Regulation of excitatory synapse development by the RhoGEF Ephexin5

The neuronal synapse is a specialized cell-cell junction that mediates communication between neurons. The formation of a synapse requires the coordinated activity of signaling molecules that can either promote or restrict synapse number and function. Tight regulation of these signaling molecules are critical to ensure that synapses form in the correct number, time and place during brain development. A number of molecular mechanisms that promote synapse formation have been elucidated, but specific mechanisms that restrict synapse formation are less well understood. The findings presented within this dissertation focus on how a specific Rho guanine nucleotide exchange factor (GEF) Ephexin5 functions to restrict early synaptic development and how perturbations in Ephexin5 signaling may lead to human neurodevelopmental disease

Hitchhiking: A Non-Canonical Mode of Microtubule-Based Transport

The long-range movement of organelles, vesicles, and macromolecular complexes by microtubule-based transport is crucial for cell growth and survival. The canonical view of intracellular transport is that each cargo directly recruits molecular motors via cargo-specific adaptor molecules. Recently, a new paradigm called 'hitchhiking' has emerged: some cargos can achieve motility by interacting with other cargos that have already recruited molecular motors. In this way, cargos are co-transported together and their movements are directly coupled. Cargo hitchhiking was discovered in fungi. However, the observation that organelle dynamics are coupled in mammalian cells suggests that this paradigm may be evolutionarily conserved. We review here the data for hitchhiking and discuss the biological significance of this non-canonical mode of microtubule-based transport

On the performance of a two-relay cooperative system in correlated lognormal channels

107 σ.A cooperative diversity system with multiple relays where the receiver com-bines the relayed and direct paths consists of a practical and useful configuration when the communication channels suffer from fading. This diploma thesis presents novel analytical formulas and numerical results on the performance of a cooperative diversity system with two relays in arbitrary positions over correlated lognormal channels (Chapter 4). The two relays form a di-amond-shaped cooperative system. The Fixed Relaying cooperative protocol and both the Time Domain Multiple Access (TDMA/half duplex relay) and the Space Do-main Multiple Access (SDMA/ full duplex relay) schemes are considered in the present contribution. The destination receiver combines the multiple versions of the received signal utilizing either the Selection Combining (SC) or the Maximal Ratio Combining (MRC) techniques. The performance of the proposed system is compared with a basic coopera-tive system with only one relay (triangle-shaped cooperative configuration) which is presented in chapter 3 and a direct link system, in order to investigate the benefits of cooperation. The proposed expressions for the outage probability can be directly applied to various propagation scenarios where the lognormal distribution is used to describe the large scale fading effects such as, indoor, urban and on body propaga-tion environments. Finally, the impact of the lognormal parameters (including correlation) on the cooperative system performance is investigated through extended numerical results. Index TermsΚωνσταντίνος Ι. Σαλογιάννη

Peroxisomes move by hitchhiking on early endosomes using the novel linker protein PxdA.

Eukaryotic cells use microtubule-based intracellular transport for the delivery of many subcellular cargos, including organelles. The canonical view of organelle transport is that organelles directly recruit molecular motors via cargo-specific adaptors. In contrast with this view, we show here that peroxisomes move by hitchhiking on early endosomes, an organelle that directly recruits the transport machinery. Using the filamentous fungus Aspergillus nidulans we found that hitchhiking is mediated by a novel endosome-associated linker protein, PxdA. PxdA is required for normal distribution and long-range movement of peroxisomes, but not early endosomes or nuclei. Using simultaneous time-lapse imaging, we find that early endosome-associated PxdA localizes to the leading edge of moving peroxisomes. We identify a coiled-coil region within PxdA that is necessary and sufficient for early endosome localization and peroxisome distribution and motility. These results present a new mechanism of microtubule-based organelle transport in which peroxisomes hitchhike on early endosomes and identify PxdA as the novel linker protein required for this coupling

Microtubules provide force to promote membrane uncoating in vacuolar escape for a cyto-invasive bacterial pathogen

Abstract Intracellular bacterial pathogens gain entry to mammalian cells inside a vacuole derived from the host membrane. Some of them escape the bacteria-containing vacuole (BCV) and colonize the cytosol. Bacteria replicating within BCVs coopt the microtubule network to position it within infected cells, whereas the role of microtubules for cyto-invasive pathogens remains obscure. Here, we show that the microtubule motor cytoplasmic dynein-1 and specific activating adaptors are hijacked by the enterobacterium Shigella flexneri. These host proteins were found on infection-associated macropinosomes (IAMs) formed during Shigella internalization. We identified Rab8 and Rab13 as mediators of dynein recruitment and discovered that the Shigella effector protein IpaH7.8 promotes Rab13 retention on moving BCV membrane remnants, thereby facilitating membrane uncoating of the Shigella-containing vacuole. Moreover, the efficient unpeeling of BCV remnants contributes to a successful intercellular spread. Taken together, our work demonstrates how a bacterial pathogen subverts the intracellular transport machinery to secure a cytosolic niche