Overwintering in North American domesticated honeybees (Apis mellifera) causes mitochondrial reprogramming while enhancing cellular immunity

Abstract

Many factors negatively impact domesticated honeybee (Apis mellifera) health causing a global decrease in their population year after year with major losses occurring during winter, and the cause remains thus far unknown. Here, we monitored for 12 months North American colonies of honeybees enduring important temperature variations throughout the year, to assess the metabolism and immune system of honeybees of summer and winter individuals. Our results show that in flight muscle, mitochondrial respiration via complex I during winter is drastically reduced compared to summer. However, the capacity for succinate and glycerol-3-phosphate (G3P) oxidation by mitochondria is increased during winter, resulting in higher mitochondrial oxygen consumption when complex I substrates, succinate and G3P were assessed altogether. Pyruvate kinase, lactate dehydrogenase, aspartate aminotransferase, citrate synthase and malate dehydrogenase tend to have reduced activity levels in winter unlike hexokinase, NADH dehydrogenase and pyruvate dehydrogenase. Transcript abundance of highly important immunity proteins like Vitellogenin and Defensin-1 were also increased in winter bees, and a stronger phagocytic response as well as a better hemocyte viability was observed during winter. Thus, a reorganization of substrate utilization favoring succinate and G3P while negatively affecting complex I of the ETS is occurring during winter. We suggest that this might be due to complex I transitioning to a dormant conformation through post-translational modification. Winter bees also have an increased response for antibacterial elimination in honeybees. Overall, this study highlights previously unknown cellular mechanisms between summer and winter honeybees that further our knowledge about this important species.

Succinate oxidation rescues mitochondrial ATP synthesis at high temperature in Drosophila melanogaster

“…The huge positive impact of G3P oxidation upon mitochondrial respiration reinforces the view that G3P might be a thermogenic substrate for endothermic insects [28,29]. However, due to the low surface‐to‐volume ratio of Drosophila , the thermogenic role of G3P is highly unlikely in these small insects, as the heat produced will be immediately dissipated.…”

Activation of innate immunity selectively compromises mitochondrial complex I, proline oxidation and flight activity in the major arbovirus vectorAedes aegypti

“…From the evolutive perspective, functional associations between complex I and immunity were reported for invertebrate organisms (Chikka et al, 2016; Cormier et al, 2022; Mello et al, 2022). In this sense, specific complex I-driven OXPHOS associates with increasing defensin expression in honeybees during winter (Cormier et al, 2022).…”

“…From the evolutive perspective, functional associations between complex I and immunity were reported for invertebrate organisms (Chikka et al, 2016; Cormier et al, 2022; Mello et al, 2022). In this sense, specific complex I-driven OXPHOS associates with increasing defensin expression in honeybees during winter (Cormier et al, 2022). Remarkably, a shift towards G3P and succinate oxidation was observed suggesting that functional suppression complex I-dependent OXPHOS is linked to immune activation (Cormier et al, 2022).…”

“…In this sense, specific complex I-driven OXPHOS associates with increasing defensin expression in honeybees during winter (Cormier et al, 2022). Remarkably, a shift towards G3P and succinate oxidation was observed suggesting that functional suppression complex I-dependent OXPHOS is linked to immune activation (Cormier et al, 2022). In C. elegans , while strong inhibition of complex I activates innate immune response and prevents neurodegeneration (Chikka et al, 2016), mild rotenone treatment down-regulated the expression of immune response genes, rendering roundworms prone to bacterial infections (Mello et al, 2022).…”

Age‐related flexibility of energetic metabolism in the honey bee Apis mellifera

“…68 In honey bees however, previous observations indicate that at flight temperature, they rely more on the oxidation of succinate and glycerol-3-phosphate (G3P) that fuel CII and the mitochondrial glycerol-3-phosphate dehydrogenase (mtG3PDH) of the ETS, respectively. 46,69,70 Although succinate contribution in mitochondrial oxygen consumption in honey bees has been challenged, 71 Hedges et al 46 , on the other hand, suggested that honey bees rely mainly on CI substrates to generate ATP and in addition, oxidize G3P as a mechanism to ensure the continuous functioning of G3P shuttle that regenerates NAD + and thus accelerates glycolytic flux that consequently fuels mitochondrial respiration through CI. 72 In addition, honey bees' mitochondria are known to be resilient to increased temperature when compared to other insects and keep functioning at extraordinary high temperatures owing to the plastic recruitment of different oxidative substrates.…”