ASD-related mouse model and autistic children present an impairment in social orientation
We employed a previously characterized orientation task (25) to investigate social orienting responses in Shank3 Knock-out (Shank3−/−) mice, a well-established ASD-related mouse model which presents impairments in social interaction (27, 28) and social preference (29). The experimental mouse was introduced and restrained in an enclosure positioned in the center of a circular arena, allowing it to turn right or left. After 5 mins habituation, we placed a sex-matched juvenile conspecific in the arena, and we calculated the orienting response toward the conspecific (Fig. 1a). Compared to littermate controls (Shank3+/+), we observed that Shank3−/− mice spent less time with the conspecific social stimulus in their respective frontal field (Fig. 1b), indicating a reduced social orientation.
Parallel rodent with humans’ data offers a unique opportunity to understand the complexity of social deficits in ASD and to reveal neuronal mechanisms underlying them. We compared the behavioral characteristics, social orientation, autistic symptoms, and developmental functioning of a human sample of ASD children with those of typically developing children (TD) participating in an ongoing longitudinal study in Geneva (30). For assessing social orientation, we employed two established paradigms (12, 14). In the first controlled visual preference paradigm (12), we quantified the time children spent observing social stimuli compared to simultaneously presented geometric stimuli. Children in the ASD group exhibited a markedly reduced interest in social content presented on the screen (Fig. 1c). In the second paradigm, we presented a more complex social scene, where we compared the gaze patterns of children with ASD to the reference gaze distribution of TD children using an in-house developed methodology (14). This comparison was conducted while both groups watched a 3-minute cartoon. We employed a data-driven approach to quantify the divergence in gaze patterns between children with ASD and TD children. Our analysis showed that children with ASD demonstrated gaze patterns that significantly diverged from those of TD children, (Fig. 1d). To obtain a standardized measure of autistic symptoms we used the ADOS-2 (Autism Diagnostic Observation Schedule, Second Edition) (31). In accordance with their diagnostic classification, children with ASD exhibited a significantly higher presence of symptoms in both the social affect (SA) and the domain of restricted and repetitive interests (RRB) (Fig. 1e-f). Finally, compared to TD children, children with ASD demonstrated greater challenges in several key developmental areas assessed using Mullen Early Learning Scales (MSEL, Mullen 1995): Visual reception, Fine motor skills, and both Receptive and Expressive language (Fig. 1g-j).
MRI revealed a lower connectivity between SC, VTA and other brain areas in children with ASD
To evaluate multivariate pattern of seed-to-whole brain functional connectivity and its association with age and autistic symptom severity, we used the partial least squares correlation (PLS-C) (32, 33). PLS-C revealed one significant component (r = 0.63, p = 0.03). This component (as depicted in Fig. 2a) showed the main (positive) effect of autistic symptomatology, while the (negative) effect of age was less important. The multivariate brain pattern indicated that in children with ASD, higher levels of symptoms co-occurred with hypoconnectivity between the SC and regions involved in reward processing and attention (clusters shown in blue in Fig. 2b). Namely, in the context of the enhanced symptoms, we found a pattern of lower connectivity between the SC and VTA (bilateral), pulvinar (bilateral), right amygdala, visual cortex (bilateral), precuneus (bilateral), vermis and left temporoparietal junction.
Shank3 −/− mice showed reduced activation of SC to VTA neurons and reduced interneural correlations.
We have previously shown that the activity of the SC to VTA neurons is involved in social orienting behavior in mice (25). Based on these findings and human samples showing changes in the SC to VTA pathway, we hypothesize that deficits in neuronal activity within the SC neurons projecting to the VTA may contribute to social interaction deficits. To test this hypothesis, we injected a retrograde cre-expressing virus (AAVrg-Ef1α-mCherry-IRES-Cre) in the VTA and cre-dependent AAV encoding GCaMP7s in the SC (AAV-hSyn1-dlox-jGCaMP7f(rev)-dlox-WPRE, Fig. 3a). We implanted a GRIN lens (Inscopix ©) in the SC (Fig. 3a) and we used miniscope to image the calcium transients of SC to VTA neurons during free social interaction with a juvenile sex-matched conspecific (Fig. 3b). We measured the frequency, decay time and amplitude of the calcium transients, the interneural correlation as well as calcium transients during specific behavioral bouts. Although we did not observe differences in amplitude and decay time between groups, the frequency of calcium transients of SC to VTA was significantly lower in Shank3−/− mice (Fig. 3c). Interestingly, the partial correlation networks estimated using the extended Bayesian information criterion (EBIC) revealed that some neurons presented significant correlations in their activity. During social interaction and the preceding habituation phase, Shank3−/− mice showed significantly lower interneural correlation than controls (Fig. 3d), suggesting a suboptimal SC-VTA pathway.
In a previous study, we demonstrated that the SC-VTA pathway is significantly activated during orientation towards a conspecific, during an ipsi-recorded orientation (25). When we aligned the peri-event time histogram (PETH) on orienting bouts, we observed a lower number of activated neurons in Shank3−/− mice compared to control mice (Fig. 3e, Shank3+/+: 49% vs Shank3−/−: 20%). These data suggest that social orienting deficits may be the consequence of neuronal deficits within the SC to VTA pathway.
SC to VTA neurons in Shank3 KO mice presented altered intrinsic properties
Using patch-clamp recordings on SC neurons projecting to the VTA, we investigated whether the neuronal activity deficits resulted from alteration in intrinsic neuronal properties. To identify the neurons, we injected a retrograde Cre-expressing virus (AAVrg-Ef1α-mCherry-IRES-Cre) in the VTA and a Cre-dependent reporter in the SC (AAV9-hEF1α-dlox-EGFP-dlox-WPRE, Fig. 4a). After at least 3 weeks, we cut coronal slices and measured whole cell recording in current clamp mode. We observed a decreased number of action potentials (Fig. 4b-c), higher input resistance (Fig. 4e), lower resting membrane potential (Fig. 4f) and a tendency of higher capacitance (Fig. 4d) in slices from Shank3−/− mice compared to wild type littermates. Furthermore, we observed an increase in AHP and in rheobase (Figs. 4g-h). The results suggest that deficits observed in the neuronal activity are at least in part the consequence of deficits in intrinsic properties.
SC-VTA impairments are correlated with social behavioral deficits in both rodents and humans.
We next verified whether the alterations observed within the SC to VTA pathway both in mice and humans were correlated with the severity of the behavioral symptoms. In male mice, the time spent in direct interaction with a social stimulus significantly correlates with the SC-VTA interneural correlation (Fig. 5a). Interestingly, we observed that interneural correlation during the habituation was positively correlated with the time of future interaction (Fig. 5b). These findings underscore the critical role of SC-VTA pathway in social behavior and propose that the strength of interneural correlations might serve as a predictive marker for social exploratory behavior towards other conspecifics.
Based on the pattern of functional connectivity (FC) between the SC and VTA revealed by our multivariate analysis, we further explored how the connectivity between the SC and VTA (ROI to ROI) related to other aspects of behavior in children with ASD (Fig. 5c). Concordant with the results revealed by the PLS-C analyses SC-VTA functional connectivity pattern showed a negative relationship with total level of autistic symptoms (Fig. 5d). The SC-VTA FC demonstrated a stronger negative correlation with symptoms in the Social Affect domain (r = -0.39, p = 0.009) compared to the Repetitive and Restricted Behavior (RRB) domain (r = -0.33, p = 0.027), as illustrated in Sup. Figure 1a-b. On the contrary, social orienting derived from the visual preference paradigm positively correlated with SC-VTA FC (Fig. 5e). Specifically, more dynamic visual exploration characterized by a higher saccade number (Fig. 5f) and stronger engagement of the ambient fixation mode (Fig. 5g) correlated positively and significantly with SC-VTA FC (r = 0.45, p = 0.004, r = 0.44, p = 0.003 respectively). No significant correlation was found between the number of focal fixation and the SC-VTA FC (Fig. 5h).
To examine the role of the SC-VTA functional connectivity in developmental changes, we used data from any follow-up visits conducted after the initial recording session with the children. In our protocol, these follow-up visits are scheduled at six-month intervals. However, due to the variability in elapsed time between the MRI session and follow-up visits – where some children were reassessed after six months and others also after one year – we devised a 'rate of change' score. This score was calculated by dividing the difference between the (furthest) follow-up and baseline assessment results (Mullen Scales of Early Learning - MSEL) by the elapsed time period. This approach allowed us to accommodate the varying follow-up time frames across participants and more accurately assess the contribution of FC connectivity to developmental changes over time. In our study, follow-up data from the MSEL were available for 28 children. Our analysis revealed a generally positive correlation between the developmental rate of change and SC-VTA connectivity (Fig. 5i). This overall effect appeared to be primarily influenced by two developmental domains: receptive language (Fig. 5j) and fine motor skill coordination (Fig. 5k). Both areas necessitate the fine and timely integration of multisensory information. This finding suggests that unaltered SC-VTA connectivity may play a significant role in the developmental acquisitions in these specific cognitive and motor domains.