Inorganic Polyphosphate, Mitochondria, and Neurodegeneration.

Depletion of mitochondrial polyP levels increase the phosphorylation of AMPK protein and compromises oxygen consumption in SH-SY5Y cells.

Inorganic polyphosphate (polyP) is an ancient polymer that is well-conserved throughout evolution. It is formed by multiple subunits of orthophosphates linked together by phosphoanhydride bonds. The presence of these bonds, which are structurally similar to those found in ATP, and the high abundance of polyP in mammalian mitochondria, suggest that polyP could be involved in the regulation of the physiology of the organelle, especially in the energy metabolism. In fact, the scientific literature shows an unequivocal role for polyP not only in directly regulating oxidative a phosphorylation; but also in the regulation of reactive oxygen species metabolism, mitochondrial free calcium homeostasis, and the formation and opening of mitochondrial permeability transitions pore. All these processes are closely interconnected with the status of mitochondrial bioenergetics and therefore play a crucial role in maintaining mitochondrial and cell physiology. In this invited review, we discuss the main scientific literature regarding the regulatory role of polyP in mammalian mitochondrial physiology, placing a particular emphasis on its impact on energy metabolism. Although the effects of polyP on the physiology of the organelle are evident; numerous aspects, particularly within mammalian cells, remain unclear and require further investigation. These aspects encompass, for example, advancing the development of more precise analytical methods, unraveling the mechanism responsible for sensing polyP levels, and understanding the exact molecular mechanism that underlies the effects of polyP on mitochondrial physiology. By increasing our understanding of the biology of this ancient and understudied polymer, we could unravel new pharmacological targets in diseases where mitochondrial dysfunction, including energy metabolism dysregulation, has been broadly described.

Inorganic polyphosphate (polyP) is an ancient, ubiquitous, and well-conserved polymer which is present in all the studied organisms. It is formed by individual subunits of orthophosphate which are linked by structurally similar bonds and isoenergetic to those found in ATP. While the metabolism and the physiological roles of polyP have already been described in some organisms, including bacteria and yeast, the exact role of this polymer in mammalian physiology still remains poorly understood. In these organisms, polyP shows a co-localization with mitochondria, and its role as a key regulator of the stress responses, including the maintenance of appropriate bioenergetics, has already been demonstrated by our group and others. Here, using Wild-type (Wt) and MitoPPX (cells enzymatically depleted of mitochondrial polyP) SH-SY5Y cells, we have conducted a comprehensive study of the status of cellular physiology, using proteomics and metabolomics approaches. Our results suggest a clear dysregulation of mitochondrial physiology, especially of bioenergetics, in MitoPPX cells when compared with Wt cells. Moreover, the effects induced by the enzymatic depletion of polyP are similar to those present in the mitochondrial dysfunction that is observed in neurodegenerative disorders and in neuronal aging. Based on our findings, the metabolism of mitochondrial polyP could be a valid and innovative pharmacological target in these conditions.

Is there a link between inorganic polyphosphate (polyP), mitochondria, and neurodegeneration?

Mitochondria play a crucial role in multiple cellular processes beyond the regulation of bioenergetics. These processes range from apoptosis to intracellular signaling. Accordingly, mitochondrial dysfunction has been broadly described in the etiopathology of multiple human diseases, including cancer, diabetes, and all the main neurodegenerative disorders. Therapeutic interventions aimed at modulating this dysfunction are promising for preventing and/or delaying the development of these pathologies. Recent research has highlighted the potential of dietary interventions to modulate mitochondrial physiology. In this text, we critically review the scientific literature available regarding the effects of different dietary interventions (such as caloric restriction, ketogenic diets, increased omega-3 fatty acid consumption, etc.) on some key components of mitochondrial physiology. Despite the significant advancements in the field that we present in this review, critical gaps remain regarding the molecular mechanisms that underlie the effects of these dietary interventions on mitochondrial physiology, especially under pathological conditions. Future research in this field could underscore these mechanisms, paving the road for the use of dietary interventions against mitochondrial dysfunction as valid pharmacological strategies in human disease.

Mitochondrial dysfunction and oxidative stress have been implicated in cellular senescence, apoptosis, aging and aging-associated pathologies. Telomere shortening and genomic instability have also been associated with replicative senescence, aging and cancer. Here we show that mitochondrial dysfunction leads to telomere attrition, telomere loss, and chromosome fusion and breakage, accompanied by apoptosis. An antioxidant prevented telomere loss and genomic instability in cells with dysfunctional mitochondria, suggesting that reactive oxygen species are mediators linking mitochondrial dysfunction and genomic instability. Further, nuclear transfer protected genomes from telomere dysfunction and promoted cell survival by reconstitution with functional mitochondria. This work links mitochondrial dysfunction and genomic instability and may provide new therapeutic strategies to combat certain mitochondrial and aging-associated pathologies.

Pathogenic variants in the human F-box and leucine-rich repeat protein 4 (FBXL4) gene result in an autosomal recessive, multisystemic, mitochondrial disorder involving variable mitochondrial depletion and respiratory chain complex deficiencies with lactic acidemia. As no FDA-approved effective therapies for this disease exist, we sought to characterize translational C. elegans and zebrafish animal models, as well as human fibroblasts, to study FBXL4–/– disease mechanisms and identify preclinical therapeutic leads. Developmental delay, impaired fecundity and neurologic and/or muscular activity, mitochondrial dysfunction, and altered lactate metabolism were identified in fbxl-1(ok3741) C. elegans. Detailed studies of a PDHc activator, dichloroacetate (DCA), in fbxl-1(ok3741)C. elegans demonstrated its beneficial effects on fecundity, neuromotor activity, and mitochondrial function. Validation studies were performed in fbxl4sa12470 zebrafish larvae and in FBXL4–/– human fibroblasts; they showed DCA efficacy in preventing brain death, impairment of neurologic and/or muscular function, mitochondrial biochemical dysfunction, and stress-induced morphologic and ultrastructural mitochondrial defects. These data demonstrate that fbxl-1(ok3741) C. elegans and fbxl4sa12470 zebrafish provide robust translational models to study mechanisms and identify preclinical therapeutic candidates for FBXL4–/– disease. Furthermore, DCA is a lead therapeutic candidate with therapeutic benefit on diverse aspects of survival, neurologic and/or muscular function, and mitochondrial physiology that warrants rigorous clinical trial study in humans with FBXL4–/– disease.

Muscling In on PGC-1α for Improved Quality of Life in ALS

Alzheimer’s Disease (AD) is the most common neurodegenerative disorder in our society, as the population ages, its incidence is expected to increase in the coming decades. The etiopathology of this disease still remains largely unclear, probably because of the highly complex and multifactorial nature of AD. However, the presence of mitochondrial dysfunction has been broadly described in AD neurons and other cellular populations within the brain, in a wide variety of models and organisms, including post-mortem humans. Mitochondria are complex organelles that play a crucial role in a wide range of cellular processes, including bioenergetics. In fact, in mammals, including humans, the main source of cellular ATP is the oxidative phosphorylation (OXPHOS), a process that occurs in the mitochondrial electron transfer chain (ETC). The last enzyme of the ETC, and therefore the ulterior generator of ATP, is the ATP synthase. Interestingly, in mammalian cells, the ATP synthase can also degrade ATP under certain conditions (ATPase), which further illustrates the crucial role of this enzyme in the regulation of cellular bioenergetics and metabolism. In this collaborative review, we aim to summarize the knowledge of the presence of dysregulated ATP synthase, and of other components of mammalian mitochondrial bioenergetics, as an early event in AD. This dysregulation can act as a trigger of the dysfunction of the organelle, which is a clear component in the etiopathology of AD. Consequently, the pharmacological modulation of the ATP synthase could be a potential strategy to prevent mitochondrial dysfunction in AD.

Role of Polyphosphate in Cancer Cell Proliferation

Mammalian Mitochondrial Inorganic Polyphosphate (polyP) and Cell Signaling: Crosstalk Between PolyP and the Activity of AMPK.

Aging is the strongest risk factor for most neurodegenerative disorders, such as Alzheimer's disease (AD), Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis. As the aging population is increasing, these neurodegenerative disorders are becoming major social issues in many countries. Interestingly, these diseases share a common pathological phenotype; the accumulation of misfolded and aggregated proteins in the brain. Mounting evidence suggests that there are mechanistic links between toxic protein aggregation and neurodegenerative diseases. With a better understanding of the pathogenesis of these disorders, discovery efforts for disease-modifying therapeutics have markedly increased in recent years. This special issue, titled ‘Neurodegenerative diseases and their therapeutic approaches', discusses the underlying pathogenic mechanisms and therapeutic implications for AD and other age-associated neurodegenerative disorders. Aaron Ciechanover and Yong Tae Kwon provide a comprehensive overview of the proteolytic pathways in neurons, with a special emphasis on the ubiquitin–proteasome system (UPS), chaperone-mediated autophagy and macroautophagy. They also discuss the role of protein quality control in the degradation of pathogenic proteins involved in several neurodegenerative diseases. The initial discovery of the UPS was published by Dr Aaron Ciechanover, who won a Nobel Prize in 20041 (http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2004/ciechanover-lecture.pdf). Among the age-associated neurodegenerative diseases, AD is the most common neurodegenerative disease. Its symptoms include cognitive impairment, disturbance of the sleep–wake cycle and circadian rhythms, and dysfunctions of metabolism, such as insulin-resistance and mitochondrial dysfunction. The pathological hallmarks of AD are intracellular neurofibrillary tangles, extracellular senile plaques and neuronal cell loss. Neurofibrillary tangles are mainly composed of hyperphosphorylated tau, and the major component of senile plaques is amyloid-β (Aβ) peptides. Many lines of evidence have shown the strong association between the two aggregated molecules, Aβ and tau, and AD pathogenesis. Holtzman's group reviews multiple molecular mechanisms by which sleep disturbance and disruption of circadian rhythms may affect AD pathogenesis. In addition, they discuss potential therapeutic strategies for AD by targeting the circadian clock and sleep–wake system. Bhumsoo Kim and Eva L Feldman focus on the dysfunction of insulin signaling in AD pathogenesis. Insulin signaling has diverse roles in the brain, such as cognition, memory, synaptic plasticity and neurogenesis. As AD patients show the disrupted glucose metabolism in the brain, it is reasonable to link insulin resistance and AD pathogenesis. The authors also suggest that several diabetes therapies targeting insulin signaling may have potential therapeutic benefits in AD patients. Mook-Jung's gourp review the mitochondrial dysfunction in several neurodegenerative diseases, including AD. Reactive oxygen species (ROS) is a well-known stress factor for the central nervous system. ROS induces mutations in mitochondrial genes, leading to more ROS generation by exacerbating mitochondrial dysfunction. This vicious cycle between ROS and mitochondrial dysfunction further stresses neurons, eventually leading to cell death. Antioxidant therapy restoring mitochondrial function might be one of potential therapeutic strategies to treat AD and other neurodegenerative disorders. Proper maintenance of neuronal circuits is critical for learning and memory. The loss of neurons and neuronal processes directly contributes to cognitive declines in AD. Because many clinical trials with small molecule approaches, as well as anti-Aβ immunotherapy has failed, cell replacement therapy using human embryonic stem cell- or induced pluripotent stem cell -derived neural cells is gaining attention as a potential treatment for AD. Huang's group reviews the current status and future prospects of stem cell therapy for AD and other related disorders. Because AD and other age-associated neurodegenerative disorders are chronic diseases, many factors might be involved in the onset and progression of these diseases. The five articles in this special issue provide comprehensive reviews of the multiple pathogenic mechanisms underlying AD and related neurodegenerative disorders. In addition, each review suggests potential therapeutic targets for the treatment of AD. Taken together, this special issue, titled ‘Neurodegenerative diseases and their therapeutic approaches', in EMM is an invaluable resource for understanding the current status and future perspectives of AD and related neurodegenerative disorders.

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Inorganic polyphosphate (polyP) is a linear polymer of Pi residues linked together by high-energy phosphoanhydride bonds as in ATP. PolyP is present in all living organisms ranging from bacteria to human and possibly even predating life of this planet. The length of polyP chain can vary from just a few phosphates to several thousand phosphate units long, depending on the organism and the tissue in which it is synthesized. PolyP was extensively studied in prokaryotes and unicellular eukaryotes by Kulaev's group in the Russian Academy of Sciences and by the Nobel Prize Laureate Arthur Kornberg at Stanford University. Recently, we reported that mitochondria of cardiac ventricular myocytes contain significant amounts (280±60pmol/mg of protein) of polyP with an average length of 25 Pi and that polyP is involved in Ca(2+)-dependent activation of the mitochondrial permeability transition pore (mPTP). Enzymatic polyP depletion prevented Ca(2+)-induced mPTP opening during ischaemia; however, it did not affect reactive oxygen species (ROS)-mediated mPTP opening during reperfusion and even enhanced cell death in cardiac myocytes. We found that ROS generation was actually enhanced in polyP-depleted cells demonstrating that polyP protects cardiac myocytes against enhanced ROS formation. Furthermore, polyP concentration was dynamically changed during activation of the mitochondrial respiratory chain and stress conditions such as ischaemia/reperfusion (I/R) and heart failure (HF) indicating that polyP is required for the normal heart metabolism. This review discusses the current literature on the roles of polyP in cardiovascular health and disease.

Mitochondria were first described in 1840 as bioblasts, elementary organisms responsible for vital cellular functions, but were subsequently named mitochondria, from the Greek names mitos (thread) and chondros (granule), which describes their appearance during spermatogenesis.1 Their discovery generated substantial interest given their structure resembling bacteria, which led in subsequent years to important scientific discoveries positioning mitochondria as the energy powerhouse of the cell. The unique architecture of mitochondria, consisting of 2 membranes (outer and inner) and compartments (intermembrane space and matrix), is crucial for their vital functions. Mitochondria serve not only as primary sources of cellular energy, but also modulate several cellular processes, including oxidative phosphorylation, calcium homeostasis, thermogenesis, oxygen sensing, proliferation, and apoptosis.2 Therefore, mitochondrial injury and dysfunction might be implicated in the pathogenesis of several diseases. Hypertension accounts for nearly 30% of patients reaching end-stage renal disease.3 Renal injury secondary to hypertension or to ischemia associated with renovascular hypertension (distal to renal artery stenosis) may have significant and detrimental effect on health outcomes. Studies have highlighted several deleterious pathways, including inflammation, oxidative stress, and fibrosis that are activated in the hypertensive kidney, eliciting functional decline.4,5 However, the precise molecular mechanisms responsible for renal injury have not been fully elucidated. Over the past few years, increasing evidence has established the experimental foundations linking mitochondrial alterations to hypertensive renal injury (Table). Mitochondriopathies, abnormalities of energy metabolism secondary to sporadic or inherited mutations in nuclear or mitochondrial DNA (mtDNA) genes, may contribute to the development and progression of hypertension and its complications. In addition, several studies have reported mitochondrial damage and dysfunction consequent to hypertensive renal disease. View this table: Table. Evidence of Renal Mitochondrial Damage in Models of Hypertension and Antihypertensive Treatment Importantly, hypertensive-induced renal injury is characterized by activation of several deleterious pathways, including oxidative stress, renin–angiotensin–aldosterone …