A transgenic mouse expressing CHMP2Bintron5 mutant in neurons develops histological and behavioural features of amyotrophic lateral sclerosis and frontotemporal dementia.

Mutations in the charged multivesicular body protein 2B (CHMP2B) are associated with frontotemporal dementia (FTD), amyotrophic lateral sclerosis (ALS), and with a mixed ALS-FTD syndrome. To model this syndrome, we generated a transgenic mouse line expressing the human CHMP2Bintron5 mutant in a neuron-specific manner. These mice developed a dose-dependent disease phenotype. A longitudinal study revealed progressive gait abnormalities, reduced muscle strength and decreased motor coordination. CHMP2Bintron5 mice died due to generalized paralysis. When paralyzed, signs of denervation were present as attested by altered electromyographic profiles, by decreased number of fully innervated neuromuscular junctions, by reduction in size of motor endplates and by a decrease of sciatic nerve axons area. However, spinal motor neurons cell bodies were preserved until death. In addition to the motor dysfunctions, CHMP2Bintron5 mice progressively developed FTD-relevant behavioural modifications such as disinhibition, stereotypies, decrease in social interactions, compulsivity and change in dietary preferences. Furthermore, neurons in the affected spinal cord and brain regions showed accumulation of p62-positive cytoplasmic inclusions associated or not with ubiquitin and CHMP2Bintron5 As observed in FTD3 patients, these inclusions were negative for TDP-43 and FUS. Moreover, astrogliosis and microgliosis developed with age. Altogether, these data indicate that the neuronal expression of human CHMP2Bintron5 in areas involved in motor and cognitive functions induces progressive motor alterations associated with dementia symptoms and with histopathological hallmarks reminiscent of both ALS and FTD.

Evaluating Behavior in Mouse Models of the Behavioral Variant of Frontotemporal Dementia: Which Test for Which Symptom?

The behavioral variant of frontotemporal dementia (bvFTD) is a neurodegenerative disease affecting people in their early sixties, characterized by dramatic changes in individual and social behavior. Despite the heterogeneity in the presentation of the clinical symptoms of bvFTD, some characteristic changes can be highlighted. Social disinhibition, changes in food preferences as well as loss of empathy and apathy are commonly described. This is accompanied by a characteristic and dramatic atrophy of the prefrontal cortex with the accumulation of protein aggregates in the neurons in this area. Several causative mutations in different genes have been discovered, allowing the development of transgenic animal models, especially mouse models. In mice, attention has been focused on the histopathological aspects of the pathology, but now studies are taking interest in assessing the behavioral phenotype of FTD models. Finding the right test corresponding to human symptoms is quite challenging, especially since the frontal cortex is much less developed in mice than in humans. Although challenging, the ability to detect relevant prefrontal cortex impairments in mice is crucial for therapeutic approaches. In this review, we aim to present the approaches that have been used to model the behavioral symptoms of FTD and to explore other relevant approaches to assess behavior involving the prefrontal cortex, as well as the deficits associated with FTD.

A metabolic switch toward lipid use in glycolytic muscle is an early pathologic event in a mouse model of amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is the most common fatal motor neuron disease in adults. Numerous studies indicate that ALS is a systemic disease that affects whole body physiology and metabolic homeostasis. Using a mouse model of the disease (SOD1(G86R)), we investigated muscle physiology and motor behavior with respect to muscle metabolic capacity. We found that at 65 days of age, an age described as asymptomatic, SOD1(G86R) mice presented with improved endurance capacity associated with an early inhibition in the capacity for glycolytic muscle to use glucose as a source of energy and a switch in fuel preference toward lipids. Indeed, in glycolytic muscles we showed progressive induction of pyruvate dehydrogenase kinase 4 expression. Phosphofructokinase 1 was inhibited, and the expression of lipid handling molecules was increased. This mechanism represents a chronic pathologic alteration in muscle metabolism that is exacerbated with disease progression. Further, inhibition of pyruvate dehydrogenase kinase 4 activity with dichloroacetate delayed symptom onset while improving mitochondrial dysfunction and ameliorating muscle denervation. In this study, we provide the first molecular basis for the particular sensitivity of glycolytic muscles to ALS pathology.

Full-length PGC-1α salvages the phenotype of a mouse model of human neuropathy through mitochondrial proliferation.

Increased mitochondrial mass, commonly termed mitochondrial proliferation, is frequently observed in many human diseases directly or indirectly involving mitochondrial dysfunction. Mitochondrial proliferation is thought to counterbalance a compromised energy metabolism, yet it might also be detrimental through alterations of mitochondrial regulatory functions such as apoptosis, calcium metabolism or oxidative stress. Here, we show that prominent mitochondrial proliferation occurs in Cramping mice, a model of hereditary neuropathy caused by a mutation in the dynein heavy chain gene Dync1h1. The mitochondrial proliferation correlates with post-prandial induction of full-length (FL) and N-terminal truncated (NT) isoforms of the transcriptional co-activator PGC-1α. The selective knock-out of FL-PGC-1α isoform, preserving expression and function of NT-PGC-1α, led to a complete reversal of mitochondrial proliferation. Moreover, FL-PGC-1α ablation potently exacerbated the mitochondrial dysfunction and led to severe weight loss. Finally, FL-PGC-1α ablation triggered pronounced locomotor dysfunction, tremors and inability to rear in Cramping mice. In summary, endogenous FL-PGC-1α activates mitochondrial proliferation and salvages neurological and metabolic health upon disease. NT-PGC-1α cannot fulfil this protective action. Activation of this endogenous salvage pathway might thus be a valuable therapeutic target for diseases involving mitochondrial dysfunction.

Phosphatidylinositol 4,5-bisphosphate is required for invasive growth in Saccharomyces cerevisiae.

Phosphatidylinositol phosphates are important regulators of processes such as the cytoskeleton organization, membrane trafficking and gene transcription, which are all crucial for polarized cell growth. In particular, phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] has essential roles in polarized growth as well as in cellular responses to stress. In the yeast Saccharomyces cerevisiae, the sole phosphatidylinositol-4-phosphate 5-kinase (PI4P5K) Mss4p is essential for generating plasma membrane PtdIns(4,5)P2. Here, we show that Mss4p is required for yeast invasive growth in low-nutrient conditions. We isolated specific mss4 mutants that were defective in cell elongation, induction of the Flo11p flocculin, adhesion and cell wall integrity. We show that mss4-f12 cells have reduced plasma membrane PtdIns(4,5)P2 levels as well as a defect in its polarized distribution, yet Mss4-f12p is catalytically active in vitro. In addition, the Mss4-f12 protein was defective in localizing to the plasma membrane. Furthermore, addition of cAMP, but not an activated MAPKKK allele, partially restored the invasive growth defect of mss4-f12 cells. Taken together, our results indicate that plasma membrane PtdIns(4,5)P2 is crucial for yeast invasive growth and suggest that this phospholipid functions upstream of the cAMP-dependent protein kinase A signaling pathway.

A steep phosphoinositide bis-phosphate gradient forms during fungal filamentous growth.

Membrane lipids have been implicated in many critical cellular processes, yet little is known about the role of asymmetric lipid distribution in cell morphogenesis. The phosphoinositide bis-phosphate PI(4,5)P(2) is essential for polarized growth in a range of organisms. Although an asymmetric distribution of this phospholipid has been observed in some cells, long-range gradients of PI(4,5)P(2) have not been observed. Here, we show that in the human pathogenic fungus Candida albicans a steep, long-range gradient of PI(4,5)P(2) occurs concomitant with emergence of the hyphal filament. Both sufficient PI(4)P synthesis and the actin cytoskeleton are necessary for this steep PI(4,5)P(2) gradient. In contrast, neither microtubules nor asymmetrically localized mRNAs are critical. Our results indicate that a gradient of PI(4,5)P(2), crucial for filamentous growth, is generated and maintained by the filament tip-localized PI(4)P-5-kinase Mss4 and clearing of this lipid at the back of the cell. Furthermore, we propose that slow membrane diffusion of PI(4,5)P(2) contributes to the maintenance of such a gradient.

CKIP-1 regulates mammalian and zebrafish myoblast fusion.

Multinucleated muscle fibres arise by fusion of precursor cells called myoblasts. We previously showed that CKIP-1 ectopic expression in C2C12 myoblasts increased cell fusion. In this work, we report that CKIP-1 depletion drastically impairs C2C12 myoblast fusion in vitro and in vivo during zebrafish muscle development. Within developing fast-twich myotome, Ckip-1 localises at the periphery of fast precursor cells, closed to the plasma membrane. Unlike wild-type myoblasts that form spatially arrayed multinucleated fast myofibres, Ckip-1-deficient myoblasts show a drastic reduction in fusion capacity. A search for CKIP-1 binding partners identified the ARPC1 subunit of Arp2/3 actin nucleation complex essential for myoblast fusion. We demonstrate that CKIP-1, through binding to plasma membrane phosphoinositides via its PH domain, regulates cell morphology and lamellipodia formation by recruiting the Arp2/3 complex at the plasma membrane. These results establish CKIP-1 as a regulator of cortical actin that recruits the Arp2/3 complex at the plasma membrane essential for muscle precursor elongation and fusion.