Expression and regulation of FRMD6 in mouse DRG neurons and spinal cord after nerve injury

Abstract

FRMD6, a member of the group of FERM-domain proteins, is involved both in communication between cells, interactions with extracellular matrix, cellular apoptotic and regenerative mechanisms. FRMD6 was first discovered in the rodent sciatic nerve, and in the present immunohistochemical study we investigated the distribution of FRMD6 in the dorsal root ganglia (DRGs), sciatic nerve and spinal cord following sciatic nerve injury. FRMD6-immunoreactivity was found in the cytoplasm, nucleus or both, and in a majority of DRG neurons. FRMD6-immunoreactivity co-existed with several wellknown neuronal markers, including calcitonin gene-related peptide, isolectin B4 and neurofilament 200 in mouse DRGs. After peripheral nerve injury, the FRMD6 mRNA levels and the overall percentage of FRMD6-positive neuron profiles (NPs) were decreased in ipsilateral lumbar DRGs, the latter mainly affecting small size neurons with cytoplasmic localization. Conversely, the proportion of NPs with nuclear FRMD6-immunoreactivity was significantly increased. In the sciatic nerve, FRMD6immunoreactivity was observed in non-neuronal cells and in axons, and accumulated proximally to a ligation of the nerve. In the spinal cord FRMD6-immunoreactivity was detected in neurons in both dorsal and ventral horns, and was upregulated in ipsilateral dorsal horn after peripheral nerve axotomy. Our results demonstrate that FRMD6 is strictly regulated by peripheral nerve injury at the spinal level. FRMD (protein 4.1, ezrin, radixin and moesin) 6, also known as Willin, is a member of the 4.1 superfamily and was first identified in the sciatic nerve of rats by northern blot analysis 1. Recently, the FRMD6 transcripts were detected in mouse lumbar dorsal root ganglia (DRGs) and spinal cord by RNAseq (see Supplementary Table 4) 2. Human FRMD6 has been shown to be a homolog of Ex in Drosophila, which is a putative tumor suppressor 3,4. FRMD6/Ex, as an upstream regulator, is a component of multiple upstream molecular complexes in the Salvador/ Warts/Hippo (Hippo) signaling pathway network that, mostly via MST1/2 (mammalian STE20-like protein kinase) and LATS1/2 (large tumour suppressor homolog), regulates the activity of YAP (Yes-associated protein) and TAZ (transcriptional co-activator with PDZ-binding motif), two downstream homologous transcriptional co-activators 5-8. This pathway, originally identified in drosophila 9,10 and more recently shown to be well conserved in mammalians 11 , regulates cellular proliferation, differentiation, apoptosis and tissue homoeostasis 5,6. In human, analysing tissues from healthy and diseased donors by immunohistochemistry, FRMD6-immunoreactivity was observed in cytoplasm, plasma membrane or nucleus of various normal tissues (e.g., intestine) and carcinoma, and sometimes with a mixed intracellular distribution 12. However, the functional associations between these subcellular localizations and physiopathologic processes are still not elucidated. Moreover, FRMD6 transcripts were detected predominantly in fibroblasts within the perine...

The Hippo pathway: Horizons for innovative treatments of peripheral nerve diseases

“…159 The transmission of nociception in unmyelinated C fibers and small thinly myelinated Aδ fibers depends on mechanosensitive ion channels at sensory terminals such as Piezo ion channels, transient receptor potential (TRP) channels and sodium channels (eg, Nav1.7-1.9) (reviewed in 160 ). While a conventional role for YAP/TAZ directly at the synaptic terminal of neurons is unlikely due to the distance between the neuronal cell bodies and synaptic terminals, a functional role for the Hippo pathway was suggested by upregulation of FRMD6 in sensory neurons cell bodies in the dorsal root ganglion after injury, 161 and further in the spinal dorsal horn of the CNS. 162 Interestingly, peripheral glia associated with these afferent fiber's terminals may also act as sensors.…”

Willin/FRMD6: A Multi-Functional Neuronal Protein Associated with Alzheimer’s Disease

“…Willin/FRMD6 is widely distributed in various tissues and cell types including epithelial cells within the uterus, placenta, cervix, brain, heart, liver, prostate, and lungs [ 1 , 19 , 20 , 21 ]; fibroblasts of the peripheral nerves [ 1 , 21 ]; neurons of the peripheral (PNS) and central nervous systems (CNS) [ 22 , 23 ].…”

“…Recent studies have provided an insight into the endogenous localization of Willin/FRMD6 in the peripheral nervous system (PNS) and central nervous system (CNS) ( Figure 2 ). In the PNS, Willin/FRMD6 expression is observed in fibroblasts and Schwann cells of rat sciatic nerves [ 21 , 23 ] and in the Schwann cell microvilli of myelinated mouse peripheral nerves [ 37 ].…”

“…As previously mentioned, Willin/FRMD6 was first identified in the rat sciatic nerve [ 1 ], where it controls fibroblast migration and proliferation [ 21 ]. In this context, Willin/FRMD6 may initiate wound closure in response to nerve injury by inducing fibroblast migration and further downregulation of receptor tyrosine kinase Ephrin B2 and epidermal growth factor receptor [ 21 ].…”

“…As previously mentioned, Willin/FRMD6 was first identified in the rat sciatic nerve [ 1 ], where it controls fibroblast migration and proliferation [ 21 ]. In this context, Willin/FRMD6 may initiate wound closure in response to nerve injury by inducing fibroblast migration and further downregulation of receptor tyrosine kinase Ephrin B2 and epidermal growth factor receptor [ 21 ]. Neuronal response to peripheral nerve injury appears to involve Willin/FRMD6 cytoplasmic-to-nuclear shuttling [ 23 ], providing evidence of its differing roles depending on subcellular localization.…”