Membrane Proteins: Pain or gain?
All cells are surrounded by a lipid membrane that is not permeable to ions and other solutes. To enter or leave a cell, therefore, ions and solutes must pass through proteins that are embedded in the lipid membrane. This gatekeeping role means that transmembrane proteins are involved in a number of physiological processes, and if they fail to work properly, the result can be a severe metabolic defect.
Membrane proteins belonging to the TMEM120 family have been associated with a wide variety of functions, and recently it was reported that one family member – TMEM120A (also referred to as TACAN) – was highly expressed in clusters of neurons called dorsal root ganglia, and was involved in the sensing of mechanical pain (Beaulieu-Laroche et al., 2020). This study suggested that TMEM120A was a channel protein that, when activated by a mechanical stimulus, allowed positive ions to cross the lipid membrane by passing through a pore within the protein. Now, in eLife, Zhenfeng Liu, Bailong Xiao, Yan Zhao and colleagues at the Chinese Academy of Sciences and Tsinghua University report the results of a series of cryo-electron microscopy and electrophysiology experiments that reveal new details about this protein and call its role in the detection of mechanical pain into question (Rong et al., 2021).
First, the researchers – who include Yao Rong, Jinghui Jiang and Jianli Guo as joint first authors – used cryo-electron microscopy to determine the three-dimensional structure of TMEM120A. This revealed a rather novel architecture: the protein is made up of two monomers, both of which contain six segments that have a helical shape (Figure 1). It is possible that ions can pass through a channel formed by the six transmembrane helices in each monomer. However, the 3D structure also revealed the presence of a molecule bound to the inside of the putative channel that would prevent the passage of ions through it. Rong et al. hypothesized that this molecule was an endogenous ligand that had remained tightly bound to the protein when it was being purified prior to the cryo-electron microscopy experiments, and then used mass spectrometry to show that the molecule was coenzyme A (CoASH). This coenzyme is primarily known for helping to synthesize and degrade various cellular compounds, including fatty acids (Gout, 2018), and it had not previously been associated with mechanosensation.
To investigate this further, Rong et al. purified TMEM120A without the coenzyme and determined its 3D structure. The two structures were similar, apart from the shape of two cytoplasmic loops: in the CoASH-bound state the loops extended into the cytoplasm of the cell; however, when CoASH was not present the loops partially closed the entrance to the channel in each monomer (Figure 1). Rong et al. then used these structures to identify which amino acid residues are involved in the binding of CoASH. Further experiments showed that mutating one of these residues, a conserved tryptophan, dramatically decreased the affinity of TMEM120A for CoASH.
By conclusively showing that TMEM120A specifically interacts with CoASH, the work of Rong et al. suggests that this membrane protein might in fact work as a membrane-embedded enzyme. It is also notable that Rong et al., along with several other groups (Ke et al., 2021; Niu et al., 2021; Parpaite et al., 2021; Rong et al., 2021; Xue et al., 2021), could not reproduce the mechanosensitive currents reported previously. Moreover, there are several lines of evidence to suggest that TMEM120A has a greater role in lipid metabolism. Originally, TMEM120A was shown to be important for fat tissue differentiation (Batrakou et al., 2015). Several recent reports have shown that C. elegans and mammalian cells need it to accumulate fat, and that disrupting the gene for TMEM120A leads to metabolic defects in mice fed a high-fat diet (Czapiewski et al., 2021; Li et al., 2021). In addition, its structure also resembles that of ELOVL7, a membrane-embedded enzyme that helps to elongate fatty acids (Nie et al., 2020; Niu et al., 2021; Xue et al., 2021). However, TMEM120A does not catalyze the same reactions as ELOVL7, and the possible substrates and end products of TMEM120A remain unknown (Niu et al., 2021).
These findings do not completely rule out a role for TMEM120A as a mechanosensitive channel. Computational modeling suggests that it might be able to act as an ion channel in the absence of CoASH (Chen et al., 2021). Alternatively, the addition of extra subunits may be required for it to work as an ion channel (Chen et al., 2021). Furthermore, TMEM120A may contribute to mechanosensation by modulating other mechanosensitive channels in dorsal root ganglia (Del Rosario et al., 2021). Undoubtedly, the work by Rong et al. provides new insights into proteins belonging to the TMEM120 family, and the controversy surrounding their role will likely fuel future scientific endeavors.
References
-
Coenzyme A, protein CoAlation and redox regulation in mammalian cellsBiochemical Society Transactions 46:721–728.https://doi.org/10.1042/BST20170506
Article and author information
Author details
Publication history
- Version of Record published: September 29, 2021 (version 1)
Copyright
© 2021, Kalienkova
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
-
- 1,386
- views
-
- 90
- downloads
-
- 0
- 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
-
- Neuroscience
Most nervous systems combine both transmitter-mediated and direct cell-cell communication, known as 'chemical' and 'electrical' synapses, respectively. Chemical synapses can be identified by their multiple structural components. Electrical synapses are, on the other hand, generally defined by the presence of a 'gap junction' (a cluster of intercellular channels) between two neuronal processes. However, while gap junctions provide the communicating mechanism, it is unknown whether electrical transmission requires the contribution of additional cellular structures. We investigated this question at identifiable single synaptic contacts on the zebrafish Mauthner cells, at which gap junctions coexist with specializations for neurotransmitter release and where the contact unequivocally defines the anatomical limits of a synapse. Expansion microscopy of these single contacts revealed a detailed map of the incidence and spatial distribution of proteins pertaining to various synaptic structures. Multiple gap junctions of variable size were identified by the presence of their molecular components. Remarkably, most of the synaptic contact's surface was occupied by interleaving gap junctions and components of adherens junctions, suggesting a close functional association between these two structures. In contrast, glutamate receptors were confined to small peripheral portions of the contact, indicating that most of the synaptic area functions as an electrical synapse. Thus, our results revealed the overarching organization of an electrical synapse that operates with not one, but multiple gap junctions, in close association with structural and signaling molecules known to be components of adherens junctions. The relationship between these intercellular structures will aid in establishing the boundaries of electrical synapses found throughout animal connectomes and provide insight into the structural organization and functional diversity of electrical synapses.
-
- Neuroscience
Negative memories engage a brain and body-wide stress response in humans that can alter cognition and behavior. Prolonged stress responses induce maladaptive cellular, circuit, and systems-level changes that can lead to pathological brain states and corresponding disorders in which mood and memory are affected. However, it is unclear if repeated activation of cells processing negative memories induces similar phenotypes in mice. In this study, we used an activity-dependent tagging method to access neuronal ensembles and assess their molecular characteristics. Sequencing memory engrams in mice revealed that positive (male-to-female exposure) and negative (foot shock) cells upregulated genes linked to anti- and pro-inflammatory responses, respectively. To investigate the impact of persistent activation of negative engrams, we chemogenetically activated them in the ventral hippocampus over 3 months and conducted anxiety and memory-related tests. Negative engram activation increased anxiety behaviors in both 6- and 14-month-old mice, reduced spatial working memory in older mice, impaired fear extinction in younger mice, and heightened fear generalization in both age groups. Immunohistochemistry revealed changes in microglial and astrocytic structure and number in the hippocampus. In summary, repeated activation of negative memories induces lasting cellular and behavioral abnormalities in mice, offering insights into the negative effects of chronic negative thinking-like behaviors on human health.