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. Overall design: 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大脑衰老过程相关。在额颞叶痴呆（frontotemporal dementia, FTD）中，高表达型TMEM106B风险单倍型可增加伴TDP-43包涵体的FTD（FTD-TDP）的发病易感性，并可改变颗粒蛋白前体（progranulin）突变携带者的疾病外显率（FTD-GRN）。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，本研究利用新近构建的Tmem106b基因纯合敲除（Tmem106b-/-）小鼠模型，对8月龄小鼠的脑组织开展了转录组学分析。本研究共纳入10只Tmem106b+/+（野生型，WT）、10只Tmem106b+/-（杂合敲除）及10只Tmem106b-/-（纯合敲除）小鼠。在Tmem106b-/-与野生型小鼠之间，共鉴定出153个下调表达基因与60个上调表达基因，其中显著富集于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相关基因。共表达分析结果进一步显示，下调最为显著的关联基因簇亦显著富集于髓鞘形成相关生物学过程。我们还在Tmem106b-/-小鼠的胼胝体中检测到Olig2阳性细胞的显著减少，该现象在21日龄幼鼠中即已出现，并持续至23月龄老年阶段且未出现进展。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的标志物无此变化。尽管未造模的小鼠未观察到髓鞘超微结构的明显异常，但经铜色酮（cuprizone，双环己酮草酰二腙）处理后发现，Tmem106b-/-小鼠对铜色酮诱导的脱髓鞘作用更为易感，且髓鞘再生能力下降；该实验结果可在新构建的Tmem106b CRISPR/Cas9基因敲除小鼠模型中重复验证。最后，本研究利用TMEM106B基因敲除的HeLa（海拉）细胞系，证实TMEM106B缺失会导致溶酶体分布及髓鞘蛋白脂质蛋白1（proteolipid protein 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（myelin oligodendrocyte glycoprotein, MOG）转运过程。综上，本研究结果揭示了TMEM106B在髓鞘形成过程中的重要功能，这为以降低TMEM106B表达为目标的治疗策略带来了潜在启示。研究设计：本研究利用新近构建的Tmem106b基因敲除小鼠模型，对8月龄小鼠的脑组织开展转录组学分析，共纳入10只Tmem106b+/+（野生型，WT）、10只Tmem106b+/-（杂合敲除）及10只Tmem106b-/-（纯合敲除）小鼠。