Lipid A is a lipid component of an endotoxin held responsible for the toxicity of gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide called endotoxin molecule, its hydrophobic nature allows it to anchor the LPS to the outer membrane. While its toxic effects can be damaging, the sensing of lipid A by the human immune system may be critical for the onset of immune responses to gram-negative infection, for the subsequent successful fight against the infection. Lipid A consists of two glucosamine units, in an β linkage, with attached acyl chains, containing one phosphate group on each carbohydrate; the optimal immune activating lipid A structure is believed to contain 6 acyl chains. Four acyl chains attached directly to the glucosamine sugars are beta hydroxy acyl chains between 10 and 16 carbons in length. Two additional acyl chains are attached to the beta hydroxy group. E. coli lipid A, as an example has four C14 hydroxy acyl chains attached to the sugars and one C14 and one C12 attached to the beta hydroxy groups.
The biosynthetic pathway for Lipid A in E. coli has been determined by the work of Christian R. H. Raetz in the past >32 years. Lipid A structure and effects on eukaryotic cells have been determined and examined, among others, by the groups of Otto Westphal, Chris Galanos, Ernst T. Rietschel and Hajime Takahashi starting in the 1960s. Many of the immune activating abilities of LPS can be attributed to the lipid A unit, it is a potent stimulant of the immune system, activating cells at picogram per milliliter quantities. When present in the body at high concentrations during a gram-negative bacterial infection, it may cause shock and death by an "out of control" excessive immune reaction. Lipid A with a reduced number of acyl chains can serve as an inhibitor of immune activation induced by Gram-negative bacteria, synthetic versions of these inhibitors were in clinical trials for the prevention of harmful effects caused by gram-negative bacterial infections. However, trials were discontinued due to lack of efficacy seen in patients suffering from severe sepsis.
On the other hand, modified versions of lipid A can be used as components of vaccines to improve their effect. Monophosphorylated lipid A is an FDA approved adjuvant that consists of a heterogeneous mixture of lipid A from Salmonella minnesota R595; the major lipid A species present in MPL lacks one of five acyl chains. Other work has shown that the removal of one or two acyl chains from native lipid A can reduce activation of inflammatory responses; the biological activity of LPS depends on the chemical structure of its lipid A. Primarily, TLR4 is required for activation of innate immunity upon recognition of LPS of Gram-negative bacteria; the ability of TLR4/MD-2 system to respond to a distinct lipid A species are clinically important. Pathogenic bacteria may employ LPS with low biological activity of its lipid A to evade proper recognition by the TLR4/MD-2 complex, dampening the host immune response and increasing the risk of bacterial dissemination. On the other hand, such lipid A would not be able to induce septic shock in susceptible patients, rendering septic complications more manageable.
Yet and understanding how the smallest structural differences between the similar lipid A species may affect the activation of the immune response may provide the mechanism for the fine tuning of the latter and new insights to immunomodulatory processes. Lipid A has been demonstrated to activate cells via Toll-like receptor 4, MD-2 and CD14 on the cell surface. Lipid A analogs like eritoran can act as TLR4 antagonists, they are being developed as drugs for the treatment of excessive inflammatory responses to infections with gram-negative bacteria. Lipid A deacylase Lipid+A at the US National Library of Medicine Medical Subject Headings The Lipid Library - Summary of Lipid A and bacterial polysaccharides
ATP-binding cassette transporter
The ATP-binding cassette transporters are a transport system superfamily, one of the largest and one of the oldest gene families. It is represented from prokaryotes to humans. ABC transporters consist of multiple subunits, one or two of which are transmembrane proteins and one or two of which are membrane-associated ATPases; the ATPase subunits utilize the energy of adenosine triphosphate binding and hydrolysis to provide the energy needed for the translocation of substrates across membranes, either for uptake or for export of the substrate. Most of the uptake systems have an extracytoplasmic receptor, a solute binding protein; some homologous ATPases function in non-transport-related processes such as translation of RNA and DNA repair. ABC transporters are considered to be an ABC superfamily based on the similarities of the sequence and organization of their ATP-binding cassette domains though the integral membrane proteins appear to have evolved independently several times, thus comprise different protein families.
ABC1 exporters evolved by intragenic triplication of a 2 TMS precursor to give 6 TMS proteins. ABC2 exporters evolved by intragenic duplication of a 3 TMS precursor, ABC3 exporters evolved from a 4 TMS precursor which duplicated either extragenicly to give two 4 TMS proteins, both required for transport function, or intragenicly to give 8 or 10 TMS proteins; the 10 TMS proteins appear to have two extra TMSs between the two 4 TMS repeat units. Like the ABC exporters, it is possible that the integral membrane proteins of ABC uptake systems evolved at least 3 times independently, based on their high resolution 3-dimensional structures. ABC uptake porters take up a large variety of nutrients, biosynthetic precursors, trace metals and vitamins, while exporters transport lipids, drugs, a large variety of primary and secondary metabolites; some of these exporters in humans are involved in tumor resistance, cystic fibrosis and a range of other inherited human diseases. High level expression of the genes encoding some of these exporters in both prokaryotic and eukaryotic organisms result in the development of resistance to multiple drugs such as antibiotics and anti-cancer agents.
Hundreds of ABC transporters have been characterized from both eukaryotes. ABC genes are essential for many processes in the cell, mutations in human genes cause or contribute to several human genetic diseases. Forty eight ABC genes have been reported in humans. Among these, many have been characterized and shown to be causally related to diseases present in humans such as cystic fibrosis, adrenoleukodystrophy, Stargardt disease, drug-resistant tumors, Dubin-Johnson syndrome, Byler’s disease, progressive familiar intrahepatic cholestasis, X-linked sideroblastic anemia and persistent and hyperinsulimenic hypoglycemia. ABC transporters are involved in multiple drug resistance, this is how some of them were first identified; when the ABC transport proteins are overexpressed in cancer cells, they can export anticancer drugs and render tumors resistant. ABC transporters utilize the energy of ATP binding and hydrolysis to transport various substrates across cellular membranes, they are divided into three main functional categories.
In prokaryotes, importers mediate the uptake of nutrients into the cell. The substrates that can be transported include ions, amino acids, peptides and other molecules that are hydrophilic; the membrane-spanning region of the ABC transporter protects hydrophilic substrates from the lipids of the membrane bilayer thus providing a pathway across the cell membrane. Eukaryotes do not possess any importers. Exporters or effluxers, which are present both in prokaryotes and eukaryotes, function as pumps that extrude toxins and drugs out of the cell. In gram-negative bacteria, exporters transport lipids and some polysaccharides from the cytoplasm to the periplasm; the third subgroup of ABC proteins do not function as transporters, but are rather involved in translation and DNA repair processes. Bacterial ABC transporters are essential in cell viability and pathogenicity. Iron ABC uptake systems, for example, are important effectors of virulence. Pathogens use siderophores, such as Enterobactin, to scavenge iron, in complex with high-affinity iron-binding proteins or erythrocytes.
These are high-affinity iron-chelating molecules that are secreted by bacteria and reabsorb iron into iron-siderophore complexes. The chvE-gguAB gene in Agrobacterium tumefaciens encodes glucose and galactose importers that are associated with virulence. Transporters are vital in cell survival such that they function as protein systems that counteract any undesirable change occurring in the cell. For instance, a potential lethal increase in osmotic strength is counterbalanced by activation of osmosensing ABC transporters that mediate uptake of solutes. Other than functioning in transport, some bacterial ABC proteins are involved in the regulation of several physiological processes. In bacterial efflux systems, certain substances that need to be extruded from the cell include surface components of the bacterial cell, proteins involved in bacterial pathogenesis, hydrolytic enzymes, S-layer proteins, competence factors, antibiotics, peptide antibiotics and siderophores, they play important roles in biosynthetic pathways, including extracellular polysaccharide biosynthesis and cytochrome biogenesis.
Although most eukaryotic ABC transporters are effluxers
A polymerase is an enzyme that synthesizes long chains of polymers or nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules by copying a DNA template strand using base-pairing interactions or RNA by half ladder replication. A DNA polymerase from the thermophilic bacterium, Thermus aquaticus is used in the polymerase chain reaction, an important technique of molecular biology. DNA polymerase Family A: DNA polymerase I. Family Y: DNA polymerase V - SOS repair polymerase, it is a major target for antiviral drugs. RNA polymerase Multi-subunit, DNA-directed: RNA polymerase I, RNA polymerase II, RNA polymerase III Single-subunit, DNA-directed: T7 RNA polymerase, POLRMT RNA replicase Primase, PrimPolIn general, viral single-subunit RNA polymerases/replicases/reverse transcriptase shares a common origin with DNA polymerase, they have a conserved "palm" domain. Multi-subunit RNA polymerase forms an unrelated group. Primases have a more complex story: bacterial primases with the Toprim domain are related to topoisomerase and mitochrondrial helicase, while archaea and eukaryotic primases form a unrelated family related to the polymerase palm.
Both families associate to the same bunch of helicases
Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane. Gram-negative bacteria are found everywhere, in all environments on Earth that support life; the gram-negative bacteria include the model organism Escherichia coli, as well as many pathogenic bacteria, such as Pseudomonas aeruginosa, Neisseria gonorrhoeae, Chlamydia trachomatis, Yersinia pestis. They are an important medical challenge, as their outer membrane protects them from many antibiotics. Additionally, the outer leaflet of this membrane comprises a complex lipopolysaccharide whose lipid A component can cause a toxic reaction when these bacteria are lysed by immune cells; this toxic reaction can include fever, an increased respiratory rate, low blood pressure — a life-threatening condition known as septic shock.
Several classes of antibiotics have been designed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins, beta-lactam-betalactamase combinations, Folate antagonists and carbapenems. Many of these antibiotics cover gram positive organisms; the drugs that target gram negative organisms include aminoglycosides and Ciprofloxacin. Gram-negative bacteria display these characteristics: An inner cell membrane is present A thin peptidoglycan layer is present Has outer membrane containing lipopolysaccharides in its outer leaflet and phospholipids in the inner leaflet Porins exist in the outer membrane, which act like pores for particular molecules Between the outer membrane and the cytoplasmic membrane there is a space filled with a concentrated gel-like substance called periplasm The S-layer is directly attached to the outer membrane rather than to the peptidoglycan If present, flagella have four supporting rings instead of two Teichoic acids or lipoteichoic acids are absent Lipoproteins are attached to the polysaccharide backbone Some contain Braun's lipoprotein, which serves as a link between the outer membrane and the peptidoglycan chain by a covalent bond Most, with few exceptions, do not form spores Along with cell shape, gram-staining is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria.
The kingdom Monera was divided into four divisions based on gram-staining: Firmacutes, Gracillicutes and Mendocutes. Since 1987, the monophyly of the gram-negative bacteria has been disproven with molecular studies; however some authors, such as Cavalier-Smith still treat them as a monophyletic taxon and refer to the group as a subkingdom "Negibacteria". Bacteria are traditionally classified based on their gram-staining response into the gram-positive and gram-negative groups, it was traditionally thought that the groups represent lineages, i.e. the extra membrane only evoved once, such that gram-negative bacteria are more related to one another than to any gram-positive bacteria. While this is true, the classification system breaks down in some cases, with lineage groupings not matching the staining result. Thus, gram-staining cannot be reliably used to assess familial relationships of bacteria. Staining gives reliable information about the composition of the cell membrane, distinguishing between the presence or absence of an outer lipid membrane.
Of these two structurally distinct groups of prokaryotic organisms, monoderm prokaryotes are thought to be ancestral. Based upon a number of different observations including that the gram-positive bacteria are the major reactors to antibiotics and that gram-negative bacteria are, in general, resistant to them, it has been proposed that the outer cell membrane in gram-negative bacteria evolved as a protective mechanism against antibiotic selection pressure; some bacteria such as Deinococcus, which stain gram-positive due to the presence of a thick peptidoglycan layer, but possess an outer cell membrane are suggested as intermediates in the transition between monoderm and diderm bacteria. The diderm bacteria can be further differentiated between simple diderms lacking lipopolysaccharide; the conventional LPS-diderm group of gram-negative bacteria are uniquely identified by a few conserved signature indel in the HSP60 protein. In addition, a number of bacterial taxa that are either part of the phylum Firmicutes or branches in its proximity are found to possess a diderm cell structure.
They lack the GroEL signature. The presence of this CSI in all se
The bacterial capsule is a large structure of many bacteria. It is a polysaccharide layer that lies outside the cell envelope, is thus deemed part of the outer envelope of a bacterial cell, it is a well-organized layer, not washed off, it can be the cause of various diseases. The capsule—which can be found in both gram negative and gram-positive bacteria—is different to the second lipid membrane – bacterial outer membrane, which contains lipopolysaccharides and lipoproteins and is found only in gram-negative bacteria; when the amorphous viscid secretion diffuses into the surrounding medium and remains as a loose undemarcated secretion, it is known as slime layer. Capsule and slime layer are sometimes summarized under the term glycocalyx, it consists of polysaccharides, but can be composed of other materials such as glycoprotein, polypeptide D-glutamic acid in B. anthracis, peptidoglycan and muramic acid found in E.coli bacterial capsule. Because most capsules are so packed, they are difficult to stain because most standard stains cannot adhere to the capsule.
For examination under the microscope, the bacteria and their background are stained darker than the capsule, which doesn't stain. When viewed, bacterial cells as well as the surface they are on, are stained dark, while the capsule remains pale or colorless and appears as a ring, or halo, around the cell; the capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease. The capsule can protect cells from engulfment such as macrophages. A capsule-specific antibody may be required for phagocytosis to occur. Capsules contain water which protects the bacteria against desiccation, they exclude bacterial viruses and most hydrophobic toxic materials such as detergents. Immunity to one capsule type does not result in immunity to the other types. Capsules help cells adhere to surfaces; as a group where the capsule is present they are known as polysaccharide encapsulated bacteria or encapsulated bacteria. The capsule is found most among gram-negative bacteria: Escherichia coli Neisseria meningitidis Klebsiella pneumoniae Haemophilus influenzae Pseudomonas aeruginosa SalmonellaHowever, some gram-positive bacteria may have a capsule: Bacillus megaterium for example, synthesizes a capsule composed of polypeptide and polysaccharides.
Streptococcus pyogenes synthesizes a hyaluronic acid capsule. Streptococcus pneumoniae has at least 91 different capsular serotypes; these serotypes are the basis for the pneumococcal vaccines. Streptococcus agalactiae produces a polysaccharide capsule of nine antigenic types that all contain sialic acid. Staphylococcus epidermidis Staphylococcus aureusThe yeast Cryptococcus neoformans, though not a bacterium, has a similar capsule. Capsules too small to be seen with an ordinary microscope, such as the M protein of Streptococcus pyogenes, are called microcapsules. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capsule. Maneval's capsule stain: the capsule appears as a clear halo between the pink-stained bacterium and the bluish-grey stained background; the background stain is the acidic stain Congo red, the pink stain is acid fuschin. Serological methods: Capsular material is antigenic and can be demonstrated by mixing it with a specific anticapsular serum.
When examined under the microscope, the capsule appears'swollen' due to an increase in its refractivity. This phenomenon is the basis of Quellung reaction. Vaccination using capsular material is effective against some organisms. However, polysaccharides are not antigenic in children, so many capsular vaccines contain polysaccharides conjugated with protein carriers, such as the tetanus toxoid or diphtheria toxoid; this stimulates a much more robust immune response. Bacterial cell structure Quellung reaction, a method to visualize capsule under a microscope
Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single, membrane-spanning, non-catalytic receptors expressed on sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes. Once these microbes have reached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell responses; the TLRs include TLR1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, TLR10, TLR11, TLR12, TLR13, though the last three are not found in humans. TLR's received their name from their similarity to the protein coded by the toll gene identified in Drosophila in 1985 by Christiane Nüsslein-Volhard and Eric Wieschaus; the ability of immune system to recognize molecules that are broadly shared by pathogens is, in part, due to the presence of Immune receptors called toll-like receptors that are expressed on the membranes of leukocytes including dendritic cells, natural killer cells, cells of the adaptive immunity and non immune cells.
The binding of ligands - either in the form of adjuvant used in vaccinations or in the form of invasive moieties during times of natural infection - to the TLR marks the key molecular events that lead to innate immune responses and the development of antigen-specific acquired immunity. Upon activation, TLRs recruit adapter proteins within the cytosol of the immune cell in order to propagate the antigen-induced signal transduction pathway; these recruited proteins are responsible for the subsequent activation of other downstream proteins, including protein kinases that further amplify the signal and lead to the upregulation or suppression of genes that orchestrate inflammatory responses and other transcriptional events. Some of these events lead to cytokine production and survival, while others lead to greater adaptive immunity. If the ligand is a bacterial factor, the pathogen might be phagocytosed and digested, its antigens presented to CD4+ T cells. In the case of a viral factor, the infected cell may shut off its protein synthesis and may undergo programmed cell death.
Immune cells that have detected a virus may release anti-viral factors such as interferons. Toll-like receptors have been shown to be an important link between innate and adaptive immunity through their presence in dendritic cells. Flagellin, a TLR5 ligand induces cytokine secretion on interacting with TLR5 on human T cells. TLRs are a type of pattern recognition receptor and recognize molecules that are broadly shared by pathogens but distinguishable from host molecules, collectively referred to as pathogen-associated molecular patterns. TLRs together with the Interleukin-1 receptors form a receptor superfamily, known as the "interleukin-1 receptor / toll-like receptor superfamily". Three subgroups of TIR domains exist. Proteins with subgroup 1 TIR domains are receptors for interleukins that are produced by macrophages and dendritic cells and all have extracellular Immunoglobulin domains. Proteins with subgroup 2 TIR domains are classical TLRs, bind directly or indirectly to molecules of microbial origin.
A third subgroup of proteins containing TIR domains consists of adaptor proteins that are cytosolic and mediate signaling from proteins of subgroups 1 and 2. TLRs are present in vertebrates, as well as in invertebrates. Molecular building blocks of the TLRs are represented in bacteria and in plants, plant pattern recognition receptors are well known to be required for host defence against infection; the TLRs thus appear to be one of the most conserved components of the immune system. In recent years TLRs were identified in the mammalian nervous system. Members of the TLR family were detected on glia, neurons and on neural progenitor cells in which they regulate cell-fate decision, it has been estimated that most mammalian species have between ten and fifteen types of toll-like receptors. Thirteen TLRs have been identified in humans and mice together, equivalent forms of many of these have been found in other mammalian species. However, equivalents of certain TLR found in humans are not present in all mammals.
For example, a gene coding for a protein analogous to TLR10 in humans is present in mice, but appears to have been damaged at some point in the past by a retrovirus. On the other hand, mice express TLRs 11, 12, 13, none of, represented in humans. Other mammals may express TLRs. Other non-mammalian species may have TLRs distinct from mammals, as demonstrated by TLR14, found in the Takifugu pufferfish; this may complicate the process of using experimental animals as models of human innate immunity. Drosophila melanogaster has only innate immune responses. Response to fungal or bacterial infection occurs through two distinct signalling cascades, one of, toll pathway and the other is immune deficiency pathway; the toll pathway is similar to mammalian TLR signalling, but unlike mammalian TLRs, toll is not activated directly by pathogen-associated molecular patterns. Its receptor ectodomain recognizes cleaved form of the cytokine Spätzle, secreted in the haemolymph as inactive dimeric precursor. Toll receptor shares the cytoplasmatic TIR domain with mammalian TLRs, but ectodomain and intracytoplasmatic tail are different.
This difference might reflect a function of these receptors as cytokine receptors rather
In cell biology, the cytoplasm is all of the material within a cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm; the main components of the cytoplasm are cytosol – a gel-like substance, the organelles – the cell's internal sub-structures, various cytoplasmic inclusions. The cytoplasm is about 80% water and colorless; the submicroscopic ground cell substance, or cytoplasmatic matrix which remains after exclusion the cell organelles and particles is groundplasm. It is the hyaloplasm of light microscopy, high complex, polyphasic system in which all of resolvable cytoplasmic elements of are suspended, including the larger organelles such as the ribosomes, the plant plastids, lipid droplets, vacuoles. Most cellular activities take place within the cytoplasm, such as many metabolic pathways including glycolysis, processes such as cell division; the concentrated inner area is called the endoplasm and the outer layer is called the cell cortex or the ectoplasm.
Movement of calcium ions in and out of the cytoplasm is a signaling activity for metabolic processes. In plants, movement of the cytoplasm around vacuoles is known as cytoplasmic streaming; the term was introduced by Rudolf von Kölliker in 1863 as a synonym for protoplasm, but it has come to mean the cell substance and organelles outside the nucleus. There has been certain disagreement on the definition of cytoplasm, as some authors prefer to exclude from it some organelles the vacuoles and sometimes the plastids; the physical properties of the cytoplasm have been contested in recent years. It remains uncertain how the varied components of the cytoplasm interact to allow movement of particles and organelles while maintaining the cell’s structure; the flow of cytoplasmic components plays an important role in many cellular functions which are dependent on the permeability of the cytoplasm. An example of such function is cell signalling, a process, dependent on the manner in which signaling molecules are allowed to diffuse across the cell.
While small signaling molecules like calcium ions are able to diffuse with ease, larger molecules and subcellular structures require aid in moving through the cytoplasm. The irregular dynamics of such particles have given rise to various theories on the nature of the cytoplasm. There has long been evidence, it is thought that the component molecules and structures of the cytoplasm behave at times like a disordered colloidal solution and at other times like an integrated network, forming a solid mass. This theory thus proposes that the cytoplasm exists in distinct fluid and solid phases depending on the level of interaction between cytoplasmic components, which may explain the differential dynamics of different particles observed moving through the cytoplasm, it has been proposed that the cytoplasm behaves like a glass-forming liquid approaching the glass transition. In this theory, the greater the concentration of cytoplasmic components, the less the cytoplasm behaves like a liquid and the more it behaves as a solid glass, freezing larger cytoplasmic components in place.
A cell's ability to vitrify in the absence of metabolic activity, as in dormant periods, may be beneficial as a defence strategy. A solid glass cytoplasm would freeze subcellular structures in place, preventing damage, while allowing the transmission of small proteins and metabolites, helping to kickstart growth upon the cell's revival from dormancy. There has been research examining the motion of cytoplasmic particles independent of the nature of the cytoplasm. In such an alternative approach, the aggregate random forces within the cell caused by motor proteins explain the non-Brownian motion of cytoplasmic constituents; the three major elements of the cytoplasm are the cytosol and inclusions. The cytosol is the portion of the cytoplasm not contained within membrane-bound organelles. Cytosol makes up about 70% of the cell volume and is a complex mixture of cytoskeleton filaments, dissolved molecules, water; the cytosol's filaments include the protein filaments such as actin filaments and microtubules that make up the cytoskeleton, as well as soluble proteins and small structures such as ribosomes and the mysterious vault complexes.
The inner and more fluid portion of the cytoplasm is referred to as endoplasm. Due to this network of fibres and high concentrations of dissolved macromolecules, such as proteins, an effect called macromolecular crowding occurs and the cytosol does not act as an ideal solution; this crowding effect alters. Organelles, are membrane-bound structures inside the cell that have specific functions; some major organelles that are suspended in the cytosol are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, in plant cells, chloroplasts. The inclusions are small particles of insoluble substances suspended in the cytosol. A huge range of inclusions exist in different cell types, range from crystals of calcium oxalate or silicon dioxide in plants, to granules of energy-storage materials such as starch, glycogen, or polyhydroxybutyrate. A widespread example are lipid droplets, which are spherical droplets composed of lipids and proteins that are used in both prokaryotes and eukaryotes as a way of storing lipids such as fatty acids and sterols.
Lipid droplets make up much of the volume of adipocytes, which are specialized lipid-st