Function of LIM-domain Transcriptional Regulators in Inner Ear Development

描述（由申请人提供）：耳聋疾病的主要原因源于内耳毛细胞的退化，尽管在许多情况下，这些疾病的机制尚不清楚。通过定义控制正常毛细胞发育的机制，我们将更好地了解这些过程在病理情况下是如何被破坏的，以及毛细胞是如何再生的。本研究的核心假设是，lim同域转录因子Isl1的表达为耳蜗腹侧上皮提供了形成感觉器官的能力，而仅lim同域转录调控因子LMO3和LMO4对Isl1功能的负调控将这种能力限制在假定的OC区。在我们的初步研究中，我们发现在E12.5 - E16.5耳蜗的感觉发育过程中，Isl1在耳蜗腹侧上皮中广泛表达，包括推定的OC。有趣的是，LMO3的表达在小上皮脊（LER）中检测到，而LMO4的表达仅限于大上皮脊（GER）和远端上皮脊（dLER）。LMO3和LMO4的联合表达域与Isl1的表达域重叠，除了在假定的OC区，Isl1单独表达。与我们的假设一致，我们已经证明LMO4的缺失导致dLER中多余毛细胞的形成，证实了LMO4作为感觉器官发育的负调节因子的作用。因此，基于LMO蛋白在抑制LIM-HD蛋白转录调控中的作用，Isl1、LMO3和LMO4的联合作用可以决定OC区的形成。为了验证这一假设并探讨lim结构域因子在内耳发育中的作用，我们提出以下三个具体目标：1)。通过靶向破坏LMO4，确定耳蜗和前庭感觉和神经元发育对LMO4的需求；2)。探讨LMO4在前感觉区的异位表达是否通过条件激活LMO4在isl1表达细胞中的表达来抑制感觉发育；和3)。通过条件缺失Isl1来确定Isl1在内耳感觉和神经元发育中的作用。Corti器官内耳毛细胞的损失是导致听力损失的主要原因，影响着全球2.78亿人，其中包括美国的2800万人。然而，由于毛细胞的丢失是一个不可逆的过程，并且哺乳动物内耳缺乏再生毛细胞的能力，因此有效的替代毛细胞的方法仍然难以捉摸。本研究将为内耳感觉器官形成的分子机制提供新的见解，并可能为通过新生毛细胞再生治疗和最终治愈耳聋提供新的方法。本研究的核心假设是，limm同源结构域转录因子的表达为耳蜗腹侧上皮提供了形成感觉器官的能力，而limm同源结构域转录调控因子抑制了limm同源结构域因子在耳蜗腹侧（Corti区除外）器官中的作用。因此，LIM-homeodomain和LIM-domain-only因子的联合作用调节了耳蜗腹侧上皮的能力，并决定了Corti推定器官的区域。

DESCRIPTION (provided by applicant): A major cause of deafness disorders stem from the degeneration of hair cells within the inner ear although in many cases the mechanisms underlying these disorders is not understood. By defining the mechanisms controlling normal hair cell development, we will gain a better understanding of how these processes are disrupted in pathological situations and how the hair cells can be regenerated. The central hypothesis of this proposal is that the expression of the LIM-homeodomain transcription factor Isl1 provides the ventral cochlear epithelium with a competence to form the sensory organ and that the negative regulation of Isl1 function by the LIM-domain-only transcriptional regulators LMO3 and LMO4 restricts the competence to the presumptive OC (OC) region. In our preliminary studies, we have shown that during the sensory development in the cochlea at E12.5 to E16.5, Isl1 is expressed in a broad domain in the ventral cochlear epithelium, including the presumptive OC. Interestingly, the expression of LMO3 is detected in the lesser epithelial ridge (LER), whereas LMO4 expression is confined to the greater epithelial ridge (GER) and to the distal LER (dLER). The combined LMO3 and LMO4 expression domain overlaps with that of Isl1 except in the presumptive OC region where Isl1 is expressed alone. Consistent with our hypothesis, we have shown that loss of LMO4 results in the formation of supernumerary hair cells in the dLER, confirming a role for LMO4 as a negative regulator of sensory organ development. Thus, based on the established roles of LMO proteins in inhibiting LIM-HD proteins' function in transcriptional regulation, the combined action of Isl1, LMO3 and LMO4 could determine the formation of the OC region. In order to test this hypothesis and investigate the roles of LIM-domain factors in the inner ear development, we propose the following three specific aims: 1). To determine the requirement for LMO4 in the sensory and neuronal development in the cochlea and vestibule by targeted disruption of LMO4; 2). To determine whether the ectopic expression of LMO4 in the presumptive prosensory region represses the sensory development by the conditional activation of LMO4 expression in the Isl1-expressing cells; and 3). To determine the role of Isl1 in the sensory and neuronal development of the inner ear by the conditional deletion of Isl1. PUBLIC HEALTH RELEVANCE Loss of the inner ear hair cells in the organ of Corti is the leading cause of hearing loss that affects 278 million people worldwide, including 28 million in the United States. However, since the loss of hair cells is an irreversible process and mammalian inner ear lacks the capability to regenerate hair cells, effective remedies to replace hair cells remain elusive. The studies proposed in this application will provide new insights into the molecular mechanisms underlying the sensory organ formation in the inner ear and could lead to novel approaches in the treatment and eventual cure of deafness by de novo hair cell regeneration. The central hypothesis of this proposal is that the expression of the LIM-homeodomain transcription factor provides the ventral cochlear epithelium with the competence to form the sensory organ and that the LIM-domain-only transcriptional regulators suppress the role of LIM-homeodomain factor in the ventral cochlea except the presumptive organ of Corti region. Thus, the combined function of LIM-homeodomain and LIM-domain-only factors regulates the competence in the ventral cochlear epithelium and determines the region of the presumptive organ of Corti.