A–E: SGNs were cultured either from JNK3+/+ (WT) or JNK3−/− (KO) littermate mice and transfected with either eGFP and empty vector plasmid (1:4 ratio of eGFP:vector) or with eGFP and dnJNK1 and dnJNK2 (1:2:2 ratio). Cotransfection of eGFP identified transfected cells. These combinations resulting in neurons expressing either all three JNK isoforms (JNK1,2,3), or only JNK1 and JNK2 (JNK1,2), or only JNK3, or no JNK (none). A–D: Representative images of cultured mouse SGNs labeled by eGFP fluorescence and transfected with vector (A,B) or dnJNK1 and dnJNK2 (C,D). A and C are from WT mice, B and D are from JNK3 KO mice. Scale bars = 50 μm. E. Neurite growth over a 24 h period was quantified. There is a significant increasing decrement in neurite growth with removal of additional JNK isoforms. F. SGNs were cultured from neonatal rats and transfected with eGFP and empty vector, dnJNK1, dnJNK2, dnJNK1 and dnJNK2 combined, or MKK-JNK1. Neurite lengths were determined after 48 h. There was no significant difference in mean neurite length between vector and a single dnJNK (horizontal bar). Combined dnJNK1 and dnJNK2 significantly reduced neurite length and constitutively-active JNK (MKK-JNK1) significantly increased neurite length. For E and F, error bar shows SEM. The number of independent replicate cultures is given within each bar and statistical significance was determined by ANOVA with a Tukey-Kramer post-hoc test, *p<0.05, **p<0.01, ***p<0.001.