doi:10.1369/jhc.5A6901.2006
Volume 54 (9): 997-1004, 2006 Copyright ©The Histochemical Society, Inc.
A Novel Fluorescent Probe That Is Brain Permeable and Selectively Binds to
Departments of Medicinal Chemistry and Pharmacognosy (CW,JW,YMW), Anethesiology (PP,AS,DLF), and Anatomy and Cell Biology (RMG), University of Illinois at Chicago, Chicago, Illinois; Department of Pharmacology, Emory University, Atlanta, Georgia (DT,YF); Biologie des Interactions Neurones/Glie, Hopital de la Salpetriere, Paris, France (BS,CL,BZ); and Department of Neurological Sciences, Rush University Medical Center, Chicago, Illinois (EJM) Correspondence and present address: Yanming Wang, Case Center for Imaging Research, Case Western University, 11100 Euclid Avenue, Cleveland, OH 44106. E-mail: ywang{at}uhrad.com
Myelin is a multilayered glial cell membrane that forms segmented sheaths around large-caliber axons of both the central nervous system (CNS) and peripheral nervous system (PNS). Myelin covering insures rapid and efficient transmission of nerve impulses. Direct visual assessment of local changes of myelin content in vivo could greatly facilitate diagnosis and therapeutic treatments of myelin-related diseases. Current histologic probes for the visualization of myelin are based on antibodies or charged histochemical reagents that do not enter the brain. We have developed a series of chemical compounds including (E,E)-1,4-bis(4'-aminostyryl)-2-dimethoxy-benzene termed BDB and the subject of this report, which readily penetrates the bloodbrain barrier and selectively binds to the myelin sheath in brain. BDB selectively stains intact myelinated regions in wild-type mouse brain, which allows for delineation of cuprizone-induced demyelinating lesions in mouse brain. BDB can be injected IV into the brain and selectively detect demyelinating lesions in cuprizone-treated mice in situ. These studies justified further investigation of BDB as a potential myelin-imaging probe to monitor myelin pathology in vivo. (J Histochem Cytochem 54:9971004, 2006)
Key Words: multiple sclerosis myelin biomarkers demyelination bloodbrain barrier
MYELIN is a specialized membrane that ensheathes neuronal axons, promoting efficient nerve impulse transmission (Morell and Quarles 1999 To study myelin histopathology in vivo, we set out to develop myelin-specific probes that readily enter the brain and selectively bind to myelin sheaths. In the present report we describe a newly developed compound, (E,E)-1,4-bis(4'-aminostyryl)-2-dimethoxy-benzene (BDB), which is a brain-permeable myelin stain. BDB is a fluorescent stilbenzene derivative that is selectively retained in white matter by binding to myelin. In the absence of myelin sheaths, as occurs in the quaking mouse brain, BDB binding was virtually undetectable. Our studies also show that BDB selectively stains intact myelin sheaths in normal mice in situ following IV injection. BDB brain uptake also allows visualization of demyelinated lesions in cuprizone-treated mice, yielding images similar to those observed in histochemical staining using antibody or other myelin dye-staining procedures. The mechanism underlying the binding of BDB to myelin is also discussed.
Chemical Synthesis and Characterization of BDB Detailed synthetic procedures of BDB will be published elsewhere. The chemical structure of BDB was confirmed by proton nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry.
Animal Preparation and Cuprizone Treatment
In Vitro Staining For IHC, sections mounted on slides were incubated in a solution containing anti-MBP MAb primary antibody (rat anti-MBP, 1:300; Chemicon, Temecula, CA) diluted in 1% normal donkey serum overnight at 4C. Following three rinses with PBS, sections were incubated in donkey anti-rabbit Rhodamine Red-X-conjugated secondary antibody or goat anti-rat IgG Texas-Red-conjugated secondary antibody (Jackson ImmunoResearch Laboratories; West Grove, PA) (diluted 1:200 in PBS with 1% normal donkey serum) for 1 hr at 37C, then washed three times for 5 min each with PBS. DAPI (400 ng/ml in PBS) staining was performed to visualize nuclei following washes with PBS. Images were obtained on an Olympus IX 51 microscope equipped with an Axiocam MRm digital camera and Axiovision 4.3 software.
Quantitation of BDB Brain Uptake
Ex Vivo Characterization of BDB
Physiochemical Properties of BDB BDB is a fluorescent compound and is soluble in CH2Cl2, DMSO, and in most other organic solvents. Excitation and emission spectra of BDB (1 µM in DMSO) recorded using a Cary Eclipse fluorescent spectrophotometer (Variant Inc.; Palo Alto, CA) are shown in Figure 1 . Maximal excitation and emission peaks were found at 426 nm and 506 nm, respectively.
BDB Stains Intact Myelin Sheaths In Vitro Myelin-binding properties of BDB were first examined by in vitro staining of frozen brain sections from wild-type mice. For comparison, immunohistochemical (IHC) staining for myelin-specific MBP was also conducted in adjacent sections. Both corpus callosum and cerebellar white matter were then examined by fluorescent microscopy. At 10 µM concentration, BDB selectively labeled intact myelin sheaths in both corpus callosum (Figure 2A ) and cerebellar white matter (Figure 2D). The pattern of myelin sheath staining detected by BDB was virtually identical to the pattern detected by MBP staining (Figures 2B and 2E). Overlap between BDB and MBP staining is shown in the merge image (Figures 2C and 2F, respectively). These observations indicated that BDB was a specific marker for myelin sheaths in the corpus callosum and cerebellum.
Specificity of BDB for myelin was tested by comparing staining in myelin-deficient quaking mice compared with age-matched control littermates. The quaking mouse is a mutant model of dysmyelination (Sidman et al. 1964
BDB Permeability in Mouse Brain Brain permeability of BDB was evaluated in normal mice using HPLC analysis. Mice were given a single IV injection of BDB solution (0.3 ml, 10 mM), sacrificed after 5, 30, and 60 min, and brain concentrations of BDB determined. As shown in Figure 4 , brain uptake already reached 4.43 ± 1.10% of the injected dose (ID) within 5 min postinjection. At 30 min, brain concentrations decreased slightly (to 2.99 ± 0.28% ID) but did not show any further decrease when measured after 60 min (2.70 ± 0.33% ID).
BDB Stains Myelin in Living Mice Following our in vitro studies, we investigated the ability of BDB to monitor myelin contents ex vivo in the mouse brain. A dose of 1.0 mg BDB (50 mg/kg) was injected via the tail vein into wild-type mice. Eighteen hr postinjection, mice were perfused (see above) and brains were removed and sectioned as described above. BDB staining of myelin was then directly examined under fluorescent microscopy. As shown in Figure 5 , BDB entered the brain and selectively labeled myelin sheaths of the corpus callosum (Figure 5A) and cerebellum (Figure 5C) of the wild-type mice. Subsequent immunostaining for MBP revealed that BDB bound more selectively to myelin fibers, because MBP stained oligodendrocyte (OL) cell soma and processes in the caudate putamen (CPu) (Figure 5B) under the same conditions. In cerebellum, BDB staining was confined to white matter tracts, whereas MBP stained fibers in the granule cell layer, suggesting that BDB binds preferentially to compacted myelin.
Following ex vivo studies with BDB, a similar study was carried out with fluoromyelin for comparison. Thus, we injected 0.5 ml of commercial fluoromyelin in an undisclosed original concentration, which was as high as 300 times that used for in vitro staining. Three hr postinjection, the mouse brain was treated under the above conditions and sectioned. No fluoromyelin was detected in various myelinated regions such as corpus callosum (Figure 6B ) and cerebellum (Figure 6E). As a result, myelinated structures could not be stained ex vivo by fluoromyelin, although the presence of myelin sheaths in both regions were confirmed by MBP immunostaining in adjacent sections (Figures 6A and 6D). This indicated that fluoromyelin is not permeable across the BBB. Thus, BDB offers a major advantage in its potential to stain myelin in vivo.
BDB Detects Demyelinated Lesions in Living Cuprizone-treated Mice We next examined whether we could use BDB to distinguish areas of demyelination from normal-appearing myelin. Wild-type C57BL/6 mice were treated with the selective neurotoxin cuprizone for 6 weeks to induce demyelination, after which we injected BDB into mice as described above and prepared brain sections 18 hr later. Under these conditions, cuprizone is known to induce significant demyelination in the corpus callosum (Matsushima and Morell 2001
Based on the above in vitro and ex vivo studies, we have demonstrated that the fluorescent probe BDB can be used as a specific histochemical stain for myelin. This is based on the following observations: (1) BDB selectively stained intact myelin sheaths present in the corpus callosum of the wild-type mouse brain. (2) BDB staining was not observed in the corpus callosum in the myelin-deficient quaking mutant mice. (3) BDB readily penetrated the BBB and accumulated in the brain following IV injection. (4) BDB readily allowed detection of demyelinated lesions found in the corpus callosum of cuprizone-treated mice in situ following IV injection but not in the cerebellum where cuprizone does not induce lesions. The precise molecular basis for the selective binding of BDB to myelin remains to be determined but most likely reflects interactions with the unique myelin structure. In the CNS, myelin sheaths formed by OL consist of concentric layers of myelin wrapped around axons. There are two major features that distinguish myelin sheaths from membranes of other cell types. First, compared with most other cell membranes that contain nearly equal amounts of lipid and proteins, myelin is lipid rich and consists of nearly 80% lipid and 20% protein. Second is the presence of high amounts of low-molecular-mass proteins, proteolipid protein (PLP) and MBP. Specificity of BDB binding may, at least in part, be due to a selective interaction with myelin-type lipids, with PLP, with MBP, or at sites of interactions between myelin lipids and these proteins. Further studies are in process to separately evaluate the binding affinity and specificity for myelin lipids, myelin proteins, including PLP and MBP, and combinations of individual myelin proteins with lipids.
The reason why BDB binds only to myelin sheaths and not components of degenerating myelin fragments also remains to be determined. Previous studies have shown that these types of compound bind preferentially to amyloid-like proteins possessing aggregated ß-sheets instead of monomeric ß-sheet structures (for a recent review, see Wu et al. 2005 Development and availability of a BBB-permeable fluorescent probe BDB complements conventional histochemical techniques. Existing myelin stains such as fluoromyelin do not penetrate the BBB and thus are limited to in vitro histopathological studies (Figure 6). BDB, therefore, provides a means to carry out myelin detection in vivo. This raises the possibility that properly labeled, brain-permeable myelin probes like BDB could potentially be used for in vivo imaging modalities like positron emission tomography and single photon emission computed tomography to detect and quantify myelin contents in vivo.
The work in part was supported by NIH/NINDS (NS054109, YMW), National Multiple Sclerosis Society (PP1136 to YMW), and the Dana Foundation (YMW).
1 These authors contributed equally to this work. Received for publication December 12, 2005; accepted April 10, 2006
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