A TAP1 null mutation leads to an enlarged olfactory bulb and supernumerary, ectopic olfactory glomeruli

Abstract

Major histocompatibility class I (MHCI) molecules are well known for their immunological role in mediating tissue graft rejection. Recently, these molecules were discovered to be expressed in distinct neuronal subclasses, dispelling the long-held tenet that the uninjured brain is immune-privileged. Here, we show that MHCI molecules are expressed in the main olfactory bulb (MOB) of adult animals. Furthermore, we find that mice with diminished levels of MHCI expression have enlarged MOBs containing an increased number of small, morphologically abnormal and ectopically located P2 glomeruli. These findings suggest that MHCI molecules may play an important role in the proper formation of glomeruli in the bulb.

Keywords: P2 glomerulus; main histocompatibility complex I; main olfactory bulb; major histocompatibility class I molecules; olfaction.

Figure 1.

Immunofluorescence labelling for MHCI (red) molecules is greatly reduced in the main olfactory bulb of TAP^−/− animals. White scale bar in (b) indicates 100 µm (both micrographs to same scale). nl, nerve layer; gl, glomerular layer; epl, external plexiform layer; gr, granule cell layer. Nuclei are labelled with DAPI (blue). MHCI is labelled in red. (a) Sample MOB tissue from a C57Bl/6 animal. (b) Comparable MOB tissue from a TAP^−/− animal.

Figure 2.

X-Gal-labelled P2 glomeruli in TAP^+/+ and TAP^−/− mice. High-magnification images of cryosections from the main olfactory bulb. These sections were cut parallel to the long axis of the bulb. The external plexiform layer is labelled epl. The olfactory nerve layer is labelled nl. Axons from P2 olfactory sensory neurons are labelled with the blue X-gal dye. (a) Two representative cryosections from a P2iTLZ animal with labelled P2 glomeruli in the ventrolateral and ventromedial domains of each olfactory bulb. The black, vertical line extending from dorsal to ventral mitral cell layer is the dorsoventral axis, as described in §5. The dashed blue lines outline the glomerular layer (gl) in each cryosection. As is typical for P2iTLZ animals, the left, more rostral image contains a ventral P2-lateral glomerulus at 150°, whereas the right, more caudal image contains a ventral P2-medial glomerulus at 205°. (b) A singlet P2-medial glomerulus in a TAP^+/+ animal. Left (TAP^+/+): typical singlet glomerular cross section from a TAP^+/+ animal. The scale bar indicates 100 µm. Right (TAP^−/−): a set of triplet P2-medial glomeruli in a TAP^−/− animal. Although these glomeruli have a normal morphology, the arrow heads in the right panel point to two exceptionally small glomeruli. (c) Two distinct, abnormally fused P2 glomeruli seen in TAP^−/− animals. (d) Serial montage of glomerular cross sections from a TAP^+/+ doublet (top panel) with one intervening, unlabelled glomerulus and a fused TAP^−/− doublet (bottom panel) with zero intervening glomeruli.

Figure 3.

TAP^−/− animals have a significant increase in the number of supernumerary P2 glomeruli. (a) The average number of TAP^−/− P2 glomeruli, 4.8 ± 1.4 (±s.e.), is significantly larger than the number of TAP^+/+ P2 glomeruli, 3 ± 0.98 (mixed-effects analysis ANOVA, p = 0.037). (b) TAP^−/− animals have a significantly higher incidence of bulbs with four or more P2 glomeruli per bulb. (c) TAP^−/− bulbs have a higher incidence of multiplet glomeruli in a given lateral (P2-lateral) or medial (P2-medial) domain. Singlet: one glomerulus in a given lateral or medial domain of a bulb. Doublet: two glomeruli per domain. Triplet or greater: three or more glomeruli per domain. (d) The number intervening glomeruli between all pairs of duplicate P2 glomeruli. TAP^+/+ animals had a total of 13 pairs of P2 glomeruli (36%) that qualified as duplicate glomeruli, whereas TAP^−/− animals had 25 pairs. Nine TAP^−/− glomeruli had the fused morphology (0 intervening glomeruli), whereas no TAP^+/+ glomerulus had a similar morphology.

Figure 4.

TAP^−/− animals have a significant increase in small P2 glomeruli. (a) Box plots of singlet (S) or multiplet (M) P2 glomerular volumes in TAP^−/− (beige) or TAP^+/+ (blue) bulbs. Triangle centres indicate 95% confidence interval. The singlet TAP^−/− glomeruli triangle interval does not overlap with triangle interval of TAP^−/− multiplets, indicating that singlet TAP^−/− glomeruli are significantly larger than multiplet TAP^−/− glomeruli at the 95% confidence level. (b) Glomerular volume normalized to the total glomerular volume of a given bulb. Median singlet glomerular normalized volume is 0.51 ± 0.03 for TAP^+/+ glomeruli and 0.44 ± 0.05 for TAP^−/− glomeruli. Median multiplet normalized volume is 0.25 ± 0.05 for TAP^+/+ glomeruli and 0.11 ± 0.02 for TAP^−/− glomeruli. Triangle ranges do not overlap for multiplet glomeruli, indicating that TAP^−/− multiplet glomeruli are significantly smaller than TAP^+/+ glomeruli. (c) Histogram of all glomerular volumes for TAP^+/+ (blue) or TAP^−/ (beige) animals highlights the large incidence of P2 TAP^−/− glomeruli with volumes calculated to be below 0.4 × 10⁶ μm³. (d) Cumulative probability of glomerular volumes, indicating significant difference in the volumes of TAP^+/+ and TAP^−/− glomeruli between 0 and 0.8 × 10⁶ μm³.

Figure 5.

TAP^−/− animals have shifted P2 glomeruli along the rostrocaudal axis and enlarged olfactory bulbs. (a) Three-dimensional representations of the IGL from the MOB. Colour maps indicate the average density of X-gal positive glomerular cross sections from all bulbs in TAP^+/+ or TAP^−/− animals. (b) Scatter plot. The cylindrical coordinates of collated P2 glomeruli from 14 TAP^+/+ and 14 TAP^−/− MOBs. The diameter of each circle is proportional to the volume of the glomerulus. Arrows: minuscule TAP^−/− glomeruli. Asterisk: an exceptional TAP^−/− P2 outlier. Notched box plots: notches that do not overlap have medians that differ at the 5% significance level. Outliers that are more than 1.5 times the interquartile range are displayed with a red + sign. Rostrocaudal positions for TAP^+/+ (blue) and TAP^−/− (beige) P2 glomeruli for P2-lateral glomeruli (top) and P2-medial glomeruli (bottom). Note the larger range of rostrocaudal positions for the TAP^−/− glomeruli. Large arrows highlight the significant shift in the median position towards the rostral end of the bulb (rank-sum test, p = 0.05). The bottom panel contains rostrocaudal plots for the P2-medial glomeruli. The right arrow highlights the significant shift in the median position of the P2-medial glomeruli towards the caudal end of the bulb (rank-sum test, p = 6.3 × 10⁻⁴). (c) Location of P2 glomeruli realigned to the median P2-lateral glomerular position in each bulb. Arrow highlights the 200+ μm caudal shift in the median position of TAP^−/− P2-medial glomeruli compared with their TAP^+/+ counterparts (rank-sum rest, p = 0.0011). (d) Location of P2 glomeruli realigned to the median P2-medial glomerular position in each bulb. Arrow highlights the approximately 257 µm rostral shift in the location of TAP^−/− P2-lateral glomeruli when compared with their TAP^+/+ counterparts (rank-sum test, p = 4.0010 × 10⁻⁶).

Figure 6.

TAP^−/− bulbs have an increase calculated surface area and volume. (a) Estimated surface area of the inner glomerular layer (IGL) of olfactory bulbs from TAP^+/+ (blue) and TAP^−/− (beige) animals. The calculated surface area is significantly larger in TAP^−/− bulbs (t-test, p = 0.01). (b) Calculated bulb volumes are significantly larger in TAP^−/− animals (t-test, p = 0.018).