More over, the dysregulated miRNAs showed consistent trends in all three of the PGRN FTLD TDP subtypes compared to PGRN FTLD TDP patients, suggesting the miRNA candidates we identified are unique to PGRN haploinsufficiency. In further support that the miRNAs dysregulated selleck chemical in our array and validation studies are under the control of systemic PGRN mediated mechan isms, we found that 5 miRNAs were also upre gulated in the cerebellum of PGRN FTLD TDP com pared to PGRN FTLD TDP patients. To further study the five candidate miRNAs, we silenced PGRN expression in SH SY5Y cells, however, none of the 3 miRNAs detectable in SY SY5Y cells dis played a significant difference in expression between con trol and PGRN silenced cells. This finding suggests that long term knockdown of PGRN may be necessary, con sistent with the late onset of symptoms in human FTLD patients.
The mechanism by which PGRN haploinsufficiency in FTLD patients leads to altered miRNA expression is currently unclear and requires future studies. Progranu lin downstream signalling involves ERK1 2 and AKT signalling and these are potential causes of altered miRNA expression. It is unlikely that the five miRNAs identified in this study are dysregulated as a result of TDP 43 aggregation since the FTLD TDP type 1 pathol ogy in the PGRN mutation carriers is indistinguishable from the pathology observed in sporadic FTLD TDP patients. It is now known that miRNAs can modulate mRNA stability and translation, therefore, we correlated publicly available mRNA expression results from spora dic FTLD TDP and PGRN FTLD TDP patients with bioinformatic miRNA target predictions for the 5 miRNAs upregulated in the frontal cortex and cerebellum.
Through this analysis, we identified 18 predicted gene targets with significantly downregulated mRNA expression profiles in PGRN FTLD TDP patients. The anti correlated expression of the upregulated miRNAs with their downregulated mRNA targets in PGRN patients parallels the estab lished miRNA mRNA regulatory relationship. Notably, Ingenuity pathway analysis of the 18 genes revealed that they have important links to biological functions involved in FTLD disease pathogenesis, including nervous system development, behavioural responses, and cell growth.
Indeed, ASTN1 is known to regulate neuronal migration in cortical regions of developing brain, SNCA is associated with neurodegeneration and dementias, including links to FTLD TDP in PGRN patients and REEP1 has been implicated in corticospinal neurode generative disorders. Brefeldin_A Importantly, only 3 genes are predicted to be targeted by 3 of the 5 miRNAs signifi cantly dysregulated in both frontal cortex and cerebel lum, including BAI3, a cell adhesion G protein coupled receptor. This finding is of significant interest since Bolli ger et al. recently reported that C1q like proteins can act as secreted signalling molecules that bind to BAI3 leading to the regulation of synapse formation and maintenance.