Cyclophilin D (CypD, encoded by Ppif) is an integral part of the mitochondrial permeability transition pore, whose opening leads to cell death. Here we show that interaction of CypD with mitochondrial amyloid-β protein (Aβ) potentiates mitochondrial, neuronal and synaptic stress. The CypD-deficient cortical mitochondria are resistant to Aβ- and Ca2+-induced mitochondrial swelling and permeability transition. Additionally, they have an increased calcium buffering capacity and generate fewer mitochondrial reactive oxygen species. Furthermore, the absence of CypD protects neurons from Aβ- and oxidative stress-induced cell death. Notably, CypD deficiency substantially improves learning and memory and synaptic function in an Alzheimer's disease mouse model and alleviates Aβ-mediated reduction of long-term potentiation. Thus, the CypD-mediated mitochondrial permeability transition pore is directly linked to the cellular and synaptic perturbations observed in the pathogenesis of Alzheimer's disease. Blockade of CypD may be a therapeutic strategy in Alzheimer's disease.

Synaptic dysfunction and the loss of synapses are early pathological features of Alzheimer's disease (AD). Synapses are sites of high energy demand and extensive calcium fluctuations; accordingly, synaptic transmission requires high levels of ATP and constant calcium fluctuation. Thus, synaptic mitochondria are vital for maintenance of synaptic function and transmission through normal mitochondrial energy metabolism, distribution and trafficking, and through synaptic calcium modulation. To date, there has been no extensive analysis of alterations in synaptic mitochondria associated with amyloid pathology in an amyloid β (Aβ)-rich milieu. Here, we identified differences in mitochondrial properties and function of synaptic vs. nonsynaptic mitochondrial populations in the transgenic mouse brain, which overexpresses the human mutant form of amyloid precursor protein and Aβ. Compared with nonsynaptic mitochondria, synaptic mitochondria showed a greater degree of agedependent accumulation of Aβ and mitochondrial alterations. The synaptic mitochondrial pool of Aβ was detected at an age as young as 4 mo, well before the onset of nonsynaptic mitochondrial and extensive extracellular Aβ accumulation. Aβ-insulted synaptic mitochondria revealed early deficits in mitochondrial function, as shown by increased mitochondrial permeability transition, decline in both respiratory function and activity of cytochrome c oxidase, and increased mitochondrial oxidative stress. Furthermore, a low concentration of Aβ (200 nM) significantly interfered with mitochondrial distribution and trafficking in axons. These results demonstrate that synaptic mitochondria, especially Aβ-rich synaptic mitochondria, are more susceptible to Aβ-induced damage, highlighting the central importance of synaptic mitochondrial dysfunction relevant to the development of synaptic degeneration in AD.synaptic AB | mitochondrial trafficking | mitochondrial oxidative stress | mitochondrial dysfunction | synaptic injury B rain mitochondria are a mixture of synaptic and nonsynaptic mitochondria. Synaptic mitochondria differ from nonsynaptic mitochondria in size, motility, life span, and other properties. Synaptic mitochondria are energy warehouses that sustain the activity of neurons/synapses (1, 2). Synaptic mitochondria are synthesized in neuronal soma; they are then transported to nerve terminals (dendrites and axons), are distributed abundantly around synapses where mitochondria modulate calcium balance (3), and actively provide energy to fuel the synaptic function (4, 5). Synaptic mitochondria thus undergo constant activation to maintain synaptic function. Defects in synaptic mitochondria obviously compromise synaptic function (1, 6, 7), and synaptic mitochondria are vulnerable to accumulative damages. Multiple studies have shown that synaptic mitochondria undergo increased oxidation during aging (8, 9). In addition, synaptic mitochondria demonstrate higher levels of cyclophilin D (CypD), and are thereby more susceptible to calcium insult (10, 11). Given the impor...

Mitochondrial stress is one of the early features of Alzheimer disease (AD). Mitochondrial Aβ has been linked to mitochondrial toxicity. Our recent study demonstrated that cyclophilin D (CypD) mediated mitochondrial permeability transition pore (mPTP) is an important mechanism for neuronal and synaptic stress induced by both Aβ and oxidative stress. In transgenic AD-type mice overexpressing mutant amyloid precursor protein (APP) and Aβ (mAPP), CypD deficiency improves mitochondrial and synaptic function and learning/memory up to 12 months old. Here we provide evidence of the protective effects of CypD deficiency in aged AD mice (22–24 months). CypD deficient mAPP mice demonstrate less calcium-induced mitochondrial swelling, increased mitochondrial calcium uptake capacity, preserved mitochondrial respiratory function and improved spatial learning/memory even in old age (known to be the age for late stage AD pathology and synaptic dysfunction). These data demonstrate that abrogation of CypD results in persistent life-long protection against Aβ toxicity in an Alzheimer’s disease mouse model, thereby suggesting that blockade of CypD may be of benefit for Alzheimer disease treatment.

F1FO-ATP synthase is critical for mitochondrial functions. The deregulation of this enzyme results in dampened mitochondrial oxidative phosphorylation (OXPHOS) and activated mitochondrial permeability transition (mPT), defects which accompany Alzheimer’s disease (AD). However, the molecular mechanisms that connect F1FO-ATP synthase dysfunction and AD remain unclear. Here, we observe selective loss of the oligomycin sensitivity conferring protein (OSCP) subunit of the F1FO-ATP synthase and the physical interaction of OSCP with amyloid beta (Aβ) in the brains of AD individuals and in an AD mouse model. Changes in OSCP levels are more pronounced in neuronal mitochondria. OSCP loss and its interplay with Aβ disrupt F1FO-ATP synthase, leading to reduced ATP production, elevated oxidative stress and activated mPT. The restoration of OSCP ameliorates Aβ-mediated mouse and human neuronal mitochondrial impairments and the resultant synaptic injury. Therefore, mitochondrial F1FO-ATP synthase dysfunction associated with AD progression could potentially be prevented by OSCP stabilization.

Amyloid-β (Aβ) peptide-binding alcohol dehydrogenase (ABAD), an enzyme present in neuronal mitochondria, exacerbates Aβ-induced cell stress. The interaction of ABAD with Aβ exacerbates Aβ-induced mitochondrial and neuronal dysfunction. Here, we show that inhibition of the ABAD-Aβ interaction, using a decoy peptide (DP) in vitro and in vivo, protects against aberrant mitochondrial and neuronal function and improves spatial learning/memory. Intraperitoneal administration of ABAD-DP [fused to the transduction of human immunodeficiency virus 1-transactivator (Tat) protein and linked to the mitochondrial targeting sequence (Mito) (TAT-mito-DP) to transgenic APP mice (Tg mAPP)] blocked formation of ABAD-Aβ complex in mitochondria, increased oxygen consumption and enzyme activity associated with the mitochondrial respiratory chain, attenuated mitochondrial oxidative stress, and improved spatial memory. Similar protective effects were observed in Tg mAPP mice overexpressing neuronal ABAD decoy peptide (Tg mAPP/mito-ABAD). Notably, inhibition of the ABAD-Aβ interaction significantly reduced mitochondrial Aβ accumulation. In parallel, the activity of mitochondrial Aβ-degrading enzyme PreP (presequence peptidase) was enhanced in Tg mAPP mitochondria expressing the ABAD decoy peptide. These data indicate that segregating ABAD from Aβ protects mitochondria/neurons from Aβ toxicity; thus, ABAD-Aβ interaction is an important mechanism underlying Aβ-mediated mitochondrial and neuronal perturbation. Inhibitors of ABAD-Aβ interaction may hold promise as targets for the prevention and treatment of Alzheimer’s disease.

Accumulation of amyloid-β (Aβ) in synaptic mitochondria is associated with mitochondrial and synaptic injury. The underlying mechanisms and strategies to eliminate Aβ and rescue mitochondrial and synaptic defects remain elusive. Presequence protease (PreP), a mitochondrial peptidasome, is a novel mitochondrial Aβ degrading enzyme. Here, we demonstrate for the first time that increased expression of active human PreP in cortical neurons attenuates Alzheimer disease's (AD)-like mitochondrial amyloid pathology and synaptic mitochondrial dysfunction, and suppresses mitochondrial oxidative stress. Notably, PreP-overexpressed AD mice show significant reduction in the production of proinflammatory mediators. Accordingly, increased neuronal PreP expression improves learning and memory and synaptic function in vivo AD mice, and alleviates Aβ-mediated reduction of long-term potentiation (LTP). Our results provide in vivo evidence that PreP may play an important role in maintaining mitochondrial integrity and function by clearance and degradation of mitochondrial Aβ along with the improvement in synaptic and behavioral function in AD mouse model. Thus, enhancing PreP activity/expression may be a new therapeutic avenue for treatment of AD.

Normal axonal mitochondrial transport and function is essential for the maintenance of synaptic function. Abnormal mitochondrial motility and mitochondrial dysfunction within axons are critical for amyloid β (Aβ)-induced synaptic stress and the loss of synapses relevant to the pathogenesis of Alzheimer’s disease (AD). However, the mechanisms controlling axonal mitochondrial function and transport alterations in AD remain elusive. Here, we report an unexplored role of cyclophilin D (CypD)-dependent mitochondrial permeability transition pore (mPTP) in Aβ-impaired axonal mitochondrial trafficking. Depletion of CypD significantly protects axonal mitochondrial motility and dynamics from Aβ toxicity as shown by increased axonal mitochondrial density and distribution and improved bidirectional transport of axonal mitochondria. Notably, blockade of mPTP by genetic deletion of CypD suppresses Aβ-mediated activation of the p38 mitogen-activated protein kinase signaling pathway, reverses axonal mitochondrial abnormalities, improves synaptic function, and attenuates loss of synapse, suggesting a role of CypD-dependent signaling in Aβ-induced alterations in axonal mitochondrial trafficking. The potential mechanisms of the protective effects of lacking CypD on Aβ-induced abnormal mitochondrial transport in axon are increased axonal calcium buffer capability, diminished reactive oxygen species (ROS), and suppressing downstream signal transduction P38 activation. These findings provide new insights into CypD-dependent mitochondrial mPTP and signaling on mitochondrial trafficking in axons and synaptic degeneration in an environment enriched for Aβ.

Accumulation of amyloid-β peptide (Aβ), the neurotoxic peptide implicated in the pathogenesis of Alzheimer's disease (AD), has been shown in brain mitochondria of AD patients and of AD transgenic mouse models. The presence of Aβ in mitochondria leads to free radical generation and neuronal stress. Recently, we identified the presequence protease, PreP, localized in the mitochondrial matrix in mammalian mitochondria as the novel mitochondrial Aβ-degrading enzyme. In the present study, we examined PreP activity in the mitochondrial matrix of the human brain's temporal lobe, an area of the brain highly susceptible to Aβ accumulation and reactive oxygen species (ROS) production. We found significantly lower hPreP activity in AD brains compared with non-AD age-matched controls. By contrast, in the cerebellum, a brain region typically spared from Aβ accumulation, there was no significant difference in hPreP activity when comparing AD samples to non-AD controls. We also found significantly reduced PreP activity in the mitochondrial matrix of AD transgenic mouse brains (Tg mAβPP and Tg mAβPP/ABAD) when compared to non-transgenic aged-matched mice. Furthermore, mitochondrial fractions isolated from AD brains and Tg mAβPP mice had higher levels of 4-hydroxynonenal, an oxidative product, as compared with those from non-AD and nonTg mice. Accordingly, activity of cytochrome c oxidase was significantly reduced in the AD mitochondria. These findings suggest that decreased PreP proteolytic activity, possibly due to enhanced ROS production, contributes to Aβ accumulation in mitochondria leading to the mitochondrial toxicity and neuronal death that is exacerbated in AD. Clearance of mitochondrial Aβ by PreP may thus be of importance in the pathology of AD.