blmp-1於線蟲表皮時間調控與細胞形態之功能

Abstract

表皮作為協助動物生存的重要保護屏障，同時也影響免疫、分泌、感知和癒傷等功能。在秀麗隱桿線蟲(C. elegans)，表皮主要由兩種皮膜細胞組成，包括接縫細胞(seam cell)與hyp7合胞體(syncytium)。接縫細胞位於身體兩側並鑲嵌於hyp7合胞體中。接縫細胞與hyp7合胞體間以特化的黏連結合(adherens junction)相接，包括黏附蛋白-鍊蛋白複合體(CCC)和DLG-1-AJM-1複合體(DAC)，並影響皮膜細胞極性、細胞黏著和先天免疫。利用具綠色螢光的AJM-1和HMR-1融合蛋白，在野生型線蟲成蟲可觀察到接縫合胞體具有兩條平行的細胞邊界。我們發現轉錄因子基因blmp-1(哺乳類BLIMP-1/PRDI-BF1同源基因)的功能缺失，會導致成蟲的黏連結合蛋白排列發生異常。這種異常的hyp7-接縫細胞邊界存在於接縫合胞體的頂層，而非中層與底層，且是開始進行L4/成蟲蛻皮的數小時內產生。這代表blmp-1參與維持成蟲接縫細胞的頂層形態。於特定細胞進行RNAi，顯示為維持正常的接縫細胞頂層形態，blmp-1在接縫合胞體與hyp7都是必要的。為測試BLMP-1是否具轉錄因子功能以控制接縫細胞頂層形態，我們用野生型與blmp-1突變株進行RNA-sequencing分析。結果顯示blmp-1突變株成蟲中某些專屬於幼蟲、耐受型幼蟲(dauer)、或蛻皮的基因轉錄量升高。因此blmp-1可能是透過避免幼蟲、耐受型幼蟲、或蛻皮基因的錯誤表現來調控成蟲表皮命運，如角質層形成。從blmp-1突變株中篩選表現量提高的候選基因和分析BLMP-1共通結合序列，我們發現參與N-連結醣基化的甘露糖轉移酶基因bus-8，與分泌型的蛻皮訊息醣蛋白基因mlt-8，當被減弱功能時可顯著抑制blmp-1突變株的接縫細胞頂層形態異常。藉由RNAi減弱數個參與在N-或O-連結醣基的基因功能亦可顯著抑制Blmp-1表現型，代表接縫細胞形態異常需要透過醣基化。在接縫合胞體或hyp7大量表現bus-8和mlt-8基因便足以造成接縫細胞頂層形態異常，且大量表現mlt-8的效果可被bus-8突變所抑制。這代表在blmp-1突變株中，訊息分子MLT-8過量產生，且可能因BUS-8作用而過度醣基化，影響接縫細胞頂層形態。於線蟲體表利用凝集素或親脂性DiI染劑染色，我們發現blmp-1功能喪失會影響到角質層完整性，造成醣質包被(glycocalyx)異常和脂質層暴露。我們的實驗結果顯示blmp-1作為異時基因控制成蟲表皮命運和角質層完整性，避免幼蟲、耐受型幼蟲和蛻皮的特定基因在成蟲錯誤表現。此外，要維持成蟲接縫細胞頂層形態，blmp-1可能須抑制N-和O-連結醣基化基因如bus-8，以及蛻皮訊息分子基因如mlt-8表現。

Epidermis is a protective barrier important for animal survival, and also functions in immunity, secretion, sensation and wound repair. In the nematode Caenorhabditis elegans, epidermis is mainly composed of two types of epithelial cells, seam cells and hyp7. Seam cells are located on the lateral sides of the body and embedded in the hyp7 syncytium. Seam and hyp7 syncytia are connected with specialized apical adherens junctions, cadherin-catenin complex (CCC) and DLG-1-AJM-1 complex (DAC), which are essential for epithelial cell polarity, adhesion and innate immunity. Using GFP-labeled AJM-1 and HMR-1 fusion proteins as reporters, two parallel boundaries outlined the seam syncytium were observed in the wild-type adult. Interestingly, we found that loss of blmp-1, which encodes a zinc finger transcription factor similar to mammalian BLIMP-1/PRDI-BF1, caused a disorganized localization pattern of the adherens junction proteins at the adult stage. This abnormal hyp7-seam cell boundary was present in the apical, but not medial or lateral, region of seam syncytium and specifically occurred within hours after entering the L4-adult molt. These results indicated that blmp-1 functions in the maintenance of seam apical morphology in adults. Cell-specific RNAi knockdown showed that blmp-1 was essential in both seam and hyp7 syncytia for normal apical seam cell morphology. To test whether BLMP-1 might function as a transcriptional factor to control the seam apical morphology, we performed an RNA-sequencing analysis of wild-type and blmp-1 mutant worms. Our data showed that several genes expressed specifically in epidermis in larvae, dauer or molt had higher transcript levels in the blmp-1 mutants than the wild-type at the adult stage. Thus, blmp-1 likely regulates the adult epidermal fate, at least in cuticle formation, by preventing inappropriate expression of larval, dauer or molting genes. Using the candidate gene approach and the BLMP-1 consensus binding site analysis among the up-regulated genes in blmp-1 mutants, we found that bus-8 and mlt-8, encoding a mannosyltransferase in N-linked glycosylation and a secreted molting signal glycoprotein, when knockdowned, significantly suppressed the abnormal apical seam cell morphology of blmp-1 mutants. RNAi inactivation of several genes in N- or O-linked glycosylation also significantly suppressed the Blmp-1 phenotype, showing that the seam morphological defect requires glycosylation. Overexpression of bus-8 or mlt-8, in either seam or hyp7 syncytium was sufficient to cause the apical seam morphology defect, and the defect resulted from mlt-8 overexpression could be suppressed by the bus-8 mutation. It is possible that in blmp-1 mutants overproduction of molting signal MLT-8 and overglycosylation mediated by BUS-8 contributes, at least in part, to seam apical morphology. Using lectin or lipophilic dye DiI to stain the surface of worms, we found that loss of blmp-1 affected cuticle integrity, resulting in defective glycocalyx and exposed lipid layer. Our data support a model that blmp-1 functions as a heterochronic gene to control the adult epidermal fate and cuticle integrity by preventing mis-expression of larval, dauer and molt-specific gene in adults. In addition, blmp-1 is required for the maintenance of seam apical morphology in adults, in part, by repressing the expression of genes in N- and O-glycosylation, such as bus-8, and the molting signal gene mlt-8.