A Role for Human Herpesvirus in Alzheimer’s disease

What's the science?

Alzheimer’s disease has previously been associated with various bacteria and viruses — in particular herpes simplex virus. However, the mechanism by which viruses may contribute to Alzheimer’s disease is not clear. This week in Neuron, Readhead and colleagues used a neuropathological network model (at the gene, transcription, protein, and histopathology levels) to understand the contribution of viruses to Alzheimer’s. 

How did they do it?

The authors obtained data from brains (after death) of healthy individuals, those with ‘pre-clinical’ Alzheimer’s (i.e. early, visible pathology but no cognitive impairment at time of death), and those with later stage Alzheimer’s disease. They first used computational modelling (they created probabilistic causal networks) to understand the differences in gene expression networks between healthy individuals and those with pre-clinical Alzheimer’s disease. They focused analyses on the entorhinal cortex and hippocampus (two regions affected by the disease). From the pre-clinical and control groups, they found genes they referred to as ‘network drivers’ that regulated a large portion of the gene expression in the network.

They then evaluated viral activity (viral RNA and DNA sequences) in patients with clinical Alzheimer’s (four independent cohorts) versus healthy controls. They first performed RNA sequencing in tissue from the superior temporal gyrus, anterior prefrontal cortex, inferior frontal gyrus, and parahippocampal gyrus obtained from one of the four cohorts, and looked for the presence of genes associated with viruses known to infect the human transcriptome. They also performed whole-exome sequencing to assess viral DNA in the same regions. The relationship between Alzheimer’s traits (Clinical Dementia Rating, Amyloid Plaque Density) and elevated viral RNA and DNA levels was also examined.

What did they find?

When the authors assessed pre-clinical Alzheimer’s versus healthy control gene networks, they found that promoters (i.e. the region of the gene that turns on transcription) for gene network drivers lost or gained in pre-clinical Alzheimer’s were enriched for C2H2 zinc factor transcription factor binding motifs. The “lost in pre-clinical Alzheimer’s disease” drivers had more G-quadruplex motifs within their genes. There was also a negative relationship between the density of G-quadruplex (co-regulatory with C2H2 transcription factor) and the expression of these genes in the entorhinal cortex in the pre-clinical Alzheimer’s and Alzheimer’s disease samples. These types of changes have been previously associated with viral biology/viral infection, suggesting that viral activity is associated with Alzheimer’s. As a second line of evidence, they found overlap between identified gene network drivers and gene targets of human microRNAs that had been previously associated with innate immunity and DNA viral activity.

When the authors assessed viral abundance in the brains of patients with Alzheimer’s, they found increased viral species in the anterior prefrontal cortex and superior temporal gyrus; in particular, HHV-7 and HHV-6A (i.e. herpesviruses). These elevated levels were also found in other brain regions in two additional cohorts of patients, suggesting that these viruses are increased across different tissues. The same findings were not present in samples of pathological aging or progressive supranuclear palsy (another neurodegenerative disorder), suggesting they are specific to Alzheimer’s. Increased viral DNA for HHV-6A was also detected. An HHV-7 gene and HHV-6A region were associated with Alzheimer’s traits (dementia ratings & plaque density) and viral abundance mediated gene expression of genes involved in disease risk and beta-amyloid processing (which form plaques). They also identified that some host genes (in particular the MIR155 host gene) regulated by HHV-6A (a herpesvirus) could form a network associated with neuronal loss, indicating that HHV-6A may be implicated in neurodegeneration. Finally, the authors performed some follow-up analyses in mice and found that MIR155 knockout mice had larger cortical amyloid plaques. Upregulated genes in MIR155 knockout mice were similar to those upregulated by the HHV-6A virus, suggesting that HHV-6A could act by inhibiting MIR155.

What's the impact?

This study provides genetic, clinical, and neuropathological evidence that there may be viral and host factors that interact to contribute to Alzheimer’s pathology. Viruses could potentially disturb biological processes (e.g. leading to plaque formation) or alter transcription or regulatory mechanisms. The contribution of viral activity in Alzheimer’s should be further investigated.

Readhead et al., Multiscale Analysis of Independent Alzheimer’s Cohorts Finds Disruption of Molecular, Genetic, and Clinical Networks by Human Herpesvirus. Neuron (2018). Access the original scientific publication here.

Oscillations in Neural Activity Travel in Waves Through the Human Brain

What's the science?

Oscillations in neural activity (sometimes referred to as ‘brain waves’) are important for brain function, as they help to coordinate activity across the brain and help spatially separated brain regions to communicate. The brain oscillates at different frequencies, including ‘alpha’ and ‘theta’ frequencies. Animal studies have shown that brain oscillations can travel across the cortex in the form of a wave, however this has not been investigated in humans. Recently in Neuron, Zhang and colleagues examine whether oscillations in neural activity can travel across the human cortex in the form of a wave and if wave propagation correlates with behavior and cognition.

How did they do it?

Electrocorticography (ECoG) - which is the measurement of neural activity from electrodes placed on the brain’s surface - was performed on 77 patients undergoing brain surgery. Neural activity was recorded while participants performed a working memory task where participants tried to memorize a list of stimuli, followed by a retrieval cue where they recalled presented stimuli. The authors used a technique designed to test whether oscillations in the brain travel across the cortex. They did this by locating electrodes where neuronal oscillations were present at the same frequency simultaneously, and showed a timing (i.e. phase) gradient across space (i.e. the cortex). Neural activity was recorded while participants performed a working memory task. They used a clustering approach to identify clusters of spatially contiguous electrodes that showed the same frequency of oscillations. They then examined the timing of the activity across each cluster to look for patterns of phase synchrony to see whether the oscillations travelled in the form of a wave. They did this by calculating the phase of the oscillations at each electrode across space. Lastly, they tested whether travelling waves in the cortex were associated with behavior.

What did they find?

Most patients (96%) had ‘clusters’ of electrodes that showed the same frequency of oscillations (with a phase gradient) across space. They found a total of 208 clusters in the 77 patients. Clusters were in frequencies ranging from 2-15 Hz (alpha and theta). Clusters at given electrodes within one patient did not necessarily have the same frequency as the (spatially) identical clusters in another patient, suggesting that neuronal oscillations vary from individual to individual. They found that frequencies within patients showed a strong spatial correlation. They also found that many of the clusters had oscillation cycles that varied systematically with the electrode location within the cluster, indicating a traveling wave. They used a circular-linear model to examine the relationship between electrode location and phase of the wave to demonstrate the direction and the robustness of the travelling wave. 140 of the clusters (67%) showed consistent travelling waves and a consistent propagation direction across multiple trials. The clusters with travelling waves were found across all lobes of the cortex. The direction of travelling waves was most consistent in the frontal and temporal lobes, and most waves demonstrated a posterior-to-anterior directionality in these regions. Direction was more varied in the parietal and occipital lobes. They authors tested whether these travelling waves were related to the working memory task and found that directional consistency (how consistently the wave propagated in one direction) was higher in the frontal and temporal lobes after the retrieval cue onset (where they were required to recall previously shown stimuli) in the working memory task. Directional consistency was also positively correlated with performance, suggesting that waves travelling in a consistent direction are associated with better working memory efficiency.

Brain, Servier Medical Art, image by BrainPost, CC BY-SA 3.0

What's the impact?

This is the first study to show that oscillations in neural activity at particular frequencies travel across the human cortex in waves. The consistency of propagation of these waves was related to an individual’s working memory. Understanding how neural activity is coordinated and associated with behavioral factors like working memory is crucial for understanding how the brain works. Future research will be need to further understand the importance of travelling waves in the brain.

Zhang et al., Theta and Alpha Oscillations Are Traveling Waves in the Human Neocortex. Neuron (2018). Access the original scientific publication here.

Stress Hormones Sensitize Fear Circuits in the Brain

What's the science?

Fear and stress can induce the ‘fight-or-flight’ active response or a passive ‘freezing’ response. Associative memories can form when a cue is associated with an aversive (i.e. negative) experience, and these cues often trigger a freezing (i.e. fear) response later on. Dysfunction of the brain circuitry involved in this response can lead to disorders such as post-traumatic-stress disorder and generalized anxiety disorder. We know that thalamus-amygdala circuitry and hormones released within the amygdala are involved in these disorders, however, we still don’t understand how it underlies ‘freezing’ behavior. This week in Molecular Psychiatry, Pliota and colleagues test whether thalamus-amygdala circuitry and hormones within the amygdala contribute to freezing behavior after stress.

How did they do it?

Part1 -- Mice underwent two elevated maze exploration task trials (a task typically used to assess anxiety behavior in mice including ‘fight or flight’ vs. freezing responses) separated by ten unpredictable foot shocks to induce fear memory. Passive freezing responses generally act as a coping mechanism when dealing with unpredictable stressful stimuli. The authors used an early gene approach (an approach that measures early gene responses to stimuli) to assess which brain regions were involved in the anxiety response after foot shock. They then performed calcium imaging (to measure neuronal activity) during unpredictable foot shock, and during a maze exploration task that followed. This imaging technique determined which brain regions were activated by these two paradigms.

Part 2 -- They then performed a series of experiments including optogenetics, electrophysiology & chemogenetics to test how the stress circuitry found underlies anxiety behavior (active vs. passive).

What did they find?

Part1 -- Mice exposed to the unpredictable foot shock later demonstrated less exploration in the maze task, indicating greater passive ‘freezing’ responses compared to control mice. They found that the periaqueductal grey area and the periventricular thalamus (previously implicated in anxiety) were brain regions recruited after foot shock. Using Calcium imaging, they found that the thalamus was 1) activated in response to foot shock, and 2) more active during the maze exploration following the unpredictable foot shock (as opposed to no previous foot shock).

Part 2 -- They used a retrograde tracer to show that thalamus projections to the amygdala were more active during the maze exploration after foot shock. They then used optogenetics to activate the thalamus-amygdala circuit and found that in later maze exploration, behavior mimicked that after foot shock, whereas deactivation of this circuit counteracted the passive behaviors seen in the maze exploration after foot shock. These results suggest that the thalamus-amygdala circuit specifically is reinforced during stress and leads to future ‘freezing behavior’. They used electrophysiology to show that activation of neurons in thalamus selectively activates neurons in the amygdala that express corticotropin releasing hormone (a stress hormone). Using designer drugs (genetically engineered receptors) to activate neurons that release corticotropin releasing hormone increased passive behaviors in the maze task. Lastly, they used microdialysis to show that corticotropin releasing hormone is actually released in the amygdala after foot shock. They blocked this hormone by injecting an antagonist to block the receptors on neurons, and found that the passive behaviors were reduced, suggesting that this hormone (typically released from the amygdala) mediates the passive freezing response to foot shock.

What's the impact?

This is the first study to show a specific thalamus-amygdala pathway and the release of corticotropin releasing hormone mediates passive anxiety behaviors. We now have a better understanding of the brain circuitry involved in the 'freezing' response to stressful events. These findings have important implications for understanding the dysfunctional circuitry in disorders like post-traumatic stress disorder and generalized anxiety disorder.

Pilota et al., Stress peptides sensitize fear circuitry to promote passive coping. Molecular Psychiatry (2018). Access the original scientific publication here.