C9orf72 and amyotrophic lateral sclerosis: GSK3β is a negative regulator of the VAPB-PTPIP51 interaction and is activated by FTD and/or ALS linked mutant TDP43, FUS, C9orf72 derived DPRs and Tau but whether GSK3β phosphorylates VAPB and/or PTPIP51 or phosphorylates some other protein to indirectly regulate their binding is not known.