Characterisation of phospholipase C-η enzymes and their relevance to disease

Phospholipase C enzymes are a class of enzymes that catalyse the cleavage of the membrane phospholipid, phosphatidylinositol bisphosphate (PtdIns(4,5)P₂) into the second messengers, inositol trisphosphate (Ins(4,5)P₃) and diacylglycerol (DAG). Six classes of PLC enzymes have been identified based on their structure and mechanism of activation. PLCηs are the most recently identified family and consist of two isozymes, PLCη1 and PLCη2. The aim of this thesis is to further understand the mechanisms of PLCη activation, the role of PLCη2 in relation to neuritogenesis and their roles in certain disease states. Both isoforms were found to be activated by physiological concentrations of intracellular Ca²⁺. Activation of PLCη2 by Gß₁γ₂ was confirmed using a bacterial 2A co-expression system to allow expression of PLCη2, Gß₁ and Gγ₂ with a single plasmid. Localisation studies show a nuclear distribution for PLCη2, but a cytoplasmic distribution for PLCη1 in a neuroblastoma cells line (Neuro2A). PLCη2 has been implicated in brain development and neurite formation. Building on this, a neuronal differentiation model using RA-treated Neuro2A cells stably expressing mutant forms of PLCη2 was utilised, revealing that PLCη2 activity is essential for neuritogenesis but that this process is independent of the enzymes high sensitivity towards Ca²⁺. Furthermore, the direct interaction of PLCη2 and LIMK-1, a previously identified PLCη2 associated protein, is confirmed in the aforementioned neuronal model. Due to the high sensitivity of PLCη enzymes to Ca²⁺ and because of their presence within neurons, they may be involved in Ca²⁺ dysregulation that occurs in certain diseases such as Alzheimer’s disease (AD). The role of PLCη2 was assessed in amyloid-ß (Aß) treated differentiated Neuro2A cells, a cellular model for AD pathogenesis. Also a developmental role for PLCη1 was investigated due to a recently identified PLCη1 polymorphism in patients with holoprosencephaly, an embryonic midline defect

Phospholipase C-η2 interacts with nuclear and cytoplasmic LIMK-1 during retinoic acid-stimulated neurite growth

We gratefully acknowledge financial support from the Wellcome Trust (Grant No. WT089803MA to J.M.L.) and the School of Medicine, University of St Andrews.Neurite growth is central to the formation and differentiation of functional neurons and recently, an essential role for phospholipase C-η2 (PLCη2) in neuritogenesis was revealed. Here we investigate the function of PLCη2 in neuritogenesis using Neuro2A cells, which upon stimulation with retinoic acid differentiate and form neurites. We first investigated the role of the PLCη2 calcium-binding EF-hand domain, a domain that is known to be required for calcium-dependent PLCη2 activation. To do this we quantified neurite outgrowth in Neuro2A cells, stably overexpressing wild-type PLCη2 and D256A (EF-hand) and H460Q (active site) PLCη2 mutants. Retinoic acid-induced neuritogenesis was highly dependent on PLCη2 activity, with the H460Q mutant exhibiting a strong dominant-negative effect. Expression of the D256A mutant had little effect on neurite growth relative to the control, suggesting that calcium-directed activation of PLCη2 is not essential to this process. We next investigated which cellular compartments contain endogenous PLCη2 by comparing immuno-electron microscopy signals over control and knockdown cell lines. When signals were analyzed to reveal specific labelling for PLCη2, it was found to be localized predominantly over the nucleus and cytosol. Furthermore in these compartments (and also in growing neurites), a proximity ligand assay revealed that PLCη2 specifically interacts with LIMK-1 in Neuro2A cells. Taken together, these data emphasize the importance of PLCη2 and its articulation with LIMK-1 in regulating neuritogenesis.Publisher PDFPeer reviewe

Phospholipase C-η2 interacts with nuclear and cytoplasmic LIMK-1 during retinoic acid-stimulated neurite growth

Neurite growth is central to the formation and differentiation of functional neurons and recently, an essential role for phospholipase C-η2 (PLCη2) in neuritogenesis was revealed. Here we investigate the function of PLCη2 in neuritogenesis using Neuro2A cells, which upon stimulation with retinoic acid differentiate and form neurites. We first investigated the role of the PLCη2 calcium-binding EF-hand domain, a domain that is known to be required for calcium-dependent PLCη2 activation. To do this we quantified neurite outgrowth in Neuro2A cells, stably overexpressing wild-type PLCη2 and D256A (EF-hand) and H460Q (active site) PLCη2 mutants. Retinoic acid-induced neuritogenesis was highly dependent on PLCη2 activity, with the H460Q mutant exhibiting a strong dominant-negative effect. Expression of the D256A mutant had little effect on neurite growth relative to the control, suggesting that calcium-directed activation of PLCη2 is not essential to this process. We next investigated which cellular compartments contain endogenous PLCη2 by comparing immuno-electron microscopy signals over control and knockdown cell lines. When signals were analyzed to reveal specific labelling for PLCη2, it was found to be localized predominantly over the nucleus and cytosol. Furthermore in these compartments (and also in growing neurites), a proximity ligand assay revealed that PLCη2 specifically interacts with LIMK-1 in Neuro2A cells. Taken together, these data emphasize the importance of PLCη2 and its articulation with LIMK-1 in regulating neuritogenesis

Phospholipase C-η2 is required for retinoic acid-stimulated neurite growth

PLCη2 is a recently identified phospholipase C (PLC) implicated in the regulation of neuronal differentiation/maturation. PLCη2 activity is triggered by intracellular calcium mobilization and likely serves to amplify Ca2+ signals by stimulating further Ca2+ release from Ins(1,4,5)P3-sensitive stores. The role of PLCη2 in neuritogenesis was assessed during retinoic acid (RA)-induced Neuro2A cell differentiation. PLCη2 expression increased 2-fold during a 4-day differentiation period. Stable expression of PLCη2-targetted shRNA led to a decrease in the number of differentiated cells and total length of neurites following RA treatment. Furthermore, RA response element (RARE) activation was perturbed by PLCη2 knockdown. Using a bacterial two-hybrid screen we identified LIM domain kinase 1 (LIMK1) as a putative interaction partner of PLCη2. Immunostaining of PLCη2 revealed significant co-localization with LIMK1 in the nucleus and growing neurites in Neuro2A cells. RA-induced phosphorylation of LIMK1 and CREB was reduced in PLCη2 knockdown cells. The phosphoinositide-binding properties of the PLCη2 PH domain, assessed using a FRET-based assay, reveal this domain to possess a high affinity toward PtdIns(3,4,5)P3. Immunostaining of PLCη2 together with PtdIns(3,4,5)P3 in the Neuro2A cells revealed a high degree of co-localization, indicating that PtdIns(3,4,5)P3 levels in cellular compartments are likely to be important for the spatial control of PLCη2 signaling

Phospholipase C-eta 2 is required for retinoic acid-stimulated neurite growth

PLCη2 is a recently identified phospholipase C (PLC) implicated in the regulation of neuronal differentiation/maturation. PLCη2 activity is triggered by intracellular calcium mobilization and likely serves to amplify Ca2+ signals by stimulating further Ca2+ release from Ins(1,4,5)P3-sensitive stores. The role of PLCη2 in neuritogenesis was assessed during retinoic acid (RA)-induced Neuro2A cell differentiation. PLCη2 expression increased 2-fold during a 4-day differentiation period. Stable expression of PLCη2-targetted shRNA led to a decrease in the number of differentiated cells and total length of neurites following RA treatment. Furthermore, RA response element (RARE) activation was perturbed by PLCη2 knockdown. Using a bacterial two-hybrid screen we identified LIM domain kinase 1 (LIMK1) as a putative interaction partner of PLCη2. Immunostaining of PLCη2 revealed significant co-localization with LIMK1 in the nucleus and growing neurites in Neuro2A cells. RA-induced phosphorylation of LIMK1 and CREB was reduced in PLCη2 knockdown cells. The phosphoinositide-binding properties of the PLCη2 PH domain, assessed using a FRET-based assay, reveal this domain to possess a high affinity toward PtdIns(3,4,5)P3. Immunostaining of PLCη2 together with PtdIns(3,4,5)P3 in the Neuro2A cells revealed a high degree of co-localization, indicating that PtdIns(3,4,5)P3 levels in cellular compartments are likely to be important for the spatial control of PLCη2 signaling