Cytoplasmic Human TDP-43 Mislocalization Induces Widespread Dendritic Spine Loss in Mouse Upper Motor Neurons

Abstract

Amyotrophic lateral sclerosis (ALS) is defined by the destruction of upper- and lower motor neurons. Post-mortem, nearly all ALS cases are positive for cytoplasmic aggregates containing the DNA/RNA binding protein TDP-43. Recent studies indicate that this pathogenic mislocalization of TDP-43 may participate in generating hyperexcitability of the upper motor neurons, the earliest detectable change in ALS patients, yet the mechanisms driving this remain unclear. We investigated how mislocalisation of TDP-43 could initiate network dysfunction in ALS. We employed a tetracycline inducible system to express either human wildtype TDP-43 (TDP-43WT) or human TDP-43 that cannot enter the nucleus (TDP-43ΔNLS) in excitatory neurons (Camk2α promoter), crossed Thy1-YFPH mice to visualize dendritic spines, the major site of excitatory synapses. In comparison to both TDP-43WT and controls, TDP-43ΔNLS drove a robust loss in spine density in all the dendrite regions of the upper motor neurons, most affecting thin spines. This indicates that TDP-43 is involved in the generation of network dysfunction in ALS likely through impacting the formation or durability of excitatory synapses. These findings are relevant to the vast majority of ALS cases, and provides further evidence that upper motor neurons may need to be protected from TDP-43 mediated synaptic excitatory changes early in disease.

Synaptic dysfunction in ALS and FTD: anatomical and molecular changes provide insights into mechanisms of disease

“…Morphological analysis of the spines in these mice revealed a significant impairment in the development of mature spines in the motor cortex and lowered synaptic transmission (Handley et al, 2017). Consistent with these findings, mice expressing TDP-43∆NLS, a form of TDP-43 that is retained in the cytoplasm, in excitatory neurons under control of the Camk2α promoter, showed TDP-43∆NLS, and not TDP-43WT expression, caused a significant loss in spine density in layer V UMN, with few mature spines and mostly thin spines (Dyer et al, 2021a). These changes corresponded with a decreased expression in AMPA receptor subunits, Gria1, Gria2, and Gria3, and NMDA receptor subunit 2A and hyperexcitability in layer V excitatory neurons of the motor cortex (Dyer et al, 2021b).…”

“…Other reported changes in different TDP-43 mouse models involve alterations in dendritic branching and spines (Fogarty et al, 2016;Handley et al, 2017;Wu et al, 2019;Dyer et al, 2021a). Conditional knockout of TDP-43 in the mouse forebrain (TDP-43cKO) using the Camk2α promoter, showed that loss of TDP-43 in neurons caused dendritic attrition in layer V neurons UMN and loss of spines (Wu et al, 2019).…”

Estrogen Enhances Dendrite Spine Function and Recovers Deficits in Neuroplasticity in the prpTDP-43A315T Mouse Model of Amyotrophic Lateral Sclerosis

“…In the motor cortex, which is a principal site of ALS pathology, we observe a loss of plasticity in the form of decreased turnover and formation of dendritic spines. Whilst early changes to dendritic spine density have now been observed in a range of disease models and across cortical layers [41,53,54,92], our results are the first to show altered plasticity in areas of the mammalian brain which are dysfunctional in ALS, which is in agreement with the previous LTP and LTD studies [25,26,90,91]. ALS and FTD, which previously have been associated through genetic and pathological overlap, are now linked through a physiological mechanism-altered dendritic spine plasticity.…”

Emerging Mechanisms Underpinning Neurophysiological Impairments in C9ORF72 Repeat Expansion-Mediated Amyotrophic Lateral Sclerosis/Frontotemporal Dementia

“…Furthermore, it appears that increased hyperexcitability can generate morphological changes. A study upon a nuclear localization sequence-deficient mouse model of TDP-43 identified that cytoplasmic mislocalization of TDP-43 drives intrinsic hyperexcitability and decreased excitatory synaptic inputs ( Dyer et al, 2021 ). Indeed, hyperexcitability may drive continued functional synaptic loss, dendritic spine loss and dendrite pathology in upper motor neurons that are commonly observed features in upper motor neurons of ALS patient post-mortem tissue ( Hammer et al, 1979 ; Genç et al, 2017 ) and other models, including TDP-43 A 315 T ( Handley et al, 2017 ), SOD1 G 93 A ( Fogarty et al, 2016b , 2017 ), and FUS R 521 G ( Sephtona et al, 2014 ).…”

“…Synaptic loss was observed in the prefrontal cortex of aged (4.5 months) transgenic mice expressing 80-repeat GR (GR 80 ) DPRs (Choi et al, 2019). TDP-43 mouse model shows intrinsic hyperexcitability and decreased excitatory synaptic inputs (Dyer et al, 2021).…”