PML nuclear bodies: new players in familial amyotrophic lateral sclerosis-frontotemporal dementia?

Anand Goswami, Serena Carra

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two closely related disorders with overlapping clinical,genetic, and neuropathological features, forming a continuous disease spectrum (Ling et al., 2013).The major pathological hallmark of ALS and FTD are the depletion from the nucleus of the RNAbinding proteins TAR DNA-binding protein 43(TDP-43) and FUsed in Sarcoma (FUS) and their abnormal accumulation in ubiquitin-positive cytoplasmic inclusions (Ling et al., 2013).TDP-43 and FUS, whose genetic mutations are associated with the familial forms of ALS-FTD, participate in the regulation of RNA maturation and DNA repair,key processes whose dysregulation is central in ALS-FTD pathogenesis, along with the impairment of protein homeostasis (proteostasis) (Ling et al.,2013).

The protein promyelocytic leukemia (PML)plays a role in many of the key processes whose dysfunction contributes to ALS-FTD.PML is spatially associated with transcription sites and can act as a transcriptional co-factor; PML also senses the damaged DNA sites and participates in repairing DNA breaks; moreover, PML is a central actor of nuclear protein quality control (PQC)and is required for the maintenance of nuclear proteostasis (Lallemand-Breitenbach and de The,2018; Mediani et al., 2019).Structurally, the PML protein contains SUMOylation sites and a SUMOinteracting motif, which regulate its assembly into spherical nuclear bodies (PML-NBs) through SUMO/SUMO-interacting motif interactions(Muller et al., 1998).The number of PML-NBs increases in response to stress conditions that induce DNA damage, protein misfolding, and aggregation.PML-NBs recruit the triple ATPase chaperone VCP/p97, which plays an important role both in PQC and DNA damage response(Meerang et al., 2011), as well as the autophagy adaptor protein SQSTM1/p62, which participates in the DNA damage repair process and mediates a crosstalk between the ubiquitin-proteasome system and autophagy in DNA repair (Hewittet al.,2016).Upon stress, PML-NBs also act as transient reservoirs that sequester misfolded proteins,comprising defective ribosomal products (DRiPs),to promote their clearance (Mediani et al., 2019;Figure 1).Based on these findings, we investigated whether the expression of PML may be affected in ALS-FTD cases.

We report that the number of PML-NBs is strongly reduced in postmortem brain and spinal cord samples from patients suffering from familial forms of FUS-related andC9orf72-related ALS-FTD (the latter showing also TDP-43 pathology; Ling et al.,2013; Antoniani et al., 2023).These findings have been substantiated using cellular models of ALSFTD, including human induced pluripotent stem cells-derived motor neurons with hexanucleotide GGGGCC/G4C2 repeat expansion in theC9orf72gene, the most common mutation associated with ALS, and non-neuronal cells overexpressing expanded G4C2 repeats (Antoniani et al., 2023;Figure 1).

Although our data identify PML defects as a novel trait of ALS-FTD, we do not yet fully understand how this occurs and how mechanistically they contribute to disease progression.PML-NB assembly is regulated both at the transcriptional level and post-translationally, via SUMO-1 conjugation of PML (Muller et al., 1998).Loss of PML-NBs in ALS-FTD cell models did not seem to be caused by a reduction of the transcription of PML or other components such as SUMO1(Antoniani et al., 2023).Future studies will need to investigate which players are responsible for the decrease in the number of PML-NBs in ALSFTD cases and cellular models.Concerning how PML-NB loss may promote cell toxicity several plausible, and not mutually exclusive mechanisms can be postulated.Mishandling of DRiPs, as a consequence of PML-NB loss, may represent a plausible mechanism contributing to cell toxicity.DRiPs are released in large amounts upon stress,when polyribosome disassemble, and are rapidly compartmentalized by the cells inside nucleoli,at PML-NBs and in cytoplasmic ubiquitin-positive foci (Mediani et al., 2019).DRiPs’ segregation and clearance require the coordinated action of the PQC, including the chaperone VCP and the ubiquitin-proteasome system.Failure to properly segregate DRiPs leads not only to the aggregation of PML within PML-NBs, but also to the sequestration of ubiquitin molecules, used to label DRiPs for ubiquitin-proteasome system-mediated clearance and to the accumulation of DRiPs inside cytoplasmic stress granules (SGs) (Ganassi et al., 2016; Mediani et al., 2019).SGs sequester translationally stalled mRNAs, as well as the RNAbinding proteins TDP-43 and FUS, and are required to protect cells during acute stress conditions.SGs are highly dynamic and rapidly disassemble during the stress-recovery phase, enabling the restoration of protein synthesis and the retrotranslocation inside the nucleus of RNA-binding proteins such as TDP-43 and FUS (Hofmann et al., 2021).Delays of SG disassembly and SG conversion into an aggregated state have been reported in ALSFTD cellular models and have been suggested to contribute to TDP-43 and FUS aggregation (Portz et al., 2021).Mechanistically, accumulation inside SGs of DRiPs promotes SG accumulation (Ganassi et al., 2016).SG conversion into an aggregatedlike state is further promoted by the accumulation of mutated TDP-43 and FUS, linked to the familial forms of ALS-FTD (Ganassi et al., 2016; Portz et al., 2021).We found a strong correlation between the loss of PML-NBs, SG disassembly failure, and the accumulation of cytoplasmic ubiquitin-positive inclusions in ALS-FTD cellular models (Antoniani et al., 2023).These data open the possibility that mishandling of DRiPs due to nuclear dysfunction(PML-NB loss) may have a negative impact on cytoplasmic protein aggregation and SG dynamics,ultimately promoting the formation of ubiquitinpositive cytoplasmic inclusion, a hallmark of ALSFTD (Figure 1).

Figure 1 ｜PML-NB loss in familial FUS and C9orf72 related ALS-FTD.Upper panel: In control (healthy) subjects, cells respond to proteotoxic and genotoxic stress by increasing the number of PML-NBs, which play an important role in the maintenance of proteostasis and genome integrity.Lower panel:Loss of PML-NB in familial FUS and C9orf72 related ALS-FTD may contribute to cell toxicity with multiple mechanisms,including: - mishandling of misfolded proteins and their accumulation in cytoplasmic stress granules, which can in turn promote their accumulation and dysfunction; - impaired ability to sense DNA damage and orchestrate DNA repair; -alteration of cytoplasmic functions of PML, including the regulation of autophagy.Created with Adobe Illustrator.ALSFTD: Amyotrophic lateral sclerosis-frontotemporal dementia; FUS: FUsed in Sarcoma; PML-NBs: promyelocytic leukemianuclear bodies.

As previously mentioned, PML-NBs participate in DNA damage sensing and repair (Lallemand-Breitenbach and de The, 2018).In addition, the ALS-linked proteins VCP and p62 participate in the maintenance of genomic integrity and are closely associated with PML (Sun et al., 2020).Thus,defects at the level of PML-NBs may influence,indirectly, the cells’ ability to sense DNA damage and orchestrate the DNA damage response.This may occur via a direct loss of PML-NBs and/or via impaired cooperation between PML, VCP, and p62.In addition, mishandling of DRiPs, because of PML-NB loss, may indirectly impact DNA repair.Mechanistically, the formation of DNA repair sites and nuclear proteostasis compete for a limiting pool of ubiquitin molecules, which are not only used to label DRiPs and other proteins for degradation, but also to identify damaged DNA sites, via H2A ubiquitination (Mediani et al., 2019).Under acute stress conditions, cells prioritize the use of ubiquitin molecules to label DRiPs that are compartmentalized at PML-NBs for later clearance, at the expense of H2A ubiquitination and 53BP1 repair foci formation (Mediani et al.,2019).Future studies will need to address whether the loss of PML-NBs in ALS-FTD is a critical event preceding DNA damage sensing and defects in the DNA repair and to what extent it may compromise specific nuclear functions of the other ALS-linked proteins VCP and p62 in both nuclear PQC and DNA repair (Figure 1).

Finally, although PML is mainly localized in the nucleus, a fraction of PML is also found in the cytoplasm.Thus, we cannot exclude that,besides an impairment of the nuclear functions of PML, defects of cytoplasmic functions of PML may also contribute to disease progression.In particular, a cytoplasmic pool of PML interacts with mitochondria-associated membranes and has been recently implicated in the negative regulation of autophagy (Missiroli et al., 2016),another process whose defects contribute to neurodegeneration, including ALS-FTD cases (Ling et al., 2013; Figure 1).

To summarize, given the interaction of PML with several proteins associated with ALS-FTD and its implication in the regulation of critical events,whose dysregulation contributes to ALS-FTD,our study opens new perspectives for a better understanding of their pathomechanisms.Since we identified PML-NB defects in familial forms of FUS-related andC9orf72-related ALS-FTD, future work should test whether PML-NB is a hallmark of ALS-FTD, including sporadic cases.Unraveling how the loss of PML function contributes to disease progression will help in the design of future potential therapeutic approaches for ALS-FTD.

This work was supported by AriSLA Foundation(MLOpathy and SUMOsolvable); Banca d’Italia;German Research Foundation (DFG; WE 1406/16-1); ALS Stichting grant“The Dutch ALS Tissue Bank”(to SC).

Presentation in part or whole at any meeting:Jacques Monod Conference“Protein phase transitions in ageing and age-related diseases:from atomic resolution to cellular solutions”,Roscoff, France.Date: October 17–21, 2022.“Research, Development and Innovation in ALS”,Milan, Italy.Date: November 2–4, 2022.ICGEB Arturo Falaschi Conference“1st biennial conference on TDP-43 function and dysfunction in disease”, Stazione Marittima, Trieste, Italy.Date:September 6–8, 2023.

Anand Goswami, Serena Carra*

Institute of Neuropathology, RWTH Aachen University Hospital, Aachen, Germany (Goswami A)Department of Neurology, Eleanor and Lou Gehrig ALS Center, Columbia University, New York, NY,USA (Goswami A)Department of Biomedical, Metabolic and Neural Sciences, University of Modena and Reggio Emilia,Modena, Italy (Carra S)

*Correspondence to:Serena Carra, PhD,serena.carra@unimore.it.

https://orcid.org/0000-0003-0939-0140(Serena Carra)

Date of submission: September 21, 2023

Date of decision: November 2, 2023

Date of acceptance: November 17, 2023

Date of web publication: December 21, 2023

https://doi.org/10.4103/1673-5374.391183

How to cite this article:Goswami A, Carra S(2024) PML nuclear bodies: new players in familial amyotrophic lateral sclerosis-frontotemporal dementia?Neural Regen Res 19(9):1875-1876.

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