Alzheimer's disease-related presenilins are key to intestinal epithelial cell function and gut immune homoeostasis.

OBJECTIVE: Mutations in presenilin genes are the major cause of Alzheimer's disease. However, little is known about their expression and function in the gut. In this study, we identify the presenilins Psen1 and Psen2 as key molecules that maintain intestinal homoeostasis. DESIGN: Human inflammatory bowel disease (IBD) and control samples were analysed for Psen1 expression. Newly generated intestinal epithelium-specific Psen1-deficient, Psen2-deficient and inducible Psen1/Psen2 double-deficient mice were used to dissect the functional role of presenilins in intestinal homoeostasis. RESULTS: Psen1 expression was regulated in experimental gut inflammation and in patients with IBD. Induced deletion of Psen1 and Psen2 in mice caused rapid weight loss and spontaneous development of intestinal inflammation. Mice exhibited epithelial barrier disruption with bacterial translocation and deregulation of key pathways for nutrient uptake. Wasting disease was independent of gut inflammation and dysbiosis, as depletion of microbiota rescued Psen-deficient animals from spontaneous colitis development but not from weight loss. On a molecular level, intestinal epithelial cells lacking Psen showed impaired Notch signalling and dysregulated epithelial differentiation. CONCLUSION: Overall, our study provides evidence that Psen1 and Psen2 are important guardians of intestinal homoeostasis and future targets for barrier-promoting therapeutic strategies in IBD.

Xenografted human microglia display diverse transcriptomic states in response to Alzheimer's disease-related amyloid-ß pathology.

Microglia are central players in Alzheimer's disease pathology but analyzing microglial states in human brain samples is challenging due to genetic diversity, postmortem delay and admixture of pathologies. To circumvent these issues, here we generated 138,577 single-cell expression profiles of human stem cell-derived microglia xenotransplanted in the brain of the AppNL-G-F model of amyloid pathology and wild-type controls. Xenografted human microglia adopt a disease-associated profile similar to that seen in mouse microglia, but display a more pronounced human leukocyte antigen or HLA state, likely related to antigen presentation in response to amyloid plaques. The human microglial response also involves a pro-inflammatory cytokine/chemokine cytokine response microglia or CRM response to oligomeric Aß oligomers. Genetic deletion of TREM2 or APOE as well as APOE polymorphisms and TREM2R47H expression in the transplanted microglia modulate these responses differentially. The expression of other Alzheimer's disease risk genes is differentially regulated across the distinct cell states elicited in response to amyloid pathology. Thus, we have identified multiple transcriptomic cell states adopted by human microglia in a multipronged response to Alzheimer's disease-related pathology, which should be taken into account in translational studies.

Functional and topological analysis of PSENEN, the fourth subunit of the γ-secretase complex.

The γ-secretase complexes are intramembrane cleaving proteases involved in the generation of the Aß peptides in Alzheimer's disease. The complex consists of four subunits, with Presenilin harboring the catalytic site. Here, we study the role of the smallest subunit, PSENEN or Presenilin enhancer 2, encoded by the gene Psenen, in vivo and in vitro. We find a profound Notch deficiency phenotype in Psenen-/- embryos confirming the essential role of PSENEN in the γ-secretase complex. We used Psenen-/- fibroblasts to explore the structure-function of PSENEN by the scanning cysteine accessibility method. Glycine 22 and proline 27, which border the membrane domains 1 and 2 of PSENEN, are involved in complex formation and stabilization of γ-secretase. The hairpin structured hydrophobic membrane domains 1 and 2 are exposed to a water-containing cavity in the complex, while transmembrane domain 3 is not water exposed. We finally demonstrate the essential role of PSENEN for the cleavage activity of the complex. PSENEN is more than a structural component of the γ-secretase complex and might contribute to the catalytic mechanism of the enzyme.

The γ-secretase substrate proteome and its role in cell signaling regulation.

γ-Secretases mediate the regulated intramembrane proteolysis (RIP) of more than 150 integral membrane proteins. We developed an unbiased γ-secretase substrate identification (G-SECSI) method to study to what extent these proteins are processed in parallel. We demonstrate here parallel processing of at least 85 membrane proteins in human microglia in steady-state cell culture conditions. Pharmacological inhibition of γ-secretase caused substantial changes of human microglial transcriptomes, including the expression of genes related to the disease-associated microglia (DAM) response described in Alzheimer disease (AD). While the overall effects of γ-secretase deficiency on transcriptomic cell states remained limited in control conditions, exposure of mouse microglia to AD-inducing amyloid plaques strongly blocked their capacity to mount this putatively protective DAM cell state. We conclude that γ-secretase serves as a critical signaling hub integrating the effects of multiple extracellular stimuli into the overall transcriptome of the cell.

Discovery of brain permeable 2-Azabicyclo[2.2.2]octane sulfonamides as a novel class of presenilin-1 selective γ-secretase inhibitors.

This paper describes the rational design, synthesis, structure-activity relationship (SAR), and biological profile of presenilin-1 (PSEN-1) complex selective γ-secretase inhibitors, assessed for selectivity using a unique set of four γ-secretase subtype complexes. A set of known PSEN-1 selective γ-Secretase inhibitors (GSIs) was analyzed to understand the pharmacophoric features required for selective inhibition. Conformational modeling suggests that a characteristic 'U' shape orientation between aromatic sulfone/sulfonamide and aryl ring is crucial for PSEN-1 selectivity and potency. Using these insights, a series of brain-penetrant 2-azabicyclo[2,2,2]octane sulfonamides was devised and synthesized as a new class of PSEN-1 selective inhibitors. Compounds 13c and 13k displayed high potency towards PSEN1-APH1B complex but moderate selectivity towards PSEN2 complexes. However, compound (+)-13b displayed low nanomolar potency towards the PSEN1-APH1B complex, little (∼4-fold) selectivity towards PSEN1-APH1A, and high selectivity (>350-fold) versus PSEN2 complexes. Excellent brain penetration, no significant CYP inhibition, or cardiotoxicity, good solubility, and permeability make (+)-13b an excellent candidate for further lead optimization.

MEG3 activates necroptosis in human neuron xenografts modeling Alzheimer's disease.

Neuronal cell loss is a defining feature of Alzheimer's disease (AD), but the underlying mechanisms remain unclear. We xenografted human or mouse neurons into the brain of a mouse model of AD. Only human neurons displayed tangles, Gallyas silver staining, granulovacuolar neurodegeneration (GVD), phosphorylated tau blood biomarkers, and considerable neuronal cell loss. The long noncoding RNA MEG3 was strongly up-regulated in human neurons. This neuron-specific long noncoding RNA is also up-regulated in AD patients. MEG3 expression alone was sufficient to induce necroptosis in human neurons in vitro. Down-regulation of MEG3 and inhibition of necroptosis using pharmacological or genetic manipulation of receptor-interacting protein kinase 1 (RIPK1), RIPK3, or mixed lineage kinase domain-like protein (MLKL) rescued neuronal cell loss in xenografted human neurons. This model suggests potential therapeutic approaches for AD and reveals a human-specific vulnerability to AD.

Selective inhibitors of the PSEN1-gamma-secretase complex.

Clinical development of γ-secretases, a family of intramembrane cleaving proteases, as therapeutic targets for a variety of disorders including cancer and Alzheimer's disease was aborted because of serious mechanism-based side effects in the phase III trials of unselective inhibitors. Selective inhibition of specific γ-secretase complexes, containing either PSEN1 or PSEN2 as the catalytic subunit and APH1A or APH1B as supporting subunits, does provide a feasible therapeutic window in preclinical models of these disorders. We explore here the pharmacophoric features required for PSEN1 versus PSEN2 selective inhibition. We synthesized a series of brain penetrant 2-azabicyclo[2,2,2]octane sulfonamides and identified a compound with low nanomolar potency and high selectivity (>250-fold) toward the PSEN1-APH1B subcomplex versus PSEN2 subcomplexes. We used modeling and site-directed mutagenesis to identify critical amino acids along the entry part of this inhibitor into the catalytic site of PSEN1. Specific targeting one of the different γ-secretase complexes might provide safer drugs in the future.

VHHs as tools for therapeutic protein delivery to the central nervous system.

BACKGROUND: The blood brain barrier (BBB) limits the therapeutic perspective for central nervous system (CNS) disorders. Previously we found an anti-mouse transferrin receptor (TfR) VHH (Nb62) that was able to deliver a biologically active neuropeptide into the CNS in mice. Here, we aimed to test its potential to shuttle a therapeutic relevant cargo. Since this VHH could not recognize the human TfR and hence its translational potential is limited, we also aimed to find and validate an anti-human transferrin VHH to deliver a therapeutic cargo into the CNS. METHODS: Alpaca immunizations with human TfR, and subsequent phage selection and screening for human TfR binding VHHs was performed to find a human TfR specific VHH (Nb188). Its ability to cross the BBB was determined by fusing it to neurotensin, a neuropeptide that reduces body temperature when present in the CNS but is not able to cross the BBB on its own. Next, the anti-ß-secretase 1 (BACE1) 1A11 Fab and Nb62 or Nb188 were fused to an Fc domain to generate heterodimeric antibodies (1A11AM-Nb62 and 1A11AM-Nb188). These were then administered intravenously in wild-type mice and in mice in which the murine apical domain of the TfR was replaced by the human apical domain (hAPI KI). Pharmacokinetic and pharmacodynamic (PK/PD) studies were performed to assess the concentration of the heterodimeric antibodies in the brain over time and the ability to inhibit brain-specific BACE1 by analysing the brain levels of Aß1-40. RESULTS: Selections and screening of a phage library resulted in the discovery of an anti-human TfR VHH (Nb188). Fusion of Nb188 to neurotensin induced hypothermia after intravenous injections in hAPI KI mice. In addition, systemic administration 1A11AM-Nb62 and 1A11AM-Nb188 fusions were able to reduce Aß1-40 levels in the brain whereas 1A11AM fused to an irrelevant VHH did not. A PK/PD experiment showed that this effect could last for 3 days. CONCLUSION: We have discovered an anti-human TfR specific VHH that is able to reach the CNS when administered systemically. In addition, both the currently discovered anti-human TfR VHH and the previously identified mouse-specific anti-TfR VHH, are both able to shuttle a therapeutically relevant cargo into the CNS. We suggest the mouse-specific VHH as a valuable research tool in mice and the human-specific VHH as a moiety to enhance the delivery efficiency of therapeutics into the CNS in human patients.

The AppNL-G-F mouse retina is a site for preclinical Alzheimer's disease diagnosis and research.

In this study, we report the results of a comprehensive phenotyping of the retina of the AppNL-G-F mouse. We demonstrate that soluble Aß accumulation is present in the retina of these mice early in life and progresses to Aß plaque formation by midlife. This rising Aß burden coincides with local microglia reactivity, astrogliosis, and abnormalities in retinal vein morphology. Electrophysiological recordings revealed signs of neuronal dysfunction yet no overt neurodegeneration was observed and visual performance outcomes were unaffected in the AppNL-G-F mouse. Furthermore, we show that hyperspectral imaging can be used to quantify retinal Aß, underscoring its potential as a biomarker for AD diagnosis and monitoring. These findings suggest that the AppNL-G-F retina mimics the early, preclinical stages of AD, and, together with retinal imaging techniques, offers unique opportunities for drug discovery and fundamental research into preclinical AD.

Computational Analysis of Alzheimer Amyloid Plaque Composition in 2D- and Elastically Reconstructed 3D-MALDI MS Images.

MALDI mass spectrometry imaging (MSI) enables label-free, spatially resolved analysis of a wide range of analytes in tissue sections. Quantitative analysis of MSI datasets is typically performed on single pixels or manually assigned regions of interest (ROIs). However, many sparse, small objects such as Alzheimer's disease (AD) brain deposits of amyloid peptides called plaques are neither single pixels nor ROIs. Here, we propose a new approach to facilitate the comparative computational evaluation of amyloid plaque-like objects by MSI: a fast PLAQUE PICKER tool that enables a statistical evaluation of heterogeneous amyloid peptide composition. Comparing two AD mouse models, APP NL-G-F and APP PS1, we identified distinct heterogeneous plaque populations in the NL-G-F model but only one class of plaques in the PS1 model. We propose quantitative metrics for the comparison of technical and biological MSI replicates. Furthermore, we reconstructed a high-accuracy 3D-model of amyloid plaques in a fully automated fashion, employing rigid and elastic MSI image registration using structured and plaque-unrelated reference ion images. Statistical single-plaque analysis in reconstructed 3D-MSI objects revealed the Aß1-42Arc peptide to be located either in the core of larger plaques or in small plaques without colocalization of other Aß isoforms. In 3D, a substantially larger number of small plaques were observed than that indicated by the 2D-MSI data, suggesting that quantitative analysis of molecularly diverse sparsely-distributed features may benefit from 3D-reconstruction. Data are available via ProteomeXchange with identifier PXD020824.

Modeling the ß-secretase cleavage site and humanizing amyloid-beta precursor protein in rat and mouse to study Alzheimer's disease.

BACKGROUND: Three amino acid differences between rodent and human APP affect medically important features, including ß-secretase cleavage of APP and Aß peptide aggregation (De Strooper et al., EMBO J 14:4932-38, 1995; Ueno et al., Biochemistry 53:7523-30, 2014; Bush, 2003, Trends Neurosci 26:207-14). Most rodent models for Alzheimer's disease (AD) are, therefore, based on the human APP sequence, expressed from artificial mini-genes randomly inserted in the rodent genome. While these models mimic rather well various biochemical aspects of the disease, such as Aß-aggregation, they are also prone to overexpression artifacts and to complex phenotypical alterations, due to genes affected in or close to the insertion site(s) of the mini-genes (Sasaguri et al., EMBO J 36:2473-87, 2017; Goodwin et al., Genome Res 29:494-505, 2019). Knock-in strategies which introduce clinical mutants in a humanized endogenous rodent APP sequence (Saito et al., Nat Neurosci 17:661-3, 2014) represent useful improvements, but need to be compared with appropriate humanized wildtype (WT) mice. METHODS: Computational modelling of the human ß-CTF bound to BACE1 was used to study the differential processing of rodent and human APP. We humanized the three pivotal residues we identified G676R, F681Y and R684H (labeled according to the human APP770 isoform) in the mouse and rat genomes using a CRISPR-Cas9 approach. These new models, termed mouse and rat Apphu/hu, express APP from the endogenous promotor. We also introduced the early-onset familial Alzheimer's disease (FAD) mutation M139T into the endogenous Rat Psen1 gene. RESULTS: We show that introducing these three amino acid substitutions into the rodent sequence lowers the affinity of the APP substrate for BACE1 cleavage. The effect on ß-secretase processing was confirmed as both humanized rodent models produce three times more (human) Aß compared to the original WT strain. These models represent suitable controls, or starting points, for studying the effect of transgenes or knock-in mutations on APP processing (Saito et al., Nat Neurosci 17:661-3, 2014). We introduced the early-onset familial Alzheimer's disease (FAD) mutation M139T into the endogenous Rat Psen1 gene and provide an initial characterization of Aß processing in this novel rat AD model. CONCLUSION: The different humanized APP models (rat and mouse) expressing human Aß and PSEN1 M139T are valuable controls to study APP processing in vivo allowing the use of a human Aß ELISA which is more sensitive than the equivalent system for rodents. These animals will be made available to the research community.

Stem-cell-derived human microglia transplanted in mouse brain to study human disease.

Although genetics highlights the role of microglia in Alzheimer's disease, one-third of putative Alzheimer's disease risk genes lack adequate mouse orthologs. Here we successfully engraft human microglia derived from embryonic stem cells in the mouse brain. The cells recapitulate transcriptionally human primary microglia ex vivo and show expression of human-specific Alzheimer's disease risk genes. Oligomeric amyloid-ß induces a divergent response in human versus mouse microglia. This model can be used to study the role of microglia in neurological diseases.

Safe targeting of T cell acute lymphoblastic leukemia by pathology-specific NOTCH inhibition.

Given the high frequency of activating NOTCH1 mutations in T cell acute lymphoblastic leukemia (T-ALL), inhibition of the γ-secretase complex remains an attractive target to prevent ligand-independent release of the cytoplasmic tail and oncogenic NOTCH1 signaling. However, four different γ-secretase complexes exist, and available inhibitors block all complexes equally. As a result, these cause severe "on-target" gastrointestinal tract, skin, and thymus toxicity, limiting their therapeutic application. Here, we demonstrate that genetic deletion or pharmacologic inhibition of the presenilin-1 (PSEN1) subclass of γ-secretase complexes is highly effective in decreasing leukemia while avoiding dose-limiting toxicities. Clinically, T-ALL samples were found to selectively express only PSEN1-containing γ-secretase complexes. The conditional knockout of Psen1 in developing T cells attenuated the development of a mutant NOTCH1-driven leukemia in mice in vivo but did not abrogate normal T cell development. Treatment of T-ALL cell lines with the selective PSEN1 inhibitor MRK-560 effectively decreased mutant NOTCH1 processing and led to cell cycle arrest. These observations were extended to T-ALL patient-derived xenografts in vivo, demonstrating that MRK-560 treatment decreases leukemia burden and increased overall survival without any associated gut toxicity. Therefore, PSEN1-selective compounds provide a potential therapeutic strategy for safe and effective targeting of T-ALL and possibly also for other diseases in which NOTCH signaling plays a role.

PARL deficiency in mouse causes Complex III defects, coenzyme Q depletion, and Leigh-like syndrome.

The mitochondrial intramembrane rhomboid protease PARL has been implicated in diverse functions in vitro, but its physiological role in vivo remains unclear. Here we show that Parl ablation in mouse causes a necrotizing encephalomyelopathy similar to Leigh syndrome, a mitochondrial disease characterized by disrupted energy production. Mice with conditional PARL deficiency in the nervous system, but not in muscle, develop a similar phenotype as germline Parl KOs, demonstrating the vital role of PARL in neurological homeostasis. Genetic modification of two major PARL substrates, PINK1 and PGAM5, do not modify this severe neurological phenotype. Parl-/- brain mitochondria are affected by progressive ultrastructural changes and by defects in Complex III (CIII) activity, coenzyme Q (CoQ) biosynthesis, and mitochondrial calcium metabolism. PARL is necessary for the stable expression of TTC19, which is required for CIII activity, and of COQ4, which is essential in CoQ biosynthesis. Thus, PARL plays a previously overlooked constitutive role in the maintenance of the respiratory chain in the nervous system, and its deficiency causes progressive mitochondrial dysfunction and structural abnormalities leading to neuronal necrosis and Leigh-like syndrome.

Generation of a human induced pluripotent stem cell-based model for tauopathies combining three microtubule-associated protein TAU mutations which displays several phenotypes linked to neurodegeneration.

INTRODUCTION: Tauopathies are neurodegenerative diseases characterized by TAU protein-related pathology, including frontotemporal dementia and Alzheimer's disease among others. Mutant TAU animal models are available, but none of them faithfully recapitulates human pathology and are not suitable for drug screening. METHODS: To create a new in vitro tauopathy model, we generated a footprint-free triple MAPT-mutant human induced pluripotent stem cell line (N279K, P301L, and E10+16 mutations) using clustered regularly interspaced short palindromic repeats-FokI and piggyBac transposase technology. RESULTS: Mutant neurons expressed pathogenic 4R and phosphorylated TAU, endogenously triggered TAU aggregation, and had increased electrophysiological activity. TAU-mutant cells presented deficiencies in neurite outgrowth, aberrant sequence of differentiation to cortical neurons, and a significant activation of stress response pathways. RNA sequencing confirmed stress activation, demonstrated a shift toward GABAergic identity, and an upregulation of neurodegenerative pathways. DISCUSSION: In summary, we generated a novel in vitro human induced pluripotent stem cell TAU-mutant model displaying neurodegenerative disease phenotypes that could be used for disease modeling and drug screening.

Inactivation of γ-secretases leads to accumulation of substrates and non-Alzheimer neurodegeneration.

γ-Secretases are a family of intramembrane cleaving aspartyl proteases and important drug targets in Alzheimer's disease. Here, we generated mice deficient for all γ-secretases in the pyramidal neurons of the postnatal forebrain by deleting the three anterior pharynx defective 1 (Aph1) subunits (Aph1abc cKO Cre+). The mice show progressive cortical atrophy, neuronal loss, and gliosis. Interestingly, this is associated with more than 10-fold accumulation of membrane-bound fragments of App, Aplp1, Nrg1, and Dcc, while other known substrates of γ-secretase such as Aplp2, Lrp1, and Sdc3 accumulate to lesser extents. Despite numerous reports linking neurodegeneration to accumulation of membrane-bound App fragments, deletion of App expression in the combined Aph1 knockout does not rescue this phenotype. Importantly, knockout of only Aph1a- or Aph1bc-secretases causes limited and differential accumulation of substrates. This was not associated with neurodegeneration. Further development of selective Aph1-γ-secretase inhibitors should be considered for treatment of Alzheimer's disease.

Phenotypic Screening Identifies Modulators of Amyloid Precursor Protein Processing in Human Stem Cell Models of Alzheimer's Disease.

Human stem cell models have the potential to provide platforms for phenotypic screens to identify candidate treatments and cellular pathways involved in the pathogenesis of neurodegenerative disorders. Amyloid precursor protein (APP) processing and the accumulation of APP-derived amyloid ß (Aß) peptides are key processes in Alzheimer's disease (AD). We designed a phenotypic small-molecule screen to identify modulators of APP processing in trisomy 21/Down syndrome neurons, a complex genetic model of AD. We identified the avermectins, commonly used as anthelmintics, as compounds that increase the relative production of short Aß peptides at the expense of longer, potentially more toxic peptides. Further studies demonstrated that this effect is not due to an interaction with the core γ-secretase responsible for Aß production. This study demonstrates the feasibility of phenotypic drug screening in human stem cell models of Alzheimer-type dementia, and points to possibilities for indirectly modulating APP processing, independently of γ-secretase modulation.

The microRNA-29 Family Dictates the Balance Between Homeostatic and Pathological Glucose Handling in Diabetes and Obesity.

The microRNA-29 (miR-29) family is among the most abundantly expressed microRNA in the pancreas and liver. Here, we investigated the function of miR-29 in glucose regulation using miR-29a/b-1 (miR-29a)-deficient mice and newly generated miR-29b-2/c (miR-29c)-deficient mice. We observed multiple independent functions of the miR-29 family, which can be segregated into a hierarchical physiologic regulation of glucose handling. miR-29a, and not miR-29c, was observed to be a positive regulator of insulin secretion in vivo, with dysregulation of the exocytotic machinery sensitizing ß-cells to overt diabetes after unfolded protein stress. By contrast, in the liver both miR-29a and miR-29c were important negative regulators of insulin signaling via phosphatidylinositol 3-kinase regulation. Global or hepatic insufficiency of miR-29 potently inhibited obesity and prevented the onset of diet-induced insulin resistance. These results demonstrate strong regulatory functions for the miR-29 family in obesity and diabetes, culminating in a hierarchical and dose-dependent effect on premature lethality.

Metabolic and Behavioural Phenotypes in Nestin-Cre Mice Are Caused by Hypothalamic Expression of Human Growth Hormone.

The Nestin-Cre driver mouse line has mild hypopituitarism, reduced body weight, a metabolic phenotype and reduced anxiety. Although several causes have been suggested, a comprehensive explanation is still lacking. In this study we examined the molecular mechanisms leading to this compound phenotype. Upon generation of the Nestin-Cre mice, the human growth hormone (hGH) minigene was inserted downstream of the Cre recombinase to ensure efficient transgene expression. As a result, hGH is expressed in the hypothalamus. This results in the auto/paracrine activation of the GH receptor as demonstrated by the increased phosphorylation of signal transducer and activator of transcription 5 (STAT5) and reduced expression of growth hormone releasing hormone (Ghrh). Low Ghrh levels cause hypopituitarism consistent with the observed mouse growth hormone (mGH) deficiency. mGH deficiency caused reduced activation of the GH receptor and hence reduced phosphorylation of STAT5 in the liver. This led to decreased levels of hepatic Igf-1 mRNA and consequently postnatal growth retardation. Furthermore, genes involved in lipid uptake and synthesis, such as CD36 and very low-density lipoprotein receptor were upregulated, resulting in liver steatosis. In conclusion, this study demonstrates the unexpected expression of hGH in the hypothalamus of Nestin-Cre mice which is able to activate both the GH receptor and the prolactin receptor. Increased hypothalamic GH receptor signaling explains the observed hypopituitarism, reduced growth and metabolic phenotype of Nestin-Cre mice. Activation of either receptor is consistent with reduced anxiety.

Deficiency of the miR-29a/b-1 cluster leads to ataxic features and cerebellar alterations in mice.

miR-29 is expressed strongly in the brain and alterations in expression have been linked to several neurological disorders. To further explore the function of this miRNA in the brain, we generated miR-29a/b-1 knockout animals. Knockout mice develop a progressive disorder characterized by locomotor impairment and ataxia. The different members of the miR-29 family are strongly expressed in neurons of the olfactory bulb, the hippocampus and in the Purkinje cells of the cerebellum. Morphological analysis showed that Purkinje cells are smaller and display less dendritic arborisation compared to their wildtype littermates. In addition, a decreased number of parallel fibers form synapses on the Purkinje cells. We identified several mRNAs significantly up-regulated in the absence of the miR-29a/b-1 cluster. At the protein level, however, the voltage-gated potassium channel Kcnc3 (Kv3.3) was significantly up-regulated in the cerebella of the miR-29a/b knockout mice. Dysregulation of KCNC3 expression may contribute to the ataxic phenotype.