Building and breeding molecules to spy on cells and tumors

Imaging of biochemical processes in living cells and organisms is essential for understanding how genes and gene products work together in space and time and in health and disease. Such imaging depends crucially on indicator molecules designed to maximize sensitivity and specificity. These molecules can be entirely synthetic, entirely genetically encoded macromolecules, or hybrid combinations, each approach having its own pros and cons. Recent examples from the author's laboratory include peptides whose uptake into cells is triggered by proteases typical of tumors, monomeric red fluorescent proteins and biarsenical-tetracysteine systems for determining the age and electron-microscopic location of proteins.     

Novel chromophores and buried charges control color in mFruits

mFruits are second-generation monomeric red fluorescent proteins (mRFPs) that have improved brightness and photostability compared to the first-generation mRFP1. The emission and excitation maxima are distributed over the remarkably large ranges of about 550-650 and 540-590 nm, respectively; however, the variations in the spectra can be traced to a few key amino acids. Spectroscopic and atomic resolution crystallographic analyses of three representatives, mOrange, mStrawberry, and mCherry, reveal that different mechanisms operate to establish the excitation and emission maxima. Evidently, they all undergo the second oxidation step to produce an acylimine linkage in the polypeptide backbone. In comparison to the progenitor DsRed, direct covalent modification to this linkage (mOrange) and indirect modification of the chromophore environment (mStrawberry and mCherry) produce strong blue- and red-shifted variants. The blue shift of mOrange is induced by an unprecedented covalent modification of the protein backbone. The electron-density map indicates the formation of a third heterocycle, 2-hydroxy-dihydrooxazole, upon the reaction of Thr 66 Ogamma with the polypeptide backbone, which in turn reduces the conjugation of the carbonyl at position 65 with the rest of the chromophore. In mStrawberry and mCherry, the movement of charged Lys 70 and protonation of Glu 215 are proposed to modify the chromophore electron-density distribution, inducing the red shift. pH-dependent spectral shifts of mCherry and mStrawberry appear to result from the titration of Glu 215, although, for mStrawberry, partial cyclization of Thr 66 may contribute at high pH.     

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.     

Effect of Temperature on the Distribution of Membrane Particles in Streptococcus faecalis as Seen by the Freeze-Fracture Technique

When cells of Streptococcus faecalis ATCC 9790 were incubated at temperatures above 10 C before being frozen for freeze-fracture, a random distribution of particles was observed on the outer fracture face of the freeze-cleaved cell membrane. However, when cells were incubated below 10 C before freezing, particleless patches were seen on this membrane surface. The size of the patches produced on chilling could be increased by centrifugation or by storing the chilled cells overnight at about 3 C. Patch formation appeared readily reversible, since the medium and large patches that formed on chilling could not be observed in cells warmed for 10 s at 25 C. However, during the transition from the patch to patchless state, smaller patches not seen in the chilled cells were observed. This suggested that the smaller patches might have been intermediate forms produced by the fragmentation of larger patches on warming.     

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.     

Signaling from synapse to nucleus: the logic behind the mechanisms

Signaling from synapse to nucleus is vital for activity-dependent control of neuronal gene expression and represents a sophisticated form of neural computation. The nature of specific signal initiators, nuclear translocators and effectors has become increasingly clear, and supports the idea that the nucleus is able to make sense of a surprising amount of fast synaptic information through intricate biochemical mechanisms. Information transfer to the nucleus can be conveyed by physical translocation of messengers at various stages within the multiple signal transduction cascades that are set in motion by a Ca(2+) rise near the surface membrane. The key role of synapse-to-nucleus signaling in circadian rhythms, long-term memory, and neuronal survival sheds light on the logical underpinning of these signaling mechanisms.     

Modelling intraspecific resting egg shape variation in a freshwater fairy shrimp Tanymastix stagnalis (L., 1758) (Crustacea, Branchiopoda)

The shape of the resting eggs of a large branchiopod crustacean, the Anostraca Tanymastix stagnalis , is represented very accurately by analytical expressions. The occurrence of atypical shape of some T. stagnalis eggs may be viewed as a simple change of the analytical expression describing the usual egg shape. Their unusual shape may be explained by a higher embryo volume within an envelope of a given size. Biological implications are briefly discussed and hypothesized in an evolutionary point of view.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 55–60.     

Towards Transgenic Primates: What can we learn from mouse genetics?

Considering the great physiological and behavioral similarities with humans, monkeys represent the ideal models not only for the study of complex cognitive behavior but also for the preclinical research and development of novel therapeutics for treating human diseases. Various powerful genetic technologies initially developed for making mouse models are being explored for generating transgenic primate models. We review the latest genetic engineering technologies and discuss the potentials and limitations for systematic production of transgenic primates. Supported by Funding from GRA, NIMH and NIA.     

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.