Monoamine oxidase A and A/B knockout mice display autistic-like features

Abstract

Converging lines of evidence show that a sizable subset of autism-spectrum disorders (ASDs) is characterized by increased blood levels of serotonin (5-hydroxytryptamine, 5-HT), yet the mechanistic link between these two phenomena remains unclear. The enzymatic degradation of brain 5-HT is mainly mediated by monoamine oxidase (MAO)A and, in the absence of this enzyme, by its cognate isoenzyme MAOB. MAOA and A/B knockout (KO) mice display high 5-HT levels, particularly during early developmental stages. Here we show that both mutant lines exhibit numerous behavioural hallmarks of ASDs, such as social and communication impairments, perseverative and stereotypical responses, behavioural inflexibility, as well as subtle tactile and motor deficits. Furthermore, both MAOA and A/B KO mice displayed neuropathological alterations reminiscent of typical ASD features, including reduced thickness of the corpus callosum, increased dendritic arborization of pyramidal neurons in the prefrontal cortex and disrupted microarchitecture of the cerebellum. The severity of repetitive responses and neuropathological aberrances was generally greater in MAOA/B KO animals. These findings suggest that the neurochemical imbalances induced by MAOA deficiency (either by itself or in conjunction with lack of MAOB) may result in an array of abnormalities similar to those observed in ASDs. Thus, MAOA and A/B KO mice may afford valuable models to help elucidate the neurobiological bases of these disorders and related neurodevelopmental problems.

Fig. 1

Monoamine oxidase (MAO)A knockout (KO) and MAOA/B KO mice display social and communication impairments. (a) Schematic diagram of the social interaction paradigm. (b, c) MAOA- and MAOA/B-deficient mice exhibit a significant reduction in social approaches and reciprocal interactions towards the foreign conspecifics compared to wild-type (WT) mice. (d–i) MAO mutant genotypes also displayed a marked decrease in overall sniffing behaviour in both number and duration in comparison to their WT counterparts. While this decrease was most pronounced in anogenital sniffing, MAOA/B-deficient mice also spent significantly less time in frontal sniffing than WT animals. (j–l) In the social investigation paradigm, both MAO mutant genotypes displayed a significant decrease in number and duration of social approaches. (m–p) Maternal separation-induced ultrasonic vocalizations (USVs) were markedly lower in MAOA KO and MAOA/B KO pups than WT pups. Values are displayed as means±S.E.M. *p<0.05, **p<0.01 and ***p<0.001 compared to WT mice. For social interactions n=7–10; social investigation n=8–10; maternal separation-induced USVs n=13–20.

Fig. 2

Monoamine oxidase (MAO)A knockout (KO) and MAOA/B KO mice exhibit perseverative behavioural responses. (a) Schematic representation of the T-maze paradigm. (b) Example of a typical set of alternations for each genotype where the change from black (left arm) to white (right arm) or vice versa indicates an alternation. (c, d) While both MAOA and MAOA/B KO mice displayed a significant reduction in spontaneous alternations compared to wild-type (WT) mice, MAOA/B-deficient mice engaged in a higher number of repetitive arm entries prior to the first alternation compared to the other genotypes. (e) Schematic representation of the hole-board task. (f–h) The 16-bar histograms depict the representative cases of percent relative dispersion of head pokes across the 16 holes of the board for each genotype. (i–l) Both MAO mutants had significantly fewer overall head pokes, as well as a lower average number of alternations per hole, but not head pokes per hole compared to WT mice. MAOA/B KO, but not MAOA KO mice, also exhibited a significantly higher coefficient of variation than their WT counterparts, signifying greater perseverative hole exploration. (m–p) In the marble-burying task, both transgenic lines displayed higher marble-burying and digging activity than WT mice; however, MAOA/B KO responses to marbles were significantly more robust than MAOA KO mice. Values are displayed as means±S.E.M. *p<0.05, **p<0.01 and ***p<0.001 compared to WT mice and ^#p<0.05 compared to MAOA KO mice. For spontaneous alternations in T-maze n=5–8; hole-board n=8; marble burying n=8–9.

Fig. 3

Monoamine oxidase (MAO)A/B, but not MAOA-deficient mice show a significant impairment in learning reversal. (a–c) Typical locomotor pathways during the initial probe trial prior to training. (d–i) All genotypes exhibited a significant decrease in the percent time to reach the platform for spatial learning. (j–o) After reversal training, only wild-type (WT) mice displayed a significant reduction in the percent time to locate the platform, signifying learning acquisition. (p–r) MAOA/B knockout (KO) mice showed a significant reduction in both the total duration and the percent locomotor activity in the platform-associated target quadrant compared to WT mice. This was not caused by alterations in locomotor activity as no changes were found between MAOA/B KO and WT mice. Values are displayed as means±S.E.M. *p<0.05, **p<0.01 and ***p<0.001 compared to controls. ^#p<0.05 compared to MAOA KO mice. ^† p<0.05, ^†† p<0.01 and ^††† p<0.001 compared to first trial within subject. For reversal learning n=6–9.

Fig. 4

Monoamine oxidase (MAO)A knockout (KO) and MAOA/B KO mice exhibit sensory and motor alterations. (a–c) Both MAO mutants display a significant reduction in haptic stimulation, as well as impairments in motor function compared to wild-type (WT) counterparts in the sticky tape and the balance beam tests respectively. In comparison to WT mice, MAOA KO, but not MAOA/B KO mice show an increase in the latency to lick paws in the hot plate test, indicating thermal insensitivity. Values are displayed as means±S.E.M. *p<0.05, **p<0.01 and ***p<0.001 compared to WT mice. For sticky-tape test n=9–10; hot-plate n=9–16; balance beam n=10.

Fig. 5

Monoamine oxidase (MAO)A knockout (KO) and MAOA/B KO mice show a reduction of callosal thickness. (a–c) Coronal images of the corpus callosum in wild-type (WT), MAOA KO, and MAOA/B KO mice. (d) MAOA- and MAOA/B-deficient mice show a significant reduction in thickness in the rostral level of the corpus callosum. Values are displayed as means±S.E.M. * p<0.05 and ** p<0.01 compared to WT mice and ^#p<0.05 compared to MAOA KO mice. For callosal thickness n=12–18.

Fig. 6

Monoamine oxidase (MAO)A knockout (KO) and MAOA/B KO mice show significant disturbances in dendritic arbourization in the orbitofrontal cortex. (a) Schematic diagram of coronal sections through mouse prefrontal cortex. The coordinates indicate position relative to bregma (Paxinos & Franklin, 2001). (b) Computer-assisted reconstructions of Golgi-stained neurons in the orbitofrontal cortex in wild-type (WT), MAOA KO and MAOA/B KO mice. Scale bar: 50 μm. (c) Both MAOA and MAOA/B KO mice display an increase in the number and length of apical dendritic branches of pyramidal neurons in the orbitofrontal cortex. (d) Conversely, MAOA KO, but not MAOA/B KO mice exhibit a significant reduction in the number and length of basilar dendritic branches in the pyramidal neurons of the orbitofrontal cortex compared to their WT counterparts. Values are displayed as means±S.E.M. *p<0.05 compared to WT mice. For dendritic analyses n=6–7.

Fig. 7

Monoamine oxidase (MAO)A/B knockout (KO) mice exhibit morphological abnormalities in the cerebellum. (a–c) Representative photographs of Nissl stained cerebellar layers. Cerebellar layers are labelled in yellow and red arrows represent the Purkinje cells. (d–f) MAOA/B KO animals have significantly fewer Purkinje cells than wild-type (WT) and MAOA KO mice. Moreover, MAOA/B KO mice show an increase in the molecular layer (ML) thickness compared to WT mice and in the granule cell layer (GCL) thickness compared to MAOA KO mice. Values are displayed as means±S.E.M. *p<0.05 and ***p<0.001 compared to WT mice. ^#p<0.05 and ^##p<0.01 compared to MAOA KO mice. Cerebellar layers n=5–8. PCL, Purkinje cell layer.