Date Published: January 5, 2016
Publisher: Public Library of Science
Author(s): Mari Sajo, Hiroki Sugiyama, Hideaki Yamamoto, Takashi Tanii, Norio Matsuki, Yuji Ikegaya, Ryuta Koyama, Roger Chammas.
Hippocampal granule cells (GCs) are generated throughout the lifetime and are properly incorporated into the innermost region of the granule cell layer (GCL). Hypotheses for the well-regulated lamination of newly generated GCs suggest that polysialic acid (PSA) is present on the GC surface to modulate GC-to-GC interactions, regulating the process of GC migration; however, direct evidence of this involvement is lacking. We show that PSA facilitates the migration of newly generated GCs and that the activity of N-acetyl-α-neuraminidase 1 (NEU1, sialidase 1) cleaves PSA from immature GCs, terminating their migration in the innermost GCL. Developing a migration assay of immature GCs in vitro, we found that the pharmacological depletion of PSA prevents the migration of GCs, whereas the inhibition of PSA degradation with a neuraminidase inhibitor accelerates this migration. We found that NEU1 is highly expressed in immature GCs. The knockdown of NEU1 in newly generated GCs in vivo increased PSA presence on these cells, and attenuated the proper termination of GC migration in the innermost GCL. In conclusion, this study identifies a novel mechanism that underlies the proper lamination of newly generated GCs through the modulation of PSA presence by neuronal NEU1.
The lamination of neuronal cell bodies and fibers is a characteristic feature of cortical organization. It has been proposed that disordered neuronal migration and differentiation result in a disturbed lamination of neurons that increases the risk of neurological disorders, including epilepsy . In the dentate gyrus, following the initial formation of the infra- and suprapyramidal blade of granule cell layer (GCL) , newly generated granule cells (GCs) are functionally incorporated into the innermost region of a densely packed cell layer throughout lifetime [3,4]. It has been suggested that the laminated organization of the GCL is necessary to receive highly laminated afferent inputs in the dentate gyrus  and that the abnormal positioning of GCs is associated with temporal lobe epilepsy . The overwhelming majority of GCs are created during the first two weeks after birth . Therefore, it is important to clarify the mechanisms underlying the migration and incorporation of newly generated GCs to the appropriate cell layer during the postnatal period.
Immunolabeling of P14 rat hippocampal slices for PSA, which is a reliable immature GC marker , and the developmentally broad GC marker Prox1 revealed the presence of PSA on GCs that are located in the innermost region of the GCL where newly generated GCs proliferate ; however, PSA presence was not observed on GCs in the middle and outer GCL (Fig 1a). In addition, these PSA+ GCs possessed immature processes with few branches (Fig 1a2 and 1a3), which is a typical feature of newly generated GCs. Although the role of PSA in GC lamination during development has been unclear, we hypothesized that the superficial localization of PSA on newly generated GCs facilitates their migration, and the degradation of PSA allows these cells to attach to PSA- mature GCs via adhesive molecules (Fig 1b).
The strong localization of PSA in immature GCs is well recognized, but the role of PSA in GC development and the mechanism of how PSA presence is regulated is unclear. In the adult dentate gyrus, where the clustering of PSA+ immature GCs are observed , removing PSA by injecting Endo-N in vivo increased the number of newly generated cells inside the cluster. These results imply that PSA facilitates the migration of newly generated cells away from the clusters . In the present study, a novel explant culture system of the hilus directly revealed that the enzymatic removal of PSA attenuates the migration of immature GCs (Fig 1c–1j). Furthermore, we have shown that NEU1 in newly generated GCs cell-autonomously regulates PSA presence. In the present study, we mainly performed immunohistochemical and immunocytochemical analysis to determine the presence and role of PSA on migrating GCs. Thus, the present study have not focused on determining whether and how the expression levels of PSA affect GC migration. One of the most important conclusions in the present study is that the presence of PSA, but not the expression levels of PSA, on the young GC surface facilitates their migration. Indeed, it has been suggested that the presence of PSA on the cell surface, but not inside a cell, is crucial for cell-cell interaction and cell migration, especially because PSA is a large volume sugar chain .