Synaptic components necessary for retrograde signaling triggered by calcium/calmodulin-dependent protein kinase II during synaptogenesis

Time- and Oil-Dependent Transcriptomic and Physiological Responses to Deepwater Horizon Oil in Mahi-Mahi (Coryphaena hippurus) Embryos and Larvae

The Deepwater Horizon (DWH) oil spill contaminated the spawning habitats for numerous commercially and ecologically important fishes. Exposure to the water accommodated fraction (WAF) of oil from the spill has been shown to cause cardiac toxicity during early developmental stages across fishes. To better understand the molecular events and explore new pathways responsible for toxicity, RNA sequencing was performed in conjunction with physiological and morphological assessments to analyze the time-course (24, 48, and 96 h post fertilization (hpf)) of transcriptional and developmental responses in embryos/larvae of mahi-mahi exposed to WAF of weathered (slick) and source DWH oils. Slick oil exposure induced more pronounced changes in gene expression over time than source oil exposure. Predominant transcriptomic responses included alteration of EIF2 signaling, steroid biosynthesis, ribosome biogenesis and activation of the cytochrome P450 pathway. At 96 hpf, slick oil exposure resulted in significant perturbations in eye development and peripheral nervous system, suggesting novel targets in addition to the heart may be involved in the developmental toxicity of DHW oil. Comparisons of changes of cardiac genes with phenotypic responses were consistent with reduced heart rate and increased pericardial edema in larvae exposed to slick oil but not source oil.

Constitutive activation of Ca2+/calmodulin-dependent protein kinase II during development impairs central cholinergic transmission in a circuit underlying escape behavior in Drosophila

Development of neural circuitry relies on precise matching between correct synaptic partners and appropriate synaptic strength tuning. Adaptive developmental adjustments may emerge from activity and calcium-dependent mechanisms. Calcium/calmodulin-dependent protein kinase II (CaMKII) has been associated with developmental synaptic plasticity, but its varied roles in different synapses and developmental stages make mechanistic generalizations difficult. In contrast, we focused on synaptic development roles of CaMKII in a defined sensory-motor circuit. Thus, different forms of CaMKII were expressed with UAS-Gal4 in distinct components of the giant fiber system, the escape circuit of Drosophila, consisting of photoreceptors, interneurons, motoneurons, and muscles. The results demonstrate that the constitutively active CaMKII-T287D impairs development of cholinergic synapses in giant fiber dendrites and thoracic motoneurons, preventing light-induced escape behavior. The locus of the defects is postsynaptic as demonstrated by selective expression of transgenes in distinct components of the circuit. Furthermore, defects among these cholinergic synapses varied in severity, while the glutamatergic neuromuscular junctions appeared unaffected, demonstrating differential effects of CaMKII misregulation on distinct synapses of the same circuit. Limiting transgene expression to adult circuits had no effects, supporting the role of misregulated kinase activity in the development of the system rather than in acutely mediating escape responses. Overexpression of wild-type transgenes did not affect circuit development and function, suggesting but not proving that the CaMKII-T287D effects are not due to ectopic expression. Therefore, regulated CaMKII autophosphorylation appears essential in central synapse development, and particular cholinergic synapses are affected differentially, although they operate via the same nicotinic receptor.

The genetics of calcium signaling in Drosophila melanogaster

BACKGROUND

Genetic screens for behavioral and physiological defects in Drosophila melanogaster, helped identify several components of calcium signaling of which some, like the Trps, were novel. For genes initially identified in vertebrates, reverse genetic methods have allowed functional studies at the cellular and systemic levels.

SCOPE OF REVIEW

The aim of this review is to explain how various genetic methods available in Drosophila have been used to place different arms of Ca2+ signaling in the context of organismal development, physiology and behavior.

MAJOR CONCLUSION

Mutants generated in genes encoding a range of Ca2+ transport systems, binding proteins and enzymes affect multiple aspects of neuronal and muscle physiology. Some also affect the maintenance of ionic balance and excretion from malpighian tubules and innate immune responses in macrophages. Aspects of neuronal physiology affected include synaptic growth and plasticity, sensory transduction, flight circuit development and function. Genetic interaction screens have shown that mechanisms of maintaining Ca2+ homeostasis in Drosophila are cell specific and require a synergistic interplay between different intracellular and plasma membrane Ca2+ signaling molecules.

GENERAL SIGNIFICANCE

Insights gained through genetic studies of conserved Ca2+ signaling pathways have helped understand multiple aspects of fly physiology. The similarities between mutant phenotypes of Ca2+ signaling genes in Drosophila with certain human disease conditions, especially where homologous genes are causative factors, are likely to aid in the discovery of underlying disease mechanisms and help develop novel therapeutic strategies. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Subunit-specific and homeostatic regulation of glutamate receptor localization by CaMKII in Drosophila neuromuscular junctions

For the efficient transfer of information across neural circuits, the number of synaptic components at synapses must be appropriately regulated. Here, we found that postsynaptic calcium/calmodulin dependent protein kinase II (CaMKII) modulates the localization of glutamate receptors (GluRs) at Drosophila larval neuromuscular junctions (NMJs). Expression of an inhibitory peptide of CaMKII, Ala, in muscle cells enhanced the density of GluRIIA, which is a major and calcium-permeable subunit of GluR, at synapses of third instar larval NMJs. On the other hand, postsynaptic expression of a constitutively active form of CaMKII (T287D) reduced synaptic GluRIIA. These results suggest that CaMKII regulates GluRIIA at NMJs. Moreover, postsynaptic expression of T287D abolished the accumulation of the scaffolding protein discs large (DLG) at synapses, while exerting no significant effects on the presynaptic area and the localization of cell adhesion molecule fasciclin II (FasII). The amplitude of excitatory junctional potentials (EJPs) was enhanced in Ala-expressing larvae, whereas it was unaffected in T287D-expressing larvae in spite of the prominent loss of GluRIIA. The amplitude of miniature EJPs (mEJPs) was significantly reduced and quantal content was significantly increased in T287D-expressing larvae. Notably, another class of GluR containing GluRIIB was enhanced by the postsynaptic expression of T287D. These results suggest that the homeostatic mechanism in T287D larvae works to maintain the level of synaptic responses. Thus, the Drosophila larval NMJs have several regulatory systems to ensure efficient muscle excitability which is necessary for proper larval movement.

Widespread changes in synaptic markers as a function of sleep and wakefulness in Drosophila

Sleep is universal, strictly regulated, and necessary for cognition. Why this is so remains a mystery, although recent work suggests that sleep, memory, and plasticity are linked. However, little is known about how wakefulness and sleep affect synapses. Using Western blots and confocal microscopy in Drosophila, we found that protein levels of key components of central synapses were high after waking and low after sleep. These changes were related to behavioral state rather than time of day and occurred in all major areas of the Drosophila brain. The decrease of synaptic markers during sleep was progressive, and sleep was necessary for their decline. Thus, sleep may be involved in maintaining synaptic homeostasis altered by waking activities.

Temperature-dependent developmental plasticity of Drosophila neurons: cell-autonomous roles of membrane excitability, Ca2+ influx, and cAMP signaling

Environmental temperature is an important factor exerting pervasive influence on neuronal morphology and synaptic physiology. In the Drosophila brain, axonal arborization of mushroom body Kenyon cells was enhanced when flies were raised at high temperature (30 degrees C rather than 22 degrees C) for several days. Isolated embryonic neurons in culture that lacked cell-cell contacts also displayed a robust temperature-induced neurite outgrowth. This cell-autonomous effect was reflected by significantly increased high-order branching and enlarged growth cones. The temperature-induced morphological alterations were blocked by the Na+ channel blocker tetrodotoxin and a Ca2+ channel mutation but could be mimicked by raising cultures at room temperature with suppressed K+ channel activity. Physiological analyses revealed increased inward Ca2+ currents and decreased outward K+ currents, in conjunction with a distal shift in the site of action potential initiation and increased prevalence of TTX-sensitive spontaneous Ca2+ transients. Importantly, the overgrowth caused by both temperature and hyperexcitability K+ channel mutations were sensitive to genetic perturbations of cAMP metabolism. Thus, temperature acts in a cell-autonomous manner to regulate neuronal excitability and spontaneous activity. Presumably, activity-dependent Ca2+ accumulation triggers the cAMP cascade to confer the activity-dependent plasticity of neuronal excitability and growth.

Neonatal exposure to decabrominated diphenyl ether (PBDE 209) results in changes in BDNF, CaMKII and GAP-43, biochemical substrates of neuronal survival, growth, and synaptogenesis

Mammals have a marked period of rapid brain growth and development (BGS), which is postnatal in mice and rats, spanning the first 3-4 weeks of life and reaching its peak around postnatal day 10. CaMKII, GAP-43 and BDNF play important roles during the BGS in mammals. One class of flame retardants, polybrominated diphenyl ethers (PBDEs), are present and increasing in the environment and in human milk, which is also true for the only congener still in use, decabrominated diphenyl ether (PBDE 209). In the present study, the brains from 1, 3, 7, 10, 14 and 28 days old mice, were analysed for CaMKII and GAP-43. The level of CaMKII increases continuously during the neonatal period, while GAP-43 has a bell-shaped ontogeny curve, which peaks around postnatal day 10, in mouse brain. Furthermore, the effects of PBDE 209 on the developmental expression of CaMKII, GAP-43 and BDNF were examined in mice. Neonatal NMRI-male mice were orally exposed on days 3-20.1mgPBDE 209/kg body weight. The animals were euthanized 7 days after exposure to PBDE 209 and levels of CaMKII, GAP-43 and BDNF were analysed in different brain regions. The protein analysis showed that CaMKII increased significantly in hippocampus, but not in cortex, in animals 7 days after exposure to PBDE 209. GAP-43 showed a significant increase in hippocampus and a significant decrease in cortex of animals 7 days after exposure to PBDE 209. BDNF decreased significantly in hippocampus, but not in cortex, in mice 7 days after exposure to PBDE 209. This shows that PBDE 209 affects important proteins involved in normal maturation of the brain and further strengthen our findings concerning PBDE 209 as a developmental neurotoxicological agent.