Premature polyadenylation-mediated loss of stathmin-2 is a hallmark of TDP-43-dependent neurodegeneration

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are associated with loss of nuclear TDP-43. Here we identify that TDP-43 regulates expression of the neuronal growth-associated factor stathmin-2. Lowered TDP-43 levels, which reduce its binding to sites within the first intron of stathmin-2 pre-mRNA, uncover a cryptic polyadenylation site whose utilization produces a truncated, non-functional mRNA. Reduced stathmin-2 expression is found in neurons trans-differentiated from patient fibroblasts expressing an ALS-causing TDP-43 mutation, in motor cortex and spinal motor neurons from sporadic ALS patients and familial ALS patients with expansion in C9orf72, and in induced pluripotent stem cell (iPSC)-derived motor neurons depleted of TDP-43. Remarkably, while reduction in TDP-43 is shown to inhibit axonal regeneration of iPSC-derived motor neurons, rescue of stathmin-2 expression restores axonal regenerative capacity. Thus, premature polyadenylation-mediated reduction in stathmin-2 is a hallmark of ALS/FTD that functionally links reduced nuclear TDP-43 function to enhanced neuronal vulnerability. Analysis of gene expression changes linked to TDP-43 loss by siRNA (n=3 biological replicates each conditon) or linked to mutant TDP-43 expression by genome editing in SH-SY5Y cells (n=2 biological replicates each condition).

肌萎缩侧索硬化症（Amyotrophic lateral sclerosis, ALS）与额颞叶痴呆（frontotemporal dementia, FTD）均与细胞核内TDP-43的缺失相关。本研究发现，TDP-43可调控神经元生长相关因子stathmin-2的表达水平。当TDP-43水平降低时，其与stathmin-2前体mRNA第一内含子内结合位点的结合能力减弱，进而暴露一处隐蔽多聚腺苷酸化位点，该位点的激活会产生截短且无功能的mRNA。
研究人员在携带ALS致病TDP-43突变的患者成纤维细胞转分化得到的神经元中、散发性ALS患者及C9orf72基因扩增型家族性ALS患者的运动皮层与脊髓运动神经元中，以及敲低TDP-43的诱导多能干细胞（induced pluripotent stem cell, iPSC）源性运动神经元中，均检测到stathmin-2表达水平的下调。值得注意的是，尽管TDP-43水平降低会抑制iPSC源性运动神经元的轴突再生能力，但恢复stathmin-2的表达可逆转这一缺陷，修复其轴突再生潜能。
综上，过早多聚腺苷酸化介导的stathmin-2表达下调是ALS/FTD的标志性事件，该机制将细胞核内TDP-43功能受损与神经元易感性增强直接关联起来。本研究还开展了相关基因表达分析：分别通过小干扰RNA（small interfering RNA, siRNA）敲低TDP-43（每组设置3个生物学重复），或在SH-SY5Y细胞中通过基因组编辑引入突变型TDP-43（每组设置2个生物学重复），以探究TDP-43功能缺失所介导的基因表达变化。