SERCaMP: a carboxy-terminal protein modification that enables monitoring of ER calcium homeostasis

Excitotoxic glutamate levels cause the secretion of resident endoplasmic reticulum proteins

Dysregulation of synaptic glutamate levels can lead to excitotoxicity such as that observed in stroke, traumatic brain injury, and epilepsy. The role of increased intracellular calcium (Ca2+ ) in the development of excitotoxicity is well established. However, less is known regarding the impact of glutamate on endoplasmic reticulum (ER)-Ca2+ -mediated processes such as proteostasis. To investigate this, we expressed a secreted ER Ca2+ modulated protein (SERCaMP) in primary cortical neurons to monitor exodosis, a phenomenon whereby ER calcium depletion causes the secretion of ER-resident proteins that perform essential functions to the ER and the cell. Activation of glutamatergic receptors (GluRs) led to an increase in SERCaMP secretion indicating that normally ER-resident proteins are being secreted in a manner consistent with ER Ca2+ depletion. Antagonism of ER Ca2+ channels attenuated the effects of glutamate and GluR agonists on SERCaMP release. We also demonstrate that endogenous proteins containing an ER retention/retrieval sequence (ERS) are secreted in response to GluR activation supporting that neuronal activation by glutamate promotes ER exodosis. Ectopic expression of KDEL receptors attenuated the secretion of ERS-containing proteins caused by GluR agonists. Taken together, our data indicate that excessive GluR activation causes disruption of neuronal proteostasis by triggering the secretion of ER-resident proteins through ER Ca2+ depletion and describes a new facet of excitotoxicity.

Pharmacological Regulation of Endoplasmic Reticulum Structure and Calcium Dynamics: Importance for Neurodegenerative Diseases

The endoplasmic reticulum (ER) is the largest organelle of the cell, composed of a continuous network of sheets and tubules, and is involved in protein, calcium (Ca2+), and lipid homeostasis. In neurons, the ER extends throughout the cell, both somal and axodendritic compartments, and is highly important for neuronal functions. A third of the proteome of a cell, secreted and membrane-bound proteins, are processed within the ER lumen and most of these proteins are vital for neuronal activity. The brain itself is high in lipid content, and many structural lipids are produced, in part, by the ER. Cholesterol and steroid synthesis are strictly regulated in the ER of the blood-brain barrier protected brain cells. The high Ca2+ level in the ER lumen and low cytosolic concentration is needed for Ca2+-based intracellular signaling, for synaptic signaling and Ca2+ waves, and for preparing proteins for correct folding in the presence of high Ca2+ concentrations to cope with the high concentrations of extracellular milieu. Particularly, ER Ca2+ is controlled in axodendritic areas for proper neurito- and synaptogenesis and synaptic plasticity and remodeling. In this review, we cover the physiologic functions of the neuronal ER and discuss it in context of common neurodegenerative diseases, focusing on pharmacological regulation of ER Ca2+ Furthermore, we postulate that heterogeneity of the ER, its protein folding capacity, and ensuring Ca2+ regulation are crucial factors for the aging and selective vulnerability of neurons in various neurodegenerative diseases. SIGNIFICANCE STATEMENT: Endoplasmic reticulum (ER) Ca2+ regulators are promising therapeutic targets for degenerative diseases for which efficacious drug therapies do not exist. The use of pharmacological probes targeting maintenance and restoration of ER Ca2+ can provide restoration of protein homeostasis (e.g., folding of complex plasma membrane signaling receptors) and slow down the degeneration process of neurons.

Prosaposin PS18 reduces dopaminergic neurodegeneration in a 6-hydroxydopamine rat model of Parkinson's disease

Saposin and its precursor prosaposin are endogenous proteins with neurotrophic and anti-apoptotic properties. Prosaposin or its analog prosaposin-derived 18-mer peptide (PS18) reduced neuronal damage in hippocampus and apoptosis in stroke brain. Its role in Parkinson's disease (PD) has not been well characterized. This study aimed to examine the physiological role of PS18 in 6-hydroxydopamine (6-OHDA) cellular and animal models of PD. We found that PS18 significantly antagonized 6-OHDA -mediated dopaminergic neuronal loss and TUNEL in rat primary dopaminergic neuronal culture. In SH-SY5Y cells overexpressing the secreted ER calcium-monitoring proteins, we found that PS18 significantly reduced thapsigargin and 6-OHDA-mediated ER stress. The expression of prosaposin and the protective effect of PS18 were next examined in hemiparkinsonian rats. 6-OHDA was unilaterally administered to striatum. The expression of prosaposin was transiently upregulated in striatum on D3 (day 3) after lesioning and returned below the basal level on D29. The 6-OHDA-lesioned rats developed bradykinesia and an increase in methamphetamine-mediated rotation, which was antagonized by PS18. Brain tissues were collected for Western blot, immunohistochemistry, and qRTPCR analysis. Tyrosine hydroxylase immunoreactivity was significantly reduced while the expressions of PERK, ATF6, CHOP, and BiP were upregulated in the lesioned nigra; these responses were significantly antagonized by PS18. Taken together, our data support that PS18 is neuroprotective in cellular and animal models of PD. The mechanisms of protection may involve anti-ER stress.

Atransgenic mouse line for assaying tissue-specific changes in endoplasmic reticulum proteostasis

Maintenance of calcium homeostasis is important for proper endoplasmic reticulum (ER) function. When cellular stress conditions deplete the high concentration of calcium in the ER, ER-resident proteins are secreted into the extracellular space in a process called exodosis. Monitoring exodosis provides insight into changes in ER homeostasis and proteostasis resulting from cellular stress associated with ER calcium dysregulation. To monitor cell-type specific exodosis in the intact animal, we created a transgenic mouse line with a Gaussia luciferase (GLuc)-based, secreted ER calcium-modulated protein, SERCaMP, preceded by a LoxP-STOP-LoxP (LSL) sequence. The Cre-dependent LSL-SERCaMP mice were crossed with albumin (Alb)-Cre and dopamine transporter (DAT)-Cre mouse lines. GLuc-SERCaMP expression was characterized in mouse organs and extracellular fluids, and the secretion of GLuc-SERCaMP in response to cellular stress was monitored following pharmacological depletion of ER calcium. In LSL-SERCaMP × Alb-Cre mice, robust GLuc activity was observed only in the liver and blood, whereas in LSL-SERCaMP × DAT-Cre mice, GLuc activity was seen in midbrain dopaminergic neurons and tissue samples innervated by dopaminergic projections. After calcium depletion, we saw increased GLuc signal in the plasma and cerebrospinal fluid collected from the Alb-Cre and DAT-Cre crosses, respectively. This mouse model can be used to investigate the secretion of ER-resident proteins from specific cell and tissue types during disease pathogenesis and may aid in the identification of therapeutics and biomarkers of disease.

Protective mechanisms by glial cell line-derived neurotrophic factor and cerebral dopamine neurotrophic factor against the α-synuclein accumulation in Parkinson's disease

Synucleinopathies constitute a disease family named after alpha-synuclein protein, which is a significant component of the intracellular inclusions called Lewy bodies. Accompanying the progressive neurodegeneration, Lewy bodies and neurites are the main histopathologies of synucleinopathies. The complicated role of alpha-synuclein in the disease pathology makes it an attractive therapeutic target for disease-modifying treatments. GDNF is one of the most potent neurotrophic factors for dopamine neurons, whereas CDNF is protective and neurorestorative with entirely different mechanisms of action. Both have been in the clinical trials for the most common synucleinopathy, Parkinson's disease. With the AAV-GDNF clinical trials ongoing and the CDNF trial being finalized, their effects on abnormal alpha-synuclein accumulation are of great interest. Previous animal studies with an alpha-synuclein overexpression model have shown that GDNF was ineffective against alpha-synuclein accumulation. However, a recent study with cell culture and animal models of alpha-synuclein fibril inoculation has demonstrated the opposite by revealing that the GDNF/RET signaling cascade is required for the protective effect of GDNF on alpha-synuclein aggregation. CDNF, an ER resident protein, was shown to bind alpha-synuclein directly. CDNF reduced the uptake of alpha-synuclein fibrils by the neurons and alleviated the behavioral deficits induced by fibrils injected into the mouse brain. Thus, GDNF and CDNF can modulate different symptoms and pathologies of Parkinson's disease, and perhaps, similarly for other synucleinopathies. Their unique mechanisms for preventing alpha-synuclein-related pathology should be studied more carefully to develop disease-modifying therapies.

Mesencephalic astrocyte-derived neurotrophic factor (MANF) prevents the neuroinflammation induced dopaminergic neurodegeneration

BACKGROUND

The excessive activation of the microglia leads to the release of inflammatory factors that contribute to neuronal cell loss and neurodegeneration in Parkinson's Disease (PD). Mesencephalic astrocyte-derived neurotrophic factor (MANF) that belongs to a newly found neurotrophic factors (NTFs) family has been reported to promote neuronal survival in the PD models. However, the effects of the MANF on neuroinflammation in PD remain unclear.

METHODS

AAV8-MANF virus was constructed to determine whether the high expression of MANF can protect the neuroinflammation-induced dopaminergic neurodegeneration in rats with 6-OHDA-induced PD. Rotarod performance test, immunofluorescent staining and western bolt were employed to evaluate the behavioral dysfunction, dopaminergic neurodegeneration, microglia activation, and signal activation. 6-OHDA treated SH-SY5Y cells and LPS treated BV-2 cells were used as the in vitro model for MANF neuroprotective and neuroinflammation mechanisms. Cell vitality and apoptosis were evaluated with MTT, CCK-8 and flow cytometric analysis. The AKT/GSK3β-Nrf2 signaling and the TNF-α/IL6 expression were measured by Western Blot.

RESULTS

Our findings indicated that the elevated MANF expression by the AAV8-MANF administration ameliorated the motor dysfunction and protected the dopaminergic neurons in the 6-OHDA treated rats. The upregulated CD11b in the rat SN caused by the 6-OHDA administration was significantly attenuated by the pretreatment of the AAV8-MANF. Furthermore, the levels of p-AKT, p-GSK3β, BCL-2, and Nrf-2 were upregulated by the high expression of the MANF. Under the oxidative stress of the 6-OHDA, the MANF significantly reduced the apoptotic effect of the TNF-α on the SH-SY5Y cells. In the LPS treated BV-2 cells, the MANF reduced the production of the TNF-α and IL-6, via enhancing the Nrf-2, p-Akt, p-GSK3β, and p-NF-κβ level.

CONCLUSIONS

These results suggested that the MANF prevented the dopaminergic neurodegeneration caused by the microglia activation in PD via activation of the AKT/GSK3β-Nrf-2 signaling axis.

Emerging trophic activities of mesencephalic astrocyte-derived neurotrophic factor in tissue repair and regeneration

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a soluble endoplasmic reticulum (ER) luminal protein and its expression and secretion can be induced by ER stress. Despite initially being classified as a neurotrophic factor, MANF has been demonstrated to have restorative and protective effects in many different cell types such as neurons, liver cells, retinal cells, cardiac myocytes, and pancreatic β cells. However, underlying molecular mechanisms are complex and remain incompletely understood. The aims of this review are to highlight the latest advances in the understanding of the trophic activities of MANF in tissue repair and regeneration as well as underlying molecular mechanisms. The structural motifs and immune modulation of MANF are also described. We therefore propose that MANF might be a promising therapeutic target for tissue repair.

Mesencephalic astrocyte-derived neurotrophic factor (MANF): Structure, functions and therapeutic potential

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a novel evolutionarily conserved protein present in both vertebrate and invertebrate species. MANF shows distinct structural and functional properties than the traditional neurotrophic factors (NTF). MANF is composed of an N-terminal saposin-like lipid-binding domain and a C-terminal SAF-A/B, Acinus and PIAS (SAP) domain connected by a short linker. The two well-described activities of MANF include (1) role as a neurotrophic factor that plays direct neuroprotective effects in the nervous system and (2) cell protective effects in the animal models of non-neuronal diseases, including retinal damage, diabetes mellitus, liver injury, myocardial infarction, nephrotic syndrome, etc. The main objective of the current review is to provide up-to-date insights regarding the structure of MANF, mechanisms regulating its expression and secretion, physiological functions in various tissues and organs, protective effects during aging, and potential clinical applications. Together, this review highlights the importance of MANF in reversing age-related dysfunction and geroprotection.

Palmitate and thapsigargin have contrasting effects on ER membrane lipid composition and ER proteostasis in neuronal cells

The endoplasmic reticulum (ER) is an organelle that performs several key functions such as protein synthesis and folding, lipid metabolism and calcium homeostasis. When these functions are disrupted, such as upon protein misfolding, ER stress occurs. ER stress can trigger adaptive responses to restore proper functioning such as activation of the unfolded protein response (UPR). In certain cells, the free fatty acid palmitate has been shown to induce the UPR. Here, we examined the effects of palmitate on UPR gene expression in a human neuronal cell line and compared it with thapsigargin, a known depletor of ER calcium and trigger of the UPR. We used a Gaussia luciferase-based reporter to assess how palmitate treatment affects ER proteostasis and calcium homeostasis in the cells. We also investigated how ER calcium depletion by thapsigargin affects lipid membrane composition by performing mass spectrometry on subcellular fractions and compared this to palmitate. Surprisingly, palmitate treatment did not activate UPR despite prominent changes to membrane phospholipids. Conversely, thapsigargin induced a strong UPR, but did not significantly change the membrane lipid composition in subcellular fractions. In summary, our data demonstrate that changes in membrane lipid composition and disturbances in ER calcium homeostasis have a minimal influence on each other in neuronal cells. These data provide new insight into the adaptive interplay of lipid homeostasis and proteostasis in the cell.

Molecular mechanisms of programmed cell death in methamphetamine-induced neuronal damage

Methamphetamine, commonly referred to as METH, is a highly addictive psychostimulant and one of the most commonly misused drugs on the planet. Using METH continuously can increase your risk for drug addiction, along with other health complications like attention deficit disorder, memory loss, and cognitive decline. Neurotoxicity caused by METH is thought to play a significant role in the onset of these neurological complications. The molecular mechanisms responsible for METH-caused neuronal damage are discussed in this review. According to our analysis, METH is closely associated with programmed cell death (PCD) in the process that causes neuronal impairment, such as apoptosis, autophagy, necroptosis, pyroptosis, and ferroptosis. In reviewing this article, some insights are gained into how METH addiction is accompanied by cell death and may help to identify potential therapeutic targets for the neurological impairment caused by METH abuse.

Identification of ER/SR resident proteins as biomarkers for ER/SR calcium depletion in skeletal muscle cells

BACKGROUND

Aberrations to endoplasmic/sarcoplasmic reticulum (ER/SR) calcium concentration can result in the departure of endogenous proteins in a phenomenon termed exodosis. Redistribution of the ER/SR proteome can have deleterious effects to cell function and cell viability, often contributing to disease pathogenesis. Many proteins prone to exodosis reside in the ER/SR via an ER retention/retrieval sequence (ERS) and are involved in protein folding, protein modification, and protein trafficking. While the consequences of their extracellular presence have yet to be fully delineated, the proteins that have undergone exodosis may be useful for biomarker development. Skeletal muscle cells rely upon tightly coordinated ER/SR calcium release for muscle contractions, and perturbations to calcium homeostasis can result in myopathies. Ryanodine receptor type-1 (RYR1) is a calcium release channel located in the SR. Mutations to the RYR1 gene can compromise calcium homeostasis leading to a vast range of clinical phenotypes encompassing hypotonia, myalgia, respiratory insufficiency, ophthalmoplegia, fatigue and malignant hyperthermia (MH). There are currently no FDA approved treatments for RYR1-related myopathies (RYR1-RM).

RESULTS

Here we examine the exodosis profile of skeletal muscle cells following ER/SR calcium depletion. Proteomic analysis identified 4,465 extracellular proteins following ER/SR calcium depletion with 1,280 proteins significantly different than vehicle. A total of 54 ERS proteins were identified and 33 ERS proteins significantly increased following ER/SR calcium depletion. Specifically, ERS protein, mesencephalic astrocyte-derived neurotrophic factor (MANF), was elevated following calcium depletion, making it a potential biomarker candidate for human samples. Despite no significant elevation of MANF in plasma levels among healthy volunteers and RYR1-RM individuals, MANF plasma levels positively correlated with age in RYR1-RM individuals, presenting a potential biomarker of disease progression. Selenoprotein N (SEPN1) was also detected only in extracellular samples following ER/SR calcium depletion. This protein is integral to calcium handling and SEPN1 variants have a causal role in SEPN1-related myopathies (SEPN1-RM). Extracellular presence of ER/SR membrane proteins may provide new insight into proteomic alterations extending beyond ERS proteins. Pre-treatment of skeletal muscle cells with bromocriptine, an FDA approved drug recently found to have anti-exodosis effects, curbed exodosis of ER/SR resident proteins.

CONCLUSION

Changes to the extracellular content caused by intracellular calcium dysregulation presents an opportunity for biomarker development and drug discovery.

Pathways Linking Nicotinamide Adenine Dinucleotide Phosphate Production to Endoplasmic Reticulum Protein Oxidation and Stress

The endoplasmic reticulum (ER) lumen is highly oxidizing compared to other subcellular compartments, and maintaining the appropriate levels of oxidizing and reducing equivalents is essential to ER function. Both protein oxidation itself and other essential ER processes, such as the degradation of misfolded proteins and the sequestration of cellular calcium, are tuned to the ER redox state. Simultaneously, nutrients are oxidized in the cytosol and mitochondria to power ATP generation, reductive biosynthesis, and defense against reactive oxygen species. These parallel needs for protein oxidation in the ER and nutrient oxidation in the cytosol and mitochondria raise the possibility that the two processes compete for electron acceptors, even though they occur in separate cellular compartments. A key molecule central to both processes is NADPH, which is produced by reduction of NADP+ during nutrient catabolism and which in turn drives the reduction of components such as glutathione and thioredoxin that influence the redox potential in the ER lumen. For this reason, NADPH might serve as a mediator linking metabolic activity to ER homeostasis and stress, and represent a novel form of mitochondria-to-ER communication. In this review, we discuss oxidative protein folding in the ER, NADPH generation by the major pathways that mediate it, and ER-localized systems that can link the two processes to connect ER function to metabolic activity.

Caffeine and MDMA (Ecstasy) Exacerbate ER Stress Triggered by Hyperthermia

Drugs of abuse can cause local and systemic hyperthermia, a known trigger of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). Another trigger of ER stress and UPR is ER calcium depletion, which causes ER exodosis, the secretion of ER-resident proteins. In rodent models, club drugs such as 3,4-methylenedioxymethamphetamine (MDMA, ‘ecstasy’) can create hyperthermic conditions in the brain and cause toxicity that is affected by the environmental temperature and the presence of other drugs, such as caffeine. In human studies, MDMA stimulated an acute, dose-dependent increase in core body temperature, but an examination of caffeine and MDMA in combination remains a topic for clinical research. Here we examine the secretion of ER-resident proteins and activation of the UPR under combined exposure to MDMA and caffeine in a cellular model of hyperthermia. We show that hyperthermia triggers the secretion of normally ER-resident proteins, and that this aberrant protein secretion is potentiated by the presence of MDMA, caffeine, or a combination of the two drugs. Hyperthermia activates the UPR but the addition of MDMA or caffeine does not alter the canonical UPR gene expression despite the drug effects on ER exodosis of UPR-related proteins. One exception was increased BiP/GRP78 mRNA levels in MDMA-treated cells exposed to hyperthermia. These findings suggest that club drug use under hyperthermic conditions exacerbates disruption of ER proteostasis, contributing to cellular toxicity.

The Pancreatic ß-cell Response to Secretory Demands and Adaption to Stress

Pancreatic β cells dedicate much of their protein translation capacity to producing insulin to maintain glucose homeostasis. In response to increased secretory demand, β cells can compensate by increasing insulin production capability even in the face of protracted peripheral insulin resistance. The ability to amplify insulin secretion in response to hyperglycemia is a critical facet of β-cell function, and the exact mechanisms by which this occurs have been studied for decades. To adapt to the constant and fast-changing demands for insulin production, β cells use the unfolded protein response of the endoplasmic reticulum. Failure of these compensatory mechanisms contributes to both type 1 and 2 diabetes. Additionally, studies in which β cells are "rested" by reducing endogenous insulin demand have shown promise as a therapeutic strategy that could be applied more broadly. Here, we review recent findings in β cells pertaining to the metabolic amplifying pathway, the unfolded protein response, and potential advances in therapeutics based on β-cell rest.

Altered neuronal physiology, development, and function associated with a common chromosome 15 duplication involving CHRNA7

BACKGROUND

Copy number variants (CNVs) linked to genes involved in nervous system development or function are often associated with neuropsychiatric disease. While CNVs involving deletions generally cause severe and highly penetrant patient phenotypes, CNVs leading to duplications tend instead to exhibit widely variable and less penetrant phenotypic expressivity among affected individuals. CNVs located on chromosome 15q13.3 affecting the alpha-7 nicotinic acetylcholine receptor subunit (CHRNA7) gene contribute to multiple neuropsychiatric disorders with highly variable penetrance. However, the basis of such differential penetrance remains uncharacterized. Here, we generated induced pluripotent stem cell (iPSC) models from first-degree relatives with a 15q13.3 duplication and analyzed their cellular phenotypes to uncover a basis for the dissimilar phenotypic expressivity.

RESULTS

The first-degree relatives studied included a boy with autism and emotional dysregulation (the affected proband-AP) and his clinically unaffected mother (UM), with comparison to unrelated control models lacking this duplication. Potential contributors to neuropsychiatric impairment were modeled in iPSC-derived cortical excitatory and inhibitory neurons. The AP-derived model uniquely exhibited disruptions of cellular physiology and neurodevelopment not observed in either the UM or unrelated controls. These included enhanced neural progenitor proliferation but impaired neuronal differentiation, maturation, and migration, and increased endoplasmic reticulum (ER) stress. Both the neuronal migration deficit and elevated ER stress could be selectively rescued by different pharmacologic agents. Neuronal gene expression was also dysregulated in the AP, including reduced expression of genes related to behavior, psychological disorders, neuritogenesis, neuronal migration, and Wnt, axonal guidance, and GABA receptor signaling. The UM model instead exhibited upregulated expression of genes in many of these same pathways, suggesting that molecular compensation could have contributed to the lack of neurodevelopmental phenotypes in this model. However, both AP- and UM-derived neurons exhibited shared alterations of neuronal function, including increased action potential firing and elevated cholinergic activity, consistent with increased homomeric CHRNA7 channel activity.

CONCLUSIONS

These data define both diagnosis-associated cellular phenotypes and shared functional anomalies related to CHRNA7 duplication that may contribute to variable phenotypic penetrance in individuals with 15q13.3 duplication. The capacity for pharmacological agents to rescue some neurodevelopmental anomalies associated with diagnosis suggests avenues for intervention for carriers of this duplication and other CNVs that cause related disorders.

Computational Modeling of C-Terminal Tails to Predict the Calcium-Dependent Secretion of Endoplasmic Reticulum Resident Proteins

The lumen of the endoplasmic reticulum (ER) has resident proteins that are critical to perform the various tasks of the ER such as protein maturation and lipid metabolism. These ER resident proteins typically have a carboxy-terminal ER retention/retrieval sequence (ERS). The canonical ERS that promotes ER retrieval is Lys-Asp-Glu-Leu (KDEL) and when an ER resident protein moves from the ER to the Golgi, KDEL receptors (KDELRs) in the Golgi recognize the ERS and return the protein to the ER lumen. Depletion of ER calcium leads to the mass departure of ER resident proteins in a process termed exodosis, which is regulated by KDELRs. Here, by combining computational prediction with machine learning-based models and experimental validation, we identify carboxy tail sequences of ER resident proteins divergent from the canonical “KDEL” ERS. Using molecular modeling and simulations, we demonstrated that two representative non-canonical ERS can stably bind to the KDELR. Collectively, we developed a method to predict whether a carboxy-terminal sequence acts as a putative ERS that would undergo secretion in response to ER calcium depletion and interacts with the KDELRs. The interaction between the ERS and the KDELR extends beyond the final four carboxy terminal residues of the ERS. Identification of proteins that undergo exodosis will further our understanding of changes in ER proteostasis under physiological and pathological conditions where ER calcium is depleted.

Neurotoxicity of methamphetamine: Main effects and mechanisms

Methamphetamine (METH) is an illicit psychostimulant that is abused throughout the world. METH addiction is also a major public health concern and the abuse of large doses of the drug is often associated with serious neuropsychiatric consequences that may include agitation, anxiety, hallucinations, paranoia, and psychosis. Some human methamphetamine users can also suffer from attention, memory, and executive deficits. METH-associated neurological and psychiatric complications might be related, in part, to METH-induced neurotoxic effects. Those include altered dopaminergic and serotonergic functions, neuronal apoptosis, astrocytosis, and microgliosis. Here we have endeavored to discuss some of the main effects of the drug and have presented the evidence supporting certain of the molecular and cellular bases of METH neurotoxicity. The accumulated evidence suggests the involvement of transcription factors, activation of dealth pathways that emanate from mitochondria and endoplasmic reticulum (ER), and a role for neuroinflammatory mechanisms. Understanding the molecular processes involved in METH induced neurotoxicity should help in developing better therapeutic approaches that might also serve to attenuate or block the biological consequences of use of large doses of the drug by some humans who meet criteria for METH use disorder.

The Function of KDEL Receptors as UPR Genes in Disease

The KDEL receptor retrieval pathway is essential for maintaining resident proteins in the endoplasmic reticulum (ER) lumen. ER resident proteins serve a variety of functions, including protein folding and maturation. Perturbations to the lumenal ER microenvironment, such as calcium depletion, can cause protein misfolding and activation of the unfolded protein response (UPR). Additionally, ER resident proteins are secreted from the cell by overwhelming the KDEL receptor retrieval pathway. Recent data show that KDEL receptors are also activated during the UPR through the IRE1/XBP1 signaling pathway as an adaptive response to cellular stress set forth to reduce the loss of ER resident proteins. This review will discuss the emerging connection between UPR activation and KDEL receptors as it pertains to ER proteostasis and disease states.

A target-agnostic screen identifies approved drugs to stabilize the endoplasmic reticulum-resident proteome

Endoplasmic reticulum (ER) dysregulation is associated with pathologies including neurodegenerative, muscular, and diabetic conditions. Depletion of ER calcium can lead to the loss of resident proteins in a process termed exodosis. To identify compounds that attenuate the redistribution of ER proteins under pathological conditions, we performed a quantitative high-throughput screen using the Gaussia luciferase (GLuc)-secreted ER calcium modulated protein (SERCaMP) assay, which monitors secretion of ER-resident proteins triggered by calcium depletion. We identify several clinically used drugs, including bromocriptine, and further characterize them using assays to measure effects on ER calcium, ER stress, and ER exodosis. Bromocriptine elicits protective effects in cell-based models of exodosis as well as in vivo models of stroke and diabetes. Bromocriptine analogs with reduced dopamine receptor activity retain similar efficacy in stabilizing the ER proteome, indicating a non-canonical mechanism of action. This study describes a strategic approach to identify small-molecule drugs capable of improving ER proteostasis in human disease conditions.

Trophic activities of endoplasmic reticulum proteins CDNF and MANF

Mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) are endoplasmic reticulum (ER) luminal proteins that confer trophic activities in a wide range of tissues under diverse pathological conditions. Despite initially being classified as neurotrophic factors, neither protein structurally nor functionally resembles bona fide neurotrophic factors. Their highly homologous structures comprise a unique globular, saposin-like domain within the N-terminus joined by a flexible linker to a C-terminus containing a SAP-like domain, CXXC motif and an ER retention sequence. Neurotrophic factors exert effects by binding to cognate receptors in the plasma membrane; however, no cell surface receptors have been identified for MANF and CDNF. Both can act as unfolded protein response (UPR) genes that modulate the UPR and inflammatory processes. The trophic activity of MANF and CDNF extends beyond the central nervous system with MANF being crucial for the development of pancreatic β cells and both have trophic effects in a variety of diseases related to the liver, heart, skeletal tissue, kidney and peripheral nervous system. In this article, the unique features of MANF and CDNF, such as their structure and mechanisms of action related to ER stress and inflammation, will be reviewed. Recently identified interactions with lipids and membrane trafficking will also be described. Lastly, their function and therapeutic potential in different diseases including a recent clinical trial using CDNF to treat Parkinson's disease will be discussed. Collectively, this review will highlight MANF and CDNF as broad-acting trophic factors that regulate functions of the endoplasmic reticulum.

Preclinical model systems of ryanodine receptor 1-related myopathies and malignant hyperthermia: a comprehensive scoping review of works published 1990-2019

BACKGROUND

Pathogenic variations in the gene encoding the skeletal muscle ryanodine receptor (RyR1) are associated with malignant hyperthermia (MH) susceptibility, a life-threatening hypermetabolic condition and RYR1-related myopathies (RYR1-RM), a spectrum of rare neuromuscular disorders. In RYR1-RM, intracellular calcium dysregulation, post-translational modifications, and decreased protein expression lead to a heterogenous clinical presentation including proximal muscle weakness, contractures, scoliosis, respiratory insufficiency, and ophthalmoplegia. Preclinical model systems of RYR1-RM and MH have been developed to better understand underlying pathomechanisms and test potential therapeutics.

METHODS

We conducted a comprehensive scoping review of scientific literature pertaining to RYR1-RM and MH preclinical model systems in accordance with the PRISMA Scoping Reviews Checklist and the framework proposed by Arksey and O'Malley. Two major electronic databases (PubMed and EMBASE) were searched without language restriction for articles and abstracts published between January 1, 1990 and July 3, 2019.

RESULTS

Our search yielded 5049 publications from which 262 were included in this review. A majority of variants tested in RYR1 preclinical models were localized to established MH/central core disease (MH/CCD) hot spots. A total of 250 unique RYR1 variations were reported in human/rodent/porcine models with 95% being missense substitutions. The most frequently reported RYR1 variant was R614C/R615C (human/porcine total n = 39), followed by Y523S/Y524S (rabbit/mouse total n = 30), I4898T/I4897T/I4895T (human/rabbit/mouse total n = 20), and R163C/R165C (human/mouse total n = 18). The dyspedic mouse was utilized by 47% of publications in the rodent category and its RyR1-null (1B5) myotubes were transfected in 23% of publications in the cellular model category. In studies of transfected HEK-293 cells, 57% of RYR1 variations affected the RyR1 channel and activation core domain. A total of 15 RYR1 mutant mouse strains were identified of which ten were heterozygous, three were compound heterozygous, and a further two were knockout. Porcine, avian, zebrafish, C. elegans, canine, equine, and drosophila model systems were also reported.

CONCLUSIONS

Over the past 30 years, there were 262 publications on MH and RYR1-RM preclinical model systems featuring more than 200 unique RYR1 variations tested in a broad range of species. Findings from these studies have set the foundation for therapeutic development for MH and RYR1-RM.

Identification of drug modifiers for RYR1-related myopathy using a multi-species discovery pipeline

Ryanodine receptor type I-related myopathies (RYR1-RMs) are a common group of childhood muscle diseases associated with severe disabilities and early mortality for which there are no available treatments. The goal of this study is to identify new therapeutic targets for RYR1-RMs. To accomplish this, we developed a discovery pipeline using nematode, zebrafish, and mammalian cell models. We first performed large-scale drug screens in C. elegans which uncovered 74 hits. Targeted testing in zebrafish yielded positive results for two p38 inhibitors. Using mouse myotubes, we found that either pharmacological inhibition or siRNA silencing of p38 impaired caffeine-induced Ca²⁺ release from wild type cells while promoting intracellular Ca²⁺ release in Ryr1 knockout cells. Lastly, we demonstrated that p38 inhibition blunts the aberrant temperature-dependent increase in resting Ca²⁺ in myotubes from an RYR1-RM mouse model. This unique platform for RYR1-RM therapy development is potentially applicable to a broad range of neuromuscular disorders.

Muscle cells have storage compartments stuffed full of calcium, which they release to trigger a contraction. This process depends on a channel-shaped protein called the ryanodine receptor, or RYR1 for short. When RYR1 is activated, it releases calcium from storage, which floods the muscle cell. Mutations in the gene that codes for RYR1 in humans cause a group of rare diseases called RYR1-related myopathies. The mutations change calcium release in muscle cells, which can make movement difficult, and make it hard for people to breathe. At the moment, RYR1 myopathies have no treatment.

It is possible that repurposing existing drugs could benefit people with RYR1-related myopathies, but trialing treatments takes time. The fastest and cheapest way to test whether compounds might be effective is to try them on very simple animals, like nematode worms. But even though worms and humans share certain genes, treatments that work for worms do not always work for humans. Luckily, it is sometimes possible to test whether compounds might be effective by trying them out on complex mammals, like mice. Unfortunately, these experiments are slow and expensive. A compromise involves testing on animals such as zebrafish. So far, none of these methods has been successful in discovering treatments for RYR1-related myopathies.

To maximize the strengths of each animal model, Volpatti et al. combined them, developing a fast and powerful way to test new drugs. The first step is an automated screening process that trials thousands of chemicals on nematode worms. This takes just two weeks. The second step is to group the best treatments according to their chemical similarities and test them again in zebrafish. This takes a month. The third and final stage is to test promising chemicals from the zebrafish in mouse muscle cells. Of the thousands of compounds tested here, one group of chemicals stood out – treatments that block the activity of a protein called p38. Volpatti et al. found that blocking the p38 protein, either with drugs or by inactivating the gene that codes for it, changed muscle calcium release. This suggests p38 blockers may have potential as a treatment for RYR1-related myopathies in mammals.

Using three types of animal to test new drugs maximizes the benefits of each model. This type of pipeline could identify new treatments, not just for RYR1-related myopathies, but for other diseases that involve genes or proteins that are similar across species. For RYR1-related myopathies specifically, the next step is to test p38 blocking treatments in mice. This could reveal whether the treatments have the potential to improve symptoms.

Post-treatment with Posiphen Reduces Endoplasmic Reticulum Stress and Neurodegeneration in Stroke Brain

Acetylcholinesterase (AChE) inhibitors have protective and anti-inflammatory actions against brain injury, mediated by nicotinic α7 cholinergic receptor activation. The use of AChE inhibitors in patients is limited by systemic cholinergic side effects. Posiphen, a stereoisomer of the AChE inhibitor Phenserine, lacks AChE inhibitor activity. The purpose of this study is to determine the protective effect of Posiphen in cellular and animal models of stroke. Both Posiphen and Phenserine reduced glutamate-mediated neuronal loss in co-cultures of primary cortical cells and microglia. Phenserine-, but not Posiphen-, mediated neuroprotection was diminished by the nicotinic α7 receptor antagonist methyllycaconitine. Posiphen antagonized NMDA-mediated Ca⁺⁺ influx, thapsigargin-mediated neuronal loss and ER stress in cultured cells. Early post-treatment with Posiphen reduced ER stress signals, IBA1 immunoreactivity, TUNEL and infarction in the ischemic cortex, as well as neurological deficits in stroke rats. These findings indicate that Posiphen is neuroprotective against stroke through regulating Ca⁺⁺i and ER stress.

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Posiphen induces protection in cell culture through noncholinergic mechanism

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Posiphen attenuates glutamate-mediated Ca⁺⁺i and ER stress in neuronal culture

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Posiphen mitigates ER stress in stroke brain

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Posiphen reduces neurodegeneration in stroke rats

KDEL Receptors Are Differentially Regulated to Maintain the ER Proteome under Calcium Deficiency

Retention of critical endoplasmic reticulum (ER) luminal proteins needed to carry out diverse functions (e.g., protein synthesis and folding, lipid metabolism) is mediated through a carboxy-terminal ER retention sequence (ERS) and its interaction with KDEL receptors. Here, we demonstrate that depleting ER calcium causes mass departure of ERS-containing proteins from cells by overwhelming KDEL receptors. In addition, we provide evidence that KDELR2 and KDELR3, but not KDELR1, are unfolded protein response (UPR) genes upregulated as an adaptive response to counteract the loss of ERS-containing proteins, suggesting previously unknown isoform-specific functions of the KDEL receptors. Overall, our findings establish that decreases in ER calcium change the composition of the ER luminal proteome and secretome, which can impact cellular functions and cell viability. The redistribution of the ER proteome from inside the cell to the outside has implications for dissecting the complex relationship of ER homeostasis with diverse disease pathologies.

Discovery of endoplasmic reticulum calcium stabilizers to rescue ER-stressed podocytes in nephrotic syndrome

Emerging evidence has established primary nephrotic syndrome (NS), including focal segmental glomerulosclerosis (FSGS), as a primary podocytopathy. Despite the underlying importance of podocyte endoplasmic reticulum (ER) stress in the pathogenesis of NS, no treatment currently targets the podocyte ER. In our monogenic podocyte ER stress-induced NS/FSGS mouse model, the podocyte type 2 ryanodine receptor (RyR2)/calcium release channel on the ER was phosphorylated, resulting in ER calcium leak and cytosolic calcium elevation. The altered intracellular calcium homeostasis led to activation of calcium-dependent cytosolic protease calpain 2 and cleavage of its important downstream substrates, including the apoptotic molecule procaspase 12 and podocyte cytoskeletal protein talin 1. Importantly, a chemical compound, K201, can block RyR2-Ser2808 phosphorylation-mediated ER calcium depletion and podocyte injury in ER-stressed podocytes, as well as inhibit albuminuria in our NS model. In addition, we discovered that mesencephalic astrocyte-derived neurotrophic factor (MANF) can revert defective RyR2-induced ER calcium leak, a bioactivity for this ER stress-responsive protein. Thus, podocyte RyR2 remodeling contributes to ER stress-induced podocyte injury. K201 and MANF could be promising therapies for the treatment of podocyte ER stress-induced NS/FSGS.

Neuronal Activation Stimulates Cytomegalovirus Promoter-Driven Transgene Expression

The cytomegalovirus (CMV) immediate early promoter has been extensively developed and exploited for transgene expression in vitro and in vivo, including human clinical trials. The CMV promoter has long been considered a stable, constitutive, and ubiquitous promoter for transgene expression. Using two different CMV-based promoters, we found an increase in CMV-driven transgene expression in the rodent brain and in primary neuronal cultures in response to methamphetamine, glutamate, kainic acid, and activation of G protein-coupled receptor signaling using designer receptors exclusively activated by designer drugs (DREADDs). In contrast, promoters derived from human synapsin 1 (hSYN1) gene or elongation factor 1α (EF1α) did not exhibit altered transgene expression in response to the same neuronal stimulations. Overall, our results suggest that the long-standing assertion that the CMV promoter confers constitutive expression in neurons should be reevaluated, and future studies should empirically determine the activity of the CMV promoter in a given application.

Canvass: A Crowd-Sourced, Natural-Product Screening Library for Exploring Biological Space

Natural products and their derivatives continue to be wellsprings of nascent therapeutic potential. However, many laboratories have limited resources for biological evaluation, leaving their previously isolated or synthesized compounds largely or completely untested. To address this issue, the Canvass library of natural products was assembled, in collaboration with academic and industry researchers, for quantitative high-throughput screening (qHTS) across a diverse set of cell-based and biochemical assays. Characterization of the library in terms of physicochemical properties, structural diversity, and similarity to compounds in publicly available libraries indicates that the Canvass library contains many structural elements in common with approved drugs. The assay data generated were analyzed using a variety of quality control metrics, and the resultant assay profiles were explored using statistical methods, such as clustering and compound promiscuity analyses. Individual compounds were then sorted by structural class and activity profiles. Differential behavior based on these classifications, as well as noteworthy activities, are outlined herein. One such highlight is the activity of (-)-2(S)-cathafoline, which was found to stabilize calcium levels in the endoplasmic reticulum. The workflow described here illustrates a pilot effort to broadly survey the biological potential of natural products by utilizing the power of automation and high-throughput screening.

Downregulation of tyrosine hydroxylase phenotype after AAV injection above substantia nigra: Caution in experimental models of Parkinson's disease

Adeno-associated virus (AAV) vector-mediated delivery of human α-synuclein (α-syn) gene in rat substantia nigra (SN) results in increased expression of α-syn protein in the SN and striatum which can progressively degenerate dopaminergic neurons. Therefore, this model is thought to recapitulate the neurodegeneration in Parkinson's disease. Here, using AAV to deliver α-syn above the SN in male and female rats resulted in clear expression of human α-syn in the SN and striatum. The protein was associated with moderate behavioral deficits and some loss of tyrosine hydroxylase (TH) in the nigrostriatal areas. However, the immunohistochemistry results were highly variable and showed little to no correlation with behavior and the amount of α-syn present. Expression of green fluorescent protein (GFP) was used as a control to monitor gene delivery and expression efficacy. AAV-GFP resulted in a similar or greater TH loss compared to AAV-α-syn and therefore an additional vector that does not express a protein was tested. Vectors with double-floxed inverse open reading frame (DIO ORF) encoding fluorescent proteins that generate RNA that is not translated also resulted in TH downregulation in the SN but showed no significant behavioral deficits. These results demonstrate that although expression of wild-type human α-syn can cause neurodegeneration, the variability and lack of correlation with outcome measures are drawbacks with the model. Furthermore, design and control selection should be considered carefully because of conflicting conclusions due to AAV downregulation of TH, and we recommend caution with having highly regulated TH as the only marker for the dopamine system.

Carboxypeptidase O is a lipid droplet-associated enzyme able to cleave both acidic and polar C-terminal amino acids

Carboxypeptidase O (CPO) is a member of the M14 family of metallocarboxypeptidases with a preference for the cleavage of C-terminal acidic amino acids. CPO is largely expressed in the small intestine, although it has been detected in other tissues such as the brain and ovaries. CPO does not contain a prodomain, nor is it strongly regulated by pH, and hence appears to exist as a constitutively active enzyme. The goal of this study was to investigate the intracellular distribution and activity of CPO in order to predict physiological substrates and function. The distribution of CPO, when expressed in MDCK cells, was analyzed by immunofluorescence microscopy. Soon after addition of nutrient-rich media, CPO was found to associate with lipid droplets, causing an increase in lipid droplet quantity. As media became depleted, CPO moved to a broader ER distribution, no longer impacting lipid droplet numbers. Membrane cholesterol levels played a role in the distribution and in vitro enzymatic activity of CPO, with cholesterol enrichment leading to decreased lipid droplet association and enzymatic activity. The ability of CPO to cleave C-terminal amino acids within the early secretory pathway (in vivo) was examined using Gaussia luciferase as a substrate, C-terminally tagged with variants of an ER retention signal. While no effect of cholesterol was observed, these data show that CPO does function as an active enzyme within the ER where it removes C-terminal glutamates and aspartates, as well as a number of polar amino acids.

Ryanodine Receptor 1-Related Myopathies: Diagnostic and Therapeutic Approaches

Ryanodine receptor type 1-related myopathies (RYR1-RM) are the most common class of congenital myopathies. Historically, RYR1-RM classification and diagnosis have been guided by histopathologic findings on muscle biopsy. Main histological subtypes of RYR1-RM include central core disease, multiminicore disease, core-rod myopathy, centronuclear myopathy, and congenital fiber-type disproportion. A range of RYR1-RM clinical phenotypes has also emerged more recently and includes King Denborough syndrome, RYR1 rhabdomyolysis-myalgia syndrome, atypical periodic paralysis, congenital neuromuscular disease with uniform type 1 fibers, and late-onset axial myopathy. This expansion of the RYR1-RM disease spectrum is due, in part, to implementation of next-generation sequencing methods, which include the entire RYR1 coding sequence rather than being restricted to hotspot regions. These methods enhance diagnostic capabilities, especially given historic limitations of histopathologic and clinical overlap across RYR1-RM. Both dominant and recessive modes of inheritance have been documented, with the latter typically associated with a more severe clinical phenotype. As with all congenital myopathies, no FDA-approved treatments exist to date. Here, we review histopathologic, clinical, imaging, and genetic diagnostic features of the main RYR1-RM subtypes. We also discuss the current state of treatments and focus on disease-modulating (nongenetic) therapeutic strategies under development for RYR1-RM. Finally, perspectives for future approaches to treatment development are broached.

Extracellular esterase activity as an indicator of endoplasmic reticulum calcium depletion

CONTEXT

Endoplasmic reticulum (ER) calcium depletion is associated with diverse diseases, including cardiac, hepatic, and neurologic diseases.

OBJECTIVE

The aim of the present study was to identify and characterize an endogenous protein that could be used to monitor ER calcium depletion comparably to a previously described exogenous reporter protein.

MATERIALS AND METHODS

The use of a selective esterase-fluorescein diester pair allowed for carboxylesterase activity in extracellular fluid to be measured using a fluorescent readout. Cell culture media from three different cell lines, rat plasma, and human serum all possess quantifiable amounts of esterase activity.

RESULTS

Fluorescence produced by the interaction of carboxylesterases with a fluorescein diester substrate tracks with pharmacological and physiological inducers of ER calcium depletion. The fluorescence measured for in vitro and in vivo samples were consistent with ER calcium depletion being the trigger for increased esterase activity.

DISCUSSION

Decreased luminal ER calcium causes ER resident esterases to be released from the cell, and, when assessed concurrently with other disease biomarkers, these esterases may provide insight into the role of ER calcium homeostasis in human diseases.

CONCLUSION

Our results indicate that carboxylesterases are putative markers of ER calcium dysfunction.

Pre-α-pro-GDNF and Pre-β-pro-GDNF Isoforms Are Neuroprotective in the 6-hydroxydopamine Rat Model of Parkinson's Disease

Glial cell line-derived neurotrophic factor (GDNF) is one of the most studied neurotrophic factors. GDNF has two splice isoforms, full-length pre-α-pro-GDNF (α-GDNF) and pre-β-pro-GDNF (β-GDNF), which has a 26 amino acid deletion in the pro-region. Thus far, studies have focused solely on the α-GDNF isoform, and nothing is known about the in vivo effects of the shorter β-GDNF variant. Here we compare for the first time the effects of overexpressed α-GDNF and β-GDNF in non-lesioned rat striatum and the partial 6-hydroxydopamine lesion model of Parkinson's disease. GDNF isoforms were overexpressed with their native pre-pro-sequences in the striatum using an adeno-associated virus (AAV) vector, and the effects on motor performance and dopaminergic phenotype of the nigrostriatal pathway were assessed. In the non-lesioned striatum, both isoforms increased the density of dopamine transporter-positive fibers at 3 weeks after viral vector delivery. Although both isoforms increased the activity of the animals in cylinder assay, only α-GDNF enhanced the use of contralateral paw. Four weeks later, the striatal tyrosine hydroxylase (TH)-immunoreactivity was decreased in both α-GDNF and β-GDNF treated animals. In the neuroprotection assay, both GDNF splice isoforms increased the number of TH-immunoreactive cells in the substantia nigra but did not promote behavioral recovery based on amphetamine-induced rotation or cylinder assays. Thus, the shorter GDNF isoform, β-GDNF, and the full-length α-isoform have comparable neuroprotective efficacy on dopamine neurons of the nigrostriatal circuitry.

In vitro modeling of HIV proviral activity in microglia

Microglia, the resident macrophages of the brain, play a key role in the pathogenesis of HIV-associated neurocognitive disorders (HAND) due to their productive infection by HIV. This results in the release of neurotoxic viral proteins and pro-inflammatory compounds which negatively affect the functionality of surrounding neurons. Because models of HIV infection within the brain are limited, we aimed to create a novel microglia cell line with an integrated HIV provirus capable of recreating several hallmarks of HIV infection. We utilized clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing technology and integrated a modified HIV provirus into CHME-5 immortalized microglia to create HIV-NanoLuc CHME-5. In the modified provirus, the Gag-Pol region is replaced with the coding region for NanoLuciferase (NanoLuc), which allows for the rapid assay of HIV long terminal repeat activity using a luminescent substrate, while still containing the necessary genetic material to produce established neurotoxic viral proteins (e.g. tat, nef, gp120). We confirmed that HIV-NanoLuc CHME-5 microglia express NanoLuc, along with the HIV viral protein Nef. We subsequently exposed these cells to a battery of experiments to modulate the activity of the provirus. Proviral activity was enhanced by treating the cells with pro-inflammatory factors lipopolysaccharide (LPS) and tumor necrosis factor alpha and by overexpressing the viral regulatory protein Tat. Conversely, genetic modification of the toll-like receptor-4 gene by CRISPR/Cas9 reduced LPS-mediated proviral activation, and pharmacological application of NF-κB inhibitor sulfasalazine similarly diminished proviral activity. Overall, these data suggest that HIV-NanoLuc CHME-5 may be a useful tool in the study of HIV-mediated neuropathology and proviral regulation.

High fat diet disrupts endoplasmic reticulum calcium homeostasis in the rat liver

BACKGROUND & AIMS

Disruption to endoplasmic reticulum (ER) calcium homeostasis has been implicated in obesity, however, the ability to longitudinally monitor ER calcium fluctuations has been challenging with prior methodologies. We recently described the development of a Gaussia luciferase (GLuc)-based reporter protein responsive to ER calcium depletion (GLuc-SERCaMP) and investigated the effect of a high fat diet on ER calcium homeostasis.

METHODS

A GLuc-based reporter cell line was treated with palmitate, a free fatty acid. Rats intrahepatically injected with GLuc-SERCaMP reporter were fed a cafeteria diet or high fat diet. The liver and plasma were examined for established markers of steatosis and compared to plasma levels of SERCaMP activity.

RESULTS

Palmitate induced GLuc-SERCaMP release in vitro, indicating ER calcium depletion. Consumption of a cafeteria diet or high fat pellets correlated with alterations to hepatic ER calcium homeostasis in rats, shown by increased GLuc-SERCaMP release. Access to ad lib high fat pellets also led to a corresponding decrease in microsomal calcium ATPase activity and an increase in markers of hepatic steatosis. In addition to GLuc-SERCaMP, we have also identified endogenous proteins (endogenous SERCaMPs) with a similar response to ER calcium depletion. We demonstrated the release of an endogenous SERCaMP, thought to be a liver esterase, during access to a high fat diet. Attenuation of both GLuc-SERCaMP and endogenous SERCaMP was observed during dantrolene administration.

CONCLUSIONS

Here we describe the use of a reporter for in vitro and in vivo models of high fat diet. Our results support the theory that dietary fat intake correlates with a decrease in ER calcium levels in the liver and suggest a high fat diet alters the ER proteome. Lay summary: ER calcium dysregulation was observed in rats fed a cafeteria diet or high fat pellets, with fluctuations in sensor release correlating with fat intake. Attenuation of sensor release, as well as food intake was observed during administration of dantrolene, a drug that stabilizes ER calcium. The study describes a novel technique for liver research and provides insight into cellular processes that may contribute to the pathogenesis of obesity and fatty liver disease.

Lateral Hypothalamic GABAergic Neurons Encode Reward Predictions that Are Relayed to the Ventral Tegmental Area to Regulate Learning

Eating is a learned process. Our desires for specific foods arise through experience. Both electrical stimulation and optogenetic studies have shown that increased activity in the lateral hypothalamus (LH) promotes feeding. Current dogma is that these effects reflect a role for LH neurons in the control of the core motivation to feed, and their activity comes under control of forebrain regions to elicit learned food-motivated behaviors. However, these effects could also reflect the storage of associative information about the cues leading to food in LH itself. Here, we present data from several studies that are consistent with a role for LH in learning. In the first experiment, we use a novel GAD-Cre rat to show that optogenetic inhibition of LH γ-aminobutyric acid (GABA) neurons restricted to cue presentation disrupts the rats' ability to learn that a cue predicts food without affecting subsequent food consumption. In the second experiment, we show that this manipulation also disrupts the ability of a cue to promote food seeking after learning. Finally, we show that inhibition of the terminals of the LH GABA neurons in ventral-tegmental area (VTA) facilitates learning about reward-paired cues. These results suggest that the LH GABA neurons are critical for storing and later disseminating information about reward-predictive cues.

Assaying the Stability and Inactivation of AAV Serotype 1 Vectors

Adeno-associated virus (AAV) vectors are a commonplace tool for gene delivery ranging from cell culture to human gene therapy. One feature that makes AAV a desirable vector is its stability, in regard to both the duration of transgene expression and retention of infectivity as a viral particle. This study examined the stability of AAV serotype 1 (AAV1) vectors under different conditions. First, transducibility after storage at 4°C decreased 20% over 7 weeks. Over 10 freeze-thaw cycles, the resulting transduction efficiency became variable at 60-120% of a single thaw. Using small stainless steel slugs to mimic a biosafety cabinet or metal lab bench surface, it was found that an AAV1 vector can be reconstituted after 6 days of storage at room temperature. The stability of AAV is a desired feature, but effective decontamination procedures must be available for safety and experimental integrity. Multiple disinfectants commonly used in the laboratory for ability to inactivate an AAV1 vector were tested, and it was found that autoclaving, 0.25% peracetic acid, iodine, or 10% Clorox bleach completely prevented AAV-mediated transgene expression. These data suggest that peracetic acid should be used for inactivating AAV1 vectors on metal-based surfaces or instruments in order to avoid inadvertent transgene expression in human cells or cross-contamination of instruments.

Near-infrared fluorescent protein iRFP713 as a reporter protein for optogenetic vectors, a transgenic Cre-reporter rat, and other neuronal studies

BACKGROUND

The use of genetically-encoded fluorescent reporters is essential for the identification and observation of cells that express transgenic modulatory proteins. Near-infrared (NIR) fluorescent proteins have superior light penetration through biological tissue, but are not yet widely adopted.

NEW METHOD

Using the near-infrared fluorescent protein, iRFP713, improves the imaging resolution in thick tissue sections or the intact brain due to the reduced light-scattering at the longer, NIR wavelengths used to image the protein. Additionally, iRFP713 can be used to identify transgenic cells without photobleaching other fluorescent reporters or affecting opsin function. We have generated a set of adeno-associated vectors in which iRFP713 has been fused to optogenetic channels, and can be expressed constitutively or Cre-dependently.

RESULTS

iRFP713 is detectable when expressed in neurons both in vitro and in vivo without exogenously supplied chromophore biliverdin. Neuronally-expressed iRFP713 has similar properties to GFP-like fluorescent proteins, including the ability to be translationally fused to channelrhodopsin or halorhodopsin, however, it shows superior photostability compared to EYFP. Furthermore, electrophysiological recordings from iRFP713-labeled cells compared to cells labeled with mCherry suggest that iRFP713 cells are healthier and therefore more stable and reliable in an ex vivo preparation. Lastly, we have generated a transgenic rat that expresses iRFP713 in a Cre-dependent manner.

CONCLUSIONS

Overall, we have demonstrated that iRFP713 can be used as a reporter in neurons without the use of exogenous biliverdin, with minimal impact on viability and function thereby making it feasible to extend the capabilities for imaging genetically-tagged neurons in slices and in vivo.

Longitudinal monitoring of Gaussia and Nano luciferase activities to concurrently assess ER calcium homeostasis and ER stress in vivo

The endoplasmic reticulum (ER) is essential to many cellular processes including protein processing, lipid metabolism and calcium storage. The ability to longitudinally monitor ER homeostasis in the same organism would offer insight into progressive molecular and cellular adaptations to physiologic or pathologic states, but has been challenging. We recently described the creation of a Gaussia luciferase (GLuc)-based secreted ER calcium-modulated protein (SERCaMP or GLuc-SERCaMP) to longitudinally monitor ER calcium homeostasis. Here we describe a complementary tool to measure the unfolded protein response (UPR), utilizing a UPRE-driven secreted Nano luciferase (UPRE-secNLuc) to examine the activating transcription factor-6 (ATF6) and inositol-requiring enzyme 1 (IRE1) pathways of the UPR. We observed an upregulation of endogenous ATF6- and XBP1-regulated genes following pharmacologically-induced ER stress that was consistent with responsiveness of the UPRE sensor. Both GLuc and NLuc-based reporters have favorable properties for in vivo studies, however, they are not easily used in combination due to overlapping substrate activities. We describe a method to measure the enzymatic activities of both reporters from a single sample and validated the approach using culture medium and rat blood samples to measure GLuc-SERCaMP and UPRE-secNLuc. Measuring GLuc and NLuc activities from the same sample allows for the robust and quantitative measurement of two cellular events or cell populations from a single biological sample. This study is the first to describe the in vivo measurement of UPRE activation by sampling blood, using an approach that allows concurrent interrogation of two components of ER homeostasis.

Unconventional neurotrophic factors CDNF and MANF: Structure, physiological functions and therapeutic potential

Cerebral dopamine neurotrophic factor (CDNF) and mesencephalic astrocyte-derived neurotrophic factor (MANF) promote the survival of midbrain dopaminergic neurons which degenerate in Parkinson's disease (PD). However, CDNF and MANF are structurally and functionally clearly distinct from the classical, target-derived neurotrophic factors (NTFs) that are solely secreted proteins. In cells, CDNF and MANF localize in the endoplasmic reticulum (ER) and evidence suggests that MANF, and possibly CDNF, is important for the maintenance of ER homeostasis. MANF expression is particularly high in secretory tissues with extensive protein production and thus a high ER protein folding load. Deletion of MANF in mice results in a diabetic phenotype and the activation of unfolded protein response (UPR) in the pancreatic islets. However, information about the intracellular and extracellular mechanisms of MANF and CDNF action is still limited. Here we will discuss the structural motifs and physiological functions of CDNF and MANF as well as their therapeutic potential for the treatment of neurodegenerative diseases and diabetes. Currently available knockout models of MANF and CDNF in mice, zebrafish and fruit fly will increase information about the biology of these interesting proteins.

Role of two sequence motifs of mesencephalic astrocyte-derived neurotrophic factor in its survival-promoting activity

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a prosurvival protein that protects the cells when applied intracellularly in vitro or extracellularly in vivo. Its protective mechanisms are poorly known. Here we studied the role of two short sequence motifs within the carboxy-(C) terminal domain of MANF in its neuroprotective activity: the CKGC sequence (a CXXC motif) that could be involved in redox reactions, and the C-terminal RTDL sequence, an endoplasmic reticulum (ER) retention signal. We mutated these motifs and analyzed the antiapoptotic effect and intracellular localization of these mutants of MANF when overexpressed in cultured sympathetic or sensory neurons. As an in vivo model for studying the effect of these mutants after their extracellular application, we used the rat model of cerebral ischemia. Even though we found no evidence for oxidoreductase activity of MANF, the mutation of CXXC motif completely abolished its protective effect, showing that this motif is crucial for both MANF's intracellular and extracellular activity. The RTDL motif was not needed for the neuroprotective activity of MANF after its extracellular application in the stroke model in vivo. However, in vitro the deletion of RTDL motif inactivated MANF in the sympathetic neurons where the mutant protein localized to Golgi, but not in the sensory neurons where the mutant localized to the ER, showing that intracellular MANF protects these peripheral neurons in vitro only when localized to the ER.

A Low Affinity GCaMP3 Variant (GCaMPer) for Imaging the Endoplasmic Reticulum Calcium Store

Endoplasmic reticulum calcium homeostasis is critical for cellular functions and is disrupted in diverse pathologies including neurodegeneration and cardiovascular disease. Owing to the high concentration of calcium within the ER, studying this subcellular compartment requires tools that are optimized for these conditions. To develop a single-fluorophore genetically encoded calcium indicator for this organelle, we targeted a low affinity variant of GCaMP3 to the ER lumen (GCaMPer (10.19)). A set of viral vectors was constructed to express GCaMPer in human neuroblastoma cells, rat primary cortical neurons, and human induced pluripotent stem cell-derived cardiomyocytes. We observed dynamic changes in GCaMPer (10.19) fluorescence in response to pharmacologic manipulations of the ER calcium store. Additionally, periodic calcium efflux from the ER was observed during spontaneous beating of cardiomyocytes. GCaMPer (10.19) has utility in imaging ER calcium in living cells and providing insight into luminal calcium dynamics under physiologic and pathologic states.

Monitoring Endoplasmic Reticulum Calcium Homeostasis Using a Gaussia Luciferase SERCaMP

The endoplasmic reticulum (ER) contains the highest level of intracellular calcium, with concentrations approximately 5,000-fold greater than cytoplasmic levels. Tight control over ER calcium is imperative for protein folding, modification and trafficking. Perturbations to ER calcium can result in the activation of the unfolded protein response, a three-prong ER stress response mechanism, and contribute to pathogenesis in a variety of diseases. The ability to monitor ER calcium alterations during disease onset and progression is important in principle, yet challenging in practice. Currently available methods for monitoring ER calcium, such as calcium-dependent fluorescent dyes and proteins, have provided insight into ER calcium dynamics in cells, however these tools are not well suited for in vivo studies. Our lab has demonstrated that a modification to the carboxy-terminus of Gaussia luciferase confers secretion of the reporter in response to ER calcium depletion. The methods for using a luciferase based, secreted ER calcium monitoring protein (SERCaMP) for in vitro and in vivo applications are described herein. This video highlights hepatic injections, pharmacological manipulation of GLuc-SERCaMP, blood collection and processing, and assay parameters for longitudinal monitoring of ER calcium.

Mesencephalic astrocyte-derived neurotrophic factor and cerebral dopamine neurotrophic factor: New endoplasmic reticulum stress response proteins

Mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) are a novel evolutionary conserved neurotrophic factor (NTF) family. There are two distinct domains in MANF and CDNF 3-dimentional structure, N-terminal saposin-like domain and C-terminal SAP-domain, which suggest their unique mode of action. Although identified for their neurotrophic activity, recent studies have shown MANF and CDNF can protect cells during endoplasmic reticulum (ER) stress. This review summarizes the unique structure and related potential protective role for cells during ER stress of MANF and CDNF.