The state of F-BAR domains as membrane-bound oligomeric platforms

F-BAR proteins CIP4 and FBP17 function in cortical neuron radial migration and process outgrowth

Neurite initiation from newly born neurons is a critical step in neuronal differentiation and migration. Neuronal migration in the developing cortex is accompanied by dynamic extension and retraction of neurites as neurons progress through bipolar and multipolar states. However, there is a relative lack of understanding regarding how the dynamic extension and retraction of neurites is regulated during neuronal migration. In recent work we have shown that CIP4, a member of the F-BAR family of membrane bending proteins, inhibits cortical neurite formation in culture, while family member FBP17 induces premature neurite outgrowth. These results beg the question of how CIP4 and FBP17 function in radial neuron migration and differentiation in vivo , including the timing and manner of neurite extension and retraction. Indeed, the regulation of neurite outgrowth is essential for the transitions between bipolar and multipolar states during radial migration. To examine the effects of modulating expression of CIP4 and FBP17 in vivo , we used in utero electroporation, in combination with our published Double UP technique, to compare knockdown or overexpression cells with control cells within the same mouse tissue of either sex. We show that either knockdown or overexpression of CIP4 and FBP17 results in the marked disruption of radial neuron migration by modulating neuronal morphology and neurite outgrowth, consistent with our findings in culture. Our results demonstrate that the F-BAR proteins CIP4 and FBP17 are essential for proper radial migration in the developing cortex and thus play a key role in cortical development.

SIGNIFICANCE STATEMENT

During embryonic development, radial migration of newly born cortical neurons is a complex process that underlies the proper formation of the neocortex, the outermost layers of neurons in the brain. Disruptions in radial migration results in profound effects on cognitive function and can lead to devastating developmental disabilities. To better understand this critical process in brain development we examined two members of the F-BAR family of membrane bending proteins, CIP4 and FBP17, which are present in the developing brain. We demonstrate that intracellular concentrations of these proteins must be tightly regulated. Increasing or decreasing levels of either protein has profound effects on neuronal morphology and proper radial migration, suggesting they are key players in cortical development.

Actin-membrane linkers: Insights from synthetic reconstituted systems

At the cell surface, the actin cytoskeleton and the plasma membrane interact reciprocally in a variety of processes related to the remodeling of the cell surface. The actin cytoskeleton has been known to modulate membrane organization and reshape the membrane. To this end, actin-membrane linking molecules play a major role in regulating actin assembly and spatially direct the interaction between the actin cytoskeleton and the membrane. While studies in cells have provided a wealth of knowledge on the molecular composition and interactions of the actin-membrane interface, the complex molecular interactions make it challenging to elucidate the precise actions of the actin-membrane linkers at the interface. Synthetic reconstituted systems, consisting of model membranes and purified proteins, have been a powerful approach to elucidate how actin-membrane linkers direct actin assembly to drive membrane shape changes. In this review, we will focus only on several actin-membrane linkers that have been studied by using reconstitution systems. We will discuss the design principles of these reconstitution systems and how they have contributed to the understanding of the cellular functions of actin-membrane linkers. Finally, we will provide a perspective on future research directions in understanding the intricate actin-membrane interaction.

Phosphorylation of the F-BAR protein Hof1 drives septin ring splitting in budding yeast

A double septin ring accompanies cytokinesis in yeasts and mammalian cells. In budding yeast, reorganisation of the septin collar at the bud neck into a dynamic double ring is essential for actomyosin ring constriction and cytokinesis. Septin reorganisation requires the Mitotic Exit Network (MEN), a kinase cascade essential for cytokinesis. However, the effectors of MEN in this process are unknown. Here we identify the F-BAR protein Hof1 as a critical target of MEN in septin remodelling. Phospho-mimicking HOF1 mutant alleles overcome the inability of MEN mutants to undergo septin reorganisation by decreasing Hof1 binding to septins and facilitating its translocation to the actomyosin ring. Hof1-mediated septin rearrangement requires its F-BAR domain, suggesting that it may involve a local membrane remodelling that leads to septin reorganisation. In vitro Hof1 can induce the formation of intertwined septin bundles, while a phosphomimetic Hof1 protein has impaired septin-bundling activity. Altogether, our data indicate that Hof1 modulates septin architecture in distinct ways depending on its phosphorylation status.

Processes Controlling the Contractile Ring during Cytokinesis in Fission Yeast, Including the Role of ESCRT Proteins

Cytokinesis, as the last stage of the cell division cycle, is a tightly controlled process amongst all eukaryotes, with defective division leading to severe cellular consequences and implicated in serious human diseases and conditions such as cancer. Both mammalian cells and the fission yeast Schizosaccharomyces pombe use binary fission to divide into two equally sized daughter cells. Similar to mammalian cells, in S. pombe, cytokinetic division is driven by the assembly of an actomyosin contractile ring (ACR) at the cell equator between the two cell tips. The ACR is composed of a complex network of membrane scaffold proteins, actin filaments, myosin motors and other cytokinesis regulators. The contraction of the ACR leads to the formation of a cleavage furrow which is severed by the endosomal sorting complex required for transport (ESCRT) proteins, leading to the final cell separation during the last stage of cytokinesis, the abscission. This review describes recent findings defining the two phases of cytokinesis in S. pombe: ACR assembly and constriction, and their coordination with septation. In summary, we provide an overview of the current understanding of the mechanisms regulating ACR-mediated cytokinesis in S. pombe and emphasize a potential role of ESCRT proteins in this process.

Toxoplasma membrane inositol phospholipid binding protein TgREMIND is essential for secretory organelle function and host infection

Apicomplexan parasites possess specialized secretory organelles called rhoptries, micronemes, and dense granules that play a vital role in host infection. In this study, we demonstrate that TgREMIND, a protein found in Toxoplasma gondii, is necessary for the biogenesis of rhoptries and dense granules. TgREMIND contains a Fes-CIP4 homology-Bin/Amphiphysin/Rvs (F-BAR) domain, which binds to membrane phospholipids, as well as a novel uncharacterized domain that we have named REMIND (regulator of membrane-interacting domain). Both the F-BAR domain and the REMIND are crucial for TgREMIND functions. When TgREMIND is depleted, there is a significant decrease in the abundance of dense granules and abnormal transparency of rhoptries, leading to a reduction in protein secretion from these organelles. The absence of TgREMIND inhibits host invasion and parasite dissemination, demonstrating that TgREMIND is essential for the proper function of critical secretory organelles required for successful infection by Toxoplasma.

Age-associated decline in RAB-10 efficacy impairs intestinal barrier integrity

The age-related decline in the ability of the intestinal barrier to maintain selective permeability can lead to various physiological disturbances. Adherens junctions play a vital role in regulating intestinal permeability, and their proper assembly is contingent upon endocytic recycling. However, how aging affects the recycling efficiency and, consequently, the integrity of adherens junctions remains unclear. Here we show that RAB-10/Rab10 functionality is reduced during senescence, leading to impaired adherens junctions in the Caenorhabditis elegans intestine. Mechanistic analysis reveals that SDPN-1/PACSINs is upregulated in aging animals, suppressing RAB-10 activation by competing with DENN-4/GEF. Consistently, SDPN-1 knockdown alleviates age-related abnormalities in adherens junction integrity and intestinal barrier permeability. Of note, the inhibitory effect of SDPN-1 on RAB-10 requires KGB-1/JUN kinase, which presumably enhances the potency of SDPN-1 by altering its oligomerization state. Together, by examining age-associated changes in endocytic recycling, our study sheds light on how aging can impact intestinal barrier permeability.

Membrane shapers from two distinct superfamilies cooperate in the development of neuronal morphology

Membrane-shaping proteins are driving forces behind establishment of proper cell morphology and function. Yet, their reported structural and in vitro properties are noticeably inconsistent with many physiological membrane topology requirements. We demonstrate that dendritic arborization of neurons is powered by physically coordinated shaping mechanisms elicited by members of two distinct classes of membrane shapers: the F-BAR protein syndapin I and the N-Ank superfamily protein ankycorbin. Strikingly, membrane-tubulating activities by syndapin I, which would be detrimental during dendritic branching, were suppressed by ankycorbin. Ankycorbin's integration into syndapin I-decorated membrane surfaces instead promoted curvatures and topologies reflecting those observed physiologically. In line with the functional importance of this mechanism, ankycorbin- and syndapin I-mediated functions in dendritic arborization mutually depend on each other and on a surprisingly specific interface mediating complex formation of the two membrane shapers. These striking results uncovered cooperative and interdependent functions of members of two fundamentally different membrane shaper superfamilies as a previously unknown, pivotal principle in neuronal shape development.

Small GTPase Cdc42, WASP, and scaffold proteins for higher-order assembly of the F-BAR domain protein

The higher-order assembly of Bin-amphiphysin-Rvs (BAR) domain proteins, including the FCH-BAR (F-BAR) domain proteins, into lattice on the membrane is essential for the formation of subcellular structures. However, the regulation of their ordered assembly has not been elucidated. Here, we show that the higher ordered assembly of growth-arrested specific 7 (GAS7), an F-BAR domain protein, is regulated by the multivalent scaffold proteins of Wiskott-Aldrich syndrome protein (WASP)/neural WASP, that commonly binds to the BAR domain superfamily proteins, together with WISH, Nck, the activated small guanosine triphosphatase Cdc42, and a membrane-anchored phagocytic receptor. The assembly kinetics by fluorescence resonance energy transfer monitoring indicated that the GAS7 assembly on liposomes started within seconds and was further increased by the presence of these proteins. The regulated GAS7 assembly was abolished by Wiskott-Aldrich syndrome mutations both in vitro and in cellular phagocytosis. Therefore, Cdc42 and the scaffold proteins that commonly bind to the BAR domain superfamily proteins promoted GAS7 assembly.

Trafficking proteins show limited differences in mobility across different postsynaptic spines

The function of the postsynaptic compartment is based on the presence and activity of postsynaptic receptors, whose dynamics are controlled by numerous scaffolding, signaling and trafficking proteins. Although the receptors and the scaffolding proteins have received substantial attention, the trafficking proteins have not been investigated extensively. Their mobility rates are unknown, and it is unclear how the postsynaptic environment affects their dynamics. To address this, we analyzed several trafficking proteins (α-synuclein, amphiphysin, calmodulin, doc2a, dynamin, and endophilin), estimating their movement rates in the dendritic shaft, as well as in morphologically distinct "mushroom" and "stubby" postsynapse types. The diffusion parameters were surprisingly similar across dendritic compartments, and a few differences between proteins became evident only in the presence of a synapse neck. We conclude that the movement of trafficking proteins is not strongly affected by the postsynaptic compartment, in stark contrast to the presynapse, which regulates strongly the movement of such proteins.

Fission yeast paxillin contains two Cdc15 binding motifs for robust recruitment to the cytokinetic ring

The F-BAR protein Cdc15 mediates attachment of the cytokinetic ring (CR) to the plasma membrane and is essential for cytokinesis in Schizosaccharomyces pombe. While its N-terminal F-BAR domain is responsible for oligomerization and membrane binding, its C-terminal SH3 domain binds other partners at a distance from the membrane. We previously demonstrated that the essential cytokinetic formin Cdc12, through an N-terminal motif, directly binds the cytosolic face of the F-BAR domain. Here, we show that paxillin-like Pxl1, which is important for CR stability, contains a motif highly related to that in formin Cdc12, and also binds the Cdc15 F-BAR domain directly. Interestingly, Pxl1 has a second site for binding the Cdc15 SH3 domain. To understand the importance of these two Pxl1-Cdc15 interactions, we mapped and disrupted both. Disrupting the Pxl1-Cdc15 F-BAR domain interaction reduced Pxl1 levels in the CR, whereas disrupting Pxl1’s interaction with the Cdc15 SH3 domain, did not. Unexpectedly, abolishing Pxl1-Cdc15 interaction greatly reduced but did not eliminate CR Pxl1 and did not significantly affect cytokinesis. These data point to another mechanism of Pxl1 CR recruitment and show that very little CR Pxl1 is sufficient for its cytokinetic function.

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.

Phosphorylation in the intrinsically disordered region of F-BAR protein Imp2 regulates its contractile ring recruitment

The F-BAR protein Imp2 is an important contributor to cytokinesis in the fission yeast, Schizosaccharomyces pombe. Because cell cycle regulated phosphorylation of the central intrinsically disordered region (IDR) of the Imp2 paralog, Cdc15, controls Cdc15 oligomerization state, localization, and ability to bind protein partners, we investigated whether Imp2 is similarly phosphoregulated. We found that Imp2 is endogenously phosphorylated on 28 sites within its IDR with the bulk of phosphorylation being constitutive. In vitro, casein kinase 1 (CK1) Hhp1 and Hhp2 can phosphorylate 17 sites and Cdk1 the remaining 11 sites. Mutations that prevent Cdk1 phosphorylation result in precocious Imp2 recruitment to the cell division site, and mutations designed to mimic these phosphorylation events delay Imp2 CR accumulation. Mutations that eliminated CK1 phosphorylation sites allowed CR sliding, and phosphomimetic substitutions at these sites reduced Imp2 protein levels and slowed CR constriction. Thus, like Cdc15, the Imp2 IDR is phosphorylated at many sites by multiple kinases. In contrast to Cdc15, for which phosphorylation plays a major cell cycle regulatory role, Imp2 phosphorylation is primarily constitutive with milder effects on localization and function.