Exosomes originate during the maturation of early endosomes through the inward budding of the endosomal membrane that generates intraluminal vesicles (ILVs), resulting in the formation of late endosomes (LEs) with multiple small vesicles termed MVB (Huotari and Helenius, 2011). During this process, cytosolic portions, proteins, and nucleic acids are enveloped into the vesicles, and the resulting MVBs can be either targeted to lysosomal degradation or fused with the plasma membrane, releasing the ILVs to the extracellular space as exosomes.

Exosome biogenesis can occur through an Endosomal Sorting Complex Required for Transport (ESCRT)-dependent mechanism or by an ESCRT-independent pathway. The first relies on multiple proteins that assemble into four ESCRT complexes (ESCRT-0, I, II, and III) and accessory proteins, such as an AAA-ATPase, VPS4 complex, and ALIX that are involved in MVB budding and loading (Babst et al., 2002; Henne et al., 2011; Colombo et al., 2014).

The exosome biogenesis also occurs through an ESCRT-independent pathway mediated by tetraspanins and ceramide-enriched lipid rafts (Trajkovic et al., 2008; Van Niel et al., 2011). Tetraspanins are recruited at early steps to endosome membranes, before ILV formation (Escola et al., 1998; Pols and Klumperman, 2009) and at least CD63, CD9, CD81, and CD82 are found in endosome and exosome membranes (Andreu and Yanez-Mo, 2014).

Ceramide and its derived metabolites are organized in raft-based microdomains that interact with proteins, such as flotillins. These lipid-enriched structures are involved not only in endosomal membrane invagination for ILV formation (Trajkovic et al., 2008) but also in cargo loading. The selective cargo loading occurs during exosome biogenesis through tetraspanin-dependent and/or ESCRT-dependent mechanisms. The composition and quantity of exosome released are dictated by specific molecules present in the membrane and lumen of ILVs and in the membrane of MVB, which is determinant for their selective trafficking toward plasma membrane for exosome release (Figure 2A).

The interaction of MVBs with actin and microtubules is essential for their transport to the plasma membrane (Villarroya-Beltri et al., 2014). The translocation of MVB toward the plasma membrane depends on several molecules via the cytoskeleton (Mittelbrunn et al., 2015). Kifc2 is a neuronal protein that moves directionally toward the plus end of microtubules and was found to be associated with MVB, suggesting a possible role in MVB transport to the neuronal plasma membrane (Saito et al., 1997). Rab GTPases such as RAB11, RAB27A/B, and RAB35 are mediators of selective sorting of MVB to the plasma membrane and exosome release (Hsu et al., 2010; Ostrowski et al., 2010; Escudero et al., 2014). RAB11 and p75 neurotrophin receptors are involved in directing MBV to the plasma membrane and exosome release in neuronal cells (Escudero et al., 2014). Also, Rab35 is involved in the mechanism of exosome release by oligodendrocytes (Fruhbeis et al., 2013). The MVBs are decorated with tethering protein complexes, such as HOPS and SNAREs, that mediate the fusion of these vesicles with the plasma membrane (Itakura et al., 2012; Jiang et al., 2014). The presence of tetraspanins (e.g., CD63) and lysosomal-associated membrane proteins LAMP1 and LAMP2 in late endosomes also facilitate the fusion of MVB with the plasma membrane (Jaiswal et al., 2002).

After secretion, the exosomes will dock into the membrane of the target cells and activate signaling events or will be internalized through specific receptor–ligand interactions (Mulcahy et al., 2014). The transmembrane proteins present in the surface of exosomes (tetraspanins) can be recognized by signaling receptors in the target cells (Munich et al., 2012), resulting in activation of transducing pathways and modulation of intracellular processes without entering the target cell. Exosomes can merge with the plasma membrane of target cells releasing its cargo directly into the cytosol by a low pH-dependent mechanism (Parolini et al., 2009). However, the main route for exosome uptake seems to be mediated by endocytic events. Exosome uptake can occur by clathrin-mediated (Escrevente et al., 2011) or caveolin-dependent endocytosis (Nanbo et al., 2013), and the presence of lipid rafts in the membrane facilitates the process (Izquierdo-Useros et al., 2009; Svensson et al., 2013). After internalization, exosomes are sorted into LE/MVB with two possible fates: to be released again to neighboring cells or to be degraded after fusion of LE/MVB with lysosomes (Tian et al., 2013).

The uptake of exosomes by brain cells seems to be cell-type dependent. For instance, neurons and glial cells seem to uptake exosomes by clathrin-mediated endocytosis (Alvarez-Erviti et al., 2011b; Fruhbeis et al., 2013). Some neurons can also use specific receptors from the SNARE family, such as SNAP25, for exosome uptake (Zhang et al., 2017a). Curiously, the uptake of exosomes seems to be a selective pathway; exosomes derived from cortical neurons were primarily internalized by hippocampal neurons whereas exosomes released by neuroblastoma cell line N2A were taken up by astrocytes and oligodendrocytes (Chivet et al., 2014). Also, exosomes derived from oligodendrocytes were mainly internalized by microglia but not by neurons or astrocytes (Fitzner et al., 2011). Thus, the mechanisms that regulate the selective targeting of exosomes to recipient cells can determine their biological effect. Moreover, the uptake of exosomes was also more active in pre-synaptic regions, which might indicate that these vesicles use constitutive endocytosis processes at these regions for neuronal cell entrance (Granseth et al., 2006; Chivet et al., 2014). Nevertheless, the exact mechanism involved in exosome entrance into neuronal cells is still uncharacterized.

The pathology of HD is caused by a CAG expansion in the HTT gene (Huntington’s Disease Collaborative Research Group, 1993), which encodes a protein with a polyglutamine expansion at the N-terminal, mutant huntingtin (mHTT). Accumulation of the mutated protein results in cognitive deficits and involuntary movements, correlated with a selective loss of striatal medium spiny neurons and cortical atrophy (Roos, 2010). The implication of the possible transfer of pathological biomolecules through a non-cell-autonomous pathway in HD has aroused attention after Cicchetti and colleagues showed that healthy fetal neural tissue engrafted in HD patients displayed mHTT aggregates several years after transplantation (Cicchetti et al., 2014). The unconventional mHTT spread throughout exosome transport was then suggested as a novel mechanism involved in HD pathology and opened new possibilities to find therapeutic targets aiming to mitigate this neurodegenerative condition.

Several studies suggested that exosomes can transport the expanded polyglutamine tract of HTT RNA and protein, as well as mHTT aggregates (Jeon et al., 2016; Zhang et al., 2016). Zhang and colleagues showed that after infecting human HEK293T cells with a lentivirus encoding mutant exon 1 huntingtin fragments, these cells secreted exosomes containing transcripts of the mutant polyglutamine tail and HTT peptides (Zhang et al., 2016). The addition of these exosomes to mouse striatal cells carrying both normal and mutant CAG repeats (Q^7/7, Q^111/111, respectively) led to the uptake of RNA but curiously not the protein (Zhang et al., 2016). However, others have acknowledged the uptake of exosomes containing mHTT by SH-SY5Y cells when exposed to conditioned media from HEK293 cells overexpressing GFP-mHtt-Q19 and -Q103 (Jeon et al., 2016). This is a topic with variable outcomes since the presence of mHTT is not always observed in exosomes, as described for astrocytes of an HD knock-in mouse model (HD140Q KI) (Hong et al., 2017). Huntingtin is a large protein with 350 kDa, which makes it difficult to be fully packed into exosomes. mHTT spreading through the exosome pathway is a complex and not yet defined mechanism that surely has an associated variability between HD models. However, strong evidence supports the idea that mHTT can be carried by exosomes reinforcing the need to better clarify how the protein is loaded into exosomes. The acknowledgment that mHTT is present in exosomes was shown when murine embryonic fibroblasts (MEFs) overexpressing the exon 1 of the Htt gene showed constitutive interaction of mHTT with exosome structural proteins Alix and TSG101 mediated by Transglutaminase 2 (Diaz-Hidalgo et al., 2016). In HD astrocytes, mHTT was shown to impair the exosome secretion by decreasing αB-crystallin levels, a glial protein involved in exosome secretion. The overexpression of αB-crystallin could revert the decrease in exosome release and simultaneously decreased the mHTT aggregates in the striatum of HD140Q KI mice (Hong et al., 2017). On the other hand, in the same study, the injection of exosomes from WT astrocytes was described to prevent aggregation of mHTT (Hong et al., 2017). The exosome transport of mHTT was confirmed when neurons differentiated from WT mice NSC displayed mHTT aggregates upon co-culture with HD fibroblasts (143 CAG repeats); moreover, mice injected with HD-derived exosomes also revealed that mHTT aggregates specifically in the DARPP-32 + medium spiny neurons of the host’s striatum. Eight weeks post-incubation, the animals exhibited motor impairment and cognitive deficits (Jeon et al., 2016). In contrast, exosomes isolated from platelets of HD patients failed to show any presence of mHTT (Table 1; Denis et al., 2018). Altogether, this data awareness on the importance of exosomes in the progression and pathophysiology of HD supports the need for deeper research in exosome-mediated mechanisms.

Alzheimer’s disease (AD), the most common form of dementia, has, as a causative mechanism, the accumulation of aggregated β-amyloid (Aβ) in the brain and phosphorylated tau in neurofibrillary tangles (Palmqvist et al., 2017). The accumulation of Aβ fibrils can precede symptoms for decades, which has long intrigued researchers looking for the mechanisms by which neurodegeneration occurs (Sperling et al., 2014). The discovery of multivesicular bodies (MVBs) as a site for the accumulation of Aβ42 in pre- and postsynaptic compartments preceded the later acknowledgment that the cleavage of β-amyloid peptides occurs in early endosomes before routing to MVB and the resulting products released from cells through exosomes (Takahashi et al., 2002; Rajendran et al., 2006). The presence of Alix associated with amyloid plaques in post-mortem AD patient’s brains, a fact not observed in healthy brains, reinforced the existence of a role for Aβ-associated exosomes in amyloid plaque formation (Rajendran et al., 2006). The presence of secretases (γ-, α-, and β-secretases), enzymes involved in the proteolytic cleavage of APP, in isolated exosomes from an AD animal model raised the possibility for these vesicles to be a potential site of APP cleavage (Sharples et al., 2008). Indeed, it was observed that APP goes through cleavage inside exosomes, with existing evidence that products resulting from APP processing are present in exosomes from AD human and mouse AD model brains, evidencing the exosome intervention in the pathological process (Vingtdeux et al., 2007; Sharples et al., 2008; Perez-Gonzalez et al., 2012; Guix et al., 2017; Laulagnier et al., 2018).

After evidence of existing Aβ in association with exosomes, the concern was to understand whether these vesicles contribute, as a vehicle, to spreading the pathology or can be protective by clearing the toxic elements associated with AD pathophysiology. Exosomes derived from neuroblastoma cells injected into the brain of the hAPP-J20 AD mouse model were able to trap Aβ bound to the glycosphingolipid surface and later transport to microglia for degradation. These findings supporting the exosome mentioned “clearance role” through the decrease in the levels of Aβ and Aβ-mediated synaptotoxicity (Yuyama et al., 2014). Altogether, these data reinforce the capacity of exosomes to carry Aβ and “deliver” their cargo to microglia, reducing the levels of synaptic toxicity and inflammatory response.

The levels of certain enzymes and structural components in exosomes promote Aβ elimination by microglial and lysosomal processes, including the insulin-degrading enzyme, ceramide, or the ganglioside GM1 (Tamboli et al., 2010; Wang et al., 2012; Yuyama et al., 2012).

Thereby, there is evidence that microglial phagocytic action on Aβ is promoted by exosomes being mediated by sphingolipids integrating their structure. Moreover, β-amyloid oligomers (Aβo) incubated with exosomes were sequestered to their superficial structure by interacting with surface proteins including the prion protein (PrP^C) (An et al., 2013). The sequestration of Aβo by exogenous exosomes can exert a protective effect against the synaptic disruption induced by Aβ (An et al., 2013). Indeed, PrP^C plays a dual role: a protective one, by capturing Aβ in exosomes, through superficial recognition, thus facilitating the formation of amyloidogenic fibrils and preventing the derivative neurotoxic complications mediated by Aβo and a neurotoxic effect when expressed in the neuronal surface as a receptor for Aβ (Falker et al., 2016). Other exosome components from the outer membrane were reported to be associated with Aβ, including glycans of glycosphingolipids and GM1 ganglioside, which was seen to induce exosome release and promote Aβ fibril formation (Yuyama et al., 2008, 2014). Hypoxia can accelerate the production of Aβ by increasing the activity of β and PS1/γ-secretase. In SH-SY5Y cells (expressing human WT APP695), maintained in hypoxic conditions, was observed an increase in Aβ40 and Aβ42 content in exosomes (Xie et al., 2018).

Although the abovementioned studies reveal an eliminatory role for exosomes to discard Aβ, C-terminal fragments, and full-length APP, recent data suggest that these vesicles can also transport Aβo, further contributing to the neurotoxicity seen in AD. The use of biological material/samples from AD patients can more accurately help to unravel what specifically happens in the development of the disease other than cellular or animal models. Recently, a study in post-mortem tissue from the temporal neocortex of AD patients showed that flotilin-1 and Aβo co-labeled and there were increased levels of oligomers in exosomes compared to controls (Sinha et al., 2018). Incubation of human-derived induced-pluripotent stem (hiPSC)-differentiated neurons and SH-SY5Y cells with labeled exosomes isolated from brain tissue of AD patients revealed the incorporation of vesicles carrying Aβo, leading to subsequent cytotoxicity when comparing to healthy brains (Sinha et al., 2018). The ablation of Aβo spreading was achieved by using siRNA to inhibit transcription of TSG101 and VPS4A, proteins involved in exosome formation and secretion (Sinha et al., 2018). In sum, these studies confirmed the transport of Aβo, APP, and its cleaved products in exosomes and their further uptake and internalization by neuronal cells. Moreover, the neurotoxic effects of adding exosomes derived from AD human plasma, cell, and animal models or previously exposed to toxicity-inducing Aβ-protofibrils has also been described (Eitan et al., 2016; Beretta et al., 2020).

Tau has also been found in exosomes isolated from CSF and brain of AD patients (Saman et al., 2012; Wang et al., 2017; Guix et al., 2018; Crotti et al., 2019; Ruan et al., 2021). In accordance, the injection into WT mice of exosomes from iPSC-derived neurons and post-mortem brain of AD patients led to the increase in tau phosphorylation and formation of inclusions (Aulston et al., 2019; Ruan et al., 2021).

Generally, the diagnosis of AD patients is based on the expression of tau and Aβ; however, the observation that exosomes from AD patients have differences in cargo has sparked curiosity in the field of biomarkers (Table 1). In accordance, exosomes isolated from plasma of AD patients exhibited decreased levels of transcription factors involved in neuronal protection from external damages like the repressor element 1-silencing transcription factor (REST), as already observed in individuals at pre-clinical stages without cognitive impairments (Goetzl et al., 2015).

The recent developments in the methods to isolate exosomes of neuronal origin from plasma have gained interest as they may include more sensitive and accurate neuronal biomarkers than total plasma exosomes (Table 1). Some reports have shown the presence of higher levels of Aβ_1–42 and TAR DNA-binding protein 43 (TDP43) and decreased levels of synaptic proteins, as synaptophysin, synaptotagmin, and 25-kDa synaptosomal-associated protein (SNAP-25), in NDEs from AD patients (Fiandaca et al., 2015; Goetzl et al., 2016, 2018). Importantly, these first two presynaptic proteins, in particular, were already diminished at the pre-clinical stage and were correlated with the cognitive decline, as evaluated by the mini-mental state examination (Goetzl et al., 2016). In accordance, the exosome-associated growth-associated protein (GAP43) neurogranin, SNAP25, and synaptotagmin 1 proteins can be detected in AD up to several years before clinical onset (Jia et al., 2020). Following the same procedure, Zhao and colleagues showed that NDE Aβ_1–42 levels allow discriminating between cognitive controls, mild cognitive impairment (MCI) patients, and AD dementia patients and predicting the risk of MCI progressing to AD dementia (Zhao et al., 2020). The levels of miR132 and miR-212 were also shown to be decreased in NDEs from AD patients (Walsh et al., 2019). Another comprehensive study involving EV isolated from post-mortem brain tissue from AD subjects has provided more insights into differential RNA biotype exosome content, namely, expression patterns of AD-associated miRNA compared with brain homogenates, adding to the pallet of possible targets for studying AD (Cheng et al., 2020). The NDEs’ novel approach, considered a liquid biopsy of the brain, resorts to exosomes released from neuronal cells found in the peripheral source, to discover more accurate biomarkers for neurodegenerative disorders. This technique has mainly been used in AD studies; however, strong proof of concept has also been assessed in other neurodegenerative diseases.

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder affecting 0.3% of the general population. The pathological hallmark is the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) of the striatum and the accumulation of α-synuclein in cytoplasmic inclusions called Lewy Bodies (Balestrino and Schapira, 2020). The formation of Lewy Bodies in healthy transplanted areas of PD patients years after grafting raised curiosity for the study of possible non-autonomous spreading mechanisms including related EVs (Kordower et al., 2008).

The first evidence of exosome involvement in PD was the presence of α-synuclein in Alix- and flotillin-positive vesicles derived from α-synuclein-expressing cells (Emmanouilidou et al., 2010). In 2012, the oligomeric form of α-synuclein was shown to be transported via exosomes, and additionally that exosomes may favor the formation of α-synuclein aggregates due to its lipid and/or protein composition thus facilitating the uptake of α-synuclein by cells (Danzer et al., 2012; Grey et al., 2015; Delenclos et al., 2017). Moreover, oligomerized α-synuclein seemed to be preferentially uptaken by cells when associated with exosomes rather than isolated, reinforcing the role of exosomes to transfer and deliver toxic oligomeric proteins to cells (Delenclos et al., 2017). Similar results were obtained when exosomes obtained from CSF of PD patients were added to H4 glioma cells, resulting in the elevated formation of α-synuclein oligomers (Stuendl et al., 2016). Surprisingly, this study reported reduced levels of α-synuclein in exosomes from CSF of early stage PD patients when compared to controls. On the other hand, others claim that the presence of α-synuclein seemed to be elevated in exosomes from plasma of PD patients, suggesting a positive correlation between exosome-associated α-synuclein levels and disease severity studies (Shi et al., 2014). In fact, the presence of α-synuclein in L1CAM-positive NDEs validated the protein as a biomarker for PD with origins in the nervous system (Shi et al., 2014). Henceforward, investigators have searched for mechanisms behind the release of exosomes aiming to manipulate it and find new outcomes to treat neurodegeneration in PD. α-synuclein was also shown to induce the release of exosomes by microglia cells in mice, thus promoting apoptosis, as well as lysosomal dysfunction in α-synuclein overexpressing SH-SY5Y cells, leading to an increase in the release of exosome-associated α-synuclein (Alvarez-Erviti et al., 2011a; Chang et al., 2013a). Additionally, when exosomes from serum of PD patients, highly enriched in α-synuclein, were injected in mice, it induced protein aggregation along with a neuroinflammatory response, reinforcing the neurotoxic role of exosome transfer in PD (Han et al., 2019). Alterations in genes’ expression like ATPase cation transporting 13A2 (ATP13A2) and β-glucocerebrosidase have also been associated with differences in the exosome-associated release of α-synuclein (Kong et al., 2014; Papadopoulos et al., 2018). While the overexpression of ATP13A2 (overexpressed in neurons of PD patients) in SH-SY5Y differentiated cells caused a decrease in intracellular α-synuclein with enhanced levels in exosomes, inhibition of β-glucocerebrosidase enzymatic activity in vivo caused the enhancement of α-synuclein release in association with exosomes and the levels of α-synuclein oligomers (Kong et al., 2014; Papadopoulos et al., 2018).

Altogether, these data suggest a transmission role for exosomes, as carriers of pathogenic/toxicity-inducing elements. The same applies to the loading of nucleic/proteic fragments that could rescue neurotoxic processes happening in PD contexts. Strikingly, a study showed that PD plasma-derived exosomes, when added to rat cortical neurons, were capable of diminishing apoptosis and increase metabolic activity when compared to the addition of exosomes from healthy individuals (Tomlinson et al., 2015). A possible explanation for the different outcomes of this study could be the incapacity to detect α-synuclein in PD-derived serum microvesicles. In line with these results, another study showed that a mouse model of PD equally benefited from injection with exosomes from blood of healthy controls, by experiencing diminished cell loss and movement alterations, demonstrating the therapeutic role of exosomes as potential rescuers in neurodegeneration (Sun et al., 2020).

Access to plasma samples from PD patients also allowed the isolation of plasma NDEs and the detection of more sensitive disease biomarkers, such as DJ1 and α-synuclein (Table 1; Zhao et al., 2019). The same increased levels of α-synuclein were, however, not confirmed in exosomes isolated from the urine of PD patients (Ho et al., 2014). More recently, in plasma L1CAM-positive vesicles, an increase of α-synuclein levels in combination with clusterin, a stress-induced chaperone, allowed the differentiation of patients with PD from those with atypical parkinsonism (Jiang et al., 2020). The α-synuclein levels in NDEs from plasma and serum of PD patients were shown to be augmented but with some divergences between studies on how this aspect correlates with disease progression (Fu et al., 2020). The number of NDEs was described to be higher in plasma of PD patients and correlated with either early or very advanced stages of the disease (Ohmichi et al., 2019). NDEs positive for L1CAM were also used as a tool to assess the concentration of proteins of interest in the serum of PD patients who participated in a clinical trial for the study of possible beneficial effects of a diabetes type 2 treatment drug (Athauda et al., 2019). The study of the mechanisms linking PD and exosome is of major interest to understand the neurotoxic events in PD contexts and brings hope for new therapeutic approaches focusing on nanovesicles as key interlocutors.

Amyotrophic lateral sclerosis (ALS) affects both upper motor neurons and lower motor neurons leading to motor and extra-motor symptoms. The majority of cases is related to alterations in four genes, namely, C9orf72, TARDBP (encoding TAR DNA-binding protein 43, TDP43), SOD1 (encoding superoxide dismutase), and FUS (encoding RNA binding protein FUS), which results in aggregation and accumulation of ubiquitinated protein inclusions in motor neurons (Mejzini et al., 2019).

ALS was first associated with exosome trafficking in 2007 when a study showed the transport of both WT and mutant Cu/Zn superoxide dismutase (SOD1) by exosomes in a cell model for ALS (mouse neuron-like NSC-34 overexpressing human mutant SOD1) (Gomes et al., 2007). Further ahead, Grad and colleagues demonstrated that the same ALS cellular model was capable of incorporating misfolded SOD1, pointing that this protein was localized in the external part of exosomes isolated from the supernatant of cells transfected with human WT and mutant GFP-SOD1 (Grad et al., 2014). Astrocytes from primary cultures overexpressing mutant SOD1 released more exosomes than controls and more efficiently transferred these proteins, resulting in the loss of motor neurons as shown by the decrease in the SMI32/NeuN ratio in cultures where exosomes were added (Basso et al., 2013). Indeed, the release of mutant SOD1 by EV has been observed in both astrocytes and neurons from an ALS animal model (transgenic SOD1^G93A mouse model) (Basso et al., 2013; Silverman et al., 2019). Moreover, mutant SOD1 was also present in EV from the brain and spinal cord of ALS patients, evidencing the spreading role of these vesicles (Silverman et al., 2019). In accordance, an EV-associated miRNA (miR-494-3p) released by astrocytes obtained through direct conversion of fibroblasts of ALS patients contributed to the loss of motor neurons in mice (Varcianna et al., 2019). Additionally, alteration of miR-124 levels associated with inflammatory processes was observed in exosomes from NSC-34 (motor neuron-like cultures) transfected with mutant SOD1 (Pinto et al., 2017). These exosomes were preferentially incorporated by microglial cells (co-culture of microglia and NSC) and lead to the decrease of microglia phagocytic ability.

The continuous search for the role of exosomes in ALS motivated concerns about the plausible spreading of other pathological agents in the disease than the mutant protein mainly implied, SOD1. Further analysis showed that TDP-43 protein was transported through exosomes isolated from the CSF of ALS patients (also suffering from frontotemporal dementia) to U251 human glioma cells (Ding et al., 2015). The uptake of these exosomes was accompanied by an increase in apoptosis- and autophagy-related proteins, which demonstrated that the transport of toxic cargo by nanovesicles can spread and promote negative effects throughout cells.

The identification of miRNA (e.g., miR-124-3p, hsa-miR-4736, and miR-146a-5p) with altered levels in both neuronal-derived and total exosomes from ALS patients (CSF and plasma) gave some insight into novel targets to overcome the pathology (Table 1; Banack et al., 2020). Nonetheless, the research regarding the role of exosome vesicles in ALS has still a long way to go to allow a better understanding of how we can use the already acquired knowledge to mitigate the progression of these neurodegenerative processes.