Despite first being described at the dawn of the 20^th century, autism was not nearly as hot a topic until the publication of Andrew Wakefield’s discredited paper linking the condition to the MMR vaccine. Consequently, media reporting on autism research has focused on the investigation between this purported link to reassure the public of vaccination’s safety. However, this is only one of many avenues of research being conducted into the condition. Last year, a team led by Dr. Sarwat Amina verified and quantified a decreased expression of chandelier cells in people diagnosed with autism. The findings support a long-standing hypothesis that autistic traits may be due to altered states of excitation as compared to neurotypical brains. Autism activists often justifiably balk when their condition is referred to as a disease in need of treatment, and this is a position to which researchers need to be sensitive. However, unravelling the neural roots of autism has the potential to alleviate the struggles of people living with its most severe forms and contributes to the wider body of knowledge surrounding the implications of cellular and circuit diversity.

Background

The catch-all term autism refers to a spectrum of disorders characterized by altered social communication and behavior; repetitive behaviors; and niche, obsessive interests. The impact autism has on the individual varies wildly: The majority are just as capable at living independent lives as their non-autistic peers. Unfortunately, those not included in that number often have severe developmental delays that impede their ability to care for themselves.^vi Autistic people also tend to share other similarities such as unusual sleeping habits, gastrointestinal issues, seizure disorders, and comorbid neuropsychological disorders.^v This interplay between brain and body is partially why researchers are so interested in puzzling out the physiological origins of autism.

Scientists in the field have suspected since at least the late 90’s that altered excitation and inhibition patterns of specific sensory, memory, social, and emotional circuits could underlie the expression of autism. Less clear was how the activity of nerve circuits was being altered. The observed excitation could be explained through changes in cell populations, changes in the circuitry itself, or both.ⁱⁱⁱ As a starting point, interneurons posed a particularly tantalizing target as they function as neuronal “brakes” to decrease and synchronize activity in the integrative areas of the brain. In previous experimentation, the UC Davis lab had determined that of the 3 general classes of inhibitory interneuron, only parvalbumin-positive (PV+) ones were decreased in autism. The lab then followed up with a second study to determine the extent of reduction in PV+ cell types and found that the reduction occurred almost entirely in chandelier cells (ChCs) as opposed to basket cells.

ChCs are striking structures. A subclass of inhibitory interneurons, their axons end in many branching terminals called cartridges.ⁱⁱⁱ Each cartridge terminates in one or more boutons that can synapse with the axon initial segment of excitatory pyramidal cells (ePCs).ⁱⁱⁱ While each ePC may only be innervated by 1 – 3 ChCs, the ChCs are capable of innervating many ePCs.ⁱⁱⁱ Through these many connections, populations of ChCs can selectively calm the activity of ePCs and synchronize their activity.ⁱⁱⁱ Based on the excitation/inhibition imbalance theory of autism, and armed with the results of their previous work, the team then set out to determine if ChCs themselves are reduced in the autistic brain or if the cartridges are present, but not expressing PV.

Methods

The team acquired postmortem samples of brain tissue from eleven autistic individuals and eleven control individuals via the Autism Tissue Program.ⁱⁱⁱ For this study, a control subject was defined as not having any neurological disorder based on medical records and information from next of kin.ⁱⁱⁱ The subjects ranged in age from 7 to 24 years old at the time of death and represented a variety of causes of death.ⁱⁱⁱ Tissue samples were taken from the Broadman areas (BA) 9, 46, and 47.ⁱⁱⁱ These regions were chosen because they represented areas of the brain that are spatially distinct. Each sample was thinly cut by a cryostat and a small portion was Nissl stained to positively identify the cortex of origin.ⁱⁱⁱ Areas surrounding the Nissl stain were then immunostained with antibodies for GABA transporter 1 protein (GAT1), a known label for ChC cartridges.ⁱⁱⁱ After staining, the researchers examined a 3mm bin encompassing all cortical layers and counted the number of cartridges present.ⁱⁱⁱ This process was replicated once more on a different tissue sample for each cortical area.ⁱⁱⁱ Since the goal of the study was to determine if there is a significant difference in the number of ChCs found in autistic versus control brains, the results were analyzed with a t-test.ⁱⁱⁱ The researchers also attempted to account for other factors such as age at death and time in storage using ANOVA.ⁱⁱⁱ

Results

The researchers found that autistic brains did indeed express fewer ChCs in all areas sampled when compared to the cell populations of controls.ⁱⁱⁱ The reduction was moderate and roughly equivalent in BA9 and BA47, but BA46 exhibited a dramatic decrease in ChC populations at a 61% reduction.ⁱⁱⁱ  Most interestingly, these results aligned quite nicely with the data from the study that aimed to quantify the reduction in PV+ interneurons. For BA9, the loss of PV+ interneurons was found to be 42% and the reduction in cartridges was 39%.ⁱⁱⁱ In BA46, there was a 60% decrease in PV+ soma and a 61% decrease in cartridges.ⁱⁱⁱ Finally, in BA47, PV+ soma were reduced by 31% and cartridges were reduced by 46% in autistic samples.ⁱⁱⁱ The authors did note that the slight differences between the studies could be due to the influence of age.ⁱⁱⁱ The older study used a larger age range of samples and ChC density changes with age.ⁱⁱⁱ Though the researchers attempted to account for any confounding factors via ANOVA, none of the analyses returned any significant findings.ⁱⁱⁱ

Significance

Not only was the UC Davis team able to determine that the reduction in PV+ interneurons seen in earlier work was due to a reduction of ChC cartridges, they also validated the findings of those studies by reporting similar results. Since the percent decreases in PV+ interneurons and ChCs

are so similar, it does strongly suggest that the dearth of PV+ soma is sufficiently explained by a loss of ChCs. This implies that the excitation/inhibition imbalance seen in autism is primarily caused by an increase of excitation in key cortical areas. Since ChCs are powerful inhibitors of excitatory neurons, their absence most likely leads to greater excitation states than one might expect in a neurotypical brain. As seen in figure 1, fewer ChCs release less GABA. They may release so little that ePCs are not inhibited and continue to elicit excitation through glutamate release. The authors are careful to note that they have not yet ascertained if the ChCs in autistic people are compensating in some way to make up for being fewer in number.ⁱⁱⁱ More research is needed, and next steps should include accounting for cellular adaptations that could augment the inhibitory power of ChCs such as an increased number of synaptic boutons, an increase in the number of vesicles released or the amount of GABA in each vesicle, and overall activity level of the ChCs. These could be investigated through genetic fluorescent labeling, electrophysiological, and biochemical investigations.^,

Researchers must be cautious not to over-pathologize people with autism. Many with the condition consider it a key part of their identity and do not wish for “treatment”.^iv We must also not discount the value of diverse perspectives from neurodivergent individuals – one must look no further than the contributions made by visionaries such as scientist Temple Grandin and jazz musician Matt Savage. However, some forms are autism are much more severe than others and can pose hardships on the individual and their family. Unearthing the mechanism of autism at scale could help lead to interventions, pharmaceutical and otherwise, that increase the quality of life for such people. Those with milder
forms of autism may also benefit if such discoveries explain some of the inconvenient comorbidities that can
accompany autism. In any case, this study serves to reminds us of the beauty of neurological variation and how our
perception of the world itself begins in our cells.

[+] References

[+] Other Work By Rachel Buzzelle