Tobias Kohl

The AML1-ETO fusion gene and the FLT3 length mutation collaborate in inducing acute leukemia in mice

The molecular characterization of leukemia has demonstrated that genetic alterations in the leukemic clone frequently fall into 2 classes, those affecting transcription factors (e.g., AML1-ETO) and mutations affecting genes involved in signal transduction (e.g., activating mutations of FLT3 and KIT). This finding has favored a model of leukemogenesis in which the collaboration of these 2 classes of genetic alterations is necessary for the malignant transformation of hematopoietic progenitor cells. The model is supported by experimental data indicating that AML1-ETO and FLT3 length mutation (FLT3-LM), 2 of the most frequent genetic alterations in AML, are both insufficient on their own to cause leukemia in animal models. Here we report that AML1-ETO collaborates with FLT3-LM in inducing acute leukemia in a murine BM transplantation model. Moreover, in a series of 135 patients with AML1-ETO-positive AML, the most frequently identified class of additional mutations affected genes involved in signal transduction pathways including FLT3-LM or mutations of KIT and NRAS. These data support the concept of oncogenic cooperation between AML1-ETO and a class of activating mutations, recurrently found in patients with t(8;21), and provide a rationale for therapies targeting signal transduction pathways in AML1-ETO-positive leukemias.

Stimulated Emission Depletion Live-Cell Super-Resolution Imaging Shows Proliferative Remodeling of T-Tubule Membrane Structures After Myocardial Infarction

Rationale: Transverse tubules (TTs) couple electric surface signals to remote intracellular Ca2+ release units (CRUs). Diffraction-limited imaging studies have proposed loss of TT components as disease mechanism in heart failure (HF). Objectives: Objectives were to develop quantitative super-resolution strategies for live-cell imaging of TT membranes in intact cardiomyocytes and to show that TT structures are progressively remodeled during HF development, causing early CRU dysfunction. Methods and Results: Using stimulated emission depletion (STED) microscopy, we characterized individual TTs with nanometric resolution as direct readout of local membrane morphology 4 and 8 weeks after myocardial infarction (4pMI and 8pMI). Both individual and network TT properties were investigated by quantitative image analysis. The mean area of TT cross sections increased progressively from 4pMI to 8pMI. Unexpectedly, intact TT networks showed differential changes. Longitudinal and oblique TTs were significantly increased at 4pMI, whereas transversal components appeared decreased. Expression of TT-associated proteins junctophilin-2 and caveolin-3 was significantly changed, correlating with network component remodeling. Computational modeling of spatial changes in HF through heterogeneous TT reorganization and RyR2 orphaning (5000 of 20 000 CRUs) uncovered a local mechanism of delayed subcellular Ca2+ release and action potential prolongation. Conclusions: This study introduces STED nanoscopy for live mapping of TT membrane structures. During early HF development, the local TT morphology and associated proteins were significantly altered, leading to differential network remodeling and Ca2+ release dyssynchrony. Our data suggest that TT remodeling during HF development involves proliferative membrane changes, early excitation-contraction uncoupling, and network fracturing.

A protease assay for two-photon crosscorrelation and FRET analysis based solely on fluorescent proteins

GFP and the red fluorescent protein, DsRed, have been combined to design a protease assay that allows not only for fluorescence resonance energy transfer (FRET) studies but also for dual-color crosscorrelation analysis, a single-molecule-based method that selectively probes the concomitant movement of two distinct tags. The measurement principle is based on a spectrally resolved detection of single molecules diffusing in and out of a diffraction-limited laser focus. Double-labeled substrate molecules are separated into two single-labeled products by specific cleavage at a protease cleavage site between the two flanking tags, DsRed and GFP, thus disrupting joint fluctuations in the two detection channels and terminating FRET between the two labels. In contrast to enzyme assays based solely on FRET, this method of dual-color crosscorrelation is not limited to a certain range of distances between the fluorophores and is much more versatile with respect to possible substrate design. To simplify the measurement setup, two-photon excitation was used, allowing for simultaneous excitation of both tags with a single infrared laser wavelength. The general concept was experimentally verified with a GFP–peptide–DsRed construct containing the cleavage site for tobacco etch virus protease. Two-photon excitation in the infrared and the use of cloneable tags make this assay easily adaptable to intracellular applications. Moreover, the combination of FRET and crosscorrelation analysis in a single-molecule-based approach promises exciting perspectives for miniaturized high-throughput screening based on fluorescence spectroscopy.

STED Live Cell Super-Resolution Imaging Shows Proliferative Remodeling of T-Tubule Membrane Structures After Myocardial Infarction

Rationale: Transverse tubules (TTs) couple electric surface signals to remote intracellular Ca2+ release units (CRUs). Diffraction-limited imaging studies have proposed loss of TT components as disease mechanism in heart failure (HF). Objectives: Objectives were to develop quantitative super-resolution strategies for live-cell imaging of TT membranes in intact cardiomyocytes and to show that TT structures are progressively remodeled during HF development, causing early CRU dysfunction. Methods and Results: Using stimulated emission depletion (STED) microscopy, we characterized individual TTs with nanometric resolution as direct readout of local membrane morphology 4 and 8 weeks after myocardial infarction (4pMI and 8pMI). Both individual and network TT properties were investigated by quantitative image analysis. The mean area of TT cross sections increased progressively from 4pMI to 8pMI. Unexpectedly, intact TT networks showed differential changes. Longitudinal and oblique TTs were significantly increased at 4pMI, whereas transversal components appeared decreased. Expression of TT-associated proteins junctophilin-2 and caveolin-3 was significantly changed, correlating with network component remodeling. Computational modeling of spatial changes in HF through heterogeneous TT reorganization and RyR2 orphaning (5000 of 20 000 CRUs) uncovered a local mechanism of delayed subcellular Ca2+ release and action potential prolongation. Conclusions: This study introduces STED nanoscopy for live mapping of TT membrane structures. During early HF development, the local TT morphology and associated proteins were significantly altered, leading to differential network remodeling and Ca2+ release dyssynchrony. Our data suggest that TT remodeling during HF development involves proliferative membrane changes, early excitation-contraction uncoupling, and network fracturing.

Superresolution Modeling of Calcium Release in the Heart

Stable calcium-induced calcium release (CICR) is critical for maintaining normal cellular contraction during cardiac excitation-contraction coupling. The fundamental element of CICR in the heart is the calcium (Ca2+) spark, which arises from a cluster of ryanodine receptors (RyR). Opening of these RyR clusters is triggered to produce a local, regenerative release of Ca2+ from the sarcoplasmic reticulum (SR). The Ca2+ leak out of the SR is an important process for cellular Ca2+ management, and it is critically influenced by spark fidelity, i.e., the probability that a spontaneous RyR opening triggers a Ca2+ spark. Here, we present a detailed, three-dimensional model of a cardiac Ca2+ release unit that incorporates diffusion, intracellular buffering systems, and stochastically gated ion channels. The model exhibits realistic Ca2+ sparks and robust Ca2+ spark termination across a wide range of geometries and conditions. Furthermore, the model captures the details of Ca2+ spark and nonspark-based SR Ca2+ leak, and it produces normal excitation-contraction coupling gain. We show that SR luminal Ca2+-dependent regulation of the RyR is not critical for spark termination, but it can explain the exponential rise in the SR Ca2+ leak-load relationship demonstrated in previous experimental work. Perturbations to subspace dimensions, which have been observed in experimental models of disease, strongly alter Ca2+ spark dynamics. In addition, we find that the structure of RyR clusters also influences Ca2+ release properties due to variations in inter-RyR coupling via local subspace Ca2+ concentration ([Ca2+]ss). These results are illustrated for RyR clusters based on super-resolution stimulated emission depletion microscopy. Finally, we present a believed-novel approach by which the spark fidelity of a RyR cluster can be predicted from structural information of the cluster using the maximum eigenvalue of its adjacency matrix. These results provide critical insights into CICR dynamics in heart, under normal and pathological conditions.

Functional KV10.1 Channels Localize to the Inner Nuclear Membrane

Ectopically expressed human KV10.1 channels are relevant players in tumor biology. However, their function as ion channels at the plasma membrane does not totally explain their crucial role in tumors. Both in native and heterologous systems, it has been observed that a majority of KV10.1 channels remain at intracellular locations. In this study we investigated the localization and possible roles of perinuclear KV10.1. We show that KV10.1 is expressed at the inner nuclear membrane in both human and rat models; it co-purifies with established inner nuclear membrane markers, shows resistance to detergent extraction and restricted mobility, all of them typical features of proteins at the inner nuclear membrane. KV10.1 channels at the inner nuclear membrane are not all transported directly from the ER but rather have been exposed to the extracellular milieu. Patch clamp experiments on nuclei devoid of external nuclear membrane reveal the existence of channel activity compatible with KV10.1. We hypothesize that KV10.1 channels at the nuclear envelope might participate in the homeostasis of nuclear K+, or indirectly interact with heterochromatin, both factors known to affect gene expression.

Characterization of Eag1 Channel Lateral Mobility in Rat Hippocampal Cultures by Single-Particle-Tracking with Quantum Dots

Voltage-gated ion channels are main players involved in fast synaptic events. However, only slow intracellular mechanisms have so far been described for controlling their localization as real-time visualization of endogenous voltage-gated channels at high temporal and spatial resolution has not been achieved yet. Using a specific extracellular antibody and quantum dots we reveal and characterize lateral mobility as a faster mechanism to dynamically control the number of endogenous ether-a-go-go (Eag)1 ion channels inside synapses. We visualize Eag1 entering and leaving synapses by lateral diffusion in the plasma membrane of rat hippocampal neurons. Mathematical analysis of their trajectories revealed how the motion of Eag1 gets restricted when the channels diffuse into the synapse, suggesting molecular interactions between Eag1 and synaptic components. In contrast, Eag1 channels switch to Brownian movement when they exit synapses and diffuse into extrasynaptic membranes. Furthermore, we demonstrate that the mobility of Eag1 channels is specifically regulated inside synapses by actin filaments, microtubules and electrical activity. In summary, using single-particle-tracking techniques with quantum dots nanocrystals, our study shows for the first time the lateral diffusion of an endogenous voltage-gated ion channel in neurons. The location-dependent constraints imposed by cytoskeletal elements together with the regulatory role of electrical activity strongly suggest a pivotal role for the mobility of voltage-gated ion channels in synaptic activity.

Tuning the electrical properties of the heart by differential trafficking of KATP ion channel complexes

ABSTRACT The copy number of membrane proteins at the cell surface is tightly regulated. Many ion channels and receptors present retrieval motifs to COPI vesicle coats and are retained in the early secretory pathway. In some cases, the interaction with COPI is prevented by binding to 14-3-3 proteins. However, the functional significance of this antagonism between COPI and 14-3-3 in terminally differentiated cells is unknown. Here, we show that ATP-sensitive K+ (KATP) channels, which are composed of Kir6.2 and SUR1 subunits, are stalled in the Golgi complex of ventricular, but not atrial, cardiomyocytes. Upon sustained &bgr;-adrenergic stimulation, which leads to activation of protein kinase A (PKA), SUR1-containing channels reach the plasma membrane of ventricular cells. We show that PKA-dependent phosphorylation of the C-terminus of Kir6.2 decreases binding to COPI and, thereby, silences the arginine-based retrieval signal. Thus, activation of the sympathetic nervous system releases this population of KATP channels from storage in the Golgi and, hence, might facilitate the adaptive response to metabolic challenges.

Rapid Internalization of the Oncogenic K+ Channel KV10.1

KV10.1 is a mammalian brain voltage-gated potassium channel whose ectopic expression outside of the brain has been proven relevant for tumor biology. Promotion of cancer cell proliferation by KV10.1 depends largely on ion flow, but some oncogenic properties remain in the absence of ion permeation. Additionally, KV10.1 surface populations are small compared to large intracellular pools. Control of protein turnover within cells is key to both cellular plasticity and homeostasis, and therefore we set out to analyze how endocytic trafficking participates in controlling KV10.1 intracellular distribution and life cycle. To follow plasma membrane KV10.1 selectively, we generated a modified channel of displaying an extracellular affinity tag for surface labeling by α-bungarotoxin. This modification only minimally affected KV10.1 electrophysiological properties. Using a combination of microscopy and biochemistry techniques, we show that KV10.1 is constitutively internalized involving at least two distinct pathways of endocytosis and mainly sorted to lysosomes. This occurs at a relatively fast rate. Simultaneously, recycling seems to contribute to maintain basal KV10.1 surface levels. Brief KV10.1 surface half-life and rapid lysosomal targeting is a relevant factor to be taken into account for potential drug delivery and targeting strategies directed against KV10.1 on tumor cells.

Transformation by Oncogenic Mutants and Ligand-Dependent Activation of FLT3 Wild-type Requires the Tyrosine Residues 589 and 591

Purpose: Mutations in the receptor tyrosine kinase FLT3 are found in up to 30% of acute myelogenous leukemia patients and are associated with an inferior prognosis. In this study, we characterized critical tyrosine residues responsible for the transforming potential of active FLT3-receptor mutants and ligand-dependent activation of FLT3-WT. Experimental Design: We performed a detailed structure-function analysis of putative autophosphorylation tyrosine residues in the FLT3-D835Y tyrosine kinase domain (TKD) mutant. All tyrosine residues in the juxtamembrane domain (Y566, Y572, Y589, Y591, Y597, and Y599), interkinase domain (Y726 and Y768), and COOH-terminal domain (Y955 and Y969) of the FLT3-D835Y construct were successively mutated to phenylalanine and the transforming activity of these mutants was analyzed in interleukin-3-dependent Ba/F3 cells. Tyrosine residues critical for the transforming potential of FLT3-D835Y were also analyzed in FLT3 internal tandem duplication mutants (FLT3-ITD)and the FLT3 wild-type (FLT3-WT) receptor. Result: The substitution of the tyrosine residues by phenylalanine in the juxtamembrane, interkinase, and COOH-terminal domains resulted in a complete loss of the transforming potential of FLT3-D835Y-expressing cells which can be attributed to a significant reduction of signal tranducer and activator of transcription 5 (STAT5) phosphorylation at the molecular level. Reintroduction of single tyrosine residues revealed the critical role of Y589 and Y591 in reconstituting interleukin-3-independent growth of FLT3-TKD-expressing cells. Combined mutation of Y589 and Y591 to phenylalanine also abrogated ligand-dependent proliferation of FLT3-WT and the transforming potential of FLT3-ITD-with a subsequent abrogation of STAT5 phosphorylation. Conclusion: We identified two tyrosine residues, Y589 and Y591, in the juxtamembrane domain that are critical for the ligand-dependent activation of FLT3-WT and the transforming potential of oncogenic FLT3 mutants.