Does acetylcholinesterase secretion involve an ADAMs-like metallosecretase?

Role of Prenatal Hypoxia in Brain Development, Cognitive Functions, and Neurodegeneration

This review focuses on the role of prenatal hypoxia in the development of brain functions in the postnatal period and subsequent increased risk of neurodegenerative disorders in later life. Accumulating evidence suggests that prenatal hypoxia in critical periods of brain formation results in significant changes in development of cognitive functions at various stages of postnatal life which correlate with morphological changes in brain structures involved in learning and memory. Prenatal hypoxia also leads to a decrease in brain adaptive potential and plasticity due to the disturbance in the process of formation of new contacts between cells and propagation of neuronal stimuli, especially in the cortex and hippocampus. On the other hand, prenatal hypoxia has a significant impact on expression and processing of a variety of genes involved in normal brain function and their epigenetic regulation. This results in changes in the patterns of mRNA and protein expression and their post-translational modifications, including protein misfolding and clearance. Among proteins affected by prenatal hypoxia are a key enzyme of the cholinergic system-acetylcholinesterase, and the amyloid precursor protein (APP), both of which have important roles in brain function. Disruption of their expression and metabolism caused by prenatal hypoxia can also result, apart from early cognitive dysfunctions, in development of neurodegeneration in later life. Another group of enzymes affected by prenatal hypoxia are peptidases involved in catabolism of neuropeptides, including amyloid-β peptide (Aβ). The decrease in the activity of neprilysin and other amyloid-degrading enzymes observed after prenatal hypoxia could result over the years in an Aβ clearance deficit and accumulation of its toxic species which cause neuronal cell death and development of neurodegeneration. Applying various approaches to restore expression of neuronal genes disrupted by prenatal hypoxia during postnatal development opens an avenue for therapeutic compensation of cognitive dysfunctions and prevention of Aβ accumulation in the aging brain and the model of prenatal hypoxia in rodents can be used as a reliable tool for assessment of their efficacy.

Characterisation of acetylcholinesterase release from neuronal cells

Although acetylcholinesterase (AChE) is primarily a hydrolytic enzyme, metabolising the neurotransmitter acetylcholine in cholinergic synapses, it also has some non-catalytic functions in the brain which are far less well characterised. AChE was shown to be secreted or shed from the neuronal cell surface like several other membrane proteins, such as the amyloid precursor protein (APP). Since AChE does not possess a transmembrane domain, its anchorage in the membrane is established via the Proline Rich Membrane Anchor (PRiMA), a transmembrane protein. Both the subunit oligomerisation and membrane anchor of AChE are shared by a related enzyme, butyrylcholinesterase (BChE), the physiological function of which in the brain is unclear. In this work, we have assayed the relative activities of AChE and BChE in membrane fractions and culture medium of three different neuronal cell lines, namely the neuroblastoma cell lines SH-SY5Y and NB7 and the mouse basal forebrain cell line SN56. In an effort to understand the shedding process of AChE, we have used several pharmacological treatments, which showed that it is likely to be mediated in part by an EDTA- and batimastat-sensitive, but GM6001-insensitive metalloprotease, with the possible additional involvement of a thiol isomerase. Cellular release of AChE by SH-SY5Y is significantly enhanced by the muscarinic acetylcholine receptor (mAChR) agonists carbachol or muscarine, with the effect of carbachol blocked by the mAChR antagonist atropine. AChE has been implicated in the pathogenesis of Alzheimer's disease and it has been shown that it accelerates formation and increases toxicity of amyloid fibrils, which have been closely linked to the pathology of AD. In light of this, greater understanding of AChE and BChE physiology may also benefit AD research.

Membrane targeting, shedding and protein interactions of brain acetylcholinesterase

The early stages of Alzheimer's disease are characterized by cholinergic deficits and the preservation of cholinergic function through the use of acetylcholinesterase inhibitors is the basis for current treatments of the disease. Understanding the causes for the loss of basal forebrain cholinergic neurons in neurodegeneration is therefore a key to developing new therapeutics. In this study, we review novel aspects of cholinesterase membrane localization in brain and propose mechanisms for its lipid domain targeting, secretion and protein-protein interactions. In erythrocytes, acetylcholinesterase (AChE) is localized to lipid rafts through a GPI anchor. However, the main splice form of AChE in brain lacks a transmembrane peptide anchor region and is bound to the 'proline-rich membrane anchor', PRiMA, in lipid rafts. Furthermore, AChE is secreted ('shed') from membranes and this shedding is stimulated by cholinergic agonists. Immunocytochemical studies on rat brain have shown that membrane-associated PRiMA immunofluorescence is located selectively at cholinergic neurons of the basal forebrain and striatum. A strong association of AChE with the membrane via PRiMA seems therefore to be a specific requirement of forebrain cholinergic neurons. α7 nicotinic acetylcholine receptors are also associated with lipid rafts where they undergo rapid internalisation on stimulation. We are currently probing the mechanism(s) of AChE shedding, and whether this process and its apparent association with α7 nicotinic acetylcholine receptors and metabolism of the Alzheimer's amyloid precursor protein is determined by its association with lipid raft domains either in normal or pathological situations.

Co-localization of PRiMA with acetylcholinesterase in cholinergic neurons of rat brain: an immunocytochemical study

In the central nervous system, acetylcholinesterase (AChE) is present in a tetrameric form that is anchored to membranes via a proline-rich membrane anchor (PRiMA). Previously it has been found that principal cholinergic neurons in the brain express high concentrations of AChE enzymic activity at their neuronal membranes. The aim of this study was to use immunocytochemical methods to determine the distribution of PRiMA in these neurons in the rat brain. Confocal laser and electron microscopic investigations showed that PRiMA immunoreactivity is associated with the membranes of the somata, dendrites and axons of cholinergic neurons in the basal forebrain, striatum and pedunculopontine nuclei, i.e. the neurons that innervate forebrain and brainstem structures. In these neurones, PRiMA also co-localizes with AChE immunoreactivity at the plasma membrane. PRiMA label was absent from neighboring GABAergic neurons, and from other neurons of the brain known to express high levels of AChE enzymic activity including cranial nerve motor neurons and dopaminergic neurons of the substantia nigra zona compacta. A strong association of AChE with PRiMA at the plasma membrane is therefore a feature specific to principal cholinergic neurons that innervate the central nervous system.