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Short Communications |





§¶
From the Departments of Pathology and Laboratory
Medicine,*
Microbiology and
Immunology,
and
Surgery,
the Neuroscience
Center,§
and the Program for Molecular Biology
of Biotechnology,¶
the University of North
Carolina, Chapel Hill, North Carolina
| Abstract |
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| Introduction |
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In this report, we describe the time course and distribution of the infiltrated donor hematogenous cells, and their morphological transformation into microglia in the twitcher mice that received BMT from GFP transgenic mice with normal galactosylceramidase activity.
| Materials and Methods |
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The transgene was engineered by subcloning the entire H-2Kb promoter region (1960 bp) into SalI and BamHI restriction sites found upstream of a variant GFP (EGFP) gene encoded by pEGFP-1 (Clontech, Palo Alto, CA). The EGFP gene encodes a protein with a single red-shifted excitation peak and fluoresces 30- to 40-fold more intensely than wild-type GFP when excited at 488 nm. Expression of the H-2KbP-EGFP recombinant was verified by transfection into EL-4 cells. A SalI-ClaI 3000-bp fragment was isolated and microinjected into C57BL/6 derived embryos. Integration of the transgene was initially determined by Southern blotting of genomic tail DNA with a GFP-specific probe, and subsequently confirmed by flow cytometry analysis of peripheral lymphocytes prepared from founder mice.
Identification of Twitcher Mice
The original breeder pairs were obtained from the Jackson Laboratory (Bar Harbor, ME) and the colony has been maintained in our institution. The genotypes of twitcher (GALCtwi/twi) on the C57BL/6 (B6) background and wild-type littermate (GALC+/+) mice were identified by polymerase chain reaction using genomic DNA from the clipped tail on postnatal days 5 to 6.19
Bone Marrow Transplantation
Either sex-matched adult GFP-transgenic mice
(C57BL/6-H-2KbP-EGFP) or wild-type C57BL/6 mice
were used as donors. The BMT was carried out in twitcher and wild-type
littermate control mice at 8 postnatal days or in 5-week-old adult
wild-type B6 mice as described previously.5
Briefly, donor
bone marrow cells were obtained by flushing the femurs of
GFP-transgenic or wild-type mice with Hanks balanced solution.
Intraperitoneal injection of 0.2 ml of the solution, containing 3 to
5 x 107
bone marrow cells, were carried out
into the mice that were irradiated 1 day before with a sublethal dosage
8.0 or 9.0 Gy of 60Co
rays for
7-postnatal-days young and 5-week-old adult mice, respectively.
FACS Analysis
To quantify the amount of engraftment, the peripheral blood mononuclear cells from 5-week-old chimeric wild-type B6 mice were analyzed at 3, 4, and 10 weeks after BMT. Briefly, the blood from the ventral tail vein was collected in Gibsons solution, an anti-coagulant, and the red blood cells were lysed with a Tris-ammonium chloride red blood cell lysis buffer. The peripheral blood mononuclear cells were then analyzed by flow cytometry using a Becton-Dickinson FACScan (San Jose, CA). The leukocytes were gated on and fluorescence measured under the fluorescein isothiocyanate channel. All analysis and quantitation were performed using the Cyclops software from Cytomation, Inc. (Fort Collins, CO).
Tissue Preparation
A total of 15 twitcher mice and nine wild-type littermate control mice transplanted with GFP+ bone marrow cells were examined at 25 to 30 postnatal days (group I), 35 to 40 postnatal days (group II), and 100 to 150 postnatal days (group III). Each group consisted of five chimeric twitcher mice and three chimeric wild-type littermate mice. Three additional adult GFP-transgenic mice were also examined. The mice were anesthetized with ether and perfused transcardially with physiological saline (0.9% NaCl), followed by cold-buffered 4% paraformaldehyde. The entire brain, spinal cord, sciatic nerve, liver, spleen, and kidney were then removed, postfixed in the same fixative for 4 hours, and immersed in the phosphate buffer containing 20% sucrose overnight at 4°C. Serial sections of the cerebrum, cerebellum, brainstem, spinal cord, liver, spleen, and kidney were cut at 40-µm thickness with a vibratome. Sciatic nerves were teased gently with fine forceps. The sections were then coverslipped with Vectashield mounting medium for fluorescence (Vector Laboratories, Burlingame, CA) and examined/photographed under a Nikon microphoto FXA microscope (Nikon, Garden City, NJ) equipped with a fluorescein isothiocyanate filter. A strong GFP fluorescence was observed in the cell bodies and the processes in the sciatic nerve, spleen, liver, and kidney. In the sections from CNS, the fluorescence was somewhat weaker and thus, sections were further immunostained with anti-GFP antibody (Clontech) to enhance the fluorescence.
Immunocytochemistry
The brain and spinal cord sections were pretreated with 10% normal goat serum with 0.2% Triton X-100, and then processed for single- and double-immunostaining. In brief, all brain and spinal cord sections were incubated with polyclonal antibody against GFP (Clontech) at a dilution of 1:200. After washing briefly, the sections were exposed to fluorescein isothiocyanate-conjugated goat anti-rabbit IgG (Sigma Chemical Co., St. Louis, MO). The immunostained sections were then examined/photographed with the microscope. Sections with or without GFP immunostaining were further processed for Mac-1 double-immunostaining. After incubating with monoclonal antibody against mouse Mac-1 (Boehringer-Mannheim, Indianapolis, IN) at a dilution of 1:500, the sections were incubated for 1 hour in biotinylated anti-rat IgG and then for 1 hour in avidin-biotin-peroxidase complex solution (Vector). After rinsing, the sections were finally reacted in a solution of 0.05% 3,3[hyph]diaminobenzidine containing 0.01% H2O2 in Tris-HCl buffer (pH 7.6). The sections were mounted and photographed again.
| Results |
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In the adult GFP-transgenic mouse (Figure 1A)
, ~90% of the peripheral blood
mononuclear cells are GFP+ cells. However, in the
brain, GFP+ cells were rarely detected even after
additional anti-GFP antibody immunostaining. Very rare ramified
microglia with a few long, delicate processes and a small cell body in
the fimbria hippocampi and Bergmann glial cells in the cerebellum were
GFP-positive (data not shown). GFP+ cells were
not detected in the spinal cord and brainstem.
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In wild-type adult B6 mice transplanted with GFP-transgenic bone
marrow at 3, 4, and 10 weeks after BMT, 65%, 70%, and 82%
GFP+ peripheral blood mononuclear cells were
detected, respectively, as mean of 5 mice (Figure 1B)
. This
demonstrates that cells from GFP-transgenic mice can be detected in the
peripheral nervous system and provides a mean to identify and
track bone marrow derived cells in the CNS.
Clinical Course of Twitcher Mice with BMT
The untreated twitcher mice could not survive beyond 37 to 45 days of age. However, after BMT, the chimeric twitcher mice were able to survive three to five times longer than nonchimeric twitcher mice with reduced neurological symptoms as reported previously.5 The body weight of these chimeric twitcher mice remained less than that of chimeric wild-type littermate mice, however.
Distribution of GFP+ Cells
Visceral Organs
Distribution of GFP+ cells in visceral
organs was similar in both chimeric twitcher and wild-type mice. In
groups I and II, almost all hematopoietic cells in the spleen expressed
GFP fluorescence (Figure 2A)
. In contrast
to the rapid engraftment in the spleen, only rare
GFP+ cells were observed in the liver, kidney,
and lung (data not shown). In group III mice, an increased number of
GFP+ cells was seen in the liver (Figure 2B)
,
kidney, and lung (data not shown).
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In wild-type chimeric mice, a few slender spindle-shaped
GFP+ cells were detected along the nerve fibers
in the sciatic nerve in all three groups (data not shown). Compared
with wild-type chimeric mice, numerous GFP+ cells
were observed in the sciatic nerve of the chimeric twitcher mice in all
three groups (Figure 2C)
. In groups I and II, the majority of the
GFP+ cells had a spheroid, round cell body. But
in group III, almost all of GFP+ cells were
slender and spindle-shaped along the nerve fibers.
Central Nervous System
Control Mice
In wild-type chimeric mice of all groups, although a few
GFP+ cells were observed in the CNS (Figure 2, JL)
, they were far less than those in other organs. They were
predominantly seen in the choroid plexus (Figure 2J)
, perivascular
region (Figure 2L)
, and leptomeninges. Most GFP+
cells in these areas were spindle-shaped but a few were oval.
Twitcher Mice
In group I chimeric twitcher mice, the distribution of
GFP+ cells was similar to the wild-type chimeric
mice, except for a few GFP+ cells with a round
cell body which were detected in the white matter of spinal cord (data
not shown). In group II chimeric twitcher mice (Figure 2, DF)
,
however, numerous GFP+ cells were detected in the
cerebrum especially in the white matter such as the corpus callosum
(Figure 2D)
, internal capsule and fimbria hippocampi, spinal white
matter (Figure 2E)
, and brainstem (Figure 2F)
. Only a few
GFP+ cells were seen in the cerebellum (data not
shown). Most of these GFP+ cells in the white
matter had a round or spheroid cell body reminiscent of ameboid cells,
whereas those in the gray matter displayed a small cell body with two
or three fine processes. In group III chimeric twitcher mice (Figure 2, GI)
, GFP+ cells were still numerous, although
more cells were detected in the white matter in the cerebrum (Figure 2G)
. Compared with group I and II chimeric twitcher mice,
GFP+ cells were more ubiquitously distributed in
both the gray and white matter, in particular, in the spinal cord
(Figure 2H)
. The majority of them had two or three fine processes with
a small cell body, although few ameboid cells were still observed. At
this time point, the increased number of GFP+
cells with both ameboid and ramified form were observed in the
cerebellum (Figure 2I)
. The double-immunostaining with Mac-1 antibody
showed that more than 50% GFP+ cells in the
group II chimeric twitcher mice expressed immunoreactivity with Mac-1
(Figure 3A)
. In group III, almost all of
GFP+ cells with fine processes were
double-immunostained with Mac-1 (Figure 3, B and C)
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| Discussion |
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and interleukin-10 mRNA by reverse transcriptase-polymerase
chain reaction determination in the group II twitcher mice but such
increase in the immunoreactivity or mRNA was not found in the group III
twitcher mice. These data are also consistent with the transformation
of macrophages/microglia from active to the quiescent state. Although
some of the donor cells were not marked by the Mac-1 antibody at the
beginning of the infiltration, eventually they became ramified cells
and labeled by the Mac-1 antibody. It might suggest, therefore, that
some undifferentiated donor cells also infiltrated into the
demyelinating CNS of twitcher mice.
The twitcher mouse is a model of GLD in humans that is caused by the
mutation of the galactosylceramidase gene. The disease is characterized
pathologically by an apoptotic death of
oligodendrocytes,22
diffuse demyelination, and a massive
increase of the reactive microglia/macrophages in the CNS and
peripheral nervous system.23,24
The demyelination
progressed in orderly fashion and affected the peripheral nervous
system earlier than the CNS.25,26
The reactive
microglia/macrophages are detected in the spinal white matter at 20
postnatal days and in the cerebral white matter at 25 postnatal days in
twitcher mice. In chimeric twitcher mice, however,
GFP+ cells were detected in the CNS only after 35
postnatal days. Although these twitcher mice received irradiation
before BMT, this apparently delayed response of hematogenous macrophage
infiltration in chimeric twitcher mice is not because of irradiation,
because our study with intraperitoneal injection of rhodamine in
nonirradiated twitcher mice resulted in delayed cellular infiltration
similarly (Y-P Wu, J Matsuda, A Kubota, K Suzuki, and K Suzuki,
unpublished observation). We hypothesize that the delayed
response may be explained as follows; in the early stage of
demyelination, phagocytosis of myelin debris is carried out primarily
by local activated microglia. These activated microglia synthesize and
secrete a variety of proinflammatory, potentially cytotoxic cytokines,
such as tumor necrosis factor-
and interleukin-1.27-31
Infiltration of the peripheral hematogenous cells may take place in
response to these cytokines and/or various chemokines. Thus, the
pattern of donor cell infiltration in the twitcher mice in the current
study seems to follow this naturally occurring demyelinating process
and is totally different from the pattern of donor cell infiltration
seen in the murine model of Sandhoff disease, in which the major
pathology is a neuronal storage and peripheral hematogenous cells are
detected mostly in the gray matter.15
Thus, natural
disease process seems to determine the site and extent of hematogenous
cell infiltration and eventual effectiveness of the BMT treatment. Our
study also indicates that unlike rapid splenic engraftment, cellular
infiltration into the CNS is a slow process because very few
GFP+ cells were detected within the CNS of
chimeric twitcher mice younger than 35 postnatal days. This slow
process may explain the ineffectiveness of BMT on the rapidly
progressive infantile form of GLD, although BMT seems effective as the
treatment of late onset slowly progressive form of
GLD.3
| Acknowledgements |
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| Footnotes |
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Supported by research grants NS-24453, AI-20288, AI-141580 and Mental Retardation Research Center core grant HD-03110 from the United States Public Health Service.
Accepted for publication February 16, 2000.
| References |
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expression in brains of twitcher, quaking and normal mice. J Neuroimmunol 1997, 73:47-56[Medline]
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