Pathogenic mechanisms of frontotemporal dementia: Measuring Activity of Dendritic CHMP2B. – MAD CHMP2B

Frontotemporal Dementia (FTD) is the most prevailing type of dementia before age 64, and second only to Alzheimer's disease in older individuals. FTD is an extremely disabling, irreversible disease; the early age of onset (average 55+/-10 years) adds to the burden. FTD arises from degeneration of cortical and occasionally hippocampal neurones. The clinical, pathological and genetic features of FTD cases are heterogeneous, and the cellular processes that drive pathogenesis are poorly understood. The identification of CHMP2B as the disease-causing gene in one large family in 2005 has provided fresh insights into possible mechanisms of neurodegeneration. CHMP2B (Charged Multivesicular body protein 2B) belongs to a family of highly conserved proteins (CHMP 1-7) which together constitute ESCRT-III, one of the four Endosomal Sorting Complexes Required for Transport. ESCRT-III plays a pivotal role in the deformation and vesiculation of cellular membranes, underlying the transit of down-regulated proteins into the internal lumen of degradative organelles, the budding of viruses at the surface of infected cells, and other cellular activities. CHMPs polymerize into 100-300 nm wide helical filaments that are thought to impose an outward-pointing tubular shape to lipid membranes. The polymerization of CHMPs is stringently regulated by their C-terminal domain; FTD-linked mutations cause the loss of this domain and generate CHMP2B variants with aberrant polymerizing potential. We have shown that when expressed at low level in cultured hippocampal neurones, CHMP2B mutants induce an impairment in the morphological maturation of dendritic spines, as well as a detectable loss of excitatory synaptic currents, without apparent degeneration. As depletion of endogenous CHMP2B by siRNA caused similar effects, pathogenic mutants may interfere with a process in which normal CHMP2B promotes spine growth or maintenance. Our recent data show that CHMP2B is unique among CHMPs in its ability to spontaneously form helical polymers that cause the plasma membrane to grow long, rigid protrusions. These CHMP2B polymers recruit other CHMPs at their base and also induce rearrangements of the actin cytoskeleton. Moreover, we find that CHMP2B and selected members of ESCRT-III associate with the post-synaptic density of excitatory synapses. We hypothesize that CHMP2B and ESCRT-III polymers are previously unsuspected factors of dendritic spine structuring, and that CHMP2B mutants block this function, resulting in synaptic defects with long-term consequences for neuronal survival. In the present project, we pursue this line of research by exploring the precise role of CHMP2B and the impact of disease-linked mutants in synaptic processes. Our aims are: 1.To examine whether and how CHMP2B polymers cooperate with regulators of actin dynamics and membrane curvature, such as those known to exist in dendritic spines, for promoting or stabilizing membrane protrusions. 2. To identify post-synaptic scaffold proteins that may interact with CHMP2B and other ESCRT-III components, and study the outcome of these interactions. 3. To confirm that CHMP2B participates in active ESCRT-III assembly at synapses, examine how this may depend on neuronal activity patterns, and how synaptic ESCRT-III may be perturbed by pathogenic CHMP2B mutants. 4. To determine the role of normal ESCRT-III and the effects of pathogenic CHMP2B variants on rapid, stimulus-dependent structural changes in filipodia and spines.5.To characterize the effects of normal and mutant CHMP2B on synaptic transmission and plasticity. The study of CHMP2B and its partners at synapses is likely to reveal novel facets of both synaptic biology and the pathological process that drives FTD.