The role of the Alzheimer related Amyloid Precursor Protein Gene Family in the Developing and Adult Nervous System

APP作为阿尔茨海默病（AD）细胞外斑块中积聚的ß-淀粉样肽的前体蛋白的作用已得到证实，处理APP的分泌酶是主要的治疗靶点。相比之下，其生理功能以及这些功能的丧失是否导致发病机制的问题在很大程度上仍未解决。APP是一个更大的基因家族的成员，包括密切相关的APP样蛋白APLP1和APLP2。为了直接研究APP基因家族的生理功能，我们之前培育了围产期死亡的APLP2-/-/APLP1-/-和APLP2-/-/APP-/-小鼠，而APLP1-/-/APP-/-小鼠和单突变体是存活的。最近，我们培育了缺乏所有三个APP/APLP家族成员的三重突变小鼠，这些小鼠在出生后不久就死亡了。致死性双突变体没有表现出明显的形态学异常，而三突变体的大脑表现出类似于人类II型无脑畸形的表型，具有异位的神经母细胞簇，这些神经母细胞簇通过基底层和枕膜迁移，这表明APP/APLPs在神经元粘附和/或迁移中的作用。总的来说，我们的数据表明APP/APLPs对正常的大脑发育和产后早期生存至关重要。然而，这种围产期致死率，排除了在产后和成年后提出的功能分析，老化的神经系统。此外，潜在的致死性机制目前尚不清楚。因此，当前项目的目的是通过两种互补的方法来避免早期致命性。为此，我们打算(A)使用四环素调控的遗传开关（tet-off）产生条件零突变体，以在出生后下调APP/APLP的表达。其次(B)，我们打算以神经元特异性的方式消融APP表达，无论是在神经祖细胞发育期间，还是在出生后有丝分裂后神经元中。这种方法将揭示是否如假设的那样，致命是由于神经系统功能受损还是由于周围器官不相关的缺陷。我们打算在神经系统形态学和功能成熟方面对这些突变系小鼠进行表征，特别是突触发生、神经元可塑性、感觉运动发育以及学习和记忆。了解APP及其同源体在成人和衰老神经系统中的生理功能，对于揭示导致阿尔茨海默病的失调和致病机制至关重要。

The role of APP as the precursor protein of the ß-amyloid peptide, which accumulates in extracellular plaques in Alzheimer¿s disease (AD), is well established and the secretases processing APP constitute major therapeutic targets. By contrast, its physiological function and the question of whether a loss of these functions contributes to pathogenesis remains largely unresolved. APP is a member of a larger gene family including the closely related APP-like proteins APLP1 and APLP2. To address the physiological functions of the APP gene family directly, we had previously generated APLP2-/-/APLP1-/-- and APLP2-/-/APP-/-- mice that die perinatally, while APLP1-/-/APP-/--mice and single mutants were viable. Recently, we generated triple mutant mice lacking all three APP/APLP family members which also died shortly after birth. Whereas lethal double mutants showed no apparent morphological abnormalities, the brains of triple mutants displayed a phenotype resembling human type II lissencephaly with ectopic clusters of neuroblasts that had migrated through the basal lamina and pial membrane, indicating a role of APP/APLPs in neruonal adhesion and/or migration. Collectively, our data demonstrate that APP/APLPs are essential for normal brain development and early postnatal survival. However, this perinatal lethality, precludes the analysis of proposed functions in the postnatal and adult, aging nervous system. Moreover, the underlying mechanism of lethality is presently unknown. The aim of the current project is thus to circumvent early lethality by two complementary approaches. To this end, we intend to (A) generate conditional null mutants using a tetracycline regulated genetic switch (tet-off) to downregulate APP/APLP expression postnatally. Secondly (B), we intend to ablate APP expression in a neuron-specific manner, either already during development in neural progenitor cells, or in postmitotic neurons postnatally. This approach will unravel whether lethality is, as hypothesized, due to compromised functions within the nervous system or due to unrelated defects in peripheral organs. We intend to characterize these mutant lines of mice with regard to the morphological and functional maturation of the nervous system, in particular synaptogenesis, neuronal plasticity, sensorimotor development, as well as learning and memory. Understanding the physiological functions of APP and its homologous in the adult and aging nervous system will be essential to unravel the misregulation and pathogenic mechanisms that lead to Alzheimer¿s disease.