Progranulin Mutations Research Articles

Frontotemporal dementia non-sense mutation of progranulin rescued by aminoglycosides.

Frontotemporal dementia (FTD) is an early onset dementia characterized by progressive atrophy of the frontal and/or temporal lobes. FTD is highly heritable with mutations in progranulin accounting for 5-26% of cases in different populations. Progranulin is involved in endocytosis, secretion and lysosomal processes, but its functions under physiological and pathological conditions remains to be defined. Many FTD-causing non-sense progranulin mutations contain a premature termination codon (PTC), thus progranulin haploinsufficiency has been proposed as a major disease mechanism. Currently, there is no effective FTD treatment or therapy. Aminoglycosides are a class of antibiotics that possess a less-known function to induce eukaryotic ribosomal readthrough of PTCs to produce a full-length protein. The aminoglycoside-induced readthrough strategy has been utilized to treat multiple human diseases caused by PTCs. In this study, we tested the only clinically approved readthrough small molecule PTC124 and 11 aminoglycosides in a cell culture system on four PTCs responsible for FTD or a related neurodegenerative disease amyotrophic lateral sclerosis. We found that the aminoglycosides G418 and gentamicin rescued the expression of the progranulin R493X mutation. G418 was more effective than gentamicin (~50% rescue versus <10%), and the effect was dose- and time-dependent. The progranulin readthrough protein displayed similar subcellular localization as the wild-type progranulin protein. These data provide an exciting proof-of-concept that aminoglycosides or other readthrough-promoting compounds are a therapeutic avenue for familial FTD caused by progranulin PTC mutations.

Impaired \u03b2-glucocerebrosidase activity and processing in frontotemporal dementia due to progranulin mutations

Loss-of-function mutations in progranulin (GRN) are a major autosomal dominant cause of frontotemporal dementia. Most pathogenic GRN mutations result in progranulin haploinsufficiency, which is thought to cause frontotemporal dementia in GRN mutation carriers. Progranulin haploinsufficiency may drive frontotemporal dementia pathogenesis by disrupting lysosomal function, as patients with GRN mutations on both alleles develop the lysosomal storage disorder neuronal ceroid lipofuscinosis, and frontotemporal dementia patients with GRN mutations (FTD-GRN) also accumulate lipofuscin. The specific lysosomal deficits caused by progranulin insufficiency remain unclear, but emerging data indicate that progranulin insufficiency may impair lysosomal sphingolipid-metabolizing enzymes. We investigated the effects of progranulin insufficiency on sphingolipid-metabolizing enzymes in the inferior frontal gyrus of FTD-GRN patients using fluorogenic activity assays, biochemical profiling of enzyme levels and posttranslational modifications, and quantitative neuropathology. Of the enzymes studied, only β-glucocerebrosidase exhibited impairment in FTD-GRN patients. Brains from FTD-GRN patients had lower activity than controls, which was associated with lower levels of mature β-glucocerebrosidase protein and accumulation of insoluble, incompletely glycosylated β-glucocerebrosidase. Immunostaining revealed loss of neuronal β-glucocerebrosidase in FTD-GRN patients. To investigate the effects of progranulin insufficiency on β-glucocerebrosidase outside of the context of neurodegeneration, we investigated β-glucocerebrosidase activity in progranulin-insufficient mice. Brains from Grn−/− mice had lower β-glucocerebrosidase activity than wild-type littermates, which was corrected by AAV-progranulin gene therapy. These data show that progranulin insufficiency impairs β-glucocerebrosidase activity in the brain. This effect is strongest in neurons and may be caused by impaired β-glucocerebrosidase processing.

Multimodal MRI of grey matter, white matter, and functional connectivity in cognitively healthy mutation carriers at risk for frontotemporal dementia and Alzheimer's disease

BackgroundFrontotemporal dementia (FTD) and Alzheimer’s disease (AD) are associated with divergent differences in grey matter volume, white matter diffusion, and functional connectivity. However, it is unknown at what disease stage these differences emerge. Here, we investigate whether divergent differences in grey matter volume, white matter diffusion, and functional connectivity are already apparent between cognitively healthy carriers of pathogenic FTD mutations, and cognitively healthy carriers at increased AD risk.MethodsWe acquired multimodal magnetic resonance imaging (MRI) brain scans in cognitively healthy subjects with (n=39) and without (n=36) microtubule-associated protein Tau (MAPT) or progranulin (GRN) mutations, and with (n=37) and without (n=38) apolipoprotein E ε4 (APOE4) allele. We evaluated grey matter volume using voxel-based morphometry, white matter diffusion using tract-based spatial statistics (TBSS), and region-to-network functional connectivity using dual regression in the default mode network and salience network. We tested for differences between the respective carriers and controls, as well as for divergence of those differences. For the divergence contrast, we additionally performed region-of-interest TBSS analyses in known areas of white matter diffusion differences between FTD and AD (i.e., uncinate fasciculus, forceps minor, and anterior thalamic radiation).ResultsMAPT/GRN carriers did not differ from controls in any modality. APOE4 carriers had lower fractional anisotropy than controls in the callosal splenium and right inferior fronto-occipital fasciculus, but did not show grey matter volume or functional connectivity differences. We found no divergent differences between both carrier-control contrasts in any modality, even in region-of-interest analyses.ConclusionsConcluding, we could not find differences suggestive of divergent pathways of underlying FTD and AD pathology in asymptomatic risk mutation carriers. Future studies should focus on asymptomatic mutation carriers that are closer to symptom onset to capture the first specific signs that may differentiate between FTD and AD.

Progranulin mutations result in impaired processing of prosaposin and reduced glucocerebrosidase activity.

Frontotemporal dementia (FTD) is a common neurogenerative disorder characterized by progressive degeneration in the frontal and temporal lobes. Heterozygous mutations in the gene encoding progranulin (PGRN) are a common genetic cause of FTD. Recently, PGRN has emerged as an important regulator of lysosomal function. Here, we examine the impact of PGRN mutations on the processing of full-length prosaposin to individual saposins, which are critical regulators of lysosomal sphingolipid metabolism. Using FTD-PGRN patient-derived cortical neurons differentiated from induced pluripotent stem cells, as well as post-mortem tissue from patients with FTLD-PGRN, we show that PGRN haploinsufficiency results in impaired processing of prosaposin to saposin C, a critical activator of the lysosomal enzyme glucocerebrosidase (GCase). Additionally, we found that PGRN mutant neurons had reduced lysosomal GCase activity, lipid accumulation and increased insoluble α-synuclein relative to isogenic controls. Importantly, reduced GCase activity in PGRN mutant neurons is rescued by treatment with saposin C. Together, these findings suggest that reduced GCase activity due to impaired processing of prosaposin may contribute to pathogenesis of FTD resulting from PGRN mutations.

Genetic predictors of survival in behavioral variant frontotemporal degeneration.

To determine autosomal dominant genetic predictors of survival in individuals with behavioral variant frontotemporal degeneration (bvFTD). A retrospective chart review of 174 cases with a clinical phenotype of bvFTD but no associated elementary neurologic features was performed, with diagnosis either autopsy-confirmed (n = 57) or supported by CSF evidence of non-Alzheimer pathology (n = 117). Genetic analysis of the 3 most common genes with pathogenic autosomal dominant mutations associated with frontotemporal degeneration was performed in all patients, which identified cases with C9orf72 expansion (n = 28), progranulin (GRN) mutation (n = 12), and microtubule-associated protein tau (MAPT) mutation (n = 10). Cox proportional hazards regressions were used to test for associations between survival and mutation status, sex, age at symptom onset, and education. Across all patients with bvFTD, the presence of a disease-associated pathogenic mutation was associated with shortened survival (hazard ratio [HR] 2.164, 95% confidence interval [CI] 1.391, 3.368). In separate models, a GRN mutation (HR 2.423, 95% CI 1.237, 4.744), MAPT mutation (HR 8.056, 95% CI 2.938, 22.092), and C9orf72 expansion (HR 1.832, 95% CI 1.034, 3.244) were each individually associated with shorter survival relative to sporadic bvFTD. A mutation on the MAPT gene results in an earlier age at onset than a C9orf72 expansion or mutation on the GRN gene (p = 0.016). Our findings suggest that autosomal dominantly inherited mutations, modulated by age at symptom onset, associate with shorter survival among patients with bvFTD. We suggest that clinical trials and clinical management should consider mutation status and age at onset when evaluating disease progression.

Microglia in frontotemporal lobar degeneration with progranulin or C9ORF72 mutations.

ObjectiveTo identify clinicopathological differences between frontotemporal lobar degeneration (FTLD) due to mutations in progranulin (FTLD‐GRN) and chromosome 9 open reading frame 72 (FTLD‐C9ORF72).MethodsWe performed quantitative neuropathologic comparison of 17 FTLD‐C9ORF72 and 15 FTLD‐GRN with a focus on microglia. For clinical comparisons, only cases with high quality medical documentation and concurring diagnoses by at least two neurologists were included (14 FTLD‐GRN and 13 FTLD‐C9ORF72). Neuropathological analyses were limited to TDP‐43 Type A to assure consistent assessment between the groups, acknowledging that Type A is a minority of C9ORF72 patients. Furthermore, only cases with sufficient tissue from all regions were studied (11 FTLD‐GRN and 11 FTLD‐C9ORF72). FTLD cases were also compared to age– and sex–matched normal controls. Immunohistochemistry was performed for pTDP‐43, IBA‐1, CD68, and GFAP. Morphological characterization of microglia was performed in sections of cortex blinded to clinical and genetic information.ResultsFTLD‐GRN patients had frequent asymmetric clinical features, including aphasia and apraxia, as well as more asymmetric cortical atrophy. Neuropathologically, FTLD‐C9ORF72 had greater hippocampal tau pathology and more TDP‐43 neuronal cytoplasmic inclusions. FTLD‐GRN had more neocortical microvacuolation, as well as more IBA‐1–positive ameboid microglia in superficial cortical layers and in subcortical white matter. FTLD‐GRN also had more microglia with nuclear condensation, possibly indicating apoptosis. Microglial morphology with CD68 immunohistochemistry in FTLD‐GRN and FTLD‐C9ORF72 differed from controls.InterpretationOur findings underscore differences in microglial response in FTLD‐C9ORF72 and FTLD‐GRN as shown by significant differences in ameboid microglia in gray and white matter. These results suggest the differential contribution of microglial dysfunction in FTLD‐GRN and FTLD‐C9ORF72 and suggest that clinical, neuroimaging and pathologic differences could in part be related to differences in microglia response.

C-terminal and full length TDP-43 specie differ according to FTLD-TDP lesion type but not genetic mutation

The transactive response DNA binding protein of 43 kDa (TDP-43) is an intranuclear protein involved in RNA splicing. Abnormally deposited TDP-43 is found in the brains of patients with frontotemporal lobar degeneration (FTLD). Different morphological characteristics of TDP-43 immunoreactive inclusions define the different variants of FTLD-TDP. Little is known about the relationships between TDP-43 specie (phosphorylated TDP-43, C-terminal fragments and full length TDP-43) and lesion types. Using novel antibodies that recognize phosphorylated TDP-43 (pTDP-43), a neoepitope in the C-terminal fragment of TDP-43 (cTDP-43) and the N-terminal, i.e. full length (nTDP-43) we assess the relative burden of pTDP-43, cTDP-43 and nTDP-43 in 8 different lesion types across FTLD-TDP type A-C. These include neuronal cytoplasmic inclusions, dystrophic neurites, neuronal intranuclear inclusions, fine neurites of the hippocampus, peri-vascular inclusions, Pick body-like inclusions, long thick dystrophic neurites and granular pre-inclusions. We also assess for associations with progranulin (GRN) and C9ORF72 genetic mutations. For all eight lesion types, the highest burden was observed for pTDP-43. In six of the eight lesions studied, cTDP-43 burden was greater than nTDP-43 burden. However, we observed a higher burden of nTDP-43 to cTDP-43 for pre-inclusions. We also noted an equal-to-slightly higher burden of nTDP-43 to cTDP-43 for peri-vascular inclusions. There was not strong evidence for associations to be driven by mutation status although for neuronal cytoplasmic inclusions and dystrophic neurites GRN+ cases had higher burden of pTDP-43, cTDP-43 and nTDP-43 compared to GRN- cases, with nTDP-43 inclusions only observed in GRN+ cases. In addition, for pre-inclusions, cTDP-43 and nTDP-43 burden tended to be higher in C9ORF72- cases compared to C9ORF72+ cases, although this was not the case for pTDP-43. There is clear evidence that phosphorylation and C terminal fragments play an important role in lesion formation in FTLD-TDP. However, for some inclusions, particularly pre-inclusions, full-length TDP-43 appears to be playing a role.

Multi-Granulin Domain Peptides Bind to Pro-Cathepsin D and Stimulate Its Enzymatic Activity More Effectively Than Progranulin in Vitro.

Progranulin (PGRN) is an evolutionarily conserved glycoprotein associated with several disease states, including neurodegeneration, cancer, and autoimmune disorders. This protein has recently been implicated in the regulation of lysosome function, whereby PGRN may bind to and promote the maturation and activity of the aspartyl protease cathepsin D (proCTSD, inactive precursor; matCTSD, mature, enzymatically active form). As the full-length PGRN protein can be cleaved into smaller peptides, called granulins, we assessed the function of these granulin peptides in binding to proCTSD and stimulating matCTSD enzyme activity in vitro. Here, we report that full-length PGRN and multi-granulin domain peptides bound to proCTSD with low to submicromolar binding affinities. This binding promoted proCTSD destabilization, the magnitude of which was greater for multi-granulin domain peptides than for full-length PGRN. Such destabilization correlated with enhanced matCTSD activity at acidic pH. The presence and function of multi-granulin domain peptides have typically been overlooked in previous studies. This work provides the first in vitro quantification of their binding and activity on proCTSD. Our study highlights the significance of multi-granulin domain peptides in the regulation of proCTSD maturation and enzymatic activity and suggests that attention to PGRN processing will be essential for the future understanding of the molecular mechanisms leading to neurodegenerative disease states with loss-of-function mutations in PGRN.

Coexisting Lewy body disease and clinical parkinsonism in frontotemporal lobar degeneration.

To investigate the prevalence of clinically relevant multiple system atrophy (MSA) and Lewy body disease (LBD) pathologies in a large frontotemporal lobar degeneration (FTLD) cohort to determine if concomitant pathologies underlie the heterogeneity of clinical features. All prospectively followed FTLD-tau and FTLD-TDP cases held by the Sydney Brain Bank (n = 126) were screened for coexisting MSA and LBD (Braak ≥ stage IV) pathology. Relevant clinical (including family history) and genetic associations were determined. MSA pathology was not identified in this series. Of the FTLD cohort, 9 cases had coexisting LBD ≥ Braak stage IV and were associated with different FTLD subtypes including Pick disease (n = 2), corticobasal degeneration (n = 2), progressive supranuclear palsy (n = 2), and TDP type A (n = 3). All FTLD-TDP cases with coexisting LBD had mutations in progranulin (n = 2) or an abnormal repeat expansion in C9orf72 (n = 1). All FTLD-tau cases with coexisting LBD were sporadic. The H1H1 MAPT haplotype was found in all cases that could be genotyped (n = 6 of 9). Seven cases presented with a predominant dementia disorder, 3 of which developed parkinsonism. Two cases presented with a movement disorder and developed dementia in their disease course. The age at symptom onset (62 ± 11 years) and disease duration (8 ± 5 years) in FTLD cases with coexisting LBD did not differ from pure FTLD or pure LBD cases in the brain bank. Coexisting LBD in FTLD comprises a small proportion of cases but has implications for clinical and neuropathologic diagnoses and the identification of biomarkers.

Review: Clinical, genetic and neuroimaging features of frontotemporal dementia.

Frontotemporal dementia (FTD) is a heterogeneous group of disorders causing neurodegeneration within a network of areas centred on the frontal and temporal lobes. Clinically, patients present with behavioural symptoms (behavioural variant FTD) or language disturbance (primary progressive aphasia), although there is an overlap with motor neurone disease and atypical parkinsonian disorders. Whilst neuroimaging commonly reveals abnormalities in the frontal and temporal lobes, a closer review identifies a more complex picture with variable asymmetry of neuronal loss, widespread subcortical involvement and in many cases more posterior cortical atrophy. An autosomal-dominant genetic disorder is found in around a third of people with mutations in progranulin, C9orf72 and the microtubule-associated protein tau being the commonest causes. In the other two-thirds, the disorder is sporadic, although recent genome-wide association studies have started to identify genetic risk factors within this group. Much of this knowledge has been understood only in the past 10years and so this review will discuss the current knowledge about the clinical, genetic and neuroimaging features of FTD.

Thalamo-cortical network hyperconnectivity in preclinical progranulin mutation carriers.

Mutations in progranulin (GRN) cause heterogeneous clinical syndromes, including behavioral variant frontotemporal dementia (bvFTD), primary progressive aphasia (PPA), corticobasal syndrome (CBS) and Alzheimer-type dementia (AD-type dementia). Human studies have shown that presymptomatic GRN carriers feature reduced connectivity in the salience network, a system targeted in bvFTD. Mice with homozygous deletion of GRN, in contrast, show thalamo-cortical hypersynchrony due to aberrant pruning of inhibitory synapses onto thalamo-cortical projection neurons. No studies have systematically explored the intrinsic connectivity networks (ICNs) targeted by the four GRN-associated clinical syndromes, or have forged clear links between human and mouse model findings. We compared 17 preclinical GRN carriers (14 “presymptomatic” clinically normal and three “prodromal” with mild cognitive symptoms) to healthy controls to assess for differences in cognitive testing and gray matter volume. Using task-free fMRI, we assessed connectivity in the salience network, a non-fluent variant primary progressive aphasia network (nfvPPA), the perirolandic network (CBS), and the default mode network (AD-type dementia). GRN carriers and controls showed similar performance on cognitive testing. Although carriers showed little evidence of brain atrophy, markedly enhanced connectivity emerged in all four networks, and thalamo-cortical hyperconnectivity stood out as a unifying feature. Voxelwise assessment of whole brain degree centrality, an unbiased graph theoretical connectivity metric, confirmed thalamic hyperconnectivity. These results show that human GRN disease and the prevailing GRN mouse model share a thalamo-cortical network hypersynchrony phenotype. Longitudinal studies will determine whether this network physiology represents a compensatory response as carriers approach symptom onset, or an early and sustained preclinical manifestation of lifelong progranulin haploinsufficiency.

Longitudinal multimodal MRI as prognostic and diagnostic biomarker in presymptomatic familial frontotemporal dementia.

Developing and validating sensitive biomarkers for the presymptomatic stage of familial frontotemporal dementia is an important step in early diagnosis and for the design of future therapeutic trials. In the longitudinal Frontotemporal Dementia Risk Cohort, presymptomatic mutation carriers and non-carriers from families with familial frontotemporal dementia due to microtubule-associated protein tau (MAPT) and progranulin (GRN) mutations underwent a clinical assessment and multimodal MRI at baseline, 2-, and 4-year follow-up. Of the cohort of 73 participants, eight mutation carriers (three GRN, five MAPT) developed clinical features of frontotemporal dementia ('converters'). Longitudinal whole-brain measures of white matter integrity (fractional anisotropy) and grey matter volume in these converters (n = 8) were compared with healthy mutation carriers ('non-converters'; n = 35) and non-carriers (n = 30) from the same families. We also assessed the prognostic performance of decline within white matter and grey matter regions of interest by means of receiver operating characteristic analyses followed by stepwise logistic regression. Longitudinal whole-brain analyses demonstrated lower fractional anisotropy values in extensive white matter regions (genu corpus callosum, forceps minor, uncinate fasciculus, and superior longitudinal fasciculus) and smaller grey matter volumes (prefrontal, temporal, cingulate, and insular cortex) over time in converters, present from 2 years before symptom onset. White matter integrity loss of the right uncinate fasciculus and genu corpus callosum provided significant classifiers between converters, non-converters, and non-carriers. Converters' within-individual disease trajectories showed a relatively gradual onset of clinical features in MAPT, whereas GRN mutations had more rapid changes around symptom onset. MAPT converters showed more decline in the uncinate fasciculus than GRN converters, and more decline in the genu corpus callosum in GRN than MAPT converters. Our study confirms the presence of spreading predominant frontotemporal pathology towards symptom onset and highlights the value of multimodal MRI as a prognostic biomarker in familial frontotemporal dementia.

Frontotemporal dementia causative CHMP2B impairs neuronal endolysosomal traffic-rescue by TMEM106B knockdown.

Mutations in the endosome-associated protein CHMP2B cause frontotemporal dementia and lead to lysosomal storage pathology in neurons. We here report that physiological levels of mutant CHMP2B causes reduced numbers and significantly impaired trafficking of endolysosomes within neuronal dendrites, accompanied by increased dendritic branching. Mechanistically, this is due to the stable incorporation of mutant CHMP2B onto neuronal endolysosomes, which we show renders them unable to traffic within dendrites. This defect is due to the inability of mutant CHMP2B to recruit the ATPase VPS4, which is required for release of CHMP2B from endosomal membranes. Strikingly, both impaired trafficking and the increased dendritic branching were rescued by treatment with antisense oligonucleotides targeting the well validated frontotemporal dementia risk factor TMEM106B, which encodes an endolysosomal protein. This indicates that reducing TMEM106B levels can restore endosomal health in frontotemporal dementia. As TMEM106B is a risk factor for frontotemporal dementia caused by both C9orf72 and progranulin mutations, and antisense oligonucleotides are showing promise as therapeutics for neurodegenerative diseases, our data suggests a potential new strategy for treating the wide range of frontotemporal dementias associated with endolysosomal dysfunction.

Reduction of microglial progranulin does not exacerbate pathology or behavioral deficits in neuronal progranulin-insufficient mice

Loss-of-function mutations in progranulin (GRN), most of which cause progranulin haploinsufficiency, are a major autosomal dominant cause of frontotemporal dementia (FTD). Individuals with loss-of-function mutations on both GRN alleles develop neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disorder. Progranulin is a secreted glycoprotein expressed by a variety of cell types throughout the body, including neurons and microglia in the brain. Understanding the relative importance of neuronal and microglial progranulin insufficiency in FTD pathogenesis may guide development of therapies. In this study, we used mouse models to investigate the role of neuronal and microglial progranulin insufficiency in the development of FTD-like pathology and behavioral deficits. Grn−/− mice model aspects of FTD and NCL, developing lipofuscinosis and gliosis throughout the brain, as well as deficits in social behavior. We have previously shown that selective depletion of neuronal progranulin disrupts social behavior, but does not produce lipofuscinosis or gliosis. We hypothesized that reduction of microglial progranulin would induce lipofuscinosis and gliosis, and exacerbate behavioral deficits, in neuronal progranulin-deficient mice. To test this hypothesis, we crossed Grnfl/fl mice with mice expressing Cre transgenes targeting neurons (CaMKII-Cre) and myeloid cells/microglia (LysM-Cre). CaMKII-Cre, which is expressed in forebrain excitatory neurons, reduced cortical progranulin protein levels by around 50%. LysM-Cre strongly reduced progranulin immunolabeling in many microglia, but did not reduce total brain progranulin levels, suggesting that, at least under resting conditions, microglia contribute less than neurons to overall brain progranulin levels. Mice with depletion of both neuronal and microglial progranulin failed to develop lipofuscinosis or gliosis, suggesting that progranulin from extracellular sources prevented pathology in cells targeted by the Cre transgenes. Reduction of microglial progranulin also did not exacerbate the social deficits of neuronal progranulin-insufficient mice. These results do not support the hypothesis of synergistic effects between progranulin-deficient neurons and microglia. Nearly complete progranulin deficiency appears to be required to induce lipofuscinosis and gliosis in mice, while partial progranulin insufficiency is sufficient to produce behavioral deficits.

Progranulin reduces insoluble TDP-43 levels, slows down axonal degeneration and prolongs survival in mutant TDP-43 mice

BackgroundTAR DNA binding protein 43 (TDP-43) is the main disease protein in most patients with amyotrophic lateral sclerosis (ALS) and about 50% of patients with frontotemporal dementia (FTD). TDP-43 pathology is not restricted to patients with missense mutations in TARDBP, the gene encoding TDP-43, but also occurs in ALS/FTD patients without known genetic cause or in patients with various other ALS/FTD gene mutations. Mutations in progranulin (GRN), which result in a reduction of ~ 50% of progranulin protein (PGRN) levels, cause FTD with TDP-43 pathology. How loss of PGRN leads to TDP-43 pathology and whether or not PGRN expression protects against TDP-43-induced neurodegeneration is not yet clear.MethodsWe studied the effect of PGRN on the neurodegenerative phenotype in TDP-43(A315T) mice.ResultsPGRN reduced the levels of insoluble TDP-43 and histology of the spinal cord revealed a protective effect of PGRN on the loss of large axon fibers in the lateral horn, the most severely affected fiber pool in this mouse model. Overexpression of PGRN significantly slowed down disease progression, extending the median survival by approximately 130 days. A transcriptome analysis did not point towards a single pathway affected by PGRN, but rather towards a pleiotropic effect on different pathways.ConclusionOur findings reveal an important role of PGRN in attenuating mutant TDP-43-induced neurodegeneration.

Cerebrospinal fluid soluble TREM2 levels in frontotemporal dementia differ by genetic and pathological subgroup

BackgroundReliable biomarkers of frontotemporal dementia (FTD) are currently lacking. FTD may be associated with chronic immune dysfunction, microglial activation and raised inflammatory markers, particularly in progranulin (GRN) mutation carriers. Levels of soluble triggering receptor expressed on myeloid cells 2 (sTREM2) are elevated in Alzheimer’s disease (AD), but they have not been fully explored in FTD.MethodsWe investigated whether cerebrospinal fluid (CSF) sTREM2 levels differ between FTD and controls, across different clinical and genetic subtypes of FTD, or between individuals with FTD due to AD versus non-AD pathology (based on CSF neurodegenerative biomarkers). We also assessed relationships between CSF sTREM2 and other CSF biomarkers (total tau [T-tau], tau phosphorylated at position threonine-181 [P-tau] and β-amyloid 1–42 [Aβ42]) and age and disease duration. Biomarker levels were measured using immunoassays in 17 healthy controls and 64 patients with FTD (behavioural variant FTD, n = 20; primary progressive aphasia, n = 44). Ten of 64 had familial FTD, with mutations in GRN (n = 3), MAPT (n = 4), or C9orf72 (n = 3). Fifteen of 64 had neurodegenerative biomarkers consistent with AD pathology (11 of whom had logopenic variant PPA). Levels were compared using multivariable linear regressions.ResultsCSF sTREM2 levels did not differ between FTD and controls or between clinical subgroups. However, GRN mutation carriers had higher levels than controls (mean ([SD] = 9.7 [2.9] vs. 6.8 [1.6] ng/ml; P = 0.028) and MAPT (3.9 [1.5] ng/ml; P = 0.003] or C9orf72 [4.6 [1.8] ng/ml; P = 0.006) mutation carriers. Individuals with AD-like CSF had higher sTREM2 levels than those with non-AD-like CSF (9.0 [3.6] vs. 6.9 [3.0] ng/ml; P = 0.029). CSF sTREM2 levels were associated with T-tau levels in control and FTD groups and also with P-tau in those with FTD and AD-like CSF. CSF sTREM2 levels were influenced by both age and disease duration in FTD.ConclusionsAlthough CSF sTREM2 levels are not raised in FTD overall or in a particular clinical subtype of FTD, levels are raised in familial FTD associated with GRN mutations and in FTD syndromes due to AD pathology. Because CSF sTREM2 levels correlate with a marker of neuronal injury (T-tau), sTREM2 should be explored as a biomarker of disease intensity in future longitudinal studies of FTD.

Partial Tmem106b reduction does not correct abnormalities due to progranulin haploinsufficiency

BackgroundLoss of function mutations in progranulin (GRN) are a major cause of frontotemporal dementia (FTD). Progranulin is a secreted glycoprotein that localizes to lysosomes and is critical for proper lysosomal function. Heterozygous GRN mutation carriers develop FTD with TDP-43 pathology and exhibit signs of lysosomal dysfunction in the brain, with increased levels of lysosomal proteins and lipofuscin accumulation. Homozygous GRN mutation carriers develop neuronal ceroid lipofuscinosis (NCL), an earlier-onset lysosomal storage disorder caused by severe lysosomal dysfunction. Multiple genome-wide association studies have shown that risk of FTD in GRN mutation carriers is modified by polymorphisms in TMEM106B, which encodes a lysosomal membrane protein. Risk alleles of TMEM106B may increase TMEM106B levels through a variety of mechanisms. Brains from FTD patients with GRN mutations exhibit increased TMEM106B expression, and protective TMEM106B polymorphisms are associated with decreased TMEM106B expression. Together, these data raise the possibility that reduction of TMEM106B levels may protect against the pathogenic effects of progranulin haploinsufficiency.MethodsWe crossed Tmem106b+/− mice with Grn+/− mice, which model the progranulin haploinsufficiency of GRN mutation carriers and develop age-dependent social deficits and lysosomal abnormalities in the brain. We tested whether partial Tmem106b reduction could normalize the social deficits and lysosomal abnormalities of Grn+/− mice.ResultsPartial reduction of Tmem106b levels did not correct the social deficits of Grn+/− mice. Tmem106b reduction also failed to normalize most lysosomal abnormalities of Grn+/− mice, except for β-glucuronidase activity, which was suppressed by Tmem106b reduction and increased by progranulin insufficiency.ConclusionsThese data do not support the hypothesis that Tmem106b reduction protects against the pathogenic effects of progranulin haploinsufficiency, but do show that Tmem106b reduction normalizes some lysosomal phenotypes in Grn+/− mice.

Pathogenic Signal Sequence Mutations in Progranulin Disrupt SRP Interactions Required for mRNA Stability

SUMMARYCells have evolved quality control pathways to prevent the accumulation of improperly localized proteins, which are often toxic. One of these pathways, regulation of aberrant protein production (RAPP), recognizes aberrant secretory proteins during translation and degrades the associated mRNA. Here, we demonstrate endogenous RAPP substrates. Haploinsufficiency of the secretory protein progranulin (GRN) is associated with the neurodegenerative disease frontotemporal lobar degeneration (FTLD). Our results show FTLD-associated GRN mutations W7R and A9D disrupt co-translational interaction with a targeting factor, signal recognition particle (SRP). This triggers RAPP and initiates specific mRNA degradation. Conversely, wild-type GRN and the naturally occurring polymorphism V5L GRN are efficiently expressed and secreted. Thus, RAPP plays a role in the molecular pathology of A9D GRN and W7R GRN.

Loss of Tmem106b is unable to ameliorate frontotemporal dementia-like phenotypes in an AAV mouse model of C9ORF72-repeat induced toxicity

Loss-of-function mutations in progranulin (GRN) and a non-coding (GGGGCC)n hexanucleotide repeat expansions in C9ORF72 are the two most common genetic causes of frontotemporal lobar degeneration with aggregates of TAR DNA binding protein 43 (FTLD-TDP). TMEM106B encodes a type II transmembrane protein with unknown function. Genetic variants in TMEM106B associated with reduced TMEM106B levels have been identified as disease modifiers in individuals with GRN mutations and C9ORF72 expansions. Recently, loss of Tmem106b has been reported to protect the FTLD-like phenotypes in Grn−/− mice. Here, we generated Tmem106b−/− mice and examined whether loss of Tmem106b could rescue FTLD-like phenotypes in an AAV mouse model of C9ORF72-repeat induced toxicity. Our results showed that neither partial nor complete loss of Tmem106b was able to rescue behavioral deficits induced by the expression of (GGGGCC)66 repeats (66R). Loss of Tmem106b also failed to ameliorate 66R-induced RNA foci, dipeptide repeat protein formation and pTDP-43 pathological burden. We further found that complete loss of Tmem106b increased astrogliosis, even in the absence of 66R, and failed to rescue 66R-induced neuronal cell loss, whereas partial loss of Tmem106b significantly rescued the neuronal cell loss but not neuroinflammation induced by 66R. Finally, we showed that overexpression of 66R did not alter expression of Tmem106b and other lysosomal genes in vivo, and subsequent analyses in vitro found that transiently knocking down C9ORF72, but not overexpression of 66R, significantly increased TMEM106B and other lysosomal proteins. In summary, reducing Tmem106b levels failed to rescue FTLD-like phenotypes in a mouse model mimicking the toxic gain-of-functions associated with overexpression of 66R. Combined with the observation that loss of C9ORF72 and not 66R overexpression was associated with increased levels of TMEM106B, this work suggests that the protective TMEM106B haplotype may exert its effect in expansion carriers by counteracting lysosomal dysfunction resulting from a loss of C9ORF72.

Pathological correlates of white matter hyperintensities in a case of progranulin mutation associated frontotemporal dementia

ABSTRACTWhite matter hyperintensities (WMH) are often seen on MRI brain scans in frontotemporal dementia (FTD) due to progranulin (GRN) mutations, but their pathological correlates are unknown. We examined the histological changes underlying WMH in a patient with GRN mutation associated behavioral variant FTD. In vivo and cadaveric MRI showed progressive, asymmetric frontotemporal and parietal atrophy, and asymmetrical WMH predominantly affecting frontal mid-zones. We first performed segmentation and localization analyses of WMH present on cadaveric MRI FLAIR images, then selected five different brain regions directly matched to differing severities of WMH for histological analysis. We used immunohistochemistry to assess vascular pathology, degree of spongiosis, neuronal and axonal loss, TDP-43, demyelination and astrogliosis, and microglial burden and morphology. Brain regions with significant WMH displayed severe cortical and white matter pathology, and prominent white matter microglial activation and microglial dystrophy, but only mild axonal loss and minimal vascular pathology. Our study suggests that WMH in GRN mutation carriers are not secondary to vascular pathology. Whilst cortical pathology induced axonal degeneration could contribute to white matter damage, individuals with GRN mutations could develop selective white matter vulnerability and myelin loss due to chronic, regional microglial dysfunction arising from GRN haploinsufficiency.