Loss of TMEM106B Ameliorates Lysosomal and Frontotemporal Dementia-Related Phenotypes in Progranulin-Deficient Mice

Microglial Progranulin: Involvement in Alzheimer's Disease and Neurodegenerative Diseases

Neurodegenerative diseases such as Alzheimer’s disease have proven resistant to new treatments. The complexity of neurodegenerative disease mechanisms can be highlighted by accumulating evidence for a role for a growth factor, progranulin (PGRN). PGRN is a glycoprotein encoded by the GRN/Grn gene with multiple cellular functions, including neurotrophic, anti-inflammatory and lysosome regulatory properties. Mutations in the GRN gene can lead to frontotemporal lobar degeneration (FTLD), a cause of dementia, and neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disease. Both diseases are associated with loss of PGRN function resulting, amongst other features, in enhanced microglial neuroinflammation and lysosomal dysfunction. PGRN has also been implicated in Alzheimer’s disease (AD). Unlike FTLD, increased expression of PGRN occurs in brains of human AD cases and AD model mice, particularly in activated microglia. How microglial PGRN might be involved in AD and other neurodegenerative diseases will be discussed. A unifying feature of PGRN in diseases might be its modulation of lysosomal function in neurons and microglia. Many experimental models have focused on consequences of PGRN gene deletion: however, possible outcomes of increasing PGRN on microglial inflammation and neurodegeneration will be discussed. We will also suggest directions for future studies on PGRN and microglia in relation to neurodegenerative diseases.

Tunable Hydrogels Derived from Genetically Engineered Extracellular Matrix Accelerate Diabetic Wound Healing

Hydrogels composed of solubilized decellularized extracellular matrix (ECM) are attractive materials because they combine the complexity of native ECM with injectability and ease of use. Nevertheless, these materials are typically only tunable by altering the concentration, which alters the ligand landscape, or by incorporating synthetic components, which can result in an unfavorable host response. Herein, we demonstrate the fabrication of genetically tunable ECM-derived materials, by utilizing wild type (WT) and (thrombospondin-2 knockout) TSP-2 KO decellularized skins to prepare hydrogels. The resulting materials exhibited distinct mechanical properties characterized by rheology and different concentrations of collagens when characterized by quantitative proteomics. Mixtures of the gels achieved intermediate effects between the WT and the KO, permitting tunability of the gel properties. In vivo, the hydrogels exhibited tunable cell invasion with a correlation between the content of TSP-2 KO hydrogel and the extent of cell invasion. Additionally, TSP-2 KO hydrogels significantly improved diabetic wound healing at 10 and 21 days. Furthermore, hydrogels derived from genetically engineered in vitro cell-derived matrix mimicked the trends observed for tissue-derived matrix, providing a platform for faster screening of novel manipulations and easier clinical translation. Overall, we demonstrate that genetic engineering approaches impart tunability to ECM-based hydrogels and can result in materials capable of enhanced regeneration.

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 Grn^fl/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.

RNA Binding Proteins and the Pathogenesis of Frontotemporal Lobar Degeneration

Frontotemporal dementia is a group of early onset dementia syndromes linked to underlying frontotemporal lobar degeneration (FTLD) pathology that can be classified based on the formation of abnormal protein aggregates involving tau and two RNA binding proteins, TDP-43 and FUS. Although elucidation of the mechanisms leading to FTLD pathology is in progress, recent advances in genetics and neuropathology indicate that a majority of FTLD cases with proteinopathy involving RNA binding proteins show highly congruent genotype-phenotype correlations. Specifically, recent studies have uncovered the unique properties of the low-complexity domains in RNA binding proteins that can facilitate liquid-liquid phase separation in the formation of membraneless organelles. Furthermore, there is compelling evidence that mutations in FTLD genes lead to dysfunction in diverse cellular pathways that converge on the endolysosomal pathway, autophagy, and neuroinflammation. Together, these results provide key mechanistic insights into the pathogenesis and potential therapeutic targets of FTLD.

Early lysosomal maturation deficits in microglia triggers enhanced lysosomal activity in other brain cells of progranulin knockout mice

Background

Heterozygous loss-of-function mutations in the progranulin gene (GRN) lead to frontotemporal lobar degeneration (FTLD) while the complete loss of progranulin (PGRN) function results in neuronal ceroid lipofuscinosis (NCL), a lysosomal storage disease. Thus the growth factor-like protein PGRN may play an important role in lysosomal degradation. In line with a potential lysosomal function, PGRN is partially localized and processed in lysosomes. In the central nervous system (CNS), PGRN is like other lysosomal proteins highly expressed in microglia, further supporting an important role in protein degradation. We have previously reported that cathepsin (Cat) D is elevated in GRN-associated FTLD patients and Grn knockout mice. However, the primary mechanism that causes impaired protein degradation and elevated CatD levels upon PGRN deficiency in NCL and FTLD remains unclear.

Methods

mRNA expression analysis of selected lysosomal hydrolases, lysosomal membrane proteins and autophagy-related genes was performed by NanoString nCounter panel. Protein expression, maturation and in vitro activity of Cat D, B and L in mouse embryonic fibroblasts (MEF) and brains of Grn knockout mice were investigated. To selectively characterize microglial and non-microglial brain cells, an acutely isolated microglia fraction using MACS microbeads (Miltenyi Biotec) conjugated with CD11b antibody and a microglia-depleted fraction were analyzed for protein expression and maturation of selected cathepsins.

Results

We demonstrate that loss of PGRN results in enhanced expression, maturation and in vitro activity of Cat D, B and L in mouse embryonic fibroblasts and brain extracts of aged Grn knockout mice. Consistent with an overall enhanced expression and activity of lysosomal proteases in brain of Grn knockout mice, we observed an age-dependent transcriptional upregulation of certain lysosomal proteases. Thus, lysosomal dysfunction is not reflected by transcriptional downregulation of lysosomal proteases but rather by the upregulation of certain lysosomal proteases in an age-dependent manner. Surprisingly, cell specific analyses identified early lysosomal deficits in microglia before enhanced cathepsin levels could be detected in other brain cells, suggesting different functional consequences on lysosomal homeostasis in microglia and other brain cells upon lack of PGRN.

Conclusions

The present study uncovers early and selective lysosomal dysfunctions in Grn knockout microglia/macrophages. Dysregulated lysosomal homeostasis in microglia might trigger compensatory lysosomal changes in other brain cells.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0281-5) contains supplementary material, which is available to authorized users.

An Overview of the Role of Lipofuscin in Age-Related Neurodegeneration

Despite aging being by far the greatest risk factor for highly prevalent neurodegenerative disorders, the molecular underpinnings of age-related brain changes are still not well understood, particularly the transition from normal healthy brain aging to neuropathological aging. Aging is an extremely complex, multifactorial process involving the simultaneous interplay of several processes operating at many levels of the functional organization. The buildup of potentially toxic protein aggregates and their spreading through various brain regions has been identified as a major contributor to these pathologies. One of the most striking morphologic changes in neurons during normal aging is the accumulation of lipofuscin (LF) aggregates, as well as, neuromelanin pigments. LF is an autofluorescent lipopigment formed by lipids, metals and misfolded proteins, which is especially abundant in nerve cells, cardiac muscle cells and skin. Within the Central Nervous System (CNS), LF accumulates as aggregates, delineating a specific senescence pattern in both physiological and pathological states, altering neuronal cytoskeleton and cellular trafficking and metabolism, and being associated with neuronal loss, and glial proliferation and activation. Traditionally, the accumulation of LF in the CNS has been considered a secondary consequence of the aging process, being a mere bystander of the pathological buildup associated with different neurodegenerative disorders. Here, we discuss recent evidence suggesting the possibility that LF aggregates may have an active role in neurodegeneration. We argue that LF is a relevant effector of aging that represents a risk factor or driver for neurodegenerative disorders.

The lysosomal function of progranulin, a guardian against neurodegeneration

Progranulin (PGRN), encoded by the GRN gene in humans, is a secreted growth factor implicated in a multitude of processes ranging from regulation of inflammation to wound healing and tumorigenesis. The clinical importance of PGRN became especially evident in 2006, when heterozygous mutations in the GRN gene, resulting in haploinsufficiency, were found to be one of the main causes of frontotemporal lobar degeneration (FTLD). FTLD is a clinically heterogenous disease that results in the progressive atrophy of the frontal and temporal lobes of the brain. Despite significant research, the exact function of PGRN and its mechanistic relationship to FTLD remain unclear. However, growing evidence suggests a role for PGRN in the lysosome-most striking being that homozygous GRN mutation leads to neuronal ceroid lipofuscinosis, a lysosomal storage disease. Since this discovery, several links between PGRN and the lysosome have been established, including the existence of two independent lysosomal trafficking pathways, intralysosomal processing of PGRN into discrete functional peptides, and direct and indirect regulation of lysosomal hydrolases. Here, we summarize the cellular functions of PGRN, its roles in the nervous system, and its link to multiple neurodegenerative diseases, with a particular focus dedicated to recent lysosome-related mechanistic developments.

Partial Tmem106b reduction does not correct abnormalities due to progranulin haploinsufficiency

Background

Loss 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.

Methods

We 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.

Results

Partial 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.

Conclusions

These 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.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0264-6) contains supplementary material, which is available to authorized users.

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.

Autophagy and lysosomal pathways in nervous system disorders

Autophagy is an evolutionarily conserved pathway for delivering cytoplasmic cargo to lysosomes for degradation. In its classically studied form, autophagy is a stress response induced by starvation to recycle building blocks for essential cellular processes. In addition, autophagy maintains basal cellular homeostasis by degrading endogenous substrates such as cytoplasmic proteins, protein aggregates, damaged organelles, as well as exogenous substrates such as bacteria and viruses. Given their important role in homeostasis, autophagy and lysosomal machinery are genetically linked to multiple human disorders such as chronic inflammatory diseases, cardiomyopathies, cancer, and neurodegenerative diseases. Multiple targets within the autophagy and lysosomal pathways offer therapeutic opportunities to benefit patients with these disorders. Here, I will summarize the mechanisms of autophagy pathways, the evidence supporting a pathogenic role for disturbed autophagy and lysosomal degradation in nervous system disorders, and the therapeutic potential of autophagy modulators in the clinic.

Molecular regulations and therapeutic targets of Gaucher disease

Gaucher disease (GD) is the most common lysosomal storage disease caused by deficiency of beta-glucocerebrosidase (GCase) resulting in lysosomal accumulation of its glycolipid substrate glucosylceramide. The activity of GCase depends on many factors such as proper folding and lysosomal localization, which are influenced by mutations in GCase encoding gene, and regulated by various GCase-binding partners including Saposin C, progranulin and heat shock proteins. In addition, proinflammatory molecules also contribute to pathogenicity of GD. In this review, we summarize the molecules that are known to be important for the pathogenesis of GD, particularly those modulating GCase lysosomal appearance and activity. In addition, small molecules that inhibit inflammatory mediators, calcium ion channels and other factors associated with GD are also described. Discovery and characterization of novel molecules that impact GD are not only important for deciphering the pathogenic mechanisms of the disease, but they also provide new targets for drug development to treat the disease.

Genetic Modifiers in Neurodegeneration

PURPOSE OF REVIEW

To review the evidence for genetic modifier effects in the neurodegenerative diseases Huntington's Disease (HD), Frontotemporal Lobar Degeneration (FTLD), Alzheimer's Disease (AD), and Parkinson's Disease (PD).

RECENT FINDINGS

Increasingly, we understand human disease genetics less through the lens of single-locus/single-trait effects, and more through that of polygenic contributions to disease risk. In addition, specific examples of genetic modifier effects of the chromosome 7 gene TMEM106B on various target genes including those causal for Mendelian classes of FTLD - GRN and c9orf72 - have emerged from both genetic cohort studies and mechanistic examinations of biological pathways.

SUMMARY

Here, we summarize the literature reporting genetic modifier effects in HD, FTLD, AD, and PD. We further contextualize reported genetic modifier effects in these diseases in terms of insight they may lend to the concept of a polygenic landscape for the major neurodegenerative diseases.

Loss of Neuroprotective Factors in Neurodegenerative Dementias: The End or the Starting Point?

Recent clinical, genetic and biochemical experimental evidences highlight the existence of common molecular pathways underlying neurodegenerative diseases. In this review, we will explore a key common pathological mechanism, i.e., the loss of neuroprotective factors, across the three major neurodegenerative diseases leading to dementia: Alzheimer's disease (AD), Frontotemporal dementia (FTD) and Lewy body dementia (LBD). We will report evidences that the Brain Derived Neurotrophic Factor (BDNF), the most investigated and characterized brain neurotrophin, progranulin, a multi-functional adipokine with trophic and growth factor properties, and cystatin C, a neuroprotective growth factor, are reduced in AD, FTD, and LBD. Moreover, we will review the molecular mechanism underlying the loss of neuroprotective factors in neurodegenerative diseases leading to dementia, with a special focus on endo-lysosomal pathway and intercellular communication mediated by extracellular vesicles. Exploring the shared commonality of disease mechanisms is of pivotal importance to identify novel potential therapeutic targets and to develop treatments to delay, slow or block disease progression.

Modifiers of GRN-Associated Frontotemporal Lobar Degeneration

Heterozygous loss-of-function (LOF) mutations in the human progranulin gene (GRN) cause frontotemporal lobar degeneration (FTLD) by a mechanism of haploinsufficiency. Patients present most frequently with frontotemporal dementia, which is the second most common neurodegenerative dementia at young age. Currently, no disease-modifying therapies are available for these patients. Stimulating GRN protein expression or inhibiting its breakdown is an obvious therapeutic strategy, and is indeed the focus of current preclinical research and clinical trials. Multiple studies have demonstrated the heterogeneity in clinical presentation and wide variability in age of onset in patients carrying a GRN LOF mutation. Recently, this heterogeneity became an opportunity to identify disease modifiers, considering that these might constitute suitable targets for developing disease-modifying or disease-delaying therapies.