Cell Cycle Visualization in Development

13 03 2010

Atsushi Miyawaki’s lab has developed a series of neat tools for visualizing cell cycle progress.

For zebrafish, the zFucci system consists of two fluorescent proteins, mKO2 and mAG, that are fused to Cdt1 and geminin genes.  Cell cycle- regulated proteolysis of these fusion proteins causes each cell to display orange fluorescence in G1 phase nuclei and green fluorescence in both the nucleus and cytoplasm of S/G2/M phase cells.

Video of cell cycle transitions in culture. Click for the video.

The last time I saw Atsushi give a talk, he showed an incredible time lapse video from the zebrafish cleavage stage that I haven’t been able to find online.  However, here is a video from later in development of the zebrafish that is still pretty remarkable.

Development of a zebrafish visualized by zFucci. Click to see the video.

This two component system has been adapted for watching the transition from neural stem cells to differentiated neurons in living mice. The Color-Timer system uses double transgenics with the fluorescent protein KOr fused to nestin and EGFP fused to doublecortin.  In this system, neural stem cells fluoresce orange, while newly differentiated neurons fluoresce green.

The cerebral cortex of an E14.5 double Tg mouse embryo of nestin/KOr was time-lapse imaged. Click for video

Sugiyama, M., Sakaue-Sawano, A., Iimura, T., Fukami, K., Kitaguchi, T., Kawakami, K., Okamoto, H., Higashijima, S., & Miyawaki, A. (2009). Illuminating cell-cycle progression in the developing zebrafish embryo Proceedings of the National Academy of Sciences, 106 (49), 20812-20817 DOI: 10.1073/pnas.0906464106

Kanki, H., Shimabukuro, M., Miyawaki, A., & Okano, H. (2010). “Color Timer” mice: visualization of neuronal differentiation with fluorescent proteins Molecular Brain, 3 (1) DOI: 10.1186/1756-6606-3-5

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CNiFERS of Acetylcholine and Attention

10 03 2010

“If you find yourself needing to reread this paragraph, perhaps it’s not that well written. Or it may be that you are low on acetylcholine.” Acetylcholine (ACh) is a major modulator of brain activity in vivo and its release strongly influences attention. If we could visualize when and where ACh is released, we could more fully understand the large trial to trial variance found in many in vivo recordings of spike activity, and perhaps correlate that to attentional and behavioral states mediated by ACh transmission.

Back in grad school, when I was desperately trying to figure out what biological question to answer with my GluSnFR glutamate sensor, I ended up in a meeting with Kleinfeld, his grad student Lee Schroder and Palmer Taylor. We plotted a strategy to make a FRET sensor for acetylcholine.  Palmer had recently solved crystal structures of an acetylcholine binding protein bound to agonists and antagonists.  Snails secrete this binding protein into their ACh synapses to modulate their potency.  The structures showed a conformational change upon agonist binding.  The hope was that by fusing CFP and YFP to the most translocated bits of the protein, they would be able to see an ACh dependent FRET change.  I was skeptical that it would work, as the translocation was much less than with calmodulin-M13 or periplasmic binding proteins used in Cameleon and GluSnFR, but thought was at least worth a shot.  FRET efficiency is highly dependent on dipole orientation, not just dipole distance, and you never know how a small conformational change might rearrange the FP dipoles…

Of course, the simple idea didn’t work.  Instead of giving up on the first dozen attempts, they kept plugging away at alternative strategies for measuring ACh release, and eventually succeeded.  In this Nature Neuroscience report, An in vivo biosensor for neurotransmitter release and in situ receptor activity, Nguyen et al demonstrate a mammalian cell based system for optically measuring ACh levels in an intact brain.  They coexpressed M1 muscarinic receptors with the genetically-encoded calcium indicator TN-XXL in HEK293 cells.  ACh binding to the M1 receptor induced IP3-mediated calcium influx.  This calcium rise was then picked up by the TN-XXL and reported as a change in CFP/YFP fluorescence.  The crazy part is that they took this cell culture assay and implanted the cells into the brains of living rats!

The CNiFER in vivo experimental paradigm

In culture, the response was highly sensitive and monotonic (for phasic response section, EC50 of 11 nM, a Hill coefficient of 1.9 and a maximum of ΔR/R = 1.1). In vivo, using two-photon imaging through a cortical window, they were able to see clear ACh responses in frontal cortex from electrical stimulation of the nucleus basalis magnocellularis, typically 200-μs current pulses of 200 μA @ 100Hz for 20-500ms.

This was essentially a in vivo proof of principal experiment, showing that one could image ACh release in spatially and temporally precise regions of the brain.  However, the imaging was done under urethane anesthesia, which is a much different brain state than an awake, behaving animal.  Are CNiFERs sensitive, powerful and stable enough to determine behavioral states via imaging in an awake animal?  Would expressing GCaMP3 (an indicator with greater fluorescence dynamic range) improve the performance of the CNiFER system? We used a very similar assay with ACh applied to HEK cells during the initial screens for better GCaMPs. Or, is the performance more limited by the properties of the M1 receptor and the adapting nature of IP3-mediated calcium dynamics?  CNiFERS provide an interesting platform for looking at ACh and potentially other G-protein mediated signaling, but it remains to be seen if labs that aren’t as technically proficient with two-photon rig will find it more useful than cyclic voltammetry for measuring acetylcholine levels.

Nature Neuroscience, 13 (1), 127-132 DOI: 10.1038/nn.2469ResearchBlogging.org
Nguyen, Q., Schroeder, L., Mank, M., Muller, A., Taylor, P., Griesbeck, O., & Kleinfeld, D. (2009). An in vivo biosensor for neurotransmitter release and in situ receptor activity





Three Cheers for GCaMP : Optogenetic Brain Reading

9 11 2009

Three papers are out online in Nature Methods that show big improvements in calcium imaging with genetically encoded sensors.  They are are based on the fluorescence intensity indicator, GCaMP.   GCaMP, first developed by Junichi Nakai, consists of a GFP that has been circularly permuted so that the N and C termini are fused and new termini are made in the middle of the protein.  Fused to one terminus is calmodulin and the other is a peptide, M13, that calmodulin (CaM) binds to in the presence of calcium. The name is supposed to look like GFP with a CaM inserted into it, G-CaM-P.  Normally the GFP is dim, as there is a hole from the outside of its barrel into the chromophore.  Upon binding calcium, this hole is plugged and fluorescence increases.

Crystal structure of GCaMP2

The first paper, A genetically encoded reporter of synaptic activity in vivo, from Leon Lagnado’s group, targets GCaMP2 to the outer surface of synaptic vesicles. This localization allows the fluorescence signal to be confined to the presynaptic terminal, where calcium fluxes in response to action potentials are high.  This targeting improves the response magnitude of GCaMP2 and permits the optical recording of synaptic inputs into whatever region of the brain one looks at.  They demonstrate the technique in live zebrafish.

In the second paper, Optical interrogation of neural circuits in Caenorhabditis elegans, from Sharad Ramanathan’s group, GCaMP2 has been combined with Channelrhodopsin-2 to perform functional circuit mapping in the worm.   Since the worm’s structural wiring diagram has been essentially solved, functional data could say much about how “thick” the wires between each cell are.  Unfortunately, with GCaMP2, the responses are too slow and weak to distinguish direct from indirect connections.

Finally, we have published a paper, Imaging neural activity in worms, flies and mice with improved GCaMP calcium indicators, describing the improved GCaMP3.  This indicator has between 2-10x better signal to noise than GCaMP2, D3cpv and TN-XXL, depending on the system you are using.  It’s kinetics are faster and it is more photostable than FRET indicators, and the responses are huge.  When expressed in motor cortex of the mouse, neuronal activity is easily seen directly in the raw data.  Furthermore, the sensor can be expressed stably for months, making it a potential tool for observing how learning reshapes the patterns of activity in the cortex.

Screen shot 2009-11-09 at 7.19.27 PM

Imaging of mouse motor cortex (M1) expressing the genetically-encoded calcium indicator GCaMP3 through a cortical window. After 72 days of GCaMP3 expression, large fluorescence transients can be seen in many neurons that are highly correlated with mouse running.

GCaMP3 is not perfect. It cannot reliably detect single action potential in vivo in mammals, though I doubt that any existing GECI can. Work continues on future generations of GCaMP that may achieve 100% fidelity in optical reading of the bits in the brain. However, there is considerable evidence from a number of groups that have been beta-testing the sensor, including the Tank lab of “quake mouse” fame, that it is a significant leap forward and unlocks much of the fantastic and fantasized potential of genetically-encoded calcium indicators.

Screen shot 2009-11-09 at 7.20.12 PM

Comparison of fluorescence changes in response to trains of action potentials in acute cortical slices.

I will try to post a more complete writeup of GCaMP3 for Brain Windows soon, with an unbiased eye to its strengths and weaknesses.  We worked very hard to carefully characterize this sensor’s effects on cellular and circuit properties.  If you have any questions about GCaMP3, please post them to the comments.

For further info about strategies for GECI use and optimization, check out our previous paper, Reporting neural activity with genetically encoded calcium indicators in Brain Cell Biology.

The official press release from HHMI regarding GCaMP3 is available here.





Optogenetic induction of memory recall

18 09 2009

Speaking of reactivating specific memories, at the 2009 Society for Neuroscience meeting, Matteo Rizzi of Michael Häusser’s lab is presenting the realization of an idea that has been floating around in some research proposals I’ve read over the last year.  Express channelrhodopsin-2 under control of the immediate early gene c-fos, induce a strong memory formation via fear conditioning, and then drive the recall of that memory by stimulating the neurons that are expressing ChR2. Immediate early genes are activated shorty after high levels activity in neurons, though the precise patterns are different depending on which promoter (c-fos, Zif268, etc) you use, making precisely HOW they reflect recent neuronal activity patterns unclear.  Nevertheless, the activation of the c-fos based pattern seems close enough to trigger an identical behavioral response as the conditioned stimulus.

Get your ass to Mars!

Not yet, but getting closer...

Electrically-induced fear conditioning is probably the most blunt instrument possible, encoding a very powerful, general ‘fear’ memory, and many things can make a mouse freeze. Thus, this is definitely the low-hanging fruit on the ‘reverse-engineering’ memories tree. Understanding how the information in a memory is distributed across participating neurons is going to take a more sophisticated approach and a lot more work. This result is still incredibly cool, and I’m somewhat surprised it worked by driving ChR2 with c-fos in a hundred cells in the dentate gyrus. That has pretty powerful implications for avenues by which memories can be recalled.  Surely the entire memory is not encoded by only the 100 neurons that were activated! How many other neurons participate, and how does the optical stimulation activate the entire ensemble? Is it even necessary to activate the entire ensemble to drive behavior? The poster will be MOBBED.  I look forward to reading the details.

Program#/Poster#: 388.8/GG103
Title: Memory recall driven by optical stimulation of functionally identified sub-populations of neurons
Location: South Hall A
Presentation Time: Monday, Oct 19, 2009, 10:00 AM -11:00 AM
Authors: *M. RIZZI, K. POWELL, J. HEFENDEHL, A. FERNANDES, M. HAUSSER;
Wolfson Inst. for Biomed. Res., UCL, London, United Kingdom
Abstract: The mammalian brain is capable of storing information in sparse populations of neurons encompassing several brain areas. Immediate recall of this information is possible upon presentation of a cue or context. Most aspects of this process remain unresolved: are the cells involved in information storage also responsible for its recall? What portion of this distributed circuit needs to be reactivated, in order to achieve successful recall? To answer these questions we selectively expressed a genetically encoded optogenetic probe (Boyden et al., 2005) in neurons engaged during the learning of a specific association. A plasmid encoding channelrhodopsin-2 and EGFP under an immediate early gene promoter (c-fos-ChR2-IRES-EGFP) was electroporated in vivo into granule cells (GCs) of the dorsal dentate gyrus of anaesthetized C57BL/6 mice. Mice were allowed to recover, and then underwent classical delay fear conditioning (consisting of 10-20 pairings of a 5 second auditory tone and a 2 second footshock). An optic fiber was implanted intra-cranially to allow optical stimulation of transfected neurons. Light stimulation (λ = 530 nm; 5 Hz) successfully induced recall of the fear memory, measured as freezing behaviour (n = 27 animals). Post-hoc analysis of the transfected tissue revealed that a remarkably small subpopulation of GCs (<~100 cells) was sufficient to cause this effect. We then tested whether any, comparatively sized, subset of GCs could be equally effective. We transfected neurons with a plasmid encoding ChR2 expression under a general promoter (pCAG-ChR2) to obtain ChR2 expression in a random population of cells. Interestingly, optical stimulation of this population was insufficient to induce memory recall (population data: n=30). Our results therefore suggest that recall of a learned association, sparsely stored in neuronal circuits distributed over several brain areas, can be achieved by the simple reactivation of a very small subset of neurons involved in learning this association. Furthermore, our strategy may also be useful for dissecting the complexities associated with memory storage and recall.
Support: Gatsby Charitable Foundation; Wellcome Trust




Light-switchable protein interactions

16 09 2009

A fully genetically-encoded approach to light-activated transcription is getting closer now that a new, generalizable method of light-switchable protein interactions has been published.  In Nature’s advance online publication, Spatiotemporal control of cell signalling using a light-switchable protein interactionAnselm Levskaya of the Voigt lab at UCSF and co-authors demonstrate inducible, reversible control of protein binding, localization and signalling in mammalian cells.  

apo-PhyB covalently binds to the chromophore phycocyanobilin (PCB) to form a light-sensitive holoprotein. PhyB undergoes conformational changes between the Pr and Pfr states catalysed by red and infrared light, reversibly associating with the PIF domain only in the Pfr state. This heterodimerization interaction can be used to translocate a YFP-tagged PIF domain to PhyB tagged by mCherry and localized to the plasma membrane by the C-terminal CAAX motif of Kras.

apo-PhyB covalently binds to the chromophore phycocyanobilin (PCB) to form a light-sensitive holoprotein. PhyB undergoes conformational changes between the Pr and Pfr states catalysed by red and infrared light, reversibly associating with the PIF domain only in the Pfr state. This heterodimerization interaction can be used to translocate a YFP-tagged PIF domain to PhyB tagged by mCherry and localized to the plasma membrane by the C-terminal CAAX motif of Kras.

When asked about the possibility that this could be used in-vivo, Levskaya said

The only real caveat for in-vivo work is delivery of the non-native PCB tetrapyrrole. From the literature and my experience with cell culture I suspect it shouldn’t be hard to just administer it directly to animals to get saturating levels for holoprotein formation. It might even be possible just to feed animals Spirulina (where it comes from). There’s nutrition literature that suggests their livers are capable of freeing PCB and getting it into the blood stream.

 

Observing light-induced Cdc42 activation with a TIRF recruitment biosensor

Observing light-induced Cdc42 activation with a TIRF recruitment biosensor

Expression of genetic tools that control neural activity (Channelrhodopsins, Halorhodopsins, DREADDs) in functionally defined populations, such as neurons that are active during a particular task or thought, is the next big leap that needs to be made in systems neuroscience. This may be achieved by combining an imaging technique to identify active neurons, such as G-CaMP3, with photo-switchable transcription. The technique presented in the above paper is one promising avenue which may lead to cell-specific photo-switchable transcription.  Once robust versions of these tools are in place, scientists will begin to work out the complex and thrilling processes of reverse-engineering and manipulation of specific thoughts and memories, at least in mice and rats.





Voltage imaging with sub-millisecond, single-action potential resolution

27 07 2009

I have been itching to post about this work since David DiGregorio presented it at a meeting at Janelia last year. His group’s results, Submillisecond Optical Reporting of Membrane Potentials In Situ Using a Neuronal Trace Dye, were published in the Journal of Neuroscience last week.  Their method of optical voltage sensing is the first one that looks like its ready for “prime-time” action outside of the labs of developers of these sorts of techniques.  It has sufficient speed (<1 ms resolution), sensitivity (25% dF/F per 100mV), and limited membrane perturbation to see single action potentials, without dramatically altering the shape of these currents.  

 

Membrane depolarization causes DPA to rapidly partition to the inner membrane leaflet, quenching DiO.

Membrane depolarization causes DPA to rapidly partition to the inner membrane leaflet, quenching DiO.

 

Like previous methods, Bradley et al. use voltage-dependent membrane partitioning of dipicrylamine (DPA), a charged small molecule, to quench a fluorophore via FRET.  Previously, high-concentrations of DPA were required to have a reasonable signal change, which caused toxicity, increased membrane capacitance and slowed voltage transients.  By using DiO, a lipophilic neuronal tracer, as the fluorophore, the DPA concentration could be reduced to 1uM, while retaining sufficient optical sensitivity for action potential detection.

Picture 9





Photoactivated Transcription Revisted

14 07 2009

Looks like there has been some new results in the field of photoactivated transcription.  Unlike the fully genetically-encoded systems reviewed in a Journal Club, this uses a hybrid genetic and small molecule approach. In Doxycycline-dependent photoactivated gene expression in eukaryotic systems, Cambridge et al. add the photolabile protecting groups to doxycyclin derivatives, which then function as photoactivatable switches in the commonly used Tet-on system. Dr. Dan O’Connor described the technique as “the path of least resistance to photoactivated transcription.” 

 

Local (left) and global (right) GFP expression following optical uncaging of cyanodoxycyclin

Local (left) and global (right) GFP expression following optical uncaging of cyanodoxycyclin

 

 

      The authors were able to get robust gene expression with standard UV irradiation, but also were able to uncage sufficient cyanodoxycycline with two-photon illumination to cause highly localized gene expression in cultures.  In live tadpoles, they stuck to UV for the greater efficiency.

     The standard caveats of the tet system apply. The off-state still has a bit of residual gene expression, which is fine for some applications (like fluorescent tagging), and a dealbreaker for others (cre induction). Drug delivery takes time and comes with diffusion, penetration and clearance issues.  UV penetration through deep tissue is going to be a big technical hurdle to overcome to apply this to full-grown mammals. Blasting living tissue with high power UV usually isn’t a good idea. Despite these caveats, the system clearly works and I’d bet the authors are already applying the system to some next-step applications and biological questions. The potential of selectively turning on genes in functionally identified neurons via light is enormous.  It is one of the most likely eventual avenues into possible optical activation or suppression of specific thought patterns (at least if you are willing to squirt virus into your brain and eat a bunch of nasty antibiotics).