Microglia that even interferes with the cerebral circulation, too

Thursday, 17 March, 2022
Tags: News

The most is known about microglia in Ádám Dénes' group. Their most recent discovery is that microglia, in addition to nerve cells, can also establish direct contact with many cells in brain blood vessels and may be involved in regulating cerebral circulation!

The most are known about microglia, the main immune cell of the central nervous system, in the Ádám Dénes' group. Their most recent discovery is that microglia, in addition to nerve cells, can also establish direct contact with many cells in brain blood vessels and may be involved in regulating cerebral circulation!

"Well, says the teacher, drawing the words thoughtfully, we'll take an interesting example..." The quotation familiar to everyone is taken from The Good Student Answers, a short story from Karinthy's masterpiece, Mr. Teacher, please.
There is no question that, in our case, Nature is the teacher, and the best researcher is only a student, however excellent. I quote the sentence because of the adjective 'interesting'. A teacher can think about whether to give his student an interesting, i.e. sufficiently difficult problem, or a less interesting one that he can solve even if he is not Steinmann. In the case of a researcher, however, even the simplest thing may soon turn out to be complicated, equations thought to have been solved may have to be recalculated because we or others have discovered new unknowns, and the textbook may have to be rewritten not because something has to be done to make a living but because our knowledge is not absolute.
"Let's take a cone", said the teacher, and "let's take the microglia", thought Adam Dénes a good ten years ago, who is making progress with his enthusiastic team like a really good student, but he is still a long way from sitting down, content and humbled, after having solved the problem.
In a recent and widely published Nature Communications article, they have revealed that in the absence of microglia, brain injury due to stroke is associated with increased cell death, the neuronal activity becomes unregulated and this leads to a worsening of stroke outcomes. This suggests that microglia play a crucial role in regulating nerve cell function. Their discovery, which describes how microglial extensions can be intimately connected to surrounding neurons through somatic connections, was published in Science in 2020. This work is co-authored by Csaba Cserép and Balázs Pósfai. The first authors of the paper, now published in the Journal of Experimental Medicine are two ladies, as Ádám Dénes politely calls them, Eszter Császár and Nikolett Lénárt. Their seminal work demonstrates that these highly motile and variable microglial cells can interact directly with brain blood vessels in addition to neurons and this has major physiological and pathological implications.
My questions were answered jointly by Eszter and Niki because they took very seriously their shared authorship.

- Why did you start to investigate the blood vessel-vasculature-circulation-microglia connection?

Niki / Eszter

- When we started exploring the role of microglia in brain injury following a stroke in 2015, we were surprised to find that in microglia-deficient animals the extent of injury, i.e. the area of the brain that died, was much greater following stroke, but we found no evidence that this was caused by increased damage to the blood-brain barrier (Szalay et al. Nat Commun. 2016).
In histological studies, we then noticed that microglia can connect to many neurons and brain vessels simultaneously. This ideal anatomical location raised the possibility that microglia could potentially be involved in the regulation of cerebral circulation. Therefore, we started parallel studies on microglia-neuron and microglia-vascular connections in two parallel threads, only to publish the former much earlier, in 2020.

- The brain is a unique organ in many terms, and its blood circulation is not quite the same as elsewhere in the body. You can read about the importance of the blood-brain barrier, but you have also studied neuro-vascular coupling, hypercapnia! Can you explain what these mean?

- The brain blood flow is regulated by complex, mostly overlapping mechanisms.
One of the most important of these mechanisms is neurovascular coupling, which is the connection between neuronal activity and cerebral blood flow. The proper functioning of this connection is ensured by additional cells of the neurovascular unit, the cerebral vascular endothelial cells, pericytes, and astrocytes. In essence, in addition to providing a continuous "basic" cerebral blood flow, the brain has a blood flow regulation mechanism that can immediately trigger a local increase in blood flow in response to locally increased neuronal activity, thus providing for the increased energy demand of the highly active neurons.
In contrast to the effect of neurovascular coupling in inducing a local increase in blood flow, hypercapnia induces a global increase in blood flow in the brain. Inhalation of air with a CO2 content higher than atmospheric CO2 (0.04%) increases the partial pressure of CO2 in the blood, and this causes immediate dilatation - dilation - of the cerebral vessels through various metabolic processes above 45 Hgmm. Since even a small change in partial CO2 induces a significant increase in blood flow in the brain, it is an excellent tool for studying global cerebral blood flow under physiological conditions.

- Thank you! What has been known so far about the role of microglia in these?

- Microglia have so far not been implicated at all as a functional factor in the regulation of cerebral blood flow. A fundamental discovery is that we have demonstrated its active involvement in the physiological processes described above, namely neurovascular coupling and hypercapnia-induced vasodilation.
The significance of this discovery is illustrated by the fact that altered microglial activity, reduced cerebral blood flow (hypoperfusion), and impaired neurovascular coupling can often be detected long before the first neurological symptoms appear in common and well-known neurological diseases such as Alzheimer's disease, vascular dementia or stroke.
Understanding and elucidating the physiological and pathological processes of microglia with cerebral blood vessels is therefore essential for the development of future therapies for all neurological diseases characterized by reduced cerebral circulation.

- What do you think is the most important-surprising result?

- The most important finding of the article is that microglia can regulate cerebral circulation through complex purinergic mechanisms and connections with neurovascular unit cells. Here, of course, many details remain unexplored.
The discovery could have important clinical implications. Among other things, altered microglial activity may influence the clinical outcome of dementia, stroke, or transient ischemic attack (TIA) through modulation of cerebral circulation or adaptation to reduced perfusion.
Our results also suggest that microglia may play an active role in two other brain events, the pathogenesis of which is still unclear.
The first is the "no-reflow" phenomenon that often occurs after clot removal that causes blockage of the cerebral arteries, where cerebral circulation is restored only in the large vessels but not in the small vessels (capillaries). The walls of the capillaries remain contracted, leading to a permanent reduction in blood flow to the brain.
The other, less common than stroke, is 'subarachnoid hemorrhage', where a rupture of the blood vessel walls leads to intracranial hemorrhage and then to spasmodic contraction of the cerebral blood vessels (cerebral vasospasm), resulting in a significant reduction in cerebral blood flow.

- If it is up to you, how will you proceed with the project?

- We would like to understand in more detail the microglia-vascular and microglia-neuron interactions and to unravel the regulatory mechanisms underlying complex cellular processes. To this end, we are already actively developing and applying new animal models, in vivo imaging methods and molecular biology approaches.

- I really like that you insisted on a joint answer, but can you tell us how this shared first authorship came about?


I started working on the project with the LSCI (Laser Speckle Contrast Imaging) blood flow imaging system that we had just acquired in our lab, which allows us to follow cortical blood flow in real-time at high spatial resolution through the intact skull bone in mice.
When we concluded from the results that we were on the right track and that further in vivo experiments, which would require a lot of time and effort, would be needed, Niki joined in with in vivo two-photon microscope measurements of smaller brain areas at higher resolution. From then on we did the in vivo mouse experiments in parallel, I with LSCI and she with two-photon microscopy.

- What do you think about your shared first authorship?

Niki :
- We feel that shared first authorship reflects the amount of experimental and intellectual work we put in. Our lab has a tradition of more easily coordinated thematic projects led by a few lab members anyway.

- And what about the tradition of journal selection? Who decides where you send your article to?

Niki / Eszter:
- Of course, there is always a consensus before submitting an article! However, there was a serious struggle before the publication in J Exp Med. Prior to that, it was rejected by another prestigious journal, after an exhaustive revision process, sometimes on unscientific grounds, without any scientific arguments. It has been a difficult and hard three years for our discovery to find its way to the scientific community!

- All's well that ends well! You all seem to work very well together, and everyone seems to be at home with several methods and even cultivating them to a high level. Are there any techniques that everyone should/needs to know for similar research these days if they want to be successful? What is your opinion, how much specialization should - can - be allowed?

- Under Adam's leadership, an inspiring and creative think-tank has developed in which it is truly an honor to participate.
As a result of his excellent professional leadership, a well-cooperative community has developed, in which everyone has individual but complementary methodological knowledge and experience. This allows room for individual development but does not require professional mastery of all the methods used in the laboratory. It allows us to work more efficiently and to deliver complex material requiring the use of many methods in a shorter time. Therefore, in our opinion, it is not necessary to specialize in too many areas but to be very good in the few areas in which one is competent.


- The girls have summarized everything very precisely and clearly, there is not much need for me here...

- So let me ask you, what was your opinion about the Canossa-walking before acceptance?

- This is really important to say. In my own career, I have not seen many publication processes that are so difficult and lengthy, not because of the professional quality of the material, but because of the novelty of the work and the outdated scientific opinions that have dominated the field, often based on decades-old articles. As Niki mentioned, we tried to publish the article three years ago. We had already gone through a nearly year-long revision process in a leading journal, while reviewers tried to 'help' us in a spirit of scientific impartiality. . .
Finally, after a professional peer review process, J Exp Med, one of the most respected and longest established medical journals, accepted the paper for publication.

- Lesson?

- What we really learned from this research is that complex physiological processes are in reality much more complex than we think, with the redundancy of biological processes and the high degree of compensation between the built-in regulatory systems being a major factor. In this case, the "built-in" stability of the cerebral circulation, which is very stably operated by autoregulatory (self-regulatory) processes over a wide, systemic blood pressure range, also makes it difficult to study the mechanisms of these processes in isolation. That brain immune cells could be so important in this was a surprising discovery for us. This is why the use of state-of-the-art imaging techniques, microglial manipulation techniques, and molecular anatomy was so necessary, in a true team effort.

- "We need a team" was never only true for sports!

- I am very grateful for the dedication and interest of my colleagues, which pushed me through all the difficulties.
That is why I also feel that special thanks are due to Eszter and Niki for their dedication, constant renewal, and resilience to challenges, without which this research could not have been published in such a complex way and in such an excellent journal. They have passed a great human test and should be proud of themselves in every respect.
I see that in science, and in many other areas of life, attitude is very important, because, on the road not traveled, interest, perseverance and persistence are as important as talent itself. It is a pleasure to see that the ladies are not lacking in any of these.

a, 3D reconstruction of the cortical vascular network (green) in vivo by two-photon microscopy. Microglia (red) cells are functionally connected to all sections of the vascular network. b, High-resolution electron tomography image of the specific anatomical connection between microglia and endothelial cells. c, In vivo two-photon microscopy images of control and dysfunctional microglia (P2Y12R KO) from animals with hypercapnia-induced vasodilation. (Red indicates the currently dilating cerebral vessel.) d, Repeated transient unilateral Carotid Communis artery occlusion during reduced cerebral perfusion in microglia-depleted and control animals by Laser Speckle Contrast Imaging.