Physiological and pathological caspase cleavage of the neuronal RasGEF GRASP-1 as detected using a cleavage site-specific antibody

Loss of Non-Apoptotic Role of Caspase-3 in the PINK1 Mouse Model of Parkinson's Disease

Caspases are a family of conserved cysteine proteases that play key roles in multiple cellular processes, including programmed cell death and inflammation. Recent evidence shows that caspases are also involved in crucial non-apoptotic functions, such as dendrite development, axon pruning, and synaptic plasticity mechanisms underlying learning and memory processes. The activated form of caspase-3, which is known to trigger widespread damage and degeneration, can also modulate synaptic function in the adult brain. Thus, in the present study, we tested the hypothesis that caspase-3 modulates synaptic plasticity at corticostriatal synapses in the phosphatase and tensin homolog (PTEN) induced kinase 1 (PINK1) mouse model of Parkinson’s disease (PD). Loss of PINK1 has been previously associated with an impairment of corticostriatal long-term depression (LTD), rescued by amphetamine-induced dopamine release. Here, we show that caspase-3 activity, measured after LTD induction, is significantly decreased in the PINK1 knockout model compared with wild-type mice. Accordingly, pretreatment of striatal slices with the caspase-3 activator α-(Trichloromethyl)-4-pyridineethanol (PETCM) rescues a physiological LTD in PINK1 knockout mice. Furthermore, the inhibition of caspase-3 prevents the amphetamine-induced rescue of LTD in the same model. Our data support a hormesis-based double role of caspase-3; when massively activated, it induces apoptosis, while at lower level of activation, it modulates physiological phenomena, like the expression of corticostriatal LTD. Exploring the non-apoptotic activation of caspase-3 may contribute to clarify the mechanisms involved in synaptic failure in PD, as well as in view of new potential pharmacological targets.

Proteomic identification of synaptic caspase substrates

The dismantling and elimination of excess neurons and their connections (pruning) is essential for brain development and may be aberrantly reactivated in some neurodegenerative diseases. Growing evidence implicates caspase-mediated apoptotic and nonapoptotic cascades in the dysfunction and death of neurons in neurodegenerative disorders such as Alzheimer's, Parkinson, and Huntington's diseases. It is the cleaved caspase substrates that are the effectors of synapse elimination. However, their identities, specific cleavage sites, and functional consequences of cleavage are largely unknown. An important gap in our knowledge is a comprehensive catalog of synapse-specific or synapse-enriched caspase targets. Traditional biochemical approaches have revealed only a small number of neuronal caspase targets. Instead, we utilized a gel-based proteomics approach to enable the first global analysis of caspase-mediated cleavage events in mammalian brain synapses, employing both an in vitro system with recombinant activated caspases and an in vivo model of ethanol-induced neuronal apoptosis. Of the more than 70 putative cleavage substrates that were identified, 22 were previously known caspase substrates. Among the novel targets identified and validated by Western blot were the proton pump ATPase subunit ATP6V1B2 and the N-ethylmaleimide-sensitive fusion protein (NSF). Our work represents the first comprehensive, proteome-wide screen for proteolytic targets of caspases in neuronal synapses. Our discoveries will have significance for both furthering basic understanding of roles of caspases in synaptic plasticity and synaptic loss in neurodegeneration, and on a more immediately practical level, may provide candidate biomarkers for measuring synapse loss in human disease states.

Caspases rule the intracellular trafficking cartel

During apoptosis, caspases feast on several hundreds of cellular proteins to orchestrate rapid cellular demise. Indeed, caspases are known to get a taste of every cellular process in one way or another, activating some, but most often shutting them down. Thus, it is not surprising that caspases proteolyze proteins involved in intracellular trafficking with particularly devastating consequences for this important process. This review article focuses on how caspases target the machinery responsible for smuggling goods within and outside the cell.

GRASP-1 regulates endocytic receptor recycling and synaptic plasticity

Remodeling of synapses is a fundamental mechanism for information storage and processing in the brain. Previous studies showed that the endosomal pathway plays a central role in synapse formation and plasticity. A popular model holds that recycling endosomes in dendrites provide the local intracellular pool of postsynaptic receptors for long-term potentiation (LTP), a widely studied cellular model for learning and memory formation. However, we are far from a complete understanding how endocytic receptor sorting and recycling is organized and coordinated in dendrites. Especially, the molecular mechanisms that couple specific endosomal trafficking routes during LTP are poorly understood. In a recent paper we discovered that the coiled-coil protein GRIP-associated protein-1 (GRASP-1) is a neuron-specific effector of the small GTPase Rab4 and key component of AMPA receptor recycling machinery in dendrites.1 GRASP-1 is essential for maintenance of spine morphology and important for LTP. GRASP-1 connects Rab4 and Rab11 recycling endosomal domains through the interaction with target (t)-SNARE syntaxin 13, which constitutes a new principle for regulating endosomal recycling. Here, we summarize our recently reported observations and further discuss their possible implications.

Caspase-3 activation via mitochondria is required for long-term depression and AMPA receptor internalization

NMDA receptor-dependent synaptic modifications, such as long-term potentiation (LTP) and long-term depression (LTD), are essential for brain development and function. LTD occurs mainly by the removal of AMPA receptors from the postsynaptic membrane, but the underlying molecular mechanisms remain unclear. Here, we show that activation of caspase-3 via mitochondria is required for LTD and AMPA receptor internalization in hippocampal neurons. LTD and AMPA receptor internalization are blocked by peptide inhibitors of caspase-3 and -9. In hippocampal slices from caspase-3 knockout mice, LTD is abolished whereas LTP remains normal. LTD is also prevented by overexpression of the anti-apoptotic proteins XIAP or Bcl-xL, and by a mutant Akt1 protein that is resistant to caspase-3 proteolysis. NMDA receptor stimulation that induces LTD transiently activates caspase-3 in dendrites, without causing cell death. These data indicate an unexpected causal link between the molecular mechanisms of apoptosis and LTD.

GRASP-1 is a neuronal scaffold protein for the JNK signaling pathway

GRASP-1 is a neuronally enriched protein that interacts with the AMPA-type glutamate receptor/GRIP complex. GRASP-1 can be cleaved by Caspase-3 in both normal and ischemic brains although the functional significance of this cleavage remains elusive. We investigated signal transduction pathways that might lie downstream of GRASP-1 and found that GRASP-1 potently activates JNK pathway signaling, with no effect on ERK signaling. Such JNK pathway activating activity requires binding of GRASP-1 to both JNK and the upstream JNK pathway activator MEKK-1. Furthermore, mutations that prevent Caspase 3-cleavage of GRASP-1 dramatically inhibit the JNK pathway activating activity of GRASP-1, suggesting a novel link between Caspase-3 activation and JNK pathway signaling. These results suggest that GRASP-1 serves as a scaffold protein to facilitate MEKK-1 activation of JNK signaling in neurons.

Posttranslational modifications and receptor-associated proteins in AMPA receptor trafficking and synaptic plasticity

AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission in the mammalian brain. It is widely believed that the long-lasting, activity-dependent changes in synaptic strength, including long-term potentiation and long-term depression, could be the molecular and cellular basis of experience-dependent plasticities, such as learning and memory. Those changes of synaptic strength are directly related to AMPAR trafficking to and away from the synapse. There are many forms of synaptic plasticity in the mammalian brain, while the prototypic form, hippocampal CA1 long-term potentiation, has received the most intense investigation. After synthesis, AMPAR subunits undergo posttranslational modifications such as glycosylation, palmitoylation, phosphorylation and potential ubiquitination. In addition, AMPAR subunits spatiotemporally associate with specific neuronal proteins in the cell. Those posttranslational modifications and receptor-associated proteins play critical roles in AMPAR trafficking and regulation of AMPAR-dependent synaptic plasticity. Here, we summarize recent studies on posttranslational modifications and associated proteins of AMPAR subunits, and their roles in receptor trafficking and synaptic plasticity.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Peptide action in stroke therapy

Glutamate receptor antagonists, although effective in preventing in vitro excitotoxic death, also block the glutamatergic signalling that is essential for normal excitatory neurotransmission and neuronal survival. This has contributed to the failure of clinical trials employing glutamate receptor antagonists as stroke therapeutics. However, recent years have seen an increased understanding of the molecular organisation of glutamate receptors in the neuronal postsynaptic density. This and a dissection of their associated intracellular signalling cascades has allowed the identification of distinct pathways responsible for excitotoxicity. It has become possible to uncouple toxic signalling cascades from glutamate receptors by targeting the interactions of membrane receptors with downstream proteins. Toxic signalling can be effectively uncoupled from glutamate receptors using targeted, cell-permeable peptides to disrupt specific protein-protein interactions. This approach does not block essential excitatory neurotransmission, but attenuates neurotoxic signals specifically and reduces stroke damage. This novel approach to blocking excitotoxic signalling in cerebral ischaemia may constitute a practical approach to stroke therapy.

Molecular mechanisms of calcium-dependent neurodegeneration in excitotoxicity

Excitotoxicity contributes to neuronal degeneration in many acute CNS diseases, including ischemia, trauma, and epilepsy, and may also play a role in chronic diseases, such as amyotrophic lateral sclerosis (ALS). Key mediators of excitotoxic damage are Ca ions (Ca(2+)), which under physiological conditions govern a multitude of cellular processes, including cell growth, differentiation, and synaptic activity. Consequently, homeostatic mechanisms exist to maintain a low intracellular Ca(2+) ion concentration so that Ca(2+) signals remain spatially and temporally localized. This permits multiple independent Ca-mediated signaling pathways to occur in the same cell. In excitotoxicity, excessive synaptic release of glutamate can lead to the disregulation of Ca(2+) homeostasis. Glutamate activates postsynaptic receptors, including the ionotropic N-methyl-D-aspartate (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) proprionate (AMPA), and kainate receptors. Upon their activation, these open their associated ion channel to allow the influx of Ca(2+) and Na(+) ions. Although physiological elevations in intracellular Ca(2+) are salient to normal cell functioning, the excessive influx of Ca(2+) together with any Ca(2+) release from intracellular compartments can overwhelm Ca(2+)-regulatory mechanisms and lead to cell death. Although Ca(2+) disregulation is paramount to neurodegeneration, the exact mechanism by which Ca(2+) ions actually mediate excitotoxicity is less clear. One hypothesis outlined in this review suggests that Ca(2+)-dependent neurotoxicity occurs following the activation of distinct signaling cascades downstream from key points of Ca(2+) entry at synapses, and that triggers of these cascades are physically co-localized with specific glutamate receptors. Thus, we summarize the importance of Ca(2+) regulation in mammalian neurons and the excitotoxicity hypothesis, and focus on the molecular determinants of glutamate receptor-mediated excitotoxic mechanisms.

Novel treatment of excitotoxicity: targeted disruption of intracellular signalling from glutamate receptors

Glutamate signalling plays key physiological roles in excitatory neurotransmission and CNS plasticity, but also mediates excitotoxicity, the process responsible for triggering neurodegeneration through glutamate receptor overactivation. Excitotoxicity is thought to be a key neurotoxic mechanism in neurological disorders, including brain ischemia, CNS trauma and epilepsy. However, treating excitotoxicity using glutamate receptor antagonists has not proven clinically viable, necessitating more sophisticated approaches. Increasing knowledge of the composition of the postsynaptic density at glutamatergic synapses has allowed us to extend our understanding of the molecular mechanisms of excitotoxicity and to dissect out the distinct signalling pathways responsible for excitotoxic damage. Key molecules in these pathways are physically linked to the cytoplasmic face of glutamate receptors by scaffolding proteins that exhibit binding specificity for some receptors over others. This imparts specificity to physiological and pathological glutamatergic signalling. Recently, we have capitalized on this knowledge and, using targeted peptides to selectively disrupt intracellular interactions linked to glutamate receptors, have blocked excitotoxic signalling in neurones. This therapeutic approach circumvents the negative consequences of blocking glutamate receptors, and may be a practical strategy for treating neurological disorders that involve excitotoxicity.

Many cuts to ruin: a comprehensive update of caspase substrates

Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.