Abbreviations: JALS: Juvenile Amyotrophic Lateral Sclerosis; FUS: Fused in Sarcoma; PRLD: Prion-Like Domain; NLS: Nuclear Localization Signal; CTD: C-Terminal Domain

Mini Review

Juvenile amyotrophic lateral sclerosis (JALS) is a degenerative neurological disease that occurs before the age of 25, involves upper and lower motor neurons and progresses progressively, with earlier onset, more rapid progression, and severe symptoms than classical ALS. So far, there is no effective treatment for JALS. The pathogenesis of JALS is still unknown and involves genetic, environmental, and biological factors. With the development of gene sequencing technology genetic factors are still dominate pathogenesis. JALS pathogenic gene spectrum is different from the classical ALS. Fused in sarcoma (FUS), which accounts for less than 1% of the causative mutation in classical ALS, is more than 60% of JALS, especially sporadic JALS [1,2]. However, its pathological mechanism is still controversial. Loss of function and Gain toxicity are the two main hypotheses used to explain the neurone death induced by FUS-NLS mutations. Based on our reading and understanding, we emphasize more that loss function cannot be ignored. FUS is a radiosensitive DNA/RNA binding protein composed of 526 amino acids encoded by the FUS gene containing 15 exons. FUS shuttles between nucleus and cytoplasm and is distributed in axons, dendrites, and dendritic spines of neurons, which is closely related to normal function maintenance and plasticity of neurons.
It consists of seven domains, including a prion-like domain (PRLD) located at the N-terminal, three intrinsically disordered Arg-gly-rich domains (RGGs), one RNA recognition motif (RRM), and one RNA-binding zinc finger (ZNF) and a C-terminal nuclear localization signal (NLS) [1]. There are more than 50 ALSassociated FUS mutations, of which approximately 40 are located in nuclear localization sequences (NLS) [1]. Current research focus on Gain toxicity hypotheses to explain the neurone death induced by FUS-NLS mutations [3,4]. FUS entry into the nucleus is mediated by binding of its NLS sequence to the nuclear entry receptor TNPO1 (transportin-1, TNPO1) [5]. Deletion of NLS fragments and mutations in sequences can lead to FUS entry barrier and abnormal aggregation of cytoplasmic proteins [6]. In cytoplasm, TNPO1 also regulates Liquid-liquid phase separation, LLPS of FUS. FUS-NLS mutations disturb the liquid-liquid phase separation equilibrium, which is one of the mechanisms leading to abnormal cytoplasmic aggregation [6,7]. However, the exudation nucleus of FUS is independent of exudation receptor XPO1 (Exportin1,XPO1), which is considered to be exudation through passive diffusion, and the binding of newly synthesized mRNA to FUS can limit its exudation [8].
It was found that the abnormal protein sequence after FUSR503fs (C. 1509-1510delag) mutation site can increase the retention of FUSR503fs in the nucleus [9]. Nucleotide sequences of FUS (low-Complexity Domains) [10] are anchored in the C-terminal domain (CTD) of RNA polymerase II (RNAPII) [11] and regulate the transcription of 2/3 of genes related to synaptic activity by preventing hyperphosphorylation of Ser2 of RNAPII [12]. FUS mutations, even in the nucleus, can affect the transcriptional activity of RNAPII and lead to some changes in biological functions [13]. Overexpression of nuclear FUS, rather than endogenous FUS knockdown, has been shown to cause neuron death, suggesting that FUS acquired nuclear toxicity plays an important role in the pathogenesis of ALS [14]. It can also bind to an active transcription region located downstream of the gene PolyA signaling, preferentially binding proteins involved in transcriptional regulation to participate in RNA level regulation [15]. In addition, FUS can inhibit viral replication, sarcoma cell proliferation, and related RNA and protein expression [16]. FUS can bind to RNAPII in the nucleus to regulate the transcriptional activity of various transcription factors [17,18]. FUS-NLS mutations lead to abnormal axon distribution and dysfunction of key NLSbinding protein factors such as SMN (Survival Motor Neuron) [19] and reduced area of nerve endplate [20]. Therefore, functional loss of FUS-NLS cannot be ignored in the pathogenesis of JALS, and we hope that more relevant studies will be carried out.

References

1. Chen L (2020) FUS mutation is probably the most common pathogenic gene for JALS, especially sporadic JALS. Rev Neurol (Paris).
2. Liu ZJ, HX Lin, GL Liu, QQ Tao, W Ni, et al. (2017) The investigation of genetic and clinical features in Chinese patients with juvenile amyotrophic lateral sclerosis. Clin Genet 92(3): 267-273.
3. Abu Dawud R, N Graffmann, S Ferber, W Wruck, J Adjaye, et al. (2018) Pluripotent stem cells: Induction and self-renewal. Philos Trans R Soc Lond B Biol Sci 373(1750): 20170213.
4. Beers DR, SH Appel (2019) Immune dysregulation in amyotrophic lateral sclerosis: mechanisms and emerging therapies. Lancet Neurol 18(2): 211-220.
5. Springhower CE, MK Rosen, YM Chook (2020) Karyopherins and condensates. Curr Opin Cell Biol 64: 112-123.
6. Agarwal S, Lorraine Potocki, Talia R Collier, Suzanne L Woodbury, Adekunle M Adesina, et al. (2016) Utility of whole exome sequencing in evaluation of juvenile motor neuron disease. Muscle Nerve 53(4): 648-652.
7. Yoshizawa T, Rustam Ali, Jenny Jiou, Ho Yee Joyce Fung, Kathleen A Burke, et al. (2018) Nuclear Import Receptor Inhibits Phase Separation of FUS through Binding to Multiple Sites. Cell 173(3): 693-705.e22.
8. Fahrenkrog B, A Harel (2028) Perturbations in Traffic: Aberrant Nucleocytoplasmic Transport at the Heart of Neurodegeneration. Cells 7(12): 232.
9. An H (2020) Frameshift peptides alter the properties of truncated FUS proteins in ALS-FUS. Mol Brain 13(1): 77.
10. Kwon I, Masato Kato, Siheng Xiang, Shanhai Xie, Jeffry L Corden, et al. (2013) Phosphorylation-regulated binding of RNA polymerase II to fibrous polymers of low-complexity domains. Cell 155(5): 1049-1060.
11. Schwartz JC, Christopher C Ebmeier, Elaine R Podell, Joseph Heimiller, Dylan J Taatjes, et al. (2012) FUS binds the CTD of RNA polymerase II and regulates its phosphorylation at Ser2. Genes Dev 26(24): 2690-2695.
12. Masuda A, Jun ichi Takeda, Tatsuya Okuno, Takaaki Okamoto, Bisei Ohkawara, et al. (2015) Position-specific binding of FUS to nascent RNA regulates mRNA length. Genes Dev 29(10): 1045-1057.
13. Schwartz JC, Elaine R Podell, Steve S W Han, James D Berry, Kevin C Eggan, et al. (2014) FUS is sequestered in nuclear aggregates in ALS patient fibroblasts. Mol Biol Cell 25(17): 2571-2578.
14. Suzuki H, M Matsuoka (2015) Overexpression of nuclear FUS induces neuronal cell death. Neuroscience 287: 113-124.
15. Luo Y, Jenny Blechingberg, Ana Miguel Fernandes, Shengting Li, Tue Fryland, et al. (2015) EWS and FUS bind a subset of transcribed genes encoding proteins enriched in RNA regulatory functions. BMC Genomics 16: 929.
16. Dunker W (2018) FUS Negatively Regulates Kaposi's Sarcoma-Associated Herpesvirus Gene Expression. Viruses 10(7): 359.
17. Burke KA (2015) Residue-by-Residue View of In Vitro FUS Granules that Bind the C-Terminal Domain of RNA Polymerase II. Mol Cell 60(2): 231-241.
18. Masuda A, J Takeda, K Ohno (2016) FUS-mediated regulation of alternative RNA processing in neurons: insights from global transcriptome analysis. Wiley Interdiscip Rev RNA 7(3): 330-340.
19. Groen EJ, Katsumi Fumoto, Anna M Blokhuis, Jooyeon Engelen Lee, Yeping Zhou, et al. (2013) ALS-associated mutations in FUS disrupt the axonal distribution and function of SMN. Hum Mol Genet 22(18): 3690-3704.
20. Almad AA, Arpitha Doreswamy, Sarah K Gross, Jean Philippe Richard, Yuqing Huo, et al. (2016) Connexin 43 in astrocytes contributes to motor neuron toxicity in amyotrophic lateral sclerosis. Glia 64(7): 1154-1169.

Abbreviations: JALS: Juvenile Amyotrophic Lateral Sclerosis; FUS: Fused in Sarcoma; PRLD: Prion-Like Domain; NLS: Nuclear Localization Signal; CTD: C-Terminal Domain