Role of Muscleblind Proteins in Health and Disease

Muscleblind (MBNL) proteins play a central role in the pathogenesis of myotonic dystrophy (DM), as MBNL depletion below a threshold level triggers a range of mRNA splicing, polyadenylation, localization, and processing defects and, thereby, the production of temporally inappropriate protein isoforms. This compromise of function for a wide range of proteins, in turn, leads to the multi-organ system phenotypes that characterize DM. To understand MBNL’s roles in normal development and function, and dysfunction in DM, it is essential to elucidate mechanisms underlying its spatial and temporal localization and steps necessary for its functional activation/inactivation.

Dissecting the Role of MBNL in the CNS

Understanding the regulation and functional roles of MBNL proteins in the CNS represents a critically important problem in determining pathogenic mechanisms underlying the nervous system phenotype in DM. Dr. Guey-Shin Wang at National Yang-Ming University and Academia Sinica (Taiwan) have published an article in Cell Reports that MBNL1 localized to the cytoplasm, but not that retained in the nucleus, plays a key role in neurite morphogenesis and show that this role is disrupted in DM1.

Using FLAG-tagged isoforms of MBNL1 (with and without exon 5) expressed in hippocampal neuron cultures, Dr. Wang and colleagues demonstrated that MBNL1 that included exon 5 was retained in the nucleus and had no impact on neuronal development, while exon 5-deleted MBNL1, which translocates to the cytoplasm, enhanced dendrite and axon morphogenesis.

Consistent with this role for MBNL1 in regulating neurite differentiation and the differentiation failure in DM1, the research team showed that either MBNL1 knockdown or overexpression of 960-repeat DMPK mRNA (DMPK-CUG⁹⁶⁰) resulted in delayed neurite maturation. Moreover, the cytoplasmic (exon 5 deleted) MBNL1 isoform, but not the nuclear-localized isoform, was found to rescue maturation defects in neurons expressing DMPK-CUG⁹⁶⁰ in a dose-dependent manner.

Mechanisms Regulating Neuronal Subcellular Distribution of MBNL1

Analysis of the developmental distribution of the two MBNL1 isoforms led to the puzzling finding that nuclear or cytoplasmic localization was independent of exon 5 inclusion/exclusion, suggesting that another factor regulated MBNL1 localization in the developing brain. The research team then demonstrated that Lys 63-linked ubiquitination of MBNL1 and neuronal activity level regulated its cellular localization. Finally, it was shown that deubiquitination caused by expanded repeat mRNA resulted in a cytoplasmic-to-nuclear translocation of MBNL1 and corresponding alterations in neurite morphogenesis—these consequences were prevented by deubiquitination inhibitors.

Taken together, Dr. Wang and colleagues have shown that MBNL1 plays a key role in neuronal differentiation and that disruption of its cellular localization alters functionally significant steps in neuronal structure and function that likely underlie the CNS consequences of DM. Moreover, their identification of the regulatory role that the ubiquitination/deubiquitination status plays in the cytoplasmic localization of MBNL1 suggests a novel potential target for therapeutic development in DM1.

Reference:

Ubiquitination of MBNL1 Is Required for Its Cytoplasmic Localization and Function in Promoting Neurite Outgrowth.
Wang PY, Chang KT, Lin YM, Kuo TY, Wang GS.
Cell Rep. 2018 Feb 27;22(9):2294-2306. doi: 10.1016/j.celrep.2018.02.025.