The gene that hijacks fear: How PTEN rewires the brain’s anxiety circuit

PTEN has emerged as one of the most significant autism risk genes. Variations in this gene are found in a significant proportion of people with autism who also exhibit brain overgrowth, making it a key player in understanding differences in brain function. To investigate the impact of PTEN misregulation, researchers have turned to animal models, where global reduction of PTEN results in altered sociability, repetitive behaviors, and increased anxiety that are often associated with ASD in humans.

But understanding how PTEN dysfunction results in specific circuit and behavioral changes has been difficult in animal models that disrupt PTEN throughout the nervous system. Therefore, MPFI research group leader Dr. McLean Bolton and her team have focused on the changes in the central lateral amygdala driven by loss of PTEN in a critical neuronal population -- somatostatin-expressing inhibitory neurons.

Alterations in the function of inhibitory neurons in the development of ASD have been seen through both human tissue studies and genetic mouse models. Moreover, the PTEN gene is known to regulate the development of inhibitory neurons. Therefore, a cell-type-specific disruption of PTEN in inhibitory neurons was a valuable target for understanding specific circuit changes associated with ASD.

"Although a cell-type specific disruption does not replicate the genome-wide changes seen in humans, it is essential to examine how genetic risk factors operate within distinct neural circuits," explained Dr. Bolton. "Understanding these mechanisms is a crucial step toward targeted interventions for specific traits such as severe anxiety."

The team, led by Dr. Tim Holford, combined a genetic model that disrupted PTEN only in somatostatin-containing inhibitory neurons with a unique circuit mapping approach previously developed in the lab. This approach measured the electrical responses of individual neurons to the sequential optogenetic activation of hundreds of nearby neurons, allowing rapid mapping of connectivity and strength with the precision of electrical recordings and the scale of imaging approaches.

"This is a powerful method that we can use to determine changes in local neuron connectivity and strength resulting from genetic variations. We were interested in uncovering how the disruption of PTEN signaling in a single cell type would change the way the brain processes information and contribute to the broad ASD phenotype," described Dr. Holford.

The scientists focused on the circuits in the central amygdala (CeL) - a brain region known to serve as an inhibitory gate on the downstream expression of fear responses - and found striking results. Deleting PTEN specifically in somatostatin-containing interneurons disrupted local inhibitory connectivity in the CeL by roughly 50% and reduced the strength of the inhibitory connections that remained. This diminished connectivity between inhibitory connections within the CeL was contrasted by an increase in the strength of excitatory inputs received from the basolateral amygdala (BLA), a nearby brain region that relays emotionally-relevant sensory information to the CeL.

Behavioral analysis of the genetic model demonstrated that this imbalance in neural signaling was linked to heightened anxiety and increased fear learning, but not alterations in social behavior or repetitive behavior traits commonly observed in ASD. The results not only confirm that PTEN loss in this specific cell type is sufficient to induce specific ASD-like behaviors, but also provide one of the most detailed maps to date of how local inhibitory networks in the amygdala are affected by genetic variations associated with neurological disorders. Importantly, the altered circuitry did not affect all ASD-relevant behaviors -- social interactions remained largely intact -- suggesting that PTEN-related anxiety and fear behaviors may stem from specific microcircuit changes.

As Dr. Holford explains, "By teasing out the local circuitry underlying specific traits, we hope to differentiate the roles of specific microcircuits within the umbrella of neurological disorders, which may one day help in developing targeted therapeutics for specific cognitive and behavioral characteristics. In future studies, we hope to evaluate these circuits in different genetic models to determine if these microcircuit alterations are convergent changes that underlie heightened fear and anxiety expression across diverse genetic profiles."