|Common pathobiochemical hallmarks of progranulin-associated frontotemporal lobar degeneration and neuronal ceroid lipofuscinosis. |
Götzl, JK; Mori, K; Damme, M; Fellerer, K; Tahirovic, S; Kleinberger, G; Janssens, J; van der Zee, J; Lang, CM; Kremmer, E; Martin, JJ; Engelborghs, S; Kretzschmar, HA; Arzberger, T; Van Broeckhoven, C; Haass, C; Capell, A
Heterozygous loss-of-function mutations in the progranulin (GRN) gene and the resulting reduction of GRN levels is a common genetic cause for frontotemporal lobar degeneration (FTLD) with accumulation of TAR DNA-binding protein (TDP)-43. Recently, it has been shown that a complete GRN deficiency due to a homozygous GRN loss-of-function mutation causes neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disorder. These findings suggest that lysosomal dysfunction may also contribute to some extent to FTLD. Indeed, Grn(-/-) mice recapitulate not only pathobiochemical features of GRN-associated FTLD-TDP (FTLD-TDP/GRN), but also those which are characteristic for NCL and lysosomal impairment. In Grn(-/-) mice the lysosomal proteins cathepsin D (CTSD), LAMP (lysosomal-associated membrane protein) 1 and the NCL storage components saposin D and subunit c of mitochondrial ATP synthase (SCMAS) were all found to be elevated. Moreover, these mice display increased levels of transmembrane protein (TMEM) 106B, a lysosomal protein known as a risk factor for FTLD-TDP pathology. In line with a potential pathological overlap of FTLD and NCL, Ctsd(-/-) mice, a model for NCL, show elevated levels of the FTLD-associated proteins GRN and TMEM106B. In addition, pathologically phosphorylated TDP-43 occurs in Ctsd(-/-) mice to a similar extent as in Grn(-/-) mice. Consistent with these findings, some NCL patients accumulate pathologically phosphorylated TDP-43 within their brains. Based on these observations, we searched for pathological marker proteins, which are characteristic for NCL or lysosomal impairment in brains of FTLD-TDP/GRN patients. Strikingly, saposin D, SCMAS as well as the lysosomal proteins CTSD and LAMP1/2 are all elevated in patients with FTLD-TDP/GRN. Thus, our findings suggest that lysosomal storage disorders and GRN-associated FTLD may share common features.
|The FTLD risk factor TMEM106B and MAP6 control dendritic trafficking of lysosomes. |
Schwenk, Benjamin M, et al.
EMBO J., (2014)
TMEM106B is a major risk factor for frontotemporal lobar degeneration with TDP-43 pathology. TMEM106B localizes to lysosomes, but its function remains unclear. We show that TMEM106B knockdown in primary neurons affects lysosomal trafficking and blunts dendritic arborization. We identify microtubule-associated protein 6 (MAP6) as novel interacting protein for TMEM106B. MAP6 over-expression inhibits dendritic branching similar to TMEM106B knockdown. MAP6 knockdown fully rescues the dendritic phenotype of TMEM106B knockdown, supporting a functional interaction between TMEM106B and MAP6. Live imaging reveals that TMEM106B knockdown and MAP6 overexpression strongly increase retrograde transport of lysosomes in dendrites. Downregulation of MAP6 in TMEM106B knockdown neurons restores the balance of anterograde and retrograde lysosomal transport and thereby prevents loss of dendrites. To strengthen the link, we enhanced anterograde lysosomal transport by expressing dominant-negative Rab7-interacting lysosomal protein (RILP), which also rescues the dendrite loss in TMEM106B knockdown neurons. Thus, TMEM106B/MAP6 interaction is crucial for controlling dendritic trafficking of lysosomes, presumably by acting as a molecular brake for retrograde transport. Lysosomal misrouting may promote neurodegeneration in patients with TMEM106B risk variants.
|Membrane orientation and subcellular localization of transmembrane protein 106B (TMEM106B), a major risk factor for frontotemporal lobar degeneration. |
Lang, Christina M, et al.
J. Biol. Chem., 287: 19355-65 (2012)
TMEM106B was identified as a major risk factor in a genome-wide association study for frontotemporal lobar degeneration (FTLD) with TAR DNA-binding protein (TDP)-43 pathology. The most significant association of TMEM106B single nucleotide polymorphisms with risk of FTLD-TDP was observed in patients with progranulin (GRN) mutations. Subsequent studies suggested an inverse correlation between TMEM106B expression and GRN levels in patient serum. However, in this study, this was not confirmed as we failed to detect a significant alteration of GRN levels upon knockdown or exogenous expression of TMEM106B in heterologous cells. To provide a basis for understanding TMEM106B function in health and disease, we investigated the membrane orientation and subcellular localization of this completely uncharacterized protein. By differential membrane extraction and sequential mutagenesis of potential N-glycosylation sites, we identified TMEM106B as a type 2 integral membrane protein with a highly glycosylated luminal domain. Glycosylation is partially required for the transport of TMEM106B beyond the endoplasmic reticulum to late cellular compartments. Endogenous as well as overexpressed TMEM106B localizes to late endosomes and lysosomes. Interestingly, the inhibition of vacuolar H(+)-ATPases significantly increased the levels of TMEM106B, a finding that may provide an unexpected biochemical link to GRN, because this protein is also strongly increased under the same conditions. Our findings provide a biochemical and cell biological basis for the understanding of the pathological role of TMEM106B in FTLD, an incurable neurodegenerative disorder.