Loss of Tmem106b leads to myelination deficits: implications for FTD treatment strategies

Genetic variants that define two distinct haplotypes at the TMEM106B locus have been implicated in multiple neurodegenerative diseases and in healthy brain aging. In frontotemporal dementia (FTD), the high expressing TMEM106B risk haplotype was shown to increase susceptibility for FTD with TDP-43 inclusions (FTD-TDP) and to modify disease penetrance in progranulin mutation carriers (FTD-GRN). To further elucidate the biological function of TMEM106B and determine whether lowering TMEM106B may be a viable therapeutic strategy, we performed brain transcriptomic analyses in 8-month-old animals from our recently developed Tmem106b-/- mouse model. We included 10 Tmem106b+/+ (WT), 10 Tmem106b+/- and 10 Tmem106-/- mice. The most differentially expressed genes (153 down-regulated and 60 upregulated) were identified between Tmem106b-/- and WT animals, with an enrichment for genes implicated in myelination-related cellular processes including axon ensheathment and oligodendrocyte differentiation. Co-expression analysis also revealed that the most downregulated group of correlated genes was enriched for myelination-related processes. We further detected a significant loss of Olig2-positive cells in the corpus callosum of Tmem106b-/- mice, which was present already in young animals (21 days) and persisted until old age (23 months), without worsening. qPCR revealed a reduction of differentiated but not undifferentiated oligodendrocytes cellular markers. While no obvious changes in myelin were observed at the ultrastructure levels in unchallenged animals, treatment with cuprizone revealed that Tmem106b-/- mice are more susceptible to cuprizone-induced de-myelination and have a reduced capacity to re-myelinate, a finding which we were able to replicate in a newly generated Tmem106b CRISPR/cas9 knock-out mouse model. Finally, using a TMEM106B HeLa knock-out cell line, we determined that loss of TMEM106B leads to abnormalities in the distribution of lysosomes and PLP1 trafficking but not to differences in MOG trafficking which is lysosome-independent. Together these findings reveal an important function for TMEM106B in myelination with possible consequences for therapeutic strategies aimed at lowering TMEM106B levels. We performed brain transcriptomic analyses in 8-month-old animals from our recently developed Tmem106b-/- mouse model. We included 10 Tmem106b+/+ (WT), 10 Tmem106b+/- and 10 Tmem106-/- mice.定位于跨膜蛋白106B（TMEM106B）基因座的两种不同单倍型的遗传变异，已被证实与多种神经退行性疾病及健康脑衰老过程密切相关。在额颞叶痴呆（frontotemporal dementia, FTD）中，高表达型TMEM106B风险单倍型被证实可增加伴TDP-43包涵体的FTD（FTD-TDP）的患病易感性，并可改变前颗粒蛋白突变携带者（FTD-GRN）的疾病外显率。为进一步阐明TMEM106B的生物学功能，并明确降低TMEM106B表达是否可作为可行的治疗策略，我们对新近构建的Tmem106b-/-小鼠模型中8月龄个体开展了脑转录组分析。我们纳入了10只Tmem106b+/+（野生型，wild type, WT）、10只Tmem106b+/-及10只Tmem106-/-小鼠。在Tmem106b-/-与野生型小鼠间鉴定出差异表达基因共213个，其中153个表达下调、60个表达上调；富集分析显示这些基因参与髓鞘形成相关细胞过程，包括轴突髓鞘化与少突胶质细胞分化。共表达分析同样显示，表达下调最显著的关联基因簇富集于髓鞘形成相关生物学过程。我们进一步在Tmem106b-/-小鼠的胼胝体中检测到少突胶质细胞转录因子2（Olig2）阳性细胞显著减少，该现象在幼龄小鼠（21日龄）中即已出现，并持续至老年阶段（23月龄）且未出现进展。实时荧光定量PCR（quantitative real-time polymerase chain reaction, qPCR）结果显示，分化型少突胶质细胞的标志物表达水平降低，而未分化型少突胶质细胞标志物无此变化。尽管在未受外界刺激的小鼠中未观察到髓鞘超微结构的明显改变，但用铜嗪（cuprizone）处理后发现，Tmem106b-/-小鼠对铜嗪诱导的脱髓鞘作用更易感，且髓鞘再生能力下降；该结果可在新构建的Tmem106b CRISPR/Cas9基因敲除小鼠模型中重复验证。最后，我们利用TMEM106B基因敲除的HeLa细胞系开展实验，证实TMEM106B缺失会导致溶酶体分布与蛋白脂蛋白1（PLP1）转运异常，但不会影响不依赖溶酶体的髓鞘少突胶质细胞糖蛋白（MOG）转运过程。综上，本研究结果揭示了TMEM106B在髓鞘形成中的重要功能，这为以降低TMEM106B表达为目标的治疗策略带来了潜在启示。我们对新近构建的Tmem106b-/-小鼠模型中8月龄个体开展了脑转录组分析。我们纳入了10只Tmem106b+/+（野生型，WT）、10只Tmem106b+/-及10只Tmem106-/-小鼠。