**3.1 Parkinson's disease**

*Neuroprotection - New Approaches and Prospects*

*Morphology and location of mitochondria change over time.*

reduces retrograde movement [42].

milton [44, 45].

**Figure 1.**

motors drive transport in two directions. Movement away from the soma or anterograde movement is conducted by the ATPase family of kinesins, with kinesin-1 being responsible for mitochondrial transport, specifically in neurons [38, 40]. Kinesin-1 consists of heavy chains (KHC) and light chains, with the heavy chains being the driving force that allows kinesin-1 to function as a motor protein [41]. Retrograde movement or movement toward the soma is driven by dynein. However, it is likely that these movements are coordinated rather than competitive toward each other, as it has been demonstrated that inhibiting kinesin-1 in *Drosophila*

*Mitochondrial movement in primary hippocampal neurons. Primary hippocampal neurons were labeled with mitoRFP, a red fluorescent tag that labels mitochondria. Micrographs were taken at 3 and 4 weeks after seeding.* 

Mitochondrial Rho-GTPase, or Miro, is an outer membrane receptor, Ca2+ sensor, and another pertinent regulator of mitochondrial motility due to its ability to anchor kinesin and dynein to the mitochondrial outer membrane [43]. Miro's anchoring role has been extensively studied in anterograde movement in the motor/ adaptor complex formed between KHC and Miro, connected by the protein adaptor

Another important component of mitochondria trafficking is stationary docking. Mitochondrial docking is largely mediated by the axonal outer membrane protein syntaphilin (SNPH) and its interaction with microtubules in the cytoskeleton. This is demonstrated in rodent models in which deletion of the SNPH gene results in an increase in mitochondria motility and reduced density, while overexpression of endogenous or exogenous SNPH abolished mobility [46]. Along with decreasing the percentage of immobile mitochondria, loss of SNPH decreases axonal branching in cortical neurons [47]. This effect is comparable to neurons lacking LKB-1 and NUAK1, which is necessary for axonal specification [48]. The removal of either of these kinases leads to a decrease in the number of stationary mitochondria along with decreased branching. However, overexpression of SNPH in the LKB-1 and NUAK1-null neurons rescued these effects. Collectively, this implies that docking of mitochondria is required for axonal branching and growth [47]. Since Bcl-xL is required for neurite outgrowth [49], it is possible that Bcl-xL exerts this effect by interacting with docking proteins such as syntaphilin. However, the exact role of

Bcl-xL in docking mechanisms must be further elucidated.

**3. Alteration of mitochondrial function in brain-associated diseases**

While various brain-associated diseases have different pathophysiologies, there is an underlying similarity that consistently occurs: mitochondrial dysfunction. Throughout these conditions, neurodegeneration is correlated with an energy deficit caused by inefficient operation of the ETC, activation of mitochondria-dependent apoptosis, and accumulation of ROS. In addition, excitotoxicity, which commonly

**288**

Parkinson's disease (PD) is a common neurodegenerative disease that has detrimental clinical effects including tremors, impaired gait, and stiffness of limbs [57]. These symptoms are often due to PD's hallmark characterization of degeneration of the dopaminergic neurons in the substantia nigra. Individuals with PD are vulnerable to increased ROS production due to reduced complex 1 activity, increased lipid oxidation, and altered antioxidant systems [58].

As several PD-specific proteins impact mitochondrial dynamics, it is possible that the neurodegeneration that occurs with PD is linked to alterations in fission and fusion [59, 60]. Dopaminergic neurons depleted of Drp1 demonstrated decreased mitochondrial mass, impaired motility, and overall neuron loss. Neurons depleted with Drp1 had less mitochondria in the soma and were almost completely depleted from the axons; by not having mitochondria at axons, this can lead to the neurodegeneration due to energy deficits, as synaptic transmission requires a high demand of ATP [61].

The PINK1/Parkin pathway has been traditionally studied with its roles in mitophagy. Under normal physiological conditions, PINK1 accumulates on the surface of dysfunctional mitochondria to signal Parkin translocation to initiate ubiquitination [62]. However, mutations in PINK1 and Parkin, which have been linked to early onset familial forms of PD, lead to loss of mitochondrial membrane potential, leading to impairment of Parkin's translocation and thus accumulation of dysfunctional mitochondria [63]. Research in recent years has begun to uncover the role of the PINK1/Parkin pathway in mitochondrial transport. Overexpression of PINK1 phosphorylates Miro, targeting it for ubiquitination and subsequent degradation. This results in the dismantling of the motor/adaptor complex, releasing kinesin and milton from the mitochondrial surface, and leads to halting of mitochondrial motility [64]. It is possible that this system may promote neuroprotection by preventing anterograde transport of mitochondria and allowing PINK1 to accumulate on damaged mitochondria to initiate mitophagy [65]. Furthermore, PINK1 may exert neuroprotection due to its interaction with Bcl-xL [66]. It has been shown that PINK1 phosphorylates Bcl-xL at its Ser62; as a result, this prevents N-terminal cleavage of Bcl-xL or formation of ΔN-Bcl-xL, which has been associated with neuronal death [67–69]. However, if altered PINK1 expression occurs as a result of genetic mutation, this may lead to dysregulated mitochondrial transport and promotion of apoptosis.

The presynaptic protein α-synuclein is a major constituent of Lewy bodies, with mutations in its encoding gene, SNCA, being linked to familial PD [70]. Some amount of α-synuclein can localize to the mitochondria, inducing mitochondrial fragmentation, dysfunction, and downregulation of complex 1 activity, potentially contributing to ROS production [71]. Overexpression of α-synuclein results in cytotoxicity due to decreased Bcl-xL expression and increased Bax expression [72]. Shaltouki's research group recently investigated the role of α-synuclein on

mitochondrial dynamics by using multiple PD models. In the postmortem brains of humans with PD, it was observed that protein levels of α-synuclein and Miro were highly upregulated compared to the control brains, while KHC, VDAC, and Mfn2 remained unchanged. Additionally, the β-subunit of ATP synthase was upregulated in the PD brains. When this was further explored in human cell lines and in *Drosophila* bearing SNCA mutations, neurodegeneration and locomotion defects occurred as a result of the upregulated α-synuclein and subsequent upregulation of Miro. These effects were rescued with a partial reduction of Miro. Interestingly, upregulation of Miro led to delayed mitophagy implying that α-synuclein's impact on Miro is probably independent of the PINK1/Parkin pathways [73].

Recently, it was shown that the β-subunit of ATP synthase binds to DJ-1 suggesting that DJ-1 plays a role in increasing ATP efficiency [74]. Mutations in DJ-1 also demonstrate inefficient ATP production, alterations in mitochondrial morphology, and enhanced membrane permeability [74–76]. Although its functions are not completely understood, DJ-1 has been noted to prevent the aggregation of α-synuclein [77]. As Lewy bodies in PD are primarily a result of α-synuclein aggregation, inhibiting this aggregation may consequently delay Lewy body formation. Thus, therapies targeting DJ-1 may serve as a multi-faceted mechanism for PD treatment.

### **3.2 Alzheimer's disease**

Alzheimer's disease (AD) is the most common neurodegenerative disease and the leading cause of dementia worldwide [78]. Two hallmark characteristics of AD are the presence of amyloid-beta peptide (Aβ) plaques and tau protein tangles. The formation of Aβ occurs as a consequence of cleavage of amyloid precursor protein (APP), where Aβ peptides can then aggregate into oligomers or fibrils [79]. In the brain, the typical role of tau proteins is to stabilize microtubules; however, in AD, it is suspected that hyperphosphorylation of tau leads to the formation of neurofibril tangles [80]. Both Aβ and tau tangles can cause impairments or interruptions in pathways essential for neuronal survival.

Reduced transport of axonal mitochondria has been documented in subjects with AD, but there may be multiple mechanisms to cause this disruption [81]. The presence of Aβ reduces bidirectional axonal mitochondrial motility but has a more significant impact on anterograde movement versus retrograde [82, 83]. This may be because Aβ activates GSK-3β, which is a negative regulator of kinesin-1. Phosphorylation of kinesin-1 by GSK-3β can lead to a reduction in mitochondria density [84]. Furthermore, mutations in presenilin-1 (PS1), which is linked to familial AD, promote GSK-3βmediated kinesin-1 phosphorylation and reductions of anterograde mitochondria transport [84]. Overexpression of tau also has the ability to redirect mitochondrial transport; kinesin-1 encounters tau and is detracted from microtubule tracks, slowing down anterograde movement and increasing the favorability of dynein-mediated retrograde movement [85, 86]. Abnormal fission mechanics have also been observed in AD as a result of alteration in Drp1 function. Neurotoxicity via tau interrupts fission, causing elongated mitochondria and mislocalization of Drp1. This occurs because hyperphosphorylation of tau causes disruptions in actin, preventing actin-based translocation of Drp1 to the mitochondria [87].

#### **3.3 Stroke**

Cerebral ischemia, or more commonly stroke, is characterized by the decrease or cessation of blood flow to the brain. Consequently, the loss of oxygen and nutrients to neurons causes ATP deficits, apoptosis, and Ca2+ influx. It is not surprising then that mitochondrial dynamics are influenced after ischemic events. This is

**291**

*Lifestyle Factors, Mitochondrial Dynamics, and Neuroprotection*

demonstrated by Zou's research group as they sought to elucidate how mechanisms of fission and mitophagy are impacted after ischemia [88]. Using a model of middle cerebral artery occlusion (MCAO), Drp1 initially increased but then decreased, implying that ischemia induced fission, but the process was disrupted due to abnormalities in translocation of Drp1 caused by MCAO. Mitophagy is also selectively induced by mild ischemia in a Drp1-dependent manner; this is evident by increased expressions of LC3B and Beclin-1 and decreased p62. Moreover, inhibition of Drp1 led to early onset of apoptotic pathways [88]. This may be supported by transient ischemia models, in which p-Drp1 is upregulated [89, 90]. Interestingly, evidence shows that p-Drp1 at Ser616 may be regulated by PINK1, establishing a link between fission and mitophagy [90]. Like fission, mechanisms of fusion are also impacted by ischemic insult. Mfn2 expression is decreased after MCAO and leads to apoptosis, but when overexpressed, Mfn2 shows an anti-apoptotic effect by modulating Bcl-2 and Bax [91]; these results are conclusive with a similar study, showing that Mfn2 expression is decreased after excitotoxic insult with a subsequent increase in

Excitotoxicity via overactivation of glutamate receptors, namely, N-methyl-Daspartate (NMDA) receptors (NMDARs), is a key player of neuronal death after cerebral ischemia [93]. Thus, uncovering mechanisms effecting NMDARs is an attractive idea for therapeutic agents. While mechanisms of mitochondrial motility are less investigated in models of cerebral ischemia, a recent study has elucidated a novel role of kinesin-1 transport. The heavy chain of kinesin-1 has been shown to bind directly to NMDARs, mediating their transport. By either disassociating this bond or suppressing kinesin-1 expression, this can improve Ca2+ influx and NMDA

Reducing excessive fission that occurs post-TBI is a potential target of neuroprotection to prevent neuronal impairments and death. In rodent models, TBI causes an increase in translocation of Drp1 to the mitochondria, increasing rates of fission. Consequently, this led to neuronal apoptosis, decreased neurogenesis, impaired cognition, and memory defects. When administered with Mdivi-1, a pharmacological inhibitor of Drp1, these negative effects were attenuated, confirming the role of increased Drp1 activity [95, 96]. Interestingly, Pietro's research group suggest that many molecular responses after severe TBI are opposite from that of mild TBI. Rodents with severe TBI presented with activation of fission as shown by overexpression of Drp1 and FIS1, a protein that binds to Drp1 for anchoring to the mitochondrial outer membrane. Furthermore, expressions of Mfn1 and Mfn2 were downregulated, and there were no changes in Opa1 gene and protein expressions, demonstrating an inhibition of fusion as a result of severe TBI. Additionally, the increase of dysfunctional mitochondria led to a subsequent overexpression of PINK1 and PARK2, triggering mitophagy. Conversely, mild TBI demonstrated activation of fusion with inhibition of fission; together, this did not change PINK1

or PARK2 gene expressions, thus showing no difference in mitophagy [97].

**4. The effect of lifestyle factors on mitochondria function and dynamics**

Traditionally observed at a large, systemic level, research in recent years has begun to investigate the effect of exercise on mitochondrial function and dynamic

*DOI: http://dx.doi.org/10.5772/intechopen.89416*

Bax translocation to the mitochondria [92].

excitotoxicity resulting from ischemia [94].

**3.4 Traumatic brain injury (TBI)**

**4.1 Exercise and mitochondria**

#### *Lifestyle Factors, Mitochondrial Dynamics, and Neuroprotection DOI: http://dx.doi.org/10.5772/intechopen.89416*

*Neuroprotection - New Approaches and Prospects*

**3.2 Alzheimer's disease**

pathways essential for neuronal survival.

translocation of Drp1 to the mitochondria [87].

mitochondrial dynamics by using multiple PD models. In the postmortem brains of humans with PD, it was observed that protein levels of α-synuclein and Miro were highly upregulated compared to the control brains, while KHC, VDAC, and Mfn2 remained unchanged. Additionally, the β-subunit of ATP synthase was upregulated in the PD brains. When this was further explored in human cell lines and in *Drosophila* bearing SNCA mutations, neurodegeneration and locomotion defects occurred as a result of the upregulated α-synuclein and subsequent upregulation of Miro. These effects were rescued with a partial reduction of Miro. Interestingly, upregulation of Miro led to delayed mitophagy implying that α-synuclein's impact

Recently, it was shown that the β-subunit of ATP synthase binds to DJ-1 suggesting that DJ-1 plays a role in increasing ATP efficiency [74]. Mutations in DJ-1 also demonstrate inefficient ATP production, alterations in mitochondrial morphology, and enhanced membrane permeability [74–76]. Although its functions are not completely understood, DJ-1 has been noted to prevent the aggregation of α-synuclein [77]. As Lewy bodies in PD are primarily a result of α-synuclein aggregation, inhibiting this aggregation may consequently delay Lewy body formation. Thus, therapies targeting DJ-1 may serve as a multi-faceted mechanism for PD treatment.

Alzheimer's disease (AD) is the most common neurodegenerative disease and the leading cause of dementia worldwide [78]. Two hallmark characteristics of AD are the presence of amyloid-beta peptide (Aβ) plaques and tau protein tangles. The formation of Aβ occurs as a consequence of cleavage of amyloid precursor protein (APP), where Aβ peptides can then aggregate into oligomers or fibrils [79]. In the brain, the typical role of tau proteins is to stabilize microtubules; however, in AD, it is suspected that hyperphosphorylation of tau leads to the formation of neurofibril tangles [80]. Both Aβ and tau tangles can cause impairments or interruptions in

Reduced transport of axonal mitochondria has been documented in subjects with AD, but there may be multiple mechanisms to cause this disruption [81]. The presence of Aβ reduces bidirectional axonal mitochondrial motility but has a more significant impact on anterograde movement versus retrograde [82, 83]. This may be because Aβ activates GSK-3β, which is a negative regulator of kinesin-1. Phosphorylation of kinesin-1 by GSK-3β can lead to a reduction in mitochondria density [84]. Furthermore, mutations in presenilin-1 (PS1), which is linked to familial AD, promote GSK-3βmediated kinesin-1 phosphorylation and reductions of anterograde mitochondria transport [84]. Overexpression of tau also has the ability to redirect mitochondrial transport; kinesin-1 encounters tau and is detracted from microtubule tracks, slowing down anterograde movement and increasing the favorability of dynein-mediated retrograde movement [85, 86]. Abnormal fission mechanics have also been observed in AD as a result of alteration in Drp1 function. Neurotoxicity via tau interrupts fission, causing elongated mitochondria and mislocalization of Drp1. This occurs because hyperphosphorylation of tau causes disruptions in actin, preventing actin-based

Cerebral ischemia, or more commonly stroke, is characterized by the decrease or cessation of blood flow to the brain. Consequently, the loss of oxygen and nutrients to neurons causes ATP deficits, apoptosis, and Ca2+ influx. It is not surprising then that mitochondrial dynamics are influenced after ischemic events. This is

on Miro is probably independent of the PINK1/Parkin pathways [73].

**290**

**3.3 Stroke**

demonstrated by Zou's research group as they sought to elucidate how mechanisms of fission and mitophagy are impacted after ischemia [88]. Using a model of middle cerebral artery occlusion (MCAO), Drp1 initially increased but then decreased, implying that ischemia induced fission, but the process was disrupted due to abnormalities in translocation of Drp1 caused by MCAO. Mitophagy is also selectively induced by mild ischemia in a Drp1-dependent manner; this is evident by increased expressions of LC3B and Beclin-1 and decreased p62. Moreover, inhibition of Drp1 led to early onset of apoptotic pathways [88]. This may be supported by transient ischemia models, in which p-Drp1 is upregulated [89, 90]. Interestingly, evidence shows that p-Drp1 at Ser616 may be regulated by PINK1, establishing a link between fission and mitophagy [90]. Like fission, mechanisms of fusion are also impacted by ischemic insult. Mfn2 expression is decreased after MCAO and leads to apoptosis, but when overexpressed, Mfn2 shows an anti-apoptotic effect by modulating Bcl-2 and Bax [91]; these results are conclusive with a similar study, showing that Mfn2 expression is decreased after excitotoxic insult with a subsequent increase in Bax translocation to the mitochondria [92].

Excitotoxicity via overactivation of glutamate receptors, namely, N-methyl-Daspartate (NMDA) receptors (NMDARs), is a key player of neuronal death after cerebral ischemia [93]. Thus, uncovering mechanisms effecting NMDARs is an attractive idea for therapeutic agents. While mechanisms of mitochondrial motility are less investigated in models of cerebral ischemia, a recent study has elucidated a novel role of kinesin-1 transport. The heavy chain of kinesin-1 has been shown to bind directly to NMDARs, mediating their transport. By either disassociating this bond or suppressing kinesin-1 expression, this can improve Ca2+ influx and NMDA excitotoxicity resulting from ischemia [94].
