8.4 Brains, the genome and medicine
Neurotransmitters in the brain
The brain contains neurones, which transmit nerve impulses to each other across synapses. Many different neurotransmitters are involved, including dopamine and serotonin.
Dopamine and Parkinson’s disease
Dopamine is a neurotransmitter secreted by neurones and is in several parts of the brain, including the midbrain. The axons of the neurones extend throughout the frontal cortex, brain stem and spinal cord. Impulses passing between neurones in this region are important for controlling muscular movement and emotional responses. Parkinson’s disease occurs when cells in the basal ganglia die, so dopamine is no longer produced, resulting in the loss of control of movement, stiffness of muscles, tremors, slowness of movement, poor balance, walking problems, depression and difficulties with speech and breathing.
Parkinson’s disease is treated using L-dopa, a precursor in the manufacture of dopamine, which enters the brain and is converted to dopamine. It is not possible to give dopamine as a drug because it cannot enter the brain. This treatment helps in the early stages of the disease but the dose must be increased as the disease progresses because the cells that convert L-dopa to dopamine gradually die. Too much L-dopa results in uncontrolled movement.
The drug selegiline slows the loss of dopamine from the brain. The drug inhibits the enzyme monoamine oxidase, responsible for breaking down dopamine in the brain.
Dopamine agonists mimic the role of dopamine, activating the dopamine receptor directly by binding to them at synapses, activating them and triggering action potentials.
Gene therapy can be used – genes for proteins that increase dopamine production and that promote the growth and survival of nerve cells are inserted into the brain.
Serotonin and depression
Serotonin is a neurotransmitter in many parts of the brain and is involved in functions including mood, appetite, temperature regulation, sensitivity to pain and sleep. Neurones secrete serotonin are in the brain stem and their axons extend into the cortex, cerebellum and spinal cord. A lack of serotonin is linked to the development of clinical depression. Symptoms include sadness, anxiety, hopelessness, loss of interest in pleasurable activities, insomnia and thoughts of death.
The causes are not completely understood and may be multifactoral:
- There may be a genetic element – it runs in families
- Could be environmental
- Fewer nerve impulses than normal are transmitted around the brain, related to low levels of neurotransmitter production
Clinical depression can be treated with drugs that increase serotonin in the brain. The drug MDMA (ecstasy) acts at a serotonin synapse. It binds to a transporter protein which removes serotonin, increasing the effect of serotonin. MDMA can produce feelings of euphoria, friendliness and energy. However side effects cause depression, confusion and anxiety. Animal research shows that regular use of MDMA causes brain damage.
SSSR drugs (selective serotonin reuptake inhibitor) e.g. Prozac inhibits the uptake of serotonin from the synaptic cleft unto the presynaptic cell. This increases the level of serotonin available to bind to the postsynaptic receptor.
- Some affect synaptic transmission
- Some are similar shapes to neurotransmitters, so mimic their effects at receptors. These are agonists and activate more receptors
- Some block receptors, stopping them being activated, these are agonists
- Some inhibit the enzyme that breaks down neurotransmitters so they are in the system for longer
- Some stimulate the release of neurotransmitters, so more receptors are activated
- Some inhibit the release of neurotransmitters, so fewer receptors are activated
How drugs affect synaptic transmission
Ecstasy effects thinking, mood and memory and can also cause anxiety and altered perceptions. It desirable effect is feelings of emotional warmth and empathy.
- Short term effects – changes in behaviour and brain chemistry, sweating, dry mouth, increased heart rate, fatigue, muscle spasms and hypothermia
- Long term effects – changes in behaviour and brain structure
Ecstasy increases the concentration of serotonin in the synaptic cleft by binding to molecules in the presynaptic membrane that are responsible for transporting the serotonin back into the cytoplasm. This prevents it being removed from the synaptic cleft. Ecstasy may also cause the transporting molecules to work in reverse, further increasing the amount of serotonin outside of the cell. These higher levels of serotonin cause the mood changes.
The human genome project and drug development
The Human genome project has worked out base sequences of all the DNA (genome) in a human cell. From this, we can work out the amino acid sequences of the proteins they produce, leading to understandings of how the proteins work. This enables researchers to develop new drugs which target specific proteins, enhancing or lessening their activity.
Not everyone responds in the same way to drugs, knowledge of difference in a person’s base sequences in genes can help us understand this. Knowledge of a particular DNA sequence will enable suitable drugs to be chosen on an individual basis.
1977 – Fred Sanger – first DNA sequencing process. DNA is used as a template to replicate a set of DNA fragments, each differing in length by one base. The fragments are separated according to size using gel electrophoresis and the base at the end of each is identified – allows the sequence of bases in the DNA chain to be determined.
However testing for genetic predisposition has implications:
- Who should decide about the use of tests and on whom should they be used?
- Making and keeping records of individual genotypes raises issues of confidentiality
- Medical treatments through the development of genetic technologies will initially be very expensive
- Restricted availability of many medical treatments will be a problem to health services in deciding who is eligible for treatment
Using genetically modified organisms to produce drugs
Genetic modification = artificial introduction of genetic material from another organism
Genes for the synthesis of particular proteins can be inserted into an organism’s DNA, so the organism expresses that gene and synthesises the protein. This involves:
- Identifying and isolating the gene that is inserted by cutting it from DNA using restriction enzymes or by reverse engineering using the sequence of amino acids in the protein to be made and constructing a length of DNA with the appropriate base sequence to code for this protein
- Inserting the gene into a vector such as bacterial plasmids or a virus
- Inserting the vector into the organism
First success in genetic engineering was with bacteria:
+ Cheap and easy to culture
+ Rapid reproduction – transferred gene copied rapidly
– Prokaryotic cells do not have the correct biochemistry to make some of the more complex human proteins – so much use eukaryotes e.g. yeast, plants and animals
Bacteria contain simple DNA structures, plasmids, which can be transferred between cells. Using restriction enzymes, the circular plasmid can be cut and using other enzymes, a piece of DNA from another species can be inserted.
Example: bacteria to produce human insulin:
- Isolated human gene, modified if necessary
- Extracted plasmid is cut with restriction enzyme
- Isolated human gene is spliced into plasmid
- Modified plasmid placed back into bacterial cells
- Cells multiply in fermenter
- Bacterium produces human insulin
- Insulin protein extracted and purified
- Bacterial cells destroyed
To ensure the inserted gene is expressed, a length of DNA called a promoter (region) is inserted for RNA polymerase to begin transcription.
Examples of GMOs used for drug production:
- Tobacco plants – produce an anti-inflammatory cytokine (interleukin-10) which may treat autoimmune diseases
- Maize plants – produce human lipase for cystic fibrosis patients
- Goats – produce antithrombin in their milk for blood clotting disorders
Genes can also be inserted by injecting DNA directly into the nucleus of a fertilised egg which is then implanted into a surrogate female. Retroviruses have also been used to introduce new genes into fertilised eggs. This virus incorporates its DNA into the host DNA.
Genetically modified plants
Genetic engineers introduce new genes with alleles for desired characteristics into a plant’s DNA, resulting in genetically modified plants.
- Plasmid carrying desired gene and an antibiotic resistance gene (marker gene) used
- DNA insertion of new gene of virus DNA used to incorporate genes into the plant DNA of some cells
- incubation in growth medium with antibiotic
- Micropropagation: cells grow in sterile culture medium containing sucrose, amino acids, inorganic ions and plant growth substances
- Plant growth substances stimulate root and shoot growth
- Transgenic plant – all new cells contain the new genes
- Plantlets separated and grown into full size plants
Genes can also be inserted into plant cells by
- A bacterium that infects the species – genes from plasmid DNA are incorporated into the plant chromosome when they infect them
- Microinjection: DNA injected directly into nucleus of a fertilised egg using a micropipette (only successful in 1% of embryos)
- Microprojectules: minute pellets carrying the desired genes are shot into the plant cells using a particle gun
- Retrovirus: virus inflects cells by inserting their DNA/RNA into host’s genome
- Liposome wrapping: gene wrapped in a lipid bilayer which can then fuse with the cell membrane and deliver the DNA into the cytoplasm
To find out which plant cells have the new gene = insert a marker gene for antibiotic resistance along with the new desired gene. The plant cells are then incubated with the antibiotic which kills unsuccessful cells that have not taken up the new genes. The only cells to survive are the ones that successfully incorporated the new genes and are resistant. The plantlets are then separated and grow into full size, transgenic plants.
|Benefits of GMOs||Risks of GMOs|
|Pest resistant crop plants – reduces the use of pesticides – increases yield, reduces risk of harming beneficial insects||Genes inserted into a crop might spread to others – cause changes in the genotypes of plant populations – affect other organisms in an ecosystem|
|Resistant crop plants allows the herbicide to kill weeds but not plant crops||Pests might develop resistance through natural selection to the substance in GM crops, resulting in ‘super-pests’|
|Crop plants can be modified to produce high quantities of nutrients||Consuming foods containing GMOs can be considered harmful to health|
|Could benefit human health||Environmental concerns – increased chemical use in crops|
|Could help to feed the developing world||Could damage organic farmers|
|GM crops are more cost-effective||Raises ethical conflicts over the control of food production|
Comparison of coordination in plants and animals
|Coordination in both involves receptors, a communication system and effectors|
|Animals have a nervous system, containing specialised neurones which transmit action potentials very rapidly||Plants do not have a nervous system or neurone, but some parts do transmit action potentials but this is slower than animals and the potential differences are less than those in mammals|
|Both use chemicals that are produced in one part and travel to other parts where they have their effects|
|These substances are called hormones and are made in endocrine glands, which secrete them directly to the blood. They affect target organs which have receptors for them||There are no glands where these chemicals are made, but plant hormones are made in one area (e.g. auxins in meristems) and travel to another part where they have their effect. Unlike animal hormones, they do not travel in vessels but move through cells by facilitated diffusion through protein channels or by active transport through protein transporters|
|Hormones are almost all small protein molecules or steroids||No protein or steroid plant hormones have been found|