生物谷报道：位于人类视网膜上数以百万计的感光细胞可采集光线，并将信号发送至大脑。 当这些采集光线的细胞死亡后，人的视力也随之而去了。 那些希望能扭转失明进程的医学研究人员们将他们的目光投向了干细胞，最近的实验显示这些细胞可替换因黄斑退化而损失的感光细胞。
黄斑退化是最常见的失明原因，在65岁以上的美国人中，有10%的人受这一病变的影响。 黄斑退化的第一个目标是一层被称作视网膜色素上皮细胞（RPE）的保护层，该保护层可将营养转运给感光细胞，对感光细胞的存活至关重要。 移植新鲜的视网膜色素上皮细胞组织能挽救正在死亡的感光细胞。 但这一方法可行性并不高，因为要治疗数以百万计的出现早期黄斑退化症状的美国人所需要的这种细胞组织数量是很大的。
在位于马塞诸塞州Worcester市的生物技术公司Advanced Cell Technology中工作的科学家们为视网膜色素上皮细胞创造了一个更丰富的来源。 2004年他们发明了一种方法可诱使胚胎干细胞转变成为可移植的视网膜色素上皮细胞组织。 在接下来的实验中，他们将转变而来的细胞注入患有可造成感光细胞死亡的视网膜色素上皮细胞基因缺陷的大鼠眼内。 据研究人员们发表在2006年12月的克隆与干细胞杂志上的报道称，数周之后，当疾病造成的后果理应出现的时候，接受过治疗的大鼠比未接受治疗的大鼠在追踪旋转园柱上条纹的实验中表现要好两倍。 它们的视力尽管得到了改善，但仍远低于正常大鼠。
但要治疗患有高度黄斑退化或其它感光细胞疾病的患者，最终仍需要修复感光细胞本身。 去年11月，在University College London及其它机构工作的研究人员们宣布，他们从处于不同发育阶段的小鼠视网膜中提取细胞，并已成功地将其移植到失明的小鼠身上。 他们发现，从新生小鼠身上提取的未成熟的感光细胞，而非从胚胎或成年小鼠身上提取的感光细胞，可迁移到视网膜的正确区域，并且继续发育成成熟的感光细胞。 接受这些细胞的瞳孔对光的敏感度也高于那些未接受移植的瞳孔。
在华盛顿大学从事视网膜发育研究的Thomas Reh表示，这些发现提示，进行细胞移植--比如感光细胞的移植时，被移植细胞的发育阶段应比干细胞要相对成熟一些。 然而，与这种从新生小鼠身上提取的未成熟感光细胞相当的人类细胞，不得不改从胚胎的视网膜上分离出来，造成一个相似的问题：寻找未成熟细胞的来源。 成人的干细胞和角膜干细胞是两个其他可能产生未成熟感光细胞的来源。
Reh在他的实验室中试图使人类胚胎干细胞转变成视网膜干细胞，目前，有大约6%的胚胎干细胞转变成了感光细胞。 这一转化率看起来很低，但不必为低转化率而感到泄气，加州La Jolla的Burnham 医学研究院的干细胞研究人员Evan Snyder认为。通过研究是什么促使这6%转变成为感光细胞，研究人员们有望找出如何造出更大数量的用于移植的细胞的方法。 他们有望找到一种方法，可将需要的细胞从大量的混合细胞中挑选出来。在位于Ann Arbor的密西根大学工作的眼科学研究人员 Anand Swaroop，正在集中精力开发一种方法，通过细胞表面的蛋白鉴别出感光细胞，并把它们挑出来。
找到制造细胞的来源，并克服与干细胞移植有关的安全方面的顾虑后，研究人员们还要面对他们最大的挑战： 展示如何将移植的感光细胞与最终连接到视神经上的其它神经元相连。 每个感光细胞都必须完成数以百计的这种关键的连接。 “仅仅拥有正确的细胞类型号并不意味着就已经拥有了正确的神经回路，”Snyder说。 从小鼠视网膜移植的未成熟的感光细胞显示其起作用了，但Swaroop谨慎地说，需要进行行为试验以证实感光细胞已经得以修复。 部分的连接即可产生在小鼠瞳孔试验中见到的那种活性，但真正的视力改善则取决于实验动物对颜色及其它视觉线索的反应能力。 毕竟，还是那句老话，眼见为实。
来源： 科学美国人杂志
原文出处：
Progress in Cell Transplants to Heal Damaged Retinas
03/06/07 -- Millions of photoreceptor cells residing in the human retina gather light and transmit signals to the brain. When these light-collecting cells die, they take the person's sight with them. Medical researchers hoping to reverse blindness have turned their gaze toward stem cells, and recent experiments have shown that these cells could replace photoreceptors lost in macular degeneration.
As the most common form of blindness, macular degeneration affects 10 percent of Americans older than 65 years. It first targets a protective lining called the retinal pigment epithelium (RPE), which shuttles nutrients to the photoreceptor cells and is vital for their survival. A transplant of fresh RPE tissue could rescue dying photoreceptors. But the approach is not feasible considering the large amounts of tissue needed to treat the millions of Americans who show signs of early macular degeneration.
Scientists at the biotechnology firm Advanced Cell Technology in Worcester, Mass., have generated a more abundant source of RPE cells. In 2004 they devised a way to coax embryonic stem cells to turn into transplantable RPE tissue. In a follow-up experiment, they injected the transformed cells into the eyes of rats that had a photoreceptor-killing genetic defect in their RPE cells. As the researchers reported in the September 2006 Cloning and Stem Cells, weeks later, when the effects of the disease would have normally set in, the rats receiving the treatment were able to track stripes on a rotating cylinder twice as well as those that did not. Their vision, though improved, was still far below normal.
But treating patients who have advanced degrees of macular degeneration or other photoreceptor diseases will ultimately require repairing the photoreceptor cells themselves. Last November researchers at University College London and other institutions announced that they had extracted cells from mouse retinas that were at different developmental stages and successfully transplanted them into blind mice. They found that immature photoreceptor cells from healthy newborn mice, rather than embryonic or adult mouse cells, migrated to the correct region of the retina and continued to develop into mature photoreceptor cells. The pupils that received these cells were also more sensitive to light than those that did not receive the transplant.
These findings have suggested the development stages at which to transplant cells--for instance, photoreceptor cells need to be relatively more mature than stem cells, according to Thomas Reh, who studies retinal development at the University of Washington. The human equivalent to the mouse cells, however, would have to be isolated from fetal retinas, posing the familiar problem of finding a source for the immature cells. Adult stem cells and cornea stem cells are two other possible sources for generating immature photoreceptor cells.
In his lab, Reh coaxes human embryonic stem cells into retinal stem cells, and currently about 6 percent of them subsequently turn into photoreceptor cells. That yield may sound small, but a low percentage is not necessarily discouraging, according to Evan Snyder, a stem cell researcher at the Burnham Institute for Medical Research in La Jolla, Calif. By studying what pushes those 6 percent into their fate as photoreceptor cells, researchers might figure out how to generate a larger number of transplantable cells. They might also come up with a way to select the right cells out of a mixed population; Anand Swaroop, an ophthalmology researcher at the University of Michigan at Ann Arbor, is working on a way to identify and weed out the photoreceptor cells by focusing on proteins present on cell surfaces.
Having generated a cell source and overcome the safety concerns associated with transplanting stem cells, researchers still face possibly their biggest challenge: showing that the transplanted photoreceptors wire up to other neurons that eventually connect to the optic nerves. Each photoreceptor must make hundreds of critical connections. "Just because you have the right cell type doesn't mean you have the right circuitry," Snyder says. The immature photoreceptors transplanted from mouse retinas show activity, but Swaroop cautions that behavioral tests must determine that the photoreceptor cells are being repaired. A partial connection could generate the activity seen in the mice's pupils, but true vision improvement depends on the animals' ability to react to color and other visual cues. Seeing, after all, is believing.
Source: Scientific America
http://www.bio.com/newsfeatures/newsfeatures_research.jhtml?cid=26800026
