Functional conservation of human Spastin in a Drosophila model of autosomal dominant-hereditary spastic paraplegia

Cul-4 inhibition rescues spastin levels and reduces defects in hereditary spastic paraplegia models

Hereditary spastic paraplegias (HSPs) are degenerative motor neuron diseases characterized by progressive spasticity and weakness in the lower limbs. The most common form of HSP is due to SPG4 gene haploinsufficiency. SPG4 encodes the microtubule severing enzyme spastin. Although, there is no cure for SPG4-HSP, strategies to induce a spastin recovery are emerging as promising therapeutic approaches. Spastin protein levels are regulated by poly-ubiquitination and proteasomal-mediated degradation, in a neddylation-dependent manner. However, the molecular players involved in this regulation are unknown. Here, we show that the Cullin-4-RING E3 ubiquitin ligase complex (CRL4) regulates spastin stability. Inhibition of CRL4 increases spastin levels by preventing its poly-ubiquitination and subsequent degradation in spastin-proficient and in patient derived SPG4 haploinsufficient cells. To evaluate the role of CRL4 complex in spastin regulation in vivo, we developed a Drosophila melanogaster model of SPG4 haploinsufficiency which show alterations of synapse morphology and locomotor activity, recapitulating phenotypical defects observed in patients. Downregulation of the CRL4 complex, highly conserved in Drosophila, rescues spastin levels and the phenotypical defects observed in flies. As a proof of concept of possible pharmacological treatments, we demonstrate a recovery of spastin levels and amelioration of the SPG4-HSP-associated defects both in the fly model and in patient-derived cells by chemical inactivation of the CRL4 complex with NSC1892. Taken together, these findings show that CRL4 contributes to spastin stability regulation and that it is possible to induce spastin recovery and rescue of SPG4-HSP defects by blocking the CRL4-mediated spastin degradation.

Ninein domains required for its localization, association with partners dynein and ensconsin, and microtubule organization

Ninein (Nin) is a microtubule (MT) anchor at the subdistal appendages of mother centrioles and the pericentriolar material (PCM) of centrosomes that also functions to organize MTs at noncentrosomal MT-organizing centers (ncMTOCs). In humans, the NIN gene is mutated in Seckel syndrome, an inherited developmental disorder. Here, we dissect the protein domains involved in Nin's localization and interactions with dynein and ensconsin (ens/MAP7) and show that the association with ens cooperatively regulates MT assembly in Drosophila fat body cells. We define domains of Nin responsible for its localization to the ncMTOC on the fat body cell nuclear surface, localization within the nucleus, and association with Dynein light intermediate chain (Dlic) and ens, respectively. We show that Nin's association with ens synergistically regulates MT assembly. Together, these findings reveal novel features of Nin function and its regulation of a ncMTOC.

Effects of DeSUMOylated Spastin on AMPA Receptor Surface Delivery and Synaptic Function Are Enhanced by Phosphorylating at Ser210

Surface trafficking of AMPA receptors (AMPARs) is one of the important mechanisms mediating synaptic plasticity which is essential for cognitive functions such as learning and memory. Spastin, as a novel binding partner for the AMPAR, has been reported to regulate AMPAR surface expression and synaptic function. Additionally, Spastin undergoes two posttranslational modifications, phosphorylation and SUMOylation, both of which are crucial for synaptic function. However, gaps exist in our knowledge of how Spastin phosphorylation cross-talks with its SUMOylation in the regulation of AMPAR surface expression and synaptic function. Here, we reported that deSUMOylation of Spastin at Lys427 increased the surface level of AMPAR GluA2 subunit, the amplitude and frequency of miniature excitatory synaptic currents (mEPSC), and facilitated the morphological maturation of dendritic spines in cultured hippocampal neurons. Further studies demonstrated that Spastin phosphorylation at Ser210 further increased the enhancement of GluA2 surface expression and synaptic function by deSUMOylated Spastin, while dephosphorylation had the opposite effect. Simultaneously, deSUMOylation at Lys427 significantly increased the promoting effect of Spastin phosphorylation on synaptic function. In conclusion, our study suggests that cooperative interactions between phosphorylated and deSUMOylated Spastin are novel pathways to enhance synaptic function.

Regulation of apical constriction via microtubule- and Rab11-dependent apical transport during tissue invagination

The formation of an epithelial tube is a fundamental process for organogenesis. During Drosophila embryonic salivary gland (SG) invagination, Folded gastrulation (Fog)-dependent Rho-associated kinase (Rok) promotes contractile apical myosin formation to drive apical constriction. Microtubules (MTs) are also crucial for this process and are required for forming and maintaining apicomedial myosin. However, the underlying mechanism that coordinates actomyosin and MT networks still remains elusive. Here, we show that MT-dependent intracellular trafficking regulates apical constriction during SG invagination. Key components involved in protein trafficking, such as Rab11 and Nuclear fallout (Nuf), are apically enriched near the SG invagination pit in a MT-dependent manner. Disruption of the MT networks or knockdown of Rab11 impairs apicomedial myosin formation and apical constriction. We show that MTs and Rab11 are required for apical enrichment of the Fog ligand and the continuous distribution of the apical determinant protein Crumbs (Crb) and the key adherens junction protein E-Cadherin (E-Cad) along junctions. Targeted knockdown of crb or E-Cad in the SG disrupts apical myosin networks and results in apical constriction defects. Our data suggest a role of MT- and Rab11-dependent intracellular trafficking in regulating actomyosin networks and cell junctions to coordinate cell behaviors during tubular organ formation.

Suppression of spastin Mutant Phenotypes by Pak3 Loss Implicates a Role for Reactive Glia in AD-HSP

Neurodegenerative mechanisms due to mutations in spastin currently center on neuronal defects, primarily in microtubule and endomembrane regulation. Spastin loss in Drosophila larvae compromises neuronal microtubule distribution, alters synaptic bouton morphology, and weakens synaptic transmission at glutamatergic neuromuscular junction (NMJ) synapses. Pak3, a p21-activated kinase that promotes actin polymerization and filopodial projections, is required for these spastin mutant defects; animals lacking both genes have normal NMJs. Here we show that Pak3 is expressed in central and peripheral glial populations, and reduction of Pak3 specifically in subperineurial glial cells is sufficient to suppress the phenotypes associated with spastin loss. Subperineurial glia in the periphery ensheathe motor neuron axons and have been shown to extend actin-based projections that regulate synaptic terminals during normal NMJ development. We find that these subperineurial glial projections are Pak3-dependent and nearly twice as frequent in spastin mutants, while in Pak3, spastin double mutants, neither glial projections nor synaptic defects are observed. Spastin deficiency thus increases Pak3-dependent subperineurial glia activity, which is in turn required for neuronal defects. Our results demonstrate a central role for Pak3-mediated, altered glial behavior in the neuronal defects due to spastin loss, and suggest that a similar reactive glia-mediated mechanism may underlie human AD-HSP pathogenesis.

NeuroD1 overexpression in spinal neurons accelerates axonal regeneration after sciatic nerve injury

Neurogenic differentiation 1 (NeuroD1) is mainlyexpressed in developing neurons where it plays critical roles in neuronal maturation and neurite elongation. The potential role and mechanism of NeuroD1 in adult axonal regeneration is not clear. The present study used synapsin (SYN) Cre and AAV9-Flex vectors to conditionally overexpress NeuroD1 in adult spinal neurons and found that NeuroD1 overexpression significantly accelerated axonal regeneration and functional recovery after sciatic nerve injury. Further in vitro and in vivo experiments suggested that the mechanism of NeuroD1 promotion on axonal regeneration was related to its regulation of the expression of neurotrophin BDNF and its receptor TrkB as well as a microtubule severing protein spastin.

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.

Scaling of cytoskeletal organization with cell size in Drosophila

Actin-rich denticle precursors are regularly distributed in the Drosophila embryo. Cytoskeletal scaling occurs through changes in denticle number and spacing. Denticle spacing scales with cell length over a 10-fold range. Accurate denticle positioning requires the microtubule cytoskeleton.

Spatially organized macromolecular complexes are essential for cell and tissue function, but the mechanisms that organize micron-scale structures within cells are not well understood. Microtubule-based structures such as mitotic spindles scale with cell size, but less is known about the scaling of actin structures within cells. Actin-rich denticle precursors cover the ventral surface of the Drosophila embryo and larva and provide templates for cuticular structures involved in larval locomotion. Using quantitative imaging and statistical modeling, we demonstrate that denticle number and spacing scale with cell length over a wide range of cell sizes in embryos and larvae. Denticle number and spacing are reduced under space-limited conditions, and both features robustly scale over a 10-fold increase in cell length during larval growth. We show that the relationship between cell length and denticle spacing can be recapitulated by specific mathematical equations in embryos and larvae and that accurate denticle spacing requires an intact microtubule network and the microtubule minus end–binding protein, Patronin. These results identify a novel mechanism of micro­tubule-dependent actin scaling that maintains precise patterns of actin organization during tissue growth.

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.

Identification and functional analysis of a SLC33A1: c.339T>G (p.Ser113Arg) variant in the original SPG42 family

Using whole-exome sequencing, we surveyed all the potential pathogenic variants in an SPG42 family and found five SNPs and four indels that are shared by two patients and lie in the mapped region. Two variants, SLC33A1 p.Ser113Arg and VEPH1 p.Gln433His, cosegregated with the disease. However, VEPH1 p.Gln433His was predicted to be tolerated, thus leaving SLC33A1 p.Ser113Arg as the most plausible causal variant in this family. We found that the phosphorylated SMAD1/5/8 (P-SMAD1/5/8) and BMP receptor type 1A (BMPR1A) were substantially upregulated in fibroblasts derived from an SPG42 individual. Slc33a1 knockdown zebrafish, which exhibited defects in morphology and axon outgrowth, also showed a significant elevation in the level of P-smad1/5/8. While the phenotypes in slc33a1 knockdown zebrafish could be rescued by human wild-type SLC33A1 mRNA, this rescuing effect was diminished by coinjected mutant mRNA encoding p.Ser113Arg, indicating that p.Ser113Arg variant acts in a dominant-negative manner. Importantly, pharmacological blockade of BMPR1 activity by dorsomorphin could efficiently rescue the phenotypic defects in slc33a1 knockdown zebrafish. These results indicate that SLC33A1 can negatively regulate BMP signaling.

Importance of gene dosage in controlling dendritic arbor formation during development

Proper dendrite morphology is crucial for normal nervous system functioning. While a number of genes have been implicated in dendrite morphogenesis in both invertebrates and mammals, it remains unclear how developing dendrites respond to changes in gene dosage and what type of patterns their responses may follow. To understand this, I review here evidence from the recent literature, focusing on the genetic studies performed in the Drosophila larval dendritic arborization class IV neuron, an excellent cell type to understand dendrite morphogenesis. I summarize how class IV arbors change morphology in response to developmental fluctuations in the expression levels of 47 genes, studied by means of genetic manipulations such as loss-of-function and gain-of-function, and for which sufficient information is available. I find that arbors can respond to changing gene dosage in several distinct ways, each characterized by a singular dose-response curve. Interestingly, in 72% of cases arbors are sensitive, and thus adjust their morphology, in response to both decreases and increases in the expression of a given gene, indicating that dendrite morphogenesis is a process particularly sensitive to gene dosage. By summarizing the parallels between Drosophila and mammals, I show that many Drosophila dendrite morphogenesis genes have orthologs in mammals, and that some of these are associated with mammalian dendrite outgrowth and human neurodevelopmental disorders. One notable disease-related molecule is kinase Dyrk1A, thought to be a causative factor in Down syndrome. Both increases and decreases in Dyrk1A gene dosage lead to impaired dendrite morphogenesis, which may contribute to Down syndrome pathoetiology.

Cold temperature improves mobility and survival in Drosophila models of autosomal-dominant hereditary spastic paraplegia (AD-HSP)

Autosomal-dominant hereditary spastic paraplegia (AD-HSP) is a crippling neurodegenerative disease for which effective treatment or cure remains unknown. Victims experience progressive mobility loss due to degeneration of the longest axons in the spinal cord. Over half of AD-HSP cases arise from loss-of-function mutations in spastin, which encodes a microtubule-severing AAA ATPase. In Drosophila models of AD-HSP, larvae lacking Spastin exhibit abnormal motor neuron morphology and function, and most die as pupae. Adult survivors display impaired mobility, reminiscent of the human disease. Here, we show that rearing pupae or adults at reduced temperature (18°C), compared with the standard temperature of 24°C, improves the survival and mobility of adult spastin mutants but leaves wild-type flies unaffected. Flies expressing human spastin with pathogenic mutations are similarly rescued. Additionally, larval cooling partially rescues the larval synaptic phenotype. Cooling thus alleviates known spastin phenotypes for each developmental stage at which it is administered and, notably, is effective even in mature adults. We find further that cold treatment rescues larval synaptic defects in flies with mutations in Flower (a protein with no known relation to Spastin) and mobility defects in flies lacking Kat60-L1, another microtubule-severing protein enriched in the CNS. Together, these data support the hypothesis that the beneficial effects of cold extend beyond specific alleviation of Spastin dysfunction, to at least a subset of cellular and behavioral neuronal defects. Mild hypothermia, a common neuroprotective technique in clinical treatment of acute anoxia, might thus hold additional promise as a therapeutic approach for AD-HSP and, potentially, for other neurodegenerative diseases.

Pathogenic mutation of spastin has gain-of-function effects on microtubule dynamics

Mutations to the SPG4 gene encoding the microtubule-severing protein spastin are the most common cause of hereditary spastic paraplegia. Haploinsufficiency, the prevalent model for the disease, cannot readily explain many of its key aspects, such as its adult onset or its specificity for the corticospinal tracts. Treatment strategies based solely on haploinsufficiency are therefore likely to fail. Toward developing effective therapies, here we investigated potential gain-of-function effects of mutant spastins. The full-length human spastin isoform called M1 or a slightly shorter isoform called M87, both carrying the same pathogenic mutation C448Y, were expressed in three model systems: primary rat cortical neurons, fibroblasts, and transgenic Drosophila. Although both isoforms had ill effects on motor function in transgenic flies and decreased neurite outgrowth from primary cortical neurons, mutant M1 was notably more toxic than mutant M87. The observed phenotypes did not result from dominant-negative effects of mutated spastins. Studies in cultured cells revealed that microtubules can be heavily decorated by mutant M1 but not mutant M87. Microtubule-bound mutant M1 decreased microtubule dynamics, whereas unbound M1 or M87 mutant spastins increased microtubule dynamics. The alterations in microtubule dynamics observed in the presence of mutated spastins are not consistent with haploinsufficiency and are better explained by a gain-of-function mechanism. Our results fortify a model wherein toxicity of mutant spastin proteins, especially mutant M1, contributes to axonal degeneration in the corticospinal tracts. Furthermore, our results provide details on the mechanism of the toxicity that may chart a course toward more effective treatment regimens.

Normal spastin gene dosage is specifically required for axon regeneration

Axon regeneration allows neurons to repair circuits after trauma; however, most of the molecular players in this process remain to be identified. Given that microtubule rearrangements have been observed in injured neurons, we tested whether microtubule-severing proteins might play a role in axon regeneration. We found that axon regeneration is extremely sensitive to levels of the microtubule-severing protein spastin. Although microtubule behavior in uninjured neurons was not perturbed in animals heterozygous for a spastin null allele, axon regeneration was severely disrupted in this background. Two types of axon regeneration-regeneration of an axon from a dendrite after proximal axotomy and regeneration of an axon from the stump after distal axotomy-were defective in Drosophila with one mutant copy of the spastin gene. Other types of axon and dendrite outgrowth, including regrowth of dendrites after pruning, were normal in heterozygotes. We conclude that regenerative axon growth is uniquely sensitive to spastin gene dosage.

Microtubule-targeting drugs rescue axonal swellings in cortical neurons from spastin knockout mice

Mutations in SPG4, encoding the microtubule-severing protein spastin, are responsible for the most frequent form of hereditary spastic paraplegia (HSP), a heterogeneous group of genetic diseases characterized by degeneration of the corticospinal tracts. We previously reported that mice harboring a deletion in Spg4, generating a premature stop codon, develop progressive axonal degeneration characterized by focal axonal swellings associated with impaired axonal transport. To further characterize the molecular and cellular mechanisms underlying this mutant phenotype, we have assessed microtubule dynamics and axonal transport in primary cultures of cortical neurons from spastin-mutant mice. We show an early and marked impairment of microtubule dynamics all along the axons of spastin-deficient cortical neurons, which is likely to be responsible for the occurrence of axonal swellings and cargo stalling. Our analysis also reveals that a modulation of microtubule dynamics by microtubule-targeting drugs rescues the mutant phenotype of cortical neurons. Together, these results contribute to a better understanding of the pathogenesis of SPG4-linked HSP and ascertain the influence of microtubule-targeted drugs on the early axonal phenotype in a mouse model of the disease.

Loss of Drosophila melanogaster p21-activated kinase 3 suppresses defects in synapse structure and function caused by spastin mutations

Microtubules are dynamic structures that must elongate, disassemble, and be cleaved into smaller pieces for proper neuronal development and function. The AAA ATPase Spastin severs microtubules along their lengths and is thought to regulate the balance between long, stable filaments and shorter fragments that seed extension or are transported. In both Drosophila and humans, loss of Spastin function results in reduction of synaptic connections and disabling motor defects. To gain insight into how spastin is regulated, we screened the Drosophila melanogaster genome for deletions that modify a spastin overexpression phenotype, eye size reduction. One suppressor region deleted p21-activated kinase 3 (pak3), which encodes a member of the Pak family of actin-regulatory enzymes, but whose in vivo function is unknown. We show that pak3 mutants have only mild synaptic defects at the larval neuromuscular junction, but exhibit a potent genetic interaction with spastin mutations. Aberrant bouton morphology, microtubule distribution, and synaptic transmission caused by spastin loss of function are all restored to wild type when pak3 is simultaneously reduced. Neuronal overexpression of pak3 induces actin-rich thin projections, suggesting that it functions in vivo to promote filopodia during presynaptic terminal arborization. pak3 therefore regulates synapse development in vivo, and when mutated, suppresses the synaptic defects that result from spastin loss.