SIPA1L1/SPAR1 Interacts with the Neurabin Family of Proteins and is Involved in GPCR Signaling

Signal-induced proliferation-associated 1 (SIPA1)-like 1 (SIPA1L1; also known as SPAR1) has been proposed to regulate synaptic functions that are important in maintaining normal neuronal activities, such as regulating spine growth and synaptic scaling, as a component of the PSD-95/NMDA-R-complex. However, its physiological role remains poorly understood. Here, we performed expression analyses using super-resolution microscopy (SRM) in mouse brain and demonstrated that SIPA1L1 is mainly localized to general submembranous regions in neurons, but surprisingly, not to PSD. Our screening for physiological interactors of SIPA1L1 in mouse brain identified spinophilin and neurabin-1, regulators of G-protein-coupled receptor (GPCR) signaling, but rejected PSD-95/NMDA-R-complex components. Furthermore, Sipa1l1^-/- mice showed normal spine size distribution and NMDA-R-dependent synaptic plasticity. Nevertheless, Sipa1l1^-/- mice showed aberrant responses to α₂-adrenergic receptor (a spinophilin target) or adenosine A1 receptor (a neurabin-1 target) agonist stimulation, and striking behavioral anomalies, such as hyperactivity, enhanced anxiety, learning impairments, social interaction deficits, and enhanced epileptic seizure susceptibility. Male mice were used for all experiments. Our findings revealed unexpected properties of SIPA1L1, suggesting a possible association of SIPA1L1 deficiency with neuropsychiatric disorders related to dysregulated GPCR signaling, such as epilepsy, attention deficit hyperactivity disorder (ADHD), autism, or fragile X syndrome (FXS).SIGNIFICANCE STATEMENT Signal-induced proliferation-associated 1 (SIPA1)-like 1 (SIPA1L1) is thought to regulate essential synaptic functions as a component of the PSD-95/NMDA-R-complex. In our screening for physiological SIPA1L1-interactors, we identified G-protein-coupled receptor (GPCR)-signaling regulators. Moreover, SIPA1L1 knock-out (KO) mice showed striking behavioral anomalies, which may be relevant to GPCR signaling. Our findings revealed an unexpected role of SIPA1L1, which may open new avenues for research on neuropsychiatric disorders that involve dysregulated GPCR signaling. Another important aspect of this paper is that we showed effective methods for checking PSD association and identifying native protein interactors that are difficult to solubilize. These results may serve as a caution for future claims about interacting proteins and PSD proteins, which could eventually save time and resources for researchers and avoid confusion in the field.