Papel del aprovisionamiento en una cadena de suministro

I posit that the endolysosomal/autophagosomal disturbances caused by TMEM106B overexpression may have direct or indirect effects on progranulin internalization, sorting, and trafficking. This may impair progranulin’s normal neuroprotective signaling pathway, i.e., the network of interconnected endosomes and lysosomes which executes spatiotemporal control over signaling events, delivering the appropriate amount of neurotrophic progranulin “signal” to different regions of the cell (Sadowski et al., 2009). Proper regulation of this network pathway is

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critical in long and polarized neurons. An example of a pathway that relies heavily on the integrity of a signaling endosome network is the EGF/EGFR signaling pathway. Endocytosis of EGF/EGFR activates the EGFR/MAPK signaling pathway, which remains activated until EGFR is inwardly budded into the intraluminal vesicles of the late endosome, removing the catalytic domain of EGFR from the cytoplasm (Sorkin and Duex, 2010; Eden et al., 2009). The neurotrophic effects of progranulin are likely to be similarly dependent on the regulated internalization of progranulin, with downstream signaling cascades and ultimate termination of signal through degradation of progranulin in the lysosome. As a consequence, TMEM106B’s effects on the endolysosomal network could affect the ability to appropriately internalize progranulin, signal through progranulin, or terminate progranulin signaling appropriately. Intriguingly, progranulin has been shown to activate signaling pathways known to be dependent on proper spatial-temporal function of signaling endosome networks (e.g., MAPK, PI3K) (He et al., 2002; Zanocco-Marani et al., 1999).

As we and others have shown, increased TMEM106B significantly disrupts the endolysosomal network and lysosomal trafficking in neurons, with knockdown of TMEM106B increasing and overexpression decreasing lysosomal trafficking in mouse cortical cultures (Stagi et al., 2014; Schwenk et al., 2014). Thus, perturbations in the endolysosomal pathway as induced by varying levels of TMEM106B could exacerbate the negative effects of progranulin deficiency by disturbing the spatio-temporal relationship of signaling endosomes and lysosomes involved in progranulin signaling pathways, thus disrupting progranulin’s neurotrophic effects. While the progranulin pathway responsible for its neurotrophic effects has yet to be determined, I note that progranulin deficiency has been shown to result in decreased mTORC1 activity in frontal cortex in a Grn-/- mouse model (Tanaka et al., 2013); thus, an intriguing possibility is that the pathway that TMEM106B overexpression negatively impacts may be mTORC1 itself.

4.4 The endolysosomal/autophagosomal pathway in disease pathogenesis

Overall, emerging data on TMEM106B, progranulin, and C9orf72 highlight a role for all three of these disease-associated proteins in endolysosomal trafficking and function. In addition,

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rarer, Mendelian forms of FTLD also point to disturbances in the endolysosomal/autophagosomal pathway and altered trafficking dynamics as a central theme in FTLD.

As shown in Figure 4.3, FTLD-TDP-associated mutations in CHMP2B may impair fusion of endosomes with autophagosomes and lysosomes; (Urwin et al., 2010; Filimonenko et al., 2007; Lee et al., 2007); disease-associated mutations in VCP may impair autophagosomal maturation and transport of ubiquitinated cargo to lysosomes (Ju et al., 2009; Ritz et al., 2011); moreover, new evidence suggests that C9orf72 expansions may result in impaired endosomal to lysosomal trafficking due to loss of C9orf72’s normal function (O'Rourke et al., 2016). In addition, progranulin deficiency has indeed been associated with significant disturbances to lysosomal and autophagosomal pathways (Petkau et al., 2010; Shacka, 2012; Gotzl et al., 2014). Thus, impairments in lysosomal function, altered fusion dynamics, and altered vesicular trafficking emerge as common themes in FTLD. I further note that endolysosomal/autophagosomal dysfunction may play an important role in other neurodegenerative diseases such as Parkinson’s, Alzheimer’s, and Huntington’s disease, as discussed in the Introduction (Wong and Cuervo, 2010).

How does TMEM106B fit within this contextual framework? I posit that disease- associated increases in TMEM106B result in altered dynamics within the endolysosomal/autophagosomal pathway. TMEM106B overexpression may do this by impairing dissolution of the secondary lysosomes (such as autolysosomes/amphisomes), potentially via impairment of autophagic lysosomal reformation. Alternatively, TMEM106B levels may modulate one or more Rab GTPases or effectors, increasing trafficking and membrane fusion dynamics to result in an increase in secondary lysosomes.

The effects of increased TMEM106B may perturb progranulin pathways that are vital to lysosomal and cellular homeostasis. In addition, TMEM106B may interact with endogenous C9orf72, and elevated levels of TMEM106B may counterbalance the loss of C9orf72 within the endolysosomal pathway.

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Figure 4.3:

Figure 4.3: Endolysosomal/autophagosomal dysfunction in neurodegeneration

Mutations in multiple genes involved in the endolysosomal/autophagosomal pathway are implicated in neurodegeneration and are shown where they have been implicated to impact. Black text: mutations associated with FTLD. Disease-associated mutations in CHMP2B have been shown to impair endolysosome-lysosome fusion. Mutations in VCP have been shown to impair lysosomal fusion with autophagosomes. C9orf72 has been implicated in late endosomal to lysosomal trafficking, with the disease-associated hexanucleotide expansion resulting in reduced levels of functional C9orf72 protein. Green text: Alzheimer’s mutations in PSEN1 impair lysosomal acidification by impairing proper trafficking of a vATPase subunit. Blue text: Parkinson’s-associated mutations in ATP13A2, a lysosomal P-type ATPase, results in impaired lysosomal acidification. Mutations in LRRK2 activate a Ca+2_{-dependent pathway that results in}

lysosomal acidification defects and increased autophagosome formation. Against this backdrop of endolysosomal/autophagosomal disruption, TMEM106B may exert its effects on the endolysosomal pathway by 1) impairing the dissolution of secondary lysosomes (such as ALs depicted) 2) increasing secondary lysosomal formation by modulation of endolysosomal Rab GTPase activity.

AL= autolysosome. AP=autophagosome.

Recent data highlight the important roles of both progranulin and C9orf72 in microglia, with C9orf72 deficient mice and Grn deficient mice exhibiting pro-inflammatory microglial cytokine profiles and neurotoxic effects (Tanaka et al., 2013; O'Rourke et al., 2016). These studies emphasize the growing importance of considering the microglia-neuron interaction in the pathogenesis of disease. Manipulation of TMEM106B within microglia, and assessment of how this influences microglial cytokine profiles as well as neuronal toxicity, should be a major focus of study in future studies attempting to unravel the relationship between TMEM106B and

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progranulin, and TMEM106B and C9orf72.

Finally, an as-yet-unanswered question within the field of FTLD-TDP concerns the relationship between TDP-43, the major protein forming the pathognomonic inclusions of the disease, and the various proteins implicated by the genetics in FTLD-TDP. Despite both being linked to FTLD-TDP ten years ago, a mechanistic link between progranulin and TDP-43 has yet to emerge. Similarly, few mechanistic connections have been found between TDP-43 and C9orf72. Our findings suggest one way that TDP-43 may be linked to these various genetic causes of FTLD-TDP is through endolysosomal/autophagosomal pathway dysfunction. Aggregates of TDP-43 are reported to be degraded by autophagy as well as by the ubiquitin proteasome system (Wang et al., 2010; Brady et al., 2011). Thus, if degradative pathways are affected by, for example TMEM106B expression levels, one major route for the cell to rid itself of TDP-43 aggregates would be impaired. I note in this context that in addition to the genes mentioned above, other ALS/FTLD-associated mutations have recently emerged in genes that are all to various extents related to autophagy. The protein products of UBQNL2, SQSTM1, OPTN – ubiquilin2, p62, and optineurin respectively – have all been shown to function in autophagy as autophagy receptors, binding ubiquitinated protein aggregates and directing them to autophagosomes for degradation (Wagner 2008, Wild 2011).

In the case of these specific, rare, Mendelian causes of ALS/FTLD, the existing evidence points to defects in autophagic proteins or related proteins that compromise the neuron’s ability to adequately degrade these protein aggregates, with subsequent toxic effects that cause neurodegeneration. Increases in TMEM106B expression may similarly compromise degradation of protein aggregates, to a lesser degree, through impairments to endolysosomal/autophagosomal function, thus contributing to increased risk for FTLD.