Methotrexate effects on synaptic plasticity: a path to female resiliency to chemotherapy-induced cognitive deficits

Abstract. Homeostatic synaptic plasticity (HSP) is an adaptive, stabilizing response to prolonged changes in network activity that is necessary for proper functioning of the nervous system. HSP is becoming recognized as a critical form of plasticity involved in neurodevelopment, cognition, and emotional regulation, that, when it becomes dysregulated, may contribute to pathological states. The chemotherapeutic agent, methotrexate (MTX), is used to treat most pediatric patients with acute lymphoblastic leukemia (ALL), the most common form of childhood cancer. The effectiveness of MTX-treatment has resulted in a growing number of survivors, yet MTX exposure is closely associated with executive function deficits in survivors, establishing a pressing need to define the mechanisms that link MTX exposure to cognitive dysfunction. To identify these mechanisms, we have developed a more translationally-relevant mouse model of early life exposure to cancer+chemotherapy. We find cognitive deficits in exposed animals (demonstrating construct and face validity of our model), that are sex and dose (of MTX) dependent, with increased vulnerability in males. Further, we find altered gene expression in astrocytes, with significantly more differentially expressed genes in females, with notable alterations in genes related to GABA signaling. Importantly, we have new preliminary data showing that prolonged MTX exposure in vitro results in an HSP-like decrease in synaptic strength. These preliminary data lead to our hypothesis that chronic exposure to MTX affects astrocyte function in a sex-dependent manner resulting in synaptic remodeling and HSP-like changes in females and not in males, which may explain female resiliency to chemotherapy-induced cognitive deficits in our animal model. The goal for this R21 application is to use functional and structural techniques (patch-clamp electrophysiology and immunohistochemistry, respectively) in vitro (the standard model for studying HSP) and in vivo to determine whether MTX drives changes in neuronal physiology, and whether these responses vary by sex. The proposed studies are conceptually innovative, as (1) effects of chemotherapy on synaptic physiology have not been studied and (2) sex differences in HSP remain wholly unexplored in the literature. Data from the proposed experiments could open new directions for the treatment and/or prevention of chemotherapy-induced cognitive deficits in children and highlight the importance of inclusion of SABV in HSP experiments.