Kristin D. Patterson

Hox11-family genes XHox11 and XHox11L2 in Xenopus: XHox11L2 expression is restricted to a subset of the primary sensory neurons

The mouse genome contains a small family of homeobox genes related to Hox11, but relatively little is known about the expression of these genes during early development. Hox11 itself is expressed in the embryonic spleen, among other tissues, and is required for its formation. No description of Hox11L2 expression has been presented previously. We have isolated the Xenopus orthologs of Hox11 and Hox11L2 and have carefully compared their expression patterns during embryogenesis. The localization of Xhox11 transcripts in the branchial arches, cranial sensory ganglia and spinal cord is similar, but not identical, to that of mouse Hox11. Xhox11 expression is not detected in the developing spleen. XHox11L2 is expressed exclusively in a portion of the primary sensory system in the frog embryo, including the cranial sensory ganglia and the Rohon‐Beard sensory neurons. There is significant overlap in the patterns of Xhox11 and XHox11L2 expression in the spinal cord and cranial sensory ganglia during early development, suggesting that they may function redundantly in these tissues. The timing of Xhox11 and Xhox11L2 expression indicates that Hox11‐family members may participate in the final stages of the differentiation process. Dev Dyn 1999;214:34–43.

Distinct expression patterns for two Xenopus Bar homeobox genes.

Abstract The BarH1 and BarH2 homeobox genes are coexpressed in cells of the fly retina and in the central and peripheral nervous systems. The fly Bar genes are required for normal development of the eye and external sensory organs. In Xenopus we have identified two distinct vertebrate Bar-related homeobox genes, XBH1 and XBH2. XBH1 is highly related in sequence and expression pattern to a mammalian gene, MBH1, suggesting that they are orthologues. XBH2 has not previously been identified but is clearly related to the DrosophilaBar genes. During early Xenopus embryogenesis XBH1 and XBH2 are expressed in overlapping regions of the central nervous system. XBH1, but not XBH2, is expressed in the developing retina. By comparing the expression of XBH1 with that of hermes, a marker of differentiated retinal ganglion cells, we show that XBH1 is expressed in retinal ganglion cells during the differentiation process, but is down-regulated as cells become terminally differentiated.

Homeobox genes in cardiovascular development.

As summarized earlier, a surprisingly large number of different homeobox genes are expressed in the developing heart. Some are clearly important, as demonstrated by mouse gene ablation studies. For example, knockout of Nkx2-5 or Hoxa-3 function is embryonic lethal due to defects in cardiovascular development. However, gene ablation studies indicate that other homeobox genes that show cardiovascular expression are either not required for heart development or their function is effectively complemented by a redundant gene activity. Given the number of closely related homeobox genes that are expressed in the heart (and the rate at which new genes are being discovered), this is very likely to be the case for at least some homeobox gene activities. At present little is known of the precise mechanism of action of homeobox genes in embryonic development. This statement applies to homeobox genes in general, not just to genes involved in cardiovascular development. There is a popular view that homeobox genes are master regulators that control expression of a large number of downstream genes. In at least some cases, e.g., the eyeless gene of Drosophila (Holder et al., 1995), homeobox genes appear to be capable of activating and maintaining a very complex developmental program. Significantly, the eyeless gene is able to initiate eye development at numerous ectopic locations. Increasing evidence, however, suggests that genes of this type may be rather rare. Certainly there is no evidence to date that any of the homeobox genes expressed in the heart are able to initiate the complete heart development pathway. This is probably best understood in the case of the tinman gene in Drosophila, which, although absolutely required for heart development, is not capable of initiating the cardiac development pathway in ectopic locations (Bodmer, 1993). This conclusion is supported by studies of the vertebrate tinman-related gene Nkx2-5. Gene ablation studies show that Nkx2-5 is essential for correct cardiac development (Lyons et al., 1995) but is not able to initiate the regulatory pathway leading to cardiac development when expressed ectopically (Cleaver et al., 1996; Chen and Fishman, 1996). If most homeodomain proteins are not direct regulators of a differentiation pathway, what is their role during organogenesis? The cardiovascular homeobox gene about which most is known at the mechanistic level is gax (Smith et al., 1997). A number of experiments indicate that the Gax protein is involved in the regulation of cell proliferation and that it interacts with components of the cell cycle regulation machinery. Indeed, over recent years, the idea that at least some homeobox genes play their role in organogenesis through regulation of proliferation has been developed in some detail by Duboule (1995). Further evidence that this mechanism of homeobox activity is important, especially during organogenesis, comes from studies of the Hox11 homeobox gene, which is absolutely required for development of the spleen in mouse (Roberts et al., 1994). Studies indicate that Hox11 is able to interact with at least two different protein phosphatases, PP2A and PP1, which in turn, are involved in cell cycle regulation (Kawabe et al., 1997). It is quite clear that research in future years will need to focus on the precise mode of action of the different homeodomain proteins if we are to understand their role in the development of the cardiovascular system.

Modulation of Developmental Signaling by the Proteostasis Network

A few signaling pathways are used repeatedly in the Drosophila larval eye disc to form the ordered array of ommatidia seen in the adult eye. Disruption of signaling by a change in either the levels or timing of expression of signaling pathway components, often leads to dramatic effects: the lack of eye tissue altogether, tumor growth, misdetermination of cell types, altered patterning, or degeneration. The level and timing of signaling in the eye are efficiently controlled by the “proteostasis network”; a collection of mechanisms responsible for the folding, modification, trafficking, and degradation of proteins. We review evidence from the Notch, EGFR, and rhodopsin pathways that the proteostasis network plays a unique and substantial role in enabling the iterative use of signaling pathways in the confines of the developing Drosophila eye and in adult vision.

Early Events in Establishment of the Vertebrate Heart

In order to understand how the heart is formed in the embryo, it is essential to understand the cellular movements and interactions that are needed for heart formation. This is the domain of classical embryology studies. It is also important to understand the molecules that mediate these interactions and direct cellular differentiation. This is the domain of molecular biology. In isolation, neither of these fields of study will provide a complete picture of how a heart is formed. Recently, a combination of these approaches has greatly increased the general understanding of embryonic heart development. We wish to present an overview of heart embryology and also to show how current molecular results are enhancing our understanding of heart formation.