Structural and functional characterization of the IgSF21-neurexin2α complex and its related signaling pathways in the regulation of inhibitory synapse organization

The prevailing model behind synapse development and specificity is that a multitude of adhesion molecules engage in transsynaptic interactions to induce pre- and postsynaptic assembly. How these extracellular interactions translate into intracellular signal transduction for synaptic assembly remains unclear. Here, we focus on a synapse organizing complex formed by immunoglobulin superfamily member 21 (IgSF21) and neurexin2α (Nrxn2α) that regulates GABAergic synapse development in the mouse brain. We reveal that the interaction between presynaptic Nrxn2α and postsynaptic IgSF21 is a high-affinity receptor-ligand interaction and identify a binding interface in the IgSF21-Nrxn2α complex. Despite being expressed in both dendritic and somatic regions, IgSF21 preferentially regulates dendritic GABAergic presynaptic differentiation whereas another canonical Nrxn ligand, neuroligin2 (Nlgn2), primarily regulates perisomatic presynaptic differentiation. To explore mechanisms that could underlie this compartment specificity, we targeted multiple signaling pathways pharmacologically while monitoring the synaptogenic activity of IgSF21 and Nlgn2. Interestingly, both IgSF21 and Nlgn2 require c-jun N-terminal kinase (JNK)-mediated signaling, whereas Nlgn2, but not IgSF21, additionally requires CaMKII and Src kinase activity. JNK inhibition diminished de novo presynaptic differentiation without affecting the maintenance of formed synapses. We further found that Nrxn2α knockout brains exhibit altered synaptic JNK activity in a sex-specific fashion, suggesting functional linkage between Nrxns and JNK. Thus, our study elucidates the structural and functional relationship of IgSF21 with Nrxn2α and distinct signaling pathways for IgSF21-Nrxn2α and Nlgn2-Nrxn synaptic organizing complexes in vitro. We therefore propose a revised hypothesis that Nrxns act as molecular hubs to specify synaptic properties not only through their multiple extracellular ligands but also through distinct intracellular signaling pathways of these ligands.

Sculpting the brain: JAK2 eliminates inactive connections

Competition between active and inactive synapses sculpts neuronal networks by activity-dependent loss of inactive connections, the mechanisms for which are poorly understood. In this issue of Neuron, Yasuda et al. (2021) demonstrate that JAK2-STAT1 signaling in inactive axons and synapses is essential for their elimination.

LRRTMs Organize Synapses through Differential Engagement of Neurexin and PTPσ

Presynaptic neurexins (Nrxs) and type IIa receptor-type protein tyrosine phosphatases (RPTPs) organize synapses through a network of postsynaptic ligands. We show that leucine-rich-repeat transmembrane neuronal proteins (LRRTMs) differentially engage the protein domains of Nrx but require its heparan sulfate (HS) modification to induce presynaptic differentiation. Binding to the HS of Nrx is sufficient for LRRTM3 and LRRTM4 to induce synaptogenesis. We identify mammalian Nrx1γ as a potent synapse organizer and reveal LRRTM4 as its postsynaptic ligand. Mice expressing a mutant form of LRRTM4 that cannot bind to HS show structural and functional deficits at dentate gyrus excitatory synapses. Through the HS of Nrx, LRRTMs also recruit PTPσ to induce presynaptic differentiation but function to varying degrees in its absence. PTPσ forms a robust complex with Nrx, revealing an unexpected interaction between the two presynaptic hubs. These findings underscore the complex interplay of synapse organizers in specifying the molecular logic of a neural circuit.

Mechanisms of PTPσ-Mediated Presynaptic Differentiation

Formation of synapses between neurons depends in part on binding between axonal and dendritic cell surface synaptic organizing proteins, which recruit components of the developing presynaptic and postsynaptic specializations. One of these presynaptic organizing molecules is protein tyrosine phosphatase σ (PTPσ). Although the protein domains involved in adhesion between PTPσ and its postsynaptic binding partners are known, the mechanisms by which it signals into the presynaptic neuron to recruit synaptic vesicles and other necessary components for regulated transmitter release are not well understood. One attractive candidate to mediate this function is liprin-α, a scaffolding protein with well-established roles at the synapse. We systematically mutated residues of the PTPσ intracellular region (ICR) and used the yeast dihydrofolate reductase (DHFR) protein complementation assay to screen for disrupted interactions between these mutant forms of PTPσ and its various binding partners. Using a molecular replacement strategy, we show that disrupting the interaction between PTPσ and liprin-α, but not between PTPσ and itself or another binding partner, caskin, abolishes presynaptic differentiation. Furthermore, phosphatase activity of PTPσ and binding to extracellular heparan sulfate (HS) proteoglycans are dispensable for presynaptic induction. Previous reports have suggested that binding between PTPσ and liprin-α is mediated by the PTPσ membrane-distal phosphatase-like domain. However, we provide evidence here that both of the PTPσ phosphatase-like domains mediate binding to liprin-α and are required for PTPσ-mediated presynaptic differentiation. These findings further our understanding of the mechanistic basis by which PTPσ acts as a presynaptic organizer.

Symbolic Shorthand System for Physiology and Medicine

The Symbolic Shorthand System (SSS) is applicable to any problem of documentation. Instead of a code designation composed of numbers and letters the SSS uses mnemonic symbols and signs reminiscent of the subjects they denote.

All possible subjects and relationships between them are first arranged in one dimension, typed separately on subject cards, and then arranged one after the other according to a fixed order of precedence.

Synonymous designations are avoided because all symbols have only one fixed meaning. The possibility of combining symbols gives this system the vital flexibility that other methods lack and it can be expanded indefinitely by establishing new symbols and signs.

In Situ Protein Binding Assay Using Fc-Fusion Proteins

This protocol describes an in situ protein-protein interaction assay between tagged recombinant proteins and cell-surface expressed synaptic proteins. The assay is arguably more sensitive than other traditional protein binding assays such as co-immunoprecipitation and pull-downs and provides a visual readout for binding. This assay has been widely used to determine the dissociation constant of binding of trans-synaptic adhesion proteins. The step-wise description in the protocol should facilitate the adoption of this method in other laboratories.

The yeast HtrA orthologue Ynm3 is a protease with chaperone activity that aids survival under heat stress

Ynm3 is the only budding yeast protein possessing a combination of serine protease and postsynaptic density 95/disc-large/zona occludens domains, a defining feature of the high temperature requirement A (HtrA) protein family. The bacterial HtrA/DegP is involved in protective stress response to aid survival at higher temperatures. The role of mammalian mitochondrial HtrA2/Omi in protein quality control is unclear, although loss of its protease activity results in susceptibility toward Parkinson's disease, in which mitochondrial dysfunction and impairment of protein folding and degradation are key pathogenetic features. We studied the role of the budding yeast HtrA, Ynm3, with respect to unfolding stresses. Similar to Escherichia coli DegP, we find that Ynm3 is a dual chaperone-protease. Its proteolytic activity is crucial for cell survival at higher temperature. Ynm3 also exhibits strong general chaperone activity, a novel finding for a eukaryotic HtrA member. We propose that the chaperone activity of Ynm3 may be important to improve the efficiency of proteolysis of aberrant proteins by averting the formation of nonproductive toxic aggregates and presenting them in a soluble state to its protease domain. Suppression studies with Deltaynm3 led to the discovery of chaperone activity in a nucleolar peptidyl-prolyl cis-trans isomerase, Fpr3, which could partly relieve the heat sensitivity of Deltaynm3.

Biochemical and immunological characterization of a glycosylated alpha-fucosidase from the invertebrate Unio: interaction of the enzyme with its in vivo binding partners

Mammalian alpha-fucosidase (EC 3.2.1.51) is a lysosomal enzyme that catalyzes the removal of fucose residues from glycosphingolipids and its absence in humans results in a rare metabolic disorder called fucosidosis. Among the invertebrates in the molluscs (Unio) two forms of the enzyme have been reported, a 68 kDa non-glycosylated form and a 56 kDa glycosylated form. The glycosylated form has been purified from the seminal fluid of Unio [Biochem. Biophys. Res. Commun. 234 (1997) 54]. In the present study, the 56 kDa glycosylated form has been purified to homogeneity from the whole body tissue of Unio using a series of chromatographic steps. The purified enzyme migrated as a single protein species in 10% SDS-PAGE. Antibodies to the purified enzyme were raised in a rabbit in order to study its biochemical and immunological properties. The purified enzyme is a glycoprotein that exhibits strong binding to Con A-Sepharose gel and can be deglycosylated by PNGase F enzyme suggesting it to be N-glycosylated. The enzyme has been shown to specifically interact with the mannose 6-phosphate receptor protein (MPR 300) purified from goat and Unio. This specific interaction is discussed in view of its possible in vivo binding partners.

Vasoconstrictor effect of the angiotensin-converting enzyme-resistant, chymase-specific substrate [Pro(11)(D)-Ala(12)] angiotensin I in human dorsal hand veins: in vivo demonstration of non-ace production of angiotensin II in humans

BACKGROUND

[Pro(11)(D)-Ala(12)] angiotensin I is an ACE-resistant substrate specific for chymase. We used this peptide to determine whether a functionally significant non-ACE angiotensin (Ang) II-generating pathway exists in human dorsal hand veins.

METHODS AND RESULTS

Using a modified Aellig technique, we studied the response to Ang I and [Pro(11)(D)-Ala(12)] Ang I in dorsal hand veins in vivo in patients with coronary heart disease. We measured the venoconstrictor effect of each peptide given before and after a 6.25-mg oral dose of the ACE inhibitor captopril or matching placebo. Placebo or captopril was given in a double-blind, randomized fashion. Ang I induced a mean+/-SEM venoconstrictor response of 45+/-11%, 40+/-10%, 55+/-8%, and 4+/-4% before placebo, after placebo, before captopril, and after captopril, respectively. Hence, the response to Ang I was reproducible and was reduced significantly only after treatment with captopril (P=0.002). [Pro(11)(D)-Ala(12)] Ang I induced a mean venoconstrictor response of 42+/-9%, 49+/-9%, 48+/-10%, and 54+/-11% before placebo, after placebo, before captopril, and after captopril, respectively. Hence, captopril had no significant effect on the response to [Pro(11)(D)-Ala(12)] Ang I.

CONCLUSIONS

We have demonstrated that [Pro(11)(D)-Ala(12)] Ang I is able to induce venoconstriction in humans in vivo. With this specific pharmacological probe, we have shown that a non-ACE pathway capable of generating Ang II exists in human veins in vivo and is potentially functionally important. This pathway is likely to involve the enzyme chymase.

Angiotensin converting enzyme (ACE) and non-ACE dependent angiotensin II generation in resistance arteries from patients with heart failure and coronary heart disease

OBJECTIVES

We sought to demonstrate non-angiotensin converting enzyme (ACE) dependent angiotensin II (AII) generating pathways in resistance arteries from patients with chronic heart failure (CHF).

BACKGROUND

Non-ACE dependent AII generation occurs in resistance arteries from normal volunteers. Inhibition of non-ACE dependent AII generation may have therapeutic potential in CHF.

METHODS

Resistance arteries were dissected from gluteal biopsies from patients with coronary heart disease (CHD) and preserved left ventricular function and from patients with CHF. Using wire myography, concentration response curves to angiotensin I (AI) and AII were constructed in the presence of 1) vehicle, 2) chymostatin [an inhibitor of chymase], 3) enalaprilat, and 4) the combination of chymostatin and enalaprilat.

RESULTS

In resistance arteries from patients with CHD, the vasoconstrictor response to AI was not inhibited by either inhibitor alone (chymostatin [p > or = 0.05] or enalaprilat [p > or = 0.05]) but was significantly inhibited by the combination (p < 0.001). In arteries from patients with CHF, AI responses were inhibited by enalaprilat (p < 0.05) but not by chymostatin alone (p > 0.05). The combination ofchymostatin and enalaprilat markedly inhibited the response to AI (p < 0.001) to a greater degree than enalaprilat alone (p < or = 0.01).

CONCLUSIONS

Non-ACE dependent AII generating pathways exist in resistance arteries from patients with both CHF and CHD. In resistance arteries from patients with CHD, inhibition of either the ACE or chymase pathway alone has no effect on AII generation, and both pathways must be blocked before the vasoconstrictor action of AI is inhibited. In CHF, blockade of ACE results in marked inhibition of responses to AI, but this is enhanced by coinhibition of chymase. These studies suggest that full suppression of the renin-angiotensin system cannot be achieved by ACE inhibition alone and provide a rationale for developing future therapeutic strategies.

Angiotensin-converting enzyme-independent contraction to angiotensin I in human resistance arteries

BACKGROUND

In vitro studies of myocardial tissue suggest that angiotensin II (Ang II) may be generated by both ACE and chymase. A similar dual pathway may exist in the vasculature. We studied the effects of ACE and chymase inhibitors on the contractile response to angiotensin I (Ang I) in human resistance arteries to investigate ACE-independent generation of Ang II.

METHODS AND RESULTS

Subcutaneous resistance arteries (250 to 350 microm) were obtained from gluteal biopsies from volunteers and New Zealand White rabbits and mounted on a wire myograph. Contractile ability was tested with high-potassium depolarization and norepinephrine 10 micromol/L and endothelial integrity by relaxation to acetylcholine 3 micromol/L. Cumulative concentration-response curves were constructed for Ang I in the presence of enalaprilat 1 micromol/L, chymostatin 10 micromol/L, or both inhibitors together. In the rabbit, enalaprilat completely inhibited the Ang I response. In human vessels, enalaprilat or chymostatin alone had no effect, but the combination of enalaprilat and chymostatin almost completely inhibited the response to Ang I.

CONCLUSIONS

A dual pathway for Ang II generation exists in human resistance arteries, mediated by ACE and a chymostatin-sensitive enzyme, probably chymase. We confirm that a marked species difference exists in the mechanism of Ang II generation between the human and the rabbit. More efficacious suppression of the renin-angiotensin system may require development of novel enzyme inhibitors or combinations of currently available drugs.

Angiotensin converting enzyme gene polymorphisms and renal disease

In the past year there has been a profusion of reports identifying a possible association between the insertion/deletion polymorphism of the angiotensin converting enzyme and renal diseases. Rather than clarifying the situation, it has become more difficult to interpret its significance owing to small sample sizes and concerns over methodology; hence, studies are frequently contradictory. Despite these concerns there is evidence for a role of this polymorphism in progressive renal disease. This review summarizes the major studies in this field and suggests future strategies that might be employed to identify useful genetic markers in nephrology, for which the angiotensin converting enzyme polymorphism has acted as an important paradigm.

The order of draw of blood specimens into additive containing tubes not affect potassium and calcium measurements

The effect of order of draw when taking blood into tubes containing additive was investigated in 47 medical inpatients; 12 of these patients acted as a control group. The samples were analysed in the order in which they were withdrawn. The results of potassium and calcium concentrations did not differ significantly between groups. Manufacturers recommend a specific order of draw when taking blood using vacuum based blood collection systems, which are routinely used in many hospitals. The results of this study, however, show that order of draw has no effect on calcium or potassium concentrations.

Separation of tubular electrical activity in amphibian skeletal muscle through temperature change

The effect of temperature on the form of the propagated action potential was investigated in frog skeletal muscle fibres. Increasing the temperature decreased the duration of the initial overshoot but a hump then appeared during a more prominent after-depolarization. Finally, at 28-30 degrees C, the after-depolarization was either noticeably enlarged or entirely absent. This all-or-none failure of tubular conduction suggests that excitation of the tubular membrane takes place through regenerative activity rather than graded electrotonic spread of depolarization. However, it is consistent with a partial electrical isolation of the tubular lumina, possibly through the access resistance proposed in earlier theoretical models for muscle membrane.

Ovarian function and adjuvant chemotherapy for early breast cancer

The effect of cyclophosphamide, methotrexate and fluorouracil (CMF) on ovarian function has been studied in 74 pre-menopausal patients with operable breast cancer. After median follow-up of 47 months, 50, 70 and 80% of 35 patients receiving CMF became permanently amenorrhoeic within 3, 6 and 12 months respectively; in contrast, only 5 in the no treatment (control) group of 39 patients became permanently amenorrhoeic within 12 months. Younger patients (less than 35 years) were more likely to retain or regain menstrual function while on or after CMF treatment. Estimation of ovarian and pituitary hormones in a subset of these women showed that CMF treatment was associated with a decrease in serum oestradiol and progesterone and an increase in serum follicle stimulating hormone and luteinizing hormone to post-menopausal levels. These hormonal changes are consistent with the induction of amenorrhoea during CMF treatment and the absence of resumption of menstrual function after completion of treatment suggests that CMF causes permanent ovarian ablation in a majority of these patients.

Monoclonal antibody HMFG-2 retains activity in vivo and binds to high molecular weight components expressed by metastatic breast cancers

The monoclonal antibody HMFG-2 was raised against components of the human milk fat globule and human milk epithelial cells. It recognizes an epitope on glycoproteins of varying molecular weight; it is tumour-associated and expressed heterogeneously by most breast cancers. An intravenous injection of 131I-labelled HMFG-2 has been given to patients with recurrent cutaneous metastasis from breast carcinomas. Clearance of radioactivity from the plasma was measured and the label was shown to be carried on intact circulating antibody that had the ability to bind in vitro to the specific determinant as expressed by both the patients own tumour and by cultured breast cancer cells. Antibody was found to be associated with biopsy material taken from skin metastases 24 hours after administration of labelled antibody. However, the amount of radioactivity was insufficient for the production of a positive isotopic scan.

Mechanism of action of adjuvant chemotherapy in early breast cancer

The relation between tumour oestrogen and progesterone receptor status, menstrual status, relapse-free survival, and overall survival was analysed in 411 patients with early breast cancer randomised to receive either postoperative adjuvant chemotherapy with cyclophosphamide, methotrexate, and fluorouracil (CMF) or no additional treatment (control). Prolongation of time to recurrence and survival was seen predominantly in premenopausal patients; these effects were seen only with tumours positive for steroid receptors, particularly progesterone. Chemotherapy led to permanent amenorrhoea in 61% of premenopausal patients. The therapeutic effects of chemotherapy were seen only when CMF induced permanent amenorrhoea in premenopausal patients. These findings support the hypothesis that the effect of adjuvant chemotherapy in early breast cancer may be mediated by ovarian suppression.

Recombinant DNA human interferon alpha 2 in advanced breast cancer: a phase 2 trial

Effectiveness of recombinant DNA (rDNA) human interferon alpha 2 (IFN alpha 2) in advanced breast cancer was evaluated in 14 patients who had received prior endocrine and/or cytotoxic therapy. After randomization, 7 patients received IFN alpha 2 two million IU m-2 day-1, s.c., 3 times a week (schedule 1) and 7 patients received 50 million IU m-2 day-1, i.v., for 5 consecutive days, every 3 weeks (schedule 2). Treatment duration was 4-21 weeks in schedule 1 and 6-24 weeks (2-8 courses) in schedule 2. Regressions were not achieved with either schedule. Treatment was associated with significant toxicity and was more severe in schedule 2. Dose limiting toxicities were leukopenia, elevation of liver enzymes, hyperglycemia and fatigue. Serum IFN activity was low or undetectable in patients on schedule 1 and high in patients on schedule 2. At 24 h, serum IFN activity was detectable in only 1/6 patients on schedule 1 as compared to 3/7 patients on schedule 2. IFN neutralizing factors were detected in the serum of only 1 patient prior to treatment but none were detected in any of the patients during or after discontinuation of treatment (4-24 weeks). IFN alpha 2 increased the expression of both HLA class 1 antigens and beta 2 microglobulin in peripheral blood lymphocytes in vivo. This effect was dose related.

Reversing flow in the aorta: a theoretical model

The viscous boundary layer in a reversing flow in a curved tube is analysed with a view to study the reversing flow in the aorta at the beginning of the diastole. The velocity in the core is taken to vary with time as in a dog's aorta. The flow is considered to be quasi-steady long before the time of reversal. Near the time of reversal, the flow is governed by the diffusion equation--a balance between the unsteady inertia terms and viscous terms. The solution which ensures the continuity of the displacement thickness at the time of take over from the quasi-steady solution to the diffusive solution, is obtained. The knowledge of the distribution of the shearing stress in the circulatory system is essential due to its relevance with regard to atheroma--a disease leading to the hardening of the arteries. With this in mind, the wall shear rate is obtained as a function of time at every cross-section of the tube. The shearing stresses at the inner bend and the outer bend are also plotted. The results are compared with those for the straight tube.

The possible role of reticulo endothelial system in hepatic regeneration

Calciphylactic blockade of Reticulo endothelial system (REB) was used to study the role of reticulo-endothelial system (RES) in hepatic regeneration. REB was induced by i.v. egg yolk administration in DHT sensitized animals, which were subsequently partially hepatectomized. Mitotic index and percentage regeneration were studied in these animals 48 hr after partial hepatectomy. REB produced a significant depression of the mitotic index and percentage regeneration in regenerating livers. This suggests that functional integrity of the RES in essential hepatic parenchymal proliferation after partial hepatectomy.

Rose bengal clearance during hepatic regeneration in rats

Intravenous I131 Rose bengal clearance was conducted in partially hepatectomized rats with a dose of 1 muCi/100 gm body weight. Blood samples were taken at regular intervals up to 25 minutes which were suitable diluted to count the remaining radioactivity. From these values t1/2 and the clearance constants were worked out upto a period of 120 hr after partial hepatectomy. An analysis of the results show that there is a linear correlation between the rose bengal clearance and the regenerating hepatic mass which may be used for quantitative assessment of functional hepatic mass.