ELFN1 is a new extracellular matrix (ECM)-associated protein.

AIMS: The ELFN1, discovered in 2007, is a single-pass transmembrane protein. Studies conducted thus far to elucidate the function of the Elfn1 have been limited only to animal studies. These studies have reported that ELFN1 is a universal binding partner of metabotropic glutamate receptors (mGluRs) in the central nervous system and its functional deficiency has been associated with the pathogenesis of neurological and neuropsychiatric diseases. In 2021, we described the first disease-associated human ELFN1 pathogenic gene mutation. Severe joint laxity, which was the most striking finding of this new disease and was clearly seen in the patients since early infancy, showed that the ELFN1 may have a possible function in the connective tissue besides the nervous system. Here, we present the first experimental evidence of the extracellular matrix (ECM)-related function of the ELFN1. MATERIALS AND METHODS: Primary skin fibroblasts were isolated from the skin biopsies of ELFN1 mutated patients and healthy foreskin donors. For the clinical trial in a dish, in vitro ECM and DEM (decellularized ECM) models were created from skin fibroblasts. All the in vitro models were comparatively characterized and analyzed. KEY FINDINGS: The mutation in the ELFN1 signal peptide region of patients resulted in a severe lack of ELFN1 expression and dramatically altered the characteristic morphology and behavior (growth, proliferation, and motility) of fibroblasts. SIGNIFICANCE: We propose that ELFN1 is involved in the cell-ECM attachment, and its deficiency is critical enough to cause a loss of cell motility and soft ECM stiffness.

m6A-related lncRNAs as potential biomarkers and the lncRNA ELFN1-AS1/miR-182-5p/BCL-2 regulatory axis in diffuse large B-cell lymphoma.

Diffuse large B-cell lymphoma (DLBCL) is the most common lymphoid subtype. However, unsatisfactory survival outcomes remain a major challenge, and the underlying mechanisms are poorly understood. N6-methyladenosine (m6A), the most common internal modification of eukaryotic mRNA, participates in cancer pathogenesis. In this study, m6A-associated long non-coding RNAs (lncRNA) were retrieved from publicly available databases. Univariate, LASSO, and multivariate Cox regression analyses were performed to establish an m6A-associated lncRNA model specific to DLBCL. Kaplan-Meier curves, principal component analysis, functional enrichment analyses and nomographs were used to study the risk model. The underlying clinicopathological characteristics and drug sensitivity predictions against the model were identified. Risk modelling based on the three m6A-associated lncRNAs was an independent prognostic factor. By regrouping patients using our model-based method, we could differentiate patients more accurately for their response to immunotherapy. In addition, prospective compounds that can target DLBCL subtypes have been identified. The m6A-associated lncRNA risk-scoring model developed herein holds implications for DLBCL prognosis and clinical response prediction to immunotherapy. In addition, we used bioinformatic tools to identify and verify the ceRNA of the m6A-associated lncRNA ELFN1-AS1/miR-182-5p/BCL-2 regulatory axis. ELFN1-AS1 was highly expressed in DLBCL and DLBCL cell lines. ELFN1-AS1 inhibition significantly reduced the proliferation of DLBCL cells and promoted apoptosis. ABT-263 inhibits proliferation and promotes apoptosis in DLBCL cells. In vitro and in vivo studies have shown that ABT-263 combined with si-ELFN1-AS1 can inhibit DLBCL progression.

ELFN1-AS1 promotes GDF15-mediated immune escape of colorectal cancer from NK cells by facilitating GCN5 and SND1 association.

The ability of colorectal cancer (CRC) cells to escape from natural killer (NK) cell immune surveillance leads to anti-tumor treatment failure. The long non-coding RNA (lncRNA) ELFN1-AS1 is aberrantly expressed in multiple tumors suggesting a role as an oncogene in cancer development. However, whether ELFN1-AS1 regulates immune surveillance in CRC is unclear. Here, we determined that ELFN1-AS1 enhanced the ability of CRC cells to escape from NK cell surveillance in vitro and in vivo. In addition, we confirmed that ELFN1-AS1 in CRC cells attenuated the activity of NK cell by down-regulating NKG2D and GZMB via the GDF15/JNK pathway. Furthermore, mechanistic investigations demonstrated that ELFN1-AS1 enhanced the interaction between the GCN5 and SND1 protein and this influenced H3k9ac enrichment at the GDF15 promotor to stimulate GDF15 production in CRC cells. Taken together, our findings indicate that ELFN1-AS1 in CRC cells suppresses NK cell cytotoxicity and ELFN1-AS1 is a potential therapeutic target for CRC.

lncRNA ELFN1-AS1 upregulates TRIM29 by suppressing miR-211-3p to promote gastric cancer progression.

Long noncoding RNA (lncRNA) extracellular leucine rich repeat and fibronectin type III domain containing 1-antisense RNA 1 (ELFN1-AS1) has been found to be upregulated in various tumors. However, the biological functions of ELFN1-AS1 in gastric cancer (GC) are not entirely understood. In the present study, the expression levels of ELFN1-AS1, miR-211-3p, and TRIM29 are determined using reverse transcription-quantitative PCR. Subsequently, CCK8, EdU, and colony formation assays are performed to determine GC cell vitality. The migratory and invasive capabilities of GC cells are further evaluated using transwell invasion and cell scratch assays. Western blot analysis is performed to quantify the levels of proteins associated with GC cell apoptosis and epithelialmesenchymal transition (EMT). The competing endogenous RNA (ceRNA) activity of ELFN1-AS1 on TRIM29 through miR-211-3p is confirmed by pull-down, RIP, and luciferase reporter assays. Our study proves that ELFN1-AS1 and TRIM29 are highly expressed in GC tissues. ELFN1-AS1 silencing inhibits GC cell proliferation, migration, invasion and EMT, and induces cell apoptosis. Rescue experiments reveal that the oncogenicity of ELFN1-AS1 is modulated by acting as a sponge for miR-211-3p, thereby increasing the expression of the target gene of miR-211-3p, TRIM29. In summary, ELFN1-AS1 maintains GC cell tumorigenicity via the ELFN1-AS1/miR-211-3p/TRIM29 axis, indicating that this axis can be directed for GC treatment in the future.

lncRNA ELFN1-AS1 enhances the progression of colon cancer by targeting miR-4270 to upregulate AURKB.

The oncogenic role of lncRNA ELFN1-AS1 has been described in different cancers, including colon cancer (CC). However, how ELFN1-AS1 regulates CC malignancy remains unclear. In this study, ELFN1-AS1, AURKB, and miR-4270 expression levels in CC cells and tissues were determined using RT-qPCR and western blotting. CCK-8 and wound healing assays were also performed to analyze alterations in CC cell proliferation and migration. The expression of apoptosis-related proteins (Bax and Bcl-2) was determined via western blot analysis. RNA immunoprecipitation (RIP) assays coupled with luciferase reporter assays were employed to verify the relationship between miR-4270, ELFN1-AS1, and AURKB. An in vivo assay was performed using xenograft tumors in mice to detect the change of tumor growth. It was found that AURKB and ELFN1-AS1 expression was upregulated, whereas miR-4270 was downregulated in CC cells and tissues. ELFN1-AS1 silencing exhibited anti-proliferative, anti-migratory, and pro-apoptotic effects in CC cells. The tumor-suppressive effect of ELFN1-AS1 silencing was verified using in vivo assays. MiR-4270 was predicted to be a target of ELFN1-AS1 and AURKB as a target of miR-4270. Their interactions were further elucidated using luciferase reporter and RNA RIP assays. More importantly, treatment with a miR-4270 inhibitor not only rescued the tumor-suppressing effect of ELFN1-AS1 silencing but also abrogated the tumor suppressor functions of AURKB silencing in CC cells. Taken together, the ELFN1-AS1/miR-4270/AURKB axis facilitates CC tumorigenesis; therefore, targeting this axis might be a promising intervention in preventing CC progression.

Hypoxia-Induced Upregulation of lncRNA ELFN1-AS1 Promotes Colon Cancer Growth and Metastasis Through Targeting TRIM14 via Sponging miR-191-5p.

Hypoxia is identified as one of the microenvironmental features of most solid tumors and is involved in tumor progression. In the present research, we demonstrate that lncRNA extracellular leucine rich repeat and fibronectin type III domain-containing 1-antisense RNA 1 (ELFN1-AS1) is upregulated by hypoxia in colon cancer cells. Knockdown of ELFN1-AS1 in hypoxic colon cancer cells can reduce cell proliferation and restore the invasion to non-hypoxic levels. Fluorescence in situ hybridization results show that ELFN1-AS1 is distributed in the cytoplasm of colon cancer cells, so we further analyze the potential targets for ELFN1-AS1 as a competing endogenous RNA (ceRNA). MiR-191-5p contains a binding sequence with ELFN1-AS1 and is downregulated by ELFN1-AS1 in colon cancer cells. Then, there is a binding site between miR-191-5p and the 3' untranslated region of tripartite motif TRIM 14 (TRIM14). The expression of TRIM14 is inhibited by ELFN1-AS1 siRNA or miR-191-5p mimics in LoVo and HT29 cells. The treatment of the miR-191-5p inhibitor in ELFN1-AS1 knockdown cells can significantly increase cell proliferation and invasion ability. Overexpression of TRIM14 in miR-191-5p-mimic-treated cells can rescue the inhibition of proliferation and invasion caused by miR-191-5p mimics. In conclusion, ELFN1-AS1 operates as a downstream target of hypoxia, promotes proliferation and invasion, and inhibits apoptosis through upregulating TRIM14 by sponging miR-191-5p in the colon cancer cells. Our results enrich our understanding of colon cancer progression and provide potential targets for clinical treatment of colon cancer.

lncRNA ELFN1-AS1 predicts poor prognosis and promotes tumor progression of non-small cell lung cancer by sponging miR-497.

BACKGROUND: More novel biomarkers need to be discovered to improve the therapeutic efficiency of non-small cell lung cancer (NSCLC). lncRNA ELFN1-AS1 (ELFN1-AS1) was proved to play crucial roles in numerous diseases, its intention in NSCLC remains unclear. OBJECTIVE: This study aimed to investigate the function of ELFN1-AS1 and its potential mechanism in NSCLC development. METHODS: A total of 117 NSCLC patients were recruited and provided paired NSCLC tissues and normal tissues. The expression of ELFN1-AS1 was analyzed by PCR. The biological function of ELFN1-AS1 was estimated by CCK8 and Transwell assay. Additionally, the potential mechanism underlying the function of ELFN1-AS1 was explored by the dual-luciferase reporter assay and western blotting. RESULTS: The significant upregulation of ELFN1-AS1 was found in NSCLC tissues and cells, which was closely associated with the TNM stage, lymph node metastasis status, and overall survival of patients. The knockdown of ELFN1-AS1 was found to inhibit the cellular processes and EMT of NSCLC. Moreover, ELFN1-AS1 was found to serve as a sponge to binding with miR-497, and CCNE1 was demonstrated to be the downstream target of miR-497, which was speculated as the potential mechanism underlying the function of ELFN1-AS1. CONCLUSIONS: ELFN1-AS1 acts as an independent prognostic biomarker and tumor promoter of NSCLC by sponging miR-497/CCNE1 axis.

Long non-coding RNA ELFN1-AS1-mediated ZBTB16 inhibition augments the progression of gastric cancer by activating the PI3K/AKT axis.

Long non-coding RNA ELFN1 antisense RNA 1 (ELFN1-AS1) has been reported as a cancer driver in many human malignancies. This study was conducted to investigate the function of ELFN1-AS1 in gastric cancer (GC) and its mechanism of action. Bioinformatics analysis revealed increased expression of ELFN1-AS1 in GC, and abundant expression of ELFN1-AS1 was observed in the acquired GC cell lines. Knockdown of ELFN1-AS1 in GC cells weakened cell proliferation, invasion, migration, and resistance to apoptosis. ELFN1-AS1 was mainly localized in the nuclei of GC cells. ELFN1-AS1 recruited DNA methyltransferases to the promoter region of ZBTB16 and induced transcriptional repression of ZBTB16 through methylation modification. Furthermore, downregulation of ZBTB16 activated the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) signaling pathway and restored the proliferation and invasiveness of GC cells. In vivo, downregulation of ELFN1-AS1 reduced the growth rate of xenograft tumors in mice. In summary, this study demonstrates that ELFN1-AS1 recruits DNA methyltransferases to the promoter region of ZBTB16 to induce its transcriptional repression, which further augments the development of GC by activating the PI3K/AKT signaling pathway.

Postmitotic Prox1 Expression Controls the Final Specification of Cortical VIP Interneuron Subtypes.

Throughout development, neuronal identity is controlled by key transcription factors that determine the unique properties of a cell. During embryogenesis, the transcription factor Prox1 regulates VIP-positive cortical interneuron migration, survival, and connectivity. Here, we explore the role of Prox1 as a regulator of genetic programs that guide the final specification of VIP interneuron subtypes in early postnatal life. Synaptic in vitro electrophysiology in male and female mice shows that postnatal Prox1 removal differentially affects the dynamics of excitatory inputs onto VIP bipolar and multipolar subtypes. RNA sequencing reveals that one of the downstream targets of Prox1 is the postsynaptic protein Elfn1, a constitutive regulator of presynaptic release probability. Further genetic, pharmacological, and electrophysiological experiments demonstrate that removing Prox1 reduces Elfn1 function in VIP multipolar but not in bipolar cells. Finally, overexpression experiments and analysis of native Elfn1 mRNA expression reveal that Elfn1 levels are differentially controlled at the post-transcriptional stage. Thus, in addition to activity-dependent processes that contribute to the developmental trajectory of VIP cells, genetic programs engaged by Prox1 control the final differentiation of multipolar and bipolar subtypes.SIGNIFICANCE STATEMENT The transcription factor Prox1 generates functional diversification of cortical VIP interneuron subtypes in early postnatal life, thus expanding the inhibitory repertoire of the cortex.

Trans-Synaptic Regulation of Metabotropic Glutamate Receptors by Elfn Proteins in Health and Disease.

Trans-regulation of G protein-coupled receptors (GPCRs) by leucine-rich repeat (LRR) transmembrane proteins has emerged as a novel type of synaptic molecular interaction in the last decade. Several studies on LRR-GPCR interactions have revealed their critical role in synapse formation and in establishing synaptic properties. Among them, LRR-GPCR interactions between extracellular LRR fibronectin domain-containing family proteins (Elfn1 and Elfn2) and metabotropic glutamate receptors (mGluRs) are particularly interesting as they can affect a broad range of synapses through the modulation of signaling by glutamate, the principal excitatory transmitter in the mammalian central nervous system (CNS). Elfn-mGluR interactions have been investigated in hippocampal, cortical, and retinal synapses. Postsynaptic Elfn1 in the hippocampus and cerebral cortex mediates the tonic regulation of excitatory input onto somatostatin-positive interneurons (INs) through recruitment of presynaptic mGluR7. In the retina, presynaptic Elfn1 binds to mGluR6 and is necessary for synapse formation between rod photoreceptor cells and rod-bipolar cells. The repertoire of binding partners for Elfn1 and Elfn2 includes all group III mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8), and both Elfn1 and Elfn2 can alter mGluR-mediated signaling through trans-interaction. Importantly, both preclinical and clinical studies have provided support for the involvement of the Elfn1-mGluR7 interaction in attention-deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD), and epilepsy. In fact, Elfn1-mGluR7-associated disorders may reflect the altered function of somatostatin-positive interneuron inhibitory neural circuits, the mesolimbic and nigrostriatal dopaminergic pathway, and habenular circuits, highlighting the need for further investigation into this interaction.

LncRNA ELFN1-AS1 Promotes Retinoblastoma Growth and Invasion via Regulating miR-4270/SBK1 Axis.

BACKGROUND: Long noncoding RNA (lncRNA) has been reported to play important roles in tumor initiation. However, how lncRNA ELFN1-AS1 affects retinoblastoma development remains unclear. Thus, we sought to elucidate its functions in retinoblastoma progression. METHODS: ELFN1-AS1 expression was measured in retinoblastoma tissues and normal tissues by qRT-PCR. CCK8, colony formation and Transwell assay were carried out to investigate the effects of ELFN1-AS1 knockdown on cell malignant behaviors. Bioinformatics analyses were performed to predict the relationship among ELFN1-AS1, miR-4270 and SBK1. RESULTS: ELFN1-AS1 was highly expressed in retinoblastoma tissues and cell lines. ELFN1-AS1 was positively correlated with retinoblastoma progression and prognosis. ELFN1-AS1 knockdown curtailed retinoblastoma proliferation, migration and invasion. ELFN1-AS1 was the competing endogenous RNA for miR-4270 and promoted SBK1expression. CONCLUSION: Altogether, our findings demonstrated that ELFN1-AS1 promotes retinoblastoma progression through mediating miR-4270/SBK1 axis and might be a promising therapeutic target.

ELFN1-AS1 accelerates cell proliferation, invasion and migration via regulating miR-497-3p/CLDN4 axis in ovarian cancer.

Previous studies indicated that long non-coding RNAs (LncRNAs) were involved in the progression of multiple cancers including ovarian cancer (OV). LncRNA ELFN1-AS1 functioned as an oncogene in many cancers, but its potential roles in OV were largely unclear. In the current study, we were aimed at clarifying the biological roles and molecular mechanisms of ELFN1-AS1 in OV. We found that ELFN1-AS1 was significantly upregulated in OV tissues and cell lines. High expression of ELFN1-AS1 was associated with poor prognosis in OV patients. Knockdown of ELFN1-AS1 inhibited the proliferation, migration and invasion of SKOV3 cell lines and repressed tumor growth in xenografted ovarian models. Mechanistically, ELFN1-AS1 promoted the proliferation, migration and invasion of SKOV3 cells by sponging miR-497-3p. Additionally, CLDN4 was verified to be the target of miR-497-3p. Rescue experiments revealed that miR-497-3p inhibition could partly reverse the inhibitory effect of ELFN1-AS1 silencing on proliferation, migration and invasion of SKOV3 cell lines. Taken together, our findings indicated that ELFN1-AS1 acted as an oncogene in ovarian cancer through regulating the expression of CLDN4 by directly interacting with miR-497-3p. The results suggested that ELFN1-AS1 might act as a promising therapeutic target for OV.

LncRNA ELFN1-AS1 promotes esophageal cancer progression by up-regulating GFPT1 via sponging miR-183-3p.

Accumulating studies highlight the critical role of long non-coding RNAs (lncRNAs) in the development of various human cancers. Extracellular leucine rich repeat and fibronectin type III domain containing 1-antisense RNA 1 (ELFN1-AS1) was shown to be a newly found lncRNA that abnormally expressed in human tumors. However, till now the specific function of this lncRNA in esophageal cancer (ESCA) remains unknown. In this study, we discovered that higher ELFN1-AS1 expression indicated shorter patient survival in pan-cancer, including ESCA, using online The Cancer Genome Atlas (TCGA) tools. The lncRNA ELFN1-AS1 was significantly up-regulated in ESCA tissues and cell lines when compared with the counterparts. Down-regulation of ELFN1-AS1 restrained cell proliferation, migration, and invasion of ESCA in vitro. In addition, we found that the expression of microRNA-183-3p (miR-183-3p) and ELFN1-AS1 or glutamine-fructose-6-phosphate transaminase 1 (GFPT1) were inversely correlated in ESCA. Both ELFN1-AS1 and GFPT1 are direct targets of miR-183-3p in ESCA. The effects of ELFN1-AS1 knockdown on ESCA progression were partially rescued by inhibition of miR-183-3p or over-expression of GFPT1. In summary, the results of this study suggest that the lncRNA ELFN1-AS1 facilitates the progression of ESCA by acting as a competing endogenous RNA (ceRNA) to promote GFPT1 expression via sponging miR-183-3p.

ELFN1-AS1 accelerates the proliferation and migration of colorectal cancer via regulation of miR-4644/TRIM44 axis.

Faced with the increasing colorectal cancer (CRC) cases, the interrogation of pivotal molecules in CRC appears to be vitally important. Long non-coding RNAs (lncRNAs) are well-known regulators of gene expression at transcriptional, post-transcriptional or epigenetic level, among which the competing endogenous RNA (ceRNA) network is a common way that lncRNAs exert their properties. The current study aimed to provide a new insight into improving the outcomes of CRC patients. Our study detected that ELFN1-AS1 expression was elevated in CRC tissues and cells, and ELFN1-AS1 upregulation was correlated with poor prognosis of CRC sufferers. Besides, it was viewed that ELFN1-AS1 knockdown impeded the proliferation and migration abilities as well as activated the apoptosis ability of CRC cells. In subsequence, mechanism assays also displayed that ELFN1-AS1 targeted miR-4644 to augment TRIM44 level. Finally, rescue experiments confirmed that TRIM44 took part in the ELFN1-AS1-medatied promotional influences on CRC cells proliferation and migration. In conclusion, ELFN1-AS1 exerted pro-proliferation, anti-apoptosis and pro-migration functions on CRC cells by acting as a sponge of miR-4644 to increase TRIM44 expression at mRNA and protein level, providing an additional molecule responsible for the carcinogenesis and progression for CRC.

Long Non-Coding RNA ELFN1-AS1 Promoted Colon Cancer Cell Growth and Migration via the miR-191-5p/Special AT-Rich Sequence-Binding Protein 1 Axis.

Long non-coding RNAs (lncRNAs) are reported to participate in tumor development. It has been manifested in previous researches that lncRNA ELFN1-AS1 is involved in early-stage colon adenocarcinoma with potential diagnostic value. However, no studies have revealed the specific mechanism of ELFN1-AS1 in colon cancer, and there are no other studies on whether ELFN1-AS1 is associated with tumorigenesis. In our study, ELFN1-AS1 with high expression in colon cancer was selected by TCGA analysis, and the survival analysis was carried out to verify it. Subsequently, qRT-PCR was adopted for validating the results in tissues and cell lines. Cell counting kit-8 (CCK8), 5-ethynyl-2'-deoxyuridine (EdU), cell colon, cell apoptosis, cell cycle, cell migration, and invasion assays were utilized to assess the role of ELFN1-AS1 in colon cancer. Results uncovered that ELFN1-AS1 expression was prominently raised in colon cancer cells and tissues. ELFN1-AS1 decrement restrained cells to grow through interfering with distribution of cell cycle and promoting apoptosis. Meanwhile, ELFN1-AS1 decrement weakened the capacity of cells to migrate and invade. What's more, ELFN1-AS1 was uncovered to act as a competing endogenous RNA (ceRNA) to decrease miR-191-5p expression, thus raising special AT-rich sequence-binding protein 1 (SATB1), a downstream target of ceRNA. To sum up, ELFN1-AS1 drives colon cancer cells to proliferate and invade through adjusting the miR-191-5p/SATB1 axis. The above results disclose that lncRNA ELFN1-AS1 is possibly a novel treatment target for colon cancer cases.

Extracellular vesicles from human umbilical cord mesenchymal stem cells treated with siRNA against ELFN1-AS1 suppress colon adenocarcinoma proliferation and migration.

Recent evidence has shown that long noncoding RNAs (lncRNAs) play major roles in tumorigenesis and cancer progression. The cancer genome atlas program (TCGA) database was used to screen colon adenocarcinoma (COAD)-related differentially expressed lncRNAs, which revealed that lncRNA ELFN1-AS1 was highly expressed in COAD. This study aimed to explore the regulatory role of ELFN1-AS1 in COAD and construct a gene delivery system based on extracellular vesicles (EVs). We found that ELFN1-AS1 levels were obviously increased in COAD patients and COAD tumor cells. Knockdown of ELFN1-AS1 expression by siRNA inhibited COAD cell proliferation and migration. Moreover, silencing ELFN1-AS1 significantly reduced the activation of extracellular signal-regulated protein kinase (Erk), up-regulated the protein expression of E-cadherin and down-regulated vimentin. In addition, we treated human umbilical cord mesenchymal stem cells (hUCMSCs) with siRNA-ELFN1-AS1 and found that EVs from siRNA-ELFN1-AS1-treated hUCMSCs could inhibit COAD cell proliferation and migration in vitro. These findings suggested that ELFN1-AS1 could promote the progression of COAD and that hUCMSC-EVs might be an attractive vehicle for the clinical administration of lncRNA-specific siRNAs in patients with COAD.

Characterization of paralogous uncx transcription factor encoding genes in zebrafish.

The paired-type homeodomain transcription factor Uncx is involved in multiple processes of embryogenesis in vertebrates. Reasoning that zebrafish genes uncx4.1 and uncx are orthologs of mouse Uncx, we studied their genomic environment and developmental expression. Evolutionary analyses indicate the zebrafish uncx genes as being paralogs deriving from teleost-specific whole-genome duplication. Whole-mount in situ mRNA hybridization of uncx transcripts in zebrafish embryos reveals novel expression domains, confirms those previously known, and suggests sub-functionalization of paralogs. Using genetic mutants and pharmacological inhibitors, we investigate the role of signaling pathways on the expression of zebrafish uncx genes in developing somites. In identifying putative functional role(s) of zebrafish uncx genes, we hypothesized that they encode transcription factors that coordinate growth and innervation of somitic muscles.

Elfn1-Induced Constitutive Activation of mGluR7 Determines Frequency-Dependent Recruitment of Somatostatin Interneurons.

Excitatory synapses onto somatostatin (SOM) interneurons show robust short-term facilitation. This hallmark feature of SOM interneurons arises from a low initial release probability that regulates the recruitment of interneurons in response to trains of action potentials. Previous work has shown that Elfn1 (extracellular leucine rich repeat and fibronectin Type III domain containing 1) is necessary to generate facilitating synapses onto SOM neurons by recruitment of two separate presynaptic components: mGluR7 (metabotropic glutamate receptor 7) and GluK2-KARs (kainate receptors containing glutamate receptor, ionotropic, kainate 2). Here, we identify how a transsynaptic interaction between Elfn1 and mGluR7 constitutively reduces initial release probability onto mouse cortical SOM neurons. Elfn1 produces glutamate-independent activation of mGluR7 via presynaptic clustering, resulting in a divergence from the canonical "autoreceptor" role of Type III mGluRs, and substantially altering synaptic pharmacology. This structurally induced determination of initial release probability is present at both layer 2/3 and layer 5 synapses. In layer 2/3 SOM neurons, synaptic facilitation in response to spike trains is also dependent on presynaptic GluK2-KARs. In contrast, layer 5 SOM neurons do not exhibit presynaptic GluK2-KAR activity at baseline and show reduced facilitation. GluK2-KAR engagement at synapses onto layer 5 SOM neurons can be induced by calmodulin activation, suggesting that synaptic function can be dynamically regulated. Thus, synaptic facilitation onto SOM interneurons is mediated both by constitutive mGluR7 recruitment by Elfn1 and regulated GluK2-KAR recruitment, which determines the extent of interneuron recruitment in different cortical layers.SIGNIFICANCE STATEMENT This study identifies a novel mechanism for generating constitutive GPCR activity through a transsynaptic Elfn1/mGluR7 structural interaction. The resulting tonic suppression of synaptic release probability deviates from canonical autoreceptor function. Constitutive suppression delays the activation of somatostatin interneurons in circuits, necessitating high-frequency activity for somatostatin interneuron recruitment. Furthermore, variations in the synaptic proteome generate layer-specific differences in facilitation at pyr → SOM synapses. The presence of GluK2 kainate receptors in L2/3 enhances synaptic transmission during prolonged activity. Thus, layer-specific synaptic properties onto somatostatin interneurons are mediated by both constitutive mGluR7 recruitment and regulated GluK2 kainate receptor recruitment, revealing a mechanism that generates diversity in physiological responses of interneurons.