Interplay between GLI3 and ARpolyQ in SBMA: friend or foe?

Spinobulbar muscular atrophy (SBMA) is a neuromuscular disease caused by a CAG triplet repeat expansion in the androgen receptor (AR) gene. The expanded CAG sequence is translated into an elongated polyglutamine tract in the AR protein, generating ARpolyQ. ARpolyQ gains toxic properties by forming aggregates in response to androgens and at the same time it partially loses its transcriptional activity. Both aberrant behaviours ultimately lead to muscle atrophy and spinobulbar motor neuron (MN) degeneration. To investigate SBMA pathogenesis, we differentiated iPSCs derived from an SBMA patient (ARQ51) and their isogenic control (ARQ1) into MNs. By performing a differential gene expression analysis, we found reduced GLI3 mRNA levels in ARQ51 MNs compared to ARQ1. GLI3 is a key transcriptional regulator of spinal cord patterning which is proteolytically processed to generate opposite gradients of a full-length GLI3 activator (GLI3-FL) and a truncated GLI3 repressor (GLI3-R). The GLI3-FL/GLI3-R ratio regulates the specification of the ventral and dorsal spinal cord regions. Specifically, upon Sonic hedgehog signalling, GLI3-FL downregulates GLI3 expression, limiting GLI3-R levels and promoting MN maturation in the ventral spinal cord. Using MN-like NSC-34 cells, we showed that both wild-type AR and ARpolyQ bind to GLI3-FL in a testosterone-independent manner. Notably, we found that the transcriptional activity of ARpolyQ is reduced to a higher extent compared to that of wild-type AR upon co-expression with GLI3, indicating transcriptional reprogramming. In parallel, we observed that GLI3 tends to form more inclusions upon co-expression with ARpolyQ compared to wild-type AR, hinting at an altered processing of GLI3-FL into GLI3-R. Together, our data suggest that ARpolyQ and GLI3 interact and may undergo reciprocal regulation in SBMA. Further investigation will define the impact of ARpolyQ-GLI3 interaction on spinal cord patterning and myogenesis, another process in which GLI3 plays a key role. Acknowledgements: PRIN 2022EFLFL8 (CUP G53D23004450006)