Mitochondrial involvement in Alzheimer’s disease (AD) and the effect of creatine supplementation. (1) Mitochondrial ROS-induced damage. AD is associated with a greater level of mitochondrially mediated oxidative damage to intracellular proteins and lipids, and this elevated oxidative stress can induce damage to mitochondrial DNA. (2) ROS-induced inactivation of mitochondrial and cytosolic creatine kinase. (2a) Elevated ROS can evoke an octameric-to-dimeric conformational shift in mitochondrial creatine kinase (MtCK-oct) leading to an inactive form of the enzyme (MtCK-dim). (2b) Elevated ROS can also cause damage to, and reduce the activity of the cytosolic creatine kinase (CK). The decreased activity of both the mitochondrial and cytosolic creatine kinases results in less generation of phosphocreatine. This leads to an excess accumulation of creatine and the formation of creatine deposits in the cytosol. (3) Cleavage of amyloid precursor peptide (APP) to Aβ inhibits chaperone-like activity to reduce mtCK in mitochondria. APP can bind to MtCK and is proposed to have chaperone-like activity that facilitates translocation of MtCK from the cytosol to mitochondria. In AD, APP is cleaved to form Aβ fragments that are incapable of this chaperone-like activity. (4) Exogenous creatine supplementation. Improvements in cellular bioenergetics in AD with exogenous creatine supplementation is dependent on (a) the extent of inactivation of the mitochondrial and cytosolic CK and, (b) the levels of reduced mitochondrial CK due to suppressed translocation to the mitochondria. Given that creatine deposition sites have found in late-stage AD, creatine supplementation would most likely be efficacious only during early stages of AD when mitochondrial and cytosolic CK might be suppressed but still active to be capable of generating and enhancing the PCr pool

Schematic illustrating the relationship of mitochondria to amyotrophic lateral sclerosis (ALS) and the potential benefit of creatine supplementation. The most common form of familial ALS (FALS) is caused by numerous different mutations in the metallo-proteinase, superoxide dismutase I (SODI). SODI is an antioxidant that is ubiquitously expressed in the cytosolic fraction of cells and catalyzes the dismutation of superoxide (a highly reactive free radical) to the less reactive hydrogen peroxide (H₂O₂) and water. (1) Elevations in ROS may occur due to mutations in SOD1, causing reduced SOD1 antioxidant activity. Although somewhat counterintuitive, data have shown that this does not play a major role in the pathogenesis associated with SOD1 mutations. (2) Alternatively, the effects of mutant SOD1 are due to toxic gain-of-function properties. While SOD1 is primarily located in the cytosolic portion of the cell, mutant SOD1 has been found in the intermembrane space and the matrix of mitochondria. Although the detailed molecular mechanisms are not completely understood, mitochondrial mutant SOD1 is associated with impaired ETC function, reduced mitochondrial membrane potential, elevated mitochondrial ROS production, mitochondrial swelling, and intra-mitochondrial vacuole formation. These alterations likely contribute to reduced ATP generation by mitochondria and creatine supplementation in ALS may serve to buffer these mutant SOD1-induced impairments in intracellular bioenergetics. Creatine supplementation was shown to be neuroprotective in animal models of ALS (G93A), but despite promise, was ineffective in clinical trials with ALS patients

Creatine kinase/phosphocreatine energy shuttle pathway and the role of mitochondria in apoptotic cell death in neurons. (1) Mitochondrial involvement in apoptosis. Mitochondria are intimately involved in apoptotic cell death since, (1) they contain pro-apoptotic proteins which can lead to cell death upon release into the cytosol (DNA fragmentation is a hallmark feature of apoptosis), and (2) they produce the majority of the potentially damaging reactive oxygen species (ROS). ROS can damage intracellular components (i.e., alter DNA, proteins, lipids) and have also been shown to increase the susceptibility to apoptosis by modulating components of the channel that traverses the inner and outer mitochondrial membrane termed the mitochondrial permeability transition pore (mtPTP). Upon a significant apoptotic insult, mitochondria can release pro-apoptotic factors to the cytosol to induce cellular death. The octameric form of the mitochondrial creatine kinase (MtCK) interacts with components of the mtPTP to suppress pore opening and potentially reduce apoptotic susceptibility. Creatine has been shown to maintain the mtCK in an octameric conformation (as opposed to dimeric) which inhibits mtPTP opening. (2) Creatine kinase/phosphocreatine energy shuttle pathway. Creatine (Cr) is taken up by the neuron via specific creatine transporters and transphosphorylated to the high-energy phosphocreatine (PCr) by either mitochondrial creatine kinases (MtCKs) or cytosolic creatine kinases (CKs). MtCK is located in the intermembrane space of mitochondria and is coupled to ATP production by mitochondrial oxidative phosphorylation to produce PCr which is then exported from the organelle. Cytosolic CK transphosphorylates ATP generated via glycolysis to PCr. This PCr can then contribute to the overall cellular PCr pool. The PCr pool is coupled to localized CKs in areas of high ATP demand such as (2a) organelle transport and/or ATP dependent cell signaling or (2b) at the cell membrane by ATP-dependent pumps and/or ATP-dependent receptors. This localized distribution of CKs allows for the efficient production of ATP from ADP to fuel ATP-dependent processes in various portions of the cell. (3) Effect of exogenous creatine supplementation (dashed lines). Exogenous creatine is transported through the cell membrane via creatine transporters to increase the overall creatine pool. (3a) Increased creatine levels may further stabilize the octameric form of mtCK to suppress opening of the mtPTP and this may reduce mitochondrially mediated apoptosis. (3b) Elevated levels of creatine within the cell can be transphosphorylated by mitochondrial and cytosolic creatine kinases (MtCKs and CKs, respectively) to enhance the overall PCr pool. Increased PCr levels can be utilized by localized creatine kinases to transphosphorylate ADP to ATP for various ATP-dependent processes within the cell (2a) and at the cell membrane (2b). (4) Potential neuroprotective effects of creatine. Creatine supplementation may improve both the bioenergetic and mitochondrial deficits associated with some neurological diseases (more efficacious in Parkinson’s and Huntington’s disease). These beneficial effects of creatine may ultimately reduce neuronal cell death and provide neuroprotection in certain neurodegenerative disorders

Mitochondrial involvement in Parkinson’s disease (PD) and the potential therapeutic efficacy of creatine supplementation. (1) PD is characterized by impaired activity of complex I in the ETC of mitochondria. In support of a mitochondrially based etiology in PD, administration of the mitochondrial-specific neurotoxin, MPTP, in animal models, inhibits complex I of the ETC and leads to PD-like pathogenesis. Suppression and/or inhibition of complex I of the ETC lowers overall mitochondrial ATP production and consequently diminishes the ATP (dashed lines) that can be potentially utilized for PCr generation by the MtCK. 2) Additionally, impaired ETC function increases ROS production to induce an octameric-to-dimeric shift of MtCK rendering it an inactive state and elevated ROS will also reduce cytosolic CK activity. (3) Data have shown that creatine supplementation is an efficacious treatment paradigm for PD animal models, but the exact molecular mechanisms are not fully elucidated. However, exogenous creatine appears to improve overall cellular bioenergetics and mitochondrial function by enhancing the PCr pool and this reduces the neuronal cell loss associated with PD pathogenesis. Given the promising results of creatine in animal models of PD, a phase III clinical study of creatine supplementation in PD patients is now underway

Mitochondrially associated mechanisms involved in Huntington’s disease and the potential effect of creatine supplementation. Mutant huntingtin has been proposed to confer toxic effects to neural tissue by a variety of different mechanisms. (1) With respect to mitochondrial dysfunction, mutant huntingtin has been shown to impair the levels of an important cofactor, PGC-1α, which is involved in regulating the expression nuclear genes encoding mitochondrial proteins (NUGEMPs). (2) Reductions in NUGEMPs can potentially lead to a decrease in the overall content and function in mitochondria. (3) As a consequence, these changes can lead to a reduction in ATP levels and enhanced ROS production. (4) Evidence has also shown that ETC complex activities are impaired in postmortem HD brain tissue. Additionally, in animal models, administration of the neurotoxin, 3-NP, inhibits complex II of the ETC, and leads to Huntington’s-like pathogenesis which underscores the importance of mitochondrial dysfunction in HD. Creatine supplementation reduces lesion volume in the mitochondrial toxin (3-NP) models of HD and provides significant neuroprotection. Given the promising neuroprotective effects of creatine found in animal and phase II clinical studies, a phase III clinical trial in HD patients is currently ongoing