Ret在視網膜退化扮演角色之探討

視網膜屬於一種神經網路，具有特化的細胞把光能轉為神經訊號，最終傳回大腦形成視覺訊息。GDNF family of ligands (GFL)為一群神經滋養因子，根據先前文獻指出GFL和神經元的發育、分化和維持有關。在GFL啟動下游的訊息傳遞前，需要透過活化Rearranged during transfection (Ret)共同接受體方能進行。過去的研究發現到Ret一旦產生基因缺失，在神經系統中會造成視網膜不正常的生理變化，同時造成視網膜的水平細胞和雙極細胞形態產生變異。然而，具有Ret基因缺失的基因轉殖鼠出生後由於腎臟的發育缺損，存活不超過21天，因此無法更進一步探討Ret對於視網膜的影響。本研究目的藉由條件敲毀Ret在視網膜的表現，進而審視正常Ret在視網膜發育完全後，甚至是面對老化時，所扮演可能的角色。本研究第一個部分先透過免疫組織化學染色了解正常老鼠視網膜的Ret表現。第二個部分是利用Chx10-Cre;C-Retlx/lx基因轉殖鼠(在視網膜條件敲毀Ret基因)探討Ret在視網膜功能。利用蘇木精染色、免疫組織化學染色和西方點墨法，檢驗出生後2個月、4個月、12個月和24個月的小鼠視網膜變化。研究結果顯示正常Ret會表現在視網膜的水平細胞的神經突出(process)和神經節的神經纖維上。在Ret缺失下，視網膜的組織結構產生變異，在出生後12和24個月時outer plexiform layer (OPL)變得較薄，且排列紊亂。深入分析組成該層的水平細胞和感光細胞出現不同形態變化：水平細胞的神經突出在出生後2個月、4個月大幅減少，在出生後12個月、24個月更是明顯。感光細胞則是在出生後12個月、24個月時出現神經突出部分縮回到outer nuclear layer (ONL)，而其突觸(synapse) 在ONL產生異位性突觸。西方點墨法分析GFL/Ret下游訊息傳遞，發現到視網膜內的pAkt和pErk的蛋白質表現並沒有因為Ret基因缺失而有明顯改變。本研究結果推斷Ret對於視網膜的水平細胞的突出甚為重要，由正常的訊息傳遞能維持其基本形態，進而鞏固OPL的組織結構，並讓感光細胞因此不易產生退化。

關鍵字

GDNF family of ligands ； Ret ；視網膜退化；水平細胞；感光細胞

並列摘要

Retina, one of neural networks, contains various specialized cells transducing photons into electric impulses to process visual information in the brain. GDNF family of ligands (GFL), a group of neurotrophic factors, is involved in development, differentiation and maintenance of neurons. GFLs could not activate the downstream signaling pathways until the receptor, rearranged during transfection (Ret), recruited. It is reported that aberrant retinal activity and abnormal morphology of horizontal cells and bipolar cells were detected in Ret mutant mice. However, Ret mutant mice died by day 21 after birth because of kidney agenesis. The aim of this study is to examine the consequences of Ret dysfunction in mature and aged retina. In our study, we identified the specific cells expressing Ret in the wild-type retina by IHC staining. Next, we examined the consequences in Chx10-Cre;C-Retlx/lx retinae with conditional knockout of Ret. The 2, 4, 12 and 24-month-old transgenic mice were analyzed by hematoxylin staining, IHC staining and western blot. The results showed Ret was expressed in the processes of horizontal cells and nerve fibers of ganglion cells of retina in the wild-type mice. In Chx10-Cre;C-Retlx/lx mice, it displayed the thinner and disorganized OPL at age of 12 months and 24 months. To examine the component in this layer, we found the number of processes of horizontal cells decreased at age of 2 and 4 months, even progressively at age of 12 and 24 months. Additionally, the synapses of photoreceptors were decreased and mislocalized at age of 12 months and 24 months Chx10-Cre;C-Retlx/lx mice. From our western blot analysis, it showed no obvious changes in pAkt and pErk by Ret dysfunction. These results suggest that Ret is important to maintain the processes of horizontal cells and then preserve the integrity of the OPL; consequently, photoreceptors are not easily induced by the transneuronal degeneration.

並列關鍵字

GDNF family of ligands ； Ret ； retinal degeneration ； horizontal cell ； photoreceptor

參考文獻

Airaksinen, M. S. and Saarma, M. (2002). The GDNF family: signalling, biological functions and therapeutic value. Nat Rev Neurosci, 3, 383-394.

Airaksinen, M. S., Titievsky, A. and Saarma, M. (1999). GDNF family neurotrophic factor signaling: four masters, one servant? Mol Cell Neurosci, 13, 313-325.

Arighi, E., Borrello, M. G. and Sariola, H. (2005). RET tyrosine kinase signaling in development and cancer. Cytokine & Growth Factor Reviews, 16, 441-467.

Bassett, E. A., Korol, A., Deschamps, P. A., Buettner, R., Wallace, V. A., Williams, T. and West-Mays, J. A. (2012). Overlapping expression patterns and redundant roles for AP-2 transcription factors in the developing mammalian retina. Dev Dyn, 241, 814-829.

Baudet, C., Pozas, E., Adameyko, I., Andersson, E., Ericson, J. and Ernfors, P. (2008). Retrograde signaling onto Ret during motor nerve terminal maturation. J Neurosci, 28, 963-975.

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Ret在視網膜退化扮演角色之探討

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