Plastic shows promise for use in human body
Engineers at The University of Texas at Austin have found a way to modify a plastic to anchor molecules that promote nerve regeneration, blood vessel growth or other biological processes.
In the study led by Dr. Christine Schmidt, the researchers identified a piece of protein from among a billion candidates that could perform the unusual feat of attaching to polypyrrole, a synthetic polymer (plastic) that conducts electricity and has shown promise in biomedical applications. When the protein piece, or peptide, was linked to a smaller protein piece that human cells like to attach to, polypyrrole gained the ability to attach to cells grown in flasks in the laboratory.
“It will be very useful from a biomedical standpoint to be able to link factors to polypyrrole in the future that stimulate nerve growth or serve other functions,” said Schmidt, an associate professor of biomedical engineering at the university.
Schmidt is the principal author for the study conducted with colleague Dr. Angela Belcher at Massachusetts Institute of Technology. It was published online May 15 by the journal Nature Materials.
Polypyrrole is of interest for tissue engineering and other purposes because it is a non-toxic plastic that conducts electricity. As a result, it could be used to extend previous experiments in Schmidt’s laboratory. The experiments involve wrapping a tiny strip of plastic around damaged, cable-like extensions of nerve cells called neurites to help them regenerate.
“We can apply an electric field to this synthetic material and enhance neurite repair,” said Schmidt. The newly gained ability to attach proteins to polypyrrole, she said, will mean that growth-enhancing factors could also be linked to this plastic wrapping, further stimulating neurite regeneration.
Working with Schmidt and Belcher, the paper’s lead authors, graduate students Archit Sanghvi and Kiley Miller identified the peptide that attaches to polypyrrole from among the billion alternatives initially analyzed. These unique peptides were displayed on the outer surface of a harmless type of virus called a bacteriophage that was purchased commercially.
To hunt for the plastic-preferring peptide, Sanghvi and Miller added a solution containing bacteriophages that displayed different peptides to a container with polypyrrole stuck on its inner surface. The bacteriophages that didn’t wash away when exposed to conditions that hinder attachment were retested on a new polypyrrole-coated container, a process that was repeated four more times.
The sticky peptide selected, known as T59, is a string of 12 amino acids. To make certain that something else on the outer surface of the bacteriophage virus wasn’t responsible for its perceived stickiness, the researchers demonstrated that T59 by itself could attach to immobilized polypyrrole, using synthetic copies of it made at the university’s Institute for Cellular and Molecular Biology. In addition, they determined that a certain amino acid, aspartic acid, had to be a part of T59 for it to attach well to the plastic.
Aspartic acid carries a negative charge, which in T59 appeared to be drawn to the positively charged surface of the polypyrrole the way magnets of opposite charges cling together. Yet other peptides containing aspartic acid didn’t attach to polypyrrole, leading the researchers to speculate that something contributed by the other amino acids in T59 influenced its 3-dimensional shape in a way that augmented its plastic preference.
“This aspartic acid is just one piece of the puzzle,” Sanghvi said. “There are still more pieces to put together.”
The researchers also evaluated how well T59 clings to polypyrrole. They attached copies of the peptide to the tip of an atomic force microscope at the university’s Center for Nano- and Molecular Science and Technology. The tip of this specialized microscope is normally passed across the surface of a material to “map” its peaks and valleys. In this case, the surface was a layer of polypyrrole, and the resistance of the peptide-coated tip to being passed across the surface revealed how well T59 clung to the plastic.
“They had a moderately strong interaction, which is useful to know,” Schmidt said, referring to the need for a stable attachment between polypyrrole and biological molecules that T59 would be used to link to.
Schmidt’s laboratory intends to study T59 as a linker to other molecules in the future, possibly including vascular endothelial growth factor, which stimulates the growth of new blood vessels. In addition, they will use the bacteriophage analysis approach, called high-throughput combinatorial screening, to look for peptide linkers for other plastics such as polyglycolic acid under study for tissue-repair or tissue-engineering purposes.
“This is a powerful technique that can be used for biomaterials modification,” Schmidt said, “and it hasn’t really been explored very much until now.”
From UT Austin
据英国《自然》杂志2005年5月15日报道，美国德克萨斯大学的科学家们将特定塑胶粘附于人体分子上，可应用于促进神经再生、血管生长等生物学领域。
该项目的研究人员对十亿个蛋白质块进行挑选，确定其中一个能有效粘附在一种名叫聚咯（polypyrrole）的塑胶上。聚咯是一种聚合体（塑胶），具有很强的导电性和生物活性。一旦蛋白质块或缩氨酸与更小的蛋白质块相连接，聚咯就能够粘附于其中的细胞上。
项目负责人施密特指出，从生物医学的角度来看，这项研究非常有用。聚咯作为一种能导电的无毒塑胶，在组织工程等方面具有很大的价值。施密特用一小条聚咯将受损的神经突包裹起来，以帮助其再生。她说：“我们可以在这种合成材料上加上电场，以此来提高神经突的再生能力。”
同时，研究人员还从数十亿个蛋白质块中确定了一种独特的缩氨酸。这种缩氨酸通常利用一种被称为抗菌素的无害病毒外表面来显示。为了寻求具有亲-塑胶性的缩氨酸，他们将一种含有抗菌素的溶液加入容器中，并将聚咯粘附在容器的内表面上。然后把没有冲掉的抗菌素倒入另一个内表面覆有聚咯的容器内，如此重复4次。
德克萨斯大学细胞和分子生物学研究所的科学家利用人造模式证明了，这种被称为T59的粘性缩氨酸具有极强的粘性。此外，他们还确定T59种有某种特定的氨酸基——天冬氨酸，有助于增强T59在塑胶上的粘附性能。
天冬氨酸带有一个负电子，因此能够与聚咯表面的正电子相互吸引。但是其它包含有天冬氨酸的缩氨酸无法粘附于聚咯上，研究人员推测可能是由于T59种其它氨基酸影响了其3-D形状，增强了其亲-塑胶的性能。
研究人员还对T59在塑胶上的粘性进行了评估。他们将缩氨酸粘附于原子显微镜的探针上，然后将探针通过一层聚咯表面，探针产生的阻力就显示出T59与塑胶的粘性。
施密特实验室准备将T59作为未来分子间连接器进行研究，此外，他们还将利用抗菌素分析方法来寻求其它可粘附有缩氨酸的塑胶。
