7DHC-induced changes of Kv1.3 operation contributes to modified T cell function in Smith-Lemli-Opitz syndrome

Abstract

In vitro manipulation of membrane sterol level affects the regulation of ion channels and consequently certain cellular functions; however, a comprehensive study that confirms the pathophysiological significance of these results is missing. The malfunction of 7-dehydrocholesterol (7DHC) reductase in Smith-Lemli-Opitz syndrome (SLOS) leads to the elevation of the 7-dehydrocholesterol level in the plasma membrane. T lymphocytes were isolated from SLOS patients to assess the effect of the in vivo altered membrane sterol composition on the operation of the voltage-gated Kv1.3 channel and the ion channel-dependent mitogenic responses. We found that the kinetic and equilibrium parameters of Kv1.3 activation changed in SLOS cells. Identical changes in Kv1.3 operation were observed when control/healthy T cells were loaded with 7DHC. Removal of the putative sterol binding sites on Kv1.3 resulted in a phenotype that was not influenced by the elevation in membrane sterol level. Functional assays exhibited impaired activation and proliferation rate of T cells probably partially due to the modified Kv1.3 operation. We concluded that the altered membrane sterol composition hindered the operation of Kv1.3 as well as the ion channel-controlled T cell functions.

Determining the target of membrane sterols on voltage-gated potassium channels

“…In our system, in vitro cholesterol loading of human lymphocytes by MβCD/C decreased the current density, shifted the voltagedependence and slowed the kinetics of K v 1.3 channels 10 . We have found similar effects of 7DHC on K v 1.3 gating 52 , a precursor of cholesterol, which only differs from cholesterol by the presence of an extra double bond. 7DHC is enriched in the cell membrane of patients suffering from Smith-Lemli-Opitz (SLO) syndrome 53 .…”

“…Our previous results on the sterol-sensitivity of K v 1.3 channel gating showed that cholesterol or 7DHC-enrichment of the membrane reduced the whole-cell current, caused a right-shift of the G-V curve and slowed activation kinetics in human lymphocytes and CHO cells 10,52 . We confirmed all these observations in the Xenopus oocyte expression system with wild-type hK v 1.3 channels indicating that despite the differences in membrane composition, the Xenopus system is a valid model for such protein-membrane interactions in mammalian cells (Suppl.…”

7‐Dehydrocholesterol (7‐DHC), But Not Cholesterol, Causes Suppression of Canonical TGF‐β Signaling and Is Likely Involved in the Development of Atherosclerotic Cardiovascular Disease (ASCVD)

“…Accumulating at high levels in lipid rafts/caveolae (in SLOS patients), 7‐DHC disturbs the structure, and function of these microdomains, resulting in alterations of stability and functions of resident proteins (including TGF‐β receptors and other membrane proteins) [Valencia et al, ]. For example, markedly reduced expression of lipid raft/caveolae‐resident proteins (including caveolin‐1, Na/K ATPase, folate transporter, and BK Ca K + channel) and reduced fibroblast proliferation activity, and altered functions of the voltage‐gated Kv1.3 channel have been found in SLOS fibroblasts and T lymphocytes, respectively [Ren et al, ; Balajthy et al, ]. Since TGF‐β is a mitogen for fibroblasts, reduced proliferation activity in SLOS fibroblasts is likely due to suppression of TGF‐β canonical signaling.…”

The C-terminal HRET sequence of Kv1.3 regulates gating rather than targeting of Kv1.3 to the plasma membrane

“…A similar conclusion can be drawn for the role of the HRET sequence in regulating channel conductance 12 , 49 : in case of Kv1.3 even an alanine substitution of the HRETE sequence restored channel function. Based on this and other recent papers the presence of the C-terminal amino acids adjacent to the activation gate in Kv1.3 are important for maintaining ion conductance, whereas distant C-terminal amino acids govern interactions with other proteins or confer cholesterol sensitivity to the Kv1.3 50 .…”