Experiment
Here, the current study describes the QTL mapping of social recognition in a panel of consomic strains, leading to the implication of protocadherin 9 (Pcdh9) in long-term social and object recognition and other phenotypes related to higher order information processing.
Two-day versions of social and object discrimination paradigms were used to measure social and object recognition capacity, using C57BL/6J, A/J, and all 21 C57BL/6J-Chr #A/J/NaJ (chromosome substitution strain [CSS]) strains. Behavioral assessments were conducted at 3 to 5 months of age and only male mice were used in the current study.
In the initial anslysis CSS strain C57BL/6J-Chr 14A/J/NaJ (CSS14) displayed impaired long-term social recognition (SRE) and the strongest decrease in SRE between testing intervals. To establish if the long-term SRE impairment of CSS14 was limited to social cues, novel object recognition (NORE) capacity was also analyyzed. Similar to SRE, CSS14 showed intact NORE following short-term intertrialtesting intervals but impaired NORE following the long-term intertrial intervals.
To localize the genetic loci on chromosome 14 associated with SRE and NORE capacity, an F2 progeny of 192 male mice (derived from an F1 intercross of C57BL/6J-Chr 14A/J/NaJ x C57BL/6J hybrids) was tested in the social and object discrimination paradigms used for CSS14. Following phenotyping and genotyping of the F2 animals, QTL analysis on the CSS14-F2 population revealed significant LOD scores for both long-term SRE and NORE, while no significant LOD scores were obtained following the short-term SRE or NORE.
QTL, Sreq (social recognition QTL) mapped to Chromosome 14 in a 1.0 LOD support interval between 90,927,318 and 106,412,167 base pair with a LOD score of 3.72.
QTL, Noreq (novel object recognition QTL) linked to 24 hour intertrial testing, Noreq mapped to Chromosome 14 in a 1.0 LOD support interval between 92,453,717 and 97,908,061 base pair with a LOD score of 4.75 [Fig 1, F]. There was a partial overlap of 5.5 Mb between the two QTL.
The QTL accounted for 11.0% and 11.1% of trait variance in the long-term social recognition and novel object recognition capacity in the F2 population respectively.
To pinpoint candidate genes contributing to long-term recognition performance in the overlapping 5.5-Mb QTL region,genes and genetic variations were scrutinized using the following selection criteria: 1) brain expression, 2) protein variants coded by one or more nonsynonymous single nucleotide polymorphisms (nsSNPs) (with http://www.jax. org/) between the progenitor strains of the CSS panel (A/J and C57BL/6J), and 3) evidence for association with neurodevelopmental disorders from human genetic studies.
Non-synoymous SNPs in two genes with predominant brain expression: kelch-like 1(Klhl1) and protocadherin 9 (Pcdh9) were revealed. Pcdh9 was selected as a promising candidate in the QTL region because Pcdh9, a member of the cadherin subfamily of protocadherins, is implicated in brain development and functioning.
Specific long-term social and object recognition deficits were confirmed in homozygous (KO) Pcdh9-deficient mice, while heterozygous mice only showed long-term social recognition impairment. The recognition deficits in KO mice were not associated with alterations in perception, multi-trial discrimination learning, sociability, behavioral flexibility, or fear memory. Rather, KO mice showed additional impairments in sensorimotor development reflected by early touch-evoked biting, rotarod performance, and sensory gating deficits. This profile emerged with structural changes in deep layers of sensory cortices, where Pcdh9 is selectively expressed.
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