Wnt Signaling Alterations in the Human Spinal Cord of Amyotrophic Lateral Sclerosis Cases: Spotlight on Fz2 and Wnt5a

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder with no cure, and elucidation of the mechanisms mediating neuronal death in this neuropathology is crucial to develop effective treatments. It has recently been demonstrated in animal models that the Wnt family of proteins is involved in this neuropathology, although its potential involvement in case of humans is almost unknown. We analyzed the expression of Wnt signaling components in healthy and ALS human spinal cords by quantitative RT-PCR, and we found that most Wnt ligands, modulators, receptors, and co-receptors were expressed in healthy controls. Moreover, we observed clear alterations in the mRNA expression of different components of this family of proteins in human spinal cord tissue from ALS cases. Specifically, we detected a significant increase in the mRNA levels of Wnt3, Wnt4, Fz2, and Fz8, together with several non-significant increases in the mRNA expression of other genes such as Wnt2b, Wnt5a, Fz3, Lrp5, and sFRP3. Based on these observations and on previous reports of studies performed in animal models, we evaluated with immunohistochemistry the protein expression patterns of Fz2 and Fz5 receptors and their main ligand Wnt5a in control samples and ALS cases. No substantial changes were observed in Fz5 protein expression pattern in ALS samples. However, we detected an increase in the amount of Fz2+ astrocytes in the borderline between gray and white matter at the ventral horn in ALS samples. Finally, Wnt5a expression was observed in neurons and astrocytes in both control and ALS samples, although Wnt5a immunolabeling in astroglial cells was significantly increased in ALS spinal cords in the same region where changes in Fz2 were observed. Altogether, these observations strongly suggest that the Wnt family of proteins, and more specifically Fz2 and Wnt5a, might be involved in human ALS pathology.

Wnt antagonist FRZB is a muscle biomarker of denervation atrophy in amyotrophic lateral sclerosis

“…Our work underscores the importance of Wnt signaling in ALS. Prior reports indicate upregulation of β-catenin, Wnt ligands and certain Wnt antagonists in astrocytes and neurons of spinal cords from human ALS subjects and the SOD1 G93A mouse [40][41][42] . Our report is the first to describe a consistent induction of FRZB in limb muscles of ALS patients, which in combination with elevated β-catenin expression, reflects the complexity of molecular responses in muscle denervation.…”

TDP-43 proteinopathy alters the ribosome association of multiple mRNAs including the glypican Dally-like protein (Dlp)/GPC6

“…dlp is translated into a GPI anchored glypican that interacts with the Wg/Wnt pathway receptor Fz2, serving as both a cofactor and competitive inhibitor for substrate binding 43 . RNAi knockdown or overexpression of a dominant negative form of Fz2 resulted in lethality prior to the third instar stage in the context of both TDP-43 WT and TDP-43 G298S disease models suggesting that the Wg/Wnt signaling pathway may be compromised by TDP-43 proteinopathy as suggested by expression studies in patient tissues 76,77 . Although overexpression of wild-type Fz2 did not mitigate TDP-43 dependent locomotor defects as would be expected based on the lethality caused by Fz2 loss of function, it is possible that cell autonomous (i.e., motor neuron) and non cell autonomous (i.e., glia, muscle) aspects of Wg/Wnt signaling 74 may be at play and confound our genetic interaction experiments.…”

“…This is consistent with the established role of Wg/Wnt signaling in motor neurons and at the neuromuscular junction 74 . Interestingly, Wg/Wnt signaling has been shown to be dysregulated in ALS 75,76,77 , although its mechanisms remain poorly understood, in part due to the complexity of Wg/Wnt signaling and cross-talk with other pathways. Given these observations, we queried the pathway for candidate targets of TDP-43 mediated translational inhibition and found dlp mRNA to be enriched in TDP-43 complexes (TDP-43 WT further substantiating the possibility that TDP-43 proteinopathy affects this signaling pathway.…”

Potential Roles of the WNT Signaling Pathway in Amyotrophic Lateral Sclerosis

“…The expression of these molecules is altered in both ALS patients and transgenic mice (Table 1). For example, the number of FZD2-positive astrocytes are significantly elevated in ALS transgenic mice and ALS patients [28,37]. Primary cultivation of astrocytes from ALS transgenic mice also reveals upregulated FZD2 expression [28].…”

“…Primary cultivation of astrocytes from ALS transgenic mice also reveals upregulated FZD2 expression [28]. Moreover, FZD2 positive astrocytes are distributed in regions where neurons are lost [37]. WNT5A acts as the ligand of FZD2, whose expression is parallel with that of FZD2 and located in the same affected areas [28,37].…”