JNK Isoforms Are Involved in the Control of Adult Hippocampal Neurogenesis in Mice, Both in Physiological Conditions and in an Experimental Model of Temporal Lobe Epilepsy.

Neurogenesis in the adult dentate gyrus (DG) of the hippocampus allows the continuous generation of new neurons. This cellular process can be disturbed under specific environmental conditions, such as epileptic seizures; however, the underlying mechanisms responsible for their control remain largely unknown. Although different studies have linked the JNK (c-Jun-N-terminal-kinase) activity with the regulation of cell proliferation and differentiation, the specific function of JNK in controlling adult hippocampal neurogenesis is not well known. The purpose of this study was to analyze the role of JNK isoforms (JNK1/JNK2/JNK3) in adult-hippocampal neurogenesis. To achieve this goal, we used JNK-knockout mice (Jnk1 ^-/- , Jnk2 ^-/- , and Jnk3 ^-/- ), untreated and treated with intraperitoneal injections of kainic acid (KA), as an experimental model of epilepsy. In each condition, we identified cell subpopulations at different stages of neuronal maturation by immunohistochemical specific markers. In physiological conditions, we evidenced that JNK1 and JNK3 control the levels of one subtype of early progenitor cells (GFAP ⁺ /Sox2 ⁺ ) but not the GFAP ⁺ /Nestin ⁺ cell subtype. Moreover, the absence of JNK1 induces an increase of immature neurons (Doublecortin ⁺ ; PSA-NCAM ⁺ cells) compared with wild-type (WT). On the other hand, Jnk1 ^-/- and Jnk3 ^-/- mice showed an increased capacity to maintain hippocampal homeostasis, since calbindin immunoreactivity is higher than in WT. An important fact is that, after KA injection, Jnk1 ^-/- and Jnk3 ^-/- mice show no increase in the different neurogenic cell subpopulation analyzed, in contrast to what occurs in WT and Jnk2 ^-/- mice. All these data support that JNK isoforms are involved in the adult neurogenesis control.