A genome-wide analysis reveals that the Drosophila transcription factor Lola promotes axon growth in part by suppressing expression of the actin nucleation factor Spire

Decreased Spire2 Expression is Involved in Epilepsy

Epilepsy is a neurological disorder caused by abnormally elevated neuronal firing and excitability. Spire2, also known as the nucleating factor of F-actin, plays an important role in long-range vesicle transport. This study showed that Spire2 was highly expressed in neurons in the cortex and hippocampus. Its knockdown significantly reduced the initiation current of the evoked action potential and the frequency of action potential, suggesting that Spire2 knockdown inhibits the threshold current of the neuron. In the cortex of patients with refractory temporal lobe epilepsy (TLE), Spire2 expression was significantly reduced. Decreased expression levels of Spire2 were also observed in kainic acid (KA) and pentylenetetrazole (PTZ) animal models. In the KA and PTZ models, Spire2-knockdown mice showed significantly increased seizures and shortened intervals between seizures, with a tendency to increase seizure duration. In contrast, Spire2-overexpressing mice showed reduced numbers of spontaneous seizures. In conclusion, this study revealed a significantly decreased expression of Spire2 in the brain tissues of epileptic individuals and an inhibitory role for this protein in the development of epilepsy. In addition, knockdown of Spire2 aggravated abnormal firing in epileptic mice, while its overexpression had the opposite effect. These findings provide new insights into the mechanism of epileptogenesis.

Evolution of Plasticity in Response to Ethanol between Sister Species with Different Ecological Histories (Drosophila melanogaster and D. simulans)

AbstractWhen populations evolve adaptive reaction norms in response to novel environments, it can occur through a process termed genetic accommodation. Under this model, the initial response to the environment is widely variable between genotypes as a result of cryptic genetic variation, which is then refined by selection to a single adaptive response. Here, I empirically test these predictions from genetic accommodation by measuring reaction norms in individual genotypes and across several time points. I compare two species of Drosophila that differ in their adaptation to ethanol (D. melanogaster and D. simulans). Both species are human commensals with a recent cosmopolitan expansion, but only D. melanogaster is adapted to ethanol exposure. Using gene expression as a phenotype and an approach that combines information about expression and alternative splicing, I find that D. simulans exhibits cryptic genetic variation in the response to ethanol, while D. melanogaster has almost no genotype-specific variation in reaction norm. This is evidence for adaptation to ethanol through genetic accommodation, suggesting that the evolution of phenotypic plasticity could be an important contributor to the ability to exploit novel resources.

Roles of the actin cytoskeleton in aging and age-associated diseases

The integrity of the cytoskeleton is essential to diverse cellular processes such as phagocytosis and intracellular trafficking. Disruption of the organization and dynamics of the actin cytoskeleton leads to age-associated symptoms and diseases, ranging from cancer to neurodegeneration. In addition, changes in the integrity of the actin cytoskeleton disrupt the functioning of not only somatic and stem cells but also gametes, resulting in aberrant embryonic development. Strategies to preserve the integrity and dynamics of the cytoskeleton are, therefore, potentially therapeutic to age-related disorders. The objective of this article is to revisit the current understanding of the roles played by the actin cytoskeleton in aging, and to review the opportunities and challenges for the transition of basic research into intervention development. It is hoped that, with the snapshot of evidence regarding changes in actin dynamics with advanced age, insights into future research directions can be attained.

A genome-wide enhancer/suppressor screen for Dube3a interacting genes in Drosophila melanogaster

The genetics underlying autism spectrum disorder (ASD) are complex. Approximately 3–5% of ASD cases arise from maternally inherited duplications of 15q11.2-q13.1, termed Duplication 15q syndrome (Dup15q). 15q11.2-q13.1 includes the gene UBE3A which is believed to underlie ASD observed in Dup15q syndrome. UBE3A is an E3 ubiquitin ligase that targets proteins for degradation and trafficking, so finding UBE3A substrates and interacting partners is critical to understanding Dup15q ASD. In this study, we take an unbiased genetics approach to identify genes that genetically interact with Dube3a, the Drosophila melanogaster homolog of UBE3A. We conducted an enhancer/suppressor screen using a rough eye phenotype produced by Dube3a overexpression with GMR-GAL4. Using the DrosDel deficiency kit, we identified 3 out of 346 deficiency lines that enhanced rough eyes when crossed to two separate Dube3a overexpression lines, and subsequently identified IA2, GABA-B-R3, and lola as single genes responsible for rough eye enhancement. Using the FlyLight GAL4 lines to express uas-Dube3a + uas-GFP in the endogenous lola pattern, we observed an increase in the GFP signal compared to uas-GFP alone, suggesting a transcriptional co-activation effect of Dube3a on the lola promoter region. These findings extend the role of Dube3a/UBE3A as a transcriptional co-activator, and reveal new Dube3a interacting genes.

Comprehensive Characterization of the Complex lola Locus Reveals a Novel Role in the Octopaminergic Pathway via Tyramine Beta-Hydroxylase Regulation

Longitudinals lacking (lola) is one of the most complex genes in Drosophila melanogaster, encoding up to 20 protein isoforms that include key transcription factors involved in axonal pathfinding and neural reprogramming. Most previous studies have employed loss-of-function alleles that disrupt lola common exons, making it difficult to delineate isoform-specific functions. To overcome this issue, we have generated isoform-specific mutants for all isoforms using CRISPR/Cas9. This enabled us to study specific isoforms with respect to previously characterized roles for Lola and to demonstrate a specific function for one variant in axon guidance via activation of the microtubule-associated factor Futsch. Importantly, we also reveal a role for a second variant in preventing neurodegeneration via the positive regulation of a key enzyme of the octopaminergic pathway. Thus, our comprehensive study expands the functional repertoire of Lola functions, and it adds insights into the regulatory control of neurotransmitter expression in vivo.

Regulation of Gonad Morphogenesis in Drosophila melanogaster by BTB Family Transcription Factors

During embryogenesis, primordial germ cells (PGCs) and somatic gonadal precursor cells (SGPs) migrate and coalesce to form the early gonad. A failure of the PGCs and SGPs to form a gonad with the proper architecture not only affects germ cell development, but can also lead to infertility. Therefore, it is critical to identify the molecular mechanisms that function within both the PGCs and SGPs to promote gonad morphogenesis. We have characterized the phenotypes of two genes, longitudinals lacking (lola) and ribbon (rib), that are required for the coalescence and compaction of the embryonic gonad in Drosophila melanogaster. rib and lola are expressed in the SGPs of the developing gonad, and genetic interaction analysis suggests these proteins cooperate to regulate gonad development. Both genes encode proteins with DNA binding motifs and a conserved protein-protein interaction domain, known as the Broad complex, Tramtrack, Bric-à-brac (BTB) domain. Through molecular modeling and yeast-two hybrid studies, we demonstrate that Rib and Lola homo- and heterodimerize via their BTB domains. In addition, analysis of the colocalization of Rib and Lola with marks of transcriptional activation and repression on polytene chromosomes reveals that Rib and Lola colocalize with both repressive and activating marks and with each other. While previous studies have identified Rib and Lola targets in other tissues, we find that Rib and Lola are likely to function via different downstream targets in the gonad. These results suggest that Rib and Lola act as dual-function transcription factors to cooperatively regulate embryonic gonad morphogenesis.

Single neuron transcriptomics identify SRSF/SR protein B52 as a regulator of axon growth and Choline acetyltransferase splicing

We removed single identified neurons from living Drosophila embryos to gain insight into the transcriptional control of developing neuronal networks. The microarray analysis of the transcriptome of two sibling neurons revealed seven differentially expressed transcripts between both neurons (threshold: log₂1.4). One transcript encodes the RNA splicing factor B52. Loss of B52 increases growth of axon branches. B52 function is also required for Choline acetyltransferase (ChAT ) splicing. At the end of embryogenesis, loss of B52 function impedes splicing of ChAT, reduces acetylcholine synthesis, and extends the period of uncoordinated muscle twitches during larval hatching. ChAT regulation by SRSF proteins may be a conserved feature since changes in SRSF5 expression and increased acetylcholine levels in brains of bipolar disease patients have been reported recently.

cGMP-Dependent Protein Kinase Encoded by foraging Regulates Motor Axon Guidance in Drosophila by Suppressing Lola Function

Correct pathfinding and target recognition of a developing axon are exquisitely regulated processes that require multiple guidance factors. Among these factors, the second messengers, cAMP and cGMP, are known to be involved in establishing the guidance cues for axon growth through different intracellular signaling pathways. However, whether and how cGMP-dependent protein kinase (PKG) regulates axon guidance remains poorly understood. Here, we show that the motor axons of intersegmental nerve b (ISNb) in the Drosophila embryo display targeting defects during axon development in the absence of foraging(for), a gene encoding PKG.In vivo tag expression revealed PKG to be present in the ventral nerve code at late embryonic stages, supporting its function in embryonic axon guidance. Mechanistic studies showed that the transcription factor longitudinal lacking(lola) genetically interacts with for.PKG physically associates with the LolaT isoform via the C-terminal zinc-finger-containing domain. Overexpression of PKG leads to the cytoplasmic retention of LolaT in S2 cells, suggesting a role for PKG in mediating the nucleocytoplasmic trafficking of Lola. Together, these findings reveal a novel function of PKG in regulating the establishment of neuronal connectivity by sequestering Lola in the cytoplasm.

SIGNIFICANCE STATEMENT

Axon pathfinding and target recognition are important processes in the formation of specific neuronal connectivity, which rely upon precise coordinated deployment of multiple guidance factors. This paper reveals the role of cGMP-dependent protein kinase (PKG) in regulating the pathfinding and targeting of the developing axons in Drosophila Moreover, our study indicates that PKG regulates the cytoplasmic-nuclear trafficking of the transcription factor LolaT, suggesting a mechanism of PKG in directing motor axon guidance. These findings highlight a new function of PKG in axon guidance by suppressing a transcription factor.

Dscam1 Forms a Complex with Robo1 and the N-Terminal Fragment of Slit to Promote the Growth of Longitudinal Axons

The Slit protein is a major midline repellent for central nervous system (CNS) axons. In vivo, Slit is proteolytically cleaved into N- and C-terminal fragments, but the biological significance of this is unknown. Analysis in the Drosophila ventral nerve cord of a slit allele (slit-UC) that cannot be cleaved revealed that midline repulsion is still present but longitudinal axon guidance is disrupted, particularly across segment boundaries. Double mutants for the Slit receptors Dscam1 and robo1 strongly resemble the slit-UC phenotype, suggesting they cooperate in longitudinal axon guidance, and through biochemical approaches, we found that Dscam1 and Robo1 form a complex dependent on Slit-N. In contrast, Robo1 binding alone shows a preference for full-length Slit, whereas Dscam1 only binds Slit-N. Using a variety of transgenes, we demonstrated that Dscam1 appears to modify the output of Robo/Slit complexes so that signaling is no longer repulsive. Our data suggest that the complex is promoting longitudinal axon growth across the segment boundary. The ability of Dscam1 to modify the output of other receptors in a ligand-dependent fashion may be a general principle for Dscam proteins.

Actin cytoskeleton in dendritic spine development and plasticity

Synapses are the basic unit of neuronal communication and their disruption is associated with many neurological disorders. Significant progress has been made towards understanding the molecular and genetic regulation of synapse formation, modulation, and dysfunction, but the underlying cellular mechanisms remain incomplete. The actin cytoskeleton not only provides the structural foundation for synapses, but also regulates a diverse array of cellular activities underlying synaptic function. Here we will discuss the regulation of the actin cytoskeleton in dendritic spines, the postsynaptic compartment of excitatory synapses. We will focus on a select number of actin regulatory processes, highlighting recent advances, the complexity of crosstalk between different pathways, and the challenges of understanding their precise impact on the structure and function of synapses.

Dendrite architecture organized by transcriptional control of the F-actin nucleator Spire

The architectures of dendritic trees are crucial for the wiring and function of neuronal circuits because they determine coverage of receptive territories, as well as the nature and strength of sensory or synaptic inputs. Here, we describe a cell-intrinsic pathway sculpting dendritic arborization (da) neurons in Drosophila that requires Longitudinals Lacking (Lola), a BTB/POZ transcription factor, and its control of the F-actin cytoskeleton through Spire (Spir), an actin nucleation protein. Loss of Lola from da neurons reduced the overall length of dendritic arbors, increased the expression of Spir, and produced inappropriate F-actin-rich dendrites at positions too near the cell soma. Selective removal of Lola from only class IV da neurons decreased the evasive responses of larvae to nociception. The increased Spir expression contributed to the abnormal F-actin-rich dendrites and the decreased nocifensive responses because both were suppressed by reduced dose of Spir. Thus, an important role of Lola is to limit expression of Spir to appropriate levels within da neurons. We found Spir to be expressed in dendritic arbors and to be important for their development. Removal of Spir from class IV da neurons reduced F-actin levels and total branch number, shifted the position of greatest branch density away from the cell soma, and compromised nocifensive behavior. We conclude that the Lola-Spir pathway is crucial for the spatial arrangement of branches within dendritic trees and for neural circuit function because it provides balanced control of the F-actin cytoskeleton.

Enhanced fear expression in Spir-1 actin organizer mutant mice

Spir proteins nucleate actin filaments at vesicle membranes and facilitate intracellular transport processes. The mammalian genome encodes two Spir proteins, namely Spir-1 and Spir-2. While the mouse spir-2 gene has a rather broad expression pattern, high levels of spir-1 expression are restricted to the nervous system, oocytes, and testis. Spir-1 mutant mice generated by a gene trap method have been employed to address Spir-1 function during mouse development and in adult mouse tissues, with a specific emphasis on viability, reproduction, and the nervous system. The gene trap cassette disrupts Spir-1 expression between the N-terminal KIND domain and the WH2 domain cluster. Spir-1 mutant mice are viable and were born in a Mendelian ratio. In accordance with the redundant function of Spir-1 and Spir-2 in oocyte maturation, spir-1 mutant mice are fertile. The overall brain anatomy of spir-1 mutant mice is not altered and visual and motor functions of the mice remain normal. Microscopic analysis shows a slight reduction in the number of dendritic spines on cortical neurons. Detailed behavioral studies of the spir-1 mutant mice, however, unveiled a very specific and highly significant phenotype in terms of fear learning in male mice. In contextual and cued fear conditioning experiments the male spir-1 mutant mice display increased fear memory when compared to their control littermates. Our data point toward a particular function of the vesicle associated Spir-1 actin organizer in neuronal circuits determining fear behavior.

The miRNA pathway controls rapid changes in activity-dependent synaptic structure at the Drosophila melanogaster neuromuscular junction

It is widely accepted that long-term changes in synapse structure and function are mediated by rapid activity-dependent gene transcription and new protein synthesis. A growing amount of evidence suggests that the microRNA (miRNA) pathway plays an important role in coordinating these processes. Despite recent advances in this field, there remains a critical need to identify specific activity-regulated miRNAs as well as their key messenger RNA (mRNA) targets. To address these questions, we used the larval Drosophila melanogaster neuromuscular junction (NMJ) as a model synapse in which to identify novel miRNA-mediated mechanisms that control activity-dependent synaptic growth. First, we developed a screen to identify miRNAs differentially regulated in the larval CNS following spaced synaptic stimulation. Surprisingly, we identified five miRNAs (miRs-1, -8, -289, -314, and -958) that were significantly downregulated by activity. Neuronal misexpression of three miRNAs (miRs-8, -289, and -958) suppressed activity-dependent synaptic growth suggesting that these miRNAs control the translation of biologically relevant target mRNAs. Functional annotation cluster analysis revealed that putative targets of miRs-8 and -289 are significantly enriched in clusters involved in the control of neuronal processes including axon development, pathfinding, and growth. In support of this, miR-8 regulated the expression of a wingless 3′UTR (wg 3′ untranslated region) reporter in vitro. Wg is an important presynaptic regulatory protein required for activity-dependent axon terminal growth at the fly NMJ. In conclusion, our results are consistent with a model where key activity-regulated miRNAs are required to coordinate the expression of genes involved in activity-dependent synaptogenesis.