Lysosomes, peroxisomes, and cytoskeleton Flashcards
Describe lysosomes and their function
Lysosomes are membrane bound organelles, with a membrane made of phospholipids. Lysosomes contain acid hydrolytic enzymes. They are produced from the Golgi system. There are approximately 50 to 100 lysosomes per cell. The primary function of lysosomes is to break down and recycle carbohydrates, lipids and proteins, although they are mainly tasked with the breakdown of (long chain) fatty acids ^[[Biochemistry Lecture 4]].
Lysosomes can be thought of as the garbage disposal of cells. They also constitute part of the cell’s defence mechanisms, breaking down invading pathogens. The pH within the lysosomes is low (4.5-5.5) in order to facilitate breakdown.
Healthy lysosomes degrade cargo to small and diffusible end products that can make their way out the cell by simple diffusion ^[[Physiology Lecture 2- Principles of membrane transport]].
However, suboptimal enzyme activity leads to accumulation of debris, partially degraded products that cannot diffuse across the membrane*, and this build-up impairs cell function.
Describe peroxisomes and their function
Peroxisomes are also membrane bound organelles, however, unlike peroxisomes, only have a single lipid bilayer membrane. Another unique feature of peroxisomes is that they can replicate independently, and are formed from budding of the ER. Both peroxisomes and lysosomes are charged with the break down of unwanted or obsolete material. Peroxisomes contain oxidative enzymes that are well -suited for their function, which is the breakdown of VLCFAs [see also [[Biochemistry Lecture 4]]] i.e. fatty acids with 20 or more carbons in their chain ^[hydrolytic enzymes wouldn’t be efficient]. Peroxisomes **also break down toxins for the cells e.g. alcohol and formaldehyde. **
There are approximately 50 oxidases found within the peroxisome. The most important of these breaks down fatty acids, and in the process **generates hydrogen peroxide H2O2, which is highly toxic to cells. Catalase is another enzyme present in the cell, and it prevents accumulation of H2O2 by breaking it down to more inert/less toxic H2O and O2. **
Peroxisomes are additionally unique in that they can get rid of the waste that they themselves produce.
Describe mechanisms of endocytosis
What sort of material is trafficked? Damaged organelles and unwanted cytosolic material. This is known as autophagy.
Additionally, material that is internalised from the extracellular space is sent to lysosomes, in a process known as endocytosis. An example of this is engulfed bacteria.
The key principles include:
- cytosolic material destined for degradation does not simply go to the lysosome
- it is first captured in a lipid vesicle, known as an autophagosome
- The autophagosome fuses with the lysosome when degradation is required
- Once fused, enzymes can work to break down contents
- This is an elegant system, as we do not want spill of acid hydrolytic enzymes
As for endocytosed material, it remains trapped in a lipid vesicle derived from the membrane. The vesicle undergoes a maturation process pre-degradation, becoming an ‘endosome’ with ligation* of ligands across vesicle in order to dictate where it goes ^[isn’t it going to the lysosome?notnecessarily…]
There are several modes of endocytosis, including:
- Phagocytosis (mostly by inflammatory cells^[Immunology]). The purpose of phagocytosis is to destroy pathogens, and avoid causing a biological response. The mature vesicle, known as a phagosome, fuses with lysosomes in order to be disposed. This process can also be used to detect and destroy cancer cells *
- Pinocytosis (‘cell drinking’) brings mostly absorbed fluids, and small hydrophilic molecules ^[are any drugs trafficked this way?] into the cell. This is because there is either no receptor available to mediate transport, or no favourable gradient. Pinocytosis is thus more efficient than waiting for fluids to diffuse across the membrane. A small amount of energy is produced in this process*. The mature vesicle (a pinosome?) binds to lysosomes for breakdown and release of components
- Receptor-mediated endocytosis. This is most utilised method to internalise material. This process requires the binding of a specific ligand to a specific receptor. Examples of ligands that bind receptors include hormones, serum proteins, growth factors, ligands (and neurotransmitters?)
- several steps are involved in receptor-mediated endocytosis
- Receptor ligand binding occurs
- The receptor-ligand complex diffuses laterally until it encounters clathrin coated pits
- The receptor-ligand complexes* accumulate at the pits
- Special proteins e.g. clathrin or other adaptor proteins like dynamin curve the membrane, causing it to invaginate, and pinch off
- The mature vesicle fuses with lysosomes to begin breakdown
Describe diseases resulting from abnormal functioning of lysosomes and peroxisomes
Tay-Sachs Disease*
- a lysosomal disease
- a result of build up of glycosphingolipid, a fatty acid, that is partially broken down
- autosomal recessive disease
- caused by mutation in HEXA gene, which encodes acid hydrolytic enzyme
- As partially degraded lipids are toxic to cells, the buildup destroys nerve cells in brain and in spinal cord, due to bulging lysosomes
- occurrence is rare i.e. 1 in 300,000
Zellweger Syndrome
- a peroxisomal disease
- neurological ^[why is this so common among lysosomal/peroxisomal abnormalities] disease, can cause secondary effects in liver and kidney
- Occurrence is 1 in 50000, and most who are born do not live past 6 months
- Caused by a mutation in PEX1 gene in 70% of cases: this gene enables other enzymes to enter including catalase and oxidase
- As a result, VLCFAs accumulate, causing disease
Accummulation causes disease
List the main filaments involved in cytoskeleton assembly
The cytoskeleton is a network of protein fibres. It provides structural support to the cells and defines shape along with the membrane; it also provides for movement of cells e.g. through the use of cilia and flagella in bacteria and sperm, enables trafficking of organelles and vesicles, and is responsible for moving chromosomes, particularly during cell division. Overall, it is essential to function.
There are three main types of filaments involved in cytoskeleton assembly. These include microtubules, intermediate filaments and microfilaments.
Describe microtubules
Microtubules are the largest of the filaments. They are straw-shaped, and are dynamic i.e. they elongate and shorten. The microtubules have two ends (+) and (-), the (+) end elongates faster ^[and the minus end elongates slower]. The microtubules grow and shrink by addition and subtraction of dimer units comprised of α and β tubulin.
The primary function of microtubules is to assist in cell shape, motility, movement of chromosomes and organelles, and cytosolic cargo movement.
The organisation of microtubules is orchestrated by MTOC (there is usually one per cell, however in cell division ^[[Cell Biology Lecture 1]?] there may be two or more). Centrosomes organise microtubules i.e. regulate how long microtubules should be, when they should be engaged for and in which direction*
(note that centrosomes are comprised of two centrioles orientated at right angles to each other, each consisting of 9 microtubules in a cylindrical array)
The negative end of the microtubules is directed at centrosomes.
Describe intermediate filaments
Intermediate filaments provide structural support and resist stress in cells. Intermediate filaments can be thought of a spring mechanism, and are often found in epithelia^[[Histology Lecture 2]] . There are 70 ^[can be classified into 6 types; other examples not included in the lecture include neurofilaments found along vertebrate axons, and desmins which as found in sarcomeres and connect different cell organelles together in order to regulate sarcomere architecture] types of intermediate filaments including keratin ^[which is found in cells such as the glia [[Histology Lecture 5]] and in retinal cells, and increased when injury is incurred in these tissues]. Other examples of intermediate filaments include GFAP in glia and astrocytes, and lamins found in the cell nucleus ^[[Cell Biology Lecture 1]].
^[side note: intermediate filaments so called as they have diameter (10nm) between actin microfilaments (7nm), and microtubules(25nm), have coiled-coil structure as building blocks. They are also dynamic, like microfilaments and microtubules, but do not go ‘treadmilling’, do not have polarity, and do not have a nucleoside triphosphate binding site]
Describe microfilaments
Microfilaments are primary actors in cell division ^[[Cell Biology Lecture 2]]
Microfilaments are involved in gross movement of the cell, exocytosis, endocytosis ^[by increasing curvature it impacts cell structure], and cell motility. They mainly work by altering cell membrane shape*
The formation of microfilaments, like microtubules, is dynamic; it consists of polymerisation and depolymerisation.
The famous group of microfilaments are the actin family of functional proteins.
In mitosis, they work to form a contractile around cells in order to constrict. The dynamics of this process are controlled by cross-talk with microtubules
- This cross-talk is initiated by signalling which occurs along microtubules. It is an interconnected system, involves kinetochores* to change actin structure and form the contractile ring
- This process enables precise control of cell function e.g. endocytosis, exocytosis, cell division
Describe the movement of cargo within the cell
In order to move cargo around the cytoplasm, motor proteins sucha s dynein and kinesinwalsk along the microtubules. A conformational change is involved*
Kinesin is involved in anterograde transport i.e. movement towards the plu send of microtubules towards the periphery of cell. Dynein is involved in retrograde transport, the movement of cargo towards the minus end of the microtubules. ^[note structure of dynein has a dynactin? component]
This action is quick, moving at1000 nm/s or using ATP per second.
The cargo is bound to the ‘tail’ of motor proteins, while the head bind microtubules.
Note that the motor proteins have a great capacity for wide range of sizes of cargo, from a single protein to complexes in vesicles.
eq links to anatomy and neurophysiology
Describe the role of cytoskeletal filaments involved in muscle contraction
There are two filaments involved in muscle contraction^[[Anatomy ‘Lecture 4’]]:
- the thin filament i.e. actin associated with troponin and tropomyosin
- thick filament mostly composed of myosin
The process of muscle contraction involves many steps. A summary of these steps is listed below:
- Nerve impulse travels down motor neurons to the neuromuscular junction
- Nerve impulse stimulates acetylcholine (neurotransmitter) release
- Acetylcholine binds receptors, activates a cascade that results in release of calcium ions from the sER
- Calcium ions bind troponin
- Tropomyosin shifts its position and exposes free binding sites on actin
- Myosin heads bind to free actin sites
- Myosin heads pivot, and pull on actin filaments towards the centre of the sarcomere i.e. the functional unit of the muscle cell
- ADP released, and the myosin head pivots
- ATP causes myosin to detach off active sites on actin i.e. induces a conformational change
To summarise in a single sentence: muscle contraction involves sliding of two filaments against each other, mediate through the use of ATP and ADP
Describe the relevance of cytoskeleton to neurodegenerative disease
Abnormalities of cytoskeleton predominate in neuronal disease.
Recall that the structure of the cytoskeleton is typically very well defined, and found prominently in axons, where transport is continual; therefore, a mutation that affects structure of filaments of the cytoskeleton will perturb its function.
Examples of neurodegenerative disease with links to cytoskeletal abnormalities include
- Parkinson’s disease: primary effect is the degradation of neurons, resulting in tumours, rigidity, and other non-motor symptoms. Proteins linked to Parkinson’s pathogenesis are associated with microtubule stability
- Alzheimer’s disease: a result of the accumulation of tau, coiled filaments which are microtuule assocoiated and provide microtubule stability. They form intracellular neurofibrillary tangles*
- Huntington’s disease: an autosomal recessive disorder*, a results of mutations in HTT gene, responsible for cell transport, endocytosis and protein degradation, thus cpaacity for these functions is reduced. Pathogenesis is linked to abnormal organisation of the cytoskeleton
Can test olf nerve for these neuro-deg diseases
