Valproic acid prevents brain injury in a canine model of hypothermic circulatory arrest: a promising new approach to neuroprotection during cardiac surgery

Duration of deep hypothermic circulatory arrest for aortic arch surgery: is it a myth, fiction, or scientific leap?

INTRODUCTION

The use of deep hypothermic circulatory arrest (DHCA) to provide aortic surgeons with a bloodless operative field while simultaneously protecting the brain and peripheries from ischemic damage revolutionized cardiac and aortic surgery, and is currently used in specialist centers across the globe. However, it is associated with manifold adverse outcomes, including neurocognitive dysfunction and mortality. This review seeks to analyze the relationship between DHCA duration and clinical outcome, and evaluate the controversies and limitations surrounding its use.

EVIDENCE ACQUISITION

We performed a review of available literature with statistical analysis to evaluate the relationship between DHCA duration (<40 min and >40 min) and key clinical outcomes, including mortality, permanent and temporary neurological deficit, renal damage, admission length, and reintervention rate. The controversies surrounding DHCA use and future directions for care are also explored.

EVIDENCE SYNTHESIS

Statistical analysis revealed no significant association (P>0.05) between DHCA duration and clinical outcomes (early and late mortality rates, neurological deficit, admission length, and reintervention rate), both with and without adjunctive perfusion techniques.

CONCLUSIONS

Available literature suggests that the relationships between DHCA duration (with and without adjunctive perfusion) and clinical outcomes are unclear, and at present not statistically significant. Alternative surgical and endovascular techniques have been identified as promising novel approaches not requiring DHCA, as have the use of biomarkers to enable early diagnosis and intervention for aortic pathologies.

HSP70-mediated neuroprotection by combined treatment of valproic acid with hypothermia in a rat asphyxial cardiac arrest model

It has been reported that valproic acid (VPA) combined with therapeutic hypothermia can improve survival and neurologic outcomes in a rat asphyxial cardiac arrest model. However, neuroprotective mechanisms of such combined treatment of valproic acid with hypothermia remains unclear. We hypothesized that epigenetic regulation of HSP70 by histone acetylation could increase HSP70-mediated neuroprotection suppressed under hypothermia. Male Sprague-Dawley rats that achieved return of spontaneous circulation (ROSC) from asphyxial cardiac arrest were randomized to four groups: normothermia (37°C ± 1°C), hypothermia (33°C ± 1°C), normothermia + VPA (300 mg/kg IV initiated 5 minutes post-ROSC and infused over 20 min), and hypothermia + VPA. Three hours after ROSC, acetyl-histone H3 was highly expressed in VPA-administered groups (normothermia + VPA, hypothermia + VPA). Four hours after ROSC, HSP70 mRNA expression levels were significantly higher in normothermic groups (normothermia, normothermia + VPA) than in hypothermic groups (hypothermia, hypothermia + VPA). The hypothermia + VPA group showed significantly higher HSP70 mRNA expression than the hypothermia group. Similarly, at five hours after ROSC, HSP70 protein levels were significantly higher in normothermic groups than in hypothermic groups. HSP70 levels were significantly higher in the hypothermia + VPA group than in the hypothermia group. Only the hypothermia + VPA group showed significantly attenuated cleaved caspase-9 levels than the normothermia group. Hypothermia can attenuate the expression of HSP70 at transcriptional level. However, VPA administration can induce hyperacetylation of histone H3, leading to epigenetic transcriptional activation of HSP70 even in a hypothermic status. Combining VPA treatment with hypothermia may compensate for reduced activation of HSP70-mediated anti-apoptotic pathway.

Nanotechnology for Neuroscience: Promising Approaches for Diagnostics, Therapeutics and Brain Activity Mapping

Unlocking the secrets of the brain is a task fraught with complexity and challenge - not least due to the intricacy of the circuits involved. With advancements in the scale and precision of scientific technologies, we are increasingly equipped to explore how these components interact to produce a vast range of outputs that constitute function and disease. Here, an insight is offered into key areas in which the marriage of neuroscience and nanotechnology has revolutionized the industry. The evolution of ever more sophisticated nanomaterials culminates in network-operant functionalized agents. In turn, these materials contribute to novel diagnostic and therapeutic strategies, including drug delivery, neuroprotection, neural regeneration, neuroimaging and neurosurgery. Further, the entrance of nanotechnology into future research arenas including optogenetics, molecular/ion sensing and monitoring, and piezoelectric effects is discussed. Finally, considerations in nanoneurotoxicity, the main barrier to clinical translation, are reviewed, and direction for future perspectives is provided.

Generation-6 hydroxyl PAMAM dendrimers improve CNS penetration from intravenous administration in a large animal brain injury model

Hypothermic circulatory arrest (HCA) provides neuroprotection during cardiac surgery but entails an ischemic period that can lead to excitotoxicity, neuroinflammation, and subsequent neurologic injury. Hydroxyl polyamidoamine (PAMAM) dendrimers target activated microglia and damaged neurons in the injured brain, and deliver therapeutics in small and large animal models. We investigated the effect of dendrimer size on brain uptake and explored the pharmacokinetics in a clinically-relevant canine model of HCA-induced brain injury. Generation 6 (G6, ~6.7nm) dendrimers showed extended blood circulation times and increased accumulation in the injured brain compared to generation 4 dendrimers (G4, ~4.3nm), which were undetectable in the brain by 48h after final administration. High levels of G6 dendrimers were found in cerebrospinal fluid (CSF) of injured animals with a CSF/serum ratio of ~20% at peak, a ratio higher than that of many neurologic pharmacotherapies already in clinical use. Brain penetration (measured by drug CSF/serum level) of G6 dendrimers correlated with the severity of neuroinflammation observed. G6 dendrimers also showed decreased renal clearance rate, slightly increased liver and spleen uptake compared to G4 dendrimers. These results, in a large animal model, may offer insights into the potential clinical translation of dendrimers.

Nanotechnology Approaches to Targeting Inflammation and Excitotoxicity in a Canine Model of Hypothermic Circulatory Arrest-Induced Brain Injury

BACKGROUND

Neurocognitive dysfunction and injury remain problematic after cardiac procedures requiring hypothermic circulatory arrest (HCA). Due to poor blood-brain barrier penetrance and toxicities associated with systemic drug therapies, clinical success has been elusive. Accordingly, we explored targeted dendrimer (a nanoparticle) drug therapies in our well-established canine model of HCA to characterize the biodistribution and cellular localization of these nanoparticles in areas of known neuronal apoptosis and necrosis.

METHODS

Class A, 27- to 30-kg male hounds were administered an initial intravenous bolus (10% of the total dose [200 mg]) of generation-six polyamidoamine dendrimer (6.7 nm) labeled with cyanine 5, and cardiopulmonary bypass with peripheral cannulation was initiated. After 90 minutes of HCA, 70% of the total dose was infused over a 6-hour period. The final 20% of the total dose was given 24 hours post-HCA. The brain was harvested 48 hours later (72 hours post-HCA) and analyzed for dendrimer 6-cyanine 5 biodistribution.

RESULTS

The dorsal hippocampus demonstrated the highest brain accumulation of dendrimer 6-cyanine 5, which closely corresponds to the distribution of apoptotic neurons evident with histologic staining and on confocal imaging. In injured brain regions, dendrimer traversed the blood-brain barrier and localized within the target cells (injured neurons and microglia).

CONCLUSIONS

These findings have exciting implications for the future development of novel therapeutics to mitigate neurocognitive deficits in this group of patients.

Brain injury in canine models of cardiac surgery

Neuropathology and neurologic impairment were characterized in a clinically relevant canine model of hypothermic (18°C) circulatory arrest (HCA) and cardiopulmonary bypass (CPB). Adult dogs underwent 2 hours of HCA (n = 39), 1 hour of HCA (n = 20), or standard CPB (n = 22) and survived 2, 8, 24, or 72 hours. Neurologic impairment and neuropathology were much more severe after 2-hour HCA than after 1-hour HCA or CPB; histopathology and neurologic deficit scores were significantly correlated. Apoptosis developed as early as 2 hours after injury and was most severe in the granule cells of the hippocampal dentate gyrus. Necrosis evolved more slowly and was most severe in amygdala and pyramidal neurons in the cornu ammonis hippocampus. Neuronal injury was minimal up to 24 hours after 1-hour HCA, but 1 dog that survived to 72 hours showed substantial necrosis in the hippocampus, suggesting that, with longer survival time, the injury was worse. Although neuronal injury was minimal after CPB, we observed rare apoptotic and necrotic neurons in hippocampi and caudate nuclei. These results have important implications for CPB in humans and may help explain the subtle cognitive changes experienced by patients after CPB.

Dendrimer brain uptake and targeted therapy for brain injury in a large animal model of hypothermic circulatory arrest

Treatment of brain injury following circulatory arrest is a challenging health issue with no viable therapeutic options. Based on studies in a clinically relevant large animal (canine) model of hypothermic circulatory arrest (HCA)-induced brain injury, neuroinflammation and excitotoxicity have been identified as key players in mediating the brain injury after HCA. Therapy with large doses of valproic acid (VPA) showed some neuroprotection but was associated with adverse side effects. For the first time in a large animal model, we explored whether systemically administered polyamidoamine (PAMAM) dendrimers could be effective in reaching target cells in the brain and deliver therapeutics. We showed that, upon systemic administration, hydroxyl-terminated PAMAM dendrimers are taken up in the brain of injured animals and selectively localize in the injured neurons and microglia in the brain. The biodistribution in other major organs was similar to that seen in small animal models. We studied systemic dendrimer-drug combination therapy with two clinically approved drugs, N-acetyl cysteine (NAC) (attenuating neuroinflammation) and valproic acid (attenuating excitotoxicity), building on positive outcomes in a rabbit model of perinatal brain injury. We prepared and characterized dendrimer-NAC (D-NAC) and dendrimer-VPA (D-VPA) conjugates in multigram quantities. A glutathione-sensitive linker to enable for fast intracellular release. In preliminary efficacy studies, combination therapy with D-NAC and D-VPA showed promise in this large animal model, producing 24 h neurological deficit score improvements comparable to high dose combination therapy with VPA and NAC, or free VPA, but at one-tenth the dose, while significantly reducing the adverse side effects. Since adverse side effects of drugs are exaggerated in HCA, the reduced side effects with dendrimer conjugates and suggestions of neuroprotection offer promise for these nanoscale drug delivery systems.

Combined diazepam and HDAC inhibitor treatment protects against seizures and neuronal damage caused by soman exposure

The occurrence of status epilepticus (SE) is considered the main cause of brain lesions and morphological alterations, such as hippocampal neuron loss, that result in chronic epilepsy. Previous work demonstrated the convulsive and widespread neuropathological effects of soman, an organophosphorus compound that causes SE and severe recurrent seizures as a result of exposure. Seizures begin rapidly after exposure, can continue for hours, and contribute to prolonged physical incapacitation of the victim. This study attempts to identify anticonvulsive and neuroprotective drugs against soman exposure. Male Sprague-Dawley rats were exposed to 1.0 LD(50) soman. EEGraphical and neuropathological (Fluoro-Jade B staining) effects were analyzed at 72 h post-exposure to soman and subsequent treatments with diazepam (DZP) alone or in combination with histone deacetylase inhibitors, suberoylanilide hydroxamic acid (SAHA) or valproic acid (VPA). The extent of brain damage was dependent on the length of SE and not on the number of recurrent seizures. DZP treatment alone decreased SE time and damage in hippocampus, amygdala, thalamus and cortex, but not in piriform nuclei. The combination of DZP and VPA 100 mg/kg showed more anticonvulsive effects, decreased SE time, and afforded more neuroprotection in the hippocampus, mainly the ventral portion. The combination DZP and SAHA 25 mg/kg was more neuroprotective, but not more anticonvulsant than DZP alone. The DZP combination with VPA HDAC inhibitor proved to be a good treatment for SE and neuronal damage caused by soman exposure.

Serum levels of neuron-specific ubiquitin carboxyl-terminal esterase-L1 predict brain injury in a canine model of hypothermic circulatory arrest

OBJECTIVES

Ubiquitin carboxyl-terminal esterase-L1 (UCHL1) is a protein highly selectively expressed in neurons and has been linked to neurodegenerative disease in humans. We hypothesize that UCHL1 would be an effective serum biomarker for brain injury as tested in canine models of hypothermic circulatory arrest (HCA) and cardiopulmonary bypass (CPB).

METHODS

Dogs were exposed to CPB (n = 14), 1 hour of HCA (1h-HCA; n = 11), or 2 hours of HCA (2h-HCA; n = 20). Cerebrospinal fluid and serum were collected at baseline, 8 hours, and 24 hours after treatment. UCHL1 levels were measured using a sandwich enzyme-linked immunosorbent assay. Neurologic function and histopathology were scored at 24 hours, and UCHL1 immunoreactivity was examined at 8 hours.

RESULTS

Baseline UCHL1 protein levels in cerebrospinal fluid and serum were similar for all groups. In serum, UCHL1 levels were elevated at 8 hours after treatment for 2h-HCA subjects compared with baseline values (P < .01) and also compared with CPB dogs at 8 hours (P < .01). A serum UCHL1 level above 3.9 ng/(mg total protein) at 8 hours had the best discriminatory power for predicting functional disability. In cerebrospinal fluid, UCHL1 was elevated in all groups at 8 hours after treatment compared with baseline (P < .01). However, UCHL1 levels in cerebrospinal fluid remained elevated at 24 hours only in 2h-HCA subjects (P < .01). Functional and histopathologic scores were closely correlated (Pearson coefficient, 0.66; P < .01) and were significantly worse in 2h-HCA animals.

CONCLUSIONS

This is the first report associating elevated serum UCHL1 with brain injury. The novel neuronal biomarker UCHL1 is increased in serum 8 hours after severe neurologic insult in 2h-HCA animals compared with CPB animals. These results support the potential for use in cardiac surgery patients and form the basis for clinical correlation in humans.

Hyperoxic reperfusion after global cerebral ischemia promotes inflammation and long-term hippocampal neuronal death

In this study we tested the hypothesis that long-term neuropathological outcome is worsened by hyperoxic compared to normoxic reperfusion in a rat global cerebral ischemia model. Adult male rats were anesthetized and subjected to bilateral carotid arterial occlusion plus bleeding hypotension for 10 min. The rats were randomized to one of four protocols: ischemia/normoxia (21% oxygen for 1 h), ischemia/hyperoxia (100% oxygen for 1 h), sham/normoxia, and sham/hyperoxia. Hippocampal CA1 neuronal survival and activation of microglia and astrocytes were measured in the hippocampi of the animals at 7 and 30 days post-ischemia. Morris water maze testing of memory was performed on days 23-30. Compared to normoxic reperfusion, hyperoxic ventilation resulted in a significant decrease in normal-appearing neurons at 7 and 30 days, and increased activation of microglia and astrocytes at 7, but not at 30, days of reperfusion. Behavioral deficits were also observed following hyperoxic, but not normoxic, reperfusion. We conclude that early post-ischemic hyperoxic reperfusion is followed by greater hippocampal neuronal death and cellular inflammatory reactions compared to normoxic reperfusion. The results of these long-term outcome studies, taken together with previously published results from short-term experiments performed with large animals, support the hypothesis that neurological outcome can be improved by avoiding hyperoxic resuscitation after global cerebral ischemia such as that which accompanies cardiac arrest.

Hawley H. Seiler Resident Award. Transcriptional profile of brain injury in hypothermic circulatory arrest and cardiopulmonary bypass

BACKGROUND

Little is known about the molecular mechanisms of neurologic complications after hypothermic circulatory arrest (HCA) with cardiopulmonary bypass (CPB). Canine genome sequencing allows profiling of genomic changes after HCA and CPB alone. We hypothesize that gene regulation will increase with increased severity of injury.

METHODS

Dogs underwent 2-hour HCA at 18 degrees C (n = 10), 1-hour HCA (n = 8), or 2-hour CPB at 32 degrees C alone (n = 8). In each group, half were sacrificed at 8 hours and half at 24 hours after treatment. After neurologic scoring, brains were harvested for genomic analysis. Hippocampal RNA isolates were analyzed using canine oligonucleotide expression arrays containing 42,028 probes.

RESULTS

Consistent with prior work, dogs that underwent 2-hour HCA experienced severe neurologic injury. One hour of HCA caused intermediate clinical damage. Cardiopulmonary bypass alone yielded normal clinical scores. Cardiopulmonary bypass, 1-hour HCA, and 2-hour HCA groups historically demonstrated increasing degrees of histopathologic damage (previously published). Exploratory analysis revealed differences in significantly regulated genes (false discovery rate < 10%, absolute fold change > or = 1.2), with increases in differential gene expression with injury severity. At 8 hours and 24 hours after insult, 2-hour HCA dogs had 502 and 1,057 genes regulated, respectively; 1-hour HCA dogs had 179 and 56 genes regulated; and CPB alone dogs had 5 and 0 genes regulated.

CONCLUSIONS

Our genomic profile of canine brains after HCA and CPB revealed 1-hour and 2-hour HCA induced markedly increased gene regulation, in contrast to the minimal effect of CPB alone. This adds to the body of neurologic literature supporting the safety of CPB alone and the minimal effect of CPB on a normal brain, while illuminating genomic results of both.

Alpha II-spectrin breakdown products serve as novel markers of brain injury severity in a canine model of hypothermic circulatory arrest

BACKGROUND

The development of specific biomarkers to aid in the diagnosis and prognosis of neuronal injury is of paramount importance in cardiac surgery. Alpha II-spectrin is a structural protein abundant in neurons of the central nervous system and cleaved into signature fragments by proteases involved in necrotic and apoptotic cell death. We measured cerebrospinal fluid alpha II-spectrin breakdown products (alphaII-SBDPs) in a canine model of hypothermic circulatory arrest (HCA) and cardiopulmonary bypass.

METHODS

Canine subjects were exposed to either 1 hour of HCA (n = 8; mean lowest tympanic temperature 18.0 +/- 1.2 degrees C) or standard cardiopulmonary bypass (n = 7). Cerebrospinal fluid samples were collected before treatment and 8 and 24 hours after treatment. Using polyacrylamide gel electrophoresis and immunoblotting, SBDPs were isolated and compared between groups using computer-assisted densitometric scanning. Necrotic versus apoptotic cell death was indexed by measuring calpain and caspase-3 cleaved alphaII-SBDPs (SBDP 145+150 and SBDP 120, respectively).

RESULTS

Animals undergoing HCA demonstrated mild patterns of histologic cellular injury and clinically detectable neurologic dysfunction. Calpain-produced alphaII-SBDPs (150 kDa+145 kDa bands-necrosis) 8 hours after HCA were significantly increased (p = 0.02) as compared with levels before HCA, and remained elevated at 24 hours after HCA. In contrast, caspase-3 alphaII-SBDP (120 kDa band-apoptosis) was not significantly increased. Animals receiving cardiopulmonary bypass did not demonstrate clinical or histologic evidence of injury, with no increases in necrotic or apoptotic cellular markers.

CONCLUSIONS

We report the use of alphaII-SBDPs as markers of neurologic injury after cardiac surgery. Our analysis demonstrates that calpain- and caspase-produced alphaII-SBDPs may be an important and novel marker of neurologic injury after HCA.

The many roles of histone deacetylases in development and physiology: implications for disease and therapy

Histone deacetylases (HDACs) are part of a vast family of enzymes that have crucial roles in numerous biological processes, largely through their repressive influence on transcription. The expression of many HDAC isoforms in eukaryotic cells raises questions about their possible specificity or redundancy, and whether they control global or specific programmes of gene expression. Recent analyses of HDAC knockout mice have revealed highly specific functions of individual HDACs in development and disease. Mutant mice lacking individual HDACs are a powerful tool for defining the functions of HDACs in vivo and the molecular targets of HDAC inhibitors in disease.

Valproic acid regulates catecholaminergic pathways by concentration-dependent threshold effects on TH mRNA synthesis and degradation

The spectrum of neurological conditions and psychiatric disorders affected by valproic acid (VPA) ranges from control of seizure and mood disorders to migraine, neuropathic pain, and even congenital malformations and autism. While widely used clinically, the mechanism(s) of action of VPA is not completely understood. Emerging evidence indicates that brain noradrenergic systems contribute to the symptoms of mood disorders and may involve regulation of tyrosine hydroxylase (TH) expression, the rate-limiting enzyme in the biosynthesis of dopamine, norepinephrine and epinephrine. We previously showed that the structurally related short chain fatty acid sodium butyrate (SB) induces TH transcription and alters TH mRNA stability in PC12 cells. The present study was undertaken to determine whether the branched short chain fatty acid VPA could also regulate TH gene expression in vitro. Similar to SB, VPA induced TH transcription at all concentrations tested. VPA-stimulated transcription was significantly attenuated by introducing point mutations in either the canonical cAMP- or in the butyrate-response elements of the TH promoter; or by co-expression of dominant-negative forms of CREB. As with SB, increasing concentrations of VPA demonstrated opposing effects on TH mRNA and protein abundance: elevation of both at low (0.1 mM) but attenuation at concentrations higher than 0.5 mM. This concentration-dependence is consistent with a novel and previously unrecognized cellular/molecular drug regulatory step at the level of TH mRNA stability. Thus, the therapeutic efficacy of VPA might be related to its ability to regulate TH mRNA and protein levels, and thereby central catecholaminergic-dependent behavioral pathways.