Neuroscience Flashcards
(112 cards)
1
Q
Describe the evolution of the nervous system
A
- due to the need for improvement in speed and specificity of signal communication
Originally, multicellular organism have sensory cells that control motor cells by releasing a chemical transmitter or hormones into a common fluid space upon receiving signals. The relay of signal to the motor cells are then dependent on diffusion across the space.
The time for a molecule to travel a distance is given on average by:
t ≈ x^2/D
Diffusion time increases with molecule size and also increases quadratically with distance, so it only works over short distances.
2
Q
What improvement did the nervous system bring
A
provide direct connections between sensory and motor cells by means of nerve axons, so communication is quicker and more specific
3
Q
What do axons have that help in transport
A
Axons are very long, so transport of material must be an active process. Axons contain microtubules, a track used by myosin-based motor proteins (kinesin, dynein) to transport components up/down the axon. The process is essential for neuron survival.
4
Q
Explain neuron signaling
A
- Most neurons signal to each other using chemical transmission at the synapses at the axon terminal to another neuron’s dendrite or cell body (soma)
- A minority of specialized neurons use electrical connections via gap junctions.
5
Q
Explain electrical excitability
A
Huge amount of energy, not feasible to obtain, is required to move a charged ion across the hydrophobic membrane.
So, membrane channels are needed to move ions across the lipid bilayer.
6
Q
How do neurons overcome the problem with electrical excitability + example
A
Neurons have a strong electrical charge due to ionic gradient across their membrane, which is set up and maintained by active pumping using ATP as an energy source. A large proportion of the energy intake of a human is devoted to the operation of ion pumps.
Eg. the sodium-potassium ATPase pump: 3 Na+ out of the cell and 2 K+ into the cell, thus building a negative charge inside the membrane – membrane potential ~-60-75 mV
7
Q
explain the distribution of ions across neuronal membranes
A
[Na+], [Ca2+], [Cl-]. is higher outside
[K+], General anions (ex. phosphate ions, carboxylate groups) is higher inside
Therefore, membrane potential = negative
8
Q
Explain equilibrium potential
A
In cells, Na+/K+ channel sets up a K+ gradient across the membrane where [K+] is higher inside. A- is also maintained higher inside.
Cells also contain K+ leak channels which allow K+ to diffuse down their concentration gradient outside. However, the membrane is not permeable to A-, so [A-] is maintained high inside the cell. This sets an excess of + charge outside the cell. An electrical potential difference then builds up across the membrane as it becomes charged.
The chemical forces causing a net diffusion of K+ outside are now countered by a growing electrical force which opposes the flow of K+ outside.
Eventually an equilibrium potential (Ek) is reached where the electrical force equals the chemical force, and no ion exchange occurs ([K+] is constant).
9
Q
Explain the Nernst equation
A
The equilibrium of charged ion concentrations across cell membrane sets up the membrane potential (voltage, Vm) which can be calculated by the Nernst Equation
* Membrane potential (voltage) across a membrane is equivalent to work per unit.
C = concentration
R is gas constant 8.314 J K-1 mol-1
T = temperature in Kelvin
(RT = thermal energy per mol)
z = valency (e.g. +1, +2 or -1)
F (Faraday’s constant) = 96,400 Coulombs/mol
Vm = RT/zf . ln(Cout/Cin)
10
Q
What is the Nernst equation derived from
A
Boltzmann distribution (the equilibrium distribution of particles with different energies in force fields)
N1/N2 = e^-(u2-u1)/kT = 1/e^”
k (Boltzmann constant) = 1.381 x 10^-23 JK-1
T = temp (K)
kT = energy of a single molecule
u1 and u2 = different amounts of energy
N1 and N2 = number of particles in each state, respectively
e = eulers constant ~ 2.718
11
Q
what equation describes the energy difference of ion gradient across a membrane
A
zFVm
zF(Vin-Vout) = zFVm (joules/mol)
12
Q
How do derive the Nernst equation from the Boltzmann distribution
A
- convert the numbers of individual entities to concentrations
- replace single-particle energies u to molar energies U
- the gas constant R= NaK where Na is avogadros number, the number of particles in a mole
N1/N2 = e^-(u2-u1)kT = C1/C2 = e^-(U2-U1)/RT
Cout/Cin = e^zFVm/RT
zFVm/RT = ln(Cout/Cin)
Vm = RT/zF . ln(Cout/Cin)
13
Q
What is used by neurons to signal information
A
A change in membrane potential
can be graded (slow) or sharp (quick).
* sharp changes = Action Potentials
* Graded changes can happen with some secondary messengers; also important (e.g. post-synaptic potentials).
14
Q
what can a change in membrane potential cause
A
Depolarization: a reduction in difference of electrical potential across the plasma membrane of a nerve or muscle cell. Potential becomes more +ve inside the cell. usually an excitatory signal.
Hyperpolarization: an increase in difference of electrical potential across the membrane. Potential becomes more -ve inside the cell. usually an inhibitory signal.
15
Q
what is the space constant λ
A
As the signal travels down the axon, according to the equation:
V = V0e^-x/λ
x= distance traveled
λ = space constant
Leakage of current into extracellular fluids along the way will result in the initial voltage decaying exponentially by the time it reaches the synaptic terminal.
16
Q
Explain the relationship between λ and V
A
When x = λ, V will be equal to V0/e which is approx. 37% of V0
Therefore, λ is the length at which the signal V decays to 37 % of its original value V0
λ is determined by the length & thickness of the axon. λ is typically 0.1 to 2 mm in nerve fibres
17
Q
Describe what results in passive spread
A
In normal conditions, a lot of current leakage in axons result in the passive spread of current in nerve cells, so the signal is not enough to trigger neurotransmitter release.
* unless in smaller organisms eg. c. elegans or if = small rod and cone cells in eye
18
Q
How do animals overcome λ (passive spread)
A
Larger animals evolved a solution by myelination & action potentials
19
Q
What is myelin
A
Myelin is formed by glial cells & creates a high-resistance, low-capacitance sheath which:
* greatly increases the space constant (λ)
* cause the action potential to jump from node to node, thus increasing the velocity of the action potential by 20x or more
* In the CNS the glial cells are oligodendrocytes, in the PNS they are Schwann cells.
20
Q
Describe the action potential
A
- A way of regenerating the voltage as it decays with length
- Require voltage-gated sodium channels
- Occurs when an initial depolarization at a specific location in nerve cells passes a certain threshold
- Initial depolarization changes structure of the voltage-gated sodium channel, causing to open
- Opening of the channel increases membrane permeability – so it depolarizes membrane nearby as well
- The sequence continues down the axon
Although the voltage generated at the initial point decreases with distance, the depolarization that spreads to the adjacent region of the axon is still above the threshold & can continue to relay signal along the axon until synaptic terminal.
21
Q
Describe the all-or-none response
A
- small depolarizing currents that do not pass the threshold will still produce a passive spread of current
- action potential does not vary in size or kinetics, no matter how much current is put in - if it passes threshold, will get a standard size of action potential (+55 mV)
- BUT can change frequency, allowing for frequency modulation
(frequency of action potentials = intensity of stimulus which provides information to generate diff. responses)
22
Q
Explain absolute and relative refractory periods
A
- Because the threshold is not fixed.
- In absolute refractory period: Voltage-gated ion channels are in a state where it cannot open, the threshold is much greater than normal, & cannot have action potential at all
- In relative refractory period: recovery of the voltage-gated ion channels lowers the threshold & allows action potential to occur if have more current stimulation
- SO action potential = only able to go in one direction down the axon (does not go backwards)
- occurs in the timespan of milliseconds
23
Q
How was the mechanism of the action potential studied
A
using the squid giant axon
* up to 1 mm in diameter – able to insert an electrode into the axon to record the voltage from inside the axon
* can also use special electronics to give precise commands to artificially control the membrane potential of the axon
24
Q
Draw the phases of an action potential
A
- flat line = resting potential
- curve up = depolarization phase (passes a threshold line)
- down = repolarization phase
-undershoot (just below the resting potential and it returns)
25
Explain what occurs in the resting potential phase of an action potential
Na+/K+ ATPase pump and leak K+ channels set up an electrochemical gradient of Na+ and K+ across the membrane, establishing the resting membrane potential of ~ -70 mV
26
Explain what occurs in the depolarization phase of an action potential
An initial depolarizing signal that causes the membrane potential at a certain region to pass the -55mV threshold will open voltage-gated Na+ channel, causing influx of Na+ into the cell (increase Na+ permeability)
* all-or-none response: if signal does not pass threshold, action potential does not occur
Na+ channels open in a positive feedback manner - increased Na+ permeability results in more depolarization which in turns opens more channels & further increase Na+ permeability
* This high influx of Na+ causes the membrane potential to increase to +30 mV
* Results in adjacent membrane regions also becoming depolarized – allows depolarization to spread forward, triggering more action potentials down the axon. Therefore, although the initial signal decays with distance, nerve cells = still able to relay the signal along the axon until the synaptic terminal
However, the Na+ channels only open transiently – has an inbuilt inactivation mechanism (molecular spring) that closes the Na+ channel and won’t be able to open again for a few milliseconds
27
Explain what occurs in the repolarization phase of an action potential
After a few milliseconds, K+ channel also transiently opens, which together with leak K+ channel, allows K+ to rapidly leave the cell which provides a counter effect against the Na+ influx, restoring a – membrane potential.
28
Explain what occurs in the undershoot/refractory phase of an action potential
Repolarization results in the membrane potential to undershoot the original -70 mV. During this period, because the Na+ channel is closed, an absolute refractory period exists in which another wave of action potential in that region cannot occur at all
* SO action potential = only able to travel down the axon (does not go backwards)
Recovery of the voltage-gated ion channels in the relative refractory period allows action potential to occur if there is more current stimulation
Recovery of the voltage-gated ion channels, along with ionic gradients re-established by Na+/K+ ATPase pump, also restores the original -70 mV membrane potential
29
what is the myelin sheath
* formed by glial cells & creates a high-resistance, low-capacitance sheath
* regions not insulated by myelin = Nodes of Ranvier (1-2 space constants apart)
30
what is the importance of myelin
multiple sclerosis
* Episodic autoimmune destruction of myelin surrounding the nerves of the CNS
* Slows down speed of communication over long axonal distance, resulting in poor nerve conduction
* leads to a progressive burden of neurological deficits from monocular blindness to total paralysis
31
why are membrane channels needed
* The membrane presents an energy barrier to ion crossing
* movement of the cation through the hydrophobic portion of the membrane bilayer is energetically unfavourable
32
how can the flow of ions through a membrane channel be detected
by the patch-clamp technique
* select for a patch by suction, and put electrode over the patch to record current changes as the channel transits between open & closed states
33
what stimuli can open channels
* change in voltage
* binding of ligand
* mechanical force
* change in temperature
34
describe the structure of a voltage-gated sodium channel
* N & C termini = intracellular
* 2000aa protein with 4 TM domains
* Each domain = 6 TM helices
* Each domain is +charged and interacts to form a closed pore
* When the membrane potential becomes +, the TM domains will move to open the pore
* Radius of channel that accepts an ion matches the hydration radius of the ion to rid of surrounding water so the ion can seamlessly move through the channel
35
describe the mechanism of a voltage-gated sodium channel
* Opening and closing of the channel are random events, but the frequency with which they occur is influenced by transmembrane voltage.
* The transition rate between open and closed states is <10 μs.
* The flux rate through the pore when it is open is of the order of 10^7 ions per second; Following opening, voltage-gated Na+ channels enter an inactivated non-conducting state in which they are refractory to subsequent depolarization
36
describe voltage-gated potassium channels (Kv1 series)
* Same gene family as Na+ channel but 4 TM domains are separated (not a single peptide)
N terminus = aids tetramerisation
S4 voltage sensor = conveys voltage sensitivity to the channel; many aa's are +charged
P region (S5-S6 loop) = hydrophobic hairpin loop; determines K+ selectivity; highly conserved; consensus sequence (GYG); K+ interact w/ the carbonyl groups of TVGYG sequence of the selectivity pore
37
describe the chemical synapse
* impulses communicate their signal across synapses, that separate nerve cells from each other and from their target destinations
38
what are the ways to pass information between neurons
Gap junctions - electrical transmission (most basic, less common)
* neurons across the synapse are joined by a physical tube
-fast – in fish, for fast escape reflexes
-no specificity - both directions
-decay of electrical current = inefficient
Chemical transmission
-slower – unidirectional
-integrative
-amplifies and regenerates the signal
39
describe the chemical transmission mechanism
1. Action potential invades axon terminal (depolarization)
2. Results in activation of Ca2+ voltage gated calcium ion channel – result in influx of Ca2+
3. Neurotransmitter mobilization & release – vesicles fuse & transport across the synapse
4. Signal continues
40
describe voltage-gated Ca2+ channel
* Ca2+ conc. outside > inside the cell, so as depolarization activates opening of Ca2+ channel, results in Ca2+ influx
* Ca2+ entry is excitatory, and depolarizes the membrane
* Ca2+ is also a 2ndary messenger - binds to target proteins & promote vesicle fusion
41
explain vesicles
* buds from endosome
* specific transporter on the vesicle membrane uses ATP to pump in each type of neurotransmitter into the vesicle
eg. glutamate pump, acetylcholine pump
* assembled in the axon terminal near the membrane, ready to dock and fuse with the membrane to release the neurotransmitters – will travel across synapse and bind to receptors on post-synaptic membrane
* are re-used via Clathrin-mediated endocytosis
-can also be via “kiss and run” recycling – vesicle briefly touches the membrane but doesn’t actually dissolve into the membrane, and then buds off again
42
describe the process of chemical neurotransmission
1. Synthesis of the neurotransmitter in the presynaptic neuron
* Ex. glutamate from Krebs cycle, decarboxylated glutamate forming GABA (inhibitory transmitter), acetylcholine & dopamine (from tyrosine) generated by a special enzyme
* Some neurons = utilize gas neurotransmitters eg. nitric oxide from nitric oxide synthase – turns arginine into gas
2. Storage of the neurotransmitter and/or its precursor in the presynaptic nerve terminal
* Some peptides are also co-released with neurotransmitters
3. Release of the neurotransmitter into the synaptic cleft
4. Binding & recognition of the neurotransmitter by target receptors (on post-synaptic membrane)
* Ex. ion channels, GPCR
5. Termination of the action of the released transmitter
* Enzymes in synaptic cleft breaks down neurotransmitters
* Also have transporters in presynaptic membrane which can suck excess neurotransmitters back up to recycle them
* Transmitted signals can be excitatory OR inhibitory (ex. GABA opening Cl- channel)
43
explain vesicle fusion
Fusing naked vesicles with other lipid membranes would require substantial energy (not energetically feasible)
* Via ATP-intensive process, specific enzymes are required to promote fusion
* common process for every cell (not just neurons)
44
explain the SNARE pin machinery
-Involves SNARE proteins via a Ca2+ dependent mechanism
* synaptobrevin on synaptic vesicle membrane
* SNAP25 & syntaxin on presynaptic membrane
* These proteins associate to tether the vesicles near the presynaptic membrane
* synaptotagmin on synaptic vesicle membrane = Ca2+ binding protein
o when Ca2+ comes in the presynaptic terminal, Ca2+ binding to synaptotagmin causes conf. change & interacts with the SNARE pin machinery, causing the vesicles to fuse
Each SNARE pin releases about 35 kBT of energy (equivalent to about 20 kcal/mol) as it zippers up. The activation energy for lipid bilayer fusion is about 50 to 100 kBT , and so three or more individual SNAREpins suitably arranged provide enough energy to drive fusion.
Cleavage of SNARE proteins by clostridial toxins result in inability to release vesicles.
45
what changes are associated with ageing
o Genomic instability
o Altered intracellular communication
o Stem cell exhaustion
o Cellular senescence
o Mitochondrial dysfunction
o Deregulated nutrient sensing
o Loss of proteolysis
o Epigenetic alterations
o Telomere attrition
46
what is dementia
* Progressive loss of cognitive and intellectual functions
* associated with structural brain disease
* Characterized by disorientation, impaired memory, judgment, and intellect
47
what is alzheimers
- dementia caused by progressive neurodegeneration
* symptoms get worse over time, and progress over several years
* Progressive failure of brain structures over time
48
What happens as dementia progresses
* Thinking, memory & decision-making all fail (all patients).
* Communication and language fail.
* Depression.
* Anxieties or phobias appear.
* Sleep problems. Disrupted circadian rhythms.
* Anger or agitation.
* Poor motor coordination.
49
What is the first stage of AD
- memory loss because during AD, there is widespread neuronal death:
* Brain white matter (containing myelin) has been attacked in Neuroinflammation
* Synapses & cell bodies in neural layer = degenerated
Both cause substantial loss of brain volume + enlarged ventricles
50
What is the effect of AD at a cellular level
formation of Amyloid plaques & Neurofibrillary tangles
Amyloid plaques
* Extracellular
* Major component β-amyloid peptide
* Also contain other proteins that bind cholesterol such as APOE
Neurofibrillary tangles
* Intracellular
* Paired helical filaments.
* Major component is the protein tau, which is “hyperphosphorylated”
51
What causes amyloid plaque formation
genetic mutations in the 3 genes:
* APP (amyloid precursor protein)
* PS1 (presenilin 1)
* PS2 (presenilin 2)
Causing autosomal-dominant, early-onset AD (account for 1% of total AD cases)
OR by inheriting:
* APOE4 allele (the strongest genetic risk factor for late-onset AD)
* TREM2 (triggering receptor expressed on myeloid cells 2) allele
52
What occurs in early-onset AD
- disease of proteolysis
* Involves the Amyloid precursor protein (APP) – found in all neurons but is of unknown function
* It is a TMD protein subject to turnover by ⍺-β- 𝜸-secretase
* The proteases cleave APP catabolic parts into constituents within the membrane, releasing a 40-42 aa fragment – the alpha beta amyloid (Aβ)
* Cleavage by:
o β-secretase generates the N-ter
o 𝜸-secretase generates the C-ter
o ⍺-secretase precludes Aβ formation
53
Describe the role of Aß in early-onset AD
* Aβ is soluble, and is then removed from the brain by unknown mechanisms
* However, too high conc. of Aβ will cause precipitation/ aggregation into amyloid plaques – a toxic process, causing Alzheimer’s (no consensus of how)
* This can be due to: Duplication of the APP gene and missense mutations
o cause inherited forms of AD and cerebral amyloid angiopathy
o missense mutations cause APP to be more susceptible to proteolysis, thus producing more Aβ
o 20 known missense mutations in APP cause human disease
54
What does y-secretase contain
contains presenilin (a membrane embedded protease)
* Mutations cause it to be more active
* cleaves aa chain inside the lipid bilayer by providing an aqueous environment within the membrane -does so by containing 2 aspartates
Inhibition of the proteases = toxic because they are also associated with the turnover of membrane proteins in muscle
55
Describe the structure of amyloid
* cross-β spine structure: elongated fibers, with spines consisting of many-stranded b sheets
* Not necessarily toxic
56
describe errors in Aß anabolism
Genetic mutations cause early-onset AD (1% of all AD) by errors in Aβ anabolism
- too much Aβ produced
* Duplication of APP gene
* Missense mutations of APP gene (more susceptible to proteolysis)
* Mutations in presenilin (increased activity)
57
What happens in late-onset AD
related to failure in Aβ catabolism (clearance)
Neurofibrillary tangles – related to Tau
* Associate with microtubules via repeated domains – but no consensus of function
* Has multiple splice forms at the N-terminus
* Hyperphosphorylation causes tau precipitation and formation of paired helical filaments in certain regions of the brain. This results in loss of tau binding & microtubule degeneration which is toxic – causes Tauopathies, a range of neurodegenerative diseases, including AD (Tau precipitate in hippocampus)
58
Describe the structure of Tau in AD
- hyperphosphorylated tau has paired helical filament
* Structure = reminiscent of amyloid structure
* In a core region of the filament, have many polypeptide chains stacked on top of each other, forming a C structure
59
What are the proteins related to late-onset AD
APOE – abundant in blood and brain & binds cholesterol, but of unknown function
* 4 variants in human gene
* Depends on which combinations of the variants you have, will affect risk of developing Alzheimer’s
o Homozygous APOE4 (4/4) = higher risk of developing Alzheimer’s
o Heterozygous with APOE2/3 or 2/4 = much lower risk
* New discovery:
o If homozygous APOE3 (3/3) = glial cells can metabolize cholesterol in a healthy way
o If homozygous APOE4 (4/4) = cholesterol transport/ metabolism in oligodendrocytes = impaired – less able to make myelin
* Has been found to associate with Amyloid plaques & intermediate amyloid aggregates
o In some plaques, E3 > E4
o But in full-scale amyloid plaques, E4 > E3
TREM2 transmembrane protein of microglia cells
Microglia cells – innate immune cells
* specialized macrophages of the brain
* trim dendrites of healthy neurons and maintain neuronal architecture
* Function in response to neuronal stress
o Repair neurons when APP synthesis & Tau phosphorylation increases
in late-onset Alzheimer’s, through an unknown mechanism, microglia are hyperactivated and attack the neurons instead of repairing them, leading to inflammation & neuronal failure
TREM2
* promotes microglia phagocytosis of protein aggregates, amyloid plaques, neuronal debris
* In late-Alzheimer’s, TREM2 is missing from microglia, so it no longer associates with amyloid plaque. Amyloid plaque will become bigger into filamentous plaque, pTau also builds up, contributing to the inflammation and neuronal failure
60
Explain how AD is multifactorial and heterogeneous
It is affected by other factors including diet, exercise, environmental factors etc. They are all factors that affect 3 core components in brain:
* Neurons
* Innate immunity (microglia cells)
* Vasculature
And all result in Aβ & pTau build up in the brain.
61
Explain vasculature in AD
* Aβ & Tau are increased in wake & decreased in sleep
* Sleep drives metabolic clearance of the brain
* The brain is supplied with a dense connection of blood vessels
* But the brain is separated from the blood vessels by a type of glial cells (astrocytes) lining the blood vessels with tight junctions – forming the blood-brain barrier
* Waste fluids is cleared from the brain by flowing through the extracellular space between glia feet and surface of capillary
* This flow increases during NREM sleep because Extracellular space increases during sleep because glia cells contract
62
What are the therapies for AD (7)
Disease modifying agents in development:
ß-secretase inhibitors = inhibit ß-amyloid production
y-secretase inhibitors = inhibit ß-amyloid production
amyloid lowering antibodies = reduce amyloid burden
neutrotrophic molecules = promote neuronal survival
anti-inflammatory drugs = reduce brain inflammation
phosphorylation inhibitors = reduce tau P
nuclear hormone receptor modulators = reduce brain inflammation
Ex. Amyloid lowering antibodies: monoclonal antibodies (eg. lecanemab) that recognize soluble amyloids, resulting in their clearance from the brain
But still currently no effective treatments
* Currently, Acetylcholinesterase inhibitors and NMDA receptor antagonists are given to patients to increase cognitive performance transiently.
AD begins to develop 10 -20 or more years prior to recognizable clinical signs.
* By the time AD is recognized, substantial synaptic, neuronal degenerative and inflammatory changes have already occurred.
* Early detection of AD is required for therapeutics to be able to delay or prevent the disease, and for disease-modifying agents given to patients to work effectively
63
What remains unknown in AD
whether the cause of idiopathic Alzheimer’s is due to amyloid plaques or not – amyloids could just be a by-product, and researchers can perhaps focus more on microglia/ Tau for studying the cause of Alzheimer’s disease.
64
What is Parkinsons
condition in which parts of the brain become progressively damaged over the years
65
what are the 2 types of PD
familial PD = caused by inherited gene mutations
sporadic/idiopathic PD = unknown cause (90% of PD)
66
PD can be...
early-onset = v.rare (<10%) -develops before 40/50
late-onset = develops between 50-70, progresses slowly to culminate in death 10-20 years later
67
what causes PD
caused by a homogenous phenotype of the degenerating neurons -the progressive loss of dopaminergic neurons (80% loss = characteristic of PD)
68
where are dopaminergic neurons located
- substantia nigra & ventral tegmental area
- spatial distribution of degenerating neurons in PD is restricted to substantia nigra pars compacta -a population that that projects to and innervates neurons in caudate and putamen (nigrostriatal pathway -produces dopamine)
69
explain the process of dopamine production
1. tyrosine hydroxylase adds OH group to L-tyrosine to produce L-DOPA, precursor of dopamine
2. DOPA decarboxylase which removes COOH group from L-DOPA to make dopamine
3. dopamine is released to other brain areas:
-nucleus accumbens, frontal cortex, olfactory tubercles
-for substantia nigra -striatum
70
what is dopamine
a neuromodulatory molecule needed for arousal, motivation and motor movements
71
explain the process of dopamine release
1. dopamine is compartmentalized into vesicles by vesicular monoamine transporter
2. they do not have a precise synaptic transmission where they are released into the synapse to post-synaptic targets like other neurotransmitters
- they have volume transmission where they are released from axonal varicosities for diffusion through a large volume of brain so they are able to reach targers 100s of nm/mm away
72
explain dopamine receptors (D1&D2)
* target cells have dopamine receptors = majority have D1 and D2 (7TM GPCRs)
* dopamine recognition by D1 = excitatory effect = causes AC to increase production of cAMP
* D2 = inhibitory effect = inhibits AC production of cAMP
73
describe the changes in cAMP due to dopamine
* changes in cAMP production lead to changes in PKA activity:
short-term response= alters the membrane potential by affecting K+/Ca+ leak channels
long-term response = through 2nd messenger pathway, changing the immediate early gene expression/synthesis of related neurons
74
how does dopamine work
* different groups of neurons (neuronal ensemble) become active/depolarized via glutamate depending on different types of stimulus
* upon recognition of stimulus, dopamine selectively enables the corresponding neuronal ensemble to ALL become activated
75
what is the significance of dopamine
signals neuronal ensembles related to reward, arousal, attention, memory, sleep regulation & motor movement
76
explain the link between dopamine & motor movement
dopamine participates in direct & indirect pathways of basal ganglia, a part of the brain responsible for motor movement by signalling to the thalamus which then signals to the motor cortex
77
explain how loss of dopaminergic neurons causes PD (non-motor symptoms)
lack of dopamine = unable to help excitation of neuronal ensembles leading to non-motor symptoms:
-autonomic defects, cognitive defects, sleep disorders, depression, dementia in ~30% of patients
78
explain how loss of dopaminergic neurons causes PD (motor symptoms)
lack of dopamine results in inability to initiate more movement via direct pathway, and inability to reduce movement via indirect pathway = leads to motor symptoms:
-tremor (4-6Hz): happens at rest, and disappears during voluntary movement
-rigidity/stiffness of the extremities & neck
-minimal facial expressions
-walking entails short steps, stopped posture, pause of associated movements eg. arm swings
-akinesia = slowness in movement execution (bradykinesia) or lack of spontaneous movement (hypokinesia)
79
what are other disease associated with dopamine
deficit of dopamine causes indecisive personality -to the point that people are unable to initiate body's own movement
excess of dopamine causes: attention deficit disorder/schizophrenia /drug addiction
80
what are the causes of degeneration of dopaminergic neurons
* due to misfolding of a-syn = aggregates into lewy bodies = appear as deposits in dopaminergic neurons
81
explain aS in dopaminergic neurons
* aS is v.abundant, especially in edges of neurons (dendrites)
* aS aggregation has toxic effects: lyse mitochondria, metabolize membrane allowing Ca2+ influx, affects autophagy and degradation of proteins
82
what is the structure of lewy bodies
cross-ß-spine = fibrils form double parallel ß sheet
83
what genetic mutations are associated with lewy body formation
* direct missense mutations in the aS-gene (SNCA) -a dominant mutation which requires only 1 mutated copy to cause the disease
* mutation in genes related to mitochondrial repair -PINK1 (mit protein kinase), GBA (glucocerebrosidase), LRRK2 (leucine rich repeat kinase 2)
* mutation that results in knockout of Parkin -an E3 involved in the destruction of aS (induces accumulation & aggregation of aS)
84
how do lewy bodies spread through the brain
* in a systematic way
* they appear in a predictable order of brain locations, resulting in 6 stages of PD pathology
85
what are the stages of PD pathology
early stages:
-lewy pathology appears first in the enteric nervous system (lines the intestine) and the olfactory bulb
-presence of aS in large intestine may be a biomarker for PD
late stages:
-lewy pathology starts to appear in the brainstem, limbic areas, & cortex as they pass through the dopamine neurons in the brain stem and kill them
-appearance in these brain areas coincides with the onset of motor, psychiatric, and cognitive symptoms
86
what are the 3 treatments for PD
L-DOPA
Cell-replacement therapy
Stem cell therapy
87
explain L-DOPA treatment for PD
* front-line treatment for PD
* inject L-DOPA (dopamine precursor) into patients blood
-can cross the blood-brain barrier
-needs 1 more step to be converted to dopamine by DOPA decarboxylase
-works because dopamine does not require specific synaptic release; the brain only needs to be supplied with a cloud of dopamine
88
what is the limitation of L-DOPA treatment
* effects of L-DOPA arent permanent & over-time, even higher doses will not give the same effect because:
-there is a timing aspect to the release of dopamine: normally, dopamine neurons do not continuously release dopamine
-when patient receives L-DOPA regularly, cell will downregulate the dopamine receptors so patient becomes desensitized to L-DOPA & will need increasing amounts
-theres also other systems in the brain which are not working properly and are beyond dopamine
89
explain cell-replacement therapy for PD
* transplantation of embryonic dopamine neurons from midbrain regions of aborted fetuses directly into the striatum of PD patients
-embryonic dopamine neurons would send their axons out into the brain of the patient and supply dopamine to the brain
-proposed to be better than supplying L-DOPA becuase it is a functional neuron releasing dopamine at the right timing
90
what are the limitations of cell-replacement therapy for PD
-ethical reasons for using fetuses
-contamination by additional cell types = major side-effects of fetal tissue transplantation
-transplanted embryonic neurons can later develop lewy bodies -lewy bodies can spread from pathological tissue to healthy tissue
91
explain stem cell therapy for PD
* expose skin fibroblasts to Yamanaka factors (oct4, klf4, sox2, c-myc) to turn them into iPSCs (self-renewing multipotent progenitors with broad developmental potential)
* expose iPSCs to certain GF at the right timing to induce formation into dopamine neurons
* transplant the induced dopamine neurons into the PD patient
92
what are limitations of stem cell therapy for PD
-stem cells have similar characteristics to cancer cells
-can have mutations initially present in skin fibroblasts
93
introduce prion disease
-happens mostly in the UK in the late 90s
-associated with prion protein
94
explain prion protein
-all our brain cell neurons have a protein transcribed from the PrnP gene, which produces prion protein (PrPc)
-280 aa; membrane-anchored, soluble, consists mostly of a-helix
95
explain the scrapie prion protein
* PrPsc pathological isoform
* protease resistant
* self-amplifying (propagates w/o need of nucleic acids)
-resistant to UV radiation & nucleases which degrade NA in contrast to traditional infectious agents like viruses or bacteria
* infectious -can induce conversion of PrPc to PrPsc
-1st case of protein acting as an infectious agent
-can cross many species barriers
* structure is mostly ß-sheet (similar to amyloid plaques/tau aggregates/aS lewy bodies)
96
how did prion disease start
-started in sheeps & goats (scrapie)
-then observed in cows (bovine spongiform encephalopathy -BSE)
-affected people who ate from infected cows (creutzfeld jakob disease)
-causes mass neuronal death
97
what led to the "protein-only" prion hypothesis
the discovery of infectious prion protein and identification of unifying causes in neurodegenerative diseases
98
explain the prion hypothesis
-views proteins as infectious agents
-hypothesizes that neurodegenerative diseases involve prion-like proteins (proteins that acquire alternative conformations that become self-propagating and are able to induce pathological forms in other normal proteins)
-the alternative conformations are usually misfolded aggregates rich in ß-sheet structures
-cause of cell-to-cell spread of misfolded proteins
99
what is the confirmation of protein only
-in diseases patient sample, can detect prion proteins using antibody but no NA detected
-disease can be inherited due to mutations in prion protein gene
-transgenic mice with mutation develops prion disease
100
how are prions formed
-prions can form via increase in ß-sheet content in WT precursor
-prions are most toxic as oligomers, and less toxic after polymerization into amyloid fibrils; depending on the protein, the amyloid fibrils aggregate into Aß plaque, Tau tangle, or aS lewy body
-pathological form of prions can then act as a template to promote misfolding of other WT precursors in the abnormal conformation
101
what is the hypothesis of how prions spread form cell to cell
-move across the synapse via exocytosis, released in vesicles called exosomes, or exported via nanotube
102
example of prions
hyperP Tau accumulates as aggregates (tauopathies)
aS accumulate as aggregates (synucleinopathies)
103
relation between different diseases and prions
-different diseases have diff misfolded conformations of prions or different strains of prions
-can figure out what disease a person has by looking at the structure of protein deposits
104
AD & prions
-caused by hyperP Tau that starts in the hippocampus and spreads to other parts of the brain & cortex
-causes massive neuronal death since it kills every cell that it spreads into
105
Picks & prions
-caused by another structure of hyperP tau that starts in the front of the brain & spread throughout the brain
106
PD & prions
-caused by aS aggregates in form of lewy bodies
-starts from brain stems and spread into the brain cortex
-restricts brain circuits from working properly, but only dopaminergic neurons die
107
what is the evidence of prion hypothesis
1. inject synthetic Aß into a healthy mouse -observe the spread of amyloid particles over time
2. inject synthetic lewy bodies into certain parts of mice brain (LB made by agitating aS in a salt solution for a few days & sonicating it to make fibrils which then form artificial LB in ß-sheet structure)
-over time, spread thorugh different parts of the brain
-after 0.5 year, enters dopamine neurons and destroys them (identified by staining tyrosine hydroxylase)
3. growth hormones extracted from pituitary gland of cadavers were given to children w/GH deficiency
-later in life, they developed AD or prion disease
-found that because hormones extracted contained prions and Aß plaques -incubation time of 10 years to later cause disease
108
what are some therapies based on prion hypothesis
they target the diff stages of prion formation
-eliminate sources of exogenous seed
-oligomeric seed removal
-blocking cell-to-cell spread
-preventing seed elongation/polymer fragmentation
109
how do prions cause neuronal death
* when prions or misfolded proteins are detected in the ER, it triggers UPR (unfolded protein response) -a complex signalling pathways which consists of 3 branches
-occurs in order to restore ER function and cell survival
110
explain the PERK branch of the UPR
1. binding of immunoglobin protein (BiP), a chaperone protein, to misfolded proteins
2. leads to activation of PERK, an RTK
3. PERK then P elF2@
4. elF2@ now acts as inhibitor of elF2ß -a GEF required to replenish the active GTP-bound form of elF2 complex for a new round of translational initiation
-P elF2@ blocks the GDP-GTP exchange reaction and reduces the dissociation rate of elF2 from elF2B
5. results in reduced translational initiation, thus protein synthesis, and neurodegeneration (cell death)
111
what is the evidence of UPR
proteins associated with UPR activation are seen in association with protein aggregates in specific brain regions of patients with neurodegenerative diseases
112
what are therapies targeting the UPR pathways
design drugs that:
-inhibit PERK P
-prevent elF2a P
-prevent elF2a binding to elF2B (trazodone)
these drugs aim to mitigate the toxic effects of misfolded proteins & reduce the progression of neurodegenerative diseases
