Disturbed neuronal connectivity is a hallmark of many neural disorders. For example, abnormal wiring of the brain during development is believed to contribute to pathophysiology of disorders such as schizophrenia and autism, while neurodegenerative disorders including Parkinson's disease and ALS are characterized by a marked loss of neuronal connections. To better understand and treat these situations of perturbed neuronal connectivity further insight is needed into the mechanisms that normally control nervous system wiring. The aim of our research is twofold: 1) to determine how neuronal circuits are formed during development at the molecular, cellular and systems level, and 2) how and why neurons and their connections are changed or lost in neurological (epilepsy) and neurodegenerative (ALS) disorders.
Molecular mechanisms of axon guidance
One of the most challenging problems in biology is to understand how the billions of neurons in the mammalian nervous system “wire up” to form functional neural circuits that underlie all behavior. This has been one of the most intensely studied areas of developmental neurobiology in the past decade, and a number of important proteins have been identified that instruct axons to project to their specific target regions (so called axon guidance proteins).
To further unveil the cellular functions of axon guidance proteins we have chosen axonal connections between dopamine neurons in the midbrain and medium spiny neurons in the striatum as our main model system. Thus far, we identified axon guidance cues that are required for the bundling, rostral growth, intermediate target navigation and intracortical targeting of dopaminergic axons. We are currently generating new mouse models to genetically label, purify, activate or ablate functionally similar subsets of neurons in the dopamine system and midbrain. This will help us to study novel aspects of striatonigral and nigrostriatal pathway development such as axonal pruning, topographic bundle organization and axonal sorting. As a second model we study axonal connectivity in the hippocampus, both during development and in adulthood. The hippocampus is one of the few regions of the adult brain were neurogenesis takes place and we are interested in the molecular cues that help to integrate new neurons in an existing network.
Axon guidance proteins are detected by a highly motile and sensitive structure at the leading tip of every growing axon, the growth cone. Receptor complexes at the growth cone cell surface detect axon guidance proteins and consequently trigger intracellular signal transduction cascades that infringe upon the cytoskeleton and induce growth cone/axon steering. Several projects in the lab focus on revealing novel signaling and regulatory mechanisms in axon guidance using cutting-edge technology such as LNA array technology, high-throughput and live cell microscopy, in utero electroporation, functional proteomics approaches, mouse genetics, FACS, biochemistry, and molecular cell biology. Examples of this research include the identification of proteins that can terminate axon guidance receptor activity or microRNAs that regulate their expression. This work on intracellular signalling mainly focuses on semaphorins, the largest family of axon guidance proteins.
Pathogenic mechanisms in ALS
Amyotrophic lateral sclerosis (ALS) is a devastating neurological disease affecting some 50,000 individuals at any time in Europe, and causing around 10,000 deaths each year. ALS is characterized by progressive degeneration of motor neurons in brain and spinal cord leading to muscle weakness. ALS can occur in any individual at anytime in adulthood. Initial manifestions are weakness of limbs, or weakness in the bulbar region leading to abnormalities of speech, swallowing difficulties and facial weakness. The patient becomes paralyzed and dies as the result of respiratory failure. The median survival is 3 years after onset of symptoms. There is no cure for ALS. The only available drug (Riluzole) is marginally effective in extending the lifespan of ALS patients with 3 to 6 months. Novel treatment options are needed for this disabling and fatal disease. The lack of treatment in ALS can be attributed to an absence of validated therapeutic targets reflecting inadequate understanding of disease mechanisms. Elucidating the ALS pathogenic mechanism is therefore essential to pave the road for therapeutic interventions. Several projects in the lab focus on further unravelling the molecular mechanisms which are disturbed in familial and sporadic ALS with a strong focus on ALS-associated proteins implicated in RNA processing such as FUS or TDP-43 or in synaptic function such as Munc13-1. Our main focus is to investigate how gene defects and subsequent protein aggregation contribute to changes in neuronal connectivity during early stages of the disease and how we can reverse these changes. An important tool here is the use of patient-derived IPSC-generated motor neurons. We run a fully equipped IPSC facility to generate, analyse and manipulate (CRISPR/TALEN) IPSC neurons. The molecular basis of ALS pathology is studied in collaboration with the Dept. Neurology, UMC Utrecht.