Effect of age on human ACE2 and ACE2-expressing alveolar type II cells levels

BACKGROUND

Angiotensin-converting enzyme 2 (ACE2) is the receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which causes COVID-19. Viral entry requires ACE2 and transmembrane protease serine 2 (TMPRSS2). Transcriptomic studies showed that children display lower ACE2 than adults, though gene expression levels do not always correlate with protein levels. We investigated the effect of age on ACE2 and TMPRSS2 protein expression in alveolar type II (AT2) cells in the lungs of children compared to adults. We also analysed the ratio of Ang-(1-7) to Ang II as a surrogate marker of ACE2 activity in the subjects' lung parenchyma.

METHODS

Ang II and Ang-(1-7) levels and ACE2 and TMPRSS2 protein expression were measured by radioimmunoassay and immunohistochemistry, respectively.

RESULTS

The amount of ACE2-expressing AT2 cells and ACE2 protein content were lower in children than in adults. Ang II levels were higher in children compared to adults and inversely correlated with the amount of ACE2-expressing AT2 cells. Children presented lower Ang-(1-7)/Ang II ratio than adult suggesting lower ACE2 activity in children. TMPRSS2 protein expression was not influenced by age.

CONCLUSIONS

These results expand on previous transcriptomic studies and may partially explain the low susceptibility of children to SARS-CoV-2 infection.

CATEGORY OF STUDY

Clinical original research IMPACT: Children display lower ACE2 protein content and activity compared to adults. Ang II levels were higher in children compared to adults and inversely correlated with the amount of ACE2-expressing AT2 cells TMPRSS2 protein expression was not influenced by age. These results expand on previous transcriptomic studies and may partially explain the low susceptibility of children to SARS-CoV-2 infection.

Plasmatic renin-angiotensin system in normotensive and hypertensive patients hospitalized with COVID-19

Background

Besides its counterbalancing role of the renin-angiotensin system (RAS), angiotensin-converting enzyme (ACE) 2 is the receptor for the type 2 coronavirus that causes severe acute respiratory syndrome, the etiological agent of COVID-19. COVID-19 is associated with increased plasmatic ACE2 levels, although conflicting results have been reported regarding angiotensin (Ang) II and Ang-(1−7) levels. We investigated plasmatic ACE2 protein levels and enzymatic activity and Ang II and Ang-(1−7) levels in normotensive and hypertensive patients hospitalized with COVID-19 compared to healthy subjects.

Methods

Ang II and Ang-(1−7), and ACE2 activity and protein levels were measured in 93 adults (58 % (n = 54) normotensive and 42 % (n = 39) hypertensive) hospitalized with COVID-19. Healthy, normotensive (n = 33) and hypertensive (n = 7) outpatient adults comprised the control group.

Results

COVID-19 patients displayed higher ACE2 enzymatic activity and protein levels than healthy subjects. Within the COVID-19 group, ACE2 activity and protein levels were not different between normotensive and hypertensive-treated patients, not even between COVID-19 hypertensive patients under RAS blockade treatment and those treated with other antihypertensive medications. Ang II and Ang-(1−7) levels significantly decreased in COVID-19 patients. When COVID-19 patients under RAS blockade treatment were excluded from the analysis, ACE2 activity and protein levels remained higher and Ang II and Ang-(1−7) levels lower in COVID-19 patients compared to healthy people.

Conclusions

Our results support the involvement of RAS in COVID-19, even when patients under RAS blockade treatment were excluded. The increased circulating ACE2 suggest higher ACE2 expression and shedding.

Renin-angiotensin system blockade on angiotensin-converting enzyme 2 and TMPRSS2 in human type II pneumocytes

AIMS

Angiotensin-converting enzyme (ACE) 2 is the receptor for severe acute respiratory syndrome coronavirus 2 which causes coronavirus disease 2019 (COVID-19). Viral cellular entry requires ACE2 and transmembrane protease serine 2 (TMPRSS2). ACE inhibitors (ACEIs) or angiotensin (Ang) receptor blockers (ARBs) influence ACE2 in animals, though evidence in human lungs is lacking. We investigated ACE2 and TMPRSS2 in type II pneumocytes, the key cells that maintain lung homeostasis, in lung parenchymal of ACEI/ARB-treated subjects compared to untreated control subjects.

MAIN METHODS

Ang II and Ang-(1-7) levels and ACE2 and TMPRSS2 protein expression were measured by radioimmunoassay and immunohistochemistry, respectively.

KEY FINDINGS

We found that the ratio Ang-(1-7)/Ang II, a surrogate marker of ACE2 activity, as well as the amount of ACE2-expressing type II pneumocytes were not different between ACEI/ARB-treated and untreated subjects. ACE2 protein content correlated positively with smoking habit and age. The percentage of TMPRSS2-expressing type II pneumocytes was higher in males than females and in subjects under 60 years of age but it was not different between ACEI/ARB-treated and untreated subjects. However, there was a positive association of TMPRSS2 protein content with age and smoking in ACEI/ARB-treated subjects, with high TMPRSS2 protein levels most evident in ACEI/ARB-treated older adults and smokers.

SIGNIFICANCE

ACEI/ARB treatment influences human lung TMPRSS2 but not ACE2 protein content and this effect is dependent on age and smoking habit. This finding may help explain the increased susceptibility to COVID-19 seen in smokers and older patients with treated cardiovascular-related pathologies.

Plasma Membrane Ca2+ Pump PMCA4z Is More Active Than Splicing Variant PMCA4x

The plasma membrane Ca²⁺ pumps (PMCA) are P-ATPases that control Ca²⁺ signaling and homeostasis by transporting Ca²⁺ out of the eukaryotic cell. Humans have four genes that code for PMCA isoforms (PMCA1-4). A large diversity of PMCA isoforms is generated by alternative mRNA splicing at sites A and C. The different PMCA isoforms are expressed in a cell-type and developmental-specific manner and exhibit differential sensitivity to a great number of regulatory mechanisms. PMCA4 has two A splice variants, the forms "x" and "z". While PMCA4x is ubiquitously expressed and relatively well-studied, PMCA4z is less characterized and its expression is restricted to some tissues such as the brain and heart muscle. PMCA4z lacks a stretch of 12 amino acids in the so-called A-M3 linker, a conformation-sensitive region of the molecule connecting the actuator domain (A) with the third transmembrane segment (M3). We expressed in yeast PMCA4 variants "x" and "z", maintaining constant the most frequent splice variant "b" at the C-terminal end, and obtained purified preparations of both proteins. In the basal autoinhibited state, PMCA4zb showed a higher ATPase activity and a higher apparent Ca²⁺ affinity than PMCA4xb. Both isoforms were stimulated by calmodulin but PMCA4zb was more strongly activated by acidic lipids than PMCA4xb. The results indicate that a PMCA4 intrinsically more active and more responsive to acidic lipids is produced by the variant "z" of the splicing site A.

The Spf1p P5A-ATPase "arm-like" domain is not essential for ATP hydrolysis but its deletion impairs autophosphorylation

The yeast Spf1p P5A-ATPase actively translocates membrane spanning peptides of mislocalized proteins from the endoplasmic reticulum. Loss of Spf1p function causes a pleiotropic ER stress-phenotype associated with alterations of homeostasis of metal ions, lipids, protein folding, glycosylation, and membrane insertion. A unique characteristic of P5A-ATPases is the presence of an extended insertion which was called the "arm-like" domain connecting the phosphorylation domain (P) with transmembrane segment M5 near the peptidyl-substrate binding pocket. Here we have constructed and characterized a Δarm mutant of Spf1p lacking a segment of 117 amino acids of the "arm-like" domain. The Δarm mutant was capable of hydrolyzing ATP at maximal rates of 50% of that of the wild type enzyme. With the non-nucleotide substrate analog pNPP, the hydrolytic activity of the mutant dropped to 10%. The mutant showed an apparent affinity for ATP similar to the wild type. When incubated with ATP the Δarm mutant produced a lower level of the catalytic phosphoenzyme in amounts proportionate to the ATPase activity. These results indicate that the "arm-like" domain is not essential for hydrolytic activity and suggest that it is needed for the stabilization of Spf1p in a phosphorylation-ready conformation.

Highly exposed segment of the Spf1p P5A-ATPase near transmembrane M5 detected by limited proteolysis

The yeast Spf1p protein is a primary transporter that belongs to group 5 of the large family of P-ATPases. Loss of Spf1p function produces ER stress with alterations of metal ion and sterol homeostasis and protein folding, glycosylation and membrane insertion. The amino acid sequence of Spf1p shows the characteristic P-ATPase domains A, N, and P and the transmembrane segments M1-M10. In addition, Spf1p exhibits unique structures at its N-terminus (N-T region), including two putative additional transmembrane domains, and a large insertion connecting the P domain with transmembrane segment M5 (D region). Here we used limited proteolysis to examine the structure of Spf1p. A short exposure of Spf1p to trypsin or proteinase K resulted in the cleavage at the N and C terminal regions of the protein and abrogated the formation of the catalytic phosphoenzyme and the ATPase activity. In contrast, limited proteolysis of Spf1p with chymotrypsin generated a large N-terminal fragment containing most of the M4-M5 cytosolic loop, and a minor fragment containing the C-terminal region. If lipids were present during chymotryptic proteolysis, phosphoenzyme formation and ATPase activity were preserved. ATP slowed Spf1p proteolysis without detectable changes of the generated fragments. The analysis of the proteolytic peptides by mass spectrometry and Edman degradation indicated that the preferential chymotryptic site was localized near the cytosolic end of M5. The susceptibility to proteolysis suggests an unexpected exposure of this region of Spf1p that may be an intrinsic feature of P5A-ATPases.

Reduction of the P5A-ATPase Spf1p phosphoenzyme by a Ca2+-dependent phosphatase

P5 ATPases are eukaryotic pumps important for cellular metal ion, lipid and protein homeostasis; however, their transported substrate, if any, remains to be identified. Ca²⁺ was proposed to act as a ligand of P5 ATPases because it decreases the level of phosphoenzyme of the Spf1p P5A ATPase from Saccharomyces cerevisiae. Repeating previous purification protocols, we obtained a purified preparation of Spf1p that was close to homogeneity and exhibited ATP hydrolytic activity that was stimulated by the addition of CaCl₂. Strikingly, a preparation of a catalytically dead mutant Spf1p (D⁴⁸⁷N) also exhibited Ca²⁺-dependent ATP hydrolytic activity. These results indicated that the Spf1p preparation contained a co-purifying protein capable of hydrolyzing ATP at a high rate. The activity was likely due to a phosphatase, since the protein i) was highly active when pNPP was used as substrate, ii) required Ca²⁺ or Zn²⁺ for activity, and iii) was strongly inhibited by molybdate, beryllium and other phosphatase substrates. Mass spectrometry identified the phosphatase Pho8p as a contaminant of the Spf1p preparation. Modification of the purification procedure led to a contaminant-free Spf1p preparation that was neither stimulated by Ca²⁺ nor inhibited by EGTA or molybdate. The phosphoenzyme levels of a contaminant-free Spf1p preparation were not affected by Ca²⁺. These results indicate that the reported effects of Ca²⁺ on Spf1p do not reflect the intrinsic properties of Spf1p but are mediated by the activity of the accompanying phosphatase.

Inhibition of the Formation of the Spf1p Phosphoenzyme by Ca2

P5-ATPases are important for processes associated with the endosomal-lysosomal system of eukaryotic cells. In humans, the loss of function of P5-ATPases causes neurodegeneration. In the yeastSaccharomyces cerevisiae, deletion of P5-ATPase Spf1p gives rise to endoplasmic reticulum stress. The reaction cycle of P5-ATPases is poorly characterized. Here, we showed that the formation of the Spf1p catalytic phosphoenzyme was fast in a reaction medium containing ATP, Mg(2+), and EGTA. Low concentrations of Ca(2+)in the phosphorylation medium decreased the rate of phosphorylation and the maximal level of phosphoenzyme. Neither Mn(2+)nor Mg(2+)had an inhibitory effect on the formation of the phosphoenzyme similar to that of Ca(2+) TheKmfor ATP in the phosphorylation reaction was ∼1 μmand did not significantly change in the presence of Ca(2+) Half-maximal phosphorylation was attained at 8 μmMg(2+), but higher concentrations partially protected from Ca(2+)inhibition. In conditions similar to those used for phosphorylation, Ca(2+)had a small effect accelerating dephosphorylation and minimally affected ATPase activity, suggesting that the formation of the phosphoenzyme was not the limiting step of the ATP hydrolytic cycle.

The Parkinson-associated human P5B-ATPase ATP13A2 protects against the iron-induced cytotoxicity

P-type ion pumps are membrane transporters that have been classified into five subfamilies termed P1-P5. The ion transported by the P5-ATPases is not known. Five genes named ATP13A1-ATP13A5 that belong to the P5-ATPase group are present in humans. Loss-of-function mutations in the ATP13A2 gene (PARK9, OMIM 610513) underlay a form of Parkinson's disease (PD) known as the Kufor-Rakeb syndrome (KRS), which belongs to the group of syndromes of neurodegeneration with brain iron accumulation (NBIA). Here we report that the cytotoxicity induced by iron exposure was two-fold reduced in CHO cells stably expressing the ATP13A2 recombinant protein (ATP13A2). Moreover, the iron content in ATP13A2 cells was lower than control cells stably expressing an inactive mutant of ATP13A2. ATP13A2 expression caused an enlargement of lysosomes and late endosomes. ATP13A2 cells exhibited a reduced iron-induced lysosome membrane permeabilization (LMP). These results suggest that ATP13A2 overexpression improves the lysosome membrane integrity and protects against the iron-induced cell damage.

Shadows of an absent partner: ATP hydrolysis and phosphoenzyme turnover of the Spf1 (sensitivity to Pichia farinosa killer toxin) P5-ATPase

The P5-ATPases are important components of eukaryotic cells. They have been shown to influence protein biogenesis, folding, and transport. The knowledge of their biochemical properties is, however, limited, and the transported ions are still unknown. We expressed in Saccharomyces cerevisiae the yeast Spf1 P5A-ATPase containing the GFP fused at the N-terminal end. The GFP-Spf1 protein was localized in the yeast endoplasmic reticulum. Purified preparations of GFP-Spf1 hydrolyzed ATP at a rate of ~0.3-1 μmol of P(i)/mg/min and formed a phosphoenzyme in a simple reaction medium containing no added metal ions except Mg(2+). No significant differences were found between the ATPase activity of GFP-Spf1 and recombinant Spf1. Omission of protease inhibitors from the purification buffers resulted in a high level of endogenous proteolysis at the C-terminal portion of the GFP-Spf1 molecule that abolished phosphoenzyme formation. The Mg(2+) dependence of the GFP-Spf1 ATPase was similar to that of other P-ATPases where Mg(2+) acts as a cofactor. The addition of Mn(2+) to the reaction medium decreased the ATPase activity. The enzyme manifested optimal activity at a near neutral pH. When chased by the addition of cold ATP, 90% of the phosphoenzyme remained stable after 5 s. In contrast, the phosphoenzyme rapidly decayed to less than 20% when chased for 3 s by the addition of ADP. The greater effect of ADP accelerating the disappearance of EP suggests that GFP-Spf1 accumulated the E1~P phosphoenzyme. This behavior may reflect a limiting countertransported substrate needed to promote turnover or a missing regulatory factor.

The plasma membrane Ca2+ pump catalyzes the hydrolysis of ATP at low rate in the absence of Ca2+

The plasma membrane Ca2+ ATPase catalyzed the hydrolysis of ATP in the presence of millimolar concentrations of EGTA and no added Ca2+ at a rate near 1.5% of that attained at saturating concentrations of Ca2+. Like the Ca-dependent ATPase, the Ca-independent activity was lower when the enzyme was autoinhibited, and increased when the enzyme was activated by acidic lipids or partial proteolysis. The ATP concentration dependence of the Ca2+-independent ATPase was consistent with ATP binding to the low affinity modulatory site. In this condition a small amount of hydroxylamine-sensitive phosphoenzyme was formed and rapidly decayed when chased with cold ATP. We propose that the Ca2+-independent ATP hydrolysis reflects the well known phosphatase activity which is maximal in the absence of Ca2+ and is catalyzed by E(2)-like forms of the enzyme. In agreement with this idea pNPP, a classic phosphatase substrate was a very effective inhibitor of the ATP hydrolysis.

High sensibility to reactivation by acidic lipids of the recombinant human plasma membrane Ca2+-ATPase isoform 4xb purified from Saccharomyces cerevisiae

The human plasma membrane Ca2+ pump (isoform 4xb) was expressed in Saccharomyces cerevisiae and purified by calmodulin-affinity chromatography. Under optimal conditions the recombinant enzyme (yPMCA) hydrolyzed ATP in a Ca2+ dependent manner at a rate of 15 micromol/mg/min. The properties of yPMCA were compared to those of the PMCA purified from human red cells (ePMCA). The mobility of yPMCA in SDS-PAGE was the expected for the hPMCA4xb protein but slightly lower than that of ePMCA. Both enzymes achieved maximal activity when supplemented with acidic phospholipids. However, while ePMCA in mixed micelles of phosphatidylcholine-detergent had 30% of its maximal activity, the yPMCA enzyme was nearly inactive. Increasing the phosphatidylcholine content of the micelles did not increase the activity of yPMCA but the activity in the presence of phosphatidylcholine improved by partially removing the detergent. The reactivation of the detergent solubilized yPMCA required specifically acidic lipids and, as judged by the increase in the level of phosphoenzyme, it involved the increase in the amount of active enzyme. These results indicate that the function of yPMCA is highly sensitive to delipidation and the restitution of acidic lipids is needed for a functional enzyme.

Intramolecular Fluorescence Resonance Energy Transfer between Fused Autofluorescent Proteins Reveals Rearrangements of the N- and C-terminal Segments of the Plasma Membrane Ca2+ Pump Involved in the Activation

The blue and green fluorescent proteins (BFP and GFP) have been fused at the N- and C-terminal ends, respectively, of the plasma membrane Ca(2+) pump (PMCA) isoform 4xb (hPMCA4xb). The fusion protein was successfully expressed in yeast and purified by calmodulin affinity chromatography. Despite the presence of the fused autofluorescent proteins BFP-PMCA-GFP performed similarly to the wild-type enzyme with respect to Ca(2+)-ATPase activity and sensitivity to calmodulin activation. In the autoinhibited state BFP-PMCA-GFP exhibited a significant intramolecular fluorescence resonance energy transfer (FRET) consistent with the location of the fluorophores at an average distance of 45A. The FRET intensity in BFP-PMCA-GFP decreased when the enzyme was activated either by Ca(2+)-calmodulin, partial proteolysis, or acidic lipids. Moreover, FRET decreased and became insensitive to calmodulin when hPMCA4xb was activated by mutation D170N in BFP-PMCA(D170N)-GFP. The results suggest that the ends of the PMCA are in close proximity in the autoinhibited conformation, and they separate or reorient when the PMCA achieves its final activated conformation.