The Laboratory of Neurobiology focuses on the mechanisms of acute and chronic axonal and neuronal degeneration and regeneration, aiming to contribute to the development of new therapeutic strategies for neurodegenerative disorders. We intensively study motor neuron diseases (amyotrophic lateral sclerosis (ALS) and hereditary motor neuropathies), frontotemporal dementia (FTD) and stroke.
ALS & FTD
Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are 2 related neurodegenerative disorders. Degeneration of motor neurons causes progressive muscle weakness in ALS, loss of cortical neurons in the frontal and anterior temporal lobes causes behavioral and cognitive impairments. ALS and FTD have overlapping molecular causes and disease mechanisms. In the lab, we aim to gain insight in the disease mechanisms of neurodegeneration using a variety of disease models.
Neurodegenerative disorders within the spectrum of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) belong to the most devastating disorders known to mankind.
ALS is a neurodegenerative disorder characterized by a rather selective motor neuron death, resulting in a progressive paralysis and death of patients 2 to 5 years after disease onset (Cleveland and Rothstein, 2001). FTD is a dementia syndrome characterized by early changes in language, personality or behaviour with marked degeneration in the prefrontal and anterior temporal cortex (Neary et al., 2005). No effective treatments exist. Both neurodegenerative disorders are familial in some patients (~10% of ALS and 20-40% of FTLD cases, mostly dominantly inherited), but in the majority of cases there is no familial history (so called ‘sporadic cases’).
The most common genetic causes of ALS are: mutations in C9orf72 (DeJesus-Hernandez et al., 2011; Renton et al., 2011), in superoxide dismutase 1 (SOD1) (Rosen et al., 1993), and in the genes of the DNA/RNA binding proteins TDP-43 (TARDBP) (Kabashi et al., 2008; Sreedharan et al., 2008) and FUS (Kwiatkowski et al., 2009; Vance et al., 2009). Mutations in the tau (MAPT) (Hutton et al., 1998), progranulin (GRN) (Baker et al., 2006; Cruts et al., 2006) and C9orf72 (DeJesus-Hernandez et al., 2011; Renton et al., 2011) gene are the most common causes of FTD. The last few years, several additional ALS-FTD genes have been discovered. Although most of them are not frequently encountered in patients, they hint at important disease pathways. At the neuropathological level, protein aggregates containing TDP-43, ubiquitin and p62 are observed in the majority of ALS patients and in about 50% of FTD patients.
In the figure, an overview of the most important ALS-FTD disease genes (panel A) is given and how the cluster in potential disease pathways. Panel B depicts the most common protein inclusions found in neurons in post-mortem studies.
Our research activities aim to identify modifiers of the disease and to better understand the mechanisms of neurodegeneration in ALS and FTD using a variety of disease models. For different forms of hereditary ALS or FTD we have previously generated small animal models using zebrafish or Drosophila. Unbiased approaches in small animal models are used as hypothesis generating models. Identified modifiers are validated in small animal, cellular and mouse models, as well as in patient-specific iPSC-derived neuronal cultures or other patient-derived materials. Targets currently studied in different forms of ALS and FTLD include the Ephrin receptor EphA4, the elongator protein ELP3, the histone deacetylase HDAC6, and the growth factor progranulin. This translational approach aims to identify and develop novel therapeutics targets for these at present incurable diseases.
· Philip Van Damme (= webfiche person)
· Ludo Van Den Bosch (= webfiche person)
· Wim Robberecht (= webfiche person)
· Drosophila models in ALS
ALS patients show a high variability in disease onset and progression. The presence of this heterogeneity in families with a monogenetic disease cause, suggests the presence of modifying genes. The identification of genetic modifiers will have a great impact on our understanding of the pathobiological mechanism(s) leading to motor neuron death and will give us the opportunity to develop novel therapeutic strategies. With the full genome of Drosophila being annotated and the availability of genetic tools, whole genome screens become very attractive to identify disease-modifying genes. This unbiased approach was started from large chromosomal deletions. As starting from these large deletions harbors the risk of missing modifying genes, a second candidate-based approach was applied. Currently the most promising targets are being validated.
Due to excellent working tools available, the fast generation time and presence of fly homologs for most of the genes implicated in familial forms of ALS, we can use the fruit fly as a powerful tool to model newly discovered disease-related genes. Recently, next-generation sequencing has led to the discovery of novel ALS-related genes. The function of these disease-related genes is still unknown and highly promising to investigate. Identifying their role in the ALS pathology can elucidate new potentially perturbed pathways and targets for the development of therapeutics. We will use Drosophila to investigate the pathogenicity of these disease-related genes and develop disease models that can be used in screening approaches established in our lab. Currently, we are developing disease models for NEK1 and C21ORF2. In the future, we will also include other identified risk genes.
EphA4 signalling in neurodegeneration and ALS
Amyotrophic lateral sclerosis is characterized by genetic and clinical heterogeneity. Family members with the same mutation in the same gene can progress very differently with the disease. This indicates that factors, environmental or genetic, may influence the phenotypic expression of the disease, and these may represent targets for therapeutic intervention. In order to screen for such genetic modifying factors, a zebrafish model for ALS was stablished (Lemmens et al., 2007). EphA4 was one of the candidate genes that were found in a screen performed in this zebrafish model (Van Hoecke et al., 2012). EphA4 is a tyrosine kinase receptor of the Eph-ephrin system that is involved in many different processes during development and adulthood. Within the nervous system, it plays an important role in neurogenesis and neuronal migration, axon guidance, and synaptic plasticity among other functions. Interestingly, genetic and pharmacological inhibition of EphA4 could not only rescue the motor neuron phenotype in zebrafish but also in rodent models of ALS. In addition, an inverse correlation between EphA4 expression and disease onset and survival was found in patients, indicating that EphA4 is an ALS disease modifier.
Current studies in the lab focus in understanding the mechanism by which EphA4 is a disease modifier and whether this modifying effect can be translated to other neurodegenerative diseases. Finally, we have developed nanobodies against EphA4 and we are assessing their potential as EphA4 antagonists (Schoonaert et al., 2017).
C9orf72 in ALS
A mutation in the C9orf72 gene is the most frequent cause of ALS. The mutation is very unconventional since it constitutes a massive repeat expansion in a non-coding region of the gene. Three possible mechanisms may explain why this repeat expansion causes degeneration of motor neurons; loss-of-function of the C9orf72 protein, repeat RNA toxicity by disturbing the normal function of several RNA-binding proteins, DPR (dipeptide repeat) toxicity. The latter concerns peculiar peptides generated from the repeat sequence itself through unconventional translation. Several of these DPRs have been shown to induce neurodegeneration in in vivo models like Drosophila (Boeynaems et al., Sci Rep 2016) with a possible involvement of nucleocytoplasmic transport in this toxicity (Boeynaems et al., Sci Rep 2016). Recently, these DPRs have also been shown to undergo liquid-liquid phase separation and may perturb essential membrane-less organelles like stress granules (Boeynaems et al., Mol Cell 2017). Current studies in the lab focus on the potential involvement of repeat RNA toxicity, which is investigated in zebrafish models.
The role Histone Deacetylase 6 (HDAC6) in ALS
Dysfunction of the motor axon seems to be a crucial and initial event in pathology of ALS. Both in ALS patients and in the mutant SOD1 animals axonal transport defects are clearly present early in life, well before onset of clinical deficits. Histone deacetylase 6 (HDAC6) is member of the class IIb HDACs and it regulates axonal transport by deacetylating α-tubulin, hence it could play a role in the pathogenesis of ALS. Inhibition of HDAC6 can reverse the axonal transport deficits induced by mutant proteins and we have shown that deletion of HDAC6 prolongs the survival of the SOD1G93A mouse model of ALS (Taes et al. 2013). The protective effect was associated with increased α-tubulin acetylation and the innervation at neuromuscular junctions was improved in the absence of HDAC6. Currently, we pursue to develop a new therapeutic strategy based on HDAC6 inhibitors that could slow down the disease progression in ALS.
The role of oligodendrocytes in ALS pathogenesis
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder defined by axonal retraction accompanied by selective upper and lower motor neuron degeneration. Although traditionally viewed as a motor neuron disease, damage developed within nonneuronal supporting cells is crucial to motor neuron dysfunction in ALS. In this regard oligodendrocytes have recently been implicated in this non-cell autonomous nature of the disorder (Philips et al., 2013).
A cycle of oligodendrocyte death and replacement precedes the onset of motor neuron death and symptoms in ALS mice. Despite there being no alteration in oligodendrocyte number, newly formed oligodendrocytes appear to be immature and dysfunctional. Both ALS patients and mutant SOD1 mice display reduced myelin basic protein (MBP) and monocarboxylate transporter-1 (MCT-1) expression, consistent with impaired oligodendrocyte differentiation.
All together motor neurons putatively lose an important source of both structural and trophic support which is normally provided by well-functioning oligodendrocytes, potentially contributing to axonal injury and motor neuron loss. Using rodent models we are therefore currently trying to further understand oligodendrocyte involvement in ALS disease onset and progression.
The role of progranulin in neurodegeneration
Heterozygous loss-of-function (LOF) mutations in the progranulin (GRN) gene are a common cause of FTD (Baker et al., 2006; Cruts et al., 2006) and give rise to ~50% reduced GRN levels, indicating that haploinsufficiency is the underlying disease mechanism (Finch et al., 2009; Ghidoni et al., 2008; Sleegers et al., 2009; Van Damme et al., 2008). Homozygous LOF mutations cause neuronal ceroid lupofuscinosis, a rare form of neurodegeneration characterized by storage of abnormal lipopigment in lysosomes (Smith et al., 2012). GRN is a multifunctional growth factor involved in cell division, inflammation, lysosomal function, neuronal survival and neurite outgrowth, tumor growth and wound healing (De Muynck and Van Damme, 2011). GRN is heavily glycosylated and secreted. It consists of 7.5 granulin domains (paragranulin and granulin A-G). We previously showed that GRN has neurotrophic properties and can stimulate neurite outgrowth (Van Damme et al., 2008). More recently, the neurotrophic effect of GRN was linked to its lysosomal role, as chaperone for the lysosomal protease cathepsin D (Beel et al., 2017). Further studies focus on the functions of GRN in the CNS and the therapeutic potential of this pleiotropic growth factor.
Understanding disease mechanisms using patient-derived induced pluripotent stem cells (iPSCs)
In collaboration with the stem cell institute Leuven (SCIL, directed by Prof. Catherine Verfaillie) we develop stem cell models for various forms of ALS-FTD. Patient-derived skin biopsies from the ALS-FTD clinic are reprogrammed with the 4 Yamanaka factors using a Sendai viral transduction. The cell lines are fully characterized for pluripotency, genetic integrity and their potential to differentiate into mature motor neurons and cortical neurons. Disease-specific phenotypes are studied and will be used to unravel disease mechanisms.
This international genetics consortium aims to unravel the genetic causes and the genetic modifiers of ALS using whole genome sequencing. To understand the genetic basis of ALS and to ultimately find a cure for this devastating, fatal neuromuscular disease, Project MinE aims to analyse the DNA of at least 15,000 ALS patients and 7,500 control subjects. The resulting 22,500 DNA profiles will be compared (www.projectmine.com).