The Hsp60-(p.V98I) mutation associated with hereditary spastic paraplegia SPG13 compromises chaperonin function both in vitro and in vivo

Asymmetric apical domain states of mitochondrial Hsp60 coordinate substrate engagement and chaperonin assembly

The mitochondrial chaperonin, mitochondrial heat shock protein 60 (mtHsp60), promotes the folding of newly imported and transiently misfolded proteins in the mitochondrial matrix, assisted by its co-chaperone mtHsp10. Despite its essential role in mitochondrial proteostasis, structural insights into how this chaperonin progresses through its ATP-dependent client folding cycle are not clear. Here, we determined cryo-EM structures of a hyperstable disease-associated human mtHsp60 mutant, V72I. Client density is identified in three distinct states, revealing interactions with the mtHsp60 apical domains and C termini that coordinate client positioning in the folding chamber. We further identify an asymmetric arrangement of the apical domains in the ATP state, in which an alternating up/down configuration positions interaction surfaces for simultaneous recruitment of mtHsp10 and client retention. Client is then fully encapsulated in mtHsp60-10, revealing prominent contacts at two discrete sites that potentially support maturation. These results identify distinct roles for the apical domains in coordinating client capture and progression through the chaperone cycle, supporting a conserved mechanism of group I chaperonin function.

Asymmetric apical domain states of mitochondrial Hsp60 coordinate substrate engagement and chaperonin assembly

The mitochondrial chaperonin, mtHsp60, promotes the folding of newly imported and transiently misfolded proteins in the mitochondrial matrix, assisted by its co-chaperone mtHsp10. Despite its essential role in mitochondrial proteostasis, structural insights into how this chaperonin binds to clients and progresses through its ATP-dependent reaction cycle are not clear. Here, we determined cryo-electron microscopy (cryo-EM) structures of a hyperstable disease-associated mtHsp60 mutant, V72I, at three stages in this cycle. Unexpectedly, client density is identified in all states, revealing interactions with mtHsp60's apical domains and C-termini that coordinate client positioning in the folding chamber. We further identify a striking asymmetric arrangement of the apical domains in the ATP state, in which an alternating up/down configuration positions interaction surfaces for simultaneous recruitment of mtHsp10 and client retention. Client is then fully encapsulated in mtHsp60/mtHsp10, revealing prominent contacts at two discrete sites that potentially support maturation. These results identify a new role for the apical domains in coordinating client capture and progression through the cycle, and suggest a conserved mechanism of group I chaperonin function.

The hereditary spastic paraplegias

The hereditary spastic paraplegias (HSPs) are a group of more than 90 genetic disorders in which lower extremity spasticity and weakness are either the primary neurologic impairments ("uncomplicated HSP") or when accompanied by other neurologic deficits ("complicated HSP"), important features of the clinical syndrome. Various genetic types of HSP are inherited such as autosomal dominant, autosomal recessive, X-linked, and maternal (mitochondrial) traits. Symptoms that begin in early childhood may be nonprogressive and resemble spastic diplegic cerebral palsy. Symptoms that begin later, typically progress insidiously over a number of years. Genetic testing is able to confirm the diagnosis for many subjects. Insights from gene discovery indicate that abnormalities in diverse molecular processes underlie various forms of HSP, including disturbance in axon transport, endoplasmic reticulum morphogenesis, vesicle transport, lipid metabolism, and mitochondrial function. Pathologic studies in "uncomplicated" HSP have shown axon degeneration particularly involving the distal ends of corticospinal tracts and dorsal column fibers. Treatment is limited to symptom reduction including amelioration of spasticity, reducing urinary urgency, proactive physical therapy including strengthening, stretching, balance, and agility exercise.

Hereditary spastic paraplegia SPG13 mutation increases structural stability and ATPase activity of human mitochondrial chaperonin

Human mitochondrial chaperonin mHsp60 is broadly associated with various human health conditions and the V72I mutation in mHsp60 causes a form of hereditary spastic paraplegia, a neurodegenerative disease. The main function of mHsp60 is to assist folding of mitochondrial proteins in an ATP-dependent manner. In this study, we unexpectedly found that mutant mHsp60^V72I was more stable structurally and more active in the ATPase activity than the wildtype. Analysis of our recently solved cryo-EM structure of mHsp60 revealed allosteric roles of V72I in structural stability and ATPase activity, which were supported by studies including those using the V72A mutation. Despite with the increases in structural stability and ATPase activity, mHsp60^V72I was less efficient in folding malate dehydrogenase, a putative mHsp60 substrate protein in mitochondria and also commonly used in chaperonin studies. In addition, although mHsp60^V72I along with its cochaperonin mHsp10 was able to substitute the E. coli chaperonin system in supporting cell growth under normal temperature of 37 °C, it was unable under heat shock temperature of 42 °C. Our results support the importance of structural dynamics and an optimal ATP turnover that mHsp60 has evolved for its function and physiology. We propose that unproductive energy utilization, or hyperactive ATPase activity and compromised folding function, not mutually exclusive, are responsible for the V72I pathology in neurodegenerative disease.

The Puzzle of Hereditary Spastic Paraplegia: From Epidemiology to Treatment

Inherited neurodegenerative pathology characterized by lower muscle tone and increasing spasticity in the lower limbs is termed hereditary spastic paraplegia (HSP). HSP is associated with changes in about 80 genes and their products involved in various biochemical pathways, such as lipid droplet formation, endoplasmic reticulum shaping, axon transport, endosome trafficking, and mitochondrial function. With the inheritance patterns of autosomal dominant, autosomal recessive, X-linked recessive, and mitochondrial inheritance, HSP is prevalent around the globe at a rate of 1–5 cases in every 100,000 individuals. Recent technology and medical interventions somewhat aid in recognizing and managing the malaise. However, HSP still lacks an appropriate and adequate therapeutic approach. Current therapies are based on the clinical manifestations observed in the patients, for example, smoothing the relaxant spastic muscle and physiotherapies. The limited clinical trial studies contribute to the absence of specific pharmaceuticals for HSPs. Our current work briefly explains the causative genes, epidemiology, underlying mechanism, and the management approach undertaken to date. We have also mentioned the latest approved drugs to summarise the available knowledge on therapeutic strategies for HSP.

Complex hereditary spastic paraplegia associated with episodic visual loss caused by ACO2 variants

Most patients with homozygous or compound heterozygous pathogenic ACO2 variants present with muscular hypotonia features, namely, infantile cerebellar-retinal degeneration. Recently, two studies reported rare familial cases of ACO2 variants presenting as complex hereditary spastic paraplegia (HSP) with broad clinical spectra. Here, we report the case of a 20-year-old Japanese woman with complex HSP caused by compound heterozygous ACO2 variants, revealing a new phenotype of episodic visual loss during febrile illness.

The Neurochaperonopathies: Anomalies of the Chaperone System with Pathogenic Effects in Neurodegenerative and Neuromuscular Disorders

The chaperone (or chaperoning) system (CS) constitutes molecular chaperones, co-chaperones, and chaperone co-factors, interactors and receptors, and its canonical role is protein quality control. A malfunction of the CS may cause diseases, known as the chaperonopathies. These are caused by qualitatively and/or quantitatively abnormal molecular chaperones. Since the CS is ubiquitous, chaperonopathies are systemic, affecting various tissues and organs, playing an etiologic-pathogenic role in diverse conditions. In this review, we focus on chaperonopathies involved in the pathogenic mechanisms of diseases of the central and peripheral nervous systems: the neurochaperonopathies (NCPs). Genetic NCPs are linked to pathogenic variants of chaperone genes encoding, for example, the small Hsp, Hsp10, Hsp40, Hsp60, and CCT-BBS (chaperonin-containing TCP-1- Bardet–Biedl syndrome) chaperones. Instead, the acquired NCPs are associated with malfunctional chaperones, such as Hsp70, Hsp90, and VCP/p97 with aberrant post-translational modifications. Awareness of the chaperonopathies as the underlying primary or secondary causes of disease will improve diagnosis and patient management and open the possibility of investigating and developing chaperonotherapy, namely treatment with the abnormal chaperone as the main target. Positive chaperonotherapy would apply in chaperonopathies by defect, i.e., chaperone insufficiency, and consist of chaperone replacement or boosting, whereas negative chaperonotherapy would be pertinent when a chaperone actively participates in the initiation and progression of the disease and must be blocked and eliminated.

Importance of lipids for upper motor neuron health and disease

Building evidence reveals the importance of maintaining lipid homeostasis for the health and function of neurons, and upper motor neurons (UMNs) are no exception. UMNs are critically important for the initiation and modulation of voluntary movement as they are responsible for conveying cerebral cortex' input to spinal cord targets. To maintain their unique cytoarchitecture with a prominent apical dendrite and a very long axon, UMNs require a stable cell membrane, a lipid bilayer. Lipids can act as building blocks for many biomolecules, and they also contribute to the production of energy. Therefore, UMNs require sustained control over the production, utilization and homeostasis of lipids. Perturbations of lipid homeostasis lead to UMN vulnerability and progressive degeneration in diseases such as hereditary spastic paraplegia (HSP) and primary lateral sclerosis (PLS). Here, we discuss the importance of lipids, especially for UMNs.

Missense Mutations of Human Hsp60: A Computational Analysis to Unveil Their Pathological Significance

Two chaperonopathies have been linked to mutations in the human hsp60 (hHsp60; HSPD1) gene, but other existing variants might cause diseases, even if there is no comprehensive information about this possibility. To fill this vacuum, which might be at the basis of misdiagnoses or simply ignorance of chaperonopathies in patients who would benefit by proper identification of their ailments, we searched the sequenced human genomes available in public databases to determine the range of missense mutations in the single hsp60 gene. A total of 224 missense mutations were identified, including those already characterized. Detailed examination of these mutations was carried out to assess their possible impact on protein structure-function, considering: (a) the properties of individual amino acids; (b) the known functions of the amino acids in the human Hsp60 and/or in the highly similar bacterial ortholog GroEL; (c) the location of the mutant amino acids in the monomers and oligomers; and (d) structure-function relationships inferred from crystal structures. And we also applied a bioinformatics tool for predicting the impact of mutations on proteins. A portion of these genetic variants could have a deleterious impact on protein structure-function, but have not yet been associated with any pathology. Are these variants causing disease with mild clinical manifestations and are, therefore, being overlooked? Or are they causing overt disease, which is misdiagnosed? Our data indicate that more chaperonopathies might occur than is currently acknowledged and that awareness of chaperonopathies among medical personnel will increase their detection and improve patient management.

Complex Destabilization in the Mitochondrial Chaperonin Hsp60 Leads to Disease

Several neurological disorders have been linked to mutations in chaperonin genes and more specifically to the HSPD1 gene. In humans, HSPD1 encodes the mitochondrial Heat Shock Protein 60 (mtHsp60) chaperonin, which carries out essential protein folding reactions that help maintain mitochondrial and cellular homeostasis. It functions as a macromolecular complex that provides client proteins an environment that favors proper folding in an ATP-dependent manner. It has been established that mtHsp60 plays a crucial role in the proper folding of mitochondrial proteins involved in ATP producing pathways. Recently, various single-point mutations in the mtHsp60 encoding gene have been directly linked to neuropathies and paraplegias. Individuals who harbor mtHsp60 mutations that negatively impact its folding ability display phenotypes with highly compromised muscle and neuron cells. Carriers of these mutations usually develop neuropathies and paraplegias at different stages of their lives mainly characterized by leg stiffness and weakness as well as degeneration of spinal cord nerves. These phenotypes are likely due to hindered energy producing pathways involved in cellular respiration resulting in ATP deprived cells. Although the complete protein folding mechanism of mtHsp60 is not well understood, recent work suggests that several of these mutations act by destabilizing the oligomeric stability of mtHsp60. Here, we discuss recent studies that highlight key aspects of the mtHsp60 mechanism with a focus on some of the known disease-causing point mutations, D29G and V98I, and their effect on the protein folding reaction cycle.

An inventory of interactors of the human HSP60/HSP10 chaperonin in the mitochondrial matrix space

The HSP60/HSP10 chaperonin assists folding of proteins in the mitochondrial matrix space by enclosing them in its central cavity. The chaperonin forms part of the mitochondrial protein quality control system. It is essential for cellular survival and mutations in its subunits are associated with rare neurological disorders. Here we present the first survey of interactors of the human mitochondrial HSP60/HSP10 chaperonin. Using a protocol involving metabolic labeling of HEK293 cells, cross-linking, and immunoprecipitation of HSP60, we identified 323 interacting proteins. As expected, the vast majority of these proteins are localized to the mitochondrial matrix space. We find that approximately half of the proteins annotated as mitochondrial matrix proteins interact with the HSP60/HSP10 chaperonin. They cover a broad spectrum of functions and metabolic pathways including the mitochondrial protein synthesis apparatus, the respiratory chain, and mitochondrial protein quality control. Many of the genes encoding HSP60 interactors are annotated as disease genes. There is a correlation between relative cellular abundance and relative abundance in the HSP60 immunoprecipitates. Nineteen abundant matrix proteins occupy more than 60% of the HSP60/HSP10 chaperonin capacity. The reported inventory of interactors can form the basis for interrogating which proteins are especially dependent on the chaperonin.

Protein Misfolding Diseases and Therapeutic Approaches

Protein folding is the process by which a polypeptide chain acquires its functional, native 3D structure. Protein misfolding, on the other hand, is a process in which protein fails to fold into its native functional conformation. This misfolding of proteins may lead to precipitation of a number of serious diseases such as Cystic Fibrosis (CF), Alzheimer's Disease (AD), Parkinson's Disease (PD), and Amyotrophic Lateral Sclerosis (ALS) etc. Protein Quality-control (PQC) systems, consisting of molecular chaperones, proteases and regulatory factors, help in protein folding and prevent its aggregation. At the same time, PQC systems also do sorting and removal of improperly folded polypeptides. Among the major types of PQC systems involved in protein homeostasis are cytosolic, Endoplasmic Reticulum (ER) and mitochondrial ones. The cytosol PQC system includes a large number of component chaperones, such as Nascent-polypeptide-associated Complex (NAC), Hsp40, Hsp70, prefoldin and T Complex Protein-1 (TCP-1) Ring Complex (TRiC). Protein misfolding diseases caused due to defective cytosolic PQC system include diseases involving keratin/collagen proteins, cardiomyopathies, phenylketonuria, PD and ALS. The components of PQC system of Endoplasmic Reticulum (ER) include Binding immunoglobulin Protein (BiP), Calnexin (CNX), Calreticulin (CRT), Glucose-regulated Protein GRP94, the thiol-disulphide oxidoreductases, Protein Disulphide Isomerase (PDI) and ERp57. ER-linked misfolding diseases include CF and Familial Neurohypophyseal Diabetes Insipidus (FNDI). The components of mitochondrial PQC system include mitochondrial chaperones such as the Hsp70, the Hsp60/Hsp10 and a set of proteases having AAA+ domains similar to the proteasome that are situated in the matrix or the inner membrane. Protein misfolding diseases caused due to defective mitochondrial PQC system include medium-chain acyl-CoA dehydrogenase (MCAD)/Short-chain Acyl-CoA Dehydrogenase (SCAD) deficiency diseases, hereditary spastic paraplegia. Among therapeutic approaches towards the treatment of various protein misfolding diseases, chaperones have been suggested as potential therapeutic molecules for target based treatment. Chaperones have been advantageous because of their efficient entry and distribution inside the cells, including specific cellular compartments, in therapeutic concentrations. Based on the chemical nature of the chaperones used for therapeutic purposes, molecular, chemical and pharmacological classes of chaperones have been discussed.

Myelin Pathology: Involvement of Molecular Chaperones and the Promise of Chaperonotherapy

The process of axon myelination involves various proteins including molecular chaperones. Myelin alteration is a common feature in neurological diseases due to structural and functional abnormalities of one or more myelin proteins. Genetic proteinopathies may occur either in the presence of a normal chaperoning system, which is unable to assist the defective myelin protein in its folding and migration, or due to mutations in chaperone genes, leading to functional defects in assisting myelin maturation/migration. The latter are a subgroup of genetic chaperonopathies causing demyelination. In this brief review, we describe some paradigmatic examples pertaining to the chaperonins Hsp60 (HSPD1, or HSP60, or Cpn60) and CCT (chaperonin-containing TCP-1). Our aim is to make scientists and physicians aware of the possibility and advantages of classifying patients depending on the presence or absence of a chaperonopathy. In turn, this subclassification will allow the development of novel therapeutic strategies (chaperonotherapy) by using molecular chaperones as agents or targets for treatment.

MitCHAP-60 and Hereditary Spastic Paraplegia SPG-13 Arise from an Inactive hsp60 Chaperonin that Fails to Fold the ATP Synthase β-Subunit

The human mitochondrial heat shock protein 60 (hsp60) is a tetradecameric chaperonin that folds proteins in the mitochondrial matrix. An hsp60 D3G mutation leads to MitCHAP-60, an early onset neurodegenerative disease while hsp60 V72I has been linked to SPG13, a form of hereditary spastic paraplegia. Previous studies have suggested that these mutations impair the protein folding activity of hsp60 complexes but the detailed mechanism by which these mutations lead the neuromuscular diseases remains unknown. It is known, is that the β-subunit of the human mitochondrial ATP synthase co-immunoprecipitates with hsp60 indicating that the β-subunit is likely a substrate for the chaperonin. Therefore, we hypothesized that hsp60 mutations cause misfolding of proteins that are critical for aerobic respiration. Negative-stain electron microscopy and DLS results suggest that the D3G and V72I complexes fall apart when treated with ATP or ADP and are therefore unable to fold denatured substrates such as α-lactalbumin, malate dehydrogenase (MDH), and the β-subunit of ATP synthase in in-vitro protein-folding assays. These data suggests that hsp60 plays a crucial role in folding important players in aerobic respiration such as the β-subunit of the ATP synthase. The hsp60 mutations D3G and V72I impair its ability to fold mitochondrial substrates leading to abnormal ATP synthesis and the development of the MitCHAP-60 and SPG13 neuromuscular degenerative disorders.

Alpha-synuclein suppresses mitochondrial protease ClpP to trigger mitochondrial oxidative damage and neurotoxicity

Both α-Synuclein (αSyn) accumulation and mitochondrial dysfunction have been implicated in the pathology of Parkinson's disease (PD). Although studies suggest that αSyn and its missense mutant, A53T, preferentially accumulate in the mitochondria, the mechanisms by which αSyn and mitochondrial proteins regulate each other to trigger mitochondrial and neuronal toxicity are poorly understood. ATP-dependent Clp protease (ClpP), a mitochondrial matrix protease, plays an important role in regulating mitochondrial protein turnover and bioenergetics activity. Here, we show that the protein level of ClpP is selectively decreased in αSyn-expressing cell culture and neurons derived from iPS cells of PD patient carrying αSyn A53T mutant, and in dopaminergic (DA) neurons of αSyn A53T mice and PD patient postmortem brains. Deficiency in ClpP induces an overload of mitochondrial misfolded/unfolded proteins, suppresses mitochondrial respiratory activity, increases mitochondrial oxidative damage and causes cell death. Overexpression of ClpP reduces αSyn-induced mitochondrial oxidative stress through enhancing the level of Superoxide Dismutase-2 (SOD2), and suppresses the accumulation of αSyn S129 phosphorylation and promotes neuronal morphology in neurons derived from PD patient iPS cells carrying αSyn A53T mutant. Moreover, we find that αSyn WT and A53T mutant interact with ClpP and suppress its peptidase activity. The binding of αSyn to ClpP further promotes a distribution of ClpP from soluble to insoluble cellular fraction in vitro and in vivo, leading to reduced solubility of ClpP. Compensating for the loss of ClpP in the substantia nigra of αSyn A53T mice by viral expression of ClpP suppresses mitochondrial oxidative damage, and reduces αSyn pathology and behavioral deficits of mice. Our findings provide novel insights into the mechanism underlying αSyn-induced neuronal pathology, and they suggest that ClpP might be a useful therapeutic target for PD and other synucleinopathies.

Toward Developing Chemical Modulators of Hsp60 as Potential Therapeutics

The 60 kDa heat shock protein (Hsp60) is classically known as a mitochondrial chaperonin protein working together with co-chaperonin 10 kDa heat shock protein (Hsp10). This chaperonin complex is essential for folding proteins newly imported into mitochondria. However, Hsp60, and/or Hsp10 have also been shown to reside in other subcellular compartments including extracellular space, cytosol, and nucleus. The proteins in these extra-mitochondrial compartments may possess a wide range of functions dependent or independent of its chaperoning activity. But the mechanistic details remain unknown. Mutations in Hsp60 gene have been shown to be associated with neurodegenerative disorders. Abnormality in expression level and/or subcellular localization have also been detected from different diseased tissues including inflammatory diseases and various cancers. Therefore, there is a strong interest in developing small molecule modulators of Hsp60. Most of the reported inhibitors were discovered through various chemoproteomics strategies. In this review, we will describe the recent progress in this area with reported inhibitors from both natural products and synthetic compounds. The former includes mizoribine, epolactaene, myrtucommulone, stephacidin B, and avrainvillamide while the latter includes o-carboranylphenoxyacetanilides and gold (III) porphyrins. The potencies of the known inhibitors range from low micromolar to millimolar concentrations. The potential applications of these inhibitors include anti-cancer, anti-inflammatory diseases, and anti-autoimmune diseases.

ACO2 homozygous missense mutation associated with complicated hereditary spastic paraplegia

Objective

To identify the clinical characteristics and genetic etiology of a family affected with hereditary spastic paraplegia (HSP).

Methods

Clinical, genetic, and functional analyses involving genome-wide linkage coupled to whole-exome sequencing in a consanguineous family with complicated HSP.

Results

A homozygous missense mutation was identified in the ACO2 gene (c.1240T>G p.Phe414Val) that segregated with HSP complicated by intellectual disability and microcephaly. Lymphoblastoid cell lines of homozygous carrier patients revealed significantly decreased activity of the mitochondrial aconitase enzyme and defective mitochondrial respiration. ACO2 encodes mitochondrial aconitase, an essential enzyme in the Krebs cycle. Recessive mutations in this gene have been previously associated with cerebellar ataxia.

Conclusions

Our findings nominate ACO2 as a disease-causing gene for autosomal recessive complicated HSP and provide further support for the central role of mitochondrial defects in the pathogenesis of HSP.

Frataxin-deficient neurons and mice models of Friedreich ataxia are improved by TAT-MTScs-FXN treatment

Friedreich ataxia (FA) is a rare disease caused by deficiency of frataxin, a mitochondrial protein. As there is no cure available for this disease, many strategies have been developed to reduce the deleterious effects of such deficiency. One of these approaches is based on delivering frataxin to the tissues by coupling the protein to trans-activator of transcription (TAT) peptides, which enables cell membranes crossing. In this study, we tested the efficiency of TAT-MTScs-FXN fusion protein to decrease neurodegeneration markers on frataxin-depleted neurons obtained from dorsal root ganglia (DRG), one of the most affected tissues. In mice models of the disease, we tested the ability of TAT-MTScs-FXN to penetrate the mitochondria and its effect on lifespan. In DRG neurons, treatment with TAT-MTScs-FXN increased cell survival, decreased neurite degeneration and reduced apoptotic markers, such as α-fodrin cleavage and caspase 9 activation. Also, we show that heat-shock protein 60 (HSP60), a molecular chaperone targeted to mitochondria, suffered an impaired processing in frataxin-deficient neurons that was relieved by TAT-MTScs-FXN addition. In mice models of the disease, administration of TAT-MTScs-FXN was able to reach muscle mitochondria, restore the activity of the succinate dehydrogenase and produce a significant lifespan increase. These results support the use of TAT-MTScs-FXN as a treatment for Friedreich ataxia.

Chaperonin of Group I: Oligomeric Spectrum and Biochemical and Biological Implications

Chaperonins play various physiological roles and can also be pathogenic. Elucidation of their structure, e.g., oligomeric status and post-translational modifications (PTM), is necessary to understand their functions and mechanisms of action in health and disease. Group I chaperonins form tetradecamers with two stacked heptameric rings. The tetradecamer is considered the typical functional complex for folding of client polypeptides. However, other forms such as the monomer and oligomers with smaller number of subunits than the classical tetradecamer, also occur in cells. The properties and functions of the monomer and oligomers, and their roles in chaperonin-associated diseases are still incompletely understood. Chaperonin I in eukaryotes occurs in various locations, not just the mitochondrion, which is its canonical place of residence and function. Eukaryotic Chaperonin I, namely Hsp60 (designated HSP60 or HSPD1 in humans) has, indeed, been found in the cytosol; the plasma-cell membrane; on the outer surface of cells; in the intercellular space; in biological liquids such as lymph, blood, and cerebrospinal fluid; and in secretions, for instance saliva and urine. Hsp60 has also been found in cell-derived vesicles such as exosomes. The functions of Hsp60 in all these non-canonical locales are still poorly characterized and one of the questions not yet answered is in what form, i.e., monomer or oligomer, is the chaperonin present in these non-canonical locations. In view of the steady increase in interest on chaperonopathies over the last several years, we have studied human HSP60 to determine its role in various diseases, its locations in cells and tissues and migrations in the body, and its post-translational modifications that might have an impact on its location and function. We also carried out experiments to characterize the oligomeric status of extramitochondrial of HSP60 in solution. Here, we provide an overview of our results, focusing on the oligomeric equilibrium and stability of the various forms of HSP60 in comparison with GroEL. We also discuss post-translational modifications associated with anti-cancer drugs to indicate the potential of Hsp60 in Medicine, as a biomarker and etiopathogenic factor.

Hereditary spastic paraplegia

The hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurologic disorders with the common feature of prominent lower-extremity spasticity, resulting from a length-dependent axonopathy of corticospinal upper motor neurons. The HSPs exist not only in "pure" forms but also in "complex" forms that are associated with additional neurologic and extraneurologic features. The HSPs are among the most genetically diverse neurologic disorders, with well over 70 distinct genetic loci, for which about 60 mutated genes have already been identified. Numerous studies elucidating the molecular pathogenesis underlying HSPs have highlighted the importance of basic cellular functions - especially membrane trafficking, mitochondrial function, organelle shaping and biogenesis, axon transport, and lipid/cholesterol metabolism - in axon development and maintenance. An encouragingly small number of converging cellular pathogenic themes have been identified for the most common HSPs, and some of these pathways present compelling targets for future therapies.

Effects of a Mutation in the HSPE1 Gene Encoding the Mitochondrial Co-chaperonin HSP10 and Its Potential Association with a Neurological and Developmental Disorder

We here report molecular investigations of a missense mutation in the HSPE1 gene encoding the HSP10 subunit of the HSP60/ HSP10 chaperonin complex that assists protein folding in the mitochondrial matrix. The mutation was identified in an infant who came to clinical attention due to infantile spasms at 3 months of age. Clinical exome sequencing revealed heterozygosity for a HSPE1 NM_002157.2:c.217C>T de novo mutation causing replacement of leucine with phenylalanine at position 73 of the HSP10 protein. This variation has never been observed in public exome sequencing databases or the literature. To evaluate whether the mutation may be disease-associated we investigated its effects by in vitro and ex vivo studies. Our in vitro studies indicated that the purified mutant protein was functional, yet its thermal stability, spontaneous refolding propensity, and resistance to proteolytic treatment were profoundly impaired. Mass spectrometric analysis of patient fibroblasts revealed barely detectable levels of HSP10-p.Leu73Phe protein resulting in an almost 2-fold decrease of the ratio of HSP10 to HSP60 subunits. Amounts of the mitochondrial superoxide dismutase SOD2, a protein whose folding is known to strongly depend on the HSP60/HSP10 complex, were decreased to approximately 20% in patient fibroblasts in spite of unchanged SOD2 transcript levels. As a likely consequence, mitochondrial superoxide levels were increased about 2-fold. Although, we cannot exclude other causative or contributing factors, our experimental data support the notion that the HSP10-p.Leu73Phe mutation could be the cause or a strong contributing factor for the disorder in the described patient.

Disease-Associated Mutations in the HSPD1 Gene Encoding the Large Subunit of the Mitochondrial HSP60/HSP10 Chaperonin Complex

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.

Novel Homozygous Missense Mutation in SPG20 Gene Results in Troyer Syndrome Associated with Mitochondrial Cytochrome c Oxidase Deficiency

Troyer syndrome is an autosomal recessive form of hereditary spastic paraplegia (HSP) caused by deleterious mutations in the SPG20 gene. Although the disease is associated with a loss of function mechanism of spartin, the protein encoded by SPG20, the precise pathogenesis is yet to be elucidated. Recent data indicated an important role for spartin in both mitochondrial maintenance and function. Here we report a child presenting with progressive spastic paraparesis, generalized muscle weakness, dysarthria, impaired growth, and severe isolated decrease in muscle cytochrome c oxidase (COX) activity. Whole exome sequencing identified the homozygous c.988A>G variant in SPG20 gene (p.Met330Val) resulting in almost complete loss of spartin in skeletal muscle. Further analyses demonstrated significant tissue specific reduction of COX 4, a nuclear encoded subunit of COX, in muscle suggesting a role for spartin in proper mitochondrial respiratory chain function mediated by COX activity. Our findings need to be verified in other Troyer syndrome patients in order to classify it as a form of HSP caused by mitochondrial dysfunction.

LON is the master protease that protects against protein aggregation in human mitochondria through direct degradation of misfolded proteins

Maintenance of mitochondrial protein homeostasis is critical for proper cellular function. Under normal conditions resident molecular chaperones and proteases maintain protein homeostasis within the organelle. Under conditions of stress however, misfolded proteins accumulate leading to the activation of the mitochondrial unfolded protein response (UPR^mt). While molecular chaperone assisted refolding of proteins in mammalian mitochondria has been well documented, the contribution of AAA+ proteases to the maintenance of protein homeostasis in this organelle remains unclear. To address this gap in knowledge we examined the contribution of human mitochondrial matrix proteases, LONM and CLPXP, to the turnover of OTC-∆, a folding incompetent mutant of ornithine transcarbamylase, known to activate UPR^mt. Contrary to a model whereby CLPXP is believed to degrade misfolded proteins, we found that LONM, and not CLPXP is responsible for the turnover of OTC-∆ in human mitochondria. To analyse the conformational state of proteins that are recognised by LONM, we examined the turnover of unfolded and aggregated forms of malate dehydrogenase (MDH) and OTC. This analysis revealed that LONM specifically recognises and degrades unfolded, but not aggregated proteins. Since LONM is not upregulated by UPR^mt, this pathway may preferentially act to promote chaperone mediated refolding of proteins.

The Chemical Biology of Molecular Chaperones--Implications for Modulation of Proteostasis

Protein homeostasis (proteostasis) is inextricably tied to cellular health and organismal lifespan. Aging, exposure to physiological and environmental stress, and expression of mutant and metastable proteins can cause an imbalance in the protein-folding landscape, which results in the formation of non-native protein aggregates that challenge the capacity of the proteostasis network (PN), increasing the risk for diseases associated with misfolding, aggregation, and aberrant regulation of cell stress responses. Molecular chaperones have central roles in each of the arms of the PN (protein synthesis, folding, disaggregation, and degradation), leading to the proposal that modulation of chaperone function could have therapeutic benefits for the large and growing family of diseases of protein conformation including neurodegeneration, metabolic diseases, and cancer. In this review, we will discuss the current strategies used to tune the PN through targeting molecular chaperones and assess the potential of the chemical biology of proteostasis.

Molecular chaperones and neuronal proteostasis

Protein homeostasis (proteostasis) is essential for maintaining the functionality of the proteome. The disruption of proteostasis, due to genetic mutations or an age-related decline, leads to aberrantly folded proteins that typically lose their function. The accumulation of misfolded and aggregated protein is also cytotoxic and has been implicated in the pathogenesis of neurodegenerative diseases. Neurons have developed an intrinsic protein quality control network, of which molecular chaperones are an essential component. Molecular chaperones function to promote efficient folding and target misfolded proteins for refolding or degradation. Increasing molecular chaperone expression can suppress protein aggregation and toxicity in numerous models of neurodegenerative disease; therefore, molecular chaperones are considered exciting therapeutic targets. Furthermore, mutations in several chaperones cause inherited neurodegenerative diseases. In this review, we focus on the importance of molecular chaperones in neurodegenerative diseases, and discuss the advances in understanding their protective mechanisms.

Delving into the complexity of hereditary spastic paraplegias: how unexpected phenotypes and inheritance modes are revolutionizing their nosology

Hereditary spastic paraplegias (HSP) are rare neurodegenerative diseases sharing the degeneration of the corticospinal tracts as the main pathological characteristic. They are considered one of the most heterogeneous neurological disorders. All modes of inheritance have been described for the 84 different loci and 67 known causative genes implicated up to now. Recent advances in molecular genetics have revealed clinico-genetic heterogeneity of these disorders including their clinical and genetic overlap with other diseases of the nervous system. The systematic analysis of a large set of genes, including exome sequencing, is unmasking unusual phenotypes or inheritance modes associated with mutations in HSP genes and related genes involved in various neurological diseases. A new nosology may emerge after integration and understanding of these new data to replace the current classification. Collectively, functions of the known genes implicate the disturbance of intracellular membrane dynamics and trafficking as the consequence of alterations of cytoskeletal dynamics, lipid metabolism and organelle structures, which represent in fact a relatively small number of cellular processes that could help to find common curative approaches, which are still lacking.

Potential involvement of the mitochondrial unfolded protein response in depressive-like symptoms in mice

Many evidences strongly suggest that a mitochondrial deficit is implicated in major depression. A mitochondrial deficit leads to mitochondrial stress responses, including the mitochondrial unfolded protein response (UPRmt), which is associated with certain brain disorders such as spastic paraplegia and Parkinson's disease. However, there is no evidence regarding the relationship between depressive disorder and UPRmt. Mice treated with chronic restraint stress showed significant depressive-like behaviors in the tail suspension and forced swim tests, decreased oxygen consumption rate, and increased levels of molecules associated with UPRmt such as Hspa9, Hspd1, Ubl5, Abcb10, and ClpP. All of the UPRmt-related molecules were significantly correlated with depressive-like behavior in the forced swim test. Thus, the present study is to reveal a relationship between the UPRmt and depressive disorder, suggesting that the UPRmt is a potential drug target for depressive disorders.

Mitochondrial protein quality control in health and disease

Progressive mitochondrial dysfunction is linked with the onset of many age-related pathologies and neurological disorders. Mitochondrial damage can come in many forms and be induced by a variety of cellular insults. To preserve organelle function during biogenesis or times of stress, multiple surveillance systems work to ensure the persistence of a functional mitochondrial network. This review provides an overview of these processes, which collectively contribute to the maintenance of a healthy mitochondrial population, which is critical for cell physiology and survival.

Loss of Miro1-directed mitochondrial movement results in a novel murine model for neuron disease

Defective mitochondrial distribution in neurons is proposed to cause ATP depletion and calcium-buffering deficiencies that compromise cell function. However, it is unclear whether aberrant mitochondrial motility and distribution alone are sufficient to cause neurological disease. Calcium-binding mitochondrial Rho (Miro) GTPases attach mitochondria to motor proteins for anterograde and retrograde transport in neurons. Using two new KO mouse models, we demonstrate that Miro1 is essential for development of cranial motor nuclei required for respiratory control and maintenance of upper motor neurons required for ambulation. Neuron-specific loss of Miro1 causes depletion of mitochondria from corticospinal tract axons and progressive neurological deficits mirroring human upper motor neuron disease. Although Miro1-deficient neurons exhibit defects in retrograde axonal mitochondrial transport, mitochondrial respiratory function continues. Moreover, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or mitochondrial calcium buffering. Our findings indicate that defects in mitochondrial motility and distribution are sufficient to cause neurological disease.

Biochemical characterization of mutants in chaperonin proteins CCT4 and CCT5 associated with hereditary sensory neuropathy

Hereditary sensory neuropathies are a class of disorders marked by degeneration of the nerve fibers in the sensory periphery neurons. Recently, two mutations were identified in the subunits of the eukaryotic cytosolic chaperonin TRiC, a protein machine responsible for folding actin and tubulin in the cell. C450Y CCT4 was identified in a stock of Sprague-Dawley rats, whereas H147R CCT5 was found in a human Moroccan family. As with many genetically identified mutations associated with neuropathies, the underlying molecular basis of the mutants was not defined. We investigated the biochemical properties of these mutants using an expression system in Escherichia coli that produces homo-oligomeric rings of CCT4 and CCT5. Full-length versions of both mutant protein chains were expressed in E. coli at levels approaching that of the WT chains. Sucrose gradient centrifugation revealed chaperonin-sized complexes of both WT and mutant chaperonins, but with reduced recovery of C450Y CCT4 soluble subunits. Electron microscopy of negatively stained samples of C450Y CCT4 revealed few ring-shaped species, whereas WT CCT4, H147R CCT5, and WT CCT5 revealed similar ring structures. CCT5 complexes were assayed for their ability to suppress aggregation of and refold the model substrate γd-crystallin, suppress aggregation of mutant huntingtin, and refold the physiological substrate β-actin in vitro. H147R CCT5 was not as efficient in chaperoning these substrates as WT CCT5. The subtle effects of these mutations are consistent with the homozygous disease phenotype, in which most functions are carried out during development and adulthood, but some selective function is lost or reduced.

Barcoding heat shock proteins to human diseases: looking beyond the heat shock response

There are numerous human diseases that are associated with protein misfolding and the formation of toxic protein aggregates. Activating the heat shock response (HSR)--and thus generally restoring the disturbed protein homeostasis associated with such diseases--has often been suggested as a therapeutic strategy. However, most data on activating the HSR or its downstream targets in mouse models of diseases associated with aggregate formation have been rather disappointing. The human chaperonome consists of many more heat shock proteins (HSPs) that are not regulated by the HSR, however, and researchers are now focusing on these as potential therapeutic targets. In this Review, we summarize the existing literature on a set of aggregation diseases and propose that each of them can be characterized or 'barcoded' by a different set of HSPs that can rescue specific types of aggregation. Some of these 'non-canonical' HSPs have demonstrated effectiveness in vivo, in mouse models of protein-aggregation disease. Interestingly, several of these HSPs also cause diseases when mutated--so-called chaperonopathies--which are also discussed in this Review.

Hsp60 chaperonopathies and chaperonotherapy: targets and agents

INTRODUCTION

Hsp60 (Cpn60) assembles into a tetradecamer that interacts with the co-chaperonin Hsp10 (Cpn10) to assist client polypeptides to fold, but it also has other roles, including participation in pathogenic mechanisms.

AREA COVERED

Hsp60 chaperonopathies are pathological conditions, inherited or acquired, in which the chaperone plays a determinant etiologic-pathogenic role. These diseases justify selection of Hsp60 as a target for developing agents that interfere with its pathogenic effects. We provide information on how to proceed.

EXPERT OPINION

The information available encourages the development of ways to improve Hsp60 activity (positive chaperonotherapy) when deficient or to block it (negative chaperonotherapy) when pathogenic. Many questions are still unanswered and obstacles are obvious. More information is needed to establish when and why autologous Hsp60 becomes a pathogenic autoantigen, or induces cytokine formation and inflammation, or favors carcinogenesis. Clarification of these points will take considerable time. However, analysis of the Hsp60 molecule and a search for active compounds aimed at structural sites that will affect its functioning should continue without interruption. No doubt that some of these compounds will offer therapeutic hopes and will also be instrumental for dissecting structure-function relationships at the biochemical and biological (using animal models and cultured cells) levels.

Hereditary spastic paraplegia: clinico-pathologic features and emerging molecular mechanisms

Hereditary spastic paraplegia (HSP) is a syndrome designation describing inherited disorders in which lower extremity weakness and spasticity are the predominant symptoms. There are more than 50 genetic types of HSP. HSP affects individuals of diverse ethnic groups with prevalence estimates ranging from 1.2 to 9.6 per 100,000. Symptoms may begin at any age. Gait impairment that begins after childhood usually worsens very slowly over many years. Gait impairment that begins in infancy and early childhood may not worsen significantly. Postmortem studies consistently identify degeneration of corticospinal tract axons (maximal in the thoracic spinal cord) and degeneration of fasciculus gracilis fibers (maximal in the cervico-medullary region). HSP syndromes thus appear to involve motor-sensory axon degeneration affecting predominantly (but not exclusively) the distal ends of long central nervous system (CNS) axons. In general, proteins encoded by HSP genes have diverse functions including (1) axon transport (e.g. SPG30/KIF1A, SPG10/KIF5A and possibly SPG4/Spastin); (2) endoplasmic reticulum morphology (e.g. SPG3A/Atlastin, SPG4/Spastin, SPG12/reticulon 2, and SPG31/REEP1, all of which interact); (3) mitochondrial function (e.g. SPG13/chaperonin 60/heat-shock protein 60, SPG7/paraplegin; and mitochondrial ATP6); (4) myelin formation (e.g. SPG2/Proteolipid protein and SPG42/Connexin 47); (5) protein folding and ER-stress response (SPG6/NIPA1, SPG8/K1AA0196 (Strumpellin), SGP17/BSCL2 (Seipin), "mutilating sensory neuropathy with spastic paraplegia" owing to CcT5 mutation and presumably SPG18/ERLIN2); (6) corticospinal tract and other neurodevelopment (e.g. SPG1/L1 cell adhesion molecule and SPG22/thyroid transporter MCT8); (7) fatty acid and phospholipid metabolism (e.g. SPG28/DDHD1, SPG35/FA2H, SPG39/NTE, SPG54/DDHD2, and SPG56/CYP2U1); and (8) endosome membrane trafficking and vesicle formation (e.g. SPG47/AP4B1, SPG48/KIAA0415, SPG50/AP4M1, SPG51/AP4E, SPG52/AP4S1, and VSPG53/VPS37A). The availability of animal models (including bovine, murine, zebrafish, Drosophila, and C. elegans) for many types of HSP permits exploration of disease mechanisms and potential treatments. This review highlights emerging concepts of this large group of clinically similar disorders.

Loss of mitochondrial peptidase Clpp leads to infertility, hearing loss plus growth retardation via accumulation of CLPX, mtDNA and inflammatory factors

The caseinolytic peptidase P (CLPP) is conserved from bacteria to humans. In the mitochondrial matrix, it multimerizes and forms a macromolecular proteasome-like cylinder together with the chaperone CLPX. In spite of a known relevance for the mitochondrial unfolded protein response, its substrates and tissue-specific roles are unclear in mammals. Recessive CLPP mutations were recently observed in the human Perrault variant of ovarian failure and sensorineural hearing loss. Here, a first characterization of CLPP null mice demonstrated complete female and male infertility and auditory deficits. Disrupted spermatogenesis already at the spermatid stage and ovarian follicular differentiation failure were evident. Reduced pre-/post-natal survival and marked ubiquitous growth retardation contrasted with only light impairment of movement and respiratory activities. Interestingly, the mice showed resistance to ulcerative dermatitis. Systematic expression studies detected up-regulation of other mitochondrial chaperones, accumulation of CLPX and mtDNA as well as inflammatory factors throughout tissues. T-lymphocytes in the spleen were activated. Thus, murine Clpp deletion represents a faithful Perrault model. The disease mechanism probably involves deficient clearance of mitochondrial components and inflammatory tissue destruction.

Opportunities and challenges for molecular chaperone modulation to treat protein-conformational brain diseases

A common pathological hallmark of protein-conformational brain diseases is the formation of disease-specific protein aggregates. In Alzheimer's disease, these are comprised of amyloid-β and Tau as opposed to α-synuclein in Parkinson's disease and N-terminal fragments of mutant huntingtin in Huntington's disease. Most aggregates also sequester molecular chaperones, a protein family that assists in the folding, refolding, stabilization, and processing of client proteins, including misfolded proteins in brain diseases. Molecular chaperone modulation has achieved remarkable therapeutic effects in some cellular and preclinical animal models of protein-conformational diseases. This has raised hope for chaperone-based strategies to combat these diseases. Here, we review briefly the functional diversity and medical significance of molecular chaperones, their therapeutic potential, and common and specific challenges towards clinical application.

Late onset motoneuron disorder caused by mitochondrial Hsp60 chaperone deficiency in mice

Cells rely on efficient protein quality control systems (PQCs) to maintain proper activity of mitochondrial proteins. As part of this system, the mitochondrial chaperone Hsp60 assists folding of matrix proteins and it is an essential protein in all organisms. Mutations in Hspd1, the gene encoding Hsp60, are associated with two human inherited diseases of the nervous system, a dominantly inherited form of spastic paraplegia (SPG13) and an autosomal recessively inherited white matter disorder termed MitCHAP60 disease. Although the connection between mitochondrial failure and neurodegeneration is well known in many neurodegenerative disorders, such as Huntington's disease, Parkinson's disease, and hereditary spastic paraplegia, the molecular basis of the neurodegeneration associated with these diseases is still ill-defined. Here, we investigate mice heterozygous for a knockout allele of the Hspd1 gene encoding Hsp60. Our results demonstrate that Hspd1 haploinsufficiency is sufficient to cause a late onset and slowly progressive deficit in motor functions in mice. We furthermore emphasize the crucial role of the Hsp60 chaperone in mitochondrial function by showing that the motor phenotype is associated with morphological changes of mitochondria, deficient ATP synthesis, and in particular, a defect in the assembly of the respiratory chain complex III in neuronal tissues. In the current study, we propose that our heterozygous Hsp60 mouse model is a valuable model system for the investigation of the link between mitochondrial dysfunction and neurodegeneration.

Heterozygosity for an in-frame deletion causes glutaryl-CoA dehydrogenase deficiency in a patient detected by newborn screening: investigation of the effect of the mutant allele

A patient with suspected glutaric aciduria type 1 (GA-1) was detected by newborn screening. GA-1 is known as an autosomal recessively inherited disease due to defects in the gene coding for glutaryl-CoA dehydrogenase (GCDH), a mitochondrial enzyme involved in the catabolism of the amino acids hydroxylysine, lysine and tryptophan. DNA and cDNA sequencing revealed a 18 bp deletion (c.553_570del18) resulting in deletion of six amino acids (p.Gly185_Ser190del) in one allele and no sequence changes in the other allele. Confirmatory biochemical analysis of blood, urine and cultured fibroblasts from the proband were consistent with a mild biochemical GA-1 phenotype. Recombinant expression of the mutant variant in E. coli showed that the GCDH-(p.Gly185_Ser190del) protein displayed severely decreased assembly into tetramers and enzyme activity. To discover a potential dominant negative effect of the mutant protein, we engineered a prokaryotic expression system in which expression of a wild type and a mutant GCDH allele is controlled by separately inducible promoters. These cells displayed decreased levels of GCDH tetramer and enzyme activity when expressing both the wild type and the mutant GCDH variant protein compared to the situation when only the wild type allele was expressed. Further experiments suggest that the major impact of the GCDH-(p.Gly185_Ser190del) protein in heterozygous cells consists of hampering the assembly of wild type GCDH into tetramers. Our experimental data are consistent with the hypothesis that heterozygosity for this mutation confers a dominant negative effect resulting in a GCDH enzyme activity that is significantly lower than the expected 50%.

Mitochondrial proteomics--a tool for the study of metabolic disorders

Mitochondria are important for a number of life and death processes, such as energy production, creation of reactive oxygen species, and elicitation of stress responses. These responses range from induction of protein quality control and antioxidant systems to mitochondria elimination and cell death. Mitochondrial dysfunctions are involved in pathologies associated with many diseases, for example metabolic disorders, diabetes, cancers, cardiovascular and neurodegenerative diseases as well as obesity and aging. Mitochondrial proteomics can be a powerful tool in the study of these diseases, especially since it can cover mitochondrial proteins from several metabolic pathways, such as the citric acid cycle, fatty acid oxidation, and respiratory chain, as well as protein networks involved in stress responses. The mitochondrial proteome can consist of more than 1,000 different proteins. However, it is difficult to define the precise number, since mitochondria are dynamic and difficult to purify, and because an unknown number of proteins possess dual or multiple localization, depending on cell type and physiological conditions. This review describes several quantitative studies of proteins from mitochondria isolated by centrifugation, separated by various methods (e.g., electrophoresis and nanoLC), and analyzed by advanced mass spectrometry. We illustrate the methods by showing that multiple pathways and networks are affected in cells from patients carrying gene variations affecting a mitochondrial protein. The study of cultured skin fibroblasts from patients with ethylmalonic aciduria associated with variations in the genes coding for short-chain acyl-CoA dehydrogenase (SCAD) or ETHE1 are two of the examples. The possibility of obtaining mitochondrial proteomics data from whole cell proteomics studies is also exemplified by the involvement of liver mitochondria in metabolic syndrome.

Mitochondria as a central sensor for axonal degenerative stimuli

Axonal degeneration is a major contributor to neuronal dysfunction in many neurological conditions and has additional roles in development. It can be triggered by divergent stimuli including mechanical, metabolic, infectious, toxic, hereditary and inflammatory stresses. Axonal mitochondria are an important convergence point as regulators of bioenergetic metabolism, reactive oxygen species (ROS), Ca²⁺ homeostasis and protease activation. The challenges likely to render axonal mitochondria more vulnerable than their cellular counterparts are reviewed, including axonal transport, replenishing nuclear-encoded proteins and maintenance of quality control, fusion and fission in locations remote from the cell body. The potential for mitochondria to act as a decision node in axon loss is considered, highlighting the need to understand the biology of axonal mitochondria and their contributions to degenerative mechanisms for novel therapeutic strategies.

Hereditary spastic paraplegias with autosomal dominant, recessive, X-linked, or maternal trait of inheritance

Hereditary spastic paraplegia (SPG) is a clinically and genetically heterogeneous group of neurodegenerative disorders that are clinically characterised by progressive spasticity and weakness of the lower-limbs (pure SPG) and, majoritorian, additional more extensive neurological or non-neurological manifestations (complex or complicated SPG). Pure SPG is characterised by progressive spasticity and weakness of the lower-limbs, and occasionally sensory disturbances or bladder dysfunction. Complex SPGs additionally include cognitive impairment, dementia, epilepsy, extrapyramidal disturbances, cerebellar involvement, retinopathy, optic atrophy, deafness, polyneuropathy, or skin lesions in the absence of coexisting disorders. Nineteen SPGs follow an autosomal-dominant (AD-SPG), 27 an autosomal-recessive (AR-SPG), 5 X-linked (XL-SPG), and one a maternal trait of inheritance. SPGs are due to mutations in genes encoding for proteins involved in the maintenance of corticospinal tract neurons. Among the AD-SPGs, 40-45% of patients carry mutations in the SPAST-gene (SPG4) and 10% in the ATL1-gene (SPG3), while the other 9 genes are more rarely involved (NIPA1 (SPG6), KIAA0196 (SPG8), KIF5A (SPG10), RNT2 (SPG12), SPGD1 (SPG13), BSCL2 (SPG17), REEP1 (SPG31), ZFYVE27 (SPG33, debated), and SLC33A1 (SPG42, debated)). Among the AR-SPGs, ~20% of the patients carry mutations in the KIAA1840 (SPG11) gene whereas the 15 other genes are rarely mutated and account for SPGs in single families yet (CYP7B1 (SPG5), SPG7 (SPG7), ZFYVE26 (SPG15), ERLIN2 (SPG18), SPG20 (SPG20), ACP33 (SPG21), KIF1A (SPG30), FA2H (SPG35), NTE (SPG39), GJA12/GJC2 (SPG44), KIAA0415 (SPG48) and 4 genes encoding for the AP4-complex (SPG47)). Among the XL-SPGs, 3 causative genes have been identified (L1CAM (SPG1), PLP1 (SPG2), and SLC16A2 (SPG22)). The diagnosis of SPGs is based on clinical, instrumental and genetic investigations. Treatment is exclusively symptomatic.

Cellular pathways of hereditary spastic paraplegia

Human voluntary movement is controlled by the pyramidal motor system, a long CNS pathway comprising corticospinal and lower motor neurons. Hereditary spastic paraplegias (HSPs) are a large, genetically diverse group of inherited neurologic disorders characterized by a length-dependent distal axonopathy of the corticospinal tracts, resulting in lower limb spasticity and weakness. A range of studies are converging on alterations in the shaping of organelles, particularly the endoplasmic reticulum, as well as intracellular membrane trafficking and distribution as primary defects underlying the HSPs, with clear relevance for other long axonopathies affecting peripheral nerves and lower motor neurons.

Changes in immunohistochemical levels and subcellular localization after therapy and correlation and colocalization with CD68 suggest a pathogenetic role of Hsp60 in ulcerative colitis

In an earlier work, the role of heat shock protein (Hsp60) in the pathogenesis of ulcerative colitis (UC) was suggested by its significant increase in the pathological mucosa parallel with an increase in inflammatory cells. More data in this direction are reported in this work. We analyzed by immunohistochemistry biopsies of colon tissue from 2 groups of patients with UC and treated with either 5-aminosalicylic acid (5-ASA) alone or in combination with a probiotic. We looked for inflammatory markers and Hsp60. Both the treatments were effective in reducing symptoms but the group treated with both 5-ASA and probiotics showed better clinical results. Amelioration of symptoms was associated with reduction of both inflammation and Hsp60, a reduction that was most marked in the group treated with 5-ASA and probiotics. The levels of Hsp60 positively correlated with those of CD68-positive cells, and double immunofluorescence showed a high index of colocalization of the chaperonin and CD68 in lamina propria. Immunoelectron microscopy showed that Hsp60-classically a mitochondrial protein-was abundantly also present in cytosol in biopsies taken at the time of diagnosis, but not after the treatment. Our data suggest that Hsp60 is an active player in pathogenesis of UC and it can be hypothesized that the chaperonin is responsible, at least in part, for initiation and maintenance of disease.