Experimental animals
This study was performed using a transgenic strain of mice (OMP-tau-lacZ mice) obtained from the Jackson Laboratory (Bar Harbor, ME, USA). This strain is derived from C57BL/6 mice and the gene sequence encoding the Olfactory Marker Protein (OMP) has been replaced with a tau-lacZ reporter gene [25]. The OMP is expressed in all mature olfactory neurons [26] and the replacement with tau-lacZ reporter gene enables the visualization of olfactory nerve fibers and their projections to olfactory bulb glomeruli. The advantage of using these mice is that a histological assessment of degenerating and regenerating olfactory nerve fibers can be performed using a standard method for staining and light microscopy. Although the functional significance of OMP is not fully understood, previous studies have shown that OMP-tau-lacZ mice are capable of recovering olfactory function after olfactory nerve injury [13–15].
Surgical procedure
Both male and female adult mice were used in this study and randomly assigned to experimental groups. Mice were anesthetized with sodium pentobarbital (80 mg/kg, ip). Under sufficient anesthesia, a frontal craniotomy was performed to expose the olfactory bulbs. An olfactory nerve transection procedure (NTx) was performed between the olfactory bulb and cribriform plate using a curved rigid stainless steel blade to generate a severe olfactory nerve injury model [12]. For histological assessments, the NTx procedure was performed only on the left side (injury side) of the animal while the right side (right olfactory bulb and nerves) remained intact and served as an internal histological control (Fig. 1). For olfactory function assessment, a bilateral NTx was performed, cutting the olfactory nerves to the right and left olfactory bulbs, resulting in a complete loss of smell (anosmia). After the NTx procedure was complete, the skin incision was sutured and the animal closely monitored until it was awake and fully recovered from anesthesia. All protocols and surgical procedures for this study were reviewed and approved by the Institutional Animal Care and Use Committee of Mie University.
IgG injection
To investigate the associations among IgG, the inflammatory reaction and nerve regeneration after injury, the IgG solution (11.4 mg/ml, Jackson ImmunoResearch, USA) was injected intraperitoneally just after the NTx. To determine if there is a dose-dependent effect of the drug, low (200 mg/kg) and high (400 mg/kg) doses of the IgG were used, making them comparable to a dose used in a previous study [22, 23]. For control animals, the vehicle (35 ml/kg of saline) was injected intraperitoneally. We collected data from 6 mice for each of the 3 treatment groups and each of 4 recovery time points (Day 5, 14, 42 and 100) for a total of 72 mice (6 mice x 3 treatments x 4 recovery time points).
Tissue preparation
For histological assays, on the assigned post recovery day mice were anesthetized with sodium pentobarbital (80 mg/kg, ip) and fixed by intracardiac perfusion using 4% paraformaldehyde in phosphate buffer after a saline rinse. The nasal cavity and anterior portion of the skull were removed en bloc and postfixed by immersion in 4% paraformaldehyde for 45 minutes and then placed in 0.5 M EDTA (ethylenediaminetetraacetic acid) for decalcification for 14 days. The tissue was cryoprotected with 30% sucrose for 2 days, then immersed in embedding compound, quickly frozen in a -80°C freezer and sectioned on a cryostat. Serial horizontal sections through the nasal cavities and olfactory bulbs along dorsum nasi were cut at 30 µm and mounted on glass slides.
X-gal staining
Tissue sections were washed at room temperature with buffer A [100 mM phosphate buffer (pH 7.4), 2 mM MgCl2 and 5 mM EGTA (ethylene glycol tetraacetic acid)] once for 5 min and then a second time for 25 min. This was followed by two 5 min washes with buffer B [100 mM phosphate buffer (pH 7.4), 2 mM MgCl2, 0.01% sodium deoxycholate, and 0.02% Nonidet P40]. The blue X-gal reaction was generated overnight in the dark by exposure to buffer C (buffer B, with 5 mM potassium ferricyanide, 5 mM potassium ferrocyanide and 1 mg/ml of X-gal). The X-gal reaction was stopped by two 5 min washes in phosphate buffer.
Measurement of injury-associated tissue and nerve recovery
After confirming the appearance of the blue X-gal reaction, tissue sections were counterstained with a 1% Neutral Red solution. Sections were examined and digitized using charge coupled device (CCD) photomicroscopy. Areas of injury-associated tissue, including inflammatory cells and glial scar tissue, were identified along with blue (X-gal) labeled olfactory nerve endings within the glomerular layer of the olfactory bulb (Fig. 1). The area of injury-associated tissue was outlined on digital images of tissue sections and quantified using ImageJ (ver. 1.53a, National Institute of Health [NIH], USA) software. For the measurement of injury-associated tissue, we targeted an area that was surrounded by the following four margins: the cribriform plate as an anterior margin, anterior edge of the olfactory bulb as a posterior margin, a line connecting posterior end of the nasal septal mucosa and the anteriomedial corner of the olfactory bulb as a medial margin, and a line connecting posteriolateral end of the most lateral sinus and the anteriolateral corner of the olfactory bulb as a lateral margin. Since this targeted area is where the olfactory nerve fibers normally run from the sinonasal mucosa to the olfactory bulb before NTx, as seen in the control side, the tissue levels in this area can be associated with the degree of nerve degeneration and regeneration.
The area (mm2) of tissue observed between the cribriform plate and olfactory bulb (Fig. 1) was measured in two representative horizontal sections (sections A and B) from each animal and averaged. Section A was selected to represent the dorsal level. At this particular level, a large olfactory nerve bundle is observed passing from endoturbinate II through the cribriform plate to the olfactory bulb (Fig. 1). Section B represented a more ventral level. At this level, endoturbinate III attaches to the cribriform plate. The area measurements from NTx mice at each of the four recovery time points were used to compare mean values for injury-associated tissue. The levels of olfactory nerve degeneration and regeneration were assessed by comparing changes in the amount of blue X-gal staining in the glomerular layer on the left (NTx injury side) to that on the right (control) side. Horizontal olfactory bulb sections (Sections A and B) were also used to obtain measurements of: (1) the glomerular layer perimeter distance (G-P distance), a continuous line passing through the center of all the glomeruli within the bulb section, and (2) the total length of glomerular segments along the perimeter that were labeled with the blue X-gal stain (G-X-gal-distance). The ratio of the X-gal-stained distance (G-X-gal-distance) to the total perimeter of the glomerular layer (G-P distance) was obtained for both the NTx injury and control sides. Changes in the blue X-gal nerve staining on the NTx injury-left side were expressed as percentage of the X-gal staining on the intact control side and were used to measure levels of olfactory nerve degeneration and regeneration within the olfactory bulb, as follows:
Immunohistochemical assessment
Immunohistochemical staining for glial fibrillary acidic protein (GFAP) and cluster of differentiation 68 (CD68) glycoprotein was performed on horizontal sections at four different time points following left NTx injury, Day 5, 14, 42 and 100. GFAP is constitutively produced by astrocytes. In the reactive glial response to central nervous system injury, hypertrophic reactive astrocytes increase their expression of GFAP [27]. CD68 staining was used to measure injury-induced inflammatory changes at different time points after NTx injury. CD68 is a lysosomal membrane-associated glycoprotein that is expressed on the surface of histiocytes, cells that are part of the immune system, including macrophages and microglia, and play an important role in phagocytic activities.
After washing with phosphate-buffer saline (PBS) for 5 min, sections were processed by immersion for 1 min intervals in a series of alcohol solutions (70, 95, 100, 95, 70% ethanol). This was followed by three 5 min washes with 0.3% Triton X-100 in PBS. Sections were then incubated with 5% normal goat serum, 1% bovine serum albumin, 0.5% Triton X-100 in PBS for 30 min and reacted with one of the following primary antibodies: rabbit anti-mouse GFAP antibody (1:500, DAKO, USA) and rat anti-mouse CD68 antibody (1:100, AbD serotec, USA). These antibodies were visualized using Cy3-conjugated goat anti-rabbit IgG (1:100, GE, USA) and Alexa Fluor 488-conjugated goat anti-rat IgG (1:100, Invitrogen, USA) under fluorescent microscope, respectively. GFAP- and CD68-positive cells were counted in five different 0.01 mm2 sampling areas located in the anterior (injured) region of the olfactory bulb (five areas: the anterior apex area, areas of anteriomedial corner and anteriolateral corner of the olfactory bulb, and fixed midpoint areas between the anterior apex area and anteriomedial and anteriolateral corner areas). The average number of GFAP and CD68-positive cells/0.01mm2 were then calculated for NTx mice at each of the four recovery time points.
Olfactory function test
To determine if olfactory function recovered after the NTx, a smell detection test using avoidance conditioning behavior to cycloheximide was administered to mice before and after the NTx as reported previously [13–15]. Cycloheximide has a peculiar odor and unpleasant taste for mice. Mice were first deprived of water for 48 hours and then trained to avoid cycloheximide solution. Before NTx surgery, mice were conditioned in two or more training sessions, each consisting of 10 trials. In each trial, the mouse was presented with bottles of 0.01% cycloheximide solution and distilled water one positioned on the left the other on the right side of a test cage. When the mouse licked the delivery tube of either bottle, the bottles were withdrawn from view and presented again. The left and right positions of the two bottles were shifted according to the Gellermann series (cycloheximide bottle position: right (R)-left (L)-L-R-L-L-R-R-R-L). Mice were considered to have learned the smell of cycloheximide when they chose the distilled water bottle 10 consecutive times out of 10 trials (percent score: 100%) on two consecutive test sessions. After NTx surgery, the test was administered every 7 days until the mouse regained its olfactory function (scored 10 out of 10 correct responses), or exceeded a 100-day cut off period. Mice that scored 100% at one of the recovery test days were considered to have fully recovered their olfactory function.
Statistical analysis
All numerical data obtained are expressed as means ± standard error of the mean (SEM). For statistical analysis of the data, the Mann–Whitney U-test was used to determine differences in average values between two groups. For three groups, the two-way analysis of variance (ANOVA) was used and post hoc comparisons were performed by the Bonferroni’s method. The chi-square (χ2) test for independence was used to test for differences in ratio. Differences were regarded as significant when p < 0.05 for two group and p < 0.0167 for three group comparisons.