Inhibition of Calcium Uptake via the Sarco/Endoplasmic Reticulum Ca2+-ATPase in a Mouse Model of Sandhoff Disease and Prevention by Treatment with N-Butyldeoxynojirimycin

Abstract

Gangliosides are found at high levels in neuronal tissues where they play a variety of important functions. In the gangliosidoses, gangliosides accumulate because of defective activity of the lysosomal proteins responsible for their degradation, usually resulting in a rapidly progressive neurodegenerative disease. However, the molecular mechanism(s) leading from ganglioside accumulation to neurodegeneration is not known. We now examine the effect of ganglioside GM2 accumulation in a mouse model of Sandhoff disease (one of the GM2 gangliosidoses), the Hexb؊/؊ mouse. Microsomes from Hexb؊/؊ mouse brain showed a significant reduction in the rate of Ca 2؉ -uptake via the sarco/endoplasmic reticulum Ca 2؉ -ATPase (SERCA), which was prevented by feeding Hexb؊/؊ mice with N-butyldeoxynojirimycin (NB-DNJ), an inhibitor of glycolipid synthesis that reduces GM2 storage. Changes in SERCA activity were not due to transcriptional regulation but rather because of a decrease in V max . Moreover, exogenously added GM2 had a similar effect on SERCA activity. The functional significance of these findings was established by the enhanced sensitivity of neurons cultured from embryonic Hexb؊/؊ mice to cell death induced by thapsigargin, a specific SERCA inhibitor, and by the enhanced sensitivity of Hexb؊/؊ microsomes to calcium-induced calcium release. This study suggests a mechanistic link among GM2 accumulation, reduced SERCA activity, and neuronal cell death, which may be of significance for delineating the neuropathophysiology of Sandhoff disease.

ER stress and neurodegenerative diseases

“…62 Recently, it was shown that in G M2 -gangliosidosis, Sandhoff's disease the accumulation is due to a reduced uptake of calcium into the ER, which is driven by the sarcoplasmic/ER calcium ATPase. 63 So far the role of ER stress in the pathogenesis of Sandhoff's disease has not bees studied.…”

Gangliosides as apoptotic signals in ER stress response

“…7,47 Progress toward this endeavor has been made by generating mice with targeted disruptions of key genes that encode glycosyltransferases or glycosylhydrolases, the enzymes that control the synthesis and degradation of gangliosides. [48][49][50] Although no human diseases have yet been identified in which pathogenesis is attributed to genetic deficiency of any glycosyltransferase, the analyses of these genetically engineered null mice are beginning to identify the functions of complex gangliosides in development and cell differentiation.…”

“…47,77,78 For instance, accumulation of GlcCer, which is the main storage product in Gaucher disease, increases calcium mobilization from intracellular stores in cultured neurons, likely via amplification of the response of the RyaR to agonists, which leads to ER calcium release. 47,77 As a result, neurons show increased sensitivity to neurotoxic agents and Ca 2 þ -mediated cell death. Isolated microsomes from the brain of a Sandhoff disease mouse model that accumulate GM2 show substantially reduced rate of Ca 2 þ uptake because the V max of SERCA is reduced in those organelles.…”

“…Isolated microsomes from the brain of a Sandhoff disease mouse model that accumulate GM2 show substantially reduced rate of Ca 2 þ uptake because the V max of SERCA is reduced in those organelles. 47 This effect of GM2 is prevented if the mice are treated with a specific inhibitor of glycolipid synthesis that reduces GM2 storage, directly implicating this ganglioside in the function of SER-CA. 47 Thus, neuronal Ca 2 þ homeostasis is perturbed in Gaucher and Sandhoff diseases resulting in elevated cytosolic Ca 2 þ levels, which may lead to enhanced sensitivity of the neurons to stress and promote cell death.…”

“…The latter possibility would be consistent with the observed effect of GM2 accumulation on the activity of SERCA in microsomal membranes from Sandhoff disease mice and the cholesterol-induced ER stress response in macrophages from Niemann-Pick A/B mice. 47,79 Ultimately, the disruption of intracellular Ca 2 þ homeostasis in ganglioside-accumulating cells could affect protein folding in the ER and induce the ER stress response through the conventional route, thereby suggesting a novel mechanism of neuronal apoptosis that could also occur in other neurodegenerative diseases. 7 …”