DECIPHERING THE ROLE OF SULFOTRANSFERASE 4A1 IN BRAIN DEVELOPMENT AND NEURONAL FUNCTIONING

Cytosolic sulfotransferases (SULTs) are enzymes that transfer a sulfuryl group from the obligate donor PAPS (3’-phosphoadenosine 5’-phosphosulfate) onto a variety of exogenous and endogenous substrates (Negishi 2001). In 2000, a novel member of this family (SULT4A1) was isolated from human and rat brain (Falany 2000). To date, the exact substrate and function of SULT4A1 are not fully addressed but since it is highly conserved and expressed extensively, and almost exclusively, in the brain, it is possible that SULT4A1 may have an important role in the central nervous system. Moreover, some recent reports have associated polymorphisms in the SULT4A1 gene with susceptibility to schizophrenia (Brennan 2005; Meltzer 2008); SULT4A1 has been suggested to be associated with neurological symptoms of Phelan-McDermid Syndrome (Disciglio 2014) and altered levels of SULT4A1 protein have been observed in bipolar and Alzheimer’s patients (Wang 2003; Ryan 2006). Given this background, we decided to investigate the still unknown role of SULT4A1 within neuron development and functioning. We started evaluating the physiological expression of SULT4A1 in the brain areas mainly involved in neuropsychiatric and neurodevelopmental disorders. To this purpose, we performed western blot analyses of total lysates of hippocampus, striatum, cerebral cortex and cerebellum dissected from adult mice (P60). Our results showed that SULT4A1 is highly expressed in all the analyzed areas, especially in cortex and in cerebellum. Moreover, area-specific expression of SULT4A1 appears to be similar between adult male and female mice. Considering the possible implication of SULT4A1 in the pathogenesis of neurodevelopmental disorders, a major point for our study was the evaluation of SULT4A1 expression during physiological neuronal maturation. To this purpose, we analyzed by western blot rat primary neuronal cultures at different stages of neuron maturation, which are Day-In-Vitro (DIV) 1, 7 and 14. From the results of these analyses, it was inferable that the expression of SULT4A1 appreciably rises during neuronal maturation, going from an almost undetectable level at DIV1 to an almost 4-fold greater level at DIV14 in cortical cultures. This result was confirmed by immunofluorescence (IF) staining of the same cultures where the protein showed a cytoplasmic localization and its level of expression steadily increased from DIV1 to DIV14. IF results also suggested that SULT4A1 is mainly expressed in GAD67-positive inhibitory neurons, in particular in Calbindin- and Parvalbumin-positive neurons. Therefore, to better determine SULT4A1 expression in human neurons, we obtained peripheral blood mononuclear cells (PBMCs) from control healthy individuals and reprogrammed them into induced Pluripotent Stem Cells (iPSCs). iPSC-derived neural stem cells (NSC) were differentiated into neurons for at least 50 days, time necessary to obtain MAP2-positive mature neurons. SULT4A1 expression was evaluated during neuronal maturation from NSC stage to mature neuron and the data from biochemical analysis suggested that the level of SULT4A1 protein rises during differentiation of NSCs into neurons. Considering that abnormalities in dendritic spines and neuronal arborization are some of the most consistent anatomical correlates of neurodevelopmental disorders (Hung 2008; Glausier 2013; Jiang 2013; Moyer 2015), we characterized the effect of SULT4A1 on spine dynamics and dendrite morphology: in particular, we overexpressed or silenced SULT4A1 in cortical cultures and, interestingly, we observed that both conditions altered neuronal arborization as well as spine density and morphology. Moreover, in light of the possibility that SULT4A1 polymorphisms may lead to a reduction of mRNA translatability (Brennan 2005) and to clarify the specific role of SULT4A1 in neuronal maturation and functioning, we further investigated the effects of SULT4A1 silencing. Biochemical and electrophysiological analyses of neurons infected or transfected with SULT4A1 shRNA demonstrated that SULT4A1 deficiency perturbs the composition and activity of excitatory and inhibitory synapses: indeed, we found an increase of GAD65 expression and a reduction of GluN1 levels. Interestingly, these data were in line with the electrophysiological recordings, where neurons lacking SULT4A1 displayed a slight augmentation of spontaneous inhibitory postsynaptic currents (sIPSC) frequency and a decrease of spontaneous excitatory postsynaptic currents (sEPSC) frequency.