How Experiencing a Negative Event Alters Responses to Others

Post by Shahin Khodaei

The takeaway

Previous negative experiences affect how mice respond to other mice in a similar state. This behavior is mediated by the corticotropin-releasing factor (CRF) system in a region of the brain called the medial prefrontal cortex.

What's the science?

As humans, we recognize the signs that another person has experienced a stressful event and respond to them, for example showing consoling or prosocial behaviors. Similar to humans, the previous experiences of a mouse can affect how it responds to another stressed mouse. These different responses may be regulated by the corticotropin-releasing factor (CRF) system in the brain, specifically in a region called the medial prefrontal cortex which is important in emotion and socialization. This week in Nature Neuroscience, Martese and colleagues investigated 1) the ways in which mice with different experiences react to a stressed mouse, and 2) the role of the CRF system in these responses. 

How did they do it?

The authors used mice as either observers or demonstrators. Observer mice were placed in a box and presented with two demonstrator mice – one that was given a stressful experience right before the test (either being restrained for 15 minutes, or getting a shock to their feet), and one that was unstressed. The authors looked at how the observer interacted with the demonstrators. In some experiments, the observer was subjected to a stressful “negative self-experience” (NSE) one day before the test. The authors compared how much the observer interacted with each of the demonstrators. 

To study whether CRF neurons in the medial prefrontal cortex played a role in the behavior of the observers, the authors used genetic techniques to add certain genes to these brain cells. These genes either 1) decreased the level of CRF, 2) caused the cells to emit light when they were activated, which could then be measured using a microscope, or 3) allowed the authors to decrease the activity of the cells using optogenetics.

What did they find?

When a naïve observer was placed in front of one naïve and one restraint-stressed demonstrator, the observer spent more time sniffing and interacting with the stressed demonstrator. In contrast, if observer mice had experienced a NSE of restraint, they spent less time with the restraint-stressed demonstrator (and sometimes completely avoided them). This change in response was experience-specific. If the type of stressful experience did not match between the observer and demonstrator (i.e., one was footshock-stressed and the other was restraint-stressed), the observer with NSE behaved just like a naïve observer, and spent more time with the stressed demonstrator. This suggests mice were responding uniquely to other mice experiencing similar negative events. For male mice, the behavior of the observer with NSE depended on the social status of the mouse: dominant mice avoided the stressed demonstrator, and non-dominant mice showed no preference between the stressed and naïve demonstrator. For female mice, the behavior of the observer with NSE depended on the phase of the estrus cycle (the mouse equivalent of the menstrual cycle). 

The authors focused on the CRF system in the medial prefrontal cortex. They showed that reducing the level of CRF in these neurons made observers with NSE behave more similarly to naïve observers. The authors then assessed how the activity of these neurons changed in observers. They found that in naïve observers, the activity of CRF neurons increased when they were near the un-stressed demonstrator. In contrast, for observers with NSE, the activity of these neurons increased near the stressed demonstrator. If they reduced this CRF neural activity using optogenetics, naïve observers started to avoid stressed demonstrators (i.e., behave more like NSE observers) and NSE observers preferred stressed demonstrators (i.e., behave more like naïve observers). These results show that the CRF system is involved in how previous experiences influence the way mice approach stressed animals. 

What's the impact?

This study sheds light on the mechanisms involved in how previous experiences influence behavioral responses to stress in others. Importantly, this research reveals a neurobiological mechanism involving the CRF system for how interactions with others may differ, based on previous negative experiences.

Access the original scientific publication here.

“Starter Cells” Mediate Stress-Induced Depression-Like Behaviors

Post by Amanda Engstrom 

The takeaway

In this study, researchers identified a small population of stress-responsive neurons, termed “starter cells”, in subregions of the hypothalamus-habenula circuit. These neurons are required for the development of depression-like behaviors in mice. 

What's the science?

Chronic stress has long been considered a major risk factor for depression. Identifying specific neurons that respond to stress could offer insights into anti-depressant interventions. Experience-responsive neuronal ensembles are small populations of neurons that are functionally involved in distinct experiences. The lateral habenula (LHb) and the lateral hypothalamus (LH), which projects into the LHb, have been implicated in encoding aversion and depression-like behaviors during chronic stress. However, the core stress-responsive neurons within this circuit have not been identified. This week in Neuron, Zheng and colleagues identified a small population of neurons within the LH-LHb circuit, investigating their role in mediating depression-like behaviors due to stress. 

How did they do it?

The authors combined c-fos immunostaining (a marker for neuronal activity) and a Robust Activity Marking (RAM) system to identify the functional neurons within the LH-LHb circuit that respond to stress. RAM is a viral strategy to capture and label experience-responsive neurons in mice. Using this approach, the mice were exposed to various types of stress, such as restraint stress, and the authors determined which cells were activated in response to stress. They then used a chemogenetic strategy where they expressed a compound to inhibit the identified neurons while the mice were exposed to stress. This enabled them to determine the effect on depression-like behaviors when these neurons were nonfunctional. In a complementary experiment, the authors used photostimulation, which uses light to activate these same cells to mimic the activity of the neurons in the absence of a stressor and determine if the mice would display depressive-like behaviors. To characterize the synaptic connections from the LH stress-responsive neurons to LHb subpopulations the authors used optogenetics - assisted electrophysiology, a technique that uses optical and genetic manipulation to control neuronal activity while simultaneously recording electrical activity. Finally, they visualized the process of neuronal recruitment by labeling stress-responsive LHb neurons via RAM at different stages of chronic stress. 

What did they find?

After exposing mice to stress the authors identified a small population of neurons in the LHb and the LH (10% and 5% respectively) that were activated and labeled via the RAM system as well as c-fos positive. In both the LH and the LHb the active cells localized to subregions, specifically the middle part of the lateral hypothalamus (mLH) and the medial part of the lateral habenula (LHbM). Chemogenetic inhibition of the stress-responsive mLH or the LHbM neurons during chronic stress stopped the development of depression-like behaviors in mice. Additionally, activating these neurons via photostimulation is sufficient to cause depression-like behaviors. These data suggest that both regions are required for the development of depression-like behaviors caused by chronic stress. Using optogenetics-assisted electrophysiology, the authors showed that the stress-responsive neurons in the mLH and LHbM formed dominant excitatory connections after one exposure to stress, and they are selectively potentiated during chronic stress. The authors termed these cells “starter cells”. LHbM starter cells propagate hyperactivity across the entire LHb via local excitatory connections. Finally, the authors visualized the progressive recruitment of LHb neurons over time due to chronic stress. Initially, very little fluorescence signified very few stress-induced neurons, but they gradually increased over time. This indicates that initially stress-irresponsive LHb neurons were gradually becoming responsive, and the intensity of this response increased over time. The LHb stress-responsive neurons were initially limited to the LHbM and gradually spread throughout the entire LHb. 

What's the impact?

This study identified a small population of stress-responsive neurons in subregions of the lateral hypothalamus (LH) and LH lateral habenula (LHb) that are critical for stress-induced depression-like behavior in mice. The authors identified and characterized a novel core functional unit within the larger LH-LHb circuit, further highlighting the importance of ensemble sparsity (a small number of functional cells within a larger ensemble) in various brain functions. Moreover, by identifying stress-responsive neurons, these data offer new insights into potential targets for antidepressant therapies.

Access the original scientific publication here. 

How Vesicles Shuttle Tau Filaments Throughout the Brain in Alzheimer's Disease

Post by Lila Metko

The takeaway

Many cell types, including neurons, release compartments called extracellular vesicles (EVs) for signaling and transportation. The cell’s transportation of tau, a protein that misfolds and propagates in Alzheimer’s disease, is carried out through these EVs. This research reveals that impairments in the function of lysosomes, organelles involved in breaking down cellular waste, may be involved in the association of tau with EVs and shows that tau filaments in EVs are short and tethered to their membranes. 

What's the science?

Tau is a protein that maintains the structural integrity of neurons in healthy individuals but becomes hyperphosphorylated, misfolded and aggregated in the brains of people with Alzheimer’s disease. Tau has a greater ability to seed new misfolded proteins when associated with EVs. It is unknown which types of tau associate with EVs, which EVs contain tau, and how tau associates with EVs. This November in Nature Neuroscience, Fowler and colleagues investigate how EVs shuttle tau throughout the brain. 

How did they do it?

The authors analyzed frontal and temporal lobe tissue that had been obtained from Alzheimer’s disease patients post-mortem. They separated the tissue using density gradient centrifugation, a type of analysis that separates molecules by density. This allows for the different subtypes of EVs to be separated into different fractions. The fractions were then analyzed by liquid chromatography-tandem mass spectrometry and immunoblotting to determine the protein content of the EV types. Additionally, they confirmed the ability of the EVs to seed further tau aggregation using cell culture and a transgenic mouse line, expressing human tau. Cryo-electron microscopy and Cryo-electron tomography were used to analyze the structure of the tau filaments found in the EVs. 

What did they find?

The authors found that only fractions 4-6, fractions with medium to high density, contained EVs with tau filaments and that these fractions also had the highest amount of lysosomal proteins. Further analysis showed that there were two types of tau filaments within the EVs, one with a symmetrical organization of its subcomponents and another with a non-symmetrical organization that was shorter than those found in neurofibrillary tangles (aggregated tau protein deposits seen in Alzheimer’s disease). Shorter tau filaments have a greater ability to seed tau assembly in animal models. The authors also found that tau filaments within EVs were either tethered to the EV membrane or tethered to a tau filament that was connected to the membrane. Additionally, these filaments were all tethered at their ends. This gives researchers insight into the tethering process of tau filaments to EVs and could potentially inform therapeutic interventions. 

What's the impact?

This research provides further insight into how tau filaments are transported out of the cell through EVs and propagate through the brain in Alzheimer’s disease. These findings can help us to develop therapeutics that target tau propagation. 

Access the original scientific publication here.