Nanoscale Signaling: Messages across time and space
Homeostasis – the ability of an organism to maintain a stable internal environment despite changes in the outside world – is essential for cells, tissues and organs to work properly. Regulating body temperature, maintaining blood pressure or monitoring blood sugar levels are all different forms of homeostasis, and they depend on specialized sensory cells. These cells monitor the environment and the body’s condition and, if necessary, chime in to keep the body functioning properly.
Sensory cells are, in turn, tightly regulated by so-called second messengers – small intracellular signaling molecules that control many different pathways within a cell. The β-cells in the pancreas, for example, are responsible for sensing elevated blood sugar levels and for releasing insulin. This hormone regulates how the body uses and stores glucose and fat. If excessive glucose concentrations are detected in the blood, second messenger molecules in the β-cells, such as calcium ions and cyclic adenosine monophosphate (cAMP), are modulated (Komatsu et al., 2013; Rorsman and Ashcroft, 2018). Temporal changes in the concentration of these two molecules cause the secretory granules (compartments within the β-cells that store insulin) to fuse with the plasma membrane, thereby releasing insulin into the blood stream (Rorsman and Ashcroft, 2018; Figure 1).
In β-cells, the concentrations of calcium ions and cAMP oscillate to integrate external and internal signaling cues and encode them into parameters such as amplitude and frequency (Parekh, 2011; Schöfl et al., 1994). Depending on these parameters, different processes are elicited by the activity of second messengers. Previous research has shown that second messengers are not only controlled at various times, they are also spatially divided within the cells. So far, however, it remains unclear how the spatial organization of these highly diffusible molecules is achieved. Now, in eLife, Jin Zhang and colleagues from the University of California, San Diego – including Brian Tenner as first author – report new insights into second messenger oscillations (Tenner et al., 2020).
Tenner et al. used targeted biosensors to measure cAMP levels in various areas within β-cells. They identified different, spatially separated pools of cAMP, which were set apart by different oscillation patterns and concentrations of cAMP. More specifically, cAMP pools situated near specific protein complexes in the plasma membrane oscillate out-of-phase with cAMP pools located freely in the plasma membrane or in the cytosol. In comparison, calcium ion levels oscillated uniformly in the entire cell.
Using super resolution microscopy, the researchers found that the enzymes that produce and degrade cAMP play an important role in the oscillation process. The enzyme that synthesizes cAMP forms clusters in the plasma membrane, while the enzyme that degrades cAMP is dispersed in the cytosol. Based on these observations, Tenner et al. compiled a mathematical model, which confirmed that the different distributions of the two enzymes enables cAMP to form compartmentalized pools close to the plasma membrane clusters, where it is produced.
An enzyme known as protein kinase A further processes the signals encoded in the second messenger concentrations by helping calcium ions to enter the cell, thereby affecting calcium ion concentrations in the entire cell. Tenner et al. demonstrated that this effect of protein kinase A is dependent on the clustering of enzymes that produce cAMP: when these clusters were disrupted, the oscillations of calcium ions in the cytosol were less sustained and asynchronous. This suggests that modulating the oscillations of second messengers in distinct signaling compartments can affect communication in the entire cell.
While this work adds important new insights into oscillatory second messenger networks, it is based on observations made using an ammonium salt to stimulate a signaling responses. This pharmacological treatment might induce responses inside the cell that could differ from those observed after a physiological stimulation. In the future, it will be important to examine how the compartments of cAMP react to relevant physiological cues such as elevated blood sugar or blood fat levels. It will also be crucial to investigate whether the observed effects on calcium ions in the cell ultimately influence the production of insulin.
Nevertheless, Tenner et al. describe a new mechanism that may change how we think about cell signaling. Oscillatory second messenger networks integrate external and internal signals and are therefore crucial for cellular information processing. Amplitude and frequency of second messenger oscillations have long been established as parameters relevant for encoding informationIt is well known that the amplitude and frequency of these oscillations can encode information (Parekh, 2011; Schöfl et al., 1994). The findings of Tenner et al., however, suggest that the relative oscillatory phase of spatially discrete cAMP pools may constitute an additional mode for encoding information. In such a conceptual framework, a third mode of information processing would significantly increase the cell’s capability to integrate information and respond to changes in its environment (Waltermann and Klipp, 2011).
References
-
Glucose-stimulated insulin secretion: a newer perspectiveJournal of Diabetes Investigation 4:511–516.https://doi.org/10.1111/jdi.12094
-
Decoding cytosolic Ca2+ oscillationsTrends in Biochemical Sciences 36:78–87.https://doi.org/10.1016/j.tibs.2010.07.013
-
Pancreatic β-Cell electrical activity and insulin secretion: of mice and menPhysiological Reviews 98:117–214.https://doi.org/10.1152/physrev.00008.2017
-
Mechanisms of cellular information processingTrends in Endocrinology & Metabolism 5:53–59.https://doi.org/10.1016/1043-2760(94)90002-7
-
Information theory based approaches to cellular signalingBiochimica Et Biophysica Acta (BBA) - General Subjects 1810:924–932.https://doi.org/10.1016/j.bbagen.2011.07.009
Article and author information
Author details
Publication history
Copyright
© 2020, Kuhn and Nadler
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 1,120
- views
-
- 100
- downloads
-
- 1
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Further reading
-
- Biochemistry and Chemical Biology
- Genetics and Genomics
Yerba mate (YM, Ilex paraguariensis) is an economically important crop marketed for the elaboration of mate, the third-most widely consumed caffeine-containing infusion worldwide. Here, we report the first genome assembly of this species, which has a total length of 1.06 Gb and contains 53,390 protein-coding genes. Comparative analyses revealed that the large YM genome size is partly due to a whole-genome duplication (Ip-α) during the early evolutionary history of Ilex, in addition to the hexaploidization event (γ) shared by core eudicots. Characterization of the genome allowed us to clone the genes encoding methyltransferase enzymes that catalyse multiple reactions required for caffeine production. To our surprise, this species has converged upon a different biochemical pathway compared to that of coffee and tea. In order to gain insight into the structural basis for the convergent enzyme activities, we obtained a crystal structure for the terminal enzyme in the pathway that forms caffeine. The structure reveals that convergent solutions have evolved for substrate positioning because different amino acid residues facilitate a different substrate orientation such that efficient methylation occurs in the independently evolved enzymes in YM and coffee. While our results show phylogenomic constraint limits the genes coopted for convergence of caffeine biosynthesis, the X-ray diffraction data suggest structural constraints are minimal for the convergent evolution of individual reactions.
-
- Biochemistry and Chemical Biology
- Structural Biology and Molecular Biophysics
The SARS-CoV-2 main protease (Mpro or Nsp5) is critical for production of viral proteins during infection and, like many viral proteases, also targets host proteins to subvert their cellular functions. Here, we show that the human tRNA methyltransferase TRMT1 is recognized and cleaved by SARS-CoV-2 Mpro. TRMT1 installs the N2,N2-dimethylguanosine (m2,2G) modification on mammalian tRNAs, which promotes cellular protein synthesis and redox homeostasis. We find that Mpro can cleave endogenous TRMT1 in human cell lysate, resulting in removal of the TRMT1 zinc finger domain. Evolutionary analysis shows the TRMT1 cleavage site is highly conserved in mammals, except in Muroidea, where TRMT1 is likely resistant to cleavage. TRMT1 proteolysis results in reduced tRNA binding and elimination of tRNA methyltransferase activity. We also determined the structure of an Mpro-TRMT1 peptide complex that shows how TRMT1 engages the Mpro active site in an uncommon substrate binding conformation. Finally, enzymology and molecular dynamics simulations indicate that kinetic discrimination occurs during a later step of Mpro-mediated proteolysis following substrate binding. Together, these data provide new insights into substrate recognition by SARS-CoV-2 Mpro that could help guide future antiviral therapeutic development and show how proteolysis of TRMT1 during SARS-CoV-2 infection impairs both TRMT1 tRNA binding and tRNA modification activity to disrupt host translation and potentially impact COVID-19 pathogenesis or phenotypes.