We study theoretically the dynamical process of yielding in cyclically sheared amorphous materials, within a thermal elastoplastic model and the soft glassy rheology model. Within both models we find ...an initially slow accumulation, over many cycles after the inception of shear, of low levels of damage in the form strain heterogeneity across the sample. This slow fatigue then suddenly gives way to catastrophic yielding and material failure. Strong strain localization in the form of shear banding is key to the failure mechanism. We characterize in detail the dependence of the number of cycles N^{*} before failure on the amplitude of imposed strain, the working temperature, and the degree to which the sample is annealed prior to shear. We discuss our finding with reference to existing experiments and particle simulations, and suggest new ones to test our predictions.
We study theoretically the dynamical process of yielding in cyclically sheared amorphous materials, within a thermal elastoplastic model and the soft glassy rheology model. Within both models we find ...an initially slow accumulation, over many cycles after the inception of shear, of low levels of damage in the form strain heterogeneity across the sample. This slow fatigue then suddenly gives way to catastrophic yielding and material failure. Strong strain localisation in the form of shear banding is key to the failure mechanism. We characterise in detail the dependence of the number of cycles N* before failure on the amplitude of imposed strain, the working temperature, and the degree to which the sample is annealed prior to shear. We discuss our finding with reference to existing experiments and particle simulations, and suggest new ones to test our predictions.
The unfolded protein response (UPR) triggered by endoplasmic reticulum (ER) stress is a feature of many neurodegenerative diseases including Alzheimer's disease, Huntington's disease and Parkinson's ...disease (PD). Although the vast majority of PD is sporadic, mutations in a number of genes including PARK7 which encodes the protein DJ-1 have been linked to early-onset, familial PD. In this regard, both PD of sporadic and genetic origins exhibit markers of ER stress-induced UPR. However, the relationship between pathogenic mutations in PARK7 and ER stress-induced UPR in PD pathogenesis remains unclear. In most contexts, DJ-1 has been shown to protect against neuronal injury. However, we find that DJ-1 deficiency ameliorates death in the context of acute ER stress in vitro and in vivo. DJ-1 loss decreases protein and transcript levels of ATF4, a transcription factor critical to the ER response and reduces the levels of CHOP and BiP, its downstream effectors. The converse is observed with DJ-1 over-expression. Importantly, we find that over-expression of wild-type and PD-associated mutant form of PARK7
, enhances ER stress-induced neuronal death by regulating ATF4 transcription and translation. Our results demonstrate a previously unreported role for wild-type and mutant DJ-1 in the regulation of UPR and provides a potential link to PD pathogenesis.
Loss of function mutations in the PTEN‐induced putative kinase 1 (Pink1) gene have been linked with an autosomal recessive familial form of early onset Parkinson's disease (PD). However, the ...underlying mechanism(s) responsible for degeneration remains elusive. Presently, using co‐immunoprecipitation in HEK (Human embryonic kidney) 293 cells, we show that Pink1 endogenously interacts with FK506‐binding protein 51 (FKBP51 or FKBP5), FKBP5 and directly phosphorylates FKBP5 at Serine in an in vitro kinase assay. Both FKBP5 and Pink1 have been previously associated with protein kinase B (AKT) regulation. We provide evidence using primary cortical cultured neurons from Pink1‐deficient mice that Pink1 increases AKT phosphorylation at Serine 473 (Ser473) challenged by 1‐methyl‐4‐phenylpyridinium (MPP+) and that over‐expression of FKBP5 using an adeno‐associated virus delivery system negatively regulates AKT phosphorylation at Ser473 in murine‐cultured cortical neurons. Interestingly, FKBP5 over‐expression promotes death in response to MPP+ in the absence of Pink1. Conversely, shRNA‐mediated knockdown of FKBP5 in cultured cortical neurons is protective and this effect is reversed with inhibition of AKT signaling. In addition, shRNA down‐regulation of PH domain leucine‐rich repeat protein phosphatase (PHLPP) in Pink1 WT neurons increases neuronal survival, while down‐regulation of PHLPP in Pink1 KO rescues neuronal death in response to MPP+. Finally, using co‐immunoprecipitation, we show that FKBP5 interacts with the kinase AKT and phosphatase PHLPP. This interaction is increased in the absence of Pink1, both in Mouse Embryonic Fibroblasts (MEF) and in mouse brain tissue. Expression of kinase dead Pink1 (K219M) enhances FKBP5 interaction with both AKT and PHLPP. Overall, our results suggest a testable model by which Pink1 could regulate AKT through phosphorylation of FKBP5 and interaction of AKT with PHLPP. Our results suggest a potential mechanism by which PINK1‐FKBP5 pathway contributes to neuronal death in PD.
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Mutations in PD‐associated genes such as Pink1 have been shown to contribute to dysregulation of numerous cellular processes. We identified that Pink1 endogenously interacts with a novel interacting partner FKBP5 and directly phosphorylates it. We provide evidence that Pink1 regulates AKT Ser473 phosphorylation in response to MPP+‐induced mitochondrial stress and FKBP5 negatively regulates this phosphorylation and sensitizes cultured neurons to cell death. Our results suggest a model in which Pink1 regulates AKT through phosphorylation of FKBP5 and its interaction with AKT and PHLPP; a potential mechanism by which Pink1‐FKBP5 pathway may contribute to neuronal death in PD.
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Dysregulation of cell cycle machinery is implicated in a number of neuronal death contexts, including stroke. Increasing evidence suggests that cyclin-dependent kinases (Cdks) are inappropriately ...activated in mature neurons under ischemic stress conditions. We previously demonstrated a functional role for the cyclin D1/Cdk4/pRb (retinoblastoma tumor suppressor protein) pathway in delayed neuronal death induced by ischemia. However, the molecular signals leading to cyclin D/Cdk4/pRb activation following ischemic insult are presently not clear. Here, we investigate the cell division cycle 25 (Cdc25) dual-specificity phosphatases as potential upstream regulators of ischemic neuronal death and Cdk4 activation. We show that a pharmacologic inhibitor of Cdc25 family members (A, B, and C) protects mouse primary neurons from hypoxia-induced delayed death. The major contributor to the death process appears to be Cdc25A. shRNA-mediated knockdown of Cdc25A protects neurons in a delayed model of hypoxia-induced death
Similar results were observed
following global ischemia in the rat. In contrast, neurons singly or doubly deficient for Cdc25B/C were not significantly protective. We show that Cdc25A activity, but not level, is upregulated
following hypoxia and global ischemic insult
Finally, we show that shRNA targeting Cdc25A blocks Ser795 pRb phosphorylation. Overall, our results indicate a role for Cdc25A in delayed neuronal death mediated by ischemia.
A major challenge in stroke is finding an effective neuroprotective strategy to treat cerebral ischemic injury. Cdc25 family member A (Cdc25A) is a phosphatase normally activated during cell division in proliferating cells. We found that Cdc25A is activated in neurons undergoing ischemic stress mediated by hypoxia
and global cerebral ischemia in rats
We show that pharmacologic or genetic inhibition of Cdc25A activity protects neurons from delayed death
and
Downregulation of Cdc25A led to reduction in retinoblastoma tumor suppressor protein (pRb) phosphorylation. An increase in pRb phosphorylation has been previously linked to ischemic neuronal death. Our results identify Cdc25A as a potential target for neuroprotectant strategy for the treatment of delayed ischemic neuronal death.
Inappropriate activation of cell cycle proteins, in particular cyclin D/Cdk4, is implicated in neuronal death induced by various pathologic stresses, including DNA damage and ischemia. Key targets of ...Cdk4 in proliferating cells include members of the E2F transcription factors, which mediate the expression of cell cycle proteins as well as death-inducing genes. However, the presence of multiple E2F family members complicates our understanding of their role in death. We focused on whether E2F4, an E2F member believed to exhibit crucial control over the maintenance of a differentiated state of neurons, may be critical in ischemic neuronal death. We observed that, in contrast to E2F1 and E2F3, which sensitize to death, E2F4 plays a crucial protective role in neuronal death evoked by DNA damage, hypoxia, and global ischemic insult both in vitro and in vivo. E2F4 occupies promoter regions of proapoptotic factors, such as B-Myb, under basal conditions. Following stress exposure, E2F4-p130 complexes are lost rapidly along with the presence of E2F4 at E2F-containing B-Myb promoter sites. In contrast, the presence of E2F1 at B-Myb sites increases with stress. Furthermore, B-Myb and C-Myb expression increases with ischemic insult. Taken together, we propose a model by which E2F4 plays a protective role in neurons from ischemic insult by forming repressive complexes that prevent prodeath factors such as Myb from being expressed.
Background: The contribution of E2F4 to hypoxic/ischemic neuronal death is understood poorly.
Results: Loss of E2F4 leads to an increase in B-Myb and contributes to hypoxic/ischemic neuronal death.
Conclusion: E2F4 is important for survival following hypoxic/ischemic neuronal death.
Significance: Targeting E2F4-repressive functions may be important in maintaining neuronal survival under hypoxic/ischemic conditions.
Abstract
Loss of function mutations in the
PTEN
‐induced putative kinase 1 (Pink1) gene have been linked with an autosomal recessive familial form of early onset Parkinson's disease (
PD
). However, ...the underlying mechanism(s) responsible for degeneration remains elusive. Presently, using co‐immunoprecipitation in
HEK
(Human embryonic kidney) 293 cells, we show that Pink1 endogenously interacts with
FK
506‐binding protein 51 (
FKBP
51 or FKBP5),
FKBP
5 and directly phosphorylates
FKBP
5 at Serine in an
in vitro
kinase assay. Both
FKBP
5 and Pink1 have been previously associated with protein kinase B (
AKT
) regulation. We provide evidence using primary cortical cultured neurons from Pink1‐deficient mice that Pink1 increases
AKT
phosphorylation at Serine 473 (Ser473) challenged by 1‐methyl‐4‐phenylpyridinium (
MPP
+
) and that over‐expression of
FKBP
5 using an adeno‐associated virus delivery system negatively regulates
AKT
phosphorylation at Ser473 in murine‐cultured cortical neurons. Interestingly,
FKBP
5 over‐expression promotes death in response to
MPP
+
in the absence of Pink1. Conversely, sh
RNA
‐mediated knockdown of
FKBP
5 in cultured cortical neurons is protective and this effect is reversed with inhibition of
AKT
signaling. In addition, sh
RNA
down‐regulation of
PH
domain leucine‐rich repeat protein phosphatase (
PHLPP
) in Pink1
WT
neurons increases neuronal survival, while down‐regulation of
PHLPP
in Pink1
KO
rescues neuronal death in response to
MPP
+
. Finally, using co‐immunoprecipitation, we show that
FKBP
5 interacts with the kinase
AKT
and phosphatase
PHLPP
. This interaction is increased in the absence of Pink1, both in Mouse Embryonic Fibroblasts (
MEF
) and in mouse brain tissue. Expression of kinase dead Pink1 (K219M) enhances
FKBP
5 interaction with both
AKT
and
PHLPP
. Overall, our results suggest a testable model by which Pink1 could regulate
AKT
through phosphorylation of
FKBP
5 and interaction of
AKT
with
PHLPP
. Our results suggest a potential mechanism by which
PINK
1‐
FKBP
5 pathway contributes to neuronal death in
PD
.
Open Science Badges
This article has received a badge for *
Open Materials
* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at
https://cos.io/our-services/open-science-badges/
.
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