Thresholds, sites, and contributions of circuit compensation following rod photoreceptorloss in mature retina

迫切需要了解细胞死亡反应的原因、程度和机制，以便采用最适合剩余回路状态的有效治疗方法，并利用建设性补偿作为一种潜在的治疗方法，在部分回路持续存在的情况下。我们的长期目标是挽救神经回路。在这里，我们将定义受控细胞死亡对特定电路、细胞类型、突触和蛋白质的影响，以了解导致建设性（例如，通过增加突触增益进行补偿）与破坏性（例如，破坏信号的异常自发活动）反应的条件。小鼠视网膜是这项研究的一个特殊平台，因为主要的感觉神经元，光感受器，可以在遗传控制下操纵；特定电路内的细胞类型是可识别和可接近的；功能读数可以解释为视觉灵敏度。我们建议在成熟视网膜中切除可变的视杆细胞群，并确定对原代视杆双极细胞通路的结构和功能影响，这是最敏感的视网膜通路：视杆→视杆双极细胞→AII无突细胞→on锥双极细胞→on持续α神经节细胞（on α）。α神经节细胞接受最多的杆状细胞输入，因此对杆状细胞丢失最为敏感。我们的中心假设是，视网膜对输入损失有建设性的反应，有能力恢复正常功能，直到一个未定义的阈值；超过这个阈值，破坏性反应就开始了。这个临界点是未知的。我们的初步数据显示，尽管失去了一半的杆状体，杆状体

There is critical need to understand the causes, extent, and mechanisms of reactions to cell death so that effective treatments most appropriate for the state of the remaining circuit can be employed, and so that constructive compensation can be harnessed as a potential treatment in conditions where a portion of the circuit endures. Our long-term goal is to salvage neuronal circuits. Here, we will define the effects of controlled cell death on specific circuits, cell types, synapses, and proteins for the purpose of understanding the conditions that result in constructive (e.g., compensation through increasing synaptic gain) vs. destructive (e.g., aberrant spontaneous activity that corrupts signal) response. The mouse retina is an exceptional platform for this study because the primary sensory neurons, photoreceptors, can be manipulated under genetic control; cell types within specific circuits are identifiable and accessible; and the functional readout can be interpreted as visual sensitivity. We propose to ablate variable populations of rods in mature retina and determine the structural and functional effects on the primary rod bipolar cell pathway, the most sensitive retinal pathway: rods→rod bipolar cells→AII amacrine cells→ON cone bipolar cells→ON sustained alpha ganglion cells (abbr. ON alpha). ON alpha ganglion cells receive the greatest number of rod inputs, thus would be the most sensitive to rod loss. Our central hypothesis is that the retina has constructive reactions to input loss with the capacity to recover normal function up to an undefined threshold; beyond this threshold, destructive reactions begin. Unknown is this tipping point. Our preliminary data show that despite loss of half the rods, rod- mediated light responses in ON alpha ganglion cell spikes are comparable to control, suggesting compensation within the primary rod bipolar cell pathway. Thus, the premise is strong for constructive compensation within the retina following rod loss, and we will determine the induction parameters, sites, and contributions of this compensation to maintaining function in the following aims: (Aim 1) to determine the degree of input loss that induces constructive vs. destructive structural and functional changes, and (Aim 2) to locate the site(s) and mechanism(s) of compensation within a well-defined neural circuit. The approach is innovative for genetic control over the timing and degree of rod death; synaptic- and cell-type specific structural and functional investigation of a well-defined retinal circuit; and molecular tools to distinguish between cell ablation and synapse disassembly in triggering compensatory mechanisms. The results will be significant for (1) determining the degree of rod death that triggers the remaining circuit to undergo destructive or constructive responses, (2) identifying the sites and contributions of structural and functional compensation to maintaining retinal function, and (3) providing knowledge essential to the optimization and deployment of therapies to treat dysfunctional photoreceptors involving stem cells, genes, and prostheses, all of which rely on a stable retinal circuit and/or extensive knowledge of the state of the surviving retinal circuit.