Macrophages maintain tissue homeostasis by removing old, damaged and apoptotic cells. During metamorphosis, the fruit fly larval fat body undergoes cell death and is eventually replaced by the adult fat body. In a new study, Adam Bajgar and colleagues find that macrophages convert dying larval adipocytes into nutrients to be utilised by other tissues during post-metamorphic development. To find out more about the story behind the paper, we caught up with first author Gabriela Krejčová and corresponding author Adam Bajgar, Associate Professor at the University of South Bohemia.

Gabriela Krejčová (left) and Adam Bajgar (right)

Adam, can you give us your scientific biography and the questions your lab is trying to answer?

AB: Within the framework of my studies, I have gone through a series of laboratories dedicated to a wide range of topics, from amphibian reproduction to the study of magnetoreception in pigeons and photoperiodic clocks in insects. It seems that, from time to time, I need to change the topic of my research to keep my passion alive. Currently, my team predominantly focuses on the immuno-metabolism of insects – that is, how the function of immune cells is linked to metabolism and vice versa. Furthermore, we aim to study various functions of immune cells in all possible physiological processes. However, you never know where the wind will take you, as they say in Czech, and I keep hoping that new fascinating topics or discoveries are waiting for us around the corner.

Gabriela, how did you come to work in Adam's lab and what drives your research today?

GK: During my undergraduate studies, I was working on cancer immunotherapy and when I was looking for a new lab, I came across Adam, who was at that time investigating the metabolic setup of activated immune cells in Drosophila. Since it is well known now that the metabolic setup of pro-inflammatory macrophages resembles the metabolism of tumour cells in many aspects, I became very interested because it was a nice link to my former research topic. Since then, I realized how crucial metabolic setup is for performing any given task the macrophage has and I became amazed by the functional versatility of these cells.

What was known about the function of macrophages during insect metamorphosis before your work?

GK: Although the presence of macrophages in the fat body during metamorphosis has been shown several times and it has also been documented that they perform efferocytosis of dying muscle cells and adipocytes, the prevailing view on their function was that they are completely dispensable for the fly's metamorphosis. However, this view seemed very odd to us. Every time we dissected a virgin fly and saw with our own eyes how big the mass of dying adipocytes is and that virtually every adipocyte is surrounded by macrophages during this period, we just could not believe that macrophage function is not important.

Can you give us the key results of the paper in a paragraph?

AB: In this article, we demonstrate that the adipose tissue of Drosophila is massively infiltrated by macrophages during metamorphosis. Virtually every adipocyte is surrounded by immune cells trying to cover their surface. When we attempted to uncover their function, we found that macrophages aid in the recycling of nutrient-rich materials from the adipocytes. This phenomenon is an integral part of the Drosophila metamorphosis. Considering that we observed similar infiltration in bees and beetles, this may be a universal physiological process inherent to all holometabolous insects. However, this must be confirmed experimentally.

Were you surprised to find a metabolic role for macrophages in post-metamorphic development?

AB: I have to say, yes. We first observed massive infiltration of adipose tissue several years ago and contemplated establishing Drosophila as a model for studying obesity, where infiltration of adipose tissue by macrophages is a characteristic sign of pathological conditions. Although there are obvious parallels, the more we understood the role of macrophages in adipose tissue remodelling in Drosophila, the more it became apparent that their role in this process is largely unique and possibly specific to metamorphosis.

What could be the mechanism behind how macrophages acquire adipocyte-like features during metamorphosis?

GK: The dying larval adipocytes contain a significant amount of electron-dense RNA–protein granules, which are formed by autophagy of various organelles, including rough endoplasmic reticulum, with translation complexes. As the macrophages engulf the adipocytes, they become enriched, not only by lipids coming from the fat body, but also by these granules. One of our theories on how macrophages acquire the adipocyte-like metabolic profile is that the RNA bound in RNA–protein granules may serve as a template for translation in macrophages. In other words, the macrophages literally become what they eat.

Late pupal stage of Drosophila melanogaster depicting the infiltration of macrophages (red) into the dying larval adipose tissue (green). Autofluorescence of the cuticle (cyan) is used for the visualisation of morphological structures.

Late pupal stage of Drosophila melanogaster depicting the infiltration of macrophages (red) into the dying larval adipose tissue (green). Autofluorescence of the cuticle (cyan) is used for the visualisation of morphological structures.

What implications do your findings have on the role of macrophages in other biological processes, such as morphogenesis and regeneration?

AB: There is a possibility that the role of macrophages, which we have uncovered in insect metamorphosis, may be conserved in vertebrates and may contribute to processes such as embryonic morphogenesis, daily turnover of dying cells, amphibian metamorphosis, etc. I hope our work will inspire other groups to study similar phenomena. That would truly make me happy.

I hope our work will inspire other groups to study similar phenomena. That would truly make me happy.

Gabriela, when doing the research, did you have any particular result or eureka moment that has stuck with you?

GK: Besides being amazed by the astonishing beauty of macrophages crawling on the spherical adipocytes, I remember when we finally crossed the flies bearing the construct for cell ablation with a strong haemocyte-specific driver line, kindly provided by the Herzig lab, and we realized that these individuals never emerge and that we were right after all that macrophages are absolutely essential during metamorphosis not only because they eliminate the microbiota.

And what about the flipside: any moments of frustration or despair?

GK: Of course! It took us an embarrassing amount of time to realize what the macrophage function in adipose tissue remodelling during metamorphosis actually is because we tried quite a lot of macrophage-specific knockdowns of various genes and they had no effect on the dying larval fat body. I think that macrophages' function is just so crucial for this process that it is unlikely that silencing one gene will result in a strong, visible phenotype. Eventually, we realized that we were looking at the wrong tissue and that we should examine the impact of these manipulations on the developing adult tissues.

Why did you choose to submit this paper to Development?

AB: Development has a reputation as a very prestigious journal among our colleagues and we rank the publication of our article in Development very high on our personal list of achievements.

Our manuscript thematically fits very well into this journal. We originally wanted to contribute to a planned Special Issue on ‘Metabolic and Nutritional Control of Development and Regeneration’, but, in the end, we didn't manage to meet the deadline for this Special Issue. I must say that I was very satisfied with the review process in Development. Yes, we received relatively challenging revisions, but I must admit that the comments were justified and truly helped to improve the original manuscript. I highly appreciate the respectful approach of the editorial team and reviewers.

Gabriela, what is next for you after this paper?

GK: I think that this paper brings a new perspective on the destiny of the matter that macrophages acquire by engulfing senescent and apoptotic cells in the body of all animals. Therefore, it would definitely be interesting to have a look at whether they recycle the nutrients also during daily cell turnover. But, this project needs to wait a little bit because I have my PhD defence coming up in a few weeks.

Adam, where will this story take your lab next?

AB: It would certainly be great to see whether this process is conserved in vertebrates and if the recycling of dying cells for nutrient reuse could be an integral part of amphibian metamorphosis, embryogenesis or regeneration. However, I'm not sure whether these topics are within our capabilities. Given that we observe infiltration of adipose tissue by macrophages during various metabolic stresses in adult Drosophila, we would like to further investigate their role in our upcoming project. However, we also aim to continue studying the diverse functions of macrophages in non-model insect species, explore the mechanisms of immune memory formation in insects, etc. I am currently fascinated by studies on flatworms and Dictyostelium and would like to incorporate these models into some of our projects.

Finally, let's move outside the lab – what do you like to do in your spare time?

AB: Here, I might like to respond for Gabriela, because I believe that besides her work in the laboratory, she engages in many absolutely amazing activities, but her natural modesty wouldn't allow her to talk about it here.

Gabriela is incredibly talented in creating jewellery (BiospheraArt). She crafts such delicate jewels from natural materials encased in resin. Additionally, she has recently started dabbling in watercolour painting, and her first attempts are simply fantastic. I hardly need to add that she occasionally participates in dance competitions, particularly oriental dance. However, she has now discovered a completely new passion, and that is mushrooms. So, she tries to spend every free moment somewhere in the forest. She not only collects edible mushrooms but also forages for various, often obscure, species.

GK: So, I'm going to say something about you…Adam has a free-spirited nature and he loves to spend his time in the mountains. I bet that he would trade this paper for being able to spend more time climbing the hills and enjoying the many adventures and beauty that nature offers. Also, he enjoys woodworking and he is very skilled in building furniture.

G.K. and A.B.: Department of Molecular Biology and Genetics, Faculty of Science, University of South Bohemia, Ceske 6 Budejovice, 37005, Czech Republic.

A.B.: Institute of Entomology, Biology Centre CAS, Ceske Budejovice, 37005, Czech Republic.

E-mail: [email protected]; [email protected]

Macrophages play a nutritive role in post-metamorphic maturation in Drosophila