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First published online 22 April 2008
doi: 10.1242/jcs.025726
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Research Article |
1 Department of Medical Genetics, Cambridge Institute for Medical Research, Wellcome/MRC Building, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 0XY, UK
2 Department of Genetics, University of Cambridge, CB2 3EH, UK
* Author for correspondence (e-mail: dcr1000{at}hermes.cam.ac.uk)
Accepted 21 February 2008
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Summary |
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 Top Summary IntroductionResults and DiscussionConclusionMaterials and MethodsReferences |
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Key words: Huntington disease, Autophagy, Rab5
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Introduction |
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TopSummaryIntroductionResults and DiscussionConclusionMaterials and MethodsReferences |
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). Full-length mutant huntingtin undergoes several proteolytic cleavages, which give rise to N-terminal fragments comprising the first 100-150 residues containing the expanded polyQ tract. These N-terminal fragments are also the toxic species found in aggregates seen in HD mouse models and human brains (Lunkes et al., 2002). Thus, HD pathogenesis is frequently modelled with fragments of exon 1 that contain expanded polyQ repeats, which cause toxicity and the formation of aggregates in cell models and in vivo (Rubinsztein, 2002).
A key pathway that regulates the degradation of aggregate-prone cytosolic proteins such as mutant huntingtin (both full-length and exon 1 forms) is macroautophagy – henceforth referred to simply as autophagy (Ravikumar et al., 2002; Ravikumar et al., 2004; Shibata et al., 2006). Wild-type huntingtin (full-length or exon 1) has a very low dependence on autophagy for its clearance. Autophagy, a process conserved from yeast to human, typically clears long-lived proteins and organelles by engulfing cytoplasmic contents in double-membrane structures called autophagosomes, which later fuse with lysosomes, where their contents are degraded (Klionsky and Ohsumi, 1999). It is believed that isolation membranes (phagophores) – which later give rise to autophagosomes – are formed at the phagophore-assembly site (also referred to as pre-autophagosomal structures, PAS) in yeast (Suzuki et al., 2001). PAS however have not been defined in mammalian autophagy. Several `Atg' (autophagy) genes that regulate yeast autophagy have been identified, and many of these genes have mammalian orthologues. A protective role of autophagy has been implicated in some neurodegenerative disorders, cancers and infectious diseases (Rubinsztein et al., 2007). Conditional knockouts of the key autophagy genes Atg5 or Atg7 in the brains of mice result in a neurodegenerative phenotype caused by aberrant accumulation of ubiquitinated proteins (Hara et al., 2006; Komatsu et al., 2006). In addition to HD, where enhanced clearance of mutant huntingtin fragments through induction of autophagy attenuates toxicity caused by mutant huntingtin in cell, fly and mouse models (Ravikumar et al., 2002; Ravikumar et al., 2004), this pathway appears to be equally important for the clearance of several other intracellular proteins that cause neurodegeneration, including the A53T point mutation in 
-synuclein that causes autosomal-dominant Parkinson disease (Berger et al., 2006; Ravikumar et al., 2002; Webb et al., 2003).
The subcellular localisation of huntingtin, the nature of many of its normal interactors and its indirect interaction with Rab5 through HAP40 (Pal et al., 2006), suggested that it may have roles in endocytosis. Rab5, a member of the small GTPase family, is a key regulator of the early endocytic pathway in mammalian cells (Bucci et al., 1992; Stenmark et al., 1994). Accordingly, we initially tested the role of Rab5 in the regulation of toxicity of mutant huntingtin. We show that Rab5 inhibition enhances toxicity induced by mutant polyQ, whereas Rab5 overexpression attenuates mutant polyQ toxicity in HD cell and fly models. Our data suggest that Rab5, in addition to its role in endocytosis, modifies aggregation and toxicity of mutant huntingtin by having a novel role in the early stage of autophagosome formation. Interestingly, inhibition of endocytosis by a variety of means also influenced the toxicity of mutant huntingtin by inhibiting autophagy at a downstream step (autophagosome-lysosome fusion) that is distinct from Rab5 inhibition. A possible therapeutic approach for HD and other proteinopathies would thus be the enhancement of Rab5 activity.
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Results and Discussion |
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) in COS-7 cells. Dominant-negative inhibition of Rab5 by overexpression of a GTP-binding-defective Rab5 mutant carrying a S34N point mutation (DN-Rab5) (Stenmark et al., 1994) increased the toxicity of Q74 (Fig. 1a), whereas overexpression of wild-type (WT) Rab5 or of a constitutively active (CA) Rab5 mutant carrying a Q79L mutation (CA-Rab5) (Stenmark et al., 1994) significantly decreased Q74-induced toxicity (Fig. 1a).
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). Also, the proportion of cells with inclusions formed by mutant huntingtin fragments generally correlates with its toxicity in cell culture models (although inclusions may not themselves be as toxic as diffusely distributed mutant huntingtin) (Arrasate et al., 2004; Ravikumar et al., 2002). DN-Rab5 increased Q74 aggregation, whereas CA-Rab5 and WT-Rab5 decreased the proportions of Q74-expressing cells with inclusions (Fig. 1b), mirroring the toxicity data (Fig. 1a). We next confirmed that the effects of DN-Rab5 were mirrored by depletion of endogenous Rab5 using RNA interference (RNAi). There are three isoforms of Rab5, namely Rab5A, Rab5B and Rab5C. Small interfering RNA (siRNA) that target individual Rab5 isoforms significantly increased Q74 aggregation, whereas simultaneous knockdown of all three isoforms increased Q74 aggregation even further (Fig. 1c, supplementary material Fig. S1). Thus, Rab5 modulation affects aggregation and toxicity of the mutant huntingtin fragments.
We next tested whether Rab5 can modify the toxicity of mutant huntingtin in vivo using a Drosophila melanogaster HD model. Fly photoreceptors that express a mutant huntingtin fragment with 120 polyQ repeats (Q120) exhibit degeneration that is not observed in flies that express the wild-type fragment with 23 polyQ repeats (Jackson et al., 1998). The Drosophila compound eye consists of many ommatidia, each comprising eight photoreceptor neurons with light-gathering parts called rhabdomeres, seven of which can be visualised by light microscopy using the pseudopupil technique (Franceschini and Kirschfeld, 1971). Neurodegeneration in the HD flies is progressive and is associated with a decrease in the number of visible rhabdomeres in each ommatidium with time (Jackson et al., 1998). We crossed the HD flies with flies transgenic for wild-type Rab5 fused to EGFP (Rab5-EGFP) and looked at the number of visible rhabdomeres in the double transgenic flies. Degeneration of photoreceptors due to Q120 overexpression was greatly rescued by overexpression of Rab5-EGFP (Fig. 1d).
Since our previous studies suggest that aggregation of mutant huntingtin can be regulated by autophagy (Ravikumar et al., 2002), we next tested whether the effects of Rab5 on Q74 aggregation are autophagy-dependent using either wild-type (Atg5+/+) or autophagy-deficient Atg5-knockout (Atg5–/–) mouse embryonic fibroblasts (MEFs) (Mizushima et al., 2001). As previously observed with autophagy inhibitors (Ravikumar et al., 2002), Q74 aggregation was higher in autophagy-incompetent Atg5–/– compared with Atg5+/+ MEFs (Fig. 1e). We observed minimal Q74-induced cell death in the MEFs under the transfection conditions we used. As expected, overexpression of DN-Rab5 caused an increase and that of CA-Rab5 a decrease in Q74 aggregation in wild-type MEFs (Fig. 1f, left), similar to the results seen in COS-7 cells (Fig. 1b). However, overexpression of DN-Rab5 or CA-Rab5 had no effects on Q74 aggregation in autophagy-incompetent Atg5–/– MEFs (Mizushima et al., 2001) (Fig. 1f, right). Furthermore, Rab5 inhibition abrogated the ability of the autophagy enhancers, the mTOR inhibitor rapamycin or the inositol monophosphatase inhibitor L-690,330, which target mTOR-dependent or -independent pathways, respectively, to clear Q74 aggregates (Fig. 1g; supplementary material Fig. S2) (Ravikumar et al., 2002; Sarkar et al., 2005). This suggests that the effect of Rab5 on Q74 aggregation and toxicity were due to alterations in the autophagic pathway rather than its role in endocytosis.
Rab5 regulates autophagosome formation in mammalian cells
Since the above data suggest a possible role of Rab5 on autophagy, we tested whether Rab5 influenced the formation of autophagosomes. Two ubiquitin-like modifications are involved in the expansion and completion of autophagosome formation. The first involves conjugation of Atg12 to Atg5 in a reaction that requires Atg7 (E1-like) and Atg10 (E2-like). Atg5-Atg12 conjugates are localised onto the PAS and dissociate upon completion of autophagosome formation. The second modification involves conjugation of microtubule-associated protein 1 light chain 3 (hereafter referred to as LC3 and also known as MAP1LC3 or Atg8) to phosphatidylethanolamine (PE). LC3 (cytosolic) is cleaved at its C-terminus by Atg4 to form LC3-I. LC3-I is covalently conjugated to PE to form LC3-II, a process requiring the activities of Atg7 and Atg3. LC3-II (membrane associated) is specifically targeted to Atg5-Atg12-associated, expanded phagophores and remains associated with autophagosomes even after fusion with lysosomes, after which LC3-II can be delipidated and recycled. LC3 is the only known protein that specifically associates with autophagosomes and not with other vesicular structures (Kabeya et al., 2000). Thus, LC3-II levels correlate with the numbers of autophagic vacuoles, which can also be assessed by counting LC3-positive vesicles (Kabeya et al., 2000). We first tested the effect of Rab5 on LC3. Whereas, inhibition of Rab5 decreased the proportion of COS-7 cells with >20 LC3-labelled autophagic vesicles (Fig. 2a), overexpression of CA-Rab5 or WT-Rab5 significantly increased the proportion of cells with >20 LC3-positive autophagic vesicles (Fig. 2a). However, we did not observe any change in the size or morphology of the LC3 vesicles under any of the above conditions (data not shown). The above results also correlated with decrease (upon DN-Rab5 overexpression) or increase (upon CA-Rab5 or WT-Rab5 overexpression) with autophagosome-associated LC3-II levels on western blots when we blocked LC3-II clearance through inhibition of autophagosome-lysosome fusion using bafilomycin A1 (BafA1) at 200 nM (Fig. 2b) (Sarkar et al., 2007). Further blockage of autophagosome-lysosome fusion through a BafA1-independent mechanism, using the dynein inhibitor erythro-9-[3-(2-hydroxynonyl)] adenine (EHNA), together with this dose of BafA1, does not increase LC3-II compared with using BafA1 alone (Sarkar et al., 2007). Results shown in Fig. 2b suggest that Rab5 increased autophagosome synthesis, rather than decreasing autophagosome-lysosome fusion and/or autophagosome degradation.
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Rab5 and its effector Vps34 regulate early steps of autophagosome formation
Autophagosome formation begins with a nucleation step during which membranes of unknown origin form phagophores; these then expand and fuse to form completed autophagosomes. The formation of autophagosome precursors is regulated by a macromolecular complex that contains phosphoinositide 3-kinase class 3 (Vps34 or PIK3C3), beclin 1 (BECN1, the human ortholog of yeast Atg6p), Atg14 and Vps15 (PIK3R4) (Kihara et al., 2001b). Vps34, which generates phosphatidylinositol-3-phosphate [PtdIns(3)P], directly interacts with beclin 1 (Kihara et al., 2001a). 3-methyl adenine (3MA) (Kovacs et al., 1998) or wortmannin (Blommaart et al., 1997) inhibit PI 3-kinases, including Vps34, and block autophagy by preventing the formation of autophagosomes. Conjugation of Atg12 with Atg5 initiates the elongation process. This conjugation however is not required for membrane targeting of Atg5, but is necessary for membrane elongation. This is followed by conjugation of PE to LC3-I to form LC3-II. Atg5-Atg12 conjugates colocalise with LC3-II on the PAS. Mutation of K130R in Atg5 (Atg5K130R) inhibits its conjugation with Atg12; as a result, membranes with unconjugated Atg5K130R that do not colocalise with LC3-II accumulate upon autophagy induction (Mizushima et al., 2001). The Atg5-Atg12 conjugate only localises to phagophores and dissociates just before or after completion of autophagic vacuole formation (Mizushima et al., 2001). So Atg5 and Atg12 are not associated with completed autophagosomes.
To investigate how Rab5 influenced early steps in autophagosome formation, we investigated the distribution of GFP-tagged Atg5 (GFP-Atg5), a marker for autophagosomal precursor structures (George et al., 2000; Mizushima et al., 2001). Cells were permeabilised with saponin to remove soluble cytosolic contents and reveal membrane-associated GFP-Atg5. After Rab5 inhibition, we noted an increased abundance of large punctate Atg5 structures (Fig. 2c). These contrast with the smaller and less abundant Atg5 puncta seen in the great majority of untreated cells (Fig. 2c). We found similar results when we immunostained for endogenous Atg5 (supplementary material Fig. S3). In control cells, however, these structures were rare (Fig. 2c; supplementary material Fig. S3). Whereas saponin extraction precludes accurate quantitation (because it does not allow visualisation of transfected versus untransfected cells), we saw these Atg5 structures in 3% of all control cells and in about 12% of Rab5-inhibited cells (an 
fourfold increase, P<0.001 from triplicate slides).
Previous studies have shown that Rab5 interacts with and activates the PI 3-kinase Vps34 (Christoforidis et al., 1999). Thus, we tested whether Vps34 inhibition had similar effects to Rab5 inhibition. As with Rab5 inhibition, inhibition of Vps34 with the PI 3-kinase inhibitors 3MA (Fig. 2c; supplementary material Fig. S3) or wortmannin (data not shown) (which are both established blockers of autophagosome formation) or siRNA knockdown of Vps34 (Fig. 2d; supplementary material Fig. S4) resulted in increases of Atg5 structures. Similar to Rab5 inhibition, inhibition of Vps34 also increased the proportion of cells with Q74 aggregates, consistent with autophagy inhibition (Fig. 2d) (Ravikumar et al., 2002). To further characterise the Atg5 structures we tested whether they colocalised with other markers or proteins associated with phagophores. A subset of the punctate Atg5 structures colocalised with a PtdIns(3)P marker, Myc-FYVE (Fig. 2e) (Gaullier et al., 1998) and they also overlapped with beclin 1 (Fig. 2f) but did not colocalise with LC3 (supplementary material Fig. S5) suggesting that these were indeed early autophagic structures. We also did not observe any colocalisation of the Atg5 structures with the Golgi markers p230 (supplementary material Fig. S6) (Derby et al., 2007) or golgin-84 (Satoh et al., 2003) and the Atg5 in these structures was not associated with the ER proteins BiP (Haas, 1994) or Grp94 (Argon and Simen, 1999) (data not shown). However, Rab5 associated with these Atg5-positive puncta, which is compatible with a role for Rab5 in early mammalian autophagy (Fig. 2g). We only observed very rare colocalisation of Atg5 and Atg12 in such puncta (Fig. 2h). It should be noted that these experiments were performed under constitutive autophagy conditions (full medium containing serum and amino acids) in cells with Rab5 inhibition. Rab5 inhibition or 3MA treatment was required to allow visualisation of sufficient numbers of these structures for characterisation. The accumulation of Atg5 structures by Vps34 or Rab5 inhibition might reflect a block in the progression of Atg5-positive membranes to the formation of autophagic vacuoles, which suggests a role of Rab5 in very early steps of autophagy. The observation that the Atg5 in these puncta was only rarely associated with Atg12 led to the hypothesis that defective Atg5-Atg12 conjugation is causally related to puncta formation.
Both Rab5 and Vps34 regulate Atg5-Atg12 conjugation
The possibility that defective Atg5-Atg12 conjugation leads to defective autophagosome formation associated with an accumulation of phagophores enriched in unconjugated Atg5 was compatible with the similarity of the above results and the phenotypes seen in apg7
or apg12 yeast strains, which typically show one to five large Atg5-positive punctate structures per cell, (a phenomenon that is very rare in wild-type cells) (George et al., 2000). Thus, when we knocked down Atg7 (an E1-like enzyme crucial for Atg5-Atg12 conjugation; supplementary material Fig. S7) in mammalian cells, we observed increased GFP-Atg5 structures, consistent with those observed in yeast (Fig. 3a). These Atg5-positive structures also colocalised with beclin 1 (supplementary material Fig. S8). Accordingly, we tested whether the increase in Atg5 structures seen with Rab5 or Vps34 inhibition was associated with aberrant Atg5-Atg12 conjugation. Consistent with this hypothesis and with the only very rare colocalisation of Atg12 and Atg5 in the puncta that accumulated following inhibition of Rab5 (Fig. 2h), we found that the ratio of Atg5-Atg12 complex formed compared with the pool of unconjugated Atg12 was significantly lower in cells expressing DN-Rab5 (Fig. 3b). 3MA treatment and siRNA knockdown of Vps34 also had similar effects on Atg5-Atg12 conjugation compared with those seen with loss-of-function of Rab5 (DN-Rab5), a Vps34 activator (Fig. 3c, left and right respectively). We also found decreased levels of Atg5-Atg12 complex compared with free Atg12 when we knocked down all Rab5 isoforms simultaneously (Fig. 3d, left). This effect was similar to the conjugation defect we observed with Atg7 knockdown (Fig. 3d, right). Thus, Rab5 inhibition and loss of activity of Vps34 (which is activated by Rab5) have a range of similar phenotypes: suppression of autophagy, increased numbers of Atg5-positive structures and decreased Atg5-Atg12 conjugation.
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As both Rab5 and Vps34 are likely to regulate Atg5-Atg12 conjugation, which, in turn, regulates formation of autophagic vacuoles from Atg5-rich phagophores, we also looked at the distribution of Atg12 in saponin-extracted cells with Rab5 inhibition. In control cells, overexpressed Atg12 was located in a single large juxtanuclear domain (Fig. 3e), which did not change with the presence or absence of Atg5 overexpression (data not shown). However, with Rab5 inhibition, Atg12 was re-distributed to several peripheral small punctate structures (Fig. 3e), a phenomenon also seen with 3MA treatment (Fig. 3e). Similarly, when we looked at the distribution of endogenous Atg12, it was distributed in a single juxtanuclear domain (reminiscent of the yeast PAS) in wild-type cells (Fig. 3f). However, neither dominant-negative inhibition of Rab5 nor 3MA treatment significantly decreased the percentage of cells with these punctate endogenous Atg12 structures (Fig. 3f). The significance of the Atg12 re-distribution is still a matter of ongoing research, but leads to the hypothesis that Rab5 inhibition or 3MA treatment affects the accessibility of Atg12 for the conjugation process, probably by re-distributing it away from its normal cellular localisation. Alternatively, the decrease in Atg5-Atg12 conjugation that we observed with Rab5 inhibition or 3MA treatment might result in aberrant accumulation of unconjugated Atg12.
Rab5 is found in a macromolecular complex containing Vps34 and beclin 1
Since a macromolecular complex containing Vps34 and beclin 1 regulates the early nucleation step in autophagy, we next tested whether Rab5 was found in such a complex. We tested whether Rab5 interacted with beclin 1, because both proteins interact with Vps34 (Christoforidis et al., 1999; Kihara et al., 2001a), beclin 1 is associated with phagophores, and our data suggests that Rab5 acts at the autophagosome precursor stage and is also associated with such structures (Fig. 2g). In 3MA-treated cells, we observed colocalisation of endogenous Rab5 and beclin 1 (Fig. 4a), particularly in structures similar to those that accumulated when Rab5 or Vps34 were inhibited (e.g. Fig. 2). These putative autophagosome-precursors are precisely where we expected such colocalisation to occur. We also found strong colocalisation of beclin 1 with an activated Rab5 mutant (Fig. 4b). We used activated Rab5 because this forms large clear vesicles that allow unambiguous visualisation of membrane colocalisation of Rab5 with other proteins, a strategy frequently used for Rab5-interactor immunocytochemistry studies (Shin et al., 2005). We next looked for interaction of endogenous Rab5 with beclin 1 by immunoprecipitation. Rab5 interacted with beclin 1 (immunoprecipitated for beclin 1 and detected for Rab5) only in the presence of Vps34 suggesting that Rab5 is part of the macromolecular complex containing beclin 1 and Vps34 (Fig. 4c). Indeed, beclin 1 knockdown (supplementary material Fig. S9) enhances mutant huntingtin (Q74) aggregation (Fig. 4d) (Shibata et al., 2006), which is compatible with the knowledge that this will impair autophagosome formation. Crucially, beclin 1 knockdown led to decreased Atg5-Atg12 conjugation (Fig. 4e), which is compatible with the concept that both beclin 1 and Rab5 activity are required for Vps34 function in autophagosome formation. Thus, Rab5 activation of Vps34 probably regulates not only endosome maturation and multivesicular body formation but also, through recruitment of beclin 1, initiation of autophagic vacuole formation.
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The effect of Rab5 on autophagy is not due to a general inhibition of the endocytic pathway
Since Rab5 is an important regulator of the endocytic pathway, we further tested whether the effects we saw with Rab5 inhibition are indirect consequences of endocytosis inhibition. We first perturbed the function of dynamin, a large GTPase essential for clathrin-mediated endocytosis, using dominant-negative dynamin (DN-Dyn). DN-Dyn increased the percentage of COS-7 cells with aggregates and cell death similar to DN-Rab5 (Fig. 5a). However, unlike Rab5, overexpression of DN-Dyn increased the size and also the number of LC3-positive autophagic vacuoles (Fig. 5b-d) similar to what we and others observe after treatment with BafA1 (Bampton et al., 2005). This suggests that DN-Dyn inhibits autophagy at the level of autophagic-lysosomal delivery similar to BafA1. Accordingly, we found decreased colocalisation of LC3-labelled vesicles with the lysosomal marker lgp120, similar to colocalisation observed in cells treated with BafA1 (Fig. 5e). Previous studies have shown that autophagosomes may fuse with endosomes to form intermediary compartments called amphisomes, which subsequently fuse with lysosomes (Berg et al., 1998). Thus, it is possible that inhibition of the endocytic flux prevents the formation of amphisomes, thereby also inhibiting a subsequent step in the autophagic pathway, amphisome-lysosome fusion. Again, unlike Rab5, we did not see any increase in the Atg5 structures (supplementary material Fig. S10) nor could we observe any change in the ratio of Atg5-Atg12 complex to free Atg12 when dynamin was inhibited (Fig. 5f).
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). Overexpression of dominant-negative Vps4 showed defects in autophagy-dependent bulk protein degradation due to an impairment in the formation of autolysosomes (Nara et al., 2002). Similar to our results for DN-Dyn we did not see an increase in Atg5 structures (data not shown) or decrease in the levels of the Atg5-Atg12 complex with DN-Vps4 (supplementary material Fig. S11) but, similar to experiments using DN-Dyn (Fig. 5a) or BafA1 (Ravikumar et al., 2002), we observed an increase in the aggregation and associated cell death of huntingtin Q74 in the presence of DN-Vps4 (Fig. 5g).
Results obtained using DN-Dyn were identical to those when using siRNA targeting clathrin heavy chain (supplementary material Fig. S12), which is involved in formation of clathrin-coated vesicles. siRNA targeting clathrin heavy chain increased the percentage of HeLa cells with aggregates (Fig. 6a), increased the number and size of LC3-labelled autophagic vacuoles (Fig. 6b,c), did not increase the number of Atg5 structures (data not shown) and had no effect on Atg5-Atg12 conjugation (Fig. 6d). Finally, we also looked at the effect of the endocytosis inhibitor methyl-β-cyclodextrin (β-CD) on LC3-II levels both in the presence or absence of BafA1 (supplementary material Fig. S13). β-CD increased the levels of LC3-II in the absence of BafA1, but had no effect when BafA1 was present (Fig. 6e); which again suggests that inhibition of endocytosis blocks autophagic-lysosomal delivery. DN-Dyn and siRNA targeting clathrin heavy chain both had similar effects on LC3-II levels in the presence of BafA1 similar to β-CD (supplementary material Figs S14, S15). Thus, Rab5 inhibition, which impairs Atg5-Atg12 conjugation, autophagosome (and LC3-II) formation, and which is associated with an accumulation of Atg5-positive structures, has different effects to a range of endocytosis inhibitors (such as β-CD, DN-Dyn, DN-Vps4 or siRNA targeting clathrin) that do not affect Atg5-Atg12 conjugation but block autophagic-lysosomal delivery and increase LC3-II levels. (The specificity of the effect of Rab5 inhibition versus endocytosis inhibition on LC3-II levels has been clearly demonstrated in our experiments where we have treated cells with BafA1.) This argues that the effect on autophagosome formation owing to the loss of Rab5 function is not due to effects on endocytosis.
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Rab5 effect on autophagy is not due to mTOR signalling
Previous studies in Drosophila have shown that disruption of endocytosis can lead to changes in the target of rapamycin (TOR) signalling, a key process regulating autophagy (Hennig et al., 2006). Since Rab5 regulates endocytosis, we tested whether the conditions of Rab5 inhibition or activation alters the mTOR-signalling pathway. mTOR directly phosphorylates at least two effectors: S6 kinase-1 and 4EBP1. S6 kinase-1 phosphorylates the ribosomal protein S6. The levels of phosphorylation of any of these downstream targets are recognised indicators of mTOR activity in the cell (Jacinto and Hall, 2003). Accordingly, we tested whether DN-Rab5 or CA-Rab5 altered the mTOR pathway by looking at the phosphorylation levels of S6. We did not observe any changes in the phosphorylation of S6 (Fig. 6f). We also did not see any changes in the phosphorylation levels of S6 kinase-1 or 4EBP1 using DN-Rab5 or CA-Rab5 (data not shown). Thus, the effects we observed with Rab5 were not owing to alterations in the mTOR signalling. Furthermore, when mTOR signaling was activated by overexpressing Ras homolog enriched in brain (Rheb; see supplementary material Fig. S16) we did not observe the same effects we observed when expressing DN-Rab5; we did not detect an increase in Atg5 structures (data not shown) and did not see any decrease in the Atg5-Atg12 conjugate compared with the pool of unbound Atg12 (Fig. 6g). Taken together, our results suggest that the role of Rab5 on autophagy is independent of its effects on endocytosis and is not because of perturbations in mTOR signalling.
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Our data do not exclude the possibility that membranes for autophagosome biogenesis are derived from endosomes. However, the effect of Rab5 inactivation on Atg5-Atg12 conjugation and autophagosome synthesis is not seen when using a wide range of molecules that inhibit endocytosis (β-CD, DN-Dyn, DN-Vps4 and siRNA targeting clathrin) that instead impede autophagic flux by inhibiting autophagosome-lysosome fusion directly or by inhibiting the autophagosome-endosome fusion step. This suggests that inhibition of endocytosis through different mechanisms will also enhance aggregation and toxicity of polyglutamine by blocking autophagy. It is possible that the loss of Rab5 activity has effects on autophagy by perturbing other unrelated or unknown membrane trafficking pathways (distinct from endocytosis). However, it is important to point out that overexpression of CA-Rab5 or WT-Rab5 enhanced autophagosome synthesis and suppressed aggregation and toxicity of mutant huntingtin in tissue culture cells and in vivo.
We believe that our data suggest a sequential model in mammalian cells, in which PtdIns(3)P generated by Vps34, in a complex that comprises at least beclin 1 and active Rab5, is a key regulator of Atg12 conjugation to Atg5, a rate-limiting step in the conversion of Atg5-positive autophagosome precursors to Atg5-negative autophagosomes. On the one hand, inhibition of this putative cascade at a number of points will lead to impaired autophagy and enhance polyglutamine toxicity. On the other hand, better understanding of the initial rate-limiting steps of autophagy may provide opportunities for the rational design of more specific and safer autophagy-inducing therapeutic drugs than rapamycin (which affects many pathways). This may be of relevance to HD and also to a range of related neurodegenerative diseases caused by intracytosolic aggregate-prone proteins.
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Materials and Methods |
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Western blot analysis
Western blot analysis was carried out applying standard techniques using the ECL detection kit (Amersham). The primary antibodies used include anti-GFP (Clontech), anti-HA (Covance), anti-Myc and anti-actin (Sigma), anti-Atg7 (Rockland Inc.), anti-Atg5 and anti-Rab5 (Abcam), anti-Vps34 (Zymed) and anti-beclin (Cell Signaling technology). Densitometry analysis was performed using Image J 1.36b or Scion Image Beta 4.02 softwares. For immunoprecipitation, cells were suspended in lysis buffer (50 mM Tris HCl pH 7.4, 150 mM NaCl, 1 mM EDTA and 1% Triton X-100) for 30 minutes on ice and supernatants were removed by centrifugation at 13,000 rpm in a tabletop centrifuge for 7 minutes at 4°C. 30 µl of anti-M2 affinity gel (Sigma) was added to the sample and incubated at 4°C for 2 hours with gentle rocking. After incubation, tubes were spun for 30 seconds at 4°C at less than 4000 g. Pellets were washed four times with 1 ml of chilled buffer A (20 mM Tris HCl pH 7.2, 2 mM MgCl2,150 mM NaCl, 0.5% Nonidet P-40). Bound protein was eluted with 100 µl of 150 ng/µl 3xFLAG peptide in TBS.
Immunocytochemistry
Immunocytochemistry was performed in HeLa cells transfected with GFP-Atg5, extracted using 0.02% saponin and fixed using 4% paraformaldehyde. Saponin was prepared using PHEM buffer (60 mM NaPIPES, 25 mM NaHEPES, 10 mM EGTA 2 mM MgCl2, pH 6.9) supplemented with 0.19 M NaCl. Primary antibodies used were anti-Myc, anti-Flag (Sigma), anti-Rab5 and anti-HA. Relevant negative controls without primary antibodies were performed alongside all experiments. Nuclei were stained with 4'6-diamidino-2-phenylindole (DAPI, 3 mg/ml; Sigma). Images were acquired using a Nikon Digital Camera DXM1200 and a Nikon Eclipse E600 fluorescence microscope. We used Nikon ACT-1 version 2.12 acquisition software. Adobe Photoshop 6.0 (Adobe Systems, Inc.) was used for subsequent image processing.
Quantification of aggregate formation, abnormal nuclear morphologies, LC3-positive vesicles
Aggregate formation and nuclear morphology were assessed using a fluorescence microscope. An example of cells expressing the mutant huntingtin with 74 polyQ repeats (Q74) with aggregates (arrows) and an aggregate-containing cell with abnormal nuclear morphology (arrowhead) is shown in supplementary material Fig. S17. Two-hundred Q74-transfected COS-7 cells or MEFs were selected and the number of cells with aggregates are counted. The identity of the slides was unavailable to the observer until all slides had been studied. The experiments were performed in triplicate and repeated twice. Cells were considered dead if the DAPI-stained nuclei showed apoptotic morphology (fragmentation or pyknosis). Pyknotic nuclei are typically <50% the diameter of normal nuclei and show obvious and increased DAPI intensity. An example of cells with abnormal nuclear morphologies (arrows) is shown in supplementary material Fig. S18. For quantification of LC3, 100 EGFP- or mRFP-positive COS-7 cells were selected and cells with 20 or more LC3-labelled vesicles were counted. The identity of the slides was unavailable to the observer until all slides had been studied. Examples of cells with (a) <20 vesicles and (b) >20 vesicles are shown in supplementary material Fig. S19.
Endocytosis assay
HeLa cells were pre-treated with or without 5 mM methyl-β-cyclodextrin (β-CD) for 24 hours. Cells were washed with serum-free medium containing 0.5% BSA and then incubated at 4°C with 10 µg/ml Alexa-Fluor-594-labeled human transferrin (Molecular Probes) for 30 minutes. Excess label was washed off, after which the cells were incubated at 37°C for 10 minutes to allow uptake of the ligand in serum-free medium containing 0.5% BSA with or without 5 mM β-CD. Cells were then fixed with 4% paraformaldehyde in 0.1 M PBS pH 7.6. Images were acquired on a Zeiss LSM510 META confocal microscope (63x1.4NA plan-apochromat oil immersion lens) at room temperature using Zeiss LSM510 v3.2 software (Carl Zeiss, Inc.).
Drosophila strains
Recombinant males of genotype w; CyO/If; UAS-Rab5-EGFP elavGal4/MKRS, carrying Rab5-EGFP (Wucherpfennig et al., 2003) and elav-Gal4 3A4 (Luo et al., 1994) on the third chromosome (gift of V. Korolchuk, Department of Genetics, University of Cambridge, UK) were crossed with y w; gmr-Htt(exon1)Q120 (Jackson et al., 1998) virgins. As a control both y w; gmr-Htt(exon1)Q120 and w; CyO/If; UAS-Rab5-EGFP-elavGal4/MKRS virgins were crossed with an isogenic w1118 line (Ryder et al., 2004). Rhabdomeres were scored using the pseudopupil technique as previously described (Franceschini and Kirschfeld, 1971). Means, standard errors and statistical comparisons using a t-test wer calculated from six independent experiments. In each experiment we evaluated ten eyes per genotype, and 15 ommatidia per eye.
Statistics
Pooled estimates for the changes in aggregate formation or cell death resulting from perturbations assessed in multiple experiments, were calculated as odds ratios with 95% confidence intervals. We have used this method frequently in the past to allow analysis of data from multiple independent experiments (Wyttenbach et al., 2001). Odds ratios and P values were determined using unconditional logistical regression analysis and the general log-linear analysis option of SPSS 9 software. 95% confidence limits for the odds ratio were obtained by using the robust variance approach. Significance levels for comparisons between groups were determined with t tests, where appropriate, for parametric data and with Mann-Whitney U tests for non-parametric data, using the STATVIEW software, version 4.53 (Abacus Concepts, Berkeley, CA).
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0.001, t-test; P
