Malonate, methylmalonate and propionate are potentially neurotoxic
metabolites in branched-chain organic acidurias. Their effects were tested
on cultured 3D rat brain cell aggregates, using dosages of 0.1, 1.0 and
10.0 mM with a short but intense (twice a day over 3 days) and a longer
but less intense treatment (every 3 rdday over 9 days).
CNS cell-specific immunohistochemical stainings allowed the follow-up of
neurons (axons, phosphorylated medium-weight neurofilament), astrocytes
(glial fibrillary acidic protein) and oligodendrocytes (myelin basic protein).
Methylmalonate and malonate were quantified by tandem mass spectrometry.
Tandem mass spectrometry analysis of harvested brain cell aggregates
revealed clear intracellular accumulation of methylmalonate and malonate.
In immunohistochemical stainings oligodendrocytes appeared the most
affected brain cells. The MBP signal disappeared already at 0.1 mM
treatment with each metabolite. Mature astrocytes were not affected by
propionate, while immature astrocytes on intense treatment with propionate
developed cell swelling. 1 mM methylmalonate induced cell swelling of
both immature and mature astrocytes , while 1 mM malonate only affected
mature astrocytes. Neurons were not affected by methylmalonate, but
10.0 mM malonate on less intense treatment and 0.1, 1.0 and 10.0 mM
propionate on intense treatment affected axonal growth.
Our study shows significant uptake and deleterious effects of these
metabolites on brain cells, principally on astrocytes and oligodendrocytes.
This may be explained by the absence of the pathway in glial cells, which
thus are not able to degrade these metabolites. Further studies are ongoing
to elucidate the underlying mechanisms of the observed neurotoxic effects.