TDP-43 and synaptic connections, a multi-protein affair
- Emanuele Buratti, Group Leader of the Molecular pathology lab at the International Centre for Genetic Engineering and Biotechnology (ICGEB), Trieste (Italy)
Pathological aggregation of TDP-43 is principally associated with Amyotrophic Lateral Sclerosis (ALS) but is also present in approximately 50% of all Frontotemporal dementia patients and is a co-pathology in ~40% of Alzheimer’s disease cases and in Chronic traumatic encephalopathy (CTE). In addition, it has also been described to occur in Inclusion Body Myositis (IBM), an acquired myopathy that displays selective slowly progressive degeneration of muscles accompanied by an inflammatory disorder. These observations suggest that TDP-43 pathological aggregation can occur in different brain areas/tissues and in many different pathological conditions. This suggests that the eventual consequences of its aggregation can be differentially affected depending on the relative abundance and expression of RNA binding proteins that are present in the local context.
To start addressing this issue, we have performed transcriptome analysis of SH-SY5Y cells silenced for DAZAP1, hnRNP Q, hnRNP D, hnRNP K and hnRNP U that have been known to affect TDP-43 pathology. After cross-comparing transcriptomic profiles of cells depleted by each of these factors, we identified seven commonly regulated transcripts: CHPF2, IGF2, IRAK2, RNF112, UBE2E3, C1orf226 and NOS1AP. Out of this list, NOS1AP (also known as CAPON) has recently emerged as an important player in brain physiology and pathophysiology. Several studies suggest that its interaction with nNOS contributes to NOS1AP-mediated excitotoxicity, the formation of neuronal processes and probably schizophrenia. Most importantly, we observed a clear correlation between the reduction of NOS1AP and the inclusion of two previously characterized cryptic exons in different brain regions of patients with TDP-43 pathology.
At the functional level, using primary mouse neuronal cultures we demonstrated that decrease of TDP-43 induces a drop in NOS1AP expression and protein levels and elicited a significant down-regulation at the mRNA level of several factors predicted to interact with NOS1AP. Specifically, we found a significant decrease in several essential component of the synaptic network: PSD93, PSD95, SynGAP, and Synapsin-3.
Most importantly, we observed that upregulation of NOS1AP in TDP-43 depleted SH-SY5Y cells can successfully rescue of many of these genes in the absence of TDP-43. In addition, downregulation of NOS1AP is also capable to rescue on its own the degenerative phenotype induced by TDP-43 overexpression in fly eyes.
Taken together, our identification of NOS1AP as a co-regulated target by several hnRNP proteins, including TDP-43, and the role of NOS1AP in the synaptic signaling can link this gene to neurological dysfunctions associated with ALS, making it a suitable candidate for the development of novel therapeutic strategies in the context of this pathology.