Microglia Ingest Myelin Sheaths During Development

Post by D. Chloe Chung

What's the science?

Myelin is an important structure in the brain that surrounds the neuronal axons and helps neurons efficiently transmit electrical signals. During the development of the central nervous system, oligodendrocytes initially produce myelin in excess which eventually gets trimmed down to an appropriate amount. While previous studies have shown that oligodendrocytes can “retract” their myelinating membranes, it is unclear whether other types of cells are also involved in removing excessive myelin sheaths. This week in Nature Neuroscience, Hughes and Appel used zebrafish models to demonstrate that microglia, a type of immune cell in the central nervous system, can actively ingest or phagocytose new myelin sheaths during normal development.

How did they do it?

The authors created a transgenic zebrafish line in which microglia and myelinating oligodendrocytes were labeled with fluorescent proteins of different colors. A few days after these zebrafish were fertilized, the authors used time-lapse imaging to monitor the interaction between microglia and oligodendrocytes during the developmental stage. They also made another zebrafish line that allows visualization of phagocytic activity of microglia via calcium imaging. To understand the dynamics among neuronal activity, microglia phagocytosis, and developmental myelin elimination, the authors genetically manipulated their zebrafish model to silence or activate a subgroup of neurons. As an additional approach, zebrafish were forced to swim against strong water currents so that their neuronal activity could be continuously increased. To evaluate the direct role of microglia in the elimination of new myelin sheaths, the authors tested multiple different methods to remove microglia in zebrafish and observed the subsequent changes in developmental myelination.

What did they find?

During live imaging of young transgenic zebrafish, the authors observed that during development, microglia actively survey dynamic myelination along axon tracts in the spinal cord. In fact, unlike previous beliefs, the authors observed that myelin sheaths that were eliminated were mostly being contacted by microglia rather than being retracted by oligodendrocytes (not associated with microglial processes). Interestingly, phagocytic microglia were found to be specifically engulfing myelin sheaths but not oligodendrocytes, leaving the majority of oligodendrocyte cell bodies intact. The authors also found an interesting link between neuronal activity and microglia-mediated removal of myelin. Upon inhibition of neuronal activity, microglia less frequently contacted the neuronal cell body while engulfing more myelin. On the contrary, when neuronal activity was increased, microglia interacted more with neuronal cell bodies and substantially decreased their phagocytic activity for myelin sheaths. These results suggest that neuronal activity can regulate the amount of phagocytosis of myelin sheaths. Importantly, excessive new myelin sheaths failed to be removed in the absence of phagocytic microglia, which further highlights the essential role of microglia in modulating myelination during development.

What’s the impact?

This study is the first one to show that phagocytic microglia are active participants in removing excessive myelin sheaths during development. These findings importantly contribute to an increasing body of literature that supports the importance of microglia not only in the context of injury or disease but also in the normal development of the central nervous system. It will be interesting for future studies to investigate specific molecular cues that can trigger microglia to engulf new myelin sheaths.

Hughes & Appel. Microglia phagocytose myelin sheaths to modify developmental myelination. Nature Neuroscience (2020). Access the original scientific publication here.

Blood Factors Can Transfer the Benefits of Exercise to the Aged Brain

Post by Elisa Guma

What's the science?

Exercise has been shown to be a promising intervention to mitigate age-related cognitive decline, and vulnerability to age-related neurodegenerative diseases, however it is not accessible to all elderly individuals due to poor health or physical frailty. Animal studies have shown that exercise can reverse age related decline in adult neurogenesis (the formation of new neurons) in the hippocampus and improve cognitive function. Interestingly, the transfer of blood from young mice to old has also been found to improve regeneration and cognitive function. This week in Science, Horowitz, Fan and colleagues tested the effects of systemic plasma administration derived from mice that exercise on regenerative and cognitive function in the aged brain.

How did they do it?

Blood was collected and plasma was isolated from aged 18-month-old mice who had exercised (i.e. had access to a running wheel for 6 weeks) and aged sedentary mice. The isolated plasma was injected into a separate cohort of aged mice. Following administration, hippocampal-dependent learning and memory were tested (using the radial arm water maze and contextual fear conditioning) and immunohistochemistry was performed to look for evidence of neurogenesis in the hippocampus. The authors also investigated whether the benefits of exercise observed in mice at younger ages could also be transferred to the aged mice through circulating blood factors.

Next, the authors measured soluble protein levels in the plasma of exercised and sedentary mice to try to understand what might be driving the beneficial effects of exercise. Once their protein of interest was identified, they characterized its expression levels in various organs of the body, including the liver, lung, fat, spleen, skin kidney, heart, muscle, cortex, hippocampus, and cerebellum. They then overexpressed the peptide in the liver (using hydrodynamic tail vein injection) and tested its ability to improve neurogenesis and cognitive performance in aged mice. In order to gain further mechanistic insight, the authors investigated the ability of the peptide to cross the blood-brain barrier, as well as the importance of enzymatic activity associated with the peptide in effects on cognition and neurogenesis.

What did they find?

When plasma from exercised mice was given to sedentary mice, there was an increase in newly born neurons and improved performance in hippocampal-dependent learning. These data indicate that plasma from exercised aged animals can transfer the beneficial effects of exercise to the regenerative capacity of the aged hippocampus and hippocampal-dependent learning and memory. They observed similar improvements when plasma from exercised mature mice (6-7 months) was administered to aged mice.

Analysis of plasma identified 30 factors in the exercised aged mice and 33 in the exercised mature mice that had greater expression than in the sedentary mice, of which 63% and 67% respectively were expressed in the liver. They identified two proteins that were overrepresented, but chose to focus on one — glycosylphosphatidylinositol (GPI)l-specific phospholipase D1 (Glpd1) (an enzyme in the liver that cleaves GPI) — as it hadn’t previously been associated with aging, neurogenesis, or cognition. The authors confirmed that Gpld1 was increased in exercised mice and that its expression levels were correlated with improved cognitive performance. Further, they demonstrated that its overexpression in the liver of aged animals improved adult neurogenesis and cognitive function. Gpld1 was found not to cross the blood-brain barrier readily, and to have low expression levels in the brain. Finally, the authors demonstrated that the peptide must be catalytically active and cleave GPI in order to induce beneficial effects on neurogenesis and cognition.

What's the impact?

The authors identified a novel role for the liver-derived factor Gpld1 as a mediator for the beneficial effects of exercise on the aged mouse brain. This provides exciting evidence for a potential liver-to-brain axis in which circulating blood factors transmit the beneficial effects of exercise to the brain. Finally, these data point to promising avenues for intervention in the aging population, as the beneficial effects of exercise could be distributed broadly across tissues via circulating blood factors.

Horowitz & Fan et al. Blood factors transfer beneficial effects of exercise on neurogenesis and cognition to the aged brain. Science (2020). Access the original scientific publication here.

Using Psychophysics and Signal Detection Theory to Improve Eyewitness Testimony

Post by Cody Walters 

What’s the science?

Eyewitness testimony plays a major role in the criminal justice system, yet 70% of those who are wrongfully convicted are imprisoned, in large part, on the basis of witness misidentification. While some research has been done to try to find ways to improve eyewitness performance, the fact remains that traditional police lineups are prone to various forms of decisional bias. This week in Nature Communications, Gepshtein and colleagues explored a more nuanced lineup methodology that avoids the pitfalls associated with traditional lineups and provides finer-grain metrics for gauging eyewitness reliability. 

How did they do it?

Simultaneous and sequential lineups have been traditionally used by law enforcement and involve the participant either being presented with all the suspects at once or one at a time, respectively. The authors introduced a third lineup type: the paired comparison design. Each participant watched a brief movie clip depicting a crime being committed. The following day, participants were shown the same six faces and told that the perpetrator (called the ‘target’) may or may not be present in the current lineup. During the paired comparison lineup, participants viewed two suspect photographs at a time and were asked to indicate which of the two more closely resembled the perpetrator. This method of relative judgment between two stimuli is a well-established technique in the field of psychophysics (i.e., the study of the relationship between physical stimuli and perception). The authors then fit a line to these voting data and constructed a voting function, where the slope of the line was used to quantify voting consistency: a voting function with a slope of zero would indicate that each face in the lineup was voted for equally often (indicating low voting consistency), whereas a large slope would indicate that each face in the lineup was not voted for equally often (indicating high voting consistency).

Lastly, the authors used receiver operator characteristic (ROC) curves to quantify the voting distribution data. To provide a simple example of how to construct an ROC curve, assume you have two partially overlapping distributions representing votes for face 1 (the perpetrator) and votes for face 2 (a filler), with the voting score represented along the x-axis. You can set an arbitrary cutoff (i.e. a decision criterion) in between those two distributions, meaning that a classifier will consider all vote scores to the right of that cutoff as a vote for the rightmost category. However, since the distributions are overlapping, this necessarily means that there will be misclassifications. Multiple such cutoffs can be positioned at varying positions along the two distributions, and the ‘hit-to-miss’ ratio for each cutoff is plotted to form a curve. The area under the ROC curve can then be used as a metric for classification accuracy.

What did they find?

The authors found that the paired comparison lineup resulted in the same rate of target identification as traditional lineups with the added benefit of having a lower lineup rejection rate (which results from participants failing to select a suspect). An additional advantage of the paired comparison design is that it provides access to information about the consistency with which each face is selected as well as the strength of participants’ recognition memory (i.e., the degree to which a given face matches their memory of the perpetrator). This is because the consistency of a participant’s votes over multiple rounds of paired comparisons is inversely proportional to the variance of the recognition memory.

The authors plotted the average voting scores for each lineup face split by subjects whose highest voting score either correctly or incorrectly identified the perpetrator. Unsurprisingly, the voting score distribution for the perpetrator was significantly above the voting score distributions for the other lineup faces among participants who correctly identified the perpetrator. However, the voting score distribution for the perpetrator was (counterintuitively) also significantly above the voting scores for the other lineup faces among subjects that incorrectly identified the perpetrator. This result is explained by the consistency with which subjects voted: though the highest voting scores among the incorrect subjects were for non-target faces, they often ranked the target face as their second choice. This unique feature of the paired comparison lineup provides access to the hidden structure of recognition memory, allowing experimenters to infer the target face from aggregate voting data even when it was not the top-ranked face among individual subjects.

The authors generated ROC curves derived from simultaneous and sequential lineup data and showed that the paired comparisons ROC curve matches (and potentially outperforms) the data from simultaneous and sequential lineups in terms of classification accuracy without the bias that comes from subjects having to make a definitive identification. Importantly, the paired comparison lineup method allows for experimenters to create ROC curves for individual subjects, an option that is unavailable when using traditional lineups.

What’s the impact?

The authors studied a novel lineup design that leverages principles from psychophysics and analyses from signal detection theory. This new approach provides a method for determining the strength of either individual or aggregate eyewitness recognition memory in a probabilistic manner, an improvement on existing methods that require definitive decision criterion. This work has the potential to pave the way for a more effective, science-based approach to eyewitness testimony.

Gepshtein et al. A perceptual scaling approach to eyewitness identification. Nature Communications (2020). Access the publication here.