animal models – Drug Discovery Opinion

Mice Show Pain, Just As We Do

mouse and man — Image: Flickr - bjormeansbear

We are used to the idea that facial expressions can tell us a lot about what our fellow humans are experiencing and researchers in Canada and the Netherlands have now found that the same is true for mice. In human volunteers, grimacing has been shown to match self-reported pain intensity and facial expression is also used to evaluate pain in patients who are unable to communicate orally.

Existing pain remedies do not work well for all patients and, in the search for more effective pain relief, mice and other rodents are used to develop new treatments. The present study has shown that mice, like humans, express pain through facial expressions and should provide a more accurate way to ensure that laboratory animals do not suffer unnecessarily and could also lead to better drugs for humans. The team have developed a ‘Mouse Grimace Scale’ by analysing high-resolution video images of mice before and during moderately painful stimuli – pain that is suggested to be comparable to a headache or that associated with an inflamed and swollen finger. The researchers, who are leaders in developing standards for facial expression to assess pain in human infants and others unable to communicate verbally, have proposed that five facial features in the mice should be scored. Several of these criteria – which are orbital tightening (eye closing), nose and cheek bulges, and ear and whisker positions – are similar to those used to assess pain in humans.

The study, which is the first to propose such a standardised and accurate scoring system linking facial expression to pain in animals, is published in the journal Nature Methods. Based on facial expression, trained researchers were able to correctly and reliably assess pain levels in real time. Previously, assessment of pain was limited to measuring withdrawal responses to pressure and heat and the ‘Mouse Grimace Scale’ may provide a more accurate way of predicting whether new treatments will eventually work in humans as well as reducing unnecessary pain in laboratory animals.

Vive la Différence

A year ago, when researchers at Purdue University argued that environmental standardisation in laboratory experiments involving mice was likely to lead to more, rather than less, variation between different laboratories, they met with some resistance since it was not clear what factors should be varied to improve reproducibility. Following an analysis of data from behavioural tests commonly used in drug discovery studies, they have now shown that introducing only two controlled environmental variables to preclinical studies using mice can greatly reduce false positives and the number of animals needed for testing. The tests, which compared behaviours between two inbred strains of mice, were repeated in four different model laboratories that varied in details such as background noise, the age of the mice, environmental enrichment, familiarity with handler, lighting levels and cage size. In each laboratory, one group of mice (standardised) were treated identically whilst the other group (heterogenised) were tested under four different sets of conditions produced by varying two environmental factors in a controlled manner. Mice of the same strain would have been expected to show the same behaviours in each laboratory but, in 33 out of 36 behavioural characteristics such as fear and curiosity, the standardised group showed as much as five times more variation between laboratories compared with the heterogenised group.

When conditions are highly standardised, the variation in data produced within a particular laboratory will be very low, but variations between laboratories will be large and unpredictable. The researchers believe that tests in mice using a heterogeneous test design more closely resemble human clinical trials and should reduce both the number of animals needed for preclinical studies and the number of false positives. A reduction in false positives could have important implications for reducing the number of compounds that fail in expensive downstream clinical trials.

The study is published in Nature Methods.

Mouse Model of Liver Disease

Although very different at a molecular level, hepatitis viruses B and C (HBV and HCV) both infect only humans and chimpanzees which means that there is a lack of suitable small animal models for studying viral lifecycles and for testing new drugs. One alternative would be to study the viruses and test new compounds in liver cells grown in vitro, but human hepatocytes are very difficult to grow and maintain in culture.

A team of researchers led by scientists at the Salk Institute has now provided a solution to the problem by generating a mouse with a liver that is almost completely ‘humanised’. The team had previously generated a mouse with a partially humanised liver but wanted to achieve more complete transformation. Around 95% of the liver cells of the new mice are human in origin and the animals are susceptible to infection by both HBV and HCV. Mice infected with HCV were shown to respond to drugs such as pegylated interferon α2a and ribavirin that are used to treat human patients. Adefovir dipivoxil, used to treat HBV patients, was found to lower viral titres in mice infected with HBV.

The mice were generated by using genetic and pharmacological pressures to lead to a growth disadvantage for mouse hepatocytes and positive selection for transplanted human hepatocytes. The mice provide a new way to study pathogens that target the human liver and to test drugs to treat human hepatitis. In the future, the mice could also be used to study other hepatotrophic pathogens such as malaria, as well as cirrhosis and liver cancer.

The study is published in the Journal of Clinical Investigation.

Mice Not a Good Model for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD), the most prevalent of more than twenty types of genetic disorder that result in progressive muscle weakness, is caused by a mutation in the gene that encodes the protein, dystrophin. Dystrophin forms one subunit of a glycoprotein complex that acts as a structural scaffold linking the extracellular matrix to the intracellular cytoplasm. Without dystrophin, the muscle fibre membranes become damaged by the large mechanical forces experienced by contractile tissues and the muscle fibres eventually die. Since the gene for dystrophin is on the X chromosome, only boys are affected and, although the disorder is predominantly one of muscle degeneration, some boys also have learning or behavioural difficulties.

Mice which have a loss-of-function mutation in the dystrophin gene (mdx mice) are used as a model for human DMD but new research led by scientists at King’s College London and funded by the Muscular Dystrophy Campaign suggests that these animals may not be suitable for studying the neurological effects of human DMD. The study looked at the genes for other proteins that make up the dystrophin glycoprotein complex and found significant differences between mice and humans. The main heterodimeric partner of dystrophin at the heart of the glycoprotein complex is α-dystrobrevin, which can exist in a substantial number of isoforms as a result of complex transcriptional and post-transcriptional regulation. The different isoforms of α-dystrobrevin influence the recruitment of other proteins into the dystrophin glycoprotein complex, leading to variations in structure and function. The researchers found that mouse, rat and hamster brains have fewer than half the number of α-dystrobrevin isoforms found in the brains of most other mammals (including humans), suggesting that there are likely to be fundamental differences between the dystrophin glycoprotein complexes of mice and humans, and calling into question the current use of mice to model neurological aspects of human DMD. Guinea pigs appear to be more similar to humans in terms of α-dystrobrevin isoforms and may provide a more suitable model.

The study is published in BMC Biology.