A critical function for the actin cytoskeleton in targeted exocytosis of prefusion vesicles during myoblast fusion.

Myoblast fusion is an essential step during muscle differentiation. Previous studies in Drosophila have revealed a signaling pathway that relays the fusion signal from the plasma membrane to the actin cytoskeleton. However, the function for the actin cytoskeleton in myoblast fusion remains unclear. Here we describe the characterization of solitary (sltr), a component of the myoblast fusion signaling cascade. sltr encodes the Drosophila ortholog of the mammalian WASP-interacting protein. Sltr is recruited to sites of fusion by the fusion-competent cell-specific receptor Sns and acts as a positive regulator for actin polymerization at these sites. Electron microscopy analysis suggests that formation of F-actin-enriched foci at sites of fusion is involved in the proper targeting and coating of prefusion vesicles. These studies reveal a surprising cell-type specificity of Sltr-mediated actin polymerization in myoblast fusion, and demonstrate that targeted exocytosis of prefusion vesicles is a critical step prior to plasma membrane fusion.

Analysis of cell migration using whole-genome expression profiling of migratory cells in the Drosophila ovary.

Cell migration contributes to normal development and homeostasis as well as to pathological processes such as inflammation and tumor metastasis. Previous genetic screens have revealed signaling pathways that govern follicle cell migrations in the Drosophila ovary, but few downstream targets of the critical transcriptional regulators have been identified. To characterize the gene expression profile of two migratory cell populations and identify Slbo targets, we purified border cells and centripetal cells expressing the mouse CD8 antigen and carried out whole-genome microarray analysis. Genes predicted to control actin dynamics and the endocytic and secretory pathways were overrepresented in the migratory cell transcriptome. Mutations in five genes, including ttk, failed to complement previously isolated mutations that cause cell migration defects in mosaic clones. Functional analysis revealed a role for the Notch-activating protease Kuzbanian in border cell migration and identified Tie as a guidance receptor for the border cells.

Multiple sclerosis symptom recrudescence at the end of the natalizumab dosing cycle.

BACKGROUND: This study was undertaken to determine how frequently patients receiving natalizumab for multiple sclerosis (MS) experience recrudescence of their MS symptoms at the end of the dosing cycle. METHODS: One hundred consecutive MS patients receiving natalizumab completed a survey evaluating changes in symptoms during the natalizumab dosing cycle. Ninety-one patients also completed questionnaires at two time points: the first week after natalizumab infusion and the last week of the dosing cycle. These included the Multiple Sclerosis Quality of Life-54 (MSQOL-54), Fatigue Visual Analog Scale (VAS), Fatigue Severity Scale (FSS), and Beck Depression Inventory-II (BDI-II). RESULTS: End of dosing interval (EDI) symptoms were reported as currently being experienced by 57% of respondents. An additional 10% reported that they previously experienced that phenomenon, but not currently, and 33% reported never experiencing this. In those with EDI symptoms, they began to occur a median of 21 days after infusion and improved again a median of 1 day after infusion. The most common symptoms reported were fatigue, weakness, walking impairment, and cognitive difficulties. No specific demographic or disease characteristics were associated with this phenomenon. In the subgroup with EDI symptoms, the MSQOL-54, Fatigue VAS, FSS, and BDI-II scores were all significantly worse in the last week of the dosing cycle when compared with the first week. No difference was seen in these scores between first and last week in the subgroup not experiencing symptom recrudescence. CONCLUSIONS: Recrudescence of fatigue, weakness, walking impairment, or cognitive difficulties at the end of the dosing cycle occurs in about two-thirds of MS patients receiving natalizumab.

Repair of the main nerve trunk of the upper limb with end-to-side neurorrhaphy: an experimental study in rabbits.

The aim of this study was to assess the effectiveness of reinnervation using end-to-side neurorrhaphy in the upper extremity of the rabbit. The cut right ulnar nerve was repaired and sutured to the side of the median nerve about 3 cm above the elbow joint. The extent of reinnervation was quantitatively evaluated, as well as the integrity of the intact donor nerve in 36 rabbits randomly treated with fresh or delayed nerve repair with or without perineurotomy. Evaluations included nerve conduction velocity (NCV) of both the ulnar and medial nerves, dry muscle weight, and histologic examination (neurofilament stain and morphometric assessment) at 3 and 6 months postoperatively. NCV recovery rates were 79% and 87% for the ulnar nerve, and 89% and 94% for the median nerve compared to contralateral intact nerves, at 3 and 6 months, respectively. Flexor carpi ulnaris muscle mass measurements revealed a recovery in dry muscle weight of about 81% and 88% at 3 and 6 months, respectively, compared to the intact contralateral flexor carpi ulnaris. Histologic studies with neurofilament staining reveal numerous axonal sprouts at the distal end of the median nerve, indicative of myelinated axonal regeneration. Morphometric analysis demonstrated no difference between fresh and delayed repairs. These results indicate that in the upper extremity of rabbits, end-to-side neurorrhaphy permits axonal regeneration from the intact donor nerve, and is associated with satisfactory recovery. The effect of the procedure on the donor nerve was negligible.

Long-term evaluation of rabbit peripheral nerve repair with end-to-side neurorrhaphy in rabbits.

This study was designed to quantitatively assess long-term end-to-side neurorrhaphy in rabbits. The cut right ulnar nerve was repaired and sutured to the median nerve, in which a perineurial window was created in an end-to-side fashion 3 cm above the elbow joint. Both the extent of the reinnervation and the integrity of the intact donor nerve were evaluated in 36 rabbits randomly treated with fresh or delayed nerve repair. Evaluations included motor nerve conduction velocity (MNCV), dry muscle weight (DMW), and histological examinations at 9 and 12 months postoperatively. The recovery rates of MNCV were 90.1% and 92.8% for the ulnar nerve, and 95.7% and 96.8% for the median nerve, compared to intact contralateral nerves at 9 and 12 months, respectively. MNCV was not detectable for the ulnar nerve in control animals, while it was normal for the median nerve. Recoveries of flexor carpi ulnaris dry muscle weight of about 90.7% and 94.5% were observed at 9 and 12 months postoperatively, respectively. However, muscle mass measurements revealed a recovery of only 31.3% and 27% for control groups at 9 and 12 months postoperatively. The differences between experimental groups and control groups were statistically significant (P < 0.01). Neurofilament and silver stains showed numerous sprouting axons originating from the median nerve to the ulnar nerve. The results indicate that end-to-side neurorrhaphy could induce axonal sprouting from the main nerve trunk of upper limbs in rabbits, leading to useful functional recovery.