PLD3 is a neuronal lysosomal phospholipase D associated with ß-amyloid plaques and cognitive function in Alzheimer's disease.

Phospholipase D3 (PLD3) is a protein of unclear function that structurally resembles other members of the phospholipase D superfamily. A coding variant in this gene confers increased risk for the development of Alzheimer's disease (AD), although the magnitude of this effect has been controversial. Because of the potential significance of this obscure protein, we undertook a study to observe its distribution in normal human brain and AD-affected brain, determine whether PLD3 is relevant to memory and cognition in sporadic AD, and to evaluate its molecular function. In human neuropathological samples, PLD3 was primarily found within neurons and colocalized with lysosome markers (LAMP2, progranulin, and cathepsins D and B). This colocalization was also present in AD brain with prominent enrichment on lysosomal accumulations within dystrophic neurites surrounding ß-amyloid plaques. This pattern of protein distribution was conserved in mouse brain in wild type and the 5xFAD mouse model of cerebral ß-amyloidosis. We discovered PLD3 has phospholipase D activity in lysosomes. A coding variant in PLD3 reported to confer AD risk significantly reduced enzymatic activity compared to wild-type PLD3. PLD3 mRNA levels in the human pre-frontal cortex inversely correlated with ß-amyloid pathology severity and rate of cognitive decline in 531 participants enrolled in the Religious Orders Study and Rush Memory and Aging Project. PLD3 levels across genetically diverse BXD mouse strains and strains crossed with 5xFAD mice correlated strongly with learning and memory performance in a fear conditioning task. In summary, this study identified a new functional mammalian phospholipase D isoform which is lysosomal and closely associated with both ß-amyloid pathology and cognition.

Blockade of the 5-HT transporter contributes to the behavioural, neuronal and molecular effects of cocaine.

BACKGROUND AND PURPOSE: The psychostimulant cocaine induces complex molecular, cellular and behavioural responses as a consequence of inhibiting presynaptic dopamine, noradrenaline and 5-HT transporters. To elucidate 5-HT transporter (SERT)-specific contributions to cocaine action, we evaluated cocaine effects in the SERT Met172 knock-in mouse, which expresses a SERT coding substitution that eliminates high-affinity cocaine recognition. EXPERIMENTAL APPROACH: We measured the effects of SERT Met172 on cocaine antagonism of 5-HT re-uptake using ex vivo synaptosome preparations and in vivo microdialysis. We assessed SERT dependence of cocaine actions behaviourally through acute and chronic locomotor activation, sensitization, conditioned place preference (CPP) and oral cocaine consumption. We used c-Fos, quantitative RT-PCR and RNA sequencing methods for insights into cellular and molecular networks supporting SERT-dependent cocaine actions. KEY RESULTS: SERT Met172 mice demonstrated functional insensitivity for cocaine at SERT. Although they displayed wild-type levels of acute cocaine-induced hyperactivity or chronic sensitization, the pattern of acute motor activation was different, with a bias toward thigmotaxis. CPP was increased, and a time-dependent elevation in oral cocaine consumption was observed. SERT Met172 mice displayed relatively higher levels of neuronal activation in the hippocampus, piriform cortex and prelimbic cortex (PrL), accompanied by region-dependent changes in immediate early gene expression. Distinct SERT-dependent gene expression networks triggered by acute and chronic cocaine administration were identified, including PrL Akt and nucleus accumbens ERK1/2 signalling. CONCLUSION AND IMPLICATIONS: Our studies reveal distinct SERT contributions to cocaine action, reinforcing the possibility of targeting specific aspects of cocaine addiction by modulation of 5-HT signalling.

Serotonin Transporter-Independent Actions of the Antidepressant Vortioxetine As Revealed Using the SERT Met172 Mouse.

Selective serotonin (5-HT, SERT) reuptake inhibitors (SSRIs) are the most commonly prescribed treatments for depression. However, they have delayed efficacy and can induce side-effects that can encourage discontinuation. Recently, agents have been developed, including vortioxetine (Trintellix), that augment SERT blockade with interactions at other targets. At therapeutic doses, vortioxetine interacts with SERT as well as 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors. We assessed the SERT-dependency of vortioxetine action using the SERT Met172 mouse model, which disrupts high-affinity interactions of many antidepressants with the transporter. We demonstrate that the SERT Met172 substitution induces an ∼19-fold loss in vortioxetine potency for SERT inhibition in midbrain synaptosomes. Moreover, in these mice, we observed reduced SERT occupancy, a diminished ability to prolong 5-HT clearance, and a reduced capacity to elevate extracellular 5-HT. Despite reduced interactions with SERT, vortioxetine maintained its ability to enhance mobility in tail suspension and forced swim tests, reduce consumption latency in the novelty induced hypophagia test, and promoted proliferation and survival of subgranular zone hippocampal stem cells. Our findings suggest that the antidepressant actions of vortioxetine may be SERT-independent, and encourage consideration of agents that mimic one or more actions of the drug in the development of improved depression treatments.

Essential Contributions of Serotonin Transporter Inhibition to the Acute and Chronic Actions of Fluoxetine and Citalopram in the SERT Met172 Mouse.

Depression is a common mental illness and a leading cause of disability. The most widely prescribed antidepressant medications are serotonin (5-HT) selective reuptake inhibitors (SSRIs). Although there is much support for 5-HT transporter (SERT) antagonism as a basis of antidepressant efficacy, this evidence is indirect and other targets and mechanisms have been proposed. In order to distinguish SERT-dependent and -independent effects of SSRIs, we developed a knock-in mouse model whereby high-affinity interactions of many antidepressants at SERT have been ablated via knock-in substitution (SERT Met172) without disrupting 5-HT recognition or uptake. Here we utilize the C57BL/6J SERT Met172 model to evaluate SERT dependence for the actions of two widely prescribed SSRIs, fluoxetine and citalopram, in tests sensitive to acute and chronic actions of antidepressants. In the tail suspension and forced swim tests, fluoxetine and citalopram fail to reduce immobility in SERT Met172 mice. In addition, SERT Met172 mice are insensitive to chronic fluoxetine and citalopram administration in the novelty induced hypophagia test (NIH) and fail to exhibit enhanced proliferation or survival of hippocampal stem cells. In both acute and chronic studies, SERT Met172 mice maintained sensitivity to paroxetine, an antidepressant that is unaffected by the Met172 mutation. Together, these studies provide definitive support for an essential role of SERT antagonism in the acute and chronic actions of two commonly used SSRIs in these tests, and reinforce the utility of the SERT Met172 model for isolating SERT/5-HT contributions of drug actions in vivo.