Confocal microscopy image showing induced neurons (red with a yellow nucleus) expressing the NeuN neuronal marker (green) within an epileptic mouse hippocampus. © Extract from: Lentini et al., Cell Stem Cell, 2021.

Many central nervous system diseases are associated with the death of neurons without the brain being able to regenerate them. A phenomenon that is observed particularly in Parkinson’s disease, Alzheimer’s disease, following stroke, and in some forms of epilepsy. How can lost neurons be regenerated? This question has been tackled by a team of researchers from Inserm, CNRS and Université Claude Bernard Lyon 1 at the Stem Cell and Brain Research Institute, in collaboration with King’s College London. Using an animal model of epilepsy, the researchers have succeeded in transforming non-neuronal cells in the brain into new inhibitory neurons that reduce chronic epileptic activity by half. This research will in time make it possible to envisage a therapeutic application of this strategy. The findings of this study have been published in Cell Stem Cell.

Our brain generally lacks the regenerative capacity to replace lost or damaged neurons. The goal of regenerative medicine is to replace lost cells in order to correct the functional disorders associated with that loss. Direct cell reprogramming (as opposed to induced pluripotent stem cell reprogramming) has emerged as an innovative strategy that consists of “reprogramming” the identity of certain non-neuronal cells present within the affected brain to transform them into neurons. If this strategy is to be effective, many challenges need to be addressed. The new neurons must be integrated into the networks of surviving neurons and take over from those they replace in order to correct the pathological disorders.

This was the strategy explored in a new study published in the journal Cell Stem Cell. A team of researchers from Inserm, CNRS and Université de Lyon have succeeded in transforming glial cells of the brain into new neurons in a mouse model with mesial temporal lobe epilepsy, the most common form of drug-resistant epilepsy in humans.

Proliferation of glial cells: a cell source from which to generate neurons

In neuronal death, as observed in mesial temporal lobe epilepsy, the most common form of adult focal epilepsy, the glial cells present in the direct environment of the damaged neurons react by multiplying themselves, albeit without resolving the problem.

In the study, the researchers had the idea of taking advantage of this proliferation and using these extra glial cells. First, they had to identify genes making it possible to transform these glial cells into inhibitory neurons, whose loss plays a key role in the onset of seizures, in order to restore the balance of the neuronal activities that had been affected. The researchers therefore selected genes known for being involved in the genesis of these inhibitory neurons during development.

By forcing the expression of these genes, they were able to reprogram the identity of the glial cells to make them so-called “induced neurons” whose properties are comparable to those lost in the disease. Through stereotactic surgery[1], the genes were inserted directly into the brains of the mice at the sites of origin of the epilepsy using deactivated viral vectors that induce reprogramming of the glial cells. Within a few weeks, the vast majority of these gene-treated glial cells had transformed into new neurons.

Functional neurons integrated into the epileptic network

The results of the study show that the induced neurons adopt an identity of inhibitory neurons that present a set of molecular characteristics comparable to those of neurons having degenerated in epilepsy.

Using electrophysiological recordings, the scientists were able to confirm that they were indeed functional neurons, capable of inhibiting the neighboring neurons responsible for seizures, thereby reducing their activity. Then, by tracing connections between the neurons, they were able to determine that the induced neurons were fully integrated into the epileptic network but also more broadly in the brain.

Finally, thanks to electroencephalographic (EEG) recordings in the sites of origin of the seizures, the researchers were able to show, in the reprogrammed mice, that the seizures had reduced by half.

“These findings reveal the therapeutic potential of this cell reprogramming strategy in fighting a pathology such as mesial temporal lobe epilepsy. This represents a blessing in the specific case of this disease at a time when 30% of patients are refractory to pharmacological treatments,” explains Christophe Heinrich, the study’s designer.

Even if much remains to be done before this research can truly be applied to the treatment of patients, this study highlights the reprogramming of glial cells into neurons as a new strategy capable of modifying not just a disease such as epilepsy, but which also could be expanded to include other devastating brain diseases.

[1] Neurosurgery technique that uses a system of 3D coordinates in space for the precision-access of brain regions. 

In purple, the tanycytes that form the brain’s cellular gateway to the hormone leptin; in yellow, the appetite-inducing neurons and, in blue, the appetite-suppressing neurons. Leptin targets both neuron types, inhibiting the former and using its appetite-suppressant signal to activate the latter. © Vincent Prévot

Diabetes, a disease in which blood sugar levels remain too high for too long, can lead to health complications in the long term. Type 2 diabetes (T2D) accounts for 90% of cases. Patients are usually obese or overweight, with risk factors that include sedentary lifestyle and unbalanced diet. To increase their understanding of the disease, a team of researchers from Inserm, Université de Lille, and Lille University Hospital in the Lille Neuroscience and Cognition laboratory[1] has for several years studied the role of leptin, a hormone involved in appetite control that sends satiety signals to the brain. In a new study published in the journal Nature Metabolism, in addition to furthering scientific knowledge of the mechanism of satiety, the scientists developed a new mouse model of diabetes that will be useful for and relevant to future research in this area.

Leptin, the satiety or appetite-suppressant hormone, is secreted by the adipose tissue at levels proportional to the body’s fat reserves and regulates appetite by controlling the feeling of fullness.  It is transported to the brain by tanycytes – cells which it enters by attaching to the LepR receptors. Tanycytes are therefore leptin’s gateway to the brain, helping it to cross the blood-brain barrier and deliver satiety information to the neurons.

Previous research has revealed that such transport is impaired in subjects who are obese or overweight. This goes some way to explaining their dysfunctional appetite regulation given that it is more difficult for the information on satiety to reach the brain. In their new study, the researchers took a closer look at this transport mechanism, and more precisely the role played by the LepR receptors.

The key role of satiety hormone receptors in glucose management

In mouse models, the researchers removed the LepR receptor that is located on the surface of the tanycytes. After three months, the mice experienced a marked increase in their fat mass (which doubled over the period) as well as a loss of muscle mass (reduced by more than half). The total amount of weight gained was only fairly moderate. The scientists also regularly measured the animals’ blood sugar levels following the injection of glucose.

They found that in order to maintain blood sugar at normal levels (between 0.70 and 1.10 g/L), the mice secreted more insulin during the first four weeks of the experiment. Three months after removing the receptor, their ability to secrete insulin from the pancreas appeared to be exhausted.

Removing the LepR receptors and impairing leptin transport to the brain therefore led the mice to initially develop a pre-diabetic state. This occurs when the body releases more insulin than usual in order to control blood sugar. Then, in the longer term, the mice became unable to secrete insulin and as such unable to control their blood sugar levels. These data therefore suggest that impaired leptin transport to the brain via the LepR receptors plays a role in the development of type 2 diabetes.

In the last part of their research, the scientists reintroduced leptin to the brain and observed the immediate resumption of its pancreatic function-promoting action – particularly the ability of the pancreas to secrete insulin to regulate blood sugar. The mice quickly regained a healthy metabolism.

This study therefore elucidates the brain’s role in type 2 diabetes and also helps to further research into a disease that until then had not been considered to involve the central nervous system.

“We show that the brain’s perception of leptin is essential for the management of energy homeostasis[2] and blood sugar. We also show that blocking the transport of leptin to the brain impairs the functioning of the neurons that control pancreatic insulin secretion,” concludes Vincent Prévot, research director at Inserm and last author of the study.

The research team started by describing the mechanism by which leptin passes through the cell gate: tanycytes (Figure opposite: cells in yellow). These cells capture circulating leptin from the blood vessels which at that location have the particularity of letting it through (step 1). Whilst in the tanycyte, the leptin captured by LepR activates the EGF receptor (or EGFR) which itself activates an ERK signaling pathway (step 2), triggering its release into the cerebrospinal fluid (step 3). The leptin then activates the brain regions that convey its anorectic (appetite suppressant) action, as well as control of pancreatic function (step 4).

[1] This research was performed in collaboration with two laboratories at Institut Cochin and Université de Strasbourg as part of a project funded by the French National Research Agency (ANR) and two European laboratories, one at Lübeck University in Germany and the other at the University of Santiago de Compostela in Spain, within the framework of European Community funding. In addition, the Lille Neuroscience and Cognition laboratory is a member of LabEx EGID (European Genomic Institute for Diabetes) and DISTALZ (Development of Innovative Strategies for a Transdisciplinary approach to ALZheimer’s disease).

[2] Stabilization, regulation in living organisms, of certain physiological characteristics (food intake, energy expenditure, etc.).

Primary culture of astrocytes © Inserm/Ruiz, Anne-Laure

Astrocytes are cells in the brain which have long been considered only as mere support cells for neurons. In recent years, the study of astrocytes has grown, gradually revealing their importance in brain function. Researchers from Inserm, CNRS and Collège de France at the Center for Interdisciplinary Research in Biology have now uncovered their crucial role in closing the period of brain plasticity that follows birth, finding them to be key to the development of sensory and cognitive faculties. Over the longer term, these findings will make it possible to envisage new strategies for reintroducing brain plasticity in adults, thereby promoting rehabilitation following brain lesions or neurodevelopmental disorders. This research has been published in Science.

Brain plasticity is a transient key period after birth in which the brain remodels the “wiring” of the neurons according to the external stimulations it receives (environment, interactions, etc.). The end – or “closure” – of this period marks the stabilization of the neural circuits, associated with efficient information processing and normal cognitive development. Plasticity is still possible in the future, although at a much lower level than at the beginning of life.

Problems occurring during the brain plasticity period could have major long-term consequences. For example, in the event of an eye condition preventing an individual from seeing correctly, such as strabismus (crossed eyes), the corresponding brain wiring will be permanently altered if it is not treated in time.

To remedy this, the researchers aim to remodel this wiring by identifying a therapy that would reintroduce brain plasticity, even once closure has occurred. To achieve this, they also seek to better characterize the biological mechanisms that underlie this closure.

Pioneering studies from the 1980s showed that transplanting immature astrocytes into the brains of adult animals reintroduced a period of major plasticity. The team of Inserm researcher and study coordinator Nathalie Rouach at the Center for Interdisciplinary Research in Biology (Inserm/CNRS/Collège de France)[1] took inspiration from this procedure to reveal the hitherto unknown cellular process responsible for the closure of plasticity.

Transplanting immature astrocytes to reintroduce brain plasticity

Through experiments on the mouse visual cortex, the researchers show that the presence of immature astrocytes is the key to brain plasticity. The astrocytes are then later involved in developing interneuron maturation[1] during the plasticity period, ultimately leading to its closure. This maturation process occurs via a novel mechanism involving the protein Connexin 30, of which the researchers found high levels in mature astrocytes during closure.

Could transplanting astrocytes into adult mice reintroduce brain plasticity?

To find out, the researchers cultured immature astrocytes from the visual cortex of young mice (1 to 3 days’ old). These immature astrocytes were transplanted into the primary visual cortex of adult mice, following which the activity of the visual cortex was evaluated after four days of monocular occlusion – a standard technique used to assess brain plasticity. They found that the mice transplanted with the immature astrocytes presented a high level of plasticity, unlike the control mice which did not receive the transplant.

 “This study is a reminder that in the neurosciences we must not only focus on neurons. The glial cells, of which the astrocytes are a subtype, regulate most of the brain’s functions. We realized that these cells have active roles. Glial cells are less fragile than neurons and so represent a more accessible means of acting on the brain, ” emphasizes Rouach.

[1]  The interneurons establish connections between an afferent neural network (which sends information to the central nervous system) and an efferent neural network (which sends this information to the organs responding to the stimulation)

Seeking a reward effect through physical exercise is thought to constitute a key component of the disease, influenced by genetics. © Bruno Nascimento on Unsplash.

In anorexia nervosa patients, the weight loss through lack of food is accompanied by fatigue and reduced physical capacity. Yet despite this they often continue to perform intense physical activity which contributes to that weight loss. Researchers from Inserm and Université de Paris at the Institute of Psychiatry and Neuroscience of Paris and the University Hospital Group Paris Psychiatry & Neurosciences show that physical exercise generates positive emotions not just in the patients (which was expected) but also – more surprisingly – in their relatives without the condition. However, this was not the case in the control subjects.

Seeking a reward effect through physical exercise is therefore thought to constitute a key component of the disease, influenced by genetics. This research, published in the International Journal of Eating Disorders, could make it possible to focus the management of anorexia nervosa patients on calories burned (sport) rather than exclusively on deficiencies (diet).

Anorexia nervosa is an eating disorder that affects mostly young women and girls between the ages of 15 and 25. Its lifetime prevalence in women is estimated to be just over 1%. Philip Gorwood and Laura Di Lodovico at the Institute of Psychiatry and Neuroscience of Paris (Inserm/Université de Paris) and the University Hospital Group Paris Psychiatry & Neurosciences have spent years trying to elucidate the disease and improve how it is managed.

In particular, their work has focused on the reward effect associated with not eating and the resultant weight loss. “We know that there is a vicious circle with anorexia nervosa, in which what makes a person lose weight is so rewarding in terms of how they feel, that they can overlook the dangers that they otherwise understand. This abnormality in the decision-making process is clearly related to the reward effect (the brain sends back messages that positively reinforce the maintenance of the disorder). But it is complicated to understand how a lack (a lack of food) can in itself be a ‘reinforcer’. That is why we have focused on the other dimension of weight loss – physical exercise,” explains Gorwood.

Based on these questions, the scientists showed in a previous study that anorexia nervosa is associated more with the pleasure of losing weight than with the fear of gaining it, and that this aspect is genetically influenced.

In their latest research, they continue to reflect on the clinical criteria of the disease and its heritability[1]by focusing on the concept of physical exercise. “This is an atypical approach because physical exertion is not considered a clinical manifestation of anorexia, even though many patients do a lot of sport, especially to manage their hunger and burn calories,” clarifies Gorwood.

The team felt that studying this aspect was all the more interesting given that yet again there is a contradiction: patients persist with exercising despite the fact that being underweight gradually reduces their physical capacities.

Patients, their relatives, and others

The protocol for this study was original because it allowed the researchers to focus not only on the emotions and perceptions of the patients following standardized physical exercise, but also on those of members of their family (particularly their mothers and sisters). Enrolled in this study were 88 female patients with anorexia nervosa, 30 of their relatives without the condition, and 89 healthy controls. Each was asked to perform standardized physical exercise and then answer questionnaires concerning the emotions they felt afterwards and the perception of their body image.

The scientists showed that for the same amounts of exertion, the emotions reported by the patients were more positive than those of the controls. “Doing sport sends the patients a message of positive reinforcement, making them continue this activity despite feeling tired or weak. The calories burned in association with this physical activity is a key factor that leads to its continuation,” explains Gorwood.

While this aspect was not found in the controls, it was present in the patients’ relatives. The study therefore suggests that this trait is shared within the families of people with anorexia.

Physical activity is associated with a rewarding effect, and this is thought to be involved in the heritability of the disease.

These findings have implications in terms of managing the condition, emphasizing the importance of focusing part of that management on physical exercise. The idea is to work with the patients so that they rediscover the pleasure of physical exercise (therefore in moderation) and lose their addiction to it, which is probably associated with the goal of weight loss. While specialized teams already considered this aspect of management to be important, the study provides firm scientific arguments in favor of pursuing this approach, legitimizing this care practice and putting it to widespread use.

[1] heritability refers to the proportion of variation in a population trait (in this case, anorexia nervosa) that can be attributed to the genetic factors we inherit.