Increased endoplasmic reticulum stress and decreased proteasomal function in lafora disease models lacking the phosphatase laforin.
BACKGROUND
Lafora progressive myoclonus epilepsy (Lafora disease; LD) is a fatal autosomal recessive neurodegenerative disorder caused by loss-of-function mutations in either the EPM2A gene, encoding the dual specificity phosphatase laforin, or the EPM2B gene, encoding the E3-ubiquitin ligase malin. Previously, we and others have shown that both proteins form a functional complex that regulates glycogen synthesis by a novel mechanism involving ubiquitination and proteasomal degradation of at least two proteins, glycogen synthase and R5/PTG. Since laforin and malin localized at the endoplasmic reticulum (ER) and their regulatory role likely extend to other proteins unrelated to glycogen metabolism, we postulated that their absence may also affect the ER-unfolded protein response pathway.


RESULTS
Here, we demonstrate that siRNA silencing of laforin in Hek293 and SH-SY5Y cells increases their sensitivity to agents triggering ER-stress, which correlates with impairment of the ubiquitin-proteasomal pathway and increased apoptosis. Consistent with these findings, analysis of tissue samples from a LD patient lacking laforin, and from a laforin knockout (Epm2a-/-) mouse model of LD, demonstrates constitutive high expression levels of ER-stress markers BIP/Grp78, CHOP and PDI, among others.


CONCLUSIONS
We demonstrate that, in addition to regulating glycogen synthesis, laforin and malin play a role protecting cells from ER-stress, likely contributing to the elimination of unfolded proteins. These data suggest that proteasomal dysfunction and ER-stress play an important role in the pathogenesis of LD, which may offer novel therapeutic approaches for this fatal neurodegenerative disorder.
Vernia
Santiago
S
Instituto de Biomedicina de Valencia, CSIC and CIBER de Enfermedades Raras, Valencia, Spain.
Rubio
Teresa
T
Heredia
Miguel
M
Rodríguez de Córdoba
Santiago
S
Sanz
Pascual
P
D000818:Animals; D002352:Carrier Proteins / Q000502:physiology; D045744:Cell Line, Tumor; D004195:Disease Models, Animal; D054637:Dual-Specificity Phosphatases / Q000502:physiology; D004721:Endoplasmic Reticulum / Q000378:metabolism / Q000473:pathology*; D000091342:Endoplasmic Reticulum Chaperone BiP; D006003:Glycogen / Q000378:metabolism; D006801:Humans; D020192:Lafora Disease / Q000235:genetics* / Q000473:pathology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D010744:Phosphoric Monoester Hydrolases / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D054558:Protein Tyrosine Phosphatases, Non-Receptor / Q000502:physiology; D025801:Ubiquitin / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases
D002352:Carrier Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; C000717247:HSPA5 protein, human; C000717248:Hspa5 protein, mouse; D025801:Ubiquitin; D006003:Glycogen; C478960:NHLRC1 protein, human; D044767:Ubiquitin-Protein Ligases; D010744:Phosphoric Monoester Hydrolases; D054637:Dual-Specificity Phosphatases; C491989:Epm2a protein, mouse; D054558:Protein Tyrosine Phosphatases, Non-Receptor; C114990:EPM2A protein, human; D046988:Proteasome Endopeptidase Complex