Complete deletion of the neurotrophin receptor p75NTR leads to long-lasting increases in the number of basal forebrain cholinergic neurons - PubMed (original) (raw)

Complete deletion of the neurotrophin receptor p75NTR leads to long-lasting increases in the number of basal forebrain cholinergic neurons

Thomas Naumann et al. J Neurosci. 2002.

Abstract

Cholinergic neurons innervating cortical structures are among the most affected neuronal populations in Alzheimer's disease. In rodents, they express high levels of the neurotrophin receptor p75NTR. We have analyzed cholinergic septohippocampal neurons of the medial septal nucleus in p75exonIII (partial p75NTR knock-out) and p75exonIV (complete p75NTR knock-out) mice, in their original genetic background and in congenic strains. At postnatal day 15, the p75exonIII mutation leads to a moderate increase (+13%) in these neurons among littermates only after back-crossing in a C57BL/6 background. In contrast, the null p75exonIV mutation, which prevents expression of both the full-length and the shorter p75NTR isoforms, results in a 28% neuronal increase, independent of genetic background. The incomplete nature of the p75NTR mutation used previously, coupled with difficulties in delineating the mouse medial septum and the impact of the genetic background on cell numbers, all contribute to explain previous difficulties in establishing the role of p75NTR in regulating cholinergic neuron numbers in the mouse forebrain.

PubMed Disclaimer

Figures

Fig. 1.

Mutations and mouse strains used in this study.A, List of the five strains analyzed and their origins.B, Schematic representation of the full-length p75NTR and the splice variant s-p75NTR proteins, depicting their different domains and their correlation to the intron–exon structure of the genomic locus. Arrows point to the approximate location of the targeting event: the p75 exonIII_mutation (Lee et al., 1992) replaced part of exon III with a selection cassette, whereas the p75exonIV_mutation inserted the cassette in reverse orientation within exon IV, thus preventing expression of both the full-length and the s-p75NTR splice isoform (von Schack et al., 2001).C, Breeding scheme for the_p75exonIII line (left) and generation of a congenic B6 strain bearing this mutation. The original line obtained from The Jackson Laboratory was maintained by heterozygous matings and crosses to Sv129, resulting in a line of mixed background in which Sv129 is most prominent. To generate the congenic B6 strain (right), originally mixed-background heterozygotes were consecutively mated to B6 males from Charles River Laboratories. The congenic_p75exonIV(B6) strain was generated according to the same scheme.

Fig. 2.

Retrograde FG tracing for septohippocampal neurons to delineate the actual boundaries of the MS. Five coronal sections of the septal complex from a p75 exonIV(Sv129/B6) mutant after intrahippocampal injections of the retrograde fluorescent tracer FG (middle columns) and subsequent immunocytochemical detection of ChAT-positive cholinergic neurons in the same sections (right columns) are shown. The actual position is indicated using section numbers according to our nomenclature described in Figure 3. Left, A schematic anatomic diagram (adapted from Franklin and Paxinos, 1997) of the estimated position. The shaded area represents the MS region in which FG-backlabeled cholinergic neurons projecting to the hippocampal formation were identified. The _dotted line_connecting the inferior edge of the anterior commissure marks the inferior boundary of the area in which neurons were counted. Note that the extent of the shaded area (and therefore the location of cholinergic MS neurons to be counted) changes substantially along the rostrocaudal axis of the MS. Oblique, dashed lines in the last panel (position 16) indicate the boundary between MS and neighboring ventrocaudal nuclei not analyzed in this study.

Fig. 3.

Variability in the localization of the MS nucleus in p75 exonIII and_p75exonIV_ mutants. Representative examples of the morphological analysis performed for each mutation in the different backgrounds and at two different ages are shown. In each_panel_, the mutant mice are represented in the top half, and the corresponding wild-type littermates are represented in the bottom half. In each individual_panel_, mice belonging to the same litter are represented by the same color. All frontal sections of the entire septal complex were collected as complete series and numbered according to their exact position from rostral (left) to caudal (right) positions (vertical lines in each_panel_). Independent of malformations of the fiber tracts, the coronal section through the septal complex was designated section c.c., where the tips of the corpus callosum were first found in close contact (Fig. 2). Because every second section of the complete series was used for quantitative stereological analysis, each position refers to the “0” position of the c.c., using even numbers in both directions. Each horizontal bar represents the complete series of the septal complex sections containing cholinergic MS neurons in an individual mouse. Numbers on each_bar_ (e.g., 4959/22) indicate the total number of ChAT-immunoreactive neurons counted in the MS (4959) and the number of sections collected through the MS (22). In some cases, virtually no cholinergic neuron was detectable in the most caudal section of the MS region (hatched region of the_horizontal bar_). Note the higher degree of variability in the p75 exonIII line than the_p75exonIV_ line, both at P15 and in the adult.

Fig. 4.

Variability in the number of ChAT-immunoreactive MS neurons in different wild-type strains. The number of MS cholinergic neurons in wild-type mice from six different lines was quantified using the criteria described in the legends to Figures 2 and 3.Numbers on each bar indicate mean values from 10 animals; error bars indicate SD. In general, a higher B6 content in the genome correlates with a lower number of ChAT-immunoreactive MS neurons.

Fig. 5.

Effect of the two_p75_ NTR mutations at P15. The effect of each mutation in a mixed background (left) or in congenic B6 strains (right) was quantified as described in Figures 2 and 3. For each line, 10 wild-type and 10 homozygous mutant mice from heterozygous matings were analyzed at P15. The moderate effect of the p75exonIII_mutation in the mixed background (6.5% increase) is enhanced in the B6 background (13% increase). In contrast, the_p75exonIV mutation, which prevents expression of both the full-length receptor and the s-p75NTR isoform, leads to comparable effects in both genetic backgrounds (28% increase in B6 and 22% increase in a mixed background).

Fig. 6.

Effect of the two p75NTRmutations at 3 months of age. The persistence of the effect of each p75NTR mutation was evaluated at 3 months of age in the congenic B6 strains as described in Figure 5. Although the effects of both mutations are less pronounced with increasing age, the larger increase seen in the p75 _exonIV_compared with the p75exonIII mutation persists.

Fig. 7.

Differential expression of the s-p75NTR isoform in different genetic backgrounds. The accumulation of the s-p75NTR mRNA was analyzed by reverse transcription-PCR in P15 whole brains and in MS from B6 and Sv129 mice. Unlike the FL-p75NTR mRNA, which accumulates at comparable levels in both strains, in whole brain the s-p75NTR transcript accumulates at much higher levels in Sv129 than in B6 animals. Shown here are the results from the MS (pooled from 5 mice of each strain), in which s-p75NTR accumulates at >10-fold higher levels in the Sv129 background.

References

1. Albanese A, Gozzo S, Iacopino C, Altavista MC. Strain-dependent variations in the number of forebrain cholinergic neurons. Brain Res. 1985;334:380–384. -PubMed
1. Bamji SX, Majdan M, Pozniak CD, Belliveau DJ, Aloyz R, Kohn J, Causing CG, Miller FD. The p75 neurotrophin receptor mediates neuronal apoptosis and is essential for naturally occurring sympathetic neuron death. J Cell Biol. 1998;140:911–923. -PMC -PubMed
1. Bartus RT, Dean RL, Beer B, Lippa AS. The cholinergic hypothesis of geriatric memory dysfunction. Science. 1982;217:408–414. -PubMed
1. Bentivoglio AR, Altavista MC, Granata R, Albanese A. Genetically determined cholinergic deficiency in the forebrain of C57BL/6 mice. Brain Res. 1994;637:181–189. -PubMed
1. Bibel M, Barde Y-A. Neurotrophins: key regulators of cell fate and cell shape in the vertebrate nervous system. Genes Dev. 2000;14:2919–2937. -PubMed

Complete deletion of the neurotrophin receptor p75NTR leads to long-lasting increases in the number of basal forebrain cholinergic neurons - PubMed (original) (raw)