Role of Grp 75 Chaperone Folding Machinery in the Maintenance of Mitochondrial Protien Quality Control

Abstract

My research focuses on understanding the importance of human mitochondrial Hsp70 (Grp75) chaperone machinery for the maintenance of protein quality control inside the mitochondrial matrix. The investigations carried out during this study have been addressed towards gaining better insights into the working of Grp75 chaperone folding machinery in association with its diverse set of co-chaperones residing in human mitochondria. Additionally, the research also focuses on explaining the various modes of Grp75 participation leading to multiple disease conditions. The thesis has been divided into the following sections as follows: Chapter I: An introduction to the mitochondrial import machinery and role of mitochondrial Hsp70 chaperone folding machinery for the maintenance of protein quality control: Mitochondrion is an essential organelle present in the eukaryotic cell and requires more than 1500 proteins for its proper functioning. Although, mitochondria harbour their own genome, it encodes for only 13 proteins in humans. The rest of the entire proteome is encoded by the nuclear genome and requires proper targeting of proteins to different compartments of mitochondria. Remarkably, mitochondrial matrix alone requires more than 60% of the proteome for its suitable functioning. Briefly, the mitochondrial matrix destined polypeptide passes through the outer membrane translocon; the ‘TOM’ complex and then enters the TIM23 translocon present in the inner membrane of mitochondria. The complete translocation of the polypeptide into the mitochondrial matrix side requires the assistance of mtHsp70 based motor system present on the matrix side which pulls the polypeptide into the matrix in an ATP-dependent manner and with the assistance of various co-chaperones. Subsequently, the unfolded polypeptide is to be folded back to its native state, which is ensured again by the mtHsp70 based chaperone folding machinery. Importantly, while 20% of mtHsp70 is involved in protein import, 80% of mtHsp70 is dedicated for protein folding. In addition to mtHsp70, the chaperone folding machinery consists of various soluble co-chaperones such as the J-proteins which stimulate the ATP hydrolysis rate of Hsp70. Furthermore, another co-chaperone termed as a nucleotide exchange factor ensures binding of fresh ATP molecule onto Hsp70 ensuring multiple rounds of folding cycles. To understand the relevance of mitochondrial Hsp70 chaperone folding machine in the maintenance of protein quality control, Chapter I of the thesis has been divided into multiple sections as follows: Briefly, the initial portion of Chapter I provide a glimpse of the translocon components present in mitochondria for targeting of proteins to outer membrane, inner membrane and inter-membrane space. Owing to the vast proteome size of the mitochondrial matrix, the following section describes the detailed mechanism and translocation process of the mitochondrial matrix targeted proteins. Additionally, subsequent sections of Chapter I provide a comprehensive description of each of the mtHsp70 chaperone folding components, which maintain the protein quality control in the matrix. The players that constitute the chaperone folding machines are mitochondrial Hsp70, J-proteins, nucleotide exchange factors and the newly discovered human escort protein. Essentially, the section provides information about the cellular distribution, structure and function of each of these players constituting the mtHsp70 chaperone folding machine. Loss of regulation between these players leads to defects in protein folding. Imbalance in protein homeostasis is one of the primary causes for mitochondrial dysfunction leading to various diseases. Importantly, recent literature has highlighted the involvement of mtHsp70 chaperone folding players in Parkinson’s disease (PD), Myelodysplastic syndrome (MDS) and cancer. In accordance, the last section of the Chapter I has been dedicated to describe the basic cell biology and proposed mechanisms for the above diseased states. Interestingly, in comparison to yeast and bacteria, the composition of mtHsp70 chaperone folding machinery in humans is unique and distinctly different. Owing to a lack of information about the functioning of human mitochondrial Hsp70 chaperone folding machinery and with an emphasis on understanding its role in various disease manifestations, the objectives that were laid for my PhD thesis are as follows: 1) Functional in vitro reconstitution of the human Grp75 chaperone folding machinery by purifying all the Grp75 chaperone folding machinery players namely; Grp75 (human mtHsp70), hTid-1L and hTid-1S (J-proteins), GrpEL1 (nucleotide exchange factor) and Human escort protein (Hep). 2) Dissection of the intrinsic biochemical defects associated with the variants of Grp75 reported in Parkinson’s disease (PD). 3) To understand the correlation between elevated levels of Grp75 and its contribution to malignancy. In conclusion, the current study has highlighted some of the key features of human Grp75 chaperone folding machinery and its regulation in the maintenance of human mitochondrial matrix protein quality control, failure of which leads to pathological conditions. Chapter II: Reconstitution of the human Grp75 chaperone folding machinery to understand the functional interplay between the multiple protein components: The mitochondrial Heat shock protein 70 (mtHsp70) machinery components are highly conserved among eukaryotes, including humans. However, the functional properties of human mtHsp70 machinery components have not been characterized among all eukaryotic families. To study the functional interactions, we have reconstituted the components of mtHsp70 chaperone machine (Hsp70/J-protein/GrpE/Hep) and systematically analyzed in vitro conditions for biochemical functions. We observed that the sequence-specific interaction of human mtHsp70 towards mitochondrial client proteins differs significantly from its yeast counterpart Ssc1. Interestingly, the helical lid of human mtHsp70 was found dispensable to the binding of P5-peptide as compared to the other Hsp70’s. We observed that the two human mitochondrial matrix J-protein splice-variants differentially regulate the mtHsp70 chaperone cycle. Strikingly, our results demonstrated that human Hep possesses a unique ability to stimulate the ATPase activity of mtHsp70 as well as to prevent the aggregation of unfolded client proteins similar to J-proteins. We observed that Hep binds with the C-terminus of mtHsp70 in a full-length context, and this interaction is distinctly different from unfolded client-specific or J-protein binding. In addition, we found that the interaction of Hep at the C-terminus of mtHsp70 is regulated by the helical lid region. However, the interaction of Hep at the ATPase domain of the human mtHsp70 is mutually exclusive with J-proteins, thereby promoting a similar conformational change that leads to ATPase stimulation. Moreover, we have also dissected out the inter-domain defective nature associated with the point mutant of Grp75 implicated in Myelodysplastic syndrome thus providing an explanation for the loss of function of Grp75 eventually leading to loss of protein quality control in the diseased state. Chapter III: Enhanced J-protein interaction and compromised protein stability of Grp75 variants leads to mitochondrial dysfunction in Parkinson’s disease: Parkinson’s disease (PD) is the second most prevalent progressive neurological disorder commonly associated with impaired mitochondrial function in dopaminergic neurons. Although familial PD is multi-factorial in nature, a recent proteomic screen involving PD-patients revealed two mitochondrial Hsp70 variants (P509S and R126W) that are implicated in PD-pathogenesis. However, molecular mechanisms underlying how mtHsp70 PD-variants are centrally involved in PD-progression is totally elusive. In this report, we provide mechanistic insights into the mitochondrial dysfunction associated with human mtHsp70 PD-variants. Biochemically, R126W variant showed severely compromised protein stability and was found highly susceptible to aggregation at physiological conditions. Strikingly, on the other hand, P509S variant exhibits significantly enhanced interaction with J-protein co-chaperones involved in folding and import machinery, thus altering the overall regulation of chaperone mediated folding cycle and protein homeostasis. To assess the impact of mtHsp70 PD-mutations at the cellular level, we have developed yeast as a model system by making analogous mutations in Ssc1 ortholog. Interestingly, PD-mutations in yeast (R103W and P486S) exhibit multiple in vivo phenotypes, which are associated with ‘mitochondrial dysfunction’ such as mitochondrial DNA (mtDNA) loss and increased susceptibility to oxidative stress recapitulating the cellular features of dopaminergic neurons similar to those reported in other PD-models. Together, our observations for both the variants strongly indicate a definite involvement of mtHsp70 as a susceptibility factor in Parkinson’s disease. Chapter IV: To understand the correlation between elevated levels of Grp75 and its contribution to malignancy: Multiple studies carried out by various groups have reported the presence of elevated levels of Grp75 in cancer cells. Furthermore, proteomic screens show a positive correlation with the higher levels of Grp75 and the aggressive or metastatic nature of cancer. Importantly, cancer cells also exhibit altered mitochondrial metabolism and are found to be under constant oxidative stress pressure. Moreover, Grp75 actively participates in maintenance of mitochondrial function and as well is reported to interact with many putative oncoproteins. However, there is little information available on the possible role of Grp75 in modulating the cellular niche which might favor towards increased malignant transformation of cells. To identify pathways for explaining the correlation between Grp75 and cancer, our initial attempts have focused on monitoring the multiple cellular changes influenced by elevated levels of Grp75 in a cell line based system. To our surprise, transient transfection of cells with Grp75 led to a tremendous increase in the reactive oxygen species levels. Furthermore, a strong positive correlation between the extent of increased levels of Grp75 and the amount of ROS generated in these cells was established. As expected, increased ROS levels observed in Grp75 overexpressing cells also resulted in reduced cell viability. Notably, mitochondrial superoxide generation was found to be the major source for the observed increment in ROS levels in Grp75 expressing cells. In addition, the localization profile of the exogenously expressed Grp75 protein highlighted the fact that the protein was found to be predominantly targeted to mitochondria. Strikingly, the elevated Grp75 levels led to an increase in mitochondrial mass and also displayed a higher proportion of circular and fragmented mitochondria in these cells. Together, the above preliminary observations hint towards a strong correlation between the levels of Grp75 and its influence on the redox biology of cells providing an additional and a possible explanation of the mode of participation of Grp75 in generation and progression of malignancy.