Project Leader

Dr. Aneta Keliris (b. Brud)
Phone: +49 7071 601-704
Fax: +49 7071 601-702
Opens window for sending emailaneta.keliris[at]tuebingen.mpg.de
 
Dr. Jörn Engelmann
Phone: +49 7071 601-704
Fax: +49 7071 601-702
joern.engelmann[at]tuebingen.mpg.de

Current and former Lab members

 
Dr. Sven Gottschalk
(former PostDoc)
 
Dr. Rajendra Joshi
(former PostDoc)
 
Dr. Deepti Jha
(former PhD student & PostDoc)
 
Dr. Ritu Mishra
(former PhD student & PostDoc)
 
Dr. Wu Su
(former PhD student)

 

Novel Contrast Agents for MRI

Molecular imaging is a rapidly growing field aimed at providing tools for the noninvasive visualization of key target molecules as specific markers of molecular and cellular processes. The assessment of such molecular targets would enable early detection of various diseases and help in revealing the disease-underlying mechanisms (e.g. cancer, ageing disorders such as Alzheimer, Parkinson), elucidation of uncharted biological processes of interest (e.g. specific brain activation pathways) as well as monitoring of novel gene/cellular therapies. Amongst different modalities, the use of MRI for in vivo molecular applications is superior given its noninvasiveness, superb spatial and temporal resolution and lack of ionizing radiation. “Peeking” noninvasively into the biology of living subjects requires, however, a development of MRI probes, so called contrast agents (CAs), which can specifically and exclusively interact with molecular targets and translate this information into changes of the acquired MR signal. In contrast to the non-specific and extracellular commercial CAs used clinically (i.e. Dotarem, Magnevist), our Chemical Biology Group has been focusing on design, synthesis, characterization, and biological testing of highly specific multimodal MRI probes. Over the past few years, we have successfully developed targeted and responsive MRI probes for diverse MRI applications.
 
Aside of MRI several other techniques, like fluorescence spectroscopy and microscopy, have been used to test the new probes in cultured cells and in vivo.

These studies are done in collaboration with the High-Field MR and Methodology group of the MRZ and the chemistry group of the Dept. Physiology of Cognitive Processes at the institute as well as external partners e.g. at the University of Tübingen, the University of Durham, and the Helmholtz Center Munich.
 
Recent or currently ongoing projects:

Enzyme responsive contrast agents

An enormous number of biochemical processes taking place in living organisms demands the action of enzymes. These highly specific essential catalysts serve as indicators of diseases (e.g. stroke, brain tumors), cellular processes and are used as essential markers of gene expression. Hence, real-time non-invasive in vivo mapping of enzyme activity can provide a means for the assessment of disease processes, evaluation of novel therapies (e.g. gene and neuronal stem cell therapies), and also better understanding of biochemical events vital for sustaining life. The aim of this project is to develop a versatile multimodal imaging platform for monitoring the enzyme activity that would allow early disease diagnosis and assessment of cellular events associated with i.e. gene expression, or monitoring neuronal stem cells after their transplantation.
 
Recently, 19F MRI and MRS are increasingly gaining an interest and relevance in biomedical and clinical research. The great advantage of 19F compared to 1H MRI is the lack of an intrinsic background signal in mammalian tissue, which allows quantitative and unambiguous detection of administered fluorine labeled probes. Accordingly, we have successfully developed a dual-modal 1H/19F MRI probe, Gd-DOMF-Gal, that can be “lightened on” from an “off” state only in the presence of the enzyme β-galactosidase [see also report of  Dr. A. Keliris. This enzyme is expressed by the LacZ gene, one of the most widely used reporter genes in transgenic studies. Specific activation of Gd-DOMF-Gal has been proven in vitro (phantoms, cells) and lately being investigated in in vivo studies with mice bearing β-galactosidase expressing tumor xenografts (in collaboration with MPI for Metabolism Research, Cologne). Another responsive 1H/19F MRI probe, which has been recently developed in this line, is sensing the activity of matrix metalloproteinase (MMP-2), an enzyme overexpressed on tumor cells.

Cell tracking by 19F MRI

Apart from low molecular weight responsive probes, we have obtained fluorinated-fluorescent nanoparticles. They did efficiently label human mesenchymal stem cells (hMSCs) as well as CD4+ T-cells. Further, the applied MR methods for efficient detection of these labeled cells by MRI were optimized in our department (G. Hagberg). Within this project, we aim at establishing a strategy that would allow an efficient assessment of information about the fate of transplanted cells in vivo, and thus provide a tool for the evaluation of efficacy of proposed cell-based therapies by 1H/19F MRI (i.e. in the treatment of urinary incontinence in collaboration with the University Hospital Tübingen). See also project descriptions of Dr. A. Keliris

β-Cell targeting

A different type of NPs (ferromagnetic metallic cobalt with a functionalized carbon coating) is used for in vivo targeting of β-cells in pancreatic islets of Langerhans in mice. A β-cell specific single chain antibody fragment (SCA), developed by our collaborators, was coupled to these NPs to obtain a targeted T2-CA. In combination with the unprecedented spatial resolution which was achievable at our 16.4T animal scanner or using a cryogenic coil at 9.4T as well as optimizing imaging sequences and image processing by our MRI experts, it was possible for the first time to visualize single islets in vivo in a freely breathing mouse after i.v. injection. This study particularly exemplifies the great advantage to have CA developers (chemists, biologists) and MR physicists working close together in one group. (See also project description of Dr. D.Z. Balla.

Receptor targeting

In a recent project, performed in collaboration with the University of Durham (UK) and the Helmholtz Center Munich, we developed and characterized in vitro 1H MRI probes responsive to the metabotropic glutamate receptor (mGluR5) that is expressed in the brain. These CAs consist of Gd3+-chelate and /or a fluorophore coupled to known antagonists of mGluR5. The primary goal of this research is the non-invasive visualization of mGluR5 receptors in the brain and potentially a more direct monitoring of neuronal activity by imaging extracellular glutamate fluctuations during activation. Extending this concept, CAs targeted to N-methyl-D-aspartate (NMDA) receptor, another specific type of ionotropic glutamate receptors in the brain, were developed and tested in vitro for their ability to visualize these receptors by optical and MR imaging.

Silica-based platforms for MR and multi modality imaging probes

One drawback of MRI is its relatively low sensitivity. A large number of contrast producing moieties is required to obtain a significant change in image contrast. To achieve a higher local accumulation, macromolecular or nanoparticle-based platforms can be used to increase the number of MR reporters per molecule. In collaboration with the University of Tübingen SiO2-based systems (silsesquioxanes, nonporous nanoparticles) functionalized with Gadolinium chelates were developed. The relatively small silsesquioxanes allow the coupling of up to eight chelates by retaining a pharmacokinetic profile of a low molecular weight CA. They undergo a slow degradation process under physiological conditions to mono-chelates readily excreted via the kidneys. In vivo, a rapid accumulation in the gastrointestinal tract after i.v. injection into mice was observed not present when the same concentration of Gadolinium as GdDOTA, a commercially available CA used in the clinics, was injected. Recently, such silsesquioxanes were functionalized with fluorine containing moieties instead of Gadolinium chelates for 19F MRI applications.
 
 
Silica-based nanoparticles (NPs) are much larger (~ 50 – 130 nm in diameter) and thus will show different biodistribution and pharmacokinetics. However, a large number of imaging reporters can be conjugated per particle and different functionalities (e.g. targeting sensors, vectors for cellular uptake) can be introduced.
NPs with a high payload of Gadolinium were synthesized and tested for their ability to enhance MR contrast in vitro. These NPs were further functionalized with fluorophores for optical imaging and a CPP to enhance cellular uptake. Such NPs can be taken up by cells and an initial in vivo study on the biodistribution after i.v. injection showed a fast accumulation particularly in lungs and liver, typical for nanoparticles. In parallel, the influence of particle size and surface charge was studied as well as the surface loading efficacy with Gadolinium chelates optimized.

Neuroanatomical tracers

Understanding brain connectivity is important to enhance the understanding of brain function and disease. To correlate brain anatomy to the functional outcome and to follow changes e.g. with development, aging or learning, non-invasive longitudinal studies are needed. MRI is providing an excellent measure to perform dynamic investigations of brain connectivity by using MR active neuroanatomical tracers. In a collaboration with former and present members of the Dept. Logothetis we are helping to develop such novel tracer molecules by providing information about the uptake efficacy into and the transport of the probes inside neural cells along their processes. Two classes of tracers, for short term (1 - 3 days) and long term (7 - 14 days) studies, were successfully tested in vitro and in vivo.

High-resolution NMR analysis of human brain tumor extracts

Recently, a collaborative project has been initiated to assess human brain tumors (gliomas) by means of 1H MRSI at 9.4T. Apart from assessing changes in important metabolites such as glutamate, glutamine, creatine or inositol, the detection (and potential quantification) of 2-hydroxyglutarate (2HG) is of high importance. An increased accumulation of this metabolite in tumor tissue is associated with mutations in the TCA cycle enzyme isocitrate dehydrogenase (IDH) that occurs frequently in grade II and III glionas. Thus, 2HG has the potential to serve as a prognostic and diagnostic biomarker.
In order to verify the detection of 2HG in human MRS spectra, tumor tissue samples were extracted and high-resolution 1H NMR spectra of the water soluble metabolites were obtained at 300 and 600 MHz. The signals for 2HG could be unambiguously identified and quantified in these spectra and were used to verify the presence of 2HG in the in vivo spectra. In addition, more than 20 different metabolites could be assigned and quantified in the NMR spectra.
Last updated: Monday, 09.02.2015