Serotonergic abnormalities in SIDS brainstems are likely to originate during prenatal development. Evidence supporting this hypothesis includes the increased risk for SIDS associated with adverse exposures during pregnancy (7
), features of abnormal 5-HT maturation (i.e., aberrant regulation of 5-HT cell number and morphology) in the SIDS medulla (25
), and altered 5-HT and nicotinic receptor binding the medulla in the postnatal period associated with adverse influences of prenatal alcohol and smoking exposure (24
). Given the important role of 5-HT as a growth factor in early cell division, migration, and differentiation in the brain (176
), 5-HT may be especially critical in the pathogenesis of the 5-HT-related abnormalities in medullary development in SIDS as well. Thus, a requisite step toward identifying underlying mechanisms in SIDS is the delineation of where, when, and how 5-HT neurons develop during embryonic and fetal life. Substantial progress has recently been made in this area of developmental neurobiology through the use of various model organisms (e.g., mouse, rat, chicken) and cutting-edge, molecular genetic approaches (e.g., gene gain- or loss-of-function studies and genetic fate mapping) (178
). Several of these advances, which are indeed shaping contemporary SIDS research, are summarized below.
Serotonergic progenitor cells reside in the embryonic hindbrain in territories that bilaterally flank the floor plate and span much of the rostrocaudal extent of the hindbrain (182
) (). Acquiring generic 5-HT neuron identity from these progenitor cells involves turning on the expression of the transcription factor–encoding genes, which include Nkx2.2, Mash1/Ascl1
, and Foxa2
). This is then followed by the activation of expression of the transcription factors Pet1, Lmx1b
, and Gata2/3
, which take descendant postmitotic precursor cells to a state of 5-HT production, the hallmark of the mature 5-HT neuron. Sonic hedgehog, a signaling molecule secreted by nearby floor plate cells, initiates this cascade of cellular and molecular events (178
). Understanding the genetic programs and signaling pathways critical to 5-HT neuron development is important because they identify avenues for exploration with respect to SIDS pathogenesis, such as alterations in various transcription factors either through genetic mutation or upon prenatal exposure to known risk factors for SIDS, such as alcohol and cigarette smoke (discussed above).
Figure 8 The organization of 5-hydroxytryptamine (5-HT) neurons as defined by embryonic origin and developmental gene expression profile. This organization differs from that based historically on anatomical architecture. (a–c) Schematics of mouse embryo (more ...)
Although 5-HT production generally de-fines a neuron as serotonergic, there are in fact many different subtypes of 5-HT neurons that are distinguishable by their different and widespread positions in the brainstem (as constituents of the nine different types of 5-HT nuclei historically referred to as B1–B9) (185
), by differences in their synaptic targets (86
), and by their coexpression of different neuropeptides and neurotransmitters (e.g., GABA, SP, and/or TRH) (186
), as well as by their different cellular, electrophysiological, and functional properties (e.g., some 5-HT neurons are chemosensitive and others are not; some 5-HT neurons are involved in pain, others in respiration) (183
). Understanding which of these parameters come together in a single cell to define a specific functional subtype of 5-HT neuron is an area of active investigation for our group, given that different 5-HT neuronal subtypes are likely associated with different disease vulnerabilities. Of interest here is the identification of those 5-HT subtypes most relevant to SIDS; achieving this goal requires knowledge of the molecular markers of these different subtypes. In working toward this goal, we recently developed an approach that takes advantage of molecular differences among 5-HT progenitor cells, rather than relying on the identification of molecular differences within mature 5-HT neurons (187
). We are investigating the possibility that each of the molecularly distinct 5-HT progenitor cell pools gives rise to a functionally distinct subtype of 5-HT neurons. Our rationale is based upon the concept that developmental programs that define the fate and function of neurons are often set in motion by the action of factors that are differentially expressed among their antecedent progenitors.
The 5-HT progenitor territory can be subdivided along the rostrocaudal axis into groups on the basis of the broader partitioning of the hindbrain into segments (rhombomeres) with distinguishing gene expression profiles (). Thus, aspects of mature 5-HT neuron subtype identity may be determined through the actions of rhombomere (r)-specific genetic programs on resident 5-HT progenitor and precursor cell subsets. Using the method of genetic fate mapping, we deconstructed the 5-HT neural system based on rhombomere-defined 5-HT sublineages, distinguishing those 5-HT neurons arising from the primordium as r1, r2, r3, r5, or r6–r8 (187
) (). [5-HT neurons are thought not to arise from r4 (183
).] Interestingly, we found that the generated developmental and molecular map of the 5-HT neural system differs from the classical anatomically defined groupings (B1–B9) of mature 5-HT neurons (187
) (). Some anatomically defined B1–B9 nuclei receive contributions from multiple (molecularly distinct) progenitor cell pools, whereas others receive contributions from only one such pool. Are these newly identified genetic sublineages of physiological relevance? That is, do genetically defined subtypes of 5-HT neurons serve different functions, despite coresiding within a single anatomical nucleus? If so, which particular subtypes serve functions that, if defective, might render an infant vulnerable to SIDS? Answering these questions has become possible because a set of tools similar to that used to identify these genetic sublineages of 5-HT neurons can be used to selectively manipulate properties of individual sublineages, such as neurotransmission, in otherwise normal mice.