Drosophila melanogaster as a model system for the study of the function of calcium/calmodulin-dependent protein kinase II in synaptic plasticity

Selective dendritic localization of mRNA in Drosophila mushroom body output neurons

Memory-relevant neuronal plasticity is believed to require local translation of new proteins at synapses. Understanding this process requires the visualization of the relevant mRNAs within these neuronal compartments. Here, we used single-molecule fluorescence in situ hybridization to localize mRNAs at subcellular resolution in the adult Drosophila brain. mRNAs for subunits of nicotinic acetylcholine receptors and kinases could be detected within the dendrites of co-labeled mushroom body output neurons (MBONs) and their relative abundance showed cell specificity. Moreover, aversive olfactory learning produced a transient increase in the level of CaMKII mRNA within the dendritic compartments of the γ5β'2a MBONs. Localization of specific mRNAs in MBONs before and after learning represents a critical step towards deciphering the role of dendritic translation in the neuronal plasticity underlying behavioral change in Drosophila.

Presynaptic activity and CaMKII modulate retrograde semaphorin signaling and synaptic refinement

Establishing synaptic connections often involves the activity-dependent withdrawal of off-target contacts. We describe an in vivo role for temporally patterned electrical activity, voltage-gated calcium channels, and CaMKII in modulating the response of Drosophila motoneurons to the chemorepellent Sema-2a during synaptic refinement. Mutations affecting the Sema-2a ligand, the plexin B receptor (plexB), the voltage-gated Ca(v)2.1 calcium channel (cac), or the voltage-gated Na(v)1 sodium channel (mle(nap-ts);tipE) each result in ectopic neuromuscular contacts. Sema-2a interacts genetically with both of the channel mutations. The cac phenotype is enhanced by the Sema-2a mutation and is suppressed by either plexB overexpression or patterned, low-frequency (0.01 Hz) bouts of electrical activity in the embryo. The calcium-dependent suppression of ectopic contacts also depends on the downstream activation of CaMKII. These results indicate a role for patterned electrical activity and presynaptic calcium signaling, acting through CaMKII, in modulating a retrograde signal during the refinement of synaptic connections.

Organ-specific distribution of the calcium sensor CaMKII in Locusta migratoria

The Ca(2+)/calmodulin-dependent kinase CaMKII is a key signaling component in Ca(2+)-dependent physiological processes. The expression and function of CaMKII in insect brain is well documented but less investigated for other tissues of insects. The present study demonstrates that in the locust Locusta migratoria CaMKII is widely expressed in various tissues. Relatively high expression levels of CaMKII were found in the brain, upper part of the digestive tract (pharynx, esophagus), and the flight and leg muscles. The different expression patterns of CaMKII in various tissues, as well as different molecular masses of CaMKII between 48 and 60 kDa indicate a tissue-specific expression of CaMKII variants. The expression was monitored with a polyclonal anti-(rat)CaMKII antibody. About 60% of total CaMKII activity in flight muscle cells is associated to the myofibril-rich, particulate fraction suggesting an important role of CaMKII in sarcomeric function.

Differential involvement of Ca(2+) channels in survival and neurite outgrowth of cultured embryonic cockroach brain neurons

The contribution of voltage-gated calcium channels (VGCC) to the development of cultured embryonic cockroach brain neurons was assessed using pharmacological agents. VGCC currents were recorded using the patch-clamp technique and were found to be blocked dose-dependently by micromolar concentrations of mibefradil. The activation and inactivation properties of the calcium channels enable a sizeable calcium current to flow at rest (about -30 and -20 mV in high-potassium culture media). As expected, the cytoplasmic-free calcium concentration was found to rise when the extracellular potassium concentration was raised from 3 to 15 and 30 mM. The effects of VGCC blockers and calcium chelators were different in fresh and in mature cultures in which the neurons were connected to each other to form a defined network. In fresh cultures, the two non-selective VGCC blockers (verapamil and mibefradil) induced a dose-dependent cell death that was proportional to their blocking effect on I(Ba). This effect could not be prevented by addition of fetal calf serum to the culture medium. A similar effect was obtained using intra- or extracellular calcium chelating agents (10 microM BAPTA-AM or 10 mM EGTA). Quite unexpectedly, blockade of the P/Q-like (omega-Aga WA-sensitive) component of the calcium current by 500 nM of omega-AgaTx IVA had no lethal effect, suggesting that the corresponding channels are not involved in the survival mechanism. As expected from their lack of effect on I(Ba), isradipine, nifedipine, and omega-CgTx GVIA did not induce cell death. When the neurons started growing neurites, their sensitivity to calcium channel blockade by mibefradil decreased, indicating a correlation between neurite outgrowth and resistance to calcium depletion. In mature cultures, the neurons became resistant to mibefradil, verapamil, and BAPTA-AM. However, these agents, as well as omega-AgaTx IVA, had a significant inhibitory effect on the increase in diameter of the connectives that linked adjacent clusters of neurons. This effect has been shown to result, in the case of mibefradil, from an inhibition of neurite outgrowth characterized by a significant reduction of the number of primary neurites and secondary branchings but not to a significant modification of the diameter of individual neurites. These results support the view that, as in vertebrates, calcium influx through VGCC plays an important role in survival and neurite outgrowth of cultured embryonic insect neurons. The differential contribution of the P/Q-like and R-like (omega-Aga WA-sensitive) calcium channels in these processes is discussed.

Electrophysiological analysis of synaptic transmission in central neurons of Drosophila larvae

We report functional neuronal and synaptic transmission properties in Drosophila CNS neurons. Whole cell current- and voltage-clamp recordings were made from dorsally positioned neurons in the larval ventral nerve cord. Comparison of neuronal Green Fluorescent Protein markers and intracellular dye labeling revealed that recorded cells consisted primarily of identified motor neurons. Neurons had resting potentials of -50 to -60 mV and fired repetitive action potentials (APs) in response to depolarizing current injection. Acetylcholine application elicited large excitatory responses and AP bursts that were reversibly blocked by the nicotinic receptor antagonist D-tubocurarine (dtC). GABA and glutamate application elicited similar inhibitory responses that reversed near normal resting potential and were reversibly blocked by the chloride channel blocker picrotoxin. Multiple types of endogenous synaptically driven activity were present in most neurons, including fast spontaneous synaptic events resembling unitary excitatory postsynaptic currents (EPSCs) and sustained excitatory currents and potentials. Sustained forms of endogenous activity ranged in amplitude from smaller subthreshold "intermediate" sustained events to large "rhythmic" events that supported bursts of APs. Electrical stimulation of peripheral nerves or focal stimulation of the neuropil evoked sustained responses and fast EPSCs similar to endogenous events. Endogenous activity and evoked responses required external Ca(2+) and were reversibly blocked by dtC application, indicating that cholinergic synaptic transmission directly underlies observed activity. Synaptic current amplitude and frequency were reduced in shibire conditional dynamin mutants and increased in dunce cAMP phosphodiesterase mutants. These results complement and advance those of recent functional studies in Drosophila embryonic neurons and demonstrate the feasibility of in-depth synaptic transmission and plasticity studies in the Drosophila CNS.

Regulation of neuronal excitability in Drosophila by constitutively active CaMKII

The ability of calcium/calmodulin-dependent protein kinase II (CaMKII) to become calcium independent after autophosphorylation makes this enzyme a temporal marker of neuronal activity. Here we show that the calcium-independent form of CaMKII has unique effects on larval viability, locomotion, and neuronal excitability in Drosophila. Expression of constitutively active T287D, but not calcium-dependent T287A, mutant CaMKII in Drosophila neurons resulted in decreased viability, behavioral defects, and failure of action potential propagation. The actions of T287D may be mediated, at least in part, by increased potassium conductances. Expression of T287D CaMKII also stimulated an increase in the number of boutons at the larval neuromuscular junction, but did not affect the mechanics of release. This study defines a role for autophosphorylation of CaMKII in the regulation of multiple neuronal functions including the intrinsic properties of neurons.

Identification and characterization of a SUMO-1 conjugation system that modifies neuronal calcium/calmodulin-dependent protein kinase II in Drosophila melanogaster

Drosophila Uba2 and Ubc9 SUMO-1 conjugation enzyme homologs (DmUba2 and DmUbc9) were isolated as calcium/calmodulin-dependent kinase II (CaMKII) interacting proteins by yeast two-hybrid screening of an adult head cDNA library. We found that at least one isoform of Drosophila neuronal CaMKII is conjugated to DmSUMO-1 in vivo. The interactions observed in the two-hybrid screen may therefore reflect catalytic events. To understand the role of SUMO conjugation in the brain, we undertook a characterization of the system. The other required components of the system, Drosophila Aos1 and SUMO-1 (DmAos1 and DmSUMO-1), were identified in expressed sequence tag data base searches. Purified recombinant DmUba2/DmAos1 dimer can activate DmSUMO-1 in vitro and transfer DmSUMO-1 to recombinant DmUbc9. DmSUMO-1 conjugation occurs in all developmental stages of Drosophila and in the adult central nervous system. Overexpression of a putative dominant negative DmUba2(C175S) mutant protein in the Drosophila central nervous system resulted in an increase in overall DmSUMO-1 conjugates and a base-sensitive p120 species, which is likely to be DmUba2(C175S) linked to endogenous DmSUMO-1 through an oxygen ester bond. Overexpression of DmUba2(wt) protein in vivo also led to increased levels of DmSUMO-1 conjugates. High level overexpression of either DmUba2(wt) or DmUba2(C175S) in the Drosophila central nervous system caused pupal and earlier stage lethality. Expression in the developing eye led to a rough eye phenotype with retinal degeneration. These results suggest that normal SUMO conjugation is essential in the differentiated nervous system and reveal a potential novel mechanism that regulates neuronal calcium/calmodulin-dependent protein kinase II function.