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Proc Natl Acad Sci U S A. Jun 16, 2009; 106(24): 9703–9708.
Published online Jun 3, 2009. doi:  10.1073/pnas.0900221106
PMCID: PMC2701000
Cell Biology

γ-Actin is required for cytoskeletal maintenance but not development

Abstract

βcyto-Actin and γcyto-actin are ubiquitous proteins thought to be essential building blocks of the cytoskeleton in all non-muscle cells. Despite this widely held supposition, we show that γcyto-actin null mice (Actg1−/−) are viable. However, they suffer increased mortality and show progressive hearing loss during adulthood despite compensatory up-regulation of βcyto-actin. The surprising viability and normal hearing of young Actg1−/− mice means that βcyto-actin can likely build all essential non-muscle actin-based cytoskeletal structures including mechanosensory stereocilia of hair cells that are necessary for hearing. Although γcyto-actin–deficient stereocilia form normally, we found that they cannot maintain the integrity of the stereocilia actin core. In the wild-type, γcyto-actin localizes along the length of stereocilia but re-distributes to sites of F-actin core disruptions resulting from animal exposure to damaging noise. In Actg1−/− stereocilia similar disruptions are observed even without noise exposure. We conclude that γcyto-actin is required for reinforcement and long-term stability of F-actin–based structures but is not an essential building block of the developing cytoskeleton.

Keywords: actin, cytoskeleton, hearing

There are six genes encoding six vertebrate actins that are classified according to where they are predominately expressed. αskeletal-Actin, αsmooth-actin, αcardiac-actin, and γsmooth-actin are primarily found in muscle cells, whereas cytoplasmic βcyto-actin and γcyto-actin are ubiquitously and highly expressed in non-muscle cells, as reviewed elsewhere (1). Athough βcyto-actin and γcyto-actin differ at only four biochemically similar amino acid residues in their N-termini, several lines of evidence suggest that each protein is functionally distinct. The amino acid sequences of βcyto- and γcyto-actin are each exactly conserved among avian and mammalian species. In addition, βcyto- and γcyto-actin proteins are differentially localized (25) and posttranslationally modified (6). Finally, although dominant missense mutations in ACTB encoding βcyto-actin are associated with syndromic phenotypes including severe developmental malformations and bilateral deafness (7), humans carrying a variety of dominant missense mutations in ACTG1 develop postlingual nonsyndromic progressive hearing loss (DFNA20, OMIM 604717) (811).

γcyto-Actin is widely expressed in the inner ear sensory epithelial cells on which mammalian hearing depends. These cells are organized in rows along the cochlea length: one row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs) (Fig. 2A). IHCs function as auditory receptors, converting sound energy into neuronal signals, whereas OHCs enhance sensitivity to sound stimuli, as reviewed elsewhere (12). The apical surface of a hair cell is topped with actin-rich microvilli-derived protrusions termed stereocilia, which deflect in response to sound stimuli, initiating mechanoelectrical transduction (Fig. 2B). βcyto- and γcyto-Actin are both thought to be essential components of the stereocilia core (24), which consists of a paracrystalline array of unidirectionally oriented actin filaments (Fig. 2C) (1315).

Fig. 2.
Differential localization of βcyto- and γcyto-actin in the mouse organ of Corti (OC). (A) The OC has three rows of outer hair cells (OHCs) and one row of inner hair cells (IHCs). Each hair cell is surrounded by non-sensory supporting cells. ...

In the mammalian organ of Corti, the precise architecture of stereocilia is preserved for the life of the organism. Meanwhile, the stereocilia actin core is reported to undergo renewal by continuous actin polymerization at filament barbed ends and depolymerization at pointed ends, which is precisely coupled to maintain stereocilia length (15, 16). The speed of stereocilia treadmilling is reported to be the same for all stereocilia of the same row and is proportional to stereocilia length (17). Immuno-electron microscopy shows that in wild-type hair cells βcyto-actin is largely restricted to stereocilia, their rootlets, and the cuticular plate (2, 3, 18), whereas γcyto-actin is reported to have more broad localization, including hair cell stereocilia and their rootlets, the cuticular plate in which stereocilia are anchored, adherens junctions, and outer hair cell lateral walls (2, 3, 18). Hair cells and their stereocilia are thus an attractive model to study the structural consequences of perturbing actin isoform composition.

βcyto- and γcyto-Actin are among the most abundant proteins in every mammalian cell, leading to the common assumption that both cytoplasmic actins are essential for function and viability. To test this supposition and to uncover the unique function of γcyto-actin, we generated a whole-body γcyto-actin knockout mouse (Actg1−/−). We show here that mice completely lacking γcyto-actin can survive to adulthood. Interestingly, Actg1−/− mice initially have normal hearing but develop progressive hearing loss during adulthood that is characterized by stereocilia actin core disruptions and stereocilia degradation. These findings led us to conclude that γcyto-actin is not necessary for the formation of actin-based structures required for organogenesis and development, but is essential for maintenance of the hair cell actin cytoskeleton.

Results

γcyto-Actin Null Mice Are Viable.

To determine whether there is a unique function of γcyto-actin that cannot be compensated by the other actin family members, we generated a γcyto-actin null (Actg1−/−) mouse. Mice entirely devoid of γcyto-actin were viable, but born at one-third of the expected Mendelian ratio, indicating that the absence of γcyto-actin caused some embryonic or perinatal lethality. Although the overall development of surviving Actg1−/− mice appeared largely normal, their body weight was ≈20% lower than wild-type (Actg1+/+) and heterozygous (Actg1+/−) littermates (Fig. 1A). In addition, some Actg1−/− mice died prematurely of unknown cause(s) (Fig. 1B). To investigate whether the observed effects were caused by a general depletion of cellular actin, we analyzed actin isoform expression in wild-type, Actg1+/− and Actg1−/− tissues by Western blot. We observed gene dose-dependent expression of γcyto-actin and compensatory up-regulation of other actin family members to maintain the total actin level in all tissues examined (Fig. 1C and [supporting information (SI) Fig. S1]). Therefore, the actin composition, but not the concentration, was altered in Actg1−/− mice.

Fig. 1.
Characterization of live-born homozygous mutant Actg1−/− mice. (A) Body mass growth curve of Actg1+/+ (wild-type, closed squares), Actg1+/− (heterozygous, open circles) and Actg1−/− (homozygous mutant, open triangles) ...

γcyto-Actin Null Mice Show Progressive Loss of Hearing.

We assessed hearing in wild-type and Actg1−/− mice by measuring auditory brainstem response (ABR) thresholds. ABR objectively measures synchronous electrical activity generated by the neurons in the ascending auditory system and can be recorded from scalp electrodes by averaging responses to short tone bursts (19, 20). We found that Actg1−/− mice up to 6 weeks of age had near-normal ABR thresholds (Fig. 1D). However, 16-week-old Actg1−/− mice demonstrated a marked hearing impairment at each frequency tested, and by 24 weeks of age were profoundly deaf (Fig. 1D). This progressive hearing loss was not found in Actg1+/− littermates, which exhibited wild-type ABR thresholds up to 24 weeks of age (Fig. S2) despite expressing only 50% of wild-type levels of γcyto-actin (Fig. 1C).

Differential Localization of βcyto- and γcyto-Actin in Developing and Adult Mouse Hair Cells Revealed Delayed Appearance of γcyto-Actin in Stereocilia.

Consistent with previous reports in postnatal chicken and mature guinea pig or rat, both βcyto- and γcyto-actin were detected in stereocilia (Fig. 2 D and E) and the cuticular plate of adult wild-type mouse hair cells. The three independently generated γcyto-actin-specific antibodies used did not stain any structures in Actg1−/− hair cells (Fig. 2F), demonstrating the specificity of these antisera for γcyto-actin. We found that during embryonic development of wild-type mice, βcyto-actin appeared in the body of hair cells and subsequently in stereocilia earlier than γcyto-actin, which accumulated first in supporting cells and only later appeared in hair cells (Fig. 2 G–P). We observed βcyto-actin in auditory hair cell stereocilia as soon as they appear around E16.5 (Fig. 2 G–I) in the basal turn of the cochlea. The first appearance of γcyto-actin within stereocilia was detected after stereocilia emerged at approximately E18.5 (Fig. 2 O–P). These data are consistent with βcyto-actin primarily contributing to the formation of the actin cytoskeleton of developing stereocilia, whereas γcyto-actin may be important for cytoskeleton maintenance and/or reinforcement.

Although both actins are found in mouse stereocilia, we observed differential localization within the stereocilia, again consistent with βcyto- and γcyto-actin having disparate functions. In the adult wild-type mouse stereocilia, βcyto-actin staining overlapped completely with rhodamine-phalloidin staining, whereas γcyto-actin was concentrated more toward the periphery of the stereocilia actin core, often only partially overlapping with rhodamine-phalloidin staining (Fig. 2 Q–T).

Phalloidin-Negative Gaps in F-Actin Stereocilia Cores Contain Core Components.

In the course of characterizing the localization of the cytoplasmic actins, we observed occasional gaps in phalloidin staining of F-actin cores of vestibular hair cell stereocilia (Fig. 3A–C). The gaps were most frequently observed at the base and along the length of stereocilia in the tallest row (Fig. 3D). Using our γcyto-actin specific antibodies, which recognize both globular (G) and filamentous (F) forms of actin (see SI Text), we found that gaps were enriched in γcyto-actin. This actin population is likely to be predominantly monomeric, because phalloidin recognizes only filamentous actin (Fig. 3 A–D). Usually gap staining was much more intense relative to that along the length of stereocilium (Figs. 3 A–D and and33 F–M), which may be caused by enhanced antibody accessibility within the gaps. Alternatively, intense gap staining could be partially explained by the redistribution of γcyto-actin to F-actin gaps from a pool of available non-filamentous actin within a stereocilium. A similar redistribution to F-actin gaps was also seen for βcyto-actin (Fig. S3). It is likely that βcyto-actin is also recruited to the gaps from a pool of non-filamentous actin, as staining intensity along a stereocilium with a gap was not different from the intensity of staining along a stereocilium without gaps (Fig. S3B). The same pattern of staining was also observed for DNase I (Fig. 3E), a marker for monomeric actin (G-actin) (21), and espin (Fig. 3F), an actin bundling protein essential for stereocilia formation, which is reported to have both F- and G-actin binding sites (22). Interestingly, only proteins that are either actin core components or directly involved in actin turnover were found to accumulate in the phalloidin-negative gaps. For example, actin-associated proteins cadherin-23, protocadherin-15-CD1, myosin-VIIa, and myosin-XVa are not present in gaps (Fig. S4 and data not shown). In contrast, cofilin, which was implicated in both severing and nucleation of F-actin (23), selectively accumulates in stereocilia gaps (Fig. S4).

Fig. 3.
γcyto-Actin concentrates at the sites of stereocilia core disruptions. (A–C) γcyto-Actin antibody highlights gaps (segments of F-actin depolymerization; arrows) in wild-type (wt) mouse vestibular hair cell (VHC) stereocilia. In ...

Together, these data suggest that (i) gaps have a different structural arrangement than the stereocilia actin core, (ii) gaps are enriched for βcyto- and γcyto-actin along with other core components, and (iii) cofilin may mediate ongoing actin remodeling in the gap to facilitate repair of local damage of the F-actin core.

γcyto-Actin Localizes to Phalloidin-Negative Gaps That Form in Response to Damage.

In contrast to vestibular hair cell stereocilia, gaps were not observed in undamaged auditory hair cell stereocilia of wild-type mice. Previously, F-actin gaps were reported in guinea pig auditory hair cell stereocilia after noise damage (24), suggesting that gaps develop in response to stereocilia damage. To assess whether damage-induced gaps are also enriched in γcyto-actin, we compared γcyto-actin localization in stereocilia from control and noise-damaged guinea pigs. Consistent with previous studies (2, 18), γcyto-actin was distributed along the length of control stereocilia (Fig. 3G). After a damaging noise exposure, γcyto-actin was enriched in both the tips and in phalloidin-negative gaps observed along the length of noise-damaged stereocilia, which were often abnormally shorter than neighboring normal appearing stereocilia of the same row (Fig. 3 H–K). In the same bundle, some stereocilia that appeared unaffected by noise still had γcyto-actin evenly distributed along their length and did not have an accumulation of γcyto-actin at the tips (Fig. 3H, inset).

Immunofluorescence Confocal Microscopy and Scanning Electron Microscopy Analyses Reveal an Unexpected Pattern of Degeneration of Actg1−/− Stereocilia.

Interestingly, F-actin gaps were occasionally observed in auditory hair cell stereocilia of Actg1−/− mice without exposure to damaging noise. These gaps lacked γcyto-actin (Fig. 3L) but contained βcyto-actin (Fig. 3M) and espin (Fig. 3 N–P). The staining of βcyto-actin within gaps of Actg−/− stereocilia was so intense that we had to turn down the gain on the confocal microscope so that the signal within gaps was not saturated (Fig. 3M and Fig. S3A). As a consequence, the βcyto-actin signal along the lengths of stereocilia was reduced to a barely detectable level (compare Fig. 3M and Fig. S3A with Fig. S3B).

The presence of F-actin gaps in the auditory hair cell stereocilia of hearing-impaired, γcyto-actin–deficient mice led us to investigate whether a stereocilia maintenance/repair mechanism is defective in Actg1−/− mice. To define the structural changes associated with a γcyto-actin-deficiency, we characterized the morphology of auditory hair cells. We examined outer hair cells of 6-week-old Actg1−/− mice by scanning electron microscopy and found that γcyto-actin deficient stereocilia were indistinguishable from OHC stereocilia of wild-type littermates (Fig. 4 A–D). However, Actg1−/− stereocilia deteriorated progressively as the animals aged (Fig. 4 E–H). By 16 weeks of age, ≈50% of stereocilia within a hair bundle were degraded or absent in Actg1−/− mice (Fig. 4I). Across all three rows within the hair bundle, we observed missing or shortened (Fig. 4 J and K) stereocilia, although the remaining stereocilia looked normal.

Fig. 4.
Morphology of stereocilia bundles in adult wild-type (Actg+/+) and γcyto-actin deficient (Actg1−/−) mice. (A–D) Scanning electron micrographs of stereocilia from (A, B) 6-week-old Actg1+/+ and (C, D) 6-week-old Actg1−/− ...

Discussion

Actg1−/− mice survive to birth and beyond, demonstrating that γcyto-actin is not strictly required for mammalian development or viability. Our observations of stereocilia from Actg1−/− mice instead indicate that γcyto-actin is necessary to maintain cytoskeletal integrity and function. The phenotype of Actg1−/− stereocilia is unique; we observed stereocilia defects ranging from simple shortening to complete loss of individual stereocilia across all three rows within the hair bundle indicating selected stereocilia disassembly (Fig. 4 E–K), whereas the remaining stereocilia within the same bundle appeared intact. The apparent independent nature of this phenomenon (affected stereocilia surrounded by normal stereocilia) indicates that the disassembly process is initiated and/or regulated at the level of an individual stereocilium. Therefore, this disassembly process appears different from actin treadmilling that normally occurs simultaneously in all stereocilia of the same row (17).

Rather, our results suggest that γcyto-actin strengthens stereocilia F-actin cores, preventing stereocilia core damage, and/or is required to repair the damaged core. Consistent with this view, in developing wild-type mice γcyto-actin appeared and accumulated in stereocilia a few days before the onset of hearing function, perhaps preparing stereocilia to withstand the rigors of acoustical stimulation. Furthermore, damaging noise induces the appearance of γcyto-actin–enriched, phalloidin-negative gaps in the stereocilia core of wild-type rodent hair cells. These gaps were not present in control stereocilia but were observed in Actg1−/− mouse auditory stereocilia indicating that stereocilia core damage was more frequent or more slowly repaired in the absence of γcyto-actin. Finally, Actg1−/− stereocilia progressively deteriorated, demonstrating that γcyto-actin is required to maintain these structures.

γcyto- and βcyto-Actin are nearly identical, featuring only four biochemically similar residue substitutions in the N terminus, suggesting likely compensation between these proteins. Indeed, βcyto-actin protein levels were elevated in Actg1−/− mice and the total actin level was equivalent in Actg1−/− and wild-type mice (Fig. 1C). However, γcyto-actin–deficient stereocilia progressively deteriorated despite the localization of βcyto-actin to gaps in the F-actin core of Actg1−/− mouse auditory stereocilia (Fig. 3M and Fig. S3). This surprising result indicates that γcyto-actin has at least some functions that are unique and cannot be compensated for by βcyto-actin. One possibility is that γcyto-actin brings to the site of damage a unique γcyto-actin protein partner that is necessary for βcyto-actin, γcyto-actin, or for βcyto- and γcyto-actin together, to repair damage to the core.

Consistent with different functions of γcyto- and βcyto-actin, we observed distinct localization patterns for each protein within wild-type auditory stereocilia. βcyto-Actin localized to stereocilia cores, exactly overlaying with phalloidin staining, whereas γcyto-actin was concentrated toward the periphery of stereocilia cores. Because the γcyto-actin antibody detects both monomeric and filamentous actin whereas phalloidin detects only filamentous actin, there appears to be a pool of monomeric γcyto-actin at the periphery of stereocilia cores. Alternatively, phalloidin-negative actin may still be filamentous but unable to bind phalloidin because of a particular F-actin conformation, which was observed in nuclear actin as previously reviewed (25), different paracrystal filament packing that excludes phalloidin, or masking by actin binding proteins. In any case, the peripheral population of γcyto-actin is distinct and may be used for stereocilia core remodeling and repair, perhaps redistributing to F-actin gaps that form as a result of stereocilia core damage.

Based on γcyto-actin localization and Actg1−/− stereocilia degradation, we envision two models of γcyto-actin function. First, γcyto-actin may have a specific role involving annealing of broken filaments or de novo polymerization, perhaps depending on an unknown actin-binding protein with specificity for γcyto-actin. Alternatively, γcyto-actin-containing filaments may have distinct biophysical and biochemical properties, such as different polymerization rates or polymer stability, which protect stereocilia from mechanical stress. Deficient repair and/or diminished structural integrity then result in the eventual loss of Actg1−/− stereocilia.

Interestingly, the gaps observed in auditory stereocilia of noise-damaged animals and untreated Actg1−/− mice (24) (Fig. 3) resemble discontinuities in actin-rich developing Drosophila bristles (26, 27). In these structures, gaps are observed both during formation, as short modules of F-actin are cross-linked to form fibers, and during disassembly, as the fibers are broken down into the original modules (26). Although mammalian stereocilia are not thought to be composed of cross-linked F-actin modules, elements of Drosophila actin regulation may nonetheless be conserved in mammalian stereocilia. Stereocilia gaps may arise through physical damage that cause F-actin bundle breakage or through the action of a protein that senses damage and severs and depolymerizes actin filaments, generating gaps similar to those that occur during modular disassembly of Drosophila bristles (28).

The accumulation of γcyto-actin at sites of damage in wild-type hair cell stereocilia after noise exposure (Figs. 3 H–K), together with the disassembly and subsequent loss of individual stereocilia in Actg1−/− hair cells (Fig. 4 G–I), are consistent with γcyto-actin being required for maintenance and/or repair of stereocilia in adult hair cells. However, γcyto-actin seems to be entirely dispensable for the proper development and functional maturation of hair cells. Indeed, the viability of Actg1−/− embryos and the normal lifespan of at least one-third of all live-born Actg1−/− mice demonstrate that γcyto-actin is not crucial for general organogenesis and thus is not necessary for the formation of actin-based structures in general. Consistent with this idea, wild-type and Actg1−/− mice intestinal brush border microvilli are morphologically indistinguishable (Fig. S5).

We previously characterized a muscle-specific γcyto-actin knockout mouse that was generated precisely because a whole-body knockout was widely presumed to be unviable. This murine model exhibited normal muscle development followed by progressive myopathy and muscle cell necrosis (29). Both muscle cells and outer hair cells must resist force generated by contractility or electromotility, respectively. Although otherwise clearly disparate in structure and function, these mechanically challenged cells seem to have a particularly evident requirement for the specialized properties of γcyto-actin necessary for cellular maintenance.

Additional γcyto-actin deficiency-based cytoskeletal pathologic conditions may exist in other organs and tissues of Actg1−/− mice that could affect long-term function. Indeed, the lower body mass of Actg1−/− mice and their occasional premature death suggests a hidden, slowly developing or partially compensated pathologic condition. We conclude that γcyto-actin is not necessary to build actin cytoskeletal structures required for organogenesis and development but, instead, functions primarily to reinforce and/or repair the actin cytoskeleton.

Methods

Generation of Actg1-Null Mice.

A targeting vector in which loxP sites flank exons 2 and 3 of the murine Actg1 gene was described previously (29). Embryonic stem cell targeting, screening, blastocyst injections, and subsequent EIIa-cre breeding were performed to generate Actg1+/− mice (29). Actg1+/− mice were backcrossed to C57BI/6 for 10 generations before N10 Actg1+/− X Actg1+/− breedings were arranged to obtain Actg1−/− mice. All genotypes were determined as previously described (29). Animals were housed and treated in accordance with the standards set by the University of Minnesota Institutional Animal Care and Use Committee.

Immunoblot Analysis.

Brain, lung, kidney, and cochlea were dissected from mice of the indicated genotypes, frozen in liquid nitrogen, ground into powder, boiled in buffer (1% SDS, EGTA, PMSF, benzamidine, leupeptin), and centrifuged to remove insoluble material. Protein concentration in the resulting lysate was determined with either a colorimetric assay (DC assay, BioRad) or by A280 measurement. Equal amounts of protein were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE), transferred to nitrocellulose membranes, and probed with the indicated antibodies (γcyto-actin; mAb 2–4 or pAb7577 (29); βcyto-actin, AC15 (Sigma); pan-actin, C4, gift of J. Lessard, University of Cincinnati; γsmooth-actin, B4 (J. Lessard, University of Cincinnati); αsmooth-actin, 1A4 (Sigma); α-tubulin B512 (Sigma). Fluorescently labeled secondary antibodies were detected and quantified from three separate experiments blotted in triplicate using an Odyssey infrared scanner and software (Li-Cor Biosciences).

Auditory Brainstem Responses.

ABRs were collected as previously described (30) or using a Tucker-Davis Technologies System3 to generate sound stimuli and to amplify and record brainstem potentials as described in the SI Text.

Antibody Validation and Immunostaining.

Polyclonal antibody pAb7577 against cytoplasmic γcyto-actin (ACTG1) was generated in the laboratory of J. Ervasti as described, and the specificity was verified (29). The second anti-γcyto-actin polyclonal antibody was a gift from C. Bulinski and was characterized previously (31). The third anti-γcyto-actin polyclonal antibody was developed in the laboratory of K. Friderici by immunizing rabbits with a peptide of the N-terminal 15 residues (Princeton Biomolecules) and affinity purified as described in the SI Text. The immunostaining is described in the SI Text. All animal care and experimental procedures were approved by the NINDS/NIDCD ACUC.

Animals and Noise Exposure.

Noise exposure methods are described in the SI Text.

Scanning Electron Microscopy.

Cochlea were rapidly dissected and fixed by perfusing 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer pH 7.3 with 1 mM CaCl2, through the round window, followed by immersion in the same solution for 2 hours. After microdissection to reveal hair cell stereocilia, cochlea were incubated in 2% arginine-HCl, glycine, glutamic acid, and sucrose followed by treatment with 2% tannic acid and 2% guanidine-HCl and were postfixed in 1% aqueous osmium tetroxide. Specimens were dehydrated in ethanol, critical point dried, sputter coated, and imaged using a cold field emission gun scanning electron microscope (Hitachi S-4700).

Supplementary Material

Supporting Information:

Acknowledgments.

We thank J. Bartles and C. Bulinski for providing anti-espin antibody and one of the anti-γcyto-actin antibodies, respectively; D. Catts and P. Diers for technical assistance; D. Drayna, R. Chadwick, N. Gavara, A. Griffith, J. Bird, and R. Morell for critically reading the manuscript; P. Belyantsev for Fig. 1 drawing; and K. Prins, S. Ikeda, and A. Ikeda for preliminary analysis of Actg1−/− mice. The work was supported by National Institutes of Health (NIH) intramural funds 1 Z01 DC000048–11 LMG (to T.B.F.), NIH intramural funds Z01-DC-000060 (to Andrew J. Griffith), funds from the DRF and NOHR Foundation (to G.I.F.), and NIH grants DC004568 (to K.H.F.), F32 DC009539 (to B.J.P.), and a R01 AR049899 (to J.M.E.).

Footnotes

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/cgi/content/full/0900221106/DCSupplemental.

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