Marco B. Rust

Background: Parkinson disease (PD) is characterized by progressive neuronal loss within defined brain regions, accompanied by α-synuclein (αSyn)-rich inclusions, termed Lewy pathology (LP). However, it is unclear which cellular factors render certain neuronal populations vulnerable, while others stay devoid of LP throughout the course of disease. Methods: This study aimed to identify and compare the cellular architecture of vulnerable and non-vulnerable neurons exposed to αSyn pathology by using a projection-based retro-AAV approach in combination with an in vivo α-synucleinopathy mouse model. To do so, a set of viral genetic, immunohistochemical, and optical tools was used in combination with the preformed αSyn fibril (PFF) model. Results: αSyn pathology propagated robustly into the input connectome of the pedunculopontine nucleus (PPN). However, we observed a marked mismatch between the anatomically expected and the actual distribution of pathology. While anatomically connected neurons in the bed nucleus of the stria terminalis (BST) and the central amygdala (CEA) accumulated substantial αSyn pathology, equally strong connected neurons of the substantia nigra pars reticulata (SNr), and the dentate nucleus (DN) were devoid of pathology. Second, cellular vulnerability and resilience were consistent and reproducible features. When PFFs were injected into alternative major output projection sites of BST, CEA, SNr, and DN, we observed similar patterns of αSyn accumulation. Third, projection-specific axonal mapping revealed that the αSyn-accumulating BST and CEA neurons possessed larger axonal arbors than the more resilient neurons in SNr and DN. Correspondingly, neurons in BST and CEA exhibited higher basal mitochondrial oxidation levels, indicating an increased bioenergetic burden. Finally, the site of initial seeding significantly influenced the extent of developing brain-wide pathology, suggesting that certain brain regions may function as "super-seeders", promoting widespread propagation of pathology, while others contribute relatively little to the global LP burden. Conclusions: αSyn pathology propagates along anatomical pathways, but cell-autonomous factors determine if a neuron exposed to misfolded αSyn will develop Lewy-like pathology or not.

Dendritic spines are the postsynaptic compartment of most excitatory synapses in the vertebrate brain. Their morphology is defined by a complex actin scaffold consisting of branched and unbranched actin filaments (F-actin), which constitute the major structural component of dendritic spines. During brain development, dendritic spines arise from dendritic filopodia, motile finger-like dendritic protrusions, whose morphology is also defined by an actin scaffold. The organization of the actin scaffold as well as its dynamic behavior in both dendritic filopodia and dendritic spines requires the coordinated activity of actin binding proteins (ABP) that promote either assembly or disassembly of F-actin. Studies of the past two decades identified a number of ABP and upstream regulatory pathways that control the morphology of dendritic spines as well as their morphological changes associated with synaptic plasticity, the cellular basis for learning and memory. Instead, much less is known about actin regulatory mechanisms that control the formation and elongation of dendritic filopodia or the structural changes associated with their transition into dendritic spines. This review article highlights recent advances in the field by summarizing and discussing studies of the past few years that provided exciting novel insights into the molecular machinery that governs dendritic filopodia initiation and their maturation into dendritic spines.

The morphology of dendritic spines, the postsynaptic compartment of most excitatory synapses, decisively modulates the function of neuronal circuits as also evident from human brain disorders associated with altered spine density or morphology. Actin filaments (F-actin) form the backbone of spines, and a number of actin-binding proteins (ABP) have been implicated in shaping the cytoskeleton in mature spines. Instead, only little is known about the mechanisms that control the reorganization from unbranched F-actin of immature spines to the complex, highly branched cytoskeleton of mature spines. Here, we demonstrate impaired spine maturation in hippocampal neurons upon genetic inactivation of cyclase-associated protein 1 (CAP1) and CAP2, but not of CAP1 or CAP2 alone. We found a similar spine maturation defect upon overactivation of inverted formin 2 (INF2), a nucleator of unbranched F-actin with hitherto unknown synaptic function. While INF2 overactivation failed in altering spine density or morphology in CAP-deficient neurons, INF2 inactivation largely rescued their spine defects. From our data we conclude that CAPs inhibit INF2 to induce spine maturation. Since we previously showed that CAPs promote cofilin1-mediated cytoskeletal remodeling in mature spines, we identified them as a molecular switch that control transition from filopodia-like to mature spines.

Serum response factor (SRF) is an essential transcription factor for brain development and function. Here, we explored how an SRF cofactor, the actin monomer-sensing myocardin-related transcription factor MRTF, is regulated in mouse cortical neurons. We found that MRTF-dependent SRF activity in vitro and in vivo was repressed by cyclase-associated protein CAP1. Inactivation of the actin-binding protein CAP1 reduced the amount of actin monomers in the cytoplasm, which promoted nuclear MRTF translocation and MRTF-SRF activation. This function was independent of cofilin1 and actin-depolymerizing factor, and CAP1 loss of function in cortical neurons was not compensated by endogenous CAP2. Transcriptomic and proteomic analyses of cerebral cortex lysates from wild-type and Cap1 knockout mice supported the role of CAP1 in repressing MRTF-SRF–dependent signaling in vivo. Bioinformatic analysis identified likely MRTF-SRF target genes, which aligned with the transcriptomic and proteomic results. Together with our previous studies that implicated CAP1 in axonal growth cone function as well as the morphology and plasticity of excitatory synapses, our findings establish CAP1 as a crucial actin regulator in the brain relevant for formation of neuronal networks.

Serum response factor (SRF) is a ubiquitously expressed transcription factor essential for brain development and function. SRF activity is controlled by two competing classes of coactivators, myocardin-related transcription factors (MRTF) and ternary complex factors (TCF), which introduce specificity into gene expression programs. To date, only few brain studies investigated upstream regulatory mechanisms, which mainly focused on TCF. Since an inhibitory function of monomeric actin towards MRTF-SRF signaling is well-established, we hypothesized a regulatory role for the key actin regulator ADF/cofilin. Surprisingly, ADF/cofilin was largely dispensable for neuronal MRTF-SRF activity. Instead, reporter assays combined with pharmacological and genetic approaches in isolated mouse neurons identified cyclase-associated protein 1 (CAP1) as an important regulator of this pathway. CAP1 promotes cytosolic MRTF retention and represses neuronal MRTF-SRF signaling via an actin-dependent mechanism that requires two specific protein domains. Further, deep RNA sequencing and mass spectrometry in mutant mice proved CAP1’s in vivo relevance for this pathway in the cerebral cortex, and led to the identification of neuronal MRTF-SRF target genes. Together, we identified CAP1 as a novel and crucial repressor of neuronal MRTF-SRF signaling.

The vast majority of excitatory synapses are formed on small dendritic protrusions termed dendritic spines. Dendritic spines vary in size and density that are crucial determinants of excitatory synaptic transmission. Aberrations in spine morphogenesis can compromise brain function and have been associated with neuropsychiatric disorders. Actin filaments (F-actin) are the major structural component of dendritic spines, and therefore, actin-binding proteins (ABP) that control F-actin dis-/assembly moved into the focus as critical regulators of brain function. Studies of the past decade identified the ABP cofilin1 as a key regulator of spine morphology, synaptic transmission, and behavior, and they emphasized the necessity for a tight control of cofilin1 to ensure proper brain function. Here, we report spine enrichment of cyclase-associated protein 1 (CAP1), a conserved multidomain protein with largely unknown physiological functions. Super-resolution microscopy and live cell imaging of CAP1-deficient hippocampal neurons revealed impaired synaptic F-actin organization and dynamics associated with alterations in spine morphology. Mechanistically, we found that CAP1 cooperates with cofilin1 in spines and that its helical folded domain is relevant for this interaction. Moreover, our data proved functional interdependence of CAP1 and cofilin1 in control of spine morphology. In summary, we identified CAP1 as a novel regulator of the postsynaptic actin cytoskeleton that is essential for synaptic cofilin1 activity.