Molecular Aspects of Mitochondrial Dysfunction in Diabetes, Pearson and Kearns-Sayre Syndromes, and Neurodegenerative Disorders

Introduction

Mitochondria have undergone a remarkable evolutionary journey. Originally free-living prokaryotic cells, they were engulfed by ancestral eukaryotes in a process of endosymbiosis. Over time, these ancient bacteria evolved into essential organelles, now serving as vital energy producers within eukaryotic cells and crucial contributors to human health and disease. The symbiotic relationship between the eukaryotic cell and mitochondria has led to the development of a double membrane architecture, with four sub-compartments, and the retention of a unique genome that encodes proteins critical for oxidative phosphorylation and ATP synthesis.^1,2 Over billions of years, extensive proteomic rewiring has occurred to accommodate the energetic and metabolic requirements of modern eukaryotic organisms, making mitochondria the most studied organelles in biomedical sciences.^1,2 However, this intricate machinery is not immune to malfunction.

Mitochondrial dysfunction refers to the inability of this energy producing organelle to produce sufficient ATP required to meet the energy demands of eukaryotic cells.³ The function of mitochondria is sensitive to increased exposure to reactive oxygen species (ROS), mitochondrial biogenesis, and underlying genetic factors.⁴ ROS are byproducts of metabolic processes within the electron transfer chain (ETC), commonly Complexes I and III.⁵ Examples of ROS include peroxides, superoxides, hydroxyl radicals, singlet oxygen, and alpha-oxygen which when produced in excess, can lead to oxidative stress, DNA damage, and impaired cellular integrity.⁶ The extracellular matrix, for example, is particularly vulnerable to oxidative stress, resulting in lipid peroxide chain reactions and protein aggregation that further compromise normal function. Decreased levels of mitochondrial activity, either due to a reduction of mitochondrial biogenesis or decreased ETC complex function, result in lipid accumulation and further stress.^7–9

The lipid accumulation results in lipotoxicity, a cellular condition that exacerbates dysfunction by inducing lipid peroxidation. The accumulation of lipid peroxidation products, such as 4-hydroxynonenal and malondialdehyde, activate inflammatory signaling pathways including NF-κB, triggering the release of pro-inflammatory cytokines. Additionally excessive lipid accumulation can trigger stress responses including the unfolded protein response (UPR),¹⁰ calcium dysregulation,¹¹ and damage to phospholipids essential to the ETC, such as cardiolipin. Cardiolipin oxidation disrupts the formation and stability of ETC supercomplexes, reducing the efficiency of electron transfer and leading to increased ROS production. The resultant destabilization further impairs ATP synthase function, worsening energy deficits.¹² Ultimately, the accumulation of these processes can trigger the opening of the mitochondrial permeability transition pore leading to loss of mitochondrial membrane potential, ATP depletion, and eventually apoptosis. This exacerbation of oxidative stress results in several downstream pathological states with significant clinical manifestations.

Clinical manifestations of mitochondrial dysfunction present more noticeably in organs with a higher energy need, including the heart, liver, brain, eyes, and skeletal muscles.^13,14 This dysfunction may result in clinical hypertension, cardiomyopathy, sensorineural hearing loss, optic atrophy, and pigmentary retinopathy.¹⁵ Due to the vast impact of mitochondrial dysfunction on different physiological systems, there are several associated diseases including rare, primary genetic disorders such as Pearson Syndrome (PS) and Kearns Sayre Syndrome (KSS), as well as the more common type 2 diabetes mellitus (T2DM) and neurodegenerative disease. While numerous mitochondrial diseases have been described, this review focuses on selected conditions such as insulin resistance, PS, KSS, and neurodegenerative disorders to illustrate the spectrum of mitochondrial dysfunction and provide a focused analysis of their molecular and clinical presentations.

Variability in symptom onset and presentation makes the diagnosis of mitochondrial disease challenging for clinicians. This results in decreased detection and further progression of disease without treatment. Successful detection requires a diverse array of clinical, biochemical, histological, and genetic approaches to ensure a definitive diagnosis.³ Notably, histochemical and histological analyses of muscle biopsies are the gold standard for the diagnosis of mitochondrial dysfunction.¹⁶ Table 1 summarizes factors and diseases associated with mitochondrial dysfunction.

Table 1 Molecular Factors and Their Associated Diseases in Mitochondrial Dysfunction

This review focuses on insulin resistance, PS, KSS, and neurodegenerative disorders to illustrate the spectrum of mitochondrial dysfunction, from prevalent metabolic disturbances to rare mitochondrial DNA deletion syndromes. Pearson Syndrome and Kearns-Sayre Syndrome are genetically and clinically linked, as PS can progress to KSS, both resulting from large-scale mtDNA deletions. This selection allows for an integrated discussion of mitochondrial dysfunction’s impact across diverse clinical contexts. The aim of this review is to synthesize current knowledge on the molecular mechanisms of mitochondrial dysfunction and to discuss their clinical implications in insulin resistance, PS, KSS, and neurodegenerative disorder.

Mitochondrial Dysfunction in Diabetes and Insulin Resistance

The pathophysiology of T2DM often involves mitochondrial dysfunction leading to insulin resistance and pancreatic β-cell dysfunction.¹² Mitochondrial impairment manifests as reduced biogenesis, oxidative capacity, and ATP production, resulting in the inability of cells to effectively utilize glucose, subsequently driving compensatory glucose oxidation. Excessive insulin resistance leads to a preference for glucose oxidation over fatty acid oxidation, a phenomenon known as the Randle cycle. This transition leads to elevated lipogenesis.^8,21,22 Additionally, reduction in mitochondrial fatty acid oxidation results in the intracellular accumulation of lipid metabolites, such as diacylglycerols (DAG) and ceramides (CER), which disrupt insulin signaling pathways and further exacerbate insulin resistance.⁴ The associated increase in ROS production from mitochondrial impairment can create a vicious cycle of oxidative stress, mitochondrial dysfunction, and disrupted insulin signaling.

The mechanism of disrupted insulin signaling due to ROS exposure involves the activation of sphingomyelinase and the generation of CER. These molecules act as activators of protein kinase C (PKC), which can then induce insulin resistance through serine/threonine phosphorylation of key insulin signaling components (Figure 1). Genetic studies have also revealed that dysfunctional mitochondria can upregulate specific microRNAs, such as miR-126, that target insulin receptor substrate-1 (ie miR-126) in hepatocytes, further contributing to the development of insulin resistance.^23,24 Additionally, high-fat diet studies have demonstrated that increased mitochondrial peroxide emission is associated with the onset of insulin resistance²⁵ This diet under conditions of estrogen deficiency, can promote cognitive decline via brain insulin resistance, mitochondrial dysfunction, and impaired hippocampal synaptic function.²⁶

Figure 1 Mechanism of mitochondrial dysfunction leading to insulin resistance in skeletal muscles. A decrease in mitochondrial biogenesis due to malfunction of PGC1 and/or a decrease in mitochondrial content can reduce the oxidation of fatty acids, leading to an increase in DAG and fatty acyl CoA. This in turn will generate a series of Serine/Threonine kinase cascade events, possibly initiated by protein kinase Cθ.27 The cascade results in the phosphorylation of serine residues (ie Ser-307) found in the SH2 domain of IRS 1 and 2, which prevents the phosphorylation of tyrosine residues.25 Studies have shown that a lack of phosphorylated tyrosine in the SH2 domain of IRS will inhibit the PI3-kinase,28 leading to low levels PIP3, less phosphorylated Akt and TBC1D4 proteins, and overall fewer GLUT 4 expression on the plasma membrane.29 The final outcome is reduced net glucose uptake, and more glucose remains in the blood. Moderate physical activity has been seen to increase insulin sensitivity by depleting muscle glycogen.28 Abbreviations: Red arrow: respective change in quantity of adjacent molecule; PGC1: peroxisome proliferator-activated receptor gamma coactivator 1; DAG and fatty acyl CoA: intracellular lipid metabolites; IRS: Insulin Receptor Substrate; Protein kinase Cθ: enzyme involved in T cell activation; (S) Serine residue; (Y) Tyrosine residue;. P13-kinase: an enzyme necessary for converting PtdIns(4,5)P2 (PIP2) to PIP3; PIP3: phosphatidylinositol (3,4,5)-trisphosphate, a crucial signaling molecule; Akt: protein kinase B; TBC1D4: a regulatory protein of GLUT4; GLUT 4: transporter protein for glucose.

Gene expression studies evaluating mitochondrial mRNA demonstrated that insulin-resistant obese patients have decreased oxidative enzyme activity, lipid metabolism rates, mitochondrial size, and mitochondrial density.^7,30 Furthermore, Kelley et al showed that NADH:O2 oxidoreductase activities are lower in obese patients compared to lean patients.³⁰ However, this study focused on obese patients who had lower mitochondrial biogenesis,³⁰ implying that obesity in these subjects was related to lack of mitochondria rather than insulin resistance.^22,23

In addition to these mechanisms, enhanced protein carbonylation is another cause of insulin resistance in skeletal muscles.^31,32 When comparing type 1 and 2 skeletal muscle fibers, type 1 muscle fibers are more oxidative and richer in mitochondria, while type 2 fibers are more glycolytic. Mechanistically, this is due to decreased expression of nuclear-encoded genes that control mitochondrial biogenesis, such as peroxisome proliferator coactivator 1 (PGC1).^33,34 Decreased PGC1 expression can subsequently decrease the expression of Nuclear Respiratory Factor (NFR)-dependent genes integral to mitochondrial function and energy metabolism. Thus, their reduction can cause insulin resistance and T2DM.^35–37

On the other hand, insulin can improve mitochondrial function, supporting the view that mitochondrial dysfunction might be secondary to insulin resistance. Stump et al showed that an 8-hour insulin infusion in humans enhanced mitochondrial oxidative capacity in skeletal muscles. This improvement was evidenced by increased mitochondrial protein synthesis rate, elevated expression of mitochondrial mRNA, and quantitative increase in cytochrome c oxidase activity, citrate synthase, and ATP production.³⁸ Another study by Karakelides et al examined type I diabetic patients after withholding insulin treatments, showing that insulin deficiency decreased muscle mitochondrial ATP production and reduced the gene expression of RASGRP1 and TYRP1 necessary for oxidative phosphorylation, concluding that lack of insulin promotes mitochondrial dysfunction.³⁹ Mitochondrial transcription factor A (TFAM) studies on mice have shown that TFAM-knockouts result in respiratory chain deficiencies and abnormal appearance of the mitochondria.^40,41 However, the respiratory chain deficiencies in skeletal muscles did not result in insulin resistance or a diabetic phenotype in a murine model despite high levels of peripheral glucose disposal during a glucose tolerance test. This suggests that mitochondrial dysfunction in skeletal muscles is a secondary phenomenon and does not directly cause insulin resistance.⁴²

Demographic studies have associated mitochondrial dysfunction with insulin resistance. Nair et al reported that Asian Indians with diabetes have the same mitochondrial capacity as non-diabetic subjects with matching body mass index (BMI), sex, and age. Furthermore, Asian Indians tend to have enhanced mitochondrial capacity and higher mtDNA content compared to Northern European Americans with matching BMI, sex, and age while being more insulin resistant, indicating a dissociation between mitochondrial dysfunction and insulin resistance.⁴³ This is intriguing, as it suggests ethnogenetic differences in fundamental cellular energetics that may have broader disease state implications that should prompt further investigation.

While no definite cure exists for insulin resistance in T2DM, pharmacotherapeutics improve symptoms and disease progression. The primary drug of choice for addressing T2DM is typically a biguanide, most notably Metformin. Biguanides have three primary functions: decrease hepatic gluconeogenesis, decrease glucose absorption in the intestinal tract, and increase peripheral insulin sensitivity. Additionally, thiazolidinediones are a traditional class that induce transcriptional changes in cultured cells and enhance insulin-dependent glucose disposal. The regulation of gene expression by Thiazolidinedione affects the nuclear receptor known as peroxisome proliferator-activated receptors (PPARs).⁴⁴ Other forms of chemicals include solute carriers (SLCs) which are protein drug targets with defined genes that can mediate passive or secondary active transport in cells.³⁸ Increasing the amount of GLUT4 through gene transcription of SLC2A4 (Figure 1) can increase the amount of GLUT4 transporters and maintain glycemic homeostasis, hence preventing insulin resistance and other associated deficits.³⁸ Dipeptidyl peptidase-4 inhibitors (DDP-4 inhibitors), also known as gliptins, inhibit glucagon release and result in an indirect increase in insulin secretion. One study evaluated the effect of DDP-4 inhibitors on mitochondrial function in rats limited to a high-fat diet. They found that twelve weeks of a high-fat diet resulted in significant increase in mitochondrial reactive oxygen species (ROS) production, mitochondrial membrane depolarization, and mitochondrial swelling. When given two different GLP-1 Agonists (vildagliptin and sitagliptin), these effects were substantially diminished.⁴⁵ Another class of antidiabetic medication includes the Sodium-Glucose Cotransporter-2 Inhibitors (SGLT-2 inhibitors, or gliflozins) which reversibly inhibit the sodium-dependent glucose transporter in the proximal nephron, resulting in polyuria and glycosuria. These medications have been shown to maintain the structural integrity of mitochondria through swelling prevention, fission inhibition, and maintenance of adequate mitochondrial size. They improve membrane potential, mitochondrial function, and suppress ROS production.⁴⁶

Mitochondrial Dysfunction in Pearson Syndrome

Pearson syndrome (PS) is a rare multisystem mitochondrial disorder resulting from mtDNA defects, typically deletions or duplications. Resulting mitochondrial dysfunction manifests as severe hematologic abnormalities including pancytopenia^29,47,48 and vacuolization of bone marrow precursors leading to significant anemia, the most common feature of PS. In infancy, PS is also characterized by exocrine pancreatic dysfunction, which may progress to acinar atrophy resulting in malabsorption, chronic diarrhea, and steatorrhea.^47–51 In some rare cases, affected infants may present with neonatal diabetes and adrenal insufficiency.⁴⁹ As the disease advances, initial abnormalities extend to involve renal, hepatic, and endocrine systems. Furthermore, impaired respiratory enzyme function contributes to metabolic disturbances such as sideroblastic anemia and lactic acidosis.⁵⁰ Due to the multisystem involvement and lack of curative treatments, most diagnosed infants do not survive past the age of three. Despite variability in size and location of mtDNA defects, the resultant phenotype consistently includes compromised hematopoietic dysfunction, underscoring the severe impacts of this disorder.^49,52

There is an established association between the size and location of mtDNA deletion and the severity of the disease, with the 4.9 kD deletion being the most common. These deletions were initially quantified with the use of external plasmids and real-time polymerase chain reaction assay.⁵³ The deletions affect genes responsible for protein coding and ribosomal transfer RNAs (tRNAs). This generates alterations in mitochondria structure and function, including oxidative phosphorylation and energy metabolism in the respiratory chain.^51,52 Defects in oxidative phosphorylation present as lactic acidemia, renal tubular and glomerular dysfunction, or hepatopathy.^50,54 Since mtDNA is inherited maternally,⁵⁵ a defect in the maternal mtDNA is suspected to be a potential cause of PS. While PS cases are typically sporadic and caused by somatic mutational events during early embryonic development, affected women with chronic progressive external ophthalmoplegia (CPEO) have an estimated one in twenty-four risk of having a child with a mtDNA deletion syndrome, highlighting a potential maternal transmission risk in certain cases. Early case reports from 1993 and 2002 support this association, documenting instances of mothers with CPEO who had children with PS.⁵⁶ Further research has shown that there has been no correlation between maternal age and the risk of the unaffected mother’s child with mtDNA mutation disease. Unlike chromosomal DNA, mtDNA mutation risk does not increase with maternal age.⁵²

The prognosis of PS is generally poor, with death often occurring within infancy. However, children who survive past the age of three have been observed to develop Kearns-Sayre Syndrome, which shares similar presentations with PS, suggesting a change over time of the location and concentration of mutated mtDNA.^28,49,52,53 A study by Yanagihara et al shows that the number of cells with deleted mtDNA decreases over time since the deletion causes a growth disadvantage, drastically decreasing the number of mitochondria.⁵⁷ This single mtDNA deletion is associated with Pearson’s Syndrome, Kearns-Sayre syndrome, and CPEO.⁵⁸ Patients have also manifested neurologic impairments that are pathologically like Leigh syndrome.⁵⁹

Without a cure for PS, treatment options are limited and primarily aimed at symptom management. Replacement therapies with hydrocortisone can temporarily improve adrenal insufficiencies.⁶⁰ Additionally, vitamin supplements, pharmacological agents, and exercise therapy are common treatment methods used in mitochondrial respiratory chain disorders like PS. Other treatments include the usage of coenzyme Q10, creatine monohydrate, and a combination of both with lipoic acid, although these trials have yielded limited results and require larger studies to validate their efficacy.⁶¹

Allogeneic hematopoietic stem cell transplantation (HSCT) has also been explored as a treatment for PS in cases of severe bone marrow failure, with mixed outcomes. While some patients, such as those receiving unrelated cord blood transplantation, showed resolution of hematologic and non-hematologic symptoms, others experienced critical complications, including graft-versus-host disease, infections, and disease progression, highlighting the risks and variable efficacy of HSCT in PS.^60–64

Mitochondrial Dysfunction in Kearns-Sayre Syndrome

The same cycle that results in mtDNA mutations in PS can also give rise to Kearns-Sayre syndrome, a rare mitochondrial myopathy that occurs due to deletions of approximately 1.1–10kb in mtDNA.^47,65,66 A study of Chinese twins with KSS identified a novel 6600 bp mtDNA deletion (positions 8702–15,302), expanding the known mutation spectrum.⁶⁷ Similar to PS, older case studies have shown a lack of correlation between the size and site of mtDNA deletion and the resultant phenotype present in KSS.^57,68,69 While most cases involve sporadic mtDNA deletions, some are linked to point mutations (eg, m.3243A>G) or rare maternal transmission.⁷⁰ However, more recent studies have shown that higher levels of heteroplasmy are associated with more severe clinical manifestations. Specifically, the distribution of such deletions among different tissues contribute to variability in symptoms. For instance, a higher burden of deleted mtDNA in skeletal muscle tissues may lead to pronounced muscle weakness and exercise intolerance, whereas greater involvement in cardiac tissues can result in significant cardiac conduction defects. Scanning electron microscopy has identified unique morphological changes in mitochondrial ultrastructure including the disappearance of cristae, dilated intracristal space, and densified matrix component.⁷¹ While the correlation between phenotype and genotype remains inconsistent, common clinical complications have been described. Current research on KSS is focused on exploring the coexistence of normal and mutated mtDNA, as their ratio can vary between tissues and even among cells within the same tissue, influencing the severity and range of clinical features observed in KSS patients.⁶⁶

KSS can present with ocular symptoms such as progressive retinal dysfunction and external ophthalmoplegia which eventually leads to ptosis.^65,72,73 Additionally, mtDNA deletions in KSS have been associated with morphologic changes in erythrocytes, fibroblasts, neural cells, and hepatocytes.⁷⁴ KSS’s clinical characteristics include short stature, growth hormone deficiency, gonadal failure, thyroid disorders, and hypomagnesemia.^75–78

Furthermore, rare cases of coexisting KSS and hypopituitarism have been reported, emphasizing the potential impact of mitochondrial dysfunction on the pituitary gland and its regulatory role in multiple endocrine axes.⁷⁹ In patients with KSS, a deficiency of the pituitary-gonadal axis resulted in delayed puberty, cryptorchidism, and hypogonadism in males; while females showed late menarche, primary or secondary amenorrhea, or oligomenorrhea.⁷² Additionally, higher risk clinical manifestations observed in KSS involve the progressive impairment of cardiac conduction, frequently resulting in ventricular arrhythmias, bundle branch blocks, and infra-Hissian atrioventricular block. Infiltrative cardiac fibrosis predisposes KSS patients to these arrhythmias, and the utilization of gadolinium enhanced cardiac magnetic resonance imaging is the gold standard for detection.⁷³

Comprehensive clinical evaluation and management strategies are crucial to address the diverse aspects of this disorder and improve patient outcomes. Despite limited treatments,⁷⁹ the use of recombinant growth hormone serves as a potential treatment for growth hormone deficiency with the risk of hypertrophic cardiomyopathy.⁸⁰ Another temporary treatment option is the administration of coenzyme Q10 which increases inner mitochondrial membrane fluidity and provides a favorable interaction between components of the ETC.⁸¹ Observed clinical improvements of KSS include lower levels of lactate and pyruvate.^82,83 Heart transplantation was successfully performed in a 16-year-old boy with KSS presenting with acute heart failure due to dilated cardiomyopathy, highlighting this as a viable treatment option for severe cardiac involvement in KSS. Although decisions remain case-specific due to the systemic and progressive nature of the disease.⁸⁴

Mitochondrial Dysfunction in Neurodegenerative Disorders

Mitochondrial dysfunction and insulin resistance are integral in the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS). Mitochondrial dysfunction commonly contributes to disruption of neuronal energy homeostasis resulting in cell damage and death.⁸⁵ Due to the link between mitochondrial dysfunction and insulin resistance, insulin’s ability to cross the blood-brain-barrier compounds its impact on brain aging, plasticity, and neurodegeneration.⁸⁶

In Alzheimer’s disease, mitochondrial dysfunction is closely associated with the accumulation of amyloid-beta (Aβ) plaques and hyperphosphorylated tau proteins, which exacerbate neuronal toxicity.⁸⁷ Mutations in Aβ production genes, such as APP, PSEN1, and PSEN2, have been shown to impair mitochondrial function by promoting the accumulation of toxic Aβ oligomers.⁸⁷ Additionally, insulin resistance impairs insulin signaling in the brain, a condition referred to as “type 3 diabetes” in the context of Alzheimer’s disease.^88–90

Insulin receptors and signaling cascades including PI3/Akt are compromised by reduced ATP production and oxidative stress.^91,92 Reduced insulin signaling in the brain disrupts glucose metabolism, promotes neuroinflammation, and impairs synaptic plasticity, all of which are critical for cognitive function.^92,93

In Parkinson’s disease, mitochondrial dysfunction is particularly devastating in the substantia nigra, where dopaminergic neurons are highly susceptible to oxidative stress and energy deficits.⁹⁴ Mutations in genes PINK1 and PRKN, which are critical for mitophagy, lead to the accumulation of damaged mitochondria and neuronal death.⁹⁴

In addition to triggering mitochondrial dysfunction, insulin resistance contributes to PD by promoting α-synuclein accumulation and disrupting PLK2 signaling, a protein kinase involved in cell division and proliferation.⁹⁵

Mutations in SOD1, which is implicated in ALS, disrupt mitochondrial dynamics and function. This often results in neuronal cell death through the disruption of axonal transport and increase in susceptibility to excitotoxicity.⁹⁶

Overall, mitochondrial dysfunction play integral roles in the development and pathophysiology of neurodegenerative diseases, making them attractive targets for therapeutics. A summary of current and future therapeutic approaches centered around mitochondria in relationship to disease states is summarized in Table 2.

Table 2 Therapeutics Targeting Mitochondrial Mediated Disease States

Taken together, these conditions exemplify the diverse clinical spectrum of mitochondrial dysfunction, from common metabolic disorders to rare genetic syndromes. Understanding their shared and unique pathophysiological mechanisms underscores the need for improved diagnostic and therapeutic strategies targeting mitochondrial health.

Conclusion

In summary, mitochondrial dysfunction is a complex pathophysiological process underlying a diverse array of clinical disorders, including type 2 diabetes mellitus, Pearson syndrome, Kearns-Sayre syndrome, and neurodegenerative diseases. While the specific mechanisms and genetic factors vary across these conditions, the common thread is an impairment in mitochondrial function, leading to disruptions in cellular energy production, oxidative stress, and organ system dysfunction. Understanding the intricate interplay between this pathology, insulin resistance, and multi-system clinical manifestations is crucial for advancing diagnostic approaches and improving patient outcomes. This review demonstrates the pervasive pathologic consequences of mitochondrial dysfunction, both in rare genetic conditions and commonly seen diseases. Continued research efforts to elucidate the molecular underpinnings of these mitochondrial disorders hold promise for uncovering novel therapeutic targets and paving the way for more effective management strategies.