Molecular investigations of the structure and function of the protein phosphatase 1-spinophilin-inhibitor 2 heterotrimeric complex.

Regulation of the major Ser/Thr phosphatase protein phosphatase 1 (PP1) is controlled by a diverse array of targeting and inhibitor proteins. Though many PP1 regulatory proteins share at least one PP1 binding motif, usually the RVxF motif, it was recently discovered that certain pairs of targeting and inhibitor proteins bind PP1 simultaneously to form PP1 heterotrimeric complexes. To date, structural information for these heterotrimeric complexes and, in turn, how they direct PP1 activity is entirely lacking. Using a combination of NMR spectroscopy, biochemistry, and small-angle X-ray scattering (SAXS), we show that major structural rearrangements in both spinophilin (targeting) and inhibitor 2 (I-2, inhibitor) are essential for the formation of the heterotrimeric PP1-spinophilin-I-2 (PSI) complex. The RVxF motif of I-2 is released from PP1 during the formation of PSI, making the less prevalent SILK motif of I-2 essential for complex stability. The release of the I-2 RVxF motif allows for enhanced flexibility of both I-2 and spinophilin in the heterotrimeric complex. In addition, we used inductively coupled plasma atomic emission spectroscopy to show that PP1 contains two metals in both heterodimeric complexes (PP1-spinophilin and PP1-I-2) and PSI, demonstrating that PSI retains the biochemical characteristics of the PP1-I-2 holoenzyme. Finally, we combined the NMR and biochemical data with SAXS and molecular dynamics simulations to generate a structural model of the full heterotrimeric PSI complex. Collectively, these data reveal the molecular events that enable PP1 heterotrimeric complexes to exploit both the targeting and inhibitory features of the PP1-regulatory proteins to form multifunctional PP1 holoenzymes.

Detailed structural characterization of unbound protein phosphatase 1 inhibitors.

Protein phosphatase 1 (PP1) is an essential and ubiquitous serine/threonine protein phosphatase that is regulated by more than 100 known inhibitor and targeting proteins. It is currently unclear how protein inhibitors distinctly and specifically regulate PP1 to enable rapid responses to cellular alterations. We demonstrate that two PP1 inhibitors, I-2 and DARPP-32, belong to the class of intrinsically unstructured proteins (IUPs). We show that both inhibitors have distinct preferences for transient local and long-range structure. These preferences are likely their structural signature for their interaction with PP1. Furthermore, we show that upon phosphorylation of Thr(34) in DARPP-32, which turns DARPP-32 into a potent inhibitor of PP1, neither local nor long-range structure of DARPP-32 is altered. Therefore, our data suggest a role for these transient three-dimensional topologies in binding mechanisms that enable extensive contacts with PP1's invariant surfaces. Together, these interactions enable potent and selective inhibition of PP1.

A PMLRARA transgene results in a retinoid-deficient phenotype associated with enhanced susceptibility to skin tumorigenesis.

The construction of transgenic FVB/N mice targeting the PMLRARA fusion gene under the control of a human MRP8 promoter recapitulated the phenotype of acute promyelocytic leukemia but had the unexpected result of multiple squamous papillomas of the skin (Brown et al., PROC: Natl. Acad. Sci. USA, 94:2551-2556, 1997). In addition, transgenic MRP8-PMLRARA mice exhibited a skin phenotype characteristic of vitamin A deficiency. The severity of the skin phenotype and spontaneous papilloma development correlated with the level of transgene expression. Papilloma formation was preceded by follicular hyperplasia and the expression of epidermal differentiation markers in the follicular epithelium. Mutations in the Ha or Ki alleles of ras were not detected in papillomas that developed on transgenic skin, and papilloma formation was accentuated on the C57/Bl6 background, unlike the usual resistance of this strain to skin tumor induction. Analysis of liver extracts from transgenic mice indicated a deficiency in the production of retinoic acid. Furthermore, affected transgenic epidermis had reduced levels of retinoic acid receptoralpha (RARalpha) and retinoic X receptor (RXRalpha), and supplementation with exogenous retinoic acid prevented the skin phenotype. When transgenic keratinocytes were grafted to nude mice, the resulting integument was normal, and conversely, when transgenic bone marrow was grafted to normal mice, a skin phenotype did not develop. Together these results suggest that local interruption of PML and RARalpha signaling in the skin, together with a systemic retinoid deficiency, initiates a tumor induction pathway that is independent of ras activation.

Status of carotenoids, vitamin A, and vitamin E in the mother-infant dyad and anthropometric status of infants in Malawi.

This prospective study was carried out during February 2000-April 2003 to characterize the relationship between the status of carotenoids, vitamin E, and retinol and anthropometric status in apparently healthy infants and their mothers in Blantyre, Malawi. Anthropometric status of infants and concentrations of carotenoids (alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin, and lycopene), retinol, and alpha-tocopherol in plasma were measured in 173 infants at 12 months of age, and concentrations of carotenoids, retinol, and a-tocopherol in plasma were measured in their mothers two weeks postpartum. In multivariate analyses, concentrations of retinol, total carotenoids, non-provitamin A carotenoids, and alpha-tocopherol in infants were associated with under-weight (p = 0.05). Concentrations of a-tocopherol were associated with wasting (p = 0.04). Concentrations in mothers and infants were all correlated (correlation coefficients from 0.230 to 0.502, p < 0.003). The findings suggest that poor status of carotenoids, retinol, and alpha-tocopherol in infants is associated with their poor anthropometric status, and status of carotenoids, retinol, and alpha-tocopherol in mothers and infants has a low-to-moderate association in the mother-infant dyad.

Vitamin A deficiency, iron deficiency, and anemia among preschool children in the Republic of the Marshall Islands.

OBJECTIVE: We investigated the co-occurrence of vitamin A deficiency, iron deficiency, and anemia among young children in the Republic of the Marshall Islands. METHODS: Hemoglobin, serum retinol, and serum ferritin were assessed in the Republic of the Marshall Islands Vitamin A Deficiency Study, a community-based survey that involved 919 children ages 1 to 5 y. RESULTS: The proportion of children with vitamin A deficiency (serum retinol concentrations < 0.70 microM/L) was 59.9%. The prevalences of anemia (hemoglobin < 110 g/L), iron deficiency (serum ferritin < 12 microg/L), and iron deficiency anemia (iron deficiency and anemia) were 36.4%, 53.5%, and 23.8%, respectively. The proportion of children who had co-occurrence of vitamin A and iron deficiencies was 33.2%. The mean ages of children with and without vitamin A deficiency were 3.2 +/- 1.4 and 2.9 +/- 1.5 y, respectively (P = 0.01), and the mean ages of those with and without iron deficiency were 2.7 +/- 1.3 and 3.5 +/- 1.4 y, respectively (P < 0.0001). CONCLUSIONS: Children in the Republic of the Marshall Islands, ages 1 to 5 y, are at high risk of anemia, vitamin A deficiency, and iron deficiency, and one-third of these children had the co-occurrence of vitamin A and iron deficiencies. Further investigation is needed to identify risk factors and evaluate interventions to address vitamin A and iron deficiencies among children.

Vitamin A deficiency and inflammatory markers among preschool children in the Republic of the Marshall Islands.

BACKGROUND: The exclusion of individuals with elevated acute phase proteins has been advocated in order to improve prevalence estimates of vitamin A deficiency in surveys, but it is unclear whether this will lead to sampling bias. The purpose of the study was to determine whether the exclusion of individuals with elevated acute phase proteins is associated with sampling bias and to characterize inflammation in children with night blindness. METHODS: In a survey in the Republic of the Marshall Islands involving 281 children, aged 1-5 years, serum retinol, C-reactive protein (CRP), and alpha1-acid glycoprotein (AGP) were measured. RESULTS: Of 281 children, 24 (8.5%) had night blindness and 165 (58.7%) had serum retinol < 0.70 micromol/L. Of 248 children with AGP and CRP measurements, 123 (49.6%) had elevated acute phase proteins (CRP > mg/L and/or AGP > 1000 mg/L). Among children with and without night blindness, the proportion with serum retinol < 0.70 micromol/L was 79.2% and 56.8% (P = 0.03) and with anemia was 58.3% and 35.7% (P = 0.029), respectively. The proportion of children with serum retinol < 0.70 micromol/L was 52.0% after excluding children with elevated acute phase proteins. Among children with and without elevated acute phase proteins, mean age was 2.8 vs 3.2 years (P = 0.016), the proportion of boys was 43.1% vs. 54.3% (P = 0.075), with no hospitalizations in the last year was 11.0% vs 23.6% (P = 0.024), and with anemia was 43.8% vs 31.7% (P = 0.05), respectively. CONCLUSIONS: Exclusion of children with inflammation in this survey of vitamin A deficiency does not improve prevalence estimates for vitamin A deficiency and instead leads to sampling bias for variables such as age, gender, anemia, and hospitalization history.

Structural diversity in free and bound states of intrinsically disordered protein phosphatase 1 regulators.

Complete folding is not a prerequisite for protein function, as disordered and partially folded states of proteins frequently perform essential biological functions. In order to understand their functions at the molecular level, we utilized diverse experimental measurements to calculate ensemble models of three nonhomologous, intrinsically disordered proteins: I-2, spinophilin, and DARPP-32, which bind to and regulate protein phosphatase 1 (PP1). The models demonstrate that these proteins have dissimilar propensities for secondary and tertiary structure in their unbound forms. Direct comparison of these ensemble models with recently determined PP1 complex structures suggests a significant role for transient, preformed structure in the interactions of these proteins with PP1. Finally, we generated an ensemble model of partially disordered I-2 bound to PP1 that provides insight into the relationship between flexibility and biological function in this dynamic complex.

Spinophilin directs protein phosphatase 1 specificity by blocking substrate binding sites.

The serine/threonine protein phosphatase 1 (PP1) dephosphorylates hundreds of key biological targets. PP1 associates with >or=200 regulatory proteins to form highly specific holoenzymes. These regulatory proteins target PP1 to its point of action within the cell and prime its enzymatic specificity for particular substrates. However, how they direct PP1's specificity is not understood. Here we show that spinophilin, a neuronal PP1 regulator, is entirely unstructured in its unbound form, and it binds PP1 through a folding-upon-binding mechanism in an elongated fashion, blocking one of PP1's three putative substrate binding sites without altering its active site. This mode of binding is sufficient for spinophilin to restrict PP1's activity toward a model substrate in vitro without affecting its ability to dephosphorylate its neuronal substrate, glutamate receptor 1 (GluR1). Thus, our work provides the molecular basis for the ability of spinophilin to dictate PP1 substrate specificity.

Structural basis for spinophilin-neurabin receptor interaction.

Neurabin and spinophilin are neuronal scaffolding proteins that play important roles in the regulation of synaptic transmission through their ability to target protein phosphatase 1 (PP1) to dendritic spines where PP1 dephosphorylates and inactivates glutamate receptors. However, thus far, it is still unknown how neurabin and spinophilin themselves are targeted to these membrane receptors. Spinophilin and neurabin contain a single PDZ domain, a common protein-protein interaction recognition motif, which are 86% identical in sequence. We report the structures of both the neurabin and spinophilin PDZ domains determined using biomolecular NMR spectroscopy. These proteins form the canonical PDZ domain fold. However, despite their high degree of sequence identity, there are distinct and significant structural differences between them, especially between the peptide binding pockets. Using two-dimensional 1H-15N HSQC NMR analysis, we demonstrate that C-terminal peptide ligands derived from glutamatergic AMPA and NMDA receptors and cytosolic proteins directly and differentially bind spinophilin and neurabin PDZ domains. This peptide binding data also allowed us to classify the neurabin and spinophilin PDZ domains as the first identified neuronal hybrid class V PDZ domains, which are capable of binding both class I and II peptides. Finally, the ability to bind to glutamate receptor subunits suggests that the PDZ domains of neurabin and spinophilin are important for targeting PP1 to C-terminal phosphorylation sites in AMPA and NMDA receptor subunits.

Injection drug use is an independent risk factor for iron deficiency and iron deficiency anemia among HIV-seropositive and HIV-seronegative women.

The risk factors for iron deficiency and iron deficiency anemia among female injection drug users are not well characterized. We measured hemoglobin and plasma ferritin and obtained demographic information and injection drug use history in the last 6 months in a cross-sectional study of 200 female injection drug users (134 HIV-positive and 66 HIV-negative). The women were participants in a natural history study, the AIDS Linked to Intravenous Experiences study in Baltimore, Maryland. In multivariate analyses adjusting for age, hepatitis C virus status, and HIV status, injection drug use within the last 6 months was associated with iron deficiency (odds ratio [OR] = 2.61, 95% confidence interval [CI]: 1.33 to 5.09) and iron deficiency anemia (OR = 6.65, 95% CI: 2.33 to 18.9). Among 134 HIV-positive women, injection drug use in the last 6 months was associated with iron deficiency (OR = 2.43, 95% CI: 1.08 to 5.48) and iron deficiency anemia (OR = 6.05, 95% CI: 1.82 to 20.1) in multivariate analyses adjusting for hepatitis C virus status and CD4 lymphocyte count. Injection drug use seems to be associated with iron deficiency and iron deficiency anemia. Further longitudinal studies are needed to gain insight into the nature of this association.

Breast milk retinol concentrations are not associated with systemic inflammation among breast-feeding women in Malawi.

The acute phase response and inflammation are associated with lower plasma retinol concentrations, but their effect on breast milk retinol concentrations is unclear. We measured plasma retinol concentrations, acute phase proteins, and breast milk retinol concentrations in 237 breast-feeding women at 2 wk postpartum in Blantyre, Malawi; 16.5% of the women had plasma retinol < 0.70 micromol/L and 14.8% had breast milk retinol < 1.05 micromol/L. Among women with and without inflammation [alpha(1)-acid glycoprotein (AGP) > 1 g/L and/or C-reactive protein (CRP) > 5 mg/L], geometric mean (95% CI) plasma retinol was 0.89 (0.84, 0.94) and 1.05 (1.01, 1.17) mumol/L, respectively (P < 0.0001). Among women with and without inflammation, geometric mean (95% CI) breast milk retinol was 2.12 (1.89, 2.36) and 2.05 (1.75, 2.39) micromol/L, respectively (P = 0.74). In multiple linear regression models adjusting for age, parity, education, BMI, and days postpartum, plasma retinol concentrations were associated with plasma AGP and CRP concentrations (P < 0.0001 and P = 0.01, respectively), whereas breast milk retinol concentrations were unaffected by plasma AGP and CRP concentrations (P = 0.22 and P = 0.86, respectively). These findings suggest that breast milk retinol concentrations are not affected by systemic inflammation.

Carotenoid status among preschool children with vitamin A deficiency in the Republic of the Marshall Islands.

Although carotenoids are known to be important dietary sources of vitamin A, there have been few epidemiological studies that have characterized the serum concentrations of major dietary carotenoids among preschool children with vitamin A deficiency. We conducted a population-based, cross-sectional study of serum pro-vitamin A carotenoids (alpha -carotene, beta-carotene, beta-cryptoxanthin), non-provitamin A carotenoids (lutein/zeaxanthin, and lycopene), and retinol among 278 children, aged 1-5 y, in the Republic of the Marshall Islands. Vitamin A deficiency was defined as serum retinol <0.70 micromol/L. Geometric mean serum concentrations of carotenoids among children with and without vitamin A deficiency were 0.003 vs 0.006 micromol/L for alpha-carotene (P = 0.0017), 0.011 vs 0.023 micromol/L for beta-carotene (P <0.0001), 0.023 vs 0.034 micromol/L for beta-cryptoxanthin (P = 0.0075), 0.007 vs 0.012 micromol/L for lycopene (P = 0.037), 0.044 vs 0.052 micromol/L for lutein/zeaxanthin (P = 0.2), and 0.045 vs 0.074 micromol/L for total provitamin A carotenoids (P <0.0001) respectively. In a multivariate analysis adjusting for sex, age (Odds Ratio [O.R.] 1.44, 95% confidence interval [C.I.] 1.16-1.78), and serum provitamin A carotenoids (O.R. 0.49, 95% C.I. 0.34-0.71) were associated with vitamin A deficiency, but serum non-provitamin A carotenoids were not associated with vitamin A deficiency (O.R. 0.93, 95% C.I. 0.67-1.28). Preschool children with vitamin A deficiency in the Republic of the Marshall Islands have extremely low serum concentrations of provitamin A carotenoids and interventions are needed to improve the dietary intake of provitamin A carotenoids among Marshallese children.

Sensitive high resolution inverse detection NMR spectroscopy of proteins in the solid state.

A new indirect detection scheme for obtaining (15)N/(1)H shift correlation spectra in crystalline proteins is described. Excellent water suppression is achieved without the need for pulsed field gradients, and using only a 2-step phase cycle. Careful attention to overall NMR instrument stability was found critical for obtaining the best resolution and sensitivity. Magnetic dilution by deuteration of the protein in combination with high-speed magic angle spinning produces (1)H resonances averaging only 0.22 ppm in width, and in some cases lines as narrow as 0.17 ppm are obtained. In application to two different polymorphs of ubiquitin, structure dependent differences in both (15)N and (1)H amide chemical shifts are observed. In one case, distinct shifts for different molecules in the asymmetric unit are seen, and all differ substantially from solution NMR shifts. A gain of 7 in sensitivity makes the method competitive with solution NMR as long as nanocrystalline samples are available.

High-sensitivity observation of dipolar exchange and NOEs between exchangeable protons in proteins by 3D solid-state NMR spectroscopy.

A highly sensitive new 1H-detected 3D solid-state NMR method is described for characterizing 1H-1H spin exchange in nanocrystalline samples of 15N- and 2H-enriched protein. Long-range contacts are observed in human ubiquitin. The method is also used to show that numerous NOEs between backbone amides and crystal water protons can be observed.