Only 6 base pairs are necessary for MBNL binding: two pyrimidine mismatches and four guanosine–cytosine base pairs that form in a helical region of a stem-loop in the endogenous

pre-mRNA target [55] (Figure 3e). In the myotonic dystrophy gene (DM1), these two regions of the RNA reside on the 3′ and 5′ sides that surround the TNR [56]. The length of the TNR tract affects only MBNL binding and impairs its function. A loss-of-function in MBNL and a gain-of-function in CELF4 tend to favor generation of the alternatively spliced forms. click here TDP-43 also binds to both the 3′ and 5′ end of the DM1 mRNA, and raises the possibility of that binding of MBNL and TDP-43 occurs at the same sites. Whether these two proteins overlap in the recognition to mRNA is unknown, but the BAY 80-6946 manufacturer common binding sites and functionality in the DM1 mRNA raise the possibility that the bi-partite mRNA binding at

the C-terminus of TDP-43 integrates translation and splicing activity. Interestingly, TDP-43 controls its own expression through a negative feedback loop involving interactions with its mRNA at the 3′ end [57]. Furthermore, the domain structure of TDP-43 is similar to that of both heterogeneous nuclear ribonucleoprotein (hnRNP) and muscleblind (MBNL) [58] (Figure 3f): an N-terminal domain (NTD) and two tandem RNA recognition motifs (RRM1 and RRM2), followed by a C-terminal glycine-rich region (G) (Figure 3a–c). The C-terminus of TDP-43 acts as a hub that regulates both splicing and translation. Indeed, TNR coding transcripts are associated with an unusual type of translation, Repeat Associated Non-ATG translation (RAN-translation) [59••]. RAN-translation does not

require an ATG translation Adenosine triphosphate start site, and random translation at TNRs occurs in all reading frames [59••]. Given its hub-like features, maintaining the C-terminus of TDP-43 would appear to be a key regulatory process. Indeed, pathological TDP-43 in the cytoplasmic and intranuclear inclusions is hyper-phosphorylated, ubiquitinated, and cleaved to ∼25 kDa C-terminal fragments in affected brain regions [60]. C-terminal-deleted TDP-43 without the glycine-rich tail is sufficient to form a head-to-head homodimer primarily via its N-terminal domain, which form fibrils in vitro [ 60]. Thus, proteolytic cleavage of TDP-43 within the RRM2 removes the N-terminal dimerization domain and produces unassembled truncated RRM2 fragments, which can abnormally oligomerize into high-order inclusions ( Figure 3). The resulting increase in oxidative DNA damage promotes expansion indirectly by RNA-mediated depletion of TDP-43/FMRP/STAU1 in the nucleus and an increase in cellular stress. Whether this type of RNA-mediated mechanism applies to all triplet repeat disorders is unknown, but there are direct links between them and mitochondrial metabolism.