A Cell Conducts Endocytosis by _____

A Cell Conducts Endocytosis by _____.

Endocytosis is used by eukaryotic cells to regulate food internalization, signal transduction, and the limerick of the plasma membrane. However, a more complex picture show is emerging, in which endocytic pathways integrate diverse signals, thereby contributing to a higher level of cellular and organismal organization. In this manner, endocytosis and cell signaling are intertwined in many biological processes, such as jail cell motion and cell fate determination.


Main Text

Although the existence of different endocytic routes is well known, their exact biological impacts are simply get-go to be understood. Clathrin-mediated endocytosis (CME) has traditionally attracted the virtually experimental attention; however, it is now clear that clathrin-contained endocytosis also plays important roles. One form of clathrin-independent endocytosis relies on cholesterol-rich membrane domains, such as lipid rafts and caveolae. Herein, we refer to this form of endocytosis as raft/caveolar endocytosis (RCE). In this review, we provide snapshots of complex situations, in which different endocytic routes orchestrate biochemical pathways and biological beliefs. This work indicates that endocytosis is a central organizer of the jail cell, which coordinates the core variables in prison cell signaling—duration, intensity, integration, and spatial distribution—to control such processes equally cell fate decision and jail cell migration.


Endocytosis Integrates and Attenuates Signaling

Some plasma-membrane receptors are internalized through CME, some through RCE, and some through both. The question is why and nether what circumstances. The answer may be that the 2 routes serve unlike purposes. As an example, when the TGF-β receptor is internalized through CME, information technology is routed to “canonical” endosomes, where information technology signals through the Smad-dependent pathway. Conversely, the fraction of receptors internalized through RCE is delivered instead to a degradative compartment (

).

Work on the epidermal growth gene receptor (EGFR) suggests a mechanism by which the selection between CME and RCE is made. When stimulated with low doses of ligand, EGFRs are near exclusively internalized through CME. However, at higher doses of EGF, RCE is also used. This shift at higher doses of EGF correlates with the monoubiquitination of EGFR. Moreover, chimeric proteins, harboring ubiquitin (Ub) as the sole intracellular signal, are internalized through RCE (

,

). One might speculate that CME is preferred under weather of scarce ligand because it sustains prolonged endosomal signaling, whereas, when abundant ligand is nowadays, backlog stimulation might be avoided by routing part of the receptor population to a degradative (RCE) pathway. This scenario finds back up in mathematical simulations showing that receptor internalization and endosomal signaling are critical for signal output only at low doses of EGF (

). Cumulatively, this work suggests that the net outcome for signaling events is dependent on the ratio of CME to RCE.

However critical questions remain. For instance, how does RCE straight receptors to degradation? Is signaling past receptors other than EGFR and TGF-β receptor likewise integrated by the decision between CME versus RCE? Also, in those cases in which CME functions as a “signaling route,” is the cargo receptor eventually targeted for degradation? At least in the instance of the TGF-β receptor, evidence suggests that CME sustains continuous shuttling of the receptor betwixt the plasma membrane and the signaling endosome rather than leading to deposition (

).


“Endocytomics” and Cellular Organization

A recent paper inaugurated the era of functional genomics of endocytosis in mammalian cells (

). By RNA interference of the man kinome, an unexpectedly loftier number of kinases were implicated in endocytosis. Ii dissimilar viruses (VSV and SV40, which are internalized via CME and RCE, respectively) were used to probe distinct endocytic routes. Surprisingly, the majority of kinases regulate only one of the two endocytic pathways, and, of the 36 kinases that touch both, 23 have opposing effects, enhancing i pathway while suppressing the other. Also, many of the “endocytic kinases” connect endocytosis to other aspects of cellular activeness, such as the cell bicycle, adhesion, and metabolism. For example, a significant negative correlation is scored betwixt prison cell proliferation and the RCE pathway. Moreover, the RCE pathway requires kinases of the integrin pathway, such as FAK, underscoring a connection between jail cell adhesion and RCE. Conversely, a number of kinases that control the CME pathway, such every bit Mylk and PKCζ, are involved in cytoskeleton-dependent send and cell polarity.

Time to come “omics” studies will likely ascertain different subgroups inside CME and RCE. A recent report analyzed the bear upon of half dozen regulatory kinases on the caveolar pathway (

). This revealed two modalities of caveolar dynamics, whereby individual caveolae undergo rapid cycles of fusion and internalization whereas multicaveolar assemblies are static and continued to the extracellular space. Interestingly, the two categories may be regulated past different kinases, and stimulation of RCE by SV40 infection could induce rapid exchange betwixt these two pools of structures.

From a wider viewpoint, these studies unveil an unexpectedly vast regulation of endocytosis past signaling and debate that the 2 programs might be so deeply intertwined that they in fact constitute a single system. Theoretical modeling has already paved the manner for such thinking. For instance, the association of signaling molecules with biological membranes is predicted to increase the number (and/or the average lifetime) of signaling complexes (

). In addition, trafficking membranes could correspond an efficient mode to deliver letters to biologically relevant locations, such equally the nucleus (

). In this framework, the kinases that control both cytoskeleton-dependent traffic and CME (

) could deliver signals to appropriate subcellular locations. Thus, theoretical modeling and experimental evidence (

) strongly advise that endocytosis provides necessary spatial and temporal dimensions to signaling. How this thought relates to the “real” biological earth is illustrated by recent discoveries in the fields of jail cell move and cell fate determination.


Endocytosis and Cell Movement

Cell motion has traditionally been regarded as a plasma-membrane-based signaling procedure in which the engagement of jail cell-surface receptors leads to actin rearrangements and the formation of motile structures such as lamellipodia and dorsal ruffles. Nonetheless, there is emerging evidence that endocytosis is an essential component of cell motility. For instance, in migrating border cells of the fruit fly
Drosophila, endocytosis is used to shuttle the receptors that interact with guidance cues to specific regions of the plasma membrane (

). In this way, endocytosis redirects molecules to regions of “high signaling.” Similarly, studies of the GTPases Rac and Rho, which regulate cell motility, advise that the recruitment and retention of signaling molecules at specific locations of the plasma membrane may exist facilitated by the differential regulation of endocytic pathways (Figure 1). Upon integrin-mediated jail cell adhesion, high-affinity binding sites for Rac go bachelor at the plasma membrane. Signaling downstream of agile Rac promotes the formation of lamellipodia, which are feature of the leading edge of migrating cells. Integrins act locally to forbid the internalization of lipid rafts by RCE, which serve every bit anchorage points for Rac. This process maintains active Rac near sites of integrin-mediated signaling (

,

). A similar mechanism, but involving different molecular effectors, might as well be responsible for localizing Rho (

).

Figure thumbnail gr1

Figure 1
Endocytosis Regulates Jail cell Movement and Invasiveness


Show full explanation

Rac is internalized via macropinosomes and recycled to rafts (1). The internalization of Rac at rafts is inhibited by integrin signaling (2) through the relocalization of phosphocaveolin to focal adhesions (

). A similar mechanism (3), merely requiring FAK (

), targets Rho to rafts. In Src-transformed cells (iv), FAK acts as a scaffold to promote the phosphorylation of endophilin-A2 by Src (

). Phosphorylation of endophilin disrupts its association with dynamin. This event may inhibit endocytosis of the matrix metalloproteinase MT1-MMP and thereby contribute to tumor invasiveness.

In addition to RCE, macropinocytosis is another form of non-clathrin-mediated endocytosis, in which membrane protrusions fuse back with the plasma membrane to produce big vesicles. Membrane spring Rac is too internalized by macropinocytosis. If this process is inhibited, lamellipodia are lost and active Rac accumulates in abnormal membrane ruffles, which could be aborted macropinosomes (

). Taken together, a complex trafficking pattern emerges: Rac is internalized through macropinosomes then recycled to specific regions of the plasma membrane where integrin-mediated inhibition of RCE makes rafts (and their components) available for binding to Rac (Figure 1). This circuitry requires that opposing effects on two endocytic pathways exist coordinated, a concept that also emerged from the contempo genomic studies (

).

Endocytosis besides contributes to the migratory and invasive behavior characteristic of transformed cells. Invasive tumor cells degrade the extracellular matrix through membrane-anchored metalloproteinases, such as MT1-MMP. In Src-transformed cells, a FAK-dependent machinery is activated, which attenuates endocytosis of MT1-MMP. This results in increased degradation of the extracellular matrix, which could contribute to tumor invasion and metastasis (

) (Effigy 1).


Endocytosis and Jail cell Fate Determination

Upon engagement by membrane bound ligands of the DSL (Delta/Serrate/Lag2) family unit, the plasma-membrane receptor Notch is cleaved in the extracellular region (the S2 cut), followed by a cleavage in the transmembrane region (the S3 cut). The liberated intracellular domain translocates to the nucleus, where it acts as a transcriptional regulator (Effigy 2A). Genetic bear witness in
Drosophila
shows that endocytosis of both DSL and Notch is required for Notch activation (Figure 2A;

). Endocytosis of DSL follows at to the lowest degree ii routes, ane dependent on ubiquitination (

) and the other relying on canonical endocytic motifs (

) (Effigy 2A). Whether the 2 pathways correspond RCE and CME, and how they are integrated, is uncertain. Notch also undergoes at to the lowest degree 2 different kinds of internalization, both dependent on ubiquitination (Effigy 2A). Unliganded Notch is continuously endocytosed, probably to forestall its sporadic activation. Conversely, ligand-engaged Notch requires endocytosis for its activation. How ubiquitination couples Notch to both a degradation/recycling route (when not leap by ligand) and activation (when ligand leap) is presently debated (

).

Figure thumbnail gr2

Figure 2
Endocytosis and Prison cell Fate


Show full caption

(A) The endocytosis of Notch and its ligands, Delta, Serrate, and Lag2 (DSL), is required for Notch action. Internalization of DSL (in
Drosophila
and zebrafish) depends on ubiquitination past the E3 ligases Neuralized and Heed flop. Downstream events require the Ub binding poly peptide epsin/lqf
(

). Models to explain why DSL endocytosis is required for Notch activation are shown. (i) Inactive DSLs are endocytosed, “activated” in endosomes, and recycled to the surface. (2) Endocytosis of DSLs generates atmospheric condition (by mechanical “pulling” forces) that unmask the Notch S2 site. (three) Ligand-engaged Notch requires endocytosis (possibly dependent on ubiquitination by Deltex) for its activation. (4) Unliganded Notch is continuously endocytosed, through ubiquitination by the E3 ligases, Su(dx)/AIP4 and Nedd4, to prevent sporadic activation. This might route Notch to an endosomal compartment where it can interact with presenilin, an effector of the S3 cut (

).

(B) Creation of disproportion in pIIa and pIIb cells in
Drosophila
is regulated by endocytosis. The SOP cell is shown with relevant molecules (dashed line, mitosis). (ane) Notch is nonfunctional in pIIb cells considering information technology is internalized/degraded or because Sanpodo is internalized. A working model (

) attempts to reconcile these two possibilities. Sanpodo itself regulates Notch endocytosis. In the absence of Numb (pIIa), Sanpodo might route Notch to an “activating” endosomal compartment (the “liganded” pathway in [A]). In the presence of Numb (pIIb), Sanpodo might participate in Notch downregulation (the “unliganded” pathway in [A]). The E3 ligase Neuralized is asymmetrically partitioned in pIIb, allowing endocytosis of Delta. (two) Following endocytosis, Delta is routed to a Rab11 endosome, and so to the plasma membrane (3). (four) In pIIa, this pathway is blocked, peradventure because a critical Rab11 partner (Nuclear fallout/Arfophilin 1) is inactivated (

). How Delta is internalized in pIIa in the absence of Neuralized is not clear, although there may also be Neuralized-independent mechanisms of Delta internalization (

). Delta might likewise be internalized before mitosis of the SOP jail cell; in pIIb, it could be recycled to the plasma membrane, whereas in pIIa cells, information technology might be destined to a degradative pathway (

). The events shown demand not exist all or none, but nonetheless occur in both cells but be biased in favor of one.

How all of these endocytic pathways converge to execute a multifaceted biological program is exemplified by studies of cell fate determination. In asymmetric cell segmentation, fate determinants are differentially partitioned between daughter cells. In the genesis of the sensory organ of
Drosophila, the forerunner cell (SOP) divides asymmetrically, generating a pIIa and a pIIb cell (Figure 2B), which have singled-out fates considering Notch signaling is activated only in pIIa. This is due to asymmetric division of Numb (an endocytic poly peptide and an antagonist of Notch) in the pIIb jail cell. Mutants of α-adaptin, a component of the endocytic adaptor AP-2, mimic Numb loss of function (

). Because Numb binds to α-adaptin and Notch, it might induce Notch endocytosis in pIIb. An alternative possibility is suggested past studies of Sanpodo, a protein required for Notch signaling. Although Sanpodo is non asymmetrically partitioned, its subcellular localization is dissimilar in pIIa and pIIb cells. In pIIa, Sanpodo is at the plasma membrane, whereas in pIIb, Sanpodo is internalized through Numb/α-adaptin-dependent endocytosis (

). Thus, Numb might regulate endocytosis of Sanpodo, making it inaccessible to Notch, thereby suppressing Notch signaling (Effigy 2B).

The endocytosis of Delta is too involved in pIIa/pIIb specification (Effigy 2B). In one case internalized, Delta passes through Rab11-positive recycling endosomes (

). The Rab11 endosome should recycle Delta to the plasma membrane of pIIb, assuasive date of Notch. This possibility is corroborated by findings that Sec15, a putative effector of Rab11, is critical in this pathway (

). Sec15 is a component of the exocyst in the secretory pathway, which mediates tethering of vesicles to the plasma membrane. In pIIa, the Rab11-based mechanism is suppressed (

), thereby hindering recycling of Delta and generating asymmetry.


Outlook

Equally our agreement comes into focus, endocytosis and signaling appear as two sides of the same money. This raises interesting questions to exist tackled in the future. For example, can the same biochemical pathways attain different biological outcomes but by being constrained by different configurations of membrane organelles and having different patterns of endocytosis? Similarly, given the high degree of overlap betwixt the pathways that are activated by diverse signaling receptors, is “coincidence detection” on endomembranes a machinery to resolve these many inputs into specific signals? There is already strong prove that this happens in phosphoinositide (PI) signaling—PIs and PI binding proteins prefer a restricted configuration on cellular membranes through a serial of signaling-mediated events (

).

Also, the need for increased signaling complexity in the form of evolution might have been met, at least in function, by increasing the complexity of the endocytic membrane arrangement. An opportunity to test this possibility is offered by studies of the GTPase dynamin (

). The primordial function of dynamins is related to mitochondrial inheritance. During development, some of the dynamins were “recruited” to the endocytic pathway, where they execute the fission of vesicles. The study by

shows that the acquisition of an endocytic role by dynamin occurred independently during the ciliate and metazoan radiation. If convergent development of this “endocytic” event is related to the independent acquisition of the same signaling-related properties or phenotypes, this would found a spectacular demonstration of the concept.

Finally, the increasing understanding of the link between endocytosis and signaling raises the possibility that targeted interference of endocytosis might modify disease-linked phenotypes, especially those that are associated with aberrant cell specification. This might lead to new means of manipulating stem cells and could increment our understanding of pathological conditions such every bit cancer.


Acknowledgments

We thank P. De Camilli and M. Gonzales Gaitan for discussions. The authors’ work is supported by the AIRC, the AICR, the EC, the Italian Ministry building of Education and University, and the Monzino Foundation.


References

    • Carlton J.G.
    • Cullen P.J.


    Trends Jail cell Biol.
    2005;

    fifteen
    :
    540-547

    • Chen H.
    • De Camilli P.


    Proc. Natl. Acad. Sci. USA.
    2005;

    102
    :
    2766-2771

    • del Pozo Thousand.A.
    • Alderson N.B.
    • Kiosses West.B.
    • Chiang H.H.
    • Anderson R.M.
    • Schwartz M.A.


    Science.
    2004;

    303
    :
    839-842

    • del Pozo M.A.
    • Balasubramanian N.
    • Alderson Due north.B.
    • Kiosses Westward.B.
    • Grande-Garcia A.
    • Anderson R.Thou.
    • Schwartz M.A.


    Nat. Cell Biol.
    2005;

    seven
    :
    901-908

    • Di Guglielmo G.M.
    • Le Roy C.
    • Goodfellow A.F.
    • Wrana J.L.


    Nat. Prison cell Biol.
    2003;

    5
    :
    410-421

    • Elde Due north.C.
    • Morgan Thou.
    • Winey M.
    • Sperling 50.
    • Turkewitz A.P.


    PLoS Genet.
    2005;

    1
    :
    e52
    https://doi.org/10.1371/journal.pgen.0010052

    • Emery G.
    • Hutterer A.
    • Berdnik D.
    • Mayer B.
    • Wirtz-Peitz F.
    • Gaitan Chiliad.G.
    • Knoblich J.A.


    Cell.
    2005;

    122
    :
    763-773

    • Hutterer A.
    • Knoblich J.A.


    EMBO Rep.
    2005;

    half-dozen
    :
    836-842

    • Jafar-Nejad H.
    • Andrews H.Chiliad.
    • Acar M.
    • Bayat Five.
    • Wirtz-Peitz F.
    • Mehta Due south.Q.
    • Knoblich J.A.
    • Bellen H.J.


    Dev. Cell.
    2005;

    9
    :
    351-363

    • Jekely Thou.
    • Sung H.H.
    • Luque C.1000.
    • Rorth P.


    Dev. Cell.
    2005;

    9
    :
    197-207

    • Kholodenko B.N.


    J. Exp. Biol.
    2003;

    206
    :
    2073-2082

    • Le Borgne R.
    • Bardin A.
    • Schweisguth F.


    Development.
    2005;

    132
    :
    1751-1762

    • Liu G.
    • Swihart G.T.
    • Neelamegham South.


    Bioinformatics.
    2005;

    21
    :
    1194-1202

    • Miaczynska One thousand.
    • Pelkmans L.
    • Zerial M.


    Curr. Opin. Prison cell Biol.
    2004;

    16
    :
    400-406

    • Palazzo A.F.
    • Eng C.H.
    • Schlaepfer D.D.
    • Marcantonio Due east.E.
    • Gundersen Thou.M.


    Science.
    2004;

    303
    :
    836-839

    • Pelkmans L.
    • Zerial 1000.


    Nature.
    2005;

    436
    :
    128-133

    • Pelkmans Fifty.
    • Fava Due east.
    • Grabner H.
    • Hannus Thou.
    • Habermann B.
    • Krausz Eastward.
    • Zerial M.


    Nature.
    2005;

    436
    :
    78-86

    • Schlunck G.
    • Damke H.
    • Kiosses W.B.
    • Rusk N.
    • Symons Grand.H.
    • Waterman-Storer C.M.
    • Schmid Due south.L.
    • Schwartz K.A.


    Mol. Biol. Jail cell.
    2004;

    xv
    :
    256-267

    • Sigismund S.
    • Woelk T.
    • Puri C.
    • Maspero E.
    • Tacchetti C.
    • Transidico P.
    • Di Fiore P.P.
    • Polo S.


    Proc. Natl. Acad. Sci. United states.
    2005;

    102
    :
    2760-2765

    • Wang W.
    • Struhl G.


    Development.
    2005;

    132
    :
    2883-2894

    • Wu X.
    • Gan B.
    • Yoo Y.
    • Guan J.L.


    Dev. Cell.
    2005;

    nine
    :
    185-196


  • View Large Image

  • Download How-do-you-do-res image

A Cell Conducts Endocytosis by _____

Source: https://www.cell.com/cell/fulltext/S0092-8674(06)00242-X