Biochemical and Biophysical Research Communications
Regular ArticleIdentification of Amyloid Precursor Protein in Synaptic Plasma Membrane
Abstract
Although the etiology of Alzheimer′s disease has not been elucidated yet, dysfunction and loss of synapses are believed to cause dementia. Recent studies suggest that the primary cause of the disease is closely related to the aberrant processing of the amyloid precursor protein (APP). To investigate the localization of APP at synaptic sites, we obtained synaptic plasma membrane and synaptic vesicles from rat brain. Enhanced chemiluminescence (ECL) western blot analysis using two specific polyclonal antibodies against APP revealed strong APP immunoreactivity in the synaptic plasama membrane, but not in the synaptic vesicle fraction. These data indicate that APP is localized at the synaptic plasma membrane and may play a role in physiological synaptic activity. Alternative localization or aberrant processing of APP at the synaptic site may cause impairment of synaptic function in Alzheimer′s disease.
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Glutamate-glutamine cycling in Alzheimer's disease
2007, Neurochemistry InternationalIn addition to its definitive pathological characteristics, neuritic plaques and neurofibrillary tangles, Alzheimer's disease (AD) brain exhibits regionally variable neuronal loss and synaptic dysfunction that are likely to underlie the symptomatic memory loss and language abnormalities. A number of mechanisms that could give rise to this localized damage have been proposed, amongst which excitotoxicity figures prominently. This is the process, well attested in experimental systems, whereby brain cells are excited to death by the pathophysiological action of the brain's most-abundant excitatory transmitter, glutamate. Glutamate transmission is mediated by a range of ionotropic and metabotropic receptors which, when activated, can lead to depolarization and increased intracellular Ca2+ ion concentration in the cells on which they are located. The action of glutamate is terminated by its removal from these receptor sites by transport into nearby cells, most commonly perisynaptic astrocytes. There it is converted to physiologically inert glutamine and shuttled back to excitatory nerve terminals. Malfunctions in components of the glutamate–glutamine cycle could result in a self-perpetuating neuronal death cascade mediated by glutamate. The approval by the FDA of an ionotropic glutamate receptor antagonist to treat late-stage AD has led to renewed interest in the contribution of altered glutamatergic neurotransmission to disease pathogenesis. This review encompasses those aspects of glutamate–glutamine cycling that are altered in AD.
Synaptic plasticity and cell cycle activation in neurons are alternative effector pathways: The 'Dr. Jekyll and Mr. Hyde concept' of Alzheimer's disease or the yin and yang of neuroplasticity
2003, Progress in NeurobiologyMental actions are based on the dynamic organization of neuronal networks. In particular, phylogenetically young brain areas (e.g., cortical associative circuits) involved in the realization of higher brain functions are continuously re-adjusted to meet environmental demands. The mechanisms of synaptic plasticity, i.e., of structural stabilization and labilization underlying a life-long synaptic remodelling, are largely based on external morphoregulatory cues and internal signalling pathways that non-neuronal cells have phylogenetically acquired to sense their relationship to the local neighbourhood and to control after development is completed proliferation and differentiation in the process of tissue repair and regeneration. After having withdrawn from the cell cycle, differentiated neurons are, thus, able to use molecular mechanisms primarily developed to control proliferation alternatively to control synaptic plasticity. The existence of these alternative effector pathways within a neuron puts it at risk to erroneously convert signals derived from plastic synaptic changes into positional cues that will activate the cell cycle. This cell cycle activation potentially links synaptic plasticity to cell death. Preventing cell cycle activation by locking neurons in a differentiated but still highly plastic phenotype will, thus, be crucial to prevent neurodegeneration.
Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization
2001, NeuroscienceMental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in “higher brain functions” such as learning, memory, perception, self-awareness and consciousness continously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary aquisition of “higher brain function” but at the same time provide the basis for a variety of neuropsychiatric disorders.
It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly “creates” and “re-creates” the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. “morphoregulatory” molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challange as it potentially interferes with those mechanisms that at the same time provide the basis for “higher brain function”.
Regulation of the dual function tissue transglutaminase/Gα(h) during murine neuromuscular development: Gene and enzyme isoform expression
2000, Neurochemistry InternationalCoagulation Factor XIII (F. VIII), a member of the transglutaminase (TGase) superfamily, is activated by thrombin, cross-links fibrin and stabilizes clots. Another member of this family, tissue TGase (tTG), having similar enzymatic activity, is implicated in neural development and synapse stabilization. Our previous studies indicated that synapse formation and maintenance at the neuromuscular junction (NMJ) involved components of the coagulation cascade in development. Others then showed that either F. XIII or tTG were localized at NMJs in a developmentally-regulated fashion. In the current studies, we addressed the temporal course of skeletal muscle tTG gene expression and found maximal expression at birth and continuing into the immediate postnatal period. Subcellular fractionation revealed a relatively constant particulate isoform of TGase activity which predominated in early embryonic muscle development. In contrast, cytosolic TGase specific activity became the major isoform in the postnatal period. The timing of muscle TGase activity correlated well with expression of tTG mRNA and we now present novel data of Tgm 2 gene expression for tTG in skeletal muscle. Confirming and extending the previous studies, TGase becomes localized at NMJs in the early, further ramifying in the late, neonatal period. These data suggest that the early pulse of particulate activity could coincide with the period of myoblast cell death in embryonic muscle. On the other hand, the peak cytosolic TGase activity occurs in the neonatal period, correlating temporally with muscle prothrombin expression during activity-dependent synapse elimination and possibly the source of the enzyme localized to the NMJ extracellular matrix resulting in synaptic stabilization.
Developmental changes of sialylation of soluble β/A4 amyloid protein precursor derivatives in human cerebrospinal fluid
1994, Molecular Brain ResearchSoluble β/A4 amyloid protein precursor derivatives (APPs) in cerebrospinal fluid from infants, children, adults and aged individuals were treated with neuraminidase. In the samples from infants, reduction of molecular weight of APPs following neuraminidase treatment was significantly less than those from adults or aged individuals. Hyposialylation of β/A4 amyloid protein precursor in infants may be relevant to a physiological role of this molecule in the development of the nervous system.
Alzheimer's disease is characterized by progressive dementia, cortical atrophy with synaptic loss, and the accumulation of neurofibrillary tangles and senile plaques containing β-amyloid. The β-amyloid protein precursor (β-APP), may normally be involved in cell adhesion related to synaptic maintenance. Loss of synapses correlates with dementia, suggesting that synaptic deficits may underlie the disease. Synapse stability may depend on the action of tissue transglutaminase (tTG), an enzyme capable of crosslinking large, multi-domain extracellular glycoproteins, that is active and present at synapses. We now show that β-APP is a substrate for tTG in vitro that results in dimers and multimers by silver staining and immunoblotting. This novel post-translational modification suggests further roles for β-APP in synaptic function as well as in Alzheimer's disease.