Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Rab7a in Endocytosis and Signaling

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_623

Synonyms

Rab7 Historical Background and Function

Mammalian Rab7 was first identified in a rat liver cell line as BRL-Ras [X12535; NM_023950] and subsequently named Rab7 when it was recognized to be a member of an emerging, separate branch of Ras-related GTPases now well known as the Rab family of GTPases [NP_004628.4; P51149; P09527]. Rab7a is the most widely studied form and encoded on human chromosome 3q21.3 (mouse chromosome 6) as two splice variants differing in the 3′ untranslated region. The most intensively studied mammalian forms of Rab7a (mouse, canine, rat, and human) are 99.5% identical with only a single conservative change among the 207 amino acids (D/E 196). A more recently discovered homolog, Rab7b/Rab7L1, is encoded on human and mouse chromosome 1q32 and functions in late endosome to Golgi trafficking [Q96AH8; Q8VEA8]. Human Rab7b is only 47% identical and 82% homologous to human Rab7a across its 199 aa length. Following the initial demonstration of Rab7a function in regulating membrane transport from early to late endosomes, Rab7a has been found to have critical roles in autophagy, lipid metabolism, growth factor signaling, bone resorption, and phagolysosome biogenesis (Fig. 1) (Agola et al. 2011).
Rab7a in Endocytosis and Signaling, Fig. 1

Rab7a-regulated pathways. Rab7a regulates endocytic transport from early to late endosomes in a process requiring Rab5 to Rab7a conversion. Rab7a also cooperates with other Rab GTPases to facilitate late endosome-lysosome fusion and phagolysosome formation (see Table 1). Key Rab7a effectors involved on individual pathways are noted in parentheses. Rab7a cooperates with Rac1 in epithelia to promote internalization of cell adhesion molecules and in osteoclasts to promote localized hydrolase secretion for bone resorption

Rab7a Activation and Localization

Typical of Ras-related GTPases, Rab7a undergoes a cycle of membrane association and dissociation that is closely linked to nucleotide binding and hydrolysis (Fig. 2) (Agola et al. 2011). In the membrane-associated state, Rab7a is GTP bound and active, while upon hydrolysis the GDP-bound Rab7a is inactive and recycles to the cytoplasm. The GDP−/GTP-dependent activation cycle is regulated by two sets of proteins, the guanine nucleotide exchange factors (GEFs) that catalyze the GTP binding and conversion to the active GTP-bound state and the GTPase-activating proteins (GAPs) that stimulate the GTPase activity of the Rab protein to convert it to the inactive GDP-bound state. Early reports suggested Vps39 functioned as a Rab7a GEF. More recently, studies in yeast and C. elegans indicate that Ypt7p/Rab7 activation is linked to endosome conversion involving coordinate inactivation and loss of the early endosomal Rab5 and acquisition and activation of the late endosomal Rab7a through large multimeric complexes with overlapping components (Fig. 3a) (Wang et al. 2011a). SAND-1 (Mon1a-Mon1b in vertebrates) binds to the CORVET components (Vps11, VPS16A, Vps18, and Vps33) and functions to displace the Rab5 GEF (RABX-5). Subsequent recruitment of Ccz1 through Mon1 and cooperation with Vps39 is central to the endosomal recruitment and activation of Ypt7p in yeast and Rab7 C. elegans enabling binding to the HOPS complex (Vps11, Vps16A, Vps18, Vps33, and Vps41) (Fig. 2). Mon1 and Ccz1 are conserved in mammals (Wang et al. 2011a), though mammalian Rab7a GEF activity remains to be demonstrated.
Rab7a in Endocytosis and Signaling, Fig. 2

Rab7a activation cycle. Newly synthesized Rab7a is prenylated by geranylgeranyl transferase (GGT) and delivered to endosomal membranes by rab escort protein (REP), thereafter Rab7a membrane cycling is facilitated by GDP dissociation inhibitor (GDI); pathways that are common to all Rab GTPases. A GDI displacement factor (GDF) has been implicated in membrane transfer of late endocytic Rab9 and Rab7a. Rab7a activation is closely linked to Rab5 inactivation in a conversion process that involves Mon1. A Vps39-Mon1-Ccz1 complex likely acts as a guanine nucleotide exchange factor (GEF) to promote activation, while TBC1D5 or TBC1D15 act as GTPase-activating proteins (GAPs) to promote hydrolysis and inactivation. Active, GTP-bound Rab7a acts as a scaffold for sequentially binding multiple effectors phosphoinositide 3-kinase (PI3K, Vps34/Vps15), HOPS, Rubicon, UVRAG, among others (see Table 1) to promote cargo selection, cytoskeletal translocation, and membrane fusion.

Rab7a in Endocytosis and Signaling, Fig. 3

Rab7a trafficking and signaling complexes. (a) Recycling from early endosomes to the Golgi. Cargo recycling from early endosomes to the Golgi entails sequential Rab5 (via CORVET) and Rab7a activation and Rab7a-mediated recruitment of a complex of Vps proteins (26/29/35) known as retromer. Rab5 activation can be positively modulated by EGF receptor signaling. (b) Transport to lysosomes. Rab7a cooperates with the Dyn2-CIN85 complex to regulate signaling and lysosomal degradation of the ligand-receptor (EGF-EGF receptor) complex. Bidirectional transport on microtubules depends on kinesin and dynein motors. (c) Transport from early to late endosomes. Growth factor receptor signaling is intimately coupled to endocytic transport. As illustrated, nerve growth factor receptor TrkA and EGF receptor associate with MAPK on endosomes are translocated bidirectionally on microtubules. Transport toward perinuclear late endosomes occurs through association with the Rab7a effector RILP and the p150/dynein motor complex. Association with HAP1/Htt contributes to perinuclear late endosome positioning. Transport to the cell periphery is mediated in association with kinesin motor complexes (Kif3a/Kinesin-2 via FYCO1 or Kinesin-1 via SKIP). (d) Distinct multi-protein complexes regulate transport to and from late endosomes. Rab5-Rab7a conversion involves coordinate inactivation of Rab5 and activation of Rab7a, transition of CORVET complex to HOPS complex, which ensures seamless cargo transport to late endosomes. Handoff to ESCRT machinery enables membrane invagination and sequestration of growth factor receptors on intraluminal vesicles of multivesicular bodies. Rab7a-retromer complex enables Golgi recycling. Rab7a HOPS complex enables late endosome-lysosome fusion

The activation of Rab7a is dynamically regulated through differential interactions of proteins first identified to be important in autophagy called Rubicon (RUN domain and cysteine-rich domain containing Beclin 1-interacting protein) and UVRAG (Liang et al. 2008; Zhong et al. 2009) (Fig. 2). Rubicon, a regulatory component of the  phosphatidylinositol 3-kinase complex (PI3KC3; hVps34/hVps15), can bind and sequester UVRAG and thereby block Rab7a-mediated transport (Sun et al. 2010, 2011; Lin and Zhong 2011). Conversely, membrane-bound, active Rab7a can relieve the inhibition of its activation by binding Rubicon (Sun et al. 2010).

Proteins of the Tre-Bub-CDC16 (TBC) family function as GTPase-activating proteins that stimulate nucleotide hydrolysis. Three family members (TBC1D2/Armus, TBC1D5, and TBC1D15) have all been shown to stimulate Rab7a nucleotide hydrolysis and may regulate Rab7a involvement in discrete functions in coordination with specific signaling (Seaman et al. 2009; Frasa et al. 2010; Peralta et al. 2010). For example, on endosomes the recycling of mannose 6-phosphate receptor to the Golgi via retromer is thought to be regulated by Rab7a/TBC1D5, while the disassembly of adherens junctions and degradation of E-cadherin depends on signal integration of Arf6 and a Rac1/TBC1D2/Rab7a complex. As illustrated by the specific examples given, the facilitated nucleotide binding and hydrolysis cycle brings about conformational changes in Rab7a that modulate its activity and localization.

Membrane localization is dependent on posttranslational modification with a lipid anchor (prenylation) (Fig. 2). Nascent Rab7a synthesized on cytosolic ribosomes is inactive and GDP bound. Prenylation on two C-terminal cysteine residues is mediated by the universal Rab geranylgeranyl transferase, through recognition of the last nine amino acid residues of Rab7a (Wu et al. 2009). The Rab escort protein (REP) serves as the intermediary for Rab7a presentation to the prenylating enzyme and first-time membrane association (Zhang et al. 2009; Agola et al. 2011). GDP dissociation inhibitor (GDI) functions as a universal Rab recycling factor, binding preferentially to doubly prenylated, GDP-bound Rab7a (Wu et al. 2007). GDI binding masks the isoprenyl anchor in the cytosol and renders Rab7a membrane association a reversible process that is closely linked to the nucleotide bound status, based on the fact that GDI has a three order of magnitude higher affinity for Rab7a-GDP than Rab7a-GTP (Wu et al. 2010). A GDI-displacement factor (GDF) has been implicated in GDI release during endosomal Rab membrane association, though GEF proteins may also perform this function in conjunction with nucleotide exchange (Wu et al. 2010). Once on the membrane and in the GTP-bound state, Rab7a interacts with diverse effectors to carry out specific functions.

Rab7a Effectors in the Control of Endocytic Trafficking

Over the years, many effector proteins have been identified to interact specifically with active GTP-bound Rab7a (Table 1). Rab7a effectors orchestrate events ranging from cargo selection to microtubule translocation to downstream membrane tethering and endosomal membrane fusion (Fig. 3). Emerging concepts are that Rab7a activation contributes to the dynamic assembly of large protein complexes in a spatially and temporally regulated manner (Wang et al. 2011a). Specific protein complexes serve discrete functions in the transport process, yet handoffs and multiple layers of regulation are common. The importance of maintaining transport fidelity is evidenced by the increasing numbers of human diseases attributable to defects in endosomal trafficking and Rab7a specifically (Charcot-Marie-Tooth disease type 2) (Cogli et al. 2009; Zhang et al. 2009; Agola et al. 2011).
Rab7a in Endocytosis and Signaling, Table 1

Rab7 GTPase regulators and effectors, and their functions

Rab7 isoform and nucleotide-bound state

Rab7 effector/binding partner

Regulator or effector functiona

Rab7a

ANKFY1 (ankyrin repeat and FYVE domain containing 1)/ANKHZN/Rabankyrin-5

Possible role in vesicular trafficking. Novel interactor of Rab7. Specific role yet to be established

Rab7a

ATP6V0A1

Component of vacuolar ATPase that regulates organelle acidification required for protein sorting, receptor-mediated endocytosis, zymogen activation, and synaptic vesicle proton gradient. Novel interactor of Rab7. Specific role yet to be established

Rab7a-GDP

Ccz1 (vacuolar protein trafficking and biogenesis-associated homolog)

Recruited to endosomes by Mon1a/Mon1b and acts as Rab7 GEF in yeast. Possible human homolog C7orf28B also some similarity to HPS4 involved in biogenesis of lysosome-related organelles

Rab7a-GTP

FYCO1 (FYVE and coiled-coil domain containing 1)

Promotes microtubule plus end transport of autophagosomes presumably by functioning as a kinesin adapter

Rab7a

GNB2L1 (guanine nucleotide binding protein (G protein), beta polypeptide)

Role in intracellular signaling and activation of protein kinase C and possible interaction with Rab7 via WD40 domain. Novel interactor of Rab7. Specific role yet to be established

Ypt7p/ Rab7a-GTP

HOPS complex (Vps11,-16,-18,-33,-39, and-41)

Involved in vacuolar tethering and fusion in yeast and conserved mammalian homologs function in mammalian endolysosomal fusion. Interfaces with CORVET complex to promote rab5 to rab7 conversion in yeast. Vps39 subunit binds Mon1-Ccz1 complex that serves as a Rab7 GEF in yeast and C. elegans

Rab7a

hVps39

In yeast Vps39p, cooperates with Mon1-Ccz1 complex to promote Ypt7p nucleotide exchange, function of mammalian protein remains to be determined

Rab7a

IMMT (Mitofilin)

Maintains mitochondrial morphology and suggested role in protein import. Novel interactor of Rab7. Specific role yet to be established

Rab7a

KIF3A (kinesin + adapter?)

Kinesin2 heavy chain associates with late endosomes along with dynein, Rab7, and dynactin. Possible mediator of Rab7-regulated anterograde transport coordinated by Rab7-interacting adapter such as FYCO1 or other as-yet-unidentified protein

Rab7a-GDP

Mon1a-Mon1b

Mammalian homologs of C. elegans SAND1. Mon1a-Mon1b causes Rab5 GEF displacement and Mon1b interacts with the HOPS complex. Mon1 is an effector of Rab5, but only interacts with Rab7 when complexed to Ccz1

Rab7a-GTP

ORP1L ([oxysterol-binding protein, OSBP]-related protein 1)

Required for cholesterol sensing and regulation of dynein/dynactin motor with Rab7 and RILP, regulates late endosome/lysosome morphogenesis and transport

Rab7a-GTP

Phosphoinositide 3-kinase complex (hVps34/hVps15)

Type III  PI 3-kinase that generates phosphoinositide 3-phosphate to control endosomal trafficking and signaling. Forms complex with myotubularins for negative regulation

Rab7a-GTP

Plekhm1 (pleckstrin homology domain containing, family M [with RUN domain] member)

Regulates lysosomal secretion in osteoclasts for bone resorption by interacting with LIS1 to control microtubule transport and Rab7 and  PI 3-kinase to recruit effectors for fusion

Rab7a

Prohibitin

Negative regulator of cell proliferation and a possible tumor suppressor. Novel interactor of Rab7, specific role yet to be established

Rab7a-GTP

Rabring7

Rab7-interacting ring finger protein, functions as E3 ligase that ubiquitinates itself and controls EGF receptor degradation

Rab7a-GDP

REP1 (Rab escort protein 1)

Presents Rab7 to Rab geranylgeranyl transferase for addition of prenyl group that acts as a membrane anchor

Rab7a-GTP

Retromer (Vps26, Vps29, Vps35)

Regulates retrograde transport from late endosome to trans-Golgi network (TGN) through direct interaction with Vps26

Rab7a-GTP

RILP (Rab7-interacting lysosomal protein)

Involved in late endosomal/lysosomal maturation. Recruits dynein-dynactin motor protein complex

Rab7a-GTP

Rubicon

Regulates endosome maturation through differential interaction with UVRAG and Rab7. Rubicon binding inhibits UVRAG. Rubicon binding to active Rab7 frees UVRAG to activate the hVps34/hVps15  PI 3-kinase and HOPS, thereby simultaneously increasing the active pool of Rab7 and PI3P signaling

Rab7a-GTP

SKIP (SifA and kinesin-interacting protein)

Homolog of PLEKHM1 that binds Rab7, Rab9 and kinesin-1, and may regulate anterograde motility of late endosomes. Target of Salmonella SifA protein

Rab7a

Spg21

Loss of function causes autosomal recessive hereditary spastic paraplegia. Involved in vesicular transport. Novel interactor of Rab7. Specific role yet to be established

Rab7a

STOML2 (Stomatin-like 2)

Negatively modulates mitochondrial sodium calcium exchange. Novel interactor of Rab7. Specific role yet to be established

Rab7a-GTP

TBC1D2 ([tre-2/USP6, BUB2, cdc16] domain family, member 5)/Armus and Rac1

Regulates cytoskeleton organization, ruffled border formation in osteoclasts, and E-cadherin/adherens junction degradation in conjunction with Rac1, inactivates Rab7 through C-terminal GAP activity

Rab7a-GTP

TBC1D5 ([tre-2/USP6, BUB2, cdc16] domain family, member 5)

Negatively regulates retromer recruitment and causes Rab7 to dissociate from membrane and may have Rab7 GAP activity

Rab7a-GTP

TBC1D15 ([tre-2/USP6, BUB2, cdc16] domain family, member 15)

Functions as Rab7 GAP and reduces interaction with RILP, fragments lysosomes, and confers resistance to growth factor withdrawal-induced cell death

Rab7a-GTP

TrkA (neurotrophic tyrosine kinase receptor)

Interacts with Rab7 and regulates endocytic trafficking and nerve growth factor signaling as well as influencing neurite outgrowth

Rab7a-GTP

UVRAG (UV radiation resistance-associated gene)/Beclin1

UVRAG/C-Vps complex positively regulates Rab7 activity via  PI 3-kinase (PI3KC) during autophagic and endocytic maturation

Rab7a-GTP

VapB ([vesicle-associated membrane protein]-associated protein B)

Involved in mediating endosome-ER interaction in response to ORP1L conformation sensing low cholesterol levels

Rab7a

Vps13c (vacuolar protein sorting 13c)

Vacuolar protein sorting and novel interactor of Rab7. Specific role yet to be established

Rab7a-GDP, GTP

XAPC7/PSMA7 (proteasome subunit, alpha type 7)

Negative regulator of late endocytic transport. Overexpression inhibits EGF receptor degradation

Rab7b

SP-A (Surfactant protein A)

Transiently enhances the expression of Rab7 and Rab7b and makes them functionally active to increase the endolysosomal trafficking in alveolar macrophages

aAgola, JO thesis provides reference listing for effectors

On early endosomes, Rab7a functions in cargo sorting by recruiting the retromer complex (Vps26/29/35), which enables retrieval of cation-independent mannose 6-phosphate receptor, TGN38, Wntless among other cargo from early endosomes to the Golgi (Rojas et al. 2008; Seaman et al. 2009; McGough and Cullen 2011) (Fig. 3a). Interaction of retromer with Snx proteins and actin-binding proteins couples sorting with membrane tubulation (Harbour et al. 2010; McGough and Cullen 2011). Dysregulation of retromer is associated with neurologic diseases, including Alzheimer’s disease, underscoring the importance of the Rab7a-retromer link.

On multivesicular bodies and late endosomes, Rab7a facilitates coordinate cargo sorting and bidirectional transport on microtubules through interactions with effectors that differentially associate with dynein or kinesin motors (Fig. 3bc). Lysosomal sorting and perinuclear transport are mediated by the Rab7a-interacting lysosomal protein (RILP) effector (Zhang et al. 2009; Wang et al. 2011b). RILP interacts with components of the endosomal sorting complex required for transport (ESCRT-II) (Vps22 and Vps36) and based on depletion studies, RILP is shown to participate in the sorting of ubiquitinated receptors into intraluminal vesicles (Zhang et al. 2009; Wang et al. 2011b). In this manner, RILP sorts and sequesters the receptors from the cytosolic signaling machinery and targets them for lysosomal degradation. RILP is also targeted by bacterial pathogens to create a specialized intracellular niche for replication (Zhang et al. 2009). In a tripartite complex, Rab7a, RILP, and a second effector known as oxysterol-binding protein-related protein 1 L (ORP1L) serve to recruit a dynein/dynactin motor complex that in association with betaIII spectrin facilitates the perinuclear transport of endosomes on microtubules (Wang et al. 2011b) (Fig. 3b).

Dynein−/dynactin-mediated perinuclear positioning of late endosomes has also been shown to depend on the membrane-associated scaffolding protein, Huntingtin (Htt), which when mutant causes Huntington’s disease, though the link between Htt and Rab7a remains unclarifed (Agola et al. 2011; Caviston et al. 2011). Htt and the Huntingtin-associated protein of 40 kDa (HAP40) are known effectors of Rab5 that facilitate transfer between microtubule-and actin-based networks (Agola et al. 2011). Parallel in vitro studies testing Rab7a did not provide evidence for a direct Rab7a-Htt or a Rab7a-HAP40 interaction, although Huntingtin-associated protein 1 (HAP1) binds dynactin p150Glued (Agola et al. 2011). Therefore, it is speculated that an Htt interaction with the Rab7a dynein/dynactin complex may occur through HAP1 (Fig. 3c). In light of the disease relevance, further study of the potential interfaces between Htt, HAP1, and Rab7a is warranted.

Anterograde movement of endosomes to the cell periphery along the microtubular network is incompletely characterized. Plus-end motility of autophagosomes is dictated by the recently identified Rab7a effector FYVE and coiled-coil domain protein 1 (FYCO1) and an unknown kinesin (Wang et al. 2011a). Late endosome movement is known to depend on kinesin-2 KIF3A heavy chain, while the Rab7a link and effector remain enigmatic (Loubery et al. 2008). Evidence from studies on Salmonella suggest that Rab9 and Rab7a associate with distinct domains on SifA and kinesin-interacting protein (SKIP), implicating kinesin-1 in anterograde motility and late endosomal sorting (Jackson et al. 2008). In sum, the function of the Rab7a-RILP complex in sorting and cytoskeletal transport is best characterized, while other Rab7a effector interactions including those with kinesins and disease relevant proteins (Htt and HAP1) await further characterization. Bacterial proteins from Salmonella with identified functions in interfering with Rab7a motor proteins or linkers may offer unique tools for further dissecting Rab7a motor protein interactions.

Endosomal lipids such as cholesterol and phosphoinositides are critical regulators of cargo sorting and transport on the late endosomal pathway that are integrated through Rab7a and associated motor proteins. In particular, cholesterol sensing is integrated with transport through the Rab7a effector ORP1L (Wang et al. 2011a). When cholesterol levels are low, ORP1L promotes the association of late endosomes with the endoplasmic reticulum via the dissociation of minus-end motor proteins. The ER protein VAPB contributes to motor dissociation and the peripheral movement of late endosomes. Being more peripherally localized, late endosomes are poised to receive cholesterol and other cargo internalized through early endosomes or association with the endoplasmic reticulum (ER). Conversely, when cholesterol levels are high, the conformation of ORP1L is altered and perinuclear transport is favored. In Niemann-Pick type C disease, where endosomal cholesterol levels are constitutively high, the bidirectional motility of endosomes/phagosomes and activation of Rab7a are perturbed (Chen et al. 2008; Zhang et al. 2009). The perturbations contribute to disease pathology and can be reversed by overexpression of Rab9 or Rab7a (Zhang et al. 2009).

Similar to Rab5 on early endosomes, GTP-bound Rab7a is required for class-III  phosphatidylinositol 3-kinase (consisting of the hVps34 catalytic, the hVps15/p150 Rab7a-binding adaptor, and the Rubicon regulatory subunits) activation on late endosomes (Agola et al. 2011; Ho et al. 2012). The local synthesis of PI(3)P on late endosomes enables the recruitment of FYVE domain–containing proteins that promote membrane remodeling (including intraluminal vesicle formation) and eventually terminate the signal. FYVE domain–containing factors include the PI(3,5)P(2)-producing kinase PIKfyve, myotubularin lipid phosphatases, among others (Table 1). Together these downstream effectors control endolysosome morphology, membrane trafficking, acidification, among other functions. Rab7a together with the early endosomal myotubularin lipid phosphatases (MTM1) and late endosomal myotubularin-related protein 2 (MTMR2) acts as a molecular switch controlling the sequential synthesis and degradation of endosomal PI(3)P (Cao et al. 2008). Direct binding of the phosphatases to the phosphatidylinositol 3-kinase complex leads to inactivation of the myotubularins. The lipid kinase-myotubularin interaction also precludes the interaction of the activated Rab7a with the lipid kinase, illustrating the importance of protein handoffs in phosphoinositide 3-phosphate homeostasis on late endosomes. Together, the examples cited provide evidence for Rab7a function in endosomal lipid homeostasis in both metabolism and signaling, the disruption of which leads to human disease.

Two Rab7a effectors participate directly in the regulation of cargo degradation. Rabring7 (Rab7a-interacting ring finger protein) functions as an E3 ligase in conjunction with the Ubc4 and Ubc5 as E2 proteins (Zhang et al. 2009; Wang et al. 2011a). Functionally, overexpression of Rabring7 increases epidermal growth factor receptor degradation and lysosome biogenesis. The proteasome alpha-subunit XAPC7 or PSMA7 in mammals has been found to interact specifically with Rab7a and is recruited to late multivesicular endosomes (Zhang et al. 2009; Agola et al. 2011). Overexpression of XAPC7 impairs late endocytic transport of EGF receptor and hence is a negative regulator of trafficking. Together, Rabring7 and XAPC7 may coordinate the degradation of ubiquitinated growth factor receptors via a link to the proteasomal degradation machinery though further studies are required to elucidate mechanistic details.

In addition to the described Rab7a effectors whose functional activities have been detailed, there are many more putative effectors whose characterization remains to be documented (Table 1). Therefore, further complexity in Rab7a-mediated regulation of cargo sorting, cytoskeletal transport, and membrane fusion will emerge through continued study. An important area for investigation is how Rab7a interactions with tethering factors and SNARE proteins control late endosomal fusion events which have primarily been characterized for the yeast homolog Ypt7p (Zhang et al. 2009; Wang et al. 2011a).

Rab7a in Endosomal Signaling

At the late endosome, Rab7a coordinately regulates intracellular signaling through special scaffolds, selective endosome positioning, and control of growth factor receptor trafficking. Epidermal growth factor receptor (EGF receptor), vascular endothelial growth factor receptor (VEGFR2), and nerve growth factor receptor (TrkA) all depend on Rab7a for their signaling and downregulation (Agola et al. 2011). For example, upon EGF stimulation, K-Ras is endocytosed and sorted to late endosomes where Rab7a and the p14-MP1-p18 scaffolding proteins recruit and activate  MEK-Erk on late endosomes (Lu et al. 2009; Nada et al. 2009).  MAP kinase signaling is further regulated by Rab7a-dependent late endosome positioning through dynactin such that peripheral mislocalization results in prolonged EGF receptor activation and downstream Erk and p38 signaling. The localization of two signaling mediators of the TGF-β superfamily to Rab7-positive late endosomes (p-Smad1 and p-Smad2) is also suggested to be critical for regulation of growth factor signaling (Rajagopal et al. 2007). At the conclusion of signaling, Rab7a may act cooperatively with dynamin 2 and CIN85 (cbl-interacting protein of 85 kDa) to promote the transfer of signaling receptors from late endosomes to lysosomes for degradation (Fig. 3b) (Schroeder et al. 2010).

In neuronal cells, the receptor tyrosine kinase, TrkA is activated by nerve growth factor (NGF). On NGF stimulation, Rab7a interacts with TrkA as it transits through early and late endosomes. Cells expressing Rab7a T22 N, which is predominantly GDP bound, showed prolonged Erk1/2 signaling due to impaired trafficking of activated TrkA (Agola et al. 2011). Disease-causing Rab7a mutants that are constitutively activated have also been shown to exhibit enhanced NGF-stimulated Erk1/2 signaling (BasuRay et al. 2010). This apparently contradictory result can be explained by the duality of Rab7a in regulating transfer of cargo to lysosomes and interacting with scaffold proteins. Thus, Rab7a plays a significant role in growth factor transport by controlling both signaling scaffolds and trafficking to degradative compartments.

Phosphorylation of Rab7a in response to growth factor suggests a further layer of regulation. Large-scale proteomics analyses have identified Rab7a to be both serine and tyrosine phosphorylated. In mouse liver extracts, Rab7a was found phosphorylated on serine 72 within a highly conserved sequence near the GTP-binding pocket (Villen et al. 2007). Rab7a was phosphorylated in response to EGF stimulation on tyrosine 183 in the C-terminal region. Enhanced tyrosine 183 phosphorylation of Rab7a was also associated with mutant EGF receptor and HER2 overexpression in non–small cell lung carcinoma and mammary epithelia, respectively (Guo et al. 2008). The functional consequences of Rab7a serine and tyrosine phosphorylation with respect to membrane trafficking, GTP binding, and hydrolysis remain to be established.

Ubiquitination of activated growth factor receptors plays a crucial role in the endosomal sorting and lysosomal targeting to downregulate receptor levels. Such ubiquitination may depend on interaction with Rab7a and the Rabring7 E3 ubiquitin ligase. The sorting of ubiquitinated receptors into luminal vesicles of multivesicular bodies depends on the ESCRT0, ESCRTI, ESCRTII, and ESCRTIII complexes (Raiborg and Stenmark 2009). EGF receptor and TrkA endolysosomal degradation are both ubiquitin and proteasome dependent. The K63-linked polyubiquitin chain on activated TrkA receptors gets shuttled by the p62 scaffolding protein, possibly in association with Rab7a/XAPC7, to the proteasome for deubiquitination prior to degradation in lysosomes (Geetha and Wooten 2008). TrkA deubiquitination prior to lysosomal degradation may allow crucial recycling of ubiquitin since the ubiquitin tagging is essential for optimum interaction of activated TrkA with the transport machinery and its delivery along the long axonal route from the tip to the cell body. Only after TrkA reaches the cell body, the termination of signaling calls for deubiquitination of the cargo prior to its lysosomal degradation. As illustrated, the reversible ubiquitination is an important component of growth factor receptor downregulation.

The Rab7a-regulated, interdependent late endocytic trafficking, and signaling pathways are indispensible for translating growth factor signals into appropriate cell responses. The development of suitable in vivo models will therefore be crucial to elucidate how impaired trafficking of growth factor receptors and consequent alterations in signaling lead to neurodegenerative diseases and cancer.

Summary

Since its discovery over 20 years ago, Rab7a and its functions in late endocytic trafficking and signaling have remained under active investigation. Unflagging interest is attributed to the diverse processes that are regulated by Rab7a together with a demonstrated role in human disease. The list of Rab7a effector proteins continues to grow, though the exact functions of many recent interacting partners remains to be elucidated. Rab7a helps to coordinate signaling through the temporal and localized assembly of signaling scaffolds and a close coupling to degradative pathways. Elucidating how Rab7 nucleotide exchange and hydrolysis are regulated and how Rab7 is selectively recruited to specific macromolecular complexes to regulate individual pathways remain important areas for further investigation.

Notes

Acknowledgments

We acknowledge PREP fellowship from NIGMS (R25GM075149) to PJ and research support from NSF (MCB0956027) to AWN and MRC (G0701444) to MNM.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Department of Pathology and Cancer Center, MSC08-4640University of New Mexico Health Sciences CenterAlbuquerqueUSA
  2. 2.Department of Clinical BiochemistryCambridge Institute for Medical ResearchCambridgeUK