Genetically modified mice in neuropharmacology

Current Genetic Techniques in Neural Circuit Control of Feeding and Energy Metabolism

The current epidemic of obesity and its associated metabolic syndromes imposes unprecedented challenges to our society. Despite intensive research focus on obesity pathogenesis, an effective therapeutic strategy to treat and cure obesity is still lacking. The obesity development is due to a disturbed homeostatic control of feeding and energy expenditure, both of which are controlled by an intricate neural network in the brain. Given the inherent complexity of brain networks in controlling feeding and energy expenditure, the understanding of brain-based pathophysiology for obesity development is limited. One key limiting factor in dissecting neural pathways for feeding and energy expenditure is unavailability of techniques that can be used to effectively reduce the complexity of the brain network to a tractable paradigm, based on which a strong hypothesis can be tested. Excitingly, emerging techniques have been involved to be able to link specific groups of neurons and neural pathways to behaviors (i.e., feeding and energy expenditure). In this chapter, novel techniques especially those based on animal models and viral vector approaches will be discussed. We hope that this chapter will provide readers with a basis that can help to understand the literatures using these techniques and with a guide to apply these exciting techniques to investigate brain mechanisms underlying feeding and energy expenditure.

Choroid plexus transport: gene deletion studies

This review examines the use of transporter knockout (KO) animals to evaluate transporter function at the choroid plexus (the blood-CSF barrier; BCSFB). Compared to the blood-brain barrier, there have been few such studies on choroid plexus (CP) function. These have primarily focused on Pept2 (an oligopeptide transporter), ATP-binding cassette (ABC) transporters, Oat3 (an organic anion transporter), Svct2 (an ascorbic acid transporter), transthyretin, ion transporters, and ion and water channels. This review focuses on the knowledge gained from such studies, both with respect to specific transporters and in general to the role of the CP and its impact on brain parenchyma. It also discusses the pros and cons of using KO animals in such studies and the technical approaches that can be used.

Recent advances in the manipulation of murine gene expression and its utility for the study of human neurological disease

Transgenic mouse models have vastly contributed to our knowledge of the genetic and molecular pathways underlying the pathogenesis of neurological disorders that affect millions of people worldwide. Not only have they allowed the generation of disease models mimicking the human pathological state but they have also permitted the exploration of the pathological role of specific genes through the generation of knock-out and knock-in models. Classical constitutive transgenic mice have several limitations however, due to behavioral adaptation process occurring and conditional mouse models are time-consuming and often lack extensive spatial or temporal control of gene manipulation. These limitations could be overcome by means of innovative methods that are now available such as RNAi, viral vectors and large cloning DNA vectors. These tools have been extensively used for the generation of mouse models and are characterized by the superior control of transgene expression that has been proven invaluable in the assessment of novel treatments for neurological diseases and to further investigate the molecular processes underlying the etiopathology of neurological disorders. Furthermore, in association with classical transgenic mouse models, they have allowed the validation of innovative therapeutic strategies for the treatment of human neurological disorders. This review describes how these tools have overcome the limitations of classical transgenic mouse models and how they have been of value for the study of human neurological diseases.

Event-related oscillations in mice: effects of stimulus characteristics

Event-related oscillations (EROs) are rhythmic changes that are evoked by sensory and/or cognitive processes that influence the dynamics of the EEG. EROs are estimated by a decomposition of the EEG signal into phase and magnitude information for a range of frequencies and then changes in those frequencies are characterized over a millisecond time scale with respect to task events. EROs have been demonstrated to be sensitive measures of both normal and abnormal cognitive functioning in humans but have not been fully described in mice. The results of these studies demonstrate that EROs can be generated in cortical sites in mice in the delta, theta, alpha/beta frequency ranges in response to auditory stimuli. Oscillations in the 7.5-40 Hz frequencies were significantly affected in the 0-50 ms time range in response to differences in tone frequency. Whereas, changes in tone loudness produced changes in oscillations in the 7.5-40 Hz frequencies in the 350-800 ms range. No significant changes in EROs were found to differences in tone probability. These studies suggest that EROs are an electrophysiological assay sensitive to tone characteristics and as such may be suitable for the exploration of the effects of genetic or neuropharmacological manipulations on neurosensory processing in mice.

Conditional gene targeting in the mouse nervous system: Insights into brain function and diseases

Conditional gene knockout represents an extremely powerful approach to study the function of single genes in the nervous system. The Cre-LoxP system is the most advanced technology for spatial and temporal control of genetic inactivation, and there is rapid progress using this methodology in neuroscience research. In this approach, mice with LoxP sites flanking the gene of interest (floxed mice) are bred with transgenic mice expressing Cre recombinase under the control of a selected promoter (Cre mice). This promoter is critical in that it determines the time and site of Cre expression. Cre enzyme, in turn, recombines the floxed gene and produces gene knockout. Here we review Cre mouse lines that have been developed to target either the entire brain, selected brain areas, or specific neuronal populations. We then summarize phenotypic consequences of conditional gene targeting in the brain for more than 40 genes, as reported to date. For many broadly expressed genes, brain-restricted knockout has overcome lethality of conventional knockout (KO) and has highlighted a specific role of the encoded protein in some aspect of brain function. In the case of neural genes, data from null mutants in specific brain sites or neurons has refined our understanding of the role of individual molecules that regulate complex behaviors or synaptic plasticity within neural circuits. Among the many developing functional genomic approaches, conditional gene targeting in the mouse has become an excellent tool to elucidate the function of the approximately 5000 known or unknown genes that operate in the nervous system.

Brain lipid metabolism in the cPLA2 knockout mouse

We examined brain phospholipid metabolism in mice in which the cytosolic phospholipase A(2) (cPLA(2,) Type IV, 85 kDa) was knocked out (cPLA(2)(-/-) mice). Compared with controls, these mice demonstrated altered brain concentrations of several phospholipids, reduced esterified linoleate, arachidonate, and docosahexaenoate in choline glycerophospholipid, and reduced esterified arachidonate in phosphatidylinositol. Unanesthetized cPLA(2)(-/-) mice had reduced rates of incorporation of unlabeled arachidonate from plasma and from the brain arachidonoyl-CoA pool into ethanolamine glycerophospholipid and choline glycerophospholipid, but elevated rates into phosphatidylinositol. These differences corresponded to altered turnover and metabolic loss of esterified brain arachidonate. These results suggests that cPLA(2) is necessary to maintain normal brain concentrations of phospholipids and of their esterified polyunsaturated fatty acids. Reduced esterified arachidonate and docosahexaenoate may account for the resistance of the cPLA(2)(-/-) mouse to middle cerebral artery occlusion, and should influence membrane fluidity, neuroinflammation, signal transduction, and other brain processes.

Long latency event-related potentials in mice: effects of stimulus characteristics and strain

The P3, or P300 component of the event-related potentials (ERPs) is a positive going waveform that can be averaged from the EEG approximately 250-500 ms following the presentation of task or context 'relevant' stimuli. This potential has been demonstrated to be a sensitive measure of both normal and abnormal cognitive functioning. P300 models have been developed in monkeys, cats and rats. The aim of the present study was to develop an auditory ERP paradigm suitable for use in mice that resembled those used in humans and other animal models. The results of the studies showed that late positive potentials in the 200-400 ms range could be generated in cortical sites in response to auditory stimuli. Additionally, like passive ERPs recorded in humans, mouse ERPs were sensitive to changes in stimulus characteristics. An earlier negative component designated the N1 was found to be sensitive to tone frequency and loudness but not to stimulus probability, whereas the mouse P300 component was sensitive to probability but not to tone frequency or loudness. C57BL/6 mice, a strain known to have a strong alcohol preference, were found to have significantly lower P300 amplitudes when compared to the DBA/2 strain. These findings also parallel human studies, and studies of selected lines of rats, demonstrating that decrements in P300 amplitude can be associated with a genetic vulnerability to alcoholism/alcohol preference. These studies further suggest that ERPs are an electrophysiological assay suitable for the exploration of the effects of genetic manipulations on neurosensory processing in mice.

Dual-modality imaging of function and physiology

Dual-modality imaging is a technique in which computed tomography (CT) or magnetic resonance imaging is combined with positron emission tomography or single-photon emission CT to acquire structural and functional images with an integated system. The data are acquired in a single procedure; the patient remains on the scanner table while undergoing both x-ray and radionuclide studies to facilitate correlation between the structural and functional images. The resulting data can aid in localization, enabling more specific diagnosis than can be obtained with a conventional imaging study. In addition, the anatomic information can be used to compensate the correlated radionuclide data for physical perturbations such as photon attenuation, scatter radiation, and partial volume errors. Thus, dual-modality imaging provides a priori information that can improve both the visual quality and the quantitative accuracy of the radionuclide images. Dual-modality imaging systems are also being developed for biologic research involving small animals. Small-animal dual-modality systems offer advantages for measurements that currently are performed invasively with autoradiography and tissue sampling. By acquiring data noninvasively, dual-modality imaging permits serial studies in a single animal, enables measurements to be performed with fewer animals, and improves the statistical quality of the data.

The ons and offs of inducible transgenic technology: a review

Classical transgenic and gene-targeted mouse mutants are powerful model systems in which to study the pathogenesis of neurodegenerative diseases. However, a number of issues of fundamental importance to neurodegenerative research cannot be addressed using classical techniques. These include identification of the earliest events in disease pathogenesis and a determination of whether a particular pathogenic protein produces a inexorable or a reversible disease process. Both of these issues have profound implications for the rational development of new therapies. To address these questions, genetic techniques that allow pathogenic proteins to be expressed or knocked out with temporal and regional specificity have been developed. We have reviewed these systems, highlighting the tetracycline-regulated system because of its demonstrated utility in mice and its reversibility. These regulatable systems are a new and powerful tool for the neurobiologist and allow one to address a new set of important questions in an in vivo setting.

Use of epitope tags for routine analysis of transgene expression

Peptide and RNA epitope tags as tools for routine analysis of transgene expression and protein accumulation in transformed plant cell cultures was evaluated using three genes that encode very structurally and functionally different proteins. A T7 peptide was introduced at the amino- and carboxyl-termini of phosphinothricin-N-acetyl transferase and avidin and at the carboxyl-terminus of galactose oxidase. An RNA sequence that forms a higher order structure that is recognized by antibodies raised against the FLAG peptide was separately introduced into the 3' nontranslated region of these genes. Constructs were introduced into maize cell cultures using particle bombardment and transgene expression, protein accumulation, protein function and presence of the tags in RNA and/or protein as appropriate were evaluated in up to approximately 25 culture lines per construct. Results indicate that, while there will likely always be a need for some empirical evaluation of any tag-protein combination, introduction of the peptide tag at the amino-terminus was generally more successful than was incorporation at the carboxyl-terminus. RNA tags show promise for this purpose, but routine application will require development of a very sensitive immunoassay.

Of mice and men: solving the molecular mysteries of Huntington's disease

Recent advances in the manipulation of mouse embryos provide opportunities for the disciplines of neuroscience and molecular genetics to join forces and tackle some previously intractable questions in this area of research. Even Huntington's disease has started to yield clues to its complex pathophysiology as a result of the recent application of transgenic technologies. This short review, while necessarily providing some background clinical information on Huntington's disease, will focus on how modifications of the mouse genome have contributed, and are continuing to contribute, to our understanding of the complex disease process. Such new insights may well turn the hope of developing the first effective treatment for this devastating disease into reality.