Abstract
Phosphatidylinositol transfer proteins (PITPs) in yeast coordinate lipid metabolism with the execution of specific membrane trafficking pathways in eukaryotic cells. While there is some evidence that the structurally unrelated metazoan-specific PITPs (mPITPs) may also be involved in membrane trafficking in higher cells, the mPITPs remain an under-investigated class of proteins. Not only is it unclear what biological activities mPITPs discharge, the mechanisms by which these proteins function are also not understood. The soluble class 1 mPITPs include the PITPα and PITPβ isoforms and, of these, the β-isoforms are particularly poorly characterized. Herein, we report use of zebrafish as model vertebrate for the study of class 1 mPITP (and particularly PITPβ) biological function. Zebrafish express PITPα and PITPβ-isoforms and a novel PITPβ-like isoform (DrPITPγ). DrPITPβ expression is particularly robust in double cone cells of the zebrafish retina, and functional interference experiments demonstrate the two DrPITPβ spliceoforms are collectively and specifically required for biogenesis/maintenance of the outer segments of double cone photoreceptor cells in developing retina. By contrast, reduced DrPITPα activity is essential for successful execution of early developmental programs. This study reports the initial characterization of the zebrafish class 1 mPITP family, and the first functional analysis of PITPβ function in a vertebrate.
Introduction
The functional compartmentalization of membrane surfaces is an essential feature of the strategies by which eukaryotic cells organize both signaling inputs and outputs. Lateral organizations of lipids within membranes represent central principles for such a biochemical compartmentalization, and a number of different lipid species are involved in facilitating such organization. Sphingomyelin (SM), cholesterol and other sphingolipids exhibit physical partitioning properties critical for formation of lateral heterogeneities (lipid rafts) that exert a broad spectrum of biological effects (Rajendran and Simons, 2005). Phosphoinositides (PIPs), the phosphorylated versions of phosphatidylinositol (PtdIns), are also major components of membrane signaling systems (Berridge and Irvine, 1989; Majerus, 1992; Fruman et al., 1998; Strahl and Thorner, 2007). The involvement of PIPs takes on multiple forms, including roles as precursors for generation of soluble and lipid second messengers (e.g. inositol phosphates and diacylglycerol, respectively; Nishizuka, 1995; Rhee, 2001), and as binding platforms for the recruitment of proteins to appropriate membrane locations(Hurley and Meyer, 2001; Lemmon, 2003). The utility of PIPs as binding platforms for the appropriate spatial and temporal
recruitment of signaling proteins is, in part, a function of the chemical diversities of PIP headgroups -- diversities encoded by the number and the positional arrangement of phosphates that decorate the inositol headgroup (Irvine and Schell, 2001). From the perspective of PIPs as binding platforms, further diversification of PIP-centric biological outcomes is realized by coincidence detection mechanisms that couple the chemical identity of a particular PIP with a second protein or lipid binding activity (Ile et al., 2006).
A newly described coincidence detection mechanism for functional specification of PIP signaling involves Sec14-like PtdIns-transfer proteins (PITPs). Whereas PITPs have been
historically interpreted as carrier proteins that mobilize individual lipid molecules (PtdIns and phosphatidylcholine; PtdCho) between membrane bilayers in cells (Hsuan and Cockcroft, 2001), this view rests solely on an operational in vitro definition of uncertain physiological significance (Helmkamp et al., 1974; Wirtz, 1991). Indeed, evidence from Sec14-like PITPs argues strongly for a lipid sensor role for these proteins that is coupled to substrate presentation functions required for sufficient PtdIns kinase activity in vivo. The coupling of sensor/presentation
activities is realized via heterotypic lipid exchange cycles that allow Sec14-like PITPs to impose an instructive regulation of PtdIns kinases – a level of control that cues activation of lipid kinases to specific metabolic inputs (Schaaf et al., 2008). This mechanism is conceptually summarized as Sec14-like PITPs serving as nanoreactors for PIP synthesis (Ile et al., 2006). Structure-based predictive bioinformatics suggest Sec14-like proteins link divergent territories of the lipid metabolome to PIP signaling in eukaryotic cells (Ryan et al., 2007; Schaaf et al., 2008).
Whereas Sec14 is an ancient and exclusively eukaryotic structural unit (Phillips et al., 2006b), expression of a second comprehensively unrelated group of PITPs is essentially restricted to the Metazoa (mPITPs; Phillips et al,, 2006b) – with Dictyostelium presenting an enigmatic exception (Swigart et al., 2000). Such an evolutionary restriction identifies these mPITPs as signatures of the most complex eukaryotes. Whether mPITPs fulfill a
sensor/presentor function resembling that of Sec14-like PITPs, or whether these function altogether differently remains an open question. Indeed, little is confidently known regarding their biological functions. The mPITPs fall into the categories of small cytosolic class 1 versions and the large tightly membrane-bound class 2 proteins (Fullwood et al., 1999). Our studies focus on the class 1 mPITPs of which there are three in mammals: PITPα, PITPβ and RdgBα
share ca. 77% primary sequence identity, but assume different localization profiles in cells and exhibit biochemical differences in their phospholipid binding/transfer properties (Phillips et al., 2006a). RdgBα is only ca 42% identical to PITPα and PITPβ and remains uncharacterized (Lu et al., 2001). Whereas Pitpα0/0 mice have been generated (these develop to birth but suffer from complex pathologies that result in neonatal death; Alb et al., 2003), essentially nothing is known regarding PITPβ function. Attempts to generate PITPβ-nullizygous mice, or cell lines, have thus far failed (Alb et al., 2002). The negative data suggest PITPβ executes essential housekeeping functions and, given the striking localization of PITPβ to the mammalian trans-Golgi complex, such functions are likely executed at the level of the Golgi complex (Phillips et al., 2006a).
To address the issue of class 1 mPITP (and particularly PITPβ) biological function more readily, we employed the experimentally tractable zebrafish (Danio rerio) as model vertebrate. Zebrafish express a set of mPITPs resembling that found in mammals with the addition of a unique DrPITPβ-like isoform designated DrPITPγ. Functional interference experiments
demonstrate DrPITPβ dysfunction evokes specific defects in biogenesis/maintenance of the outer segments of double cone photoreceptor cells in developing retina. These defects are associated with altered status of a retinal-specific arrestin (Arr3L) in double cone cells, although reduced Arr3L activity does not evoke reciprocal effects; i.e. loss of DrPITPβ, structural derangements in double cone cells, or compromised structural integrity of double cone outer segments. We propose DrPITPβ is required for optimal activity of the robust membrane trafficking system required for photoreceptor outer segment biogenesis and maintenance. Finally, in contrast to the specific developmental defects scored in DrPITPβ morphants, DrPITPα insufficiencies result in early developmental failure. The collective data describe the initial description of the zebrafish class 1 mPITP family, and document the first functional analysis of a PITPβ in a vertebrate.