Introduction
Hearing loss is one of the most common disabilities found worldwide; the severity can range from modest difficulty with speech comprehension to profound deafness. In over 80% of cases, the cause of hearing loss is related to the degeneration and death of auditory hair cells and their associated spiral ganglion neurons.1) The damaged hair cells and spiral ganglion cells have a limited capacity for self-repair.2)
The bone marrow contains two types of stem cell populations, the hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). HSCs are the source of blood cells; the MSCs have the capacity for self-renewal. MSCs have the ability to differentiate into stromal cells including muscle, epithelium, and neural cells.3-8) The ability of MSCs to divide (self-renewal) and differentiate (plasticity) suggests that they might be useful for cell repair.
Regeneration or replacement of damaged hair cells and neurons in the cochlea might provide ideal treatment for sensorineural hearing loss (SNHL). Recent studies have demonstrated that auditory hair cells and neurons can differentiate from endogenous stem cells of the inner ear and from embryonic stem cells.9,10) Therefore, bone marrow-derived MSCs might be used to treat irreversible inner ear disease, especially SNHL.
The purpose of the present study was to investigate whether MSCs derived from the bone marrow of rats could differentiate into auditory hair cells and neurons.
Materials and Methods
Animals
The Animal Ethics Committee, at the Medical College of the Catholic University of Korea, approved all experimental protocols (CUMC 2008-0115-02). A total of 10 Sprague-Dawley (SD) rats were used in this study.
The isolation and culture of mesenchymal stem cells from bone marrow
Ten SD rats were anesthetized with an intraperitoneal injection of sodium pentobarbital (4 mg/100 g). A small skin incision was made to expose the femur and tibia. Approximately 2-3 mL of bone marrow cells were aspirated and mixed with an equal volume of phosphate buffered saline (PBS) with heparin (10 units/mL). The cells were then centrifuged at 1000 rpm for 10 minutes. The cell pellets were resuspended with bone marrow-derived mesenchymal stem cells (BM-MSCs) medium composed of Dulbecco's Modified Eagle Medium (Gibco, Carlsbad, CA, USA) supplemented with 10% fetal bovine serum (FBS; Biowest, Nuaille, France), 100 units/mL penicillin-streptomycin (Gibco, Carlsbad, CA, USA), and 0.1 mg/mL of streptomycin (Gibco, Carlsbad, CA, USA). Five days after culture, the suspension was discarded. Only the cells that attached to the culture dish were isolated and rinsed. These cells were then cultured in minimum essential media alpha(+)glucose, (Gibco, Carlsbad, CA, USA) containing 10% FBS and 1% P/S. The culture medium was replaced at 3-day intervals. Only MSCs that were subcultured to the third generation were used for the experiments (Fig. 1A).
Differentiation of bone marrow-derived mesenchymal stem cells into neuronal progenitor cells, hair cells and neurons
Bone marrow-derived MSCs were isolated by density gradient centrifugation and sub-cultured three times. The BM-MSCs were cultured separately and aliquots of the cells were transferred into a 4 well chamber slide (Nalgen Nunc International, Rochester, NY, USA) at a concentration of 1×104 cells/well. Variable concentrations of growth factors were added to the culture medium and the differentiation into neuronal progenitor cells, hair cells and neurons was induced. All these neural differentiation medium changes were replaced at 3-day intervals.
The differentiation into neurospheres
To determine whether the bone marrow-derived MSCs could differentiate into neurospheres, the MSCs were cultured in a medium containing 20 ng/mL of epidermal growth factor (EGF, invitrogen) and 10 ng/mL of basic fibroblast growth factor (bFGF, invitrogen) for 14 days. At 3-day intervals, 10 ng/mL of bFGF was added to the culture medium three times (Table 1).
The differentiation into hair cells and neurons
The differentiation into hair cells and neurons was induced by altering the composition of the culture medium used for the BM-MSCs. The basic medium was a neurobasal medium. The nerve growth factors added included glial-derived neurotrophic factor (GDNF, invitrogen), brain-derived neurotrophic factor (BDNF, invitrogen) and neurotrophin-3 (NT-3, invitrogen) (Table 1).
Confirmation of the presence of the hair cells and neurons using immunocytochemistry
Bone marrow-derived MSCs were differentially cultured; aliquots of the cells were transferred into a Lab-Tec 4-well Chamber Slide (Nalgene Nunc International, Rochester, NY, USA) at a concentration of 1×104 cells/well. Then, the differentiation into hair cells and neurons was induced. Then immunocytochemistry was performed 14 days after cells differentiation. First, to confirm specific cellular proliferation, BrdU (5-Bromo-2'-deoxy-uridine Labeling and Detection Kit, Roche, Indianapolis, IN, USA), a marker for cellular proliferation was used; BrdU labeling was performed over 24 hours. The washing buffer was PBS containing 3% Bovine Serum Albumin (BSA, Ogilvie ST Essendon, Australia). The labeled cells were rinsed three times. Thereafter, fixation was performed using 4% paraformaldehyde for 15 minutes. To permeate, the samples were treated with 0.5% Triton X-100 (Promega Corporation, Madison, WI, USA) at room temperature for 20 minutes. The samples were treated with primary antibody, anti-BrdU working solution (1 : 10), overnight, and then rinsed three times with PBS containing 3% BSA.
Next, the samples were treated with a secondary antibody, anti-mouse-Ig-Fluorescein working solution (1 : 200), at room temperature for a few hours. Following this, blocking was performed with 5% normal goat serum. In each sample, specific cellular markers were used; glial cell markers such as Glial Fibrillary Acidic Protein (GFAP, Abcam, Cambridge, UK) and S-100 (Abcam), neuronal cell markers such as βIII-tubulin (Abcam) and NeuN (Abcam), neuronal progenitor markers such as nestin (Abcam), and hair cell markers such as myosin VIIA (Abcam).
A double staining procedure was performed. All of the antibodies that underwent double staining were processed overnight and then rinsed three times with PBS containing 3% BSA. The secondary antibody, Alexa 555, was added to the samples at room temperature at a ratio of 1 : 200 for 1 hr. Then, expression was confirmed. In addition to the nuclear staining, the cells were mounted with DAPI (4',6-diamidino-2-phenylindole)-conjugated mounting medium (Vectashild, Burlingame, CA, USA). Expression was confirmed using a Fluorescence Attached Microscope (Olympus corporation, Shinjuku, Tokyo, Japan). Based on the evaluation of cells, the number of expressed cells per total number of cells was calculated.
The expression of hair cells and neuronpositive cells on RT-PCR
The bone marrow-derived-MSCs were sub-cultured three times in a 100 mm dish at a concentration of 1×104 cells/well. Then, differentiation into hair cells and neurons was induced. The cells were studied to determine whether they differentiated into neuronal progenitor cells, hair cells and neurons over time. The cells were divided into two groups: 7-day and 14-day differentiation. RNA was extracted from these cells and PCR performed by reverse transcription.
RNA was isolated from the cultured cells, using the AXYGEN RNA mini prep kit (Axygen Scientific, Inc., Union City, CA, USA). The isolated RNA was used for the reverse transcription at 42℃ for 60 minutes and at 95℃ for 5 minutes with the RT Premix Kit (Intron Biotechnology, Seongnam, Korea). Then, cDNA was synthesized. The synthesized cDNA was quantified to 100 ng. Then, using the RT-PCR premix (Bioneer, Daejeon, Korea) and GFAP, S100, βIII-tubulin, nestin, Bone Morphology Protein 4 (BMP4), BMP7, myosin VIIA primer, DNA denaturation was performed at 94℃ for 5 minutes. Each of the gene annealing temperatures required different conditions; the reaction was performed for 30 seconds. At 72℃, DNA polymerization and extension were performed for 5 minutes. The House keeping gene, GlycerAldehyde-3-Phosphate Dehydrogenase was used as a control. Expression was confirmed by electrophoresis, using a 2% agarose gel. The expressed genes were confirmed by band analysis using the Image Analysis System (Bio-Rad, Herculesus, CA, USA). The primer compositions for each gene are listed in Table 2.
Results
Confirmation of differentiation of hair cells and neurons using immunocytochemistry
For differentiation into neuronal positive cells the isolated MSCs were cultured in medium containing BDNF, GDNF and NT-3. 14 days after, spherical-shaped cells were observed (Fig. 1-3). To confirm whether the cells were neuron, hair cell or schwann cell, the cells were treated with specific neural lineage markers. The expression of neuronal precursor cell markers (nestin) and cellular proliferation markers (BrdU) in the neuronal positive cells were confirmed by immunocytochemistry (Fig. 1A-D). These cells had the capacity for subculture and self-renewal. The sphere cells (Fig. 4A) became spherical after 1 week. After 2 and 3 weeks, the cells were larger (Fig. 4B, C), and the sphere measured 50-150 µm. Subsequently, the size of the neuronal progenitor cells did not increase further.
Neuronal positive cells expressed βIII-tubulin (Fig. 3A-D) and NeuN (Fig. 3E-H), as a markers for neurons. Moreover, S-100, a marker for glial cells (Fig. 2A-D), was also observed. And the hair cells markers myosin VIIA positive cells were seen in approximately 3-4% (400±100) of 1×104 cells in the neuronal medium (Fig. 1E-H).
Neurons are characterized by the presence of axons and dendrites, which were observed after culture (Fig. 3). Although the number of cells did not increase, axon-like and dendrite-like cells were developed neuronal differentiation (Fig. 2, 3). The neuron-like cells stained for BrdU (Fig. 3B, F), and all of the cells expressed specific markers for neurons, such as, NeuN and βIII-tubulin (Fig. 3). Overall, 7-8% (700±100) of 1×104 cells in neuronal medium expressed the neuronal cell markers. Also another cell group also stained for BrdU (Fig. 1B, F) and expressed neuronal progenitor cell markers, such as nestin, and glial cell markers, such as S100 and GFAP (Fig. 1, 2). These cells morphology were more elongated. And GFAP and s100 positive cells were seen in approximately about 20% (2000±50) of 1×104 cells in the neuronal medium expressed.
Confirmation of differentiation of neuronal cells using RT-PCR
In 14-day cultures, the differentiation from BM-MSCs to neurospheres and neurons were confirmed based on the expression of genes such as: nestin, BMP4, GFAP, S-100, NF, and βIII-tubulin (Fig. 5). However, the expression of Myosin VIIa (a hair cell marker) was extremely low, until two weeks. The proportion of cells that differentiated into Schwann cells and neurons was much higher than cells that differentiated into hair cells.
Discussion
The stem cells located in the bone marrow are the primary source of blood cells. In addition to these hematopoietic stem cells, the bone marrow contains MSCs that can differentiate into various cell types including all three embryonic germ layers. Recent studies have demonstrated that bone marrow-derived MSCs can differentiate into cartilage, insulin-producing cells, muscle cells, and neurons.3-8) Furthermore, bone marrow-derived MSCs have been reported to differentiate into inner ear sensory cells.11) The results of this study show that bone marrow-derived MSCs, from the rat, can differentiate into neuronal progenitor cells, and be used for the treatment of injured cells.
Although there are several methods for the isolation of stromal cells from bone marrow, there is currently no optimal method. The known methods include plastic adherence,12) gradient density centrifugation13) and immunomagnetic selection.14,15) In this study, the MSCs were isolated and cultured from bone marrow using the gradient density centrifugation method.
Jeon, et al.11) reported that MSCs from bone marrow could differentiate into inner ear sensory cells. These authors noted that over-expression of the pro-sensory transcription factor, Math1, in sensory epithelial precursor cells, induced the expression of developing sensory epithelial markers and mature hair cell markers; in addition, they detected the expression of supporting cell markers.11) In the current study, bone marrow-derived MSCs were isolated and cultured in neurosphere medium containing bFGF and EGF, and as a result, neurospheres developed. EGF and bFGF are known to play a role in promoting the proliferation and differentiation of neurons.16,17) The neurospheres identified in this study expressed nestin and BrdU, both neural progenitor cell markers. Nestin is an intermediate filament protein expressed in dividing cells during early development in the central nervous system, peripheral nervous system and in myogenic and other tissues; it is expressed by neuronal precursor cells; however, it can also be expressed by many other types of cells during development.18) BrdU, a thymidine analog that incorporates the DNA of dividing cells during the S-phase of the cell cycle, is a marker for neurogenesis.19)
The differentiation of induced neuronal progenitor cells from mature hair cells and neurons was induced by neurotrophins including BDNF, GDNF and NT-3. BDNF is a neurotrophin that is present most abundantly in the brain. It promotes the growth of neurons and regulates the synthesis, metabolism and excretion of neuro-transmitters and the activation of neurons. BDNF and NT-3 play an important role in the maturation of inner ear neurons, and in the differentiation of neural stem cells to neurons.20,21) GDNF and NT-3 are also known to promote the growth of glial cells, astrocytes and neurons. Incubation with these neurotrophic factors resulted in enrichment of progenitors that could be converted to hair cells and neurons.
When the bone marrow-derived MSCs were cultured using neurosphere medium and neuronal medium, the pattern of expression of specific genes was confirmed. Both the differentiation into neuronal progenitor cells and into hair cells and neurons confirmed the presence of gene expression over time. However, there was a difference in the differentiation observed in the neurosphere medium compared to the neuronal medium. The results of this study confirmed the expression of specific types of genes.
Staining properties were identified using BrdU, one of the specific markers of cell proliferation potential. However, the results confirmed the proliferation potential of neuronal progenitor cells. Bone marrow-derived MSCs, present in the neuronal medium, showed complete differentiation with cell proliferation. Compared to the neuronal progenitor cells, BrdU, a specific marker for proliferation potential, showed a relatively low degree of staining in the differentiated cells. These results suggest an inverse proportional relationship; the differentiated cells showed a lower degree of proliferation potential.
Conclusion
The results of this study demonstrated that bone marrow-derived MSCs could differentiate into neuronal progenitor cells. These findings suggest the possibility that such cells might be used for further differentiation and treatment of inner ear disease, such as SNHL, on the cellular level.
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