Ubiquilin 2 mutations in Italian patients with amyotrophic lateral sclerosis and frontotemporal dementia

Physiological and pathological effects of phase separation in the central nervous system

Phase separation, also known as biomolecule condensate, participates in physiological processes such as transcriptional regulation, signal transduction, gene expression, and DNA damage repair by creating a membrane-free compartment. Phase separation is primarily caused by the interaction of multivalent non-covalent bonds between proteins and/or nucleic acids. The strength of molecular multivalent interaction can be modified by component concentration, the potential of hydrogen, posttranslational modification, and other factors. Notably, phase separation occurs frequently in the cytoplasm of mitochondria, the nucleus, and synapses. Phase separation in vivo is dynamic or stable in the normal physiological state, while abnormal phase separation will lead to the formation of biomolecule condensates, speeding up the disease progression. To provide candidate suggestions for the clinical treatment of nervous system diseases, this review, based on existing studies, carefully and systematically represents the physiological roles of phase separation in the central nervous system and its pathological mechanism in neurodegenerative diseases.

Pathogenic mutations in UBQLN2 exhibit diverse aggregation propensity and neurotoxicity

The ubiquitin-adaptor protein UBQLN2 promotes degradation of several aggregate-prone proteins implicated in neurodegenerative diseases. Missense UBQLN2 mutations also cause X-linked amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Previously we demonstrated that the liquid-like properties of UBQLN2 molecular assemblies are altered by a specific pathogenic mutation, P506T, and that the propensity of UBQLN2 to aggregate correlated with neurotoxicity. Here, we systematically assess the effects of multiple, spatially distinct ALS/FTD-linked missense mutations on UBQLN2 aggregation propensity, neurotoxicity, phase separation, and autophagic flux. In contrast to what we observed for the P506T mutation, no other tested pathogenic mutant exhibited a clear correlation between aggregation propensity and neurotoxicity. These results emphasize the unique nature of pathogenic UBQLN2 mutations and argue against a generalizable link between aggregation propensity and neurodegeneration in UBQLN2-linked ALS/FTD.

Primary Lateral Sclerosis: An Overview

Primary lateral sclerosis (PLS) is a rare neurodegenerative disorder which causes the selective deterioration of the upper motor neurons (UMNs), sparing the lower motor neuron (LMN) system. The clinical course is defined by a progressive motor disability due to muscle spasticity which typically involves lower extremities and bulbar muscles. Although classically considered a sporadic disease, some familiar cases and possible causative genes have been reported. Despite it having been recognized as a rare but distinct entity, whether it actually represents an extreme end of the motor neuron diseases continuum is still an open issue. The main knowledge gap is the lack of specific biomarkers to improve the clinical diagnostic accuracy. Indeed, the diagnostic imprecision, together with some uncertainty about overlap with UMN-predominant ALS and Hereditary Spastic Paraplegia (HSP), has become an obstacle to the development of specific therapeutic trials. In this study, we provided a comprehensive analysis of the existing literature, including neuropathological, clinical, neuroimaging, and neurophysiological features of the disease, and highlighting the controversies still unsolved in the differential diagnoses and the current diagnostic criteria. We also discussed the current knowledge gaps still present in both diagnostic and therapeutic fields when approaching this rare condition.

Autophagy Markers Are Altered in Alzheimer's Disease, Dementia with Lewy Bodies and Frontotemporal Dementia

The accumulation of protein aggregates defines distinct, yet overlapping pathologies such as Alzheimer's disease (AD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD). In this study, we investigated ATG5, UBQLN2, ULK1, and LC3 concentrations in 66 brain specimens and 120 plasma samples from AD, DLB, FTD, and control subjects (CTRL). Protein concentration was measured with ELISA kits in temporal, frontal, and occipital cortex specimens of 32 AD, 10 DLB, 10 FTD, and 14 CTRL, and in plasma samples of 30 AD, 30 DLB, 30 FTD, and 30 CTRL. We found alterations in ATG5, UBQLN2, ULK1, and LC3 levels in patients; ATG5 and UBQLN2 levels were decreased in both brain specimens and plasma samples of patients compared to those of the CTRL, while LC3 levels were increased in the frontal cortex of DLB and FTD patients. In this study, we demonstrate alterations in different steps related to ATG5, UBQLN2, and LC3 autophagy pathways in DLB and FTD patients. Molecular alterations in the autophagic processes could play a role in a shared pathway involved in the pathogenesis of neurodegeneration, supporting the hypothesis of a common molecular mechanism underlying major neurodegenerative dementias and suggesting different potential therapeutic targets in the autophagy pathway for these disorders.

Simple models to understand complex disease: 10 years of progress from Caenorhabditis elegans models of amyotrophic lateral sclerosis and frontotemporal lobar degeneration

The nematode Caenorhabditis elegans are a powerful model system to study human disease, with numerous experimental advantages including significant genetic and cellular homology to vertebrate animals, a short lifespan, and tractable behavioral, molecular biology and imaging assays. Beginning with the identification of SOD1 as a genetic cause of amyotrophic lateral sclerosis (ALS), C. elegans have contributed to a deeper understanding of the mechanistic underpinnings of this devastating neurodegenerative disease. More recently this work has expanded to encompass models of other types of ALS and the related disease frontotemporal lobar degeneration (FTLD-TDP), including those characterized by mutation or accumulation of the proteins TDP-43, C9orf72, FUS, HnRNPA2B1, ALS2, DCTN1, CHCHD10, ELP3, TUBA4A, CAV1, UBQLN2, ATXN3, TIA1, KIF5A, VAPB, GRN, and RAB38. In this review we summarize these models and the progress and insights from the last ten years of using C. elegans to study the neurodegenerative diseases ALS and FTLD-TDP.

Current challenges in primary lateral sclerosis diagnosis

INTRODUCTION

Primary lateral sclerosis (PLS) is a rare, adult-onset and slowly progressive motor neuron disorder whose clinical core is characterized by upper motor neuron (UMN) dysfunction. Its formal diagnosis is clinically based and disease duration-dependent. Differentiating PLS from other disorders involving UMN can be challenging, particularly in the early stages.

AREAS COVERED

Our review covers and discusses different aspects of the PLS field, including the diagnostic criteria and its limitations, its differential diagnosis and their major pitfalls, and the actual role of neurophysiology, neuroimaging, genetics, and molecular biomarkers. Symptomatic treatment of the different manifestations is also addressed. The authors searched MEDLINE and Scopus. They also searched the reference lists of articles identified by our search strategy and reviewed and selected those deemed relevant. They selected papers and studies based on the quality of the report, significance of the findings, and on the author's critical appraise and expertise.

EXPERT OPINION

It is important to investigate novel molecular biomarkers and plan multicenter clinical trials for PLS. However, this will require a large international project to recruit enough patients, particularly given the diagnostic uncertainty of the current clinical criteria. A better understanding of PLS pathophysiology is crucial for designing disease-targeted therapies.

Axon guidance genes modulate neurotoxicity of ALS-associated UBQLN2

Mutations in the ubiquitin (Ub) chaperone Ubiquilin 2 (UBQLN2) cause X-linked forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) through unknown mechanisms. Here, we show that aggregation-prone, ALS-associated mutants of UBQLN2 (UBQLN2ALS) trigger heat stress-dependent neurodegeneration in Drosophila. A genetic modifier screen implicated endolysosomal and axon guidance genes, including the netrin receptor, Unc-5, as key modulators of UBQLN2 toxicity. Reduced gene dosage of Unc-5 or its coreceptor Dcc/frazzled diminished neurodegenerative phenotypes, including motor dysfunction, neuromuscular junction defects, and shortened lifespan, in flies expressing UBQLN2ALS alleles. Induced pluripotent stem cells (iPSCs) harboring UBQLN2ALS knockin mutations exhibited lysosomal defects while inducible motor neurons (iMNs) expressing UBQLN2ALS alleles exhibited cytosolic UBQLN2 inclusions, reduced neurite complexity, and growth cone defects that were partially reversed by silencing of UNC5B and DCC. The combined findings suggest that altered growth cone dynamics are a conserved pathomechanism in UBQLN2-associated ALS/FTD.

C9orf72 expansions are the most common cause of genetic frontotemporal dementia in a Southeast Asian cohort

OBJECTIVE

Frontotemporal dementia (FTD) encompasses a spectrum of neurodegenerative disorders, including behavioural variant FTD (bvFTD), semantic variant primary progressive aphasia (svPPA) and non-fluent variant PPA (nfvPPA). While a strong genetic component is implicated in FTD, genetic FTD in Asia is less frequently reported. We aimed to investigate the frequency of Southeast Asian FTD patients harbouring known genetic FTD variants.

METHODS

A total of 60 FTD-spectrum patients (25 familial and 35 sporadic) from Singapore and the Philippines were included. All underwent next-generation sequencing and repeat-primed PCR for C9orf72 expansion testing. Neurofilament light chain (NfL) levels were measured in a subset of patients.

RESULTS

Overall, 26.6% (16/60 cases) carried pathogenic or likely pathogenic variants in a FTD-related gene, including: MAPT Gln351Arg (n = 1); GRN Cys92Ter (n = 1), Ser301Ter (n = 2), c.462 + 1G > C (n = 1); C9orf72 expansion (35-70 repeats; n = 8); TREM2 Arg47Cys (n = 1); and OPTN frameshift insertion (n = 2). Genetic mutations accounted for 48% (12/25) of patients with familial FTD, and 11.4% (4/35) of patients with sporadic FTD. C9orf72 repeat expansions were the most common genetic mutation (13.3%, 8/60), followed by GRN (6.7%, 4/60) variants. Within mutation carriers, plasma NfL was highest in a C9orf72 expansion carrier, and CSF NfL was highest in a GRN splice variant carrier.

INTERPRETATION

In our cohort, genetic mutations are present in one-quarter of FTD-spectrum cases, and up to half of those with family history. Our findings highlight the importance of wider implementation of genetic testing in FTD patients from Southeast Asia.

Amyotrophic lateral sclerosis

The scientific landscape surrounding amyotrophic lateral sclerosis has shifted immensely with a number of well-defined ALS disease-causing genes, each with related phenotypical and cellular motor neuron processes that have come to light. Yet in spite of decades of research and clinical investigation, there is still no etiology for sporadic amyotrophic lateral sclerosis, and treatment options even for those with well-defined familial syndromes are still limited. This chapter provides a comprehensive review of the genetic basis of amyotrophic lateral sclerosis, highlighting factors that contribute to its heritability and phenotypic manifestations, and an overview of past, present, and upcoming therapeutic strategies.

Mutation spectrum of chinese amyotrophic lateral sclerosis patients with frontotemporal dementia

Background

Studies have reported that a noncoding hexanucleotide repeat in C9ORF72, is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) among Caucasian population, nevertheless it is rare in Chinese population. Therefore, we aimed to investigate the mutation spectrum of Chinese ALS patients with FTD (ALS-FTD).

Methods

ALS patients with and without cognitive impairments were enrolled. Clinical features were collected including age, sex, disease duration, ALSFRS-r, family history and cognitive evaluation. Thirty-six ALS genes were screened by whole exome sequencing (WES) and repeat-primed polymerase chain reaction (PCR) were used for detection of and abnormal repeat expansions of C9ORF72.

Results

A total of 1208 patients, including 66 familial ALS (FALS) and 1142 sporadic ALS (SALS) patients were included. Twenty-three patients with sporadic ALS and one familial ALS index had concomitant FTD, which accounts for 1.99% (24/1208) of patients with ALS. In sporadic ALS-FTD, one case harboring C9ORF72 expansion variant, two cases harboring ANXA11 variants and one individual carrying CCNF variant were identified. A recurrent UBQLN2 variant was detected in a familial ALS-FTD patient. All of the ALS-FTD patients carrying variants in known causative genes manifested motor symptom onset (two bulbar onset and three limb onset) and developed cognitive impairment thereafter. It is not easy to draw a conclusion of the genotype-phenotype association in ALS-FTD with certain variants, limited by the small number of patients.

Conclusion

Our findings provide an overview of spectrum of genetic variants in Chinese ALS-FTD patients. Variants of uncertain significance in UBQLN2, ANXA11 and CCNF were identified and further studies are required for causal relations of these variants with ALS-FTD.

UBQLN proteins in health and disease with a focus on UBQLN2 in ALS/FTD

Ubiquilin (UBQLN) proteins are a dynamic and versatile family of proteins found in all eukaryotes that function in the regulation of proteostasis. Besides their canonical function as shuttle factors in delivering misfolded proteins to the proteasome and autophagy systems for degradation, there is emerging evidence that UBQLN proteins play broader roles in proteostasis. New information suggests the proteins function as chaperones in protein folding, protecting proteins prior to membrane insertion, and as guardians for mitochondrial protein import. In this review, we describe the evidence for these different roles, highlighting how different domains of the proteins impart these functions. We also describe how changes in the structure and phase separation properties of UBQLNs may regulate their activity and function. Finally, we discuss the pathogenic mechanisms by which mutations in UBQLN2 cause amyotrophic lateral sclerosis and frontotemporal dementia. We describe the animal model systems made for different UBQLN2 mutations and how lessons learnt from these systems provide fundamental insight into the molecular mechanisms by which UBQLN2 mutations drive disease pathogenesis through disturbances in proteostasis.

Primary Lateral Sclerosis: Clinical, radiological and molecular features

Primary Lateral Sclerosis (PLS) is an uncommon motor neuron disorder. Despite the well-recognisable constellation of clinical manifestations, the initial diagnosis can be challenging and therapeutic options are currently limited. There have been no recent clinical trials of disease-modifying therapies dedicated to this patient cohort and awareness of recent research developments is limited. The recent consensus diagnostic criteria introduced the category 'probable' PLS which is likely to curtail the diagnostic journey of patients. Extra-motor clinical manifestations are increasingly recognised, challenging the view of PLS as a 'pure' upper motor neuron condition. The post mortem literature of PLS has been expanded by seminal TDP-43 reports and recent PLS studies increasingly avail of meticulous genetic profiling. Research in PLS has gained unprecedented momentum in recent years generating novel academic insights, which may have important clinical ramifications.

ALS-linked mutations impair UBQLN2 stress-induced biomolecular condensate assembly in cells

Mutations in Ubiquilin-2 (UBQLN2), a ubiquitin-binding shuttle protein involved in several protein quality control processes, can lead to amyotrophic lateral sclerosis (ALS). We previously found that wild-type UBQLN2 forms dynamic, membraneless biomolecular condensates upon cellular stress, and undergoes liquid-liquid phase separation in vitro. However, the impact of ALS-linked mutations on UBQLN2 condensate formation in cells is unknown. Here, we employ live-cell imaging and photokinetic analysis to investigate how five patient-derived ALS-linked mutations in UBQLN2 impact stress-induced UBQLN2 condensate assembly and condensate material properties. Both wild-type and mutant UBQLN2 condensates are generally cytoplasmic and liquid-like. However, cells transfected with mutant UBQLN2 contain fewer stress-induced UBQLN2 condensates than those with wild-type UBQLN2. Most strikingly, exogenously expressed P506T UBQLN2 forms the lowest number of stress-induced condensates of all UBQLN2 mutants, and these condensates are significantly smaller than those of wild-type UBQLN2. Fluorescence recovery after photobleaching (FRAP) analysis of UBQLN2 condensates revealed higher immobile fractions for UBQLN2 mutants, especially P506T. P497S and P497H mutations differentially impact condensate properties, demonstrating that the effects of ALS-linked mutations are both position- and amino acid-dependent. Collectively, our data show that disease mutations hinder assembly and alter viscoelastic properties of stress-induced UBQLN2 condensates, potentially leading to aggregates commonly observed in ALS.

Human Ubiquilin 2 and TDP-43 co-pathology drives neurodegeneration in transgenic C. elegans

Amyotrophic lateral sclerosis (ALS) is a debilitating, fatal neurodegenerative disease that causes rapid muscle wasting. It shares a spectrum of symptoms and pathology with frontotemporal lobar degeneration (FTLD). These diseases are caused by aberrant activity of a set of proteins including TDP-43 and UBIQUILIN-2 (UBQLN2). UBQLN2 encodes an ubiquitin-like adaptor protein involved in the ubiquitin-proteasome protein degradation pathway. Mutations in the PXX domain of UBQLN2 cause familial ALS. UBQLN2 aggregates in skein-like inclusions with other ALS and FTLD associated proteins including TDP-43 and ubiquitin. To facilitate further investigation of UBQLN2-mediated mechanisms of neurodegeneration, we made Caenorhabditis elegans transgenic lines pan-neuronally expressing human UBQLN2 cDNAs carrying either the wild-type UBQLN2 sequence or UBQLN2 with ALS causing mutations. Transgenic animals exhibit motor dysfunction accompanied by neurodegeneration of GABAergic motor neurons. At low levels of UBQLN2 expression, wild-type UBQLN2 causes significant motor impairment and neurodegeneration that is exacerbated by ALS associated mutations in UBQLN2. At higher levels of UBQLN2 expression, both wild-type and ALS mutated versions of UBQLN2 cause severe impairment. Molecular genetic investigation revealed that UBQLN2 dependent locomotor defects do not require the involvement of the endogenous homolog of TDP-43 in C. elegans (tdp-1). However, co-expression of wild-type human TDP-43 exacerbates UBQLN2 deficits. This model of UBQLN2-mediated neurodegeneration may be useful for further mechanistic investigation into the molecular cascades driving neurodegeneration in ALS and ALS-FTLD.

Exploring dementia and neuronal ceroid lipofuscinosis genes in 100 FTD-like patients from 6 towns and rural villages on the Adriatic Sea cost of Apulia

Frontotemporal dementia (FTD) refers to a complex spectrum of clinically and genetically heterogeneous disorders. Although fully penetrant mutations in several genes have been identified and can explain the pathogenic mechanisms underlying a great portion of the Mendelian forms of the disease, still a significant number of families and sporadic cases remains genetically unsolved. We performed whole exome sequencing in 100 patients with a late-onset and heterogeneous FTD-like clinical phenotype from Apulia and screened mendelian dementia and neuronal ceroid lipofuscinosis genes. We identified a nonsense mutation in SORL1 VPS domain (p.R744X), in 2 siblings displaying AD with severe language problems and primary progressive aphasia and a near splice-site mutation in CLCN6 (p.S116P) segregating with an heterogeneous phenotype, ranging from behavioural FTD to FTD with memory onset and to the logopenic variant of primary progressive aphasia in one family. Moreover 2 sporadic cases with behavioural FTD carried heterozygous mutations in the CSF1R Tyrosin kinase flanking regions (p.E573K and p.R549H). By contrast, only a minority of patients carried pathogenic C9orf72 repeat expansions (1%) and likely moderately pathogenic variants in GRN (p.C105Y, p.C389fs and p.C139R) (3%). In concert with recent studies, our findings support a common pathogenic mechanisms between FTD and neuronal ceroid lipofuscinosis and suggests that neuronal ceroid lipofuscinosis genes should be investigated also in dementia patients with predominant frontal symptoms and language impairments.

Emerging Evidence Highlighting the Importance of Redox Dysregulation in the Pathogenesis of Amyotrophic Lateral Sclerosis (ALS)

The cellular redox state, or balance between cellular oxidation and reduction reactions, serves as a vital antioxidant defence system that is linked to all important cellular activities. Redox regulation is therefore a fundamental cellular process for aerobic organisms. Whilst oxidative stress is well described in neurodegenerative disorders including amyotrophic lateral sclerosis (ALS), other aspects of redox dysfunction and their contributions to pathophysiology are only just emerging. ALS is a fatal neurodegenerative disease affecting motor neurons, with few useful treatments. Hence there is an urgent need to develop more effective therapeutics in the future. Here, we discuss the increasing evidence for redox dysregulation as an important and primary contributor to ALS pathogenesis, which is associated with multiple disease mechanisms. Understanding the connection between redox homeostasis, proteins that mediate redox regulation, and disease pathophysiology in ALS, may facilitate a better understanding of disease mechanisms, and lead to the design of better therapeutic strategies.

Amyotrophic lateral sclerosis-linked UBQLN2 mutants inhibit endoplasmic reticulum to Golgi transport, leading to Golgi fragmentation and ER stress

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are fatal neurodegenerative diseases that are related genetically and pathologically. Mutations in the UBQLN2 gene, encoding the ubiquitin-like protein ubiquilin2, are associated with familial ALS/FTD, but the pathophysiological mechanisms remain unclear. Here, we demonstrate that ALS/FTD UBQLN2 mutants P497H and P506T inhibit protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus in neuronal cells. In addition, we observed that Sec31-positive ER exit sites are clustered in UBQLN2T487I patient spinal cord tissues. Both the ER-Golgi intermediate (ERGIC) compartment and the Golgi become disorganised and fragmented. This activates ER stress and inhibits ER-associated degradation. Hence, this study highlights perturbations in secretory protein trafficking and ER homeostasis as pathogenic mechanisms associated with ALS/FTD-associated forms of UBQLN2.

Structure, dynamics and functions of UBQLNs: at the crossroads of protein quality control machinery

Cells rely on protein homeostasis to maintain proper biological functions. Dysregulation of protein homeostasis contributes to the pathogenesis of many neurodegenerative diseases and cancers. Ubiquilins (UBQLNs) are versatile proteins that engage with many components of protein quality control (PQC) machinery in cells. Disease-linked mutations of UBQLNs are most commonly associated with amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and other neurodegenerative disorders. UBQLNs play well-established roles in PQC processes, including facilitating degradation of substrates through the ubiquitin-proteasome system (UPS), autophagy, and endoplasmic-reticulum-associated protein degradation (ERAD) pathways. In addition, UBQLNs engage with chaperones to sequester, degrade, or assist repair of misfolded client proteins. Furthermore, UBQLNs regulate DNA damage repair mechanisms, interact with RNA-binding proteins (RBPs), and engage with cytoskeletal elements to regulate cell differentiation and development. Important to the myriad functions of UBQLNs are its multidomain architecture and ability to self-associate. UBQLNs are linked to numerous types of cellular puncta, including stress-induced biomolecular condensates, autophagosomes, aggresomes, and aggregates. In this review, we focus on deciphering how UBQLNs function on a molecular level. We examine the properties of oligomerization-driven interactions among the structured and intrinsically disordered segments of UBQLNs. These interactions, together with the knowledge from studies of disease-linked mutations, provide significant insights to UBQLN structure, dynamics and function.

Neuroimaging in genetic frontotemporal dementia and amyotrophic lateral sclerosis

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) have a strong clinical, genetic and pathological overlap. This review focuses on the current understanding of structural, functional and molecular neuroimaging signatures of genetic FTD and ALS. We overview quantitative neuroimaging studies on the most common genes associated with FTD (MAPT, GRN), ALS (SOD1), and both (C9orf72), and summarize visual observations of images reported in the rarer genes (CHMP2B, TARDBP, FUS, OPTN, VCP, UBQLN2, SQSTM1, TREM2, CHCHD10, TBK1).

Modeling UBQLN2-mediated neurodegenerative disease in mice: Shared and divergent properties of wild type and mutant UBQLN2 in phase separation, subcellular localization, altered proteostasis pathways, and selective cytotoxicity

The ubiquitin-binding proteasomal shuttle protein UBQLN2 is implicated in common neurodegenerative disorders due to its accumulation in disease-specific aggregates and, when mutated, directly causes familial frontotemporal dementia/amyotrophic lateral sclerosis (FTD/ALS). Like other proteins linked to FTD/ALS, UBQLN2 undergoes phase separation to form condensates. The relationship of UBQLN2 phase separation and accumulation to neurodegeneration, however, remains uncertain. Employing biochemical, neuropathological and behavioral assays, we studied the impact of overexpressing WT or mutant UBQLN2 in the CNS of transgenic mice. Expression of UBQLN2 harboring a pathogenic mutation (P506T) elicited profound and widespread intraneuronal inclusion formation and aggregation without prominent neurodegenerative or behavioral changes. Both WT and mutant UBQLN2 formed ubiquitin- and P62-positive inclusions in neurons, supporting the view that UBQLN2 is intrinsically prone to phase separate, with the size, shape and frequency of inclusions depending on expression level and the presence or absence of a pathogenic mutation. Overexpression of WT or mutant UBQLN2 resulted in a dose-dependent decrease in levels of a key interacting chaperone, HSP70, as well as dose-dependent profound degeneration of the retina. We conclude that, at least in mice, robust aggregation of a pathogenic form of UBQLN2 is insufficient to cause neuronal loss recapitulating that of human FTD/ALS. Our results nevertheless support the view that altering the normal cellular balance of UBQLN2, whether wild type or mutant protein, has deleterious effects on cells of the CNS and retina that likely reflect perturbations in ubiquitin-dependent protein homeostasis.

Multifaceted Genes in Amyotrophic Lateral Sclerosis-Frontotemporal Dementia

Amyotrophic lateral sclerosis and frontotemporal dementia are two progressive, adult onset neurodegenerative diseases, caused by the cell death of motor neurons in the motor cortex and spinal cord and cortical neurons in the frontal and temporal lobes, respectively. Whilst these have previously appeared to be quite distinct disorders, in terms of areas affected and clinical symptoms, identification of cognitive dysfunction as a component of amyotrophic lateral sclerosis (ALS), with some patients presenting with both ALS and FTD, overlapping features of neuropathology and the ongoing discoveries that a significant proportion of the genes underlying the familial forms of the disease are the same, has led to ALS and FTD being described as a disease spectrum. Many of these genes encode proteins in common biological pathways including RNA processing, autophagy, ubiquitin proteasome system, unfolded protein response and intracellular trafficking. This article provides an overview of the ALS-FTD genes before summarizing other known ALS and FTD causing genes where mutations have been found primarily in patients of one disease and rarely in the other. In discussing these genes, the review highlights the similarity of biological pathways in which the encoded proteins function and the interactions that occur between these proteins, whilst recognizing the distinctions of MAPT-related FTD and SOD1-related ALS. However, mutations in all of these genes result in similar pathology including protein aggregation and neuroinflammation, highlighting that multiple different mechanisms lead to common downstream effects and neuronal loss. Next generation sequencing has had a significant impact on the identification of genes associated with both diseases, and has also highlighted the widening clinical phenotypes associated with variants in these ALS and FTD genes. It is hoped that the large sequencing initiatives currently underway in ALS and FTD will begin to uncover why different diseases are associated with mutations within a single gene, especially as a personalized medicine approach to therapy, based on a patient's genetics, approaches the clinic.

A Systematic Review of Genotype-Phenotype Correlation across Cohorts Having Causal Mutations of Different Genes in ALS

Amyotrophic lateral sclerosis is a rare and fatal neurodegenerative disease characterised by progressive deterioration of upper and lower motor neurons that eventually culminates in severe muscle atrophy, respiratory failure and death. There is a concerning lack of understanding regarding the mechanisms that lead to the onset of ALS and as a result there are no reliable biomarkers that aid in the early detection of the disease nor is there an effective treatment. This review first considers the clinical phenotypes associated with ALS, and discusses the broad categorisation of ALS and ALS-mimic diseases into upper and lower motor neuron diseases, before focusing on the genetic aetiology of ALS and considering the potential relationship of mutations of different genes to variations in phenotype. For this purpose, a systematic review is conducted collating data from 107 original published clinical studies on monogenic forms of the disease, surveying the age and site of onset, disease duration and motor neuron involvement. The collected data highlight the complexity of the disease's genotype-phenotype relationship, and thus the need for a nuanced approach to the development of clinical assays and therapeutics.

HIV Nef and Antiretroviral Therapy Have an Inhibitory Effect on Autophagy in Human Astrocytes that May Contribute to HIV-Associated Neurocognitive Disorders

A significant number of people living with HIV (PLWH) develop HIV-associated neurocognitive disorders (HAND) despite highly effective antiretroviral therapy (ART). Dysregulated macroautophagy (autophagy) is implicated in HAND pathogenesis. The viral protein Nef, expressed even with suppressive ART, and certain antiretrovirals affect autophagy in non-CNS cells. Astrocytes, vital for CNS microenvironment homeostasis and neuronal health, require autophagy for their own homeostasis. We hypothesized that extracellular Nef and/or ART impact astrocyte autophagy, thus contributing to HAND. We studied in-bulk and selective autophagic flux in primary human astrocytes treated with extracellular Nef and/or a combination of tenofovir+emtricitabine+raltegravir (ART) using Western blotting, a tandem fluorescent LC3 reporter, and transmission electron microscopy/morphometry. We show that after 24 h treatment, Nef and ART decrease autophagosomes through different mechanisms. While Nef accelerates autophagosome degradation without inducing autophagosome formation, ART inhibits autophagosome formation. Combination Nef+ART further depletes autophagosomes by inducing both abnormalities. Additionally, extracellular Nef and/or ART inhibit lysosomal degradation of p62, indicating Nef and/or ART affect in-bulk and selective autophagy differently. Dysregulation of both autophagic processes is maintained after 7 days of Nef and/or ART treatment. Persistent autophagy dysregulation due to chronic Nef and/or ART exposure may ultimately result in astrocyte and neuronal dysfunction, contributing to HAND.

Implications of Selective Autophagy Dysfunction for ALS Pathology

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder that progressively affects motor neurons in the brain and spinal cord. Due to the biological complexity of the disease, its etiology remains unknown. Several cellular mechanisms involved in the neurodegenerative process in ALS have been found, including the loss of RNA and protein homeostasis, as well as mitochondrial dysfunction. Insoluble protein aggregates, damaged mitochondria, and stress granules, which contain RNA and protein components, are recognized and degraded by the autophagy machinery in a process known as selective autophagy. Autophagy is a highly dynamic process whose dysregulation has now been associated with neurodegenerative diseases, including ALS, by numerous studies. In ALS, the autophagy process has been found deregulated in both familial and sporadic cases of the disease. Likewise, mutations in genes coding for proteins involved in the autophagy machinery have been reported in ALS patients, including selective autophagy receptors. In this review, we focus on the role of selective autophagy in ALS pathology.

ALS Genetics, Mechanisms, and Therapeutics: Where Are We Now?

The scientific landscape surrounding amyotrophic lateral sclerosis (ALS) continues to shift as the number of genes associated with the disease risk and pathogenesis, and the cellular processes involved, continues to grow. Despite decades of intense research and over 50 potentially causative or disease-modifying genes identified, etiology remains unexplained and treatment options remain limited for the majority of ALS patients. Various factors have contributed to the slow progress in understanding and developing therapeutics for this disease. Here, we review the genetic basis of ALS, highlighting factors that have contributed to the elusiveness of genetic heritability. The most commonly mutated ALS-linked genes are reviewed with an emphasis on disease-causing mechanisms. The cellular processes involved in ALS pathogenesis are discussed, with evidence implicating their involvement in ALS summarized. Past and present therapeutic strategies and the benefits and limitations of the model systems available to ALS researchers are discussed with future directions for research that may lead to effective treatment strategies outlined.

Key role of UBQLN2 in pathogenesis of amyotrophic lateral sclerosis and frontotemporal dementia

Ubiquilin-2 (UBQLN2) is a member of the ubiquilin family, actively implicated in the degradation of misfolded and redundant proteins through the ubiquitin-proteasome system and macroautophagy. UBQLN2 received much attention after the discovery of gene mutations in amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). The abnormal presence of positive UBQLN2 inclusion in the cytosol of degenerating motor neurons of familial and sporadic forms of ALS patients has been newly related to neurodegeneration. Only recently, data have emerged on its role in liquid-liquid phase separation, in stress granule development and in the formation of secondary amyloid structures. Furthermore, several animal models are available to investigate its involvement in TDP-43 pathology and neuroinflammation in ALS. This review addresses the molecular pathogenetic pathways involving UBQLN2 abnormalities which are converging toward defects in clearance mechanisms. UBQLN2.

ALS-Linked Mutations Affect UBQLN2 Oligomerization and Phase Separation in a Position- and Amino Acid-Dependent Manner

Proteasomal shuttle factor UBQLN2 is recruited to stress granules and undergoes liquid-liquid phase separation (LLPS) into protein-containing droplets. Mutations to UBQLN2 have recently been shown to cause dominant X-linked inheritance of amyotrophic lateral sclerosis (ALS) and ALS/dementia. Interestingly, most of these UBQLN2 mutations reside in its proline-rich (Pxx) region, an important modulator of LLPS. Here, we demonstrated that ALS-linked Pxx mutations differentially affect UBQLN2 LLPS, depending on both amino acid substitution and sequence position. Using size-exclusion chromatography, analytical ultracentrifugation, microscopy, and NMR spectroscopy, we determined that those Pxx mutants that enhanced UBQLN2 oligomerization decreased saturation concentrations needed for LLPS and promoted solid-like and viscoelastic morphological changes to UBQLN2 liquid assemblies. Ubiquitin disassembled all LLPS-induced mutant UBQLN2 aggregates. We postulate that the changes in physical properties caused by ALS-linked Pxx mutations modify UBQLN2 behavior in vivo, possibly contributing to aberrant stress granule morphology and dynamics, leading to formation of inclusions, pathological characteristics of ALS.

Alzheimer's Disease and Frontotemporal Dementia: The Current State of Genetics and Genetic Testing Since the Advent of Next-Generation Sequencing

The advent of next-generation sequencing has changed genetic diagnostics, allowing clinicians to test concurrently for phenotypically overlapping conditions such as Alzheimer's disease (AD) and frontotemporal dementia (FTD). However, to interpret genetic results, clinicians require an understanding of the benefits and limitations of different genetic technologies, such as the inability to detect large repeat expansions in such diseases as C9orf72-associated FTD and amyotrophic lateral sclerosis. Other types of mutations such as large deletions or duplications and triple repeat expansions may also go undetected. Additionally, the concurrent testing of multiple genes or the whole exome increases the likelihood of discovering variants of unknown significance. Our goal here is to review the current knowledge about the genetics of AD and FTD and suggest up-to-date guidelines for genetic testing for these dementias. Despite the improvements in diagnosis due to biomarkers testing, AD and FTD can have overlapping symptoms. When used appropriately, genetic testing can elucidate the diagnosis and specific etiology of the disease, as well as provide information for the family and determine eligibility for clinical trials. Prior to ordering genetic testing, clinicians must determine the appropriate genes to test, the types of mutations that occur in these genes, and the best type of genetic test to use. Without this analysis, interpretation of genetic results will be difficult. Patients should be counseled about the benefits and limitations of different types of genetic tests so they can make an informed decision about testing.

Impaired Autophagy in Motor Neurons: A Final Common Mechanism of Injury and Death

Autophagy is a cellular digestion process that contributes to cellular homeostasis and adaptation by the elimination of proteins and damaged organelles. Evidence suggests that dysregulation of autophagy plays a role in neurodegenerative diseases, including motor neuron disorders. Herein, we review emerging evidence indicating the roles of autophagy in physiological motor neuron processes and its function in specific compartments. Moreover, we discuss the involvement of autophagy in the pathogenesis of motor neuron diseases, including spinal cord injury and aging, and recent developments that offer promising therapeutic approaches to mitigate effects of dysregulated autophagy in health and disease.

The Emerging Role of DNA Damage in the Pathogenesis of the C9orf72 Repeat Expansion in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal, rapidly progressing neurodegenerative disease affecting motor neurons, and frontotemporal dementia (FTD) is a behavioural disorder resulting in early-onset dementia. Hexanucleotide (G4C2) repeat expansions in the gene encoding chromosome 9 open reading frame 72 (C9orf72) are the major cause of familial forms of both ALS (~40%) and FTD (~20%) worldwide. The C9orf72 repeat expansion is known to form abnormal nuclei acid structures, such as hairpins, G-quadruplexes, and R-loops, which are increasingly associated with human diseases involving microsatellite repeats. These configurations form during normal cellular processes, but if they persist they also damage DNA, and hence are a serious threat to genome integrity. It is unclear how the repeat expansion in C9orf72 causes ALS, but recent evidence implicates DNA damage in neurodegeneration. This may arise from abnormal nucleic acid structures, the greatly expanded C9orf72 RNA, or by repeat-associated non-ATG (RAN) translation, which generates toxic dipeptide repeat proteins. In this review, we detail recent advances implicating DNA damage in C9orf72-ALS. Furthermore, we also discuss increasing evidence that targeting these aberrant C9orf72 confirmations may have therapeutic value for ALS, thus revealing new avenues for drug discovery for this disorder.

Striking phenotypic variation in a family with the P506S UBQLN2 mutation including amyotrophic lateral sclerosis, spastic paraplegia, and frontotemporal dementia

Analysis of 226 exome-sequenced UK cases of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia identified 2 individuals who harbored a P497H and P506S UBQLN2 mutation, respectively (n = 0.9%). The P506S index case presented with behavioral variant frontotemporal dementia at the age of 54 years then progressed to ALS surviving 3 years. Three sons presented with (1) slowly progressive pure spastic paraplegia with an onset at 25 years and (2) ALS with disease onset of 25 years and survival of 2 years, and (3) ALS presenting symptoms at the age of 26 years, respectively. Analysis of postmortem tissue from the index case revealed frequent neuronal cytoplasmic UBQLN2-positive inclusions in the dentate gyrus and TDP-43-positive neuronal cytoplasmic inclusions in the frontal and temporal cortex and granular cell layer of the dentate gyrus of the hippocampus. Furthermore, a comprehensive analysis of published UBQLN2 mutations demonstrated that only proline-rich domain mutations contribute to a significantly earlier age of onset in male patients (p = 0.0026).

Revisiting the concept of amyotrophic lateral sclerosis as a multisystems disorder of limited phenotypic expression

PURPOSE OF REVIEW

The current review will examine the contemporary evidence that amyotrophic lateral sclerosis (ALS) is a syndrome in which the unifying feature is a progressive loss of upper and lower motor neuron function.

RECENT FINDINGS

Although ALS is traditionally viewed as a neurodegenerative disorder affecting the motor neurons, there is considerable phenotypic heterogeneity and widespread involvement of the central nervous system. A broad range of both causative and disease modifying genetic variants are associated with both sporadic and familial forms of ALS. A significant proportion of ALS patients have an associated frontotemporal dysfunction which can be a harbinger of a significantly shorter survival and for which there is increasing evidence of a fundamental disruption of tau metabolism in those affected individuals. Although the traditional neuropathology of the degenerating motor neurons in ALS is that of neuronal cytoplasmic inclusions composed neuronal intermediate filaments, the presence of neuronal cytoplasmic inclusions composed of RNA binding proteins suggests a key role for RNA dysmetabolism in the pathogenesis of ALS.

SUMMARY

ALS is a complex multisystem neurodegenerative syndrome with marked heterogeneity at not only the level of clinical expression, but also etiologically.

Frontotemporal dementia

Frontotemporal dementia (FTD) is a neurodegenerative disorder characterized by progressive changes in behavior, personality, and language with involvement of the frontal and temporal regions of the brain. About 40% of FTD cases have a positive family history, and about 10% of these cases are inherited in an autosomal-dominant pattern. These gene defects present with distinct clinical phenotypes. As the diagnosis of FTD becomes more recognizable, it will become increasingly important to keep these gene mutations in mind. In this chapter, we review the genes with known associations to FTD. We discuss protein functions, mutation frequencies, clinical phenotypes, imaging characteristics, and pathology associated with these genes.

Emerging understanding of the genotype-phenotype relationship in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive, noncurable neurodegenerative disorder of the upper and lower motor neurons causing weakness and death within a few years of symptom onset. About 10% of patients with ALS have a family history of the disease; however, ALS-associated genetic mutations are also found in sporadic cases. There are over 100 ALS-associated mutations, and importantly, several genetic mutations, including C9ORF72, SOD1, and TARDBP, have led to mechanistic insight into this complex disease. In the clinical realm, knowledge of ALS genetics can also help explain phenotypic heterogeneity, aid in genetic counseling, and in the future may help direct treatment efforts.

A new Drosophila model of Ubiquilin knockdown shows the effect of impaired proteostasis on locomotive and learning abilities

Ubiquilin (UBQLN) plays a crucial role in cellular proteostasis through its involvement in the ubiquitin proteasome system and autophagy. Mutations in the UBQLN2 gene have been implicated in amyotrophic lateral sclerosis (ALS) and ALS with frontotemporal lobar dementia (ALS/FTLD). Previous studies reported a key role for UBQLN in Alzheimer's disease (AD); however, the mechanistic involvement of UBQLN in other neurodegenerative diseases remains unclear. The genome of Drosophila contains a single UBQLN homolog (dUbqn) that shows high similarity to UBQLN1 and UBQLN2; therefore, the fly is a useful model for characterizing the role of UBQLN in vivo in neurological disorders affecting locomotion and learning abilities. We herein performed a phenotypic and molecular characterization of diverse dUbqn RNAi lines. We found that the depletion of dUbqn induced the accumulation of polyubiquitinated proteins and caused morphological defects in various tissues. Our results showed that structural defects in larval neuromuscular junctions, abdominal neuromeres, and mushroom bodies correlated with limited abilities in locomotion, learning, and memory. These results contribute to our understanding of the impact of impaired proteostasis in neurodegenerative diseases and provide a useful Drosophila model for the development of promising therapies for ALS and FTLD.

Novel UBQLN2 mutations linked to amyotrophic lateral sclerosis and atypical hereditary spastic paraplegia phenotype through defective HSP70-mediated proteolysis

Mutations in UBQLN2 have been associated with rare cases of X-linked juvenile and adult forms of amyotrophic lateral sclerosis (ALS) and ALS linked to frontotemporal dementia (FTD). Here, we report 1 known (c.1489C>T, p.Pro497Ser, P497S) and 3 novel (c.1481C>T, p.Pro494Leu, P494L; c.1498C>T, p.Pro500Ser, P500S; and c.1516C>G, p.Pro506Ala, P506A) missense mutations in the PXX domain of UBQLN2 in familial motor neuron diseases including ALS and spastic paraplegia (SP). A novel missense mutation (c.1462G>A, p.Ala488Thr, A488T) adjacent to this hotspot UBQLN2 domain was identified in a sporadic case of ALS. These mutations are conserved in mammals, are absent from ExAC and gnomAD browsers, and are predicted to be deleterious by SIFT in silico analysis. Patient lymphoblasts carrying a UBQLN2 mutation showed absence of ubiquilin-2 accumulation, disrupted binding with HSP70, and impaired autophagic pathway. Our results confirm the role of PXX repeat in ALS pathogenesis, show that UBQLN2-linked disease can manifest like a SP phenotype, evidence a highly reduced disease penetrance in females carrying UBQLN2 mutations, which is important information for genetic counseling, and underline the pivotal role of ubiquilin-2 in proteolysis regulation pathways.

Protein Quality Control and the Amyotrophic Lateral Sclerosis/Frontotemporal Dementia Continuum

Protein homeostasis, or proteostasis, has an important regulatory role in cellular function. Protein quality control mechanisms, including protein folding and protein degradation processes, have a crucial function in post-mitotic neurons. Cellular protein quality control relies on multiple strategies, including molecular chaperones, autophagy, the ubiquitin proteasome system, endoplasmic reticulum (ER)-associated degradation (ERAD) and the formation of stress granules (SGs), to regulate proteostasis. Neurodegenerative diseases are characterized by the presence of misfolded protein aggregates, implying that protein quality control mechanisms are dysfunctional in these conditions. Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases that are now recognized to overlap clinically and pathologically, forming a continuous disease spectrum. In this review article, we detail the evidence for dysregulation of protein quality control mechanisms across the whole ALS-FTD continuum, by discussing the major proteins implicated in ALS and/or FTD. We also discuss possible ways in which protein quality mechanisms could be targeted therapeutically in these disorders and highlight promising protein quality control-based therapeutics for clinical trials.

Clinical and genetic analyses of familial and sporadic frontotemporal dementia patients in Southern Italy

INTRODUCTION

We investigated the clinical differences between familial and sporadic frontotemporal dementia (FTD), screening for mutations in known FTD genes.

METHODS

We diagnosed 22 affected individuals belonging to eight families and 43 sporadic cases with FTD in Apulia, Southern Italy, in 2 years. Mutations in common causative FTD genes (GRN, MAPT, VCP, and TARDBP) and C9ORF72 expansions were screened.

RESULTS

Behavioral variant of FTD was the most common clinical subtype (50% and 69% in familial and sporadic cases, respectively). Social conduct impairment/disinhibition, loss of insight, and inflexibility were the most frequent clinical features observed at onset. One new mutation was identified in GRN in family A.

DISCUSSION

Disease onset in sporadic FTD was more frequently characterized by a clustering of behavioral symptoms with apathy and loss of personal hygiene. Mutations in common causative FTD genes are not a major cause of familial and sporadic FTD in the Southern Italian population.

No Evidence for Pathogenic Role of UBQLN2 Mutations in Sporadic Amyotrophic Lateral Sclerosis in the Mainland Chinese Population

Mutations in the UBQLN2 gene, which encodes a member of the ubiquitin-like protein family (ubiquilin-2), have been identified in patients with dominant X-linked amyotrophic lateral sclerosis (ALS) and ALS with frontotemporal dementia (FTD). We analyzed mutations in the UBQLN2 gene in a Chinese cohort of 515 patients with sporadic ALS (sALS). A novel missense mutation (p.M392V) was detected in one sALS patient. The p.M392V mutation substitutes a highly conserved residue, has not been reported in the population databases, and previously, at the same residue, a missense mutation p.M392I was detected in two Turkey ALS patients and was considered to be pathogenic, so the M392V is a variant of uncertain significance (VOUS) for ALS. We also found a deletion mutation (p.P500_G502del), which seems to be benign. In conclusion, our data suggest that mutations in the UBQLN2 gene are rare in Chinese sALS patients.

Amyotrophic lateral sclerosis - frontotemporal spectrum disorder (ALS-FTSD): Revised diagnostic criteria

This article presents the revised consensus criteria for the diagnosis of frontotemporal dysfunction in amyotrophic lateral sclerosis (ALS) based on an international research workshop on frontotemporal dementia (FTD) and ALS held in London, Canada in June 2015. Since the publication of the Strong criteria, there have been considerable advances in the understanding of the neuropsychological profile of patients with ALS. Not only is the breadth and depth of neuropsychological findings broader than previously recognised - - including deficits in social cognition and language - but mixed deficits may also occur. Evidence now shows that the neuropsychological deficits in ALS are extremely heterogeneous, affecting over 50% of persons with ALS. When present, these deficits significantly and adversely impact patient survival. It is the recognition of this clinical heterogeneity in association with neuroimaging, genetic and neuropathological advances that has led to the current re-conceptualisation that neuropsychological deficits in ALS fall along a spectrum. These revised consensus criteria expand upon those of 2009 and embrace the concept of the frontotemporal spectrum disorder of ALS (ALS-FTSD).

Frontotemporal Dementias

PURPOSE OF REVIEW

This article reviews the common behavioral and cognitive features of frontotemporal dementia (FTD) and related disorders as well as the distinguishing clinical, genetic, and pathologic features of the most common subtypes.

RECENT FINDINGS

Advances in clinical phenotyping, genetics, and biomarkers have enabled improved predictions of the specific underlying molecular pathology associated with different presentations of FTD. Evaluation of large international cohorts has led to recent refinements in diagnostic criteria for several of the FTD subtypes.

SUMMARY

The FTDs are a group of neurodegenerative disorders featuring progressive deterioration of behavior or language and associated pathology in the frontal or temporal lobes. Based on anatomic, genetic, and neuropathologic categorizations, the six clinical subtypes of FTD or related disorders are: (1) behavioral variant of FTD, (2) semantic variant primary progressive aphasia, (3) nonfluent agrammatic variant primary progressive aphasia, (4) corticobasal syndrome, (5) progressive supranuclear palsy, and (6) FTD associated with motor neuron disease. Recognition and accurate diagnoses of FTD subtypes will aid the neurologist in the management of patients with FTD.

Motor neuron disease, TDP-43 pathology, and memory deficits in mice expressing ALS-FTD-linked UBQLN2 mutations

Missense mutations in ubiquilin 2 (UBQLN2) cause ALS with frontotemporal dementia (ALS-FTD). Animal models of ALS are useful for understanding the mechanisms of pathogenesis and for preclinical investigations. However, previous rodent models carrying UBQLN2 mutations failed to manifest any sign of motor neuron disease. Here, we show that lines of mice expressing either the ALS-FTD-linked P497S or P506T UBQLN2 mutations have cognitive deficits, shortened lifespans, and develop motor neuron disease, mimicking the human disease. Neuropathologic analysis of the mice with end-stage disease revealed the accumulation of ubiquitinated inclusions in the brain and spinal cord, astrocytosis, a reduction in the number of hippocampal neurons, and reduced staining of TAR-DNA binding protein 43 in the nucleus, with concomitant formation of ubiquitin⁺ inclusions in the cytoplasm of spinal motor neurons. Moreover, both lines displayed denervation muscle atrophy and age-dependent loss of motor neurons that correlated with a reduction in the number of large-caliber axons. By contrast, two mouse lines expressing WT UBQLN2 were mostly devoid of clinical and pathological signs of disease. These UBQLN2 mouse models provide valuable tools for identifying the mechanisms underlying ALS-FTD pathogenesis and for investigating therapeutic strategies to halt disease.

The clinical spectrum of sporadic and familial forms of frontotemporal dementia

The term frontotemporal dementia (FTD) describes a clinically, genetically and pathologically diverse group of neurodegenerative disorders. Symptoms of FTD can present in individuals in their 20s through to their 90s, but the mean age at onset is in the sixth decade. The most common presentation is with a change in personality and impaired social conduct (behavioural variant FTD). Less frequently patients present with language problems (primary progressive aphasia). Both of these groups of patients can develop motor features consistent with either motor neuron disease (usually the amyotrophic lateral sclerosis variant) or parkinsonism (most commonly a progressive supranuclear palsy or corticobasal syndrome). In about a third of cases FTD is familial, with mutations in the progranulin, microtubule-associated protein tau and chromosome 9 open reading frame 72 genes being the major causes. Mutations in a number of other genes including TANK-binding kinase 1 are rare causes of familial FTD. This review aims to clarify the often confusing terminology of FTD, and outline the various clinical features and diagnostic criteria of sporadic and familial FTD syndromes. It will also discuss the current major challenges in FTD research and clinical practice, and potential areas for future research. This review clarifies the terminology of frontotemporal dementia (FTD) and summarizes the various clinical features and most recent diagnostic criteria of sporadic and familial FTD syndromes. It also discusses the current major challenges in FTD research and clinical practice, and highlights potential areas for future research.

The genetics of amyotrophic lateral sclerosis: current insights

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder that results in loss of the upper and lower motor neurons from motor cortex, brainstem, and spinal cord. While the majority of cases are sporadic, approximately 10% show familial inheritance. ALS is usually inherited in an autosomal dominant manner, although autosomal recessive and X-linked inheritance do occur. To date, 24 of the genes at 26 loci have been identified; these include loci linked to ALS and to frontotemporal dementia-ALS, where family pedigrees contain individuals with frontotemporal dementia with/without ALS. The most commonly established genetic causes of familial ALS (FALS) to date are the presence of a hexanucleotide repeat expansion in the C9ORF72 gene (39.3% FALS) and mutation of SOD1, TARDBP, and FUS, with frequencies of 12%-23.5%, 5%, and 4.1%, respectively. However, with the increasing use of next-generation sequencing of small family pedigrees, this has led to an increasing number of genes being associated with ALS. This review provides a comprehensive review on the genetics of ALS and an update of the pathogenic mechanisms associated with these genes. Commonly implicated pathways have been established, including RNA processing, the protein degradation pathways of autophagy and ubiquitin-proteasome system, as well as protein trafficking and cytoskeletal function. Elucidating the role genetics plays in both FALS and sporadic ALS is essential for understanding the subsequent cellular dysregulation that leads to motor neuron loss, in order to develop future effective therapeutic strategies.

Autophagy in motor neuron disease: Key pathogenetic mechanisms and therapeutic targets

Autophagy is a lysosome-dependant intracellular degradation process that eliminates long-lived proteins as well as damaged organelles from the cytoplasm. An increasing body of evidence suggests that dysregulation of this system plays a pivotal role in the etiology and/or progression of neurodegenerative diseases including motor neuron disorders. Herein, we review the latest findings that highlight the involvement of autophagy in the pathogenesis of amyotrophic lateral sclerosis (ALS) and the potential role of this pathway as a target of therapeutic purposes. Autophagy promotes the removal of toxic, cytoplasmic aggregate-prone pathogenetic proteins, enhances cell survival, and modulates inflammation. The existence of several drugs targeting this pathway can facilitate the translation of basic research to clinical trials for ALS and other motor neuron diseases.

Ubiquilin-2 drives NF-κB activity and cytosolic TDP-43 aggregation in neuronal cells

Background

Mutations in the gene encoding Ubiquilin-2 (UBQLN2) are linked to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). UBQLN2 plays a central role in ubiquitin proteasome system (UPS) and UBQLN2 mutants can form cytoplasmic aggregates in vitro and in vivo.

Results

Here, we report that overexpression of WT or mutant UBQLN2 species enhanced nuclear factor κB (NF-κB) activation in Neuro2A cells. The inhibition of NF-κB stress-mediated activation with SB203580, a p38 MAPK inhibitor, demonstrated a role for MAPK in NF-κB activation by UBQLN2 species. Live cell imaging and microscopy showed that UBQLN2 aggregates are dynamic structures that promote cytoplasmic accumulation of TAR DNA-binding protein (TDP-43), a major component of ALS inclusion bodies. Furthermore, up-regulation of UBQLN2 species in neurons caused an ER-stress response and increased their vulnerability to death by toxic mediator TNF-α. Withaferin A, a known NF-κB inhibitor, reduced mortality of Neuro2A cells overexpressing UBQLN2 species.

Conclusions

These results suggest that UBQLN2 dysregulation in neurons can drive NF-κB activation and cytosolic TDP-43 aggregation, supporting the concept of pathway convergence in ALS pathogenesis. These Ubiquilin-2 pathogenic pathways might represent suitable therapeutic targets for future ALS treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0162-6) contains supplementary material, which is available to authorized users.

Frontotemporal Dysfunction and Dementia in Amyotrophic Lateral Sclerosis

Although amyotrophic lateral sclerosis (ALS) is classically considered a disorder exclusively affecting motor neurons, there is substantial clinical, neuroimaging, and neuropathologic evidence that more than half of patients have an associated syndrome of frontotemporal dysfunction. These syndromes range from frontotemporal dementia to behavioral or cognitive syndromes. Neuroimaging and neuropathologic findings are consistent with frontotemporal lobar degeneration that underpins alterations in network connectivity. Future clinical trials need to be stratified based on the presence or absence of frontotemporal dysfunction on the disease course of ALS.

Phenotypic Heterogeneity of Monogenic Frontotemporal Dementia

Frontotemporal dementia (FTD) is a genetically and pathologically heterogeneous disorder characterized by personality changes, language impairment, and deficits of executive functions associated with frontal and temporal lobe degeneration. Different phenotypes have been defined on the basis of presenting clinical symptoms, i.e., the behavioral variant of FTD, the agrammatic variant of primary progressive aphasia, and the semantic variant of PPA. Some patients have an associated movement disorder, either parkinsonism, as in progressive supranuclear palsy and corticobasal syndrome, or motor neuron disease (FTD-MND). A family history of dementia is found in 40% of cases of FTD and about 10% have a clear autosomal-dominant inheritance. Genetic studies have identified several genes associated with monogenic FTD: microtubule-associated protein tau, progranulin, TAR DNA-binding protein 43, valosin-containing protein, charged multivesicular body protein 2B, fused in sarcoma, and the hexanucleotide repeat expansion in intron 1 of the chromosome 9 open reading frame 72. Patients often present with an extensive phenotypic variability, even among different members of the same kindred carrying an identical disease mutation. The objective of the present work is to review and evaluate available literature data in order to highlight recent advances in clinical, biological, and neuroimaging features of monogenic frontotemporal lobar degeneration and try to identify different mechanisms underlying the extreme phenotypic heterogeneity that characterizes this disease.