marmite defines a novel conserved neuropeptide family mediating nutritional homeostasis

Background

Energy homeostasis and feeding behavior are tightly regulated across several species. This homeostatic regulation depends on releasing neuropeptides and hormones from different organs and integrating these signals into the nervous system. Some examples of neuropeptides that play a major role in controlling energy homeostasis are NPY, AgRP, and POMC, which mediate hunger and satiety signals (Schwartz et al., 2000). The small size and low abundance of neuropeptides make their identification using proteomic techniques difficult (Anderson & Anderson, 2002), suggesting there are neuropeptides to be discovered, which may play a significant role in energy homeostasis.

Macronutrient intake is also a tightly regulated process in animals, particularly for proteins, because some amino acids cannot be synthesized and need to be obtained from food (Leitão-Gonçalves et al., 2017). Different theories exist in the literature regarding the role of dietary protein in metabolic health. Several lines of evidence suggest that protein restriction improves metabolic health (Levine et al., 2014). In contrast, extensive research on multiple species has shown that low protein intake leads to an overconsumption of other macronutrients, such as fats and carbohydrates, as a compensatory mechanism to meet the protein intake requirements (Protein Leverage Hypothesis) (Simpson & Raubenheimer, 2005). This hypothesis would explain the higher rates of obesity worldwide in the last decades, caused by abundant exposure to ultra-processed, low-protein foods. However, the mechanisms regulating specific nutrient feeding remain poorly characterized. Therefore, identifying novel neuropeptides, such as marmite, with relevant physiological functions in controlling protein intake, may help better understand feeding behavior and nutrient choice.

Main findings

In this preprint, Francisco and colleagues (Francisco et al., 2022) discovered marmite (mmt), a new neuropeptide-encoding gene that regulates protein consumption in Drosophila melanogaster. They observed that the loss-of-function mutation or knockdown of mmt leads to an increase in protein intake. On the other hand, the overexpression of mmt reduces the protein appetite.

Nutritional choices were analyzed in Drosophila using the flyPAD technology (Itskov et al., 2014), quantifying the number of yeast sips (protein choice) or sucrose. mmt KO flies, generated using a CRISPR-Cas9-based technique, showed an increase of sips on yeast (protein-rich) and no differences in sucrose preference. Neuronal-specific mmt knockdown resulted in a similar phenotype (increased number of sips on yeast), suggesting that mmt acts as a neuropeptide. On the contrary, mmt overexpression in neurons led to a decrease of protein appetite (reduced number of sips) in females deprived of the essential amino acid isoleucine, that normally have higher protein appetite. Overall, the authors convincingly demonstrated that mmt modulates protein intake acting as a protein-satiety signal, and showed that the product of the mmt gene has the characteristics of a neuropeptide.

One of the main findings of Francisco et al. is the distribution of the mmt neuropeptide in 28 cell bodies in the fly brain, as was shown by using a specific antibody. The most prominently detected neurons were named Medial Marmite Neurons (MMNs) and were situated in large somata in the medial part of the brain. The activation of these neurons led to a decrease in protein appetite, which correlates with mmt overexpression. In contrast, silencing this neuronal population, marked with a specific Gal4 line, led to a significant increase in yeast appetite.

The authors expanded the relevance of their findings by showing that marmite belongs to a conserved family of neuropeptides that mediate nutritional homeostasis, which in mammals includes neuropeptides B (NPB) and W (NPW). The overexpression of the human NPB and NPW in fly neurons led to a decrease in yeast intake that phenocopied the effect observed with mmt overexpression. On the other hand, intracerebroventricular (ICV, a method that bypasses the blood-brain barrier and other mechanisms that limit drug distribution into the brain) administration of mmt in mice increased food intake, showing that this family of neuropeptides controls food intake across different species.

Why we picked this preprint:

We picked this preprint because we are very interested in understanding energy homeostasis regulation and how nutritional choices are made. The current limitations of proteomic identification of peptide hormones and neuropeptides suggest that there are some of them left to be discovered. Their finding might help to better understand energy homeostasis and macronutrient intake regulation. Interestingly, this preprint combines robust gain and loss-of-function experiments in Drosophila with phylogenetic studies to uncover a novel family of neuropeptides with conserved effects on nutrient regulation across species. These results showcase the relevance of studies in different models that can lead to a better understanding of mechanisms that are relevant to human physiology.

Questions for the authors:

1. Could you clarify the difference between mmt “Deficiency” and mmt KO in the graphs of Fig. 1?
2. One of the most intriguing findings was the opposite effect found in mice after injection of marmite or NPB. Are you considering other experiments to unravel the role of this family of neuropeptides in mammals? Choosing a wide range of diets with different amounts of proteins could help understand the role of these peptides in regulating protein intake in mammals.
3. Would you expect that the control of protein intake by marmite is restricted to changes in isoleucine? Did you try modulating the content of other essential amino acids?

References

Anderson, N. L., & Anderson, N. G. (2002). The human plasma proteome: History, character, and diagnostic prospects. Molecular & Cellular Proteomics: MCP, 1(11), 845–867. https://doi.org/10.1074/mcp.r200007-mcp200

Francisco, A., Tastekin, I., Fernandes, A., Ezra-Nevo, G., Deplancke, B., Oliveira-Maia, A., Gontijo, A., & Ribeiro, C. (2022). Marmite defines a novel conserved neuropeptide family mediating nutritional homeostasis. BioRxiv, 2022.12.12.520095. https://doi.org/10.1101/2022.12.12.520095

Itskov, P. M., Moreira, J.-M., Vinnik, E., Lopes, G., Safarik, S., Dickinson, M. H., & Ribeiro, C. (2014). Automated monitoring and quantitative analysis of feeding behaviour in Drosophila. Nature Communications, 5(1), Article 1. https://doi.org/10.1038/ncomms5560

Leitão-Gonçalves, R., Carvalho-Santos, Z., Francisco, A. P., Fioreze, G. T., Anjos, M., Baltazar, C., Elias, A. P., Itskov, P. M., Piper, M. D. W., & Ribeiro, C. (2017). Commensal bacteria and essential amino acids control food choice behavior and reproduction. PLoS Biology, 15(4), e2000862. https://doi.org/10.1371/journal.pbio.2000862

Levine, M. E., Suarez, J. A., Brandhorst, S., Balasubramanian, P., Cheng, C.-W., Madia, F., Fontana, L., Mirisola, M. G., Guevara-Aguirre, J., Wan, J., Passarino, G., Kennedy, B. K., Wei, M., Cohen, P., Crimmins, E. M., & Longo, V. D. (2014). Low protein intake is associated with a major reduction in IGF-1, cancer, and overall mortality in the 65 and younger but not older population. Cell Metabolism, 19(3), 407–417. https://doi.org/10.1016/j.cmet.2014.02.006

Schwartz, M. W., Woods, S. C., Porte, D., Seeley, R. J., & Baskin, D. G. (2000). Central nervous system control of food intake. Nature, 404(6778), Article 6778. https://doi.org/10.1038/35007534

Simpson, S. J., & Raubenheimer, D. (2005). Obesity: The protein leverage hypothesis. Obesity Reviews: An Official Journal of the International Association for the Study of Obesity, 6(2), 133–142. https://doi.org/10.1111/j.1467-789X.2005.00178.x

Tags: energy balance, neuropeptide, nutrient, obesity

Posted on: 22 March 2023

doi: https://doi.org/10.1242/prelights.34143

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