Assessment of R18, COG1410, and APP96-110 in Excitotoxicity and Traumatic Brain Injury

Neurobiochemical characteristics of arginine-rich peptides explain their potential therapeutic efficacy in neurodegenerative diseases

Neurodegenerative diseases, including Alzheimer̕ s disease (AD), Parkinson̕ s disease (PD), Huntington̕ s disease (HD), and Amyotrophic Lateral Sclerosis (ALS) require special attention to find new potential treatment methods. This review aims to summarize the current knowledge of the relationship between the biochemical properties of arginine-rich peptides (ARPs) and their neuroprotective effects to deal with the harmful effects of risk factors. It seems that ARPs have portrayed a promising and fantastic landscape for treating neurodegeneration-associated disorders. With multimodal mechanisms of action, ARPs play various unprecedented roles, including as the novel delivery platforms for entering the central nervous system (CNS), the potent antagonists for calcium influx, the invader molecules for targeting mitochondria, and the protein stabilizers. Interestingly, these peptides inhibit the proteolytic enzymes and block protein aggregation to induce pro-survival signaling pathways. ARPs also serve as the scavengers of toxic molecules and the reducers of oxidative stress agents. They also have anti-inflammatory, antimicrobial, and anti-cancer properties. Moreover, by providing an efficient nucleic acid delivery system, ARPs can play an essential role in developing various fields, including gene vaccines, gene therapy, gene editing, and imaging. ARP agents and ARP/cargo therapeutics can be raised as an emergent class of neurotherapeutics for neurodegeneration. Part of the aim of this review is to present recent advances in treating neurodegenerative diseases using ARPs as an emerging and powerful therapeutic tool. The applications and progress of ARPs-based nucleic acid delivery systems have also been discussed to highlight their usefulness as a broad-acting class of drugs.

Are We Ready for Clinical Therapy based on Cerebral Autoregulation? A Pro-con Debate

Cerebral autoregulation (CA) is a physiological mechanism that maintains constant cerebral blood flow regardless of changes in cerebral perfusion pressure and prevents brain damage caused by hypoperfusion or hyperperfusion. In recent decades, researchers have investigated the range of systemic blood pressures and clinical management strategies over which cerebral vasculature modifies intracranial hemodynamics to maintain cerebral perfusion. However, proposed clinical interventions to optimize autoregulation status have not demonstrated clear clinical benefit. As future trials are designed, it is crucial to comprehend the underlying cause of our inability to produce robust clinical evidence supporting the concept of CA-targeted management. This article examines the technological advances in monitoring techniques and the accuracy of continuous assessment of autoregulation techniques used in intraoperative and intensive care settings today. It also examines how increasing knowledge of CA from recent clinical trials contributes to a greater understanding of secondary brain injury in many disease processes, despite the fact that the lack of robust evidence influencing outcomes has prevented the translation of CA-guided algorithms into clinical practice.

APP96-110 Elicits Neuroprotective Effects Following Ischemic Insult in Animal Models

Competitive amyloidogenic pathways play an important role in many neurological diseases such as the onset of various degenerative diseases and ischemic stroke. Overexpression of amyloid precursor protein (APP) and amyloid-beta is modulated via the amyloidogenic pathway, which plays a crucial role in neuroinflammation. During ischemic conditions, the activity of the anti-inflammatory non-amyloidogenic pathway decreases, thus increasing the activity of amyloidogenic pathway. The soluble alpha form of APP (sAPPα), formed via the non-amyloidogenic pathway, exhibits neuroprotective effects against neurological diseases. sAPPα is thought to have a modulatory effect on several cell survival pathways, including its ability to inhibit the phosphoinositide 3-kinases (PI3K) pathway, thereby inhibiting the inflammatory response. The APP derivative, APP96-110, could act as a functional substitute for native sAPPα. Herein, we investigated whether APP96-110 has neuroprotective effects against neuroinflammation and damage following cerebral ischemic stroke. Treatment with diluted APP96-110 (0.005 mg/kg) in mice after 30 min of transient middle cerebral artery occlusion (tMCAO) showed improved motor function and reduced expression of the inflammatory marker CD86. APP96-110 decreased the infarct size and induced an anti-inflammatory response by inhibiting the PI3K pathway. These results suggest that the treatment of APP96-110 is efficacious in reducing neuroinflammation and infarct size in ischemic stroke.

ApoE Mimetic Peptide COG1410 Exhibits Strong Additive Interaction with Antibiotics Against Mycobacterium smegmatis

Background

Drug-resistant tuberculosis (TB) is an emerging threat to public health worldwide. Antimicrobial peptide (AMP) is a promising solution to solve the antimicrobial resistance crisis. The apolipoprotein E mimetic peptide COG1410 has been confirmed to simultaneously have neuroprotective, anti-inflammatory, and antibacterial activity. However, whether it is effective to inhibit growth of mycobacteria has not been investigated yet.

Methods

The peptide COG1410 was synthesized with conventional solid-phase peptide synthesis and qualified by HPLC and mass spectrometry. Micro-dilution method was used to determine the minimal inhibitory concentration. A time-kill assay was used to determine the bactericidal dynamics of antimicrobial peptide and relative antibiotics. Static biofilm formation was conducted in 24-well plate and the biofilm was separated from planktonic cells and collected. The mechanism of action of COG1410 was explored by TEM observation and ATP leak assay. The localization of COG1410 was observed by confocal laser scan microscopy. The drug-drug interaction was determined by a checkerboard assay.

Results

COG1410 was a potent bactericidal agent against M. smegmatis in vitro and within the macrophages with MIC 16 μg/mL, but invalid against M. abscess and M. tuberculosis. A time-kill assay showed that COG1410 killed M. smegmatis as potent as clarithromycin, but faster than LL-37, another short synthetic cationic peptide. 1× MIC COG1410 almost reduced 90% biofilm formation of M. smegmatis. Additionally, COG1410 was able to penetrate the cell membrane of macrophage and inhibit intracellular M. smegmatis growth. TEM observation and ATP leak assay found that COG1410 disrupted cell membrane and caused release of cell contents. Confocal fluorescence microscopy showed that FITC-COG1410 aggregated around cell membrane instead of entering the cytoplasm. Although COG1410 had relative high cytotoxicity, it exhibited strong additive interaction with regular anti-TB antibiotics, which reduced the working concentration of COG1410 and expanding safety window. After 30 passages, there was no induced drug resistance for COG1410.

Conclusion

COG1410 was a novel and potent AMP against M. smegmatis by disrupting the integrity of cell membrane.

The role of the microbiota-gut-brain axis in long-term neurodegenerative processes following traumatic brain injury

Traumatic brain injury (TBI) can be a devastating and debilitating disease to endure. Due to improvements in clinical practice, declining mortality rates have led to research into the long-term consequences of TBI. For example, the incidence and severity of TBI have been associated with an increased susceptibility of developing neurodegenerative disorders, such as Parkinson's or Alzheimer's disease. However, the mechanisms linking this alarming association are yet to be fully understood. Recently, there has been a groundswell of evidence implicating the microbiota-gut-brain axis in the pathogenesis of these diseases. Interestingly, survivors of TBI often report gastrointestinal complaints and animal studies have demonstrated gastrointestinal dysfunction and dysbiosis following injury. Autonomic dysregulation and chronic inflammation appear to be the main driver of these pathologies. Consequently, this review will explore the potential role of the microbiota-gut-brain axis in the development of neurodegenerative diseases following TBI.

Assessment of the safety of the cationic arginine-rich peptides (CARPs) poly-arginine-18 (R18 and R18D) in ex vivo models of mast cell degranulation and red blood cell hemolysis

Our laboratory focuses on the development of novel neuroprotective cationic peptides, such poly-arginine-18 (R18: 18-mer of l-arginine; net charge +18) and its d-enantiomer R18D in stroke and other brain injuries. In the clinical development of R18/R18D, their cationic property raises potential safety concerns on their non-specific effects to induce mast cell degranulation and hemolysis. To address this, we first utilised primary human cultured mast cells (HCMCs) to examine anaphylactoid effects. We also included as controls, the well-characterised neuroprotective TAT-NR2B9c peptide and the widely used heparin reversal peptide, protamine. Degranulation assay based on β-hexosaminidase release demonstrated that R18 and R18D did not induce significant mast cell degranulation in both untreated (naïve) and IgE-sensitised HCMCs in a dose-response study to a maximum peptide concentration of 16 μM. Similarly, TAT-NR2B9c and protamine did not induce significant mast cell degranulation. To examine hemolytic effects, red blood cells (RBCs), were incubated with the peptides at a concentration range of 1–16 μM in the absence or presence of 2% plasma. Measurement of hemoglobin absorbance revealed that only R18 induced a modest, but significant degree of hemolysis at the 16 μM concentration, and only in the absence of plasma. This study addressed the potential safety concern of the application of the cationic neuroprotective peptides, especially, R18D, on anaphylactoid responses and hemolysis. The findings indicate that R18, R18D, TAT-NR2B9c and protamine are unlikely to induce histamine mediated anaphylactoid reactions or RBC hemolysis when administered intravenously to patients.

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Cationic peptides R18, R18D, TAT-NR2B9c and protamine do not stimulate the degranulation of ex vivo untreated (naïve) and IgE-sensitised primary human cultured mast cells (HCMCs).

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R18, R18D, TAT-NR2B9c and protamine do not induce significant hemolysis of human red blood cells (RBCs).

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R18, R18D, TAT-NR2B9c and protamine are unlikely to induce histamine mediated anaphylactoid reactions or hemolysis when administered intravenously to patients.

The role of L-arginine metabolism in neurocritical care patients

Nitric oxide is an important mediator of vascular autoregulation and is involved in pathophysiological changes after acute neurological disorders. Nitric oxide is generated by nitric oxide synthases from the amino acid L-arginine. L-arginine can also serve as a substrate for arginases or lead to the generation of dimethylarginines, asymmetric dimethylarginine, and symmetric dimethylarginine, by methylation. Asymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase and can lead to endothelial dysfunction. This review discusses the role of L-arginine metabolism in patients suffering from acute and critical neurological disorders often requiring neuro-intensive care treatment. Conditions addressed in this review include intracerebral hemorrhage, aneurysmal subarachnoid hemorrhage, and traumatic brain injury. Recent therapeutic advances in the field are described including current randomized controlled trials for traumatic brain injuries and hemorrhagic stroke.

Effects of amyloid precursor protein peptide APP96-110, alone or with human mesenchymal stromal cells, on recovery after spinal cord injury

Delivery of a peptide (APP96-110), derived from amyloid precursor protein (APP), has been shown to elicit neuroprotective effects following cerebral stroke and traumatic brain injury. In this study, the effect of APP96-110 or a mutant version of this peptide (mAPP96-110) was assessed following moderate (200 kdyn, (2 N)) thoracic contusive spinal cord injury (SCI) in adult Nude rats. Animals received a single tail vein injection of APP96-110 or mAPP96-110 at 30 minutes post-SCI and were then assessed for functional improvements over the next 8 weeks. A cohort of animals also received transplants of either viable or non-viable human mesenchymal stromal cells (hMSCs) into the SC lesion site at one week post-injury to assess the effect of combining intravenous APP96-110 delivery with hMSC treatment. Rats were perfused 8 weeks post-SCI and longitudinal sections of spinal cord analyzed for a number of factors including hMSC viability, cyst size, axonal regrowth, glial reactivity and macrophage activation. Analysis of sensorimotor function revealed occasional significant differences between groups using Ladderwalk or Ratwalk tests, however there were no consistent improvements in functional outcome after any of the treatments. mAPP96-110 alone, and APP96-110 in combination with both viable and non-viable hMSCs significantly reduced cyst size compared to SCI alone. Combined treatments with donor hMSCs also significantly increased βIII tubulin⁺, glial fibrillary acidic protein (GFAP⁺) and laminin⁺ expression, and decreased ED1⁺ expression in tissues. This preliminary study demonstrates that intravenous delivery of APP96-110 peptide has selective, modest neuroprotective effects following SCI, which may be enhanced when combined with hMSC transplantation. However, the effects are less pronounced and less consistent compared to the protective morphological and cognitive impact that this same peptide has on neuronal survival and behaviour after stroke and traumatic brain injury. Thus while the efficacy of a particular therapeutic approach in one CNS injury model may provide justification for its use in other neurotrauma models, similar outcomes may not necessarily occur and more targeted approaches suited to location and severity are required. All animal experiments were approved by The University of Western Australia Animal Ethics Committee (RA3/100/1460) on April 12, 2016.

Bidentatide, a Novel Plant Peptide Derived from Achyranthes bidentata Blume: Isolation, Characterization, and Neuroprotection through Inhibition of NR2B-Containing NMDA Receptors

Increasing attention is being focused on the use of polypeptide-based N-methyl-d-aspartate (NMDA) receptor antagonists for the treatment of nervous system disorders. In our study on Achyranthes bidentata Blume, we identified an NMDA receptor subtype 2B (NR2B) antagonist that exerts distinct neuroprotective actions. This antagonist is a 33 amino acid peptide, named bidentatide, which contains three disulfide bridges that form a cysteine knot motif. We determined the neuroactive potential of bidentatide by evaluating its in vitro effects against NMDA-mediated excitotoxicity. The results showed that pretreating primary cultured hippocampal neurons with bidentatide prevented NMDA-induced cell death and apoptosis via multiple mechanisms that involved intracellular Ca²⁺ inhibition, NMDA current inhibition, and apoptosis-related protein expression regulation. These mechanisms were all dependent on bidentatide-induced inhibitory regulation of NR2B-containing NMDA receptors; thus, bidentatide may contribute to the development of neuroprotective agents that would likely possess the high selectivity and safety profiles inherent in peptide drugs.

PTD4 Peptide Increases Neural Viability in an In Vitro Model of Acute Ischemic Stroke

Ischemic stroke is a disturbance in cerebral blood flow caused by brain tissue ischemia and hypoxia. We optimized a multifactorial in vitro model of acute ischemic stroke using rat primary neural cultures. This model was exploited to investigate the pro-viable activity of cell-penetrating peptides: arginine-rich Tat(49–57)-NH₂ (R⁴⁹KKRRQRRR⁵⁷-amide) and its less basic analogue, PTD4 (Y⁴⁷ARAAARQARA⁵⁷-amide). Our model included glucose deprivation, oxidative stress, lactic acidosis, and excitotoxicity. Neurotoxicity of these peptides was excluded below a concentration of 50 μm, and PTD4-induced pro-survival was more pronounced. Circular dichroism spectroscopy and molecular dynamics (MD) calculations proved potential contribution of the peptide conformational properties to neuroprotection: in MD, Tat(49–57)-NH₂ adopted a random coil and polyproline type II helical structure, whereas PTD4 adopted a helical structure. In an aqueous environment, the peptides mostly adopted a random coil conformation (PTD4) or a polyproline type II helical (Tat(49–57)-NH₂) structure. In 30% TFE, PTD4 showed a tendency to adopt a helical structure. Overall, the pro-viable activity of PTD4 was not correlated with the arginine content but rather with the peptide’s ability to adopt a helical structure in the membrane-mimicking environment, which enhances its cell membrane permeability. PTD4 may act as a leader sequence in novel drugs for the treatment of acute ischemic stroke.

Advances in D-Amino Acids in Neurological Research

D-amino acids have been known to exist in the human brain for nearly 40 years, and they continue to be a field of active study to today. This review article aims to give a concise overview of the recent advances in D-amino acid research as they relate to the brain and neurological disorders. This work has largely been focused on modulation of the N-methyl-D-aspartate (NMDA) receptor and its relationship to Alzheimer’s disease and Schizophrenia, but there has been a wealth of novel research which has elucidated a novel role for several D-amino acids in altering brain chemistry in a neuroprotective manner. D-amino acids which have no currently known activity in the brain but which have active derivatives will also be reviewed.

Poly-arginine-18 (R18) Confers Neuroprotection through Glutamate Receptor Modulation, Intracellular Calcium Reduction, and Preservation of Mitochondrial Function

Recent studies have highlighted that a novel class of neuroprotective peptide, known as cationic arginine-rich peptides (CARPs), have intrinsic neuroprotective properties and are particularly effective anti-excitotoxic agents. As such, the present study investigated the mechanisms underlying the anti-excitotoxic properties of CARPs, using poly-arginine-18 (R18; 18-mer of arginine) as a representative peptide. Cortical neuronal cultures subjected to glutamic acid excitotoxicity were used to assess the effects of R18 on ionotropic glutamate receptor (iGluR)-mediated intracellular calcium influx, and its ability to reduce neuronal injury from raised intracellular calcium levels after inhibition of endoplasmic reticulum calcium uptake by thapsigargin. The results indicate that R18 significantly reduces calcium influx by suppressing iGluR overactivation, and results in preservation of mitochondrial membrane potential (ΔΨm) and ATP production, and reduced ROS generation. R18 also protected cortical neurons against thapsigargin-induced neurotoxicity, which indicates that the peptide helps maintain neuronal survival when intracellular calcium levels are elevated. Taken together, these findings provide important insight into the mechanisms of action of R18, supporting its potential application as a neuroprotective therapeutic for acute and chronic neurological disorders.

Effect of Polyarginine Peptide R18D Following a Traumatic Brain Injury in Sprague-Dawley Rats

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Cationic arginine-rich peptides (CARPs) have previously demonstrated neuroprotective efficacy in a rat model of traumatic brain injury (TBI).

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The CARP R18D significantly reduced the extent of axonal injury at a high dose of 1000 nmol/kg.

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Both high (1000 nmol/kg) and low (100 nmol/kg) doses only showed a trend in functional improvement.

Background

Despite extensive studies, there are still no clinically available neuroprotective treatments for traumatic brain injury.

Objectives

In previous studies we demonstrated beneficial treatment effects of polyarginine peptides R18 (18-mer of arginine; 300 nmol/kg) and R18D (18-mer of D-arginine; 1000 nmol/kg) in a rat model of impact-acceleration closed-head injury.

Methods

We examined the efficacy of R18D when intravenously administered at a low (100 nmol/kg) and high (1000 nmol/kg) dose, 30 minutes after a closed-head injury in male Sprague-Dawley rats.

Results

At postinjury day 3, treatment with R18D at the high dose significantly reduced axonal injury (P = 0.044), whereas the low-dose treatment of R18D showed a trend for reduced axonal injury. Following assessment in the Barnes maze, both doses of R18D treatment appeared to improve learning and memory recovery compared with vehicle treatment at postinjury days 1 and 3, albeit not to a statistically significant level. Rotarod assessment of vestibulomotor recovery did not differ between R18D and the vehicle treatment groups.

Conclusions

R18D modestly decreased axonal injury only at the highest dose used but had no significant effect on functional recovery. These findings warrant further studies with additional doses to better understand peptide pharmacodynamics and provide information to guide optimal dosing.

Tissue distribution of intravenously administrated poly-arginine peptide R18D in healthy male Sprague–Dawley rats

Aim: R18D is a poly-arginine peptide that has demonstrated neuroprotection in preclinical models of excitotoxicity, stroke, hypoxic-ischemic encephalopathy and traumatic brain injury. Here, we examined the peptide’s uptake in serum. Materials & methods: Healthy, male Sprague–Dawley rats were intravenously administered either 1000 nmol/kg R18D (D-enantiomer of R18) or approximately 2.5 nmol/kg (36 ± 9 MBq) [18F]R18D, for serum and organ tissue uptake, respectively. Serum samples underwent mass spectrometric analysis to detect unbound R18D peptide. Animals administered [18F]R18D were subjected to positron emission tomography imaging. Results & conclusion: Free R18D was detected at 5 min post-infusion in serum samples. [18F]R18D was rapidly distributed to the kidney (6–7%ID/g), and a small fraction localized to the brain (0.115–0.123%ID/g) over a 60-min acquisition period.

Cationic Arginine-Rich Peptides (CARPs): A Novel Class of Neuroprotective Agents With a Multimodal Mechanism of Action

There are virtually no clinically available neuroprotective drugs for the treatment of acute and chronic neurological disorders, hence there is an urgent need for the development of new neuroprotective molecules. Cationic arginine-rich peptides (CARPs) are an expanding and relatively novel class of compounds, which possess intrinsic neuroprotective properties. Intriguingly, CARPs possess a combination of biological properties unprecedented for a neuroprotective agent including the ability to traverse cell membranes and enter the CNS, antagonize calcium influx, target mitochondria, stabilize proteins, inhibit proteolytic enzymes, induce pro-survival signaling, scavenge toxic molecules, and reduce oxidative stress as well as, having a range of anti-inflammatory, analgesic, anti-microbial, and anti-cancer actions. CARPs have also been used as carrier molecules for the delivery of other putative neuroprotective agents across the blood-brain barrier and blood-spinal cord barrier. However, there is increasing evidence that the neuroprotective efficacy of many, if not all these other agents delivered using a cationic arginine-rich cell-penetrating peptide (CCPPs) carrier (e.g., TAT) may actually be mediated largely by the properties of the carrier molecule, with overall efficacy further enhanced according to the amino acid composition of the cargo peptide, in particular its arginine content. Therefore, in reviewing the neuroprotective mechanisms of action of CARPs we also consider studies using CCPPs fused to a putative neuroprotective peptide. We review the history of CARPs in neuroprotection and discuss in detail the intrinsic biological properties that may contribute to their cytoprotective effects and their usefulness as a broad-acting class of neuroprotective drugs.

Poly-arginine-18 peptides do not exacerbate bleeding, or improve functional outcomes following collagenase-induced intracerebral hemorrhage in the rat

Background

Cationic arginine-rich peptides (CARPs) have demonstrated neuroprotective and/or behavioural efficacy in ischemic and hemorrhagic stroke and traumatic brain injury models. Therefore, in this study we investigated the safety and neuroprotective efficacy of the CARPs poly-arginine-18 (R18; 18-mer of arginine) and its D-enantiomer R18D given in the acute bleeding phase in an intracerebral hemorrhage (ICH) model.

Methods

One hundred and fifty-eight male Sprague-Dawley rats received collagenase-induced ICH. Study 1 examined various doses of R18D (30, 100, 300, or 1000 nmol/kg) or R18 (100, 300, 1000 nmol/kg) administered intravenously 30 minutes post-collagenase injection on hemorrhage volume 24 hours after ICH. Study 2 examined R18D (single intravenous dose) or R18 (single intravenous dose, plus 6 daily intraperitoneal doses) at 300 or 1000 nmol/kg commencing 30 minutes post-collagenase injection on behavioural outcomes (Montoya staircase test, and horizontal ladder test) in the chronic post-ICH period. A histological assessment of tissue loss was assessed using a Nissl stain at 28 days after ICH.

Results

When administered during ongoing bleeding, neither R18 or R18D exacerbated hematoma volume or worsened functional deficits. Lesion volume assessment at 28 days post-ICH was not reduced by the peptides; however, animals treated with the lower R18D 300 nmol/kg dose, but not with the higher 1000 nmol/kg dose, demonstrated a statistically increased lesion size compared to saline treated animals.

Conclusion

Overall, both R18 and R18D appeared to be safe when administered during a period of ongoing bleeding following ICH. Neither peptide appears to have any statistically significant effect in reducing lesion volume or improving functional recovery after ICH. Additional studies are required to further assess dose efficacy and safety in pre-clinical ICH studies.

Proteomic analysis of cortical neuronal cultures treated with poly-arginine peptide-18 (R18) and exposed to glutamic acid excitotoxicity

Abstract

Poly-arginine peptide-18 (R18) has recently emerged as a highly effective neuroprotective agent in experimental stroke models, and is particularly efficacious in protecting cortical neurons against glutamic acid excitotoxicity. While we have previously demonstrated that R18 can reduce excitotoxicity-induced neuronal calcium influx, other molecular events associated with R18 neuroprotection are yet to investigated. Therefore, in this study we were particularly interested in protein expression changes in R18 treated neurons subjected to excitotoxicity.

Proteomic analysis was used to compare protein expression patterns in primary cortical neuronal cultures subjected to: (i) R18-treatment alone (R18); (ii) glutamic acid excitotoxic injury (Glut); (iii) R18-treatment and glutamic acid injury (R18 + Glut); (iv) no treatment (Cont). Whole cell lysates were harvested 24 h post-injury and subjected to quantitative proteomic analysis (iTRAQ), coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) and subsequent bioinformatic analysis of differentially expressed proteins (DEPs).

Relative to control cultures, R18, Glut, and R18 + Glut treatment resulted in the detection of 5, 95 and 14 DEPs respectively. Compared to Glut alone, R18 + Glut revealed 98 DEPs, including 73 proteins whose expression was also altered by treatment with Glut and/or R18 alone, as well as 25 other uniquely regulated proteins. R18 treatment reversed the up- or down-regulation of all 73 Glut-associated DEPs, which included proteins involved in mitochondrial integrity, ATP generation, mRNA processing and protein translation. Analysis of protein-protein interactions of the 73 DEPs showed they were primarily associated with mitochondrial respiration, proteasome activity and protein synthesis, transmembrane trafficking, axonal growth and neuronal differentiation, and carbohydrate metabolism. Identified protein pathways associated with proteostasis and energy metabolism, and with pathways involved in neurodegeneration.

Collectively, the findings indicate that R18 neuroprotection following excitotoxicity is associated with preservation of neuronal protein profiles, and differential protein expression that assists in maintaining mitochondrial function and energy production, protein homeostasis, and membrane trafficking.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1186/s13041-019-0486-8) contains supplementary material, which is available to authorized users.

Mitochondrial dysfunctioning and neuroinflammation: Recent highlights on the possible mechanisms involved in Traumatic Brain Injury

Traumatic brain injury (TBI) is the injury to the vasculature of brain while trauma caused by physical, chemical and biological stimuli. TBI is the leading cause of mortality and morbidity around the world. In this, primary insult leads to secondary injury through the involvement and initiation of various pathological processes. The most citable includes excitotoxicity, Blood Brain Barrier (BBB) dysfunction, inflammation, mitochondrial dysfunction, oxidative stress, calcium efflux, microglial mediated release of proinflammatory mediators (cytokine, chemokines, interleukin, tissue necrosis factor etc.). The morphological changes in TBI are proportional to mitochondrial dysfunctioning and microglial activation, which play an assorted role in neurodegeneration following traumatic brain injury. It is also assumed that the release of nitric oxide, activation of microglial cells plays a diversive role in maintaining the physiological and pathological balance. This review cites different pathophysiological mechanisms that are involved in progenesis of secondary injury after primary insult. These targets further are useful to explore the deep molecular mechanisms and to analyse the effectiveness of available drugs. Moreover, the present review reflects the underlying inflammatory cascade responsible for neuronal loss and neurological deficit in TBI.

Effect of poly-arginine R18 on neurocyte cell growth via autophagy in traumatic brain injury

The present study assessed the effects of poly-arginine R18 and its promotion of neurocyte cell growth via autophagy in traumatic brain injury (TBI), and aimed to determine the possible mechanism by which this occurs. Brain water content was measured to analyze the effects of poly-arginine R18 in TBI. MTT and lactate dehydrogenase activity assays were performed to measure N2A cell growth. Western blotting and immunofluorescence staining were also performed to determine the protein expression of Bcl-2 associated X, LC3, Beclin-1 and p62. The results demonstrated that poly-arginine R18 treatment reduced neurocyte apoptosis and promoted neurocyte cell growth via the activation of autophagy in a rat model of TBI. Furthermore, poly-arginine R18 treatment promoted neurocyte cell growth, reduced apoptosis, induced the protein expression of LC3 and Beclin-1, and suppressed p62 expression by promoting autophagy in vitro. In addition, the inhibition of autophagy attenuated the effects of poly-arginine R18 on cell growth in vitro. Collectively, the results demonstrate the effects of poly-arginine R18 on neurocyte cell growth via autophagy activation in a model of TBI, and poly-arginine R18 is therefore a potential therapeutic target in TBI.

Mitochondria and neuroprotection in stroke: Cationic arginine-rich peptides (CARPs) as a novel class of mitochondria-targeted neuroprotective therapeutics

Stroke is the second leading cause of death globally and represents a major cause of devastating long-term disability. Despite sustained efforts to develop clinically effective neuroprotective therapies, presently there is no clinically available neuroprotective agent for stroke. As a central mediator of neurodamaging events in stroke, mitochondria are recognised as a critical neuroprotective target, and as such, provide a focus for developing mitochondrial-targeted therapeutics. In recent years, cationic arginine-rich peptides (CARPs) have been identified as a novel class of neuroprotective agent with several demonstrated mechanisms of action, including their ability to target mitochondria and exert positive effects on the organelle. This review provides an overview on neuronal mitochondrial dysfunction in ischaemic stroke pathophysiology and highlights the potential beneficial effects of CARPs on mitochondria in the ischaemic brain following stroke.

Perinatal Hypoxic-Ischemic Encephalopathy and Neuroprotective Peptide Therapies: A Case for Cationic Arginine-Rich Peptides (CARPs)

Perinatal hypoxic-ischemic encephalopathy (HIE) is the leading cause of mortality and morbidity in neonates, with survivors suffering significant neurological sequelae including cerebral palsy, epilepsy, intellectual disability and autism spectrum disorders. While hypothermia is used clinically to reduce neurological injury following HIE, it is only used for term infants (>36 weeks gestation) in tertiary hospitals and improves outcomes in only 30% of patients. For these reasons, a more effective and easily administrable pharmacological therapeutic agent, that can be used in combination with hypothermia or alone when hypothermia cannot be applied, is urgently needed to treat pre-term (≤36 weeks gestation) and term infants suffering HIE. Several recent studies have demonstrated that cationic arginine-rich peptides (CARPs), which include many cell-penetrating peptides [CPPs; e.g., transactivator of transcription (TAT) and poly-arginine-9 (R9; 9-mer of arginine)], possess intrinsic neuroprotective properties. For example, we have demonstrated that poly-arginine-18 (R18; 18-mer of arginine) and its D-enantiomer (R18D) are neuroprotective in vitro following neuronal excitotoxicity, and in vivo following perinatal hypoxia-ischemia (HI). In this paper, we review studies that have used CARPs and other peptides, including putative neuroprotective peptides fused to TAT, in animal models of perinatal HIE. We critically evaluate the evidence that supports our hypothesis that CARP neuroprotection is mediated by peptide arginine content and positive charge and that CARPs represent a novel potential therapeutic for HIE.

Poly-arginine R18 and R18D (D-enantiomer) peptides reduce infarct volume and improves behavioural outcomes following perinatal hypoxic-ischaemic encephalopathy in the P7 rat

We examined the neuroprotective efficacy of the poly-arginine peptide R18 and its D-enantiomer R18D in a perinatal hypoxic-ischaemic (HI) model in P7 Sprague-Dawley rats. R18 and R18D peptides were administered intraperitoneally at doses of 30, 100, 300 or 1000 nmol/kg immediately after HI (8% O₂/92%N₂ for 2.5 h). The previously characterised neuroprotective JNKI-1-TATD peptide at a dose of 1000 nmol/kg was used as a control. Infarct volume and behavioural outcomes were measured 48 h after HI. For the R18 and R18D doses examined, total infarct volume was reduced by 25.93% to 43.80% (P = 0.038 to < 0.001). By comparison, the JNKI-1-TATD reduced lesion volume by 25.27% (P = 0.073). Moreover, R18 and R18D treatment resulted in significant improvements in behavioural outcomes, while with JNKI-1-TATD there was a trend towards improvement. As an insight into the likely mechanism underlying the effects of R18, R18D and JNKI-1-TATD, the peptides were added to cortical neuronal cultures exposed to glutamic acid excitotoxicity, resulting in up to 89, 100 and 71% neuroprotection, respectively, and a dose dependent inhibition of neuronal calcium influx. The study further confirms the neuroprotective properties of poly-arginine peptides, and suggests a potential therapeutic role for R18 and R18D in the treatment of HIE.