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Since the first discovery of an extrasolar planet around a solar-type star, observers have detected over 500 planets outside the solar system. Many of these planets have Jovian masses and orbit their host stars in orbits of only a few days, the so-called 'Hot Jupiters'. At such close proximity to their parent stars, strong tidal interactions between the two bodies are expected to cause significant secular spin-orbit evolution. This thesis tackles two problems regarding the tidal evolution of short-period extrasolar planets. In the first part, we adopt a simple model of the orbit-averaged effects of tidal friction, to study the tidal evolution of planets on inclined orbits. We also analyse the effects of stellar magnetic braking. We then discuss the implications of our results for the importance of Rossiter-Mclaughlin effect observations. In the second part, we study the mechanisms of tidal dissipation in solar-type stars. In particular, internal gravity waves are launched at the interface of the convection and radiation zones of such a star, by the tidal forcing of a short-period planet. The fate of these waves as they approach the centre of the star is studied, primarily using numerical simulations, in both two and three dimensions. We find that the waves undergo instability and break above a critical amplitude. A model for the tidal dissipation that results from this process is presented, and its validity is verified by numerical integrations of the linear tidal response, in an extensive set of stellar models. The dissipation is efficient, and varies by less than an order of magnitude between all solar-type stars, throughout their main-sequence lifetimes, for a given planetary orbit. The implications of this mechanism for the survival of short-period extrasolar planets is discussed, and we propose a possible explanation for the survival of all of the extrasolar planets currently observed in short-period orbits around F, G and K stars. We then perform a stability analysis of a standing internal gravity wave near the centre of a solar-type star, to understand the early stages of the wave breaking process in more detail, and to determine whether the waves are subject to weaker parametric instabilities, below the critical amplitude required for wave breaking. We discuss the relevance of our results to our explanation for the survival of short-period planets presented in the second part of this thesis. Finally, we propose an alternative mechanism of tidal dissipation, involving the gradual radiative damping of the waves. Based on a simple estimate, it appears that this occurs even for low mass planets. However, it is in conflict with current observations since it would threaten the survival of all planets in orbits shorter than 2 days. We discuss some hydrodynamic instabilities and magnetic stresses which may prevent this process.

Tidal interactions impact the long-term evolution of orbits and spins of planet-satellite systems. Observations of these orbits and spins for giant planets in our solar system suggest efficient tidal dissipation. We also know that tidal dissipation is strongly influenced by the internal structure of a planet. This, coupled with evidence for stable stratification or semi-convective layers within giant planets, motivates considering the effect of stratification on tidal interactions. In this thesis we analyse how stable stratification within giant planets can alter tidal dissipation rates through excitation and subsequent dissipation of internal and inertial waves. We combine a mixture of analytical calculations with numerical results in a global spherical Boussinesq model, both with and without rotation. We analyse the free modes and transmission of waves through layered staircase structures, as well as studying the dissipation rates of tidally forced systems where stably stratified layers form. We find that a staircase density structure can alter the free modes and the transmission of waves through such a medium. We find our results tend towards the behaviour of a continuously stratified medium as the number of steps in the staircase increases, and that the transmission of short wavelength waves through a staircase is only efficient when they are resonant with the free modes. Enhanced tidal dissipation arises when the tidal forcing frequency is close to a resonance of the system. By varying parameters, we find overarching trends in dissipation rates. In particular, that an extended core can enhance the inertial wave response, but such a response is not strongly altered by the properties of the core itself. Therefore, we find that as well as introducing additional internal gravity and gravito-inertial wave resonances into the system, stable stratification can enhance the inertial wave response, all of which can contribute to the resultant tidal dissipation.

Tidal interactions are important in driving spin and orbital evolution in various astrophysical systems such as hot Jupiters, close binary stars, planetary satellites, and more. However, the fluid dynamical mechanisms responsible for tidal dissipation in giant planets and stars remain poorly understood. One key mechanism is the interaction between tidal flows and turbulent convection which is thought to act as an eddy viscosity dampening the large-scale tidal flow. The efficiency of this mechanism has long been debated, particularly in the regime of fast tides, when the tidal frequency exceeds the turnover frequency of the dominant convective eddies. The pioneering work of Zahn (1966) proposed that the effective viscosity scales as the inverse of the tidal frequency while Goldreich & Nicholson (1977) found that the effective viscosity scales as the inverse squared of the tidal frequency. Using hydrodynamical simulations we investigate the dissipation of the large-scale (non-wavelike) equilibrium tide as a result of its interaction with convection. Our approach is to conduct a wide parameter survey (over a number of parameters) in order to study the interaction between an oscillatory background shear flow, which represents a large-scale tidal flow, and the convecting fluid inside a small patch of a star or planet. We simulate Rayleigh-Bénard convection in this Cartesian model and explore how the effective viscosity depends on the tidal (shear) frequency in both laminar and turbulent regimes. We also provide a complementary asymptotic analysis which is an extension of the work of Ogilvie & Lesur (2012) which supports our findings in the laminar cases. We will present the results from our simulations to determine the effective viscosity, and its dependence on the tidal frequency in both laminar and weakly turbulent regimes. The main results are: a new scaling law for the frequency dependence of the effective viscosity which has not previously been observed in simulations or predicted by theory and occurs for shear frequencies smaller than those in the fast tides regime; the possibility of anti-dissipation (which could result in inverse-tides); and a strong agreement with the frequency dependence of Goldreich & Nicholson (1977) (despite disagreement with the fundamental mechanism). These results have important implications for tidal dissipation in convection zones of stars and planets which we will discuss. The results of this work indicate that the classical tidal theory of the equilibrium tide in stars and giant planets should be revisited.

Protease-activated receptor-2 is a G-protein-coupled receptor involved in inflammatory and pain responses, as well as pathogenesis in the gastrointestinal and cardiovascular systems, among others. Efforts to identify the serine protease(s) involved in activation of Par2 in vivo have so far been unsuccessful. Here we describe novel roles for Par2 in two animal models: mouse and zebrafish, as well as identify matriptase as a key activator of Par2 in vivo. We report an unexpected role for protease signaling in neural tube closure and formation of the central nervous system. Mouse embryos lacking protease-activated receptor 1 and 2 showed defective hindbrain and posterior neuropore closure and developed exencephaly and spina bifida, important human congenital anomalies. Par1 and Par2 were expressed in surface ectoderm, Par2 selectively along the line of closure. Ablation of Gi/z and Rac1 function in these Par2-expressing cells disrupted neural tube closure, further implicating G protein-coupled receptors and identifying a likely effector pathway. Cluster analysis of protease and Par2 expression patterns revealed a group of membrane-tethered proteases often co-expressed with Par2. Among these, matriptase activated Par2 with picomolar potency, and hepsin and prostasin activated matriptase. Together, our results suggest a role for protease-activated receptor signaling in neural tube closure and identify a local protease network that may trigger Par2 signaling and monitor and regulate epithelial integrity in this context. We also describe a role for Par2 in zebrafish embryos. Hai1a morphants exhibit skin defects including keratinocyte aggregation and shedding, as well as chronic inflammation and ultimately death. Co-knockdown of matriptase or Par2b rescues this phenotype. Further examination of Hai1a morphants reveals keratinocytes that are extruded from the skin, characterized by contraction of an actin-myosin ring around the ejected cell; Par2b knockdown rescues this. Cell extrusion in this context is not apoptosis dependent as extruding cells were negative for caspase-3 and TUNEL stain. Our data taken together suggests important roles for Par2 in epithelial biology as well as identifies matriptase as an important activator of Par2 in animal models.