Cadherin family is classified into classical cadherins, desmosomal cadherins and protocadherins (PCDHs). Genomic structures distinguish between PCDHs and other cadherins, and between clustered and non-clustered PCDHs. The phylogenetic analysis with full sequences of non-clustered PCDHs enabled them to be further classified into three subgroups: δ1 (PCDH1, PCDH7, PCDH9, PCDH11 and PCDH20), δ2 (PCDH8, PCDH10, PCDH12, PCDH17, PCDH18 and PCDH19) and ε (PCDH15, PCDH16, PCDH21 and MUCDHL). ε-PCDH members except PCDH21 have either higher or lower numbers of cadherin repeats than those of other PCDHs. Non-clustered PCDHs are expressed predominantly in the nervous system and have spatiotemporally diverse expression patterns. Especially, the region-specific expressions of non-clustered PCDHs have been observed in cortical area of early postnatal stage and in caudate putaman and/or hippocampal formation of mature brains, suggesting that non-clustered PCDHs play roles in the circuit formation and maintenance. The non-clustered PCDHs appear to have homophilic/heterophilc cell-cell adhesion properties, and each member has diverse cell signaling partnership distinct from those of other members (PCDH7/TAF1; PCDH8/TAO2β; PCDH10/Nap1; PCDH11/β-catenin; PCDH18/mDab1). Furthermore, each PCDH has several isoforms with differential cytoplasmic sequences, suggesting that one PCDH isoform could activate intracellular signaling differential from other isoforms. These facts suggest that non-clustered PCDHs play roles as a mediator of a regulator of other molecules as well as cell-cell adhesion. Furthermore, some non-clustered PCDHs have been considered to be involved in neuronal diseases such as autism-spectrum disorders, schizophrenia, and female-limited epilepsy and cognitive impairment, suggesting that they play multiple, tightly regulated roles in normal brain function. In addition, some non-clustered PCDHs have been suggested as candidate tumor suppressor genes in several tissues. Although molecular adhesive and regulatory properties of some PCDHs began to be unveiled, the endeavor to understand the molecular mechanism of non-clustered PCDH is still in its infancy and requires future study.

Pentylenetetrazole (PTZ) is a GABA-A receptor antagonist. An intraperitoneal injection of PTZ into an animal induces an acute, severe seizure at a high dose, whereas sequential injections of a subconvulsive dose have been used for the development of chemical kindling, an epilepsy model. A single low-dose injection of PTZ induces a mild seizure without convulsion. However, repetitive low-dose injections of PTZ decrease the threshold to evoke a convulsive seizure. Finally, continuous low-dose administration of PTZ induces a severe tonic-clonic seizure. This method is simple and widely applicable to investigate the pathophysiology of epilepsy, which is defined as a chronic disease that involves repetitive seizures. This chemical kindling protocol causes repetitive seizures in animals. With this method, vulnerability to PTZ-mediated seizures or the degree of aggravation of epileptic seizures was estimated. These advantages have led to the use of this method for screening anti-epileptic drugs and epilepsy-related genes. In addition, this method has been used to investigate neuronal damage after epileptic seizures because the histological changes observed in the brains of epileptic patients also appear in the brains of chemical-kindled animals. Thus, this protocol is useful for conveniently producing animal models of epilepsy.

Epilepsy is one of the most common chronic brain disorders worldwide, affecting 1% of people across different ages and backgrounds. Epilepsy is defined as the sporadic occurrence of spontaneous recurrent seizures. Accumulating preclinical and clinical evidence suggest that there is a positive feedback cycle between epileptogenesis and brain inflammation. Epileptic seizures increase key inflammatory mediators, which in turn cause secondary damage to the brain and increase the likelihood of recurrent seizures. Cytokines and prostaglandins are well-known inflammatory mediators in the brain, and their biosynthesis is enhanced following seizures. Such inflammatory mediators could be therapeutic targets for the development of new antiepileptic drugs. In this review, we discuss the roles of inflammatory mediators in epileptogenesis.

Aberrant branch formation of granule cell axons (mossy fiber sprouting) is observed in the dentate gyrus of many patients with temporal lobe epilepsy and in animal models of epilepsy. However, the mechanisms underlying mossy fiber sprouting remain elusive. Based on the hypothesis that seizure-mediated gene expression induces abnormal mossy fiber growth, we screened activity-regulated genes in the hippocampus and found that neuritin, an extracellular protein anchored to the cell surface, was rapidly upregulated after electroconvulsive seizures. Overexpression of neuritin in the cultured rat granule cells promoted their axonal branching. Also, kainic acid-dependent axonal branching was abolished in the cultured granule cells fromneuritinknock-out mice, suggesting that neuritin may be involved in activity-dependent axonal branching. Moreover,neuritinknock-out mice showed less-severe seizures in chemical kindling probably by reduced mossy fiber sprouting and/or increased seizure resistance. We found that inhibition of the fibroblast growth factor (FGF) receptor attenuated the neuritin-dependent axonal branching. FGF administration also increased branching in granule neurons, whereasneuritinknock-out mice did not show FGF-dependent axonal branching. In addition, FGF and neuritin treatment enhanced the recruitment of FGF receptors to the cell surface. These findings suggest that neuritin and FGF cooperate in inducing mossy fiber sprouting through FGF signaling. Together, these results suggest that FGF and neuritin-mediated axonal branch induction are involved in the aggravation of epilepsy.SIGNIFICANCE STATEMENTThis study reveals the molecular mechanism underlying mossy fiber sprouting. Mossy fiber sprouting is the aberrant axonal branching of granule neurons in the hippocampus, which is observed in patients with epilepsy. Excess amounts of neuritin, a protein upregulated by neural activity, promoted axonal branching in granule neurons. A deficiency of neuritin suppressed mossy fiber sprouting and resulted in mitigation of seizure severity. Neuritin and fibroblast growth factor (FGF) cooperated in stimulating FGF signaling and enhancing axonal branching. Neuritin is necessary for FGF-mediated recruitment of FGF receptors to the cell surface. The recruitment of FGF receptors would promote axonal branching. The discovery of this new mechanism should contribute to the development of novel antiepileptic drugs to inhibit axonal branching via neuritin-FGF signaling.

Mutations in the Tsc1 or Tsc2 genes cause tuberous sclerosis complex (TSC). Tsc1 and Tsc2 proteins form a complex that inhibits mammalian target of rapamycin complex 1 (mTORC1) signalling through Rheb-GTPase. We found that Tsc2+/- neurons showed impaired spine synapse formation, which was resistant to an mTORC1 inhibitor. Knockdown of mTOR also failed to restore these abnormalities, suggesting mTORC may not participate in impaired spinogenesis in Tsc2+/- neurons. To address whether Rheb activation impairs spine synapse formation, we expressed active and inactive forms of Rheb in WT and Tsc2+/- neurons, respectively. Expression of active Rheb abolished dendritic spine formation in WT neurons, whereas inactive Rheb restored spine synapse formation in Tsc2+/- neurons. Moreover, inactivation of Rheb with farnesyl transferase inhibitors recovered spine synapse morphogenesis in Tsc2+/- neurons. In conclusion, dendritic spine abnormalities in TSC neurons may be caused through activation of Rheb, but not through of mTORC1.

Syntenin is an adaptor-like molecule that has two adjacent tandem postsynaptic density protein 95/Discs large protein/Zonula occludens 1 (PDZ) domains. The PDZ domains of syntenin recognize multiple peptide motifs with low to moderate affinity. Many reports have indicated interactions between syntenin and a plethora of proteins. Through interactions with various proteins, syntenin regulates the architecture of the cell membrane. As a result, increases in syntenin levels induce the metastasis of tumor cells, protrusion along the neurite in neuronal cells, and exosome biogenesis in various cell types. Here, we review the updated data that support various roles for syntenin in the regulation of neuronal synapses, tumor cell invasion, and exosome control.