Fundamental role of microvilli in the main functions of differentiated cells: Outline of an universal regulating and signaling system at the cell periphery

Abstract

Until now, the general importance of microvilli present on the surface of almost all differentiated cells has been strongly underestimated and essential functions of these abundant surface organelles remained unrecognized. Commonly, the role of microvilli has been reduced to their putative function of cell-surface enlargement. In spite of a large body of detailed knowledge about the specific functions of microvilli in sensory receptor cells for sound, light, and odor perception, their functional importance for regulation of basic cell functions remained obscure. Here, a number of microvillar mechanisms involved in fundamental cell functions are discussed. Two structural features enable the extensive functional competence of microvilli: First, the exclusive location of almost all functional important membrane proteins on microvilli of differentiated cells and second, the function of the F-actin-based cytoskeletal core of microvilli as a structural diffusion barrier modulating the flow of low molecular substrates and ions into and out of the cell. The specific localization on microvilli of important functional membrane proteins such as glucose transporters, ion channels, ion pumps, and ion exchangers indicate the importance and diversity of microvillar functions. In this review, the microvillar mechanisms of audioreceptor, photoreceptor, and olfactory/taste receptor cells are discussed as highly specialized adaptations of a general type of microvillar mechanisms involved in regulation of important basic cell functions such as glucose transport/energy metabolism, ion channel regulation, generation and modulation of the membrane potential, volume regulation, and Ca signaling. Even the constitutive cellular defence against cytotoxic compounds, also called "multidrug resistance (MDR)," is discussed as a microvillar mechanism. A comprehensive examination of the specific properties of "cable-like" ion conduction along the microvillar core structure of F-actin allows the proposal that microvilli are specifically designed for using ionic currents as cellular signals. In view of the multifaceted gating and signaling properties of TRP channels, the possible role of microvilli as a universal gating device for TRP channel regulation is discussed. Combined with the role of the microvillar core bundle of actin filaments as high-affinity Ca store, microvilli may turn out as highly specialized Ca signaling organelle involved in store-operated Ca entry (SOCE) and initiation of nonlinear Ca signals such as waves and oscillations.

Sphingomyelin clustering is essential for the formation of microvilli

“…Microvilli are membrane protrusions that increase the surface area of cells and essential for perception and Ca 2+ signaling (Lange, 2011). Microvilli consist of a core of bundled actin filaments tethered laterally to the plasma membrane by a member of the ERM family (Fehon et al, 2010).…”

NHERF2 Protein Mobility Rate Is Determined by a Unique C-terminal Domain That Is Also Necessary for Its Regulation of NHE3 Protein in OK Cells

“…For enterocytes and kidney epithelial cells, the microvillus expands the surface over which absorption and secretion occur (1). NHE3 (SLC9A3) is an Na ϩ /H ϩ exchanger that is primarily localized to the microvilli, is responsible for the majority of small intestinal and renal Na ϩ absorption (2,3), and is highly regulated as part of physiologic function, which includes both stimulation and inhibition (4).…”

“…Multiple classes of membrane proteins attach to these cytoskeletal structures, and it is assumed that they are relatively immobilized (1,18). FRAP studies with NHE3 have suggested that there are mobile as well as immobile pools of NHE3 in the microvilli and that under basal conditions NHERF1 and NHERF2 immobilize NHE3 and reduce its mobile fraction (19).…”

Podocalyxin in the Diagnosis and Treatment of Cancer

“…Strikingly, expression of PC in MCF7 (and MDCK) potently induces formation of apical microvilli. Epithelial microvilli are common features in specialized epithelial layers involved in the secretion or absorption of nutrients, vesicles and proteins from the extracellular space (Lange, 2011). As we anticipated, full-length, exogenously expressed PC recruits NHERF-1, ezrin and Factin to apical membrane domains and forms microvilli in MCF7 cells.…”