Functional fluorescent Ca2+ indicator proteins in transgenic mice under TET control

Genetically encoded fluorescent calcium indicator proteins (FCIPs) are promising tools to study calcium dynamics in many activity-dependent molecular and cellular processes. Great hopes—for the measurement of population activity, in particular—have therefore been placed on calcium indicators derived from the green fluorescent protein and their expression in (selected) neuronal populations. Calcium transients can rise within milliseconds, making them suitable as reporters of fast neuronal activity. We here report the production of stable transgenic mouse lines with two different functional calcium indicators, inverse pericam and camgaroo-2, under the control of the tetracycline-inducible promoter. Using a variety of in vitro and in vivo assays, we find that stimuli known to increase intracellular calcium concentration (somatically triggered action potentials (APs) and synaptic and sensory stimulation) can cause substantial and rapid changes in FCIP fluorescence of inverse pericam and camgaroo-2.     

All-optical histology using ultrashort laser pulses

As a means to automate the three-dimensional histological analysis of brain tissue, we demonstrate the use of femtosecond laser pulses to iteratively cut and image fixed as well as fresh tissue. Cuts are accomplished with 1 to 10 microJ pulses to ablate tissue with micron precision. We show that the permeability, immunoreactivity, and optical clarity of the tissue is retained after pulsed laser cutting. Further, samples from transgenic mice that express fluorescent proteins retained their fluorescence to within microns of the cut surface. Imaging of exogenous or endogenous fluorescent labels down to 100 microm or more below the cut surface is accomplished with 0.1 to 1 nJ pulses and conventional two-photon laser scanning microscopy. In one example, labeled projection neurons within the full extent of a neocortical column were visualized with micron resolution. In a second example, the microvasculature within a block of neocortex was measured and reconstructed with micron resolution.     

Linking Hebb's coincidence-detection to memory formation

The theoretical foundations of learning and memory were laid by Donald Hebb 50 years ago. Recent genetic experiments that enhanced coincidence-detection of the NMDA receptor (a molecular master-switch in implementing Hebb's rule) and that led to better learning and memory in adult animals have substantially validated Hebb's rule in memory formation in the brain.     

Calcium-dependent regulation of protein kinase D revealed by a genetically encoded kinase activity reporter

Protein kinase D (PKD) regulates many diverse cellular functions in response to diacylglycerol. To monitor PKD signaling in live cells, we generated a genetically encoded fluorescent reporter for PKD activity, DKAR (D kinase activity reporter). DKAR expressed in mammalian cells undergoes reversible fluorescence resonance energy transfer changes upon activation and inhibition of endogenous PKD. Surprisingly, we find that agonist-evoked activation of PKD is driven not only by diacylglycerol production, but by Ca(2+). Furthermore, elevation of intracellular Ca(2+), in the absence of any other stimulus, is sufficient to activate PKD. Concurrent imaging of Ca(2+), diacylglycerol, and PKD activity reveals that thapsigargin-mediated elevation of intracellular Ca(2+) is closely followed by a robust increase in diacylglycerol production, in turn followed by PKD activation. The Ca(2+)-induced production of diacylglycerol and accompanying PKD activation is dependent on phospholipase C activity. These data reveal that Ca(2+) is a major contributor to the initiation of PKD signaling through positive feedback regulation of diacylglycerol production, unveiling a new mechanism in PKD activation.     

Roger Tsien: bringing color to cell biology. A passion for chemistry, color, and conversations with cells is what drives Roger Tsien. Interview by Ruth Williams

A passion for chemistry, color, and conversations with cells is what drives Roger Tsien.     

NMDA receptors in dopaminergic neurons are crucial for habit learning

Dopamine is crucial for habit learning. Activities of midbrain dopaminergic neurons are regulated by the cortical and subcortical signals among which glutamatergic afferents provide excitatory inputs. Cognitive implications of glutamatergic afferents in regulating and engaging dopamine signals during habit learning, however, remain unclear. Here, we show that mice with dopaminergic neuron-specific NMDAR1 deletion are impaired in a variety of habit-learning tasks, while normal in some other dopamine-modulated functions such as locomotor activities, goal-directed learning, and spatial reference memories. In vivo neural recording revealed that dopaminergic neurons in these mutant mice could still develop the cue-reward association responses; however, their conditioned response robustness was drastically blunted. Our results suggest that integration of glutamatergic inputs to DA neurons by NMDA receptors, likely by regulating associative activity patterns, is a crucial part of the cellular mechanism underpinning habit learning.     

A genetically encoded tag for correlated light and electron microscopy of intact cells, tissues, and organisms

Electron microscopy (EM) achieves the highest spatial resolution in protein localization, but specific protein EM labeling has lacked generally applicable genetically encoded tags for in situ visualization in cells and tissues. Here we introduce "miniSOG" (for mini Singlet Oxygen Generator), a fluorescent flavoprotein engineered from Arabidopsis phototropin 2. MiniSOG contains 106 amino acids, less than half the size of Green Fluorescent Protein. Illumination of miniSOG generates sufficient singlet oxygen to locally catalyze the polymerization of diaminobenzidine into an osmiophilic reaction product resolvable by EM. MiniSOG fusions to many well-characterized proteins localize correctly in mammalian cells, intact nematodes, and rodents, enabling correlated fluorescence and EM from large volumes of tissue after strong aldehyde fixation, without the need for exogenous ligands, probes, or destructive permeabilizing detergents. MiniSOG permits high quality ultrastructural preservation and 3-dimensional protein localization via electron tomography or serial section block face scanning electron microscopy. EM shows that miniSOG-tagged SynCAM1 is presynaptic in cultured cortical neurons, whereas miniSOG-tagged SynCAM2 is postsynaptic in culture and in intact mice. Thus SynCAM1 and SynCAM2 could be heterophilic partners. MiniSOG may do for EM what Green Fluorescent Protein did for fluorescence microscopy.     

CaMKII tethers to L-type Ca2+ channels, establishing a local and dedicated integrator of Ca2+ signals for facilitation

Ca2+-dependent facilitation (CDF) of voltage-gated calcium current is a powerful mechanism for up-regulation of Ca2+ influx during repeated membrane depolarization. CDF of L-type Ca2+ channels (Ca(v)1.2) contributes to the positive force-frequency effect in the heart and is believed to involve the activation of Ca2+/calmodulin-dependent kinase II (CaMKII). How CaMKII is activated and what its substrates are have not yet been determined. We show that the pore-forming subunit alpha(1C) (Ca(v)alpha1.2) is a CaMKII substrate and that CaMKII interaction with the COOH terminus of alpha1C is essential for CDF of L-type channels. Ca2+ influx triggers distinct features of CaMKII targeting and activity. After Ca2+-induced targeting to alpha1C, CaMKII becomes tightly tethered to the channel, even after calcium returns to normal levels. In contrast, activity of the tethered CaMKII remains fully Ca2+/CaM dependent, explaining its ability to operate as a calcium spike frequency detector. These findings clarify the molecular basis of CDF and demonstrate a novel enzymatic mechanism by which ion channel gating can be modulated by activity.