TDP-43 regulates retinoblastoma protein phosphorylation through the repression of cyclin-dependent kinase 6 expression

Abstract

TDP-43 (for TAR DNA binding protein) is a highly conserved heterogeneous nuclear ribonucleoprotein (hnRNP) involved in specific pre-mRNA splicing and transcription events. TDP-43 recently has been identified as the main component of cytoplasmic inclusions in frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS), two neurodegenerative disorders. The cellular role of this protein remains to be identified. Here, we show that loss of TDP-43 results in dysmorphic nuclear shape, misregulation of the cell cycle, and apoptosis. Removal of TDP-43 in human cells significantly increases cyclin-dependent kinase 6 (Cdk6) protein and transcript levels. The control of Cdk6 expression mediated by TDP-43 involves GT repeats in the target gene sequence. Cdk6 up-regulation in TDP-43-depleted cells is accompanied by an increase in phosphorylation of two of its major targets, the retinoblastoma protein pRb and pRb-related protein pRb2/p130. TDP-43 silencing also is followed by changes in the expression levels of several factors that control cell proliferation. Morphological nuclear defects and increased apoptosis upon TDP-43 loss are mediated via the pRb pathway because pRb-negative cells (Saos-2) do not undergo programmed cell death or nuclear shape deformation upon TDP-43 removal. Our results identify a regulatory target of TDP-43 and show that TDP-43 depletion has important consequences in essential metabolic processes in human cells.