Original research
Impaired Autophagy in Hilar Mossy Cells of the Dentate Gyrus and Its Implication in Schizophrenia

https://doi.org/10.1016/j.jgg.2014.12.001Get rights and content

Abstract

Schizophrenia (SCZ) is a complex disease that has been regarded as a neurodevelopmental, synaptic or epigenetic disorder. Here we provide evidence that neurodegeneration is implicated in SCZ. The DTNBP1 (dystrobrevin-binding protein 1) gene encodes dysbindin-1 and is a leading susceptibility gene of SCZ. We previously reported that the dysbindin-1C isoform regulates the survival of the hilar glutamatergic mossy cells in the dentate gyrus, which controls the adult hippocampal neurogenesis. However, the underlying mechanism of hilar mossy cell loss in the dysbindin-1-deficient sandy (sdy) mice (a mouse model of SCZ) is unknown. In this study, we did not observe the apoptotic signals in the hilar mossy cells of the sdy mice by using the TUNEL assay and immunostaining of cleaved caspase-3 or necdin, a dysbindin-1- and p53-interacting protein required for neuronal survival. However, we found that the steady-state level of LC3-II, a marker of autophagosomes, was decreased in the hippocampal formation in the mice lacking dysbindin-1C. Furthermore, we observed a significant reduction of the cytosolic LC3-II puncta in the mossy cells of sdy mice. In addition, overexpression of dysbindin-1C, but not 1A, in cultured cells increased LC3-II level and the LC3 puncta in the transfected cells. These results suggest that dysbindin-1C deficiency causes impaired autophagy, which is likely implicated in the pathogenesis of SCZ.

Introduction

Schizophrenia (SCZ) is a chronic debilitating disorder characterized by positive and negative symptoms, disorganization and cognitive deficits. Etiologically, both genetic and environmental factors contribute to the development of SCZ. To date, its pathogenesis has not been well defined. Developmental or synaptic alternations have been documented in SCZ, and especially the impaired adult neurogenesis has been implicated in SCZ (Kempermann et al., 2008, Balu and Coyle, 2011, Ming and Song, 2011). DTNBP1 (dystrobrevin-binding protein 1), which encodes dysbindin-1, is a leading susceptibility gene of SCZ (Straub et al., 2002, Fanous and Kendler, 2008). The sandy (sdy) mutant, which carries a spontaneously occurring large deletion in the Dtnbp1 gene on the DBA/2J inbred strain (Li et al., 2003), is an appropriate mouse model of SCZ (Feng et al., 2008, Hattori et al., 2008, Takao et al., 2008, Bhardwaj et al., 2009).

Three isoforms of dysbindin-1 (dysbindin-1A, 1B and 1C) in primates have been identified (Talbot et al., 2009). Human dysbindin-1A, the longest isoform with 351 amino acids (aa), differs from 1B in the C-terminus, and differs from 1C in the N-terminus. However, there is no known mouse or rat ortholog of human dysbindin-1B (Talbot et al., 2009). In the temporal lobe, about 92% of the schizophrenia cases displayed synaptic dysbindin-1A reductions without alterations in other dysbindin-1 isoforms in the auditory association cortices. In contrast, in the hippocampal formation (HF), most SCZ cases displayed normal levels of synaptic dysbindin-1A, but showed reductions in 1B and 1C (Talbot et al., 2011). Our recent findings revealed that dysbindin-1C, not 1A, is required for the survival of hilar mossy cells and the maturation of adult newborn neurons in the dentate gyrus (DG), suggesting that reduced dysbindin-1C in the schizophrenic HF is important for the development of cognitive deficits (Wang et al., 2014). However, the underlying mechanism of hilar mossy cell loss is unknown.

Dysbindin-1A is a subunit of the biogenesis of lysosome-related organelles complex 1 (BLOC-1) (Li et al., 2003, Wang et al., 2014). BLOC-1 complex functions in lysosomal trafficking and biogenesis of lysosome-related organelles (Wei and Li, 2013). Mouse dysbindin-1C mRNA lacks exons 1–4 of 1A and contains an alternative promoter in intron 4, thus resulting in a downstream start codon on exon 5 of 1A and a deletion of the N-terminal 81 residues (Li et al., 2003). Dysbindin-1C does not reside in the BLOC-1 complex and has a distinct spatial and temporal expression pattern in the central nervous system compared with 1A due to the utilization of a different promoter (Wang et al., 2014). In addition, dysbindin-1C regulates adult hippocampal neurogenesis in both non-cell autonomous and BLOC-1 independent manners (Wang et al., 2014). This suggests that dysbindin-1C regulates the survival of hilar mossy cells independent of BLOC-1 complex. A default nucleus localization signal may lie on the N-termini of dysbindin-1A and 1B as lack of this sequence in 1C leads to its cytosolic retention (Fei et al., 2010). The different subcellular distribution may underlie their difference in function, which requires further investigation. Both dysbindin-1A and 1C contain the 97–118 aa region that is required for its interaction with necdin (Ma et al., 2011). Necdin is a p53-interacting protein expressed predominantly in post-mitotic neurons and is required for neuronal survival (Hasegawa and Yoshikawa, 2008). Whether dysbindin-1C is involved in this pathway for the regulation of hilar mossy cell survival is unknown.

In this study, by using sdy mice, in which both dysbindin-1A and 1C are deleted (1A/1C) (Li et al., 2003), we explore the molecular mechanism of dysbindin-1C in regulating the hilar mossy cell survival by comparing with the wild-type controls (1A+/1C+), and the muted (mu) (Zhang et al., 2002) or pallid (pa) mice (Huang et al., 1999), in which only dysbindin-1A is destabilized while 1C is unaltered (1A/1C+) (Wang et al., 2014). We found that dysbindin-1C, but not 1A, is involved in autophagy which may be required for the survival of hilar mossy cells.

Section snippets

No positive TUNEL signals in the DG of sdy mice

We recently reported that the number of hilar mossy cells in the DG of sdy mice was significantly decreased compared with wild-type controls. In contrast, there was no significant difference in the number of hilar GABA interneurons in the DG between sdy mice and controls (Wang et al., 2014). This result indicates that loss of dysbindin-1C selectively compromises the survival of hilar mossy cells in the DG. To explore the underlying mechanism of mossy cell loss, we tested whether the mossy cells

Discussion

In this study, we found that dysbindin-1C, which is localized in glutamatergic mossy cells of the hilus in the DG (Wang et al., 2014), is required for the maintenance of their survival by functioning in regulating autophagy. This highlights the relationship between impaired autophagy and the pathogenesis of SCZ. Our results support the importance of decreased dysbindin-1C found in the schizophrenic HF (Talbot et al., 2011) for the development of mossy cell degeneration. In this sense,

Mice

The sdy mutant (sdy/sdy, loss of dysbindin-1) and its wild-type control DBA/2J (DBA) mice, the mu mutant (mu/mu, loss of muted protein) and its wild-type control CHMU/Le, and the pa mutant (pa/pa, loss of pallidin) and its wild-type control C57BL/6J (B6) were originally obtained from the Jackson Laboratory, transferred from Dr. Richard T. Swank's laboratory and bred in the animal facility of the Institute of Genetics and Developmental Biology (IGDB), Chinese Academy of Sciences. All procedures

Acknowledgements

This work was partially supported by grants from the National Natural Science Foundation of China (Nos. 91332116 and 31230046), from the National Basic Research Program of China (No. 2014CB942803) and from Chinese Academy of Sciences (No. KJZD-EW-L08).

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