Cilia/flagella are cell organelles that protrude from the surface of many eukaryotic cells and perform various functions ranging from cell locomotion to sensing environmental stimuli (Satir, 2017). They are divided into primary cilia and motile cilia. Primary cilia have “9 + 0” core axoneme structure and are present in most mammalian cells; the motile cilia have “9 + 2” core axoneme structure and are present in specific cells in the testis, brain, trachea, oviduct, efferent ductules (S. Khan & Scholey, 2018). Cilia are assembled and maintained by a conserved mechanism called intraflagellar transport (IFT), a bidirectional transport process originally discovered in Chlamydomonas (Kozminski et al., 1993). Twenty‐two IFT components have been identified so far and these components form IFT‐A and IFT‐B protein complexes, which contain at least 6 and 16 polypeptides, respectively (Prevo et al., 2017; Rosenbaum & Witman, 2002). These complexes further form a large non‐membrane‐bound protein complex, termed IFT particles, that lie in close proximity to the basal body and move from the base to the tip of the flagellum, and then back to the base. The BBSome, identified as a protein complex containing eight subunits (Loktev et al., 2008; Nachury et al., 2007), is described to associate with IFT particles to mediate ciliary trafficking of membrane proteins (Wingfield et al., 2018). Including the eight core subunits, 25 Bardet‐Biedl Syndrom (BBS)‐associated genes have been identified and deficiency of any of them leads to BBS, a special ciliopathy (Rohrschneider & Bolz, 2020). Those IFT particles are thought to carry precursors needed for ciliary/flagellar assembly from the site of synthesis in the cell body to the site of assembly in the cilium/flagellum and the IFT complexes serve as adaptors to mediate contacts between cargo and motor protein.

Genetic mouse models provide powerful tools to study the role of IFT in vivo. Like cilia defects, which cause a spectrum of diseases, also called ciliapathies (Ishikawa & Marshall, 2011), disruption of IFT also gives rise to various genetic and developmental disorders (Finetti et al., 2020). Given that cilia play an essential role in embryonic development, global disruption of IFT components causes embryonic lethality, which makes it impossible to use global knockout mice to study the role of IFT in reproduction. An exception is the Ift88 gene. The Oak Ridge Polycystic Kidney (orpk) insertional mutation of the Ift88 gene (Moyer et al., 1994; Pazour et al., 2000), is hypomorphic and is reported to result in the expression of what may be an alternatively spliced messenger RNA (Moyer et al., 1994; Taulman et al., 2001) and a reduced amount of a smaller‐than‐normal IFT88 protein. This apparently supports sufficient residual IFT to allow the embryo to pass through critical stages in its development, so that some mice homozygous for the mutation survive to birth and even reach adulthood. This mutation is highly disruptive to the ciliary assembly in other organs. The surviving Ift88^−/− mice are completely sterile. They produce ∼350‐fold fewer sperm than wild‐type mice and the remaining sperm completely lack or have very short flagella (Kierszenbaum et al., 2011; San Agustin et al., 2015).

The tissue‐/cell‐specific knockout technique is a game‐changer. Using the Stra8‐cre transgenic mice, we successfully disrupted Ift20, Ift25, Ift27, Ift140, Ift74, Ift81, and Ift172 (Liu et al., 2017; Qu et al., 2020; Shi et al., 2019; Z. Zhang et al., 2016; Y. Zhang et al., 2017, 2018; S. Zhang et al., 2020) in male germ cells. All these mutant mice were grossly normal, but spermiogenesis and fertility were affected. Studies from these individual IFT knockout mice strongly suggest that functions of these IFTs do not compensate each other. Each IFT component might be responsible for carrying specific cargo proteins for sperm flagella formation. For example, IFT74 and IFT81 are likely more important for carrying tubulin for the sperm core axoneme structure formation (Shi et al., 2019; S. Zhang et al., 2020). IFT25 and IFT27 are likely uninvolved in core axoneme formation, but they are more likely to carry proteins for assembling sperm accessory structures (Liu et al., 2017; Y. Zhang et al., 2017). These two IFT components, IFT25 and IFT27, play unique roles in sperm flagella formation. They are not required for primary and motile cilia formation in somatic cells in mice (Eguether et al., 2014; Keady et al., 2012). However, they are indispensable for sperm flagella formation and function (Liu et al., 2017; Y. Zhang et al., 2017). The major differences between cilia in somatic cells and sperm flagella lie in that sperm flagella have accessory structures, including the mitochondria sheath, the fibrous sheath and outer dense fiber, and unique ion channels (Chung & Wang, 2020; Lehti & Sironen, 2017). Thus, IFT25 and IFT27 might play important roles in carrying unique proteins to assemble sperm accessory structures and ion channel proteins for functional sperm. Even though IFT25 and IFT27 form a heterodimer to conduct their functions, the lipid raft on the sperm flagella was disrupted only in the conditional Ift25 knockout mice but not the conditional Ift27 knockout mice (Y. Zhang et al., 2017), indicating that IFT25, but not IFT27, is also involved in transporting lipid to sperm flagella. Mouse Ift172 gene translates two proteins in the testis, a full‐length 172‐kDa protein, and a truncated protein, as supported by our finding that the antibody targeting the N‐terminus of the full‐length IFT172 cross‐reacted with two proteins in the testis (S. Zhang et al., 2020). The truncated protein seems to be a germ cell‐specific isoform because the isoform was more affected in the germ cell‐specific conditional Ift172 knockout mice. Antibodies should be generated to specifically cross‐react the individual isoform to further characterize the two IFT172 isoforms in the future and knockout models should be generated to specifically target the two Ift172 transcripts. It is not yet known if other IFT components are required for germ cell development. From our previous studies, we are not surprised to see that each individual IFT component is essential for spermatogenesis and male fertility.

In the testis, germ cell development needs support from other somatic cells (Zhou et al., 2019). Primary cilia are present in Sertoli cells (Ponzio et al., 1997), Leydig cells (Nygaard et al., 2014), and peritubular myoid cells (Gaytan et al., 1986). It is still unclear whether IFT is essential for physiological functions in these somatic cells. Studies that cross the floxed Ift mice with cell‐type‐specific transgenic cre mice will answer these questions. Male reproductive tracks, including epididymis (Girardet et al., 2019), efferent ductules (Hess, 2015), and vas deferens (Bruňanská et al., 2011) also have cilia. It has been shown that disruption of micro RNAs in the efferent ductules affected ciliogenesis and fluid reabsorption, which ultimately resulted in impaired spermatogenesis (Yuan et al., 2019). We expect that IFT should have a similar function in the efferent ductules. Whether or not IFT plays a role in epididymis and vas deferens needs generation cell‐/tissue type‐specific knockout mice using specific transgenic cre mice.

Compared to male reproduction, even less is known about the role of IFT in female reproduction. Primary and motile cilia are present in female reproductive organs (Ghosh et al., 2017; Koyama et al., 2019; Lobo et al., 2009; Teilmann & Christensen, 2013). It has been reported that IFT88 was highly expressed in the granulosa cells of antral follicles and it is also present in immature follicles, oocytes, the surface epithelium, the epithelium of the oviduct, theca cells, and stromal cells (Johnson et al., 2010). The gene was disrupted by using Prx1‐Cre transgenic mice. The Cre is expressed in the early limb mesenchyme and cranial mesoderm, in the follicle cells of the ovary. The mutant mice showed alterations in the estrous cycle and impaired ovulation, suggesting that IFT also plays a role in ovarian function (Johnson et al., 2010). More specific transgenic cre lines are required to further investigate the role of other IFT components in female reproduction. Particularly, some transgenic cre lines express Cre in both somatic cells and in the gonads. For example, Amh‐cre is expressed in the Sertoli cells in the testis and the granulosa cells in the ovary. The functions of IFT can be studied in both males and females when this transgenic cre line is used.

Recently, a number of candidate genes associated with male infertility were identified using whole‐exome sequencing (Capalbo, 2020) and IFT gene was among these genes (Ni et al., 2020; Wang et al., 2019). Interestingly, human IFT components are present in the epididymal sperm (Ni et al., 2020; Wang et al., 2019), this is different from mouse IFT (San Agustin et al., 2015). However, IFT signals were discovered in human sperm as shown by immunofluorescence staining. Human IFT proteins might play a unique role in sperm function. However, Western blot analysis should be conducted to further validate if IFT is truly present in matured human sperm because immunofluorescence staining might cause nonspecific signals on sperm. Interestingly, more genetic mutations were identified in patients with BBS (S. A. Khan et al., 2016). It is likely that the core IFT A and B components play more important roles in embryonic development than core BBSome components. Mutations of the core IFT components might cause embryonic lethality as observed in the mouse models. However, mutations of core human BBSome components will not cause early mortality. This is also supported by mouse models, which showed that no embryonic lethality was found in the homozygous mutant mice when BBSome genes were disrupted globally (Bales et al., 2020; Mykytyn et al., 2004; Q. Zhang et al., 2011, 2013). Spermatogenesis was affected in these global knockout mice, but it is not known which cells contributed to the failure of sperm formation. It is also unclear if female reproduction is affected. Conditional knockout models are required to further characterize the functions of the BBSome components in male and female reproduction.

Finally, IFT components are not only present in ciliated cells but also in nonciliated cells and these IFT components still play key functions beyond their role in ciliogenesis (Vitre et al., 2020). One example is IFT20. Lymphoid and myeloid cells, which lack primary cilia, express IFT proteins including IFT20 and IFT20 is a new regulator of immune synapse assembly in T cells and is involved in membrane trafficking in cells lacking primary cilia (Finetti et al., 2009). Thus, we should study the role of IFT in the reproductive cells/tissues with an open mind. Even though some cells do not have cilia, that does not mean IFT components do not have functions in these cells.

Overall, even though IFT has been extensively studied in some somatic systems, little is known about the role of these IFT components in reproduction. Effective cell‐/tissue‐specific transgenic cre lines are the key to explore the paradise.

纤毛/鞭毛是从许多真核细胞表面突出的细胞器，具有从细胞运动到感知环境刺激的各种功能（Satir，  2017 年）。它们分为初级纤毛和运动纤毛。初级纤毛具有“9 + 0”核心轴丝结构，存在于大多数哺乳动物细胞中；活动纤毛具有“9 + 2”核心轴丝结构，存在于睾丸、大脑、气管、输卵管、传出小管的特定细胞中（S. Khan & Scholey，  2018 年）。纤毛通过称为鞭毛内运输 (IFT) 的保守机制组装和维持，这是一种最初在衣藻中发现的双向运输过程(Kozminski et al.,  1993）。迄今为止，已鉴定出 22 种 IFT 成分，这些成分形成 IFT-A 和 IFT-B 蛋白复合物，分别包含至少 6 和 16 种多肽（Prevo 等人，  2017 年；Rosenbaum 和 Witman，  2002 年）。这些复合物进一步形成了一个大的非膜结合蛋白复合物，称为 IFT 颗粒，它靠近基体并从基部移动到鞭毛尖端，然后回到基部。BBSome 被鉴定为包含八个亚基的蛋白质复合物（Loktev 等人，  2008 年；Nachury 等人，  2007 年），被描述为与 IFT 颗粒结合以介导膜蛋白的纤毛运输（Wingfield 等人，  2018 年））。包括 8 个核心亚基在内，已鉴定出 25 个 Bardet-Biedl 综合征 (BBS) 相关基因，其中任何一个基因的缺乏都会导致 BBS，一种特殊的纤毛病 (Rohrschneider & Bolz,  2020 )。这些 IFT 颗粒被认为携带纤毛/鞭毛组装所需的前体，从细胞体内的合成部位到纤毛/鞭毛的组装部位，并且 IFT 复合物充当调节货物和运动蛋白之间接触的适配器。

遗传小鼠模型为研究 IFT 在体内的作用提供了强大的工具。就像纤毛缺陷会导致一系列疾病，也称为纤毛病 (Ishikawa & Marshall,  2011 )，IFT 的破坏也会导致各种遗传和发育障碍 (Finetti et al.,  2020 )。鉴于纤毛在胚胎发育中发挥着重要作用，IFT 成分的全球破坏会导致胚胎致死，这使得无法使用全球基因敲除小鼠来研究 IFT 在生殖中的作用。Ift88基因是一个例外。Ift88基因的橡树岭多囊肾( orpk ) 插入突变（Moyer 等人，  1994 年；Pazour 等人， 2000 年），是亚型的，据报道会导致表达可能是可变剪接的信使 RNA（Moyer 等人，  1994 年；Taulman 等人，  2001 年）和比正常的 IFT88 蛋白量减少. 这显然支持足够的剩余 IFT 以允许胚胎通过其发育的关键阶段，因此一些纯合突变的小鼠存活到出生甚至成年。这种突变对其他器官的纤毛组装具有高度破坏性。幸存的Ift88 ^-/-小鼠完全不育。它们产生的精子比野生型小鼠少约 350 倍，而剩余的精子完全没有鞭毛或鞭毛非常短（Kierszenbaum 等，  2011; 圣奥古斯丁等人，  2015 年）。

组织/细胞特异性敲除技术改变了游戏规则。使用Stra8-cre转基因小鼠，我们成功破坏了Ift20、Ift25、Ift27、Ift140、Ift74、Ift81和Ift172（Liu 等人，  2017；Qu 等人，  2020；Shi 等人，  2019；Z. Zhang等人，  2016；Y. Zhang 等人 ，  2017，2018 ；S. Zhang等人，  2020) 在雄性生殖细胞中。所有这些突变小鼠都非常正常，但精子发生和生育能力受到影响。来自这些个体 IFT 敲除小鼠的研究强烈表明，这些 IFT 的功能不会相互补偿。每个 IFT 组件可能负责携带特定的货物蛋白以形成精子鞭毛。例如，IFT74 和 IFT81 可能更重要的是携带微管蛋白以形成精子核心轴丝结构（Shi 等人，  2019 年；S. Zhang 等人，  2020 年）。IFT25 和 IFT27 可能不参与核心轴丝的形成，但它们更可能携带用于组装精子附属结构的蛋白质（Liu et al.,  2017 ; Y. Zhang et al.,  2017）。这两个 IFT 成分 IFT25 和 IFT27 在精子鞭毛形成中发挥着独特的作用。它们不是小鼠体细胞中初级和运动纤毛形成所必需的（Eguether 等人，  2014；Keady 等人，  2012）。然而，它们对于精子鞭毛的形成和功能是必不可少的（Liu et al.,  2017 ; Y. Zhang et al.,  2017）。体细胞纤毛与精子鞭毛的主要区别在于精子鞭毛具有辅助结构，包括线粒体鞘、纤维鞘和外部致密纤维，以及独特的离子通道（Chung & Wang,  2020 ; Lehti & Sironen,  2017）。因此，IFT25 和 IFT27 可能在携带独特的蛋白质以组装精子辅助结构和功能性精子的离子通道蛋白方面发挥重要作用。尽管 IFT25 和 IFT27 形成异二聚体来发挥其功能，但精子鞭毛上的脂筏仅在条件性 Ift25敲除小鼠中被破坏，而不是在条件性 Ift27敲除小鼠中被破坏（Y. Zhang 等人，  2017 年），表明 IFT25 ，但不是 IFT27，也参与将脂质运输到精子鞭毛。鼠标Ift172基因翻译睾丸中的两种蛋白质，一种全长 172kDa 的蛋白质和一种截短的蛋白质，我们发现靶向全长 IFT172 的 N 端的抗体与睾丸中的两种蛋白质发生交叉反应。 （S. Zhang 等人，  2020 年）。截短的蛋白质似乎是一种生殖细胞特异性同种型，因为这种同种型在生殖细胞特异性条件Ift172敲除小鼠中受到的影响更大。应生成抗体以特异性地交叉反应单个异构体，以在未来进一步表征两种 IFT172 异构体，并应生成敲除模型以专门针对两种Ift172成绩单。目前尚不清楚生殖细胞发育是否需要其他 IFT 组件。从我们之前的研究中，我们并不惊讶地发现每个单独的 IFT 成分对于精子发生和男性生育能力都是必不可少的。

在睾丸中，生殖细胞的发育需要其他体细胞的支持（Zhou et al.,  2019）。初级纤毛存在于支持细胞（Ponzio 等人，  1997）、Leydig 细胞（Nygaard 等人，  2014）和管周肌样细胞（Gaytan 等人，  1986）中。目前尚不清楚 IFT 是否对这些体细胞的生理功能至关重要。将 floxed Ift 小鼠与细胞类型特异性转基因 cre 小鼠交叉的研究将回答这些问题。男性生殖轨迹，包括附睾 (Girardet et al.,  2019 )、传出小管 (Hess,  2015 ) 和输精管 (Bruňanská et al.,  2011 )) 也有纤毛。已经表明，传出小管中微小 RNA 的破坏会影响纤毛发生和液体重吸收，最终导致精子发生受损（Yuan 等人，  2019 年）。我们预计 IFT 在传出导管中应该具有类似的功能。IFT 是否在附睾和输精管中发挥作用需要使用特定的转基因 cre 小鼠产生细胞/组织类型特异性敲除小鼠。

与男性生殖相比，人们对 IFT 在女性生殖中的作用知之甚少。女性生殖器官中存在初级纤毛和活动纤毛（Ghosh 等人，  2017 年；Koyama 等人，  2019 年；Lobo 等人，  2009 年；Teilmann & Christensen，  2013 年）。据报道，IFT88 在窦状卵泡的颗粒细胞中高度表达，它也存在于未成熟卵泡、卵母细胞、表面上皮、输卵管上皮、膜细胞和基质细胞中（Johnson 等，  2010）。使用 Prx1-Cre 转基因小鼠破坏了该基因。Cre 在早期的肢体间充质和颅中胚层、卵巢的卵泡细胞中表达。突变小鼠表现出动情周期的改变和排卵受损，这表明 IFT 在卵巢功能中也起作用（Johnson 等，  2010）。需要更具体的转基因 cre 系来进一步研究其他 IFT 成分在女性生殖中的作用。特别是，一些转基因 cre 系在体细胞和性腺中都表达 Cre。例如，Amh-cre 在睾丸的支持细胞和卵巢的颗粒细胞中表达。当使用这种转基因 cre 系时，可以在男性和女性中研究 IFT 的功能。

最近，使用全外显子组测序（Capalbo，  2020 年）鉴定了一些与男性不育症相关的候选基因，IFT 基因就是其中之一（Ni 等人，  2020 年；Wang 等人，  2019 年）。有趣的是，人类 IFT 成分存在于附睾精子中（Ni et al.,  2020 ; Wang et al.,  2019），这与小鼠 IFT 不同（San Agustin et al.,  2015）。然而，如免疫荧光染色所示，在人类精子中发现了 IFT 信号。人类 IFT 蛋白可能在精子功能中发挥独特作用。然而，应该进行蛋白质印迹分析以进一步验证 IFT 是否真的存在于成熟的人类精子中，因为免疫荧光染色可能会导致精子出现非特异性信号。有趣的是，在 BBS 患者中发现了更多的基因突变（SA Khan 等人，  2016）。IFT A 和 B 的核心成分在胚胎发育中的作用可能比 BBSome 的核心成分更重要。如在小鼠模型中观察到的，核心 IFT 组件的突变可能导致胚胎致死率。然而，核心人类 BBSome 成分的突变不会导致早期死亡。小鼠模型也支持这一点，该模型表明，当 BBSome 基因在全球范围内被破坏时，在纯合突变小鼠中未发现胚胎致死率（Bales 等人，  2020；Mykytyn 等人，  2004；Q. Zhang 等人，  2011 ,  2013）。这些全球基因敲除小鼠的精子发生受到影响，但尚不清楚哪些细胞导致精子形成失败。目前还不清楚女性生殖是否受到影响。需要条件淘汰模型来进一步表征 BBSome 组件在男性和女性生殖中的功能。

最后，IFT 成分不仅存在于纤毛细胞中，还存在于非纤毛细胞中，并且这些 IFT 成分仍然发挥着超出其在纤毛发生中的作用的关键功能（Vitre 等人，  2020 年）。一个例子是 IFT20。缺乏初级纤毛的淋巴和骨髓细胞表达 IFT 蛋白，包括 IFT20 和 IFT20 是 T 细胞中免疫突触组装的新调节剂，并参与缺乏初级纤毛的细胞的膜运输 (Finetti et al.,  2009 )。因此，我们应该以开放的心态研究IFT在生殖细胞/组织中的作用。尽管有些细胞没有纤毛，但这并不意味着 IFT 成分在这些细胞中没有功能。

总体而言，尽管 IFT 已在某些体细胞系统中得到广泛研究，但对这些 IFT 组件在生殖中的作用知之甚少。有效的细胞/组织特异性转基因 cre 系是探索天堂的关键。