Renato X. Santos

Insulin is a Two-Edged Knife on the Brain

Insulin, long known as an important regulator of blood glucose levels, plays important and multifaceted roles in the brain. It has been reported that insulin is an important neuromodulator, contributing to several neurobiological processes in particular energy homeostasis and cognition. Dysregulation of insulin signaling has been linked to aging and metabolic and neurodegenerative disorders. The first part of this review is devoted to discussion of the critical role of insulin signaling in normal brain function. Then the involvement of impaired insulin signaling in the pathophysiology of diabetes, Alzheimers, Parkinsons and Huntingtons diseases and amyotrophic lateral sclerosis will be discussed. Finally, the potential therapeutic effect of insulin and insulin sensitizers will be examined.

Insulin signaling, glucose metabolism and mitochondria: Major players in Alzheimer's disease and diabetes interrelation

Many epidemiological studies have shown that diabetes, particularly type 2 diabetes, significantly increases the risk to develop Alzheimers disease. Both diseases share several common abnormalities including impaired glucose metabolism, increased oxidative stress, insulin resistance and deposition of amyloidogenic proteins. It has been suggested that these two diseases disrupt common cellular and molecular pathways and each disease potentiates the progression of the other. This review discusses clinical and biochemical features shared by Alzheimers disease and diabetes, giving special attention to the involvement of insulin signaling, glucose metabolism and mitochondria. Understanding the key mechanisms underlying this deleterious interaction may provide opportunities for the design of effective therapeutic strategies.

A synergistic dysfunction of mitochondrial fission/fusion dynamics and mitophagy in Alzheimer's disease

Alzheimers disease (AD), the most common form of dementia in the elderly, can have a late-onset sporadic or an early-onset familial origin. In both cases, the neuropathological hallmarks are the same: senile plaques and neurofibrillary tangles. Despite AD having a proteinopathic nature, there is strong evidence for an organelle dysfunction-related neuropathology, namely dysfunctional mitochondria. In this regard, dysfunctional mitochondria and associated exacerbated generation of reactive oxygen species are among the earliest events in the progression of the disease. Since the maintenance of a healthy mitochondrial pool is essential given the central role of this organelle in several determinant cellular processes, mitochondrial dysfunction in AD would be predicted to have profound pluripotent deleterious consequences. Mechanistically, recent reports suggest that mitochondrial fission/fusion and mitophagy are altered in AD and in in vitro models of disease, and since both processes are reported to be protective, this review will discuss the role of mitochondrial fission/fusion and mitophagy in the pathogenesis of AD.

Metabolic Alterations Induced by Sucrose Intake and Alzheimer’s Disease Promote Similar Brain Mitochondrial Abnormalities

Evidence shows that diabetes increases the risk of developing Alzheimer’s disease (AD). Many efforts have been done to elucidate the mechanisms linking diabetes and AD. To demonstrate that mitochondria may represent a functional link between both pathologies, we compared the effects of AD and sucrose-induced metabolic alterations on mouse brain mitochondrial bioenergetics and oxidative status. For this purpose, brain mitochondria were isolated from wild-type (WT), triple transgenic AD (3xTg-AD), and WT mice fed 20% sucrose-sweetened water for 7 months. Polarography, spectrophotometry, fluorimetry, high-performance liquid chromatography, and electron microscopy were used to evaluate mitochondrial function, oxidative status, and ultrastructure. Western blotting was performed to determine the AD pathogenic protein levels. Sucrose intake caused metabolic alterations like those found in type 2 diabetes. Mitochondria from 3xTg-AD and sucrose-treated WT mice presented a similar impairment of the respiratory chain and phosphorylation system, decreased capacity to accumulate calcium, ultrastructural abnormalities, and oxidative imbalance. Interestingly, sucrose-treated WT mice presented a significant increase in amyloid β protein levels, a hallmark of AD. These results show that in mice, the metabolic alterations associated to diabetes contribute to the development of AD-like pathologic features.

Mitochondrial DNA Oxidative Damage and Repair in Aging and Alzheimer's Disease

SIGNIFICANCE Mitochondria are fundamental to the life and proper functioning of cells. These organelles play a key role in energy production, in maintaining homeostatic levels of second messengers (e.g., reactive oxygen species and calcium), and in the coordination of apoptotic cell death. The role of mitochondria in aging and in pathophysiological processes is constantly being unraveled, and their involvement in neurodegenerative processes, such as Alzheimers disease (AD), is very well known. RECENT ADVANCES A considerable amount of evidence points to oxidative damage to mitochondrial DNA (mtDNA) as a determinant event that occurs during aging, which may cause or potentiate mitochondrial dysfunction favoring neurodegenerative events. Concomitantly to reactive oxygen species production, an inefficient mitochondrial base excision repair (BER) machinery has also been pointed to favor the accumulation of oxidized bases in mtDNA during aging and AD progression. CRITICAL ISSUES The accumulation of oxidized mtDNA bases during aging increases the risk of sporadic AD, an event that is much less relevant in the familial forms of the disease. This aspect is critical for the interpretation of data arising from tissue of AD patients and animal models of AD, as the major part of animal models rely on mutations in genes associated with familial forms of the disease. FUTURE DIRECTIONS Further investigation is important to unveil the role of mtDNA and BER in aging brain and AD in order to design more effective preventive and therapeutic strategies.

The role of endoplasmic reticulum in amyloid precursor protein processing and trafficking: implications for Alzheimer's disease

The endoplasmic reticulum (ER) is the principal organelle responsible for the proper folding/processing of nascent proteins and perturbed ER function leads to a state known as ER stress. Mammalian cells try to overcome ER stress through a set of protein signaling pathways and transcription factors termed the unfolded protein response (UPR). However, under unresolvable ER stress conditions, the UPR is hyperactivated inducing cell dysfunction and death. The accumulation of misfolded proteins in the brain of Alzheimers disease (AD) patients suggests that alterations in ER homeostasis might be implicated in the neurodegenerative events that characterize this disorder. This review discusses the involvement of ER stress in the pathogenesis of AD, focusing the processing and trafficking of the AD-related amyloid precursor protein (APP) during disease development. The potential role of ER as a therapeutic target in AD will also be debated.

Mitochondrial Abnormalities in a Streptozotocin-Induced Rat Model of Sporadic Alzheimer's Disease

This study aimed to show that the rat model of sporadic Alzheimers disease (sAD) generated by the intracerebroventricular (icv) injection of a sub-diabetogenic dose of streptozotocin (icvSTZ) is characterized by brain mitochondrial abnormalities. Three-month-old male Wistar rats were investigated 5 weeks after a single bilateral icv injection of STZ (3 mg/ Kg) or vehicle. icvSTZ administration induced a decrease in brain weight and cognitive decline, without affecting blood glucose levels. icvSTZ administration also resulted in a significant increase in hippocampal amyloid beta peptide 1-42 (Aβ(1-42)) levels as well as in cortical and hippocampal hyperphosphorylated tau protein levels. Brain mitochondria from icvSTZ rats revealed deficits in their function, as shown by a decrease in mitochondrial transmembrane potential, repolarization level, ATP content, respiratory state 3, respiratory control ratio and ADP/O index and an increase in lag phase of repolarization. Mitochondria from icvSTZ rats also displayed a decrease in pyruvate and α-ketoglutarate dehydrogenases and cytochrome c oxidase activities and an increase in the susceptibility to calcium-induced mitochondrial permeability transition. An increase in hydrogen peroxide and lipid peroxidation levels and a reduction in glutathione content were also observed in mitochondria from icvSTZ rats. These results demonstrate that the insulin-resistant brain state that characterizes this rat model of sAD is accompanied by the occurrence of mitochondrial abnormalities reinforcing the validity of this animal model to study sAD pathogenesis and potential therapies.

Autophagy in Alzheimer’s disease

Autophagy is a degradation pathway for the turnover of dysfunctional organelles or aggregated proteins in cells. Extensive literature exists supporting a causative role of mitochondrial dysfunction and amyloid-β protein in the pathogenesis of Alzheimer’s disease (AD). Furthermore, a link between mitochondrial dysfunction, amyloid-β levels and autophagy has been reported to occur in AD. However, it is not yet clear if autophagy plays a causative role, a protective role or is a consequence of the disease process itself. Understanding the exact role of autophagy in different stages of AD progression may help to design more effective therapeutic strategies. A central issue in developing therapies for neurodegenerative diseases involves understanding why and when responses to stress or injury can help prevent neuronal degeneration and death.

Doxorubicin increases the susceptibility of brain mitochondria to Ca2+-induced permeability transition and oxidative damage

This study was aimed at investigating the effects of subchronic administration of doxorubicin (DOX) on brain mitochondrial bioenergetics and oxidative status. Rats were treated with seven weekly injections of vehicle (sc, saline solution) or DOX (sc, 2 mg kg(-1)), and 1 week after the last administration of the drug the animals were sacrificed and brain mitochondrial fractions were obtained. Several parameters were analyzed: respiratory chain, phosphorylation system, induction of the permeability transition pore (PTP), mitochondrial aconitase activity, lipid peroxidation markers, and nonenzymatic antioxidant defenses. DOX treatment induced an increase in thiobarbituric acid-reactive substances and vitamin E levels and a decrease in reduced glutathione content and aconitase activity. Furthermore, DOX potentiated PTP induced by Ca2+. No statistical differences were observed in the other parameters analyzed. Altogether our results show that DOX treatment increases the susceptibility of brain mitochondria to Ca(2+)-induced PTP opening and oxidative stress, predisposing brain cells to degeneration and death.

Insulin-induced recurrent hypoglycemia exacerbates diabetic brain mitochondrial dysfunction and oxidative imbalance

Intensive insulin therapy can prevent or slow the progression of long-term diabetes complications but, at the same time, it increases the risk for episodes of severe hypoglycemia. In our study, we used a protocol intended to mimic the levels of blood glucose that occur in type 1 diabetic patients under an intensive insulin therapy. Streptozotocin (STZ)-induced diabetic rats were treated subcutaneously with twice-daily insulin injections for 2weeks to induce hypoglycemic episodes. Brain cortical and hippocampal mitochondria were isolated and mitochondrial bioenergetics (respiratory chain and phosphorylation system) and oxidative status parameters (malondialdehyde (MDA) levels, mitochondrial aconitase activity and enzymatic and non-enzymatic antioxidant defenses) were analyzed. The protein levels of synaptophysin, a marker of synaptic integrity, and caspase 9 activity were also evaluated in cortical and hippocampal homogenates. Brain cortical mitochondria isolated from hyper- and recurrent hypoglycemic animals presented higher levels of MDA and α-tocopherol together with an increased glutathione disulfide reductase activity, lower manganese superoxide dismutase (MnSOD) activity and glutathione-to-glutathione disulfide (GSH/GSSG) ratio. No significant alterations were found in cortical mitochondrial respiratory chain and oxidative phosphorylation system. Hippocampal mitochondria from both experimental groups presented an impaired oxidative phosphorylation system characterized by a decreased mitochondrial energization potential and ATP levels and higher repolarization lag phase. In addition, higher MDA levels and decreased GSH/GSSG, α-tocopherol levels, and aconitase, glutathione peroxidase and MnSOD activities were observed in both groups of animals. Hippocampal mitochondria from recurrent hypoglycemic animals also showed an impairment of the respiratory chain characterized by a lower state 3 of respiration, respiratory control ratio and ADP/O index, and a higher state 4 of respiration. Additionally, a non-statistically significant decrease in synaptophysin protein levels was observed in cortical homogenates from recurrent hypoglycemic rats as well as in hippocampal homogenates from hyperglycemic and recurrent hypoglycemic rats. An increase in caspase 9 activity was also observed in hippocampal homogenates from hyperglycemic and recurrent hypoglycemic animals. Our results show that mitochondrial dysfunction induced by long-term hyperglycemic effects is exacerbated by recurrent hypoglycemia, which may compromise the function and integrity of brain cells.