Telling porkies: Dietary studies on cloned pigs show poor metabolome consistency

Model cloned pigs

Animals are often used as models to study human medical conditions, with rodents and primates to the fore, but pigs are an attractive alternative for some research projects due to the similarity of their nutritional and digestive systems to ours. So, studies of obesity and various metabolic syndromes might be better placed using pigs than other animals.

The advent of cloned pigs promised even better studies as it was expected that cloned animals would show fewer metabolic variations than normal pigs, allowing for easier comparisons. However, this may not be the case in practice, according to a recent joint Danish-Dutch study involving scientists from Aarhus University and the University of Copenhagen that was published in 2011.  

Using NMR spectroscopy, it confirmed that cloned and normal pigs exist as different phenotypes but added that “it cannot be concluded that the use of cloned animals will reduce the inter-individual variation in intervention studies, though this is based on a limited number of animals.”

Now, some of the Danish scientists have taken a different angle, comparing the metabolites of cloned and normal pigs feeding on restricted or free diets to see if the inter-individual variations were reduced. Kirstine Christensen, Mette Hedemann, Henry Jorgensen, Jan Stagsted and Knud Erik Knudsen from Aarhus University wanted to see if cloned pigs would be suitable for studying obesity.

Feeding differential

A total of 17 female clones from 5 surrogates and 19 female controls from 6 litters were fed the same diet for 3 months and ended up the same average weights. Thereafter, they were fed the same high-energy diet based on wheat. Approximately half of each group were allowed to eat freely while the remainder were fed 60% of the total feed that the first group consumed.

Blood samples were taken regularly and several standard metabolites such as glucose, lactate, triglycerides, cholesterol and low- and high-density lipoproteins were measured. The metabolomes were studied by LC-tandem MS with electrospray ionisation in positive and negative modes, to reveal 850 and 469 metabolites, respectively.

The metabolites were identified from their accurate masses and fragmentation patterns by searching several standard databases. Their abundances were compared using multivariate analysis to identify those ions of metabolites that differed between the cloned and normal pigs.

Cloned pigs still display metabolite variations

Both the cloned pigs and normal animals that were fed on the unrestricted diet displayed differences to those on the restricted diet but their responses were not the same. For instance, the cloned pigs had reduced energy intake and lower weight gain than the normal pigs.

The freely fed cloned pigs had raised levels of plasma lactate compared to the normal animals and several explanations were offered. One of these involved an increased anaerobic metabolism which would be accompanied by reduced levels of creatinine, which were observed in ptractice.

The cloned animals also had levels of insulin-like growth factor 1 (IGF-1) that were about half those of the normal pigs, which might explain their reduced weight gain. However, the cloned pigs on the restricted diet did not have reduced IGF-1.

Changes in the levels of tryptophan and 4-sulphobenzyl alcohol in the freely fed cloned pigs point to changes in the microbiota of the gut and/or changes in metabolism. Two other amino acids, isoleucine and valine, were increased and decreased, respectively, in the clones on a restricted diet, although reasons for this were not forthcoming.

So, there were a number of differences between the metabolomes of the cloned and normal pigs when fed either diet but there did not appear to be fewer variations in the metabolite abundances for the cloned pigs compared with the normal pigs, confirming the earlier NMR study.

Nevertheless, the cloned animals might be useful for studying particular disorders of the metabolism but more metabolomes need to be evaluated more closely if the clones are to be used for the study of obesity, say the researchers.