Membrane lipids influence protein complex assembly-disassembly

Discovery that cells have plasma membrane portals called porosomes that govern secretion

A large number of products are synthesized and packaged in membrane vesicles to be secreted from cells to carry out essential physiological functions such as nerve transmission, digestion and immune response. How do cells secrete with great precision a portion of the vesicle contents?These questions have been answered through the work of Dr. Bhanu P. Jena, a cell physiologist and chemist at Wayne State University School of Medicine in Detroit Already in the mid 1990s he discovered that pancreatic acinar cells possess secretory portals (porosomes) at the cell plasma membrane that govern the transport and secretion of digestive enzymes. During the next twenty-five years, Jena characterized in great detail the molecular mechanisms underlying this secretory process. He also showed that similar "secretory portals", or "porosomes", are present in all cell types including endocrine cells secreting hormones and brain neurons secreting neurotransmitters.The principles discovered and described by Bhanu P. Jena turned out to be universal, operating similarly in all animal cells. A number of human hereditary diseases are caused by mutations in some of the nearly 30 proteins composing the porosome complex. Jena's discovery of the porosome, in addition to providing a deep understanding of cell secretion, has also contributed to the establishment of a drug development platform (https://www.porosome.com) for the treatment of a wide range of diseases. Among the therapeutic application is porosome reconstitution in stem cell derived beta cells, for the treatment of Type 1 diabetes and holds great promise for the cure of cystic fibrosis.

Experimental envenomation with honeybee venom melittin and phospholipase A2 induced multiple ultrastructural changes in adrenocortical mitochondria

Bee stings represent a public health subject, but the mechanisms involved in bee venom toxicity are not yet fully understood. To evaluate the reactions of adrenocortical cells, through which organisms respond to stress, two honeybee venom components: melittin (Mlt) and phospholipase A2 (PLA2) were tested as potential chemical stressors. Modifications were investigated with transmission electron microscopy and microanalysis. A single dose of Mlt (31 mg/kg) or PLA2 (9.3 mg/kg) was injected in rats of groups ML and PL; daily doses of Mlt (350 μg/kg) or PLA2 (105 μg/kg) were injected 30 days in rats of groups M30 and P30. Adrenocortical cells in ML group showed ultrastructural degenerative alterations of nuclei, endoplasmic reticulum, and mitochondria that exhibited lipid inclusions and mitochondrial cristae (MC) re-organized into mono- or multimembrane large vesicles, and whorls of membranes. Many MC were degenerated. In the M30 group, similar ultrastructural changes, but of lower amplitude were noted; lipid cytosolic droplets were heterogenous. MC diameters in Mlt groups (melittin treated groups) were significantly higher than in control (C) group. In PL group, mitochondria contained large lipid inclusions, vesicular MC of different sizes and multiple membranes, and debris, or whorl structures. In P30 group MC were tubular with increased diameters. In both PLA2 groups (PLA2 treated groups) MC were significantly larger than in C group. We concluded that Mlt and PLA2 were powerful stressors, toxic at the tested doses, cellular reactions concerning in all groups mainly mitochondria, but also other cellular compartments. Apart from degenerative regression of MC, the rearrangement of tubular MC occurred into one or multiple large multimembrane vesicular MC. Reactions to the high doses were more pronounced, with the highest amplitude in ML group, and the lowest in P30 group.

Neuronal Porosome Complex: Secretory Machinery at the Nerve Terminal

Neuronal porosomes are 15 nm cup-shaped lipoprotein secretory machines composed of nearly 30 proteins present at the presynaptic membrane, that have been investigated using multiple imaging modalities, such as electron microscopy, atomic force microscopy, and solution X-ray. Synaptic vesicles transiently dock and fuse at the base of the porosome cup facing the cytosol, by establishing a fusion pore for neurotransmitter release. Studies on the morphology, dynamics, isolation, composition, and reconstitution of the neuronal porosome complex provide a molecular understanding of its structure and function. In the past twenty years, a large body of evidence has accumulated on the involvement of the neuronal porosome proteins in neurotransmission and various neurological disorders. In light of these findings, this review briefly summarizes our current understanding of the neuronal porosome complex, the secretory nanomachine at the nerve terminal.

RIBEYE(B)-domain binds to lipid components of synaptic vesicles in an NAD(H)-dependent, redox-sensitive manner

Synaptic ribbons are needed for fast and continuous exocytosis in ribbon synapses. RIBEYE is a main protein component of synaptic ribbons and is necessary to build the synaptic ribbon. RIBEYE consists of a unique A-domain and a carboxyterminal B-domain, which binds NAD(H). Within the presynaptic terminal, the synaptic ribbons are in physical contact with large numbers of synaptic vesicle (SV)s. How this physical contact between ribbons and synaptic vesicles is established at a molecular level is not well understood. In the present study, we demonstrate that the RIBEYE(B)-domain can directly interact with lipid components of SVs using two different sedimentation assays with liposomes of defined chemical composition. Similar binding results were obtained with a SV-containing membrane fraction. The binding of liposomes to RIBEYE(B) depends upon the presence of a small amount of lysophospholipids present in the liposomes. Interestingly, binding of liposomes to RIBEYE(B) depends on NAD(H) in a redox-sensitive manner. The binding is enhanced by NADH, the reduced form, and is inhibited by NAD⁺, the oxidized form. Lipid-mediated attachment of vesicles is probably part of a multi-step process that also involves additional, protein-dependent processes.

Phospholipase A2-Induced Remodeling Processes on Liquid-Ordered/Liquid-Disordered Membranes Containing Docosahexaenoic or Oleic Acid: A Comparison Study

Vesicle cycling, which is an important biological event, involves the interplay between membrane lipids and proteins, among which the enzyme phospholipase A2 (PLA2) plays a critical role. The capacity of PLA2 to trigger the budding and fission of liquid-ordered (L(o)) domains has been examined in palmitoyl-docosahexaenoylphosphatidylcholine (PDPC) and palmitoyl-oleoylphosphatidylcholine (POPC)/sphingomyelin/cholesterol membranes. They both exhibited a L(o)/liquid-disordered (L(d)) phase separation. We demonstrated that PLA2 was able to trigger budding in PDPC-containing vesicles but not POPC ones. The enzymatic activity, line tension, and elasticity of the membrane surrounding the L(o) domains are critical for budding. The higher line tension of Lo domains in PDPC mixtures was assigned to the greater difference in order parameters of the coexisting phases. The higher amount of lysophosphatidylcholine generated by PLA2 in the PDPC-containing mixtures led to a less-rigid membrane, compared to POPC. The more elastic L(d) membranes in PDPC mixtures exert a lower counteracting force against the L(o) domain bending.

The neuronal porosome complex in health and disease

Cup-shaped secretory portals at the cell plasma membrane called porosomes mediate the precision release of intravesicular material from cells. Membrane-bound secretory vesicles transiently dock and fuse at the base of porosomes facing the cytosol to expel pressurized intravesicular contents from the cell during secretion. The structure, isolation, composition, and functional reconstitution of the neuronal porosome complex have greatly progressed, providing a molecular understanding of its function in health and disease. Neuronal porosomes are 15 nm cup-shaped lipoprotein structures composed of nearly 40 proteins, compared to the 120 nm nuclear pore complex composed of >500 protein molecules. Membrane proteins compose the porosome complex, making it practically impossible to solve its atomic structure. However, atomic force microscopy and small-angle X-ray solution scattering studies have provided three-dimensional structural details of the native neuronal porosome at sub-nanometer resolution, providing insights into the molecular mechanism of its function. The participation of several porosome proteins previously implicated in neurotransmission and neurological disorders, further attest to the crosstalk between porosome proteins and their coordinated involvement in release of neurotransmitter at the synapse.

Function Suggests Nano-Structure: Quantitative Structural Support for SNARE-Mediated Pore Formation

Granule secretory content is released in either basal or calcium-activated complete exocytosis mode. A vital element in these processes is the establishment of a fusion pore between the granule membrane and the plasma membrane, initiated by the formation of a circular rosette docking arrangement of SNARE protein complexes. The controversially disputed number of SNARE complexes needed for granule priming leading to the formation of the fusion pore, is granule-size dependent and varies between secretion modes. Resorting to a statistical mechanics approach that views SNARE complexes and Ca(2+) ions as interacting particles, we have developed a relationship that links secretion rate to SNARE rosette size, Ca(2+) concentration and Ca(2+) ion cooperativity. Data are presented and discussed which suggest this SNARE-dependent generalization of existing narrow-range biophysical models that correlate secretion rate with Ca(2+) concentration and maximal Ca(2+) ion cooperativity. Evidence from dozens of examples in the literature advocate for this relation, which holds through the entire biological range. The coalescence of so many areas of diverse research methodologies has greatly augmented our understanding of so many different sequences of granule life cycle. Accordingly, these new tools may become valuable in a variety of electrophysiological experiments.

'Porosome' discovered nearly 20 years ago provides molecular insights into the kiss-and-run mechanism of cell secretion

Secretion is a fundamental cellular process in living organisms, from yeast to cells in humans. Since the 1950s, it was believed that secretory vesicles completely merged with the cell plasma membrane during secretion. While this may occur, the observation of partially empty vesicles in cells following secretion suggests the presence of an additional mechanism that allows partial discharge of intra-vesicular contents during secretion. This proposed mechanism requires the involvement of a plasma membrane structure called ‘porosome’, which serves to prevent the collapse of secretory vesicles, and to transiently fuse with the plasma membrane (Kiss-and-run), expel a portion of its contents and disengage. Porosomes are cup-shaped supramolecular lipoprotein structures at the cell plasma membrane ranging in size from 15 nm in neurons and astrocytes to 100–180 nm in endocrine and exocrine cells. Neuronal porosomes are composed of nearly 40 proteins. In comparison, the 120 nm nuclear pore complex is composed of >500 protein molecules. Elucidation of the porosome structure, its chemical composition and functional reconstitution into artificial lipid membrane, and the molecular assembly of membrane-associated t-SNARE and v-SNARE proteins in a ring or rosette complex resulting in the establishment of membrane continuity to form a fusion pore at the porosome base, has been demonstrated. Additionally, the molecular mechanism of secretory vesicle swelling, and its requirement for intra-vesicular content release during cell secretion has also been elucidated. Collectively, these observations provide a molecular understanding of cell secretion, resulting in a paradigm shift in our understanding of the secretory process.

Lipid molecules influence early stages of yeast SNARE-mediated membrane fusion

Lipid molecules, structural components of biomembranes, have been proposed for an important role in membrane fusion. Through various techniques based on a protein-reconstituted vesicle-vesicle fusion system, we investigated the influence of several lipid molecules on different stages of a yeast soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion process. Lipid compositions played a significant role in the early stages, docking and lipid mixing, while only exhibiting a minor effect on fusion pore formation and dilation phases, indicated by both small and large content mixing.

Neuronal Porosome-The Secretory Portal at the Nerve Terminal: It's Structure-Function, Composition, and Reconstitution

Cup-shaped secretory portals at the cell plasma membrane called porosomes mediate secretion from cells. Membrane bound secretory vesicles transiently dock and fuse at the cytosolic compartment of the porosome base to expel intravesicular contents to the outside during cell secretion. In the past decade, the structure, isolation, composition, and functional reconstitution of the neuronal porosome complex has been accomplished providing a molecular understanding of its structure-function. Neuronal porosomes are 15 nm cup-shaped lipoprotein structures composed of nearly 40 proteins. Being a membrane-associated supramolecular complex has precluded determination of the atomic structure of the porosome. However recent studies using small-angle X-ray solution scattering (SAXS), provide at sub-nanometer resolution, the native 3D structure of the neuronal porosome complex associated with docked synaptic vesicle at the nerve terminal. Additionally, results from the SAXS study and earlier studies using atomic force microscopy, provide the possible molecular mechanism involved in porosome-mediated neurotransmitter release at the nerve terminal.

Porosome in Cystic Fibrosis

Macromolecular structures embedded in the cell plasma membrane called ‘porosomes’, are involved in the regulated fractional release of intravesicular contents from cells during secretion. Porosomes range in size from 15 nm in neurons and astrocytes to 100-180 nm in the exocrine pancreas and neuroendocrine cells. Porosomes have been isolated from a number of cells, and their morphology, composition, and functional reconstitution well documented. The 3D contour map of the assembly of proteins within the porosome complex, and its native X-ray solution structure at sub-nm resolution has also advanced. This understanding now provides a platform to address diseases that may result from secretory defects. Water and ion binding to mucin impart hydration, critical for regulating viscosity of the mucus in the airways epithelia. Appropriate viscosity is required for the movement of mucus by the underlying cilia. Hence secretion of more viscous mucus prevents its proper transport, resulting in chronic and fatal airways disease such as cystic fibrosis (CF). CF is caused by the malfunction of CF transmembrane conductance regulator (CFTR), a chloride channel transporter, resulting in viscous mucus in the airways. Studies in mice lacking functional CFTR secrete highly viscous mucous that adhered to the epithelium. Since CFTR is known to interact with the t-SNARE protein syntaxin-1A, and with the chloride channel CLC-3, which are also components of the porosome complex, the interactions between CFTR and the porosome complex in the mucin-secreting human airway epithelial cell line Calu-3 was hypothesized and tested. Results from the study demonstrate the presence of approximately 100 nm in size porosome complex composed of 34 proteins at the cell plasma membrane in Calu-3 cells, and the association of CFTR with the complex. In comparison, the nuclear pore complex measures 120 nm and is comprised of over 500 protein molecules. The involvement of CFTR in porosome-mediated mucin secretion is hypothesized, and is currently being tested.

Oxidative-stress induced increase in circulating fatty acids does not contribute to phospholipase A2-dependent appetitive long-term memory failure in the pond snail Lymnaea stagnalis

Background

Reactive oxygen species (ROS) are essential for normal physiological functioning of the brain. However, uncompensated increase in ROS levels may results in oxidative stress. Phospholipase A₂ (PLA₂) is one of the key players activated by elevated ROS levels resulting in the hydrolysis of various products from the plasmamembrane such as peroxidized fatty acids. Free fatty acids (FFAs) and fatty acid metabolites are often implicated to the genesis of cognitive impairment. Previously we have shown that age-, and experimentally induced oxidative stress causes PLA₂-dependent long-term memory (LTM) failure in an aversive operant conditioning model in Lymnaea stagnalis. In the present study, we investigate the effects of experimentally induced oxidative stress and the role of elevated levels of circulating FFAs on LTM function using a non-aversive appetitive classical conditioning paradigm.

Results

We show that intracoelomic injection of exogenous PLA₂ or pro-oxidant induced PLA₂ activation negatively affects LTM performance in our learning paradigm. In addition, we show that experimental induction of oxidative stress causes significant temporal changes in circulating FFA levels. Importantly, the time of training coincides with the peak of this change in lipid metabolism. However, intracoelomic injection with exogenous arachidonic acid, one of the main FFAs released by PLA₂, does not affect LTM function. Moreover, sequestrating circulating FFAs with the aid of bovine serum albumin does not rescue pro-oxidant induced appetitive LTM failure.

Conclusions

Our data substantiates previous evidence linking lipid peroxidation and PLA₂ activation to age- and oxidative stress-related cognitive impairment, neuronal dysfunction and disease. In addition however, our data indicate that lipid peroxidation induced increased levels of circulating (per)oxidized FFAs are not a factor in oxidative stress induced LTM impairment.

Atomic force microscopy: High resolution dynamic imaging of cellular and molecular structure in health and disease

The atomic force microscope (AFM), invented in 1986, and a member of the scanning probe family of microscopes, offers the unprecedented ability to image biological samples unfixed and in a hydrated environment at high resolution. This opens the possibility to investigate biological mechanisms temporally in a heretofore unattainable resolution. We have used AFM to investigate: (1) fundamental issues in cell biology (secretion) and, (2) the pathological basis of a human thrombotic disease, the antiphospholipid syndrome (APS). These studies have incorporated the imaging of live cells at nanometer resolution, leading to discovery of the "porosome," the universal secretory portal in cells, and a molecular understanding of membrane fusion from imaging the interaction and assembly of proteins between opposing lipid membranes. Similarly, the development of an in vitro simulacrum for investigating the molecular interactions between proteins and lipids has helped define an etiological explanation for APS. The prime importance of AFM in the success of these investigations will be presented in this manuscript, as well as a discussion of the limitations of this technique for the study of biomedical samples.

Using neurolipidomics to identify phospholipid mediators of synaptic (dys)function in Alzheimer's Disease

Not all of the mysteries of life lie in our genetic code. Some can be found buried in our membranes. These shells of fat, sculpted in the central nervous system into the cellular (and subcellular) boundaries of neurons and glia, are themselves complex systems of information. The diversity of neural phospholipids, coupled with their chameleon-like capacity to transmute into bioactive molecules, provides a vast repertoire of immediate response second messengers. The effects of compositional changes on synaptic function have only begun to be appreciated. Here, we mined 29 neurolipidomic datasets for changes in neuronal membrane phospholipid metabolism in Alzheimer's Disease (AD). Three overarching metabolic disturbances were detected. We found that an increase in the hydrolysis of platelet activating factor precursors and ethanolamine-containing plasmalogens, coupled with a failure to regenerate relatively rare alkyl-acyl and alkenyl-acyl structural phospholipids, correlated with disease severity. Accumulation of specific bioactive metabolites [i.e., PC(O-16:0/2:0) and PE(P-16:0/0:0)] was associated with aggravating tau pathology, enhancing vesicular release, and signaling neuronal loss. Finally, depletion of PI(16:0/20:4), PI(16:0/22:6), and PI(18:0/22:6) was implicated in accelerating Aβ₄₂ biogenesis. Our analysis further suggested that converging disruptions in platelet activating factor, plasmalogen, phosphoinositol, phosphoethanolamine (PE), and docosahexaenoic acid metabolism may contribute mechanistically to catastrophic vesicular depletion, impaired receptor trafficking, and morphological dendritic deformation. Together, this analysis supports an emerging hypothesis that aberrant phospholipid metabolism may be one of multiple critical determinants required for Alzheimer disease conversion.

Lysophosphatidylcholine inhibits membrane-associated SNARE complex disassembly

In cells, N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors called SNAREs are involved in membrane fusion. In neurons, for example, target membrane proteins SNAP-25 and syntaxin called t-SNAREs present at the pre-synaptic membrane, and a synaptic vesicle-associated membrane protein (VAMP) or v-SNARE, is part of the conserved protein complex involved in neurotransmission. Cholesterol and LPC (L-α-lysophosphatidylcholine) are known to contribute to the negative and positive curvature respectively of membranes. In this study, using purified recombinant neuronal membrane-associated SNAREs, we demonstrate for the first time that membrane-curvature-influencing lipids profoundly influence SNARE complex disassembly. Exposure of cholesterol-associated t-SNARE and v-SNARE liposome mixtures to NSF-ATP results in dissociated vesicles. In contrast, exposure of LPC-associated t-SNARE and v-SNARE liposome mixtures to NSF-ATP, results in inhibition of t-/v-SNARE disassembly and the consequent accumulation of clustered vesicles. Similarly, exposure of isolated rat brain slices and pancreas to cholesterol or LPC, also demonstrates LPC-induced inhibition of SNARE complex disassembly. Earlier studies demonstrate a strong correlation between altered plasma LPC levels and cancer. The altered plasma LPC levels observed in various cancers may in part contribute to defects in SNARE assembly-disassembly and membrane fusion, consequently affecting protein maturation and secretion in cancer cells.

3D organization and function of the cell: Golgi budding and vesicle biogenesis to docking at the porosome complex

Insights into the three-dimensional (3D) organization and function of intracellular structures at nanometer resolution, holds the key to our understanding of the molecular underpinnings of cellular structure-function. Besides this fundamental understanding of the cell at the molecular level, such insights hold great promise in identifying the disease processes by their altered molecular profiles, and help determine precise therapeutic treatments. To achieve this objective, previous studies have employed electron microscopy (EM) tomography with reasonable success. However, a major hurdle in the use of EM tomography is the tedious procedures involved in fixing, high-pressure freezing, staining, serial sectioning, imaging, and finally compiling the EM images to obtain a 3D profile of sub-cellular structures. In contrast, the resolution limit of EM tomography is several nanometers, as compared to just a single or even sub-nanometer using the atomic force microscope (AFM). Although AFM has been hugely successful in 3D imaging studies at nanometer resolution and in real time involving isolated live cellular and isolated organelles, it has had limited success in similar studies involving 3D imaging at nm resolution of intracellular structure-function in situ. In the current study, using both AFM and EM on aldehyde-fixed and semi-dry mouse pancreatic acinar cells, new insights on a number of intracellular structure-function relationships and interactions were achieved. Golgi complexes, some exhibiting vesicles in the process of budding were observed, and small vesicles were caught in the act of fusing with larger vesicles, possibly representing either secretory vesicle biogenesis or vesicle refilling following discharge, or both. These results demonstrate the power and scope of the combined engagement of EM and AFM imaging of fixed semi-dry cells, capable of providing a wealth of new information on cellular structure-function and interactions.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

Membrane-directed molecular assembly of the neuronal SNARE complex

Since the discovery and implication of N-ethylmaleimide-sensitive factor (NSF)-attachment protein receptor (SNARE) proteins in membrane fusion almost two decades ago, there have been significant efforts to understand their involvement at the molecular level. In the current study, we report for the first time the molecular interaction between full-length recombinant t-SNAREs and v-SNARE present in opposing liposomes, leading to the assembly of a t-/v-SNARE ring complex. Using high-resolution electron microscopy, the electron density maps and 3D topography of the membrane-directed SNARE ring complex was determined at nanometre resolution. Similar to the t-/v-SNARE ring complex formed when 50 nm v-SNARE liposomes meet a t-SNARE-reconstituted planer membrane, SNARE rings are also formed when 50 nm diameter isolated synaptic vesicles (SVs) meet a t-SNARE-reconstituted planer lipid membrane. Furthermore, the mathematical prediction of the SNARE ring complex size with reasonable accuracy, and the possible mechanism of membrane-directed t-/v-SNARE ring complex assembly, was determined from the study. Therefore in the present study, using both lipososome-reconstituted recombinant t-/v-SNARE proteins, and native v-SNARE present in isolated SV membrane, the membrane-directed molecular assembly of the neuronal SNARE complex was determined for the first time and its size mathematically predicted. These results provide a new molecular understanding of the universal machinery and mechanism of membrane fusion in cells, having fundamental implications in human health and disease.

Phospholipases A2 and neural membrane dynamics: implications for Alzheimer's disease

Phospholipases A(2) (PLA(2)s) are essential enzymes in cells. They are not only responsible for maintaining the structural organization of cell membranes, but also play a pivotal role in the regulation of cell functions. Activation of PLA(2) s results in the release of fatty acids and lysophospholipids, products that are lipid mediators and compounds capable of altering membrane microdomains and physical properties. Although not fully understood, recent studies have linked aberrant PLA(2) activity to oxidative signaling pathways involving NADPH oxidase that underlie the pathophysiology of a number of neurodegenerative diseases. In this paper, we review studies describing the involvement of cytosolic PLA(2) in oxidative signaling pathways leading to neuronal impairment and activation of glial cell inflammatory responses. In addition, this review also includes information on the role of cytosolic PLA(2) and exogenous secretory PLA(2) on membrane physical properties, dynamics, and membrane proteins. Unraveling the mechanisms that regulate specific types of PLA(2)s and their effects on membrane dynamics are important prerequisites towards understanding their roles in the pathophysiology of Alzheimer's disease, and in the development of novel therapeutics to retard progression of the disease.