Rhabdomyosarcoma (RMS) is a paediatric neoplasm of skeletal muscle in dire need of novel therapeutic strategies. We explored the role of pannexin 1 (PANX1), an emerging regulator of skeletal muscle differentiation, in the two major histological subtypes of RMS: embryonal RMS (eRMS) and alveolar RMS (aRMS). We found that PANX1 levels are down-regulated in eRMS and aRMS tissue specimens and patient-derived cell lines. While PANX1 transcripts were expressed in RMS, we showed that the majority lacked the putative 5’ untranslated region (5’UTR) and the first ATG start codon, which contributed to low PANX1 expression. Re-introduction of PANX1 in eRMS and aRMS patient-derived cell lines induced their partial differentiation and suppressed their malignant properties in vitro and in vivo. Notably, our data suggest that the PANX1-mediated tumour-inhibitory function in RMS does not depend on its canonical channel function. Furthermore, our genome-wide transcriptomic study found that PANX1 over-expression in RMS cells induced changes in expression of genes in various cellular processes and signaling pathways involved in tumour-suppression including connexin 43 (Cx43), which was previously shown to induce RMS cell differentiation. At the protein level, we showed that the PANX1 interactome in RMS cells involved plasma membrane and cytoskeleton associated proteins including the neuroblast differentiation-associated protein AHNAK, also known as desmoyokin. We demonstrated that PANX1-mediated tumour-suppression in RMS cells depended on its interaction with AHNAK. We further constructed the first searchable PANX1 transcriptome and interactome databases and made them available for the scientific community. We then searched for a means by which to increase PANX1 levels as a new therapeutic approach for RMS. We found quercetin, a clinical approved natural flavonoid, as a PANX1 up-regulator in RMS cells. This induction of PANX1 expression involved alternative transcription of a PANX1 mRNA variant containing a translationally active 5’UTR, which depended, in part, on binding of the transcription factor ETV4 (ETS Variant Transcription Factor 4) in the PANX1 promoter. Moreover, quercetin treatment induced RMS cell differentiation, inhibited tumour spheroid growth, and induced regression of established tumour spheroids. Collectively, we demonstrate down-regulation of PANX1 in RMS and the dramatic tumour-inhibitory effects of restoring PANX1 expression in this malignancy. We show that PANX1-induced tumour-suppression does not rely on its canonical channel function but rather by signaling transduction via interaction with AHNAK. We reveal that PANX1 expression can be regulated at the translational level by the 5’UTR of its mRNA via quercetin-induced alternative transcription. We further demonstrate quercetin as a potential therapeutic agent for RMS by showing its tumour-suppressive effects in RMS. These studies not only substantiate our current understanding of PANX1, but also establish PANX1 as a therapeutic target in RMS and provide potential therapeutic options for its treatment.

Type II diabetes is a major cause of blindness according to the World Health Organization (WHO, 2018). Diabetics are at risk of developing corneal diseases such as recurrent abrasions, ulcers, and erosions due to dysfunctional wound healing. Corrective surgeries or corneal transplants may be considered as a treatment in some, but not all, cases. The purinoreceptor P2X7 has been shown to be involved in cell-cell communication and in the restructuring of cytoskeletal actin, a necessary process for cell migration in wound healing. P2X7 relies on the binding of extracellular ATP for activation. Panx1 is a transmembrane protein whose primary role is for the release of intracellular ATP into the extracellular space. In healthy corneal epithelium, Panx1 localizes to the wound edge and forms clusters with the P2X7, which augments the wound healing response. This thesis looks at the localization of Panx1 in pre-diabetic murine corneal tissue. It was found that Panx1 is less expressed and does not localize to the wound edge to the extent as control corneas, therefore, creating less clusters with P2X7. Furthermore, preliminary studies that inhibit Panx1 with probenecid reduce the communication between cells, which is hypothesized as critical for migration of the tissue sheet and proper wound healing. / 2019-12-17T00:00:00Z

Pannexins represent a recently discovered second family of gap junction proteins in vertebrates. However, instead of forming intercellular gap junction channels like connexins, pannexins operate as unpaired pannexons, allowing the flux of molecules from the cytoplasm to the extracellular space and vice versa. Pannexins appear to play a vital role in the local control loop of blood perfusion and oxygen delivery. The properties of Panx1 channels indicate that this protein is the most probable candidate for an ATP release channel and is involved in the propagation of intercellular calcium waves. It is also proposed to mediate the large pore formation of the P2X7 receptor death complex. Prolonged activation of this receptor can lead to cell death. There must be some mechanisms to stop this ATP-induced ATP release and opening of the lethal pore. Here we describe a negative feedback loop controlling pannexin 1 channel activity. ATP, permeant to pannexin 1 channels, was found to inhibit its permeation pathway when applied extracellularly. ATP analogues, including BzATP, suramine, and BBG were even more effective inhibitors of pannexin 1 currents than ATP. These compounds also attenuated the uptake of dyes by erythrocytes, which express pannexin 1. The rank order of the compounds in attenuation of pannexin 1 currents was similar to their binding affinities to the P2X7 receptor, except that receptor agonists and antagonists both were inhibitory to the channel. The ATP inhibitory effect is largely decreased when R75 on the first extracellular loop of Pannexin1 is mutated to alanine, strongly indicating that the ATP regulates this channel through binding. To further investigate the structural property of the ATP binding, we did alanine scanning mutagenesis of the extracellular loops and found that mutations on W74, S237, S240, I247 and L266 on the extracellular loops severely impair the BzATP inhibitory effect indicating that they might be direct binding partners for the ligands. Mutations on R75, S82, S93, L94, D241, S249, P259 and I267 have largely decreased BzATP sensitivity. Mutations on other residues didn't change the BzATP sensitivity compared to the wild type except for some nonfunctional mutants. All these data demonstrate that some amino acid residues on the extracellular loop of Pannexin 1 mediate ATP sensitivity. However, how these residues form the ATP-binding pocket remains to be elucidated.

The ATP-gated P2X7 receptors (P2X7Rs) play a key role in the release of pro-inflammatory cytokines in response to immunological challenges. Pannexin 1 (Panx1), conventionally described as a hemichannel forming protein, was suggested to be involved in the formation of the P2X7 large pore, which provides a conduit for large molecules such as fluorescent dyes. Firstly, this thesis demonstrated that the P2X7R-mediated dye uptake, a phenomenon attributed to the activation of Panx1, was suppressed by acidic pH and this inhibition was abolished in a P2X7 mutant (aspartic acid 197 to alanine) that was insensitive to extracellular pH. Then, the functional properties of human or mouse Panx1 in HEK293 cells were analysed in the absence of P2X7. The Panx1 currents were not affected by extracellular/intracellular calcium, but were reversibly inhibited by adenosine triphosphate (ATP) and non-specific anion channel blockers. Ion substitution experiments showed that Panx1 was permeable only to monovalent anions and single channel studies revealed a medium sized unitary conductance of Panx1 (~65 pS). Based on the evidence, this thesis concluded that Panx1 is an anion channel but not a hemichannel as originally proposed.

The wound healing response within corneal epithelium cells is controlled by the purinergic P2X7 receptor, purinergic P2Y2 receptor, and Pannexin1 transmembrane proteins. The response is initiated by the release of ATP into the extracellular space by damaged cells and by the Pannexin1 channel protein and limited by the hydrolysis of extracellular ATP by ecto-ATPases. P2X7 and P2Y2 receptors are necessary for the initiation of cell migration, actin reorganization, and cell to cell communication required for the sheet like migration found within the corneal epithelium wound response. ARL 67156 Trisodium Salt Hydrate, an inhibitor for human and murine ecto-ATPases, was used to inhibit the hydrolysis of extracellular ATP during the wound response to prolong and further elevate extracellular ATP concentrations to increase the cellular activity and cell to cell communication of the wound response. The goal was to compare primary diabetic murine and human cells. However, there were issues with culturing these cells and diabetic murine and non-diabetic primary and established human cells were used for these experiments. Significant increases in cellular activity were found in both primary human cells with a 100 ARL to 10 ATP concentration ratio while no significant changes in cellular activity were found with the murine diabetic cells at any ARL concentration. In summary, with increased ATP in the wound response due to the inhibition of hydrolysis enzymes, human cells displayed increased levels and rates of cell-to-cell communication during the delayed wound response. / 2024-01-27T00:00:00Z

Calcium (Ca2+) influx through N-methyl-D-asparate receptors (NMDARs) is widely held to be the requisite step initiating delayed neuronal death following ischemic stroke. However, blocking NMDARs fails to prevent the accumulation of intracellular Ca2+ ([Ca2+]i) and subsequent neurotoxicity. This suggests that alternate, as yet uncharacterized Ca2+-influx pathways exist in neurons. Transient receptor melastatin 2 (TRPM2) is a Ca2+-permeable member of the transient receptor potential melastatin family of cation channels whose activation by reactive oxygen/nitrogen species (ROS/RNS) and ADP-ribose (ADPR) is linked to cell death. While these channels are broadly expressed in the central nervous system (CNS), the presence of TRPM2 in neurons remains controversial and more specifically, whether they are expressed in neurons of the hippocampus is an open question. Here, I employ a combination of molecular, biochemical and electrophysiological approaches to demonstrate that functional TRPM2 channels are expressed in pyramidal neurons of the hippocampus. Unlike in heterologous expression systems, the ADPR-dependent activation of TRPM2 in neurons required a concomitant rise in [Ca2+]i via either voltage-dependent Ca2+ channels or NMDARs. While short, repeated NMDA applications activated a TRPM2-like current in the absence of exogenous ADPR, sustained NMDA application to hippocampal neurons resulted in the activation of a pannexin1 (Px1) hemichannel. Px1 hemichannels are large conductance, nonjunctional gap junction channels that can be activated following periods of oxygen-glucose deprivation (OGD) in neurons. Activation of Px1 required the influx of Ca2+ through NMDARs. Supplementing the intracellular milieu with adenosine triphosphate (ATP) prevented Px1 activation, suggesting that hemichannels may be activated during periods of mitochondrial dysfunction and metabolic failure. Our findings have potential implications for the treatment of diseases such as cerebral ischemia and Alzheimer’s disease (AD) as they implicate two novel ion channels in the excitotoxic signaling cascade activated downstream of NMDARs.

Calcium (Ca2+) influx through N-methyl-D-asparate receptors (NMDARs) is widely held to be the requisite step initiating delayed neuronal death following ischemic stroke. However, blocking NMDARs fails to prevent the accumulation of intracellular Ca2+ ([Ca2+]i) and subsequent neurotoxicity. This suggests that alternate, as yet uncharacterized Ca2+-influx pathways exist in neurons. Transient receptor melastatin 2 (TRPM2) is a Ca2+-permeable member of the transient receptor potential melastatin family of cation channels whose activation by reactive oxygen/nitrogen species (ROS/RNS) and ADP-ribose (ADPR) is linked to cell death. While these channels are broadly expressed in the central nervous system (CNS), the presence of TRPM2 in neurons remains controversial and more specifically, whether they are expressed in neurons of the hippocampus is an open question. Here, I employ a combination of molecular, biochemical and electrophysiological approaches to demonstrate that functional TRPM2 channels are expressed in pyramidal neurons of the hippocampus. Unlike in heterologous expression systems, the ADPR-dependent activation of TRPM2 in neurons required a concomitant rise in [Ca2+]i via either voltage-dependent Ca2+ channels or NMDARs. While short, repeated NMDA applications activated a TRPM2-like current in the absence of exogenous ADPR, sustained NMDA application to hippocampal neurons resulted in the activation of a pannexin1 (Px1) hemichannel. Px1 hemichannels are large conductance, nonjunctional gap junction channels that can be activated following periods of oxygen-glucose deprivation (OGD) in neurons. Activation of Px1 required the influx of Ca2+ through NMDARs. Supplementing the intracellular milieu with adenosine triphosphate (ATP) prevented Px1 activation, suggesting that hemichannels may be activated during periods of mitochondrial dysfunction and metabolic failure. Our findings have potential implications for the treatment of diseases such as cerebral ischemia and Alzheimer’s disease (AD) as they implicate two novel ion channels in the excitotoxic signaling cascade activated downstream of NMDARs.

Neuroblastoma (NB) is the most common extracranial solid tumour in childhood. NB is thought to arise from the failed differentiation of neural crest progenitor cells that would normally form tissues of the adrenal gland and sympathetic nervous system. These neural crest progenitors then uncontrollably proliferate forming a tumour. Despite aggressive surgery and chemotherapy, the cure rate of high-risk NB patients remains below 30%. Our laboratory has shown that human NB tumour specimens and high-risk patient derived cell lines express pannexin 1 (PANX1), and that treatment with the PANX1 channel blockers carbenoxolone or probenecid constitute reduce NB progression in vitro and in vivo. PANX1 is a glycoprotein that forms single membrane channels best known to serve as conduits for ATP release. Interestingly, while PANX1 was also detected in control neurons by western blotting, its banding pattern was strikingly different as a band at around 50 kDa was found in all NB cell lines, but not in neurons. Using shRNA targeting PANX1 and deglycosylation enzymes, I have shown that this band corresponds to a PANX1 glycosylated species. PANX1 has been reported to be phosphorylated in NB at amino acid Y10. PANX1 is also predicted to be glycosylated at N255. In order to study the role of these post-translational modifications, myc-tagged Y10F and N255A PANX1 mutants were engineered by site-directed mutagenesis. Immunolocalization and cell surface biotinylation assays suggest that the localization both mutants at the cell surface is reduced compared to that of myc-PANX1. Dye uptake assays revealed that myc-Y10F has significantly reduced channel activity. Expression of myc-Y10F and myc-N255A in NB cells inhibited cell proliferation and decreased metastatic potential in vitro. Further analysis of NB tumour specimens revealed that there is a missense mutation in PANX1 resulting in the formation of truncated peptide (amino acid 1-99). Interestingly, I have found that when co-expressed with myc-PANX1, PANX11-99, reduced PANX1 channel activity. Taken together, these findings indicate that phosphorylation on Y10 and glycosylation on N255 regulate PANX1 channel activity and exacerbate NB malignancy, while the expression of PANX11-99 in NB may be beneficial.

Neurons are generated from unspecialized neural precursor cells (NPCs) in a process termed neurogenesis. This neuronal development continues throughout life in the ventricular zone (VZ) of the lateral ventricles, and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus. NPCs undergo a complex and highly regulated set of behaviours in order to ultimately integrate into the existing brain circuitry as fully functional neurons. Recently the pannexin (Panx) large-pore channel proteins were discovered. One family member, Panx1 is expressed in the nervous system in mature neurons, and acts as an ATP release channel in various cell types throughout the body. Post-natal NPCs are responsive to ATP via activation of purinergic receptors, which modulate a variety of NPC behaviours. I therefore investigated the hypothesis that Panx1 was expressed in post-natal VZ NPCs, where it functioned as an ATP release channel and regulated neuronal development. In the course of my studies, I found that Panx1 positively regulated NPC proliferation and migration, and negatively regulated neurite outgrowth in vitro. Using an NPC-specific Panx1 knock-out strategy, I showed that Panx1 expression was required for maintenance of a consistent population of VZ NPCs in vivo in both healthy and injured brain. Together these data indicated that Panx1 directed NPC behaviours associated with neuronal development both in vitro and in vivo. To further understand the molecular underpinnings of this regulation, I examined the Panx1 interactome, and uncovered a novel association with collapsin response mediator protein 2 (Crmp2). Functional studies suggested that this interaction likely was at least in part responsible for Panx1’s negative impact on neurite outgrowth. Overall, my results represent important novel findings that contribute to our understanding of post-natal neuronal development and the molecular function of Panx1 within the brain. / Graduate / 0317 / 0379 / leighws@uvic.ca

Pannexin 1 (Panx1) is a metabolite channel enriched in the brain and known to localize to the cell surface, where it is involved in a variety of neuronal processes including cell proliferation and differentiation. The mechanisms through which Panx1 is trafficked or stabilized at the surface, however, are not fully understood. The proximal Panx1 C-terminus (Panx1CT), upstream of a caspase-cleavage site has been demonstrated to be required for Panx1 cell-surface expression. We discovered a previously unreported putative leucine-rich repeat (LRR) motif within the proximal Panx1CT. I investigated the involvement of this putative LRR motif on Panx1 localization and oligomerization. Deletion of the putative LRR motif or uniquely the highly conserved segment of the putative LRR motif resulted in a significant loss of Panx1 cell surface expression. Finally, ectopic expression of Panx1-EGFP in HEK293T cells increased cell proliferation, which was not recapitulated by a Panx1 deletion mutant lacking the putative LRR motif. Overall the findings presented in this thesis provide new insights into the molecular determinants of Panx1 trafficking and oligomerization. / Graduate / 2020-02-14