**2. Improving mitochondrial health as a valid therapy for AD**

axonal transport induced by Aβ were prevented in CycD‐depleted neurons obtained from CycD knockout mice (Ppif‐/‐). In addition, restoration of mitochondrial dynamics was repli‐

Overall, defects of mitochondrial transport through axons include the reduced anterograde or/and retrograde movement, increased stationary mitochondria and reduced mitochondrial density in synaptic terminals [45]. These alterations affect neuronal function including

Mitochondrial defects in dynamics, bioenergetics and transport are tightly related (**Figure 1**). Morphology alterations impair mitochondrial bioenergetics, and this deficiency generates fragmented and dysfunctional mitochondria. Also, defects in both transport and dysfunctional mitochondria could affect the energy and bioavailability of fresh mitochondria in demand zones such as nervous terminals. Together with an increased oxidative stress and reduced mitophagy may affect synaptic communication. Altogether, these alterations in mitochondrial health suggest the possibility that modulating mitochondrial function could be a key strategy

**Figure 1. Mitochondrial function defects in AD**. (**A**). **Dynamics/morphology**. The regulation of mitochondrial dy‐ namics, such as fusion, fission, biogenesis and mitophagy, represents an important mechanism that control neuronal fate. Mitochondrial morphological alterations are present in all levels in AD, and the consequence is the accumulation of fragmented and dysfunctional mitochondria in all the cell body (**B**). **Transport**. Kinesin and dynein proteins mediate axonal transport of mitochondria. Generally, this movement is bidirectional in an anterograde (kinesin) and retrograde direction (dynein). In several models of AD, a deregulated mitochondrial movement together with an increase of im‐ mobile mitochondria population associated with syntaphilin has been reported. This alteration generates a decrease in the total mitochondria movement and their distribution to the synaptic space (**C**) **Bioenergetics**. Neuronal models of AD present a severe mitochondrial dysfunction with an increase in oxidative stress. This alteration leads to a bioener‐ getic imbalance that affects ATP levels in the presynaptic neuron, with an increase in calcium overload and a conse‐

cated using the CycD inhibitor cyclosporin A in the same neuronal model [57].

autophagy, vesicle transport and energy supply leading to synaptic failure [45].

to prevent or retard the progression of AD (**Figure 1**).

354 Update on Dementia

quent synaptic dysfunction.

AD is one of the most common forms of dementia in elderly and one of the biggest health problems worldwide [59, 60]. This disease represents a high monetary, personal and family cost, and despite the large number of investigations and tremendous progress that has been made in understanding the molecular mechanism underlying the disease progression, currently there are no available therapies to cure AD. Nowadays, existing treatments for AD are only symptomatic [60]. The current therapies are palliatives that focus on reducing symptoms, but they do not delay the progression of the disease [60].

Currently, the most used drugs to treat AD are the inhibitors of the enzyme acetylcholines‐ terase [61, 62], as donepezil [62–66], which acts by increasing the availability of acetylcholine in the synaptic space of cholinergic neurons [62, 67, 68]. Another drug used is memantine, which is a pharmacological antagonist of glutamatergic receptor N‐methyl‐D‐aspartate (NMDA) [62, 69, 70]. Both drugs protect neurons against glutamate excitotoxicity, which is considered a major player in the neuronal damage observed in AD progression [70]. However, the approval of these drugs has not been based on their ability to slow down the disease progression but to improve the clinical symptomatology [62]. Therefore, only symptomatic drugs with transient benefits have been approved for clinical use in AD patients by the US Food and Drug Administration (FDA) [62].

Today, multiple therapies for AD are being studied [62, 70, 71]. The progress in the knowledge of the molecular characteristics of the disease and the availability of several animal models for study, it has open the boundaries to test and develop new therapies [61, 62, 72], for example, strategies for modifying AD progression include reducing neuroinflammation, metabolic approaches such as lipid‐lowering agents, estrogen, antioxidants, anti‐Aβ immunotherapy and recent neurotrophin‐based approaches [62, 69, 72, 73]. In this scenario, and given the importance and the temporality of mitochondrial damage in AD, we believe that mitochon‐ drial‐targeted therapeutic strategies are one of the most promising areas of interest.

Mitochondria‐targeted protective compounds that prevent or minimize mitochondrial dysfunction represent a potential target in the prevention and treatment of the pathogenesis of ageing‐related diseases [4, 10, 74–79]. Recently, it have been reported several progresses in the use of mitochondrial therapies against several neurodegenerative diseases [44]. These strategies include preventing mitochondrial fragmentation, reducing ROS levels and increas‐ ing ATP production in the brain [4, 10, 36, 79].

#### **2.1. Reducing defects of mitochondrial dynamics as a therapeutic target against AD**

As mentioned earlier, mitochondrial dynamics is an essential mitochondrial process for the maintenance of cell viability [20, 59, 79], and apparently, it is involved in the development of many neurodegenerative diseases [44, 80]. Mitochondrial dynamics defects may result in an impaired bioenergetics and reduced mitochondrial localization in the synaptic area [20, 78, 80]. In AD, extensive researches based on the analysis of post‐mortem brains, cell and animal models have reported several defects in mitochondrial dynamics [44, 81]. Therefore, increasing mitophagy and mitochondrial biogenesis may represent a promising therapeutic strategy in the treatment and prevention of common neurodegenerative diseases [82].

Preventing defects in mitochondrial dynamics reduce neuronal injury in neurodegenerative diseases [83]. For example, in Parkinson's disease (PD) the use of different compounds that regulate mitochondrial dynamics as Mdivi‐1 (mitochondrial division inhibitor‐1), an inhibitor of Drp1 activity, restored dopamine release, reduced mitochondrial fragmentation and prevented cell death in dopaminergic neurons [78]. In C57BL/6 mice hippocampal neurons incubated with Aβ25–35, the use of the antioxidant peptide SS31 decreased the levels of both mitochondrial fissions proteins, Drp1 and Fis1, and managed to increase the number of healthy and intact mitochondria [44, 78]. Mitochondria plays several key roles in synaptic communi‐ cation [81, 84], and to exert their synaptic roles, mitochondria must be actively transported from the soma to distal synapses zones through cytoskeleton [80, 85–87]. Interestingly, the treatment with SS31 peptide was able to reverse both the trafficking deficit and the occurrence of excess mitochondrial fission [88], restoring mitochondrial transport defects and increasing mitophagy of defective mitochondria in dopaminergic neurons [78].

Stimulation of mitophagy can also equilibrate the dysfunctional mitochondria in AD; in fact, the use of candidate drugs that increase mitophagy appears to be a promising target against many neurodegenerative diseases [89]. PINK1 is a key molecule in the signal transduction of mitophagy [90], and drugs enhancing the activity of this pathway increase the elimination of depolarizing mitochondria, which seems to be an interesting alternative for mitochondrial therapy [89, 91]. Also, the use of autophagy inducers such as rapamycin presents another tool to increase the mitophagy [90, 91]. For example, treatment with rapamycin prevented from mitochondrial fragmentation and bioenergetics defects in a rat model of PD [92].

Mitochondrial biogenesis seems to be an interesting alternative to reduce or prevent mito‐ chondrial dynamics defects in AD. Peroxisome proliferators‐activated receptors gamma (PPARγ) are nuclear receptors that, together with PGC1‐alpha, participate in lipid metabolism, and they are key players in the control of energy metabolism and mitochondrial biogenesis [93, 94]. PPARγ are significantly reduced in AD as the severity of the disease increases. [93, 95] and, interestingly, improvement of neuronal mitochondrial biogenesis through PPARγ activation has been suggested to be a potential therapeutic target to reduce mitochondrial dysfunction in AD [94]. In fact, activation of those receptors using antidiabetic drugs called thiazolidinediones (TZDs) reduced mitochondrial dysfunction, decreased oxidative stress and improved memory impairment in AD mice models and patients with mild to moderate AD [22, 96, 97].

#### **2.2. Improving mitochondrial bioenergetics in AD**

Neurodegeneration and synaptic damage in AD are primarily mediated by defective mito‐ chondrial function [31, 57, 59, 98]. This mitochondrial alteration, together with the progressive accumulation of Aβ and pathological tau, affects mitochondrial membrane potential, respira‐ tion and energy metabolism and calcium homeostasis; promotes mPTP opening; and increase oxidative stress [57, 99]. Because the bioenergetics functions are closely related to each other, overall treatments of mitochondrial‐targeted compounds will generate a general improvement in several aspects of this organelle performance [79].

mitophagy and mitochondrial biogenesis may represent a promising therapeutic strategy in

Preventing defects in mitochondrial dynamics reduce neuronal injury in neurodegenerative diseases [83]. For example, in Parkinson's disease (PD) the use of different compounds that regulate mitochondrial dynamics as Mdivi‐1 (mitochondrial division inhibitor‐1), an inhibitor of Drp1 activity, restored dopamine release, reduced mitochondrial fragmentation and prevented cell death in dopaminergic neurons [78]. In C57BL/6 mice hippocampal neurons incubated with Aβ25–35, the use of the antioxidant peptide SS31 decreased the levels of both mitochondrial fissions proteins, Drp1 and Fis1, and managed to increase the number of healthy and intact mitochondria [44, 78]. Mitochondria plays several key roles in synaptic communi‐ cation [81, 84], and to exert their synaptic roles, mitochondria must be actively transported from the soma to distal synapses zones through cytoskeleton [80, 85–87]. Interestingly, the treatment with SS31 peptide was able to reverse both the trafficking deficit and the occurrence of excess mitochondrial fission [88], restoring mitochondrial transport defects and increasing

Stimulation of mitophagy can also equilibrate the dysfunctional mitochondria in AD; in fact, the use of candidate drugs that increase mitophagy appears to be a promising target against many neurodegenerative diseases [89]. PINK1 is a key molecule in the signal transduction of mitophagy [90], and drugs enhancing the activity of this pathway increase the elimination of depolarizing mitochondria, which seems to be an interesting alternative for mitochondrial therapy [89, 91]. Also, the use of autophagy inducers such as rapamycin presents another tool to increase the mitophagy [90, 91]. For example, treatment with rapamycin prevented from

Mitochondrial biogenesis seems to be an interesting alternative to reduce or prevent mito‐ chondrial dynamics defects in AD. Peroxisome proliferators‐activated receptors gamma (PPARγ) are nuclear receptors that, together with PGC1‐alpha, participate in lipid metabolism, and they are key players in the control of energy metabolism and mitochondrial biogenesis [93, 94]. PPARγ are significantly reduced in AD as the severity of the disease increases. [93, 95] and, interestingly, improvement of neuronal mitochondrial biogenesis through PPARγ activation has been suggested to be a potential therapeutic target to reduce mitochondrial dysfunction in AD [94]. In fact, activation of those receptors using antidiabetic drugs called thiazolidinediones (TZDs) reduced mitochondrial dysfunction, decreased oxidative stress and improved memory impairment in AD mice models and patients with mild to moderate AD

Neurodegeneration and synaptic damage in AD are primarily mediated by defective mito‐ chondrial function [31, 57, 59, 98]. This mitochondrial alteration, together with the progressive accumulation of Aβ and pathological tau, affects mitochondrial membrane potential, respira‐ tion and energy metabolism and calcium homeostasis; promotes mPTP opening; and increase oxidative stress [57, 99]. Because the bioenergetics functions are closely related to each other,

mitochondrial fragmentation and bioenergetics defects in a rat model of PD [92].

the treatment and prevention of common neurodegenerative diseases [82].

mitophagy of defective mitochondria in dopaminergic neurons [78].

[22, 96, 97].

356 Update on Dementia

**2.2. Improving mitochondrial bioenergetics in AD**

Several groups have reported that enhanced antioxidant capacity lowers the risk of develop‐ ment and progression of neurodegenerative diseases [60, 100–102]. At the same time, other studies have explored the use of mitochondrial antioxidants in order to reduce neurodegen‐ eration in AD [4, 10, 103, 104]. Mitochondrial‐targeted antioxidants have been developed in this regard and they are currently undergoing preclinical testing [106]. For example, treatment with CoQ10 decreased oxidative stress, Aβ42 levels and β‐amyloid burden, and improved cognitive impairment in AD transgenic mice [4, 10, 105]. CoQ10 is an essential biologic factor of the ETC, where it accepts electrons from complexes I and II, and also serves as an important antioxidant molecule in mitochondrial lipid membranes [10, 103].

Another example of mitochondrial targeted antioxidant is the MitoQ drug, a lipophilic cation compound with strong antioxidant actions that has been successfully targeted to mitochon‐ dria, where it reduce ROS levels, leading to the protection of neurons in AD [78, 106]. MitoQ and MitoE, both are mito‐targeted compounds and they accumulate in the mitochondria, enhancing ETC function and preventing oxidation of an important lipidic component of the mitochondrial membrane called cardiolipin [78, 107].

Also, in experiments with AD mice models and neuronal cultures treated with MitoQ, it was shown that mitochondria maintain their integrity and function, decrease CycD expression and prevent mitochondrial depolarization, with an additional prevention of the caspases activation [105]. In addition, in N2a cells treated with Aβ, MitoQ decreased abnormal expression of mitochondrial structural genes and reduced mitochondrial population [106]. Other studies showed that in primary cortical neurons treated with Aβ and in the 3xTg‐AD mice, MitoQ showed prevention of Aβ‐induced oxidative stress, reduced Aβ accumulation, improved synaptic loss and caspase activation in the brain [105]. Additionally, in a PD pharmacological model, treatment with MitoQ inhibited the activation of mitochondrial apoptotic pathway, decreasing the levels of Bax and Drp1 protein, which suggests a possible role in the control of mitochondrial dynamics [78, 108].

Another bioenergetics feature that is significantly affected in AD mitochondria is the calcium homeostasis and the opening of mPTP [99, 109, 110]. Research has demonstrated that mito‐ chondria isolated from the hippocampus of AD patients showed elevated levels of CypD [109, 110]. CypD is a necessary component of mPTP formation, triggering the opening of mPTP by translocation of CypD to the inner membrane [57]. Studies of the genetic deletion of CypD showed a decrease in the probability of mPTP opening and a great increase in mitochondrial capacity to buffer calcium [57, 87, 110–112].

Evidence indicates that the use of CypD inhibitors may improve mitochondrial function, and even if these inhibitors can cross the blood‐brain barrier, it can have considerable potential as prevention and treatment drugs against AD [57]. Additionally, it has been shown that the treatment with CsA could have mitochondrial protective effects in neurons [99, 113]. That is, because treatment with this drug enhances mitochondrial transmembrane potential, the releasing of cytochrome c outside the mitochondria is prevented and superoxide dismutase activity is increased [113], suggesting an important role of the mPTP in mitochondrial injury in AD [113, 114].

**Figure 2. Improving mitochondrial health as a valid therapy for AD**. (**A**) **Mitochondrial morphology therapy**. Ma‐ nipulating the processes of mitochondrial dynamics has a considerable potential for treating neurodegenerative diseas‐ es. Therapies that increase mitochondrial biogenesis and fission/fusion cycle may improve mitochondrial function and decreased oxidative stress. Mitophagy is a selective autophagy process that removes dysfunctional mitochondria and maintains adequate mitochondria quality control. Increasing PINK1‐mediated mitophagy improves mitochondrial in‐ tegrity and function. (**B**) **Mitochondrial bioenergetics therapy**. Several agents that boost bioenergetics could have effi‐ cacy in improving mitochondrial function. These compounds show neuroprotective effects, which may be a useful target for treating neurodegenerative diseases. Treatment with CoQ10, MitoQ and MitoE prevented oxidative stress; cyclosporine A, a substance that blocks the opening of mPTP, prevented mitochondrial depolarization, blocks cyto‐ chrome c release and increased superoxide dismutase activity. Drugs that mediate the activation of Nrf2 induce the expression of antioxidant enzymes and improve mitochondrial function and biogenesis.

In that context, several groups have found that some compounds not only improve one aspect of mitochondrial damage but also improve several alterations at once by the activation of several pathways like nuclear factor E2‐related factor 2 (Nrf2) [10, 101, 102]. The Nfr2 and the Nrf2‐Are pathways have been studied in mitochondrial dysfunction and neurodegeneration [10, 115]. In response to oxidative stress, the Nrf2 translocate from the cytoplasm into the nucleus and activates the expression of several antioxidant genes [116]. Nrf2 is the principal regulator of the antioxidant cellular response and seems to be a promising target in the treatment of age‐related neurodegenerative diseases [10, 101, 102, 117]. Nrf2 activation induces changes in mitochondrial structure and function, which is of particular importance under conditions of oxidative stress [10, 102, 118]. In primary murine cortical cultures, neurons lacking Nrf2 are more susceptible to oxidative stress induced by H2O2 and glutamate [116, 119] and overexpression of Nrf2, totally prevented these changes [116]. Furthermore, overex‐ pression of Nrf2 can rescue neurons from mitochondrial complex II inhibition and ischemic insult in animal models of Huntington disease and stroke [116, 120, 121].

Interestingly, it has been suggested that Nrf2 may play a role in the pathogenesis of AD [102, 116]. Positive outcomes of Nrf2 activation include decreasing oxidative stress, reducing inflammation and increasing autophagy [115, 122]. Studies from human AD brains showed a decrease in Nrf2 levels in the cytoplasm of hippocampal neurons [115, 116]. In addition, studies in neuronal cultures derived from Nrf2 knockout mice show increased susceptibility to oxidative damage, as well as damage produced by mitochondrial electron transport gene complex inhibitors such as MPP+ and rotenone [10, 102]. Interestingly, small food‐derived molecule such as sulforaphane (SFN) is a nutritional and natural activator of Nrf2 and presented neuroprotective effects and attenuated oxidative damage induced by Aβ 25–35 [102].

Overall, improving mitochondrial defects using the strategies mentioned above could have a potential impact reducing neurodegeneration in AD (**Figure 2**).
