Caspase-3 immunoreactivity in different cortical areas of young and aging macaque (Macaca mulatta) monkeys

Pathological Impact of Tau Proteolytical Process on Neuronal and Mitochondrial Function: a Crucial Role in Alzheimer's Disease

Tau protein plays a pivotal role in the central nervous system (CNS), participating in microtubule stability, axonal transport, and synaptic communication. Research interest has focused on studying the role of post-translational tau modifications in mitochondrial failure, oxidative damage, and synaptic impairment in Alzheimer's disease (AD). Soluble tau forms produced by its pathological cleaved induced by caspases could lead to neuronal injury contributing to oxidative damage and cognitive decline in AD. For example, the presence of tau cleaved by caspase-3 has been suggested as a relevant factor in AD and is considered a previous event before neurofibrillary tangles (NFTs) formation.Interestingly, we and others have shown that caspase-cleaved tau in N- or C- terminal sites induce mitochondrial bioenergetics defects, axonal transport impairment, neuronal injury, and cognitive decline in neuronal cells and murine models. All these abnormalities are considered relevant in the early neurodegenerative manifestations such as memory and cognitive failure reported in AD. Therefore, in this review, we will discuss for the first time the importance of truncated tau by caspases activation in the pathogenesis of AD and how its negative actions could impact neuronal function.

Caspase-cleaved tau is senescence-associated and induces a toxic gain of function by putting a brake on axonal transport

The microtubule-associated protein tau plays a central role in tauopathies such as Alzheimer's disease (AD). The exact molecular mechanisms underlying tau toxicity are unclear, but aging is irrefutably the biggest risk factor. This raises the question of how cellular senescence affects the function of tau as a microtubule regulator. Here we report that the proportion of tau that is proteolytically cleaved at the caspase-3 site (TauC3) doubles in the hippocampus of senescent mice. TauC3 is also elevated in AD patients. Through quantitative live-cell imaging, we show that TauC3 has a drastically reduced dynamics of its microtubule interaction. Single-molecule tracking of tau confirmed that TauC3 has a longer residence time on axonal microtubules. The reduced dynamics of the TauC3-microtubule interaction correlated with a decreased transport of mitochondria, a reduced processivity of APP-vesicle transport and an induction of region-specific dendritic atrophy in CA1 neurons of the hippocampus. The microtubule-targeting drug Epothilone D normalized the interaction of TauC3 with microtubules and modulated the transport of APP-vesicles dependent on the presence of overexpressed human tau. The results indicate a novel toxic gain of function, in which a post-translational modification of tau changes the dynamics of the tau-microtubule interaction and thus leads to axonal transport defects and neuronal degeneration. The data also introduce microtubule-targeting drugs as pharmacological modifiers of the tau-microtubule interaction with the potential to restore the physiological interaction of pathologically altered tau with microtubules.

Mitochondria, motor neurons and aging

While the role of mitochondria in aging has been well characterized, their involvement in motor neuron aging remains poorly understood. Thus, we performed an exhaustive ultrastructural study of mitochondria in motor neurons from aged rats that revealed dramatic alterations in the mitochondria of axon terminals at neuromuscular junctions, characterized by swelling, mitochondrial fusion and the presence of megamitochondria. These alterations were not observed in ventral horn motor neurons in the spinal cord of aged rats, which were only altered by the appearance of electron-dense bodies in the dilated matrix cristae. Using X-ray microanalytical techniques we demonstrated the presence of calcium in these bodies, suggesting Ca(2+) overload. Moreover, in motor neurons from aged rats, cytochrome c and activated caspase 3 were detected in the cytoplasm of axon terminals at neuromuscular junctions, factors implicated in the apoptosis. Active caspase 3 was also found in the nucleus, soma and axons of aged alpha motor neuron neurons, where it mainly associated with microtubules. The colocalization of dynein and cleaved caspase 3 in neuromuscular junctions is strongly suggestive of the retrograde transport of apoptotic factors to the soma. These results are consistent with the early stages of degeneration in neuromuscular junctions during aging, which is followed by dying back. Given that aging is a key risk factor for neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), the identification of age-related motor neuron degeneration initiated at the distal end of the axon may provide a new therapeutic target for early intervention.

Aging is associated with altered inflammatory, arachidonic acid cascade, and synaptic markers, influenced by epigenetic modifications, in the human frontal cortex

Aging is a risk factor for Alzheimer's disease (AD) and is associated with cognitive decline. However, underlying molecular mechanisms of brain aging are not clear. Recent studies suggest epigenetic influences on gene expression in AD, as DNA methylation levels influence protein and mRNA expression in postmortem AD brain. We hypothesized that some of these changes occur with normal aging. To test this hypothesis, we measured markers of the arachidonic acid (AA) cascade, neuroinflammation, pro- and anti-apoptosis factors, and gene specific epigenetic modifications in postmortem frontal cortex from nine middle-aged [41 ± 1 (SEM) years] and 10 aged subjects (70 ± 3 years). The aged compared with middle-aged brain showed elevated levels of neuroinflammatory and AA cascade markers, altered pro and anti-apoptosis factors and loss of synaptophysin. Some of these changes correlated with promoter hypermethylation of brain derived neurotrophic factor (BDNF), cyclic AMP responsive element binding protein (CREB), and synaptophysin and hypomethylation of BCL-2 associated X protein (BAX). These molecular alterations in aging are different from or more subtle than changes associated with AD pathology. The degree to which they are related to changes in cognition or behavior during normal aging remains to be evaluated.

Treadmill exercise prevents aging-induced failure of memory through an increase in neurogenesis and suppression of apoptosis in rat hippocampus

Aging leads to functional changes in the hippocampus, and consequently induces cognitive deficits, such as failure of memory. Neurogenesis in the hippocampal dentate gyrus continues throughout life, but steadily declines from early adulthood. Apoptosis occurs under various pathologic and physiologic conditions, and excessive apoptotic cell death can cause a number of functional disorders in humans. Apoptosis in the hippocampus also disturbs cognitive functions. In this study, we examined the effect of treadmill exercise on memory in relation to neurogensis and apoptosis in the hippocampal dentate gyrus of old-aged rats. The present results showed that loss of memory by aging was associated with a decrease in neurogenesis and an increase in apoptosis in the hippocampal dentate gyrus. Treadmill exercise improved short-term and spatial memories by enhancing neurogenesis and suppressing apoptosis in the hippocampal dentate gyrus of old-aged rats. In the present study, we showed that treadmill exercise is a very useful strategy for preventing failure of memory in the elderly.

Immunohistochemical analysis of active caspase-3 expression in structures of neonatal brain

The hippocampal fields of neonatal rats differ by the level of active caspase-3: dentate gyrus >CA3>CA1>CA2. In the dentate gyrus it was 70% of its maximum value in the cortex, while in CA2 it corresponded to the minimum level in the brain stem. Taking into account the role of caspase-3 in apoptosis, these differences can indicate different intensity of programmed cell death in different fields of the forming hippocampus.

Differential response of apoptosis-regulatory Bcl-2 and Bax proteins to an inflammatory challenge in the cerebral cortex and hippocampus of aging mice

Apoptosis plays a key role in normal aging and neurodegeneration. It is now known that normal aging implies low-grade inflammation and increases susceptibility to neurodegenerative diseases, which, in turn, include a neuroinflammatory component. We here investigated, using mice of 2-3 months, 10-11 months, or 18-21 months of age, the expression of apoptosis-regulatory proteins in cortical brain regions in response to intracerebroventricular administration of pro-inflammatory cytokines. A mixture of interferon-gamma and tumor necrosis factor-alpha was injected, using vehicle (phosphate-buffered saline) as control. At 4 days, levels of the anti-apoptotic Bcl-2 and pro-apoptotic Bax proteins in the cerebral cortex and hippocampus, examined with Western blotting, were markedly upregulated by cytokine exposure in mice of all age groups with respect to controls. Interestingly, cytokine-elicited Bcl-2 upregulation was aging-dependent, with significant enhancement paralleling the animals' age. Cytokine-elicited Bax expression did not exhibit instead significant aging-related variation. Using the same paradigm and 1 or 2 day survival, Bcl-2 immunoreactivity was observed mainly in neurons of cortex and hippocampus of both control and cytokine-treated mice of all age groups. Furthermore, immunohistochemistry confirmed the enhancement of cytokine-elicited Bcl-2 expression in the cerebral cortex and hippocampus of old mice, and showed that this finding was already evident in the second day after cytokine exposure. The data point out the novel finding that Bcl-2 and Bax expression in cortical brain regions is differentially regulated during senescence in response to an acute inflammatory challenge. Aging-related Bcl-2 increases in neurons after cytokine exposure could contribute to amplify neuroprotective mechanisms in the old brain.

Perinatal hypoxia induces anterior chamber changes in the eyes of offspring fish

Hypoxia is a consistent challenge for aquatic animals. It is a pressing environmental problem; hypoxia can cause cranial edema and ovarium dysfunction in fish. Although several studies have reported the effect of hypoxic insult to the visual system, the hypoxic effect on perinatal animals and in particular their offspring has yet to be elucidated. In this study, activated caspase-3 activity was investigated using immunohistochemistry in order to examine the perinatal hypoxic damage in offspring fish. Offspring were divided into groups based on different time points of sacrifice. This allowed assessment of ocular development for different age groups. The results indicated that perinatal hypoxia induced ocular developmental defects in the offspring. The defects took the form of trabecular cell death and fibre degeneration, corneal thinning and lens fibre derangement. A concomitant change in intraocular pressure was recorded by tonometer in the experimental animals compared with the controls. Further investigation should be initiated to develop strategies to prevent developmental disability due to perinatal hypoxia and to increase survivability of the offspring.