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Dendrites have a unique microtubule organization. In vertebrates, dendritic microtubules are organized in antiparallel bundles, oriented with their plus ends either pointing away or toward the soma. The mixed microtubule arrays control intracellular trafficking and local signaling pathways, and are essential for dendrite development and function. The organization of microtubule arrays largely depends on the combined function of different microtubule regulatory factors or generally named microtubule-associated proteins (MAPs). Classical MAPs, also called structural MAPs, were identified more than 20 years ago based on their ability to bind to and copurify with microtubules. Most classical MAPs bind along the microtubule lattice and regulate microtubule polymerization, bundling, and stabilization. Recent evidences suggest that classical MAPs also guide motor protein transport, interact with the actin cytoskeleton, and act in various neuronal signaling networks. Here, we give an overview of microtubule organization in dendrites and the role of classical MAPs in dendrite development, dendritic spine formation, and synaptic plasticity.

The accumulation of cellular damage, including DNA damage, is hypothesized to contribute to aging-related neurodegenerative changes. DNA excision repair cross-complementing group 1 (Ercc1) knock-out mice represent an accepted model of neuronal aging, showing gradual neurodegenerative changes, including loss of synaptic contacts and cell body shrinkage. Here, we used the Purkinje cell-specific Ercc1 DNA-repair knock-out mouse model to study aging in the mouse cerebellum. We performed an in-depth quantitative proteomics analysis, using stable isotope dimethyl labeling, to decipher changes in protein expression between the early (8 weeks), intermediate (16 weeks), and late (26 weeks) stages of the phenotypically aging Ercc1 knock-out and healthy littermate control mice. The expression of over 5,200 proteins from the cerebellum was compared quantitatively, whereby 79 proteins (i.e. 1.5%) were found to be substantially regulated during aging. Nearly all of these molecular markers of the early aging onset belonged to a strongly interconnected network involved in excitatory synaptic signaling. Using immunohistological staining, we obtained temporal and spatial profiles of these markers confirming not only the proteomics data but in addition revealed how the change in protein expression correlates to synaptic changes in the cerebellum. In summary, this study provides a highly comprehensive spatial and temporal view of the dynamic changes in the cerebellum and Purkinje cell signaling in particular, indicating that synapse signaling is one of the first processes to be affected in this premature aging model, leading to neuron morphological changes, neuron degeneration, inflammation, and ultimately behavior disorders.

The human brain is the most complicated organ, that processes the information from the sensory organs, forms memories and controls motor functions. Neurons are the fundamental units of the brain. These highly specialized cells are connected via synapses. There are 100 billion neurons in the human brain that form the enormous complexity. To understand how the brain works, it is essential to study the fundamental units, the neurons. Neurons are highly polarized cells and critically rely on their cytoskeletal filaments to acquire and maintain their specialized morphology and functions. Microtubules are the major cytoskeletal components of neurons and they are important for neurodevelopment from differentiation, migration and development to synapses connection. They serve as tracks for long distance transport, support neuronal morphology, and control local signaling events. In neurons, microtubules are organized into a specialized architecture. Neurons express abundant microtubule-associated proteins (MAPs). It is becoming clear that MAPs are critical for the establishment and maintenance of the microtubule architecture. To date, our understanding on MAPs and their function is still quite limited, and it is what we aim to explore in this thesis.

The microtubule cytoskeleton is essential for nearly every stage of neurodevelopment, fromdifferentiation,migration, polarization, axon and dendrite development to synapse connections.During these processes, microtubules serves as ‘roads’ for motor proteins mediated cargotransport over long distances. As the neuron from human PNS can have axons with a totallength ranging from 10 mm to 100 m, the microtubule based motors transport life sustainingmaterials such as organelles, proteins or mRNAs from the soma to the axon terminal. Neurons express abundant MAPs to regulate the microtubule cytoskeleton. MAPs were initially defined as proteins that bind to and stabilize microtubules. More recently it is becoming clear that MAPs can regulate the microtubule network in many other ways, which will be explored in this thesis. In chapter 2 we describe the emerging concept that regionally confined MAPs act as a “MAP code” regulating motor activity. We found that MAP7 family members MAP7/7D1 are specifically enriched in somatodendritic compartment of the neuron, whereas MAP7D2 localizes in the proximal axon and promotes Kinesin-1 mediated cargo transport into axons. In chapter 3 we discussed how the E3 ubiquitin ligase family members MID1 and MID2 associate with microtubule cytoskeleton and control axon and dendrite development. In chapter 4 we show how MAPs act as microtubule crosslinkers to form the proximal axon specific microtubule fascicles. In chapter 5 we discuss the cross-talk between TRIM46 induced parallel microtubules and the AnkG mediated submembrane domain coordinate to drive theAIS assembly to establish neuronal polarity. In chapter 6 we discuss the mechanisms that underlie the specific microtubule organization in axons; the “MAP code” that guide Kinesin-1 with cargos out of dendrites and into axons; the mechanisms of how MAPs are confined to different compartments of neurons and how MAPs regulate axon branching.