Functional complementation of Drosophila itpr mutants by rat Itpr1

Modulation of flight and feeding behaviours requires presynaptic IP3Rs in dopaminergic neurons

Innate behaviours, although robust and hard wired, rely on modulation of neuronal circuits, for eliciting an appropriate response according to internal states and external cues. Drosophila flight is one such innate behaviour that is modulated by intracellular calcium release through inositol 1,4,5-trisphosphate receptors (IP3Rs). Cellular mechanism(s) by which IP3Rs modulate neuronal function for specific behaviours remain speculative, in vertebrates and invertebrates. To address this, we generated an inducible dominant negative form of the IP3R (IP3RDN). Flies with neuronal expression of IP3RDN exhibit flight deficits. Expression of IP3RDN helped identify key flight-modulating dopaminergic neurons with axonal projections in the mushroom body. Flies with attenuated IP3Rs in these presynaptic dopaminergic neurons exhibit shortened flight bouts and a disinterest in seeking food, accompanied by reduced excitability and dopamine release upon cholinergic stimulation. Our findings suggest that the same neural circuit modulates the drive for food search and for undertaking longer flight bouts.

Store-Operated Ca2+ Entry in Drosophila Primary Neuronal Cultures

Intracellular calcium signals in neurons frequently derive from the stimulation of G protein-coupled receptors (GPCR) by neurotransmitters, neuropeptides, and neurohormones. GPCR stimulation in neurons leads to generation of inositol 1,4,5-trisphosphate (IP₃), which in turn activates endoplasmic reticulum (ER)-localized IP₃ receptors. The IP₃ receptor (IP₃R) is a ligand-gated Ca²⁺ channel, which releases Ca²⁺ from ER stores. In Drosophila neurons it has been shown that depletion of ER Ca²⁺ store is followed by store-operated Ca²⁺ entry (SOCE) through STIM and Orai, the ER Ca²⁺ sensor and the plasma membrane Ca²⁺ channel respectively. The elucidation of this Ca²⁺ signaling pathway in neurons has in part been possible due to the ease of genetic manipulation in Drosophila, which has allowed neuron-specific knockdown of various proteins of interest. This has been followed by standardization of conditions for culturing neurons from dissected brains of the relevant genotypes, such that they could be used for robust Ca²⁺ measurements by imaging with standard Ca²⁺ indicator dyes. Protocols for measurement of IP₃-mediated Ca²⁺ release, passive depletion of ER Ca²⁺ store, and SOCE in primary cultures of Drosophila neurons are described here.

Control of protein translation by IP3R-mediated Ca2+ release in Drosophila neuroendocrine cells

The inositol 1,4,5-trisphosphate receptor (IP3R) is one of two Ca2+ channels that gates Ca2+ release from ER-stores. The ligand IP3, generated upon specific G-protein coupled receptor activation, binds to IP3R to release Ca2+ into the cytosol. IP3R also mediates ER-store Ca2+ release into the mitochondria, under basal as well as stimulatory conditions; an activity that influences cellular bioenergetics and thus, cellular growth and proliferation. In Drosophila neuroendocrine cells expressing a hypomorphic mutant of IP3R, we observed reduced protein translation levels. Here, we discuss the possible molecular mechanism for this observation. We hypothesise that the cellular energy sensor, AMPK connects IP3R mediated Ca2+ release into the mitochondria, to protein translation, via the TOR pathway.

Spontaneous Ca2+ Influx in Drosophila Pupal Neurons Is Modulated by IP3-Receptor Function and Influences Maturation of the Flight Circuit

Inositol 1,4,5-trisphosphate receptors (IP₃R) are Ca²⁺ channels on the neuronal endoplasmic reticulum (ER) membrane. They are gated by IP₃, produced upon external stimulation and activation of G protein-coupled receptors on the plasma membrane (PM). IP₃-mediated Ca²⁺ release, and the resulting depletion of the ER store, triggers entry of extracellular Ca²⁺ by store-operated Ca²⁺ entry (SOCE). Mutations in IP₃R attenuate SOCE. Compromised IP₃R function and SOCE during pupal development of Drosophila leads to flight deficits and mimics suppression of neuronal activity during pupal or adult development. To understand the effect of compromised IP₃R function on pupal neuronal calcium signaling, we examined the effects of mutations in the IP₃R gene (itpr) on Ca²⁺ signals in cultured neurons derived from Drosophila pupae. We observed increased spontaneous Ca²⁺ influx across the PM of isolated pupal neurons with mutant IP₃R and also a loss of SOCE. Both spontaneous Ca²⁺ influx and reduced SOCE were reversed by over-expression of dOrai and dSTIM, which encode the SOCE Ca²⁺ channel and the ER Ca²⁺-sensor that regulates it, respectively. Expression of voltage-gated Ca²⁺ channels (cac, Ca-α1D and Ca-αT) was significantly reduced in itpr mutant neurons. However, expression of trp mRNAs and transient receptor potential (TRP) protein were increased, suggesting that TRP channels might contribute to the increased spontaneous Ca²⁺ influx in neurons with mutant IP₃R. Thus, IP₃R/SOCE modulates spontaneous Ca²⁺ influx and expression of PM Ca²⁺ channels in Drosophila pupal neurons. Spontaneous Ca²⁺ influx compensates for the loss of SOCE in Drosophilaitpr mutant neurons.

Store-independent modulation of Ca(2+) entry through Orai by Septin 7

Orai channels are required for store-operated Ca²⁺ entry (SOCE) in multiple cell types. Septins are a class of GTP-binding proteins that function as diffusion barriers in cells. Here we show that Septin 7 acts as a ‘molecular brake’ on activation of Orai channels in Drosophila neurons. Lowering Septin 7 levels results in dOrai-mediated Ca²⁺ entry and higher cytosolic Ca²⁺ in resting neurons. This Ca²⁺ entry is independent of depletion of endoplasmic reticulum Ca²⁺ stores and Ca²⁺ release through the inositol-1,4,5-trisphosphate receptor. Importantly, store-independent Ca²⁺ entry through Orai compensates for reduced SOCE in the Drosophila flight circuit. Moreover, overexpression of Septin 7 reduces both SOCE and flight duration, supporting its role as a negative regulator of Orai channel function in vivo. Septin 7 levels in neurons can, therefore, alter neural circuit function by modulating Orai function and Ca²⁺ homeostasis.

Orai channels are well known to mediate store-operated calcium entry. Here authors show that in neurons of the Drosophila flight circuit, Septin 7 acts as a negative regulator of Orai channels, surprisingly, by modulating store-independent calcium entry through Orai.

Store-Operated Calcium Entry through Orai Is Required for Transcriptional Maturation of the Flight Circuit in Drosophila

Store operated calcium entry (SOCE) is thought to primarily regulate calcium homeostasis in neurons. Subsequent to identification of Orai as the SOCE channel in nonexcitable cells, investigation of Orai function in neurons demonstrated a requirement for SOCE in Drosophila flight. Here, by analysis of an Orai mutant and by controlled expression of a dominant-negative Drosophila Orai transgene, we show that Orai-mediated SOCE is required in dopaminergic interneurons of the flight circuit during pupal development. Expression of dominant-negative Orai in dopaminergic neurons of pupae abolished flight. The loss of Orai-mediated SOCE alters transcriptional regulation of dopaminergic neurons, leading to downregulation of the enzyme tyrosine hydroxylase, which is essential for dopamine synthesis, and the dopamine transporter, which is required for dopamine uptake after synaptic release. These studies suggest that modulation of SOCE could serve as a novel mechanism for restoring dopamine levels in dopaminergic neurons.

SIGNIFICANCE STATEMENT

The specificity of an animal's response to an environmental stimulus is determined in part by the release of neurotransmitters, which are sensed by responding neurons through cognate receptors on their surface. One way by which neurons respond is through release of calcium from intracellular stores followed by store refilling from extracellular calcium sources. This mechanism is called store-operated calcium entry (SOCE). The function of SOCE in neurons has been debated. Here we describe a new function for SOCE in the regulation of neurotransmitter levels in Drosophila flight neurons. This cell-signaling mechanism is required to maintain optimal levels of a key enzyme for dopamine synthesis and may serve as a mechanism for restoring dopamine levels in relevant pathological conditions.