Teneurins and Alzheimer's disease: a suggestive role for a unique family of proteins

Morphological neurite changes induced by porcupine inhibition are rescued by Wnt ligands

Background

Wnt signaling plays key roles in cellular and physiological processes, including cell proliferation, differentiation and migration during development and tissue homeostasis in adults. This pathway can be defined as Wnt/β-catenin-dependent or β-catenin-independent or “non-canonical”, both signaling are involved in neurite and synapse development/maintenance. Porcupine (PORCN), an acylase that o-acylates Wnt ligands, a major modification in secretion and interaction with its receptors. We use Wnt-C59, a specific PORCN inhibitor, to block the secretion of endogenous Wnts in embryonic hippocampal neurons (DIV 4). Under these conditions, the activity of exogenous Wnt ligands on the complexity of the dendritic tree and axonal polarity were evaluated

Methods

Cultured primary embryonic hippocampal neurons obtained from Sprague–Dawley rat fetuses (E18), were cultured until day in vitro (DIV) 4 (according to Banker´s protocol) and treated with Wnt-C59 for 24 h, Wnt ligands were added to the cultures on DIV 3 for 24 h. Dendritic arbors and neurites were analysis by fluorescence microscopy. Transfection with Lipofectamine 2000 on DIV 2 of plasmid expressing eGFP and KIF5-Cherry was carried out to evaluate neuronal polarity. Immunostaining was performed with MAP1B and Tau protein. Immunoblot analysis was carried out with Wnt3a, β-catenin and GSK-3β (p-Ser9). Quantitative analysis of dendrite morphology was carried out with ImageJ (NIH) software with Neuron J Plugin.

Results

We report, here, that Wnt-C59 treatment changed the morphology of the dendritic arbors and neurites of embryonic hippocampal neurons, with decreases β-catenin and Wnt3a and an apparent increase in GSK-3β (p-Ser9) levels. No effect was observed on axonal polarity. In sister cultures, addition of exogenous Wnt3a, 5a and 7a ligands rescued the changes in neuronal morphology. Wnt3a restored the length of neurites to near that of the control, but Wnt7a increased the neurite length beyond that of the control. Wnt5a also restored the length of neurites relative to Wnt concentrations.

Conclusions

Results indicated that Wnt ligands, added exogenously, restored dendritic arbor complexity in embryonic hippocampal neurons, previously treated with a high affinity specific Porcupine inhibitor. We proposed that PORCN is an emerging molecular target of interest in the search for preclinical options to study and treat Wnt-related diseases.

Human HINT1 Mutant Proteins that Cause Axonal Motor Neuropathy Exhibit Anomalous Interactions with Partner Proteins

The 14 kDa histidine triad nucleotide-binding protein 1 (HINT1) is critical to maintain the normal function of motor neurons. Thus, a series of human HINT1 mutants cause autosomal recessive axonal neuropathy with neuromyotonia. HINT1 establishes a series of regulatory interactions with signaling proteins, some of which are enriched in motor neurons, such as the type 1 sigma receptor or intracellular domain (ICD) of transmembrane teneurin 1, both of which are also implicated in motor disturbances. In a previous study, we reported the capacity of HINT1 to remove the small ubiquitin-like modifier (SUMO) from a series of substrates and the influence of HINT1 mutants on this activity. We now report how human HINT1 mutations affect the interaction of HINT1 with the regulator of its SUMOylase activity, calcium-activated calmodulin, and its substrate SUMO. Moreover, HINT1 mutants exhibited anomalous interactions with G protein coupled receptors, such as the mu-opioid, and with glutamate N-methyl-D-aspartate receptors as well. Additionally, these HINT1 mutants showed impaired associations with transcriptional regulators such as the regulator of G protein signaling Z2 protein and the cleaved N-terminal ICD of teneurin 1. Thus, the altered enzymatic activity of human HINT1 mutants and their anomalous interactions with partner proteins may disrupt signaling pathways essential to the normal function of human motor neurons.

The Axonal Motor Neuropathy-Related HINT1 Protein Is a Zinc- and Calmodulin-Regulated Cysteine SUMO Protease

Aims: Histidine triad nucleotide-binding protein 1 (HINT1) exhibits proapoptotic and tumor-suppressive activity. HINT1 binds to transcription factors such as teneurin1 and to the regulator of G protein signaling 17 (RGS) (Z2) protein, which incorporates the small ubiquitin-like modifier (SUMO), and is implicated in several types of cancer. HINT1 interacts with proteins such as PKCγ and Raf-1 through zinc ions provided by the cysteine-rich domain of RGSZ2 and the coupled neural nitric oxide synthase (nNOS). Recently, a series of HINT1 mutants have been reported to cause human autosomal recessive axonal neuropathy with neuromyotonia (ARAN-NM). However, the specific alteration in the function of HINT1 induced by these mutants remains to be elucidated. Because sumoylation modifies protein association and transcriptional regulation, we investigated whether HINT1 exhibits zinc- and redox-regulated sumoylase activity, which may be altered in those mutants. Results: HINT1 exhibits cysteine protease activity to remove SUMO from a variety of signaling proteins. HINT1 sumoylase activity is blocked by zinc, and it is released by nitric oxide or calcium-activated calmodulin (CaM). HINT1 contains a SUMO-interacting motif (110-116 HIHLHVL) and the catalytic triad Cys84-Asp87-His114 in the C-terminal region. Thus, zinc probably provided by the RGSZ2-nNOS complex may bind to Cys84 to block HINT1 isopeptidase activity. Innovation: To date, HINT1 is the only sumoylase that is regulated by two alternate pathways, redox- and calcium-activated CaM. Conclusion: The 15 human HINT1 mutants reported to cause ARAN-NM exhibited altered sumoylase activity, which may contribute to the onset of this human motor disease.

A Putative Role of Teneurin-2 and Its Related Proteins in Astrocytes

Teneurins are type II transmembrane proteins comprised of four phylogenetically conserved homologs (Ten-1-4) that are highly expressed during neurogenesis. An additional bioactive peptide named teneurin C-terminal-associated peptide (TCAP-1-4) is present at the carboxyl terminal of teneurins. The possible correlation between the Ten/TCAP system and brain injuries has not been explored yet. Thus, this study examined the expression of these proteins in the cerebral cortex after mechanical brain injury. Adult rats were subjected to cerebral cortex injury by needle-insertion lesion and sacrificed at various time points. This was followed by analysis of the lesion area by immunohistochemistry and conventional RT-PCR techniques. Control animals (no brain injury) showed only discrete Ten-2-like immunoreactive pyramidal neurons in the cerebral cortex. In contrast, Ten-2 immunoreactivity was significantly up-regulated in the reactive astrocytes in all brain-injured groups (p < 0.0001) when compared to the control group. Interestingly, reactive astrocytes also showed intense immunoreactivity to LPHN-1, an endogenous receptor for the Ten-2 splice variant named Lasso. Semi-quantitative analysis of Ten-2 and TCAP-2 expression revealed significant increases of both at 48 h, 3 days and 5 days (p < 0.0001) after brain injury compared to the remaining groups. Immortalized cerebellar astrocytes were also evaluated for Ten/TCAP expression and intracellular calcium signaling by fluorescence microscopy after TCAP-1 treatment. Immortalized astrocytes expressed additional Ten/TCAP homologs and exhibited significant increases in intracellular calcium concentrations after TCAP-1 treatment. This study is the first to demonstrate that Ten-2/TCAP-2 and LPHN-1 are upregulated in reactive astrocytes after a mechanical brain injury. Immortalized cerebellar astrocytes expressed Ten/TCAP homologs and TCAP-1 treatment stimulated intracellular calcium signaling. These findings disclose a new functional role of the Ten/TCAP system in astrocytes during tissue repair of the CNS.

Wnt Signaling Upregulates Teneurin-3 Expression via Canonical and Non-canonical Wnt Pathway Crosstalk

Teneurins (Tens) are a highly conserved family of proteins necessary for cell-cell adhesion. Tens can be cleaved, and some of their proteolytic products, such as the teneurin c-terminal associated-peptide (TCAP) and the intracellular domain (ICD), have been demonstrated to be biologically active. Although Tens are considered critical for central nervous system development, they have also been demonstrated to play important roles in adult tissues, suggesting a potential link between their deregulation and various pathological processes, including neurodegeneration and cancer. However, knowledge regarding how Ten expression is modulated is almost absent. Relevantly, the functions of Tens resemble several of the effects of canonical and non-canonical Wnt pathway activation, including the effects of the Wnt pathways on neuronal development and function as well as their pivotal roles during carcinogenesis. Accordingly, in this initial study, we decided to evaluate whether Wnt signaling can modulate the expression of Tens. Remarkably, in the present work, we used a specific inhibitor of porcupine, the key enzyme for Wnt ligand secretion, to not only demonstrate the involvement of Wnt signaling in regulating Ten-3 expression for the first time but also reveal that Wnt3a, a canonical Wnt ligand, increases the expression of Ten-3 through a mechanism dependent on the secretion and activity of the non-canonical ligand Wnt5a. Although our work raises several new questions, our findings seem to demonstrate the upregulation of Ten-3 by Wnt signaling and also suggest that Ten-3 modulation is possible because of crosstalk between the canonical and non-canonical Wnt pathways.

Patent highlights: December 2015-January 2016

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

Systems genetic analysis of hippocampal neuroanatomy and spatial learning in mice

Variation in hippocampal neuroanatomy correlates well with spatial learning ability in mice. Here, we have studied both hippocampal neuroanatomy and behavior in 53 isogenic BXD recombinant strains derived from C57BL/6J and DBA/2J parents. A combination of experimental, neuroinformatic and systems genetics methods was used to test the genetic bases of variation and covariation among traits. Data were collected on seven hippocampal subregions in CA3 and CA4 after testing spatial memory in an eight-arm radial maze task. Quantitative trait loci were identified for hippocampal structure, including the areas of the intra- and infrapyramidal mossy fibers (IIPMFs), stratum radiatum and stratum pyramidale, and for a spatial learning parameter, error rate. We identified multiple loci and gene variants linked to either structural differences or behavior. Gpc4 and Tenm2 are strong candidate genes that may modulate IIPMF areas. Analysis of gene expression networks and trait correlations highlight several processes influencing morphometrical variation and spatial learning.