An aldehyde-crosslinking mitochondrial probe for STED imaging in fixed cells

Fluorescence labeling of chemically fixed specimens, especially immunolabeling, plays a vital role in super-resolution imaging as it offers a convenient way to visualize cellular structures like mitochondria or the distribution of biomolecules with high detail. Despite the development of various distinct probes that enable super-resolved stimulated emission depletion (STED) imaging of mitochondria in live cells, most of these membrane-potential-dependent fluorophores cannot be retained well in mitochondria after chemical fixation. This lack of suitable mitochondrial probes has limited STED imaging of mitochondria to live cell samples. In this study, we introduce a mitochondria-specific probe, PK Mito Orange FX (PKMO FX), which features a fixation-driven cross-linking motif and accumulates in the mitochondrial inner membrane. It exhibits high fluorescence retention after chemical fixation and efficient depletion at 775 nm, enabling nanoscopic imaging both before and after aldehyde fixation. We demonstrate the compatibility of this probe with conventional immunolabeling and other strategies commonly used for fluorescence labeling of fixed samples. Moreover, we show that PKMO FX facilitates correlative super-resolution light and electron microscopy, enabling the correlation of multicolor fluorescence images and transmission EM images via the characteristic mitochondrial pattern. Our probe further expands the mitochondrial toolkit for multimodal microscopy at nanometer resolutions.

Enhanced mitochondrial fusion during a critical period of synaptic plasticity in adult-born neurons

Integration of new neurons into adult hippocampal circuits is a process coordinated by local and long-range synaptic inputs. To achieve stable integration and uniquely contribute to hippocampal function, immature neurons are endowed with a critical period of heightened synaptic plasticity, yet it remains unclear which mechanisms sustain this form of plasticity during neuronal maturation. We found that as new neurons enter their critical period, a transient surge in fusion dynamics stabilizes elongated mitochondrial morphologies in dendrites to fuel synaptic plasticity. Conditional ablation of fusion dynamics to prevent mitochondrial elongation selectively impaired spine plasticity and synaptic potentiation, disrupting neuronal competition for stable circuit integration, ultimately leading to decreased survival. Despite profuse mitochondrial fragmentation, manipulation of competition dynamics was sufficient to restore neuronal survival but left neurons poorly responsive to experience at the circuit level. Thus, by enabling synaptic plasticity during the critical period, mitochondrial fusion facilitates circuit remodeling by adult-born neurons.

Correction: Winter et al. Z-Disk-Associated Plectin (Isoform 1d): Spatial Arrangement, Interaction Partners, and Role in Filamin C Homeostasis. Cells 2023, 12, 1259

The authors wish to make the following changes to their paper [...].

Docking and stability defects in mitofusin highlight the proteasome as a potential therapeutic target

Defects in mitochondrial fusion are at the base of many diseases. Mitofusins power membrane-remodeling events via self-interaction and GTP hydrolysis. However, how exactly mitofusins mediate fusion of the outer membrane is still unclear. Structural studies enable tailored design of mitofusin variants, providing valuable tools to dissect this stepwise process. Here, we found that the two cysteines conserved between yeast and mammals are required for mitochondrial fusion, revealing two novel steps of the fusion cycle. C381 is dominantly required for the formation of the trans-tethering complex, before GTP hydrolysis. C805 allows stabilizing the Fzo1 protein and the trans-tethering complex, just prior to membrane fusion. Moreover, proteasomal inhibition rescued Fzo1 C805S levels and membrane fusion, suggesting a possible application for clinically approved drugs. Together, our study provides insights into how assembly or stability defects in mitofusins might cause mitofusin-associated diseases and uncovers potential therapeutic intervention by proteasomal inhibition.

Z-Disk-Associated Plectin (Isoform 1d): Spatial Arrangement, Interaction Partners, and Role in Filamin C Homeostasis

Plectin, a highly versatile cytolinker protein, is crucial for myofiber integrity and function. Accordingly, mutations in the human gene (PLEC) cause several rare diseases, denoted as plectinopathies, with most of them associated with progressive muscle weakness. Of several plectin isoforms expressed in skeletal muscle and the heart, P1d is the only isoform expressed exclusively in these tissues. Using high-resolution stimulated emission depletion (STED) microscopy, here we show that plectin is located within the gaps between individual α-actinin-positive Z-disks, recruiting and bridging them to desmin intermediate filaments (Ifs). Loss of plectin in myofibril bundles led to a complete loss of desmin Ifs. Loss of Z-disk-associated plectin isoform P1d led to disorganization of muscle fibers and slower relaxation of myofibrils upon mechanical strain, in line with an observed inhomogeneity of muscle ultrastructure. In addition to binding to α-actinin and thereby providing structural support, P1d forms a scaffolding platform for the chaperone-assisted selective autophagy machinery (CASA) by directly interacting with HSC70 and synpo2. In isoform-specific knockout (P1d-KO) mouse muscle and mechanically stretched plectin-deficient myoblasts, we found high levels of undigested filamin C, a bona fide substrate of CASA. Similarly, subjecting P1d-KO mice to forced swim tests led to accumulation of filamin C aggregates in myofibers, highlighting a specific role of P1d in tension-induced proteolysis activated upon high loads of physical exercise and muscle contraction.

Capsazepine (CPZ) Inhibits TRPC6 Conductance and Is Protective in Adriamycin-Induced Nephropathy and Diabetic Glomerulopathy

Reactive oxygen species (ROS), which excessively arise in diabetes and systemic inflammatory diseases, modify cellular lipids and cellular lipid composition leading to altered biophysical properties of cellular membranes. The impact of lipid peroxidation on transmembrane signaling routes is not yet well studied. The canonical transient receptor potential channel 6 (TRPC6) is implicated in the pathogenesis of several forms of glomerular diseases. TRPC6 is sensitive to membrane stretch and relies on a distinct lipid environment. This study investigates the effect of oxidative alterations to plasma membrane lipids on TRPC6 activity and the function of the glomerular filter. Knockout of the anti-oxidative, lipid modifying enzyme paraoxonase 2 (PON2) leads to altered biophysical properties of glomerular epithelial cells, which are called podocytes. Cortical stiffness, quantified by atomic force microscopy, was largely increased in PON2-deficient cultured podocytes. PON2 deficiency markedly enhanced TRPC6 channel currents and channel recovery. Treatment with the amphiphilic substance capsazepine in micromolar doses reduced cortical stiffness and abrogated TRPC6 conductance. In in vivo studies, capsazepine reduced the glomerular phenotype in the model of adriamycin-induced nephropathy in PON2 knockout mice and wildtype littermates. In diabetic AKITA mice, the progression of albuminuria and diabetic kidney disease was delayed. In summary, we provide evidence that the modification of membrane characteristics affects TRPC6 signaling. These results could spur future research to investigate modification of the direct lipid environment of TRPC6 as a future therapeutic strategy in glomerular disease.

Extraocular Muscle Reveals Selective Vulnerability of Type IIB Fibers to Respiratory Chain Defects Induced by Mitochondrial DNA Alterations

Purpose

The purpose of this study was to gain insights on the pathogenesis of chronic progressive external ophthalmoplegia, thus we investigated the vulnerability of five extra ocular muscles (EOMs) fiber types to pathogenic mitochondrial DNA deletions in a mouse model expressing a mutated mitochondrial helicase TWINKLE.

Methods

Consecutive pairs of EOM sections were analyzed by cytochrome C oxidase (COX)/succinate dehydrogenase (SDH) assay and fiber type specific immunohistochemistry (type I, IIA, IIB, embryonic, and EOM-specific staining).

Results

The mean average of COX deficient fibers (COX-) in the recti muscles of mutant mice was 1.04 ± 0.52% at 12 months and increased with age (7.01 ± 1.53% at 24 months). A significant proportion of these COX- fibers were of the fast-twitch, glycolytic type IIB (> 50% and > 35% total COX- fibers at 12 and 24 months, respectively), whereas embryonic myosin heavy chain-expressing fibers were almost completely spared. Furthermore, the proportion of COX- fibers in the type IIB-rich retractor bulbi muscle was > 2-fold higher compared to the M. recti at both 12 (2.6 ± 0.78%) and 24 months (20.85 ± 2.69%). Collectively, these results demonstrate a selective vulnerability of type IIB fibers to mitochondrial DNA (mtDNA) deletions in EOMs and retractor bulbi muscle. We also show that EOMs of mutant mice display histopathological abnormalities, including altered fiber type composition, increased fibrosis, ragged red fibers, and infiltration of mononucleated nonmuscle cells.

Conclusions

Our results point to the existence of fiber type IIB-intrinsic factors and/or molecular mechanisms that predispose them to increased generation, clonal expansion, and detrimental effects of mtDNA deletions.

Microtubule-dependent and independent roles of spastin in lipid droplet dispersion and biogenesis

Lipid droplets (LDs) are metabolic organelles that store neutral lipids and dynamically respond to changes in energy availability by accumulating or mobilizing triacylglycerols (TAGs). How the plastic behavior of LDs is regulated is poorly understood. Hereditary spastic paraplegia is a central motor axonopathy predominantly caused by mutations in SPAST, encoding the microtubule-severing protein spastin. The spastin-M1 isoform localizes to nascent LDs in mammalian cells; however, the mechanistic significance of this targeting is not fully explained. Here, we show that tightly controlled levels of spastin-M1 are required to inhibit LD biogenesis and TAG accumulation. Spastin-M1 maintains the morphogenesis of the ER when TAG synthesis is prevented, independent from microtubule binding. Moreover, spastin plays a microtubule-dependent role in mediating the dispersion of LDs from the ER upon glucose starvation. Our results reveal a dual role of spastin to shape ER tubules and to regulate LD movement along microtubules, opening new perspectives for the pathogenesis of hereditary spastic paraplegia.

Effect of Maternal Obesity in Mice on IL-6 Levels and Placental Endothelial Cell Homeostasis

Obesity during pregnancy is a known health risk for mother and child. Since obesity is associated with increased inflammatory markers, our objectives were to determine interleukin-6 (IL-6) levels in obese mice and to examine the effect of IL-6 on placental endothelial cells. Placentas, blood, and adipose tissue of C57BL/6N mice, kept on high fat diet before and during pregnancy, were harvested at E15.5. Serum IL-6 levels were determined and endothelial cell markers and IL-6 expression were measured by qRT-PCR and western blot. Immunostaining was used to determine surface and length densities of fetal capillary profiles and placental endothelial cell homeostasis. Human placental vein endothelial cells were cultured and subjected to proliferation, apoptosis, senescence, and tube formation assays after stimulation with hyperIL-6. Placental endothelial cell markers were downregulated and the percentage of senescent endothelial cells was higher in the placental exchange zone of obese dams and placental vascularization was strongly reduced. Additionally, maternal IL-6 serum levels and IL-6 protein levels in adipose tissue were increased. Stimulation with hyperIL-6 provoked a dose dependent increase of senescence in cultured endothelial cells without any effects on proliferation or apoptosis. Diet-induced maternal obesity led to an IUGR phenotype accompanied by increased maternal IL-6 serum levels. In the placenta of obese dams, this may result in a disturbed endothelial cell homeostasis and impaired fetal vasculature. Cell culture experiments confirmed that IL-6 is capable of inducing endothelial cell senescence.

Injured Podocytes Are Sensitized to Angiotensin II-Induced Calcium Signaling

BACKGROUND

Inhibition of angiotensin II (AngII) signaling, a therapeutic mainstay of glomerular kidney diseases, is thought to act primarily through regulating glomerular blood flow and reducing filtration pressure. Although extravascular actions of AngII have been suggested, a direct effect of AngII on podocytes has not been demonstrated in vivo.

METHODS

To study the effects of AngII on podocyte calcium levels in vivo, we used intravital microscopy of the kidney in mice expressing the calcium indicator protein GCaMP3.

RESULTS

In healthy animals, podocytes displayed limited responsiveness to AngII stimulation. In contrast, in animals subjected to either adriamycin-induced acute chemical injury or genetic deletion of the podocin-encoding gene Nphs2, the consequent podocyte damage and proteinuria rendered the cells responsive to AngII and resulted in AngII-induced calcium transients in significantly more podocytes. The angiotensin type 1 receptor blocker losartan could fully inhibit this response. Also, responsiveness to AngII was at least partly mediated through the transient receptor potential channel 6, which has been implicated in podocyte calcium handling. Interestingly, loss of a single Nphs2 allele also increased podocytes' responsiveness to AngII signaling. This direct effect of AngII on injured podocytes results in increased calcium transients, which can further aggravate the underlying kidney disease.

CONCLUSIONS

Our discovery that podocytes become sensitized to AngII-induced calcium signaling upon injury might explain results from large, randomized, controlled trials in which improved renal outcomes occur only in the subgroup of patients with proteinuria, indicating podocyte damage. Our findings also emphasize the need to treat every patient with a glomerular disease with either an angiotensin-converting enzyme inhibitor or an angiotensin type 1 receptor blocker.

Plasticity in salt bridge allows fusion-competent ubiquitylation of mitofusins and Cdc48 recognition

Mitofusins are dynamin-related GTPases that drive mitochondrial fusion by sequential events of oligomerization and GTP hydrolysis, followed by their ubiquitylation. Here, we show that fusion requires a trilateral salt bridge at a hinge point of the yeast mitofusin Fzo1, alternatingly forming before and after GTP hydrolysis. Mutations causative of Charcot-Marie-Tooth disease massively map to this hinge point site, underlining the disease relevance of the trilateral salt bridge. A triple charge swap rescues the activity of Fzo1, emphasizing the close coordination of the hinge residues with GTP hydrolysis. Subsequently, ubiquitylation of Fzo1 allows the AAA-ATPase ubiquitin-chaperone Cdc48 to resolve Fzo1 clusters, releasing the dynamin for the next fusion round. Furthermore, cross-complementation within the oligomer unexpectedly revealed ubiquitylated but fusion-incompetent Fzo1 intermediates. However, Cdc48 did not affect the ubiquitylated but fusion-incompetent variants, indicating that Fzo1 ubiquitylation is only controlled after membrane merging. Together, we present an integrated model on how mitochondrial outer membranes fuse, a critical process for their respiratory function but also putatively relevant for therapeutic interventions.

The Cdkn1aSUPER Mouse as a Tool to Study p53-Mediated Tumor Suppression

Cdkn1a, which encodes p21, functions as a major route for p53-mediated cell-cycle arrest. However, the consequence of Cdkn1a gene dosage on tumor suppression has not been systematically investigated. Here, we employed BAC transgenesis to generate a Cdkn1a^SUPER mouse, which harbors an additional Cdkn1a allele within its natural genomic context. We show that these mice display enhanced cell-cycle arrest and reduced apoptosis in response to genotoxic stress. Furthermore, using a chemically induced skin cancer model and an autochthonous Kras-driven lung adenocarcinoma model, we show that Cdkn1a^SUPER mice display a cancer protection phenotype that is indistinguishable from that observed in Tp53^SUPER animals. Moreover, we demonstrate that Tp53 and Cdkn1a cooperate in mediating cancer resistance, using a chemically induced fibrosarcoma model. Overall, our Cdkn1a^SUPER allele enabled us to assess the contribution of Cdkn1a to Tp53-mediated tumor suppression.

Hollow silica capsules for amphiphilic transport and sustained delivery of antibiotic and anticancer drugs

Hollow mesoporous silica capsules were used as amphiphilic drug delivery vehicles and sustained release systems for antimicrobial and anticancer drugs.

Characterization of tumor-associated T-lymphocyte subsets and immune checkpoint molecules in head and neck squamous cell carcinoma

The composition of tumor-infiltrating lymphocytes (TIL) reflects biology and immunogenicity of cancer. Here, we characterize T-cell subsets and expression of immune checkpoint molecules in head and neck squamous cell carcinoma (HNSCC). We analyzed TIL subsets in primary tumors (n = 34), blood (peripheral blood mononuclear cells (PBMC); n = 34) and non-cancerous mucosa (n = 7) of 34 treatment-naïve HNSCC patients and PBMC of 15 healthy controls. Flow cytometry analyses revealed a highly variable T-cell infiltration mainly of an effector memory phenotype (CD45RA-/CCR7-). Naïve T cells (CD45RA+/CCR7+) were decreased in the microenvironment compared to PBMC of patients, while regulatory T cells (CD4+/CD25+/CD127low and CD4+/CD39+) were elevated. Furthermore, we performed digital image analyses of entire cross sections of HNSCC to define the 'Immunoscore' (CD3+ and CD8+ cell infiltration in tumor core and invasive margin) and quantified MHC class I expression on tumor cells by immunohistochemistry. Immune checkpoint molecules cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death 1 (PD-1) and programmed cell death 1 ligand 1 (PD-L1) were increased in TILs compared to peripheral T cells in flow-cytometric analysis. Human papillomavirus (HPV) positive tumors showed higher numbers of TILs, but a similar composition of T-cell subsets and checkpoint molecule expression compared to HPV negative tumors. Taken together, the tumor microenvironment of HNSCC is characterized by a strong infiltration of regulatory T cells and high checkpoint molecule expression on T-cell subsets. In view of increasingly used immunotherapies, a detailed knowledge of TILs and checkpoint molecule expression on TILs is of high translational relevance.

Advanced light microscopy core facilities: Balancing service, science and career

Core Facilities (CF) for advanced light microscopy (ALM) have become indispensable support units for research in the life sciences. Their organizational structure and technical characteristics are quite diverse, although the tasks they pursue and the services they offer are similar. Therefore, throughout Europe, scientists from ALM-CFs are forming networks to promote interactions and discuss best practice models. Here, we present recommendations for ALM-CF operations elaborated by the workgroups of the German network of ALM-CFs, German Bio-Imaging (GerBI). We address technical aspects of CF planning and instrument maintainance, give advice on the organization and management of an ALM-CF, propose a scheme for the training of CF users, and provide an overview of current resources for image processing and analysis. Further, we elaborate on the new challenges and opportunities for professional development and careers created by CFs. While some information specifically refers to the German academic system, most of the content of this article is of general interest for CFs in the life sciences. Microsc. Res. Tech. 79:463-479, 2016. © 2016 THE AUTHORS MICROSCOPY RESEARCH AND TECHNIQUE PUBLISHED BY WILEY PERIODICALS, INC.

A novel multiplex bead-based platform highlights the diversity of extracellular vesicles

The surface protein composition of extracellular vesicles (EVs) is related to the originating cell and may play a role in vesicle function. Knowledge of the protein content of individual EVs is still limited because of the technical challenges to analyse small vesicles. Here, we introduce a novel multiplex bead-based platform to investigate up to 39 different surface markers in one sample. The combination of capture antibody beads with fluorescently labelled detection antibodies allows the analysis of EVs that carry surface markers recognized by both antibodies. This new method enables an easy screening of surface markers on populations of EVs. By combining different capture and detection antibodies, additional information on relative expression levels and potential vesicle subpopulations is gained. We also established a protocol to visualize individual EVs by stimulated emission depletion (STED) microscopy. Thereby, markers on single EVs can be detected by fluorophore-conjugated antibodies. We used the multiplex platform and STED microscopy to show for the first time that NK cell–derived EVs and platelet-derived EVs are devoid of CD9 or CD81, respectively, and that EVs isolated from activated B cells comprise different EV subpopulations. We speculate that, according to our STED data, tetraspanins might not be homogenously distributed but may mostly appear as clusters on EV subpopulations. Finally, we demonstrate that EV mixtures can be separated by magnetic beads and analysed subsequently with the multiplex platform. Both the multiplex bead-based platform and STED microscopy revealed subpopulations of EVs that have been indistinguishable by most analysis tools used so far. We expect that an in-depth view on EV heterogeneity will contribute to our understanding of different EVs and functions.

Immune checkpoints programmed death 1 ligand 1 and cytotoxic T lymphocyte associated molecule 4 in gastric adenocarcinoma

Remarkable efficacy of immune checkpoint inhibition has been reported for several types of solid tumors and early studies in gastric adenocarcinoma are promising. A detailed knowledge about the natural biology of immune checkpoints in gastric adenocarcinoma is essential for clinical and translational evaluation of these drugs. This study is a comprehensive analysis of cytotoxic T lymphocyte associated molecule 4 (CTLA-4) and programmed death 1 ligand 1 (PD-L1) expression in gastric adenocarcinoma. PD-L1 and CTLA-4 were stained on tumor sections of 127 Caucasian patients with gastric adenocarcinoma by immunohistochemistry (IHC) and somatic mutation profiling was performed using targeted next-generation sequencing. Expression of PD-L1 and CTLA-4 on lymphocytes in tumor sections, tumor-draining lymph nodes (TDLN) and peripheral blood were studied by flow-cytometry and immune-fluorescence microscopy in an additional cohort. PD-L1 and CTLA-4 were expressed in 44.9% (57/127) and 86.6% (110/127) of the analyzed gastric adenocarcinoma samples, respectively. Positive tumor cell staining for PD-L1 or CTLA-4 was associated with inferior overall survival. Somatic mutational analysis did not reveal a correlation to expression of PD-L1 or CTLA-4 on tumor cells. Expression of PD-1 (52.2%), PD-L1 (42.2%) and CTLA-4 (1.6%) on tumor infiltrating T cells was significantly elevated compared to peripheral blood. Of note, PD-1 and PD-L1 were expressed far higher by tumor-infiltrating lymphocytes than CTLA-4. In conclusion, specific immune checkpoint-inhibitors should be evaluated in this disease and the combination with molecular targeted therapies might be of benefit. An extensive immune monitoring should accompany these studies to better understand their mode of action in the tumor microenvironment.

CALM regulates clathrin-coated vesicle size and maturation by directly sensing and driving membrane curvature

The size of endocytic clathrin-coated vesicles (CCVs) is remarkably uniform, suggesting that it is optimized to achieve the appropriate levels of cargo and lipid internalization. The three most abundant proteins in mammalian endocytic CCVs are clathrin and the two cargo-selecting, clathrin adaptors, CALM and AP2. Here we demonstrate that depletion of CALM causes a substantial increase in the ratio of “open” clathrin-coated pits (CCPs) to “necked”/“closed” CCVs and a doubling of CCP/CCV diameter, whereas AP2 depletion has opposite effects. Depletion of either adaptor, however, significantly inhibits endocytosis of transferrin and epidermal growth factor. The phenotypic effects of CALM depletion can be rescued by re-expression of wild-type CALM, but not with CALM that lacks a functional N-terminal, membrane-inserting, curvature-sensing/driving amphipathic helix, the existence and properties of which are demonstrated. CALM is thus a major factor in controlling CCV size and maturation and hence in determining the rates of endocytic cargo uptake.

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CALM loss increases size and frequency of early endocytic clathrin-coated structures

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Depletion of CALM slows endocytic clathrin-coated pit maturation and endocytic rate

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CALM possesses an N-terminal, membrane-curvature-sensing/driving amphipathic helix

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Clathrin-coated pit maturation is regulated by CALM’s N-terminal amphipathic helix

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.

Hereditary spastic paraplegia (HSP) is a genetically heterogeneous neurological disease characterized by weakness and spasticity of the lower limbs, caused by progressive retrograde degeneration of the corticospinal axons, the longest in the central nervous system. The most commonly mutated gene in autosomal dominant forms of HSP, SPAST, encodes for spastin, a microtubule-severing protein. Spastin has been implicated in several processes involving remodeling of membrane structures. We now show that the longest spastin form, spastin-M1, harbors a lipid droplet targeting sequence, which allows targeting of the protein to the surface of lipid droplets, the organelles where cells store neutral lipids. Furthermore, we demonstrate that depletion of the homologous spastin proteins in both flies and worms affects lipid droplet number and triacylglycerol content. Our study adds to recent discoveries that implicate other HSP proteins in lipid droplet and lipid metabolism, and strongly suggests that lipid droplet dysfunction in neurons should be investigated to understand pathogenesis of HSP.

Ugo1 and Mdm30 act sequentially during Fzo1-mediated mitochondrial outer membrane fusion

Dynamin-related GTPase proteins (DRPs) are main players in membrane remodelling. Conserved DRPs called mitofusins (Mfn1/Mfn2/Fzo1) mediate the fusion of mitochondrial outer membranes (OM). OM fusion depends on self-assembly and GTPase activity of mitofusins as well as on two other proteins, Ugo1 and Mdm30. Here, we define distinct steps of the OM fusion cycle using in vitro and in vivo approaches. We demonstrate that yeast Fzo1 assembles into homo-dimers, depending on Ugo1 and on GTP binding to Fzo1. Fzo1 homo-dimers further associate upon formation of mitochondrial contacts, allowing membrane tethering. Subsequent GTP hydrolysis is required for Fzo1 ubiquitylation by the F-box protein Mdm30. Finally, Mdm30-dependent degradation of Fzo1 completes Fzo1 function in OM fusion. Our results thus unravel functions of Ugo1 and Mdm30 at distinct steps during OM fusion and suggest that protein clearance confers a non-cycling mechanism to mitofusins, which is distinct from other cellular membrane fusion events.

The SUMO protease SENP5 is required to maintain mitochondrial morphology and function

Mitochondria are dynamic organelles that undergo regulated fission and fusion events that are essential to maintain metabolic stability. We previously demonstrated that the mitochondrial fission GTPase DRP1 is a substrate for SUMOylation. To further understand how SUMOylation impacts mitochondrial function, we searched for a SUMO protease that may affect mitochondrial dynamics. We demonstrate that the cytosolic pool of SENP5 catalyzes the cleavage of SUMO1 from a number of mitochondrial substrates. Overexpression of SENP5 rescues SUMO1-induced mitochondrial fragmentation that is partly due to the downregulation of DRP1. By contrast, silencing of SENP5 results in a fragmented and altered morphology. DRP1 was stably mono-SUMOylated in these cells, suggesting that SUMOylation leads to increased DRP1 mediated fission. In addition, the reduction of SENP5 levels resulted in a significant increase in the production of free radicals. Reformation of the mitochondrial tubules by expressing the dominant interfering DRP1 or by RNA silencing of endogenous DRP1 protein rescued both the morphological aberrations and the increased production of ROS induced by downregulation of SENP5. These data demonstrate the importance of SENP5 as a new regulator of SUMO1 proteolysis from mitochondrial targets, impacting mitochondrial morphology and metabolism.

[Femoral neck fractures in the growing skeleton]

Prognosis and therapy of fractures of the neck of the femur are identical in children and adults. The results of injuries to the neck of the femur in children are determined by the path of vessels to the femoral head and the neck of the femur. 28 cases have been followed up, partly for 25 years. These observations allow to draw the conclusion that fractures of the neck of the femur in children have to be treated as acute lesions in order to avoid severe late consequences for the hip joint. The fractures of the neck of the femur are classified according to Ratliff. The transcervical fracture is the most frequent injury. Late consequences are treated by correction osteotomy, chip implantation, and boring, according to the extent of necrosis and deformity.