LIM-HD Transcription Factors in Forebrain Patterning

描述(申请人提供)：支持成年哺乳动物前脑解剖和功能组织的基本原理是将新皮质划分成不同的区域，并将丘脑背侧核(DTH)分割成核。新皮质向区域的发育模式，即区域化的过程，既受内在机制和外部影响两种机制的控制，内在机制在这里被定义为指定皮质神经元位置或区域特性的调节基因，外部影响主要被认为是区域特异性丘脑皮质轴突(TCA)输入或由其传递的信息，它来自DTH核，部分由新皮质固有的遗传框架建立。关于区域化内在遗传调控的证据在过去几年才开始出现，到目前为止已经发现同源结构域转录因子EMX2和成对框转录因子PAX6控制区域化。人们对DTH形成核型并指定核特有特性的遗传机制知之甚少。我们假设LIM同源结构域(LIM-HD)转录因子LHX2和LHX9通过一种直接的机制指定皮层中的区域同一性和DTH中的核同一性，其基础是LIM-HD基因的已知功能、LHX2在新皮质中的唯一分级表达以及LHX2和LHX9在DTH核亚群中的组合表达。我们还提出，它们通过控制DTH进入核团的模式并指定DTH投射神经元自主的属性来间接但关键地影响新皮质区域化，这些属性决定了它们的TCA投射到特定新皮质区域的发展，这反过来又影响皮质模式。为了研究LHX2和LHX9的这些作用，我们将采用一系列方法，包括使用组织特异性转基因表达和靶向基因失活的条件获得和功能丧失分析，这些分析旨在使小鼠存活到成年。我们将分析皮质和DTH神经元的区域和核特异性表型，包括识别标记的表达，特定的输入和输出投射，以及新皮质组织成区域和DTH成核。提出的多层次分析和替代策略将有助于规避与更有限的方法相关的潜在问题，提供补充的发现来支持解释，并有助于建立对前脑模式的调控等级。

DESCRIPTION (provided by applicant): A fundamental principle that underlies the anatomical and functional organization of the adult mammalian forebrain is the patterning of the neocortex into distinct areas and the parcellation of the dorsal thalamus (dTh) into nuclei. The developmental patterning of the neocortex into areas, i.e. the process of arealization, is controlled by both intrinsic mechanisms, defined here as regulatory genes that specify positional or area identities of cortical neurons, and extrinsic influences, mainly considered to be area-specific thalamocortical axon (TCA) input, or the information being relayed by it, that arises from dTh nuclei and is established in part by the genetic framework intrinsic to the neocortex. Evidence for the intrinsic genetic regulation of arealization has only begun to emerge over the past few years and thus far has implicated the homeodomain transcription factor EMX2 and the paired-box transcription factor PAX6 in controlling arealization. Less is known about the genetic mechanisms that pattern the dTh into nuclei and specify nuclei-unique properties. We hypothesize that the LIM-homeodomain (LIM-HD) transcription factors LHX2 and LHX9 specify area identities in the cortex and nuclei identities in the dTh through a direct mechanism, based on the known functions of LIM-HD genes, the uniquely graded expression of LHX2 in the neocortex, and the combinatorial expression of LHX2 and LHX9 in subsets of dTh nuclei. We also propose that they indirectly, but critically, influence neocortical arealization by controlling patterning of the dTh into nuclei and specifying properties autonomous to dTh projection neurons that determine the development of their TCA projections to specific neocortical areas, which in turn influence cortical patterning. To study these roles for LHX2 and LHX9, we will employ a range of approaches including conditional gain- and loss-of-function analyses using tissue specific transgene expression and targeted gene inactivation in mice designed to survive to adulthood. We will analyze area- and nuclei-specific phenotypes of cortical and dTh neurons, including expression of markers of identity, specific input and output projections, and the organization of neocortex into areas and dTh into nuclei. The multilevel analyses and alternative strategies proposed will help circumvent potential problems associated with more limited approaches, provide complementary findings to support interpretations, and help establish a hierarchy of regulation of forebrain patterning.