Science Focus (issue 25)

the nanobody, scientists devised a smart solution of tagging the nanobody with radioisotopes like fluorine-18 or zirconium-89 [6]. The reason why nanobodies are preferred over conventional antibodies is because of their small sizes. Being small allows nanobodies to easily penetrate tumor tissue, thus potentially revealing a larger number of cancer cells in hiding. Similar to conventional antibodies, nanobodies can be applied as therapeutic agents. In 2020, a group of scientists from Sweden reported an exciting discovery that an alpacaderived nanobody could neutralize SARSCoV-2 by blocking its interaction with a host cell receptor, hence preventing the virus from entering and infecting the host cell [7]. Nanobodies also hold promise as potential cancer therapies. Scientists have been developing nanobodybased drugs for colon, breast and liver cancer [4]. They believe these drugs could block important cancer cell signals, or act as delivery vectors of chemotherapy and radiotherapy to deliver molecular drugs or radioactive compounds to the tumor, to kill cancer cells. Beyond disease-related applications, scientists also use nanobodies for live cell imaging. By fusing alpaca nanobody with a green fluorescent protein, researchers were able to visualize the actions of target proteins during immune response in real-time [8]. Structural biologists are fascinated by nanobodies, too. They have used nanobodies to help determine protein structures by X-ray crystallography (footnote 1) and cryo-electron microscopy (footnote 2) [9-11]. The above list is definitely not exhaustive; scientists are still actively exploring other amazing applications of nanobodies. Next time when you visit an alpaca farm with your family and friends, don’t forget to share with them the wonders inside these cute creatures! 1 X-ray crystallography: A common technique used to find out the three-dimensional molecular and atomic arrangement of a crystallized sample. The sample is exposed to X-rays and the resulting X-ray diffraction pattern can be used to determine the sample’s structure. However, preparation of crystallized sample could be challenging, and nanobodies is a tool to increase crystallization probability [10]. 2 Cryo-electron microscopy (cryo-EM): A method that uses frozen samples and less intense electron beams compared to traditional transmission electron microscopy, in which biomolecules may be burned or destroyed by the high energy electrons [12]. Nanobodies allow the study of small proteins (<100 kDa) by cryoEM, which was technically challenging in the past [11]. 談及羊駝,我們通常會想到假日農場或羊駝絨,但如果 你問生物醫學範疇的科學家的話,他們卻可能會想起一種 特殊抗體的片段 — 奈米抗體。 要明白奈米抗體的特殊之處,我們須先了解抗體的結構 (圖一)。傳統抗體是由兩條重鏈和兩條輕鏈通過二硫鍵 連接形成的 Y 形分子。Y形抗體的兩個尖端稱為可變區,負 責與目標抗原結合。一如其名,可變區可以有不同的變化, 它們決定抗體與甚麼抗原結合。除傳統抗體外,羊駝、駱駝 和駱馬等駱駝科動物也會製造一種僅由兩條重鏈組成的特 殊抗體 [1, 2],而奈米抗體就是這些特殊抗體的可變區。 在 1993 年發現奈米抗體後不久,科學家便注意到它的 巨大潛力 [1]。這些抗原結合域(與抗原結合的部分)不但 有著非凡的專一性、穩定性和溶解度,它們的大小也只有 傳統抗體的十分之一 [3],以上特性均使奈米抗體有望成 為新一代藥物和顯影劑……等等,怎樣才能生產我們想要 的奈米抗體呢? 你心中或許已有答案。是的,就是羊駝(儘管透過其他 駱駝科動物來生產奈米抗體也行)!在典型的篩選過程中 [4],科學家會透過注射不同抗原,誘導羊駝的 B 細胞產生 相應的抗體。在提取B細胞及其mRNA後,科學家會把 圖一 (a) 傳統抗體、(b) 駱駝科動物只有重鏈的抗體及(c) 奈米 抗體(L:輕鏈、H:重鏈、灰色:可變區、紫色:恆定區)