Effect of transection on choline acetyltransferase, thyrotropin releasing hormone and substance P in the cat cervical cord.

The effects of a transection on the choline acetyltransferase activity, the thyrotropin releasing hormone and substance P contents in the cat cervical spinal cord have been investigated. Seven days after the hemitransection at the C1 level, the grey matter of the C6-7 levels of the spinal cord were dissected for the biochemical measurements. The choline acetyltransferase activity and the thyrotropin releasing hormone content remained unchanged in any regions in the grey matter following the high cervical transection. On the other hand, the substance P content was decreased by approx. 70% in the ventral horn. These results suggest that the fibers originating the supraspinal structures and terminating in the grey matter of the spinal cord, contain the substance P-releasing fibers, whereas there seems to be little cholinergic or thyrotropin releasing hormone-containing fibers.     

Activation of Hepatic Stellate Cells Requires Dissociation of E-Cadherin–Containing Adherens Junctions with Hepatocytes

Hepatic stellate cells (HSCs) are resident mesenchymal cells in the space of Disse interposed between liver sinusoidal endothelial cells and hepatocytes. Thorn-like microprojections, or spines, project out from the cell surface of HSCs, crossing the space of Disse, to establish adherens junctions with neighboring hepatocytes. Although HSC activation is initiated largely from stimulation by adjacent cells, isolated HSCs also activate spontaneously in primary culture on plastic. Therefore, other unknown HSC-initiating factors apart from paracrine stimuli may promote activation. The dissociation of adherens junctions between HSCs and hepatocytes as an activating signal for HSCs was explored, establishing epithelial cadherin (E-cadherin) as an adhesion molecule linking hepatocytes and HSCs. In vivo, following carbon tetrachloride–induced liver injury, HSCs lost their spines and dissociated from adherens junctions in the early stages of injury, and were subsequently activated along with an increase in YAP/TAZ expression. After abrogation of liver injury, HSCs reconstructed their spines and adherens junctions. In vitro, reconstitution of E-cadherin–containing adherens junctions by forced E-cadherin expression quiesced HSCs and suppressed TAZ expression. Additionally, increase of TAZ expression leading to the activation of HSCs by autocrine stimulation of transforming growth factor-β, was revealed as a mechanism of spontaneous activation. Thus, we have uncovered a critical event required for HSC activation through enhanced TAZ-mediated mechanotransduction after the loss of adherens junctions between HSCs and hepatocytes.

Hepatic stellate cells (HSCs) are resident mesenchymal cells localized within the space of Disse, interposed between liver sinusoidal endothelial cells (LSECs) and hepatocytes. Quiescent HSCs store vitamin A lipid droplets in their cytoplasm and encircle the endothelial cells with their long, branching cellular process cells to form hepatic sinusoids. Characteristically, many thorn-like microprojections, or spines, project out from the cell surface of HSCs, crossing the space of Disse, to establish adherens junctions with neighboring hepatocytes. In response to hepatic injury of any etiology, quiescent HSCs transdifferentiate into activated myofibroblasts to produce extracellular matrix (ECM), including collagen and fibronectin, and proliferate, migrate toward regions of injury, and lose vitamin A lipid droplets. After cessation of injury, activated HSCs either undergo apoptosis or revert to an inactivated phenotype,^1,2 which reduces production of ECM. However, persistent activation of HSCs during chronic liver injury due to viral infection, alcohol, or in nonalcoholic steatohepatitis provokes accumulation of collagen in the space of Disse, leading to capillarization of sinusoids, followed by panlobular fibrosis. Furthermore, cirrhosis, the most advanced form of fibrosis, is a critical risk factor for hepatic carcinogenesis.³ Therefore, clarifying the extracellular signals that suppress activated HSCs is an important step toward identifying novel therapeutic targets for cirrhosis and hepatocellular carcinoma, although none has yet been identified.Canonically, HSC activation has been conceived as a two-phase event: initiation followed by perpetuation. Initiation refers to early changes in gene expression and phenotype that render cells responsive to stimulants for perpetuation.⁴ The initiation of HSC activation results primarily from paracrine stimulation produced by neighboring cells, including hepatocytes, macrophages, LSECs, inflammatory cells, platelets, and HSCs themselves.⁵ After initiation, several features of the perpetuation phase, including proliferation, ECM production, chemotaxis, and increased contractility may occur, depending on the type of stimulus. For instance, platelet-derived growth factor is the most potent HSC mitogen identified,^6,7 and transforming growth factor beta (TGF-β) induces the production of collagen and other ECM molecules and down-regulates the degradation of ECM by matrix metalloproteinases in HSCs.8, 9, 10 However, isolated HSCs activate spontaneously in the absence of these exogenous initiation factors, and culture on uncoated plastic or other stiff matrices.^11,12 This raises the prospect that initiating factors other than paracrine signals may also contribute to cellular activation.Although HSCs surround LSECs in normal liver, their contact with LSECs is smooth and does not constitute direct cell adhesion.¹³ Conversely, HSCs extend protrusions called spines, that establish adherens junction with hepatocytes.¹⁴The current study focused on the biological significance of contact between HSCs and hepatocytes through these adherens junctions. It was found that early activation of HSCs requires dissociation of these adherens junctions after injury to hepatocytes, which constitutes a critical event required for HSC activation through enhanced mechanotransduction.