De Lorenzo, Francesca
(2020)
Neurodegenerative diseases are characterized by the dysfunction and death of specific neuronal populations. Parkinson’s disease (PD) is caused by the progressive loss of dopamine neurons in the substantia nigra, whereas motor neurons (MNs) in the motor cortex, brain stem, and spinal cord degenerate and die in amyotrophic lateral sclerosis (ALS). Accumulation of misfolded proteins and endoplasmic reticulum (ER) stress are some common hallmarks in the pathophysiology of neurodegenerative diseases.
ER stress triggers the unfolded protein response (UPR), a physiological response that aims at restoring the ER homeostasis by degrading misfolded proteins, attenuating protein translation, and increasing the expression of ER chaperones important for protein folding. Initially the UPR is protective, but, upon prolonged ER stress, the UPR switches from an adaptive to a pro-apoptotic response.
Cerebral dopamine neurotrophic factor (CDNF) is an ER resident protein with neurotrophic properties that is protective and restorative in preclinical models of PD. The mechanism underlying CDNF’s action is still unclear, but experimental data suggest a possible involvement of CDNF in the ER homeostasis.
The aim of this thesis work was to study the therapeutic potential of CDNF in PD and ALS rodent models and investigate CDNF mode of action, with a special focus on the ER stress response.
Herein, we report that co-administration of CDNF and glial cell line-derived neurotrophic factor (GDNF) showed an additive neurorestorative effect in the unilateral 6-hydroxydopamine rat model of PD, suggesting a different mechanism of action for these two proteins. We found that GDNF activated the pro-survival MAPK/ERK and PI3K/AKT pathways in the striatal dopamine neurons within 1 hour from protein administration. In contrast, CDNF activated only the PI3K/AKT pathway and at 4 hours upon treatment. Furthermore, CDNF, but not GDNF, reduced the expression of UPR markers ATF6, p-eIF2α, and GRP78.
Therefore, the ability of CDNF to regulate ER stress was thoroughly investigated in three rodent models of ALS with different genetic etiology and disease progression. We showed that CDNF decreased the ER stress response specifically in MNs, by attenuating all three branches of the UPR, initiated by transducers inositol-requiring enzyme 1 (IRE1), protein kinase R (PKR)-like ER kinase (PERK), and activating transcription factor 6 (ATF6). CDNF treatment was effective in all three models, indicating that CDNF’s therapeutic effect was independent of disease etiology. CDNF rescued MNs from ER-stressed induce cell death, halting the progression of the disease and ameliorating the motor deficit in the SOD1-G93A mouse model and in the TDP43-M337V rat model.
Finally, we identified that depleting endogenous CDNF from the SOD1-G93A model worsened the motor symptoms in the mice, but did not affect their lifespan. The ER stress response in the Cdnf -/- SOD1-G93A mice was especially exacerbated in the skeletal muscle, where CDNF is normally highly expressed, and an overexpression of homologous protein mesencephalic astrocyte-derived neurotrophic factor (MANF) was detected in the same tissue. We observed a reduction in the number of lumbar MNs in Cdnf -/- SOD1-G93A compared to classical SOD1-G93A mice, which would explain the aggravated motor impairment. At this point, however, we could not determine whether the increase in MNs loss was caused by CDNF depletion in MNs, or rather a consequence of CDNF-deficiency in the degenerating muscle cells, targets of MNs.
It was previously reported that, in mice, endogenous CDNF is important for the development and maintenance of enteric submucosal neurons, as well as for the regulation of gastrointestinal transit. Remarkably, we found that Cdnf -/- mice had less lumbar MNs at 4 months, compared to WT littermates, although this decrease did not result in any motor deficit. These findings suggest that CDNF may also have a role in the development and/or survival of MNs.
Altogether, these studies indicate that ER stress is an important therapeutic target for neurodegenerative diseases, such as PD and ALS, and that CDNF is a promising drug candidate, due to its ability to attenuate all three pathways of UPR.