Bone Abstracts

Hypophosphatasia (HPP) is a rare disease characterized by low enzymatic activity of tissue-nonspecific alkaline phosphatase (TNSALP) resulting in an accumulation of its endogenous substrates like pyridoxal phosphate (PLP) and inorganic pyrophosphate (PP_i). The ectoenzyme plays an important role during bone mineralization and might contribute to proper function of kidney and muscle. Neurological symptoms of HPP like seizures, anxiety disorders and depression provide an indication of the enzyme’s relevance in the nervous system. The severity of symptoms varies strongly depending on the respective mutation in the ALPL gene and the residual alkaline phosphatase activity.

The neuroblastoma cell line SH-SY5Y was chosen as an in vitro model for unraveling the role of TNSALP in the nervous system and SH-SY5Y–TNSALP which is stably overexpressing TNSALP was created by lipofection. A comparative microarray in combination with Gostat analysis highlighted differences in genes relating to neurogenesis and axonal growth. Cells with higher TNSALP activity reacted quicker to treatment with differentiation media containing 1 mm all-trans retinoic acid and showed a stronger growth of projections compared to cells with lower activity. A possible explanation might be the differential expression of neuropilin1 (Nrp1). Quantitative real-time PCR confirmed the microarray analysis which indicated a negative correlation of Nrp1 and TNSALP expression. Nrp1 is a receptor for semaphorin3A and located in the plasma membrane. Nrp1 forms a complex with plexinA in order to transfer the extracellular signal towards a modulation of the cytoskeleton, finally resulting in an inhibition of neurite outgrowth.

As a conclusion the comparison of neuronal cell lines with different expression levels of tissue-nonspecific alkaline phosphatase reveals possible molecular explanations for its role during brain development and the transmission of neuronal signals in addition to its function during bone mineralization. Therefore the described data might indicate putative targets for therapies of the neuronal symptoms caused by HPP.