Signaling endosomes and growth cone motility in axon regeneration

FIGNL1 associates with KIF1Bβ and BICD1 to restrict dynein transport velocity during axon navigation

Atkins et al. identify a new role for Fidgetin-like 1 in motor axon navigation via its regulation of bidirectional axonal transport. They show that Fidgetin-like 1 binds Kif1bβ and the opposed polarity-directed motor dynein/dynactin in a molecular complex and controls circuit wiring by reducing dynein velocity in developing motor axons.

Neuronal connectivity relies on molecular motor-based axonal transport of diverse cargoes. Yet the precise players and regulatory mechanisms orchestrating such trafficking events remain largely unknown. We here report the ATPase Fignl1 as a novel regulator of bidirectional transport during axon navigation. Using a yeast two-hybrid screen and coimmunoprecipitation assays, we showed that Fignl1 binds the kinesin Kif1bβ and the dynein/dynactin adaptor Bicaudal D-1 (Bicd1) in a molecular complex including the dynactin subunit dynactin 1. Fignl1 colocalized with Kif1bβ and showed bidirectional mobility in zebrafish axons. Notably, Kif1bβ and Fignl1 loss of function similarly altered zebrafish motor axon pathfinding and increased dynein-based transport velocity of Rab3 vesicles in these navigating axons, pinpointing Fignl1/Kif1bβ as a dynein speed limiter complex. Accordingly, disrupting dynein/dynactin activity or Bicd1/Fignl1 interaction induced motor axon pathfinding defects characteristic of Fignl1 gain or loss of function, respectively. Finally, pharmacological inhibition of dynein activity partially rescued the axon pathfinding defects of Fignl1-depleted larvae. Together, our results identify Fignl1 as a key dynein regulator required for motor circuit wiring.

HDAC5 promotes optic nerve regeneration by activating the mTOR pathway

Neurons in the central nervous system (CNS) regenerate poorly compared to their counterparts in the peripheral nervous system. We previously showed that, in peripheral sensory neurons, nuclear HDAC5 inhibits the expression of regenerative associated genes. After nerve injury, HDAC5 is exported to the cytoplasm to promote axon regeneration. Here we investigated the role of HDAC5 in retinal ganglion cells (RGCs), a CNS neuron which fails to survive and regenerate axons after injury. In contrast to PNS neurons, we found that HDAC5 is mostly cytoplasmic in naïve RGCs and its localization is not affected by optic nerve injury, suggesting that HDAC5 does not directly suppress regenerative associated genes in these cells. Manipulation of the PKCμ pathway, the canonical pathway that regulates HDAC5 localization in PNS neurons by phosphorylating serine 259 and 498, and other pathways that regulate nuclear/cytoplasmic transport, did not affect HDAC5 cytoplasmic localization in RGC. Also, an HDAC5 mutant whose serine 259 and 488 were replaced by alanine (HDAC5^AA) to prevent phosphorylation and nuclear export showed a predominantly cytoplasmic localization, suggesting that HDAC5 resides mostly in the cytoplasm in RGCs. Interestingly, expression of HDAC5^AA, but not HDAC5 wild type, in RGCs in vivo promoted optic nerve regeneration and RGC survival. Mechanistically, we found that HDAC5^AA stimulated the survival and regeneration of RGCs by activating the mTOR pathway. Consistently, the combination of HDAC5^AA expression and the stimulation of the immune system by zymosan injection had an additive effect in promoting robust axon regeneration. These results reveal the potential of manipulating HDAC5 phosphorylation state to activate the mTOR pathway, offering a new therapeutic target to design drugs that promote axon regeneration in the optic nerve.

A novel homozygous AP4B1 mutation in two brothers with AP-4 deficiency syndrome and ocular anomalies

Adaptor protein complex-4 (AP-4) is a heterotetrameric protein complex which plays a key role in vesicle trafficking in neurons. Mutations in genes affecting different subunits of AP-4, including AP4B1, AP4E1, AP4S1, and AP4M1, have been recently associated with an autosomal recessive phenotype, consisting of spastic tetraplegia, and intellectual disability (ID). The overlapping clinical picture among individuals carrying mutations in any of these genes has prompted the terms "AP-4 deficiency syndrome" for this clinically recognizable phenotype. Using whole-exome sequencing, we identified a novel homozygous mutation (c.991C>T, p.Q331*, NM_006594.4) in AP4B1 in two siblings from a consanguineous Pakistani couple, who presented with severe ID, progressive spastic tetraplegia, epilepsy, and microcephaly. Sanger sequencing confirmed the mutation was homozygous in the siblings and heterozygous in the parents. Similar to previously reported individuals with AP4B1 mutations, brain MRI revealed ventriculomegaly and white matter loss. Interestingly, in addition to the typical facial gestalt reported in other AP-4 deficiency cases, the older brother presented with congenital left Horner syndrome, bilateral optic nerve atrophy and cataract, which have not been previously reported in this condition. In summary, we report a novel AP4B1 homozygous mutation in two siblings and review the phenotype of AP-4 deficiency, speculating on a possible role of AP-4 complex in eye development.

ESCRT-II controls retinal axon growth by regulating DCC receptor levels and local protein synthesis

Endocytosis and local protein synthesis (LPS) act coordinately to mediate the chemotropic responses of axons, but the link between these two processes is poorly understood. The endosomal sorting complex required for transport (ESCRT) is a key regulator of cargo sorting in the endocytic pathway, and here we have investigated the role of ESCRT-II, a critical ESCRT component, in Xenopus retinal ganglion cell (RGC) axons. We show that ESCRT-II is present in RGC axonal growth cones (GCs) where it co-localizes with endocytic vesicle GTPases and, unexpectedly, with the Netrin-1 receptor, deleted in colorectal cancer (DCC). ESCRT-II knockdown (KD) decreases endocytosis and, strikingly, reduces DCC in GCs and leads to axon growth and guidance defects. ESCRT-II-depleted axons fail to turn in response to a Netrin-1 gradient in vitro and many axons fail to exit the eye in vivo. These defects, similar to Netrin-1/DCC loss-of-function phenotypes, can be rescued in whole (in vitro) or in part (in vivo) by expressing DCC. In addition, ESCRT-II KD impairs LPS in GCs and live imaging reveals that ESCRT-II transports mRNAs in axons. Collectively, our results show that the ESCRT-II-mediated endocytic pathway regulates both DCC and LPS in the axonal compartment and suggest that ESCRT-II aids gradient sensing in GCs by coupling endocytosis to LPS.

Shank2 Regulates Renal Albumin Endocytosis

Albuminuria is a strong and independent predictor of kidney disease progression but the mechanisms of albumin handling by the kidney remain to be fully defined. Previous studies have shown that podocytes endocytose albumin. Here we demonstrate that Shank2, a large scaffolding protein originally identified at the neuronal postsynaptic density, is expressed in podocytes in vivo and in vitro and plays an important role in albumin endocytosis in podocytes. Knockdown of Shank2 in cultured human podocytes decreased albumin uptake, but the decrease was not statistically significant likely due to residual Shank2 still present in the knockdown podocytes. Complete knockout of Shank2 in podocytes significantly diminished albumin uptake in vitro. Shank2 knockout mice develop proteinuria by 8 weeks of age. To examine albumin handling in vivo in wild-type and Shank2 knockout mice we used multiphoton intravital imaging. While FITC-labeled albumin was rapidly seen in the renal tubules of wild-type mice after injection, little albumin was seen in the tubules of Shank2 knockout mice indicating dysregulated renal albumin trafficking in the Shank2 knockouts. We have previously found that caveolin-1 is required for albumin endocytosis in cultured podocytes. Shank2 knockout mice had significantly decreased expression and altered localization of caveolin-1 in podocytes suggesting that disruption of albumin endocytosis in Shank2 knockouts is mediated via caveolin-1. In summary, we have identified Shank2 as another component of the albumin endocytic pathway in podocytes.

Regulation of intrinsic axon growth ability at retinal ganglion cell growth cones

PURPOSE

Mammalian central nervous system neurons fail to regenerate after injury or disease, in part due to a progressive loss in intrinsic axon growth ability after birth. Whether lost axon growth ability is due to limited growth resources or to changes in the axonal growth cone is unknown.

METHODS

Static and time-lapse images of purified retinal ganglion cells (RGCs) were analyzed for axon growth rate and growth cone morphology and dynamics without treatment and after manipulating Kruppel-like transcription factor (KLF) expression or applying mechanical tension.

RESULTS

Retinal ganglion cells undergo a developmental switch in growth cone dynamics that mirrors the decline in postnatal axon growth rates, with increased filopodial adhesion and decreased lamellar protrusion area in postnatal axonal growth cones. Moreover, expressing growth-suppressive KLF4 or growth-enhancing KLF6 transcription factors elicits similar changes in postnatal growth cones that correlate with axon growth rates. Postnatal RGC axon growth rate is not limited by an inability to achieve axon growth rates similar to embryonic RGCs; indeed, postnatal axons support elongation rates up to 100-fold faster than postnatal axonal growth rates. Rather, the intrinsic capacity for rapid axon growth is due to both growth cone pausing and retraction, as well as to a slightly decreased ability to achieve rapid instantaneous rates of forward progression. Finally, we observed that RGC axon and dendrite growth are regulated independently in vitro.

CONCLUSIONS

Together, these data support the hypothesis that intrinsic axon growth rate is regulated by an axon-specific growth program that differentially regulates growth cone motility.