CSHL Meeting Session VI

24 03 2007

Novel Methods to Dissect Neural Circuits – Saturday afternoon

Dmitri Chklovskii, Janelia Farm

Reconstruction of neuronal wiring diagram from automated serial EM. Must be able to track identity of segments between slices, determine synapses and the cells they belong to. Wiring diagram draft was done in c. elegans (~7000 synapses, 279 neurons) in 1986, Mitya’s student finished it in 2006.

How do we do it? Automated alignment of serial sections by translation, slight rotation and elastic stretching. Automated segmentation of color coding, makes a draft that must be reviewed by human editor. State of art is 10x faster than manual tracing, reconstructed complete 10x10x10um^3 volume two man-months. 1000 synapses, 1000 axons, 100 dendrites.

Biological results : If there are an equal # of spines and axons neighboring a single segment of dendrite, no significant wiring rearrangement possible. But connectivity fraction is actually 0.1-0.3 so plenty of room for structural plasticity. For optimal info storage, there should be equal volume of axons and dendrites, which is shown to be true. Axons appear to be concentrated near other axons, dendrites far from other dendrites, but this actually fits random packing of processes.

Questions : Can you see systematic slicing errors from alignment?
A: Only errors from people walking by.
Q: How much shrinkage do you see from fixation?
A: Significant uniform volume shrinkage, but not worried about that. Loss of extracellular space, may effect shape of processes.

Jeff Lichtman, Harvard
Connectomics : Brief definition – Map neural circuits.
Naturally occurring synapse elimination in the developing brain.
Three changes in synaptic connectivity:
1. Decreased axon connectivity
a. Imaging NMJ, decreased convergence with compensatory synaptic takeover by the remaining input
b. Non-monotonic process, appears to be competition
c. Axons are branches, other branches of same axon innervates other targets, these effect the competition
2. Decreased axonal divergance
a. E18 – 80% NMJ innervation. P13 – 4.2% innervation
3. “Synchronization” or rewiring process
a. When two axons compete on multiple terminals, same axon loses in both
b. Is there a deeper hierarchical structure?
Each outcome of synapse elimination causes unique pattern of synapse innervation in each axon.
Automated Tape-collecting lathe ultramicrotome (ATLUM). Grad student made a homemade one with 15uM thickness. Now building 50nM thickness with $200K McKnight. http://www.extremeneuroanatomy.com

Clay Reid, Harvard Med – New tools for imaging the functional anatomy of the visual system.
Originally an electophysiologist only, mapping functional connectivity with electrodes. Now doing functional imaging.
Calcium imaging of the visual cortex. Bulk loading of calcium indications in cerebral cortex, look at 300uM cubes. What is the function of each of these cells? Excite with visual stimuli of anesthetized animal, 2p imaging of 2/3 rat visual cortex. Find orientation selectivity without clustering : salt and pepper. No apparent functional microorginaization. However, in the cat, similar neurons types (horizantal, vertical) cluster together with sharp cutoff between cells in orientation pinwheels. How do they do this?

Are functionally similar groups of cells:
Spontaneously co-active?
Correlated with cell type?
Anatomically/functionally connected?
What is the wiring diagram?
Tracing individual connections with viruses
Tracing many/all connections with serial electron microscopy
Use conventional sectioning and imaging with high throughput camera array.
Large volumes up to 500uM cubes at 5nm x-y resolution
Large datasets of 10-100 terabytes
Record everything but analyze only a bit, a very relevant bit
Showing preliminary data of automated serial em collection and analysis

Andre Fiala – Optophysiological techniques for the dissection of neuronal circuits underlying learning and memory in Drosophila
[Great talk content, but my notes are poor.]
In vivo monitoring of neural activity
Glue fly under coverslip. 1p DualView with Cameleon expressed in dopaminergic neurons, which have extensive innervation throughout brain. Following 8 training sessions, dopamine neurons show prolonged activity that persists thru conditioned stimulus, suggesting predictive abilities.
Expresses ChR2 in fly larva and can control contraction on larva with light. Can substitute light stimulation in octopamine neurons for appetitive odor stimulus in learning paradigm. Substitute ChR2 dopamine light stimulation for aversive stimulus. Express ChR2 in gustatory neurons, flash light, proboscis extends. “The light tastes sweet.”

Tamily Weissman, Harvard – Mapping neural circuitry in the cerebellum using multicolor fluorescent “Brainbow” mice
Gain neuronal identity in labeling by using combinations of fluorescent proteins “Technicolor Golgi stain”.
Thy-1 promoter-L1-L2-RFP-L1-mYFP-L2-mCFP with incompatible Lox sites. PreCre get RFP, Post Cre get YFP or CFP. Since multiple copies per cell, get blends of colors. [I doubt there is any FRET since they are using monomeric (A206K) mutants of C/YFP.] How many colors? Hard to say, conservative estimate for 100% confidence by eye is 78 colors eye can descriminate. [Why limit by eye? What is the limit using spectral deconvolution?] Limiting 20% mossy fiber, 5% granual cell and can do total reconstruction of this fairly dense labeling. Appears there is some convergence in circuit of mossy fibers onto granual cells by looking at ratio of filled terminals. Granual cells sometimes innervate same presynaptic mossy fiber at two distinct terminals on different dendrites.

Wei Chen – In vivo two photon imaging of firing and wiring of local neuronal circuits.
[Speaker is the lead author on the in vivo electroporation paper we recently covered, see the paper for more details.] Understanding the brain depends on sparse labeling of neurons. Konnerth, Reid using bulk loading, but this obscures fine neuronal structures. Tried bulk loading, G-CaMP2 mouse, now trying local electroporation. Following electroporation, only very small change in field recording. Hey but aren’t only a small proportion of the neurons electroporated? Hmm….

Ian Wickersham, Salk – Transcomplemented transsynaptic tracing : mediation by helper viruses
[I was planning on covering this work in the recent publication in Neuron, but will just do it here.] How do we determine what cell is monosynaptically connected to other cell types? Classic transsynaptic tracers pass at different rates due to connection strength and can move through strong polysynaptic connection steps. Enter transcomplemented tracing.

Component 1 – Deletion mutant tracing virus
Component 2 – Complement of the deletion, activates virus.

Rabies virus, RNA virus (can’t use Cre recombinase)
Replace the glycoprotein of rabies virus with GFP. Virus can replicate core but cannot cross membrane. Pseudotype virus with coat glycoprotein to avian ASLV’s membrane protein. Express gene of ASLV receptor, dsRed and native virus coat protein complementation gene in single neuron in the brain. Then pseudotyped virus infects that single cell and can cross 1 step. But, since complementation gene only exists in single cell, virus stops crossing after 1 step.

Day 1 : shoot in triple gene coated particles with genegun
Day 2 : Apply pseudotyped rabies virus
Get 1 red cell, and many sparse green cells that are monosynaptically connected.

Aravinthan Samuel, Harvard – Brain and behavior in freely moving worms
Thermotaxis exhibits long-term plasticity. Thermosensation occurs at tip of nose. Side to side wiggles and net forward movements could contribute to perception of thermogradients. Express cameleon in AFD neuron using cell-specific promoters.

Worm wants 2 pieces of info:
Is temp higher than it likes?
Is temp rising or falling?

Immobilized worm subjected to defined thermosensory inputs. Increasing T in a linear rate with wiggle induces a phased locked ratio change to the wiggle that starts above about 18C. Getting 150% dR with YC3.60 in response to wiggles. Ratio in AFD is directly correlated to T in tail fixed worms moving head around on a temp gradient. Turning off gradient kills correlation, reversing grad reverses side correlation.
AFD detects the temp variations driven by self-movement in a spatial gradient.


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