Postnatal Development of Entorhinodentate Projection of the <i>Reeler</i> Mutant Mouse

Abstract

We anterogradely labeled entorhinodentate axons by the injection of biotin dextran amine into the entorhinal cortex of adult wildtype and reeler mice to clarify whether the course and terminal endings of the reeler entorhinal projection are normal or not. We found that in the reeler mouse, biotin dextran amine-labeled entorhinodentate fibers arising from the entorhinal cortex curved around the hippocampal fissure instead of crossing it, whereas in the wildtype mouse, they crossed the fissure as a perforant pathway. Next, we examined carbocyanine dye (DiI) labeling of the immature entorhinodentate projection and the developmental changes of the hippocampal fissure during early postnatal days based on the laminin and glial fibrillary acidic protein (GFAP) immunohistochemistry. Injection of DiI into the entorhinal area of the wildtype and reeler mice at postnatal day 1 resulted in anterograde labeling of pioneer axons passing through the hippocampal fissure. However, follower axons could not penetrate through the hippocampal fissure in reeler mice, whereas in the normal controls, many DiI-labeled axons continued to pass through the fissure. GFAP immunohistochemistry demonstrated that GFAP-immunopositive astrocytes were abundant along the hippocampal fissure both in the wildtype and reeler mice at birth. In the wildtype mouse, GFAP-positive neurons nearby the fissure were decreasing in number during the early postnatal days, whereas in the reeler mouse, many GFAP-positive astrocytes were continuing to accumulate there. This barrier made of astrocytes in the reeler mouse may obstruct the ingrowth of the follower axons arising from the entorhinal cortex through the hippocampal fissure, resulting in the abnormal course of the entorhinodentate axons in this mutant.

Integrity of Cajal–Retzius cells in thereeler‐mouse hippocampus

“…SDF‐1 is, inter alia, produced by astrocytes, which are abundantly located at the HF. Concordant with previous studies, we observed astrogliosis in the vicinity of the HF in the reeler mouse (Muraoka et al, ; this study). The higher number of astrocytes in reeler mice may thus account for increased SDF‐1‐levels, causing enhanced chemoattractive signaling onto CR‐cells.…”

“…Thus, quantification of SDF‐1 immunoreactivity revealed a significantly higher SDF‐1 immunoreactivity in the reeler mice (Figure c3). Furthermore, using confocal microscopy, a higher number of GFAP‐positive cells and processes were observed in the reeler mice, suggesting that gliosis could have contributed to the observed phenotype (Muraoka, Katsuyama, Kikkawa, & Terashima, ; Supporting Information Figure S2).…”

“…As an important function, CR-cells guide the ingrowth of hippocampal afferents, by mediating the attraction of entorhinodentate (Del Rio et al, 1997;Frotscher & Heimrich, 1993;Frotscher et al, 1995;Li, Field, Starega, Li, & Raisman, 1993) and the repulsion of commissural fibers (Borrell, Ruiz, Del Rio, & Soriano, 1999). Indeed, in reeler hippocampi, both fiber-tracts showed signs of disorganization (Borrell, Ruiz, et al, 1999;Muraoka et al, 2007). However, Reelin was highlighted as an important, but not the only signal involved, leaving additional molecular cues to be determined (Squarzoni, Thion, & Garel, 2015).…”

Developmental anatomy of reeler mutant mouse

“…The perforant pathway is the major input to the hippocampal formation from the entorhinal cortex, and it crosses the hippocampal fissure to terminate in the hippocampus proper and the dentate gyrus (van Groen et al . 2003; Muraoka et al . 2007) (Fig.…”

“…4F). Glial Fibrillary Acidic Protein (GFAP) immunostaining suggested that increased gliosis both in the reeler and SRK prevents penetration of the axons from the entorhinal cortex across the hippocampal fissure (Woodhams and Terashima 1999, 2000; Muraoka et al . 2007) (Fig.…”