Lost in translation: absence of KIAA1324/ELAPOR1 protein in pathological TDP-43-affected neurons in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a movement disorder lacking effective diagnostics and therapeutics, largely due to its clinical and etiological heterogeneity. The unifying hallmark of TDP-43 pathology is found in approximately 97% of ALS patients, and 50% of frontotemporal dementia (FTD) patients. Indeed, TDP-43 has a central role in ALS/FTD disease mechanisms. An mRNA target of TDP-43 loss of function, KIAA1324/ELAPOR1, is consistently upregulated in various RNA-sequencing datasets from systems with TDP-43 depletion.

This study sought to investigate the TDP-43 target gene, KIAA1324, in the context of human brain tissue. We performed immunohistochemistry and image analysis on 10 ALS and 10 control brains to quantify the protein levels of KIAA1324 in TDP-43 pathology-affected cells. We then used immunocytochemistry of iPSC-derived neurons and mass spectroscopy of SH-SY5Y cells to investigate the relationship between KIAA1324 mRNA and the function of its cognate protein KIAA1324.

KIAA1324 expression was enriched in neurons in the human brain. While KIAA1324 mRNA increased in iPSC-derived neurons with TDP-43 depleted from the nucleus in vitro, in human post-mortem brain neurons, KIAA1324 protein was significantly decreased (p < 0.05) in cells with pathological TDP-43 (nuclear-cleared TDP-43 and cytoplasmic, phosphorylated TDP-43). This may be due to the alternative polyadenylation of KIAA1324 detected with TDP-43 depletion from iPSC-derived neurons, hypothesised to affect translation efficiency. Mass spectrometry of SH-SY5Y cells revealed that overexpression of KIAA1324 protein affects a network of mitochondrial proteins.

The clear inverse relationship between KIAA1324 mRNA levels and TDP-43 function, and the near complete absence of KIAA1324 protein from neurons with pathological TDP-43 in post-mortem brain tissue, suggests KIAA3142 function is impaired in TDP-43 proteinopathies. Therefore, in addition to there being various disease mechanisms implicated in ALS, and TDP-43 being a challenging disease target to restore, KIAA1324 emerges as another of the many targets downstream of TDP-43 that may need to be addressed to demonstrate a therapeutic effect in ALS/FTD.