The involvement of mitochondrial fission, fusion and biogenesis and autophagy in the diabetic brain

Abstract

Diabetes mellitus has become a global epidemic. Etiologically, diabetes can be classified in two main forms, type 1 and type 2 diabetes (T1D and T2D, respectively). Both forms of diabetes have been associated with a number of complications including neurodegeneration. Additionally, increasing evidence supports the idea that diabetes increases the risk of cognitive decline and dementia. Insulin, a widely used therapeutic agent in the treatment of diabetes, which assumes critical importance in the treatment of T1D, has also been proved to exert beneficial effects in the brain. Mitochondria sit at a strategic position in brain cells, particularly in neurons, that present a high energy demand. Besides energy production, mitochondria play other key roles such as cell life or death decision and homeostasis of second messengers (Ca2+ and reactive oxygen species). The maintenance of a healthy mitochondrial pool requires an equilibrium between mitochondrial fission/fusion, biogenesis and degradation by autophagy (mitophagy). Although previous studies demonstrated the involvement of mitochondrial dysfunction in diabetes‐associated brain damage, the knowledge concerning the role of mitochondrial fission/fusion and biogenesis and autophagy in diabetic brains is insufficient. So, the main aim of this work was to elucidate these mitochondrial parameters in T1D and T2D rat brain.

Studies were conducted in brain cortices of six‐month‐old Wistar (control) and Goto‐Kakizaki (GK) rats (Chapter 4). GK rats are a model of nonobese T2D considered by some authors a good animal model of prediabetes or initial stages of T2D since it does not present the full clinical setting of this pathology. In fact, these rats are characterized by mild hyperglycemia and insulin resistance. Concerning brain cortical mitochondria, no significant differences were observed in respiratory chain function and oxidative phosphorylation efficiency. Despite the inexistence of functional alterations, increased mitochondrial fission was observed in T2D rats. Furthermore, autophagy was significantly decreased, whereas mitochondrial biogenesis remained unaltered in the brain cortex of T2D rats. These observations suggest the occurrence of compensatory mechanisms that may help prevent alterations in mitochondrial function. Insulin is critical for the survival of T1D individuals and has proven to have beneficial effects in the brain. Therefore, to evaluate the effects of T1D and insulin treatment in mitochondrial function, fission/fusion and biogenesis, autophagy and tau protein phosphorylation we used streptozotocin (STZ)‐ induced T1D rats (3 months of diabetes duration) and STZ rats treated with a daily injection of insulin during the last month of the experimental protocol (Chapter 5). Vehicle‐treated Wistar rats were used as control animals. STZ-induced T1D and insulin treatment did not affect mitochondrial function. No significant changes were observed in brain cortical levels of glucose and pyruvate in T1D rats. However, insulin treatment increased significantly brain cortical glucose levels, despite no significant alterations in pyruvate levels occurred in the brains of these animals. An increase in mitochondrial biogenesis as well as a shift towards increased mitochondrial fission were observed in the brain cortex of T1D animals. T1D animals also presented, an increased phosphorylation of the tau protein at Ser396 residue, this effect being partially reversed by insulin treatment. This effect of insulin was associated with a modest decrease in the active form of glycogen synthase kinase 3β (GSK3β) and significant increase in protein phosphatase 2A (PP2A) activity. Insulin was also able to slightly decrease LC3‐II levels, a marker of autophagy. No significant alterations were found in apoptotic cell death and synaptic integrity. Overall the results presented in this thesis suggest the existence of mitochondrial readjustments in brain cortex of diabetic rats in order to preserve mitochondrial function and, consequently, the integrity and functionality of brain cells. In general, insulin therapy was shown to have positive effects against T1D-induced brain cortical alterations reinforcing the idea that insulin is an effective therapeutic agent against diabetes‐associated complications. Further research must be done to evaluate the behavior of the brain mitochondrial network in later stages of diabetes since it has been described that mitochondrial dysfunction is intimately associated with diabetes‐associated neurodegenerative conditions.