Unit 4: Applied Biotechnology I: Animal Tissue Culture, Gene Therapy, and Recombinant Proteins

1. Animal Tissue Culture

Introduction to Animal Tissue Culture

Animal tissue culture refers to the in vitro cultivation of animal cells, tissues, or organs in a sterile, artificial environment. Under precise temperature, pH, and nutrient conditions, these cells can survive, proliferate, and perform metabolic functions outside their host organism.

Culture Media

Culture media provide the necessary chemical and nutritional factors to sustain animal cells outside their native environment. Because animal cells lack cell walls and are highly sensitive, their nutrient demands are highly specific and complex.

Animal tissue culture media must mimic the physiological environment of the animal body, providing essential nutrients, maintaining an osmotic balance, and regulating pH.

Essential Components of Culture Media:

• Carbon Source: Glucose or galactose provides the primary source of energy.
• Amino Acids: Essential amino acids (which cells cannot synthesize) are critical for protein production.
• Vitamins: Function as coenzymes in crucial metabolic pathways.
• Inorganic Salts: Maintain osmotic pressure, cellular membrane potential, and provide essential ions such as Na⁺, K⁺, Ca²⁺, and Mg²⁺.
• Buffering Systems: Usually a bicarbonate/CO₂ buffering system, designed to keep the pH strictly between 7.2 and 7.4. Carbon dioxide levels in the incubator are typically maintained at 5% to 10%.
• Serum: Commonly Bovine Serum (BS) or Fetal Bovine Serum (FBS). Serum is a complex mixture containing growth factors, hormones, attachment factors, and transport proteins. It promotes cell attachment, division, and viability.

Classification of Culture Media:

Cell Lines and Applications

When cells are isolated directly from animal tissue and cultured, they form a primary culture. Once the primary cells grow to cover the entire surface of the culture vessel (reaching confluence), they must be subcultured (passed) into new vessels. This subcultured population is known as a cell line.

Key Applications of Cell Lines:

1. Virology and Vaccine Production: Host cells for viral replication to manufacture vaccines (such as polio, measles, rabies).
2. Toxicity Testing: Evaluating the safety and side effects of pharmaceutical drugs, cosmetics, and environmental chemicals without animal testing.
3. Cancer Research: Studying cell cycle regulation, signaling pathways, and testing potential chemotherapeutic agents.
4. Production of Biopharmaceuticals: Utilizing mammalian expression systems to produce therapeutic monoclonal antibodies, growth factors, and hormones.

Transgenic Animals

A transgenic animal is an animal whose genome has been stably modified by the integration of foreign DNA (referred to as a transgene). This introduced gene can be passed down to subsequent generations.

Methods of Gene Transfer:

• Microinjection: Directly injecting the transgene into the male pronucleus of a fertilized egg (zygote) using a glass micropipette under a high-power microscope. The egg is then implanted into a pseudopregnant surrogate female.
• Embryonic Stem (ES) Cell-Mediated Transfer: Introducing the foreign DNA into pluripotent ES cells in vitro, selecting the successfully transfected cells, and injecting them into a developing blastocyst.
• Retroviral Vectors: Using disabled retroviruses to infect early-stage embryos and insert the transgene into the host genome.

Key Applications:

1. Biopharming: Using transgenic animals as bioreactors to produce pharmaceutical proteins in their milk or blood (such as transgenic goats producing antithrombin III).
2. Disease Models: Developing animals that mimic human disease states to study pathogenesis and test therapies (such as the OncoMouse developed for breast cancer research).
3. Agricultural Improvement: Developing livestock with enhanced meat yield, resistance to diseases, or improved wool quality.

Gene Knockout

A gene knockout is a genetic engineering technique where an organism's specific gene is made entirely inoperative (inactive). This allows researchers to study the function of a gene by observing what biological processes fail in its absence.

Gene knockout utilizes homologous recombination to replace an active endogenous gene with an inactive, mutated version of the same sequence inside embryonic stem cells.

Step-by-Step Knockout Process (in Mice):

1. An inactive version of the target gene is designed in vitro, containing marker genes (like neomycin resistance) for selection.
2. The vector is introduced into mouse embryonic stem (ES) cells via electroporation.
3. Homologous recombination occurs, replacing the wild-type gene with the knocked-out construct.
4. Modified ES cells are selected using antibiotics and injected into a mouse blastocyst.
5. The blastocyst is implanted into a surrogate mother, producing chimeric offspring.
6. Chimeric mice are bred to generate homozygous knockout mice that express the mutated phenotype in all their cells.

Animal Cloning

Animal cloning is the process of generating a genetically identical copy of a multicellular animal. This is typically achieved through a technique known as Somatic Cell Nuclear Transfer (SCNT).

Somatic Cell Nuclear Transfer (SCNT) Workflow:

1. Enucleation: The haploid nucleus of an unfertilized oocyte (egg cell) is carefully removed.
2. Nuclear Transfer: The diploid nucleus from a donor somatic cell (such as a skin cell or mammary gland cell) is extracted and inserted into the enucleated oocyte. Alternatively, the whole somatic cell is fused with the oocyte using electric pulses.
3. Activation and Culturing: An electric shock or chemical treatment initiates cell division, mimicking fertilization, prompting the reconstructed egg to develop into an embryo.
4. Implantation: The developing embryo is transferred into the uterus of a surrogate mother, where it grows to term.

Famous Example: Dolly the Sheep (1996) was the first mammal successfully cloned from an adult somatic cell (mammary gland cell).

2. Gene Therapy

Gene therapy is an experimental medical field that seeks to treat, cure, or prevent disease by modifying or manipulating gene expression, or altering the biological properties of living cells for therapeutic use.

Types of Gene Therapy:

Delivery Strategies:

• Ex Vivo Gene Therapy: Cells are harvested from the patient's body, genetically corrected in vitro, and then infused back into the patient. This is safer because gene transfer occurs outside the body, allowing for screening of successfully modified cells.
• In Vivo Gene Therapy: The gene delivery vector is administered directly into the patient's body (such as inhalation or direct tissue injection).

Vectors Used in Gene Therapy:

1. Viral Vectors:
   • Retroviruses / Lentiviruses: Integrate their genome directly into the host chromosome, providing long-term expression, but carry a risk of insertional mutagenesis (which can trigger oncogenes).
   • Adenoviruses: Do not integrate into the host genome (remain episomal). They offer high transient gene expression but can elicit strong host immune responses.
   • Adeno-Associated Viruses (AAV): Do not integrate into the host genome but provide stable expression with minimal immune response. Ideal for targeting non-dividing cells like neurons and muscle fibers.
2. Non-Viral Vectors:
   • Liposomes: Lipid bilayers that encapsulate therapeutic DNA, facilitating entry through endocytosis. They have low immunogenicity but lower transfection efficiency compared to viral vectors.
   • Naked DNA: Direct injection of plasmid DNA. Simple and safe, but inefficient for widespread gene delivery.

3. Production of Recombinant Proteins

Recombinant proteins are proteins encoded by recombinant DNA that has been cloned into an expression vector and expressed within a host cell. These proteins are vital for industrial, diagnostic, and therapeutic purposes.

General Workflow of Recombinant Protein Production

1. Gene Cloning: The gene of interest (usually cDNA without introns) is isolated and inserted into a suitable expression vector behind a strong promoter.
2. Host Transformation: The expression vector is introduced into host cells (via transformation in bacteria, transfection in animal cells, or transduction using viruses).
3. Expression and Culturing: The recombinant cells are cultivated in bioreactors under optimal temperature, pH, and oxygen conditions to maximize cell density and protein expression.
4. Downstream Processing: The host cells are harvested (and lysed if the protein is intracellular), and the target protein is purified using techniques like chromatography (affinity, ion-exchange, size-exclusion) and ultrafiltration.

Host Expression Systems

Common Mistakes and Key Observations in Exams:

• Maturity of the gene construct: Prokaryotes (E. coli) cannot splice introns. Therefore, when expressing eukaryotic genes in bacteria, always use complementary DNA (cDNA) obtained from reverse transcribing processed mRNA, rather than genomic DNA.
• Inclusion Bodies: If E. coli expresses a protein too quickly, the protein aggregates into insoluble clumps called inclusion bodies. Resolving this requires denaturation and refolding steps during downstream processing.