Dosage changes of a segment at 17p13.1 lead to intellectual disability and microcephaly as a result of complex genetic interaction of multiple genes.

The 17p13.1 microdeletion syndrome is a recently described genomic disorder with a core clinical phenotype of intellectual disability, poor to absent speech, dysmorphic features, and a constellation of more variable clinical features, most prominently microcephaly. We identified five subjects with copy-number variants (CNVs) on 17p13.1 for whom we performed detailed clinical and molecular studies. Breakpoint mapping and retrospective analysis of published cases refined the smallest region of overlap (SRO) for microcephaly to a genomic interval containing nine genes. Dissection of this phenotype in zebrafish embryos revealed a complex genetic architecture: dosage perturbation of four genes (ASGR1, ACADVL, DVL2, and GABARAP) impeded neurodevelopment and decreased dosage of the same loci caused a reduced mitotic index in vitro. Moreover, epistatic analyses in vivo showed that dosage perturbations of discrete gene pairings induce microcephaly. Taken together, these studies support a model in which concomitant dosage perturbation of multiple genes within the CNV drive the microcephaly and possibly other neurodevelopmental phenotypes associated with rearrangements in the 17p13.1 SRO.

Plasma kallikrein and thrombin are cleared through unrelated hepatic pathways.

Plasma kallikrein (PK) and thrombin (TH), serine proteinases formed from inactive precursors, participate in important body defence mechanisms. The isolated hepatocyte recognizes TH, and the liver clears PK by calcium-independent receptors through mechanisms that are not yet clearly understood. It is known that heparin impairs the binding of TH to isolated liver cells through the inhibition of high affinity receptors. Using an isolated, exsanguinated and perfused rat liver preparation we confirmed that the TH hepatic clearance is calcium-independent and affected by heparin; PK clearance rates both in the presence (t1/2 10 +/- 2 min) or the absence (t1/2 10 +/- 1 min) of heparin were similar; the presence of beta-galactosides does not impair the TH clearance but adversely affects the PK clearance and a large excess of TH does not impair the PK clearance rate (t1/2 6 +/- 1 min). These results indicate that PK and TH are cleared by calcium-independent but otherwise unrelated hepatic pathways and suggest that TH may indeed facilitate the PK clearance by the liver.

The recognition site for hepatic clearance of plasma kallikrein is on its heavy chain and is latent on prokallikrein.

We partially purified the glycoproteins prokallikrein and kallikrein from rat plasma. The purification of rat plasma kallikrein may result in two forms: an intact form (alpha, M(r) 84-87 kDa) and a partially degraded form (beta, M(r) 46-51 kDa). The alpha-form is composed of a heavy chain (M(r) 50 kDa) and a light chain (M(r) 34-37 kDa) linked by a disulfide bond. The catalytic site is found on the light chain. The beta-form has a partially degraded heavy chain (M(r) 28 kDa). Using a preparation of exsanguinated and perfused rat liver, we verified that rat plasma prokallikrein is not activated by the liver and that neither the proenzyme nor the light chain is removed by the organ. Both forms (alpha and beta) of the active enzyme are similarly removed from the perfusate. We also observed that the clearance of plasma kallikrein is temperature-dependent, and not affected by substances that inhibit binding to galactosyl-, mannosyl-, fucosyl- or phosphomannosyl-specific lectins, but inhibited by beta-galactosides. We suggest that: (a) the binding site to hepatocytes is latent on prokallikrein and is located on its heavy chain, more specifically on the 28-kDa fragment still present in the beta form of the active enzyme and (b) plasma kallikrein is recognized by an S-type lectin.

On the mechanism of rat uterus desensitization to kallikrein.

An inactive form of kallikrein prepared by iodination with cold iodine, did not show any enzymatic or oxytocic action. However, a competitive pattern between this inactive and active kallikrein was observed in rat uterus preparation: When the inactive form was applied several times in the muscle, a single dose of active kallikrein was unable to cause contraction, but a double dose elicited a response. The rhythmic movement caused by a singular dose of active kallikrein, had its time curtailed by adding the inactive kallikrein to the bath. The inactive kallikrein did not interfere with bradykinin activity.

Chloramine T impairs the receptor-mediated endocytosis of plasma kallikrein by the liver.

Treatment of rat plasma-kallikrein with chloramine T (which does not affect neither its amidolytic activity nor its Mr) impairs the hepatic clearance of the enzyme in a dose-related manner. Preperfusion of the liver with chloramine T before the addition of plasma-kallikrein also diminishes the hepatic clearance of the enzyme.

Identification of receptors in the liver that mediate endocytosis of circulating tissue kallikreins.

The liver plays an important role in the clearance, by receptor-mediated endocytosis, of circulating glycoproteins. It has been demonstrated that tissue kallikreins, which are acid glycoproteins, circulate in plasma, where they are poorly inhibited by plasma proteins. We have shown that the liver is the main organ that clears tissue kallikreins from the circulation. We now report the identification of receptors involved in this clearance. Using a perfused rat-liver system, and as models, pig pancreatic (PPK) and horse urinary (HoUK) kallikreins, we have found that: (a) the binding of PPK to the perfused liver was inhibited by 50 mM methyl alpha-D-mannoside and 20 microM mannan, was partially inhibited by 50 mM mannose and was unaffected by 1.5 microM asialofetuin; (b) binding of HoUK to the perfused liver was inhibited by 1.5 microM asialofetuin, 50 mM galactose and 50 mM lactose and was unaffected by 50 mM mannose; (c) the clearance rate of both kallikreins followed the equation y = a.xb; (d) their binding was Ca2+-dependent and their clearance was inhibited by 3 mM chloroquine and 10 mM methylamine. Using isolated liver cells and tritiated HoUK, we calculated that 500,000 receptors/cell were present and the Scatchard plot showed that there were two apparent affinity constants: 0.24.10(9) l/M) (high-affinity) and 0.3.10(8) l/M (low-affinity). These results show that PPK is recognized by a liver mannose receptor and HoUK by the galactose receptor. The liver uptake of native and circulating tissue kallikreins thus emerges as a mechanism by which their levels in plasma are regulated.