Itt írjon a(z) MembraneContact-ról/ről

Signalling at Membrane Contact Sites

Introduction and Definition:

“Regions of close apposition between two organelles often referred to as membrane contact sites (MCSs), mostly form between the endoplasmic reticulum and a second organelle, although contact between mitochondria and other organelles have also begun to be characterised.”(Prinz, 2014):

Membrane contact sites are close connections between two organelles in a cell that facilitate both signalling and the exit and entry of metabolites and other small molecules from one cellular compartment to another (Elbaz et al, 2011). Membrane Contact Sites between organelles are a characteristic of eukaryotic cells due to their individually membraned organelle systems within one cell. These organelles provide the optimum conditions for specific cellular reactions, and membrane contact sites are suggested to play a role in collaboration and communication between these individual networks, leading to complete cellular homeostasis and development (Elbaz et al, 2011).

Role:

The role of MCSs is vast with some areas being well investigated but other areas requiring more research. Some examples of MCS roles include: critical roles in metabolism; intracellular signalling; trafficking of metabolites; organelle inheritance, division and transport. Most widely known is the involvement of MCSs in intracellular exchange of calcium (Lewis, 2007) and lipids (Voelker, 2009), which allows direct channeling between organelles via these membrane contact sites. More recently studies by English & Voeltz (2013) and Helle et al, (2013) have investigated the importance of MCS roles in intracellular signalling, organelle trafficking and inheritance and have identified that MCSs are specialised regions where regulatory complexes are assembled.

Properties of MCSs:

The main properties of MCSs include: close apposition (usually within 30nm) of two intracellular membranes; membranes are unfused; enrichment of MCSs with specific lipids and/or protein; and functioning of one or more organelles is affected by the formation of MCSs. These properties are necessary for characterisation, as cases exist whereby membranes interact with one another or are “tethered” to one another but are not considered to be true membrane contact sites (Prinz, 2014).

Tethers and Membrane Contact Sites:

Certain proteins or protein complexes exist at membrane contact sites, their role being two-fold. Firstly, their involvement in binding of the two opposing membranes and, secondly, their role in maintenance of the site (English & Voeltz 2013; Helle et al, 2013). Due to a lack of evidence and supported results it is unclear how necessary such tethering proteins are in sustaining contact (Prinz, 2014).

MCSs, Organelle Trafficking, Inheritance, Organelle Division:

A further possible role of MCSs is their involvement in inheritance, as a result of their regulatory effects on membrane trafficking. This process can be seen in “budding yeast”, which is the mechanism by which yeast asexually reproduce. ER-PM associations are vital for transport of Perioxisomes (involved in the breakdown of long-chain fatty acids through beta-oxidation) and mitochondria. This transport is necessary for successful organelle inheritance (Prinz, 2014). The inheritance (by daughter cells) and equal sharing (ensuring mother cell’ supply) of perioxisomes is maintained by tethering proteins. Accumulation of peroxisomes can be seen in daughter cells when a lack of ER-peroxisome tether exists (Prinz, 2014) thus exhibiting their necessity. The inheritance of mitochondria in yeast also requires membrane contacts, notably between the ER and PM. Such contacts are important in ensuring correct distribution of mitochondria between mother and daughter cells (similar to peroxisome distribution) and especially in the retention of mitochondria in the mother cells (Prinz, 2014). Furthermore, tethering between the ER and mitochondria are also shown to be involved in Mitochondrial inheritance (Swayne et al, 2013). Finally, MCSs have recently been shown to be involved in organelle division, namely mitochondrial fission through contact between the ER and mitochondria (Friedman et al, 2011). The mechanism involves tethering complexes, detailed info on the entire process however is lacking.

Methods of Communication:

1. ER-PM Communication

It has been found in mammalian cells that a series of specific sensors and channels can be stimulated by stresses enforced on the cells to form MCS for organelle-organelle communication (Toulmay & Prinz et al, 2011). These stresses can include nutrient starvation, blockage of channels leading to impacted cellular processes, or dangerous levels of metabolites threatening cellular homeostasis. (Rocha et al, 2009). It is worth noting that this specific type of MCS communication has come into light in recent years for being directly linked with maintaining a calcium homeostasis in the cell in particular in cardiomyocytes, cells that show excitement and activity depending on their calcium influx (Elbaz et al, 2011). Calcium sensor STIM1 on the surface of the ER can stimulate the opening of PM calcium channel ORAI1, activated to increase calcium levels in the cell during times of imbalance. These two organelle areas form a site of tightly regulated communication (Deng et al, 2009) As for their communication mechanism, ORAI1 and STIM1 sites arrange themselves across from each other on their corresponding membrane surfaces. This allows for local CRAC channels to bind the two sites for a direct MCS flow of CA²⁺ ions (Wu et al, 2006). Although this seems like an optimum way to maintain sufficient metabolite levels in the cells, studies have shown that only one specific channel can be activated at any time (Park et al, 2010), as a method of calcium regulation.

2. ER-Mitochondria Communication

Mitochondrial MCS are a new concept with favorable results. 5-20% of HeLa cells have been found to have surface contact networks between the ER and their mitochondria (Rizzuto et al, 1998). Another yeast study has shown over 100 membrane contact sites between these two organelles (Achleitner et al, 1999). The main idea projected about ER-mitochondria communication regards narrow distances between communications, around 10-30nm (Csordas G, et al, 2006) and the protein tethering factor of mitochondria associated membrane (MAM) fraction (Morre et al, 1971; de Brito et al, 2010). This MAM factor has been observed in lipid delivery between organelles. In the journey of newly synthesized phosphatidylserine, produced in the ER, it is transported to the mitochondria for further catalysation into phosphatidylethanolamin, before going back to the ER to yield levels of PC. This transport and movement is all facilitated by MAM factor and MCS between the two organelles (Daum et al, 1997). Sterols can also be synthesized and shuttled between organelles via MCS in particular ER-mitochondria. Cholesterol is transported from the mitochondria to the ER where it is synthesized into pregnenol, a precursor for many other sterols, and then transported back to the ER via MCS adjacent cellular structures (Miller et al, 1998)

3. The Nucleus-Vacuole Junction

Although vacuoles are primarily associated with plants and not eukaryotic cells, they too have displayed evidence of MCS, with counterparts in higher eukaryotes discussed later on. Studies for this theory have been carried out in Saccaromyces cerevisiae yeast, and the NVJs, or nucleus vacuole junction sites have been the most promising find. Three membranes are connected in these contact sites: the vacuole membrane, the ER membrane, and the nucleus membrane in the yeast cell (Elbaz et al, 2011). NVJ have shown strong evidence supporting the process of microautophagy, a type of pathway that is mediated by the vacuolar engulfment of any cytoplasmic material via membrane invagination that may be threatening to the cell. This is carried out in the presence of an electrochemical gradient across the vacuole membrane (Dawaliby et al, 2010). Due to genome studies, there is evidence to suggest a direct link between the vacuole protein Vac8 and the ER protein Nvj1. Through studies of elimination, the absence of either of these proteins results in the loss in MCS or communication activity between NVJs (Pan et al, 2010)

4. ER-Lysozymes or Endosome Communication

Since mammalian cells do not possess vacuoles for nutrient storage, Lysozymes and endosome are organelles that are the yeast counterpart in higher eukaryotes (Rocha et al, 2009). Like mentioned before, cellular stresses activate MCS and metabolite transport. This type of transport has been studies in higher eukaryotic cells. Studies have shown that protein ORP1L mediated the communication between the two organelles under the cellular state of low cholesterol (Rocha et al, 2009) ORP1L enlists the small GTPase Rab7 and Rab-7 interacting lysosomal protein found on endosomes (Johansson et al, 2007). This mechanism is cholesterol sensitive, and works in the following way.

• “ORP1L protein undergoes a conformational change that allows its FFAT domain to interact with VAP proteins on the ER, leading to the release of p150GLUED snf to the formation of close contacts between the ER and endosomes” (Rocha et al, 2009).

This type of communication is of strong interest to the theory developers, although it is yet to be determined if cholesterol is actually exchanged at these newly formed MCS (Prinz et al, 2011).

Calcium Signalling

The SERCA (Sarcoplasmic/Endoplasmic reticulum Ca2+ ATPase) pumps Ca²⁺ into the ER lumen. The PMCA (Plasma membrane Ca²⁺ ATPase) pump Ca²⁺ out of the cell. This gradient is then used in signalling event (Helle et al, 2013). Calcium signalling administers a variety of different processes such as memory, vision, fertilization, muscle contraction, proliferation, cell migration, immune response and transcription (Helle et al, 2013).

Excitation - Contraction in Muscle Cells

The signal that generates muscle contraction comes from the contact site of the muscle ER and PM. An influx of Ca²⁺ into the cytosol activates myosin movement, which triggers muscle contraction. Synergetic activation of two mechanisms causes this calcium influx. These 2 mechanisms are:

1. “The opening of voltage-gated PM Ca²⁺ channels, such as the dihydropyridine receptor (DHPR), which respond to a change in PM potential originating at the neuromuscular synapse” (Brette et al, 2007)

2. “The synchronous opening of the main SR Ca²⁺ channel RyR (Ryanodine Receptors” (Brette et al, 2007)

Rapid and precise muscular contraction is the result of the coordination of cellular events, which is permitted because of the close proximity of SR and PM membrane contact sites.

Calcium - Dependent Respiration

In the Krebs cycle there are three mitochondrial enzymes that are activated by Ca²⁺, these enzymes are Pyruvate dehydrogenase, NAD+- isocitrate dehydrogenase, and 2-oxoglutarate dehydrogenase (Denton, 2009). Therefore mitochondrial energy production is the result of mitochondrial Ca²⁺ uptake via MCSs. Membrane contact sites in lipid exchange Lipid transport protein functions include the exchange and transport of specific lipids across the aqueous phase of the cell and monitor the intercompartmental lipid levels. (Helle et al, 2013) Energy-dependent processes provide lipid exchange directionality but membrane contact sites provide specificity. Several lipid transport proteins are targeted to membrane contact sites. For example “Both CERT (ceramide transporter) and several ORPs (Oxysterol-binding protein related proteins) have two targeting domains: one PH-domain and one FFAT motif (diphenylalanine-in-an-acidic-tract motif). The FFAT-motif anchors these lipid transport proteins to the ER by interacting with the ER-protein VAP (VAMP-associated-protein), while the PH domain binds to PIPs (phosphatidylinositol phosphates) on closely apposed membranes” (Toulmay et al, 2011).

Lipid Storage

Energy storage is one of the main functions of lipids. Lipids store energy in the form of neutral lipids such as triglycerides, which are stored in intracellular lipid droplets. “Lipid droplets (LDs) are thought to emanate from the ER by triglyceride and steryl esters accumulation in between the two leaflets of the ER membrane” (Walther et al, 2012; Sturley et al, 2012). Lipid droplets core is made up of neutral lipids surrounded by “single leaflet of amphipathic lipids”(Helle et al, 2013). The continuity between the ER and single leaflet surrounding lipid droplets is unclear (Jacquier et al, 2011). Although it is clear that lipid droplets are covered with ER cisternae on their surface (Blanchette-Mackie et al, 1995). These ER-LD contact sites likely have a translocation function (Zhang et al, 2012).

Examples of Membrane Contact Sites:

As well as MCSs existing between organelles (inter-), they may also be present within organelles (intra-), i.e. between specific compartments of a singular organelle (Prinz, 2013). Examples of these include within Golgi cisternae, absence of which causes disassembly of the Golgi stacks demonstrating their importance (Xiang & Wang, 2010). The presence of internal membranes seen, for example, in mitochondria, chloroplasts, and multi vesicular bodies also presents the opportunity for the formation of MCSs (Prinz, 2014). In this case MCSs may be formed either:

⁽ⁱ⁾ Between internal membranes and the outer membrane of the organelle or;

⁽ⁱⁱ⁾ Within the organelle between membranes. An example of which was found between the mitochondrial cisternae and also between the mitochondrial outer membrane and the cisternae (Harner et al, 2011; Hoppins et al, 2011; von der Malsburg et al, 2011)

Further examples of MCSs, identified by Toulmay & Prinz (2011), in yeast Saccharomyces cerevisiae include between Endoplasmic Reticulum (ER) and mitochondria; between plasma membrane and the ER; and the NVJ, which is the nucleus-vacuole junction and exists between the nucleus and the vacuole. ER-mitochondria contacts are facilitated by the ER-mitochondrion encounter structure (ERMES), Kornmann et al, (2009).

The role of membrane contact sites are vast and a promising area for many reasons. It is an area that warrants further research as certain areas examined lacked supporting evidence.

References

• Achleitner, G; Gaigg, B; Krasser, A; Kainersdorfer, E; Kohlwein, SD; Perktold, A; Zellnig, G; Daum, G. (1999): Association between the endoplamsic reticulum and mitochondria of yeast facilitates interorganelle transport of phospholipids through membrane contact. Eur J Biochem. 264: 545-553. [PubMed: 21220505]

• Alexandre Toulmay; William, A. Prinz (2011): Lipid Transfer and signalling at organelle contact sites: the tip of the iceberg. Laboratory of cell biochemistry and biology, National institute of diabetes and digestive and kidney diseases, National institute of health, Bethesda, MD 20892, USA.
• Blanchette-Mackie, E.J; Dwyer, N.K; Barber, T; Coxey, R.A; Takeda, T; Rondinone, C.M; Theodorakis, J.L; Greenberg, A.S; Londos, C. (1995): Perilipin is located on the surface layer of intracellular lipid droplets in adipocytes, J. Lipid Res. 36 1211–1226.
• Brette, F; Orchard, C. (2007): Resurgence of cardiac t-tubule research, Physiology (Bethesda) 22 167–173.
• Csordas, G. et al. (2006): Structural and functional features and significance of the physical linkage between ER and mitochondria. J. Cell Biol. 174, 915-921
• Daum, G. and Vance, J.E. (1997): Import of lipids in mitochondria. Prog. Lipid Res. 36, 103-130
• Dawaliby, R. and Mayer, A. (2010): Microautophagy of nucleus coincides with a vacuolar diffusion barrier at nuclear-vacoular junctions. Mol. Biol. Cell 21, 4173-4183.
• De Brito , O.M. and Scorrano, L. (2010): An intimate liaison: spatial organiganization of the endoplasmic reticulum-mitochondria relationship. EMBO J. 29, 2715-2723

• Deng, X; Wang, Y; Zhou, Y; Soboloff, J; Gill DL. STIM and Orai (2009): Dynamic intermembrane coupling to control cellular calcium signals. J Biol Chem. 284:22501-22505 [PubMed:19472984]

• Denton, R.M. (2009): Regulation of mitochondrial dehydrogenases by calcium ions, Biochim. Biophys. Acta 1787 1309–1316.
• Elbaz, Y; Schuldiner, M. (2011): Staying in touch: the molecular era of organelle contact sites. Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel

• English, A.R. and G.K. Voeltz. (2013): Endoplasmic reticulum structure and interconnections with other organelles. Cold Spring Harb. Prospect. Bill. 5:a013227. http://dx.doi.org/10.1101/chsperspect.a013227

• Friedman, J.R.; L.L., Lackner; M., West; J.R., BiBenedetto; J., Nunnari and G.K. Voeltz (2011): ER tubules mark sites of mitochondrial division. Science. 334:358-362. http://dx.doi.org/10.1126/science.1207385

• Harner, M.; C., Körner; D., Walther; D., Mokranjac; J., Kaesmacher; U., Welsch; J., Griffith; M., Mann; F., Reggiori; and W. Neuport (2011): The mitochondrial contact site complex, a determinant of mitochondrial architecture. EMBO J. 30:4356-4370. http://dx.doi.org/10.1038/emboj.2011.379

• Helle, S.C.; G., Kanfer; K., Kolar; A., Lang; A.h., Michel; and B. Kornamm (2013): Organization and function of membrane contact sites. Biochem. Biophys. Acta. 1833:2526-2541. http://dx.doi.org/10.1016/j.bbamcr.2013.01.028

• Hoppins, S; S.R., Collins; A., Cassidy-Stone; E., Hummel; R.m., Devay; L.L, Lackner; B., Westermann; M., Schuldiner; J.S., Weissman; and J. Nunnari (2011): A mitochondrial-focused genetic interaction map reveals a scaffold-like complex required for inner membrane organization in mitochondria. J. Cell Biol. 195:323-340. http://dx.doi.org/10.1083/jcb.201107053

• Jacquier, N; Choudhary, V; Mari, M; Toulmay, A; Reggiori, F; Schneiter, R. (2011): Lipid droplets are functionally connected to the endoplasmic reticulum in Saccharo- myces cerevisiae, J. Cell Sci. 124 2424–2437.

• Johnasson, M; Rocha, N; Zwart, W; Jordens, I; Janssen, L; Kuijl, C; Olkkonen, VM; Neefjes, J. (2007): Activation of endosomal dynein motors by stepwise assembly of Rab7-RILP-p150glued, ORP1L, and the receptor betall1 spectrin. J Cell Biol. 176;459-471 [PubMed: 17283181]

• Kornamm, B.; Currie, E.; Collins, S. R.; Schuldiner, M.; Nunnari, J.; Weissman, J.S.; and Walter, P. (2009): An ER-mitochondria tethering complex revealed by a synthetic biology screen. Science 325, 477-481
• Lewis, R. S. (2007): The molecular choreography of a store operated calcium channel. Nature 446, 284-287
• Miller, W.L. (1988): Molecular biology of steriod hormone synthesis. Endocr. Rev 9, 295-318
• Morre, DJ. et al. (1971): Connections between mitochondria and endoplasmic reticulum in rat liver and onion stem. Protoplasma 73, 43-49.
• Pan, X. et al (2010): Nucleus-vacoule junctions in Saccharomycese cerevisiae are formed through the direct interaction of Vac8p with Nvj1p. Mol. Biol. Cell 11, 2445-2457.
• Park, C.Y. et al, (2010): The CRAC channel activator STIM1 binds and inhibits L-type voltage gated calcium channels. Science 330, 101-105.
• Prinz, WA. (2014): Bridging the gap: Membrane contact sites in signalling, metabolism, and organelle dynamics. J.Cell.Biol. Vol 205. Number 6. 759-769.

• Rizzuto, R; Pinton, P; Carrington, W; Fay, FS; Fogarty, KE; Liftshitz, LM; Tuft, RA; Pozzan, T. (1998): Close contacts with the endoplamic reticulum as determinants of mitochondrial CA2+ responses. Science. 280: 1763-1766. [PubMed: 9624056]

• Rocha, N; Kuijl, C; van der Kant, R; Janseen, L; Houben, D; Janssen, H; Zwart, W; Neefjes, J. (2009): Cholesterol sensor ORP1L contacts the ER protein VAP to control Rab7-RILP-p150 Glued and late endosome positioning. J Cell Biol. 185;1209-1225. [PubMed: 19564404]

• Sturley, S.L.; Hussain, M.M. (2012): Lipid droplet formation on opposing sides of the endoplasmic reticulum, J. Lipid Res. 53 1800–1810.

• Swayne, T.C.; C, Zhou; I.R., Boldogh; J.K., Charalel; J.R., McFaline-Figueroa; S., Thoms; C., Yang; G., Leung; J., McInnes; R., Erdmann; and L.A. Pon (2011): Role for cER and Mmr1p in anchorage of mitochondria at sites of polarized surface growth in budding yeast. Curr. Biol. 21:1994-1999. http://dx.doi.org/10.1016/j.cub.2011.10.019

• Toulmay, A; Prinz, W.A. (2011): Lipid transfer and signaling at organelle contact sites: the tip of the iceberg, Curr. Opin. Cell Biol. 23 458–463.
• Toulmay, A; Prinz, W.A. (2011): A conserved membrane-binding domain targets proteins to organelle contact sites. J.Cell Science. 125. 49-58.
• Voelker, D.R. (2009): Genetic and biochemical analysis of non-vesicular lipid traffic. Ann. Rev. Biochem. 78, 827-856

• Von der Malsburg, K.; J.M., Müller; M., Bohnert; S., Oeljeklaus; P., Kwiatkowska; T., Becker; A., Loniewska-Lwowska; S., Wiese; S., Rao; D., Milenkovic et al. (2011): Dual role of mitofilin in mitochondrial membrane organization and protein biogenesis. Dev. Cell. 21:694-707. http://dx.doi.org/10.1016/j.devcel.2011.08.026

• Walther, T.C; Farese, R.V. (2012): Lipid droplets and cellular lipid metabolism, Annu. Rev. Biochem. 81 687–714.
• Wu, M.M. et al (2006): Ca2+ store depletion causes STIm1 to accumulate in ER regions closely associated with the plasma membrane. J. Cell Biol. 174, 803-813.

• Xiang, Y. and Y, Wang. (2010): GRASP55 and GRASP65 play complementary and essential roles in Golgi cisternal stacking. J.Cell Biol. 188:237-251. http://dx.doi.org/10.1083/jcb.200907132

• Xu, N; Zhang, S.O; Cole, R.A; McKinney, S.A; Guo, F; Haas, J.T; Bobba, S; Farese, R.V; Mak, H.Y. (2012): The FATP1–DGAT2 complex facilitates lipid droplet expansion at the ER–lipid droplet interface, J. Cell Biol. 198 895–911.