Dendrite branching Flashcards
In developing drosophila, how many of class I-IV sensory neurons are there in each abdominal segment?
3 class I
4 class II
5 class III
3 class IV
Classes (I-IV) of sensory neurons in developing drosophila have different functions, what are these possible functions? (3)
- Proprioception
- Sensing muscle contractions
- Nociception
Why is dendrite branching in drosophila a useful model system?
All the neurons in drosophila have been well-characterized, and they have a very stereotypical fomation
During development, what qualities of drosophila dendrites are stereotypical? (2)
Position and shape
Each class of drosophila sensory neurons (class I - IV) have a distinct…
Dendritic morphology
Class I-IV neurons of the same class have (overlapping/distinct) dendritic fields
Distinct
Why do neurons have distinct dendritic fields?
This simplifies processing: if dendritic fields overlapped too much this would overcomplicate the system
What are fundamental rules that are commonly adhered to during dendrite development
- dendrites within the same neuron spread out by avoiding each other (self-avoidance)
- dendrites of a certain type avoid dendrites of the same type, while dendrites of different types can overlap
What is tiling with respect to dendritic fields?
The distinction between neighbouring dendritic fields between neruons of the same type
Why is coexistence of neurons of difference types necessary?
Need the ability for a given tissue to detect a variety of stimuli
Describe why the leech system was used to study colonization of dendritic fields following ablation
Leech have nicely characterized neurons that have two distinct branches with “mirror-esque” morphology, ablation of one side would demonstrate nicely what happens after
When one part of a dendritic arbor is ablated, does the remaining dendrite react? How?
Ablation of an axon of a minor field allows axons from the major territory to spread into the minor field territory
Starburst amacrine cells are responsible for this property of vision
Directional selectivity
How many kinds of amacrine cells are there?
Upwards of 20 types
Define directional selectivity
Stimuli moving across a receptive field in a particular direction will elicit a neuronal response, but movement in the inverse (negative vector) direction will not elicit the same response
Describe isoneuronal interaction
Interaction among self dendrites
Describe heteroneuronal interaction
Interaction between neighbouring dendrites
What is the key mechanism starburst amacrine cells use to prevent dendrites of the same cell overlapping each other?
Protocadherin recognition
Protocadherins are expressed (2) in single neurons
Stochastically (randomly) and combinatorially
Protocadherins which mediate self-self interactions among dendrites exhibit…
Isoform-specific homophilic adhesion
Do protocadherins always mediate repulsive interactions?
No, sometimes they mediate attractive interactions, this just depends on the cadherin and the context
What is happening in the bottom panel when a specific protocadherin is being knocked out?
See that there is much more overlapping in dendrites, there has been a loss of self-repulsive interactions
What region of the protocadherin protein is being knocked out here?
The variable extracellular domain – all protocadherins “same” now
What is the takeaway of this image?
When these specific isoforms are knocked out, there is no dramatic change in phenotype
No single isoform is necessary for self-avoidance
What is the takeaway of this image?
When only one isoform of protocadherin is being expressed, get extreme self avoidance
Any isoform is sufficient for dendrite self-avoidance
Describe how the bar code hypothesis pertains to self-avoidance and protocadherins in the CNS
Allows interaction of non-self dendrites and provides high specificity for self-avoidance
Bar code - distinct code which allows neurons to recognize themselves/their neighbours
The bar code hypothesis informs how neurons (2)
Self avoid
Form synaptic targets and circuits
Neuron self-avoidance is mediated by these kinds of molecules in drosophila
DsCams
DsCams are hypothesized to mediate these kinds of interactions in drosophila
Homotypic interactions
In drosophila, retinal ON and OFF ganglion cells exhibit what kind of pattern with regards to overlap
Tiling: on and off overlap with each other, but they exhibit tiling within their own types
What does the Math5 mutant do to ganglion cells?
Results in an individual having only about 5% of the normal amount of RGCs – death in 95%
How are dendritic fields affected by the Math5 mutant?
They are not
What does this image tell us about the timing of dendritic repulsion interactions?
May be an end-stage fine-tuning mechanism
End stage because they develop their fields independent of interaction from other dendrites – interaction later might cause repulsion (fine-tuning these established fields)
Based on the data from this image, is dendritic field formation a cell autonomous or cell non-autonomous process?
Cell-autonomous: fields develop without input from other cells
What mechanisms regulate dendritic branching? (6)
- Transcription factor signaling
- Secreted proteins/receptor pathways (morphogens, axon guidance molecules)
- Cytoskeletal regulators
- Secretory pathway proteins (think vesicles)
- Cell adhesion molecules
- RNA targeting and local translation
How many neurons does C. elegans have?
302
How many somatic cells does C. elegans have?
959
Describe the central “nerve hub” of C. elegans
Has a nerve ring (NR) at its anterior end
Describe the hypodermis of C. elegans
Kind of like a germ layer, envelops the worm and all the worm’s sensory neurons are embedded within this region
Why is PVD a good model neuron to study?
There are only 2 of them and their branching is very stereotypical
Describe the role of PVD
Involved in detection of noxious stimuli: specifically harsh touch (mechanosens) and noxious cold
It is common for a lot of mechanosensory neurons in C. elegans to have this property
Multisensory
The stereotypical branching pattern of PVD is in a “(1)”-like shape
Menorah
How many levels of branching do PVD neurons undergo?
5: 1o, 2o… etc
Harsh touch on the quaternary branches of PVD dendrites causes what channels to open?
Mechanically-gated epithelial sodium channels: DEG/ENaC on the primary and tertiary branches
Do quaternary PVD dendrites contain ion channels
Idk maybe but they don’t have ENaC channels which are they only ones we care about in the context of the discussion paper
Describe the opening of ENaC channels on 3o and 1o PVD dendrites
Stretch opens them allowing the passage of ion channels
At what stage during C. elegans development do the PVD neurons appear?
The 2nd larval stage
