Summary An intronic GGGGCC repeat expansion in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but its pathogenic mechanism remains unclear. Here we use human induced motor neurons (iMNs) to show that repeat-expanded C9ORF72 is haploinsufficient in ALS. We show that C9ORF72 interacts with endosomes and is required for normal vesicle trafficking and lysosomal biogenesis in motor neurons. Repeat expansion reduces C9ORF72 expression, triggering neurodegeneration through two mechanisms: accumulation of glutamate receptors leading to excitotoxicity, and impaired clearance of neurotoxic dipeptide repeat proteins derived from the repeat expansion. Thus, cooperativity between gain- and loss-of-function mechanisms leads to neurodegeneration. Restoring C9ORF72 levels or augmenting its function with constitutively active RAB5 or chemical modulators of RAB5 effectors rescues patient neuron survival and ameliorates neurodegenerative processes in both gain- and loss-of function C9ORF72 mouse models. Thus, modulating vesicle trafficking can rescue neurodegeneration caused by the C9ORF72 repeat expansion. Coupled with rare mutations in ALS2, FIG4, CHMP2B, OPTN, and SQSTM1, our results reveal mechanistic convergence on vesicle trafficking in ALS/FTD.

PICALM is highly validated genetic risk factor for Alzheimer’s disease (AD). Here, we report that PICALM reductions in AD and murine brain endothelium correlate with amyloid–β (Aβ) pathology and cognitive impairment. Moreover, Picalm deficiency diminishes Aβ clearance across the murine blood–brain barrier (BBB) and accelerates Aβ pathology that is reversible by endothelial PICALM re–expression. Using human brain endothelial monolayer, we show that PICALM regulates PICALM/clathrin–dependent internalization of Aβ bound to the low density lipoprotein receptor related protein–1, a key Aβ clearance receptor, and guides Aβ trafficking to Rab5 and Rab11 leading to Aβ endothelial transcytosis and clearance. PICALM levels and Aβ clearance were reduced in AD–derived endothelial monolayers, which was reversible by adenoviral–mediated PICALM transfer. iPSC–derived human endothelial cells carrying the rs3851179 protective allele exhibited higher PICALM levels and enhanced Aβ clearance. Thus, PICALM regulates Aβ BBB transcytosis and clearance that has implications for Aβ brain homeostasis and clearance therapy.

Divergent natural selection acting on ecological traits, which also affect mate choice, is a key element of ecological speciation theory, but has not previously been demonstrated at the molecular gene level to our knowledge. Here we demonstrate parallel evolution in two cichlid genera under strong divergent selection in a gene that affects both. Strong divergent natural selection fixed opsin proteins with different predicted light absorbance properties at opposite ends of an environmental gradient. By expressing them and measuring absorbance, we show that the reciprocal fixation adapts populations to divergent light environments. The divergent evolution of the visual system coincides with divergence in male breeding coloration, consistent with incipient ecological by-product speciation.