Bruce A. Yankner

Caspase-12 mediates endoplasmic-reticulum-specific apoptosis and cytotoxicity by amyloid-|[beta]|

Apoptosis, or cellular suicide, is important for normal development and tissue homeostasis, but too much or too little apoptosis can also cause disease. The family of cysteine proteases, the so-called caspases, are critical mediators of programmed cell death, and thus far 14 family members have been identified. Some of these, such as caspase-8 (refs 4, 5), mediate signal transduction downstream of death receptors located on the plasma membrane. Others, such as caspase-9 (ref. 6), mediate apoptotic signals after mitochondrial damage. Stress in the endoplasmic reticulum (ER) can also result in apoptosis. Here we show that caspase-12 is localized to the ER and activated by ER stress, including disruption of ER calcium homeostasis and accumulation of excess proteins in ER, but not by membrane- or mitochondrial-targeted apoptotic signals. Mice that are deficient in caspase-12 are resistant to ER stress-induced apoptosis, but their cells undergo apoptosis in response to other death stimuli. Furthermore, we show that caspase-12-deficient cortical neurons are defective in apoptosis induced by amyloid-β protein but not by staurosporine or trophic factor deprivation. Thus, caspase-12 mediates an ER-specific apoptosis pathway and may contribute to amyloid-β neurotoxicity.

Apoptosis in the nervous system

Neuronal apoptosis sculpts the developing brain and has a potentially important role in neurodegenerative diseases. The principal molecular components of the apoptosis programme in neurons include Apaf-1 (apoptotic protease-activating factor 1) and proteins of the Bcl-2 and caspase families. Neurotrophins regulate neuronal apoptosis through the action of critical protein kinase cascades, such as the phosphoinositide 3-kinase/Akt and mitogen-activated protein kinase pathways. Similar cell-death-signalling pathways might be activated in neurodegenerative diseases by abnormal protein structures, such as amyloid fibrils in Alzheimers disease. Elucidation of the cell death machinery in neurons promises to provide multiple points of therapeutic intervention in neurodegenerative diseases.

Gene regulation and DNA damage in the ageing human brain.

The ageing of the human brain is a cause of cognitive decline in the elderly and the major risk factor for Alzheimers disease. The time in life when brain ageing begins is undefined. Here we show that transcriptional profiling of the human frontal cortex from individuals ranging from 26 to 106 years of age defines a set of genes with reduced expression after age 40. These genes play central roles in synaptic plasticity, vesicular transport and mitochondrial function. This is followed by induction of stress response, antioxidant and DNA repair genes. DNA damage is markedly increased in the promoters of genes with reduced expression in the aged cortex. Moreover, these gene promoters are selectively damaged by oxidative stress in cultured human neurons, and show reduced base-excision DNA repair. Thus, DNA damage may reduce the expression of selectively vulnerable genes involved in learning, memory and neuronal survival, initiating a programme of brain ageing that starts early in adult life.

Mechanisms of Neuronal Degeneration in Alzheimer's Disease

Although a consensus on the primary mechanism(s) of neuronal degeneration in AD has not yet been reached, several potential pathogenic mechanisms have emerged. The involvement of APP and Aβ in the neurodegenerative process and their relationship to the tau-related neurofibrillary pathology are central issues. The pathogenic mechanism associated with inheritance of the ApoE4 allele remains to be determined, although initial investigations implicate effects on Aβ or possibly tau. Increasing evidence implicates oxidative stress in the neurodegenerative process, although this has yet to be convincingly linked to specific molecular mechanisms. Most importantly, the recent identification of new AD susceptibility genes promises to rapidly advance our understanding of the primary neurodegenerative mechanisms. In the broader context of human neurobiology, AD poses fundamental questions about how the brain ages and why the systems subserving memory and cognition are selectively vulnerable. The potential to answer these questions and treat this devastating illness makes this an exciting time in AD research.

Neural mechanisms of ageing and cognitive decline

During the past century, treatments for the diseases of youth and middle age have helped raise life expectancy significantly. However, cognitive decline has emerged as one of the greatest health threats of old age, with nearly 50% of adults over the age of 85 afflicted with Alzheimers disease. Developing therapeutic interventions for such conditions demands a greater understanding of the processes underlying normal and pathological brain ageing. Recent advances in the biology of ageing in model organisms, together with molecular and systems-level studies of the brain, are beginning to shed light on these mechanisms and their potential roles in cognitive decline.

Dopamine-dependent neurotoxicity of alpha-synuclein: a mechanism for selective neurodegeneration in Parkinson disease.

The mechanism by which dopaminergic neurons are selectively lost in Parkinson disease (PD) is unknown. Here we show that accumulation of α-synuclein in cultured human dopaminergic neurons results in apoptosis that requires endogenous dopamine production and is mediated by reactive oxygen species. In contrast, α-synuclein is not toxic in non-dopaminergic human cortical neurons, but rather exhibits neuroprotective activity. Dopamine-dependent neurotoxicity is mediated by 54–83-kD soluble protein complexes that contain α-synuclein and 14-3-3 protein, which are elevated selectively in the substantia nigra in PD. Thus, accumulation of soluble α-synuclein protein complexes can render endogenous dopamine toxic, suggesting a potential mechanism for the selectivity of neuronal loss in PD.

SIRT1 redistribution on chromatin promotes genomic stability but alters gene expression during aging.

Genomic instability and alterations in gene expression are hallmarks of eukaryotic aging. The yeast histone deacetylase Sir2 silences transcription and stabilizes repetitive DNA, but during aging or in response to a DNA break, the Sir complex relocalizes to sites of genomic instability, resulting in the desilencing of genes that cause sterility, a characteristic of yeast aging. Using embryonic stem cells, we show that mammalian Sir2, SIRT1, represses repetitive DNA and a functionally diverse set of genes across the mouse genome. In response to DNA damage, SIRT1 dissociates from these loci and relocalizes to DNA breaks to promote repair, resulting in transcriptional changes that parallel those in the aging mouse brain. Increased SIRT1 expression promotes survival in a mouse model of genomic instability and suppresses age-dependent transcriptional changes. Thus, DNA damage-induced redistribution of SIRT1 and other chromatin-modifying proteins may be a conserved mechanism of aging in eukaryotes.

β-Amyloid fibrils induce tau phosphorylation and loss of microtubule binding

A central issue in the pathogenesis of Alzheimers disease (AD) is the relationship between amyloid deposition and neurofibrillary tangle formation. To determine whether amyloid fibril formation affects the phosphorylation state of tau, primary cultures of fetal rat hippocampal and human cortical neurons were treated with beta-amyloid (beta A) in a soluble, amorphous-aggregated, or fibrillar form. Fibrillar beta A, but not soluble or amorphous-aggregated beta A, markedly induces the phosphorylation of tau at Ser-202 and Ser-396/Ser-404, resulting in a shift in the tau M(r) in human cortical neurons. Hyperphosphorylated tau accumulates in the somatodendritic compartment of fibrillar beta A-treated neurons in a soluble form that is not associated with microtubules and is incapable of binding to microtubules in vitro. Dephosphorylation of beta A-induced tau restores its capacity to bind to microtubules. Thus, amyloid fibril formation alters the phosphorylation state of tau, resulting in the loss of microtubule binding capacity and somatodendritic accumulation, properties also exhibited by tau in the AD brain. Amyloid fibril formation may therefore be a cause of abnormal tau phosphorylation in AD.

Destabilization of beta-catenin by mutations in presenilin-1 potentiates neuronal apoptosis.

Mutations of the presenilin-1 gene are a major cause of familial early-onset Alzheimers disease. Presenilin-1 can associate with members of the catenin family of signalling proteins, but the significance of this association is unknown,. Here we show that presenilin-1 forms a complex with β-catenin in vivo that increases β-catenin stability. Pathogenic mutations in the presenilin-1 gene reduce the ability of presenilin-1 to stabilize β-catenin, and lead to increased degradation of β-catenin in the brains of transgenic mice. Moreover, β-catenin levels are markedly reduced in the brains of Alzheimers disease patients with presenilin-1 mutations. Loss of β-catenin signalling increases neuronal vulnerability to apoptosis induced by amyloid-β protein. Thus, mutations in presenilin-1 may increase neuronal apoptosis by altering the stability of β-catenin, predisposing individuals to early-onset Alzheimers disease.

Synaptic NMDA receptor activity boosts intrinsic antioxidant defenses.

Intrinsic antioxidant defenses are important for neuronal longevity. We found that in rat neurons, synaptic activity, acting via NMDA receptor (NMDAR) signaling, boosted antioxidant defenses by making changes to the thioredoxin-peroxiredoxin (Prx) system. Synaptic activity enhanced thioredoxin activity, facilitated the reduction of overoxidized Prxs and promoted resistance to oxidative stress. Resistance was mediated by coordinated transcriptional changes; synaptic NMDAR activity inactivated a previously unknown Forkhead box O target gene, the thioredoxin inhibitor Txnip. Conversely, NMDAR blockade upregulated Txnip in vivo and in vitro, where it bound thioredoxin and promoted vulnerability to oxidative damage. Synaptic activity also upregulated the Prx reactivating genes Sesn2 (sestrin 2) and Srxn1 (sulfiredoxin), via C/EBPβ and AP-1, respectively. Mimicking these expression changes was sufficient to strengthen antioxidant defenses. Trans-synaptic stimulation of synaptic NMDARs was crucial for boosting antioxidant defenses; chronic bath activation of all (synaptic and extrasynaptic) NMDARs induced no antioxidative effects. Thus, synaptic NMDAR activity may influence the progression of pathological processes associated with oxidative damage.