Study Suggests Disruptive Effects of Anesthesia on Brain Cell Connections Are Temporary

A study of juvenile rat brain cells suggests that the effects of a commonly used anesthetic drug on the connections between brain cells are temporary.

The study, published in this week’s issue of the journal PLOS ONE, was conducted by biologists at the University of California, San Diego and Weill Cornell Medical College in New York in response to concerns, arising from multiple studies on humans over the past decade, that exposing children to general anesthetics may increase their susceptibility to long-term cognitive and behavioral deficits, such as learning disabilities.

An estimated six million children, including 1.5 million infants, undergo surgery in the United States requiring general anesthesia each year and a least two large-scale clinical studies are now underway to determine the potential risks to children and adults.

“Since these procedures are unavoidable in most cases, it’s important to understand the mechanisms associated with the potentially toxic effects of anesthetics on the developing brain, and on the adult brain as well,” said Shelley Halpain, a professor of biology at UC San Diego and the Sanford Consortium for Regenerative Medicine, who co-headed the investigation. “Because the clinical studies haven’t been completed, preclinical studies, such as ours, are needed to define the effects of various anesthetics on brain structure and function.”

“There is concern now about cognitive dysfunction from surgery and anesthesia—how much these effects are either permanent or slowly reversible is very controversial,” said Hugh Hemmings, Jr., chair of anesthesiology at Weill Cornell and the study’s other senior author. “It has been suggested recently that some of the effects of anesthesia may be more lasting than previously thought. It is not clear whether the residual effects after an operation are due to the surgery itself, or the hospitalization and attendant trauma, medications and stress—or a combination of these issues.”

However, he added, “There is evidence that some of the delayed or persistent cognitive effects after surgery are not primarily due to anesthesia itself, but more importantly to brain inflammation resulting from the surgery. But this is not yet clear.”

The team of biologists examined one of the most commonly used general anesthetics, a derivative of ether called “isoflurane” used to maintain anesthesia during surgery.

“Previous studies in cultured neurons and in the intact brains of rodents provided evidence suggesting that exposure to anesthetics might render neurons more susceptible to cell death through a process called ‘apoptosis’,” said Halpain. “While overt cell death could certainly be one way to explain any long-lasting neurocognitive consequences of general anesthesia, we hypothesized that there could be other cellular mechanisms that disrupt neural circuits without inducing cell death per se.”

One such mechanism, she added, is known as “synaptotoxicity.” In this mechanism of neural-circuit disruption, the “synapses,” or junctions between neurons, become weakened or shrink away due to some factor that injures the neurons locally along their axons (the long processes of neurons that transmit signals) and dendrites (the threadlike extensions of neurons that receive nerve signals) without inducing the neurons themselves to die.

In the experiments at UC San Diego headed by Jimcy Platholi, a postdoctoral researcher in Halpain’s lab who is now at Weill Cornell, the scientists used neurons from embryonic rats taken from the hippocampus, a part of the mammalian forebrain essential for encoding newly acquired memories and ensuring that short-term memories are converted into long-term memories. The researchers cultured these brain cells in a laboratory dish for three weeks, allowing the neurons time to mature and to develop a dense network of synaptic connections and “dendritic spines”—specialized structures that protrude from the dendrites and are essential mediators of activity throughout neural networks.

“Evidence from animal studies indicates that new dendritic spines emerge and existing spines expand in size during learning and memory,” explained Halpain. “Therefore, the overall numbers and size of dendritic spines can profoundly impact the strength of neural networks. Since neural network activity underlies all brain function, changes in dendritic spine number and shape can influence cognition and behavior.”

Using neurons in culture, rather than intact animal brains, allowed the biologists to take images of the synapses at high spatial resolution using techniques called fluorescence light microscopy and confocal imaging. They also used time-lapse microscopy to observe structural changes in individual dendritic spines during exposure to isoflurane. Karl Herold, a research associate in the Hemmings laboratory and a co-author of the study, performed some of the image analysis.

“Imaging of human brain synapses at this level of detail is impossible with today’s technology and it remains very challenging even in laboratory rodents,” said Halpain. “It was important that we performed our study using rodent neurons in a culture dish, so that we could really drill down into the subcellular and molecular details of how anesthetics work.”

The researchers wondered whether brief exposure to isoflurane would alter the numbers and size of dendritic spines, so they applied the anesthetic to the cultured rat cells at concentrations and durations (up to 60 minutes) that are frequently used during surgery.

“We observed detectable decreases in dendritic spine numbers and shape within as little as 10 minutes,” said Halpain. “However this spine loss and shrinkage was reversible after the anesthetic was washed out of the culture.”

“Our study was reassuring in the sense that the effects are not irreversible and this fits in with known clinical effects,” said Hemmings. “For the most part, we find that the effects are reversible.”

“We clearly see an effect—a very marked effect on the dendritic spines—from use of this drug that was reversible, suggesting that it is not a toxic effect, but something more relevant to the pharmacological actions of the drug,” he added. “Connecting what we found to the cognitive effects of isoflurane will require much more detailed analysis.”

The team plans to follow up its study with future experiments to probe the molecular mechanisms and long-lasting consequences of isoflurane’s effects on neuron synapses and examine other commonly-used anesthetics for surgery.

(Image caption: In brain cancer cells, the protein PARC plays a key role in long-term cell survival. In both images, the red represents the protein cytochrome c, which is released when mitochondria are damaged and trigger apoptosis – cell suicide. At left, injured brain cancer cells exhibit little cytochrome c; they use the protein PARC to degrade the released cytochrome c, allowing the cancer cells to survive. At right, when researchers reduced PARC, cytochrome c accumulated, allowing apoptosis to carry on)

Neurons, brain cancer cells require the same little-known protein for long-term survival

Researchers at the UNC School of Medicine have discovered that the protein PARC/CUL9 helps neurons and brain cancer cells override the biochemical mechanisms that lead to cell death in most other cells. In neurons, long-term survival allows for proper brain function as we age. In brain cancer cells, though, long-term survival contributes to tumor growth and the spread of the disease.

These results, published in the journal Science Signaling, not only identify a previously unknown mechanism used by neurons for their much-needed survival, but show that brain cancer cells hijack the same mechanism for their own survival.

The discovery will lead to new investigations of brain cancer treatments and provides insight into Parkinson’s disease, including a potential new research tool for scientists.

“PARC is very similar to Parkin, a protein that’s mutated in Parkinson’s disease,” said Mohanish Deshmukh, PhD, a professor of cell biology and physiology and senior author of the Science Signaling paper. “We think they might work in tandem to protect neurons.”

If so, researchers can investigate the interplay between these proteins to create better drugs to treat the second-most prevalent neurodegenerative disease after Alzheimer’s disease.

Vivian Gama, PhD, a postdoctoral fellow in Deshmukh’s lab, led the experiments in cell cultures and animal models. First, she used external stimuli to promote the damage of mitochondria – the energy sources for cells. In most cell types, when mitochondria are damaged, they release a protein called cytochrome c, which triggers a cascade of biochemical steps that end in cell death – a process known as apoptosis.

Working with neurons, though, Gama found that the protein PARC/CUL9 blocked this process; it degraded cytochrome c, halted apoptosis, and allowed for long-term cell survival. “In this setting, we want PARC to do that because we want neurons to survive as long as possible,” said Gama, first author of the Science Signaling paper.

Deshmukh, a member of the UNC Neuroscience Center and the UNC Lineberger Comprehensive Cancer Center, said, “In Parkinson’s disease, we know that Parkin targets damaged mitochondria for degradation. However, exactly what happens to the proteins, such as cytochrome c, that are released from the damaged mitochondria has been unknown. Now, we think PARC plays a role in this process.”

Deshmukh and Gama’s work could lead to an alternative way to study Parkinson’s disease. Other researchers have created mouse models that lack the Parkin gene, but Gama said these models don’t have many of the hallmark symptoms that human patients have, making the model less than desirable for researchers. “Our hypothesis is that in the absence of Parkin, PARC still does the job,” Gama said, “as it may allow cells to survive.”

Gama and Deshmukh are now creating a model that lacks both the Parkin and PARC genes.

They will also investigate PARC as a target for cancer treatment.

“We tested several cancer cell lines and found that PARC degrades cytochrome c in medulloblastoma, a cancer of the central nervous system and in neuroblastoma, a cancer of the peripheral nervous system,” Gama said. “Not all cytochrome c is degraded; there are likely other factors involved. But PARC is an important player.”

When Gama and colleagues triggered the apoptotic process in brain cancer cells, they found that PARC allowed the cells to survive. When PARC was inhibited, the cells were more vulnerable to stress and damage, which means they could be more vulnerable to compounds aimed at destroying them.

Deshmukh said, “We show that brain cancer cells co-opt PARC to bypass apoptosis in the same way that neurons do and for the exact same purpose.”

Researchers identify how zinc regulates a key enzyme involved in cell death

Findings may help develop targeted drug interventions and fight cancer and neurodegenerative diseases

The molecular details of how zinc, an essential trace element of human metabolism, interacts with the enzyme caspase-3, which is central to apoptosis or cell death, have been elucidated in a new study led by researchers at Virginia Commonwealth University. The study is featured on the cover of the April issue of the journal Angewandte Chemie’s International Edition.

Dysregulation of apoptosis is implicated in cancer and neurodegenerative disease such as Alzheimer’s disease. Zinc is known to affect the process by inhibiting the activity of caspases, which are important drug targets for the treatment of the above conditions. The findings may help researchers design therapeutic agents that target zinc-caspase interaction to specifically control the activity of caspases, and hence, apoptosis.

“The work is unique in helping to open up a broad new area of research which we call the bioinorganic chemistry of apoptosis – understanding the role of essential metal ions in one of life’s fundamental processes,” said corresponding author Nicholas P. Farrell, Ph.D., member of the Developmental Therapeutics program at VCU Massey Cancer Center and professor of chemistry in the VCU College of Humanities and Sciences.

“Indeed, the zinc inhibition of apoptosis in fact contrasts with the role of its closely related neighbor copper, which is understood to enhance apoptosis,” he said.

In the study, Farrell and his research team, A. Gerard Daniel, Ph.D., and Erica J. Peterson, used conventional enzymology and biophysical techniques combined with state-of-the-art computational methods, to show evidence for a hitherto unrecognized interaction site with caspase-3.

According to Farrell, caspases were discovered in the mid-1990s. There are 11 caspases known humans, and seven of these are involved in cell death. The study suggests a regulatory zinc site that may be common to all caspases. Previous findings have shown other zinc binding sites in caspase-6 and -9. Now, Farrell said, the generality of the team’s observations must be extended and verified in other caspases.

“The [journal] cover epitomizes the contrasting but interdependent roles of the metal ions copper/zinc in the regulation of apoptosis and perfectly captures the duality of this most fundamental of biological processes,” Farrell said.

Study Expands Concerns About Anesthesia’s Impact on the Brain

As pediatric specialists become increasingly aware that surgical anesthesia may have lasting effects on the developing brains of young children, new research suggests the threat may also apply to adult brains.

Researchers from Cincinnati Children’s Hospital Medical Center report June 5 the Annals of Neurology that testing in laboratory mice shows anesthesia’s neurotoxic effects depend on the age of brain neurons – not the age of the animal undergoing anesthesia, as once thought.

Although more research is needed to confirm the study’s relevance to humans, the study suggests possible health implications for millions of children and adults who undergo surgical anesthesia annually, according to Andreas Loepke, MD, PhD, a physician and researcher in the Department of Anesthesiology.

“We demonstrate that anesthesia-induced cell death in neurons is not limited to the immature brain, as previously believed,” said Loepke. “Instead, vulnerability seems to target neurons of a certain age and maturational stage. This finding brings us a step closer to understanding the phenomenon’s underlying mechanism”.

New neurons are generated abundantly in most regions of the very young brain, explaining why previous research has focused on that developmental stage. In a mature brain, neuron formation slows considerably, but extends into later life in dentate gyrus and olfactory bulb.

The dentate gyrus, which helps control learning and memory, is the region Loepke and his research colleagues paid particular attention to in their study. Also collaborating were researchers from the University of Cincinnati College of Medicine and the Children’s Hospital of Fudan University, Shanghai, China.

Researchers exposed newborn, juvenile and young adult mice to a widely used anesthetic called isoflurane in doses approximating those used in surgical practice. Newborn mice exhibited widespread neuronal loss in forebrain structures – confirming previous research – with no significant impact on the dentate gyrus. However, the effect in juvenile mice was reversed, with minimal neuronal impact in the forebrain regions and significant cell death in the dentate gyrus.

The team then performed extensive studies to discover that age and maturational stage of the affected neurons were the defining characteristics for vulnerability to anesthesia-induced neuronal cell death. The researchers observed similar results in young adult mice as well.

Research over the past 10 years has made it increasingly clear that commonly used anesthetics increase brain cell death in developing animals, raising concerns from the Food and Drug Administration, clinicians, neuroscientists and the public. As well, several follow-up studies in children and adults who have undergone surgical anesthesia show a link to learning and memory impairment.

Cautioning against immediate application of the current study’s findings to children and adults undergoing anesthesia, Loepke said his research team is trying to learn enough about anesthesia’s impact on brain chemistry to develop protective therapeutic strategies, in case they are needed. To this end, their next step is to identify specific molecular processes triggered by anesthesia that lead to brain cell death.

“Surgery is often vital to save lives or maintain quality of life and usually cannot be performed without general anesthesia,” Loepke said. “Physicians should carefully discuss with patients, parents and caretakers the risks and benefits of procedures requiring anesthetics, as well as the known risks of not treating certain conditions.”

Loepke is also collaborating with researchers from the Pediatric Neuroimaging Research Consortium at Cincinnati Children’s Hospital Medical Center to examine anesthesia’s impact on children’s brain using non-invasive magnetic resonance imaging (MRI) technology.

The Secret Lives (and Deaths) of Neurons

As the human body fine-tunes its neurological wiring, nerve cells often must fix a faulty connection by amputating an axon — the “business end” of the neuron that sends electrical impulses to tissues or other neurons. It is a dance with death, however, because the molecular poison the neuron deploys to sever an axon could, if uncontained, kill the entire cell.

Researchers from the University of North Carolina School of Medicine have uncovered some surprising insights about the process of axon amputation, or “pruning,” in a study published May 21 in the journal Nature Communications. Axon pruning has mystified scientists curious to know how a neuron can unleash a self-destruct mechanism within its axon, but keep it from spreading to the rest of the cell. The researchers’ findings could offer clues about the processes underlying some neurological disorders.

“Aberrant axon pruning is thought to underlie some of the causes for neurodevelopmental disorders, such as schizophrenia and autism,” said Mohanish Deshmukh, PhD, professor of cell biology and physiology at UNC and the study’s senior author. “This study sheds light on some of the mechanisms by which neurons are able to regulate axon pruning.”

Axon pruning is part of normal development and plays a key role in learning and memory. Another important process, apoptosis — the purposeful death of an entire cell — is also crucial because it allows the body to cull broken or incorrectly placed neurons. But both processes have been linked with disease when improperly regulated.

The research team placed mouse neurons in special devices called microfluidic chambers that allowed the researchers to independently manipulate the environments surrounding the axon and cell body to induce axon pruning or apoptosis.

They found that although the nerve cell uses the same poison — a group of molecules known as Caspases — whether it intends to kill the whole cell or just the axon, it deploys the Caspases in a different way depending on the context.

“People had assumed that the mechanism was the same regardless of whether the context was axon pruning or apoptosis, but we found that it’s actually quite distinct,” said Deshmukh. “The neuron essentially uses the same components for both cases, but tweaks them in a very elegant way so the neuron knows whether it needs to undergo apoptosis or axon pruning.”

In apoptosis, the neuron deploys the deadly Caspases using an activator known as Apaf-1. In the case of axon pruning, Apaf-1 was simply not involved, despite the presence of Caspases. “This is really going to take the field by surprise,” said Deshmukh. “There’s very little precedent of Caspases being activated without Apaf-1. We just didn’t know they could be activated through a different mechanism.”

In addition, the team discovered that neurons employ other molecules as safety brakes to keep the “kill” signal contained to the axon alone. “Having this brake keeps that signal from spreading to the rest of the body,” said Deshmukh. “Remarkably, just removing one brake makes the neurons more vulnerable.”

Deshmukh said the findings offer a glimpse into how nerve cells reconfigure themselves during development and beyond. Enhancing our understanding of these basic processes could help illuminate what has gone wrong in the case of some neurological disorders.