cGMP-Dependent Protein Kinase Inhibition Extends the Upper Temperature Limit of Stimulus-Evoked Calcium Responses in Motoneuronal Boutons of Drosophila melanogaster Larvae

Characterization of a novel stimulus-induced glial calcium wave in Drosophila larval peripheral segmental nerves and its role in PKG-modulated thermoprotection

Insects, as poikilotherms, have adaptations to deal with wide ranges in temperature fluctuation. Allelic variations in the foraging gene that encodes a cGMP dependent protein kinase, were discovered to have effects on behavior in Drosophila by Dr. Marla Sokolowski in 1980. This single gene has many pleiotropic effects and influences feeding behavior, metabolic storage, learning and memory and has been shown to affect stress tolerance. PKG regulation affects motoneuronal thermotolerance in Drosophila larvae as well as adults. While the focus of thermotolerance studies has been on the modulation of neuronal function, other cell types have been overlooked. Because glia are vital to neuronal function and survival, we wanted to determine if glia play a role in thermotolerance as well. In our investigation, we discovered a novel calcium wave at the larval NMJ and set out to characterize the wave's dynamics and the potential mechanism underlying the wave prior to determining what effect, if any, PKG modulation has on the thermotolerance of glia cells. Using pharmacology, we determined that calcium buffering mechanisms of the mitochondria and endoplasmic reticulum play a role in the propagation of our novel glial calcium wave. By coupling pharmacology with genetic manipulation using RNA interference (RNAi), we found that PKG modulation in glia alters thermoprotection of function as well as glial calcium wave dynamics.

A cGMP-dependent protein kinase, encoded by the Drosophila foraging gene, regulates neurotransmission through changes in synaptic structure and function

A cGMP-dependent protein kinase (PKG) encoded by the Drosophila foraging (for) gene regulates both synaptic structure (nerve terminal growth) and function (neurotransmission) through independent mechanisms at the Drosophila larval neuromuscular junction (nmj). Glial for is known to restrict nerve terminal growth, whereas presynaptic for inhibits synaptic vesicle (SV) exocytosis during low frequency stimulation. Presynaptic for also facilitates SV endocytosis during high frequency stimulation. for's effects on neurotransmission can occur independent of any changes in nerve terminal growth. However, it remains unclear if for's effects on neurotransmission affect nerve terminal growth. Furthermore, it's possible that for's effects on synaptic structure contribute to changes in neurotransmission. In the present study, we examined these questions using RNA interference to selectively knockdown for in presynaptic neurons or glia at the Drosophila larval nmj. Consistent with our previous findings, presynaptic knockdown of for impaired SV endocytosis, whereas knockdown of glial for had no effect on SV endocytosis. Surprisingly, we found that knockdown of either presynaptic or glial for increased neurotransmitter release in response to low frequency stimulation. Knockdown of presynaptic for did not affect nerve terminal growth, demonstrating that for's effects on neurotransmission does not alter nerve terminal growth. In contrast, knockdown of glial for enhanced nerve terminal growth. This enhanced nerve terminal growth was likely the cause of the enhanced neurotransmitter release seen following knockdown of glial for. Overall, we show that for can affect neurotransmitter release by regulating both synaptic structure and function.

Insect responses to heat: physiological mechanisms, evolution and ecological implications in a warming world

Surviving changing climate conditions is particularly difficult for organisms such as insects that depend on environmental temperature to regulate their physiological functions. Insects are extremely threatened by global warming, since many do not have enough physiological tolerance even to survive continuous exposure to the current maximum temperatures experienced in their habitats. Here, we review literature on the physiological mechanisms that regulate responses to heat and provide heat tolerance in insects: (i) neuronal mechanisms to detect and respond to heat; (ii) metabolic responses to heat; (iii) thermoregulation; (iv) stress responses to tolerate heat; and (v) hormones that coordinate developmental and behavioural responses at warm temperatures. Our review shows that, apart from the stress response mediated by heat shock proteins, the physiological mechanisms of heat tolerance in insects remain poorly studied. Based on life-history theory, we discuss the costs of heat tolerance and the potential evolutionary mechanisms driving insect adaptations to high temperatures. Some insects may deal with ongoing global warming by the joint action of phenotypic plasticity and genetic adaptation. Plastic responses are limited and may not be by themselves enough to withstand ongoing warming trends. Although the evidence is still scarce and deserves further research in different insect taxa, genetic adaptation to high temperatures may result from rapid evolution. Finally, we emphasize the importance of incorporating physiological information for modelling species distributions and ecological interactions under global warming scenarios. This review identifies several open questions to improve our understanding of how insects respond physiologically to heat and the evolutionary and ecological consequences of those responses. Further lines of research are suggested at the species, order and class levels, with experimental and analytical approaches such as artificial selection, quantitative genetics and comparative analyses.

Distinct functions of a cGMP-dependent protein kinase in nerve terminal growth and synaptic vesicle cycling

Sustained neurotransmission requires the tight coupling of synaptic vesicle (SV) exocytosis and endocytosis. The mechanisms underlying this coupling are poorly understood. We tested the hypothesis that a cGMP-dependent protein kinase (PKG), encoded by the foraging (for) gene in Drosophila melanogaster, is critical for this process using a for null mutant, genomic rescues, and tissue specific rescues. We uncoupled FOR's exocytic and endocytic functions in neurotransmission using a temperature-sensitive shibire mutant in conjunction with fluorescein-assisted light inactivation of FOR. We discovered a dual role for presynaptic FOR, where FOR inhibits SV exocytosis during low frequency stimulation by negatively regulating presynaptic Ca2+ levels and maintains neurotransmission during high frequency stimulation by facilitating SV endocytosis. Additionally, glial FOR negatively regulated nerve terminal growth through TGF-β signaling and this developmental effect was independent from FOR's effects on neurotransmission. Overall, FOR plays a critical role in coupling SV exocytosis and endocytosis, thereby balancing these two components to maintain sustained neurotransmission.

Contribution of a natural polymorphism in protein kinase G modulates electroconvulsive seizure recovery in Drosophila melanogaster

Drosophila melanogaster is a well-characterized model for neurological disorders and is widely used for investigating causes of altered neuronal excitability leading to seizure-like behavior. One method used to analyze behavioral output of neuronal perturbance is recording the time to locomotor recovery from an electroconvulsive shock. Based on this behavior, we sought to quantify seizure susceptibility in larval D. melanogaster with differences in the enzymatic activity levels of a major protein, cGMP-dependent protein kinase (PKG). PKG, encoded by foraging, has two natural allelic variants and has previously been implicated in several important physiological characteristics including: foraging patterns, learning and memory, and environmental stress tolerance. The well-established NO/cGMP/PKG signaling pathway found in the fly, which potentially targets downstream K⁺ channel(s), ultimately impacts membrane excitability, leading to our hypothesis: altering PKG enzymatic activity modulates time to recovery from an electroconvulsive seizure. Our results show that by both genetically and pharmacologically increasing PKG enzymatic activity, we can decrease the locomotor recovery time from an electroconvulsive seizure in larval D. melanogaster.