ER biogenesis: self-assembly of tubular topology by protein hairpins

Sequences flanking the transmembrane segments facilitate mitochondrial localization and membrane fusion by mitofusin

Mitochondria constantly connect through membrane fusion. The merging of the outer mitochondrial membrane requires mitofusin (MFN) proteins. MFN is a membrane-anchored GTPase, but whether it is sufficient to achieve fusion, and if so how, is largely unknown. We have taken advantage of a similar GTPase named atlastin (ATL), which mediates fusion of the endoplasmic reticulum (ER), as its mechanism is better understood. Domain swapping experiments show that MFN is capable of fusing membranes, even on the ER. The C-terminal tail of MFN contains an amphipathic helix that promotes fusion. MFN is properly inserted into the mitochondrial membrane with the help of the helix and neighboring hydrophobic residues. These findings provide insight into how mitochondria fuse.

Mitochondria constantly divide and fuse. Homotypic fusion of the outer mitochondrial membranes requires the mitofusin (MFN) proteins, a family of dynamin-like GTPases. MFNs are anchored in the membrane by transmembrane (TM) segments, exposing both the N-terminal GTPase domain and the C-terminal tail (CT) to the cytosol. This arrangement is very similar to that of the atlastin (ATL) GTPases, which mediate fusion of endoplasmic reticulum (ER) membranes. We engineered various MFN-ATL chimeras to gain mechanistic insight into MFN-mediated fusion. When MFN1 is localized to the ER by TM swapping with ATL1, it functions in the maintenance of ER morphology and fusion. In addition, an amphipathic helix in the CT of MFN1 is exchangeable with that of ATL1 and critical for mitochondrial localization of MFN1. Furthermore, hydrophobic residues N-terminal to the TM segments of MFN1 play a role in membrane targeting but not fusion. Our findings provide important insight into MFN-mediated membrane fusion.

Formation and Stability of Lipid Membrane Nanotubes

Lipid membrane nanotubes are abundant in living cells, even though tubules are energetically less stable than sheet-like structures. According to membrane elastic theory, the tubular endoplasmic reticulum (ER), with its high area-to-volume ratio, appears to be particularly unstable. We explore how tubular membrane structures can nevertheless be induced and why they persist. In Monte Carlo simulations of a fluid-elastic membrane model subject to thermal fluctuations and without constraints on symmetry, we find that a steady increase in the area-to-volume ratio readily induces tubular structures. In simulations mimicking the ER wrapped around the cell nucleus, tubules emerge naturally as the membrane area increases. Once formed, a high energy barrier separates tubules from the thermodynamically favored sheet-like membrane structures. Remarkably, this barrier persists even at large area-to-volume ratios, protecting tubules against shape transformations despite enormous driving forces toward sheet-like structures. Molecular dynamics simulations of a molecular membrane model confirm the metastability of tubular structures. Volume reduction by osmotic regulation and membrane area growth by lipid production and by fusion of small vesicles emerge as powerful factors in the induction and stabilization of tubular membrane structures.

Dysfunctional tubular endoplasmic reticulum constitutes a pathological feature of Alzheimer's disease

Pathological features in Alzheimer's brains include mitochondrial dysfunction and dystrophic neurites (DNs) in areas surrounding amyloid plaques. Using a mouse model that overexpresses reticulon 3 (RTN3) and spontaneously develops age-dependent hippocampal DNs, here we report that DNs contain both RTN3 and REEPs, topologically similar proteins that can shape tubular endoplasmic reticulum (ER). Importantly, ultrastructural examinations of such DNs revealed gradual accumulation of tubular ER in axonal termini, and such abnormal tubular ER inclusion is found in areas surrounding amyloid plaques in biopsy samples from Alzheimer's disease (AD) brains. Functionally, abnormally clustered tubular ER induces enhanced mitochondrial fission in the early stages of DN formation and eventual mitochondrial degeneration at later stages. Furthermore, such DNs are abrogated when RTN3 is ablated in aging and AD mouse models. Hence, abnormally clustered tubular ER can be pathogenic in brain regions: disrupting mitochondrial integrity, inducing DNs formation and impairing cognitive function in AD and aging brains.

Functional architecture of the cell's nucleus in development, aging, and disease

In eukaryotes, the function of the cell's nucleus has primarily been considered to be the repository for the organism's genome. However, this rather simplistic view is undergoing a major shift, as it is increasingly apparent that the nucleus has functions extending beyond being a mere genome container. Recent findings have revealed that the structural composition of the nucleus changes during development and that many of these components exhibit cell- and tissue-specific differences. Increasing evidence is pointing to the nucleus being integral to the function of the interphase cytoskeleton, with changes to nuclear structural proteins having ramifications affecting cytoskeletal organization and the cell's interactions with the extracellular environment. Many of these functions originate at the nuclear periphery, comprising the nuclear envelope (NE) and underlying lamina. Together, they may act as a "hub" in integrating cellular functions including chromatin organization, transcriptional regulation, mechanosignaling, cytoskeletal organization, and signaling pathways. Interest in such an integral role has been largely stimulated by the discovery that many diseases and anomalies are caused by defects in proteins of the NE/lamina, the nuclear envelopathies, many of which, though rare, are providing insights into their more common variants that are some of the major issues of the twenty-first century public health. Here, we review the contributions that mouse mutants have made to our current understanding of the NE/lamina, their respective roles in disease and the use of mice in developing potential therapies for treating the diseases.

The overexpression of nuclear envelope protein Lap2β induces endoplasmic reticulum reorganisation via membrane stacking

Some nuclear envelope proteins are localised to both the nuclear envelope and the endoplasmic reticulum; therefore, it seems plausible that even small amounts of these proteins can influence the organisation of the endoplasmic reticulum. A simple method to study the possible effects of nuclear envelope proteins on endoplasmic reticulum organisation is to analyze nuclear envelope protein overexpression. Here, we demonstrate that Lap2β overexpression can induce the formation of cytoplasmic vesicular structures derived from endoplasmic reticulum membranes. Correlative light and electron microscopy demonstrated that these vesicular structures were composed of a series of closely apposed membranes that were frequently arranged in a circular fashion. Although stacked endoplasmic reticulum cisternae were highly ordered, Lap2β could readily diffuse into and out of these structures into the surrounding reticulum. It appears that low-affinity interactions between cytoplasmic domains of Lap2β can reorganise reticular endoplasmic reticulum into stacked cisternae. Although the effect of one protein may be insignificant at low concentrations, the cumulative effect of many non-specialised proteins may be significant.

Association with endoplasmic reticulum promotes proteasomal degradation of GADD34 protein

Stress-induced endogenous and ectopically expressed GADD34 proteins were present both in the cytoplasm and in membranes, with their membrane association showing similar biochemical properties. Deletion of N-terminal sequences in GADD34-GFP proteins highlighted an amphipathic helix, whose hydrophobic surface, specifically valine 25 and leucine 29, mediated endoplasmic reticulum (ER) localization. Substitution of leucines for three arginines on the polar surface indicated that the same helix also mediated the association of GADD34 with mitochondria. Fluorescence protease protection and chemical modification of cysteines substituted in the membrane-binding domain pointed to a monotopic insertion of GADD34 into the outer layer of the ER membrane. Fluorescence recovery after photobleaching showed that ER association retards the mobility of GADD34 in living cells. Both WT GADD34 and the mutant, V25R, effectively scaffolded the α-isoform of protein phosphatase-1 (PP1α) and enabled eIF2α dephosphorylation. However, the largely cytosolic V25R protein displayed a reduced rate of proteasomal degradation, and unlike WT GADD34, whose ectopic expression resulted in a dilated or distended ER, V25R did not modify ER morphology. These studies suggested that the association of with ER modulates intracellular trafficking and proteasomal degradation of GADD34, and in turn, its ability to modify ER morphology.

Endothelial reticulon-4B (Nogo-B) regulates ICAM-1-mediated leukocyte transmigration and acute inflammation

The reticulon (Rtn) family of proteins are localized primarily to the endoplasmic reticulum (ER) of most cells. The Rtn-4 family, (aka Nogo) consists of 3 splice variants of a common gene called Rtn-4A, Rtn-4B, and Rtn-4C. Recently, we identified the Rtn-4B (Nogo-B) protein in endothelial and smooth muscle cells of the vessel wall, and showed that Nogo-B is a regulator of cell migration in vitro and vascular remodeling and angiogenesis in vivo. However, the role of Nogo-B in inflammation is still largely unknown. In the present study, we use 2 models of inflammation to show that endothelial Nogo-B regulates leukocyte transmigration and intercellular adhesion molecule-1 (ICAM-1)-dependent signaling. Mice lacking Nogo-A/B have a marked reduction in neutrophil and monocyte recruitment to sites of inflammation, while Nogo-A/B(-/-) mice engrafted with wild-type (WT) bone marrow still exhibit impaired inflammation compared with WT mice engrafted with Nogo-A/B(-/-) bone marrow, arguing for a critical role of host Nogo in this response. Using human leukocytes and endothelial cells, we show mechanistically that the silencing of Nogo-B with small interfering RNA (siRNA) impairs the transmigration of neutrophils and reduces ICAM-1-stimulated phosphorylation of vascular endothelial-cell cadherin (VE-cadherin). Our results reveal a novel role of endothelial Nogo-B in basic immune functions and provide a key link in the molecular network governing endothelial-cell regulation of diapedesis.

Caveolin-1 induces formation of membrane tubules that sense actomyosin tension and are inhibited by polymerase I and transcript release factor/cavin-1

We found that PTRF/cavin-1 is lost coordinately with caveolin-1 in some cancer cells. When reexpressed in these cells, caveolin-1 formed membrane tubules that were under actomyosin-induced tension and recruited Rab8 and EHD proteins. PTRF/cavin-1 inhibited tubule formation by caveolin-1, showing a new function for the protein.

Caveolin-1 and caveolae are often lost in cancer. We found that levels of caveolin-1 and polymerase I and transcript release factor (PTRF)/cavin-1 correlated closely in a panel of cancer and normal cells. Caveolin-1 reexpression in cancer cells lacking both proteins induced formation of long membrane tubules rarely seen in normal cells. PTRF/cavin-1 inhibited tubule formation when coexpressed with caveolin-1 in these cells, whereas suppression of PTRF/cavin-1 expression in cells that normally expressed both genes stimulated tubule formation by endogenous caveolin-1. Caveolin-1 tubules shared several features with previously described Rab8 tubules. Coexpressed Rab8 and caveolin-1 labeled the same tubules (as did EHD proteins), and synergized to promote tubule formation, whereas a dominant-interfering Rab8 mutant inhibited caveolin-1 tubule formation. Both overexpression and inhibition of dynamin-2 reduced the abundance of caveolin-1 tubules. Caveolin-1 reexpression in SK-BR-3 breast cancer cells also induced formation of short membrane tubules close to cortical actin filaments, which required actin filaments but not microtubules. Actomyosin-induced tension destabilized both long and short tubules; they often snapped and resolved to small vesicles. Actin filament depolymerization or myosin II inhibition reduced tension and stabilized tubules. These data demonstrate a new function for PTRF/cavin-1, a new functional interaction between caveolin-1 and Rab8 and that actomyosin interactions can induce tension on caveolin-1-containing membranes.

Domain-driven morphogenesis of cellular membranes

Cellular membrane systems delimit and organize the intracellular space. Most of the morphological rearrangements in cells involve the coordinated remodeling of the lipid bilayer, the core of the membranes. This process is generally thought to be initiated and coordinated by specialized protein machineries. Nevertheless, it has become increasingly evident that the most essential part of the geometric information and energy required for membrane remodeling is supplied via the cooperative and synergistic action of proteins and lipids, as cellular shapes are constructed using the intrinsic dynamics, plasticity and self-organizing capabilities provided by the lipid bilayer. Here, we analyze the essential role of proteo-lipid membrane domains in conducting and coordinating morphological remodeling in cells.

Reticulon 4B (Nogo-B) is necessary for macrophage infiltration and tissue repair

Blood vessel formation during ischemia and wound healing requires coordination of the inflammatory response with genes that regulate blood vessel assembly. Here we show that the reticulon family member 4B, aka Nogo-B, is upregulated in response to ischemia and is necessary for blood flow recovery secondary to ischemia and wound healing. Mice lacking Nogo-B exhibit reduced arteriogenesis and angiogenesis that are linked to a decrease in macrophage infiltration and inflammatory gene expression in vivo. Bone marrow-derived macrophages isolated from Nogo knock-out mice have reduced spreading and chemotaxis due to impaired Rac activation. Bone marrow reconstitution experiments show that Nogo in myeloid cells is necessary to promote macrophage homing and functional recovery after limb ischemia. Thus, endogenous Nogo coordinates macrophage-mediated inflammation with arteriogenesis, wound healing, and blood flow control.

Fraying at the edge mouse models of diseases resulting from defects at the nuclear periphery

Eukaryotic cells compartmentalize their genetic material within the nucleus. The boundary separating the genetic material from the cytoplasm is the nuclear envelope (NE) and lamina. Historically, the NE was perceived as functioning primarily as a barrier regulating the entry and exit of macromolecules between the nucleus and cytoplasm via the nuclear pore complexes (NPCs) that traverse the nuclear membranes. However, recent findings have caused a fundamental reassessment with regard to NE and lamina functions. Evidence now points to the NE and lamina functioning as a "hub" in regulating and perhaps integrating critical cellular functions that include chromatin organization, transcriptional regulation, mechanical integrity of the cell, signaling pathways, as well as acting as a key component of the cytoskeleton. Such an integral role for the nuclear boundary has emerged from increased interest into the functions of the NE/lamina, which has been largely stimulated by the discovery that some 24 different diseases and anomalies are caused by defects in proteins of the NE and lamina.