The effects of ER morphology on synaptic structure and function in Drosophila melanogaster.

Hereditary spastic paraplegia (HSP) is a set of genetic diseases caused by mutations in one of 72 genes that results in age-dependent corticospinal axon degeneration accompanied by spasticity and paralysis. Two genes implicated in HSPs encode proteins that regulate endoplasmic reticulum (ER) morphology. Atlastin 1 (ATL1, also known as SPG3A) encodes an ER membrane fusion GTPase and reticulon 2 (RTN2, also known as SPG12) helps shape ER tube formation. Here, we use a new fluorescent ER marker to show that the ER within wild-type Drosophila motor nerve terminals forms a network of tubules that is fragmented and made diffuse upon loss of the atlastin 1 ortholog atl. atl or Rtnl1 loss decreases evoked transmitter release and increases arborization. Similar to other HSP proteins, Atl inhibits bone morphogenetic protein (BMP) signaling, and loss of atl causes age-dependent locomotor deficits in adults. These results demonstrate a crucial role for ER in neuronal function, and identify mechanistic links between ER morphology, neuronal function, BMP signaling and adult behavior.

Spastin binds to lipid droplets and affects lipid metabolism.

Mutations in SPAST, encoding spastin, are the most common cause of autosomal dominant hereditary spastic paraplegia (HSP). HSP is characterized by weakness and spasticity of the lower limbs, owing to progressive retrograde degeneration of the long corticospinal axons. Spastin is a conserved microtubule (MT)-severing protein, involved in processes requiring rearrangement of the cytoskeleton in concert to membrane remodeling, such as neurite branching, axonal growth, midbody abscission, and endosome tubulation. Two isoforms of spastin are synthesized from alternative initiation codons (M1 and M87). We now show that spastin-M1 can sort from the endoplasmic reticulum (ER) to pre- and mature lipid droplets (LDs). A hydrophobic motif comprised of amino acids 57 through 86 of spastin was sufficient to direct a reporter protein to LDs, while mutation of arginine 65 to glycine abolished LD targeting. Increased levels of spastin-M1 expression reduced the number but increased the size of LDs. Expression of a mutant unable to bind and sever MTs caused clustering of LDs. Consistent with these findings, ubiquitous overexpression of Dspastin in Drosophila led to bigger and less numerous LDs in the fat bodies and increased triacylglycerol levels. In contrast, Dspastin overexpression increased LD number when expressed specifically in skeletal muscles or nerves. Downregulation of Dspastin and expression of a dominant-negative variant decreased LD number in Drosophila nerves, skeletal muscle and fat bodies, and reduced triacylglycerol levels in the larvae. Moreover, we found reduced amount of fat stores in intestinal cells of worms in which the spas-1 homologue was either depleted by RNA interference or deleted. Taken together, our data uncovers an evolutionarily conserved role of spastin as a positive regulator of LD metabolism and open up the possibility that dysfunction of LDs in axons may contribute to the pathogenesis of HSP.

EFHC1 variants in juvenile myoclonic epilepsy: reanalysis according to NHGRI and ACMG guidelines for assigning disease causality.

PURPOSE: EFHC1 variants are the most common mutations in inherited myoclonic and grand mal clonic-tonic-clonic (CTC) convulsions of juvenile myoclonic epilepsy (JME). We reanalyzed 54 EFHC1 variants associated with epilepsy from 17 cohorts based on National Human Genome Research Institute (NHGRI) and American College of Medical Genetics and Genomics (ACMG) guidelines for interpretation of sequence variants. METHODS: We calculated Bayesian LOD scores for variants in coinheritance, unconditional exact tests and odds ratios (OR) in case-control associations, allele frequencies in genome databases, and predictions for conservation/pathogenicity. We reviewed whether variants damage EFHC1 functions, whether efhc1-/- KO mice recapitulate CTC convulsions and "microdysgenesis" neuropathology, and whether supernumerary synaptic and dendritic phenotypes can be rescued in the fly model when EFHC1 is overexpressed. We rated strengths of evidence and applied ACMG combinatorial criteria for classifying variants. RESULTS: Nine variants were classified as "pathogenic," 14 as "likely pathogenic," 9 as "benign," and 2 as "likely benign." Twenty variants of unknown significance had an insufficient number of ancestry-matched controls, but ORs exceeded 5 when compared with racial/ethnic-matched Exome Aggregation Consortium (ExAC) controls. CONCLUSIONS: NHGRI gene-level evidence and variant-level evidence establish EFHC1 as the first non-ion channel microtubule-associated protein whose mutations disturb R-type VDCC and TRPM2 calcium currents in overgrown synapses and dendrites within abnormally migrated dislocated neurons, thus explaining CTC convulsions and "microdysgenesis" neuropathology of JME.Genet Med 19 2, 144-156.

Homotypic fusion of ER membranes requires the dynamin-like GTPase atlastin.

Establishment and maintenance of proper architecture is essential for endoplasmic reticulum (ER) function. Homotypic membrane fusion is required for ER biogenesis and maintenance, and has been shown to depend on GTP hydrolysis. Here we demonstrate that Drosophila Atlastin--the fly homologue of the mammalian GTPase atlastin 1 involved in hereditary spastic paraplegia--localizes on ER membranes and that its loss causes ER fragmentation. Drosophila Atlastin embedded in distinct membranes has the ability to form trans-oligomeric complexes and its overexpression induces enlargement of ER profiles, consistent with excessive fusion of ER membranes. In vitro experiments confirm that Atlastin autonomously drives membrane fusion in a GTP-dependent fashion. In contrast, GTPase-deficient Atlastin is inactive, unable to form trans-oligomeric complexes owing to failure to self-associate, and incapable of promoting fusion in vitro. These results demonstrate that Atlastin mediates membrane tethering and fusion and strongly suggest that it is the GTPase activity that is required for ER homotypic fusion.

Membrane fusion by the GTPase atlastin requires a conserved C-terminal cytoplasmic tail and dimerization through the middle domain.

The biogenesis and maintenance of the endoplasmic reticulum (ER) requires membrane fusion. ER homotypic fusion is driven by the large GTPase atlastin. Domain analysis of atlastin shows that a conserved region of the C-terminal cytoplasmic tail is absolutely required for fusion activity. Atlastin in adjacent membranes must associate to bring the ER membranes into molecular contact. Drosophila atlastin dimerizes in the presence of GTPγS but is monomeric with GDP or without nucleotide. Oligomerization requires the juxtamembrane middle domain three-helix bundle, as does efficient GTPase activity. A soluble version of the N-terminal cytoplasmic domain that contains the GTPase domain and the middle domain three-helix bundle serves as a potent, concentration-dependent inhibitor of membrane fusion both in vitro and in vivo. However, atlastin domains lacking the middle domain are without effect. GTP-dependent dimerization of atlastin generates an enzymatically active protein that drives membrane fusion after nucleotide hydrolysis and conformational reorganization.

GTP-dependent packing of a three-helix bundle is required for atlastin-mediated fusion.

The mechanisms governing atlastin-mediated membrane fusion are unknown. Here we demonstrate that a three-helix bundle (3HB) within the middle domain is required for oligomerization. Mutation of core hydrophobic residues within these helices inactivates atlastin function by preventing membrane tethering and the subsequent fusion. GTP binding induces a conformational change that reorients the GTPase domain relative to the 3HB to permit self-association, but the ability to hydrolyze GTP is required for full fusion, indicating that nucleotide binding and hydrolysis play distinct roles. Oligomerization of atlastin stimulates its ability to hydrolyze GTP, and the energy released drives lipid bilayer merger. Mutations that prevent atlastin self-association also abolish oligomerization-dependent stimulation of GTPase activity. Furthermore, increasing the distance of atlastin complex formation from the membrane inhibits fusion, suggesting that this distance is crucial for atlastin to promote fusion.

Defhc1.1, a homologue of the juvenile myoclonic gene EFHC1, modulates architecture and basal activity of the neuromuscular junction in Drosophila.

Mutations in the EFHC1 gene have been linked to juvenile myoclonic epilepsy. To understand EFHC1 function in vivo, we generated knockout Drosophila for the fly homolog Defhc1.1. We found that the neuromuscular junction synapse of Defhc1.1 mutants displays an increased number of satellite boutons resulting in increased spontaneous neurotransmitter release. Defhc1.1 binds to microtubules in vitro and overlaps in vivo with axonal and synaptic microtubules. Elimination of Defhc1.1 from synaptic terminals reduces the number of microtubule loops, suggesting that Defhc1.1 is a negative regulator of microtubule dynamics. In fact, pharmacological treatment of Defhc1.1 mutants with vinblastine, an inhibitor of microtubule dynamics, suppresses the satellite bouton phenotype. Furthermore, Defhc1.1 mutants display overgrowth of the dendritic arbor and Defhc1.1 overexpression reduces dendrite elaboration. These results suggest that Defhc1.1 functions as an inhibitor of neurite growth by finely tuning the microtubule cytoskeleton dynamics and that EFHC1-dependent juvenile myoclonic epilepsy may result from augmented spontaneous neurotransmitter release due to overgrowth of neuronal processes.

ER Morphology in the Pathogenesis of Hereditary Spastic Paraplegia.

The endoplasmic reticulum (ER) is the most abundant and widespread organelle in cells. Its peculiar membrane architecture, formed by an intricate network of tubules and cisternae, is critical to its multifaceted function. Regulation of ER morphology is coordinated by a few ER-specific membrane proteins and is thought to be particularly important in neurons, where organized ER membranes are found even in the most distant neurite terminals. Mutation of ER-shaping proteins has been implicated in the neurodegenerative disease hereditary spastic paraplegia (HSP). In this review we discuss the involvement of these proteins in the pathogenesis of HSP, focusing on the experimental evidence linking their molecular function to disease onset. Although the precise biochemical activity of some ER-related HSP proteins has been elucidated, the pathological mechanism underlying ER-linked HSP is still undetermined and needs to be further investigated.

In vivo Analysis of CRISPR/Cas9 Induced Atlastin Pathological Mutations in Drosophila.

The endoplasmic reticulum (ER) is a highly dynamic network whose shape is thought to be actively regulated by membrane resident proteins. Mutation of several such morphology regulators cause the neurological disorder Hereditary Sp astic Paraplegia (HSP), suggesting a critical role of ER shape maintenance in neuronal activity and function. Human Atlastin-1 mutations are responsible for SPG3A, the earliest onset and one of the more severe forms of dominant HSP. Atlastin has been initially identified in Drosophila as the GTPase responsible for the homotypic fusion of ER membrane. The majority of SPG3A-linked Atlastin-1 mutations map to the GTPase domain, potentially interfering with atlastin GTPase activity, and to the three-helix-bundle (3HB) domain, a region critical for homo-oligomerization. Here we have examined the in vivo effects of four pathogenetic missense mutations (two mapping to the GTPase domain and two to the 3HB domain) using two complementary approaches: CRISPR/Cas9 editing to introduce such variants in the endogenous atlastin gene and transgenesis to generate lines overexpressing atlastin carrying the same pathogenic variants. We found that all pathological mutations examined reduce atlastin activity in vivo although to different degrees of severity. Moreover, overexpression of the pathogenic variants in a wild type atlastin background does not give rise to the loss of function phenotypes expected for dominant negative mutations. These results indicate that the four pathological mutations investigated act through a loss of function mechanism.

Microtubules Stabilization by Mutant Spastin Affects ER Morphology and Ca2+ Handling.

The endoplasmic reticulum (ER) extends as a network of interconnected tubules and sheet-like structures in eukaryotic cells. ER tubules dynamically change their morphology and position within the cells in response to physiological stimuli and these network rearrangements depend on the microtubule (MT) cytoskeleton. Store-operated calcium entry (SOCE) relies on the repositioning of ER tubules to form specific ER-plasma membrane junctions. Indeed, the tips of polymerizing MTs are supposed to provide the anchor for ER tubules to move toward the plasma membrane, however the precise role of the cytoskeleton during SOCE has not been conclusively clarified. Here we exploit an in vivo approach involving the manipulation of MT dynamics in Drosophila melanogaster by neuronal expression of a dominant-negative variant of the MT-severing protein spastin to induce MT hyper-stabilization. We show that MT stabilization alters ER morphology, favoring an enrichment in ER sheets at the expense of tubules. Stabilizing MTs has a negative impact on the process of SOCE and results in a reduced ER Ca2+ content, affecting the flight ability of the flies. Restoring proper MT organization by administering the MT-destabilizing drug vinblastine, chronically or acutely, rescues ER morphology, SOCE and flight ability, indicating that MT dynamics impairment is responsible for all the phenotypes observed.

Proteasome dysfunction induces excessive proteome instability and loss of mitostasis that can be mitigated by enhancing mitochondrial fusion or autophagy.

The ubiquitin-proteasome pathway (UPP) is central to proteostasis network (PN) functionality and proteome quality control. Yet, the functional implication of the UPP in tissue homeodynamics at the whole organism level and its potential cross-talk with other proteostatic or mitostatic modules are not well understood. We show here that knock down (KD) of proteasome subunits in Drosophila flies, induced, for most subunits, developmental lethality. Ubiquitous or tissue specific proteasome dysfunction triggered systemic proteome instability and activation of PN modules, including macroautophagy/autophagy, molecular chaperones and the antioxidant cncC (the fly ortholog of NFE2L2/Nrf2) pathway. Also, proteasome KD increased genomic instability, altered metabolic pathways and severely disrupted mitochondrial functionality, triggering a cncC-dependent upregulation of mitostatic genes and enhanced rates of mitophagy. Whereas, overexpression of key regulators of antioxidant responses (e.g., cncC or foxo) could not suppress the deleterious effects of proteasome dysfunction; these were alleviated in both larvae and adult flies by modulating mitochondrial dynamics towards increased fusion or by enhancing autophagy. Our findings reveal the extensive functional wiring of genomic, proteostatic and mitostatic modules in higher metazoans. Also, they support the notion that age-related increase of proteotoxic stress due to decreased UPP activity deregulates all aspects of cellular functionality being thus a driving force for most age-related diseases. Abbreviations: ALP: autophagy-lysosome pathway; ARE: antioxidant response element; Atg8a: autophagy-related 8a; ATPsynß: ATP synthase, ß subunit; C-L: caspase-like proteasomal activity; cncC: cap-n-collar isoform-C; CT-L: chymotrypsin-like proteasomal activity; Drp1: dynamin related protein 1; ER: endoplasmic reticulum; foxo: forkhead box, sub-group O; GLU: glucose; GFP: green fluorescent protein; GLY: glycogen; Hsf: heat shock factor; Hsp: Heat shock protein; Keap1: kelch-like ECH-associated protein 1; Marf: mitochondrial assembly regulatory factor; NFE2L2/Nrf2: nuclear factor, erythroid 2 like 2; Opa1: optic atrophy 1; PN: proteostasis network; RNAi: RNA interference; ROS: reactive oxygen species; ref(2)P: refractory to sigma P; SQSTM1: sequestosome 1; SdhA: succinate dehydrogenase, subunit A; T-L: trypsin-like proteasomal activity; TREH: trehalose; UAS: upstream activation sequence; Ub: ubiquitin; UPR: unfolded protein response; UPP: ubiquitin-proteasome pathway.

Dynamic constriction and fission of endoplasmic reticulum membranes by reticulon.

The endoplasmic reticulum (ER) is a continuous cell-wide membrane network. Network formation has been associated with proteins producing membrane curvature and fusion, such as reticulons and atlastin. Regulated network fragmentation, occurring in different physiological contexts, is less understood. Here we find that the ER has an embedded fragmentation mechanism based upon the ability of reticulon to produce fission of elongating network branches. In Drosophila, Rtnl1-facilitated fission is counterbalanced by atlastin-driven fusion, with the prevalence of Rtnl1 leading to ER fragmentation. Ectopic expression of Drosophila reticulon in COS-7 cells reveals individual fission events in dynamic ER tubules. Consistently, in vitro analyses show that reticulon produces velocity-dependent constriction of lipid nanotubes leading to stochastic fission via a hemifission mechanism. Fission occurs at elongation rates and pulling force ranges intrinsic to the ER, thus suggesting a principle whereby the dynamic balance between fusion and fission controlling organelle morphology depends on membrane motility.

Disease-related phenotypes in a Drosophila model of hereditary spastic paraplegia are ameliorated by treatment with vinblastine.

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative diseases characterized by progressive weakness and spasticity of the lower limbs. Dominant mutations in the human SPG4 gene, encoding spastin, are responsible for the most frequent form of HSP. Spastin is an ATPase that binds microtubules and localizes to the spindle pole and distal axon in mammalian cell lines. Furthermore, its Drosophila homolog, Drosophila spastin (Dspastin), has been recently shown to regulate microtubule stability and synaptic function at the Drosophila larval neuromuscular junction. Here we report the generation of a spastin-linked HSP animal model and show that in Drosophila, neural knockdown of Dspastin and, conversely, neural overexpression of Dspastin containing a conserved pathogenic mutation both recapitulate some phenotypic aspects of the human disease, including adult onset, locomotor impairment, and neurodegeneration. At the subcellular level, neuronal expression of both Dspastin RNA interference and mutant Dspastin cause an excessive stabilization of microtubules in the neuromuscular junction synapse. In addition, we provide evidence that administration of the microtubule targeting drug vinblastine significantly attenuates these phenotypes in vivo. Our findings demonstrate that loss of spastin function elicits HSP-like phenotypes in Drosophila, provide novel insights into the molecular mechanism of spastin mutations, and raise the possibility that therapy with Vinca alkaloids may be efficacious in spastin-associated HSP and other disorders related to microtubule dysfunction.

Transgenic fruit-flies expressing a FRET-based sensor for in vivo imaging of cAMP dynamics.

3'-5'-cyclic adenosine monophosphate (cAMP) is a ubiquitous intracellular second messenger that mediates the action of various hormones and neurotransmitters and influences a plethora of cellular functions. In particular, multiple neuronal processes such as synaptic plasticity underlying learning and memory are dependent on cAMP signalling cascades. It is now well recognized that the specificity and fidelity of cAMP downstream effects are achieved through a tight temporal as well as spatial control of the cAMP signals. Approaches relying on real-time imaging and Fluorescence Resonance Energy Transfer (FRET)-based biosensors for direct visualization of cAMP changes as they happen in intact living cells have recently started to uncover the fine details of cAMP spatio-temporal signalling patterns. Here we report the generation of transgenic fruit-flies expressing a FRET-based, GFP-PKA sensor and their use in real-time optical recordings of cAMP signalling both ex vivo and in vivo in adult and developing organisms. These transgenic animals represent a novel tool for understanding the physiology of the cAMP signalling pathway in the context of a functioning body.

Manipulation of Mitochondria Dynamics Reveals Separate Roles for Form and Function in Mitochondria Distribution.

Mitochondria shape is controlled by membrane fusion and fission mediated by mitofusins, Opa1, and Drp1, whereas mitochondrial motility relies on microtubule motors. These processes govern mitochondria subcellular distribution, whose defects are emphasized in neurons because of their polarized structure. We have studied how perturbation of the fusion/fission balance affects mitochondria distribution in Drosophila axons. Knockdown of Marf or Opa1 resulted in progressive loss of distal mitochondria and in a distinct oxidative phosphorylation and membrane potential deficit. Downregulation of Drp1 rescued the lethality and bioenergetic defect caused by neuronal Marf RNAi, but induced only a modest restoration of axonal mitochondria distribution. Surprisingly, Drp1 knockdown rescued fragmentation and fully restored aberrant distribution of axonal mitochondria produced by Opa1 RNAi; however, Drp1 knockdown did not improve viability or mitochondria function. Our data show that proper morphology is critical for proper axonal mitochondria distribution independent of bioenergetic efficiency. The health of neurons largely depends on mitochondria function, but does not depend on shape or distribution.

The hereditary spastic paraplegia gene, spastin, regulates microtubule stability to modulate synaptic structure and function.

BACKGROUND: Hereditary Spastic Paraplegia (HSP) is a devastating neurological disease causing spastic weakness of the lower extremities and eventual axonal degeneration. Over 20 genes have been linked to HSP in humans; however, mutations in one gene, spastin (SPG4), are the cause of >40% of all cases. Spastin is a member of the ATPases associated with diverse cellular activities (AAA) protein family, and contains a microtubule interacting and organelle transport (MIT) domain. Previous work in cell culture has proposed a role for Spastin in regulating microtubules. RESULTS: Employing Drosophila transgenic methods for overexpression and RNA interference (RNAi), we have investigated the role of Spastin in vivo. We show that Drosophila Spastin (D-Spastin) is enriched in axons and synaptic connections. At neuromuscular junctions (NMJ), Dspastin RNAi causes morphological undergrowth and reduced synaptic area. Moreover, Dspastin overexpression reduces synaptic strength, whereas Dspastin RNAi elevates synaptic currents. By using antibodies against posttranslationally modified alpha-Tubulin, we find that Dspastin regulates microtubule stability. Functional synaptic defects caused by Dspastin RNAi and overexpression were pharmacologically alleviated by agents that destabilize and stabilize microtubules, respectively. CONCLUSIONS: Loss of Dspastin in Drosophila causes an aberrantly stabilized microtubule cytoskeleton in neurons and defects in synaptic growth and neurotransmission. These in vivo data strongly support previous reports, providing a probable cause for the neuronal dysfunction in spastin-linked HSP disease. The role of Spastin in regulating neuronal microtubule stability suggests therapeutic targets for HSP treatment and may provide insight into neurological disorders linked to microtubule dysfunction.

The first ALS2 missense mutation associated with JPLS reveals new aspects of alsin biological function.

Primary lateral sclerosis (PLS) is a rare progressive paralytic disorder that results from dysfunction of the upper motoneurons. Although PLS is a sporadic disorder of adult middle age, it has also been described in children as juvenile PLS or JPLS. The causative gene for JPLS was found to be ALS2, which is also responsible for a recessive form of amyotrophic lateral sclerosis, for infantile onset ascending hereditary spastic paralysis (IAHSP) and for a form of complicated hereditary spastic paraplegia (cHSP). ALS2 gene encodes a protein termed alsin, containing multiple guanine nucleotide exchange factor domains, specifically binding to small GTPase Rab5 and acting as a GEF for Rab5. In vitrostudies performed with full-length and truncating forms of alsin protein support its role in endosomal dynamics and trafficking of mitochondria. All ALS2 mutations so far reported generate alsin protein truncation. Here, we describe the first homozygous missense mutation in ALS2, p.G540E. The mutation, which falls within the RCC1 domain, was identified in a 34-year-old patient with typical signs of JPLS such as ascending generalized and severe spasticity involving the limbs and the bulbar region, dysphagia, limb atrophy, preserved cognition and sensation. The father and two proband's sisters were found to be heterozygous carriers of the mutation with no signs of the disease. Studies in the neuronal cell line SK-N-BE indicated that the known subcellular localization of wild-type alsin with the early endosome antigen 1, in enlarged endosomal structures, and transferrin receptor is completely lost by the mutant protein, thus indicating that this mutation leads to protein delocalization. Mutant alsin induced neuronal death itself and also significantly enhanced the apoptogenic effect of NMDA and staurosporine. This effect was associated with decreased Bcl-xL : Bax ratio. In contrast, wild-type alsin was neuroprotective and increased Bcl-xL : Bax ratio. Our results provide the first demonstration that a missense mutation in alsin is cytotoxic. In addition, the identification of Bcl-xL/Bax as target of protection by alsin and of cytotoxicity by the mutant form provides a new signalling event regulated by alsin protein that may be important to define its role in neuronal physiology and neurodegeneration. Finally, the phenotype-genotype correlation in our patient, in view of all other ALS2 mutant cases reported previously, suggests a functional interplay of long and short forms of alsin in relation to disease onset and progression.

Eight novel mutations in SPG4 in a large sample of patients with hereditary spastic paraplegia.

BACKGROUND: Hereditary spastic paraplegia (HSP) is a group of genetically heterogeneous disorders characterized by progressive spasticity of the lower limbs. Mutations in the SPG4 gene, which encodes spastin protein, are responsible for up to 45% of autosomal dominant cases. OBJECTIVE: To search for disease-causing mutations in a large series of Italian patients with HSP. DESIGN: Samples of DNA were analyzed by direct sequencing of all exons in SPG4. Samples from a subset of patients were also analyzed by direct sequencing of all exons in SPG3A, SPG6, SPG10, and SPG13. SETTING: Molecular testing facility in Italy. PATIENTS: Sixty unrelated Italian patients with pure (n = 50) and complicated (n = 10) HSP. MAIN OUTCOME MEASURES: Mutations in SPG4, SPG3A, SPG6, SPG10, and SPG13. RESULTS: We identified 12 different mutations, 8 of which were novel, in 13 patients. No mutations of any of the other HSP genes tested were found in 15 patients with sporadic pure HSP who did not have mutations in the SPG4 gene. CONCLUSIONS: The overall rate of mutation in the SPG4 gene within our sample was 22%, rising to 26% when only patients with pure HSP were considered. The negative result obtained in 15 patients without mutations in SPG4 in whom 4 other genes were analyzed (SPG3A, SPG6, SPG10, and SPG13) indicate that these genes are not frequently mutated in sporadic pure HSP.

Reduction of endoplasmic reticulum stress attenuates the defects caused by Drosophila mitofusin depletion.

Ablation of the mitochondrial fusion and endoplasmic reticulum (ER)-tethering protein Mfn2 causes ER stress, but whether this is just an epiphenomenon of mitochondrial dysfunction or a contributor to the phenotypes in mitofusin (Mfn)-depleted Drosophila melanogaster is unclear. In this paper, we show that reduction of ER dysfunction ameliorates the functional and developmental defects of flies lacking the single Mfn mitochondrial assembly regulatory factor (Marf). Ubiquitous or neuron- and muscle-specific Marf ablation was lethal, altering mitochondrial and ER morphology and triggering ER stress that was conversely absent in flies lacking the fusion protein optic atrophy 1. Expression of Mfn2 and ER stress reduction in flies lacking Marf corrected ER shape, attenuating the developmental and motor defects. Thus, ER stress is a targetable pathogenetic component of the phenotypes caused by Drosophila Mfn ablation.

Balancing ER dynamics: shaping, bending, severing, and mending membranes.

The endoplasmic reticulum is a multifunctional organelle composed of functionally and morphologically distinct domains. These include the relatively planar nuclear envelope and the peripheral ER, a network of sheet-like cisternae interconnected with tubules that spread throughout the cytoplasm. The ER is highly dynamic and the shape of its domains as well as their relative content are in constant flux. The multiple forces driving these morphological changes depend on the interaction between the ER and microtubules, membrane fusion and fission events and the action of proteins capable of actively shaping membranes. The interplay between these forces is ultimately responsible for the dynamic morphology of the ER, which in turn is crucial for properly executing the varied functions of this organelle.