Membrane Structures: Cellular fingers take hold

  1. Yukiko M Yamashita  Is a corresponding author
  1. Howard Hughes Medical Institute, University of Michigan, United States

Cells in textbooks tend to have simple shapes, with surfaces that merely separate the contents of the cell from the outside world. However, this is far from the truth. Sperm cells and nerve cells, for example, have quite complex shapes, and the cell surface can play host to a range of organelles and structures, including primary cilia and a variety of other protrusions from the cell membrane (Singla and Reiter, 2006; Kornberg and Roy, 2014; Gerdes and Carvalho, 2008; Buszczak et al., 2016).

Now, in eLife, Takefumi Negishi, Hitoyoshi Yasuo, Naoto Ueno and colleagues report the discovery of a new membrane structure that forms in the embryos of a marine creature commonly called a sea squirt (Negishi et al., 2016). This structure involves a thin finger-like protrusion from one cell’s membrane inserting itself into a pocket (or invagination) formed in the membrane of the cell in front (Figure 1).

Negishi et al. – who are based at National Institutes of Natural Sciences in Japan and Sorbonne University in France – found that the membrane invagination always projects toward an organelle in the cell known as the centrosome. This organelle serves as an organizing center for protein filaments called microtubules. As such, the centrosome has an integral role in the assembly of the mitotic spindle: the macromolecular machine (composed of microtubules) that segregates the chromosomes during cell division.

Negishi et al. suggest that the invagination holds the centrosome in place to ensure that the mitotic spindle is oriented correctly. This conclusion is based upon three lines of evidence. First, the centrosome is closely connected to the tip of the invagination via microtubules. Second, cutting this tip with a laser caused the invagination to quickly retract, demonstrating that it was under mechanical tension. Third, Negishi et al. show that mutant sea squirts that fail to orient their spindles correctly tended to form the membrane invaginations in the wrong direction as well. These mutants included those with defects in a developmental phenomenon known as “planar cell polarity” (often shortened to PCP), which instructs how animal cells in specific tissues become oriented in the same direction. These findings indicate that the direction in which the invagination forms is under the control of the same signaling pathway that controls planar cell polarity.

This circumstantial evidence is strong. However, Negishi et al. were not able to directly test the role of the invagination in orienting the spindle because it regrew rapidly after being cut with the laser. Confirming that the invagination does indeed anchor the centrosome will require further study, in particular to identify the molecular components that govern how the invagination forms.

Further experiments are also needed to answer a number of other questions. For example, how does the membrane know the position of the centrosome in order to project towards it? Although the microtubules that emanate from the centrosome might provide the cue, this mechanism does not explain how the membrane invagination is carved into a thin finger-like tube. Also, is the composition of the invagination different to that of the rest of the cell surface? And if the answer to this question is yes, is there a barrier that keeps the two membranes distinct (as is the case for the primary cilia; Hu et al., 2010)?

Also, how does the invagination orient the spindle? This remains unclear because, by the time the spindle forms and a cell begins to divide, the invagination (like the primary cilium) has been retracted or otherwise removed (Figure 1). Perhaps, the invagination pulls the centrosome when it retracts as the process of cell division begins.

Membrane invaginations and cell division.

Schematic diagram showing two neighboring cells in the developing embryo of a sea squirt. (A) Before the cells divide an invagination forms in the membrane at the rear (posterior) side of both cells, while a finger-like protrusion forms on the front (anterior) side of each cell and inserts itself into the invagination of the cell in front of it. The tip of the invagination is attached to the centrosome by microtubules. Specifically, the invagination attaches to one of the two centrioles that make up the centrosome – the same centriole that also grows a primary cilium. (B) When the cells begin to divide, the invagination/protrusions and primary cilia have disappeared and the mitotic spindles are oriented along the anterior-posterior axis. Negishi et al. propose that the membrane invagination holds the centrosome to orient the spindle.

Many other membrane protrusions have roles in cell signaling: is this the case for membrane invaginations too? Negishi et al. show that the formation of the invagination is clearly downstream of the PCP signaling pathway (Negishi et al., 2016), so it is tempting to speculate that PCP signaling acts through the invagination itself.

Finally, the membrane invagination discovered by Negishi et al. adds to an expanding list of membrane protrusions and organelles. As such we cannot help but wonder how many similar structures have been missed in the cells of other organisms and therefore are still waiting to be discovered.

References

Article and author information

Author details

  1. Yukiko M Yamashita

    Life Sciences Institute, Department of Cell and Developmental Biology, Howard Hughes Medical Institute, University of Michigan, Ann Arbor, United States
    For correspondence
    yukikomy@umich.edu
    Competing interests
    The author declares that no competing interests exist.
    ORCID icon "This ORCID iD identifies the author of this article:" 0000-0001-5541-0216

Publication history

  1. Version of Record published: August 9, 2016 (version 1)

Copyright

© 2016, Yamashita

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

  • 1,302
    Page views
  • 102
    Downloads
  • 0
    Citations

Article citation count generated by polling the highest count across the following sources: Crossref, PubMed Central, Scopus.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

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)

  1. Yukiko M Yamashita
(2016)
Membrane Structures: Cellular fingers take hold
eLife 5:e19405.
https://doi.org/10.7554/eLife.19405

Further reading

    1. Biochemistry and Chemical Biology
    2. Cell Biology
    Dirk H Siepe, Lukas T Henneberg ... Kenan Christopher Garcia
    Tools and Resources

    Secreted proteins, which include cytokines, hormones and growth factors, are extracellular ligands that control key signaling pathways mediating cell-cell communication within and between tissues and organs. Many drugs target secreted ligands and their cell-surface receptors. Still, there are hundreds of secreted human proteins that either have no identified receptors ('orphans') and are likely to act through cell surface receptors that have not yet been characterized. Discovery of secreted ligand-receptor interactions by high-throughput screening has been problematic, because the most commonly used high-throughput methods for protein-protein interaction (PPI) screening do not work well for extracellular interactions. Cell-based screening is a promising technology for definition of new ligand-receptor interactions, because multimerized ligands can enrich for cells expressing low affinity cell-surface receptors, and such methods do not require purification of receptor extracellular domains. Here, we present a proteo-genomic cell-based CRISPR activation (CRISPRa) enrichment screening platform employing customized pooled cell surface receptor sgRNA libraries in combination with a magnetic bead selection-based enrichment workflow for rapid, parallel ligand-receptor deorphanization. We curated 80 potentially high value orphan secreted proteins and ultimately screened 20 secreted ligands against two cell sgRNA libraries with targeted expression of all single-pass (TM1) or multi-pass (TM2+) receptors by CRISPRa. We identified previously unknown interactions in 12 of these screens, and validated several of them using surface plasmon resonance and/or cell binding. The newly deorphanized ligands include three receptor tyrosine phosphatase (RPTP) ligands and a chemokine like protein that binds to killer cell inhibitory receptors (KIR's). These new interactions provide a resource for future investigations of interactions between the human secreted and membrane proteomes.

    1. Cell Biology
    Meng Zhao, Niels Banhos Dannieskiold-Samsøe ... Katrin J Svensson
    Research Article

    The secreted protein Isthmin-1 (Ism1) mitigates diabetes by increasing adipocyte and skeletal muscle glucose uptake by activating the PI3K-Akt pathway. However, while both Ism1 and insulin converge on these common targets, Ism1 has distinct cellular actions suggesting divergence in downstream intracellular signaling pathways. To understand the biological complexity of Ism1 signaling, we performed phosphoproteomic analysis after acute exposure, revealing overlapping and distinct pathways of Ism1 and insulin. We identify a 53 % overlap between Ism1 and insulin signaling and Ism1-mediated phosphoproteome-wide alterations in ~ 450 proteins that are not shared with insulin. Interestingly, we find several unknown phosphorylation sites on proteins related to protein translation, mTOR pathway and, unexpectedly, muscle function in the Ism1 signaling network. Physiologically, Ism1 ablation in mice results in altered proteostasis, including lower muscle protein levels under fed and fasted conditions, reduced amino acid incorporation into proteins, and reduced phosphorylation of the key protein synthesis effectors Akt and downstream mTORC1 targets. As metabolic disorders such as diabetes are associated with accelerated loss of skeletal muscle protein content, these studies define a non-canonical mechanism by which this anti-diabetic circulating protein controls muscle biology.