The membrane form of acetylcholinesterase from rat brain contains a 20 kDa hydrophobic anchor

Cholinesterase inhibitors: a patent review (2007 - 2011)

INTRODUCTION

Cholinesterase inhibitors participate in the maintenance of the levels of the neurotransmitter acetylcholine by inhibiting the enzymes implicated in its degradation, namely, butyrylcholinesterase and acetylcholinesterase. This pharmacological action has an important role in several diseases, including neurodegenerative diseases such as Alzheimer's.

AREAS COVERED

This article reviews recent advances in the development of cholinesterase enzyme inhibitors, covering the development of new chemical entities, new pharmaceutical formulations with known inhibitors or treatments in combination with other drug families.

EXPERT OPINION

The development of cholinesterase inhibitors has to face several issues, including the fact that the principal indication for these drugs, Alzheimer's disease, is not currently believed to derivate from a cholinergic deficiency, although most of the drugs clinically used for these disease are cholinesterase inhibitors. Moreover, the adverse effects found when administering cholinesterase inhibitors limit their use in other diseases, such as gastrointestinal diseases, glaucoma, or analgesia.

Assembly of acetylcholinesterase tetramers by peptidic motifs from the proline-rich membrane anchor, PRiMA: competition between degradation and secretion pathways of heteromeric complexes

The membrane-bound form of acetylcholinesterase (AChE) constitutes the major component of this enzyme in the mammalian brain. These molecules are hetero-oligomers, composed of four AChE catalytic subunits of type T (AChE(T)), associated with a transmembrane protein of type 1, called PRiMA (proline-rich membrane anchor). PRiMA consists of a signal peptide, an extracellular domain that contains a proline-rich motif (14 prolines with an intervening leucine, P4LP10), a transmembrane domain, and a cytoplasmic domain. Expression of AChE(T) subunits in transfected COS cells with a truncated PRiMA, without its transmembrane and cytoplasmic domains (P(stp54) mutant), produced secreted heteromeric complexes (T4-P(stp54)), instead of membrane-bound tetramers. In this study, we used a series of deletions and point mutations to analyze the interaction between the extracellular domain of PRiMA and AChE(T) subunits. We confirmed the importance of the polyproline stretches and defined a peptidic motif (RP4LP10RL), which induces the assembly and secretion of a heteromeric complex with four AChE(T) subunits, nearly as efficiently as the entire extracellular domain of PRiMA. It is noteworthy that deletion of the N-terminal segment preceding the prolines had little effect. Interestingly, short PRiMA mutants, truncated within the proline-rich motif, reduced both cellular and secreted AChE activity, suggesting that their interaction with AChE(T) subunits induces their intracellular degradation.

Development and application of diazirines in biological and synthetic macromolecular systems

Many different reagents and methodologies have been utilised for the modification of synthetic and biological macromolecular systems. In addition, an area of intense research at present is the construction of hybrid biosynthetic polymers, comprised of biologically active species immobilised or complexed with synthetic polymers. One of the most useful and widely applicable techniques available for functionalisation of macromolecular systems involves indiscriminate carbene insertion processes. The highly reactive and non-specific nature of carbenes has enabled a multitude of macromolecular structures to be functionalised without the need for specialised reagents or additives. The use of diazirines as stable carbene precursors has increased dramatically over the past twenty years and these reagents are fast becoming the most popular photophors for photoaffinity labelling and biological applications in which covalent modification of macromolecular structures is the basis to understanding structure-activity relationships. This review reports the synthesis and application of a diverse range of diazirines in macromolecular systems.

Design, synthesis, and pharmacological profile of novel fused pyrazolo[4,3-d]pyridine and pyrazolo[3,4-b][1,8]naphthyridine isosteres: a new class of potent and selective acetylcholinesterase inhibitors

A new family of tacrine (THA) analogues (7-9, 12), containing the azaheterocyclic pyrazolo[4,3-d]pyridine or pyrazolo[3,4-b][1,8]naphthyridine systems as isosteres of the quinoline ring of THA, has been synthesized. The compounds were tested in rat brain cholinesterases using Ellman's method, and all were fully efficacious in inhibiting the enzymes. Compounds 9 and 12b were the most potent against acetylcholinesterase (AChE), showing IC(50) of 6.0 and 6.4 microM, and were less active against rat brain butyrylcholinesterase, showing selectivity indexes of 5.3 and 20.9, respectively. Compounds 7-9 and 12 were also tested for their acute neurotoxicity in vitro, using cultured rat cortical cells. Compounds 7 and 8 were not significantly toxic; 9 was toxic at 500 microM, but not at 100 microM. The naphthyridine derivatives 12a and 12b showed a significant concentration-dependent neurotoxicity, being able to kill most cells at 500 microM. Molecular dynamic simulation using the X-ray crystal structure of AChE from Torpedo californica was used to explain the possible binding mode of these new THA isosteres.

A four-to-one association between peptide motifs: four C-terminal domains from cholinesterase assemble with one proline-rich attachment domain (PRAD) in the secretory pathway

The major type of acetylcholinesterase in vertebrates (AChET) is characterized by the presence of a short C-terminal domain of 40 residues, the 'tryptophan amphiphilic tetramerization' (WAT) domain. The presence of this domain is not necessary for catalytic activity but is responsible for hydrophobic interactions and for the capacity of AChET subunits to form quaternary associations with anchoring proteins, thereby conditioning their functional localization. In the collagen tail of asymmetric forms, we characterized a small conserved region that is sufficient for binding an AChET tetramer, the proline-rich attachment domain (PRAD). We show that the WAT domain alone is sufficient for association with the PRAD, and that it can attach foreign proteins (alkaline phosphatase, GFP) to a PRAD-containing construct with a glycophosphatidylinositol anchor (GPI), and thus anchor them to the cell surface. Furthermore, we show that isolated WAT domains, or proteins containing a WAT domain, can replace individual AChET subunits in PRAD-linked tetramers. This suggests that the four WAT domains interact with the PRAD in a similar manner. These quaternary interactions can form without intercatenary disulfide bonds. The common catalytic domains of AChE are not necessary for tetrameric assembly, although they may contribute to the stability of the tetramer.

Acetylcholinesterase: C-terminal domains, molecular forms and functional localization

Acetylcholinesterase (AChE) possesses short C-terminal peptides that are not necessary for catalytic activity. These peptides belong to different classes (R, H, T, S) and define the post-translational processing and targeting of the enzyme. In vertebrates, subunits of type H (AChEH) and of type T (AChET) are the most important: AChEH subunits produce glycolipid (GPI)-anchored dimers and AChET subunits produce hetero-oligomeric forms such as membrane-bound tetramers in the mammalian brain (containing a 20 kDa hydrophobic protein) and asymmetric collagen-tailed forms in neuromuscular junctions (containing a specific collagen, ColQ). The T peptide allows the formation of tetrameric assemblies with a proline-rich attachment domain (PRAD) of collagen ColQ. These complex molecular structures condition the functional localization of the enzyme in the supramolecular architecture of cholinergic synapses.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Regulation of acetylcholinesterase expression during neuronal differentiation

We have examined the developmental expression of acetylcholinesterase (AChE) during the process of neuronal differentiation from a pluripotent stem cell. P19 embryonic carcinoma cells form embryoid bodies, which, when cultured with retinoic acid, are induced to differentiate into neurons and glia. No AChE activity is present in the undifferentiated stem cells, and mRNA protection analyses do not detect AChE mRNA. Commitment to a neuronal differentiation pathway results in increased levels of AChE mRNA, production of a tetrameric form of the enzyme, and secretion of AChE into the culture medium. Concomitant with subsequent morphological differentiation into neurons, enzyme secretion diminishes and AChE becomes largely tethered to the neuronal cell membranes. The enzyme is attached to the cell surface as a globular tetramer. Its hydrodynamic properties are consistent with association through a noncatalytic hydrophobic subunit rather than anchorage by a glycophospholipid tail. No change in the rate of transcription of the Ache gene was detected during the course of differentiation, suggesting that the gene is actively transcribed at very early stages of development. Results suggest that stabilization of a labile mRNA governs the increase in AChE mRNA and gene product. The studies presented indicate that an early event in neuronal differentiation is the stabilization of the mRNA leading to expression of a secreted form of AChE. A subsequent step associated with neurite outgrowth results in a transition from secretion of the tetrameric enzyme to its localization on the cell membrane.