Revisiting I-BAR Proteins at Central Synapses

Dendritic spines, the distinctive postsynaptic feature of central nervous system (CNS) excitatory synapses, have been studied extensively as electrical and chemical compartments, as well as scaffolds for receptor cycling and positioning of signaling molecules. The dynamics of the shape, number, and molecular composition of spines, and how they are regulated by neural activity, are critically important in synaptic efficacy, synaptic plasticity, and ultimately learning and memory. Dendritic spines originate as outward protrusions of the cell membrane, but this aspect of spine formation and stabilization has not been a major focus of investigation compared to studies of membrane protrusions in non-neuronal cells. We review here one family of proteins involved in membrane curvature at synapses, the BAR (Bin-Amphiphysin-Rvs) domain proteins. The subfamily of inverse BAR (I-BAR) proteins sense and introduce outward membrane curvature, and serve as bridges between the cell membrane and the cytoskeleton. We focus on three I-BAR domain proteins that are expressed in the central nervous system: Mtss2, MIM, and IRSp53 that promote negative, concave curvature based on their ability to self-associate. Recent studies suggest that each has distinct functions in synapse formation and synaptic plasticity. The action of I-BARs is also shaped by crosstalk with other signaling components, forming signaling platforms that can function in a circuit-dependent manner. We discuss another potentially important feature—the ability of some BAR domain proteins to impact the function of other family members by heterooligomerization. Understanding the spatiotemporal resolution of synaptic I-BAR protein expression and their interactions should provide insights into the interplay between activity-dependent neural plasticity and network rewiring in the CNS.

Dendritic Spine Initiation in Brain Development, Learning and Diseases and Impact of BAR-Domain Proteins

Dendritic spines are small, bulbous protrusions along neuronal dendrites where most of the excitatory synapses are located. Dendritic spine density in normal human brain increases rapidly before and after birth achieving the highest density around 2–8 years. Density decreases during adolescence, reaching a stable level in adulthood. The changes in dendritic spines are considered structural correlates for synaptic plasticity as well as the basis of experience-dependent remodeling of neuronal circuits. Alterations in spine density correspond to aberrant brain function observed in various neurodevelopmental and neuropsychiatric disorders. Dendritic spine initiation affects spine density. In this review, we discuss the importance of spine initiation in brain development, learning, and potential complications resulting from altered spine initiation in neurological diseases. Current literature shows that two Bin Amphiphysin Rvs (BAR) domain-containing proteins, MIM/Mtss1 and SrGAP3, are involved in spine initiation. We review existing literature and open databases to discuss whether other BAR-domain proteins could also take part in spine initiation. Finally, we discuss the potential molecular mechanisms on how BAR-domain proteins could regulate spine initiation.

Comparing physical mechanisms for membrane curvature-driven sorting of BAR-domain proteins

We review current theoretical models for curvature sensing of BAR-domain proteins, test the models on 2 proteins, and present new electron microscopy data on the organization of BAR domains on tubes.

Light-inducible generation of membrane curvature in live cells with engineered BAR domain proteins

Nanoscale membrane curvature is now understood to play an active role in essential cellular processes such as endocytosis, exocytosis and actin dynamics. Previous studies have shown that membrane curvatures directly affect protein functions and intracellular signaling. However, few methods are able to precisely manipulate membrane curvature in live cells. Here, we report the development of a new method of generating nanoscale membrane curvature in live cells that is controllable, reversible, and capable of precise spatial and temporal manipulation. For this purpose, we make use of BAR domain proteins, a family of well-studied membrane-remodeling and membrane-sculpting proteins. Specifically, we engineered two optogenetic systems, opto-FBAR and opto-IBAR, that allow light-inducible formation of positive and negative membrane curvature respectively. Using opto-FBAR, blue light activation results in the formation of tubular membrane invaginations (positive curvature), controllable down to the subcellular level. Using opto-IBAR, blue light illumination results in the formation of membrane protrusions or filopodia (negative curvature). These systems present a novel approach for light-inducible manipulation of nanoscale membrane curvature in live cells.HighlightsOpto-FBAR enables light-inducible positive membrane curvature formation.Opto-IBAR enables light-inducible negative membrane curvature formation.Light-inducible activation enables precise spatial and temporal control.Opto-BAR systems present a new approach for studying membranes in live cells.

A putative N-BAR-domain protein is crucially required for the development of hyphae tip appressorium-like structure and its plant infection in Magnaporthe oryzae

Membrane remodeling modulates many biological processes. The binding of peripheral proteins to lipid membranes results in membrane invaginations and protrusions, which regulate essential intra-cellular membrane and extra-cellular trafficking events. Proteins that bind and re-shape bio-membranes have been identified and extensively investigated. The Bin/Amphiphysin/Rvs (BAR) domain proteins are crescent-shape and play a conserved role in tubulation and sculpturing of cell membranes. We deployed targeted gene replacement technique to functionally characterize two hypothetical proteins (MoBar-A and MoBar-B) containing unitary N-BAR domain in Magnaporthe oryzae. The results obtained from phenotypic examinations showed that MoBAR-A deletion exerted a significant reduction in the growth of the defective ∆Mobar-A strain. Also, MoBAR-A disruption exclusively compromised hyphae-mediated infection. Additionally, the targeted replacement of MoBAR-A suppressed the expression of genes associated with the formation of hyphae tip appressorium-like structure in M. oryzae. Furthermore, single as well as combined deletion of MoBAR-A and MoBAR-B down-regulated the expression of nine different membrane-associated genes. From these results, we inferred that MoBAR-A plays a key and unique role in the pathogenesis of M. oryzae through direct or indirect regulation of the development of appressorium-like structures developed by hyphae tip. Taken together, these results provide unique insights into the direct contribution of the N-BAR domain proteins to morphological, reproduction, and infectious development of M. oryzae.

Phagocytosis is mediated by two-dimensional assemblies of the F-BAR protein GAS7

Phagocytosis is a cellular process for internalization of micron-sized large particles including pathogens. The Bin-Amphiphysin-Rvs167 (BAR) domain proteins, including the FCH-BAR (F-BAR) domain proteins, impose specific morphologies on lipid membranes. Most BAR domain proteins are thought to form membrane invaginations or protrusions by assembling into helical submicron-diameter filaments, such as on clathrin-coated pits, caveolae, and filopodia. However, the mechanism by which BAR domain proteins assemble into micron-scale phagocytic cups was unclear. Here, we show that the two-dimensional sheet-like assembly of Growth Arrest-Specific 7 (GAS7) plays a critical role in phagocytic cup formation in macrophages. GAS7 has the F-BAR domain that possesses unique hydrophilic loops for two-dimensional sheet formation on flat membranes. Super-resolution microscopy reveals the similar assemblies of GAS7 on phagocytic cups and liposomes. The mutations of the loops abolishes both the membrane localization of GAS7 and phagocytosis. Thus, the sheet-like assembly of GAS7 plays a significant role in phagocytosis.