Altered endochondral bone development in matrix metalloproteinase 13-deficient mice

The assembly and degradation of extracellular matrix (ECM) molecules are crucial processes during bone development. In this study, we show that ECM remodeling is a critical rate-limiting step in endochondral bone formation. Matrix metalloproteinase (MMP) 13 (collagenase 3) is poised to play a crucial role in bone formation and remodeling because of its expression both in terminal hypertrophic chondrocytes in the growth plate and in osteoblasts. Moreover, a mutation in the human MMP13 gene causes the Missouri variant of spondyloepimetaphyseal dysplasia. Inactivation of Mmp13 in mice through homologous recombination led to abnormal skeletal growth plate development. Chondrocytes differentiated normally but their exit from the growth plate was delayed. The severity of the Mmp13- null growth plate phenotype increased until about 5 weeks and completely resolved by 12 weeks of age. Mmp13-null mice had increased trabecular bone, which persisted for months. Conditional inactivation of Mmp13 in chondrocytes and osteoblasts showed that increases in trabecular bone occur independently of the improper cartilage ECM degradation caused by Mmp13 deficiency in late hypertrophic chondrocytes. Our studies identified the two major components of the cartilage ECM, collagen type II and aggrecan, as in vivo substrates for MMP13. We found that degradation of cartilage collagen and aggrecan is a coordinated process in which MMP13 works synergistically with MMP9. Mice lacking both MMP13 and MMP9 had severely impaired endochondral bone, characterized by diminished ECM remodeling, prolonged chondrocyte survival, delayed vascular recruitment and defective trabecular bone formation (resulting in drastically shortened bones). These data support the hypothesis that proper ECM remodeling is the dominant rate-limiting process for programmed cell death, angiogenesis and osteoblast recruitment during normal skeletal morphogenesis.     

MMP and TIMP temporal gene expression during osteocytogenesis

Osteocytes within bone differentiate from osteoblast precursors which reside in a mineralised extracellular matrix (ECM). Fully differentiated osteocytes are critical for bone development and function but the factors that regulate this differentiation process are unknown. The enzymes primarily responsible for ECM remodelling are matrix metalloproteinases (MMP); however, the expression and role of MMPs during osteocytogenesis is undefined. Here we used MLO-A5 cells to determine the temporal gene expressions of the MMP family and their endogenous inhibitors – tissue inhibitors of metalloproteinases (TIMPs) during osteocytogenesis. RT-qPCR revealed expression of 14 Mmps and 3 Timps in MLO-A5 cells. Mmp2, Mmp23 and Mmp28 were decreased concurrent with mineralisation onset (P < 0.05*). Mmp14 and Mmp19 mRNAs were also significantly increased at day 3 (P < 0.05*) before returning to baseline levels at day 6. Decreased expressions of Timp1, Timp2 and Timp3 mRNA were observed by day 6 compared to day 0 (P < 0.05*). To examine whether these changes are linked to osteocytogenesis, we determined Mmp/Timp mRNA expressions in mineralisation-limited conditions. RT-qPCR revealed that the previously observed decreases in Mmp2, Mmp23 and Mmp28 were not observed in these mineralisation-limited cultures, therefore closely linking these MMPs with osteocyte differentiation. Similarly, we found differential expression of Timp1, Timp2 and Timp3 mRNA in mineralisation-restricted cultures (P < 0.05*). In conclusion, we have identified several members of the MMP/TIMP families as regulators of ECM remodelling necessary for the acquisition of the osteocyte phenotype.     

Molecular analysis of SMN1, SMN2, NAIP, GTF2H2, and H4F5 genes in 157 Chinese patients with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a common and lethal autosomal recessive neurodegenerative disorder, which is caused by mutations of the survival motor neuron 1 (SMN1) gene. Additionally, the phenotype is modified by several genes nearby SMN1 in the 5q13 region. In this study, we analyzed mutations in SMN1 and quantified the modifying genes, including SMN2, NAIP, GTF2H2, and H4F5 by polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP), multiplex ligation-dependent probe amplification (MLPA), TA cloning, allele-specific long-range PCR, and Sanger sequencing in 157 SMA patients. Most SMA patients (94.90%) possessed a homozygous SMN1 deletion, while 10 patients demonstrated only the absence of exon 7, but the presence of exon 8. Two missense mutations (c.689 C > T and c.844 C > T) were identified in 2 patients who both carried a single copy of SMN1. We found inverse correlations between SMN2, the NAIP copy number, and the clinical severity of the disease. Furthermore, 7 severe type I patients possessed large-scale deletions, including SMN1, NAIP, and GTF2H2. We conclude that SMN1 gene conversion, SMN1 subtle mutations, SMN2 copy number, and the extent of deletion in the 5q13 region should all be considered in the genotype–phenotype analysis of SMA.     

A novel ADAMTS17 variant that causes Weill-Marchesani syndrome 4 alters fibrillin-1 and collagen type I deposition in the extracellular matrix

Weill-Marchesani syndrome (WMS) is a rare genetic disorder that affects the musculoskeletal system, the eye, and the cardiovascular system. Individuals with WMS present with short stature, joint contractures, thick skin, microspherophakia, small and dislocated lenses, and cardiac valve anomalies. WMS can be caused by recessive mutations in ADAMTS10 (WMS 1), ADAMTS17 (WMS 4), or LTBP2 (WMS 3), or by dominant mutations in fibrillin-1 (FBN1) (WMS 2); all genes encode secreted extracellular matrix (ECM) proteins. Individuals with WMS 4 due to ADAMTS17 mutations appear to have less severe cardiac involvement and present predominantly with the musculoskeletal and ocular features of WMS. ADAMTS17 is a member of the ADAMTS family of secreted proteases and directly binds to fibrillins. Here we report a novel pathogenic variant in ADAMTS17 that causes WMS 4 in an individual with short stature, brachydactyly, and small, spherical, and dislocated lenses. We provide biochemical and cell biological insights in the pathomechanisms of WMS 4, which also suggest potential biological functions for ADAMTS17. We show that the variant in ADAMTS17 prevents its secretion and we found intracellular accumulation of fibrillin-1 and collagen type I in patient-derived skin fibroblasts. In accordance, transmission electron microscopy revealed elastic fiber abnormalities, decreased collagen fibril diameters, and intracellular collagen accumulation in the dermis of the proband. Together, the data indicate a possible role for ADAMTS17 in the secretion of fibrillin-1 and collagen type I or in their early assembly in the pericellular matrix or the ECM.     

Role of matrix metalloproteinase 13 in both endochondral and intramembranous ossification during skeletal regeneration

Extracellular matrix (ECM) remodeling is important during bone development and repair. Because matrix metalloproteinase 13 (MMP13, collagenase-3) plays a role in long bone development, we have examined its role during adult skeletal repair. In this study we find that MMP13 is expressed by hypertrophic chondrocytes and osteoblasts in the fracture callus. We demonstrate that MMP13 is required for proper resorption of hypertrophic cartilage and for normal bone remodeling during non-stabilized fracture healing, which occurs via endochondral ossification. However, no difference in callus strength was detected in the absence of MMP13. Transplant of wild-type bone marrow, which reconstitutes cells only of the hematopoietic lineage, did not rescue the endochondral repair defect, indicating that impaired healing in Mmp13-/- mice is intrinsic to cartilage and bone. Mmp13-/- mice also exhibited altered bone remodeling during healing of stabilized fractures and cortical defects via intramembranous ossification. This indicates that the bone phenotype occurs independently from the cartilage phenotype. Taken together, our findings demonstrate that MMP13 is involved in normal remodeling of bone and cartilage during adult skeletal repair, and that MMP13 may act directly in the initial stages of ECM degradation in these tissues prior to invasion of blood vessels and osteoclasts.     

A matrix metalloproteinase mediates airway remodeling in Drosophila

Organ size typically increases dramatically during juvenile growth. This growth presents a fundamental tension, as organs need resiliency to resist stresses while still maintaining plasticity to accommodate growth. The extracellular matrix (ECM) is central to providing resiliency, but how ECM is remodeled to accommodate growth is poorly understood. We investigated remodeling of Drosophila respiratory tubes (tracheae) that elongate continually during larval growth, despite being lined with a rigid cuticular ECM. Cuticle is initially deposited with a characteristic pattern of repeating ridges and valleys known as taenidia. We find that for tubes to elongate, the extracellular protease Mmp1 is required for expansion of ECM between the taenidial ridges during each intermolt period. Mmp1 protein localizes in periodically spaced puncta that are in register with the taenidial spacing. Mmp1 also degrades old cuticle at molts, promotes apical membrane expansion in larval tracheae, and promotes tube elongation in embryonic tracheae. Whereas work in other developmental systems has demonstrated that MMPs are required for axial elongation occurring in localized growth zones, this study demonstrates that MMPs can also mediate interstitial matrix remodeling during growth of an organ system.     

Matrix Metalloproteinase 9 Exerts Antiviral Activity against Respiratory Syncytial Virus

Increased lung levels of matrix metalloproteinase 9 (MMP9) are frequently observed during respiratory syncytial virus (RSV) infection and elevated MMP9 concentrations are associated with severe disease. However little is known of the functional role of MMP9 during lung infection with RSV. To determine whether MMP9 exerted direct antiviral potential, active MMP9 was incubated with RSV, which showed that MMP9 directly prevented RSV infectivity to airway epithelial cells. Using knockout mice the effect of the loss of Mmp9 expression was examined during RSV infection to demonstrate MMP9’s role in viral clearance and disease progression. Seven days following RSV infection, Mmp9 ^-/- mice displayed substantial weight loss, increased RSV-induced airway hyperresponsiveness (AHR) and reduced clearance of RSV from the lungs compared to wild type mice. Although total bronchoalveolar lavage fluid (BALF) cell counts were similar in both groups, neutrophil recruitment to the lungs during RSV infection was significantly reduced in Mmp9 ^-/- mice. Reduced neutrophil recruitment coincided with diminished RANTES, IL-1β, SCF, G-CSF expression and p38 phosphorylation. Induction of p38 signaling was required for RANTES and G-CSF expression during RSV infection in airway epithelial cells. Therefore, MMP9 in RSV lung infection significantly enhances neutrophil recruitment, cytokine production and viral clearance while reducing AHR.     

Association of APOE, GCPII and MMP9 polymorphisms with common diseases and lipid levels in an older adult/elderly cohort

Background and aims

The characterization of candidate gene polymorphisms in elderly populations is an important tool for the identification of risk factors for age-related diseases and conditions. We aimed to genotype the APOE polymorphisms (rs429358 and rs7412), rs61886492 (1561C>T) and rs202720 of GCPII gene and rs3918242 (− 1562C>T) of MMP9 gene in an older-adult/elderly cohort from Cuiabá city, Mato Grosso Brazil as well as to characterize risk factors for morbidities and conditions affecting this cohort.

Methods

The studied population consisted of 570 subjects from Cuiabá city, Brazil, who were subjected to clinical interviews and blood collection for laboratory examinations and DNA extraction. Restriction Fragment Length Polymorphism Polymerase Chain Reaction (RFLP-PCR), sequence-specific primer PCR (SSP-PCR) and TaqMan® allelic discrimination assay were used for genotyping.

Results

The frequencies of APOE ε2 and ε4 were 6.6% and 14.8%, respectively, and the frequencies of GCPII rs61886492 T allele, GCPII rs202720 C allele and MMP9 rs3918242 T allele were, respectively, 3.0%, 26.6% and 10.1%. Significant associations between APOE ε2 allele with lower total cholesterol and LDL-cholesterol were found. In addition, MMP9 rs3918242 T allele was associated with higher LDL-cholesterol levels, suggesting a link between lipid metabolism alteration and cardiovascular disease.

Conclusions

The present findings contributed to characterize risk factors specific for the studied population and to better understand the molecular physiopathology of common morbidities and conditions affecting older-adult/elderly people.     

Loss of Timp3 Gene Leads to Abdominal Aortic Aneurysm Formation in Response to Angiotensin II

Aortic aneurysm is dilation of the aorta primarily due to degradation of the aortic wall extracellular matrix (ECM). Tissue inhibitors of metalloproteinases (TIMPs) inhibit matrix metalloproteinases (MMPs), the proteases that degrade the ECM. Timp3 is the only ECM-bound Timp, and its levels are altered in the aorta from patients with abdominal aortic aneurysm (AAA). We investigated the causal role of Timp3 in AAA formation. Infusion of angiotensin II (Ang II) using micro-osmotic (Alzet) pumps in Timp3^−/− male mice, but not in wild type control mice, led to adverse remodeling of the abdominal aorta, reduced collagen and elastin proteins but not mRNA, and elevated proteolytic activities, suggesting excess protein degradation within 2 weeks that led to formation of AAA by 4 weeks. Intriguingly, despite early up-regulation of MMP2 in Timp3^−/−Ang II aortas, additional deletion of Mmp2 in these mice (Timp3^−/−/Mmp2^−/−) resulted in exacerbated AAA, compromised survival due to aortic rupture, and inflammation in the abdominal aorta. Reconstitution of WT bone marrow in Timp3^−/−/Mmp2^−/− mice reduced inflammation and prevented AAA in these animals following Ang II infusion. Treatment with a broad spectrum MMP inhibitor (PD166793) prevented the Ang II-induced AAA in Timp3^−/− and Timp3^−/−/Mmp2^−/− mice. Our study demonstrates that the regulatory function of TIMP3 is critical in preventing adverse vascular remodeling and AAA. Hence, replenishing TIMP3, a physiological inhibitor of a number of metalloproteinases, could serve as a therapeutic approach in limiting AAA development or expansion.     

Irradiation of rat mesangial cells alters the expression of gene products associated with the development of renal fibrosis.

To determine the ability of radiation to modulate mesangial cell expression of various molecules involved in promoting extracellular matrix (ECM) accumulation [fibronectin, plasminogen activator-inhibitor 1 (Pai1), and tissue inhibitor of metalloproteinase-2 (Timp2)] and degradation (Tgfb, plasminogen activators u-PA or t-PA, matrix metalloproteinases Mmp2 and Mmp9), primary cultures of rat mesangial cells (passage number 6-11) were placed in serum-free medium 24 h prior to irradiation with single doses of 0.5-20 Gy (137)Cs gamma rays. After irradiation, cells were maintained in serum-free medium for a further 48 h. Irradiation of quiescent mesangial cells resulted in significant (P < 0.05) time- and dose-dependent increases in Fn and Pai1 mRNA and/or immunoreactive protein. Despite an increase in Tgfb1 mRNA, there was little evidence for an increase in total Tgfb protein. Indeed, active levels remained unaltered after irradiation. Irradiation led to differential changes in MMP expression; active Mmp2 levels increased, while Mmp9 levels appeared unaltered. In addition, secretion of plasminogen activators into the medium was unchanged after irradiation, while secretion of Timp2 increased. We conclude that irradiating mesangial cells leads to altered production of various molecules involved in accumulation and degradation of extracellular matrix.     

Novel mutations in SAR1B and MTTP genes in Tunisian children with chylomicron retention disease and abetalipoproteinemia

Monogenic hypobetalipoproteinemias include three disorders: abetalipoproteinemia (ABL) and chylomicron retention disease (CMRD) with recessive transmission and familial hypobetalipoproteinemia (FHBL) with dominant transmission. We investigated three unrelated Tunisian children born from consanguineous marriages, presenting hypobetalipoproteinemia associated with chronic diarrhea and retarded growth. Proband HBL-108 had a moderate hypobetalipoproteinemia, apparently transmitted as dominant trait, suggesting the diagnosis of FHBL. However, she had no mutations in FHBL candidate genes (APOB, PCSK9 and ANGPTL3). The analysis of MTTP gene was also negative, whereas SAR1B gene resequencing showed that the patient was homozygous for a novel mutation (c.184G>A), resulting in an amino acid substitution (p.Glu62Lys), located in a conserved region of Sar1b protein. In the HBL-103 and HBL-148 probands, the severity of hypobetalipoproteinemia and its recessive transmission suggested the diagnosis of ABL. The MTTP gene resequencing showed that probands HBL-103 and HBL-148 were homozygous for a nucleotide substitution in the donor splice site of intron 9 (c.1236+2T>G) and intron 16 (c.2342+1G>A) respectively. Both mutations were predicted in silico to abolish the function of the splice site. In vitro functional assay with splicing mutation reporter MTTP minigenes showed that the intron 9 mutation caused the skipping of exon 9, while the intron 16 mutation caused a partial retention of this intron in the mature mRNA. The predicted translation products of these mRNAs are non-functional truncated proteins.     

Loss of MT1‐MMP causes cell senescence and nuclear defects which can be reversed by retinoic acid

MT1-MMP (MMP14) is a collagenolytic enzyme located at the cell surface and implicated in extracellular matrix (ECM) remodeling. Mmp14^−/− mice present dwarfism, bone abnormalities, and premature death. We demonstrate herein that the loss of MT1-MMP also causes cardiac defects and severe metabolic changes, and alters the cytoskeleton and the nuclear lamina structure. Moreover, the absence of MT1-MMP induces a senescent phenotype characterized by up-regulation of p16^INK4a and p21^CIP1/WAF1, increased activity of senescence-associated β-galactosidase, generation of a senescence-associated secretory phenotype, and somatotroph axis alterations. Consistent with the role of retinoic acid signaling in nuclear lamina stabilization, treatment of Mmp14^−/− mice with all-trans retinoic acid reversed the nuclear lamina alterations, partially rescued the cell senescence phenotypes, ameliorated the pathological defects in bone, skin, and heart, and extended their life span. These results demonstrate that nuclear architecture and cell senescence can be modulated by a membrane protease, in a process involving the ECM as a key regulator of nuclear stiffness under cell stress conditions.     

Site-Specific Expression of Gelatinolytic Activity during Morphogenesis of the Secondary Palate in the Mouse Embryo

Morphogenesis of the secondary palate in mammalian embryos involves two major events: first, reorientation of the two vertically oriented palatal shelves into a horizontal position above the tongue, and second, fusion of the two shelves at the midline. Genetic evidence in humans and mice indicates the involvement of matrix metalloproteinases (MMPs). As MMP expression patterns might differ from sites of activity, we used a recently developed highly sensitive in situ zymography technique to map gelatinolytic MMP activity in the developing mouse palate. At embryonic day 14.5 (E14.5), we detected strong gelatinolytic activity around the lateral epithelial folds of the nasopharyngeal cavity, which is generated as a consequence of palatal shelf elevation. Activity was concentrated in the basement membrane of the epithelial fold but extended into the adjacent mesenchyme, and increased in intensity with lateral outgrowth of the cavity at E15.5. Gelatinolytic activity at this site was not the consequence of epithelial fold formation, as it was also observed in Bmp7-deficient embryos where shelf elevation is delayed. In this case, gelatinolytic activity appeared in vertical shelves at the exact position where the epithelial fold will form during elevation. Mmp2 and Mmp14 (MT1-MMP), but not Mmp9 and Mmp13, mRNAs were expressed in the mesenchyme around the epithelial folds of the elevated palatal shelves; this was confirmed by immunostaining for MMP-2 and MT1-MMP. Weak gelatinolytic activity was also found at the midline of E14.5 palatal shelves, which increased during fusion at E15.5. Whereas MMPs have been implicated in palatal fusion before, this is the first report showing that gelatinases might contribute to tissue remodeling during early stages of palatal shelf elevation and formation of the nasopharynx.     

Mutations in LTBP4 Cause a Syndrome of Impaired Pulmonary, Gastrointestinal, Genitourinary, Musculoskeletal, and Dermal Development

We report recessive mutations in the gene for the latent transforming growth factor-β binding protein 4 (LTBP4) in four unrelated patients with a human syndrome disrupting pulmonary, gastrointestinal, urinary, musculoskeletal, craniofacial, and dermal development. All patients had severe respiratory distress, with cystic and atelectatic changes in the lungs complicated by tracheomalacia and diaphragmatic hernia. Three of the four patients died of respiratory failure. Cardiovascular lesions were mild, limited to pulmonary artery stenosis and patent foramen ovale. Gastrointestinal malformations included diverticulosis, enlargement, tortuosity, and stenosis at various levels of the intestinal tract. The urinary tract was affected by diverticulosis and hydronephrosis. Joint laxity and low muscle tone contributed to musculoskeletal problems compounded by postnatal growth delay. Craniofacial features included microretrognathia, flat midface, receding forehead, and wide fontanelles. All patients had cutis laxa. Four of the five identified LTBP4 mutations led to premature termination of translation and destabilization of the LTBP4 mRNA. Impaired synthesis and lack of deposition of LTBP4 into the extracellular matrix (ECM) caused increased transforming growth factor-β (TGF-β) activity in cultured fibroblasts and defective elastic fiber assembly in all tissues affected by the disease. These molecular defects were associated with blocked alveolarization and airway collapse in the lung. Our results show that coupling of TGF-β signaling and ECM assembly is essential for proper development and is achieved in multiple human organ systems by multifunctional proteins such as LTBP4.     

Ribosomal Protein Genes RPS10 and RPS26 Are Commonly Mutated in Diamond-Blackfan Anemia

Diamond-Blackfan anemia (DBA), an inherited bone marrow failure syndrome characterized by anemia that usually presents before the first birthday or in early childhood, is associated with birth defects and an increased risk of cancer. Although anemia is the most prominent feature of DBA, the disease is also characterized by growth retardation and congenital malformations, in particular craniofacial, upper limb, heart, and urinary system defects that are present in ∼30%–50% of patients. DBA has been associated with mutations in seven ribosomal protein (RP) genes, RPS19, RPS24, RPS17, RPL35A, RPL5, RPL11, and RPS7, in about 43% of patients. To continue our large-scale screen of RP genes in a DBA population, we sequenced 35 ribosomal protein genes, RPL15, RPL24, RPL29, RPL32, RPL34, RPL9, RPL37, RPS14, RPS23, RPL10A, RPS10, RPS12, RPS18, RPL30, RPS20, RPL12, RPL7A, RPS6, RPL27A, RPLP2, RPS25, RPS3, RPL41, RPL6, RPLP0, RPS26, RPL21, RPL36AL, RPS29, RPL4, RPLP1, RPL13, RPS15A, RPS2, and RPL38, in our DBA patient cohort of 117 probands. We identified three distinct mutations of RPS10 in five probands and nine distinct mutations of RPS26 in 12 probands. Pre-rRNA analysis in lymphoblastoid cells from patients bearing mutations in RPS10 and RPS26 showed elevated levels of 18S-E pre-rRNA. This accumulation is consistent with the phenotype observed in HeLa cells after knockdown of RPS10 or RPS26 expression with siRNAs, which indicates that mutations in the RPS10 and RPS26 genes in DBA patients affect the function of the proteins in rRNA processing.     

Avicularin suppresses cartilage extracellular matrix degradation and inflammation via TRAF6/MAPK activation

BackgroundOsteoarthritis (OA) is an intractable degenerative disease of the whole joint, which is characterized by synovitis inflammation, cartilage damage, and chronic pain. Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) performs an important role in OA.PurposeWe aim to investigate avicularin to protect cartilage extracellular matrix degradation (ECM) and suppresses inflammation both in rat and human chondrocytes.Methods5-Ethynyl-2′-deoxyuridine (EdU) staining, Quantitative real-time PCR, TRAF6 plasmid transfection, Western blot, Measurement of nitric oxide (NO), ROS detection and Immunofluorescence were utilized in vitro. micro-CT scanning, Safranin O-Fast Green, toluidine blue and immunohistochemistry staining were performed in vivo.ResultsIn vitro, avicularin attenuates the degradation of ECM and inflammation, which could inhibit the activation of TRAF6/MAPK pathway via targeting TRAF6. Increased MMP3 and MMP13 expressions and decreased Aggrecan and Collagen Ⅱ levels were observed in anterior cruciate ligament transection (ACLT) induced osteoarthritic rats. Interestingly, intra-articular injection of avicularin attenuates this phenomenon.ConclusionsTaken together, our results indicate that avicularin suppresses cartilage extracellular matrix degradation and inflammation via TRAF6/MAPK activation by targeting TRAF6. These observations identify TRAF6 as a relevant drug target, and avicularin may as a potential therapeutic agent in osteoarthritis.     

CNTNAP2 and NRXN1 Are Mutated in Autosomal-Recessive Pitt-Hopkins-like Mental Retardation and Determine the Level of a Common Synaptic Protein in Drosophila

Heterozygous copy-number variants and SNPs of CNTNAP2 and NRXN1, two distantly related members of the neurexin superfamily, have been repeatedly associated with a wide spectrum of neuropsychiatric disorders, such as developmental language disorders, autism spectrum disorders, epilepsy, and schizophrenia. We now identified homozygous and compound-heterozygous deletions and mutations via molecular karyotyping and mutational screening in CNTNAP2 and NRXN1 in four patients with severe mental retardation (MR) and variable features, such as autistic behavior, epilepsy, and breathing anomalies, phenotypically overlapping with Pitt-Hopkins syndrome. With a frequency of at least 1% in our cohort of 179 patients, recessive defects in CNTNAP2 appear to significantly contribute to severe MR. Whereas the established synaptic role of NRXN1 suggests that synaptic defects contribute to the associated neuropsychiatric disorders and to severe MR as reported here, evidence for a synaptic role of the CNTNAP2-encoded protein CASPR2 has so far been lacking. Using Drosophila as a model, we now show that, as known for fly Nrx-I, the CASPR2 ortholog Nrx-IV might also localize to synapses. Overexpression of either protein can reorganize synaptic morphology and induce increased density of active zones, the synaptic domains of neurotransmitter release. Moreover, both Nrx-I and Nrx-IV determine the level of the presynaptic active-zone protein bruchpilot, indicating a possible common molecular mechanism in Nrx-I and Nrx-IV mutant conditions. We therefore propose that an analogous shared synaptic mechanism contributes to the similar clinical phenotypes resulting from defects in human NRXN1 and CNTNAP2.