Girós Amparo, Morante Javier, Gil-Sanz Cristina, Fairén Alfonso, Costell Mercedes
Department of Biochemistry and Molecular Biology, Universitat de València, Burjassot, Spain.
BMC Dev Biol. 2007 Apr 5;7:29. doi: 10.1186/1471-213X-7-29.
Perlecan is a proteoglycan expressed in the basal lamina of the neuroepithelium during development. Perlecan absence does not impair basal lamina assembly, although in the 55% of the mutants early disruptions of this lamina conducts to exencephaly, impairing brain development. The rest of perlecan-null brains complete its prenatal development, maintain basal lamina continuity interrupted by some isolated ectopias, and are microcephalic. Microcephaly consists of thinner cerebral walls and underdeveloped ganglionic eminences. We have studied the mechanisms that generate brain atrophy in telencephalic areas where basal lamina is intact.
Brain atrophy in the absence of perlecan started in the ventral forebrain and extended to lateral and dorsal parts of the cortex in the following stages. First, the subpallial forebrain developed poorly in early perlecan-null embryos, because of a reduced cell proliferation: the number of cells in mitosis decreased since the early stages of development. This reduction resulted in a decreased tangential migration of interneurons to the cerebral cortex. Concomitant with the early hypoplasia observed in the medial ganglionic eminences, Sonic Hedgehog signal decreased in the perlecan-null floor plate basal lamina at E12.5. Second, neurogenesis in the pallial neuroepithelium was affected in perlecan deficient embryos. We found reductions of nearly 50% in the number of cells exiting the cell cycle at E12-E13. The labeling index, which was normal at this age, significantly decreased with advancing corticogenesis. Moreover, nestin+ or PCNA+ progenitors increased since E14.5, reaching up to about 150% of the proportion of PCNA+ cells in the wild-type at E17.5. Thus, labeling index reduction together with increased progenitor population, suggests that atrophy is the result of altered cell cycle progression in the cortical progenitors. Accordingly, less neurons populated the cortical plate and subplate of perlecan-null neocortex, as seen with the neuronal markers beta-tubulin and Tbr1.
As a component of the basal lamina, perlecan both maintains this structure and controls the response of the neuroepithelium to growth factors. Less mitotic cells in the early medial ganglionic eminences, and impaired cell cycle progression in the late neocortex, suggests insufficient recruitment and signaling by neurogenic morphogens, such as SHH or FGF2.
基底膜聚糖是一种蛋白聚糖，在发育过程中表达于神经上皮的基底膜。缺乏基底膜聚糖并不损害基底膜组装，尽管在55%的突变体中，该基底膜的早期破坏会导致无脑畸形，损害大脑发育。其余基底膜聚糖缺失的大脑完成其产前发育，维持基底膜连续性，但有一些孤立的异位，且为小头畸形。小头畸形表现为脑壁变薄和神经节隆起发育不全。我们研究了在基底膜完整的端脑区域产生脑萎缩的机制。
缺乏基底膜聚糖时的脑萎缩始于腹侧前脑，并在随后阶段扩展至皮质的外侧和背侧部分。首先，在早期基底膜聚糖缺失的胚胎中，皮质下前脑发育不良，原因是细胞增殖减少：自发育早期起，有丝分裂细胞数量就减少。这种减少导致中间神经元向大脑皮质的切向迁移减少。与内侧神经节隆起中观察到的早期发育不全同时出现的是，在E12.5时，基底膜聚糖缺失的底板基底膜中的音猬因子信号减少。其次，基底膜聚糖缺陷胚胎中的皮质神经上皮神经发生受到影响。我们发现在E12 - E13时，退出细胞周期的细胞数量减少了近50%。在此年龄时正常的标记指数随着皮质发生的进展而显著降低。此外，自E14.5起，巢蛋白阳性或增殖细胞核抗原阳性祖细胞增加，在E17.5时达到野生型中增殖细胞核抗原阳性细胞比例的约150%。因此，标记指数降低以及祖细胞群体增加表明，萎缩是皮质祖细胞中细胞周期进程改变的结果。相应地，如用神经元标志物β - 微管蛋白和Tbr1所见，基底膜聚糖缺失的新皮质的皮质板和皮质下板中的神经元数量减少。
作为基底膜的一个成分，基底膜聚糖既能维持该结构，又能控制神经上皮对生长因子的反应。内侧神经节隆起早期有丝分裂细胞较少，以及新皮质后期细胞周期进程受损，提示神经源性形态发生素（如音猬因子或成纤维细胞生长因子2）的募集和信号传导不足。
