Mislocalization of TAR DNA binding protein 43 kDa (TAPDBP, or TDP-43) is a principle pathological hallmark identified in cases of neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). As an RNA binding protein, TDP-43 serves in the nuclear compartment to repress nonconserved cryptic exons to ensure the generation of a normal transcriptome. Since nuclear depletion of TDP-43 is frequently observed independent of its cytosolic accumulation, a loss-of-function mechanism has been proposed to contribute to neurodegeneration. Multiple lines of evidence from animal models and human studies support the view that loss of TDP-43 leads to neuron loss, independent of its cytosolic aggregation. However, the underlying pathogenic pathways driven by the loss-of-function mechanism are still poorly defined.
We employed a genetic approach to determine the impact of TDP-43 loss in pyramidal neurons of the prefrontal cortex (PFC). Young adult male homozygous floxed Tdp-43F/F mice were bilaterally injected into the PFC with mixed viruses containing a Cre virus driven by the CaMKII promoter and a Cre-dependent virus expressing a genetically encoded green fluorescent calcium indicator, GCaMP6f. This approach allowed us to specifically deplete TDP-43 and express GCaMP6f in pyramidal neurons of the PFC. We also bilaterally injected a virus expressing GCaMP6f driven by the CaMKII promoter into the PFC of age-matched male Tdp-43F/F mice as controls. Using a custom-built miniscope imaging system, we performed repetitive in vivo calcium imaging from freely behaving mice for up to 7 months, to monitor the dynamic changes of calcium activity in PFC pyramidal neurons of TDP-43 depleted and TDP-43 intact mice.
By comparing calcium activity in PFC pyramidal neurons between TDP-43 depleted and TDP-43 intact mice, we demonstrated remarkably increased numbers of pyramidal neurons exhibiting hyperactive calcium activity after short-term TDP-43 depletion, followed by rapid activity declines during disease progression. We further demonstrated that long-term TDP-43 depletion was accompanied by a significant reduction in neuron numbers and increased gliosis in the PFC.
Our results suggest that TDP-43 loss-of-function drives dynamic changes in neural activity prior to neurodegeneration, highlighting the role of aberrant neural activity and dysfunctional calcium signaling in the pathogenesis of neurodegenerative diseases including ALS and FTD.