Drosophila spichthyin inhibits BMP signaling and regulates synaptic growth and axonal microtubules

Bone morphogenetic protein signaling: the pathway and its regulation

In the mid-1960s, bone morphogenetic proteins (BMPs) were first identified in the extracts of bone to have the remarkable ability to induce heterotopic bone. When the Drosophila gene decapentaplegic (dpp) was first identified to share sequence similarity with mammalian BMP2/BMP4 in the late-1980s, it became clear that secreted BMP ligands can mediate processes other than bone formation. Following this discovery, collaborative efforts between Drosophila geneticists and mammalian biochemists made use of the strengths of their respective model systems to identify BMP signaling components and delineate the pathway. The ability to conduct genetic modifier screens in Drosophila with relative ease was critical in identifying the intracellular signal transducers for BMP signaling and the related transforming growth factor-beta/activin signaling pathway. Such screens also revealed a host of genes that encode other core signaling components and regulators of the pathway. In this review, we provide a historical account of this exciting time of gene discovery and discuss how the field has advanced over the past 30 years. We have learned that while the core BMP pathway is quite simple, composed of 3 components (ligand, receptor, and signal transducer), behind the versatility of this pathway lies multiple layers of regulation that ensures precise tissue-specific signaling output. We provide a sampling of these discoveries and highlight many questions that remain to be answered to fully understand the complexity of BMP signaling.

Beyond C9orf72: repeat expansions and copy number variations as risk factors of amyotrophic lateral sclerosis across various populations

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder which is characterized by the loss of both upper and lower motor neurons in the central nervous system. In a significant fraction of ALS cases - irrespective of family history- a genetic background may be identified. The genetic background of ALS shows a high variability from one ethnicity to another. The most frequent genetic cause of ALS is the repeat expansion of the C9orf72 gene. With the emergence of next-generation sequencing techniques and copy number alteration calling tools the focus in ALS genetics has shifted from disease causing genes and mutations towards genetic susceptibility and risk factors.In this review we aimed to summarize the most widely recognized and studied ALS linked repeat expansions and copy number variations other than the hexanucleotide repeat expansion in the C9orf72 gene. We compare and contrast their involvement and phenotype modifying roles in ALS among different populations.

LncRNA NIPA1-SO confers atherosclerotic protection by suppressing the transmembrane protein NIPA1

Long non-coding RNAs (lncRNAs) are emerging as important players in gene regulation and cardiovascular diseases. However, the roles of lncRNAs in atherosclerosis are poorly understood. In the present study, we found that the levels of NIPA1-SO were decreased while those of NIPA1 were increased in human atherosclerotic plaques. Furthermore, NIPA1-SO negatively regulated NIPA1 expression in human umbilical vein endothelial cells (HUVECs). Mechanistically, NIPA1-SO interacted with the transcription factor FUBP1 and the NIPA1 gene. The effect of NIPA1-SO on NIPA1 protein levels was reversed by the knockdown of FUBP1. NIPA1-SO overexpression increased, whilst NIPA1-SO knockdown decreased BMPR2 levels; these effects were enhanced by the knockdown of NIPA1. The overexpression of NIPA1-SO reduced while NIPA1-SO knockdown increased monocyte adhesion to HUVECs; these effects were diminished by the knockdown of BMPR2. The lentivirus-mediated-overexpression of NIPA1-SO or gene-targeted knockout of NIPA1 in low-density lipoprotein receptor-deficient mice reduced monocyte-endothelium adhesion and atherosclerotic lesion formation. Collectively, these findings revealed a novel anti-atherosclerotic role for the lncRNA NIPA1-SO and highlighted its inhibitory effects on vascular inflammation and intracellular cholesterol accumulation by binding to FUBP1 and consequently repressing NIPA1 expression.

Presynaptic Ube3a E3 ligase promotes synapse elimination through down-regulation of BMP signaling

Inactivation of the ubiquitin ligase Ube3a causes the developmental disorder Angelman syndrome, whereas increased Ube3a dosage is associated with autism spectrum disorders. Despite the enriched localization of Ube3a in the axon terminals including presynapses, little is known about the presynaptic function of Ube3a and mechanisms underlying its presynaptic localization. We show that developmental synapse elimination requires presynaptic Ube3a activity in Drosophila neurons. We further identified the domain of Ube3a that is required for its interaction with the kinesin motor. Angelman syndrome-associated missense mutations in the interaction domain attenuate presynaptic targeting of Ube3a and prevent synapse elimination. Conversely, increased Ube3a activity in presynapses leads to precocious synapse elimination and impairs synaptic transmission. Our findings reveal the physiological role of Ube3a and suggest potential pathogenic mechanisms associated with Ube3a dysregulation.

Drosophila Atlastin regulates synaptic vesicle mobilization independent of bone morphogenetic protein signaling

BACKGROUND

The endoplasmic reticulum (ER) contacts endosomes in all parts of a motor neuron, including the axon and presynaptic terminal, to move structural proteins, proteins that send signals, and lipids over long distances. Atlastin (Atl), a large GTPase, is required for membrane fusion and the structural dynamics of the ER tubules. Atl mutations are the second most common cause of Hereditary Spastic Paraplegia (HSP), which causes spasticity in both sexes' lower extremities. Through an unknown mechanism, Atl mutations stimulate the BMP (bone morphogenetic protein) pathway in vertebrates and Drosophila. Synaptic defects are caused by atl mutations, which affect the abundance and distribution of synaptic vesicles (SV) in the bouton. We hypothesize that BMP signaling, does not cause Atl-dependent SV abnormalities in Drosophila.

RESULTS

We show that atl knockdown in motor neurons (Atl-KD) increases synaptic and satellite boutons in the same way that constitutively activating the BMP-receptor Tkv (thick veins) (Tkv-CA) increases the bouton number. The SV proteins Cysteine string protein (CSP) and glutamate vesicular transporter are reduced in Atl-KD and Tkv-CA larvae. Reducing the activity of the BMP receptor Wishful thinking (wit) can rescue both phenotypes. Unlike Tkv-CA larvae, Atl-KD larvae display altered activity-dependent distributions of CSP staining. Furthermore, Atl-KD larvae display an increased FM 1-43 unload than Control and Tkv-CA larvae. As decreasing wit function does not reduce the phenotype, our hypothesis that BMP signaling is not involved is supported. We also found that Rab11/CSP colocalization increased in Atl-KD larvae, which supports the concept that late recycling endosomes regulate SV movements.

CONCLUSIONS

Our findings reveal that Atl modulates neurotransmitter release in motor neurons via SV distribution independently of BMP signaling, which could explain the observed SV accumulation and synaptic dysfunction. Our data suggest that Atl is involved in membrane traffic as well as formation and/or recycling of the late endosome.

Gbb glutathionylation promotes its proteasome-mediated degradation to inhibit synapse growth

Glutathionylation is a posttranslational modification involved in various molecular and cellular processes. However, it remains unknown whether and how glutathionylation regulates nervous system development. To identify critical regulators of synapse growth and development, we performed an RNAi screen and found that postsynaptic knockdown of glutathione transferase omega 1 (GstO1) caused significantly more synaptic boutons at the Drosophila neuromuscular junctions. Genetic and biochemical analysis revealed an increased level of glass boat bottom (Gbb), the Drosophila homolog of mammalian bone morphogenetic protein (BMP), in GstO1 mutants. Further experiments showed that GstO1 is a critical regulator of Gbb glutathionylation at cysteines 354 and 420, which promoted its degradation via the proteasome pathway. Moreover, the E3 ligase Ctrip negatively regulated the Gbb protein level by preferentially binding to glutathionylated Gbb. These results unveil a novel regulatory mechanism in which glutathionylation of Gbb facilitates its ubiquitin-mediated degradation. Taken together, our findings shed new light on the crosstalk between glutathionylation and ubiquitination of Gbb in synapse development.

GTPase-activating protein TBC1D5 coordinates with retromer to constrain synaptic growth by inhibiting BMP signaling

Formation and plasticity of neural circuits rely on precise regulation of synaptic growth. At Drosophila neuromuscular junction (NMJ), Bone Morphogenetic Protein (BMP) signaling is critical for many aspects of synapse formation and function. The evolutionarily conserved retromer complex and its associated GTPase-activating protein TBC1D5 are critical regulators of membrane trafficking and cellular signaling. However, their functions in regulating the formation of NMJ are less understood. Here, we report that TBC1D5 is required for inhibition of synaptic growth, and loss of TBC1D5 leads to abnormal presynaptic terminal development, including excessive satellite boutons and branch formation. Ultrastructure analysis reveals that the size of synaptic vesicles and the density of subsynaptic reticulum are increased in TBC1D5 mutant boutons. Disruption of interactions of TBC1D5 with Rab7 and retromer phenocopies the loss of TBC1D5. Unexpectedly, we find that TBC1D5 is functionally linked to Rab6, in addition to Rab7, to regulate synaptic growth. Mechanistically, we show that loss of TBC1D5 leads to upregulated BMP signaling by increasing the protein level of BMP type II receptor Wishful Thinking (Wit) at NMJ. Overall, our data establish that TBC1D5 in coordination with retromer constrains synaptic growth by regulating Rab7 activity, which negatively regulates BMP signaling through inhibiting Wit level.

Clinical and genetic characterization of NIPA1 mutations in a Taiwanese cohort with hereditary spastic paraplegia

OBJECTIVE

NIPA1 mutations have been implicated in hereditary spastic paraplegia (HSP) as the cause of spastic paraplegia type 6 (SPG6). The aim of this study was to investigate the clinical and genetic features of SPG6 in a Taiwanese HSP cohort.

METHODS

We screened 242 unrelated Taiwanese patients with HSP for NIPA1 mutations. The clinical features of patients with a NIPA1 mutation were analyzed. Minigene-based splicing assay, RT-PCR analysis on the patients' RNA, and cell-based protein expression study were utilized to assess the effects of the mutations on splicing and protein expression.

RESULTS

Two patients were identified to carry a different heterozygous NIPA1 mutation. The two mutations, c.316G>A and c.316G>C, are located in the 3' end of NIPA1 exon 3 near the exon-intron boundary and putatively lead to the same amino acid substitution, p.G106R. The patient harboring NIPA1 c.316G>A manifested spastic paraplegia, epilepsy and schizophrenia since age 17 years, whereas the individual carrying NIPA1 c.316G>C had pure HSP since age 12 years. We reviewed literature and found that epilepsy was present in multiple individuals with NIPA1 c.316G>A but none with NIPA1 c.316G>C. Functional studies demonstrated that both mutations did not affect splicing, but only the c.316G>A mutation was associated with a significantly reduced NIPA1 protein expression.

INTERPRETATION

SPG6 accounted for 0.8% of HSP cases in the Taiwanese cohort. The NIPA1 c.316G>A and c.316G>C mutations are associated with adolescent-onset complex and pure form HSP, respectively. The different effects on protein expression of the two mutations may be associated with their phenotypic discrepancy.

AP2 Regulates Thickveins Trafficking to Attenuate NMJ Growth Signaling in Drosophila

Compromised endocytosis in neurons leads to synapse overgrowth and altered organization of synaptic proteins. However, the molecular players and the signaling pathways which regulate the process remain poorly understood. Here, we show that σ2-adaptin, one of the subunits of the AP2-complex, genetically interacts with Mad, Medea and Dad (components of BMP signaling) to control neuromuscular junction (NMJ) growth in Drosophila Ultrastructural analysis of σ2-adaptin mutants show an accumulation of large vesicles and membranous structures akin to endosomes at the synapse. We found that mutations in σ2-adaptin lead to an accumulation of Tkv receptors at the presynaptic membrane. Interestingly, the level of small GTPase Rab11 was significantly reduced in the σ2-adaptin mutant synapses. However, expression of Rab11 does not restore the synaptic defects of σ2-adaptin mutations. We propose a model in which AP2 regulates Tkv internalization and endosomal recycling to control synaptic growth.

Vav independently regulates synaptic growth and plasticity through distinct actin-based processes

Modulation of presynaptic actin dynamics is fundamental to synaptic growth and functional plasticity; yet the underlying molecular and cellular mechanisms remain largely unknown. At Drosophila NMJs, the presynaptic Rac1-SCAR pathway mediates BMP-induced receptor macropinocytosis to inhibit BMP growth signaling. Here, we show that the Rho-type GEF Vav acts upstream of Rac1 to inhibit synaptic growth through macropinocytosis. We also present evidence that Vav-Rac1-SCAR signaling has additional roles in tetanus-induced synaptic plasticity. Presynaptic inactivation of Vav signaling pathway components, but not regulators of macropinocytosis, impairs post-tetanic potentiation (PTP) and enhances synaptic depression depending on external Ca²⁺ concentration. Interfering with the Vav-Rac1-SCAR pathway also impairs mobilization of reserve pool (RP) vesicles required for tetanus-induced synaptic plasticity. Finally, treatment with an F-actin–stabilizing drug completely restores RP mobilization and plasticity defects in Vav mutants. We propose that actin-regulatory Vav-Rac1-SCAR signaling independently regulates structural and functional presynaptic plasticity by driving macropinocytosis and RP mobilization, respectively.

Dichotomous cis-regulatory motifs mediate the maturation of the neuromuscular junction by retrograde BMP signaling

Retrograde bone morphogenetic protein (BMP) signaling at the Drosophila neuromuscular junction (NMJ) has served as a paradigm to study TGF-β-dependent synaptic function and maturation. Yet, how retrograde BMP signaling transcriptionally regulates these functions remains unresolved. Here, we uncover a gene network, enriched for neurotransmission-related genes, that is controlled by retrograde BMP signaling in motor neurons through two Smad-binding cis-regulatory motifs, the BMP-activating (BMP-AE) and silencer (BMP-SE) elements. Unpredictably, both motifs mediate direct gene activation, with no involvement of the BMP derepression pathway regulators Schnurri and Brinker. Genome editing of candidate BMP-SE and BMP-AE within the locus of the active zone gene bruchpilot, and a novel Ly6 gene witty, demonstrated the role of these motifs in upregulating genes required for the maturation of pre- and post-synaptic NMJ compartments. Our findings uncover how Smad-dependent transcriptional mechanisms specific to motor neurons directly orchestrate a gene network required for synaptic maturation by retrograde BMP signaling.

Effects of copy number variations on brain structure and risk for psychiatric illness: Large-scale studies from the ENIGMA working groups on CNVs

The Enhancing NeuroImaging Genetics through Meta-Analysis copy number variant (ENIGMA-CNV) and 22q11.2 Deletion Syndrome Working Groups (22q-ENIGMA WGs) were created to gain insight into the involvement of genetic factors in human brain development and related cognitive, psychiatric and behavioral manifestations. To that end, the ENIGMA-CNV WG has collated CNV and magnetic resonance imaging (MRI) data from ~49,000 individuals across 38 global research sites, yielding one of the largest studies to date on the effects of CNVs on brain structures in the general population. The 22q-ENIGMA WG includes 12 international research centers that assessed over 533 individuals with a confirmed 22q11.2 deletion syndrome, 40 with 22q11.2 duplications, and 333 typically developing controls, creating the largest-ever 22q11.2 CNV neuroimaging data set. In this review, we outline the ENIGMA infrastructure and procedures for multi-site analysis of CNVs and MRI data. So far, ENIGMA has identified effects of the 22q11.2, 16p11.2 distal, 15q11.2, and 1q21.1 distal CNVs on subcortical and cortical brain structures. Each CNV is associated with differences in cognitive, neurodevelopmental and neuropsychiatric traits, with characteristic patterns of brain structural abnormalities. Evidence of gene-dosage effects on distinct brain regions also emerged, providing further insight into genotype-phenotype relationships. Taken together, these results offer a more comprehensive picture of molecular mechanisms involved in typical and atypical brain development. This "genotype-first" approach also contributes to our understanding of the etiopathogenesis of brain disorders. Finally, we outline future directions to better understand effects of CNVs on brain structure and behavior.

Insights into Clinical, Genetic, and Pathological Aspects of Hereditary Spastic Paraplegias: A Comprehensive Overview

Hereditary spastic paraplegias (HSP) are a heterogeneous group of motor neurodegenerative disorders that have the core clinical presentation of pyramidal syndrome which starts typically in the lower limbs. They can present as pure or complex forms with all classical modes of monogenic inheritance reported. To date, there are more than 100 loci/88 spastic paraplegia genes (SPG) involved in the pathogenesis of HSP. New patterns of inheritance are being increasingly identified in this era of huge advances in genetic and functional studies. A wide range of clinical symptoms and signs are now reported to complicate HSP with increasing overall complexity of the clinical presentations considered as HSP. This is especially true with the emergence of multiple HSP phenotypes that are situated in the borderline zone with other neurogenetic disorders. The genetic diagnostic approaches and the utilized techniques leave a diagnostic gap of 25% in the best studies. In this review, we summarize the known types of HSP with special focus on those in which spasticity is the principal clinical phenotype ("SPGn" designation). We discuss their modes of inheritance, clinical phenotypes, underlying genetics, and molecular pathways, providing some observations about therapeutic opportunities gained from animal models and functional studies. This review may pave the way for more analytic approaches that take into consideration the overall picture of HSP. It will shed light on subtle associations that can explain the occurrence of the disease and allow a better understanding of its observed variations. This should help in the identification of future biomarkers, predictors of disease onset and progression, and treatments for both better functional outcomes and quality of life.

Analysis of Diffusion Tensor Imaging Data From the UK Biobank Confirms Dosage Effect of 15q11.2 Copy Number Variation on White Matter and Shows Association With Cognition

BACKGROUND

Copy number variations at the 15q11.2 BP1-BP2 locus are present in 0.5%-1.0% of the population, and the deletion is associated with several neurodevelopmental disorders. Previously, we showed a reciprocal effect of 15q11.2 copy number variation on fractional anisotropy, with widespread increases in deletion carriers. We aim to expand these findings using a larger sample of participants (N = 29,166) and higher resolution imaging and by examining the implications for cognitive performance.

METHODS

Diffusion tensor imaging measures from participants with no neurological or psychiatric diagnoses were obtained from the UK Biobank database. We compared 15q11.2 BP1-BP2 deletion (n = 102) and duplication (n = 113) carriers to a large cohort of control individuals with no neuropsychiatric copy number variants (n = 28,951). Additionally, we assessed how changes in white matter mediated the association between carrier status and cognitive performance.

RESULTS

Deletion carriers showed increases in fractional anisotropy in the internal capsule and cingulum and decreases in the posterior thalamic radiation compared with both duplication carriers and control subjects (who had intermediate values). Compared with control subjects, deletion carriers had lower scores across cognitive tasks, which were partly influenced by white matter. Reduced fractional anisotropy in the posterior thalamic radiation partially contributed to worse cognitive performance in deletion carriers.

CONCLUSIONS

These results, together with our previous findings, provide convergent evidence for an effect of 15q11.2 BP1-BP2 on white matter microstructure, this being more pronounced in deletion carriers. Additionally, changes in white matter were found to partially mediate cognitive ability in deletion carriers, providing a link between white matter changes in 15q11.2 BP1-BP2 carriers and cognitive function.

Current Knowledge of Endolysosomal and Autophagy Defects in Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) refers to a group of neurological disorders involving the degeneration of motor neurons. Due to their clinical and genetic heterogeneity, finding common effective therapeutics is difficult. Therefore, a better understanding of the common pathological mechanisms is necessary. The role of several HSP genes/proteins is linked to the endolysosomal and autophagic pathways, suggesting a functional convergence. Furthermore, impairment of these pathways is particularly interesting since it has been linked to other neurodegenerative diseases, which would suggest that the nervous system is particularly sensitive to the disruption of the endolysosomal and autophagic systems. In this review, we will summarize the involvement of HSP proteins in the endolysosomal and autophagic pathways in order to clarify their functioning and decipher some of the pathological mechanisms leading to HSP.

The conserved alternative splicing factor caper regulates neuromuscular phenotypes during development and aging

RNA-binding proteins play an important role in the regulation of post-transcriptional gene expression throughout the nervous system. This is underscored by the prevalence of mutations in genes encoding RNA splicing factors and other RNA-binding proteins in a number of neurodegenerative and neurodevelopmental disorders. The highly conserved alternative splicing factor Caper is widely expressed throughout the developing embryo and functions in the development of various sensory neural subtypes in the Drosophila peripheral nervous system. Here we find that caper dysfunction leads to aberrant neuromuscular junction morphogenesis, as well as aberrant locomotor behavior during larval and adult stages. Despite its widespread expression, our results indicate that caper function is required to a greater extent within the nervous system, as opposed to muscle, for neuromuscular junction development and for the regulation of adult locomotor behavior. Moreover, we find that Caper interacts with the RNA-binding protein Fmrp to regulate adult locomotor behavior. Finally, we show that caper dysfunction leads to various phenotypes that have both a sex and age bias, both of which are commonly seen in neurodegenerative disorders in humans.

ESCRT-III-associated proteins and spastin inhibit protrudin-dependent polarised membrane traffic

Mutations in the gene encoding the microtubule severing ATPase spastin are the most frequent cause of hereditary spastic paraplegia, a genetic condition characterised by length-dependent axonal degeneration. Here, we show that HeLa cells lacking spastin and embryonic fibroblasts from a spastin knock-in mouse model become highly polarised and develop cellular protrusions. In HeLa cells, this phenotype was rescued by wild-type spastin, but not by forms unable to sever microtubules or interact with endosomal ESCRT-III proteins. Cells lacking the spastin-interacting ESCRT-III-associated proteins IST1 or CHMP1B also developed protrusions. The protrusion phenotype required protrudin, a RAB-interacting protein that interacts with spastin and localises to ER-endosome contact sites, where it promotes KIF5-dependent endosomal motility to protrusions. Consistent with this, the protrusion phenotype in cells lacking spastin also required KIF5. Lack or mutation of spastin resulted in functional consequences for receptor traffic of a pathway implicated in HSP, as Bone Morphogenetic Protein receptor distribution became polarised. Our results, therefore, identify a novel role for ESCRT-III proteins and spastin in regulating polarised membrane traffic.

The 15q11.2 BP1-BP2 Microdeletion (Burnside-Butler) Syndrome: In Silico Analyses of the Four Coding Genes Reveal Functional Associations with Neurodevelopmental Phenotypes

The 15q11.2 BP1-BP2 microdeletion (Burnside–Butler) syndrome is emerging as the most frequent pathogenic copy number variation (CNV) in humans associated with neurodevelopmental disorders with changes in brain morphology, behavior, and cognition. In this study, we explored functions and interactions of the four protein-coding genes in this region, namely NIPA1, NIPA2, CYFIP1, and TUBGCP5, and elucidate their role, in solo and in concert, in the causation of neurodevelopmental disorders. First, we investigated the STRING protein-protein interactions encompassing all four genes and ascertained their predicted Gene Ontology (GO) functions, such as biological processes involved in their interactions, pathways and molecular functions. These include magnesium ion transport molecular function, regulation of axonogenesis and axon extension, regulation and production of bone morphogenetic protein and regulation of cellular growth and development. We gathered a list of significantly associated cardinal maladies for each gene from searchable genomic disease websites, namely MalaCards.org: HGMD, OMIM, ClinVar, GTR, Orphanet, DISEASES, Novoseek, and GeneCards.org. Through tabulations of such disease data, we ascertained the cardinal disease association of each gene, as well as their expanded putative disease associations. This enabled further tabulation of disease data to ascertain the role of each gene in the top ten overlapping significant neurodevelopmental disorders among the disease association data sets: (1) Prader–Willi Syndrome (PWS); (2) Angelman Syndrome (AS); (3) 15q11.2 Deletion Syndrome with Attention Deficit Hyperactive Disorder & Learning Disability; (4) Autism Spectrum Disorder (ASD); (5) Schizophrenia; (6) Epilepsy; (7) Down Syndrome; (8) Microcephaly; (9) Developmental Disorder, and (10) Peripheral Nervous System Disease. The cardinal disease associations for each of the four contiguous 15q11.2 BP1-BP2 genes are NIPA1- Spastic Paraplegia 6; NIPA2—Angelman Syndrome and Prader–Willi Syndrome; CYFIP1—Fragile X Syndrome and Autism; TUBGCP5—Prader–Willi Syndrome. The four genes are individually associated with PWS, ASD, schizophrenia, epilepsy, and Down syndrome. Except for TUBGCP5, the other three genes are associated with AS. Unlike the other genes, TUBGCP5 is also not associated with attention deficit hyperactivity disorder and learning disability, developmental disorder, or peripheral nervous system disease. CYFIP1 was the only gene not associated with microcephaly but was the only gene associated with developmental disorders. Collectively, all four genes were associated with up to three-fourths of the ten overlapping neurodevelopmental disorders and are deleted in this most prevalent known pathogenic copy number variation now recognized among humans with these clinical findings.

Drosophila VCP/p97 Mediates Dynein-Dependent Retrograde Mitochondrial Motility in Axons

Valosin-containing protein (VCP), also called p97, is an evolutionarily conserved and ubiquitously expressed ATPase with diverse cellular functions. Dominant mutations in VCP are found in a late-onset multisystem degenerative proteinopathy. The neurological manifestations of the disorder include frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). In these patients, long motor neuron axons could be particularly susceptible to defects in axonal transport. However, whether VCP has a physiological function in maintaining axonal transport and whether this role is impaired by disease-causing mutations remains elusive. Here, by employing live-imaging methods in Drosophila larval axons and performing genetic interaction experiments, we discover that VCP regulates the axonal transport of mitochondria. Downregulation of VCP enhances the retrograde transport of mitochondria and reduces the density of mitochondria in larval axons. This unidirectional motility phenotype is rescued by removing one copy of the retrograde motor dynein heavy chain (DHC), or elevating Miro which facilitates anterograde mitochondrial movement by interacting with the anterograde motor kinesin heavy chain (KHC). Importantly, Miro upregulation also significantly improves ATP production of VCP mutant larvae. We investigate human VCP pathogenic mutations in our fly system. We find that expressing these mutations affects mitochondrial transport in the same way as knocking down VCP. Our results reveal a new role of VCP in mediating axonal mitochondrial transport, and provide evidence implicating impaired mitochondrial motility in the pathophysiology of VCP-relevant neurodegenerative diseases.

Association of Copy Number Variation of the 15q11.2 BP1-BP2 Region With Cortical and Subcortical Morphology and Cognition

Importance

Recurrent microdeletions and duplications in the genomic region 15q11.2 between breakpoints 1 (BP1) and 2 (BP2) are associated with neurodevelopmental disorders. These structural variants are present in 0.5% to 1.0% of the population, making 15q11.2 BP1-BP2 the site of the most prevalent known pathogenic copy number variation (CNV). It is unknown to what extent this CNV influences brain structure and affects cognitive abilities.

Objective

To determine the association of the 15q11.2 BP1-BP2 deletion and duplication CNVs with cortical and subcortical brain morphology and cognitive task performance.

Design, Setting, and Participants

In this genetic association study, T1-weighted brain magnetic resonance imaging were combined with genetic data from the ENIGMA-CNV consortium and the UK Biobank, with a replication cohort from Iceland. In total, 203 deletion carriers, 45 247 noncarriers, and 306 duplication carriers were included. Data were collected from August 2015 to April 2019, and data were analyzed from September 2018 to September 2019.

Main Outcomes and Measures

The associations of the CNV with global and regional measures of surface area and cortical thickness as well as subcortical volumes were investigated, correcting for age, age2, sex, scanner, and intracranial volume. Additionally, measures of cognitive ability were analyzed in the full UK Biobank cohort.

Results

Of 45 756 included individuals, the mean (SD) age was 55.8 (18.3) years, and 23 754 (51.9%) were female. Compared with noncarriers, deletion carriers had a lower surface area (Cohen d = -0.41; SE, 0.08; P = 4.9 × 10-8), thicker cortex (Cohen d = 0.36; SE, 0.07; P = 1.3 × 10-7), and a smaller nucleus accumbens (Cohen d = -0.27; SE, 0.07; P = 7.3 × 10-5). There was also a significant negative dose response on cortical thickness (β = -0.24; SE, 0.05; P = 6.8 × 10-7). Regional cortical analyses showed a localization of the effects to the frontal, cingulate, and parietal lobes. Further, cognitive ability was lower for deletion carriers compared with noncarriers on 5 of 7 tasks.

Conclusions and Relevance

These findings, from the largest CNV neuroimaging study to date, provide evidence that 15q11.2 BP1-BP2 structural variation is associated with brain morphology and cognition, with deletion carriers being particularly affected. The pattern of results fits with known molecular functions of genes in the 15q11.2 BP1-BP2 region and suggests involvement of these genes in neuronal plasticity. These neurobiological effects likely contribute to the association of this CNV with neurodevelopmental disorders.

Spastin MIT Domain Disease-Associated Mutations Disrupt Lysosomal Function

The hereditary spastic paraplegias (HSPs) are genetic motor neuron diseases characterized by progressive degeneration of corticospinal tract axons. Mutations in SPAST, encoding the microtubule-severing ATPase spastin, are the most common causes of HSP. The broad SPAST mutational spectrum indicates a haploinsufficiency pathogenic mechanism in most cases. Most missense mutations cluster in the ATPase domain, where they disrupt the protein's ability to sever microtubules. However, several putative missense mutations in the protein's microtubule interacting and trafficking (MIT) domain have also been described, but the pathogenicity of these mutations has not been verified with functional studies. Spastin promotes endosomal tubule fission, and defects in this lead to lysosomal enzyme mistrafficking and downstream lysosomal abnormalities. We investigated the function of three disease-associated spastin MIT mutants and found that none was able to promote normal endosomal tubule fission, lysosomal enzyme receptor trafficking, or lysosomal morphology. One of the mutations affected recruitment of spastin to endosomes, a property that requires the canonical function of the MIT domain in binding endosomal sorting complex required for transport (ESCRT)-III proteins. However, the other mutants did not affect spastin's endosomal recruitment, raising the possibility of pathologically important non-canonical roles for the MIT domain. In conclusion, we demonstrate that spastin MIT mutants cause functional abnormalities related to the pathogenesis of HSP. These mutations do not directly affect spastin's microtubule-severing capacity, and so we identify a new molecular pathological mechanism by which spastin mutations may cause disease.

Axonal transport defects and neurodegeneration: Molecular mechanisms and therapeutic implications

Because of the extremely polarized morphology, the proper functioning of neurons largely relies on the efficient cargo transport along the axon. Axonal transport defects have been reported in multiple neurodegenerative diseases as an early pathological feature. The discovery of mutations in human genes involved in the transport machinery provide a direct causative relationship between axonal transport defects and neurodegeneration. Here, we summarize the current genetic findings related to axonal transport in neurodegenerative diseases, and we discuss the relationship between axonal transport defects and other pathological changes observed in neurodegeneration. In addition, we summarize the therapeutic approaches targeting the axonal transport machinery in studies of neurodegenerative diseases. Finally, we review the technical advances in tracking axonal transport both in vivo and in vitro.

Neuropeptide F regulates courtship in Drosophila through a male-specific neuronal circuit

Male courtship is provoked by perception of a potential mate. In addition, the likelihood and intensity of courtship are influenced by recent mating experience, which affects sexual drive. Using Drosophila melanogaster, we found that the homolog of mammalian neuropeptide Y, neuropeptide F (NPF), and a cluster of male-specific NPF (NPFM) neurons, regulate courtship through affecting courtship drive. Disrupting NPF signaling produces sexually hyperactive males, which are resistant to sexual satiation, and whose courtship is triggered by sub-optimal stimuli. We found that NPFM neurons make synaptic connections with P1 neurons, which comprise the courtship decision center. Activation of P1 neurons elevates NPFM neuronal activity, which then act through NPF receptor neurons to suppress male courtship, and maintain the proper level of male courtship drive.

Cyfip1 haploinsufficient rats show white matter changes, myelin thinning, abnormal oligodendrocytes and behavioural inflexibility

The biological basis of the increased risk for psychiatric disorders seen in 15q11.2 copy number deletion is unknown. Previous work has shown disturbances in white matter tracts in human carriers of the deletion. Here, in a novel rat model, we recapitulated low dosage of the candidate risk gene CYFIP1 present within the 15q11.2 interval. Using diffusion tensor imaging, we first showed extensive white matter changes in Cyfip1 mutant rats, which were most pronounced in the corpus callosum and external capsule. Transmission electron microscopy showed that these changes were associated with thinning of the myelin sheath in the corpus callosum. Myelin thinning was independent of changes in axon number or diameter but was associated with effects on mature oligodendrocytes, including aberrant intracellular distribution of myelin basic protein. Finally, we demonstrated effects on cognitive phenotypes sensitive to both disruptions in myelin and callosal circuitry.

People with a genetic deletion of the 15q11.2 locus are at increased risk for psychiatric disorders and white matter disturbances, but the gene(s) responsible are unclear. Here, the authors show that low dosage of CYFIP1, present in the human 15q11.2 region, alters white matter structure and cognition in rats.

Regulation of subcellular dendritic synapse specificity by axon guidance cues

Neural circuit assembly occurs with subcellular precision, yet the mechanisms underlying this precision remain largely unknown. Subcellular synaptic specificity could be achieved by molecularly distinct subcellular domains that locally regulate synapse formation, or by axon guidance cues restricting access to one of several acceptable targets. We address these models using two Drosophila neurons: the dbd sensory neuron and the A08a interneuron. In wild-type larvae, dbd synapses with the A08a medial dendrite but not the A08a lateral dendrite. dbd-specific overexpression of the guidance receptors Unc-5 or Robo-2 results in lateralization of the dbd axon, which forms anatomical and functional monosynaptic connections with the A08a lateral dendrite. We conclude that axon guidance cues, not molecularly distinct dendritic arbors, are a major determinant of dbd-A08a subcellular synapse specificity.

Reciprocal White Matter Changes Associated With Copy Number Variation at 15q11.2 BP1-BP2: A Diffusion Tensor Imaging Study

BACKGROUND

The 15q11.2 BP1-BP2 cytogenetic region has been associated with learning and motor delays, autism, and schizophrenia. This region includes a gene that codes for the cytoplasmic FMR1 interacting protein 1 (CYFIP1). The CYFIP1 protein is involved in actin cytoskeletal dynamics and interacts with the fragile X mental retardation protein. Absence of fragile X mental retardation protein causes fragile X syndrome. Because abnormal white matter microstructure has been reported in both fragile X syndrome and psychiatric disorders, we looked at the impact of 15q11.2 BP1-BP2 dosage on white matter microstructure.

METHODS

Combining a brain-wide voxel-based approach and a regional-based analysis, we analyzed diffusion tensor imaging data from healthy individuals with the deletion (n = 30), healthy individuals with the reciprocal duplication (n = 27), and IQ-matched control subjects with no large copy number variants (n = 19), recruited from a large genotyped population sample.

RESULTS

We found global mirror effects (deletion > control > duplication) on fractional anisotropy. The deletion group showed widespread increased fractional anisotropy when compared with duplication. Regional analyses revealed a greater effect size in the posterior limb of the internal capsule and a tendency for decreased fractional anisotropy in duplication.

CONCLUSIONS

These results show a reciprocal effect of 15q11.2 BP1-BP2 on white matter microstructure, suggesting that reciprocal chromosomal imbalances may lead to opposite changes in brain structure. Findings in the deletion overlap with previous white matter differences reported in fragile X syndrome patients, suggesting common pathogenic mechanisms derived from disruptions of cytoplasmic CYFIP1-fragile X mental retardation protein complexes. Our data begin to identify specific components of the 15q11.2 BP1-BP2 phenotype and neurobiological mechanisms of potential relevance to the increased risk for disorder.

BMP-dependent synaptic development requires Abi-Abl-Rac signaling of BMP receptor macropinocytosis

Retrograde BMP trans-synaptic signaling is essential for synaptic development. Despite the importance of endocytosis-regulated BMP receptor (BMPR) control of this developmental signaling, the mechanism remains unknown. Here, we provide evidence that Abelson interactor (Abi), a substrate for Abl kinase and component of the SCAR/WAVE complex, links Abl and Rac1 GTPase signaling to BMPR macropinocytosis to restrain BMP-mediated synaptic development. We find that Abi acts downstream of Abl and Rac1, and that BMP ligand Glass bottom boat (Gbb) induces macropinocytosis dependent on Rac1/SCAR signaling, Abl-mediated Abi phosphorylation, and BMPR activation. Macropinocytosis acts as the major internalization route for BMPRs at the synapse in a process driven by Gbb activation and resulting in receptor degradation. Key regulators of macropinocytosis (Rabankyrin and CtBP) control BMPR trafficking to limit BMP trans-synaptic signaling. We conclude that BMP-induced macropinocytosis acts as a BMPR homeostatic mechanism to regulate BMP-mediated synaptic development.

BMP ligands act as retrograde signalling molecules to regulate presynaptic development, and regulation of BMP receptors by endocytosis may be an important component of this signalling pathway. Here, the authors show that Abi-mediated macropinocytosis of BMP receptors in Drosophila larva and contributes to neuromuscular development.

Coordinated Regulation of Axonal Microtubule Organization and Transport by Drosophila Neurexin and BMP Pathway

Neurexins are well known trans-synaptic cell adhesion molecules that are required for proper synaptic development and function across species. Beyond synapse organization and function, little is known about other roles Neurexins might have in the nervous system. Here we report novel phenotypic consequences of mutations in Drosophila neurexin (dnrx), which alters axonal microtubule organization and transport. We show that dnrx mutants display phenotypic similarities with the BMP receptor wishful thinking (wit) and one of the downstream effectors, futsch, which is a known regulator of microtubule organization and stability. dnrx has genetic interactions with wit and futsch. Loss of Dnrx also results in reduced levels of other downstream effectors of BMP signaling, phosphorylated-Mad and Trio. Interestingly, postsynaptic overexpression of the BMP ligand, Glass bottom boat, in dnrx mutants partially rescues the axonal transport defects but not the synapse undergrowth at the neuromuscular junctions. These data suggest that Dnrx and BMP signaling are involved in many diverse functions and that regulation of axonal MT organization and transport might be distinct from regulation of synaptic growth in dnrx mutants. Together, our work uncovers a novel function of Drosophila Neurexin and may provide insights into functions of Neurexins in vertebrates.

BMP- and neuropilin 1-mediated motor axon navigation relies on spastin alternative translation

Functional analyses of genes responsible for neurodegenerative disorders have unveiled crucial links between neurodegenerative processes and key developmental signalling pathways. Mutations in SPG4-encoding spastin cause hereditary spastic paraplegia (HSP). Spastin is involved in diverse cellular processes that couple microtubule severing to membrane remodelling. Two main spastin isoforms are synthesised from alternative translational start sites (M1 and M87). However, their specific roles in neuronal development and homeostasis remain largely unknown. To selectively unravel their neuronal function, we blocked spastin synthesis from each initiation codon during zebrafish development and performed rescue analyses. The knockdown of each isoform led to different motor neuron and locomotion defects, which were not rescued by the selective expression of the other isoform. Notably, both morphant neuronal phenotypes were observed in a CRISPR/Cas9 spastin mutant. We next showed that M1 spastin, together with HSP proteins atlastin 1 and NIPA1, drives motor axon targeting by repressing BMP signalling, whereas M87 spastin acts downstream of neuropilin 1 to control motor neuron migration. Our data therefore suggest that defective BMP and neuropilin 1 signalling may contribute to the motor phenotype in a vertebrate model of spastin depletion.

Drosophila globin1 is required for maintenance of the integrity of F-actin based cytoskeleton during development

Hemoglobins (Hbs) are evolutionarily conserved small globular proteins with characteristic 3-over-3 α-helical sandwich structure that is typically known as "globin fold". Hbs have been found to be involved in diverse biological functions and the characteristic property of oxygen transportation is relatively a recent adaptation. Drosophila genome possesses three globin genes (glob1, glob2, and glob3) and it was previously reported that adequate expression of glob1 is required for various aspects of development, and also to regulate the cellular level of reactive oxygen species (ROS). The present study illustrates the explicit role of glob1 gene in Drosophila development. We demonstrate a dynamic expression pattern of glob1 in larval tissues which largely concentrate around F-actin rich structures and also co-precipitate. Reduced expression of glob1 leads to developmental abnormalities which appeared to be largely mediated by inappropriately formed F-actin based cytoskeletal structures. Our subsequent analysis in FLP/FRT mediated somatic clones establishes specific role of Drosophila glob1 in maintenance of the integrity of F-actin based cytoskeleton during development. For the first time, we report interaction between Glob1 and actin, and propose a novel role of glob1 in maintenance of F-actin based cytoskeleton in Drosophila.

Hereditary spastic paraplegia

The hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurologic disorders with the common feature of prominent lower-extremity spasticity, resulting from a length-dependent axonopathy of corticospinal upper motor neurons. The HSPs exist not only in "pure" forms but also in "complex" forms that are associated with additional neurologic and extraneurologic features. The HSPs are among the most genetically diverse neurologic disorders, with well over 70 distinct genetic loci, for which about 60 mutated genes have already been identified. Numerous studies elucidating the molecular pathogenesis underlying HSPs have highlighted the importance of basic cellular functions - especially membrane trafficking, mitochondrial function, organelle shaping and biogenesis, axon transport, and lipid/cholesterol metabolism - in axon development and maintenance. An encouragingly small number of converging cellular pathogenic themes have been identified for the most common HSPs, and some of these pathways present compelling targets for future therapies.

The role of TGF-β superfamily signaling in neurological disorders

The TGF-β superfamily signaling is involved in a variety of biological processes during embryogenesis and in adult tissue homeostasis. Faulty regulation of the signaling pathway that transduces the TGF-β superfamily signals accordingly leads to a number of ailments, such as cancer and cardiovascular, metabolic, urinary, intestinal, skeletal, and immune diseases. In recent years, a number of studies have elucidated the essential roles of TGF-βs and BMPs during neuronal development in the maintenance of appropriate innervation and neuronal activity. The new advancement implicates significant roles of the aberrant TGF-β superfamily signaling in the pathogenesis of neurological disorders. In this review, we compile a number of reports implicating the deregulation of TGF-β/BMP signaling pathways in the pathogenesis of cognitive and neurodegenerative disorders in animal models and patients. We apologize in advance that the review falls short of providing details of the role of TGF-β/BMP signaling or mechanisms underlying the pathogenesis of neurological disorders. The goal of this article is to reveal a gap in our knowledge regarding the association between TGF-β/BMP signaling pathways and neuronal tissue homeostasis and development and facilitate the research with a potential to develop new therapies for neurological ailments by modulating the pathways.

The Rap activator Gef26 regulates synaptic growth and neuronal survival via inhibition of BMP signaling

In Drosophila, precise regulation of BMP signaling is essential for normal synaptic growth at the larval neuromuscular junction (NMJ) and neuronal survival in the adult brain. However, the molecular mechanisms underlying fine-tuning of BMP signaling in neurons remain poorly understood. We show that loss of the Drosophila PDZ guanine nucleotide exchange factor Gef26 significantly increases synaptic growth at the NMJ and enhances BMP signaling in motor neurons. We further show that Gef26 functions upstream of Rap1 in motor neurons to restrain synaptic growth. Synaptic overgrowth in gef26 or rap1 mutants requires BMP signaling, indicating that Gef26 and Rap1 regulate synaptic growth via inhibition of BMP signaling. We also show that Gef26 is involved in the endocytic downregulation of surface expression of the BMP receptors thickveins (Tkv) and wishful thinking (Wit). Finally, we demonstrate that loss of Gef26 also induces progressive brain neurodegeneration through Rap1- and BMP signaling-dependent mechanisms. Taken together, these results suggest that the Gef26-Rap1 signaling pathway regulates both synaptic growth and neuronal survival by controlling BMP signaling.

Neuroligin 4 regulates synaptic growth via the bone morphogenetic protein (BMP) signaling pathway at the Drosophila neuromuscular junction

The neuroligin (Nlg) family of neural cell adhesion molecules is thought to be required for synapse formation and development and has been linked to the development of autism spectrum disorders in humans. In Drosophila melanogaster, mutations in the neuroligin 1–3 genes have been reported to induce synapse developmental defects at neuromuscular junctions (NMJs), but the role of neuroligin 4 (dnlg4) in synapse development has not been determined. Here, we report that the Drosophila neuroligin 4 (DNlg4) is different from DNlg1–3 in that it presynaptically regulates NMJ synapse development. Loss of dnlg4 results in reduced growth of NMJs with fewer synaptic boutons. The morphological defects caused by dnlg4 mutant are associated with a corresponding decrease in synaptic transmission efficacy. All of these defects could only be rescued when DNlg4 was expressed in the presynapse of NMJs. To understand the basis of DNlg4 function, we looked for genetic interactions and found connections with the components of the bone morphogenetic protein (BMP) signaling pathway. Immunostaining and Western blot analyses demonstrated that the regulation of NMJ growth by DNlg4 was due to the positive modulation of BMP signaling by DNlg4. Specifically, BMP type I receptor thickvein (Tkv) abundance was reduced in dnlg4 mutants, and immunoprecipitation assays showed that DNlg4 and Tkv physically interacted in vivo. Our study demonstrates that DNlg4 presynaptically regulates neuromuscular synaptic growth via the BMP signaling pathway by modulating Tkv.

TGF-β Family Signaling in Drosophila

The transforming growth factor β (TGF-β) family signaling pathway is conserved and ubiquitous in animals. In Drosophila, fewer representatives of each signaling component are present compared with vertebrates, simplifying mechanistic study of the pathway. Although there are fewer family members, the TGF-β family pathway still regulates multiple and diverse functions in Drosophila. In this review, we focus our attention on several of the classic and best-studied functions for TGF-β family signaling in regulating Drosophila developmental processes such as embryonic and imaginal disc patterning, but we also describe several recently discovered roles in regulating hormonal, physiological, neuronal, innate immunity, and tissue homeostatic processes.

Drosophila Atlastin in motor neurons is required for locomotion and presynaptic function

Hereditary spastic paraplegias (HSPs) are characterized by spasticity and weakness of the lower limbs, resulting from length-dependent axonopathy of the corticospinal tracts. In humans, the HSP-related atlastin genes ATL1-ATL3 catalyze homotypic membrane fusion of endoplasmic reticulum (ER) tubules. How defects in neuronal Atlastin contribute to axonal degeneration has not been explained satisfactorily. Using Drosophila, we demonstrate that downregulation or overexpression of Atlastin in motor neurons results in decreased crawling speed and contraction frequency in larvae, while adult flies show progressive decline in climbing ability. Broad expression in the nervous system is required to rescue the atlastin-null Drosophila mutant (atl² ) phenotype. Importantly, both spontaneous release and the reserve pool of synaptic vesicles are affected. Additionally, axonal secretory organelles are abnormally distributed, whereas presynaptic proteins diminish at terminals and accumulate in distal axons, possibly in lysosomes. Our findings suggest that trafficking defects produced by Atlastin dysfunction in motor neurons result in redistribution of presynaptic components and aberrant mobilization of synaptic vesicles, stressing the importance of ER-shaping proteins and the susceptibility of motor neurons to their mutations or depletion.

Notum coordinates synapse development via extracellular regulation of Wingless trans-synaptic signaling

Synaptogenesis requires orchestrated communication between pre- and postsynaptic cells via coordinated trans-synaptic signaling across the extracellular synaptomatrix. The first Wnt signaling ligand discovered, Drosophila Wingless (Wg; Wnt1 in mammals), plays crucial roles in synaptic development, regulating synapse architecture as well as functional differentiation. Here, we investigate synaptogenic functions of the secreted extracellular deacylase Notum, which restricts Wg signaling by cleaving an essential palmitoleate moiety. At the glutamatergic neuromuscular junction (NMJ) synapse, we find that Notum secreted from the postsynaptic muscle acts to strongly modulate synapse growth, structural architecture, ultrastructural development and functional differentiation. In Notum null flies, we find upregulated extracellular Wg ligand and nuclear trans-synaptic signal transduction, as well as downstream misregulation of both pre- and postsynaptic molecular assembly. Structural, functional and molecular synaptogenic defects are all phenocopied by Wg overexpression, suggesting that Notum acts solely by inhibiting Wg trans-synaptic signaling. Moreover, these synaptic development phenotypes are suppressed by genetically correcting Wg levels in Notum null mutants, indicating that Notum normally functions to coordinate synaptic structural and functional differentiation via negative regulation of Wg trans-synaptic signaling in the extracellular synaptomatrix.

MCTP is an ER-resident calcium sensor that stabilizes synaptic transmission and homeostatic plasticity

Presynaptic homeostatic plasticity (PHP) controls synaptic transmission in organisms from Drosophila to human and is hypothesized to be relevant to the cause of human disease. However, the underlying molecular mechanisms of PHP are just emerging and direct disease associations remain obscure. In a forward genetic screen for mutations that block PHP we identified mctp (Multiple C2 Domain Proteins with Two Transmembrane Regions). Here we show that MCTP localizes to the membranes of the endoplasmic reticulum (ER) that elaborate throughout the soma, dendrites, axon and presynaptic terminal. Then, we demonstrate that MCTP functions downstream of presynaptic calcium influx with separable activities to stabilize baseline transmission, short-term release dynamics and PHP. Notably, PHP specifically requires the calcium coordinating residues in each of the three C2 domains of MCTP. Thus, we propose MCTP as a novel, ER-localized calcium sensor and a source of calcium-dependent feedback for the homeostatic stabilization of neurotransmission.

spict, a cyst cell-specific gene, regulates starvation-induced spermatogonial cell death in the Drosophila testis

Tissues are maintained in a homeostatic state by balancing the constant loss of old cells with the continued production of new cells. Tissue homeostasis can shift between high and low turnover states to cope with environmental changes such as nutrient availability. Recently, we discovered that the elimination of transit-amplifying cells plays a critical role in maintaining the stem cell population during protein starvation in the Drosophila testis. Here, we identify spict, a gene expressed specifically in differentiating cyst cells, as a regulator of spermatogonial death. Spict is upregulated in cyst cells that phagocytose dying spermatogonia. We propose that phagocytosis and subsequent clearance of dead spermatogonia, which is partly promoted by Spict, contribute to stem cell maintenance during prolonged protein starvation.

Beneficial effects of rapamycin in a Drosophila model for hereditary spastic paraplegia

The locomotor deficits in the group of diseases referred to as hereditary spastic paraplegia (HSP) reflect degeneration of upper motor neurons, but the mechanisms underlying this neurodegeneration are unknown. We established a Drosophila model for HSP, atlastin (atl), which encodes an ER fusion protein. Here, we show that neuronal atl loss causes degeneration of specific thoracic muscles that is preceded by other pathologies, including accumulation of aggregates containing polyubiquitin, increased generation of reactive oxygen species and activation of the JNK-Foxo stress response pathway. We show that inhibiting the Tor kinase, either genetically or by administering rapamycin, at least partially reversed many of these pathologies. atl loss from muscle also triggered muscle degeneration and rapamycin-sensitive locomotor deficits, as well as polyubiquitin aggregate accumulation. These results indicate that atl loss triggers muscle degeneration both cell autonomously and nonautonomously.

S113R mutation in SLC33A1 leads to neurodegeneration and augmented BMP signaling in a mouse model

The S113R mutation (c.339T>G) (MIM #603690.0001) in SLC33A1 (MIM #603690), an ER membrane acetyl-CoA transporter, has been previously identified in individuals with hereditary spastic paraplegia type 42 (SPG42; MIM #612539). SLC33A1 has also been shown to inhibit the bone morphogenetic protein (BMP) signaling pathway in zebrafish. To better understand the function of SLC33A1, we generated and characterized Slc33a1^S113R knock-in mice. Homozygous Slc33a1^S113R mutant mice were embryonic lethal, whereas heterozygous Slc33a1 mutant mice (Slc33a1^wt/mut) exhibited behavioral abnormalities and central neurodegeneration, which is consistent with hereditary spastic paraplegia (HSP) phenotypes. Importantly, we found an upregulation of BMP signaling in the nervous system and mouse embryonic fibroblasts of Slc33a1^wt/mut mice. Using a sciatic nerve crush injury model in vivo and dorsal root ganglion (DRG) culture in vitro we showed that injury-induced axonal regeneration in Slc33a1^wt/mut mice was accelerated and mediated by upregulated BMP signaling. Exogenous addition of BMP signaling antagonist, noggin, could efficiently alleviate the accelerated injury-induced axonal regrowth. These results indicate that SLC33A1 can negatively regulate BMP signaling in mice, further supporting the notion that upregulation of BMP signaling is a common mechanism of a subset of hereditary spastic paraplegias.

Summary:Slc33a1^wt/mut knock-in mice with a S113R mutation and exhibiting hereditary spastic paraplegia-related phenotypes show that SLC33A1 negatively regulates BMP signaling and axonal regeneration.

Scribbled Optimizes BMP Signaling through Its Receptor Internalization to the Rab5 Endosome and Promote Robust Epithelial Morphogenesis

Epithelial cells are characterized by apical-basal polarity. Intrinsic factors underlying apical-basal polarity are crucial for tissue homeostasis and have often been identified to be tumor suppressors. Patterning and differentiation of epithelia are key processes of epithelial morphogenesis and are frequently regulated by highly conserved extrinsic factors. However, due to the complexity of morphogenesis, the mechanisms of precise interpretation of signal transduction as well as spatiotemporal control of extrinsic cues during dynamic morphogenesis remain poorly understood. Wing posterior crossvein (PCV) formation in Drosophila serves as a unique model to address how epithelial morphogenesis is regulated by secreted growth factors. Decapentaplegic (Dpp), a conserved bone morphogenetic protein (BMP)-type ligand, is directionally trafficked from longitudinal veins (LVs) into the PCV region for patterning and differentiation. Our data reveal that the basolateral determinant Scribbled (Scrib) is required for PCV formation through optimizing BMP signaling. Scrib regulates BMP-type I receptor Thickveins (Tkv) localization at the basolateral region of PCV cells and subsequently facilitates Tkv internalization to Rab5 endosomes, where Tkv is active. BMP signaling also up-regulates scrib transcription in the pupal wing to form a positive feedback loop. Our data reveal a unique mechanism in which intrinsic polarity genes and extrinsic cues are coupled to promote robust morphogenesis.

Epithelial morphogenesis is one of the key processes in animal development. Evolutionarily conserved growth factors frequently instruct patterning and differentiation in morphogenesis. However, little is known about how extracellular cues and epithelial morphogenesis are mutually coordinated in vivo. Wing posterior crossvein (PCV) development in Drosophila provides an excellent system for understanding how bone morphogenetic protein (BMP) signaling regulates patterning and differentiation of epithelia. We find that the apical-basal polarity gene Scribbled (Scrib) is required for PCV formation by optimizing BMP signaling in the PCV region as follows. First, Scrib regulates BMP type-I receptor Thickveins (Tkv) localization basally. Second, Scrib facilitates Tkv internalization to the Rab5 endosomes to optimize signal transduction after receptor-ligand binding. Third, BMP signaling up-regulates scrib transcription in the pupal wing to form a positive feedback loop. These results suggest that coupling between epithelial polarity genes and conserved growth factors play crucial roles in patterning and differentiation of epithelia.

The Role of BAR Domain Proteins in the Regulation of Membrane Dynamics

Many cellular processes are associated with membrane remodeling. The BAR domain protein family plays a key role in the formation and detection of local membrane curvatures and in attracting other proteins, including the regulators of actin dynamics. Based on their structural and phylogenetic properties, BAR domains are divided into several groups which affect membrane in various ways and perform different functions in cells. However, recent studies have uncovered evidence of functional differences even within the same group. This review discusses the principles underlying the interactions of different groups of BAR domains, and their individual representatives ,with membranes.

Drosophila glob1 is required for the maintenance of cytoskeletal integrity during oogenesis

BACKGROUND

Hemoglobins (Hbs) are evolutionarily conserved heme-containing metallo-proteins of the Globin protein family that harbour the characteristic "globin fold." Hemoglobins have been functionally diversified during evolution and their usual property of oxygen transport is rather a recent adaptation. Drosophila genome possesses three globin genes (glob1, glob2, and glob3), and we have reported earlier that adequate expression of glob1 is required for various aspects of development, as well as to regulate the cellular level of reactive oxygen species (ROS). The present study illustrates the explicit role of Drosophila globin1 in progression of oogenesis.

RESULTS

We demonstrate a dynamic expression pattern of glob1 in somatic and germ cell derivatives of developing egg chambers during various stages of oogenesis, which largely confines around the F-actin-rich cellular components. Reduced expression of glob1 leads to various types of abnormalities during oogenesis, which were primarily mediated by the inappropriately formed F-actin-based cytoskeleton. Our subsequent analysis in the somatic and germ line clones shows cell autonomous role of glob1 in the maintenance of the integrity of F-actin-based cytoskeleton components in the somatic and germ cell derivatives.

CONCLUSIONS

Our study establishes a novel role of glob1 in maintenance of F-actin-based cytoskeleton during progression of oogenesis in Drosophila. Developmental Dynamics 245:1048-1065, 2016. © 2016 Wiley Periodicals, Inc.

The Presynaptic Microtubule Cytoskeleton in Physiological and Pathological Conditions: Lessons from Drosophila Fragile X Syndrome and Hereditary Spastic Paraplegias

The capacity of the nervous system to generate neuronal networks relies on the establishment and maintenance of synaptic contacts. Synapses are composed of functionally different presynaptic and postsynaptic compartments. An appropriate synaptic architecture is required to provide the structural basis that supports synaptic transmission, a process involving changes in cytoskeletal dynamics. Actin microfilaments are the main cytoskeletal components present at both presynaptic and postsynaptic terminals in glutamatergic synapses. However, in the last few years it has been demonstrated that microtubules (MTs) transiently invade dendritic spines, promoting their maturation. Nevertheless, the presence and functions of MTs at the presynaptic site are still a matter of debate. Early electron microscopy (EM) studies revealed that MTs are present in the presynaptic terminals of the central nervous system (CNS) where they interact with synaptic vesicles (SVs) and reach the active zone. These observations have been reproduced by several EM protocols; however, there is empirical heterogeneity in detecting presynaptic MTs, since they appear to be both labile and unstable. Moreover, increasing evidence derived from studies in the fruit fly neuromuscular junction proposes different roles for MTs in regulating presynaptic function in physiological and pathological conditions. In this review, we summarize the main findings that support the presence and roles of MTs at presynaptic terminals, integrating descriptive and biochemical analyses, and studies performed in invertebrate genetic models.

Cellular Organization of Neuroimmune Interactions in the Gastrointestinal Tract

The gastrointestinal (GI) tract is the largest immune organ; in vertebrates, it is the only organ whose function is controlled by its own intrinsic enteric nervous system (ENS), but it is additionally regulated by extrinsic (sympathetic and parasympathetic) innervation. The GI nervous and immune systems are highly integrated in their common goal, which is to unite digestive functions with protection from ingested environmental threats. This review discusses the physiological relevance of enteric neuroimmune integration by summarizing the current knowledge of evolutionary and developmental pathways, cellular organization, and molecular mechanisms of neuroimmune interactions in health and disease.

Human Senataxin Modulates Structural Plasticity of the Neuromuscular Junction in Drosophila through a Neuronally Conserved TGFβ Signalling Pathway

BACKGROUND

Mutations in the human Senataxin (hSETX) gene have been shown to cause two forms of neurodegenerative disorders - a dominant form called amyotrophic lateral sclerosis type 4 (ALS4) and a recessive form called ataxia with oculomotor apraxia type 2 (AOA2). SETX is a putative DNA/RNA helicase involved in RNA metabolism. Although several dominant mutations linked with ALS4 have been identified in SETX, their contribution towards ALS4 pathophysiology is still elusive.

METHOD

In order to model ALS4 in Drosophila and to elucidate the morphological, physiological and signalling consequences, we overexpressed the wild-type and pathological forms of hSETX in Drosophila.

RESULTS AND CONCLUSIONS

The pan-neuronal expression of wild-type or mutant forms of hSETX induced morphological plasticity at neuromuscular junction (NMJ) synapses. Surprisingly, we found that while the NMJ synapses were increased in number, the neuronal function was normal. Analysis of signalling pathways revealed that hSETX modulates the Highwire (Hiw; a conserved neuronal E3 ubiquitin ligase)-dependent bone morphogenetic protein/TGFβ pathway. Thus, our study could pave the way for a better understanding of ALS4 progression by SETX through the regulation of neuronal E3 ubiquitin pathways.