What Can We Learn About Fertilization from Cystic Fibrosis?

COMMENTARY

What can we learn about fertilization from cystic fibrosis?

Harvey M. Florman*, Melissa K. Jungnickel, and Keith A. Sutton Department of Cell Biology, University of Massachusetts Medical School, Worcester, MA 01655

t is a curious and intriguing situa- tion that mammalian sperm are introduced into the female reproductive tract in an infertile state Iand must be educated there before they are able to fertilize oocytes. The recog- nition that mammalian sperm must first be switched into a competent state, or capacitated, was exploited in the devel- opment of in vitro fertilization methods and has proved essential for the depen- dent technologies of clinical assisted reproduction and infertility treatments. Yet many aspects of the mechanisms of capacitation remain unclear (for recent reviews, see refs. 1 and 2). In a recent issue of PNAS, Xu et al. (3) advanced our understanding of capacitation by showing that CFTR, the cystic fibrosis transmembrane regulator, plays an essential role in some aspects of this process. During capacitation there is a complex alteration of the biochemical, biophysical, and cell biological properties Fig. 1. Capacitation and fertilization. (A) As a result of capacitation, sperm develop hyperactivated motility, the ability to respond to chemotactic signals and acrosome reaction-inducing signals. Capaci- of sperm. These include (but are not tated sperm penetrate the cumulus and reach the zona pellucida. Contact with the zona pellucida triggers restricted to) alterations in membrane acrosome reactions, permitting sperm to penetrate to the oocyte surface and fuse with the oocyte. potential and membrane sterol content, Multiple steps in this process are controlled by capacitation. (B) Simple model of CFTR function during Ϫ a rise in pHi and changes in other intra- capacitation, in which HCO3 entry through CFTR stimulates an sAC/PKA cascade leading to the activation cellular ion activities, and the enhanced of downstream effectors and to capacitation. It is unresolved whether this pathway can fully account for tyrosine phosphorylation of an array of the modulation of sperm function during capacitation and of how CFTR conductance is controlled to drive sperm proteins. As a result of this re- this cascade. programming, sperm become competent to fertilize (1, 2). Capacitation is now understood as a functional chemotactic signaling capacitation, resulting in a robust physiological transformation that ren- pathways are present in sperm (5). secretory response when capaci- ders sperm better able to reach the The molecular nature of the active tated sperm contact either the zona oocyte surface. In this regard, the task factor(s) has not yet been deter- pellucida or purified ZP3. Further- confronting the mammalian sperm may mined. This pathway may assist more, the acrosome reaction is a be summarized as follows (Fig. 1A). Fer- sperm guidance to the site of fertil- prerequisite for penetration of the tilization typically occurs in the ampulla ization (4). zona pellucida and also for fusion of the oviduct. Sperm must reach the (ii) Flagellar motility switches from an with oocytes, and so these aspects vicinity of the oocyte; penetrate be- activated mode, characterized by of gamete interaction are indirectly tween the several thousand cumulus symmetric beating with shallow regulated by capacitation (2). oophorus cells; contact and penetrate bends, to a hyperactivated mode in the oocyte extracellular matrix, or zona which the beat is asymmetric and Unresolved is the question of how pellucida; and finally adhere to and fuse exhibits deep bends. Hyperactivated those biochemical and biophysical with the oocyte plasma membrane (2). motility may be required for sperm changes result in the maturation of Uncapacitated sperm carry out these ascent of the oviduct and penetra- sperm functional capacity. Previous in tasks inefficiently and fail at various tion through the zona pellucida vitro studies have consistently pointed to (6, 7). Ϫ steps in the process. the importance of HCO3 as a medium (iii) Sperm acquire the ability to inter- ϩ Three modifications in the functional component that, in concert with Ca2 properties of sperm occur during capaci- act with oocytes. To penetrate the tation that may contribute to efficient zona pellucida, sperm must first fertilization. complete an exocytotic event, the Author contributions: H.M.F., M.K.J., and K.A.S. wrote the acrosome reaction. This is triggered paper. (i) Sperm develop the ability to re- by one of the zona pellucida glyco- The authors declare no conﬂict of interest spond to chemotactic signals. Che- proteins, ZP3, following sperm con- See companion article on page 9816 in issue 23 of volume motactic activities are associated tact. The efficiency of the signal 104. with oocytes, cumulus cells, and transducing pathways in sperm that *To whom correspondence should be addressed. E-mail: possibly other components of the are activated by ZP3 and drive exo- harvey.ﬂ[email protected]. female reproductive tract (4), and cytosis is enhanced as a result of © 2007 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0703626104 PNAS ͉ July 3, 2007 ͉ vol. 104 ͉ no. 27 ͉ 11123–11124 Downloaded by guest on September 27, 2021 Ϫ and a cholesterol-binding element (usu- with other HCO transport pathways determined and might include either 3 Ϫ ally BSA), is required for capacitation (15). Inhibition of CFTR in sperm other HCO3 transporters (18) or Ϫ (1, 2). One model of HCO action has results in a failure of capacitation, an basal PKA activity, but in either case 3 Ϫ emerged from recent studies. Mamma- apparent reduction in HCO3 influx, the downstream activation of sAC and lian sperm are enriched in the atypical reduced cAMP responses, and the loss PKA provides an obvious amplification of a number of the anticipated down- mechanism. soluble adenylyl cyclase (sAC), which is Ϫ not regulated by G proteins but rather stream targets of HCO3 /cAMP. In addi- This simple model may account for Ϫ tion, heterozygote Cftrϩ/Ϫ male mice certain aspects of capacitation, such as by HCO3 (8, 9). Downstream events following sAC activation include stimu- enhanced protein tyrosine phosphoryla- lation of protein kinase A (PKA) and tion. However, capacitation is more lead to the enhancement of the tyrosine During complex than just that. Loss of sAC phosphorylation status of an array of activity, either through targeted gene sperm proteins (10, 11). Several lines of capacitation there is deletion or by pharmacological inhibi- evidence support the importance of this tion, results in sperm that fail to exhibit Ϫ a complex alteration either enhanced protein tyrosine pathway, including removal of HCO3 from the medium, inhibition of sAC or phosphorylation or hyperactivated motility but are able to undergo a zona targeted deletion of the sAC gene, and of the biochemical, pellucida-evoked acrosome reaction inhibition of downstream effectors (10– Ϫ biophysical, and cell (11, 12). Thus, HCO may modulate 12). In addition, there is circumstantial 3 Ϫ only some aspects of capacitation by evidence for a role of HCO in capaci- 3 biological properties acting through an sAC pathway. tation in vivo (13). An interesting but unresolved But how is this capacitation cascade of sperm. question is whether CFTR function is initiated? Although there is agreement Ϫ similarly linked to only a subset of that HCO3 is essential, there was no capacitation-associated events or acts consensus as to how intracellular levels show reduced fertility in vivo and in more broadly. Such broader effects, of this anion were regulated. In fact, vitro (3). These observations indicate on events such as acrosome reaction given the presence of carbonic anhy- that CFTR may drive some early events responses, may indicate that CFTR drase in sperm (14), it could plausibly of capacitation (Fig. 1B). action is not restricted to an sAC/PKA be argued that the elevation of pHi that The mechanism of CFTR activation pathway and may point to a role of accompanies capacitation (1, 2) could Ϫ Ϫ during capacitation has not been deter- other HCO3 effectors. In addition, it generate intracellular HCO3 in situ in mined. Previous studies have established should not be forgotten that the major the absence of anion influx. This situa- that this channel opens in response conductance of CFTR, ClϪ (16), may tion has been clarified by the work of to PKA phosphorylation and that this also play a role. In any case, the recog- Xu et al. (3) on CFTR. Although ini- may be regulated dynamically by nition that this channel may play a role tially described as a ClϪ channel, it is protein–protein interactions (16, 17). in the early events of capacitation understood that CFTR can both directly The specific pathway that initiates points to the elaboration and testing Ϫ conduct an HCO3 current and interact CFTR opening in sperm has not been of new models of fertilization.

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