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.

Quantitative expression of MMPs 2, 9, 14, and collagen IV in LCIS and paired normal breast tissue.

Certain matrix metalloproteinases (MMPs) have the ability to degrade collagen IV, a main component of the breast lobular basement membrane. In this cross-sectional study, we evaluated expression of MMPs 2, 9, and 14 and collagen IV in LCIS and adjacent normal breast tissue among LCIS patients without invasive breast cancer to determine whether expression differed between benign and preinvasive breast epithelial tissue. A total of 64 LCIS patients, diagnosed 2004-2014, were included; 44 had sufficient paired normal tissue for analysis. Marker epithelial expression was measured using immunofluorescence and quantified using the H score (MMPs) or pixel intensity (collagen IV). Associations were evaluated using the Spearman correlation or the Wilcoxon signed-rank test. In LCIS and normal tissue, there was a strong correlation between MMP2 and MMP14 expression (LCIS r = 0.69, normal r = 0.81, both P < 0.01). Other pairwise correlations were moderate to weak (range: LCIS r = 0.32-0.47, normal r = 0.19-0.32). For all markers, expression was lower in LCIS vs. normal tissue (all P ≤ 0.05). In sum, collagenase MMPs were expressed in normal breast and LCIS lesions of LCIS patients. However, expression was not higher in LCIS compared with normal tissue, suggesting collagenase MMP expression does not increase as breast tissue gains a more proliferative phenotype.

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.

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

Mutations in spastin are the most frequent cause of the neurodegenerative disease autosomal dominant-hereditary spastic paraplegia (AD-HSP). Drosophila melanogaster lacking spastin exhibit striking behavioral similarities to human patients suffering from AD-HSP, suggesting conservation of Spastin function between the species. Consistent with this, we show that exogenous expression of wild-type Drosophila or human spastin rescues behavioral and cellular defects in spastin null flies equivalently. This enabled us to generate genetically representative models of AD-HSP, which arises from dominant mutations in spastin rather than a complete loss of the gene. Flies co-expressing one copy of wild-type human spastin and one encoding the K388R catalytic domain mutation in the fly spastin null background, exhibit aberrant distal synapse morphology and microtubule distribution, similar to but less severe than spastin nulls. R388 or a separate nonsense mutation act dominantly and are furthermore sufficient to confer partial rescue, supporting in vitro evidence for additional, non-catalytic Spastin functions. Using this model, we tested the observation from human pedigrees that S44L and P45Q are trans-acting modifiers of mutations affecting the Spastin catalytic domain. As in humans, both L44 and Q45 are largely silent when heterozygous, but exacerbate mutant phenotypes when expressed in trans with R388. These transgenic 'AD-HSP' flies therefore provide a powerful and tractable model to enhance our understanding of the cellular and behavioral consequences of human spastin mutations and test hypotheses directly relevant to the human disease.

Stall encodes an ADAMTS metalloprotease and interacts genetically with Delta in Drosophila ovarian follicle formation.

Ovarian follicle formation in Drosophila melanogaster requires stall (stl) gene function, both within and outside the ovary, for follicle individualization, stalk cell intercalation, and oocyte localization. We have identified the stl transcript as CG3622 and confirmed the presence of three alternatively spliced isoforms, contrary to current genome annotation. Here we show that the gene is expressed in both ovarian and brain tissues, which is consistent with previous evidence of an ovary nonautonomous function. On the basis of amino acid sequence, stl encodes a metalloprotease similar to the "a disintegrin and metalloprotease with thrombospondin" (ADAMTS) family. Although stl mutant ovaries fail to maintain the branched structure of the fusome and periodically show improperly localized oocytes, stl mutants do not alter oocyte determination. Within the ovary, stl is expressed in pupal basal stalks and in adult somatic cells of the posterior germarium and the follicular poles. Genetically, stl exhibits a strong mutant interaction with Delta (Dl), and Dl mutant ovaries show altered stl expression patterns. Additionally, a previously described genetic interactor, daughterless, also modulates stl expression in the somatic ovary and may do so directly in its capacity as a basic helix-loop-helix (bHLH) transcription factor. We propose a complex model of long-range extraovarian signaling through secretion or extracellular domain shedding, together with local intraovarian protein modification, to explain the dual sites of Stl metalloprotease function in oogenesis.

stall-mediated extrinsic control of ovarian follicle formation in Drosophila.

Complex patterns of morphogenesis require intricate coordination of multiple, regulatory processes that control cellular identities, shapes, and behaviors, both locally and over vast distances in the developing organism or tissue. Studying Drosophila oogenesis as a model for tissue morphogenesis, we have discovered extraovarian regulation of follicle formation. Clonal analysis and ovary transplantation have demonstrated that long-range control of follicle individualization requires stall gene function in cells outside of the ovary. Although tissue nonautonomous regulation has been shown to govern follicle maturation and survival, this is the first report of an extraovarian pathway involved in normal follicle formation.