Author:

Abstract:

EphA4 is a tyrosine kinase receptor of the ephrin system, which is highly expressed in the nervous system. During development of the nervous system, EphA4 mediates axon repulsion during the formation of the corticospinal tract and the innervation pattern of the hindlimbs. In amyotrophic lateral sclerosis (ALS), a motor neuron degenerative disorder, EphA4 has been identified as an interesting disease modifier in zebrafish models, rodent models and in patients. Inhibition of EphA4 signaling slowed down disease onset and/or progression, and improved motor function in rodent models for ALS by reducing the vulnerability of motor neurons and enhancing the neuromuscular junction innervation. The latter is most likely caused by increased sprouting and re-innervation capacity of motor axons upon EphA4 loss. Interestingly, knockdown of EphA4 also rescued the axonal deficits in a zebrafish model for spinal muscular atrophy (SMA), suggesting that the neuroprotective effect of EphA4 inhibition could translate to other motor neuron diseases. SMA is a neurodegenerative disorder characterized by the degeneration of motor neurons of the spinal cord, resulting in hypotonia, progressive muscle weakness and atrophy, and in the most severe cases paralysis, respiratory failure and death. The diseases segregates in an autosomal recessive manner and is caused by reduced levels of survival of motor neuron protein (SMN) resulting from deletions or loss-of-function mutations in the SMN1 gene. Nusinersen and Zolgensma are the only approved therapies and both increase the production of functional SMN protein. However, they do not cure the disease and disease outcomes remain variable. Therefore, non-SMN therapies could provide additional support to patients, or could be of importance for patients that are intolerant, not responsive to or excluded from SMN-targeting strategies. In the first part of this doctoral thesis, we investigated whether the modifying potential of EphA4 in a zebrafish model could translate to a mouse model for SMA. We heterozygously deleted EphA4 in the SMNΔ7 mouse model for severe SMA and showed that this strategy did not ameliorate disease progression. Although these results hint towards a limited therapeutic potential for EphA4, the severe clinical phenotype and fast clinical progression in SMNΔ7 mice might impede beneficial effects of EphA4 loss and further studies need to clarify whether EphA4 might still be an interesting target in mouse models of milder forms of the disease or in combination therapy. In the adult nervous system, EphA4 is highly expressed in high-plasticity regions, such as the cortex and hippocampus, where it is a critical mediator of synapse morphology, functionality and plasticity. In addition, EphA4 modulates neuroinflammation as shown in mouse models for spinal cord injury (SCI). Hence, EphA4 is an interesting target for diseases characterized by synaptic dysfunction and neuroinflammation such as Alzheimer's disease (AD). Interestingly, inhibition of EphA4 signaling rescues Amyloid-β (Aβ)-induced dendritic spine loss and long-term potentiation deficits in vitro. AD is a neurodegenerative disorder, representing the underlying cause of 60-70% of the dementia cases. AD is characterized by progressive memory loss, together with multiple cognitive impairments that compromise the quality of life for patients and their relatives. The key pathological features of AD are the extracellular amyloid depositions of the Aβ peptide into senile plaques and the intraneuronal aggregates of misfolded hyperphosphorylated tau protein. In addition, several neuroinflammatory alterations are present, including changes in astrocyte and microglial reactivity, which appear to be important drivers of disease pathophysiology. Synapse dysfunction and loss occur early in AD and are suggested to be the underlying cause of cognitive deficits in patients. Current treatments are the acetylcholinesterase inhibitors and an NMDA receptor antagonist, which are symptomatic and aim to enhance cognitive function in patients. Unfortunately the efficacy of these drugs is limited. As the socioeconomic burden of the disease is high and still increasing, the need for disease-modifying therapies is urging. Several research strategies exist, designed to ameliorate Aβ and tau pathology, but therapies focused on neuroinflammation and synapse dysfunction are also of interest. In the second part of this doctoral thesis, we studied whether the beneficial effects of EphA4 reduction on Aβ-induced spine pathology and on neuroinflammation, would translate to a mouse model for AD, and improve cognitive function. In order to do so, we profoundly reduced EphA4 levels in the forebrain of the APPPS1 mouse model for AD. Our work demonstrates that loss of EphA4 selectively improves social memory, in association with alterations in spine morphology and microglial phenotype. However, further studies are needed to clarify how these changes contribute to the observed social memory improvement and whether these results are of interest for the development of therapeutic strategies. In conclusion, we explored the modifying potential of EphA4 in two neurodegenerative disorders, SMA and AD. We have shown that loss of EphA4 did not improve the disease in a mouse model for severe SMA, while a similar strategy could partially improve cognitive function in a mouse model for AD.