Time-Lapse Imaging Reveals Symmetric Neurogenic Cell Division of GFAP-Expressing Progenitors for Expansion of Postnatal Dentate Granule Neurons

Granule cells in the hippocampus, a region critical for memory and learning, are generated mainly during the early postnatal period but neurogenesis continues in adulthood. Postnatal neuronal production is carried out by primary progenitors that express glial fibrillary acidic protein (GFAP) and they are assumed to function as stem cells. A central question regarding postnatal dentate neurogenesis is how astrocyte-like progenitors produce neurons. To reveal cell division patterns and the process of neuronal differentiation of astrocyte-like neural progenitors, we performed time-lapse imaging in cultured hippocampal slices from early postnatal transgenic mice with mouse GFAP promoter-controlled enhanced green fluorescent protein (mGFAP-eGFP Tg mice) in combination with a retrovirus carrying a red fluorescent protein gene. Our results showed that the majority of GFAP-eGFP+ progenitor cells that express GFAP, Sox2 and nestin divided symmetrically to produce pairs of GFAP+ cells (45%) or pairs of neuron-committed cells (45%), whereas a minority divided asymmetrically to generate GFAP+ cells and neuron-committed cells (10%). The present results suggest that a substantial number of GFAP-expressing progenitors functions as transient amplifying progenitors, at least in an early postnatal dentate gyrus, although a small population appears to be stem cell-like progenitors. From the present data, we discuss possible cell division patterns of adult GFAP+ progenitors.海马是记忆与学习功能的关键脑区，其中的颗粒细胞主要在出生后早期生成，但成年后仍可发生神经发生。出生后早期的神经元生成由表达胶质纤维酸性蛋白（glial fibrillary acidic protein, GFAP）的初级祖细胞完成，这类细胞被认为具备干细胞功能。针对出生后齿状回神经发生的核心科学问题之一，是星形胶质细胞样祖细胞如何产生神经元。为揭示星形胶质细胞样神经祖细胞的细胞分裂模式与神经元分化过程，我们对出生后早期转基因小鼠的体外培养海马脑片开展了延时成像实验。该转基因小鼠携带由小鼠GFAP启动子调控的增强型绿色荧光蛋白（mGFAP-eGFP Tg小鼠），并联合使用了携带红色荧光蛋白基因的逆转录病毒。实验结果显示，绝大多数同时表达GFAP、Sox2与巢蛋白的GFAP-eGFP阳性祖细胞以对称分裂方式增殖，分别产生成对的GFAP阳性细胞（45%）与成对的神经元定向分化细胞（45%）；仅少数祖细胞以不对称分裂方式生成GFAP阳性细胞与神经元定向分化细胞（10%）。本研究结果表明，至少在出生后早期的齿状回中，大量表达GFAP的祖细胞发挥瞬时扩增祖细胞的功能，尽管仍有一小部分群体表现出干细胞样祖细胞的特性。基于上述实验数据，我们对成年GFAP阳性祖细胞可能存在的细胞分裂模式展开了讨论。