Distinct roles for non-muscle myosin II isoforms in mouse hepatic stellate cells

Background & Aims

Upon liver injury, hepatic stellate cells (HSCs) undergo dramatic morphological and functional changes including migration and contraction. In the present study, we investigated the role of myosin II isoforms in the development of the contractile phenotype of mouse HSCs, which are considered therapeutic targets to decrease portal hypertension and fibrosis.

Methods

We characterized the expression of myosin IIA and IIB in primary mouse HSCs and addressed their function by gene knock-down using isoform-specific siRNAs.

Results

We found that myosin IIA and IIB are differentially expressed and localized and have clearly different functions in HSCs. Myosin IIA is mainly located in the subcortical area of quiescent HSCs and at α-SMA-containing stress fibres after activation, while myosin IIB is located in the cytoplasm and at the edge of protrusions of quiescent HSCs, at stress fibres of activated cells, and at the leading edge of lamellipodia. Knock-down of myosin IIA in HSCs influences cell size and shape, results in the disruption of stress fibres and in a decrease of focal adhesions, and inhibits contractility and intra-cellular Ca²⁺ release but increases cell migration. Myosin IIB contributes to the extension of lamellipodia and cell spreading but has no direct role in stress fibres and focal adhesion formation, contraction, or intra-cellular Ca²⁺ signalling.

Conclusions

In mouse HSCs, myosin IIA and IIB clearly fulfil distinct roles. Our results provide an insight into the contractile machinery of HSCs, that could be important in the search for new molecules to treat portal hypertension.

Abbreviations: HSCs, hepatic stellate cells, Myosin IIA (B), myosin II isoform, non-muscle myosin heavy chain IIA (B), Myh9 (10), α-SMA, α-smooth muscle actin, ET-1, endothelin-1, PBS, phosphate buffered saline, [Ca²⁺]_i, intra-cellular Ca²⁺, FA, focal adhesion

Keywords: Hepatic stellate cells, Myosin II isoforms, Contraction, Migration, Intra-cellular calcium