干擾素引起小鼠憂鬱行為之細胞分子機制研究

Abstract

臨床研究中發現，長期的發炎與憂鬱症有緊密的關聯。然而，發炎與重度憂鬱症(Major Depression Disorder, MDD)之間的因果關係與病理機制尚未釐清。干擾素-α (IFN-α)屬於第一類型的干擾素。常被用於治療自體免疫性疾病，病毒性疾病和癌症。然而長期使用IFN-α也會導致憂鬱。長期接受IFN-α治療的患者中大約有15％至40％被診斷患有憂鬱症，由於使用IFN-α也會誘導發炎，因此長期使用IFN-α導致憂鬱的分子機制可能與發炎引起憂鬱的機制相似。長期注射IFN-α的小鼠也會引起類似憂鬱的行為表現，因此可用做IFN-α導致憂鬱的動物模式。前人研究發現，大腦中發炎及微膠質細胞的活化，及神經細胞的IFN-α受體的訊息傳導被活化，均與此異常行為的發生有密切的關聯。本研究旨在了解第一類型干擾素誘導憂鬱症的細胞分子機制，透過為期四周的腹腔注射IFN-α來誘導小鼠出現憂鬱行為，包括懸尾實驗以及強迫游泳中的僵直時間會更長，糖水偏好測試中糖水的飲用量減少，相比之下透過施打IFN-α處理的小鼠在高腳十字迷宮和開放場域的行為測試中並未出現焦慮行為。我們透過文獻回顧挑選在憂鬱症的致病中幾個關鍵的大腦核區，包括前額葉皮質、依核、海馬迴、杏仁核以及下視丘，並檢驗在各腦區干擾素是否可直接作用，及是否造成發炎。我們檢驗在這些腦區中干擾素訊息傳遞的下游基因IFNRA、IRF1和STAT1，以及發炎因子TNF-α和iNOS等的基因表現。我們發現干擾素可直接作用在下視丘中，並引起發炎。另在海馬迴中干擾素訊息傳遞的下游基因表現也顯著上升，但並未有發炎的現象。因此干擾素引起憂鬱可能是先作用在下視丘或海馬迴，再進一步影響血清素及多巴胺等系統。我們進一步對施打了IFN-α的下視丘中數種神經胜肽的表現進行分析，發現許多神經胜肽的mRNA表現量有所抑制。另外在海馬迴以及杏仁核中我們發現PKA活性受到抑制。總而言之我們的實驗結果顯示，IFN-α在腹腔施打在大腦中的初級作用點應為下視丘或海馬迴，可能經由造成神經胜肽的功能低下後進一步造成憂鬱的症狀，我們的發現將會提供有關IFN-α與憂鬱症之間的致病機制的重要資訊及新的研究方向。

Clinical studies revealed that inflammation is highly associated with the pathogenesis of major depression disorder (MDD). However, the causative relationship and the cellular and molecular mechanisms of inflammation induced MDD have not been established. Type I interferons, such as Interferon-alpha (IFN-α), function as an inflammatory regulator that displays a unique pharmacological profile, including induce pro-inflammatory cytokines and prevent the virus producing and replicating the RNA and DNA. Based on these pharmacological characteristics, IFN-α is often used for treating the autoimmune disease, viral disease, and various cancers. Chronic treatment of IFN-α leads to one notable adverse side effect: depression. About 15% to 40% of patients were diagnosed with depression after receiving IFN-α treatment for several months. The mechanisms of IFN-α induced depression might be similar to the mechanisms underlying inflammation induced depression. Therefore, the IFN-α can be used as an animal model of depression. Previous studies suggested that the activation of myeloid cells, including microglia and perivascular macrophages, contributes to IFN-α induced depression. Additionally, the phenotypes of conditional mutants indicated that IFN-α Signaling in neurons is essential for the onset of the pathological behavior. This study aimed to understand the cellular and molecular mechanism of type-I interferon induced depression. Daily injected with IFN-α through intraperitoneal route for four weeks induced depressive like behavior, including longer immobility time in tail suspension and forced swim test, as well as reduced sugar intake in sucrose preference test. In comparison, these IFN-α treated mice did not exhibit anxiety-like behavior in elevated plus maze and open field test. We first identify the primary target brain regions of IFN-α. Five brain regions that are involved in depression, including hypothalamus, amygdala, nucleus accumbens, hippocampus and prefrontal cortex, were examined. The expression of inflammatory genes, TNF-α and iNOS, as well as the downstream genes of interferon signaling, IFNRA, IRF1 and STAT1, were determined. As we expected, our preliminary data suggested that inflammatory genes and interferon signaling were only upregulated in hypothalamus, possibly due to the higher permeability to blood brain barrier as a circumventricular organ. Based on the above observations, we hypothesized that chronic treatment of IFN-α induces hypothalamic inflammation, disrupts the normal functions of neuropeptide system and trigger depressive like behavior or indirectly affect mesolimbic pathways to generate anhedonia. The expressions of various neuropeptide genes in the IFN-α treated hypothalamus were further evaluated. Numbers of neuropeptide mRNA level are decrease. In addition, IFN-α treatment downregulated the activation of protein kinase A, which is a major downstream target of monoaminergic receptors, in the hippocampus and amygdala. In conclusion, our results suggested IFN-α can directly suppress the functions of neuroendocrine system, and possibly in turn affect serotonergic or dopaminergic system. Our finding will provide important information about the pathological mechanisms of IFN-α induced depression.