创新背景

酶是一种特殊的蛋白质，充当生物催化剂——也就是说，它们促进细胞内的化学反应，创造新的生物化合物并分解其他化合物，它们几乎参与细胞在其生命过程中的每一个方面。由于酶在生物中扮演的核心角色，研究人员一直热衷于了解它们在细胞内的行为。

创新过程

研究人员描述了他们如何利用一种蛋白质的基因，帮助细菌漂浮在水中，创造出一种显微镜超声标记，表明细胞内某些酶何时活跃。

这一进展建立在实验室之前对所谓的声学报告基因进行的研究基础上，这种基因编码的蛋白质可以从细胞外通过超声波看到。研究人员从一种浮力细菌中借用了这些蛋白质。在这些细菌中，蛋白质形成了微小的充满空气的蛋白质壳，称为气囊，帮助细菌漂浮在水中。研究团队发现，囊泡在超声成像中表现强烈，包括编码它们的基因被纳入其他细菌或哺乳动物细胞时。

气泡是流行的绿色荧光蛋白的超声波等价物，正如它的名字一样，是一种发出明亮绿色光的蛋白质。研究人员通常使用绿色荧光蛋白的基因作为所谓的报告基因。如果细胞在光学显微镜下呈绿色发光，则该基因处于活性状态。荧光报告基因的主要缺点是，它们的产物不能在肌肉等不透明的组织中被观察到——人体的大多数组织都是如此。相比之下，声波报告基因可以被超声波看到，它可以渗透到活体组织深处。

报告基因的表达模式对于监测细胞的内部行为是有用的。例如，为了进一步了解细胞是如何变成神经元的，研究人员可以将报告基因与神经元基因一起插入胚胎的DNA中。当胚胎的细胞打开神经元基因时，它们也会表达报告基因，细胞会显示出一个特征性的信号。这使得研究人员很容易看到编码神经元形成的遗传程序是活跃的。

酶在细胞周围追逐气体囊泡

荧光蛋白的变体已被设计成改变其荧光，以响应细胞内的分子，如钙、脂类或酶。这些荧光“生物传感器”在显微镜下研究细胞生物学的研究人员中很受欢迎。但是，和其他荧光蛋白一样，它们在活体组织中很难被发现。

在最新研究中，研究团队开发了第一个与超声波一起使用的生物传感器，它使研究人员能够分辨出位于身体深处的细胞中特定的酶何时在忙碌地工作。研究人员在之前的囊泡研究的基础上改进了囊泡，使它们能够被细胞使用的酶靶向。他们改变了形成囊泡的蛋白质，这样它们就可以像一些常见的、经过充分研究的酶的天然靶标一样被识别出来，从而将表面蛋白质切成碎片。

创新关键点

研究人员在之前的囊泡研究的基础上改进了囊泡，使它们能够被细胞使用的酶靶向。

创新价值

研究团队开发了第一个与超声波一起使用的生物传感器，它使研究人员能够分辨出位于身体深处的细胞中特定的酶何时在忙碌地工作。

Innovative development of "biosensors" that combine ultrasound imaging of enzyme activity

The researchers described how they used genes for a protein that helps bacteria float in water to create a microscopic ultrasound marker that indicates when certain enzymes are active inside cells.

The progress builds on previous lab work on so-called acoustic reporter genes, which encode proteins that can be seen from outside cells by ultrasound. The researchers borrowed the proteins from a species of buoyant bacteria. In these bacteria, the proteins form tiny air-filled protein shells, called air sacs, that help the bacteria float in the water. The team found that vesicles show up strongly on ultrasound imaging, including when the genes encoding them are incorporated into other bacterial or mammalian cells.

Bubble is the ultrasonic equivalent of the popular green fluorescent protein, which, as its name suggests, is a protein that emits a bright green light. Researchers often use the gene for green fluorescent protein as a so-called reporter gene. If the cell glows green under a light microscope, the gene is active. The main drawback of fluorescent reporters is that their products cannot be observed in opaque tissues such as muscle -- the same is true of most tissues in the human body. By contrast, acoustic reporter genes can be seen by ultrasound and can penetrate deep into living tissue.

The expression pattern of reporter genes is useful for monitoring the internal behavior of cells. For example, to learn more about how cells become neurons, researchers could insert a reporter gene into the DNA of an embryo along with a neuron gene. When embryonic cells turn on neuronal genes, they also express reporter genes, and the cells show a characteristic signal. This makes it easy for researchers to see that the genetic program encoding the formation of neurons is active.

Variants of fluorescent proteins have been designed to change their fluorescence in response to intracellular molecules such as calcium, lipids or enzymes. These fluorescent "biosensors" are popular among researchers studying cell biology under the microscope. But, like other fluorescent proteins, they are hard to spot in living tissues.

In the latest study, the team developed the first biosensor to be used with ultrasound, which allows researchers to tell when specific enzymes are busily working in cells located deep within the body. The researchers improved on previous vesicle studies so that they could be targeted by enzymes used by cells. They altered the vesicle-forming proteins so that they could be identified like natural targets of some common, well-studied enzymes, thus slicing surface proteins into pieces.