Labeling and identifying cell-specific proteomes in the mouse brain

Cyclopropeniminium Ions Exhibit Unique Reactivity Profiles with Bioorthogonal Phosphines

We report a new ligation of cyclopropeniminium ions with bioorthogonal phosphines. Cyclopropeniminium scaffolds are sufficiently stable in biological media and, unlike related isomers, react with functionalized phosphines via formal 1,2-addition to a π-system. The ligation can be performed in aqueous solution and is compatible with existing bioorthogonal transformations. Such mutually compatible reactions are useful for multicomponent labeling.

Total synthesis of Escherichia coli with a recoded genome

Nature uses 64 codons to encode the synthesis of proteins from the genome, and chooses 1 sense codon-out of up to 6 synonyms-to encode each amino acid. Synonymous codon choice has diverse and important roles, and many synonymous substitutions are detrimental. Here we demonstrate that the number of codons used to encode the canonical amino acids can be reduced, through the genome-wide substitution of target codons by defined synonyms. We create a variant of Escherichia coli with a four-megabase synthetic genome through a high-fidelity convergent total synthesis. Our synthetic genome implements a defined recoding and refactoring scheme-with simple corrections at just seven positions-to replace every known occurrence of two sense codons and a stop codon in the genome. Thus, we recode 18,214 codons to create an organism with a 61-codon genome; this organism uses 59 codons to encode the 20 amino acids, and enables the deletion of a previously essential transfer RNA.

Construction of ligand assay systems by protein-based semisynthetic biosensors

Proteins as causative agents of diseases such as cancers, diabetes and neurological disorders are attractive drug targets. For developing chemicals selectively acting on key disease-causing proteins, one useful concept is the direct conversion of such target proteins into biosensors. This approach provides ligand-binding assay systems based on protein-based biosensors, which can quantitatively evaluate interactions between the protein and a specific ligand in many environments. Site-specific chemical modifications are used widely for the creation of protein-based semisynthetic biosensors in vitro. Notably, a few bio-orthogonal approaches capable of selectively modifying drug-targets have been developed, allowing conversion of specific target proteins into semisynthetic biosensors in live cells. These biosensors can be used for quantitative drug binding analyses in native environments. In this review, we discuss recent efforts for the construction of ligand assay systems using semisynthetic protein-based biosensors and their application to quantitative analysis and high-throughput screening of small molecules for drug discovery.

Cell-Type-Specific Proteomics: A Neuroscience Perspective

Cell-type-specific analysis has become a major focus for many investigators in the field of neuroscience, particularly because of the large number of different cell populations found in brain tissue that play roles in a variety of developmental and behavioral disorders. However, isolation of these specific cell types can be challenging due to their nonuniformity and complex projections to different brain regions. Moreover, many analytical techniques used for protein detection and quantitation remain insensitive to the low amounts of protein extracted from specific cell populations. Despite these challenges, methods to improve proteomic yield and increase resolution continue to develop at a rapid rate. In this review, we highlight the importance of cell-type-specific proteomics in neuroscience and the technical difficulties associated. Furthermore, current progress and technological advancements in cell-type-specific proteomics research are discussed with an emphasis in neuroscience.

Translational switching of Cry1 protein expression confers reversible control of circadian behavior in arrhythmic Cry-deficient mice

Circadian rhythms dominate our lives through our daily cycle of sleep and wakefulness. They are controlled by a brain master clock: the suprachiasmatic nucleus (SCN). SCN timekeeping pivots around a molecular loop incorporating Cryptochrome (Cry) proteins; global loss of these proteins disables the clock. We developed a biologically appropriate translational switch based on genetic code expansion to achieve reversible control of Cry1 expression. Cry1 translation in neurons of arrhythmic Cry-null SCN slices immediately, reversibly, and dose-dependently initiated circadian molecular rhythms. Cry1 translation in SCN neurons was sufficient to initiate circadian behavior rapidly and reversibly in arrhythmic Cry-null mice. This demonstrates control of mammalian behavior using translational switching, a method of broad applicability.

The suprachiasmatic nucleus (SCN) is the principal circadian clock of mammals, coordinating daily rhythms of physiology and behavior. Circadian timing pivots around self-sustaining transcriptional–translational negative feedback loops (TTFLs), whereby CLOCK and BMAL1 drive the expression of the negative regulators Period and Cryptochrome (Cry). Global deletion of Cry1 and Cry2 disables the TTFL, resulting in arrhythmicity in downstream behaviors. We used this highly tractable biology to further develop genetic code expansion (GCE) as a translational switch to achieve reversible control of a biologically relevant protein, Cry1, in the SCN. This employed an orthogonal aminoacyl-tRNA synthetase/tRNA_CUA pair delivered to the SCN by adeno-associated virus (AAV) vectors, allowing incorporation of a noncanonical amino acid (ncAA) into AAV-encoded Cry1 protein carrying an ectopic amber stop codon. Thus, translational readthrough and Cry1 expression were conditional on the supply of ncAA via culture medium or drinking water and were restricted to neurons by synapsin-dependent expression of aminoacyl tRNA-synthetase. Activation of Cry1 translation by ncAA in neurons of arrhythmic Cry-null SCN slices immediately and dose-dependently initiated TTFL circadian rhythms, which dissipated rapidly after ncAA withdrawal. Moreover, genetic activation of the TTFL in SCN neurons rapidly and reversibly initiated circadian behavior in otherwise arrhythmic Cry-null mice, with rhythm amplitude being determined by the number of transduced SCN neurons. Thus, Cry1 does not specify the development of circadian circuitry and competence but is essential for its labile and rapidly reversible activation. This demonstrates reversible control of mammalian behavior using GCE-based translational switching, a method of potentially broad neurobiological interest.

An Evolved Methanomethylophilus alvus Pyrrolysyl-tRNA Synthetase/tRNA Pair Is Highly Active and Orthogonal in Mammalian Cells

We recently characterized a new class of pyrrolysyl-tRNA synthetase (PylRS)/^PyltRNA pairs from Methanomassiliicocales that are active and orthogonal in Escherichia coli. The aminoacyl-tRNA synthetases (aaRSs) of these pairs lack the N-terminal domain that is essential for tRNA recognition and in vivo activity in the Methanosarcina mazei ( Mm) PylRS but share a homologous active site with MmPylRS; this facilitates the transplantation of mutations discovered with existing PylRS systems into the new PylRS systems to reprogram their substrate specificity for the incorporation of noncanonical amino acids (ncAAs). Several of the new PylRS/^PyltRNA pairs, or their evolved variants [including Methanomethylophilus alvus ( Ma) PylRS/ Ma^PyltRNA(6)_CUA], are mutually orthogonal to the MmPylRS/ Mm^PyltRNA pair, and the active sites of the Mm pair and Ma pair can be diverged to enable the incorporation of distinct ncAAs in response to distinct codons via orthogonal translation in E. coli. Here we demonstrate that MaPylRS/ Ma^PyltRNA(6)_CUA is orthogonal to the aaRSs and tRNAs in mammalian cells and directs efficient incorporation of ncAAs into proteins. Moreover, we confirm that the MaPylRS/ Ma^PyltRNA(6) and MmPylRS/ Mm^PyltRNA pairs are mutually orthogonal in mammalian cells and demonstrates that these pairs can be used to encode distinct ncAAs into a protein in mammalian cells. Thus, the MaPylRS/ Ma^PyltRNA(6)_CUA pair provides an additional pair that is orthogonal in both E. coli and mammalian systems and is mutually orthogonal to the most widely used system for genetic code expansion. Our results provide a foundation for expanding the scope of genetic code expansion and may also facilitate strategies for proteome-wide ncAA tagging with mutually orthogonal systems.

Genetic Code Expansion in Animals

Expanding the genetic code to enable the incorporation of unnatural amino acids into proteins in biological systems provides a powerful tool for studying protein structure and function. While this technology has been mostly developed and applied in bacterial and mammalian cells, it recently expanded into animals, including worms, fruit flies, zebrafish, and mice. In this review, we highlight recent advances toward the methodology development of genetic code expansion in animal model organisms. We further illustrate the applications, including proteomic labeling in fruit flies and mice and optical control of protein function in mice and zebrafish. We summarize the challenges of unnatural amino acid mutagenesis in animals and the promising directions toward broad application of this emerging technology.

Genetically encoded fluorescent indicators for live cell pH imaging

BACKGROUND

Intracellular pH underlies most cellular processes. There is emerging evidence of a pH-signaling role in plant cells and microorganisms. Dysregulation of pH is associated with human diseases, such as cancer and Alzheimer's disease.

SCOPE OF REVIEW

In this review, we attempt to provide a summary of the progress that has been made in the field during the past two decades. First, we present an overview of the current state of the design and applications of fluorescent protein (FP)-based pH indicators. Then, we turn our attention to the development and applications of hybrid pH sensors that combine the capabilities of non-GFP fluorophores with the advantages of genetically encoded tags. Finally, we discuss recent advances in multicolor pH imaging and the applications of genetically encoded pH sensors in multiparameter imaging.

MAJOR CONCLUSIONS

Genetically encoded pH sensors have proven to be indispensable noninvasive tools for selective targeting to different cellular locations. Although a variety of genetically encoded pH sensors have been designed and applied at the single cell level, there is still much room for improvements and future developments of novel powerful tools for pH imaging. Among the most pressing challenges in this area is the design of brighter redshifted sensors for tissue research and whole animal experiments.

GENERAL SIGNIFICANCE

The design of precise pH measuring instruments is one of the important goals in cell biochemistry and may give rise to the development of new powerful diagnostic tools for various diseases.

Small Unnatural Amino Acid Carried Raman Tag for Molecular Imaging of Genetically Targeted Proteins

Raman has been implemented to image biological systems for decades. However, Raman microscopy along with Raman probes is restricted to image metabolites or a few intracellular organelles so far and lacks genetic specificity for imaging proteins of interest, which significantly hinders their application. Here, we report the Raman spectra-based protein imaging method, which incorporates a small phenyl ring enhanced Raman tag (total of ∼0.55 kDa) with a single unnatural amino acid (UAA) to genetically label specific proteins. We further demonstrate hyperspectral stimulated Raman scattering (SRS) imaging of the Histone3.3 protein in the nucleus, Sec61β protein in the endoplasmic reticulum of HeLa cells, and Huntingtin protein Htt74Q in mutant huntingtin-induced cells. Genetic encoding of a small, stable, sensitive, and narrow-band Raman tag took one key step forward to enable SRS or Raman imaging of specific proteins and could further facilitate quantitative Raman spectra-based supermultiplexing microscopy in the future.

Uncovering Discrete Synaptic Proteomes to Understand Neurological Disorders

The mammalian nervous system is an immensely heterogeneous organ composed of a diverse collection of neuronal types that interconnect in complex patterns. Synapses are highly specialized neuronal cell-cell junctions with common and distinct functional characteristics that are governed by their protein composition or synaptic proteomes. Even a single neuron can possess a wide-range of different synapse types and each synapse contains hundreds or even thousands of proteins. Many neurological disorders and diseases are caused by synaptic dysfunction within discrete neuronal populations. Mass spectrometry (MS)-based proteomic analysis has emerged as a powerful strategy to characterize synaptic proteomes and potentially identify disease driving synaptic alterations. However, most traditional synaptic proteomic analyses have been limited by molecular averaging of proteins from multiple types of neurons and synapses. Recently, several new strategies have emerged to tackle the ‘averaging problem’. In this review, we summarize recent advancements in our ability to characterize neuron-type specific and synapse-type specific proteomes and discuss strengths and limitations of these emerging analysis strategies.

A Bifunctional Noncanonical Amino Acid: Synthesis, Expression, and Residue-Specific Proteome-wide Incorporation

Mapping of weak and hence transient interactions between low-abundance interacting molecules is still a major challenge in systems biology and protein biochemistry. Therefore, additional system-wide acting tools are needed to determine protein interactomics. Most important are reagents that can be applied at any kind of protein interface and the possibility to enrich cross-linked fragments with high efficiency. In this study, we report the synthesis of a novel noncanonical amino acid that features a diazirine group for ultraviolet cross-linking as well as an alkyne group for labeling by click chemistry. This bifunctional amino acid, called PrDiAzK, may be inserted into almost any protein interface with minimal structural perturbation using genetic code expansion. We demonstrate that PrDiAzK can be site-selectively incorporated into proteins in both bacterial and mammalian cell cultures, and we show that PrDiAzK allows protein labeling as well as cross-linking. In addition, we tested PrDiAzK for proteome-wide incorporation via stochastic orthogonal recoding of translation, implying potential applications in system-wide mapping of protein-protein interactions in the future.

Multiple Click-Selective tRNA Synthetases Expand Mammalian Cell-Specific Proteomics

Bioorthogonal tools enable cell-type-specific proteomics, a prerequisite to understanding biological processes in multicellular organisms. Here we report two engineered aminoacyl-tRNA synthetases for mammalian bioorthogonal labeling: a tyrosyl ( ScTyr_Y43G) and a phenylalanyl ( MmPhe_T413G) tRNA synthetase that incorporate azide-bearing noncanonical amino acids specifically into the nascent proteomes of host cells. Azide-labeled proteins are chemoselectively tagged via azide-alkyne cycloadditions with fluorophores for imaging or affinity resins for mass spectrometric characterization. Both mutant synthetases label human, hamster, and mouse cell line proteins and selectively activate their azido-bearing amino acids over 10-fold above the canonical. ScTyr_Y43G and MmPhe_T413G label overlapping but distinct proteomes in human cell lines, with broader proteome coverage upon their coexpression. In mice, ScTyr_Y43G and MmPhe_T413G label the melanoma tumor proteome and plasma secretome. This work furnishes new tools for mammalian residue-specific bioorthogonal chemistry, and enables more robust and comprehensive cell-type-specific proteomics in live mammals.