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#cell fusion

17 public questions tagged with this topic.

Plant tissue culture 2

Which of the following is NOT a fusogen?

Colchicine is not considered fusogen, unlike true fusogenic agents that promote membrane merging of protoplasts. Legitimate fusogens include polyethylene glycol PEG high molecular weight polymer that dehydrates membrane interface creating calcium bridges neutralizing negative charge inducing lipid mixing upon dilution, high concentration calcium ions at alkaline pH around 10.5 similar mechanism reducing electrostatic repulsion, and inactivated Sendai virus bearing fusogenic HN and F glycoproteins that merge animal and plant membranes albeit rarely used now due to biohazard handling. Electrical pulses via electrofusion also physically fuse membranes through reversible dielectric breakdown not chemical fusogen definition but effective. Colchicine mechanism distinct: it binds tubulin dimer with high affinity blocking microtubule assembly disrupting spindle formation during mitosis leading to polyploidy chromosome doubling, classified as antimitotic alkaloid rather than membrane fusogen. It does not destabilize lipid bilayer nor create intermembrane coalescence step necessary for protoplast fusion. Hence in lists evaluating fusogenic compounds colchicine stands out as non fusogenic used for ploidy manipulation rather than heterokaryon formation, correctly identified as answer for which is NOT fusogen.

Ref: Anne & Harada 1991 fusogens list; Kao 1977 colchicine not fusogen.

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Plant tissue culture 2

Somatic hybridization can transfer:

Somatic hybridization can transfer extensive genetic material ranging from blocks of linked genes to entire chromosomes or whole genomes, contrasting with single gene transformation using plasmid vectors. When two protoplasts fuse, initial heterokaryon contains cytoplasm of both parents and nuclei that may fuse producing allotetraploid somatic hybrid carrying full complement of chromosomes from both species, allowing introgression of polygenic quantitative trait loci QTL controlled by multiple genes interacting for disease resistance, abiotic stress tolerance, quality attributes that cannot be transferred via single gene approach. Through asymmetric fusion where donor protoplasts irradiated with gamma rays fragment chromosomes before fusion, partial genome transfer achieved moving chromosome segments or single added chromosomes into recipient background, verifiable by genomic in situ hybridization GISH. This permits wide hybridization circumventing sexual incompatibility barriers pre zygotic and post zygotic including endosperm abortion. Iconic examples include transfer of late blight resistance gene cluster from wild Solanum brevidens to cultivated potato conferring durable resistance dependent on multiple R genes clustered in genomic block, demonstrating somatic hybridization power for moving complex agronomic traits impossible by conventional crossing.

Ref: Helgeson 1979 potato somatic hybrid; Bhat & Bhat 2011 gene blocks transfer.

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Plant tissue culture 2

Electrofusion induces fusion by:

Electrofusion induces protoplast fusion by applying controlled electrical pulses that create reversible membrane breakdown at points of contact allowing lipid bilayers to merge and cytoplasms to coalesce, offering higher fusion frequency viability and selectivity than chemical PEG method. Protocol consists of two electrical phases: first low strength high frequency alternating current 50 to 200 kilohertz field causes dielectrophoresis polarizing protoplasts forming induced dipoles aligning them in pearl chains bringing membranes into intimate contact at poles. Second short high intensity direct current pulses 0.5 to 2 kilovolts per centimeter lasting microseconds generate transient pores dielectric breakdown in contacting membranes that upon resealing fuse into single membrane encircling two protoplasts. Post fusion alternating current field maintained briefly stabilizes heterokaryons. Advantages include ability to monitor individual pair fusion under microscope enabling one to one fusion selection, reduced chemical toxicity preserving division capacity, and high heterokaryon yield up to 60 percent. Parameters voltage duration number of pulses optimized per species to avoid Joule heating irreversible electroporation and cell death. Electrofusion combined with fluorescence activated sorting and microfluidic devices now facilitates precise cybrid and somatic hybrid production for citrus potato Brassica rootstock improvement and organelle transfer programs.

Ref: Zimmermann & Vienken 1982 electrofusion; Tempelaar et al., Plant Sci electroporation.

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Immunological Applications

Myeloma cells used in hybridoma technology are selected to be:

Establishing stringent selection requires myeloma background auxotrophic for purine salvage, achieved by exposure to 8-azaguanine, purine analogue structurally similar to guanine. Salvage competent cells convert 8-azaguanine via HGPRT to 8-azaguanosine monophosphate incorporated into mRNA and rRNA causing miscoding, translational errors, and lethal proteotoxicity triggering unfolded protein response. Spontaneous HPRT1 mutants survive because they cannot metabolize analogue, accumulating no toxic nucleotides. Surviving subclones harbor frameshift, nonsense, or missense mutations in coding exons 2-9, leading to premature stop codon and nonsense mediated mRNA decay, enzymatic activity undetectable via radiometric assay. In complete medium containing folate, deficiency tolerated because de novo purine synthesis via amidophosphoribosyltransferase using glutamine nitrogen supplies sufficient IMP via ten-step pathway. Once switched to HAT where aminopterin blocks that pathway, reliance shifts entirely to salvage, now impossible in mutants. ATP depletion, accumulation of PRPP, activation of AMPK, inhibition of mTORC1, and caspase 9 mediated apoptosis eliminate myeloma. Clones SP2/0-Ag14, P3X63-Ag8.653 routinely used worldwide ensure background below 1 per million, maximizing hybrid recovery efficiency.

Ref: Szybalski & Szybalska 1962 HGPRT negative mutants 8-azaguanine; ATCC SP2/0 Ag14 datasheet selection mechanism.

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Immunological Applications

PEG (polyethylene glycol) is used in hybridoma technology for:

Efficient cell fusion must overcome hydration repulsion keeping lipid bilayers approximately two nanometers apart due to ordered water molecules bound to phosphatidylcholine headgroups. Polyethylene glycol with molecular weight 1450 to 4000 daltons is extremely hygroscopic linear polymer containing repeating ether oxygens that strongly bind water through hydrogen bonding, lowering water activity dramatically and creating osmotic stress that forces membranes into close proximity below one nanometer. At concentrated 40 to 50 percent weight per volume, PEG disrupts lipid packing, induces inverted hexagonal phase propensity, promotes hemifusion stalk formation between apposed outer leaflets as demonstrated by electron microscopy, then upon rapid dilution with serum-free medium stalk expands into fusion pore allowing mixing of cytoplasm, mitochondria, and endoplasmic reticulum. Unlike electrofusion requiring specialized cuvettes delivering kilovolt pulses or Sendai virus envelope hemagglutinin-neuraminidase introducing viral antigens that provoke immune response and hinder clinical use, PEG is chemically inert, inexpensive, and easily washed after one minute exposure. Cytotoxicity minimized by controlled exposure timing. Yield typically one stable hybrid per ten thousand B cells, sufficient selection amplifies later.

Ref: Lentz J Mol Biol PEG fusion membrane dehydration; Alberts MBoC membrane fusion hemifusion stalk; Nature Protocols PEG hybridoma.

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Immunological Applications

Hybridoma technology involves fusion of:

Hybridoma formation relies on merging two complementary cellular programs within one cytoplasm to create self-renewing antibody factory. Splenic B lymphocytes from immunized donor possess productively rearranged heavy chain locus on chromosome 14 with VDJ recombination mediated by RAG1/2 and light chain kappa or lambda locus with VJ recombination, encoding high-affinity combining site selected through germinal center affinity maturation involving activation-induced deaminase, but they senesce within days after extraction due to lack of telomerase, dependence on BAFF survival signal, and activation of mitochondrial apoptosis. Myeloma cells are malignant plasma cell tumors adapted to suspension culture, thriving in RPMI-1640 with 10 percent serum, exhibiting constitutively active PI3K-AKT, NF-kB, and c-Myc driving endless cell cycle progression bypassing G1/S checkpoint. When mixed at optimized ratio of one myeloma to five B cells and exposed to 50 percent polyethylene glycol 1450, plasma membranes dehydrate, outer leaflets merge forming hemifusion stalk, expansion creates fusion pore allowing cytoplasm mixing, endoplasmic reticula intermix, binucleated heterokaryons undergo mitosis to become mononucleated hybrids retaining immunoglobulin genes from B partner and proliferative machinery from tumor, describing core fusion of antibody-producing B cell and myeloma cell.

Ref: Janeway Immunobiology 9th ed monoclonal generation; J Genet Eng Biotechnol 2020 PMC7414427 hybridoma fusion B myeloma.

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Immunological Applications

PEG (polyethylene glycol) is used in hybridoma technology for:

Polyethylene glycol polymer repeating ethylene oxide units average MW 1500 Da widely employed fusogen hybridoma generation due unique physicochemical properties affecting membrane biophysics. Mechanism involves strong hygroscopic nature binding water around phospholipid headgroups phosphatidylcholine sphingomyelin reducing hydration repulsion normally maintaining 2-3 nm separation bilayers enabling close apposition

Ref: Kohler Milstein PEG Fusion Method 1976 Detailed; Alberts Membrane Fusion PEG Mechanism Biophysical; Janeway Hybridoma PEG Fusion Laboratory Protocol Chapter 5.

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Immunological Applications

Hybridoma technology involves fusion of:

Somatic cell fusion underlying hybridoma merges two differentiated cell types each contributing specialized traits necessary monoclonal antibody manufacturing. Antigen-experienced B lymphocyte harvested spleen four days after final booster carries functionally rearranged Ig heavy VDJ and light VJ genes formed RAG1/RAG2 recombination junctional diversity somatic hypermutation conferring high affinity specificity target epitope active transcription intronic enhancer Emu 3 prime regulatory region capacity synthesizing Ig up to 2000 molecules per second but limited lifespan due lack telomerase dependence T-cell help CD40L IL-4 IL-21 undergoing Fas-FasL activation-induced cell death few divisions. Plasmacytoma myeloma line P3X63Ag8.653 lacking HPRT enzyme secreting no endogenous Ig provides immortal growth due chromosomal translocation t(12;15) placing c-myc under IgH enhancer driving continuous cycle plus HAT selection sensitivity enabling elimination unfused myeloma. PEG induced membrane fusion allows cytoplasmic mixing forming heterokaryon subsequently nuclear fusion mitotic segregation retaining chromosomes encoding antibody variable regions B cell while myeloma chromosomes sustain uncontrolled proliferation. Selection eliminates unfused myeloma primary B cells permitting only hybridoma colonies producing monoclonal antibody exquisite specificity and immortality essential scale manufacturing.

Ref: Janeway Hybridoma Fusion B cell Myeloma Mechanism 9th ed; Alberts Cell Fusion Mechanism Chapter 19; NCBI Kohler Milstein Hybridoma Methodology Original.

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