Bending the Rules of Cell Protrusions : Molecular Mechanisms and Biological Roles of Inverse-BAR Proteins in Cell Morphogenesis

REVIEW TO THE LITERATURE . 1 1.1. Plasma membrane . 1 1.2. Phosphoinositides act as sub-cellular signposts . 2 1.3. The Cytoskeleton . 3 1.4. The Actin Cytoskeleton . 4 1.5. Regulation of the actin cytoskeleton by the Rho GTPases . 5 1.6. Regulation of actin dynamics by actin-binding proteins . 6 1.6. 1. Actin nucleation and elongation factors . 7 1.6. 1.1. WH2-domain mediated fi lament nucleation/elongation . 8 1.6. 1.2. Formin mediated fi lament nucleation/elongation . 10 1.6. 2. Actin fi lament capping by heterodimeric capping protein . 10 1.6. 3. Profi lin recharges actin monomers . 10 1.6. 4. ADF/cofi lins depolymerize and sever actin fi laments . 10 1.7. BAR protein superfamily mediates interactions between the actin cytoskeleton and plasma membrane . 10 1.7. 1. BAR/N-BAR Domains.13 1.7. 2. F-BAR/IF-BAR . 13 1.7. 3. I-BAR Domains . 15 1.7. 3.1. IRSp53 . 16 1.7. 3.2. IRTKS . 17 1.7. 3.3. FLJ22582 . 17 1.7. 3.4. MIM . 17 1.7. 3.5. ABBA . 19 2. Protrusive cellular events involving membrane deformation and actin cytoskeleton remodeling . 19 2.1. Cell migration and invasion . 19 2.1.1. Lamellipodium leads the way . 19 2.1.2. Blebs– dissolution of the holy marriage . 21 2.1.3. Filopodia . 21 2.1.4. Podosomes and Invadopodia . 23 2.2. Cell-cell interactions and intercellular communication . 24 2.2.1. Adherens Junctions . 24 2.2.2. Tunneling or membrane nanotubes (TNT’s) /cytonemes . 25 2.3. Endocytosis . 26 2.3.1. Phagocytosis . 26 2.3.2. Macropinocytosis . 27 2.4. Bacteria-induced pedestals . 27 2.5. BAR proteins in membrane protrusions . 28 AIMS OF THE STUDY . 29 METHODS . 30 RESULTS . 31 3. Identifi cation of a novel class of BAR proteins . 31 3.1. The formation of cell protrusions by I-BAR domains is independent of small GTPase binding and does not involve actin bundling by I-BARs . 31 3.2. Inverse-BAR domains bind phosphoinositide-rich membranes and bend them in to the opposite direction as canonical BAR domains . 32 3.3. The binding of I-BAR domains to the membranes involves clustering of PI(4,5)P 2 . 33 3.4. A sub-group of I-BAR domains insert an amphipathic helix into the membrane . 34 3.5. I-BAR domains induce cell-protrusions that are dependent on intact membrane-binding interface of the domain and the actin cytoskeleton . 34 3.6. I-BAR domain induced cell protrusions contain fi lopodial markers . 35 3.7. Mammalian I-BAR domains display dynamic interaction with membranes whereas the nematode homolog forms more stable and rigid structures . 36 4. Cellular and physiological roles of I-BAR proteins in mammals . 37 4.1. ABBA is highly expressed in glial cells during development. 37 4.2. ABBA is localized to the interface between the plasma membrane and the cortical actin cytoskeleton through its I-BAR domain . 38 4.3. ABBA binds ATP-actin monomers with a high affi nity . 38 4.4. ABBA regulates the extension of glial cells through its membrane binding/deforming activity . 38 4.5. MIM is dispensable for mouse development . 39 4.6. MIM defi ciency leads to compromised renal functions and consequent bone abnormalities . 40 4.7. MIM defi cient mice display morphological alterations in the kidney . 40 4.8. MIM displays dynamic localization to adheres junctions where it promotes F-actin assembly . 41 DISCUSSION . 43 FUTURE PERSPECTIVES . 50 ACKNOWLEDGEMENTS . 52 REFERENCES . 54