Olfactory mapping Flashcards
Where is olfactory information detected?
What receptors?
In the olfactory epithelium
By olfactory receptors
How many different olfactory receptors are there?
How many olfactory receptors does on neuron express?
1000s of different OR
One neuron expresses ONE TYPE of olfactory receptor
How are receptors organised in the nasal epithelium vs the olfactory bulb?
How does this change occur?
NE - dispersed
OB - organised
Occurs because neurons expressing one type of receptor go the the SAME glomerulus in the olfactory bulb
How is the mapping of the OE to the OB different to retinotectal mapping?
OE –> OB maintains DISCRETE information
RT - maintains TOPOLOGICAL information
How can the convergence of neurons to the OB be visualised?
Using Lac Z (reporter gene)
How use Lac Z to visualise convergence?
Why do it like this?
Use internal ribosome entry site:
- Express more than one gene on a single mRNA transcript
- Olfactory receptor and reporter share the SAMe RNA
So that the olfactory receptor ISN’T KO - can see the NORMAL BEHAVIOUR of the olfactory receptor
How are glomeruli arranged in the olfactory bulb?
Glomeruli responding to certain odours are CLUSTERED to particular part of the OB
What governs the guidance of olfactory neurons to the olfactory bulb?
How is this seen?
The receptor expressed (coding region)
Seen through receptor swap experiments
What is the structure of an olfactory receptor?
7 transmembrane pass GPCR-like molecules
What does OR activity determine?
How?
Guidance response state:
NO activation of OR (no ligand bound) –> first stage of guidance (to the olfactory bulb)
ACTIVATION of the OR (ligand bound) –> second stage of guidance (sorted into glomeruli)
How does early guidance TO the OB occur?
How are the axons sorted?
Activity-INDEPENDANT (no binding of ligand and no action potentials)
Sorted due to type I cue/receptor interactions
Describe the activity in of the OR in the ABSENCE of a ligand?
What causes this activity?
Each receptor - CHARACTERISTIC basal signalling activity
Caused by:
- Activation of adenylate cyclase and normal G protein activation and a specific level of cAMP
What determines the mapping to the olfactory bulb?
- Specific levels of cAMP generated through adenylate cyclase activation
- Via CREB, cAMP levels determine the level of type I molecules
- This results in CHARACTERISTIC type I protein LEVEL being associated with the expression of a particular OR
(eg. when cAMP is low, Sema expression is high and Nrp level is low)
(eg. when cAMP is high, Sema expression is low and Nrp level is high)
- -> High level of receptor = repelled further away from axons expressing high level of repellant
- LEVEL of type I molecule transcript in the axon forms a gradient on the way to the olfactory bulb which then drives a gradient in the OB
(Direct relationship between the level of transcript in the axon and the position along the DV axis in the OB
- MAP emerges in the axons BEFORE they reach the OB (sort depending on the levels of type I molecule –> determines LEVELS of type I cue/receptor interactions)
- LEVELS of type I cue/receptor determined by INITIAL RECEPTOR expressed –> determines point in gradient and therefore DV axis
What is CREB?
How does it activate different levels type I molecules?
cAMP response element binding protein (TF)
Activates different levels of type I molecules, dependant on the levels of cAMP
- In a GRADED way
What are type I molecules?
Examples?
Familiar guidance cues and their receptors
Robo/Slit
Neuropilin/Sema
What happens to mapping in the OB when guidance cue expression is disrupted?
Disrupts mapping
What happens to cue expression in the axons going to the OB over time?
Why?
Cue expression SWITCHES:
- From Robo/Slit
- To Nrp/Sema
So that EARLY entering axons can guide later entering axons
How are neurons sorted into the glomeruli?
ACTIVITY-DEPENDANT (action potentials):
- Action potentials drive HIGHER cAMP LEVELS –> turns on expression of type II cues
- Neurons expressing SAME adhesion molecule –> stick together and are REPELLED from ephs and ephrins
What happens to sorting into the glomeruli when block electrical activity?
Blocked
How can type II signals be used to sort neurons into the glomeruli?
In many different COMBINATIONS
What are type II molecules?
HOMOPHILLIC ADHESION MOLECULES (Kirrels and contactins (TAG1))
or
MUTUAL REPELLANTS (ephs and ephrins)
What is the ‘early’ sorting?
‘Late’ sorting?
Early:
- Activity-independant
- Pre-target sorting
- Using Type I molecules
Late:
- Activity-dependant
- Glomerulus sorting
- Using Type II molecules
Where do olfactory signals go after the olfactory bulb?
Relayed to HIGHER brain centres:
- Piriform cortex
- Amygdala
- Olfactory tubercle
How are the projections from the OB to the piriform cortex different to projections from superior colliculus to LGN?
OB --> PC = NO spatial organisation Mosaic organisation (individual odorants activate subpopulations of neurons DISTRIBUTED across the PC)
What cells output from the OB?
Mitral cells
What odorants do PC neurons respond to?
MULTIPLE, DISSIMILAR odorants
Describe the distribution of information from nasal epithelium to PC
Nasal epithelium - DISPERSED set of neurons activated
OB - Specific ensemble
PC - DISPERSED set of neurons
How does the brain know which odorant is which in a chaotic situation in the PC?
Describe this
ASSOCIATIVE LEARNING:
- Odours drive behaviour AFTER LEARNING
- Significance of odours is learnt by ASSOCIATION
How was it tested if the PC was the site of olfactory learning?
OPTOGENETICS:
- Introduce ‘channelrhodopsin’ (ChR2) into subset of PC neurons using tissue specific promoters
- Stimulate ChR2 with light - activates PC without any input from mitral cells
- Stimulate ChR+ subset of neurons with light and PAIR with either AVERSIVE of APPETITIVE stimulus in naive animals
- Then, test is light alone can elicit the appropriate behavioural response
What is ChR2?
Light-activated CATION channel - produces action potentials under light
What are ‘niave’ animals?
Animals that are unconditioned
What are the 3 ways that ChR2 can be introduced into PC neurons?
1) Use SYNAPSIN PROMOTER injected into the cortex
2) Infect floxed ChR2 into mouse in which cre driven from Emx1 promoter (excitatory neuron-restricted)
3) Infect floxed ChR2 at the same time as the viruse containing synapsin driving cre
Describe ‘using SYNAPSIN PROMOTER injected into the cortex’ to introduce Chr2 into PC neurons
Synapsin expressed in ALL neurons
Hits 50% of cells at the injection site
‘Infect floxed ChR2 into mouse in which cre driven from Emx1 promoter’?
Process?
Hits 50% of cells but ONLY in EXCITATORY NEURONS
Cre lox sites used
Lox sites FACE each other in ‘flip’ orientation
Cre INVERTS the gene
‘Infect floxed ChR2 at the same time as the viruse containing synapsin driving cre’
Work in ALL neurons
Much lower Chr2 expression rate (10%)
How can ChR2 activation condition AVERSIVE behaviours? (experiment)
Photostimulation of ChR2-expressing neurons in PC (C stimulus) paired with FOOT-SHOCK (UC stimulus) on ONE side of the chamber
Animals then exhibited behaviour with PS alone
What is an ‘unconditioned stimulus’?
A stimulus that naturally and normally evokes a response
What is a ‘conditioned stimulus’?
Stimulus that is paired with an unconditioned stimulus, neutral and doesn’t normally provoke a response
When using ChR2 activation to conditions aversive behaviours, when is the only time PS with elicit the response alone?
ONLY when ChR2 was present in the PC and a MINIMUM of 200 neurons transfected with ChR2
What can conditioning odorants and PS together cause?
EITHER PS or ODORANTS can elicit flight
How can ChR2 activation condition APPETITIVE behaviours? (experiment)
Mice trained to take water in response to odorant:
- Can be paired with PS or not
Male mice can be trained to associate presence of a female with either PS or odour
How are the PC neurons plastic in their associative capacity?
The SAME set of ChR2-expressing PC neurons (in the same mouse) can be RETRAINED in either direction
DIFFERENT sets of ChR2-expressing neurons can be trained to have DIFFERENT behaviours
Do the previous experiments prove that PC is the site of odorant learning?
What DO the experiments show?
NO, it just shows that PC can be used for associative learning (can happen here, but doesn’t mean it IS the site)
Shows that PC is very plastic - ANY group of 200 neurons can be used to elicit REVERSIBLE, DIVERSE behavioural responses
Why is the PC plastic, whereas other regions of the cortex (eg. somatosensory) isnt?
Because the PC doesn’t have any spatial mapping, whereas the somatosensory cortex does (has a SPECIFIC behavioural out put according to location)
What are random connections from the OB into the PC used for?
To ASSOCIATE odours with PARTICULAR EXPERIENCES (learned odours)
How can it be tested if the PC is the site of associative learning?
LESION the PC and see if it works (get associative learning?)
What are innate responses?
How are they triggered?
Inborn responses - happen without seeing a stimulus
Specific odours (eg. fox smell)
Where are innate odours mapped to?
The AMYGDALA from the olfactory bulb
Describe the mapping from the OB to the amygdala
Why?
Mapping is BIAST to particular regions from the OB
SPATIAL mapping to get FEAR response
Can defects in axon guidance cause neurological disease?
Yes
What is L1CAM?
L1 cell adhesion molecule (immunoglobulin-like) related to TAG-1 and NrCam
What encodes L1CAM?
X-linked gene
How does L1CAM adhese to things?
1) Bind homophillically
2) Bind to other UNRELATED cell adhesion molecules
What are mutations in L1CAM associated with?
How is this known?
A broad spectrum of neurological disease:
- Hydrocephalus
- Mental retardation (MASA syndrome)
- Spastic paraplegia (SPG1)
Known through human mapping studies
How is spastic paraplegia caused?
By faliure of the CSTs to project beyond the cervical regions
What is L1CAM mutations characterised by?
How is this seen?
Defects in axon pathfinding at the pyramidal decussation of the CST
Seen by: KO of L1CAM and following the axons using a tracer
What is HGPPS?
What are the symptoms?
Horizontal Gaze Palzy with Progressive Scoliosis:
- Monogenic disease
Symptoms:
- Spinal twisting
- Inability to control coordination between the 2 eyes (inability to control motor systems innervating the eye)
What controls the motor systems that innervate the eye?
Pons and inferior olive
What mutation causes HGPPS?
- Mutations in Robo3 which normally inhibits robo1 as commisural axons cross the midline
- No commissurless at the floor plate
What commissures does HGPPS affect?
Several commissures (not just in the spinal cord)
Crossed fibre tracts in the pons that coordinate eye movement
What do conditional KOs in HGPPS do?Why?
What does this show?
Mimics different aspects of the disease due to KO of different commissures
Shows coordination between different motor systems are disrupted
What is the cause of scoliosis?
Not understood
What is Kallmann’s syndrome?
Congenital anosmia (inability to smell) and hypogonadism (infertility)
What causes Kallmann’s syndrome?
Mutations in several different genes:
- Adhesion molecules (KAL1)
- FGFR1 and FGF8
- Sema 3A and Sema7A
Causes:
- GnRH (gonadotropin releasing hormone) neurons fail to migrate to the hypothalamus from the VNO (The vomeronasal organ - in the olfactory system)
- Inability to regulate the development of the gonads to develop properly
Why do mutations in Sema3A cause Killmann’s syndrome?
Sema3A is required for the olfactory epithelial axon guidance to the OB
- NORMALLY, GnRH neurons migrate along these axons to the hypothalamus - SO, absence of OE axon guidance to the OB –> GnRH migrate to the wrong place
Why do mutations in Sema7A cause Killmann’s syndrome?
Sema7A is required for GnRH neuron migration:
- In absence, GnRH neurons FALL OFF OB axons projecting into the brain
What is unusual about Sema7A use in GnRH migration?
How?
Acts as an attractant
Repellants can act as attractants in certain situations (PREVIOUS LECTURE)
Describe the genetics of ASD, SZ, bipolar and depression
- CLEAR genetic components (siblings likely to have the same condition)
- COMPLEX genetics (NOT monogenic)
- De novo copy number variations, mutations in multiple different genes
- Lots of genes with low penetrance add up OR rare disease states add up
- HUNDREDS of genes implicated
- Overlap in the genetics of each disease type (many genes involved in one disease are involved in another)
Describe the genetic risk factors for ASD, SZ, bipolar and depression
MANY genetic risk factors, EACH contributing to <1% risk
What are examples of genes mutated in ASD, SZ, bipolar and depression?
Involved in:
- Axon guidance
- Synapse formation
- Wnt signalling and polarity
- Cell migration
What is genetic redundancy?
What does it allow?
Biochemical function is encoded by 2 or more genes - compensation if there is a mutation
- Allows the systems to function by pathway substitution
- Allows to carry a lot of genetic damage/variation
What may constitute to the wide variety of human ‘characters’ in the normal population?
The ability to survive well with many genetic and neurodevelopmental disturbances due to genetic redundancy
What does variation between individuals mean (in regards to environmental insult)
Every individual is DIFFERENTIALLY vulnerable to environmental insult
What are CST disruptions partly compensated by?
The RST (rubro-spinal tract)
