The serotonin 5-HT4 receptor and the amyloid precursor protein processing
Introduction
Alzheimer's disease (AD) is the most prevalent progressive neurological disorder in the aged population and affects millions of people in the world. The main clinical features of this disease are cognitive decline and mental retardation which are a consequence of a massive neuronal loss in several regions of the brain. Postmortem histopathological studies of AD patients typically show two pathological hallmarks, i.e. the senile plaques and neurofibrillary tangles (Dickson, 1997). Senile plaques are mainly composed of a 40-42 amino acid peptide called amyloid β-protein (Aβ) and most evidence gathered over the past decade indicates that accumulation of the neurotoxic Aβ is a key event in the pathogenesis of AD (for review see Selkoe, 1999). Aβ derives from a larger transmembrane glycoprotein, the amyloid precursor protein (APP) by the action of β-and γ-secretases. Aβ can be directly neurotoxic via oxidative stress and can induce lipid peroxidation in cultured primary neurons with hydrogen peroxide as a mediator (Behl et al., 1994, Nunan and Small, 2000). On the other hand, the α-secretase pathway splits APP within Aβ domain producing a large amino-terminal non-amyloidogenic soluble APP (sAPPα), which is secreted in the extracellular medium and has neurotrophic activities (Mattson et al., 1993, Schubert and Behl, 1993, Mattson, 1997). In addition, sAPPα stimulates neurite outgrowth, regulates neuronal excitability and has potent memory-enhancing effects (Furukawa et al., 1996, Meziane et al., 1998).
The brain of AD patients is also characterised by a dramatic loss of cholinergic neurons in central regions involved in cognitive functions. Besides cholinergic deficit, loss of additional neurotransmitters has been reported in AD and there is increasing evidence that a deficit in serotonergic neurotransmission may play a role in cognitive impairment in AD (Meltzer et al., 1998). Since neuronal activity may be involved in the regulation of APP metabolism, deregulation of neuronal signalling involving, for instance 5-HT may lead to abnormal APP processing and Aβ overproduction. It is therefore crucial to determine the regulatory mechanisms of APP metabolism by neurotransmitter receptors.
Section snippets
Involvement of the serotonin 5-Ht4 receptor in cognitive processes
Among the long list of 5-HT receptor subtypes, the 5-HT4 receptor has gained a lot of attention for its physiological effects on the brain (for review, Barnes and Sharp, 1999). Indeed, with the availability of selective 5-HT4 ligands, radioligand binding and autoradiography studies have been performed in the brain of such species as the rat, pig and human (Grossman et al., 1993, Waeber et al., 1993, Bonaventure et al., 2000). A consistent cross-species finding is the relatively high expression
Molecular identity of the human serotonin 5-Ht4 receptor
To better understand the pathological implication of the 5-HT4 receptor in human disorders, considerable efforts have been made to elucidate its nucleic acid sequence. Initially, the molecular identity of the 5-HT4 receptor was determined in the rat where two C-terminal splice variants were isolated by PCR from brain tissues and named 5-HT4s and 5-HT4L for the short and long isoforms, respectively (Gerald et al., 1995). Using degenerate oligonucleotide primers designed from the sequence of the
The human serotonin 5-Ht4 receptor and the amyloid precursor processing
An important observation towards the understanding of the pharmacological regulation of APP metabolism comes from the fact that GPCRs through a complex network of second messengers may regulate the α cleavage of APP (for review see Racchi and Govoni, 1999). Early work (Nitsch et al., 1992) showed that the muscarinic acetylcholine M1 and M3 receptor subtypes stably transfected into HEK cells increased extracellular release of sAPPα with a concomitant reduction in the levels of Aβ (Hung et al.,
Conclusion
Our results elucidate the involvement of the h5-HT4 receptor in APP metabolism. We provide in vitro evidence that 5-HT4 ligands enhance sAPPα secretion via a cAMP-dependent and PKA-independent signalling pathway. Non-amyloidogenic secreted sAPPα has potent neuroprotective functions against neurotoxic insults such as glutamate and Aβ. Furthermore, sAPPα promotes neuron outgrowth, regulates neuronal excitability and has memory-enhancing effects in behavioural paradigms. This observation together
Acknowledgements
We wish to thank Dr Christian Haass for providing the cDNA encoding the human APP695 (Ludwig-Maximilians-University, Munich). Prucalopride (4-amino-5-chloro-2,3-dihydro-N-[1-(3-methoxypropyl)-4-piperidinyl]-7-benzofurancarboxamide monohydrochloride) was generously given by Janssen Research Foundation (Beerse, Belgium). We are grateful to Drs Rodolphe Fischmeister and Alain Gardier for the critical reading of the manuscript and ongoing support.
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