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a Laboratory of Neuropharmacology, National Neurological Institute "C. Besta," Milan, Italy;
b Schering-Plough Research Institute, San Raffaele Science Park, Milan, Italy;
c University of Chieti "G. D'Annunzio," Department of Oncology and Neuroscience, Chieti, Italy;
d I Division of Clinical Neurosurgery, National Neurological Institute "C. Besta," Milan, Italy;
e II Division of Clinical Neurosurgery, National Neurological Institute "C. Besta," Milan, Italy
Key Words. Neural stem cell • Human adult olfactory bulb • Transplantation • Parkinson's disease • Multipotent precursor • Cell migration
Eugenio Parati M.D., Istituto Neurologico Nazionale "C. Besta," Via Celoria, 11, 20133 Milano, Italy. Telephone: 39-02-2394387; Fax: 39-02-70638217; e-mail: parati{at}istituto-besta.it
| ABSTRACT |
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| INTRODUCTION |
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Widespread cell migrations are the hallmark of vertebrate brain development. In the early embryo, morphogenetic movements of precursor cells establish the rhombomeres of the hindbrain and the region bound by the forebrain. In mid-gestation, individual postmitotic cells undergo directed migrations along the glial fiber system, and radial migration establishes the neuronal layers. In the postnatal period, a wave of secondary neurogenesis produces large numbers of interneurons destined for the cerebral cortex and olfactory bulb [1, 2].
De novo neurogenesis has been reported in the subgranular region of the dentate gyrus, the cortex [3, 4], and olfactory bulb of various adult rodent brains [5-7]. In 1992, Reynolds and Weiss reported the presence of subventricular zone (SVZ) neural stem cells during rodent brain development [8].
Neural stem cells can only be characterized on a critical functional basis in terms of their undifferentiated features, capacity for self-renewal, pluripotentiality, and ability to regenerate damaged tissue [9]. Neural stem cells with these characteristics have been found in cultures of embryonic and adult murine brain [10-14], as well as in human embryonic brain explants after epigenetic stimulation [15, 16]. The SVZ is an intensive proliferative zone and the progeny of neural stem cells can either die or give rise to "neural progenitors" that migrate to the olfactory bulb [17].
We have for the first time isolated stem cells from the olfactory bulbs of adult patients undergoing particularly invasive neurosurgery. Under our optimized conditions, stem cells isolated from the olfactory bulb proliferate in culture in the same way as human embryonic stem cells after epigenetic stimulation and retain all of the typical characteristics of neural stem cells: like human embryonic cells, they proliferate in response to mitotic factors (basic fibroblast growth factor, bFGF, and epidermal growth factor, EGF) and have the ability to give rise to neurons, astrocytes, and oligodendrocytes [15].
The novel aspects of this discovery are the combination of the intense regeneration potential of the olfactory bulb and the possibility of explanting autologous neural stem cells from patients affected by different neurodegenerative disorders by means of simple partial bulbectomy. Finally, as a result of our many years of experience in manipulating these cells, we are able to induce their expansion in vitro and improve their differentiation in neurons, thus representing the ideal biological source for autotransplantation in these patients.
| MATERIAL AND METHODS |
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Two weeks later, the cultures were harvested, mechanically dissociated and replated under the same conditions. After performing this procedure twice in order to eliminate short-term dividing precursors, bulk cultures were generated by passing the cells at a higher density (104 cells/cm2) in the same growth medium every 10 days. Cell counts and viability tests were performed at every passage by means of trypan blue exclusion (as previously reported) [14]. In order to demonstrate multipotentiality after differentiation, the stem cell progeny were plated at very low density onto laminine-coated (Boehringer Mannhneim; Indianapolis, IN) chamber slides (Labtek®; NUNK; Naperville, IL; http://www.labtek.net) in DMEM/F12 medium with 2% fetal calf serum for four days before immunocytochemical analysis.
Improving Adult Human Olfactory Bulb Stem Cell-Derived Neuronal Progeny Using Leukemia Inhibitory Factor (LIF)
The stem cell progenies obtained from olfactory bulb explantation were plated at 500 cells/cm2 onto laminine-coated (Boehringer Mannhneim) chamber slides (Labtek®) in mitotic factor and serum-free DMEM/F12 medium containing LIF (20 ng/ml) for 15 days. After differentiation, the neurons were counted every five days and the result compared with the total cell number.
Immunocytochemistry
The cell composition was analyzed by means of immunostaining with lineage-specific antibodies. The cells were fixed for 20 min in 4% paraformaldehyde in phosphate-buffered saline (PBS), pH 7.4, and then washed and incubated with PBS/0.1% Triton-X containing 10% normal goat serum and the appropriate antibody mixture for 90 min at 37°C. The primary antibodies were rabbit anti-Glial fibrillary acidic protein (ready to use; Incstar Corporation; Stillwater, MN), mouse anti-ß-tubulinIII (1:1000; Sigma) and anti-Galactocerebroside (1:300; Sigma). After some washing, the cultures were incubated for 45 min at room temperature with secondary fluorescein isothiocyanate- or rhodamine isothiocyanate-conjugate goat antimouse or antirabbit IgG antibodies (1:100; Boehringer Mannheim), washed, incubated with 4', 6-diamidino2-phenylinole dihydrochcloride (1 mg/ml in methanol, 15 min at 37°C [14]), and finally mounted using FluorsaveTM (Calbiochem; La Jolla, CA; http://www.calbiochem.com) and viewed under a Zeiss (Oberkochen, Germany; http://www.zeiss.com) Axiophot-2 microscope.
| RESULTS |
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We have for the first time established the conditions for isolating and propagating neural stem cells from the olfactory bulb of adult human subjects. This was a difficult process because of the intrinsic characteristics of the cells (the small number of stem cells and degraded state of the tissue) and the small number of vital cells obtained from a single adult subject. The individual cells were then plated in growth medium containing 20 ng/ml of EGF and 10 ng/ml of fibroblast growth factor (FGF). When exposed to these mitotic factors, approximately 1% of the cells are "real" stem cells, 70% died, and the others are transiently proliferating progenitors, mostly stuck on the flask surface. Early daughter cells undergo transient proliferation, followed by cell differentiation after a few weeks and eventual death. Conversely, in these condition, the surviving floating cells began to divide and gave rise to spherical clusters which were composed of undifferentiated neural precursor, as demonstrated by the lack of any specific morphology, the absence of neural and glial antigens, and the expression of central nervous system precursor marker nestin (not shown) [15]. Self-renewal was demonstrated by serial subcloning experiments. Single spheres were dissociated and replated in growth medium at low density (less than 100 cells/cm2) to prevent cell aggregation. When individual primary spheres were dissociated and replated as single cells under clonal condition in growth medium, some cells either differentiated to acquire the typical morphology of neuronal/glial cells or died. However, a subset of cells proliferated and gave rise to an average of 6 ± 2 secondary spheres (approximately 150 µm of diameter). Thus the number of 2° neurospheres is equal to the neural stem cells in the original culture [14, 15]. The process is maintained constantly even at late passages in vitro (not shown).
In order to amplify the neural stem cells number, the floating neurospheres were isolated by low-speed centrifugation and then plated in untreated 25 cm2 tissue culture flasks containing growth medium.
The entire process was repeated four times, with the cultures being mechanically dissociated to single-cell suspensions every five days and then replated in fresh growth medium. Using this approach, the cultures proliferated efficiently giving rise to "olfactory bulb stem cell lines." At each passage, the culture expansions were monitored using the trypan-blue method. As shown in Figure 1
, the number of olfactory bulb neural stem cells increased significantly after 15 days in vitro (DIV) and the proliferation rate became exponentially higher after about 20 DIV. Observation of single proliferating cells revealed that the first division always occurred after 3-4 DIV in cells from either early fifth or late fifteenth passages. Accordingly, the slope of the stem cells growth curve remained constant even after a significant number of serial passages. Giving the slow rate of proliferation of adult olfactory bulb neural stem cells, we have successfully maintained cell lines in culture until twentieth passages in vitro. They are cryopreservable and retain full proliferation and differentiation capacity even in long-term cultures (Table 1
) [14, 15]. As already reported in other literature, our cell lines possess the critical stem characteristics such as: A) undifferentiated features (as defined by the lack of differentiated markers); B) self-renewing capacity, and C) pluripotentiality [9, 18], so they can really be defined as olfactory bulb neural stem cells [19]. Thus, likewise in embryonic stem cells, the adult olfactory bulb derived stem cells can be amplified in vitro like embryonic neural stem cells, and it is possible to significantly increase their small starting number.
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In agreement with the significant expression of the transcript for the LIF-ß-receptor in human stem cell cultures [16], exposure to LIF had a striking effect on neuronal differentiation. In comparison with control cultures, there was a twofold increase in the number of cells immunolabeled with ß-tubulinIII after only five days of exposure to LIF, and this difference was maintained after ten and fifteen days in vitro (Table 2
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| DISCUSSION |
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For the first time we have successfully isolated neural stem cells from the olfactory bulb of adult human subjects. The isolation and characterization of neural stem cells from the human olfactory bulb open up a further interesting therapeutic perspective. The high regenerative potential of this area suggests that the olfactory bulb is an ideal autologous source for neurodegenerative disease. Under our optimized conditions, the stem cells obtained from the olfactory bulb, like embryonic stem cells, proliferate and are capable of differentiating into the three classical neural phenotypes. We suggest that stem cells deriving from this area can be simply explanted by means of partial bulbectomy in patients (with few damaging effects such as anosmia). As a result of our many years of experience in manipulating neural stem cells, we are able to expand these cells considerably and, by means of the addition of LIF, ensure their differentiation into neurons [16], the elective biological source for autotransplantation in various neurodegenerative disorders. In particular, the discovery of a large number of immunoreactive tyrosine hydroxylase structures in the olfactory bulbs and peduncles of elderly humans [21] suggests that the olfactory bulb is a hypothetical source for the autotransplantation therapy in Parkinson's disease.
In support of the idea of using olfactory bulb neural stem cells for autologous transplantation in patients with Parkinson's disease, we have recently used an experimental model of Parkinson's disease. Lesions in the nigrostriatal zone were induced by inoculation of 6-hidroxy-dopamine in CD1 mice, and we investigated the ability of these cells to elicit functional recovery after intrastriatal transplantation. These data [22, 23] show that inoculation of these cells induces functional recovery in comparison with control untransplanted mice, and detailed biochemical and immunohistochemical evaluations are currently under way. These are fundamental confirmatory data for the future use of these cells for transplant therapy in patients with Parkinson's disease and other neurodegenerative disorders.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| REFERENCES |
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