Graduate Student Literature Review: Mitochondrial response to heat stress and its implications on dairy cattle bioenergetics, metabolism, and production.

The dairy industry depends upon the cow's successful lactation for economic profitability. Heat stress compromises the economic sustainability of the dairy industry by reducing milk production and increasing the risk of metabolic and pathogenic disease. Heat stress alters metabolic adaptations, such as nutrient mobilization and partitioning, that support the energetic demands of lactation. Metabolically inflexible cows are unable to enlist the necessary homeorhetic shifts that provide the needed nutrients and energy for milk synthesis, thereby impairing lactation performance. Mitochondria provide the energetic foundation that enable a myriad of metabolically demanding processes, such as lactation. Changes in an animal's energy requirements are met at the cellular level through alterations in mitochondrial density and bioenergetic capacity. Mitochondria also act as central stress modulators and coordinate tissues' energetic responses to stress by integrating endocrine signals, through mito-nuclear communication, into the cellular stress response. In vitro heat insults affect mitochondria through a compromise in mitochondrial integrity, which is linked to a decrease in mitochondrial function. However, limited evidence exists linking the in vivo metabolic effects of heat stress with parameters of mitochondrial behavior and function in lactating animals. This review summarizes the literature describing the cellular and subcellular effects of heat stress, with a focus on the effect of heat stress on mitochondrial bioenergetics and cellular dysfunction in livestock. Implications for lactation performance and metabolic health are also discussed.

Graduate Student Literature Review: Mitochondrial adaptations across lactation and their molecular regulation in dairy cattle.

As milk production in dairy cattle continues to increase, so do the energetic and nutrient demands on the dairy cow. Difficulties making the necessary metabolic adjustments for lactation can impair lactation performance and increase the risk of metabolic disorders. The physiological adaptations to lactation involve the mammary gland and extramammary tissues that coordinately enhance the availability of precursors for milk synthesis. Changes in whole-body metabolism and nutrient partitioning are accomplished, in part, through the bioenergetic and biosynthetic capacity of the mitochondria, providing energy and diverting important substrates, such as AA and fatty acids, to the mammary gland in support of lactation. With increased oxidative capacity and ATP production, reactive oxygen species production in mitochondria may be altered. Imbalances between oxidant production and antioxidant activity can lead to oxidative damage to cellular structures and contribute to disease. Thus, mitochondria are tasked with meeting the energy needs of the cell and minimizing oxidative stress. Mitochondrial function is regulated in concert with cellular metabolism by the nucleus. With only a small number of genes present within the mitochondrial genome, many genes regulating mitochondrial function are housed in nuclear DNA. This review describes the involvement of mitochondria in coordinating tissue-specific metabolic adaptations across lactation in dairy cattle and the current state of knowledge regarding mitochondrial-nuclear signaling pathways that regulate mitochondrial proliferation and function in response to shifting cellular energy need.

Life History Trade-offs within the Context of Mitochondrial Hormesis.

Evolutionary biologists have been interested in the negative interactions among life history traits for nearly a century, but the mechanisms that would create this negative interaction remain poorly understood. One variable that has emerged as a likely link between reproductive effort and longevity is oxidative stress. Specifically, it has been proposed that reproduction generates free radicals that cause oxidative stress and, in turn, oxidative stress damages cellular components and accelerates senescence. We propose that there is limited support for the hypothesis because reactive oxygen species (ROS), the free radicals implicated in oxidative damage, are not consistently harmful. With this review, we define the hormetic response of mitochondria to ROS, termed mitochondrial hormesis, and describe how to test for a mitohormetic response. We interpret existing data using our model and propose that experimental manipulations will further improve our knowledge of this response. Finally, we postulate how the mitohormetic response curve applies to variation in animal performance and longevity.

Interspecific and intraspecific variation in proximate, mineral, and fatty acid composition of milk in Old World fruit bats (Chiroptera: Pteropodidae).

We examine the effect of body mass on milk composition among Old World fruit bats, including Pteropus pumilus (0.175 kg), Pteropus rodricensus (0.265 kg), Pteropus hypomelanus (0.571 kg), and Pteropus vampyrus (1.133 kg). We describe intra- and interspecific differences in the proximate composition of milk among these four species and the minerals and fatty acids in the milk of the latter two species. There were no differences between species in the concentrations of dry matter, fat, or lactose in milk. However, there were significant, although small, differences in the protein content of milk among species, with protein being significantly greater in P. rodricensus than in P. pumilus and P. hypomelanus and protein being significantly less in P. hypomelanus than in P. rodricensus and P. vampyrus. There were no differences in mineral content between P. hypomelanus and P. vampyrus in milk minerals, but minor differences were evident in fatty acids 12:0, 14:0, 18:0, 18:1n11, and 18:2n6. Our findings suggest that milk composition is relatively constant across lactation for most proximate, mineral, and fatty acid components. We found a significant increase in dry matter and energy across lactation in the concentration of dry matter and energy in P. pumilus and fat in P. hypomelanus. In P. hypomelanus, we found a significant increase in the concentration of fatty acids 10:0 and 20:1n9 and a significant decrease in Iso15 and 20:1n7. No other differences associated with day of lactation were found. These findings suggest that milk composition is generally similar within the genus Pteropus, despite a 6.5-fold difference in body mass between species that we evaluated.

Plasma leptin decreases during lactation in insectivorous bats.

We previously demonstrated high leptin levels during late pregnancy in little brown bats (Myotis lucifugus). We now extend these observations to a second species, the big brown bat (Eptesicus fuscus), and also report that leptin increases after the first trimester of pregnancy. Leptin decreased to baseline 1 week following parturition, with a half-time decay of 2 days. During lactation, leptin was significantly correlated with body mass in E. fuscus, but not in M. lucifugus. No circadian pattern of leptin was observed in M. lucifugus. The decrease in post-partum leptin in bats may be partly explained by loss of putative placental leptin. The continued decrease may reflect depletion of body fat during this energy demanding period, at least in Eptesicus. Changes in leptin during lactation appeared to be independent of circadian effects and time of sampling. Our study provides additional evidence that leptin increases during pregnancy and declines during lactation in a free-ranging mammal, supporting the hypothesis that leptin plays important but yet undetermined roles in reproduction.