Cholinergic Receptor Knockout Mice

Excerpt

The cholinergic system’s essential involvement in both preclinical and clinical aspects of cognition processes has been proposed and reviewed extensively elsewhere. Terry and Buccafusco [1] provide the latest review on this subject. According to this review, all currently approved FDA drugs for the treatment of Alzheimer’s disease are cholinesterase inhibitors, which exert their efficacy apparently through stimulation of both muscarinic acetylcholine receptors (mAChRs) and nicotinic acetylcholine receptors (nAChRs).

Gene targeting (knockout) technologies allow us to replace the gene of interest with one that is inactive, altered, or irrelevant [2]. In the case of a completed deletion, a gene is knocked out in vivo, and a mutant organism with a deficit in the gene product is generated. The lack of suitable embryonic stem (ES) cell lines prevents the application of these technologies into rats [3]. However, mice and humans are both mammals, and both species contain a similar number of genes that show a high degree of similarity [4,5]. Therefore, in rodents, mouse is a better species for employing the knockout approach compared with rat. In theory, knockout (KO) mice contain a targeted gene that is deleted; therefore, no product of the mutated gene is synthesized in the null mutants [6]. These mutant mouse lines, which have inactivating mutations of the individual genes, can be studied in a battery of physiological, pharmacological, behavioral, biochemical, and neurochemical tests to confirm or invalidate a hypothesis on the effects of specific proteins in the function of the brain.

Müller [7] provided an excellent review article regarding targeted mouse mutants from vector design to phenotype analysis. The cited article provides detailed technical information on how to generate KO mice, including selection markers and screening strategies, potential problems and pitfalls, as well as construct design. Moreover, Bolivar et al. [8] have summarized behavioral profiles in all available knockout mice. They also provide updated information on available KO mice through their Web site, which is identified in their article. Additional information about Internet resources regarding transgenic rodent production is provided by Wells and Carter [3].

Because gene-targeting techniques enable us to analyze diverse aspects of gene function in whole animals, measurable phenotypes relevant to human pathology could be obtained in a good mouse model of human disease. However, for most diseases in the central nervous system (CNS), the coexistence of malfunctions in multiple subtypes of the same receptor or in multiple neurotransmitters typically contribute to a complex phenotype such as cognition. For example, Buccafusco and Terry [9] demonstrate that multiple CNS targets are needed to elicit beneficial effects on memory and cognition. Therefore, generating multiple mouse mutants that mimic all facets of a multifactorial disease would help us to address individual subsets of symptoms of the disease.

In short, knockout technologies provide a powerful tool to reveal and refine treatment strategies for human disorders by building bridges between genetics and the pathogenesis of disease. Behavioral phenotypes discovered in the mutant can be used to evaluate (screen) the efficacy of potential new pharmacological therapies. For example, by evaluating the results of specific behavioral tests — including learning and memory tests in mAChR, nAChR, and acetylcholinesterase (AChE) KO mice — our knowledge of cognitional impairment would be broadened. This, in turn, will improve our ability to identify potential new drug therapies for the treatment of cognitional aspects of such diseases as Alzheimer’s disease, attention deficit hyperactivity disorder (ADHD), and schizophrenia.

This chapter focuses on:

1. Available cholinergic receptor KO mouse models, including muscarinic acetylcholine receptor (mAChR) KO mice, nicotinic acetylcholine receptor (nAChR) KO mice, and acetylcholinesterase (AChE) KO mice

2. Cognitional-related data for mAChR KO mice

3. Limitations on KO mouse models and future directions