Click-EM for imaging metabolically tagged nonprotein biomolecules.

EM has long been the main technique for imaging cell structures with nanometer resolution but has lagged behind light microscopy in the crucial ability to make specific molecules stand out. Here we introduce click-EM, a labeling technique for correlative light microscopy and EM imaging of nonprotein biomolecules. In this approach, metabolic labeling substrates containing bioorthogonal functional groups are provided to cells for incorporation into biopolymers by endogenous biosynthetic machinery. The unique chemical functionality of these analogs is exploited for selective attachment of singlet oxygen-generating fluorescent dyes via bioorthogonal 'click chemistry' ligations. Illumination of dye-labeled structures generates singlet oxygen to locally catalyze the polymerization of diaminobenzidine into an osmiophilic reaction product that is readily imaged by EM. We describe the application of click-EM in imaging metabolically tagged DNA, RNA and lipids in cultured cells and neurons and highlight its use in tracking peptidoglycan synthesis in the Gram-positive bacterium Listeria monocytogenes.

PKMζ, but not PKCλ, is rapidly synthesized and degraded at the neuronal synapse.

Synthesizing, localizing, and stabilizing new protein copies at synapses are crucial factors in maintaining the synaptic changes required for storing long-term memories. PKMζ recently emerged as a molecule putatively responsible for maintaining encoded memories over time because its presence correlates with late LTP and because its inhibition disrupts LTP in vitro and long-term memory storage in vivo. Here we investigated PKMζ stability in rat neurons to better understand its role during information encoding and storage. We used TimeSTAMP reporters to track the synthesis and degradation of PKMζ as well as a related atypical PKC, PKCλ. These reporters revealed that both PKMζ and PKCλ were upregulated after chemical LTP induction; however, these new PKMζ copies exhibited more rapid turnover than basally produced PKMζ, particularly in dendritic spines. In contrast to PKMζ, new PKCλ copies exhibited elevated stability. Stable information storage over long periods of time is more challenging the shorter the metabolic lifetime of the candidate molecules.

Improved PeT molecules for optically sensing voltage in neurons.

VoltageFluor (VF) dyes have the potential to measure voltage optically in excitable membranes with a combination of high spatial and temporal resolution essential to better characterize the voltage dynamics of large groups of excitable cells. VF dyes sense voltage with high speed and sensitivity using photoinduced electron transfer (PeT) through a conjugated molecular wire. We show that tuning the driving force for PeT (ΔGPeT + w) through systematic chemical substitution modulates voltage sensitivity, estimate (ΔGPeT + w) values from experimentally measured redox potentials, and validate the voltage sensitivities in patch-clamped HEK cells for 10 new VF dyes. VF2.1(OMe).H, with a 48% ΔF/F per 100 mV, shows approximately 2-fold improvement over previous dyes in HEK cells, dissociated rat cortical neurons, and medicinal leech ganglia. Additionally, VF2.1(OMe).H faithfully reports pharmacological effects and circuit activity in mouse olfactory bulb slices, thus opening a wide range of previously inaccessible applications for voltage-sensitive dyes.

αßT cell receptors expressed by CD4(-)CD8αß(-) intraepithelial T cells drive their fate into a unique lineage with unusual MHC reactivities.

Coreceptor CD4 and CD8αß double-negative (DN) TCRαß(+) intraepithelial T cells, although numerous, have been greatly overlooked and their contribution to the immune response is not known. Here we used T cell receptor (TCR) sequencing of single cells combined with retrogenic expression of TCRs to study the fate and the major histocompatibility complex (MHC) restriction of DN TCRαß(+) intraepithelial T cells. The data show that commitment of thymic precursors to the DN TCRαß(+) lineage is imprinted by their TCR specificity. Moreover, the TCRs they express display a diverse and unusual pattern of MHC restriction that is nonoverlapping with that of CD4(+) or CD8αß(+) T cells, indicating that they sense antigens that are not recognized by the conventional T cell subsets. The new insights indicate that DN TCRαß(+) T cells form a third lineage of TCRαß T lymphocytes expressing a variable TCR repertoire, which serve nonredundant immune functions.