Chronic pain causes Tau-mediated hippocampal pathology and memory deficits.

Persistent pain has been recently suggested as a risk factor for dementia. Indeed, chronic pain is frequently accompanied by maladaptive brain plasticity and cognitive deficits whose molecular underpinnings are poorly understood. Despite the emerging role of Tau as a key regulator of neuronal plasticity and pathology in diverse brain disorders, the role of Tau has never been studied in the context of chronic pain. Using a peripheral (sciatic) neuropathy to model chronic pain in mice-spared nerve injury (SNI) for 4 months-in wildtype as well as P301L-Tau transgenic mice, we hereby demonstrate that SNI triggers AD-related neuropathology characterized by Tau hyperphosphorylation, accumulation, and aggregation in hippocampus followed by neuronal atrophy and memory deficits. Molecular analysis suggests that SNI inhibits autophagy and reduces levels of the Rab35, a regulator of Tau degradation while overexpression of Rab35 or treatment with the analgesic drug gabapentin reverted the above molecular changes leading to neurostructural and memory recovery. Interestingly, genetic ablation of Tau blocks the establishment of SNI-induced hippocampal morphofunctional deficits supporting the mediating role of Tau in SNI-evoked hippocampal pathology and memory impairment. These findings reveal that exposure to chronic pain triggers Tau-related neuropathology and may be relevant for understanding how chronic pain precipitates memory loss leading to dementia.

LRRK2 functions in synaptic vesicle endocytosis through a kinase-dependent mechanism.

Mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with Parkinson's disease, but the precise physiological function of the protein remains ill-defined. Recently, our group proposed a model in which LRRK2 kinase activity is part of an EndoA phosphorylation cycle that facilitates efficient vesicle formation at synapses in the Drosophila melanogaster neuromuscular junctions.Flies harbor only one Lrrk gene, which might encompass the functions of both mammalian LRRK1 and LRRK2. We therefore studied the role of LRRK2 in mammalian synaptic function and provide evidence that knockout or pharmacological inhibition of LRRK2 results in defects in synaptic vesicle endocytosis, altered synaptic morphology and impairments in neurotransmission. In addition, our data indicate that mammalian endophilin A1 (EndoA1,also known as SH3GL2) is phosphorylated by LRRK2 in vitro at T73 and S75, two residues in the BAR domain. Hence, our results indicate that LRRK2 kinase activity has an important role in the regulation of clathrin-mediated endocytosis of synaptic vesicles and subsequent neurotransmission at the synapse.

Evidence for lack of direct causality between pain and affective disturbances in a rat peripheral neuropathy model.

Chronic pain is frequently accompanied by the manifestation of emotional disturbances and cognitive deficits. While a causality relation between pain and emotional/cognitive disturbances is generally assumed, several observations suggest a temporal dissociation and independent mechanisms. We therefore studied Sprague-Dawley rats that presented a natural resistance to pain manifestation in a neuropathy model (spared nerve injury [SNI]) and compared their performance in a battery of behavioral paradigms-anxiety, depression and fear memory-with animals that presented a pain phenotype. Afterward, we performed an extensive volumetric analysis across prefrontal, orbitofrontal and insular cortical areas. The majority of SNI animals manifested mechanical allodynia (low threshold [LT]), but 13% were similar to Sham controls (high threshold [HT]). Readouts of spontaneous hypersensivity (paw flinches) were also significantly reduced in HT and correlated with allodynia. To increase the specificity of our findings, we segregated the SNI animals in those with left (SNI-L) and right (SNI-R) lesions and the lack of association between pain and behavior still remains. Left-lesioned animals, independent of the LT or HT phenotype, presented increased anxiety-like behaviors and decreased well-being. In contrast, we found that the insular cortex (agranular division) was significantly smaller in HT than in LT. To conclude, pain and emotional disturbances observed following nerve injury are to some extent segregated phenomena. Also, HT and LT SNI presented differences in insular volumes, an area vastly implicated in pain perception, suggesting a supraspinal involvement in the manifestation of these phenotypes.

Pawedness Trait Test (PaTRaT)-A New Paradigm to Evaluate Paw Preference and Dexterity in Rats.

In rodents, dexterity is commonly analyzed in preference paradigms in which animals are given the chance to use either the left or the right front paws to manipulate food. However, paw preference and dexterity at population and individual levels are controversial as results are incongruent across paradigms. We have therefore developed a semi-quantitative method-the pawdeness trait test (PaTRaT)-to evaluate paw preference degree in rats. The PaTRaT consists in a classification system, ranging from +4 to -4 where increasingly positive and negative values reflect the bias for left or right paw use, respectively. Sprague-Dawley male rats were confined into a metal rectangular mesh cylinder, from which they can see, smell and reach sugared rewards with their paws. Due to its size, the reward could only cross the mesh if aligned with its diagonal, imposing additional coordination. Animals were allowed to retrieve 10 rewards per session in a total of four sessions while their behavior was recorded. PaTRaT was repeated 4 and 8 weeks after the first evaluation. To exclude potential bias, rats were also tested for paw fine movement and general locomotion in other behavioral paradigms as well as impulsivity (variable delay-to-signal, VDS), memory and cognitive flexibility (water maze). At the population level 54% of the animals presented a rightward bias. Individually, all animals presented marked side-preferences, >2 and <-2 for left- and right-sided bias, respectively, and this preference was stable across the three evaluations. Inter-rater consistency was very high between two experienced raters and substantial when two additional inexperienced raters were included. Left- and right-biased animals presented no differences in the ability to perform fine movements with any of the forelimbs (staircase) and general locomotor performance. Additionally, these groups performed similarly in executive function and memory tasks. In conclusion, PaTRaT is able to reliably classify rats' pawedness direction and degree.

Absence of Tau triggers age-dependent sciatic nerve morphofunctional deficits and motor impairment.

Dementia is the cardinal feature of Alzheimer's disease (AD), yet the clinical symptoms of this disorder also include a marked loss of motor function. Tau abnormal hyperphosphorylation and malfunction are well-established key events in AD neuropathology but the impact of the loss of normal Tau function in neuronal degeneration and subsequent behavioral deficits is still debated. While Tau reduction has been increasingly suggested as therapeutic strategy against neurodegeneration, particularly in AD, there is controversial evidence about whether loss of Tau progressively impacts on motor function arguing about damage of CNS motor components. Using a variety of motor-related tests, we herein provide evidence of an age-dependent motor impairment in Tau-/- animals that is accompanied by ultrastructural and functional impairments of the efferent fibers that convey motor-related information. Specifically, we show that the sciatic nerve of old (17-22-months) Tau-/- mice displays increased degenerating myelinated fibers and diminished conduction properties, as compared to age-matched wild-type (Tau+/+) littermates and younger (4-6 months) Tau-/- and Tau+/+ mice. In addition, the sciatic nerves of Tau-/- mice exhibit a progressive hypomyelination (assessed by g-ratio) specifically affecting large-diameter, motor-related axons in old animals. These findings suggest that loss of Tau protein may progressively impact on peripheral motor system.