mVps34 is activated following high-resistance contractions

Following resistance exercise in the fasted state, both protein synthesis and degradation in skeletal muscle are increased. The addition of essential amino acids potentiates the synthetic response suggesting that an amino acid sensor, which is involved in both synthesis and degradation, may be activated by resistance exercise. One such candidate protein is the class 3 phosphatidylinositol 3OH-kinase (PI3K) Vps34. To determine whether mammalian Vps34 (mVps34) is modulated by high-resistance contractions, mVps34 and S6K1 (an index of mTORC1) activity were measured in the distal hindlimb muscles of rats 0.5, 3, 6 and 18 h after acute unilateral high-resistance contractions with the contralateral muscles serving as a control. In the lengthening tibialis anterior (TA) muscle, S6K1 (0.5 h = 366.3 +/- 112.08%, 3 h = 124.7 +/- 15.96% and 6 h = 129.2 +/- 0%) and mVps34 (3 h = 68.8 +/- 15.1% and 6 h = 36.0 +/- 8.79%) activity both increased, whereas in the shortening soleus and plantaris (PLN) muscles the increase was significantly lower (PLN S6K1 0.5 h = 33.1 +/- 2.29% and 3 h = 47.0 +/- 6.65%; mVps34 3 h = 24.5 +/- 7.92%). HPLC analysis of the TA demonstrated a 25% increase in intramuscular leucine concentration in rats 1.5 h after exercise. A similar level of leucine added to C2C12 cells in vitro increased mVps34 activity 3.2-fold. These data suggest that, following high-resistance contractions, mVps34 activity is stimulated by an influx of essential amino acids such as leucine and this may prolong mTORC1 signalling and contribute to muscle hypertrophy.

Melanism and disease resistance in insects

There is growing evidence that insects in high-density populations invest relatively more in pathogen resistance than those in low-density populations (i.e. density-dependent prophylaxis). Such increases in resistance are often accompanied by cuticular melanism, which is characteristic of the high-density form of many phase polyphenic insects. Both melanism and pathogen resistance involve the prophenoloxidase enzyme system. In this paper the link between resistance, melanism and phenoloxidase activity is examined in Spodoptera lanae. In S. exempta, cuticular melanism was positively correlated with phenoloxidase activity in the cuticle, haemolymph and midgut. Melanic S. exempta larvae were found to melanize a greater proportion of eggs of the ectoparasitoid Euplectrus laphygmae than non-melanic larvae, and melanic S. littoralis were more resistant to the entomopathogenic fungus Beauveria bassiana (in S. exempta the association between melanism and fungal resistance was non-signficant). These results strengthen the link between melanism and disease resistance and implicate the involvement of phenoloxidase.

Can the protein costs of bacterial resistance be offset by altered feeding behaviour?

1. Mounting an immune response is likely to be costly in terms of energy and nutrients, and so it is predicted that dietary intake should change in response to infection to offset these costs. The present study focuses on the interactions between a specialist grass-feeding caterpillar species, the African armyworm Spodoptera exempta, and an opportunist bacterium, Bacillus subtilis.
2. The main aims of the study were (i) to establish the macronutrient costs to the insect host of surviving a systemic bacterial infection, (ii) to determine the relative importance of dietary protein and carbohydrate to immune system functions, and (iii) to determine whether there is an adaptive change in the host's normal feeding behaviour in response to bacterial challenge, such that the nutritional costs of resisting infection are offset.
3. We show that the survival of bacterially infected larvae increased with increasing dietary protein-to-carbohydrate (P:C) ratio, suggesting a protein cost associated with bacterial resistance. As dietary protein levels increased, there was an increase in antibacterial activity, phenoloxidase (PO) activity and protein levels in the haemolymph, providing a potential source for this protein cost. However, there was also evidence for a physiological trade-off between antibacterial activity and phenoloxidase activity, as larvae whose antibacterial activity levels were elevated in response to immune activation had reduced PO activity.
4. When given a choice between two diets varying in their P:C ratios, larvae injected with a sub-lethal dose of bacteria increased their protein intake relative to control larvae whilst maintaining similar carbohydrate intake levels. These results are consistent with the notion that S. exempta larvae alter their feeding behaviour in response to bacterial infection in a manner that is likely to enhance the levels of protein available for producing the immune system components and other factors required to resist bacterial infections (‘self-medication’).

Costs of resistance: genetic correlations and potential trade-offs in an insect immune system

Theory predicts that natural selection will erode additive genetic variation in fitness-related traits. However, numerous studies have found considerable heritable variation in traits related to immune function, which should be closely linked to fitness. This could be due to trade-offs maintaining variation in these traits. We used the Egyptian cotton leafworm, Spodoptera littoralis, as a model system to examine the quantitative genetics of insect immune function. We estimated the heritabilities of several different measures of innate immunity and the genetic correlations between these immune traits and a number of life history traits. Our results provide the first evidence for a potential genetic trade-off within the insect immune system, with antibacterial activity (lysozyme-like) exhibiting a significant negative genetic correlation with haemocyte density, which itself is positively genetically correlated with both haemolymph phenoloxidase activity and cuticular melanization. We speculate on a potential trade-off between defence against parasites and predators, mediated by larval colour, and its role in maintaining genetic variation in traits under natural selection.