Abstract
Heat shock protein 60 (HSP60) forms together with heat shock protein 10 (HSP10) double-barrel chaperonin complexes that are essential for folding to the native state of proteins in the mitochondrial matrix space. Two extremely rare monogenic disorders have been described that are caused by missense mutations in the HSPD1 gene that encodes the HSP60 subunit of the HSP60/HSP10 chaperonin complex. Investigations of the molecular mechanisms underlying these disorders have revealed that different degrees of reduced HSP60 function produce distinct neurological phenotypes. While mutations with deleterious or strong dominant negative effects are not compatible with life, HSPD1 gene variations found in the human population impair HSP60 function and depending on the mechanism and degree of HSP60 dys- and mal-function cause different phenotypes. We here summarize the knowledge on the effects of disturbances of the function of the HSP60/HSP10 chaperonin complex by disease-associated mutations.
Introduction
The type I chaperonins, a subclass of the molecular chaperone family of proteins, assist folding of proteins in the bacterial cytosol, the mitochondrial matrix space, and the chloroplast stroma. Like its bacterial and chloroplast homologs the mitochondrial HSP60/HSP10 complex is composed of two seven-meric rings of the large subunit (HSP60) stacked back to back (Nisemblat et al., ; Figure 1A). The HSP60 ring structures enclose an inner cavity that is sealed by lids formed by seven-meric rings of the small subunit (HSP10). With the exception of a few endosymbionts, homologs of these proteins are abundantly expressed in mitochondria, chloroplasts and bacteria. The functional folding cycle of the mammalian HSP60/HSP10 complex (Nielsen and Cowan, ; Levy-Rimler et al., , ) has to a large degree been elucidated in analogy to detailed studies of the homologous GroEL/GroES complex of E. coli bacteria (Horwich, ; Hayer-Hartl et al., ). Cycles including binding of proteins undergoing folding to the HSP60 rings, their encapsulation by association of HSP10 rings and dissociation of both the HSP10 ring and the enclosed protein are orchestrated by ATP binding, hydrolysis and release of ADP by the HSP60 subunits. These cycles promote folding of proteins to the native state, but not every cycle results in successful folding. Some proteins may require several rounds. Knock-out experiments have shown that the genes encoding homologs of the HSP60/HSP10 complex are essential in organisms from bacteria to mice (Cheng et al., ; Fayet et al., ; Perezgasga et al., ; Christensen et al., ). In humans only few gene variations altering the amino acid sequence of HSP60 and HSP10 have been described (Table 1). However, very rare disease-associated missense mutations in HSP60 have been associated with a dominant form of hereditary spastic paraplegia (HSP; Hansen et al., ) and a recessively inherited white matter disorder called MitCHAP60 disease (Magen et al., ). In another article published under this research topic we describe the first potentially disease associated mutation in HSP10 that has been identified in a patient with a neurological and developmental disorder (Bie et al., submitted).
Figure 1
Table 1
| Variation | Disease association | Growth in genetic complementation assay | ExAC allele count | PolyPhen-2 prediction# | References |
|---|---|---|---|---|---|
| p.Asp29Gly | MitCHAP60 | Slow, temperature-sensitive | 0 | benign | Magen et al., |
| p.Leu46Phe | Not tested | 29 | possibly damaging | ExAC | |
| p.Val98Ile | SPG13 | No growth | 0 | possibly damaging | Hansen et al., |
| p.Arg142Lys | Not tested | 56 | benign | ExAC | |
| p.Lys156Arg | Not tested | 17 | benign | ExAC | |
| p.Asn184Ser | Unaffected | 61 | benign | Hansen et al., | |
| p.Asn265Ser | Not tested | 23 | possibly damaging | ExAC | |
| p.Leu291Val | Not tested | 12 | probably damaging | ExAC | |
| p.Arg370His | Not tested | 13 | benign | ExAC | |
| p.Asp379Gly | Unaffected | 0 | benign | Bross et al., | |
| p.Gln461Glu | SPG13* | Impaired | 0 | probably damaging | Hansen et al., |
| p.Gly559Asp | Unaffected | 0 | possibly damaging | Bross et al., | |
| p.Gly563Ala | Unaffected | 1904 | probably damaging | Bross et al., |
Missense variations in the HSPD1 gene encoding HSP60.
Missense variations recorded in the literature and/or in the ExAC consortium database with ≥10 alleles are shown. The basis for scoring growth in the genetic complementation assay is explained in Section Functional analysis in vivo. Prediction of possible impact of amino acid substitutions was performed using the PolyPhen-2 webserver (Adzhubei et al.,
PolyPhen-2 has three prediction output options, from the highest probability score for being damaging to the lowest: “probably damaging,” “possibly damaging” and “benign.” The asterisk denotes that the variation/disease relationship for the p.Gln461Glu variation is not fully established. For further details see text.
Amino acid sequence variations in HSP60
The genomic structure for the human HSPD1 and HSPE1 genes encoding the proteins HSP60 and HSP10, respectively, has been characterized experimentally (Hansen et al.,
Functional analyses of HSP60 variant proteins
Table 1 lists both the HSPD1 missense variations described in the literature and those present in ≥10 alleles in the ExAC Consortium Website. The functional consequences for some of these variations have been investigated previously and the results are summarized in this review.
In vitro analyses of disease-associated HSP60 variant proteins
Expression in E. coli had indicated that the two disease-associated variant proteins HSP60-p.Val98Ile and HSP60-p.Asp29Gly displayed similar stability as wild type HSP60 suggesting that the amino acid replacements caused functional impairment (Hansen et al.,
Functional analysis in vivo
A sophisticated genetic complementation assay for analyzing the function of variants of the human HSP60/HSP10 chaperonin complex has been developed in the lab of Costa Georgopoulos (Richardson et al.,
Seven human HSP60 missense variants have so-far been investigated using this functional assay (Table 1). These studies showed that the SPG13-associated mutant HSP60-p.Val98Ile was unable to functionally replace the bacterial chaperonin. However, cells expressing the HSP60-p.Asp29Gly variant displayed slow and temperature-sensitive growth. Cells with the HSP60-p.Gln461Glu variation found in a sporadic spastic paraplegia patient also displayed impaired growth. The four other HSP60 variants studied: p.Asn184Ser, p.Asp379Gly, p.Gly559Asp, and p.Gly563Ala, behaved like wild type HSP60 in the genetic complementation assay suggesting that they have no significant effect.
Bioinformatics prediction of the effects of the variations using the PolyPhen-2 tool (Adzhubei et al.,
The SPG13-associated mutation HSP60-p.Val98Ile is dominantly inherited, i.e., the patient cells express both a wild type and a mutant allele and these two variant proteins are likely on equal terms incorporated into HSP60 ring structures resulting in heteromeric rings with stochastically distributed content of the two variants. To test whether incorporation of mutant HSP60-p.Val98Ile subunits together with wild type HSP60 subunits into HSP60 ring complexes would cause a dominant negative effect, the complementation assay was further engineered. E. coli cells with the deletion of the endogenous groESgroEL operon and containing a plasmid with an IPTG-inducible operons comprising HSP10 and wild type HSP60 were transformed with a second plasmid comprising an arabinose-inducible operon with the respective mutant HSP60 variant and HSP10 (Bross et al.,
Clinical phenotypes of SPG13 and MitCHAP60 disease
Notwithstanding the rarity, studies of the very large index family that led to the discovery of the association of the HSP60-p.Val98Ile mutation with HSP has given a firm basis for the mutation/disease relationship (Hansen et al.,
Potential disease-association of other missense variations in HSP60
In spite of widely spread genetic screening, so far only one single additional spastic paraplegia patient heterozygous for another mutation in HSP60 (HSP60-p.Gln461Glu) has been reported (Hansen et al.,
Cellular and mouse models for HSP60 deficiency
Effects of expressing the SPG13- and MitCHap60-associated mutant proteins on mitochondrial morphology have been assessed in Cos-7 cells transfected with cDNAs encoding the disease-associated mutant proteins (Miyamoto et al.,
ShRNA-mediated knock-down of HSP60 in human HEK293 cells decreased the steady state levels of the mitochondrial medium-chain acyl-CoA dehydrogenase (Corydon et al.,
Knock-out of both HSP60-encoding alleles in mice is not compatible with life. Such embryos died early during development (Christensen et al.,
Proteins like UQCRC1 or SOD2 thus appear to depend more than others proteins on folding assistance by the HSP60/HSP10 complex. For E. coli 85 proteins that display obligate dependence on folding assistance by the bacterial chaperonin complex have been characterized (Kerner et al.,
Perspectives
Different mutations in HSP60 or its partner protein HSP10 lead to distinct phenotypes of neurological disorders with a clear mitochondrial dysfunction pattern. These diseases are very rare as deleterious effects of mutations in these essential genes are not compatible with normal embryonal development (Christensen et al.,
Besides being affected by mutations, the activity and function of the HSP60/HSP10 complex can also be regulated by its expression levels. The regulation of the transcription levels of both proteins occurs via different elements in the bidirectional promoter. SP1 elements provide robust house-keeping levels of expression and on top of that heat-shock elements, mitochondrial unfolding protein response elements, and STAT3 elements further modulate expression adapting it to specific situations (Zhao et al.,
Indeed, dysregulation of HSP60 expression in hypothalamus has been implicated with type 2 diabetes mellitus (Kleinridders et al.,
Finally, posttranslational modifications of the HSP60/HSP10 complex may regulate the activity of the complex. Like other molecular chaperones HSP60 is a highly modified protein with a long list of PTMs recorded in the UniProt database (Consortium,
Statements
Author contributions
PF and PB have both contributed to writing the draft and producing the final version.
Acknowledgments
The research of PB has been supported by the Ludvig and Sara Elsass Foundation. The authors would like to thank the Exome Aggregation Consortium and the groups that provided exome variant data for comparison. A full list of contributing groups can be found at http://exac.broadinstitute.org/about.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
HSP60, chaperonin, neurological disease, protein folding problem, mitochondria, gene variation
Citation
Bross P and Fernandez-Guerra P (2016) Disease-Associated Mutations in the HSPD1 Gene Encoding the Large Subunit of the Mitochondrial HSP60/HSP10 Chaperonin Complex. Front. Mol. Biosci. 3:49. doi: 10.3389/fmolb.2016.00049
Received
15 July 2016
Accepted
22 August 2016
Published
31 August 2016
Volume
3 - 2016
Edited by
Alberto J. L. Macario, University of Maryland at Baltimore, USA
Reviewed by
Carlos H. Ramos, State University of Campinas, Brazil; Konstantin K. Turoverov, Institute of Cytology (RAS), Russia
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© 2016 Bross and Fernandez-Guerra.
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*Correspondence: Peter Bross peter.bross@clin.au.dk
This article was submitted to Protein Folding, Misfolding and Degradation, a section of the journal Frontiers in Molecular Biosciences
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