Varicose projection astrocytes (VP-As) are found in the cerebral cortex and have been described to be specific to humans and chimpanzees. To further examine the phylogenetic distribution of this cell type, we analyzed cortical tissue from several primates ranging from primitive primates to primates evolutionary closer to human such as apes. We specifically analyzed tissue from four strepsirrhine species, one tarsier, six species of platyrrhine monkeys, ten species of cercopithecoid monkeys, two hylobatid ape species, four to six cases each of chimpanzee, bonobo, gorilla, and orangutan, and thirteen human. We found that VP-As were present only in human and other apes (hominoids) and were absent in all other species. We showed that VP-As are localized to layer VI and the superficial white matter of the cortex. The presence of VP-As co-occured with interlaminar astrocytes that also had varicosities in their processes. Due to their location, their long tangential processes, and their irregular presence within species, we propose that VP-As are astrocytes that develop varicosities under specific conditions and that are not a distinct astrocyte type.

Autism spectrum disorder is a neurodevelopmental condition characterized by deficits in social communication and repetitive behaviors. How specific anatomical alterations contribute to the clinical profile of autism spectrum disorder remains largely uncharacterized. We have previously shown that parvalbumin-positive Chandelier cells, a specific type of GABAergic interneuron, are reduced in number in the autism spectrum disorder prefrontal cortex. Here, we assessed the relationship between interneuron pathology with autism spectrum disorder symptom severity and comorbidity. We collected clinical records from autism ( n = 20) and control ( n = 19) brain donors, from whom we previously characterized GABAergic interneuron pathology in three regions of the prefrontal cortex (BA9, 46, and 47). We assessed the relationship between the severity of core symptoms, as indicated by Autism Diagnostic Interview—Revised scores, and Chandelier cell pathology in autism spectrum disorder, and also differences in interneuron pathology associated with autism spectrum disorder comorbidities. Total GABAergic interneuron number was significantly reduced in autism spectrum disorder cases with intellectual disability in the prefrontal cortex (PFC )—by 36.6% relative to autism spectrum disorder without intellectual disability and by 38.7% relative to neurotypical controls. The severity of autism spectrum disorder motor stereotypies was correlated with the severity of Chandelier cell loss in BA47, as indicated by reductions in parvalbumin+ interneurons and GABA transporter 1+ cartridges. Chandelier cell loss is associated with the core autism spectrum disorder symptom domain of restricted repetitive behaviors and likely plays a role in stereotypic motor mannerisms. Intellectual impairment in autism spectrum disorder reflects a more severe form of a common underlying neuropathology-cortical GABAergic interneuron loss. Lay Abstract Autism spectrum disorder is a neurodevelopmental condition characterized by deficits in sociability and communication and the presence of repetitive behaviors. How specific pathological alterations of the brain contribute to the clinical profile of autism spectrum disorder remains unknown. We previously found that a specific type of inhibitory interneuron is reduced in number in the autism spectrum disorder prefrontal cortex. Here, we assessed the relationship between interneuron reduction and autism spectrum disorder symptom severity. We collected clinical records from autism spectrum disorder ( n = 20) and assessed the relationship between the severity of symptoms and interneuron number. We found that the reduced number of inhibitory interneurons that we previously reported is linked to specific symptoms of autism spectrum disorder, particularly stereotypic movements and intellectual impairments.

Chandelier (Ch) cells are cortical interneurons with axon terminal structures known as cartridges that synapse on the axon initial segment of excitatory pyramidal neurons. Previous studies indicate that the number of Ch cells is decreased in autism, and that GABA receptors are decreased in the Ch cell synaptic target in the prefrontal cortex. To further identify Ch cell alterations, we examined whether the length of cartridges, and the number, density, and size of Ch cell synaptic boutons, differed in the prefrontal cortex of cases with autism versus control cases. We collected samples of postmortem human prefrontal cortex (Brodmann Area (BA) 9, 46, and 47) from 20 cases with autism and 20 age- and sex-matched control cases. Ch cells were labeled using an antibody against parvalbumin, a marker that labeles soma, cartridges, and synaptic boutons. We found no significant difference in the average length of cartridges, or in the total number or density of boutons in control subjects vs. subjects with autism. However, we found a significant decrease in the size of Ch cell boutons in those with autism. The reduced size of Ch cell boutons may result in reduced inhibitory signal transmission and impact the balance of excitation to inhibition in the prefrontal cortex in autism.