Transaldolase deficiency influences the pentose phosphate pathway, mitochondrial homoeostasis and apoptosis signal processing

TAL (transaldolase) was originally described in the yeast as an enzyme of the PPP (pentose phosphate pathway). However, certain organisms and mammalian tissues lack TAL, and the overall reason for its existence is unclear. Recently, deletion of Ser¹⁷¹ (TALΔS171) was found in five patients causing inactivation, proteasome-mediated degradation and complete deficiency of TAL. In the present study, microarray and follow-up Western-blot, enzyme-activity and metabolic studies of TALΔS171 TD (TAL-deficient) lymphoblasts revealed co-ordinated changes in the expression of genes involved in the PPP, mitochondrial biogenesis, oxidative stress, and Ca²⁺ fluxing. Sedoheptulose 7-phosphate was accumulated, whereas G6P (glucose 6-phosphate) was depleted, indicating a failure to recycle G6P for the oxidative branch of the PPP. Nucleotide analysis showed depletion of NADPH and NAD⁺ and accumulation of ADP-ribose. TD cells have diminished Δψ_m (mitochondrial transmembrane potential) and increased mitochondrial mass associated with increased production of nitric oxide and ATP. TAL deficiency resulted in enhanced spontaneous and H₂O₂-induced apoptosis. TD lymphoblasts showed increased expression of CD38, which hydrolyses NAD⁺ into ADP-ribose, a trigger of Ca²⁺ release from the endoplasmic reticulum that, in turn, facilitated CD20-induced apoptosis. By contrast, TD cells were resistant to CD95/Fas-induced apoptosis, owing to a dependence of caspase activity on redox-sensitive cysteine residues. Normalization of TAL activity by adeno-associated-virus-mediated gene transfer reversed the elevated CD38 expression, ATP and Ca²⁺ levels, suppressed H₂O₂- and CD20-induced apoptosis and enhanced Fas-induced cell death. The present study identified the TAL deficiency as a modulator of mitochondrial homoeostasis, Ca²⁺ fluxing and apoptosis.