Inability to activate Rac1-dependent forgetting contributes to behavioral inflexibility in mutants of multiple autism-risk genes

Abstract

The etiology of autism is so complicated because it involves the effects of variants of several hundred risk genes along with the contribution of environmental factors. Therefore, it has been challenging to identify the causal paths that lead to the core autistic symptoms such as social deficit, repetitive behaviors, and behavioral inflexibility. As an alternative approach, extensive efforts have been devoted to identifying the convergence of the targets and functions of the autism-risk genes to facilitate mapping out causal paths. In this study, we used a reversal-learning task to measure behavioral flexibility in Drosophila and determined the effects of loss-of-function mutations in multiple autism-risk gene homologs in flies. Mutations of five autism-risk genes with diversified molecular functions all led to a similar phenotype of behavioral inflexibility indicated by impaired reversal-learning. These reversal-learning defects resulted from the inability to forget or rather, specifically, to activate Rac1 (Ras-related C3 botulinum toxin substrate 1)-dependent forgetting. Thus, behavior-evoked activation of Rac1-dependent forgetting has a converging function for autism-risk genes.A utism spectrum disorders are common polygenic neurodevelopmental disorders diagnosed by a cluster of core clinical syndromes characterized by impaired social interaction, communication deficits, and behavioral inflexibility (1-3). A large number of autism-risk genes have been identified through different genetic approaches (4-8). These candidate genes play highly diversified roles in synaptic organization, transmission, intracellular signaling pathways, and protein expression regulation, among others (9). How do variants of these risk genes lead to the core symptoms of autism? Are there causal paths that can determine the onset of autism? Answering these questions is difficult because the disorder is multifactorial in nature with multiple genetic variants and environmental factors contributing to the core symptoms (10-13). To face this challenge, extensive efforts have been made to identify the targeting and functional convergence of the autism-risk genes, such as those involved in translation regulation (14-16) and long-range connectivity among different brain regions (17)(18)(19). These targets and functions provide a better biological basis for elucidating the causal paths between risk genes and the disorders of autism.The aim of the this study was to determine whether Rac1 (Rasrelated C3 botulinum toxin substrate 1)-dependent forgetting is a functional converging point for autism-risk genes. This idea was inspired by two clues. First, inhibition of Rac1 activity in Drosophila leads to the failure to activate Rac1-dependent forgetting, resulting in behavioral inflexibility as assayed through a reversal-learning task (20). Behavioral inflexibility has been observed frequently in the clinical studies of autism patients (21,22), and children with autism are reported to have impaired reversal-learning, although they can learn new beh...

The Biology of Forgetting—A Perspective

“…In Drosophila , evidence suggests that Rac1 mediates at least four types of active forgetting of olfactory memories, including intrinsic forgetting, interference-induced forgetting, trace memory forgetting, and reversal learning-activated forgetting (Figure 1; Shuai et al, 2010; Shuai et al, 2011a; Dong et al, 2016). The fundamental observations are that expressing a transgene carrying a dominant negative form of Rac1 in the mushroom body neurons (MBn) inhibits intrinsic forgetting of odor:shock pairing, whereas expressing a constitutively active form in the same neurons accelerates forgetting (Shuai et al, 2010).…”

“…Dong et al, (2016) probed the question of whether ASD risk genes confer impairments in Rac1-dependent active forgetting. Drosophila mutants in five ASD-associated genes, including Fmr1 ( Fragile X mental retardation 1 ), Ube3a ( Ubiquitin-protein ligase E3A ), Nrx-1 ( Neurexin-1 ), Nlg4 ( Neuroligin 4 ), and Tsc1 ( Tuberous sclerosis complex 1 ), were chosen to measure behavioral flexibility using reversal learning tests.…”

Hippocampal Activation of Rac1 Regulates the Forgetting of Object Recognition Memory

“…Molecular mechanisms underlying the active forgetting remain to be determined for memories in vertebrates. Recent progress has begun to unravel such mechanisms underlying the active forgetting [3][4][5][6][7][8][9][10][11] that is induced through the behavior-dependent activation of intracellular signaling pathways. In Drosophila, training-induced activation of the small G protein Rac1 mediates natural memory decay and interference-based forgetting of aversive conditioning memory [3].…”

Basic reversal-learning capacity in flies suggests rudiments of complex cognition

“…A large number of studies of learning focus on single-step saturation training [ 20 ]; for instance [ 17 , 18 , 36 ]. Reversal learning evaluation is typically the comparison of single-estimate post-training, and post-reversal levels of conditioning (including [ 8 , 21 ]). In other experiments, time course data is aggregated, and these single metrics are compared [ 13 , 25 ].…”

“…Reversal-learning is often more rapid than initial acquisition, and as such it is increasingly being studied as an important indicator of behavioral flexibility (e.g. [ 6 , 8 , 9 ]).…”