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CF innervation of Purkinje cells in ORP150 genetically manipulated mice. A , Sam- ple traces of CF-EPSCs recorded from ORP150 ϩ / Ϫ , ORP150 ϩ / ϩ , and Tg ORP150 Purkinje cells.
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The endoplasmic reticulum (ER) stress response contributes to neuronal survival in ischemia and neurodegenerative processes. ORP150 (oxygen-regulated protein 150)/HSP12A (heat shock protein 12A), a novel stress protein located in the ER, was markedly induced in Purkinje cells maximally at 4-8 d after birth, a developmental period corresponding to t...
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... to ischemic stress (Sieber et al., 1995; Yoshida et al., 2002), as well as other neurodegenerative-associated conditions (Dove et al., 2000). A likely final common pathway for such toxicity is elevation of free [Ca 2 ϩ ]i. Increased [Ca 2 ϩ ]i is associated with a number of cytotoxic events, including chronic ethanol intoxica- tion (Netzeband et al., 1999), diseases characterized by accumu- lation of proteins with polyglutamine repeats (Clark and Orr, 2000), and traumatic brain injury (Netzeband et al., 1999). Molecular chaperones in the ER have the capacity to function as a buffer system to suppress elevated [Ca 2 ϩ ]i by the maintaining the complex metabolic and biosynthetic properties of this organelle (Yu et al., 1999). In this context, we have demonstrated that ORP150 also suppresses elevations of [Ca 2 ϩ ]i in cultured hippocampal neurons exposed to excitatory amino acids (Kitao et al., 2001). Our preliminary observations reveal expression of ORP150 in Purkinje cells in the setting of human stroke and a primate model of experimental brain ischemia. Taken together, these observations further support the concept that expression of ORP150 in Purkinje cells during development (P4 – P8; Figs. 1, 2) is indicative of the presence of environmental stress, potentially ischemic, excitotoxic, or both (see below). Our current results demonstrate selective upregulation of ORP150 in the developing cerebellum, whereas levels of other molecular chaperones, such as GRP78, remain unchanged. From an evolutionary point of view, these two stress proteins (ORP150 and GRP78) have overlapping functions in yeast. Null mutant strains of lumenal Hsp seventy (LHS)1, the yeast homolog of ORP150, display “ compensatory ” upregulation of Kar2p, the yeast homolog of GRP78. Although each gene alone is not essential for yeast viability, lethality is observed when inactivating mutations are introduced into both Kar2p and LHS1 (Craven et al., 1996). In contrast, our previous study demonstrated embryonic lethality in homozygous ORP150 Ϫ / Ϫ embryos (in which the ORP150 gene had been deleted by homologous recombination and replaced by an inactive, truncated form). Thus, it appears that properties of ORP150 and GRP78 have diverged over time; the function of ORP150 cannot be complemented by increased expression of GRP78, and ORP150 appears to be essential for survival in mammalian embryogenesis (Craven et al., 1996). A similar critical role for the HSP47, another molecular chaperone in the ER, in embryonic development has been shown; a genomic mutant of HSP47 also results in embryonic lethality (Nagai et al., 2000). Mechanisms underlying the vulnerability of cerebellar Purkinje cells to environmental stress remain to be clarified. Cell differentiation and synaptogenesis in cerebellum come in different waves depending on the neuronal populations and afferents and are highly interactive mutually (Altman and Bayer, 1997). In rats, the first postnatal wave comes at P4, characterized with rapid growth in cortex. Most PCs were already multiply innervated by CFs as early as 3 d. The multiple innervation culminated on P5, which rapidly regressed later on (Zhao et al., 1998; Miranda- Contreras et al., 1999). Our results demonstrate increased Purkinje cell death at P4 – P6. Though the synaptic formation of climbing fibers from the inferior olive are still immature and confined in somatic regions (Altman and Bayer, 1997), strong immunostaining of glutamate receptor subunits 2 and 3 could be observed at postnatal days 1 – 3 within Purkinje cell bodies and primary dendrites (Bergmann et al., 1996; Hafidi and Hillman, 1997). At this stage of development, vesicular glutamate transporter is expressed in terminals around PC soma at P1 – P10 (Miyazaki T et al., 2003). Because this transporter mainly medi- ates the filling of cytoplasmic glutamate into synaptic vesicles in terminals, its expression indicates that glutamate release at CF 3 PC synapses is functional from the molecular point of view, suggesting that polyinnervation of CFs during the first postnatal week could be glutamate stress to developing PCs. A certain extent of Purkinje cell death at this point in development appears to be essential for optimal cerebellar function. Although synaptic properties of Purkinje cells in Tg ORP150 mice appeared normal (Fig. 7), and the number of these neurons was increased (Fig. 4), cerebellar function was clearly suboptimal (Fig. 6). Thus, it is possible that the agility with which certain neuronal populations mount an ER stress response may have important implications for their vulnerability to a range of environmental perturbations. Consistent with this concept, mutations in the presenilin-1 gene, a cause of familial Alzheimer ’ s disease, also renders neurons more sensitive to glutamate stress (Guo et al., 1999), probably via mod- ification of the ER stress response (Katayama et al., 1999). We have demonstrated that expression of ORP150 in developing brain most likely serves a cytoprotective function in Purkinje cells. Levels of ORP150 induced during brain development are carefully balanced to allow the appropriate amount of Purkinje cell death but to preserve the necessary number of these cerebellar neurons for normal function. The subtle nature of the system was revealed by overexpression of the ORP150 transgene in Purkinje cells; the number of Purkinje cells increased in Tg ORP150 animals, but cerebellar function was suboptimal. These observations with ORP150 emphasize the importance of ER stress in the Purkinje cell response to ischemia and, most likely, a range of environmental perturbations. It is intriguing to specu- late that the same mechanisms that may contribute to neuronal vulnerability to ischemia and excitotoxicity (and, potentially, other stresses), such as insufficient induction of ORP150 to uni- formly prevent cell death in the larger population of neurons, may be carefully programmed to prevent excess cell survival during development, at which time such additional neurons would compromise brain ...
Citations
... For example, the ER-resident protein ORP150/ HSP12A is involved in cerebellum development. Transgenic expression of this protein in neurons reduces PCs' apoptotic death and their vulnerability to hypoxic and excitotoxic stress, which subsequently leads to maintaining the survival of these cells during cerebellar development [88]. Defects in BAP (SIL1), another regulator of UPR, can also cause damage in PCs [166,167] and cerebral ataxia disease [10,133]. ...
Development is an evolutionary process that is tightly regulated in mammalian species. Several different cascades are involved in various stages of development. Among these mechanisms, apoptosis, autophagy, and unfolded protein response play critical roles in regulating development by affecting cell fate. All of these pathways are involved in the regulation of cell numbers via determining the life and death cycles of the cells. In this chapter, we first explain the brief mechanisms that are involved in the regulation of apoptosis, autophagy, and unfolded protein response, and later, we briefly describe how these mechanisms play roles in general development. We next address the critical role of these pathways in cerebellar development regulation and how they will aid in our knowledge of the processes behind neurodevelopmental disorders. Additionally, we summarize the present findings on neurological symptoms and disorders related to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and their linkage to autophagy pathways in the cerebellum.
... Particularly, Hsp12a and Hsp90aa1 are inducible stress proteins that prevent protein aggregation and enhance cell survival (Wynn et al., 1994). Notably, Hsp12a is an endoplasmic reticulum (ER) stress protein (Kitao et al., 2004). Hsp90aa1 (inducible isoform of Hsp90) mediates nucleus-cytoplasm trafficking of glucocorticoid receptors through interaction with cytoskeletal proteins (Pratt et al., 2006). ...
EFhd2 is a conserved calcium‐binding protein that is highly expressed in the central nervous system. We have shown that EFhd2 interacts with tau protein, a key pathological hallmark in Alzheimer's disease and related dementias. However, EFhd2’s physiological and pathological functions in the brain are still poorly understood. To gain insights into its physiological function, we identified proteins that co‐immunoprecipitated with EFhd2 from mouse forebrain and hindbrain, using tandem mass spectrometry (MS). In addition, quantitative mass spectrometry was used to detect protein abundance changes due to the deletion of the Efhd2 gene in mouse forebrain and hindbrain regions. Our data show that mouse EFhd2 is associated with cytoskeleton components, vesicle trafficking modulators, cellular stress response‐regulating proteins, and metabolic proteins. Moreover, proteins associated with the cytoskeleton, vesicular transport, calcium signaling, stress response, and metabolic pathways showed differential abundance in Efhd2(−/−) mice. This study presents, for the first time, an EFhd2 brain interactome that it is associated with different cellular and molecular processes. These findings will help prioritize further studies to investigate the mechanisms by which EFhd2 modulates these processes in physiological and pathological conditions of the nervous system.
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... Also typical for an Hsp70, both BiP activities involve an ATPase cycle, where the chaperone goes through states of substrate trapping in its ADP-bound state and substrate release in its ATP-bound state (Figure 5b). Furthermore, these activities involve JDPs, termed ERj-or ERdj-proteins, which stimulate the hydrolysis of BiP-bound ATP [84,199], and NEFs [200][201][202][203][204][205][206], which stimulate the exchange of ADP for ATP, thereby affecting BiP conformations allosterically [193,194]. Following the same principles and interactions, BiP also plays a central role in folding and assembly of newlyimported polypeptides, such as heavy and light chains of immunoglobulins in plasma cells of the immune system [85,185,186], and supports efficient Sec61 channel closing to preserve Ca 2+ homeostasis [69,207]. ...
... There are two Hsp70-type chaperones in the human ER (BiP and Grp170) but, more accurately, Grp170 is a Hsp110 protein family member [78,82,[201][202][203][204]. Hsp70-type molecular chaperones, such as BiP, bind reversibly to substrate polypeptides via their carboxyterminal substrate-binding domains (SBDs) (Figure 5b). ...
... Grp170 does not only act as a Hsp70-type chaperone, it also acts as one of the NEFs for BiP [201][202][203][204]. There is another functional homolog to bacterial GrpE in the ER lumen (termed Sil1 or BAP) [200,204], i.e., there is redundancy also at the level of the NEFs, which may explain the non-lethal phenotype of loss of Sil1 function that is associated with the neurodegenerative disease, Marinesco-Sjögren syndrome (see below and Table 1). ...
The rough endoplasmic reticulum (ER) of nucleated human cells has crucial functions in protein biogenesis, calcium (Ca2+) homeostasis, and signal transduction. Among the roughly one hundred components, which are involved in protein import and protein folding or assembly, two components stand out: The Sec61 complex and BiP. The Sec61 complex in the ER membrane represents the major entry point for precursor polypeptides into the membrane or lumen of the ER and provides a conduit for Ca2+ ions from the ER lumen to the cytosol. The second component, the Hsp70-type molecular chaperone immunoglobulin heavy chain binding protein, short BiP, plays central roles in protein folding and assembly (hence its name), protein import, cellular Ca2+ homeostasis, and various intracellular signal transduction pathways. For the purpose of this review, we focus on these two components, their relevant allosteric effectors and on the question of how their respective functional cycles are linked in order to reconcile the apparently contradictory features of the ER membrane, selective permeability for precursor polypeptides, and impermeability for Ca2+. The key issues are that the Sec61 complex exists in two conformations: An open and a closed state that are in a dynamic equilibrium with each other, and that BiP contributes to its gating in both directions in cooperation with different co-chaperones. While the open Sec61 complex forms an aqueous polypeptide-conducting- and transiently Ca2+-permeable channel, the closed complex is impermeable even to Ca2+. Therefore, we discuss the human hereditary and tumor diseases that are linked to Sec61 channel gating, termed Sec61-channelopathies, as disturbances of selective polypeptide-impermeability and/or aberrant Ca2+-permeability.
... Although BiP deficiency is embryonic lethal, the non-lethal phenotype of SIL1-deficient mice was hypothesized to be due to the redundancy of SIL1 with GRP170, which also demonstrates nucleotide exchange activity towards BiP [78][79][80][81]. Consistent with this possibility, the reduction of GRP170 expression worsens the cerebellar phenotype in Sil1disrupted mice and adversely impacts the previously unaffected IX and X lobules. ...
Cell surface and secreted proteins provide essential functions for multicellular life. They enter the endoplasmic reticulum (ER) lumen co-translationally, where they mature and fold into their complex three-dimensional structures. The ER is populated with a host of molecular chaperones, associated co-factors, and enzymes that assist and stabilize folded states. Together, they ensure that nascent proteins mature properly or, if this process fails, target them for degradation. BiP, the ER HSP70 chaperone, interacts with unfolded client proteins in a nucleotide-dependent manner, which is tightly regulated by eight DnaJ-type proteins and two nucleotide exchange factors (NEFs), SIL1 and GRP170. Loss of SIL1′s function is the leading cause of Marinesco-Sjögren syndrome (MSS), an autosomal recessive, multisystem disorder. The development of animal models has provided insights into SIL1′s functions and MSS-associated pathologies. This review provides an in-depth update on the current understanding of the molecular mechanisms underlying SIL1′s NEF activity and its role in maintaining ER homeostasis and normal physiology. A precise understanding of the underlying molecular mechanisms associated with the loss of SIL1 may allow for the development of new pharmacological approaches to treat MSS.
... The 12 genes were Gpr12, Hyou1, Rgma, Rad51, Dusp4, Chac1, Ier5, Ybx3, Pard6g, Bloc1s3, Sephs2, and Il7. Of these 12 genes, Hyou1, Chac1 and Bloc1s3 exhibit an apoptotic function (18)(19)(20)(21)(22). miRNAs upstream of these three mRNAs are miR-3591 (Hyou1), miR-181a-2-3p and miR-6321 (Chac1), miR-181a-2-3p (Bloc1s3). ...
... Hyou1 is involved in protein folding in the endoplasmic reticulum (ER) and suppresses ER stress (18). Chac1 plays a role in inducing cells under high ER stress to undergo apoptosis (19). ...
Background/aim:
miRNA molecules have been attracting attention as genetic modifiers between organs. We examined the relationship between serum miRNA and targeted liver mRNA profiles in a periodontitis rat model, and the influence of periodontitis on the liver.
Materials and methods:
Male Wistar rats (n=16, 8 weeks old) were randomly divided into two groups (8 rats each): control and periodontitis (ligature placement for 4 weeks). Serum miRNA and liver mRNA profiles were compared.
Results:
Periodontal destruction and hepatocyte apoptosis were induced in the periodontitis group. Microarray analysis indicated that 52 serum miRNAs and 33 liver mRNAs were expressed with a >1.5-fold change (FC) and a >2.0-FC (p<0.05), respectively, between the two groups. From the miRNA target genes, 12 genes equivalented to liver mRNAs with a >2.0-FC, among which, Hyou1, Chac1, and Bloc1s3 have apoptotic functions in our model. miRNAs upstream of these 3 mRNAs are miR-3591, miR-181a-2-3p and miR-6321.
Conclusion:
miR-3591, miR-181a-2-3p and miR-6321 induced hepatocyte apoptosis in our periodontitis rat model.
... On the other hand, induction of HSPA5 in the piriform cortex, may just reflect its particular susceptibility to neuronal damage following an epileptogenic insult (Vismer et al., 2015). In this context, it is of interest that ER stress is discussed as a relevant contributor to neuronal damage (Scheper and Hoozemans, 2015) and that the ER chaperone can serve a protective function (Kitao et al., 2001b;Kitao et al., 2004;Casas, 2017;Louessard et al., 2017). ...
Unfolded protein response is a signaling cascade triggered by misfolded proteins in the endoplasmic reticulum. Heat shock protein H4 (HSPH4) and A5 (HSPA5) are two chaperoning proteins present within the organelle, which target misfolded peptides during prolonged stress conditions. Epileptogenic insults and epileptic seizures are a notable source of stress on cells. To investigate whether they influence expression of these chaperones, we performed immunohistochemical stainings in brains from rats that experienced a status epilepticus (SE) as a trigger of epileptogenesis and from canine epilepsy patients. Quantification of HSPA5 and HSPH4 revealed alterations in hippocampus and parahippocampal cortex. In rats, SE induced up-regulation of HSPA5 in the piriform cortex and down-regulation of HSPA5 and HSPH4 in the hippocampus. Regionally restricted increases in expression of the two proteins has been observed in the chronic phase with spontaneous recurrent seizures. Confocal microscopy revealed a predominant expression of both proteins in neurons, no expression in microglia and circumscribed expression in astroglia. In canine patients, only up-regulation of HSPH4 expression was observed in Cornu Ammonis 1 region in animals diagnosed with structural epilepsy. This characterization of HSPA5 and HSPH4 expression provided extensive information regarding spatial and temporal alterations of the two proteins during SE-induced epileptogenesis and following epilepsy manifestations. Up-regulation of both proteins implies stress exerted on ER during these disease phases. Taken together suggest a differential impact of epileptogenesis on HSPA5 and HSPH4 expression and indicate them as a possible target for pharmacological modulation of unfolded protein response.
... Interestingly, SIL1 is widely expressed in many regions of the brain with the highest expression in the inferior olive and choroid plexus, yet the loss of SIL1 particularly affected the cerebellar Purkinje cells in lobules I-VIII and spared the lobule X and caudal lobule IX of the vestibulocerebellum, which represents a developmentally and functionally distinct region of the cerebellum (87). While BiP-deficiency is embryonic lethal, the non-lethal phenotype of SIL1-deficient mice was attributed to the redundancy of SIL1 with GRP170, which also demonstrates nucleotide exchange activity towards BiP (44,50,53,200). Consistent with this explanation, the reduction of GRP170 gene dosage worsens the cerebellar phenotype in SIL1-null mice and impacts the previously unaffected lobules IX and X, whereas its transgenic overexpression is able to completely rescue the Purkinje cell degeneration, proving it to be a modifier of this disease along with ERdj6 (91). Although the transgenic overexpression of GRP170 seemed to completely ameliorate the cerebellar pathology, the endogenous ability of different tissue systems to dramatically upregulate GRP170 might be capped since it has been associated with cerebellar dysfunction and vacuolar degeneration in skeletal muscles (200,201). ...
... Consistent with this explanation, the reduction of GRP170 gene dosage worsens the cerebellar phenotype in SIL1-null mice and impacts the previously unaffected lobules IX and X, whereas its transgenic overexpression is able to completely rescue the Purkinje cell degeneration, proving it to be a modifier of this disease along with ERdj6 (91). Although the transgenic overexpression of GRP170 seemed to completely ameliorate the cerebellar pathology, the endogenous ability of different tissue systems to dramatically upregulate GRP170 might be capped since it has been associated with cerebellar dysfunction and vacuolar degeneration in skeletal muscles (200,201). Mild to moderate intellectual disability is a cardinal feature of MSS that precipitates relatively early in the course of this disease (78). Inaguma et al. demonstrated data suggesting that SIL1 is regulated in a spatiotemporal manner during brain development, and SIL1-depletion in ventricular zone progenitor cells of embryonic murine brains caused an abnormal cellular morphology, aberrant neuronal migration pattern with delayed kinetics, and slower axonal growth, resembling the phenotype observed upon BiP-knockdown in these cells and mutant-BiP knock-in mice (78,90,202). ...
Marinesco-Sjögren syndrome (MSS) is a rare, autosomal recessive, multisystem disorder, which is characterized by cerebellar ataxia, early-onset bilateral cataracts, and progressive myopathy amongst other symptoms. MSS has been attributed to mutations in the SIL1 gene, which encodes a nucleotide exchange factor for the endoplasmicreticulum- resident Hsp70 chaperone, BiP. To date, there are 46 MSS-associated mutations that have been reported in SIL1, which occur throughout this gene and are predicted to result in a loss of SIL1’s function. The large majority of these mutations cause deletions of large fractions of the SIL1 protein. Nine MSS-associated mutations are particularly interesting because they alter less than six amino acids, yet are associated with a phenotype indistinguishable from a near-full length deletion of SIL1. The mechanisms by which these nine mutations lead to a loss of SIL1’s function are not well understood. On the other hand, it remains unclear how the loss of SIL1 leads to the multisystem defects observed in MSS, selectively affecting certain tissues while sparing others. Our goal was to answer these two questions.
We have shown that the selected nine MSS-associated SIL1 mutations may dramatically alter the protein microenvironment and disrupt intramolecular interactions, such that it alters the folding properties of SIL1 and renders it aggregation-prone. This offers a potential mechanism by which mutations in SIL1 cause a loss of its function. We validated that the C57BL/6 Sil1Gt mouse model, which harbors a genetic disruption of Sil1, phenocopies numerous aspects of the MSS-phenotype and represents a valid preclinical model system to investigate the MSS-associated pathology and explore pharmacotherapeutic strategies. Using a combination of the Sil1Gt mice and SIL1- deficient MSS-patient-derived lymphoblastoid cell lines, we explored the biosynthesis and secretion of immunoglobulins (Ig), which are the best characterized substrate of BiP to date. In vivo antigen-specific immunizations and ex vivo LPS stimulation of splenic B cells revealed that the Sil1Gt mouse was indistinguishable from wild-type age-matched controls, in terms of both the kinetics and magnitude of antigen-specific antibody responses. There was no significant accumulation of BiP-associated Ig assembly intermediates or evidence that another molecular chaperone system was used for antibody production in the LPS-stimulated splenic B cells from Sil1Gt mice. ER chaperones were expressed at the same level in wild-type and Sil1Gt mice, indicating that there was no evident compensation for the disruption of Sil1. These results were confirmed and extended in lymphoblastoid cell lines from individuals with MSS, leading us to conclude that, surprisingly, the SIL1 was dispensable for antibody production.
Using Sil1Gt mice, we next characterized the molecular aspects of progressive myopathy associated with MSS. Proteomic-profiling of quadriceps at the onset of myopathy revealed that SIL1 deficiency affected multiple pathways critical to muscle physiology. We observed an increase in ER chaperones prior to the onset of muscle weakness, which was complemented by up-regulation of multiple protein degradation pathways. These responses were inadequate to maintain normal expression of secretory pathway proteins, including insulin and IGF-1 receptors. There was a paradoxical downstream PI3K-AKT signaling and glucose uptake in Sil1-disrupted skeletal muscles, all of which were insufficient to maintain systemic glucose homeostasis and muscle mass. Together, these data reveal defects in maintaining ER homeostasis upon SIL1 loss, which are countered by multiple compensatory responses that are ultimately unsuccessful, leading to trans-organellar proteostasis collapse and myopathy.
... For example, the ER-resident protein ORP150/HSP12A is involved in cerebellum development. Transgenic expression of this protein in neurons reduces Purkinje cells apoptotic cell death and their vulnerability to hypoxic and excitotoxic stress, which subsequently leads to maintaining survival of these cells during cerebellar development [59]. Defects in BAP (SIL1), another regulator of UPR, can also cause damage in cerebellar Purkinje cells [128,129] and cerebral ataxia disease [5,103]. ...
Development is an evolutionary process that is tightly regulated in mammalian species. Several different cascades are involved in different stages of development. Among these mechanisms, apoptosis, autophagy, and unfolded protein response play critical roles in regulation of development by affecting the cell fate. All of these pathways are involved in regulation of cell number via determining the life and death cycles of the cells. In this chapter, we first explain the brief mechanisms that are involved in regulation of apoptosis, autophagy, and unfolded protein response, and later, we briefly describe how these mechanisms play roles in general development. We then discuss the importance of these pathways in regulation of cerebellar development.
... As expected, stimulators of the BiP/GRP78 ATPase activity also influence ER Ca 2+ homeostasis: The BiP/Grp78 nucleotide exchange factor Grp170/ORP150 is upregulated during hypoxia [131], increasing resistance to apoptosis from a variety of triggers in a Ca 2+ -dependent manner [132]. This function is perhaps especially important for neuronal survival [133]. Under conditions of accumulated misfolded proteins, the translocon-mediated leak increases, suggesting that BiP/Grp78 exerts its inhibitory effect on Ca 2+ leak only when it is not participating in protein folding. ...
The folding of secretory proteins is a mechanism that is well understood today, based on decades of research on endoplasmic reticulum (ER) chaperones. These chaperones interact with newly imported polypeptides close to the ER translocon. Classic examples for these proteins include the immunoglobulin binding protein (BiP/GRP78), and the lectins calnexin and calreticulin. Although not considered chaperones per se, the ER oxidoreductases of the protein disulfide isomerase (PDI) family subsequently complete the folding job by catalyzing the formation of disulfide bonds through cysteine oxidation. Research from the past decade has demonstrated that ER chaperones are multifunctional proteins. The regulation of ER-mitochondria Ca²⁺ crosstalk is one of their additional functions, as shown for calnexin, BiP/GRP78 or the oxidoreductases Ero1α and TMX1. This function depends on interactions of this group of proteins with the ER Ca²⁺ handling machinery. This novel function makes perfect sense for two reasons: i. It allows ER chaperones to control mitochondrial apoptosis instantly without a lengthy bypass involving the upregulation of pro-apoptotic transcription factors via the unfolded protein response (UPR); and ii. It allows the ER protein folding machinery to fine-tune ATP import via controlling the speed of mitochondrial oxidative phosphorylation. Therefore, the role of ER chaperones in regulating ER-mitochondria Ca²⁺ flux identifies the progression of secretory protein folding as a central regulator of cell survival and death, at least in cell types that secrete large amount of proteins. In other cell types, ER protein folding might serve as a sentinel mechanism that monitors cellular well-being to control cell metabolism and apoptosis. The selenoprotein SEPN1 is a classic example for such a role. Through the control of ER-mitochondria Ca²⁺-flux, ER chaperones and folding assistants guide cellular apoptosis and mitochondrial metabolism.
... Two time windows have been reported: in the first of these, apoptotic figures have been found at E15 [2]. In the second, dying PCs are seen from birth until P10 [16,19,25]. Thus, whereas the timing and extent of dying PCs have been addressed [16], to our knowledge, no attempts have been made to determine whether physiological PC death is related to its time of origin. ...
Natural cell death by apoptosis was studied in two neuronal populations of BALB/c, C57BL/6 and B6CBA-Aw-j/A hybrid stock mice: (I) dopaminergic (DA) neurons in choosing coronal levels throughout the anteroposterior extent of the substantia nigra pars compacta (SNc), and (II) Purkinje cells (PCs) in each vermal lobe of the cerebellar cortex. Mice were collected at postnatal day (P) 2 and P14 for the midbrain study, and at P4 and P7 for the analysis of the cerebellum. No DA cells with morphologic criteria for apoptosis were found. Moreover, when the combination of tyrosine hydroxylase and TUNEL or tyrosine hydroxylase and active caspase-3 immunohistochemistry were performed in the same tissue section, no DA cells TUNEL positives or active caspase-3-stained DA neurons were seen. On the other hand, when PCs were considered, data analysis revealed that more dying PCs were observed at P4 than at P7. Values of neuron death were highest in the central lobe; this was followed by the posterior and anterior lobes and then by the inferior lobe. To determine if apoptotic death of PCs is linked to their time-of-origin profiles, pregnant dams were administered with [3H]TdR on embryonic days 11-12, 12-13, 13-14 and 14-15. When TUNEL and [3H]TdR autoradiography or active caspase-3 immunohistochemistry and [3H]TdR autoradiography were combined in the same tissue section, results reveal that the naturally occurring PC death is not related to its time of origin but, rather, is random across age.
