Loss-of-function mutations in SHANK2 are associated with autism spectrum disorder (ASD), and past studies in mouse models have suggested that SHANK2 and other ASD-risk mutations cause deficits in synaptic connectivity. Yet analyses of neuronal mechanisms in humans have been difficult due to heterogeneity among neurons generated from different induced pluripotent stem cell (iPSC) lines.

To overcome these issues, SFARI Investigator James Ellis and colleagues adapted a sparse co-culture protocol (Shcheglovitov et al., Nature, 2013) to generate neuronal cultures with consistent densities that are suitable for connectivity assessments. Essentially this involved differentially labeling and sparsely seeding control and mutant neurons together on a lawn of unlabeled control neurons.

Ellis and his team applied this co-culture system to iPSC-derived cortical neurons from two ASD individuals that have mutations in SHANK2 that lead to loss of one functional copy of the gene. They observed increases in many synaptic measures (e.g., dendritic length and complexity, synapse number and frequency of spontaneous excitatory postsynaptic currents). Furthermore, they demonstrated that these cellular phenotypes could be rescued when the SHANK2 mutation was corrected.

These results are in line with the demonstration of increased synaptic density in postmortem tissue from individuals with ASD but are in contrast to studies in mice harboring mutations in Shank2, Shank3 and other ASD-risk genes that have largely demonstrated decreases in connectivity.

Combined, these data support the idea that alterations to neuronal connectivity are likely a key mechanism in ASD but highlight the importance of assessing connectivity in well-defined experimental systems.