The role of the circadian system in the etiology of depression.

Circadian rhythms have evolved in almost all organisms enabling them to anticipate alternating changes in the environment. As a consequence, the circadian clock controls a broad range of bodily functions including appetite, sleep, activity and cortisol levels. The circadian clock synchronizes itself to the external world mainly by environmental light cues and can be disturbed by a variety of factors, including shift-work, jet-lag, stress, ageing and artificial light at night. Interestingly, mood has also been shown to follow a diurnal rhythm. Moreover, circadian disruption has been associated with various mood disorders and patients suffering from depression have irregular biological rhythms in sleep, appetite, activity and cortisol levels suggesting that circadian rhythmicity is crucially involved in the etiology and pathophysiology of depression. The aim of the present review is to give an overview and discuss recent findings in both humans and rodents linking a disturbed circadian rhythm to depression. Understanding the relation between a disturbed circadian rhythm and the etiology of depression may lead to novel therapeutic and preventative strategies.

Doublecortin-like expressing astrocytes of the suprachiasmatic nucleus are implicated in the biosynthesis of vasopressin and influences circadian rhythms.

We have recently identified a novel plasticity protein, doublecortin-like (DCL), that is specifically expressed in the shell of the mouse suprachiasmatic nucleus (SCN). DCL is implicated in neuroplastic events, such as neurogenesis, that require structural rearrangements of the microtubule cytoskeleton, enabling dynamic movements of cell bodies and dendrites. We have inspected DCL expression in the SCN by confocal microscopy and found that DCL is expressed in GABA transporter-3 (GAT3)-positive astrocytes that envelope arginine vasopressin (AVP)-expressing cells. To investigate the role of these DCL-positive astrocytes in circadian rhythmicity, we have used transgenic mice expressing doxycycline-induced short-hairpin (sh) RNA's targeting DCL mRNA (DCL knockdown mice). Compared with littermate wild type (WT) controls, DCL-knockdown mice exhibit significant shorter circadian rest-activity periods in constant darkness and adjusted significantly faster to a jet-lag protocol. As DCL-positive astrocytes are closely associated with AVP-positive cells, we analyzed AVP expression in DCL-knockdown mice and in their WT littermates by 3D reconstructions and transmission electron microscopy (TEM). We found significantly higher numbers of AVP-positive cells with increased volume and more intensity in DCL-knockdown mice. We found alterations in the numbers of dense core vesicle-containing neurons at ZT8 and ZT20 suggesting that the peak and trough of neuropeptide biosynthesis is dampened in DCL-knockdown mice compared to WT littermates. Together, our data suggest an important role for the astrocytic plasticity in the regulation of circadian rhythms and point to the existence of a specific DCL+ astrocyte-AVP+ neuronal network located in the dorsal SCN implicated in AVP biosynthesis.

Earlier chronotype in patients with rheumatoid arthritis.

OBJECTIVES: Rheumatoid arthritis (RA) patients show an earlier circadian rhythm (i.e. serum melatonin peaks earlier during the night, indicating an earlier timing of the internal circadian pacemaker). In the current study, we examined whether the chronotype, which is influenced by the circadian rhythm, is also earlier. In addition, we explored whether chronotype is related to disease activity and patient-reported outcomes. METHODS: The chronotype (Munich Chronotype Questionnaire) of patients with RA (n = 121; mean age 60 years, 73% female) was compared with that of subjects from the general population (norm group; n = 1695) with a one-sample t test. In addition, we investigated chronotype in relation to disease activity (Disease Activity Score; DAS), reported morning stiffness, fatigue (Checklist Individual Strength), and health-related quality of life (RAND-36). RESULTS: The chronotype of patients with RA was, on average, 23 min (95% CI, 15 to 31 min) earlier than that of the norm group (t(115) = - 5.901, p < 0.001, d = 0.55). Chronotype was not related to disease activity or patient-reported outcomes (p > 0.05). CONCLUSION: As expected, chronotype was earlier in RA patients. However, in this correlational study, chronotype was not related to disease activity or patient-reported outcomes. An experimental study is needed to examine whether delaying the circadian rhythm has a positive influence on these outcomes. This insight could improve our understanding of the pathophysiology of RA and contribute to exploring new treatment possibilities. Key Points • This is the first study examining chronotype in patients with rheumatoid arthritis, and how chronotype relates to disease activity and patient-reported outcomes. • We found an earlier chronotype in patients with rheumatoid arthritis than in subjects from the general population. • In this correlational study, chronotype was not related to disease activity or patient-reported outcomes. An experimental study is needed to examine whether delaying the circadian rhythm positively influences these outcomes.

Doublecortin-Like Is Implicated in Adult Hippocampal Neurogenesis and in Motivational Aspects to Escape from an Aversive Environment in Male Mice.

Doublecortin (DCX)-like (DCL) is a microtubule (MT)-associated protein (MAP) that is highly homologous to DCX and is crucially involved in embryonic neurogenesis. Here, we have investigated the in vivo role of DCL in adult hippocampal neurogenesis by generating transgenic mice producing inducible shRNA molecules that specifically target DCL but no other splice variants produced by the DCLK gene. DCL knock-down (DCL-KD) resulted in a significant increase in the number of proliferating BrdU+ cells in the subgranular zone (SGZ) 1 d after BrdU administration. However, the number of surviving newborn adult NeuN+/BrdU+ neurons are significantly decreased when inspected four weeks after BrdU administration suggesting a blockade of neuronal differentiation after DCL-KD. In line with this, we observed an increase in the number of proliferating cells, but a significant decrease in postmitotic DCX+ cells that are characterized by long dendrites spanning all dentate gyrus layers. Behavioral analysis showed that DCL-KD strongly extended the escape latency of mice on the circular hole board (CHB) but did not affect other aspects of this behavioral task. Together, our results indicate a function for DCL in adult neurogenesis and in the motivation to escape from an aversive environment. In contrast to DCX, its pivotal role in the maturation of postmitotic neuronal progenitor cells (NPCs) marks DCL as a genuine adult neurogenesis indicator in the hippocampus.

Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?

Adult neurogenesis, the birth of new neurons in the mature brain, has attracted considerable attention in the last decade. One of the earliest identified and most profound factors that affect adult neurogenesis both positively and negatively is stress. Here, we review the complex interplay between stress and adult neurogenesis. In particular, we review the role of the glucocorticoid receptor, the main mediator of the stress response in the proliferation, differentiation, migration, and functional integration of newborn neurons in the hippocampus. We review a multitude of mechanisms regulating glucocorticoid receptor activity in relationship to adult neurogenesis. We postulate a novel concept in which the level of glucocorticoid receptor expression directly regulates the excitation-inhibition balance, which is key for proper neurogenesis. We furthermore argue that an excitation-inhibition dis-balance may underlie aberrant functional integration of newborn neurons that is associated with psychiatric and paroxysmal brain disorders.

ß1 integrin inhibition elicits a prometastatic switch through the TGFß-miR-200-ZEB network in E-cadherin-positive triple-negative breast cancer.

Interactions with the extracellular matrix (ECM) through integrin adhesion receptors provide cancer cells with physical and chemical cues that act together with growth factors to support survival and proliferation. Antagonists that target integrins containing the ß1 subunit inhibit tumor growth and sensitize cells to irradiation or cytotoxic chemotherapy in preclinical breast cancer models and are under clinical investigation. We found that the loss of ß1 integrins attenuated breast tumor growth but markedly enhanced tumor cell dissemination to the lungs. When cultured in three-dimensional ECM scaffolds, antibodies that blocked ß1 integrin function or knockdown of ß1 switched the migratory behavior of human and mouse E-cadherin-positive triple-negative breast cancer (TNBC) cells from collective to single cell movement. This switch involved activation of the transforming growth factor-ß (TGFß) signaling network that led to a shift in the balance between miR-200 microRNAs and the transcription factor zinc finger E-box-binding homeobox 2 (ZEB2), resulting in suppressed transcription of the gene encoding E-cadherin. Reducing the abundance of a TGFß receptor, restoring the ZEB/miR-200 balance, or increasing the abundance of E-cadherin reestablished cohesion in ß1 integrin-deficient cells and reduced dissemination to the lungs without affecting growth of the primary tumor. These findings reveal that ß1 integrins control a signaling network that promotes an epithelial phenotype and suppresses dissemination and indicate that targeting ß1 integrins may have undesirable effects in TNBC.

Silencing of doublecortin-like (DCL) results in decreased mitochondrial activity and delayed neuroblastoma tumor growth.

Doublecortin-like (DCL) is a microtubule-binding protein crucial for neuroblastoma (NB) cell proliferation. We have investigated whether the anti-proliferative effect of DCL knockdown is linked to reduced mitochondrial activity. We found a delay in tumor development after DCL knockdown in vivo in doxycycline-inducible NB tumor xenografts. To understand the mechanisms underlying this tumor growth retardation we performed a series of in vitro experiments in NB cell lines. DCL colocalizes with mitochondria, interacts with the mitochondrial outer membrane protein OMP25/ SYNJ2BP and DCL knockdown results in decreased expression of genes involved in oxidative phosphorylation. Moreover, DCL knockdown decreases cytochrome c oxidase activity and ATP synthesis. We identified the C-terminal Serine/Proline-rich domain and the second microtubule-binding area as crucial DCL domains for the regulation of cytochrome c oxidase activity and ATP synthesis. Furthermore, DCL knockdown causes a significant reduction in the proliferation rate of NB cells under an energetic challenge induced by low glucose availability. Together with our previous studies, our results corroborate DCL as a key player in NB tumor growth in which DCL controls not only mitotic spindle formation and the stabilization of the microtubule cytoskeleton, but also regulates mitochondrial activity and energy availability, which makes DCL a promising molecular target for NB therapy.

Doublecortin and doublecortin-like are expressed in overlapping and non-overlapping neuronal cell population: implications for neurogenesis.

We have characterized the expression of doublecortin-like (DCL), a microtubule-associated protein involved in embryonic neurogenesis that is highly homologous to doublecortin (DCX), in the adult mouse brain. To this end, we developed a DCL-specific antibody and used this to compare DCL expression with DCX. In the neurogenic regions of the adult brain like the subventricular zone (SVZ), the rostral migratory stream (RMS), the olfactory bulb (OB), and the hippocampus, DCL colocalizes with DCX in immature neuronal cell populations. In contrast to DCX, we also found high DCL expression in three other brain regions with suspected neurogenesis or neuronal plasticity. First, the radial glia-like, hypothalamic tanycytes show high DCL expression that partly colocalizes with the neural stem cell marker vimentin. Second, DCL expression is found in cells of the suprachiasmatic nucleus (SCN), which lacks expression of the adult neuron marker NeuN. Third, a novel region exhibiting DCL expression is part of the olfactory tubercle where DCL is found in the neuropil of the islands of Calleja (ICj). Our findings define DCL as a novel marker for specific aspects of adult neurogenesis, which partly overlap with DCX. In addition, we propose unique roles for DCL in adult neurogenesis and we suggest high levels of neuronal plasticity in tanycytes, SCN, and ICj.

Combining doublecortin-like kinase silencing and vinca alkaloids results in a synergistic apoptotic effect in neuroblastoma cells.

Microtubule-destabilizing agents, such as vinca alkaloids (VAs), are part of the treatment currently applied in patients with high-risk neuroblastoma (NB). However, the development of drug resistance and toxicity make NB difficult to treat with these drugs. In this study we explore the combination of VAs (vincristine or vinblastine) with knockdown of the microtubule-associated proteins encoded by the doublecortin-like kinase (DCLK) gene by using short interference RNA (siRNA). We examined the effect of VAs and DCLK knockdown on the microtubule network by immunohistochemistry. We performed dose-response studies on cell viability and proliferation. By combining VA with DCLK knockdown we observed a strong reduction in the EC(50) to induce cell death: up to 7.3-fold reduction of vincristine and 21.1-fold reduction of vinblastine. Using time-lapse imaging of phosphatidylserine translocation and a terminal deoxynucleotidyl transferase dUTP nick-end labeling-based assay, we found a significant increase of apoptosis by the combined treatment. Induction of caspase-3 activity, as detected via cleavage of N-acetyl-Asp-Glu-Val-Asp-7-amido-4-methylcoumarin, showed a 3.3- to 12.0-fold increase in the combined treatment. We detected significant increases in caspase-8 activity as well. Moreover, the multidrug dose effect calculated by using the median effect method showed a strong synergistic inhibition of proliferation and induction of apoptosis at most of the combined concentrations of siRNAs and VAs. Together, our data demonstrate that the silencing of DCLK sensitizes NB cells to VAs, resulting in a synergetic apoptotic effect.

Leptin administration restores the fasting-induced increase of hepatic type 3 deiodinase expression in mice.

BACKGROUND: Decreased serum leptin has been proposed as a critical signal initiating the neuroendocrine response to fasting. Leptin administration partially reverses the fasting-induced suppression of the hypothalamus-pituitary-thyroid axis at the central level. It is, however, unknown to what extent leptin affects peripheral thyroid hormone metabolism. The aim of this study was to evaluate the effect of leptin administration on starvation-induced alterations of peripheral thyroid hormone metabolism in mice. METHODS: Three types of experiments were performed: (i) mice were fasted for 24 hours while leptin was administered twice (at 0 and 8 hours, 1 µg/g body weight [BW]), (ii) mice were fasted for 24 hours and, subsequently, leptin was given once at 24 hours (killed at 28 and 32 hours), and (iii) mice were fasted for 48 hours. All groups had appropriate controls. Serum triiodothyronine and thyroxine, liver type 1 deiodinase (D1), type 3 deiodinase (D3), thyroid hormone receptor (TR)ß1, TRα1 and α2 mRNA expression, and liver D1 and D3 activity were measured. RESULTS: Twenty-four hours of fasting decreased liver TRß1 mRNA expression, while liver TRα1, TRα2, and D1 mRNA expression and activity did not change. In contrast, 24 hours of fasting increased liver D3 mRNA. Leptin administration after fasting restored liver D3 expression, while serum thyroid hormone levels and liver TRß1 expression remained low. CONCLUSION: Leptin administration selectively restores starvation-induced increased hepatic D3 expression independently of serum thyroid hormone concentrations. The present study shows that fasting-induced changes in mRNA expression of genes involved in hepatic hormone metabolism are influenced not only by decreased serum thyroid hormone levels but also by serum leptin.

Neuroblastoma therapy: what is in the pipeline?

Despite the expansion of knowledge about neuroblastoma (NB) in recent years, the therapeutic outcome for children with a high-risk NB has not significantly improved. Therefore, more effective therapies are needed. This might be achieved by aiming future efforts at recently proposed but not yet developed targets for NB therapy. In this review, we discuss the recently proposed molecular targets that are in clinical trials and, in particular, those that are not yet explored in the clinic. We focus on the selection of these molecular targets for which promising in vitro and in vivo results have been obtained by silencing/inhibiting them. In addition, these selected targets are involved at least in one of the NB tumorigenic processes: proliferation, anti-apoptosis, angiogenesis and/or metastasis. In particular, we will review a recently proposed target, the microtubule-associated proteins (MAPs) encoded by doublecortin-like kinase gene (DCLK1). DCLK1-derived MAPs are crucial for proliferation and survival of neuroblasts and are highly expressed not only in NB but also in other tumours such as gliomas. Additionally, we will discuss neuropeptide Y, its Y2 receptor and cathepsin L as examples of targets to decrease angiogenesis and metastasis of NB. Furthermore, we will review the micro-RNAs that have been proposed as therapeutic targets for NB. Detailed investigation of these not yet developed targets as well as exploration of multi-target approaches might be the key to a more effective NB therapy, i.e. increasing specificity, reducing toxicity and avoiding long-term side effects.

ERK-regulated double cortin-like kinase (DCLK)-short phosphorylation and nuclear translocation stimulate POMC gene expression in endocrine melanotrope cells.

We tested whether double cortin-like kinase-short (DCLK-short), a microtubule-associated Ser/Thr kinase predominantly expressed in the brain, is downstream of the ERK signaling pathway and is involved in proopiomelanocortin gene (POMC) expression in endocrine pituitary melanotrope cells of Xenopus laevis. Melanotropes form a well-established model to study physiological aspects of neuroendocrine plasticity. The amphibian X. laevis adapts its skin color to the background light intensity by the release of α-MSH from the melanotrope cell. In frogs on a white background, melanotropes are inactive but they are activated during adaptation to a black background. Our results show that melanotrope activation is associated with an increase in DCLK-short mRNA and with phosphorylation of DCLK-short at serine at position 30 (Ser-30). Upon cell activation phosphorylated Ser-30-DCLK-short was translocated from the cytoplasm into the nucleus, and the ERK blocker U0126 inhibited this process. The mutation of Ser-30 to alanine also inhibited the translocation and reduced POMC expression, whereas overexpression stimulated POMC expression. This is the first demonstration of DCLK-short in a native endocrine cell. We conclude that DCLK-short is physiologically regulated at both the level of its gene expression and protein phosphorylation and that the kinase is effectively regulating POMC gene expression upon its ERK-mediated phosphorylation.

Hippocampal CARP over-expression solidifies consolidation of contextual fear memories.

The Doublecortin-Like Kinase (DCLK) gene is involved in neuronal migration during development. Through alternative splicing the DCLK gene also produces a transcript called Ca(2+)/calmodulin dependent protein kinase (CaMK)-related peptide (CARP) that is expressed exclusively during adulthood in response to neuronal activity. The function of CARP, however, is poorly understood. To study CARP function, we have generated transgenic mice with over-expression of the CARP transcript in, amongst other brain areas, the hippocampus. We aimed to characterize possible behavioral adaptations of these mice by using a Pavlovian fear conditioning approach. This type of fear conditioning, in which both the hippocampus and amygdala are critically involved, allows studying the formation and extinction of fear related memories. We here report on the behavioral adaptations of two distinct transgenic lines: one with high levels of CARP in the hippocampus and amygdala, whilst the other has high levels of CARP in the hippocampal formation, but not in the amygdala. We tested both mouse lines separately by comparing them to their wild-type littermate controls. We provide evidence suggesting consolidation of contextual fear memories is strengthened in mice of both transgenic lines.

Over-expression of δC-DCLK-short in mouse brain results in a more anxious behavioral phenotype.

Products of the Doublecortin-Like Kinase (DCLK) gene are associated with cortical migration and hippocampal maturation during embryogenesis. However, the functions of those DCLK gene transcripts that encode kinases and are expressed during adulthood are incompletely understood. To elucidate potential functions of these DCLK gene splice variants we have generated and analyzed transgenic mice with neuronal over-expression of a truncated, constitutively active form of DCLK-short, designated δC-DCLK-short. Previously, we have performed an extensive molecular characterization of these transgenic δC-DCLK-short mice and established that a specific subunit of the GABA(A) receptor, which is involved in anxiety-related GABAergic neurotransmission, is down-regulated in the hippocampus. Here we show that δC-DCLK-short mRNA is highly expressed in the hippocampus, cortex and amygdala of transgenic mice. We provide evidence that the δC-DCLK-short protein is expressed and functional. In addition, we examined anxiety-related behavior in δC-DCLK-short mice in the elevated plus maze. Interestingly, δC-DCLK-short mice spend less time, move less in the open arms of the maze and show a reduction in the number of rim dips. These behaviors indicate that δC-DCLK-short mice display a more anxious behavioral phenotype.

Over-expression of the DCLK gene transcript CARP decreases CA3/CA1 network excitability.

Products of the Doublecortin Like Kinase (DCLK) gene are implicated in cortical migration and hippocampal maturation during embryogenesis. However, one of its splice variants, called CaMK Related Peptide (CARP), is expressed during adulthood in response to neurological stimuli, such as kainic acid-induced seizures and BDNF-LTP. The function of this transcript of the DCLK gene is poorly understood. To elucidate its function during adulthood we have created transgenic mice with over-expression of CARP in the brain. To study potential functions of CARP in the hippocampus we performed an electrophysiological characterization of the CA3/CA1 network of transgenic and wild-type mice and showed that field excitatory post synaptic potentials (fEPSPs) are highly increased in transgenic mice, while population spike amplitudes (PSAs) remained equal between genotypes. Consequently, hippocampal CA3/CA1 network excitability was decreased in transgenic mice. In addition we show a 2-fold up-regulation of the Ca(2+)-binding protein calretinin and a down-regulation of Rapgef4, a guanine exchange factor for Rap1, in the hippocampus. Given previously established conditions during which CARP is induced and our current data, we propose that this DCLK gene product affects glutamatergic neuronal transmission in response to neurological stimuli.

An adeno-associated viral vector transduces the rat hypothalamus and amygdala more efficient than a lentiviral vector.

BACKGROUND: This study compared the transduction efficiencies of an adeno-associated viral (AAV) vector, which was pseudotyped with an AAV1 capsid and encoded the green fluorescent protein (GFP), with a lentiviral (LV) vector, which was pseudotyped with a VSV-G envelop and encoded the discosoma red fluorescent protein (dsRed), to investigate which viral vector transduced the lateral hypothalamus or the amygdala more efficiently. The LV-dsRed and AAV1-GFP vector were mixed and injected into the lateral hypothalamus or into the amygdala of adult rats. The titers that were injected were 1 x 108 or 1 x 109 genomic copies of AAV1-GFP and 1 x 105 transducing units of LV-dsRed. RESULTS: Immunostaining for GFP and dsRed showed that AAV1-GFP transduced significantly more cells than LV-dsRed in both the lateral hypothalamus and the amygdala. In addition, the number of LV particles that were injected can not easily be increased, while the number of AAV1 particles can be increased easily with a factor 100 to 1000. Both viral vectors appear to predominantly transduce neurons. CONCLUSIONS: This study showed that AAV1 vectors are better tools to overexpress or knockdown genes in the lateral hypothalamus and amygdala of adult rats, since more cells can be transduced with AAV1 than with LV vectors and the titer of AAV1 vectors can easily be increased to transduce the area of interest.

Acute pain increases phosphorylation of DCLK-long in the Edinger-Westphal nucleus but not in the hypothalamic paraventricular nucleus of the rat.

UNLABELLED: The doublecortin-like kinase (DCLK) gene is crucially involved in neuronal plasticity and microtubule-guided retrograde transport of signaling molecules. We have explored the possibility that DCLK is involved in pain-induced signaling events in adult male Wistar rats. Our results show that both DCLK-short and DCLK-long splice variants are present in the cell body and proximal dendrites of neurons in stress-related nuclei, ie, the paraventricular nucleus of the hypothalamus (PVN) and the non-preganglionic Edinger-Westphal nucleus (npEW) in the rostroventral periaqueductal grey. We found that DCLK-long but not DCLK-short is phosphorylated in its serine/proline-rich domain. Furthermore, we demonstrate that phosphorylation of DCLK-long in the npEW is increased by acute pain, whereas DCLK-long phosphorylation in the PVN remains unaffected. This is the first report revealing that DCLK isoforms in the PVN and npEW occur in the adult mammalian brain and that pain differentially affects DCLK-long-mediated neuronal plasticity in these 2 stress-sensitive brain centers. PERSPECTIVE: Pain is a burden for society and the individual, and although the mechanisms underlying pain are relatively well known, its treatment remains difficult and incomplete. Pain stress can lead to diseases like chronic pain and depression. The differential DCLK-phosphorylation in stress-sensitive brain areas is a potential novel therapeutic target in pain research.

Silencing of the microtubule-associated proteins doublecortin-like and doublecortin-like kinase-long induces apoptosis in neuroblastoma cells.

Doublecortin-like kinase-long (DCLK-long) and doublecortin-like (DCL) are two splice variants of DCLK gene. DCL and DCLK-long are microtubule-associated proteins with specific expression in proliferative neural progenitor cells. We have tested the hypothesis that knockdown of DCL/DCLK-long by RNA interference technology will induce cell death in neuroblastoma (NB) cells. First, we analyzed the expression of DCL and DCLK-long in several human neuroblastic tumors, other tumors, and normal tissues, revealing high expression of both DCL and DCLK-long in NB and glioma. Secondly, gene expression profiling revealed numerous differentially expressed genes indicating apoptosis induction after DCL/DCLK-long knockdown in NB cells. Finally, apoptosis was confirmed by time-lapse imaging of phosphatidylserine translocation, caspase-3 activation, live/dead double staining assays, and fluorescence-activated cell sorting. Together, our results suggest that silencing DCL/DCLK-long induces apoptosis in NB cells.

The doublecortin gene family and disorders of neuronal structure.

The doublecortin gene family is associated with subcortical band heterotopia, lissencephaly, epilepsy, developmental dyslexia and retinitis pigmentosa. At least 11 genes homologous to the doublecortin gene exist in humans and mice. Cellular processes regulated by different members of the doublecortin family involve neuronal migration, neurogenesis and eye receptor development. Underlying mechanisms include regulation of cytoskeletal structure and microtubule-based transport. Through their doublecortin-domains, doublecortin proteins can bind microtubules and regulate microtubule-dependent processes. However, this regulation is complex and involves many interacting proteins. Moreover, different spatiotemporal expression patterns and the generation of splice variants further contribute to this complexity. The doublecortin-like kinase 1 gene in particular, produces splice variants with different protein domains such as doublecortin-domains, a serine, threonine and proline-rich domain and a serine/threonine kinase-domain. Here, we review our current knowledge on the doublecortin gene family with an emphasis on proteins interacting with doublecortin domains and other domains. In addition, to generate new hypotheses for further research, we analyzed the serine, threonine and proline-rich domain for predicted protein interactions.