pathway plays a central role in the regulation of different neuronal functions by neurotrophic factors, and there is evidence of its involvement in the reduction of Aβ production and tau hyperphosphorylation,[127] the modulation of neuroinflammation,[128] the increase of brain IGF-1 levels,[129] the uptake of cellular glucose,[130] the protection against neuronal damage, apoptosis and death,[131–134] the promotion of neuroplasticity and neurogenesis[133,134] and the improvement of cognitive deficits in different experimental conditions.[129] GSK-3β inhibition and Akt activation were also found to contribute to the enhancement neuronal survival, learning and memory after TBI.[48]

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