Background

Due to the unmatched complexity of the human brain, it is difficult to model features of psychiatric and neurodevelopmental diseases using animal models. However, in spite of some limitations, in mice an impressive battery of tests targeting numerous individual aspects of these disorders has been developed in recent years (Seong et al., 2002; Ricceri et al., 2007; Sukoff Rizzo and Crawley, 2017). Many of these assays are now widely accepted as valid tools to study disease mechanisms or therapeutic approaches. Here, we demonstrate two simple tests, nestlet shredding and nest building, widely used to study potential features of psychiatric and, particularly, neurodevelopmental disorders that we have – among others – used recently to characterize the behavioral phenotype of mice with a genetically caused deficiency to synthesize ether lipids (Dorninger et al., 2019a) (cf. Figure 1). The latter are compounds performing a wide range of biological activities, ranging from structural maintenance of cellular membranes to signal transduction (Dorninger et al., 2017a; Dorninger et al., 2020), and are particularly important for correct functioning of the brain (Berger et al., 2016). Accordingly, ether lipid-deficient mice present with complex behavioral abnormalities including motor disturbances, hyperactivity and memory deficits as well as restricted social interaction and interest in novel objects (Dorninger et al., 2017b; Dorninger et al., 2019a; Dorninger et al., 2019b).

Both of the presented assays have several obvious advantages: They are low-cost experiments, by which a significant amount of data can be obtained in a relatively short time frame. Furthermore, they can be reliably performed by investigators with only modest experience in behavioral analysis. Both tests are hardly dependent on physical abilities of the animals, thus allowing the use of genetically modified mouse models with marked phenotypes. For example, we used mice with considerable developmental defects and visual impairments (the ether lipid-deficient Gnpat knockout (KO) mouse model) (Rodemer et al., 2003), which precluded the application of many behavioral tests. However, as demonstrated by their performance in the nest building assay (Dorninger et al., 2019a), the animals are perfectly capable to utilize nestlets. Certainly, the simplicity of the assays also comes with some caveats: Both tests provide only an initial estimation on potential features of neurodevelopmental disorders and should not be used isolated. Also, they do not yield numeric data that can be directly related to the extent of human disease. Instead, they need to be complemented by additional behavioral tests assessing similar features (Silverman et al., 2010) and, optimally, embedded in a battery of assays allowing a comprehensive description of the behavioral phenotype. Furthermore, results need to be interpreted with care: Excessive nestlet shredding is generally seen as an indicator of repetitive behavior, a symptom also seen in human autism spectrum disorders or obsessive-compulsive disorder (Angoa-Perez et al., 2013). On the contrary, reduced engagement in the task may be viewed as restricted interest in novel objects, another feature commonly associated with autism (Bernard et al., 2015; Dorninger et al., 2019a). We feel that both interpretations are valid but need to be considered in the context of other aspects of behavior of a certain mouse model. Nest building, in turn, can be indicative of general well-being, distress or pain (Jirkof, 2014). At the same time, impairments in this task could hint towards structural or functional deficits in the brain, as demonstrated by mouse models of brain injury or with genetic modifications (Lijam et al., 1997; Deacon et al., 2002; Sager et al., 2010).

Our procedures for both tests are based on the previous protocols (Deacon, 2006; Angoa-Perez et al., 2013; Neely et al., 2019) that we have slightly modified. Furthermore, we provide more detailed descriptions of the testing protocols, including novel recommendations on the testing environment and extended illustration of the procedures and results, and share our experiences in test execution and analysis.