Two factors associated with increased uptake of calcium in platelets from essential hypertensive patients

Evolving concepts in progressive supranuclear palsy and other 4-repeat tauopathies

Tauopathies are classified according to whether tau deposits predominantly contain tau isoforms with three or four repeats of the microtubule-binding domain. Those in which four-repeat (4R) tau predominates are known as 4R-tauopathies, and include progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, globular glial tauopathies and conditions associated with specific MAPT mutations. In these diseases, 4R-tau deposits are found in various cell types and anatomical regions of the brain and the conditions share pathological, pathophysiological and clinical characteristics. Despite being considered 'prototype' tauopathies and, therefore, ideal for studying neuroprotective agents, 4R-tauopathies are still severe and untreatable diseases for which no validated biomarkers exist. However, advances in research have addressed the issues of phenotypic overlap, early clinical diagnosis, pathophysiology and identification of biomarkers, setting a road map towards development of treatments. New clinical criteria have been developed and large cohorts with early disease are being followed up in prospective studies. New clinical trial readouts are emerging and biomarker research is focused on molecular pathways that have been identified. Lessons learned from failed trials of neuroprotective drugs are being used to design new trials. In this Review, we present an overview of the latest research in 4R-tauopathies, with a focus on progressive supranuclear palsy, and discuss how current evidence dictates ongoing and future research goals.

Structure-based classification of tauopathies

The ordered assembly of tau protein into filaments characterizes several neurodegenerative diseases, which are called tauopathies. It was previously reported that, by cryo-electron microscopy, the structures of tau filaments from Alzheimer's disease^1,2, Pick's disease³, chronic traumatic encephalopathy⁴ and corticobasal degeneration⁵ are distinct. Here we show that the structures of tau filaments from progressive supranuclear palsy (PSP) define a new three-layered fold. Moreover, the structures of tau filaments from globular glial tauopathy are similar to those from PSP. The tau filament fold of argyrophilic grain disease (AGD) differs, instead resembling the four-layered fold of corticobasal degeneration. The AGD fold is also observed in ageing-related tau astrogliopathy. Tau protofilament structures from inherited cases of mutations at positions +3 or +16 in intron 10 of MAPT (the microtubule-associated protein tau gene) are also identical to those from AGD, suggesting that relative overproduction of four-repeat tau can give rise to the AGD fold. Finally, the structures of tau filaments from cases of familial British dementia and familial Danish dementia are the same as those from cases of Alzheimer's disease and primary age-related tauopathy. These findings suggest a hierarchical classification of tauopathies on the basis of their filament folds, which complements clinical diagnosis and neuropathology and also allows the identification of new entities-as we show for a case diagnosed as PSP, but with filament structures that are intermediate between those of globular glial tauopathy and PSP.

A vasoactive serum component and reactivity of isolated rat aorta after chronic alcohol administration

Norepinephrine-induced contractility and relaxation to acetylcholine of isolated aortas taken from normal and chronic alcoholic rats were measured. Similar experiments were performed adding serum from normal (NS) or alcoholic rats (AS), with or without norepinephrine and acetylcholine. Responses of normal and alcoholic aortas to agonists were comparable. NS and AS induced contractions on normal aorta, but the responses of alcoholic aortas were lower; contractions induced by AS on both aortas were larger than those induced by NS. AS decreased verapamil-induced relaxation more than NS. A pressor circulating factor might be responsible.

Increased calcium uptake in vascular tissue by plasma of patients with essential hypertension

Plasma factors have been implicated in causing increased calcium uptake and cytosolic (Ca++) in vascular tissue leading to hypertension. We compared the effect of plasma from hypertensive and normotensive subjects on rat aortic calcium uptake. Plasma from hypertensive subjects was fractionated and the fractions assessed for their activity on aortic calcium uptake. Aortic calcium uptake was significantly higher in plasma from hypertensives as compared to normotensives (p less than 0.05). There was a significant positive correlation between diastolic blood pressure and aortic calcium uptake (r = 0.645; p = 0.002). There was a significant positive correlation between percentage ideal body weight and aortic calcium uptake in normotensives and hypertensives (r = 0.522; p less than 0.05). The calcium uptake stimulatory activity in plasma of hypertensives was found in nonesterified fatty acid and cholesterol ester fractions.

Effect of circulating factors on vascular smooth muscle contraction and its calcium uptake in uremia

Uremia is often associated with alterations in calcium metabolism and vascular smooth muscle function in hypertension and atherosclerosis. The ways in which these conditions inter-relate are not clearly understood. In order to study the possibility that circulating factors might influence smooth muscle function, experiments were performed on rat aortic strips. The serum from both uremic patients and rats enhanced the norepinephrine-induced contraction (NEIC) and net 45-calcium uptake in rat aortic strips. In a similar manner, the serum of parathyroidectomized uremic rats also increased the NEIC, whereas verapamil reduced the aortic response to levels below those of the control, in the presence of uremic serum. These findings suggest that in both chronic (patients) and early (rats) stages of uremia, there is a circulating factor, different from parathyroid hormone, that affects calcium uptake and vascular smooth muscle contraction.

Parathyroid origin of a new hypertensive factor

Many physiological abnormalities have been described in essential hypertension, yet the cause of this condition remains unknown. Included among the reported abnormalities are alterations in serum and tissue calcium levels, abnormalities in calcium regulating hormones, and the involvement of the parathyroid gland in some forms of hypertension. In the current study, the authors review evidence suggesting that a newly described hypertensive factor may explain a number of these abnormalities. This factor was first described in spontaneously hypertensive rat (SHR) plasma and is characterized by its ability to raise blood pressure in a delayed manner in normotensive rats, as well as by its ability to increase calcium uptake in vascular smooth muscle. The factor seems to be produced by the parathyroid gland, yet it is distinct from parathyroid hormone. Histological studies suggest that the factor may be produced by a specific cell type in the parathyroid glands. Given the parathyroid gland dependency of this factor, the authors have tentatively named it "parathyroid hypertensive factor," or "PHF."

In vivo potentiation of vasopressors by spontaneously hypertensive rat plasma: correlation with blood pressure and calcium uptake

Previous studies have shown that plasma from Spontaneously Hypertensive Rats (SHR) can potentiate the action of various vasopressors. Recently, we described a novel circulating hypertensive factor in the plasma of SHR rats which increases calcium uptake in vascular smooth muscle. In order to determine whether this factor might be responsible for the previously described pressor potentiation, the effect of dialyzed SHR plasma on the in vivo sensitivity of normotensive rats to norepinephrine, angiotensin II and arginine vasopressin was examined. Results of this study showed that SHR plasma potentiated the effects of all three pressor agents with peak potentiation occurring approximately 45-60 min post-plasma injection. The time course of pressor potentiation was similar to those for the hypertensive and cellular calcium effects of the plasma. These results suggest that a single factor may be responsible for the hypertensive, calcemic and pressor-potentiating effects of SHR plasma.