Description

DNM1 encephalopathy in humans is a neurological disorder that causes severe epilepsy with mutations in the gene DNM1 (dynamin 1). This syndrome begins in childhood and persists through adulthood, requiring life-long management of severe seizures. The underlying cause of the seizures is not well understood. However, missense mutations associated with DNM1 encephalopathy are clustered within either the GTPase domain or the middle domain, which, along with the GTPase effector domain (GED), is required for oligomerization and stimulation of GTPase activity (Fig. 1A) (Von Spiczak et al., 2017). The nematode, Caenorhabditis elegans, is a proven model organism for studying epilepsy due to the ability to induce epileptic-like convulsions when mutant animals are treated with pentylenetetrazole (PTZ) (Williams et al., 2004). C. elegans possess a homolog of human DNM1, termed dyn-1. Notably, there is a mutant, dyn-1(ky51), with a missense substitution (P70S) in the GTPase domain (Fig. 1A) (Clark et al., 1997). The nematode proline residue is conserved in human DNM1 (P68). Through detailed analyses in multiple organisms, dynamin 1 has been characterized to function in endocytosis and synaptic transmission (Ferguson and De Camilli, 2012). In this context, we hypothesized that we could induce epileptic-like convulsions in C. elegans dyn-1(ky51) animals following treatment with the drug PTZ. Accordingly, we observed a dose-dependent convulsive effect in dyn-1(ky51) animals (Fig. 1B). These convulsions were similar to the positive control unc-25(e156). UNC-25 is an ortholog of human glutamate decarboxylase and is involved in GABAergic synaptic transmission. The dyn-1 animals exhibit a singular phenotype following exposure to PTZ; they displayed posterior paralysis with repetitive anterior convulsions at all tested concentrations, a phenotype we termed “head bobbing” that we associate with a tonic-clonic convulsion phenotype (Williams et al., 2004). Furthermore, the dyn-1 animals displayed the tonic-clonic phenotype almost immediately upon exposure to PTZ. The unc-25 worms, however, displayed two different convulsion phenotypes following PTZ exposure. When exposed to 8 and 10 mg/mL PTZ, these animals also displayed head bobbing (tonic-clonic) convulsions and they occurred fairly quickly upon exposure to the drug. However, at a lower concentration, 6 mg/mL of PTZ, the phenotype of unc-25 worms was a mixture of both heading bobbing (tonic-clonic) and full body paralysis (tonic convulsions) and it took longer for these phenotypes to appear (40 minutes on average); in this scenario the animal is stiff and will not respond to stimulus, but is still viable, as evidenced by pharyngeal activity. In summary, while dyn-1(ky51) animals displayed a consistent and immediate convulsion phenotype regardless of the concentration, phenotypes for unc-25(e156) worms were dependent on PTZ exposure concentration. It cannot be ruled out that the convulsive differences observed between these strains arise because different temperatures were used to raise the dyn-1 animals (16°C) vs. the unc-25 (20^oC) animals.