Dynamic Changes of CD44 Expression from Progenitors to Subpopulations of Astrocytes and Neurons in Developing Cerebellum

We previously reported that CD44-positive cells were candidates for astrocyte precursor cells in the developing cerebellum, because cells expressing high levels of CD44 selected by fluorescence-activated cell sorting (FACS) gave rise only to astrocytes in vitro. However, whether CD44 is a specific cell marker for cerebellar astrocyte precursor cells in vivo is unknown. In this study, we used immunohistochemistry, in situ hybridization, and FACS to analyze the spatial and temporal expression of CD44 and characterize the CD44-positive cells in the mouse cerebellum during development. CD44 expression was observed not only in astrocyte precursor cells but also in neural stem cells and oligodendrocyte precursor cells (OPCs) at early postnatal stages. CD44 expression in OPCs was shut off during oligodendrocyte differentiation. Interestingly, during development, CD44 expression was limited specifically to Bergmann glia and fibrous astrocytes among three types of astrocytes in cerebellum, and expression in astrocytes was shut off during postnatal development. CD44 expression was also detected in developing Purkinje and granule neurons but was limited to granule neurons in the adult cerebellum. Thus, at early developmental stages of the cerebellum, CD44 was widely expressed in several types of precursor cells, and over the course of development, the expression of CD44 became restricted to granule neurons in the adult.

我们此前曾报道，CD44阳性细胞可作为发育中小脑内星形胶质细胞前体细胞（astrocyte precursor cells）的候选细胞群，因为通过荧光激活细胞分选术（fluorescence-activated cell sorting, FACS）筛选出的高表达CD44的细胞在体外仅能分化为星形胶质细胞。然而，CD44是否可作为体内小脑星形胶质细胞前体细胞的特异性细胞标志物，目前尚不明确。本研究采用免疫组织化学、原位杂交及荧光激活细胞分选术（FACS）技术，分析了小鼠发育过程中小脑内CD44的时空表达特征，并对CD44阳性细胞进行了表征。在出生后早期阶段，CD44的表达不仅见于星形胶质细胞前体细胞，还可见于神经干细胞（neural stem cells）及少突胶质细胞前体细胞（oligodendrocyte precursor cells, OPCs）。少突胶质细胞分化过程中，OPCs内的CD44表达会被终止。有趣的是，在发育过程中，小脑三类星形胶质细胞中，CD44的表达仅特异性局限于伯格曼胶质细胞（Bergmann glia）与纤维性星形胶质细胞；且星形胶质细胞内的CD44表达在出生后发育阶段会被终止。发育中的浦肯野神经元（Purkinje neurons）与颗粒神经元中也可检测到CD44的表达，但在成年小脑中，CD44的表达仅局限于颗粒神经元。因此，在小脑发育早期，CD44在多种前体细胞中广泛表达；而随着发育进程，CD44的表达最终局限于成年小脑中的颗粒神经元。