血球細胞中顆粒性細胞-巨噬細胞聚落刺激因子/第三介白質/第五介白質受體共同乙型鏈結合蛋白(CBAP)鑑別與功能之探討

Abstract

人類顆粒性細胞-巨噬細胞聚落刺激因子/第三介白質/第五介白質受體共同乙型鏈靠近細胞膜的細胞質內部分，有一段對於受體傳遞促細胞增生活性所必須的區域。這段靠近細胞膜的區域內含兩個和其他細胞激素家族成員非常相似的特殊段落，稱為區塊1(box1)和區塊2(box2)，區塊1是乙型鏈吸引及磷酸化兩面激酶2 (Janus kinase 2, JAK2) 所必須，而區塊2的功能目前仍有很多部分不清楚。在本篇研究中我藉由酵母菌雙雜交法發現一個可能的膜蛋白，名為共同乙型鏈結合蛋白 (CBAP)，我利用麩胱甘肽S轉移酶下拉法(GST pull-down)證明CBAP靠著區塊2這個段落和共同乙型鏈直接連接。CBAP和共同乙型鏈結合所必需的區域包含了CBAP的可能膜間區域，並且在缺乏細胞激素時可以明顯增加CBAP和共同乙型鏈在細胞內的結合。當我們額外表現CBAP在造血細胞中時，會破壞粒線體功能導致細胞凋亡，這樣的現象會受到B細胞白血病/淋巴瘤蛋白2 (B-cell leukemia/lymphoma 2，Bcl-2)的大量表現而被抑制。另外，藉由小干擾核甘酸(siRNA)降低細胞內生的CBAP，則可明顯抑制因缺乏人類顆粒性細胞-巨噬細胞聚落刺激因子所引起的細胞凋亡，但不影響其他刺激造成的死亡和激素本身傳遞的增殖訊息。這些發現推論共同乙型鏈-CBAP複合體傳遞的訊息參與調控因缺乏人類顆粒性細胞-巨噬細胞聚落刺激因子所引起的細胞凋亡。最後藉由cbap剔除小鼠，來進一步了解CBAP在生理上扮演的角色。目前觀察的結果發現cbap缺失的小鼠可以存活、具有生育能力且外觀正常，無顯著異常的性狀。 目前對於細胞受體傳遞的促細胞存活和增生的訊息已有相當完整的研究，然而當細胞激素缺乏時，細胞受體傳遞的訊息則尚未被探討。在這篇研究當中，我們發現在缺乏細胞激素時，細胞受體乙型鏈會和一新穎致死蛋白結合，並傳遞訊息調控細胞走向死亡。這樣的發現，不僅讓我們了解一新穎蛋白的功能，更開啟了細胞受體研究一個新的方向 -細胞受體的多效性，在有無細胞激素的刺激下，傳遞正面及負面的訊息。

The cytoplasmic domain of common β chain (βc) of human granulocyte- macrophage colony-stimulating factor (GM-CSF)/ Interleukin-3 (IL-3)/ Interleukin-5 (IL-5) receptor contains a region, proximal to the transmembrane domain, which is sufficient to mediate ligand- dependent mitogenic activity. This membrane proximal region has homology with members of the cytokine receptor superfamily and is designated as box 1 and box 2 motifs. Whereas box 1 motif is required for the recruitment and phosphorylation of Janus kinase 2 (JAK2) kinase, the function of box 2 motif remains largely unknown. Here I report the identification of the common beta chain associating protein (CBAP), a putative transmembrane protein, by yeast two-hybrid selection. GST pull-down experiments show that CBAP directly associates with βc via the box 2 motif. Association of CBAP with βc increases in the absence of GM-CSF in vivo and requires the region containing the putative transmembrane domain. Ectopic overexpression of CBAP in hematopoietic cells triggered apoptosis via mitochondria dysfunction, which could be negated by Bcl-2 overexpression. In addition, reduced expression of endogenous CBAP by siRNA significantly inhibited apoptosis induced by GM-CSF deprivation, but did not inhibit other death stimuli or proliferation signals. These findings suggest that the mitochondrial-dependent βc/CBAP pathway is involved in and modulates GM-CSF-deprivation induced apoptosis. Lastly, the physiology role of CBAP was investigated by knocking out mouse cbap gene expression. Cbap deficient mice were viable, fertile and normal appearance with no significant phenotype in my observation. At present, cytokine receptors mediated survival signals were well established, while the signaling transduced by cytokine receptors during cytokine deprivation has not yet been identified. In this study, we identified a novel apoptogenic protein CBAP which interacts with GM-CSF receptor βc and modulates cellular apoptosis in the absence of GM-CSF. These discoveries not only let us to know the function of the novel protein CBAP but also shed light on the pleiotropism of receptor βc which transduce both positive and negative signaling upon presence and absence of cytokine-a new study field of the cytokine receptors.