Sequential acquisition of cacophony calcium currents, sodium channels and voltage-dependent potassium currents affects spike shape and dendrite growth during postembryonic maturation of an identified Drosophila motoneuron

Differential expression of Phlebotomus tobbi Adler, Theodor & Lourie, 1930 (Diptera: Psychodidae) genes under different environmental conditions

Phlebotomus tobbi is a widely distributed sand fly species in Turkey and is the proven vector of Leishmania infantum and several Phleboviruses. Information regarding the genetic basis of phenotypic plasticity is crucial for managing vector-borne diseases, as the changing environmental conditions have consequences for the survival of arthropods and the disease agents they transmit. However, limited data is available on the impacts of environmental conditions on the traits associated with sand fly survival, reproduction, and vectorial competence. The present study aimed to reveal the changes in the expression levels of three selected P. tobbi genes using laboratory-reared and wild-caught populations. A nervous system protein, Cacophony (PtCac), related to the life history traits of sand flies, and two sand fly salivary protein genes, PtSP32 and PtSP38, influence the infection of the vertebrate hosts, were assessed. Sand flies were maintained at 23 °C and 27 °C in the laboratory to evaluate the relationship between temperature and the expressed phenotypes. Field collections were carried out in three climatically distinct regions of Turkey to establish the regional differences in the gene expression levels of natural P. tobbi populations. In the laboratory, PtCac expression increased with the temperature. However, PtCac expression was negatively correlated with local temperature and humidity conditions. No differences were detected in the PtSP32 gene expression levels of both laboratory-reared and wild-caught females, but a negative correlation was observed with relative humidity in natural populations. Although the expression levels of PtSP38 did not differ among the females collected from distinct regions, a positive correlation was detected in the laboratory-reared colony. The findings indicated that changes in environmental conditions could drive the expression levels of P. tobbi genes, which influence population dynamics and the transmission risk of the disease.

A chromosome-level genome of Semiothisa cinerearia provides insights into its genome evolution and control

BACKGROUND

Semiothisa cinerearia belongs to Geometridae, which is one of the most species-rich families of lepidopteran insects. It is also one of the most economically significant pests of the Chinese scholar tree (Sophora japonica L.), which is an important urban greenbelt trees in China due to its high ornamental value. A genome assembly of S. cinerearia would facilitate study of the control and evolution of this species.

RESULTS

We present a reference genome for S. cinerearia; the size of the genome was ~ 580.89 Mb, and it contained 31 chromosomes. Approximately 43.52% of the sequences in the genome were repeat sequences, and 21,377 protein-coding genes were predicted. Some important gene families involved in the detoxification of pesticides (P450) have expanded in S. cinerearia. Cytochrome P450 gene family members play key roles in mediating relationships between plants and insects, and they are important in plant secondary metabolite detoxification and host-plant selection. Using comparative analysis methods, we find positively selected gene, Sox15 and TipE, which may play important roles during the larval-pupal metamorphosis development of S. cinerearia.

CONCLUSION

This assembly provides a new genomic resource that will aid future comparative genomic studies of Geometridae species and facilitate future evolutionary studies on the S. cinerearia.

Identification of Sodium Transients Through NaV1.5 Channels as Regulators of Differentiation in Immortalized Dorsal Root Ganglia Neurons

Neuronal differentiation is a complex process through which newborn neurons acquire the morphology of mature neurons and become excitable. We employed a combination of functional and transcriptomic approaches to deconvolute and identify key regulators of the differentiation process of a DRG neuron-derived cell line, and we focused our study on the NaV1.5 ion channel (encoded by Scn5a) as a channel involved in the acquisition of DRG neuronal features. Overexpression of Scn5a enhances the acquisition of neuronal phenotypic features and increases the KCl-elicited hyperexcitability response in a DRG-derived cell line. Moreover, pharmacologic inhibition of the NaV1.5 channel during differentiation hinders the acquisition of phenotypic features of neuronal cells and the hyperexcitability increase in response to changes in the extracellular medium ionic composition. Taken together, these data highlight the relevance of sodium transients in regulating the neuronal differentiation process in a DRG neuron-derived cell line.

Different functions of two putative Drosophila α2δ subunits in the same identified motoneurons

Voltage gated calcium channels (VGCCs) regulate neuronal excitability and translate activity into calcium dependent signaling. The α₁ subunit of high voltage activated (HVA) VGCCs associates with α₂δ accessory subunits, which may affect calcium channel biophysical properties, cell surface expression, localization and transport and are thus important players in calcium-dependent signaling. In vertebrates, the functions of the different combinations of the four α₂δ and the seven HVA α₁ subunits are incompletely understood, in particular with respect to partially redundant or separate functions in neurons. This study capitalizes on the relatively simpler situation in the Drosophila genetic model containing two neuronal putative α₂δ subunits, straightjacket and CG4587, and one Ca_v1 and Ca_v2 homolog each, both with well-described functions in different compartments of identified motoneurons. Straightjacket is required for normal Ca_v1 and Ca_v2 current amplitudes and correct Ca_v2 channel function in all neuronal compartments. By contrast, CG4587 does not affect Ca_v1 or Ca_v2 current amplitudes or presynaptic function, but is required for correct Ca_v2 channel allocation to the axonal versus the dendritic domain. We suggest that the two different putative α₂δ subunits are required in the same neurons to regulate different functions of VGCCs.

Differential localization of voltage-gated potassium channels during Drosophila metamorphosis

Neuronal excitability is determined by the combination of different ion channels and their sub-neuronal localization. This study utilizes protein trap fly strains with endogenously tagged channels to analyze the spatial expression patterns of the four Shaker-related voltage-gated potassium channels, K_v1-4, in the larval, pupal, and adult Drosophila ventral nerve cord. We find that all four channels (Shaker, K_v1; Shab, K_v2; Shaw, K_v3; and Shal, K_v4) each show different spatial expression patterns in the Drosophila ventral nerve cord and are predominantly targeted to different sub-neuronal compartments. Shaker is abundantly expressed in axons, Shab also localizes to axons but mostly in commissures, Shaw expression is restricted to distinct parts of neuropils, and Shal is found somatodendritically, but also in axons of identified motoneurons. During early pupal life expression of all four Shaker-related channels is markedly decreased with an almost complete shutdown of expression at early pupal stage 5 (∼30% through metamorphosis). Re-expression of K_v1-4 channels at pupal stage 6 starts with abundant channel localization in neuronal somata, followed by channel targeting to the respective sub-neuronal compartments until late pupal life. The developmental time course of tagged K_v1-4 channel expression corresponds with previously published data on developmental changes in single neuron physiology, thus indicating that protein trap fly strains are a useful tool to analyze developmental regulation of potassium channel expression. Finally, we take advantage of the large diameter of the giant fiber (GF) interneuron to map channel expression onto the axon and axon terminals of an identified interneuron. Shaker, Shaw, and Shal but not Shab channels localize to the non-myelinated GF axonal membrane and axon terminals. This study constitutes a first step toward systematically analyzing sub-neuronal potassium channel localization in Drosophila. Functional implications as well as similarities and differences to K_v1-4 channel localization in mammalian neurons are discussed.

Distinctions among electroconvulsion- and proconvulsant-induced seizure discharges and native motor patterns during flight and grooming: quantitative spike pattern analysis in Drosophila flight muscles

In Drosophila, high-frequency electrical stimulation across the brain triggers a highly stereotypic repertoire of spasms. These electroconvulsive seizures (ECS) manifest as distinctive spiking discharges across the nervous system and can be stably assessed throughout the seizure repertoire in the large indirect flight muscles dorsal longitudinal muscles (DLMs) to characterize modifications in seizure-prone mutants. However, the relationships between ECS-spike patterns and native motor programs, including flight and grooming, are not known and their similarities and distinctions remain to be characterized. We employed quantitative spike pattern analyses for the three motor patterns including: (1) overall firing frequency, (2) spike timing between contralateral fibers, and (3) short-term variability in spike interval regularity (CV₂) and instantaneous firing frequency (ISI^-1). This base-line information from wild-type (WT) flies facilitated quantitative characterization of mutational effects of major neurotransmitter systems: excitatory cholinergic (Cha), inhibitory GABAergic (Rdl) and electrical (ShakB) synaptic transmission. The results provide an initial glimpse on the vulnerability of individual motor patterns to different perturbations. We found marked alterations of ECS discharge spike patterns in terms of either seizure threshold, spike frequency or spiking regularity. In contrast, no gross alterations during grooming and a small but noticeable reduction of firing frequency during Rdl mutant flight were found, suggesting a role for GABAergic modulation of flight motor programs. Picrotoxin (PTX), a known pro-convulsant that inhibits GABA_A receptors, induced DLM spike patterns that displayed some features, e.g. left-right coordination and ISI^-1 range, that could be found in flight or grooming, but distinct from ECS discharges. These quantitative techniques may be employed to reveal overlooked relationships among aberrant motor patterns as well as their links to native motor programs.

Intra-neuronal Competition for Synaptic Partners Conserves the Amount of Dendritic Building Material

Brain development requires correct targeting of multiple thousand synaptic terminals onto staggeringly complex dendritic arbors. The mechanisms by which input synapse numbers are matched to dendrite size, and by which synaptic inputs from different transmitter systems are correctly partitioned onto a postsynaptic arbor, are incompletely understood. By combining quantitative neuroanatomy with targeted genetic manipulation of synaptic input to an identified Drosophila neuron, we show that synaptic inputs of two different transmitter classes locally direct dendrite growth in a competitive manner. During development, the relative amounts of GABAergic and cholinergic synaptic drive shift dendrites between different input domains of one postsynaptic neuron without affecting total arbor size. Therefore, synaptic input locally directs dendrite growth, but intra-neuronal dendrite redistributions limit morphological variability, a phenomenon also described for cortical neurons. Mechanistically, this requires local dendritic Ca²⁺ influx through Dα7nAChRs or through LVA channels following GABA_AR-mediated depolarizations. VIDEO ABSTRACT.

Apoptotic Activity of MeCP2 Is Enhanced by C-Terminal Truncating Mutations

Methyl-CpG binding protein 2 (MeCP2) is a widely abundant, multifunctional protein most highly expressed in post-mitotic neurons. Mutations causing Rett syndrome and related neurodevelopmental disorders have been identified along the entire MECP2 locus, but symptoms vary depending on mutation type and location. C-terminal mutations are prevalent, but little is known about the function of the MeCP2 C-terminus. We employ the genetic efficiency of Drosophila to provide evidence that expression of p.Arg294* (more commonly identified as R294X), a human MECP2 E2 mutant allele causing truncation of the C-terminal domains, promotes apoptosis of identified neurons in vivo. We confirm this novel finding in HEK293T cells and then use Drosophila to map the region critical for neuronal apoptosis to a small sequence at the end of the C-terminal domain. In vitro studies in mammalian systems previously indicated a role of the MeCP2 E2 isoform in apoptosis, which is facilitated by phosphorylation at serine 80 (S80) and decreased by interactions with the forkhead protein FoxG1. We confirm the roles of S80 phosphorylation and forkhead domain transcription factors in affecting MeCP2-induced apoptosis in Drosophila in vivo, thus indicating mechanistic conservation between flies and mammalian cells. Our findings are consistent with a model in which C- and N-terminal interactions are required for healthy function of MeCP2.

Voltage-gated calcium channels are abnormal in cultured spinal motoneurons in the G93A-SOD1 transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of motoneurons. Hyperexcitability and excitotoxicity have been implicated in the early pathogenesis of ALS. Studies addressing excitotoxic motoneuron death and intracellular Ca(2+) overload have mostly focused on Ca(2+) influx through AMPA glutamate receptors. However, intrinsic excitability of motoneurons through voltage-gated ion channels may also have a role in the neurodegeneration. In this study we examined the function and localization of voltage-gated Ca(2+) channels in cultured spinal cord motoneurons from mice expressing a mutant form of human superoxide dismutase-1 with a Gly93→Ala substitution (G93A-SOD1). Using whole-cell patch-clamp recordings, we showed that high voltage activated (HVA) Ca(2+) currents are increased in G93A-SOD1 motoneurons, but low voltage activated Ca(2+) currents are not affected. G93A-SOD1 motoneurons also have altered persistent Ca(2+) current mediated by L-type Ca(2+) channels. Quantitative single-cell RT-PCR revealed higher levels of Ca1a, Ca1b, Ca1c, and Ca1e subunit mRNA expression in G93A-SOD1 motoneurons, indicating that the increase of HVA Ca(2+) currents may result from upregulation of Ca(2+) channel mRNA expression in motoneurons. The localizations of the Ca1B N-type and Ca1D L-type Ca(2+) channels in motoneurons were examined by immunocytochemistry and confocal microscopy. G93A-SOD1 motoneurons had increased Ca1B channels on the plasma membrane of soma and dendrites. Ca1D channels are similar on the plasma membrane of soma and lower on the plasma membrane of dendrites of G93A-SOD1 motoneurons. Our study demonstrates that voltage-gated Ca(2+) channels have aberrant functions and localizations in ALS mouse motoneurons. The increased HVA Ca(2+) currents and PCCa current could contribute to early pathogenesis of ALS.

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.

Distal spike initiation zone location estimation by morphological simulation of ionic current filtering demonstrated in a novel model of an identified Drosophila motoneuron

Studying ion channel currents generated distally from the recording site is difficult because of artifacts caused by poor space clamp and membrane filtering. A computational model can quantify artifact parameters for correction by simulating the currents only if their exact anatomical location is known. We propose that the same artifacts that confound current recordings can help pinpoint the source of those currents by providing a signature of the neuron’s morphology. This method can improve the recording quality of currents initiated at the spike initiation zone (SIZ) that are often distal to the soma in invertebrate neurons. Drosophila being a valuable tool for characterizing ion currents, we estimated the SIZ location and quantified artifacts in an identified motoneuron, aCC/MN1-Ib, by constructing a novel multicompartmental model. Initial simulation of the measured biophysical channel properties in an isopotential Hodgkin-Huxley type neuron model partially replicated firing characteristics. Adding a second distal compartment, which contained spike-generating Na⁺ and K⁺ currents, was sufficient to simulate aCC’s in vivo activity signature. Matching this signature using a reconstructed morphology predicted that the SIZ is on aCC’s primary axon, 70 μm after the most distal dendritic branching point. From SIZ to soma, we observed and quantified selective morphological filtering of fast activating currents. Non-inactivating K⁺ currents are filtered ∼3 times less and despite their large magnitude at the soma they could be as distal as Na⁺ currents. The peak of transient component (NaT) of the voltage-activated Na⁺ current is also filtered more than the magnitude of slower persistent component (NaP), which can contribute to seizures. The corrected NaP/NaT ratio explains the previously observed discrepancy when the same channel is expressed in different cells. In summary, we used an in vivo signature to estimate ion channel location and recording artifacts, which can be applied to other neurons.

The study of ion channels is essential both for understanding normal brain function and for finding drug targets to treat neurological disease. Traditional experimental techniques remain challenging for recording ion channel currents accurately because of their locations in the neuron. Computer modeling of the three dimensional structure of neurons can provide a correction estimate for the measurement error introduced by neuronal membranes. To achieve this, we developed a modeling approach to localize, and correct for, distant ion channels. We demonstrated this approach by constructing novel computer models of an identified insect motor neuron, which provides a powerful model for studying the central nervous system. Through the study of electrical activity and genetic manipulations, it has been found that the persistent sodium current contributes to seizure. By modeling three dimensional structure, we were able to predict the location of these currents in the neuron, which were more distal than expected. Localizing sodium channels allowed us to predict their properties at origin, which favorably matched isolated recordings of these channels in more compact cells. This result is important in validating the use of heterologous compact cells to study insect sodium channels, and also demonstrates the usefulness of the presented modeling approach for studying channel physiology more generally.

Dendrites are dispensable for basic motoneuron function but essential for fine tuning of behavior

Dendrites are highly complex 3D structures that define neuronal morphology and connectivity and are the predominant sites for synaptic input. Defects in dendritic structure are highly consistent correlates of brain diseases. However, the precise consequences of dendritic structure defects for neuronal function and behavioral performance remain unknown. Here we probe dendritic function by using genetic tools to selectively abolish dendrites in identified Drosophila wing motoneurons without affecting other neuronal properties. We find that these motoneuron dendrites are unexpectedly dispensable for synaptic targeting, qualitatively normal neuronal activity patterns during behavior, and basic behavioral performance. However, significant performance deficits in sophisticated motor behaviors, such as flight altitude control and switching between discrete courtship song elements, scale with the degree of dendritic defect. To our knowledge, our observations provide the first direct evidence that complex dendrite architecture is critically required for fine-tuning and adaptability within robust, evolutionarily constrained behavioral programs that are vital for mating success and survival. We speculate that the observed scaling of performance deficits with the degree of structural defect is consistent with gradual increases in intellectual disability during continuously advancing structural deficiencies in progressive neurological disorders.