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Presenillin and CIC7

Presenillin, often referred to by its gene name PSEN1, is a subunit in the gamma secretase complex, which cleaves integral membrane proteins. This image was obtained from the UniProt link.

The UniProt and Go annotations categorize PSEN1 as a mult-pass membrane protein found in all stages of endosomes,axon projections, and the synapse. The structure in the lower left shows blue transmembrane passes that we are pretty sure of the stucture and some cytoplasmic domains that we are not so sure of. As we are about to learn, PSEN1 functions as a sorter of proteins in the lyosome too that include CIC7, expressing a familial Alzheimer’s Disease version of PSEN1 results in CIC7 not getting propering loaded in to the lysosome in addition to the v-ATPase H+ pump.. [1]

The Bose review covers the supply differences between the various isoforms of CIC proteins, mutations associated with disease, and so on.  The featured image stops short of sorting out when these various isoforms appear in early versus sorting and late endosomes. [2] Note that CIC family members pretty much follow PSEN1 around.

Part of the idea is that there are ways of packing lysosomes with CIC7 when PSEN1 is not functional. This strategy might be something to keep on the side while be explore why the PSEN1 and autophagy are so important in the brain and muscle. It seems to have to do with mitophagy specifically.

These images were obtained from ProteinAtlas

PSEN1 also facilitates endoplasmic reticulum delivery of  the V0a1 subunit, the critical 10 transmembrane span protein subunit of the vATPase V0 sector. PSEN1-FAD mutations and PSEN1 deficiency impair the glycosylation of V0a1 and increase its vulnerability to ER associated protein degradation (ERAD) leading to its premature degradation and to reduced vATPase assembly and acidification of lysosomes. [3]

This particular image is from teh Lee2010 study in which the authors used a technique called immunoprecipitation to demonstrate association between PSEN1 with various complexes in ghe Golgi that assemble newly synthesized proteins as they come off the ribosome. Glycosylation is the attachments of polysaccharides to the newly synthesized proteins. This study reinforces the UniProt annotation notion that PSEN1 is also a Golgi protein.

Failure of proper assembly of VO-ATPase / H+ pump is associated with

These are some points from the introduction to the 2017 Lee study [1]

  • Reduced CIC7 recruitment to the lysosomes, not in the Lee introduction, but in their study… CIC7 seems to follow the VO-ATPase / H+ pump
  • impaired lysosome acidification
  • accumulation of lysosome substrates
  • efflux of lysosomal Ca2+ from TRPML1 channels
  • reduced cell survival

Lee and coworkers obtained fibroblasts from patients with familial Azheimer’s Disease driven by mutations in PSEN1.  Fibroblasts in which PSEN1 was knocked out were also studied.  For reasons not disclosed in the introduction, β2-adrenergic receptor (β2-AR) agonists were evaluated as a way to normalize elevated pH in PSEN1 KO cells and primary fibroblasts derived from patients with familial AD.  β2-AR  adrenergic receptor agonists in particular would be considered as therapies.   Skimming through the reference section, it became evident that β2-AR  have been explored as Alzheimer’s Disease treatments and that Alzheimer’s Disease.  Dysfunctional autophagy is also a hallmark of neurodegenerative diseases in general.

These compounds, as

represented by the clinically used agent isoproterenol (ISO), reversed impairments of

lysosomal hydrolase activity and autophagic flux, and substantially reduced abnormal

accumulation of autophagic vesicles (AV).

Lee and coworkers found a lowered lysosomal Cl-  content in PSEN1-deficient cells explained by a deficiency of ClC-7 activity selectively in lysosomes.  This resulted from stalled ER-to-lysosome delivery of ClC-7 and a down regulation of gene transcriptional programs regulating this transport process. These

Most of the authors of the Lee 2020 are elsewhere.  The corresponding author, Dr Ralph Nixon, has positions at multiple research institutes in New York State.   Three years prior to the Lee publication,[1] collaborators in Spain and New York State (Columbia and Cornell Universities) induced mitochondrial damage in fibroblasts and iPSC  (induced pluripotent stem cells) that were induced to resemble neurons. [3]  While this publication was focused on cells from patients with familial Alzheimer’s Disease ( FAD), one would think that the techniques could be used for any diseases in which mitophagy is defective. [3] iPSC lines were generated by Andrew Sproul and Scott Noggle from New York Stem Cell Foundation from skin fibroblast cell lines.  This group used cells from three FAD patients who all had A264E PSEN1 gene.  In lay terms this is a replacement of the hydrophobic alanine amino acid with a charged glutamate.  Two of the FAD patients had only moderate dementia whereas the third had severe dementia.  Three other samples were obtained from normal age and gender matched controls.

Autophagy Flux Study
Fibroblasts and iPSC-derived neurons were treated with 20 mM carbonyl cyanide m-chlorophenylhydrazone (CCCP) for 24 h to cause sustained desolution of the H+ electrochemical gradient.  This was followed by  PBS or NH4Cl (15 mM) for 6 hours still in CCCP.  The NH4Cl was used to prevent acidification of the lysosomes.  The PINK1/PARK2 system that ubiquitin tags depolarized, dysfunctional mitochondria was given some discussion.  Defects in the tagging of dysfunctional mitochondria may account for sporadic cases of AD. In this particular version of FAD, the defect appeared to be in the lysosomal digestion process.   This post will not discuss the various protein components of the lysosome in normal versus FAD in response to mitochondrial damage. 

The ryanodine receptor is a calcium release channel in the sarcoplasmic reticulum found in cardiac and skeletal muscle.  Calcium release from the SR is a necessary event for contraction of these muscle groups.  In order for relaxation to occur, the RYR Ca2+ channel must close so that the SERCA Ca2+ can reload the SR storage.    During ischemic reperfusion injury proteins are damaged.  Pedrozo introduced three terms to describe removal processes for damaged proteins.

  1. Macro (auto) phagy appears to be autophagocygtosis of large amounts of material without fusing to the autolysosome.
  2. The proteosome is a non-organelle bound complex that digests misfolded proteins with a ubiquitin tag.  This is the preferred pathway during ischemic reperfusion injury.
  3. Chaperone mediated autophagy is the preferred pathway for removal of RyR damaged by a reactive oxygen species generator.  RyR2 contains numerous cysteines that may become oxidized in response to H2O2 and other reactive oxygen species.  These conformational changes may expose cryptic amino acid sequences that can be recognized by the heat shock protein 70 chaperone that recruits misfolded proteins to the autophagosome. 

References

  1. Lee JH, Wolfe DM, Darji S, McBrayer MK, Colacurcio DJ, Kumar A, Stavrides P, Mohan PS, Nixon RA. β2-adrenergic Agonists Rescue Lysosome Acidification and Function in PSEN1 Deficiency by Reversing Defective ER-to-lysosome Delivery of ClC-7. J Mol Biol. 2020 Apr 3;432(8):2633-2650. PMC free article
  2. Bose S, He H, Stauber T. Neurodegeneration Upon Dysfunction of Endosomal/Lysosomal CLC Chloride Transporters. Front Cell Dev Biol. 2021 Feb 23;9:639231. PMC free paper
  3. Lee JH, Yu WH, Kumar A, Lee S, Mohan PS, Peterhoff CM, Wolfe DM, Martinez-Viente M, Massey AC, Sovak G, Uchiyama Y, Westaway D, Cuervo AM, Nixon RA. Lysosomal proteolysis and autophagy require presenilin 1 and are disrupted by Alzheimer-related PS1 mutations. Cell. 2010 Jun 25;141(7):1146-58. PMC free paper

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